WO2014024534A1 - Negative electrode active material for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

Description

Negative electrode active material for lithium ion secondary battery and lithium ion secondary battery

The present invention relates to a negative electrode active material for a lithium ion secondary battery and a lithium ion secondary battery.

Conventionally, lithium ion secondary batteries using a silicon component-containing powder and graphite powder as a negative electrode active material are known. For example, Patent Documents 1 to 5 exemplify silicon particles, SiO ₂ particles, SiO particles, silicide particles, and the like as silicon component-containing powders.

JP 2008-1212710 A JP 2004-185984 A JP 2003-238992 A JP 2009-238663 A JP 2010-92934 A JP 2006-164952 A

However, conventional batteries have insufficient capacity and cycle characteristics. This invention is made | formed in view of the said subject, and it aims at providing the negative electrode active material which can implement | achieve the lithium ion secondary battery excellent in the capacity | capacitance and cycling characteristics.

The negative electrode active material for a lithium ion secondary battery according to the present invention includes a silicon oxide powder having a Si phase and a SiO ₂ phase and a graphite powder, and the silicon oxide powder with respect to a total of the silicon oxide powder and the graphite powder. The silicon oxide powder has a particle size D50 of 4 μm or more.

According to the present invention, the capacity and cycle characteristics of the lithium ion secondary battery are improved.

Here, the mass ratio of the silicon oxide powder to the total of the silicon oxide powder and the graphite powder is preferably 0.12 to 0.45.

Moreover, it is preferable that the particle size D50 of the graphite powder is 8 μm or more.

Moreover, it is preferable that the particle size D50 of the graphite powder is 15 μm or more. According to this, the peel strength of the negative electrode active material layer is improved.

Moreover, it is preferable that the particle size D50 of the graphite powder is 30 μm or less.

Moreover, it is preferable that the silicon oxide powder D50 has a particle size of 20 μm or less.

The lithium ion secondary battery according to the present invention includes a negative electrode including the negative electrode active material described above and a positive electrode.

According to the present invention, a lithium ion secondary battery excellent in capacity and cycle characteristics is provided.

FIG. 1A is a schematic cross-sectional view of a negative electrode according to an embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

(Negative electrode active material)
The negative electrode active material according to the present embodiment includes silicon oxide powder and graphite powder having a Si phase and a SiO ₂ phase.

(Silicon oxide powder)
The silicon oxide powder has a Si phase and a SiO ₂ phase in each particle. The Si phase is very fine and is dispersed in the SiO ₂ phase. Further, the SiO ₂ phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution.

Here, when the number of oxygen atoms with respect to the number of silicon atoms in the silicon oxide powder is x, when x is less than 0.5, the proportion of the Si phase increases, so the volume change during charge / discharge is large. Therefore, the cycle characteristics are difficult to improve. Moreover, when x exceeds 1.5, the ratio of Si phase may fall and energy density may fall. Therefore, x in the silicon oxide powder is preferably 0.5 to 1.5, and more preferably 0.7 to 1.2.

Such a silicon oxide can be obtained by disproportionating SiO, which is an amorphous silicon oxide obtained using silicon dioxide (SiO ₂ ) and simple silicon (Si) as raw materials, by heat treatment or the like. The disproportionation reaction is a reaction in which SiO is decomposed into a Si phase and a SiO ₂ phase. In general, when oxygen is turned off, it is said that almost all SiO is disproportionated and separated into two phases at 800 ° C. or higher. Specifically, non-crystalline SiO powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as in a vacuum or in an inert gas. A silicon oxide powder containing two phases of _two phases and a crystalline Si phase is obtained.

The particle size D50 of the silicon oxide powder is 4 μm or more. The particle diameter D50 is a median diameter, and can be obtained based on a volume-based particle size distribution by a laser diffraction method. The particle size D50 of the silicon oxide powder is preferably 20 μm or less, and preferably 15 μm or less. If the particle size D50 is too small, there are many active points, so SEI (Solid
Electrolyte Interphase) is often generated and the cycle characteristics tend to be remarkably deteriorated. On the other hand, if the particle size D50 is too large, the conductivity of the silicon oxide is poor, so that the conductivity of the entire electrode becomes non-uniform, resulting in an increase in resistance and a decrease in output.

(Graphite powder)
Examples of the graphite powder include natural graphite powder, artificial graphite powder, spherulite graphite powder (graphitized mesophase carbon microspheres), graphite-based carbon material powder, and the like. Examples of the graphite-based carbon material are powders of thermal decomposition products of condensed polycyclic hydrocarbon compounds such as pitch and coke.

