RU2836819C2 - Polytetrafluoroethylene powder, electrode binder, electrode mixture, electrode and secondary storage battery - Google Patents

Abstract

FIELD: electric elements.

SUBSTANCE: invention relates to polytetrafluoroethylene powders, electrode binder, electrode mixtures, electrodes and secondary storage batteries. Invention provides polytetrafluoroethylene powder intended for use as electrode binder, wherein polytetrafluoroethylene powder is characterized by standard specific gravity of 2.200 or less and does not contain water.

EFFECT: reduction or prevention of gas release inside the battery element, improved characteristics of the battery, increased strength of the electrode.

10 cl, 5 tbl, 11 ex

Description

AREA OF TECHNOLOGY

[0001] The invention relates to polytetrafluoroethylene powders, electrode binders, electrode mixtures, electrodes and secondary batteries.

LEVEL OF TECHNOLOGY

[0002] Secondary batteries such as lithium-ion batteries are used in small and portable electrical and electronic devices such as laptops, cell phones, smart phones, tablet PCs and ultrabooks, and they are also presented in the market as a variety of power sources, including power supplies for cars and the like, as well as high-power stationary power supplies. The reason is that secondary batteries are high-voltage batteries with high energy density, low self-discharge, low memory effect, and can be very light. At present, secondary batteries with even greater energy density are in demand, and improvement in other parameters is also desirable.

[0003] Patent Document 1 describes an energy storage device in which at least a cathode or an anode comprises a polytetrafluoroethylene composite binder.

[0004] Each of Patent Documents 2 and 3 describes the use of an aqueous dispersion of polytetrafluoroethylene as a binder for batteries.

List of references

Patent literature

[0005]

Patent Document 1: JP 2017-517862 T.

Patent Document 2: JP 2004-31179 A.

Patent Document 3: JP H11-343317 A.

ESSENCE OF THE INVENTION

Technical task

[0006] An object of the present invention is to provide a polytetrafluoroethylene powder for an electrode binder, which is capable of not only reducing or preventing gas evolution inside a battery element and deterioration of battery performance, but also increasing the strength of the electrode; electrode binder, electrode mixture, electrode and secondary storage battery.

Solution of the given problem

[0007] In paragraph (1) of the disclosure, there is provided a polytetrafluoroethylene powder for use as an electrode binder, wherein the polytetrafluoroethylene powder has a standard specific gravity of 2.200 or less and is substantially free of water.

[0008] In paragraph (2), the invention provides an electrode binder consisting essentially of a polytetrafluoroethylene powder, wherein the polytetrafluoroethylene powder has a standard specific gravity of 2.200 or less and is essentially free of water.

[0009] In paragraph (3), the invention relates to an electrode binder of paragraph (2), in which the polytetrafluoroethylene powder contains water in an amount of 0.050% by weight or less.

[0010] In paragraph (4), the invention relates to an electrode binder according to paragraph (2) or (3), wherein the polytetrafluoroethylene powder is characterized by an extrusion pressure at a compression ratio of 100 10 MPa or more.

[0011] In paragraph (5), the invention relates to an electrode binder in any combination with any of paragraphs (2) to (4), in which the polytetrafluoroethylene powder is expandable.

[0012] In paragraph (6), the invention relates to an electrode binder in any combination with any of paragraphs (2) to (5), in which the polytetrafluoroethylene comprises a tetrafluoroethylene unit and a modifying monomer unit, wherein the modifying monomer is capable of copolymerization with the tetrafluoroethylene.

[0013] In paragraph (7), the invention relates to an electrode binder according to paragraph (6), wherein the modifying monomer comprises at least one monomer selected from the group consisting of perfluoro(methyl vinyl ether) and hexafluoropropylene.

[0014] In paragraph (8), the invention relates to an electrode binder in any combination with any of paragraphs (2) to (7), wherein the polytetrafluoroethylene powder is characterized by an average primary particle size of from 100 to 350 nm.

[0015] In paragraph (9), the invention relates to an electrode mixture comprising: polytetrafluoroethylene powder of paragraph (1) or an electrode binder in any combination with any of paragraphs (2) to (8); and an active electrode material.

[0016] In paragraph (10), the invention provides an electrode comprising: polytetrafluoroethylene powder of paragraph (1) or an electrode binder in any combination with any of paragraphs (2) to (8); an active electrode material; and a current collector.

[0017] In paragraph (11), the invention provides a secondary storage battery comprising an electrode of paragraph (10).

Advantages of the invention

[0018] The present invention provides a polytetrafluoroethylene powder for an electrode binder capable of not only reducing or preventing gas generation inside a battery element and deterioration of battery performance, but also improving the strength of an electrode; an electrode binder, an electrode mixture, an electrode, and a secondary storage battery.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0019] The invention is now described in detail.

[0020] The invention provides a polytetrafluoroethylene (PTFE) powder intended for use as an electrode binder, wherein the PTFE powder has a standard specific gravity of 2.200 or less and is substantially free of water.

The PTFE powder of the present invention corresponding to the above, when used as an electrode binder, can reduce or prevent gas evolution inside a battery element and deterioration of battery characteristics (for example, a decrease in capacity during storage at a high temperature) and improve the strength of the electrode. Since the PTFE powder of the present invention substantially does not contain water, the active material of the electrode used in combination with it can be selected from a wide range of materials having advantages in terms of production technology. The PTFE powder of the present invention can be mixed with the active material of the electrode as is, so there is no need to use organic solvents, thereby simplifying the process and eliminating the cost of organic solvents. In addition, the PTFE powder of the present invention has a very high adhesive force with active materials, so a smaller amount of PTFE powder can be used.

[0021] The PTFE powder of the present invention has a standard specific gravity (SSG) of 2.200 or less. The PTFE powder with an SSG in the specified range is capable of further reducing or preventing gas generation inside a battery element and deterioration of battery performance, and increasing the strength of an electrode.

The SSG is preferably 2.180 or less, more preferably 2.170 or less, still more preferably 2.160 or less, still more preferably 2.150 or less, still more preferably 2.145 or less, particularly preferably 2.140 or less.

Preferably, the SSG is 2.130 or more.

SSG is determined by the water displacement method in accordance with ASTM D792 using a specimen cast in accordance with ASTM D4895.

[0022] The PTFE powder of the present invention is preferably characterized by an average primary particle size of 100 to 350 nm. With an average primary particle size in the specified range, PTFE has a high molecular weight, which provides a high adhesive force and increased flexibility of the electrode.

The average particle size of the primary particles is more preferably 330 nm or less, even more preferably 320 nm or less, even more preferably 300 nm or less, even more preferably 280 nm or less, particularly preferably 250 nm or less, while more preferably 150 nm or more, even more preferably 170 nm or more, even more preferably 200 nm or more.

The average size of primary particles is measured by the following method.

An aqueous dispersion of PTFE is diluted with water to a solid phase concentration of 0.15% by weight. The transmission of light with a wavelength of 550 nm per unit length of the diluted dispersion is measured. Feret diameters are measured on the image obtained using a transmission electron microscope and the numerical average particle size by length is determined based on them. These values are used to plot a calibration curve. Using the calibration curve, based on the transmission of incident light with a wavelength of 550 nm measured for each sample, the average numerical particle size is determined, which is taken as the average size of the primary particles.

[0023] The PTFE powder of the present invention may have an average secondary particle size of 350 μm or more, preferably 400 μm or more, more preferably 450 μm or more, still more preferably 500 μm or more, still more preferably 550 μm or more, particularly preferably 600 μm or more, further preferably 1000 μm or less, more preferably 900 μm or less, still more preferably 800 μm or less, still more preferably 700 μm or less.

The average size of secondary particles is measured in accordance with JIS K 6891.

[0024] In view of further reducing or preventing gas evolution and deterioration of battery performance, and also to provide greater adhesive force, strength and flexibility of the electrode, the PTFE powder of the present invention is preferably characterized by an extrusion pressure at a reduction ratio (RR) of 100 10 MPa or more, more preferably 12 MPa or more, even more preferably 15 MPa or more, even more preferably 16 MPa or more, particularly preferably 17 MPa or more.

From the viewpoint of greater processability, the extrusion pressure at RR 100 is preferably 50 MPa or less, more preferably 40 MPa or less, still more preferably 35 MPa or less, still more preferably 30 MPa or less, still more preferably 25 MPa or less, still more preferably 21 MPa or less, particularly preferably 20 MPa or less.

[0025] In view of further reducing or preventing gas evolution and deterioration of battery performance, and also to provide greater adhesive force, strength and flexibility of the electrode, the PTFE powder of the present invention preferably has an extrusion pressure at RR 300 of preferably 18 MPa or more, more preferably 23 MPa or more, even more preferably 25 MPa or more, even more preferably 28 MPa or more, even more preferably 30 MPa or more, particularly preferably 32 MPa or more.

From the viewpoint of greater processability, the extrusion pressure at RR 300 is preferably 45 MPa or less, more preferably 40 MPa or less.

[0026] The extrusion pressure at RR 100 is measured by the following method.

Fifty grams of PTFE powder and 10.25 g of hydrocarbon oil (Isopar-E, ExxonMobil Corporation) as an extrusion aid are mixed for three minutes in a polyethylene container. The extruder barrel is filled with the mixture at room temperature (25±2°C). A load of 0.47 MPa is applied to the piston inserted into the barrel and maintained for one minute. The mixture is then extruded through the extruder head at a piston speed of 18 mm/min. The ratio of the cross-sectional area of the barrel to the cross-sectional area of the extruder head (compression ratio) is 100. The value obtained by dividing the load (N) at which the pressure in the second half of the extrusion operation is balanced by the cross-sectional area of the extruder is taken as the extrusion pressure (MPa).

[0027] The extrusion pressure at RR 300 is measured by the following method.

Fifty grams of PTFE powder and 11.00 g of hydrocarbon oil (Isopar-E, ExxonMobil Corporation) as an extrusion aid are mixed for three minutes in a polyethylene container. The extruder barrel is filled with the mixture at room temperature (25±2°C). A load of 0.47 MPa is applied to the piston inserted into the barrel and maintained for one minute. The mixture is then extruded through the extruder head at a piston speed of 18 mm/min. The ratio of the cross-sectional area of the barrel to the cross-sectional area of the extruder head (compression ratio) is 300. The value obtained by dividing the load (N) at which the pressure in the second half of the extrusion operation is balanced by the cross-sectional area of the extruder is taken as the extrusion pressure (MPa).

[0028] In view of further reducing or preventing gas evolution and deterioration of battery performance, and also to provide greater adhesive force, strength and flexibility of the electrode, the PTFE powder of the present invention is preferably expandable.

The term "expandable" means that an expanded product can be obtained by the expansion test described below.

The rod obtained by extrusion of the paste at the RR of 100 described above is dried at 230°C for 30 min to remove the slip. The dried rod is cut into pieces of the appropriate length and placed in an oven heated to 300°C. In the oven, the pieces are subjected to expansion at a rate of 100%/sec.

[0029] In view of further reducing or preventing gas evolution and deterioration of battery performance, and also to provide greater adhesive force, strength and flexibility of the electrode, the PTFE powder of the present invention is preferably expanded to increase in size by 25 times.

Whether it has the ability to expand 25 times or not can be determined by the following expansion test.

The rod obtained by extruding the paste at RR equal to 100 as described above is dried at 230°C for 30 min to remove the slip. The dried rod is cut into pieces of the appropriate length and placed in an oven heated to 300°C. In the oven, the pieces are expanded at a rate of 100%/s until their length increases to 25 times their length before the expansion test. If no fracture occurs during the expansion, the rod is considered capable of expanding to 25 times its size.

[0030] From the viewpoint of processability, the aspect ratio of the PTFE powder of the present invention may be 2.0 or less, preferably 1.8 or less, more preferably 1.7 or less, still more preferably 1.6 or less, still more preferably 1.5 or less, still more preferably 1.4 or less, still more preferably 1.3 or less, particularly preferably 1.2 or less, most preferably 1.1 or less. The average aspect ratio may also be 1.0 or more.

The average aspect ratio is determined as follows. PTFE powder or an aqueous dispersion of PTFE powder diluted to a solid phase concentration of about 1% by weight is examined using a scanning electron microscope (SEM) and an image of 200 or more randomly selected particles is obtained. The average ratio of the major axis to the minor axis of these particles is determined and taken as the average aspect ratio.

[0031] From the viewpoint of workability, the apparent density of the PTFE powder of the present invention is preferably 0.40 g/ml or more, more preferably 0.43 g/ml or more, still more preferably 0.45 g/ml or more, still more preferably 0.48 g/ml or more, particularly preferably 0.50 g/ml or more. The upper limit is not limited and may be 0.70 g/ml.

The apparent density is determined in accordance with JIS K 6892.

[0032] The PTFE powder of the present invention is preferably recyclable without melting. Recyclable without melting means a property of the polymer that the melt flow rate at a temperature higher than the melting point cannot be measured in accordance with ASTM D1238 and D2116, in other words, the polymer does not have sufficient flowability even at a temperature in the melting range.

[0033] PTFE may be a homopolymer of tetrafluoroethylene (TFE) or a modified PTFE comprising a polymerized TFE-based unit (TFE unit) and a polymerized modifying monomer-based unit (hereinafter also referred to as a "modifying monomer unit"). The modified PTFE may contain 99.0% by weight or more TFE units and 1.0% by weight or more modifying monomer units. The modified PTFE may consist of TFE units and modifying monomer units.

In view of further reducing or preventing gas evolution and deterioration of battery performance, and to provide greater adhesive force, strength and flexibility of the electrode, the PTFE is preferably modified PTFE.

[0034] In view of further reducing or preventing gas evolution and deterioration of battery performance, and to provide greater adhesive force, strength and flexibility of the electrode, PTFE preferably contains modifying monomer units in an amount ranging from 0.00001 to 1.0 mass % relative to all polymerized units. The lower limit of the amount of modifying monomer units is more preferably 0.0001 mass %, even more preferably 0.001 mass %, even more preferably 0.005 mass %, even more preferably 0.010 mass %. The upper limit of the amount of units of the modifying monomer is preferably 0.90% by mass, more preferably 0.80% by mass, even more preferably 0.50% by mass, even more preferably 0.40% by mass, even more preferably 0.30% by mass, even more preferably 0.20% by mass, even more preferably 0.15% by mass, even more preferably 0.10% by mass, even more preferably 0.08% by mass, particularly preferably 0.05% by mass, most preferably 0.03% by mass.

In this case, the links of the modifying monomer are understood to be the part of the PTFE molecular structure formed from the modifying monomer.

[0035] The above amounts of polymerized units can be determined by any combination of NMR, FTIR, elemental analysis and X-ray fluorescence analysis methods according to the types of monomers.

[0036] The modifying monomer may be any monomer capable of copolymerization with TFE. Examples thereof are perfluoroolefins such as hexafluoropropylene (HFP); hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride (VDF); perhaloolefins such as chlorotrifluoroethylene; perfluorovinyl ether; perfluoroallyl ether; (perfluoroalkyl)ethylene; and ethylene. These modifying monomers may be used alone or in combination.

[0037] The perfluorovinyl ether may be, inter alia, an unsaturated perfluoro compound corresponding to the following formula (A):

_CF2 =CF-ORf (A)

where Rf denotes a perfluoroorganic group. The term "perfluoroorganic group" refers to an organic group in which all hydrogen atoms bonded to any carbon atom are replaced by fluorine atoms. The perfluoroorganic group may include ether oxygen.

[0038] One example of a perfluorovinyl ether is perfluoro(alkyl vinyl ether) (PAVE) corresponding to formula (A), in which Rf is a C1-C10 perfluoroalkyl group. The perfluoroalkyl group preferably includes from 1 to 5 carbon atoms.

[0039] Examples of the perfluoroalkyl group in PAVE are perfluoromethyl group, perfluoroethyl group, perfluoropropyl group, perfluorobutyl group, perfluoropentyl group, and perfluorohexyl group.

[0040] Examples of perfluorovinyl ether also include:

compounds of formula (A) in which Rf is a perfluoro(alkoxyalkyl) group;

compounds of formula (A) in which Rf is a group corresponding to the following formula:

[0041] Scheme 1

[0042] where m is zero or an integer from 1 to 4; and

compounds of formula (A) in which Rf is a group corresponding to the following formula:

[0043] Scheme 2

[0044] where n is an integer from 1 to 4.

[0045] Examples of (perfluoroalkyl)ethylene (PFAE) include, but are not limited to, (perfluorobutyl)ethylene (PFBE) and (perfluorohexyl)ethylene.

[0046] An example of a perfluoroallyl ether is a fluoromonomer corresponding to the following formula (B):

_CF2 =CF- _CF2 - ^ORf1 (B)

where Rf ¹ denotes a perfluoroorganic group.

[0047] Rf ¹ is preferably a C1-C10 perfluoroalkyl group or a C1-C10 perfluoroalkoxyalkyl group. The perfluoroallyl ether preferably comprises at least _one compound selected from the group consisting of _CF2 =CF- _CF2 -O- _CF3 , CF2= _CF -CF2 _{- OC2F5} , _CF2 =CF- _{CF2 - OC3F7} and _CF2 =CF- _CF2 - _OC4F9 , more preferably comprises at least _one compound selected from _the group consisting of _CF2 =CF- _CF2 - _OC2F5 , _CF2 =CF- _CF2 - _OC3F7 and _{CF2 =} CF- _CF2 - _{OC4F9 ,} even more preferably is _CF2 =CF _{- CF2} - _O - _CF2CF2CF3 .

[0048] In view of providing increased expandability, greater adhesive strength and improved flexibility of the electrode, the modifying monomer preferably comprises at least one compound selected from the group consisting of PAVE and HFP, more preferably at least one compound selected from the group consisting of perfor(methyl vinyl ether) (PMVE) and HFP.

[0049] To create an electrode mixture sheet of even greater strength, the other modifying monomer preferably comprises at least one compound selected from the group consisting of VDF, HFP, CTFE and PAVE, more preferably at least one compound selected from the group consisting of VDF, HFP and CTFE.

In view of providing improved heat resistance, in one preferred embodiment, PTFE comprises a TFE unit, a VDF unit and an HFP unit, wherein the total amount of VDF and HFP units is 1.0 mass% or less relative to all polymerized units.

[0050] PTFE may have a core-shell structure. One example of PTFE with a core-shell structure is a modified PTFE comprising a particle core of high-molecular PTFE and a shell of low-molecular PTFE or modified PTFE. One example of such a modified PTFE is the PTFE described in JP 2005-527652 T.

[0051] In view of creating an electrode mixture sheet of even greater strength, PTFE preferably has an endothermic peak temperature of 320°C or higher, more preferably 325°C or higher, even more preferably 330°C or higher, even more preferably 335°C or higher, even more preferably 340°C or higher, even more preferably 342°C or higher, particularly preferably 344°C or higher. The endothermic peak temperature is preferably 350°C or lower.

The endothermic peak temperature corresponds to the temperature at the minimum point on the heat of fusion curve obtained by differential scanning calorimetry (DSC) at a temperature increase rate of 10 °C/min for fluororubber that has never been heated before, to a temperature of 300 °C or higher. If there are two or more minimum points in one melting peak, the temperatures corresponding to these points are considered to be endothermic peak temperatures.