The particle size D50 of the graphite powder is preferably 8 μm or more, and preferably 15 μm or more. Moreover, it is preferable that the particle size D50 of graphite powder is 30 micrometers or less.
If the particle size of the graphite powder is too large, the silicon oxide powder not in contact with the graphite increases, the conductivity becomes poor, and the resistance tends to increase. On the other hand, if the particle size of the graphite powder is too small, the active points of the graphite increase, so that the capacity decreases during high-temperature storage.

(Mixing ratio)
In the negative electrode active material, the mass ratio of the silicon oxide powder to the total of the silicon oxide powder and the graphite powder is 0.1 to 0.5. This mass ratio is more preferably 0.12 to 0.45. If the mass ratio of the silicon oxide powder is too large, the discharge capacity retention rate during cycling tends to deteriorate and the resistance increases. On the other hand, if the mass ratio of the silicon oxide powder is too small, the energy density tends to be low.

(Negative electrode)
Next, the negative electrode 10 according to this embodiment will be described with reference to FIG. The negative electrode 10 includes a negative electrode current collector 12 and a negative electrode active material layer 14 provided on the negative electrode current collector 12. The negative electrode active material layer 14 may be provided only on one surface of the negative electrode current collector 12, or may be provided on both surfaces of the negative electrode current collector 12 as indicated by a dotted line in FIG.

The negative electrode current collector 12 is made of a conductive material. An example of the material of the negative electrode current collector 12 is a metal material such as stainless steel, titanium, nickel, aluminum, or copper, or a conductive resin. In particular, copper is suitable as a material for the negative electrode current collector 12. The thickness of the negative electrode current collector 12 is not particularly limited, but can be, for example, 5 to 25 μm. Further, the thickness of the negative electrode active material layer 14 is not particularly limited, but may be, for example, 40 to 100 μm.

The negative electrode active material layer 14 includes the negative electrode active material described above and a binder.

The binder fixes the active material to the current collector. Examples of the binder are fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, and alkoxysilanol group-containing resins. The amount of the binder can be 1 to 30 parts by mass with respect to 100 parts by mass of the active material.

The negative electrode active material layer 14 can further contain a conductive additive as necessary. Examples of the conductive aid are carbon-based particles such as carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (Vapor Grown Carbon Fiber: VGVG). These can be added alone or in combination of two or more. The amount of the conductive aid used is not particularly limited, but for example, it can be 1 to 30 parts by mass with respect to 100 parts by mass of the active material.

Such a negative electrode can be obtained by applying a slurry containing an active material, a binder, and a conductive additive added as necessary to a current collector and drying it. Examples of the solvent for the slurry are N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK). After drying, the active material layer may be pressed.

The negative electrode current collector 12 has a tab portion 12t at the end thereof where the negative electrode active material layer 14 is not formed. A lead 16 described later is electrically connected to the tab portion 12t.

(Lithium ion secondary battery)
Next, an example of the lithium ion secondary battery 100 according to the embodiment of the present invention will be described with reference to FIG.

The lithium ion secondary battery 100 mainly includes a negative electrode 10, a separator 20, a positive electrode 30, a case 70, and an electrolytic solution.

The positive electrode 30 includes a positive electrode current collector 32 and a positive electrode active material layer 34 provided on the positive electrode current collector 32. The positive electrode current collector 32 is made of a conductive material. An example of the material of the positive electrode current collector is a metal such as aluminum.

The positive electrode active material layer 34 has a positive electrode active material and a binder. The positive electrode active material layer 34 may include a conductive additive as necessary. Examples and blending amounts of binders and conductive assistants can be the same as those described for the negative electrode.

The positive electrode active material is not particularly limited as long as it is a positive electrode active material for a lithium secondary battery. An example of the positive electrode active material is a lithium compound. For example, lithium metal composite oxides such as lithium cobalt composite oxide, lithium nickel composite oxide, and lithium manganese composite oxide can be used. Other metal compounds or polymer materials can also be used as the positive electrode active material. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, or disulfides such as titanium sulfide and molybdenum sulfide. Examples of the polymer material include conductive polymers such as polyaniline and polythiophene.

In particular, the positive electrode active material has a general formula: Li _a Co _p Ni _q Mn _r O ₂ (0.9 <a <1.1, p + q + r = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r < The lithium metal composite oxide represented by 1) is preferably included. Since the composite oxide is excellent in thermal stability and low in cost, by including the composite oxide, an inexpensive lithium ion secondary battery having good thermal stability can be obtained.
Examples of the composite metal oxide include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 CO _0.2 Mn _0.3 O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , and LiCoMnO ₂ can be used. Among them, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ is preferable from the viewpoint of thermal stability.

Also, the positive electrode active material, the general _{formula: Li a Co p Ni q Mn} r D s O 2 (0.9 <a <1.1, p + q + r + s = 1,0 <p ≦ 1,0 ≦ q <1,0 It is also preferable to include a lithium metal composite oxide represented by ≦ r <1, 0 <s <1). D is at least one element selected from the group consisting of Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na. An example of such a positive electrode active material is LiNi _0.48 Co _0.2 Mn _0.3 Mg _0.02 O ₂ .

The manufacturing method of the positive electrode is the same as that of the negative electrode except that the active material is different.

The positive electrode current collector 32 has a tab portion 32t at the end thereof where the positive electrode active material layer 34 is not formed. A lead 36 described later is electrically connected to the tab portion 32t.

(Separator)
The separator 20 separates the negative electrode 10 and the positive electrode 30 and allows lithium ions to pass through while preventing a short circuit of current due to contact between both electrodes. As the separator 20, for example, a porous film made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics can be used. The negative electrode active material layer 14 of the negative electrode 10 and the positive electrode active material layer 34 of the positive electrode 30 are in contact with each surface of the separator.

(Electrolyte)
The electrolytic solution includes an electrolyte and a solvent that dissolves the electrolyte. The electrolyte is impregnated in the negative electrode active material layer 14, the separator 20, and the positive electrode active material layer 34.

Examples of the electrolyte are lithium salts such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ) ₂ .

Examples of the solvent are cyclic esters, chain esters, and ethers. Two or more of these solvents can be mixed.

Examples of cyclic esters are ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma ptyrolactone, and gamma valerolactone. Examples of the chain esters are methyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers are tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane.

The concentration of the electrolyte in the electrolytic solution can be, for example, 0.5 to 1.7 mol / L. The electrolytic solution may contain a gelling agent.

(Case)
The case 70 accommodates the negative electrode 10, the separator 20, the positive electrode, and the electrolytic solution. The material and form of the case are not particularly limited, and various known materials such as resins and metals can be used.

Leads 16 and 36 are connected to the tab portion 12t of the negative electrode current collector 12 and the tab portion 32t of the positive electrode current collector 32, respectively. One ends of the leads 16 and 36 are out of the case 70.

Such a lithium ion secondary battery is excellent in capacity and cycle capacity maintenance rate. The reason for this is not clear, but by optimizing the blending amount of silicon oxide powder and graphite powder and the particle size of silicon oxide powder, the generation of SEI formed on the surface of the negative electrode can be suppressed, and the consumption of electrolyte Therefore, it is considered that the cycle capacity maintenance ratio can be improved.

It should be noted that the lithium ion secondary battery according to the present invention is not limited to the above embodiment, and can be used for various modifications. For example, a plurality of positive electrodes, negative electrodes, and separators may be provided, the positive electrodes and the negative electrodes may be alternately disposed, and the separators may be disposed so as to be disposed between the negative electrodes covering the positive electrodes.

(Silicon oxide powder 1)
Commercially available SiO powder was heat-treated in an inert gas atmosphere at a temperature of 900 ° C. for 2 hours to disproportionate the SiO. When the obtained silicon oxide powder 1 was subjected to X-ray diffraction (XRD) measurement using CuKα, a peak derived from simple silicon and silicon dioxide was obtained. The silicon oxide was composed of Si phase, SiO 2 It was confirmed to contain _two phases.

When the particle size distribution of the obtained silicon oxide powder was examined, the particle size D50 was 4.4 μm.

(Silicon oxide powder 2)
The silicon oxide powder obtained in the same manner as the silicon oxide powder 1 is classified by a classifier (forced vortex centrifugal precision air classifier, turbo classifier, manufactured by Nisshin Engineering) to remove fine powder, and the particle size D50 is 5. 7 μm of silicon oxide powder 2 was obtained. The classification conditions were a rotor rotation speed of 7000 rpm, a supply speed of 1.0 kg / h, and an air volume of 2.0 m ³ / min.

(Silicon oxide powder 3)
Silicon oxide powder obtained in the same manner as silicon oxide powder 1 was classified with a cyclone to remove fine powder, and silicon oxide powder 3 having a particle diameter D50 of 6.4 μm was obtained. The classification conditions were a rotor rotation speed of 6000 rpm, a supply speed of 1.0 kg / h, and an air volume of 2.0 m ³ / min.

(Silicon oxide powder 4)
Silicon oxide powder obtained in the same manner as silicon oxide powder 1 was classified with a cyclone to remove fine powder, and silicon oxide powder 4 having a particle diameter D50 of 7.2 μm was obtained. The classification conditions were a rotor rotational speed of 4000 rpm, a supply speed of 1.0 kg / h, and an air volume of 2.0 m ³ / min.