[0052] Preferably, the PTFE is characterized by at least one endothermic peak in the range from 333°C to 347°C in a heat of fusion curve obtained by increasing the temperature at a rate of 10°C/min using a differential scanning calorimeter (DSC), and an enthalpy of fusion calculated from the heat of fusion curve of 62 mJ/mg or higher at a temperature from 290°C to 350°C.

[0053] In view of creating an electrode mixture sheet of even greater strength, PTFE preferably has a number average molecular weight (Mn) of 3.0×10 ⁶ or more, more preferably 3.2×10 ⁶ or more, still more preferably 3.5×10 ⁶ or more, still more preferably 3.7×10 ⁶ or more, particularly preferably 4.0×10 ⁶ or more. The number average molecular weight is preferably 7.0×10 ⁶ or less, more preferably 6.5×10 ⁶ or less, still more preferably 6.0×10 ⁶ or less, still more preferably 5.5×10 ⁶ or less, particularly preferably 5.0×10 ⁶ or less.

The number average molecular weight is the molecular weight determined by the method described in the article below, based on the heat of crystallization estimated by measuring the temperature drop using DSC on molten fluororubber. The measurements are carried out five times and the average value of three measurements is used, excluding the maximum and minimum values.

Article: Suwa, T.; Takehisa, M.; Machi, S., J. Appl. Polym. Sci. vol. 17, pp. 3253 (1973).

[0054] The PTFE powder of the present invention substantially does not contain water. Such PTFE powder can further reduce or prevent gas emission and deterioration of battery performance and increase the strength of the electrode. In addition, the active material of the electrode used in combination therewith can be selected from a wide range of materials having advantages in terms of production technology. The expression "substantially does not contain water" means that the water content of the PTFE powder is less than 0.050% by mass.

The water content is preferably 0.040% by mass or less, more preferably 0.020% by mass or less, still more preferably 0.010% by mass or less, still more preferably 0.005% by mass or less, particularly preferably 0.002% by mass or less.

Water content is determined by the following method.

PTFE powder is weighed before and after heating at 150ºC for two hours and the water content is calculated using the following formula. A powder sample is taken three times, the calculation is carried out for each sample and the obtained values are averaged.

Water content (% by mass) = [(mass (g) of PTFE powder before heating) - (mass (g) of PTFE powder after heating)] / (mass (g) of PTFE powder before heating) × 100

[0055] The PTFE powder of the present invention preferably substantially does not contain fluorine-containing compounds with a molecular weight of 1,000 or less. The expression "substantially does not contain fluorine-containing compounds" means that the amount of the fluorine-containing compound in the PTFE powder is 25 parts per billion by mass or less.

The amount of the fluorine-containing compound is more preferably less than 25 parts per billion by mass, even more preferably 10 parts per billion by mass or less, even more preferably 5 parts per billion by mass or less, particularly preferably 3 parts per billion by mass or less, even more preferably 1 part per billion by mass or less. The lower limit is not limited, it may be an amount less than the detection limit.

[0056] The amount of a fluorine-containing compound with a molecular weight of 1000 or less is measured by the following method.

To 1 g of the weighed sample, 10 g (12.6 ml) of methanol are added, followed by sonication for 60 minutes, thereby obtaining an extract. The concentration of this extract is appropriately increased by purging with nitrogen, and the fluorine-containing compound in the concentrated extract is analyzed by a liquid chromatograph/mass spectrometer (LC/MS). Based on the molecular weight information determined from the obtained LC/MS spectrum, the compatibility with the structural formulas of possible fluorine-containing compounds is checked. Aqueous solutions with five or more different concentrations of the standard substance are prepared, and LC/MS analysis of these aqueous solutions is carried out. A plot of the ratio between amount and area versus amount is plotted, and a calibration curve is obtained. Based on the obtained calibration curve, the peak area of the fluorine-containing compound in the LC/MS chromatogram of the extract is converted into the amount of the fluorine-containing compound.

The lower detection limit of this measurement method is 10 parts per billion by mass.

[0057] The amount of a fluorine-containing compound with a molecular weight of 1000 or less can also be measured by the following method.

To 1 g of the weighed sample, 10 g (12.6 ml) of methanol are added, then sonicated for 2 hours, kept at room temperature, and the solid phase is removed, thereby obtaining an extract. The concentration of this extract is appropriately increased by purging with nitrogen, and the fluorine-containing compound in the concentrated extract is subjected to LC/MS analysis. Based on the molecular weight information established from the obtained LC/MS spectrum, compatibility with structural formulas of possible fluorine-containing compounds is checked. Standard solutions in methanol with five known concentrations of the fluorine-containing compound are prepared and analyzed by LC/MS. A calibration curve is constructed by first-order approximation based on the concentration of the standard solutions in methanol and the integral values of the peaks in each concentration range. The amount of fluorine-containing compound in the extract is determined from the calibration curve and converted into the amount of fluorine-containing compound in the sample.

The lower detection limit of this measurement method is 1 part per billion by mass.

[0058] An example of a fluorine-containing compound having a molecular weight of 1,000 or less is a fluorine-containing compound including a hydrophilic group and having a molecular weight of 1,000 g/mol or less. The molecular weight of this fluorine-containing compound is preferably 800 or less, more preferably 500 or less.

Polymerized particles obtained by polymerization in the presence of a fluorinated surfactant usually include a fluorinated surfactant in addition to PTFE. In this case, the fluorinated surfactant is used in the polymerization.

The fluorine-containing compound having a molecular weight of 1,000 or less may be a compound that is not added during polymerization, such as a compound formed as a by-product during polymerization.

When the fluorine-containing compound includes an anionic part and a cationic part, the fluorine-containing compound with a molecular weight of 1,000 or less is called a fluorine-containing compound in which the anionic part has a molecular weight of 1,000 or less. The fluorine-containing compound with a molecular weight of 1,000 or less does not include PTFE.

[0059] The hydrophilic group may be, for example, -COOM, -SO ₂ M or -SO ₃ M, examples thereof are anionic groups such as -COOM and -SO ₃ M, in which M is H, a metal atom, NR ¹ ₄ , imidazole, optionally substituted, pyridine, optionally substituted, or phosphonium, optionally substituted; R ¹ is H or an organic group.

[0060] The fluorosurfactant may also be a fluorosurfactant in which the anionic portion has a molecular weight of 1000 or less (anionic fluorosurfactant). The "anionic portion" means a portion of the fluorosurfactant without a cation. For example, in the case of F(CF ₂ ) _n1 COOM, the anionic portion is called "F(CF ₂ ) _n1 COO".

One example of a fluorinated anionic surfactant is a compound corresponding to the following formula (N ⁰ ):

X ⁿ⁰ -Rf ⁿ⁰ -Y ⁰ (N ⁰ )

where X ⁿ⁰ is H, Cl and/or F; Rf ⁿ⁰ is a linear, branched or cyclic alkylene group C3-C20 in which part or all of the H atoms are replaced by F atoms, the alkylene group possibly comprising one or more ether linkages with one or more H atoms optionally replaced by Cl; Y ⁰ is an anionic group.

The anionic group ^Y0 may be -COOM, _-SO2M , or _-SO3M , and may be -COOM or _-SO3M .

M is H, a metal atom, NR ¹ ₄ , imidazole, optionally substituted, pyridine, optionally substituted, or phosphonium, optionally substituted; R ¹ is H or an organic group.

Examples of metal atom are alkali metal (Group 1) and alkaline earth metal (Group 2), such as Na, K and Li.

R ¹ may be H or an organic group C1-C10, may be H or an organic group C1-C4, may be H or an alkyl group C1-C4.

M may be H, a metal atom, or NR ¹ ₄ , may be H, an alkali metal (Group 1), an alkaline earth metal (Group 2), or NR ¹ ₄ , and may be H, Na, K, Li, NH ₄ .

In Rf ⁿ⁰ 50% or more of the H atoms can be replaced by fluorine.

[0061] The fluorosurfactant may be a single fluorosurfactant or a mixture of two or more fluorosurfactants.

[0062] Examples of fluorinated surfactants are compounds corresponding to the following formulas. A fluorinated surfactant may be a mixture of these compounds:

F( _CF2 ) _7COOM ,

F( _CF2 ) _5COOM ,

H( _CF2 ) _6COOM ,

H( _CF2 ) _7COOM ,

CF ₃ O(CF ₂ ) ₃ OCHFCF ₂ COOM,

C ₃ F ₇ OCF(CF ₃ )CF ₂ OCF(CF ₃ )COOM,

CF ₃ CF ₂ CF ₂ OCF(CF ₃ )COOM,

CF ₃ CF ₂ OCF ₂ CF ₂ OCF ₂ COOM,

C ₂ F ₅ OCF(CF ₃ )CF ₂ OCF(CF ₃ )COOM,

CF ₃ OCF(CF ₃ )CF ₂ OCF(CF ₃ )COOM,

CF ₂ ClCF ₂ CF ₂ OCF(CF ₃ )CF ₂ OCF ₂ COOM,

CF ₂ ClCF ₂ CF ₂ OCF ₂ CF(CF ₃ )OCF ₂ COOM,

CF ₂ ClCF(CF ₃ )OCF(CF ₃ )CF ₂ OCF ₂ COOM,

CF ₂ ClCF(CF ₃ )OCF ₂ CF(CF ₃ )OCF ₂ COOM and

Scheme 3

In each of the formulas, M represents H, a metal atom, NR ¹ ₄ , imidazole, optionally substituted, pyridine, optionally substituted, or phosphonium, optionally substituted; R ¹ represents H or an organic group.

The PTFE powder of the present invention preferably substantially does not contain any of the fluorine-containing compounds corresponding to the above formulae.

[0063] In each of the above formulas, M may be H, a metal atom, or NR ¹ ₄ , may be H, an alkali metal (Group 1), an alkaline earth metal (Group 2), or NR ¹ ₄ , or may be H, Na, K, Li, NH ₄ .

R ¹ may be H or an organic group C1-C10, may be H or an organic group C1-C4, may be H or an alkyl group C1-C4.

[0064] The PTFE powder of the present invention, substantially free from any of the fluorine-containing compounds corresponding to the above formulas, is capable of further reducing or preventing gas emission and deterioration of battery performance and increasing the strength of the electrode.

The expression "substantially free of any of the fluorine-containing compounds corresponding to the above formulas" means that the amount of any of these fluorine-containing compounds in the PTFE powder is 25 parts per billion by weight or less.

The amount of any of these fluorine-containing compounds is preferably less than 25 parts per billion by weight, more preferably 10 parts per billion by weight or less, even more preferably 5 parts per billion by weight or less, particularly preferably 3 parts per billion by weight or less, still more preferably 1 part per billion by weight or less. The lower limit is not limited and may be less than the detection limit.

[0065] The PTFE powder of the present invention also preferably substantially does not include a fluorine-containing compound corresponding to the following formula:

[C _n-1 F _2n-1 COO ^- ]M ⁺

where n is an integer from 9 to 14, preferably from 9 to 12; M ⁺ is a cation. Such PTFE powder is capable of further reducing or preventing gas emission and deterioration of battery performance and increasing the strength of the electrode.

The M that forms the M ⁺ cation in this formula is similar to the M described above.

The expression “substantially does not include a fluorine-containing compound corresponding to the formula” means that the amount of this fluorine-containing compound in the PTFE powder is 25 parts per billion by weight or less.

The amount of the fluorine-containing compound is preferably less than 25 parts per billion by mass, more preferably 10 parts per billion by mass or less, even more preferably 5 parts per billion by mass or less, particularly preferably 3 parts per billion by mass or less, still more preferably less than 1 parts per billion by mass or less. The lower limit is not limited, it may be an amount less than the detection limit.

[0066] The PTFE powder of the present invention can be produced, for example, by a method comprising the following steps: (A) preparing an aqueous dispersion of PTFE; (B) coagulating the aqueous dispersion to obtain a wet PTFE powder; and (C) drying the wet powder.

[0067] In step (A), the aqueous dispersion can be obtained, for example, by emulsion polymerization.

[0068] Emulsion polymerization can be carried out by a known method. For example, emulsion polymerization of monomers required for forming PTFE is carried out in the presence of a fluorine-containing anionic surfactant and a polymerization initiator in an aqueous medium, thereby an aqueous dispersion containing PTFE particles (primary particles) can be obtained. In the emulsion polymerization, additives such as a chain transfer agent, a buffer, a pH adjuster, an auxiliary stabilizer, a dispersion stabilizer, or a radical scavenger can be used if necessary.

By properly selecting the emulsion polymerization conditions, it is possible to achieve SSG within the above range.

[0069] Step (A) may be a step of emulsion polymerization of TFE and, if desired, a modifying monomer.

[0070] Emulsion polymerization can be carried out, for example, in an aqueous medium in the presence of a fluorine-containing anionic surfactant and a polymerization initiator.

Emulsion polymerization can be carried out, for example, by loading an aqueous medium, a fluorine-containing anionic surfactant, a monomer, and, if necessary, other additives into a polymerization reactor, stirring the contents of the reactor, maintaining a predetermined polymerization temperature in the reactor, and then adding a predetermined amount of a polymerization initiator to start the polymerization reaction. Depending on the needs, after the polymerization reaction has started, a monomer, a polymerization initiator, a chain transfer agent, a surfactant, etc. can be added.

[0071] The polymerization initiator may be any polymerization initiator capable of generating radicals at the polymerization temperature; any known oil-soluble and/or water-soluble polymerization initiator may be used. In addition, the initiator may be combined with a reducing agent, for example in the form of an oxidation-reduction agent, to initiate the polymerization. The concentration of the polymerization initiator is appropriately determined in accordance with the type of monomer, the target molecular weight of PTFE and the reaction rate.

[0072] The polymerization initiator used may be an oil-soluble radical polymerization initiator or a water-soluble radical polymerization initiator.

[0073] The oil-soluble radical polymerization initiator may be a known oil-soluble peroxide. Typical examples thereof are dialkyl peroxycarbonates such as diisopropyl peroxydicarbonate and di-sec-butyl peroxydicarbonate, peroxyesters such as tert-butyl peroxyisobutyrate and tert-butyl peroxypivalate, dialkyl peroxides such as di-tert-butyl peroxide, and di(perfluoroacyl) or di(fluorochloro)acyl peroxides such as di(ω-hydrododecafluoroheptanoyl) peroxide, di(ω-hydrotetradecafluoroheptanoyl) peroxide, di(ω-hydrohexadecafluorononanoyl) peroxide, di(perfluorobutyryl) peroxide, di(perfluorovaleryl) peroxide, di(perfluorohexanoyl) peroxide, di(perfluoroheptanoyl) peroxide, di(perfluorooctanoyl) peroxide, di(perfluorononanoyl) peroxide, Di(ω-chlorohexafluorobutyryl)peroxide, di(ω-chlorodecafluorohexanoyl)peroxide, di(ω-chlorotetradecafluorooctanoyl)peroxide, ω-hydrododecafluoroheptanoyl-ω-hydrohexadecafluorononanoyl peroxide, ω-chlorohexafluorobutyryl-ω-chlorodecafluorohexanoyl peroxide, ω-hydrododecafluoroheptanoyl-perfluorobutyryl peroxide, di(dichloropentafluorobutanoyl)peroxide, di(trichlorooctafluorohexanoyl)peroxide, di(tetrachloroundecafluorooctanoyl)peroxide, di(pentachlorotetradecafluorodecanoyl)peroxide and di(undecachlorotriacontafluorodocosanoyl)peroxide.

[0074] The water-soluble radical polymerization initiator may be a known water-soluble peroxide. Examples thereof are ammonium, potassium and sodium salts and any of the following acids: persulfuric acid, perboric acid, perchloric acid, perphosphoric acid and percarboxylic acid, tert-butyl permaleate, tert-butyl hydroperoxide and disuccinic acid peroxide. Of these, ammonium persulfate and disuccinic acid peroxide are preferred. The initiator may also include a reducing agent such as sulfite or a salt of sulfurous acid. Its amount may be a multiple of 0.1 to 20 of the amount of peroxide.

[0075] The water-soluble radical polymerization initiator can be added in any amount. For example, it can be added in an amount that does not lead to a significant decrease in the polymerization rate (for example, in a concentration of several parts per million relative to the amount of water), or more at one time at the initial stage of polymerization or periodically or continuously. The upper limit of this amount corresponds to the range in which the reaction temperature can increase until the heat released during the polymerization reaction is removed through the surfaces of the device. More preferably, the upper limit corresponds to the range in which the heat of the polymerization reaction can be removed through the surfaces of the device.

In order to easily ensure the physical conditions described above, the added amount of the polymerization initiator is preferably equivalent to 0.1 ppm or more, more preferably equivalent to 1.0 ppm or more, and, further, preferably equivalent to 100 ppm or less, more preferably equivalent to 10 ppm or less, relative to an aqueous medium.

[0076] For example, in the case of polymerization at a low temperature of 30°C or less, the polymerization initiator used is preferably a redox initiator which is a combination of an oxidizing agent and a reducing agent. Examples of the oxidizing agent are persulfate, organic peroxide, potassium permanganate, manganese triacetate, cerium ammonium nitrate and bromate. Examples of the reducing agent are sulfite, bisulfite, bromate, diimine and oxalic acid. Examples of the persulfate are ammonium persulfate and potassium persulfate. Examples of the sulfite are sodium sulfite and ammonium sulfite. In order to increase the decomposition rate of the initiator, a copper salt or an iron salt is also preferably added to the combination of the redox initiator. An example of the copper salt is copper (II) sulfate, and an example of the iron salt is iron (II) sulfate.

[0077] The oxidizing agent of the redox initiator is preferably permanganic acid or its salt, persulfate, manganese triacetate, cerium (IV) salt, bromic acid or its salt. The reducing agent of the redox initiator is preferably a dicarboxylic acid or its salt, or a diamine.

More preferably, the oxidizing agent is permanganic acid or its salt, persulfate, or bromic acid or its salt, and the reducing agent is a dicarboxylic acid or its salt.

[0078] Examples of a redox initiator include combinations of potassium permanganate/oxalic acid, potassium permanganate/ammonium oxalate, manganese triacetate/oxalic acid, manganese triacetate/ammonium oxalate, ammonium cerium nitrate/oxalic acid, and ammonium cerium nitrate/ammonium oxalate.

In the case of using a redox initiator, the polymerization reactor may be pre-loaded with an oxidizer or a reductant, and then the rest may be added continuously or periodically to it, which allows the polymerization to start. For example, in the case of using a potassium permanganate/ammonium oxalate combination, it is preferable to load ammonium oxalate into the polymerization reactor and continuously add potassium permanganate to it.

In this case, in relation to the oxidation-reduction initiator, "potassium permanganate/ammonium oxalate" means a combination of potassium permanganate and ammonium oxalate. The same applies to other compounds.

[0079] Particularly preferably, the redox initiator is a combination of an oxidizing agent in the form of a salt and a reducing agent in the form of a salt.

For example, the oxidizing agent in the form of a salt more preferably comprises at least one salt selected from the group consisting of persulfates, permanganates, cerium (IV) salts, bromates, even more preferably, it is permanganate, particularly preferably potassium permanganate.