(Silicon oxide powder 5)
The fine powder was recovered by classification with a classifier (forced vortex centrifugal precision air classifier, turbo classifier, manufactured by Nisshin Engineering Co., Ltd.) to obtain silicon oxide powder 5 having a particle diameter D50 of 1.4 μm.

(Example 1)
(Manufacture of negative electrode)
Silicon oxide powder 1, natural graphite powder (particle size D50: 20 μm), conductive additive (acetylene black), binder (polyamideimide) are mixed at a mass ratio of 32: 50: 8: 10, respectively, A solvent (N-methyl-2-pyrrolidone (NMP)) was added to obtain a slurry.

The slurry was formed into a film on one side of a copper foil, the solvent was dried on a hot plate at 80 ° C. for 15 minutes, pressed, and further heated at 200 ° C. for 2 hours. In this way, a negative electrode having a negative electrode active material layer of 25 mm × 30 mm was obtained.

(Manufacture of positive electrode)
LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , acetylene black, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 94: 3: 3, and the solvent (N-methyl-2- Pyrrolidone (NMP)) was added to obtain a slurry. This slurry was applied to one side of an aluminum foil, the solvent was dried on a hot plate at 80 ° C. for 30 minutes, pressed, and further heated at 120 ° C. for 6 hours. In this way, a positive electrode having a positive electrode active material layer of 25 mm × 30 mm was obtained.

(Manufacture of batteries)
A polypropylene porous membrane (27 mm × 32 mm, thickness 25 μm) was prepared and sandwiched between a positive electrode and a negative electrode to obtain a laminate. This laminated body was accommodated in a case in which both surfaces of an aluminum foil were laminated with a resin. Further, an electrolytic solution was supplied into the case, and then the case was sealed to obtain a lithium ion secondary battery. The electrolytic solution contained a solvent and an electrolyte (LiPF ₆ ), and the solvent contained ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a mass ratio of 3: 3: 4, and the electrolyte concentration was 1 mol / dm ³ . Note that leads were connected to the positive electrode and the negative electrode, respectively, and both the leads extended out of the case.

(Example 2)
Example 1 was performed except that silicon oxide powder 2 was used.

(Example 3)
The same procedure as in Example 1 was performed except that silicon oxide powder 3 was used.

Example 4
Example 1 was performed except that silicon oxide powder 4 was used.

(Example 5)
Example 2 was performed except that the blending mass ratio of the silicon oxide powder and the graphite powder was 12:70.

(Example 6)
It replaced with natural graphite powder (particle diameter D50: 20micrometer), and was carried out similarly to Example 5 except having used natural graphite powder (particle diameter D50: 10micrometer).

(Comparative Example 1)
The procedure was the same as in Example 1 except that silicon oxide powder 5 was used.

In each of the above examples, a pair of lithium ion secondary batteries was prepared for a cycle test at 25 ° C. and for a cycle test at 55 ° C.

(conditioning)
About each obtained lithium ion secondary battery, the conditioning process was performed and the aging process was performed after that.

(Discharge capacity evaluation)
The cycle test was performed on the conditioned battery at 25 ° C. and 55 ° C. The charge / discharge cycle conditions were 1C, 4.2V CC (constant current) charge, and 1C, 2.5V CC (constant current) discharge. The discharge capacity at the first cycle was defined as the initial discharge capacity. Further, a value obtained by dividing the discharge capacity at the 150th cycle by the discharge capacity at the first cycle was defined as a 150 cycle capacity retention rate. The results are shown in Table 1. For Examples 5 and 6, there is no data on 150 cycle capacity retention at 55 ° C.

For Examples 5 and 6, the peel strength of the negative electrode active material layer was also measured. The peel strength was measured using LTS-200N-S20 manufactured by Minebea. The conditions and results are shown in Table 1.

It was confirmed that the battery of the example was superior in capacity and cycle capacity maintenance rate as compared with the comparative example.

10 ... positive electrode, 20 ... separator, 30 ... negative electrode, 100 ... lithium ion secondary battery.

Claims (7)

Including silicon oxide powder and graphite powder having Si phase and SiO ₂ phase,
The mass ratio of the silicon oxide powder to the total of the silicon oxide powder and the graphite powder is 0.1 to 0.5,
A negative electrode active material for a lithium ion secondary battery, wherein the silicon oxide powder has a particle size D50 of 4 µm or more. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein a mass ratio of the silicon oxide powder to a total of the silicon oxide powder and the graphite powder is 0.12 to 0.45. The negative electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the graphite powder has a particle size D50 of 8 µm or more. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein a particle diameter D50 of the graphite powder is 15 µm or more. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the graphite powder has a particle size D50 of 30 µm or less. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein a particle diameter D50 of the silicon oxide powder is 20 µm or less. A lithium ion secondary battery comprising a negative electrode having the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6 and a positive electrode.

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