The reducing agent in the form of a salt more preferably comprises at least one salt selected from the group consisting of oxalates, malonates, succinates, glutarates and bromates, even more preferably it is an oxalate, particularly preferably ammonium oxalate.

[0080] More specifically, the redox initiator preferably comprises at least one combination selected from the group consisting of potassium permanganate/oxalic acid, potassium permanganate/ammonium oxalate, potassium bromate/ammonium sulfite, manganese triacetate/ammonium oxalate and ammonium cerium nitrate/ammonium oxalate, more preferably at least one combination selected from the group consisting of potassium permanganate/oxalic acid, potassium permanganate/ammonium oxalate, potassium bromate/ammonium sulfite and ammonium cerium nitrate/ammonium oxalate, even more preferably potassium permanganate/oxalic acid.

[0081] When using a redox initiator, the oxidizing agent and the reducing agent may be added simultaneously at the initial stage of the polymerization, or the reducing agent may be added simultaneously at the initial stage of the polymerization and the oxidizing agent may be added continuously. Alternatively, the oxidizing agent may be added simultaneously at the initial stage of the polymerization and the reducing agent may be added continuously, or both the oxidizing agent and the reducing agent may be added continuously.

[0082] When one component of the redox initiator is added at the initial stage of polymerization and the other is added continuously, it is preferable to gradually decrease the addition rate in order to obtain PTFE with a low SSG. In addition, the addition is preferably stopped in the middle of the polymerization. The addition is preferably stopped before 20% - 40% by weight of the total TFE intended for the polymerization reaction is consumed.

[0083] When using a redox initiator as the polymerization initiator, the added amount of the oxidizing agent is preferably 0.1 ppm or more, more preferably 0.3 ppm or more, even more preferably 0.5 ppm or more, even more preferably 1.0 ppm or more, particularly preferably 5 ppm or more, even more preferably 10 ppm or more, and wherein it is preferably 10,000 ppm or less, more preferably 1,000 ppm or less, even more preferably 100 ppm or less, even more preferably 10 ppm or less, with respect to the aqueous medium.

The amount of the reducing agent added is preferably 0.1 ppm or more, more preferably 3 ppm or more, even more preferably 5 ppm or more, particularly preferably 10 ppm or more, and further preferably 10,000 ppm or less, more preferably 1,000 ppm or less, even more preferably 100 ppm or less, even more preferably 10 ppm or less.

When using a redox initiator for emulsion polymerization, the polymerization temperature is preferably 100°C or less, more preferably 95°C or less, even more preferably 90°C or less, and further preferably 10°C or more, more preferably 20°C or more, even more preferably 30°C or more, even more.

[0084] In order to provide greater adhesive force, strength and flexibility of the electrode, preferred examples of the polymerization initiator include a water-soluble radical polymerization initiator and a redox initiator.

[0085] The aqueous medium is a reaction medium for carrying out the polymerization and is a liquid containing water. The aqueous medium may be any medium containing water and may contain water and, for example, a fluorine-free organic solvent such as an alcohol, ether or ketone, and/or a fluorine-containing organic solvent with a boiling point of 40°C or less.

[0086] If necessary, additives such as a nucleating agent, a chain transfer agent, a buffer, a pH regulator, an auxiliary stabilizer, a dispersion stabilizer, a radical scavenger, a polymerization initiator decomposer, or a dicarboxylic acid can be used during the emulsion polymerization.

[0087] To control the particle size during emulsion polymerization, a nucleating agent is preferably added. Preferably, the nucleating agent is added before the polymerization reaction begins.

The nucleating agent may be one of those known. The nucleating agent preferably comprises at least one substance selected from the group consisting of fluoropolyethers, nonionic surfactants and chain transfer agents, more preferably a nonionic surfactant.

[0088] The fluoropolyether may be, for example, perfluoropolyether (PFPE) acid or a salt thereof.

PFPE acid or its salt may have any chain structure in which the oxygen atoms of the main chain of the molecule are separated by saturated fluorocarbon groups comprising from 1 to 3 carbon atoms. In addition, two or more types of fluorocarbon groups may be present in the molecule. A typical structure includes repeating units corresponding to any of the following formulas:

(-CFCF ₃ -CF ₂ -O-) _n ,

( _-CF2 - _CF2 - _CF2- O-) _n ,

(-CF ₂ -CF ₂ -O-) _n -(-CF ₂ -O-) _m and

(-CF ₂ -CFCF ₃ -O-) _n -(-CF ₂ -O-) _m .

[0089] These structures are described by Kasai in J. Appl. Polymer Sci. 57, 797 (1995). According to this publication, the PFPE acid or salt thereof may include a carboxylic acid group or a salt thereof at one end or at each end. The PFPE acid or salt thereof may also include a sulfonic acid or phosphonic acid group or a salt thereof at one end or at each end. In addition, the PFPE acid or salt thereof may include different groups at each end. In the case of a single-functional PFPE, the other end of the molecule is typically perfluorinated, but may also include a hydrogen or chlorine atom. The PFPE acid or salt thereof includes at least two ether oxygens, preferably at least four ether oxygens, more preferably at least six ether oxygen atoms. Preferably, at least one of the fluorocarbon groups separating the ether oxygen atoms, more preferably at least two of such fluorocarbon groups comprise two or three carbon atoms. Even more preferably, at least 50% of the fluorocarbon groups separating the ether oxygen atoms comprise two or three carbon atoms. Preferably, the PFPE acid or salt thereof comprises at least 15 carbon atoms in total and, for example, the preferred minimum value of n or n+m in the repeat unit structure is at least 5. In the production method according to the invention, two or more PFPE acids or salts thereof having an acid group at one end or at each end can be used. The PFPE acid or salt thereof preferably has a number average molecular weight of less than 6000 g/mol.

[0090] In order to further increase the molecular weight of PTFE and increase the strength of the electrode mixture sheet, a radical scavenger or a polymerization initiator decomposition agent is preferably added during the emulsion polymerization. The radical scavenger or the polymerization initiator decomposition agent is added after the start of the polymerization reaction, preferably before the polymerization of 10% by weight, preferably 20% by weight of the total TFE to be consumed in the polymerization reaction, and preferably before the polymerization of 50% by weight, preferably 40% by weight of the total TFE to be consumed in the polymerization reaction. The radical scavenger or the polymerization initiator decomposition agent is preferably added after the pressure release and pressure recovery described below, if they are carried out.

[0091] The radical scavenger used is a compound that does not have the ability to be reinitiated after addition or the ability to transfer a chain to free radicals in a polymerization system. More specifically, a compound that performs the function of initiating a chain transfer reaction with a primary radical or a growing macroradical followed by the formation of a stable radical that does not react with the monomer or a compound that performs the function of initiating a reaction of a compound with a primary radical or a growing macroradical followed by the formation of a stable radical is used.

In general, the so-called chain transfer agents are distinguished by their activity, described by the chain transfer constant and the reinitiation efficiency, and those characterized by a reinitiation efficiency of 0% are called radical scavengers.

A radical scavenger may also be described, for example, as a compound for which the chain transfer constant to TFE at the polymerization temperature is greater than the polymerization rate constant and which is characterized by a reinitiation efficiency of substantially 0%. The expression "reinitiation efficiency of substantially 0%" means that the radicals formed convert the radical scavenger into a stable radical.

Preferably, the radical scavenger is a compound characterized by a chain transfer constant (Cs) (=chain transfer rate constant (kc)/polymerization rate constant (kp)) to TFE at a polymerization temperature of more than 0.1. This compound is characterized by a Cs of more preferably 0.5 or more, even more preferably 1.0 or more, even more preferably 5.0 or more, particularly preferably 10 or more.

[0092] The radical scavenger preferably comprises, for example, at least one compound selected from the group consisting of aromatic hydroxy compounds, aromatic amines, n,n-diethylhydroxyamine, quinone compounds, terpenes, thiocyanates and copper chloride (CuCl ₂ ).

Examples of aromatic hydroxy compounds are unsubstituted phenols, polyhydric phenols, salicylic acid, m- or p-salicylic acid, gallic acid and naphthol.

Examples of unsubstituted phenols are o-, m-, or p-nitrophenol, o-, m-, or p-aminophenol, and p-nitrosophenol. Examples of polyhydric phenols are catechol, resorcinol, hydroquinone, pyrogallol, fluoroglucinol, and naphtresorcinol.

Examples of aromatic amines are o-, m-, or p-phenylenediamine and benzidine.

Examples of quinones are o-, m-, or p-benzoquinone, 1,4-naphthoquinone, and alizarin.

Examples of thiocyanates are ammonium thiocyanate (NH ₄ SCN), potassium thiocyanate (KSCN), and sodium thiocyanate (NaSCN).

Of these compounds, a preferred radical scavenger is an aromatic hydroxy compound, more preferably an unsubstituted phenol or a polyhydric phenol, even more preferably hydroquinone.

[0093] From the viewpoint of mildly reducing the standard specific gravity, the addition amount of the radical scavenger is preferably equivalent to 3% - 500% (in molar terms) of the concentration of the polymerization initiator. The lower limit is preferably 10% (in molar terms), still more preferably 15% (in molar terms). The upper limit is more preferably 400% (in molar terms), still more preferably 300% (in molar terms).

[0094] The substance for decomposing the polymerization initiator can be any compound capable of decomposing the polymerization initiator used, and includes, for example, at least one compound selected from the group consisting of sulfites, bisulfites, bromates, diimines, diimine salts, oxalic acid, oxalates, copper salts and iron salts. Examples of sulfites are sodium sulfite and ammonium sulfite. An example of a copper salt is copper (II) sulfate, an example of an iron salt is iron (II) sulfate.

From the viewpoint of softly reducing the standard specific gravity, the added amount of the decomposing agent is preferably equivalent to 3% - 500% (in molar terms) of the concentration of the polymerization initiator. The lower limit is more preferably 10% (in molar terms), still more preferably 15% (in molar terms). The upper limit is more preferably 400% (in molar terms), still more preferably 300% (in molar terms).

[0095] In order to reduce the amount of lumps formed during polymerization, emulsion polymerization can be carried out in the presence of a dicarboxylic acid in an amount of 5 to 500 parts per million, preferably 10 to 200 parts per million in an aqueous medium. If the amount of the dicarboxylic acid is too small relative to the aqueous medium, the desired effect cannot be achieved. Conversely, if the amount is too large, a chain transfer reaction is likely, and the resulting polymer may have a low molecular weight. The amount of the dicarboxylic acid is more preferably 150 parts per million or less. The dicarboxylic acid can be introduced before the polymerization reaction or can be added during the polymerization.

[0096] The dicarboxylic acid preferably corresponds to the formula HOOCRCOOH (where R is a C1-C5 alkylene group), more preferably it is succinic acid, malonic acid, glutaric acid, adipic acid or pimelic acid, even more preferably succinic acid.

[0097] When carrying out emulsion polymerization, the polymerization temperature and pressure are appropriately selected according to the type of monomer used, the target molecular weight of PTFE and the reaction rate. Generally, the polymerization temperature is 5°C to 150°C. Preferably, the polymerization temperature is 10°C or higher, more preferably 30°C or higher, still more preferably 50°C or higher, while more preferably 120°C or lower, still more preferably 100°C or lower.

The polymerization pressure is in the range of 0.05 to 10 MPaG. The polymerization pressure is more preferably 0.3 MPaG or higher, still more preferably 0.5 MPaG or higher, wherein it is more preferably 5.0 MPaG or lower, still more preferably 3.0 MPaG or lower.

[0098] When VDF is used as a modifying monomer for emulsion polymerization, in order to achieve the physical properties described above, the concentration of VDF in the gas present in the reactor at the time of starting the polymerization (when adding the initiator) is preferably set to 0.001 mol% or higher, more preferably 0.01 mol% or higher. The concentration of VDF may be 15 mol% or lower, more preferably 6.0 mol% or lower, still more preferably 5.0 mol% or lower, still more preferably 3.0 mol% or lower, particularly preferably 1.0 mol% or lower. After this, the concentration of VDF may be maintained until the end of the polymerization reaction, or the pressure may be released during the process. Preferably, VDF is added all at once before the start of polymerization. However, a part of it may be continuously or intermittently added also after the start of polymerization.

[0099] When VDF is used as a modifying monomer for emulsion polymerization, depressurization is preferably not performed until polymerization is completed after adding VDF to the polymerization reactor. Due to this, VDF remains in the system until polymerization is completed and makes it possible to achieve even greater strength of the electrode mixture sheet containing PTFE.

[0100] When HFP is used as a modifying monomer for emulsion polymerization, in order to achieve the above-described physical properties, the concentration of HFP in the gas in the reactor at the time of starting the polymerization (when adding the initiator) is preferably set to 0.01 to 3.0 mol%. In addition, the concentration of HFP in the gas in the reactor at the time when 40 mass% of TFE to be consumed in the polymerization reaction has been polymerized is preferably greater than 0 mol% and 0.2 mol% or less. After that, the concentration of HFP is preferably maintained until the end of the polymerization reaction. HFP may be added all at once before the start of the polymerization, or a part of it may be introduced before the start of the polymerization and the rest may be added continuously or intermittently after the start of the polymerization. By keeping TFE in the reactor until the end of the polymerization reaction, the extrusion pressure is reduced, although the subsequently obtained sheet of electrode mixture containing PTFE has high strength.

[0101] When HFP is used as a modifying monomer for emulsion polymerization, in order to achieve higher strength of the subsequently obtained sheet of electrode mixture containing PTFE, it is preferable to depressurize and then restore the pressure of only TFE before polymerization of 5% to 40% by weight of the total TFE that is to be consumed in the polymerization reaction.

The pressure release is preferably carried out so that the pressure in the reactor is 0.2 MPaG or less, more preferably 0.1 MPaG or less, even more preferably 0.05 MPaG or less. The pressure release is preferably carried out so that the pressure in the reactor is 0.0 MPaG or more.

Pressure relief and restoration can be performed multiple times. Pressure relief to vacuum can be performed using a vacuum pump.

[0102] When CTFE is used as a modifying monomer for emulsion polymerization, in order to achieve the above-described physical properties, the concentration of CTFE in the gas in the reactor at the time of starting the polymerization (when adding the initiator) is preferably set to 0.001 mol% or higher, more preferably 0.01 mol% or higher. The concentration of CTFE may be preferably 0.3 mol% or lower, more preferably 1.0 mol% or lower. Thereafter, the concentration of CTFE may be maintained until the end of the polymerization reaction, or the pressure may be released during the polymerization reaction. Preferably, CTFE is added all at once before the start of polymerization. However, a part of it may also be continuously or intermittently added after the start of polymerization.

[0103] When CTFE is used as a modifying monomer for emulsion polymerization, depressurization is preferably not performed until polymerization is completed after adding CTFE to the polymerization reactor. Due to this, CTFE remains in the system until polymerization is completed and makes it possible to achieve even greater strength of the electrode mixture sheet containing PTFE.

[0104] Coagulation in step (B) can be carried out in a known manner.

[0105] Drying in step (C) is usually carried out using a vacuum, high-frequency radiation or hot air; at this time, the wet powder is in a state that prevents flow, preferably in a stationary state. Friction between powder particles, especially at high temperature, usually has an adverse effect on fine PTFE powder. The reason is that PTFE particles of this type are easily fibrillated even under low shear stress and lose the original stable particle structure.

From the viewpoint of reducing the extrusion pressure, the drying temperature is preferably 300°C or less, more preferably 250°C or less, still more preferably 230°C or less, still more preferably 210°C or less, still more preferably 190°C or less, particularly preferably 170°C or less. From the viewpoint of increasing the tensile strength, the drying temperature is preferably 10°C or higher, more preferably 100°C or higher, still more preferably 150°C or higher, still more preferably 170°C or higher, still more preferably 190°C or higher, particularly preferably 210°C or higher. From the viewpoint of further increasing the strength coefficient, the drying temperature is preferably appropriately adjusted in the above range.

[0106] The drying step (C) preferably comprises placing the wet powder obtained in step (B) in a container with an air-permeable bottom and/or walls and heating the powder to a temperature of 130°C to 300°C for two hours or more. The heat treatment under the extremely limited conditions described above makes it possible to effectively remove water and a fluorine-containing compound with a molecular weight of 1000 or less so that the amounts of the fluorine-containing compound and water correspond to the ranges specified above.

[0107] In view of improving the efficiency of removing water and the fluorine-containing compound, the heat treatment temperature in step (C) is preferably 140°C or more, more preferably 150°C or more, even more preferably 160°C or more, even more preferably 180°C or more, even more preferably 200°C or more, particularly preferably 220°C or more, and further preferably 280°C or less, more preferably 250°C or less.

[0108] In view of improving the efficiency of removing water and the fluorine-containing compound, the duration of the heat treatment in step (C) is preferably 5 hours or more, more preferably 10 hours or more, still more preferably 15 hours or more. The upper limit is not limited. However, it is preferably 100 hours or less, more preferably 50 hours or less, still more preferably 30 hours, for example.

[0109] In view of improving the efficiency of removing water and the fluorine-containing compound, the flow velocity in step (C) is preferably 0.01 m/s or more, more preferably 0.03 m/s or more, still more preferably 0.05 m/s or more, still more preferably 0.1 m/s or more. In order to reduce or prevent dispersion of the powder, the flow velocity is preferably 50 m/s or less, more preferably 30 m/s or less, still more preferably 10 m/s or less.

[0110] The heat treatment in step (C) may be carried out using an electric furnace or a steam furnace. For example, the heat treatment may be carried out in an electric furnace such as a direct-flow chamber furnace, a ventilated chamber furnace, a ventilated belt furnace, a belt furnace, a radiation belt furnace, a fluidized bed furnace, a vacuum furnace, a stirred furnace, a convection furnace, or a circulating hot air furnace; or in a steam furnace corresponding to any of the mentioned electric furnaces (in a device of the type described by replacing "electric furnace" in the name of the electric furnaces in the given list with "steam furnace"). For more efficient removal of water, a straight-through chamber furnace, a ventilated chamber furnace, a ventilated conveyor furnace, a belt furnace, a fluidized bed furnace, a circulating hot air furnace or corresponding steam furnaces (devices whose type is described by replacing “electric furnace” in the name of the electric furnaces in the above list with “steam furnace”) are preferable.

[0111] For the heat treatment in step (C), the wet powder is placed in a container with an air-permeable bottom and/or walls. Any container capable of withstanding the heat treatment temperature can be used as a container with an air-permeable bottom and/or walls. Preferably, the container is made of a metal such as stainless steel.

The container with an air-permeable bottom and/or machines is preferably a tray with an air-permeable bottom and/or machines, more preferably a tray (lattice tray) with a bottom and/or machines made of a lattice.

The grating is preferably either woven mesh or perforated metal.

The mesh size is preferably 2000 μm or less (No. 10 mesh or more according to ASTM), more preferably 595 μm or less (No. 30 mesh or more), still more preferably 297 μm or less (No. 50 mesh or more), still more preferably 177 μm or less (No. 80 mesh or more), still more preferably 149 μm or less (No. 100 mesh or more), particularly preferably 74 μm or less (No. 200 mesh or more). The mesh size is preferably 25 μm or less (No. 500 mesh or less).

When the lattice is made as a woven mesh, it can be, for example, a braid mesh, a twill mesh, a plain Dutch mesh or a twill Dutch mesh.

When the grating is formed as a perforated metal, the porosity is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, and, in this case, preferably 95% or less.

[0112] In view of more efficient removal of water and a fluorine-containing compound, the amount of wet powder processed in step (C) is preferably 10 g/ ^cm2 or less, more preferably 8 g/ ^cm2 or less, even more preferably 5 g/ ^cm2 or less, even more preferably 3 g/ ^cm2 or less, and further preferably 0.01 g/ ^cm2 or more, more preferably 0.1 g/ ^cm2 or more.

[0113] In view of more efficient removal of water and a fluorine-containing compound, the water content of the wet powder to be heated in step (C) is preferably 10 mass% or more, more preferably 20 mass% or more, even more preferably 30 mass% or more, and further preferably 150 mass% or less, more preferably 100 mass% or less, relative to the weight of the wet powder.

[0114] The PTFE powder of the present invention is intended for use in an electrode binder. In the electrode binder, the PTFE powder of the present invention may be used alone or in a mixture with other materials (for example, polymers other than PTFE). However, it is preferable to use the PTFE powder of the present invention substantially alone, more preferably, individually. The expression "the PTFE powder of the present invention is used substantially alone" means that the amount of PTFE powder used in the electrode binder corresponds to the range described below.

[0115] The invention also provides an electrode binder consisting essentially of PTFE powder, wherein the PTFE powder has a standard specific gravity of 2.200 or less and is substantially free of water.

The binder of the present invention containing a special PTFE powder is capable of reducing or preventing gas evolution inside a battery element and deterioration of battery characteristics (for example, a decrease in capacity during storage at a high temperature), and also increasing the strength of the electrode. Since the binder of the present invention essentially does not contain water, the active material of the electrode used in combination with it can be selected from a wide range of materials having advantages in terms of production technology. The binder of the present invention in powder form can be mixed with the active material of the electrode as is, so there is no need to use organic solvents, thereby simplifying the process and eliminating the cost of organic solvents. In addition, the binder of the present invention has a very high adhesive force with active materials, so a smaller amount of the binder can be used.

[0116] The PTFE powder in the binder of the present invention may be the same PTFE powder as the PTFE powder described above, and the same embodiments are preferable.

[0117] The binder of the present invention essentially contains PTFE powder. Due to this, the effect attributed to the PTFE powder is clearly demonstrated. The expression "essentially consists of PTFE powder" means that the amount of PTFE powder is 95.0 mass% or more relative to the mass of the binder.

The amount of PTFE powder is preferably 98.0% by mass or more, more preferably 99.0% by mass or more, even more preferably 99.5% by mass or more, particularly preferably 99.9% by mass or more, most preferably 99.95% by mass or more relative to the mass of the binder.

The binder of the present invention preferably consists of PTFE powder.

[0118] Preferably, the binder of the present invention is substantially free of organic solvents. This simplifies the process and eliminates the cost of organic solvents. The expression "substantially free of organic solvents" means that the content of the organic solvent in the binder is 5% by mass or less.

The content of the organic solvent is preferably 3% by mass or less, more preferably 1% by mass or less, still more preferably 0.1% by mass or less, still more preferably 0.01% by mass or less, particularly preferably 0.001% by mass or less.

[0119] The binder of the present invention is preferably in powder form.

[0120] The binder of the present invention can be suitably used as an electrode binder for secondary batteries such as lithium-ion batteries.

[0121] The present invention also provides an electrode mixture containing the PTFE powder of the present invention or the electrode binder of the present invention and an active electrode material. Using the electrode mixture of the present invention, an electrode can be obtained that can reduce or prevent gas generation inside a battery element and deterioration of battery characteristics (for example, a decrease in capacity during storage at a high temperature). In addition, the strength of the electrode can be increased. In addition, the active electrode material can be firmly held even with a small amount of the binder.

[0122] Examples of an active electrode material are a positive electrode active material and a negative electrode active material.

[0123] The positive electrode active material may be any material capable of electrochemically absorbing and releasing alkali metal ions. However, the positive electrode material is preferably, for example, a material containing an alkali metal and at least one transition metal. Specific examples are complex oxides of a transition metal containing an alkali metal, phosphate compounds of a transition metal containing an alkali metal, and conductive polymers. Of these, complex oxides of a transition metal containing an alkali metal that generate a high voltage are particularly preferable for use as the positive electrode active material. Examples of alkali metal ions are lithium ions, sodium ions, and potassium ions. In one preferred embodiment, the alkali metal ions may be lithium ions. That is, in this embodiment, the alkali ion secondary battery is a lithium-ion secondary battery.

[0124] Examples of complex transition metal oxides containing an alkali metal are complex oxides of the lithium-manganese spinel type, corresponding to the formula:

M _a Mn _2-b M ¹ _b O ₄

where M comprises at least one metal selected from the group consisting of Li, Na and K; 0.9 ≤ a; 0 ≤ b ≤ 1.5; and M ¹ comprises at least one metal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge;

complex lithium-nickel oxides corresponding to the formula:

MNi _1-c M ² _c O ₂

where M comprises at least one metal selected from the group consisting of Li, Na and K; 0 ≤ c ≤ 0.5; and M ² comprises at least one metal selected from the group consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge); and

complex lithium-cobalt oxides corresponding to the formula:

MCo _1-d M ³ _d O ₂

where M comprises at least one metal selected from the group consisting of Li, Na and K; 0 ≤ d ≤ 0.5; and M ³ comprises at least one metal selected from the group consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge). In the above formulas, M preferably represents a metal selected from the group consisting of Li, Na and K, more preferably Li or Na, even more preferably Li.

[0125] From the viewpoint of creating secondary batteries characterized by high energy density and high capacity, preferred compounds among these are MCoO ₂ , MMnO ₂ , MNiO ₂ , MMn ₂ O ₄ , MNi _0.8 Co _0.15 Al _0.05 O ₂ and MNi _1/3 Co _1/3 Mn _1/3 O ₂ , and also preferred is a compound corresponding to formula (3):

MNi _h Co _i Mn _j M ⁵ _k O ₂ (3)

where M comprises at least one metal selected from the group consisting of Li, Na and K; M ⁵ comprises at least one metal selected from the group consisting of Fe, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge; and (h+i+j+k)=1.0, 0 ≤ h ≤ 1.0, 0 ≤ i ≤ 1.0, 0 ≤ j ≤ 1.5 and 0 ≤ k ≤ 0.2.

[0126] Examples of alkali metal-containing transition metal phosphate compounds are compounds corresponding to the following formula (4):

M _e M ⁴ _f (PO ₄ ) _g (4)

where M comprises at least one metal selected from the group consisting of Li, Na and K; M ⁴ comprises at least one metal selected from the group consisting of V, Ti, Cr, Mn, Fe, Co, Ni and Cu; and 0.5 ≤ e ≤ 3, 1 ≤ f ≤ 2 and 1 ≤ g ≤ 3. In this formula, M preferably denotes a metal selected from the group consisting of Li, Na and K, more preferably Li or Na, even more preferably Li.

[0127] The transition metal in the lithium-containing transition metal phosphate compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. Specific examples are iron phosphate such as LiFePO ₄ , Li ₃ Fe ₂ (PO ₄ ) ₃ and LiFeP ₂ O ₇ , cobalt phosphates such as LiCoPO ₄ , and those obtained by replacing some transition metal atoms that are the main components of lithium and transition metal phosphate compounds with other elements such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb or Si. Preferably, the lithium-containing transition metal phosphate compound has an olivine structure.

[0128] Other examples of the positive electrode active material include MFePO ₄ , MNi _0.8 Co _0.2 O ₂ , M _1.2 Fe _0.4 Mn _0.4 O ₂ , MNi _0.5 Mn _1.5 O ₂ , MV ₃ O ₆ and M ₂ MnO ₃ . In particular, positive electrode active materials such as M ₂ MnO ₃ or MNi _0.5 Mn _1.5 O ₂ are preferable because their crystal structure is not destroyed even when the secondary battery is operated at a voltage of more than 4.4 V or at a voltage of 4.6 V or more. An electrochemical device such as a secondary battery with a positive electrode active material including the active material of the above examples is preferable for the following reasons. It is less likely to have a decrease in residual capacity or a change in the resistance increase rate even after storage at high temperature, and the battery performance will not deteriorate even if the device is operated at high voltage.

[0129] Other examples of the positive electrode active material include materials that are solid solutions of M ₂ MnO ₃ and MM ⁶ O ₂ (where M comprises at least one metal selected from the group consisting of Li, Na and K; and M ⁶ is a transition metal such as Co, Ni, Mn or Fe).

[0130] The solid solution is, for example, an oxide of manganese and an alkali metal corresponding to the formula Mx[Mn _(1-y) M ⁷ _y ]O _z . In this formula, M comprises at least one metal selected from the group consisting of Li, Na and K; and M ⁷ means at least one metallic element other than M or Mn, including, for example, one or more elements selected from the group consisting of Co, Ni, Fe, Ti, Mo, W, Cr, Zr and Sn. The values of x, y, z in this formula lie in the ranges 1 < x < 2, 0 ≤ y < 1 and 1.5 < z < 3, respectively. In particular, a manganese-containing solid solution in which LiNiO ₂ or LiCoO ₂ is dissolved in a Li ₂ MnO ₃ base, for example, Li _1.2 Mn _0.5 Co _0.14 Ni _0.14 O ₂ , is preferable, since the alkaline ion secondary battery including it has a high energy density.

[0131] From the viewpoint of maintaining charging characteristics, a positive electrode active material containing lithium phosphate is preferable. Lithium phosphate may be introduced by any method, and it is preferable to use it in the form of an impurity to the positive electrode active material. The lower limit of the amount of lithium phosphate to be used is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, still more preferably 0.5 mass% or more, relative to the total mass of the positive electrode active material and lithium phosphate. The upper limit is preferably 10 mass% or less, more preferably 8 mass% or less, still more preferably 5 mass% or less.

[0132] Examples of conductive polymers include acceptor dopant-doped polymers and donor dopant-doped polymers. Examples of conductive polymers include polyacetylene-based polymers, polyphenylene-based polymers, heterocyclic polymers, ionic polymers, ladder polymers, and network polymers.

[0133] A substance other than the positive electrode active material may be bonded to the surface of the positive electrode active material. Examples of the substance bonded to the surface of the positive electrode active material are oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, magnesium carbonate; and carbon.

[0134] These substances can be applied to the surface of the active material of the positive electrode, for example, by dissolving or suspending the substance in a solvent, impregnating the active material of the positive electrode with the solution or suspension and drying the impregnated material; by dissolving or suspending a precursor of a given substance in a solvent, impregnating the active material of the positive electrode with the solution or suspension, heating the material and the precursor and thereby carrying out a reaction between them; or by adding the substance to the precursor of the active material of the positive electrode while simultaneously sintering these materials. In the case where the substance to be attached is carbon, a carbonaceous material in the form of activated carbon, for example, can subsequently be applied to the surface.

[0135] Regarding the amount of the substance bonded to the surface, expressed as a mass relative to the amount of the positive electrode active material, the lower limit of the amount is preferably 0.1 ppm or more, more preferably 1 ppm or more, even more preferably 10 ppm or more, while the upper limit of the amount is preferably 20% or less, more preferably 10% or less, even more preferably 5% or less. The substance bonded to the surface prevents the electrolyte solution on the surface of the positive electrode active material from being oxidized, thereby increasing the service life of the battery. If the said amount is too small, this effect may be insufficient. If it is too large, the entry/exit of lithium ions may be difficult, causing the resistance to increase.

[0136] The shape of the particles of the active material of the positive electrode may be any of those commonly used, such as volumetric, polyhedral, spherical, ellipsoidal, plate-shaped, needle-shaped, or columnar. Primary particles may be combined into secondary particles.

[0137] The positive electrode active material has a tapped density of preferably 0.5 g/cm ³ or more, more preferably 0.8 g/cm ³ or more, even more preferably 1.0 g/cm ³ or more. If the tapped density of the positive electrode active material is less than the lower limit, a larger amount of dispersion medium and a larger amount of conductive material and binder may be required to form the positive electrode active material layer, which may also limit the packing coefficient of the positive electrode active material in the layer, and thus limit the capacity of the battery. If the powdered complex oxide has a large tapped density, a very dense positive electrode active material layer can be formed from it. The tapped density is preferably as large as possible and is generally not limited. However, when the density after tapping is too large, the diffusion of lithium ions in the positive electrode active material layer with the electrolyte as the diffusion medium may become a process speed determining step, which significantly deteriorates the loading characteristics. Therefore, the upper limit of the density after tapping is preferably 4.0 g/cm ³ or less, more preferably 3.7 g/cm ³ or less, still more preferably 3.5 g/cm ³ or less.

The tapped density is defined as the density of the powder in g/ ^cm3 after 5-50 g of the powdered positive electrode active material is placed in a 10 ml graduated glass cylinder and the cylinder is tapped 200 times with a stroke of about 20 mm.

[0138] The positive electrode active material particles have a median particle size d50 (or a secondary particle size when the primary particles are combined to form secondary particles) of preferably 0.3 μm or more, more preferably 0.5 μm or more, even more preferably 0.8 μm or more, most preferably 1.0 μm or more, and further preferably 30 μm or less, more preferably 27 μm or less, even more preferably 25 μm or less, most preferably 22 μm or less. If the median particle size is less than the lower limit, it is difficult to obtain a product with a high density after tapping. If the median particle size is greater than the upper limit, the diffusion of lithium in the particles may be prolonged, which deteriorates the performance of the battery, and stripes may occur when forming the positive electrode (for example, when the active material and components such as a conductive material and a binder are mixed with a solvent to form a slurry and a film is formed from the slurry). Mixing two or more positive electrode active materials with different median d50 size can improve the packing when forming the positive electrode.

[0139] The median size d50 is determined using a known laser diffraction particle size analyzer. In the case of using LA-920 (Horiba, Ltd.) as the particle size analyzer, the dispersion medium used in the measurement is an aqueous solution of 0.1% by mass of sodium hexametaphosphate, and the refractive index after 5 min of ultrasonic dispersion is set to be 1.24.

[0140] When the primary particles are combined to form secondary particles, the average primary particle size of the positive electrode active material is preferably 0.05 μm or more, more preferably 0.1 μm or more, even more preferably 0.2 μm or more. The upper limit is preferably 5 μm or less, more preferably 4 μm or less, even more preferably 3 μm or less, most preferably 2 μm or less. Primary particles with an average primary particle size exceeding the said upper limit may poorly form spherical secondary particles, which is unfavorable for powder packaging. In addition, such primary particles may have a significantly smaller specific surface area, which greatly deteriorates battery performance such as output characteristics. In contrast, primary particles with an average primary particle size smaller than the said lower limit are usually insufficiently grown crystals, which causes, for example, deterioration in charge and discharge reversibility.

The size of primary particles is measured using SEM. Specifically, the size of primary particles is determined as follows. First, an image is obtained with a magnification of 10,000x. Any 50 primary particles are selected and the maximum distance between the left and right boundary lines of each primary particle is measured horizontally. Then, the average maximum length is calculated, which is taken as the size of the primary particle.

[0141] The positive electrode active material has a BET (Brunauer-Emmett-Teller) specific surface area of preferably 0.1 ^m2 /g or more, more preferably 0.2 ^m2 /g or more, even more preferably 0.3 ^m2 /g or more. The upper limit of this value is preferably 50 ^m2 /g or less, more preferably 40 ^m2 /g or less, still more preferably 30 ^m2 /g or less. Using a positive electrode active material with a BET specific surface area less than the specified range may result in deterioration in battery performance. A positive electrode active material with a BET specific surface area greater than the specified range may not have sufficient density after tapping, which causes difficulty in applying the material when forming a positive electrode active material layer.

The specific surface area according to BET corresponds to the value obtained by the single-point nitrogen adsorption method in a gas flow using a BET surface area analyzer (e.g., a fully automatic surface area measuring device of Ohkura Riken Co., Ltd.) for a sample pre-dried in a nitrogen flow at 150°C for 30 min, using a gaseous mixture of nitrogen and helium with a nitrogen pressure relative to the atmospheric pressure exactly equal to 0.3.

[0142] When the secondary battery of the present invention is used as a large-scale battery for a hybrid vehicle or in a distributed power generation system, it is required to have a high output power. Therefore, the positive electrode active material particles are preferably mainly secondary particles. The secondary particles of the positive electrode active material include 0.5 to 0.7 vol.% of fine particles with an average particle size of 40 μm or less and an average primary particle size of 1 μm or less. The presence of fine particles with an average primary particle size of 1 μm or less makes it possible to increase the contact area with the electrolyte solution and ensures faster diffusion of lithium ions between the electrode mixture and the electrolyte solution, thereby improving the output characteristics of the battery.

[0143] The active material of the positive electrode can be produced by any known method for producing an inorganic compound. In particular, spherical or ellipsoidal particles of the active material can be produced by various methods. For example, a transition metal compound is dissolved or ground and dispersed in a solvent such as water, the pH of the solution or dispersion is adjusted with stirring to obtain a spherical precursor. The precursor is recovered and, if necessary, dried. Then, a Li source such as LiOH, Li ₂ CO ₃ or LiNO ₃ is added thereto, the mixture is sintered at a high temperature, thereby obtaining the active material.

[0144] For the production of the positive electrode, one of the above-mentioned positive electrode materials may be used individually, or two or more of the above-mentioned materials of different composition may be used in any combination and in any ratio. Preferred examples of combinations in this case are: a combination of LiCoO ₂ with a ternary system, for example, LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , a combination of LiCoO ₂ with either LiMn ₂ O ₄ or LiMn ₂ O ₄ , in which one or more Mn atoms are replaced by another transition metal, and a combination of LiFePO ₄ with either LiCoO ₂ or LiCoO ₂ , in which one or more Co atoms are replaced by another transition metal.

[0145] From the viewpoint of ensuring a high capacity of the battery, the amount of the positive electrode active material is preferably 50 to 99.5 mass %, more preferably 80 to 99 mass %, relative to the positive electrode electrode mixture. The amount of the positive electrode active material in the positive electrode active material layer is preferably 80 mass % or more, more preferably 82 mass % or more, particularly preferably 84 mass % or more. The upper limit of this amount is preferably 99 mass % or less, more preferably 98 mass % or less. If the amount of the positive electrode active material in the positive electrode active material layer is too small, insufficient electrical capacity may be observed. On the contrary, too large an amount may cause insufficient strength of the positive electrode.

[0146] The active material of the negative electrode is not limited; it may be, for example, any material selected from the following: metallic lithium; a material containing a carbonaceous material such as artificial graphite, graphitized carbon fiber, carbon sintered with resin, pyrolytic carbon grown from the gas phase, coke, mesocarbon microbeads (MCMB), carbon sintered with furyl alcohol resin, polyacene, pitch-based carbon fiber, carbon fiber grown from the gas phase, natural graphite or non-graphitizable carbon; a silicon-containing compound such as silicon or a silicon alloy; and Li ₄ Ti ₅ O ₁₂ or a mixture of two or more of them. Of these materials, a material containing, at least in part, a carbonaceous material or a silicon-containing compound may be advantageously used in particular.

[0147] The active material of the negative electrode used in the context of the invention preferably contains silicon as its component. The use of an active material of the negative electrode containing silicon as a component makes it possible to produce a high-capacity battery.

[0148] Preferred silicon-containing materials are silicon particles, particles in the structure of which finely dispersed silicon particles are distributed in a silicon-based compound, silicon oxide particles corresponding to the formula SiOx (0.5 ≤ x ≤ 1.6), and mixtures thereof. Using any of these materials, an electrode mixture of a negative electrode for lithium-ion batteries with higher initial charge/discharge efficiency, high capacity, and excellent cyclability is obtained.

[0149] In the present context, "silicon oxide" is used as a collective term for amorphous silicon oxide, and silicon oxide before disproportionation corresponds to the formula SiOx (0.5 ≤ x ≤ 1.6). In this formula, x preferably satisfies the relationship 0.8 ≤ x < 1.6, more preferably 0.8 ≤ x < 1.3. Such silicon oxide can be obtained, for example, by cooling and precipitating gaseous silicon monoxide formed by heating a mixture of silicon dioxide and metallic silicon.

[0150] Particles with a structure in which finely dispersed silicon particles are distributed in a silicon-based compound can be obtained, for example, by a method comprising sintering a mixture of finely dispersed silicon particles with a silicon-based compound, or by heating silicon oxide particles to disproportionation corresponding to the formula SiOx to a temperature of 400 °C or more, for example from 800 °C to 1100 °C in an inert, non-oxidizing atmosphere, for example in an argon atmosphere, in order to carry out a disproportionation reaction. The material obtained by the latter method is particularly suitable, since it is characterized by uniformly distributed silicon microcrystals. The above-mentioned disproportionation reaction makes it possible to adjust the size of silicon nanoparticles to 1-10 nm. The silicon oxide in the particle structure in which silicon nanoparticles are distributed in silicon oxide is preferably silicon dioxide. The dispersion of silicon nanoparticles (crystals) in amorphous silicon oxide can be confirmed using a transmission electron microscope.

[0151] The physical properties of the silicon-containing particles can be appropriately set according to the properties of the target composite particles. For example, the average particle size is preferably 0.1 to 50 μm. The lower limit is more preferably 0.2 μm or more, even more preferably 0.5 μm or more. The upper limit is more preferably 30 μm or less, even more preferably 20 μm or less. The average particle size is expressed as the weight average particle size determined by measuring the particle size distribution by a laser diffraction method.

[0152] The specific surface area according to BET is preferably 0.5 to 100 ^m2 /g, more preferably 1 to 20 ^m2 /g. With a specific surface area according to BET of 0.5 ^m2 /g or more, the risk of deterioration of battery performance due to poor adhesion of the active material of the negative electrode when applied to the electrode can be eliminated. With a specific surface area according to BET of 100 ^m2 /g or less, the proportion of silicon dioxide on the surface of the particle increases, thereby eliminating the risk of decreasing the battery capacity when using fine silicon particles as a material of the negative electrode of lithium-ion secondary batteries.

[0153] Applying carbon to silicon-containing particles imparts electrical conductivity to them, thereby improving battery performance. Examples of methods for imparting electrical conductivity include mixing silicon-containing particles with electrically conductive particles such as graphite particles, applying carbon films to the surface of silicon-containing particles, and a combination of these methods. A method of applying carbon films to the surface of silicon-containing particles is preferred, and a chemical vapor deposition (CVD) method is more preferred.

[0154] In view of increasing the capacity of the obtained electrode mixture, the amount of the active material of the negative electrode in the electrode mixture is preferably 40 mass% or more, more preferably 50 mass% or more, particularly preferably 60 mass% or more. The upper limit is preferably 99 mass% or less, more preferably 98 mass% or less.

[0155] The electrode mixture of the present invention preferably also contains an electrically conductive additive.

Any known conductive material may be used as the conductive additive. Specific examples thereof are metallic materials such as copper and nickel, and carbon materials such as graphite-containing natural graphite and artificial graphite, carbon black including acetylene black, Ketjenblack, channel black, furnace black, lamp black and thermal black, amorphous carbon including needle coke, carbon nanotubes, fullerenes and vapor-grown carbon fibers (VGCF). One of these materials may be used individually; or two or more materials may be used in any combination and in any ratio.

[0156] The conductive additive is used in the electrode mixture in an amount of usually 0.01 mass% or more, preferably 0.1 mass% or more, more preferably 1 mass% or more, wherein it is usually 50 mass% or less, preferably 30 mass% or less, more preferably 15 mass% or less. If the amount of the conductive additive is less than the specified range, the conductivity may be insufficient. On the contrary, if the amount of the conductive additive is greater than the specified range, the capacity of the battery may decrease.

[0157] The electrode mixture of the present invention may also include a thermoplastic resin. Examples of thermoplastic resins are polyvinylidene fluoride, polypropylene, polyethylene, polystyrene, polyethylene terephthalate and polyethylene oxide. One thermoplastic resin may be used individually, two or more thermoplastic resins may be used in any combination and any ratio.

[0158] The proportion of the thermoplastic resin relative to the active material of the electrode is usually 0.01 mass% or more, preferably 0.05 mass% or more, more preferably 0.10 mass% or more, wherein it is usually 3.0 mass% or less, preferably 2.5 mass% or less, more preferably 2.0 mass% or less. The introduction of the thermoplastic resin makes it possible to increase the mechanical strength of the electrode. If the proportion of the thermoplastic resin exceeds the specified range, the proportion of the active material of the electrode in the electrode mixture may decrease, which results in a decrease in the capacity of the battery and an increase in the resistance between the active materials.

[0159] The amount of the binder in the electrode mixture of the present invention may be 0.1% by mass or more, preferably 0.2% by mass or more, more preferably 0.5% by mass or more relative to the weight of the electrode mixture. This amount may be 50% by mass or less, preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 10% by mass or less, particularly preferably 5% by mass or less, most preferably 3% by mass or less relative to the weight of the electrode mixture. When the proportion of the binder is too small, the fixation of the active material of the electrode mixture may be insufficient, causing low mechanical strength of the electrode mixture sheet, adversely affecting battery characteristics such as cyclability. Too large a proportion of the binder may result in a decrease in battery capacity and electrical conductivity. Since the binder of the present invention has excellent adhesive force, the active material of the electrode can be reliably fixed even with a low binder content.

[0160] In the electrode mixture of the present invention, the binder component preferably consists essentially of PTFE powder, more preferably consists of PTFE powder. The expression "the binder component consists essentially of PTFE powder" means that the amount of PTFE powder in the binder included in the electrode mixture is 95.0 mass% or more relative to the mass of the binder. The amount of PTFE powder is preferably 98.0 mass% or more, more preferably 99.0 mass% or more, still more preferably 99.5 mass% or more, particularly preferably 99.9 mass% or more, most preferably 99.95 mass% or more relative to the mass of the binder.

[0161] The electrode mixture of the present invention is preferably in the form of a sheet.

[0162] The electrode mixture of the present invention can be suitably used as an electrode mixture for a secondary storage battery. In particular, the electrode mixture of the present invention is suitable for lithium-ion secondary storage batteries. In the secondary storage battery, the electrode mixture of the present invention is usually used in the form of a sheet.

[0163] An example of a specific method for producing a sheet of electrode mixture comprising the electrode mixture of the present invention is described below. The sheet of electrode mixture can be produced by a production method comprising the steps of: (1) mixing a starting material composition comprising an active electrode material, a binder and, if necessary, an electrically conductive additive; (2) giving the starting material composition obtained in step (1) a three-dimensional shape; and (3) rolling the three-dimensional starting material composition obtained in step (2) to form a sheet.

[0164] In the initial material composition mixed in step (1), materials such as the electrode active material, the binder, etc. are simply mixed with each other and do not have a fixed shape. Specific examples of mixing methods include mixing using a W-shaped stirrer, a V-shaped stirrer, a drum stirrer, a ribbon-spiral stirrer, a conical screw mixer, a single-screw mixer, a twin-screw mixer, mixing runners, a mechanical stirrer, or a planetary stirrer.

[0165] Regarding the mixing conditions of the binder in step (1), the rotational speed is preferably 1000 rpm or less. The rotational speed is preferably 10 rpm or more, more preferably 15 rpm or more, even more preferably 20 rpm or more, and further preferably 900 rpm or less, more preferably 800 rpm or less, even more preferably 700 rpm or less. If the rotational speed is less than the specified range, the mixing time increases, which adversely affects the productivity. If the rotational speed exceeds the specified range, excessive fibrillation is likely to occur, which may cause the electrode mixture sheet to be weak and insufficiently flexible.

[0166] Giving the initial composition of materials a three-dimensional form in step (2) means transforming the initial composition of materials into a single mass. Specific examples of methods for giving a three-dimensional form are extrusion molding and pressing. The term "three-dimensional" does not imply a specific form and means any state of a single mass, including in the form of a rod, sheet, sphere, cube, etc.

[0167] A specific example of the rolling method in step (3) is a method of rolling a bulk initial material composition using, for example, a roller press, a flat press or a flattening roller.

[0168] After step (3), step (4) of applying a greater load to the obtained rolled sheet is preferably carried out in order to obtain a thinner product. It is also preferable to repeat step (4). In this way, greater flexibility is achieved not by a single reduction in the sheet thickness, but by stepwise rolling. The number of repetitions of step (4) is preferably two or more and 10 or less, more preferably three or more and nine or less. A specific example of a rolling method is a method using two or a plurality of rotating shafts between which the rolled sheet is passed in order to obtain a thinner sheet product.

[0169] In order to control the degree of fibrillation, after the step (3) or the step (4), a step (5) of coarsely crushing the rolled sheet, re-shaping the coarsely crushed product into a three-dimensional shape, and rolling the three-dimensional product to obtain a sheet product is preferably also carried out. Repeating the step (5) is also preferable. The number of repetitions of the step (5) is preferably one or more and 12 or less, more preferably two or more and 11 or less.

[0170] Specific examples of coarsely crushing the rolled sheet and re-shaping the coarsely crushed product into a three-dimensional shape in step (5) include a method of folding the sheet, a method of shaping the sheet into a rod or thin sheet, and a method of dividing the sheet into small pieces. The term "coarsely crushing" in the present context means changing the shape of the rolled sheet obtained in step (3) or step (4) to obtain a product of a different shape, which is rolled into a sheet shape in a subsequent step, and also includes simply folding the rolled sheet.

[0171] Step (5) may be followed by step (4); or step (5) may be repeated. Uniaxial or biaxial stretching may be performed in any of steps (2), (3), (4) and (5). The degree of fibrillation may also be adjusted according to the intensity of coarse grinding in step (5).

[0172] In step (3), (4) or (5), the rolling ratio expressed as a percentage is preferably 10% or more, more preferably 20% or more, and preferably 80% or less, more preferably 65% or less, still more preferably 50% or less. If the rolling ratio is less than the specified range, a greater number of rolling operations with a longer duration may be required, which is unfavorable from the viewpoint of productivity. If the rolling ratio exceeds the specified range, excessive fibrillation may occur, resulting in the electrode mixture sheet being insufficiently strong and insufficiently flexible. In the present context, the "rolling ratio" means a decrease in the thickness of the sample after rolling relative to the thickness before rolling. The sample before rolling may be a bulk raw material composition or a sheet raw material composition. The thickness of the sample is the thickness in the direction of application of the load during rolling.

[0173] The electrode mixture sheet may also be suitably produced by a method comprising the steps of:

(a) mixing the powder component and the binder to obtain an electrode mixture; and

(b) calendering or extrusion forming the electrode mixture to form a sheet,

wherein the mixing in step (a) comprises: (a1) homogenizing the powder component and the binder to a powder; and (a2) mixing the powder obtained in step (a1) to form an electrode mixture.

[0174] For example, PTFE has two transition temperatures, approximately 19°C and approximately 30°C. At temperatures below 19°C, PTFE can withstand agitation without losing its shape. However, at temperatures above 19°C, the structure of PTFE particles loosens and becomes more sensitive to mechanical shear. At temperatures above 30°C, more significant fibrillation occurs.

[0175] Thus, the homogenization in step (a1) is carried out preferably at 19°C or less, preferably at a temperature of from 0°C to 19°C.

In other words, step (a1) is preferably carried out so that the materials are mixed and thereby homogenized, but with little fibrillation.

The stirring in the next step (a2) is preferably carried out at a temperature of 30°C or more to enhance fibrillation.

[0176] Step (a2) is carried out preferably at a temperature of from 30°C to 150°C, more preferably from 35°C to 120°C, even more preferably from 40°C to 80°C.

In one embodiment of the invention, the calendering or extrusion moulding in step (b) is carried out at a temperature of from 30°C to 150°C, preferably from 35°C to 120°C, more preferably from 40°C to 100°C.

[0177] Mixing in step (a) is preferably carried out by applying shear force.

Specific examples of mixing methods include mixing using a W-type mixer, V-type mixer, drum mixer, ribbon-spiral mixer, conical screw mixer, single screw mixer, twin screw mixer, mixing runners, mechanical mixer, planetary mixer, Henkel mixer, or high-speed mixer.

[0178] Regarding the mixing conditions, the rotation speed and the mixing time are appropriately selected. For example, the rotation speed is 15,000 rpm or less, preferably 10 rpm or more, more preferably 1,000 rpm or more, even more preferably 3,000 rpm or more, and further preferably 12,000 rpm or less, more preferably 11,000 rpm or less, even more preferably 10,000 rpm or less. If the rotation speed is less than the specified range, the mixing time increases, which adversely affects the productivity. If the rotation speed exceeds the specified range, excessive fibrillation is likely to occur, which may cause the electrode mixture sheet to be weak.

Step (a1) is preferably carried out at a lower shear force than in step (a2).

[0179] In step (a2), the starting material composition preferably does not contain a liquid solvent, but a small amount of a lubricating additive may be used. In other words, a lubricating additive may be added to the powder starting material composition obtained in step (a1), thereby obtaining a paste.

[0180] Examples of a lubricating additive include, but are not limited to, water, ether compounds, alcohols, ionic liquids, carbonates, aliphatic hydrocarbons (e.g., weakly polar solvents such as heptane and xylene), isoparaffinic hydrocarbons, petroleum distillates (e.g., gasoline (C4-C10), naphtha (C4-C11), kerosene/paraffin (C10-C16), and any mixtures thereof).

[0181] The lubricating additive is characterized by a water content of preferably 1000 ppm or less.

A water content of 1000 ppm or less is preferable in view of preventing deterioration of the electrochemical device. More preferably, the water content is 500 ppm or less.

[0182] Preferably, the lubricating additive, if used, is a low polar solvent such as heptane or xylene, or an ionic liquid.

[0183] The amount of the lubricating additive, if used, is from 5.0 to 35.0 parts by weight, preferably from 10.0 to 30.0 parts by weight, more preferably from 15.0 to 25.0 parts by weight, relative to the total weight of the composition supplied to step (a1).

[0184] Preferably, the starting material composition substantially does not contain a liquid solvent. According to a conventional method for producing an electrode mixture, a solvent with a binder dissolved therein is usually used to obtain a suspension containing an electrode mixture component in the form of a dispersed powder, the suspension is applied to a plane and dried, obtaining a sheet of the electrode mixture. In this case, a solvent is used to dissolve the binder. However, a solvent capable of dissolving the binder usually used in conventional cases is selected from a limited number of solvents; for example, butyl butyrate is used. Solvents react with a solid electrolyte, deteriorate its quality and can lead to a decrease in the performance characteristics of the battery. In addition, low-polarity solvents such as heptane can dissolve very few types of binders and are characterized by a low flash point, which can complicate their handling.

[0185] Eliminating the use of a solvent and using a powdered binder with a lower water content when creating a sheet of electrode mixture makes it possible to produce a battery that is less likely to damage the solid electrolyte. The above-described production method produces a sheet of electrode mixture with a binder having a fine-fiber structure, while the production process is simplified by eliminating the stage of making the suspension.

[0186] Step (b) comprises calendering or extrusion. Calendering and extrusion can be carried out by known methods. In this way, the material can be given the form of a sheet of electrode mixture.

Step (b) preferably comprises (b1) giving the electrode mixture obtained in step (a) a bulk form and (b2) calendering or extrusion forming the bulk electrode mixture.

[0187] Giving the electrode mixture a volumetric form means forming a single mass from the electrode mixture.

Specific examples of methods for forming a single mass are extrusion molding and pressing.

The term "bulk" does not imply a specific shape and means any state of a single mass, including in the form of a rod, sheet, sphere, cube, etc. As for its size, the diameter or minimum side of the cross-section is preferably 10,000 μm or more, more preferably 20,000 μm or more.

[0188] One specific example of calendering or extrusion molding in step (b2) is the method of rolling the electrode mixture using a roller press or a flattening roller.

[0189] Step (b) is preferably carried out at a temperature of from 30°C to 150°C. As indicated above, PTFE has a glass transition temperature of about 30°C, and at a temperature of 30°C or more, it undergoes significant fibrillation. Therefore, step (b) is preferably carried out at the indicated temperature.

[0190] During calendering or extrusion molding, a shear force is applied, causing the PTFE to fibrillate and be shaped.

[0191] After step (b), step (c) can preferably be carried out in which a greater load is applied to the obtained rolled sheet in order to obtain a thinner sheet product. It is also preferable to repeat step (c). As indicated above, greater flexibility can be achieved not by a single reduction in the sheet thickness, but by stepwise rolling.

The number of repetitions of step (c) is preferably two or more and 10 or less, more preferably three or more and nine or less.

A specific example of a rolling method is a method using two or more rotating rollers between which the sheet being rolled is passed in order to obtain a thinner sheet product.

[0192] In order to adjust the strength of the sheet, after the step (b) or the step (c), it is preferable to also perform the step (d) of roughly crushing the rolled sheet, re-shaping the roughly crushed product into a three-dimensional shape, and rolling the three-dimensional product to obtain a sheet product. It is also preferable to repeat the step (d). The number of repetitions of the step (d) is preferably one or more and 12 or less, more preferably two or more and 11 or less.

[0193] Specific examples of coarsely grinding the rolled sheet and re-shaping the coarsely ground product into a three-dimensional form in step (d) are a method of folding the sheet, a method of shaping the sheet into a rod or thin sheet product, and a method of dividing the sheet into small pieces. The term "coarsely grinding" in the present context means changing the shape of the rolled sheet obtained in step (b) or step (c) to obtain a product of a different shape, which is rolled into a sheet form in a subsequent step, and also covers simply folding the rolled sheet.

[0194] Step (d) may be followed by step (c) or may be repeated.

At any of stages (a), (b), (c) and (d) uniaxial or biaxial tension may be performed.

The sheet strength can also be adjusted according to the intensity of coarse grinding in step (d).

[0195] In step (b), (c) or (d), the rolling ratio expressed as a percentage is preferably 10% or more, more preferably 20% or more, and preferably 80% or less, more preferably 65% or less, still more preferably 50% or less. If the rolling ratio is less than the specified range, a greater number of rolling operations with a longer duration may be required, which is unfavorable from the viewpoint of productivity. If the rolling ratio exceeds the specified range, excessive fibrillation may occur, causing the electrode mixture sheet to be insufficiently strong and insufficiently flexible.

In the present context, "degree of rolling" means the decrease in the thickness of the specimen after rolling relative to the thickness before rolling. The specimen before rolling may be a bulk material composition or a sheet material composition. The thickness of the specimen is the thickness in the direction of application of the load during rolling.

Steps (c) and (d) are carried out at a temperature of preferably 30°C or more, more preferably 60°C or more, and preferably 150°C or less.

[0196] The electrode mixture sheet can be used as an electrode mixture sheet for a secondary storage battery and can serve as either a negative electrode or a positive electrode. In particular, the electrode mixture sheet is suitable for a lithium-ion battery.

[0197] The present invention also provides an electrode comprising the PTFE powder of the present invention or the electrode binder of the present invention, an electrode active material and a current collector. The electrode of the present invention is capable of reducing or preventing gas evolution inside a battery cell and deterioration of battery performance (e.g., capacity reduction during storage at high temperature). The electrode of the present invention also has high strength.

[0198] The electrode of the present invention may comprise the above-described electrode mixture of the present invention (preferably, a sheet of electrode mixture) and a current collector.

[0199] The electrode of the present invention may be a positive electrode or a negative electrode.

[0200] The positive electrode is preferably formed by a current collector and a sheet of electrode mixture containing an active material of the positive electrode. Examples of the material of the positive electrode current collector include metallic materials such as aluminum, titanium, tantalum, stainless steel and nickel, as well as their alloys; and carbon materials such as carbon cloth and carbon paper. Any metallic material is preferable, especially aluminum or its alloy.

[0201] When made of a metal material, the current collector may have the form of a metal foil, a metal cylinder, a metal spiral, a metal plate, an expanded metal sheet, a perforated metal sheet, a foamed metal, etc. In the case of a carbon material, it may have the form of a carbon plate, a carbon film, a carbon cylinder, etc. Of these, the most preferable is the form of a metal foil. If appropriate, the metal foil may be a mesh. The metal foil may have any thickness, the thickness thereof is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and wherein it is usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thickness of the metal foil is less than the specified range, the current collector may not be strong enough. On the contrary, a metal foil of a greater thickness than the specified range may be inconvenient to process.

[0202] In order to reduce the resistance of the electrical contact between the current collector and the active material layer of the positive electrode, an electrically conductive additive is preferably applied to the surface of the current collector. Examples of the electrically conductive additive are carbon and noble metals such as gold, platinum and silver.

[0203] The ratio of the thicknesses of the current collector and the positive electrode electrode mixture layer may be any; the value of the ratio {(thickness of the positive electrode electrode mixture layer on one side immediately before the introduction of the electrolyte solution)/(thickness of the current collector)} is preferably 20 or less, more preferably 15 or less, most preferably 10 or less. This ratio is also preferably 0.5 or more, more preferably 0.8 or more, most preferably 1 or more. If the ratio of the thicknesses of the current collector and the positive electrode electrode mixture is greater than the specified range, the current collector may be heated by Joule heating during charging/discharging at a high current density. If the ratio of the thicknesses of the current collector and the positive electrode electrode mixture is less than the specified range, the volume of the current collector may be too large relative to the active material of the positive electrode, thereby reducing the capacity of the battery.

[0204] The positive electrode can be manufactured in a conventional manner. In one example of a manufacturing method, a sheet of electrode mixture and a current collector are layered on top of each other using an adhesive, after which vacuum drying is performed.

[0205] The density of the positive electrode composite sheet is preferably 3.00 g/cm ³ or more, more preferably 3.10 g/cm ³ or more, even more preferably 3.20 g/cm ³ or more, and further preferably 3.80 g/cm ³ or less, more preferably 3.75 g/cm ³ or less, still more preferably 3.70 g/cm ³ or less. A positive electrode composite sheet of a higher density than the above may make it difficult for the electrolyte solution to penetrate to the interface between the current collector and the active material and cause inadequate charge and discharge characteristics, especially at a high current density, without ensuring a high output power. A positive electrode composite sheet of a lower density than the above may cause insufficient electrical conductivity between the active materials and an increase in the resistance of the battery, without ensuring a high output power.

[0206] In order to improve the stability at a high output power and a high temperature, the area of the positive electrode mixture sheet is preferably larger than the area of the outer surface of the battery outer case. Namely, the total area of the positive electrode mixture is preferably 15 times larger, more preferably 40 times larger than the surface area of the outer case of the secondary battery. In the case of closed square-section cases, the outer surface area of the battery outer case is the total area calculated based on dimensions including the length, width and thickness of the case portion in which the energy generating element is accommodated, excluding the protruding portion of the terminal. In the case of closed cylindrical cases, the outer surface area of the battery outer case is the geometric surface area of the approximately cylindrical shape of the case portion in which the energy generating element is accommodated, excluding the protruding portion of the terminal. Here, the total area of the positive electrode mixture is the geometric surface area of the positive electrode mixture layer opposite the mixture layer containing the active material of the negative electrode. In designs that include a foil-shaped current collector and layers of positive electrode mixture on both sides of the current collector, the total area of the positive electrode is equal to the sum of the areas calculated for the corresponding sides.

[0207] The positive electrode may have any thickness. In view of achieving a large capacity and a high output power, the lower limit of the thickness of the mixture layer on one side of the current collector excluding the thickness of the metal foil of the current collector is preferably 10 μm or more, more preferably 20 μm or more, and further preferably 500 μm or less, more preferably 450 μm or less.

[0208] A substance other than the composition of the positive electrode may be bonded to the surface of the positive electrode. Examples of the substance bonded to the surface are oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, magnesium carbonate; and carbon.

[0209] The negative electrode is preferably formed by a current collector and a sheet of electrode mixture containing a negative electrode active material. Examples of the negative electrode current collector material include metallic materials such as copper, nickel, titanium, tantalum and stainless steel, as well as their alloys; and carbon materials such as carbon cloth and carbon paper. Any metallic material is preferable, especially copper, nickel or their alloy.

[0210] When made of a metal material, the current collector may have the form of a metal foil, a metal cylinder, a metal spiral, a metal plate, an expanded metal sheet, a perforated metal sheet, a foamed metal, etc. In the case of a carbon material, it may have the form of a carbon plate, a carbon film, a carbon cylinder, etc. The most preferable of these is the form of a metal foil. If appropriate, the metal foil may be a mesh. The metal foil may have any thickness, the thickness thereof is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and herein, usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thickness of the metal foil is less than the specified range, the current collector may not be strong enough. On the contrary, a metal foil of a greater thickness than the specified range may be inconvenient to process.

[0211] The negative electrode can be manufactured in a conventional manner. In one example of a manufacturing method, a sheet of electrode mixture and a current collector are layered on top of each other using an adhesive, after which vacuum drying is performed.

[0212] The density of the negative electrode mixture is preferably 1.3 g/cm ³ or more, more preferably 1.4 g/cm ³ or more, even more preferably 1.5 g/cm ³ or more, and further preferably 2.0 g/cm ³ or less, more preferably 1.9 g/cm ³ or less, still more preferably 1.8 g/cm ³ or less. A negative electrode mixture of a higher density than the above may make it difficult for the electrolyte solution to penetrate to the interface between the current collector and the active material and cause inadequate charge and discharge characteristics, especially at a high current density, without ensuring a high output power. A negative electrode mixture of a higher density than the above may cause insufficient electrical conductivity between the active materials and an increase in the resistance of the battery, without ensuring a high output power.

[0213] The negative electrode may have any thickness. In view of achieving a large capacity and a high output power, the lower limit of the thickness of the mixture layer on one side of the current collector excluding the thickness of the metal foil of the current collector is preferably 10 μm or more, more preferably 20 μm or more, and further preferably 500 μm or less, more preferably 450 μm or less.

[0214] The present invention provides a secondary storage battery including the above-described electrode of the present invention.

[0215] The secondary battery of the present invention may be a battery with a liquid electrolyte or a solid electrolyte.

[0216] A liquid electrolyte battery can be obtained by using conventional components of a secondary battery, such as an electrolyte solution and a separator. This is explained in detail below.

[0217] Preferably, a non-aqueous electrolyte solution is used as the electrolyte solution. The non-aqueous electrolyte solution used may be a solution obtained by dissolving a known electrolyte salt in a known organic solvent.

[0218] Any organic solvent can be used to dissolve the electrolyte salt. One or more known hydrocarbon solvents can be used, for example, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; and fluorine-containing solvents, for example, fluoroethylene carbonate, fluoroether and fluorinated carbonate.

[0219] Examples of the electrolyte salt are LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN(SO ₂ CF ₃ ) ₂ and LiN(SO ₂ C ₂ F ₅ ) ₂ . From the standpoint of achieving favorable cycling parameters, LiPF ₆ , LiBF ₄ , LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ and any combination thereof are particularly preferred.

[0220] The concentration of the electrolyte salt is preferably 0.8 mol/L or more, more preferably 1.0 mol/L or more. The upper limit is usually 1.5 mol/L, but it depends on what organic solvent is used to dissolve the electrolyte salt.

[0221] The secondary battery obtained using the electrolyte solution preferably also includes a separator. The separator may be made of any known material and may have any known shape, provided that the separator is stable in the electrolyte solution and has sufficient liquid retention capacity. Preferably, the separator has the form of a porous sheet or non-woven fabric made of a material that is stable in the electrolyte solution, such as a polymer, fiberglass, or inorganic material, and has sufficient liquid retention capacity.

[0222] Examples of the material for producing the separator made of polymer or glass fiber include polyolefins such as polyethylene and polypropylene, aromatic polyamide, polytetrafluoroethylene, polyethersulfone, and glass fiber. One of these materials may be used alone; or two or more of them may be used in any combination and any ratio, for example, in the form of a two-layer polypropylene/polyethylene film or a three-layer polypropylene/polyethylene/polypropylene film. In view of ensuring proper penetration of the electrolyte solution and a sufficient separating effect, the separator is preferably in the form of a porous sheet or a non-woven fabric made of a polyolefin such as polyethylene or polypropylene.

[0223] The separator may have any thickness, usually the thickness is 1 μm or more, preferably 5 μm or more, more preferably 8 μm or more, and usually 50 μm or less, preferably 40 μm or less, more preferably 30 μm or less. A separator with a thickness less than the specified range may have insufficient insulating ability and mechanical strength. A separator with a thickness greater than the specified range may cause not only low battery performance such as rated parameters, but also low energy density of the battery as a whole.

[0224] A porous separator such as a porous sheet or a non-woven fabric may have any porosity. Typically, the porosity is 20% or more, preferably 35% or more, more preferably 45% or more, and typically 90% or less, preferably 85% or less, more preferably 75% or less. A separator with a porosity less than the specified range is usually characterized by high film resistance, which deteriorates the performance. If the porosity of the separator is greater than the specified range, it has insufficient mechanical strength and therefore poor insulating ability.

[0225] The separator may also be characterized by any average pore size. The average pore size is typically 0.5 μm or less, preferably 0.2 μm or less, with typically 0.05 μm or more. A separator with an average pore size greater than the specified range may cause a short circuit. A separator with an average pore size less than the specified range may have high film resistance, causing deterioration in performance.

[0226] Examples of inorganic material include oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate, each of which may be in the form of particles or fibers.

[0227] The separator has the form of a thin film, for example, a non-woven fabric, a woven fabric or a microporous film. It is advantageous that the thin film includes pores of a size from 0.01 to 1 μm and has a thickness of from 5 to 50 μm. Instead of the separate thin film described above, the separator can have a structure in which a composite porous layer containing particles of the said inorganic material is applied to the surface of one of the electrodes or each of the positive and negative electrodes using a polymer binder. For example, aluminum oxide particles including 90% of particles smaller than 1 μm in size can be applied to the corresponding surfaces of the positive electrode using fluororubber as a binder to obtain a porous layer.

[0228] The functional group with the electrode mixture may have either a layered structure including the above-described positive and negative electrodes and a separator located therebetween, or a structure obtained by winding including the above-described positive and negative electrodes wound in a spiral and a separator located therebetween. The proportion of the internal volume of the battery occupied by the functional group with the electrode mixture (hereinafter referred to as the proportion of the functional group with the electrode mixture) is usually 40% or more, preferably 50% or more, and usually 90% or less, preferably 80% or less.

[0229] If the proportion of the functional group with the electrode mixture is less than the specified range, the capacity of the battery may be low. If the proportion of the functional group with the electrode mixture is greater than the specified range, the free volume inside the battery may be insufficient. That is, if the temperature of the battery rises greatly, causing the components to swell, and the internal pressure increases due to the increase in the vapor pressure of the liquid fraction of the electrolyte solution, the battery performance such as charge and discharge reproducibility and suitability for storage at high temperatures deteriorates, and the gas relief valve designed to reduce the internal pressure may operate.

[0230] The current collector may have any design. In view of the increase in the efficiency of the electrolyte solution in the process of charging and discharging at a high current density, the design of the current collector preferably allows for a decrease in the resistance in the wiring and connection sections. Such a decrease in the internal resistance can particularly favorably influence the effects achieved when using the electrolyte solution.

[0231] In a functional group with an electrode mixture having a layered structure, the metal base elements of the respective layers of the electrode mixture are preferably bundled and welded to a terminal. If the electrode mixture has a large area, the internal resistance is high. Therefore, it is preferable to form a plurality of terminals in the electrode mixture in order to reduce the resistance. In a functional group with an electrode mixture having a structure obtained by winding, a plurality of terminals may be formed on the positive and negative electrodes, brought together in a bundle at one common terminal. Due to this, the internal resistance is reduced.

[0232] The outer casing may be made of any material that is resistant to the electrolyte solution. Specific examples thereof are metals such as nickel-plated steel plates, stainless steel, aluminum and aluminum alloys and magnesium alloys, and a multilayer (laminated) film of a polymer and aluminum foil. In view of the reduction in weight, it is preferable to use a metal such as aluminum or an aluminum alloy, or a multilayer film.

[0233] The outer casing made of metal may have a hermetic structure obtained by welding metal using a laser welding method, contact welding or ultrasonic welding; or a sealable structure with a polymer gasket between metal parts. The outer casing made of a multilayer film may have a hermetic structure obtained by thermally fusing the polymer layers. To improve the hermeticity, a polymer different from that included in the multilayer film may be placed between the polymer layers. In particular, in the case of creating a hermetic structure by thermally fusing the polymer layers with the current collector leads between them, it is necessary to connect the metal and the polymer. Therefore, the polymer placed between the polymer layers is preferably a polymer including a polar group, or a modified polymer into which a polar group is introduced.

[0234] The lithium-ion secondary battery using the electrolyte solution may have any shape, such as cylindrical, square, multi-layer, coin-shaped, or large-sized. The shapes and structures of the positive electrode, the negative electrode, and the separator may be changed according to the shape of the battery.

[0235] The secondary battery with a solid electrolyte is preferably a fully solid-state secondary battery. The solid-state secondary battery is preferably a lithium-ion battery or, preferably, a fully solid-state sulfide-based secondary battery.

The solid-state secondary battery preferably includes a positive electrode, a negative electrode, and a solid electrolyte layer between the positive electrode and the negative electrode.

[0236] The solid electrolyte used in the solid-state mixture of the secondary storage battery may be a sulfide-based solid electrolyte or an oxide-based solid electrolyte. In particular, when a sulfide-based solid electrolyte is used, the solid-state mixture of the secondary storage battery is advantageously flexible.

[0237] The sulfide-based solid electrolyte has no specific limitations. The sulfide-based solid electrolyte used may be any electrolyte selected from the following: Li ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₃ , Li ₂ SP ₂ S ₃ -P ₂ S ₅ , Li ₂ S-SiS ₂ , LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , LiI-Li ₂ S-SiS ₂ -P ₂ S ₅ , Li ₂ S-SiS ₂ -Li ₄ SiO ₄ , Li ₂ S-SiS ₂ -Li ₃ PO ₄ , Li ₃ PS ₄ -Li ₄ GeS ₄ , Li _3.4 P _0.6 Si _0.4 S ₄ , Li _3.25 P _0.25 Ge _0.76 S ₄ , Li _4-x Ge _1-x P _x S ₄ (X=from 0.6 to 0.8), Li _4+y Ge _1-y Ga _y S ₄ (y=from 0.2 to 0.3), LiPSCl, LiCl, Li _7-x-2y PS _6-xy Cl _x (0.8 ≤ x ≤ 1.7, 0 < y ≤ -0.25x+0.5) or a mixture of two or more of them.

[0238] Preferably, the sulfide-based solid electrolyte contains lithium. Lithium-containing sulfide-based solid electrolytes are used in those solid-state batteries where the carriers are lithium ions, and are particularly preferred because they make it possible to create electrochemical devices characterized by a high current density.

[0239] The solid oxide-based electrolyte is preferably a compound comprising an oxygen atom (O), has the ability to conduct ions of a metal of Group 1 or Group 2 of the Periodic Table of Elements and electrical insulating properties.

[0240] Specific examples of such a compound are Li _xa La _ya TiO ₃ (xa = 0.3 to 0.7, ya = 0.3 to 0.7) (LLT), Li _xb La _yb Zr _zb M ^bb _mb O _nb (where M ^bb includes at least one element selected from the following: Al, Mg, Ca, Sr, V, Nb, Ta, Ti, Ge, In and Sn; xb satisfies the relationship 5 ≤ xb ≤ 10; yb satisfies the relationship 1 ≤ yb ≤ 4; zb satisfies the relationship 1 ≤ zb ≤ 4; mb satisfies the relationship 0 ≤ mb ≤ 2; and nb satisfies the relationship 5 ≤ nb ≤ 20), Li _xc B _yc M ^cc _zc O _nc (where M ^cc includes at least one element selected from the following: C, S, Al, Si, Ga, Ge, In and Sn; xc satisfies the relation 0 ≤ xc ≤ 5; yc satisfies the relation 0 ≤ yc ≤ 1; zc satisfies the relation 0 ≤ zc ≤ 1; and nc satisfies the relation 0 ≤ nc ≤ 6), Li _xd (Al, Ga) _yd (Ti, Ge) _zd Si _ad P _md O _nd (1 ≤ xd ≤ 3, 0 ≤ yd ≤ 2, 0 ≤ zd ≤ 2, 0 ≤ ad ≤ 2, 1 ≤ md ≤ 7, and 3 ≤ nd ≤ 15), Li _(3-2xe) M ^ee _xe D ^ee O (where xe is a number from 0 or more and 0.1 or less, M ^ee means a divalent metal atom, D ^ee means a halogen atom or a combination of two or more halogen atoms), Li _xf Si _yf O _zf (1 ≤ xf ≤ 5, 0 < yf ≤ 3 and 1 ≤ zf ≤ 10), Li _xg S _yg O _zg (1 ≤ xg ≤ 3, 0 < yg ≤ 2 and 1 ≤ zg ≤ 10), Li ₃ BO ₃ -Li ₂ SO ₄ , Li ₂ OB ₂ O ₃ -P ₂ O ₅ , Li ₂ O-SiO ₂ , Li ₆ BaLa ₂ Ta ₂ O ₁₂ , Li ₃ PO _(4-3/2w) N _w (w < 1), Li _3.5 Zn _0.25 GeO ₄ with lithium superionic conductor crystal structure (LISICON), La _0.51 Li _0.34 TiO _2.94 with perovskite crystal structure, La _0.55 Li _0.35 TiO ₃ , LiTi ₂ P ₃ O ₁₂ with sodium superionic conductor crystal structure (NASICON), Li _1+xh+yh (Al, Ga) _xh (Ti, Ge) _2-xh Si _yh P _3-yh O ₁₂ (0 ≤ xh ≤ 1 and 0 ≤ yh ≤ 1), and Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ) with garnet crystal structure. Also known are ceramic materials which are LLZ with substituted elements. Examples are LLZ-based ceramic materials in which the substitution in LLZ is performed using at least one element from magnesium (Mg) and A (A includes at least one element selected from the group consisting of calcium (Ca), strontium (Sr) and barium (Ba)). It is also possible and preferable to use phosphorus compounds containing Li, P and O. Examples thereof are lithium phosphate (Li ₃ PO ₄ ), LiPON in which one or more oxygen atoms of lithium phosphate are substituted by nitrogen, and LiPOD ¹ (where D ¹ includes at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Ag, Ta, W, Pt, Au, etc.). LiA ¹ ON (where A ¹ includes at least one element selected from Si, B, Ge, Al, C, Ga, etc.). Specific examples are Li ₂ O-Al ₂ O ₃ -SiO ₂ -P ₂ O ₅ -TiO ₂ -GeO ₂ and Li ₂ O-Al ₂ O ₃ -SiO ₂ -P ₂ O ₅ -TiO ₂ .

[0241] The solid oxide-based electrolyte preferably contains lithium. Solid oxide-based electrolytes containing lithium are used in those solid-state batteries where the carriers are lithium ions, they are particularly preferred because they make it possible to create electrochemical devices characterized by a high current density.

[0242] The oxide-based solid electrolyte is preferably an oxide with a crystalline structure. Oxides with a crystalline structure are particularly preferable due to good lithium ion conductivity. The oxide with a crystalline structure may be of the perovskite type (La _0.51 Li _0.34 TiO _2.94 , etc.), the NASICON type (Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , etc.), or the garnet type (Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ), etc.). Of these, the NASICON type is preferable.

[0243] The volume average particle size of the oxide solid electrolyte is not particularly limited. However, the volume average particle size is preferably 0.01 μm or more, more preferably 0.03 μm or more. The upper limit is preferably 100 μm or less, more preferably 50 μm or less. The volume average particle size of the oxide-based solid electrolyte is measured as follows. A 1 mass % dispersion of the oxide-based solid electrolyte particles is prepared by diluting with water (or heptane in the case of a substance unstable in water) in a 20 ml test tube. A sample of the diluted dispersion is irradiated with ultrasonic waves of 1 kHz for 10 minutes and immediately used for testing. For this dispersion sample, measurements were performed 50 times using a laser diffraction particle size analyzer LA-920 (Horiba, Ltd.) using a quartz measuring cell at a temperature of 25°C, and the volume average particle size was calculated. If necessary, detailed description of other measurement conditions can be found in JIS Z8828:2013 "Particle size analysis - Dynamic light scattering". For each concentration level, five samples were used and the average value was calculated.

[0244] The solid-state secondary battery may include a separator located between the positive electrode and the negative electrode. Examples of the separator include porous films such as polyethylene and polypropylene; a non-woven fabric made of a polymer such as polypropylene; and a non-woven fabric made of fiberglass.

[0245] The solid-state secondary battery may also include a housing. The battery housing may have any shape that can accommodate the above-mentioned components, that is, the positive electrode, the negative electrode, and the solid electrolyte layer. More specifically, the battery may be of a cylindrical type, a square type, a coin type, or a multilayer type.

[0246] A solid-state secondary battery can be manufactured, for example, by layering a positive electrode, a sheet layer of solid electrolyte, and a negative electrode in that order and then pressing.

EXAMPLES

[0247] The present invention will now be described in more detail with reference to examples, but the invention is not limited to these examples.

[0248] Various physical properties were measured by the following methods.

[0249] Average primary particle size

An aqueous dispersion of PTFE was diluted with water to a solid phase concentration of 0.15% by weight. The transmission of light with a wavelength of 550 nm per unit length of the diluted dispersion was measured. Feret diameters were measured in the image obtained using a transmission electron microscope and the numerical average particle size along the length was determined based on them. These values were used to plot a calibration curve. Using the calibration curve, the average number particle size was determined based on the transmission of incident light with a wavelength of 550 nm measured for each sample and was taken as the average size of the primary particles.

[0250] Apparent Density

The apparent density was measured in accordance with JIS K6892.

[0251] Average secondary particle size

The average secondary particle size was measured according to JIS K6891.

[0252] Standard Specific Gravity (SSG)

Standard specific gravity was determined by the water displacement method in accordance with ASTM D792 using a specimen cast in accordance with ASTM D4895.

[0253] PMVE Quantity

PTFE powder was melted at 370°C and analyzed by ¹⁹ F-NMR. The amount of PMVE was calculated based on the obtained functional group signals using the following equation.

PMVE content (% mass) = (664B/(300A+364B)) × 100

(A: The total integral value of the CF ₂ signal is approximately -120 ppm and the CF signal is approximately -136 ppm; B: The integral value of the CF ₃ PMVE signal is approximately -54 ppm.)

The chemical shift value used was obtained for the peak apex of the CF ₂ signal of the polymer backbone at -120 ppm.

[0254] CTFE Contents

The PTFE powder was pressed into a thin film disc, which was measured by Fourier transform infrared spectroscopy (FT-IR) to obtain IR absorption data. The CTFE content was determined by multiplying the ratio (absorption at 957 cm ^-1 )/(absorption at 2360 cm ^-1 ) by 0.58.

[0255] HFP Contents

The PTFE powder was pressed into a thin film disc, which was measured by FT-IR and IR absorption data were obtained. The HFP content was determined by multiplying the ratio (absorption at 982 cm ^-1 )/(absorption at 935 cm ^-1 ) by 0.3.

[0256] RR100 Extrusion (Extrusion at Compression Ratio 100)

Fifty grams of PTFE powder and 10.25 g of hydrocarbon oil (Isopar-E, ExxonMobil Corporation) as an extrusion aid were mixed for three minutes in a polyethylene container. The extruder barrel was filled with the mixture at room temperature (25±2°C). A load of 0.47 MPa was applied to the piston inserted into the barrel and maintained for one minute. The mixture was then extruded through the extruder die at a piston speed of 18 mm/min. The ratio of the cross-sectional area of the barrel to the cross-sectional area of the extruder die was 100. The value obtained by dividing the load (N) at which the pressure in the second half of the extrusion operation is equilibrated by the cross-sectional area of the extruder was taken as the extrusion pressure (MPa).

[0257] Water content

PTFE powder in an amount of about 20 g was weighed before and after heating at 150 °C for two hours, and the water content was calculated using the following formula. A powder sample was taken three times, the calculation was carried out for each sample, and the obtained values were averaged.

Water content (% by mass) = [(mass (g) of PTFE powder before heating) - (mass (g) of PTFE powder after heating)] / (mass (g) of PTFE powder before heating) × 100

[0258] Expansion Test

The rod obtained by extruding the paste as described above was dried at 230°C for 30 minutes to remove the slip. The dried rod was cut into pieces of appropriate length and placed in an oven heated to 300°C. In the oven, the pieces were expanded at a rate of 100%/s until their length increased to 25 times their length before the expansion test. If no fracture occurred during the expansion, the rod was considered expandable, and the fractured rods were considered as not expandable.

[0259] Battery Rating (1)

According to the methods described below, sheets of the mixture of examples 1-9 and comparative examples 1 and 2 were produced and an evaluation of the sheets of electrodes and batteries was carried out.

Manufacturing of positive electrode electrode mixture sheet

Li(Ni _0.6 Mn _0.2 Co _0.2 )O ₂ (NMC622) as the active material of the positive electrode and carbon black as the conductive additive were weighed and stirred at a speed of 30 rpm for 300 s using a pressure mixer, thereby obtaining a mixture.

The PTFE powder obtained in each of the examples and comparative examples was added to this mixture as a binder, then stirred at a speed of 50 rpm for 300 s, obtaining a mixture. In this case, the mass ratio (positive electrode active material: binder: conductive additive) was set to 95:2:3.

The resulting mixture was given a three-dimensional shape and rolled into a sheet.

The rolled sheet was folded in half, performing a coarse grinding operation, again given a three-dimensional shape and rolled into a sheet using metal rollers and a flat plate, thereby promoting fibrillation. These actions were repeated four times. Then, the resulting sheet was further rolled, obtaining a sheet of the electrode mixture of the positive electrode with a thickness of about 500 μm. The sheet of the electrode mixture of the positive electrode was cut into pieces of 5 cm x 5 cm, placed them in a roller press and rolled. For further fibrillation, a load of 2 kN was repeatedly applied, adjusting the thickness. The distance between the rollers was adjusted so that the final thickness of the electrode mixture layer of the positive electrode was 90 μm, and the density was 3.30 g/cm ³ . In examples 6-9, the thickness was 90 μm, and the density was 3.60 g/cm ³ .

[0260] Measurement of the strength of the positive electrode electrode mixture sheet

The positive electrode mixture sheet was cut into strips of 4 mm in width to obtain test specimens. A tensile testing machine (AGS-100NX, manufactured by Shimadzu Corp.) was used to determine the strength of the test specimen at a tensile stress of 100 mm/min. The length of the section fixed in the chuck was 30 mm. The specimen was stretched until failure, and the maximum tensile stress measured was taken as the strength of each specimen. The obtained result was related to the strength of the specimen of Comparative Example 2, which was taken as 100.

[0261] Making a positive electrode

The positive electrode electrode mixture sheet was connected to 20 μm thick aluminum foil as follows.

The adhesive used was a suspension obtained by dissolving polyvinylidene fluoride (PVDF) in n-methylpyrrolidone (NMP) and dispersing carbon nanotubes (CNT) in it. This adhesive was applied to aluminum foil and dried on an electric hotplate at 120ºC for 15 min, thereby obtaining a current collector with an adhesive layer.

After that, a sheet of the positive electrode electrode mixture was placed on the current collector with a layer of adhesive and they were connected to each other using a roller press heated to 180ºC. The workpiece was cut into samples of a given size and a label was attached to each. These samples were used as a positive electrode.

[0262] Making a negative electrode

First, 98 parts by weight of carbon material (graphite) were mixed with 1 part by weight of aqueous dispersion of sodium carboxymethylcellulose (sodium carboxymethylcellulose concentration 1% by weight) and 1 part by weight of aqueous dispersion of styrene-butadiene rubber (styrene-butadiene rubber concentration 50% by weight), which acted as a thickener and a binder, respectively. The components were mixed using a dispersing device and a suspension was obtained. This suspension was applied to copper foil 10 μm thick and dried. The workpiece was rolled out using a roller press, cut into samples of a given size, and labels were attached to them. These samples were used as a negative electrode.

[0263] Preparation of electrolyte solution

A mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC=30:70 (volume ratio)) in an organic solvent was weighed into a test tube, fluoroethylene carbonate (FEC) and vinylene carbonate were added and dissolved in it, each in an amount of 1% by weight, to obtain a mixed solution. LiPF ₆ was mixed with it at 23ºC in such an amount that its concentration in the electrolyte solution was 1.0 mol/L.

[0264] Fabrication of a multilayer cell with aluminum foil

A porous polyethylene film (separator) 20 μm thick was placed between the positive electrode and negative electrode prepared as described above. The electrolyte solution prepared as described above was added and the non-aqueous electrolyte solution was allowed to sufficiently soak the components, including the separator. The workpiece was then sealed and pre-charged, thereby obtaining a lithium-ion secondary battery.

[0265] Evaluation of storage parameters (residual capacity coefficient, amount of released gas)

The manufactured lithium-ion secondary battery was charged under constant current/constant voltage (hereinafter referred to as CC/CV charging) (0.1C cutoff) to 4.3 V at a current corresponding to 0.5 C at 25 °C, and then discharged to 3 V at a constant current of 0.5 C. This was taken as one cycle; the discharge capacity in the third cycle was used to determine the initial discharge capacity.

The battery for which the initial resistance was determined was again subjected to CC/CV charging (0.1C cutoff) at 25ºC to 4.3 V, and the battery volume was determined. After the battery volume was determined, it was stored at 60ºC for 30 days. After storage at high temperature, the battery was cooled sufficiently, and its volume was determined at 25ºC. Based on the difference in the battery volume before and after the storage test, the amount of gas released was determined. This amount was compared with the amount of gas released in Comparative Example 1, which was taken as 100.

After determining the amount of gas released, the battery was discharged at 0.5C and 25ºC to 3 V and the residual capacity was determined.

The ratio of the residual capacity after high temperature storage to the initial discharge capacity was calculated and the obtained value was used as the residual capacity coefficient (%).

(Residual capacity)/(Initial charging capacity)×100=Residual capacity ratio (%)

[0266] Synthesis example

White solid A was obtained by the method described in synthesis example 1 of document WO 2021/045228.

[0267] 1 production example

Into a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle and a temperature control jacket, 3480 g of deionized water, 100 g of paraffin wax and 5.25 g of white solid A were charged. The inside of the autoclave was purged with nitrogen gas heated to 70 °C to remove oxygen. By introducing TFE, the pressure in the system was adjusted to 0.78 MPa g, the temperature in the system was maintained at 70 °C, and stirring was performed. Then, an aqueous solution obtained by dissolving 15.0 mg of ammonium persulfate in 20 g of water was introduced into the system together with TFE, starting the polymerization reaction. As the polymerization reaction progressed, the pressure in the system decreased. Accordingly, TFE was added while maintaining the temperature in the system at 70 °C and the pressure in the system at 0.78 MPa g.

When 400 g of TFE were consumed after the start of polymerization, an aqueous solution obtained by dissolving 18.0 mg of hydroquinone, which acted as a radical acceptor, in 20 g of water was introduced into the system together with TFE. The polymerization was continued. When the amount of TFE polymerized after the start of the reaction reached 1200 g, stirring and feeding of TFE were stopped. Immediately after this, the gas was released from the system to normal pressure and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 25.3% by weight and an average primary particle size of 310 nm.

[0268] Example 1

The aqueous dispersion of PTFE obtained in the 1st production example was diluted to a solid phase concentration of 13% by weight. The PTFE was coagulated by shaking the container, and the water was filtered to obtain a wet powder.

The resulting wet PTFE powder was placed in a stainless steel grid tray, which was heated in an electric furnace with hot air circulation at 210ºC. After five hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder.

The SSG of the obtained PTFE powder was 2.159. The extrusion pressure at RR10 was 15.1 MPa, and the PTFE powder was expandable.

[0269] Comparative Example 1

PTFE powder was obtained as in Example 1, except that a solid tray (the bottom and walls of which were impermeable to air) was used instead of a lattice tray.

[0270] 2 production example

Into a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle and a temperature control jacket, 3560 g of deionized water, 100 g of paraffin wax and 5.4 g of white solid A were charged. The inside of the autoclave was purged with nitrogen gas heated to 70 °C to remove oxygen. When introducing TFE, the pressure in the system was adjusted to 0.60 MPaG, the temperature in the system was maintained at 70 °C, and stirring was performed. Then, 0.60 g of perfluoro(methyl vinyl ether) (PMVE) was added to TFE. Then, an aqueous solution obtained by dissolving 15.0 mg of ammonium persulfate in 20 g of water was introduced into the system together with TFE, the pressure in the system was adjusted to 0.78 MPaG, and the polymerization reaction was started. As the polymerization reaction proceeded, the pressure in the system decreased. Accordingly, TFE was added, maintaining the temperature in the system equal to 70ºС and the pressure in the system equal to 0.78 MPa excess.

When 429 g of TFE were consumed after the start of polymerization, an aqueous solution obtained by dissolving 14.0 mg of hydroquinone, which acted as a radical acceptor, in 20 g of water was introduced into the system together with TFE. The polymerization was continued. When the amount of TFE polymerized after the start of the reaction reached 1225 g, stirring and feeding of TFE were stopped. Immediately after this, the gas was released from the system to normal pressure and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 25.4 wt. % and an average primary particle size of 243 nm.

[0271] Example 2

The aqueous dispersion of PTFE obtained in the 2nd production example was diluted to a solid phase concentration of 13% by weight. The PTFE was coagulated by shaking the container, and the water was filtered to obtain a wet powder.

The resulting wet PTFE powder was placed in a stainless steel grid tray, which was heated in an electric furnace with hot air circulation at 160ºC. After 18 hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder.

The PMVE content of the obtained PTFE powder was 0.046% by mass, the SSG of the PTFE powder was 2.145. The extrusion pressure at RR10 was 17.4 MPa, and the PTFE powder was expandable.

[0272] Example 3

PTFE powder was obtained as in Example 2, except that drying was carried out for five hours instead of 18.

The PMVE content of the obtained PTFE powder was 0.046% by mass, the SSG of the PTFE powder was 2.145. The extrusion pressure at RR10 was 16.8 MPa, and the PTFE powder was expandable.

[0273] The physical properties of the PTFE powder obtained in each of the examples were analyzed using the methods described above. In addition, using each of these PTFE powders, a sheet of the electrode mixture of the positive electrode, an electrode and a lithium-ion secondary battery were produced and evaluated using the methods described above. The results are presented in Table 1.

[0274] Table 1

Example 1 Comparative example 1 Example 2 Example 3 PTFE aqueous dispersion 1 production example 1 production example 2 production example 2 production example [Physical properties of PTFE powder] Average size of secondary particles microns 540 540 540 540 Apparent density g/ml 0.48 0.47 0.43 0.41 Water content % mass. 0.002 0.064 0,000 0.023 [Battery Rating (1)] Electrode strength - 850 850 1100 1100 Residual capacity coefficient % 72 70 73 72 Amount of gas released - 86 100 84 92

[0275] The results presented in Table 1 show that the amount of gas released in the battery in Examples 1 and 2, where PTFE powder containing substantially no water was used, is less than in Comparative Examples 1 and 2, where PTFE powder containing water was used.

[0276] 3 production example

A 6 L stainless steel polymerization reactor equipped with a stainless steel stirring paddle and a temperature control jacket was charged with 3600 g of deionized water, 180 g of paraffin wax, 5.4 g of white solid A, and 0.025 g of oxalic acid. The interior of the polymerization reactor was purged with nitrogen gas heated to 70 °C to remove oxygen. The reactor temperature was maintained at 70 °C with stirring, while TFE was added to bring the system pressure to 2.7 MPa g.

Deionized water containing 3.5 mg of dissolved potassium permanganate was continuously added to the reactor at a constant rate while the reactor contents were stirred. TFE was continuously fed so that the pressure in the polymerization reactor was constant at 2.7 MPa g. When the amount of TFE consumed reached 184 g, 5.3 g of white solid substance A was added. When the amount of TFE consumed reached 900 g, the addition of all the deionized water containing 3.5 mg of dissolved potassium permanganate was completed. When the amount of TFE consumed reached 1540 g, stirring and feeding of TFE were stopped. TFE was discharged from the polymerization reactor, the polymerization reaction was stopped. The resulting aqueous dispersion was withdrawn and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous PTFE dispersion had a solids concentration of 29.7 wt% and an average primary particle size of 296 nm.

[0277] 4 production example

Into a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle and a temperature control jacket, 3500 g of deionized water, 100 g of paraffin wax and 5.3 g of white solid A were charged. The inside of the autoclave was purged with nitrogen gas heated to 85 °C to remove oxygen. Then, an aqueous solution obtained by dissolving 260 mg of disuccinic acid peroxide in 20 g of water was introduced together with TFE. Then, an aqueous solution obtained by dissolving 15 mg of ammonium persulfate in 20 g of water was introduced into the system together with TFE, setting the pressure in the system to 0.8 MPaG, thereby starting the polymerization reaction. As the polymerization reaction progressed, the pressure in the system decreased. Accordingly, TFE was added, maintaining the temperature in the system equal to 85ºС and the pressure in the system equal to 0.8 MPa excess.

When the amount of TFE polymerized after the polymerization reaction started reached 1140 g, stirring and feeding of TFE were stopped. Immediately after that, the gas was released from the system to normal pressure and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 24.3 mass% and an average primary particle size of 330 nm.

[0278] 5 production example

In a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle, 3500 g of deionized water, 100 g of paraffin wax and 5.3 g of white solid A were charged. The inside of the system was purged with TFE. The inside of the autoclave was purged with nitrogen gas heated to 70 °C to remove oxygen. TFE was supplied so that the internal pressure was 0.78 MPaG, and 10 g of an aqueous solution of 0.6 wt % ammonium persulfate (APS) was supplied to start the reaction. As the polymerization reaction progressed, the pressure in the system decreased. Accordingly, TFE was continuously supplied to maintain the internal pressure at 0.78 MPaG, and the reaction was continued.

When the amount of TFE polymerized after the polymerization reaction started reached 1200 g, stirring and feeding of TFE were stopped. Immediately after that, the gas was released from the system to normal pressure and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 25.3 mass % and an average primary particle size of 256 nm.

[0279] Example 4

The aqueous dispersion of PTFE obtained in the 3rd production example was diluted to a solid phase concentration of 13% by weight. The PTFE was coagulated by shaking the container, and the water was filtered to obtain a wet powder.

The resulting wet PTFE powder was placed in a stainless steel grid tray, which was heated in an electric furnace with hot air circulation at 210ºC. After 18 hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder.

[0280] Example 5

The aqueous dispersion of PTFE obtained in the 4th production example was diluted to a solid phase concentration of 13% by weight. The PTFE was coagulated by shaking the container, and the water was filtered to obtain a wet powder.

The resulting wet PTFE powder was placed in a stainless steel grid tray, which was heated in an electric furnace with hot air circulation at 160ºC. After 18 hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder.

[0281] Comparative Example 2

The aqueous dispersion of PTFE obtained in the 5th production example was diluted to a solid phase concentration of 13% by weight. The PTFE was coagulated by shaking the container, and the water was filtered to obtain a wet powder.

The resulting wet PTFE powder was placed in a stainless steel grid tray, which was heated in an electric furnace with hot air circulation at 160ºC. After 18 hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder.

[0282] The physical properties of the PTFE powder obtained in each of the examples were analyzed using the methods described above. In addition, using each of these PTFE powders, a sheet of the electrode mixture of the positive electrode, an electrode, and a lithium-ion secondary battery were produced and evaluated using the methods described above. The results are presented in Table 2.

[0283] Table 2

Example 4 Example 5 Comparative example 2 PTFE aqueous dispersion 3 production example 4 production example 5 production example [Physical properties of PTFE powder] Average size of secondary particles microns 580 460 450 Apparent density g/ml 0.50 0.48 0.51 Water content % mass. 0,000 0,000 0,000 SSG - 2,152 2,177 2,210 Extrusion pressure at RR100 MPa 15.9 10.3 8.9 Ability to expand - Non-extensible Non-extensible Non-extensible [Battery Rating (1)] Electrode strength - 950 250 100 Residual capacity coefficient % 72 72 72 Amount of gas released - 84 84 85

[0284] The results shown in Table 2 show that when using PTFE powder with SSG of 2,200 or less, the strength of the electrode is increased. In particular, when using expandable PTFE powder, the strength of the electrode can be significantly increased, and the battery including it has excellent performance.

[0285] 6 production example

A 6 L stainless steel reactor equipped with a stirrer was charged with 3600 g of deionized water, 180 g of paraffin wax, 5.4 g of white solid A, and 26.5 mg of oxalic acid. The reactor contents were then heated to 70 °C while being evacuated, while purging the reactor with tetrafluoroethylene to remove oxygen and stirring the contents. Together with TFE, 2.60 g of chlorotrifluoroethylene (CTFE) were fed into the reactor, then additional TFE was added until a pressure of 2.70 MPa g was reached. An aqueous solution of potassium permanganate containing 3.4 mg of potassium permanganate dissolved in deionized water was continuously fed into the reactor as an initiator. After feeding the initiator, the pressure decreased and the onset of polymerization was observed. TFE was fed into the reactor so that the pressure was constant at 2.70 MPa g. When the fed amount of TFE reached 430 g, the feeding of the aqueous solution of potassium permanganate was stopped. When the fed amount of TFE reached 1660 g, the feeding of TFE and stirring were stopped, and the reaction was terminated. After that, the pressure in the reactor was released to normal and purged with nitrogen. The contents were taken out of the reactor and cooled. The paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 31.4 mass % and an average primary particle size of 248 nm.

[0286] Example 6

The aqueous PTFE dispersion obtained in the 6th production example was diluted to a solid phase concentration of 13% by weight. The diluted dispersion was vigorously stirred in a container equipped with a stirrer for coagulation, and water was filtered to obtain a wet powder. The resulting wet powder was placed in a stainless steel lattice tray, which was heated in an electric furnace with hot air circulation at 210 °C. After 18 hours, the lattice tray was taken out and cooled with air, thereby obtaining PTFE powder. The CTFE content in the finished PTFE powder was 0.100% by weight. Various physical properties of the obtained PTFE powder were analyzed. In addition, using the thus obtained PTFE powder, a sheet of electrode mixture of a positive electrode, an electrode and a lithium-ion secondary battery were manufactured by the above-described methods and evaluated. The results are presented in Table 3.

[0287] 7 production example

An aqueous PTFE dispersion was obtained by polymerization under the same conditions as in Production Example 6, except that the feed amount of CTFE was changed to 1.28 g, the feed amount of potassium permanganate was changed to 3.87 mg, and the final feed amount of TFE was changed to 1790 g. The solid phase concentration in the obtained aqueous PTFE dispersion was 33.0 mass %, and the average primary particle size was 263 nm.

[0288] Example 7

PTFE powder was obtained in the same way as in Example 6, using the PTFE aqueous dispersion obtained in Production Example 7. Various physical properties of the obtained PTFE powder were analyzed. The CTFE content in the finished PTFE powder was 0.050% by weight. In addition, using the PTFE powder thus obtained, a positive electrode mixture sheet, an electrode, and a lithium-ion secondary battery were manufactured by the above-described methods and evaluated. The results are presented in Table 3.

[0289] 8 production example

In a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle and a temperature control jacket, 3580 g of deionized water, 100 g of paraffin wax and 5.4 g of white solid A were charged. The inside of the autoclave was purged with nitrogen gas heated to 70 °C to remove oxygen. Then, 0.50 g of HFP was added together with TFE, TFE was supplied until the system pressure reached 0.78 MPaG, the system temperature was maintained at 70 °C, and stirring was performed. Then, an aqueous solution obtained by dissolving 15.4 mg of ammonium persulfate in 20 g of water was added into the system together with TFE, starting the polymerization reaction. As the polymerization reaction progressed, the pressure in the system decreased. Accordingly, TFE was added, maintaining the temperature in the system equal to 70ºС and the pressure in the system equal to 0.78 MPa excess.

When 430 g of TFE were consumed after the start of polymerization, an aqueous solution obtained by dissolving 18.0 mg of hydroquinone, which acted as a radical acceptor, in 20 g of water was introduced into the system together with TFE. The polymerization was continued. When the amount of TFE polymerized after the start of the reaction reached 1540 g, stirring and feeding of TFE were stopped. Immediately after this, the gas was released from the system to normal pressure and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous dispersion of PTFE. This aqueous dispersion of PTFE was characterized by a solid phase concentration of 29.6% by weight and an average primary particle size of 246 nm.

[0290] Example 8

The aqueous dispersion of PTFE obtained in the 8th production example was diluted to a solid phase content concentration of 13% by mass. The diluted dispersion was stirred in a container equipped with a stirrer to coagulate PTFE, and water was filtered to obtain a wet powder. The resulting wet powder was placed in a stainless steel lattice tray, which was heated in an electric furnace with hot air circulation at 180ºC. After 18 hours, the lattice tray was taken out and cooled with air, thereby obtaining PTFE powder.

The various physical properties of the obtained PTFE powder were analyzed. The HFP content in the finished PTFE powder was 0.019% by weight. In addition, using the PTFE powder obtained in this way, a sheet of the electrode mixture of the positive electrode, an electrode and a lithium-ion secondary battery were manufactured by the methods described above and evaluated. The results are presented in Table 3.

[0291] 9 production example

The aqueous PTFE dispersion was obtained in the same manner as in Production Example 8, except that the fed amount of HFP was changed to 0.06 g.

The concentration of the solid phase in the obtained aqueous PTFE dispersion was 29.2% by weight, the average size of the primary particles was 274 nm.

[0292] Example 9

PTFE powder was obtained as in Example 8, except that the aqueous PTFE dispersion obtained in Production Example 9 was heated to a temperature of 160°C.

The various physical properties of the obtained PTFE powder were analyzed. The HFP content in the finished PTFE powder was 0.002% by weight. In addition, using the PTFE powder obtained in this way, a sheet of the electrode mixture of the positive electrode, an electrode and a lithium-ion secondary battery were manufactured and evaluated using the methods described above. The results are presented in Table 3.

[0293] Table 3

Example 6 Example 7 Example 8 Example 9 PTFE aqueous dispersion 6 production example 7 production example 8 production example 9 production example [Physical properties of PTFE powder] Average size of secondary particles microns 510 520 610 530 Apparent density g/ml 0.48 0.48 0.43 0.46 Water content % mass. 0,000 0,000 0,000 0,000 SSG - 2,150 2,150 2,146 2,154 Ability to expand - Expandable Expandable Expandable Expandable [Battery Rating (1)] Electrode strength - 1403 1190 1530 1254 Residual capacity coefficient % 72 72 73 73 Amount of gas released - 84 84 85 85

[0294] Battery Rating (2)

Sheets of the mixture of examples 10 and 11 and comparative example 3 were prepared as follows.

Manufacturing of positive electrode electrode mixture sheet

The positive electrode active material, conductive additive and binder were weighed in a weight ratio of 97:2:1 and placed in a container of a high-speed stirrer (Wonder Crusher (WC-3) manufactured by Osaka Chemical Co., Ltd.).

After that, the whole container was cooled sufficiently in a constant temperature bath (at 5ºC for eight hours). Then, the cooled container was placed in a high-speed mixer, the materials were stirred at 10,000 rpm for three minutes to disperse, and thus an electrode mixture was obtained.

The following materials were used.

Li(Ni _0.8 Mn _0.1 Co _0.1 )O ₂ , (hereinafter referred to as NMC811)

Conductive additive: SuperP carbon black

Binder: PTFE powders obtained in Examples 10 and 11 and Comparative Example 3

Then the obtained electrode mixture was placed in a pressure mixer heated to 50ºС and stirred at a speed of 30 rpm for 100 s to enhance fibrillation, thereby obtaining an electrode mixture.

This electrode mixture was passed through parallel metal rollers (80ºC) and thin pieces of about 1 by 2 cm ² were obtained. These pieces were again passed several times through rollers heated to 80ºC and a sheet was obtained.

The sheet was rolled out several times using rollers, thereby obtaining an independent sheet of electrode mixture. The temperature of the metal rollers was set to 80ºС. The distance between the rollers was gradually reduced, and as a result, a sheet of electrode mixture of the positive electrode was obtained. The manufactured sheet of electrode mixture of the positive electrode had a thickness of 80 µm and a density of 3.50 g/cm ³ .

Measurement of the strength of the positive electrode electrode mixture sheet

The evaluation of the positive electrode mixture sheets of Examples 10 and 11 and Comparative Example 3 was carried out in accordance with the following procedure.

The results of examples 10 and 11 and comparative example 3 were compared with the evaluation result of comparative example 3, taken as 100.

[0295] Making a positive electrode

The electrode sheets and batteries of examples 10 and 11 and comparative example 3 were manufactured using the following method.

The positive electrode electrode mixture sheet was connected to a 20 µm thick aluminum foil as follows.

The adhesive used was a suspension obtained by dissolving polyvinylidene fluoride (PVDF) in n-methylpyrrolidone (NMP) and dispersing carbon black therein at a solid phase content ratio of 80:20. This adhesive was applied to aluminum foil and dried on an electric hotplate at 120ºC for 15 min, thereby obtaining a current collector with an adhesive layer of 4 μm thickness.

After this, a sheet of positive electrode electrode mixture was placed on the current collector with a layer of adhesive and they were connected to each other using a roller press heated to 150ºС. The workpiece was cut according to the specified size and a label was attached, thereby obtaining a positive electrode.

Manufacturing of multilayer cell with aluminum foil

The negative electrode, separator and positive electrode were connected layer by layer as in Example 1 and impregnated with a non-aqueous electrolyte solution, then pre-charged and subjected to aging, thereby obtaining a lithium-ion secondary battery. The battery was evaluated.

[0296] The evaluation of the batteries of examples 10 and 11 and comparative measurement 3 was carried out in accordance with the following procedure.

Evaluation of storage parameters (residual capacity coefficient, amount of released gas)

The evaluation of the parameters during storage (residual capacity coefficient, amount of released gas) was carried out in the same way as in example 1. The amount of released gas was compared with the amount of gas released in comparative example 3, taken as 100.

[0297] 10 production example

Into a 6 L stainless steel autoclave equipped with a stainless steel stirring paddle and a temperature control jacket, 3600 g of deionized water, 100 g of paraffin wax and 5.4 g of white solid A were charged. The inside of the polymerization reactor was purged with nitrogen gas heated to 85 °C to remove oxygen. The inside temperature of the reactor was maintained at 85 °C with stirring, and TFE was supplied so that the pressure in the system was 2.4 MPa g. Deionized water with 468 mg of disuccinic acid peroxide dissolved therein was added to the contents of the reactor with stirring to start the polymerization. As the polymerization reaction progressed, the pressure in the system decreased. Accordingly, TFE was added to maintain the pressure at 2.4 MPa g.

When the amount of TFE consumed reached 1580 g, stirring and feeding of TFE were stopped. TFE was purged from the polymerization reactor and the polymerization reaction was stopped. The resulting aqueous dispersion was drained and cooled, paraffin wax was separated, thereby obtaining an aqueous PTFE dispersion. This aqueous PTFE dispersion was characterized by an average primary particle size of 294 nm and a solid phase concentration of 30.4 mass%.

[0298] Example 10

The aqueous dispersion of PTFE obtained in the 10th production example was diluted to a solid phase concentration of 13% by mass. The diluted dispersion was vigorously stirred in a container equipped with a stirrer for coagulation, and the water was filtered off to obtain a wet powder. The water content of the wet powder was about 40% by mass.

The resulting wet powder was placed in a stainless steel grid tray (in an amount of 2.0 g/cm ² ), which was heated in an electric furnace with hot air circulation at 170 ° C. After 18 hours, the grid tray was taken out and cooled with air, thereby obtaining PTFE powder. Various physical properties of the obtained PTFE powder were analyzed. The results are shown in Table 4. In addition, using the thus obtained PTFE powder, a positive electrode mixture sheet, an electrode, and a lithium-ion secondary battery were manufactured and evaluated as described in the Battery Evaluation section (2). The results are shown in Table 5.

Table 4

Example 10 PTFE aqueous dispersion 10 production examples [Physical properties of PTFE powder] Average size of secondary particles microns 570 Apparent density g/ml 0.50 Water content % mass. 0.002 SSG - 2,160 Extrusion pressure at RR100 MPa 13.0 Ability to expand - Expandable

[0299] Example 11

Using the PTFE powder obtained in Example 4, a positive electrode electrode mixture sheet, an electrode, and a lithium-ion secondary battery were produced by the method described in the Battery Evaluation section (2), and evaluated. The results are shown in Table 5.

[0300] Comparative Example 3

Using the PTFE powder obtained in Comparative Example 2, a positive electrode electrode mixture sheet, an electrode, and a lithium-ion secondary battery were produced by the method described in the Battery Evaluation (2) section, and evaluated. The results are shown in Table 5.

Table 5

[Battery Rating (2)] Example 10 Example 11 Comparative example 3 Electrode strength - 500 750 100 Residual capacity coefficient % 70 71 68 Amount of gas released - 86 83 100

[0301] The results shown in Table 5 show that the use of the polytetrafluoroethylene powder of the present invention makes it possible to obtain a sheet of electrode mixture of a positive electrode with high strength. Due to this, the amount of the binder added can be reduced while maintaining excellent battery performance. It was also found that its use makes it possible to produce batteries that are advantageous in terms of strength, low gas emission and high capacity retention rate.

Claims (19)

1. The use of polytetrafluoroethylene powder in electrode binder, wherein the polytetrafluoroethylene powder is characterized by a standard specific gravity of 2.200 or less and a water content of 0.050% by weight or less, wherein the polytetrafluoroethylene powder is used in such a manner that the content of the polytetrafluoroethylene powder in the electrode binder is in the range of 95.0% by mass or more of the binder. 2. Electrode binder including polytetrafluoroethylene powder, wherein the amount of polytetrafluoroethylene powder is 95.0% by weight or more of the weight of the binder, wherein the polytetrafluoroethylene powder is characterized by a standard specific gravity of 2.200 or less and a water content of 0.050% by mass or less. 3. The electrode binder according to claim 2, wherein the polytetrafluoroethylene powder is characterized by an extrusion pressure at a compression ratio of 100 10 MPa or more. 4. An electrode binder according to claim 2 or 3, wherein the polytetrafluoroethylene powder is expandable. 5. An electrode binder according to any one of paragraphs 2-4, in which the polytetrafluoroethylene comprises a tetrafluoroethylene unit and a modifying monomer unit, wherein the modifying monomer is capable of copolymerization with the tetrafluoroethylene. 6. The electrode binder according to claim 5, wherein the modifying monomer comprises at least one monomer selected from the group consisting of perfluoro(methyl vinyl ether) and hexafluoropropylene. 7. An electrode binder according to any one of paragraphs 2-6, wherein the polytetrafluoroethylene powder is characterized by an average primary particle size of 100 to 350 nm. 8. Electrode mixture containing: an electrode binder according to any one of paragraphs 2-7; and active electrode material. 9. Electrode, including: an electrode binder according to any one of paragraphs 2-7; active electrode material; and current collector. 10. A secondary storage battery including an electrode according to item 9.