CN100533816C - Battery diaphragm, preparation method thereof, and lithium-ion secondary battery containing the diaphragm - Google Patents

Abstract

A battery separator includes the substrate and holes distributed on it, in which, the substrate contains polyimide. The battery separator of this invention uses polyimide which is a new material bearing high temperature as substrate, so that it has excellent chemical stability, high temperature, good permeability and high mechanical strength. The lithium-ion rechargeable batteries using this invention don't appear short circuit even in 150 deg. C, thus this battery separator can be used for high-capacity and power batteries.

Description

Battery diaphragm, preparation method thereof, and lithium-ion secondary battery containing the diaphragm

technical field

The invention relates to a battery diaphragm, a preparation method thereof and a lithium ion secondary battery containing the diaphragm.

Background technique

With its high specific energy, high voltage, small size, light weight, and no memory, lithium-ion secondary batteries have achieved tremendous development in the past ten years and have become one of the main energy sources for communication electronic products. However, under abuse conditions, lithium-ion secondary batteries are prone to safety hazards such as smoke, fire, explosion, and even personal injury, so high-capacity and power lithium-ion batteries have not been widely used. Therefore, improving the safety of lithium-ion batteries is the key to the development of lithium-ion batteries.

The battery separator plays a role in preventing the positive and negative electrodes from directly contacting the short circuit in the lithium-ion battery. In order to improve the safety of the battery, not only the battery separator is required to prevent the positive and negative electrodes from directly contacting short circuit at room temperature, but also the battery separator is required to prevent the positive and negative electrodes from directly contacting short circuit at high temperature. However, commonly used battery separators, such as polyethylene and polypropylene, are difficult to guarantee the integrity at high temperature. In safety tests such as furnace heat, internal short circuits and thermal runaway problems often occur due to the shrinkage of battery separators. Therefore, improving the high-temperature resistance of the battery separator is the key to improving the high-temperature safety performance of the lithium-ion secondary battery.

JP-A-63-308866 discloses a method of obtaining a microporous film having high strength and high temperature characteristics by laminating a single-layer film comprising low melting point polyethylene and high melting point polypropylene. However, the internal resistance of the separator increases due to lamination, and thus the resulting separator is not suitable for high-output, high-performance batteries.

JP-A-10-298325 discloses a method for producing a porous film containing low molecular weight polyethylene, low molecular weight polypropylene and high molecular weight polyethylene. This porous film has poor mechanical strength and relatively poor thermal shock resistance. In addition, due to the low mechanical strength of the separator, when the internal electrolyte of the battery generates a strong pressure due to thermal vaporization, it is easy to cause the membrane to rupture. Therefore, there is still danger when the porous film prepared by this method is used in lithium ion secondary battery.

JP-A-2001-59036 proposes a porous film having an excellent balance between shutdown characteristics (SD characteristics) and heat resistance, which distributes polybornene in a sheet or leaf shape in a matrix polymer . However, although this invention obtains a porous film with high heat resistance and high mechanical strength, the use of polynorbornene not only increases the cost, but also has by-products because the porous film is synthesized by a special reaction, resulting in the Electrolytes are affected. In addition, the heat resistance and mechanical strength of porous membranes may still be poor due to the difficulty in adequately controlling the reaction conditions such that polynorbornene may be cross-linked as a monogel.

CN 1512607A provides a kind of improved porous membrane on the basis of the porous membrane that JP-A-2001-59036 proposes, and this porous membrane contains ethylene-propylene-ethylene norbornene terpolymer, through the intersection of three Combined effect improves strength and heat resistance. But also because of the use of ultra-high molecular weight polyethylene and norbornene, the cost is increased, and the traditional thermoplastic material PE/PP is mainly used, and the thermal shrinkage rate is relatively large, which cannot fundamentally solve the problem of shutdown characteristics and durability. The balance relationship between thermal properties, and from what it describes, the use of ultra-high molecular weight materials will also cause the so-called "static" phenomenon.

Contents of the invention

The purpose of the present invention is to overcome the disadvantages of poor heat resistance and poor mechanical strength of the porous membrane in the prior art, and provide a battery separator with good mechanical strength and good heat resistance, its preparation method and lithium ion secondary battery.

The battery separator provided by the invention includes a substrate and pores distributed on the substrate, wherein the substrate contains polyimide.

The preparation method of the battery separator provided by the present invention includes forming a film from a solution containing a substrate, a pore-forming substance and a solvent, and removing the pore-forming substance at a temperature lower than the glass transition temperature of the substrate, wherein the substrate contains polyimide amine.

The lithium-ion secondary battery provided by the present invention includes a pole core and an electrolyte, the pole core includes a positive electrode, a negative electrode and a battery separator separating the positive electrode and the negative electrode, wherein the battery separator is the battery separator provided by the present invention.

The battery diaphragm provided by the invention has excellent chemical stability, high temperature resistance, good permeability and high mechanical strength because the novel high temperature resistant material polyimide is used as the base material. The battery separator obtained in the embodiment of the present invention does not rupture when heated to a high temperature of 400°C; the thermal shrinkage rate of the battery separator at 150°C is less than 0.5%, and the thermal shrinkage rate at 400°C is not more than 1.5%, which is much smaller than that of the prior art The thermal shrinkage rate of 3% and 5%; the puncture strength is also far less than the puncture strength of the battery separator in the prior art; the average diameter and porosity of the holes all meet the conductivity requirements, and have suitable and excellent air permeability, Moreover, the heat shrinkage rate is much smaller than that of the battery separator in the prior art. The lithium-ion secondary battery using the battery diaphragm provided by the invention does not have a short circuit phenomenon even at a high temperature of 150° C., so the battery diaphragm provided by the invention can be used in high-capacity and power batteries.

Detailed ways

The invention provides a battery separator, which comprises a substrate and pores distributed on the substrate, wherein the substrate contains polyimide.

In the present invention, the polyimide can be a polymer containing imide groups in various repeating units, preferably, the polyimide is a polyimide having the following structural formula:

Wherein, R ₁ and R ₂ are the same or different, and can be various substituted alkyl groups and substituted aryl groups, and the value of the degree of polymerization n makes the tensile strength of the battery separator containing the polyimide be 30-100 MPa, The glass transition temperature is 150-420°C.

Preferably, _{both R1} and _R2 are groups containing an aromatic ring, and the carbonyl is directly bonded to the aromatic ring of _R1 ; the nitrogen atom on the imide group is directly bonded to the aromatic ring of _R2 , and n Values are chosen so that the tensile strength of the battery separator containing the polyimide is 50-100 MPa, and the glass transition temperature is 380-420°C. In this case, the separator has better heat resistance and greater Tensile Strength.

The aromatic ring may be a benzene ring or a naphthalene ring. More preferably, _R is selected from one or more of substituted phenyl, substituted biphenyl, and substituted benzophenone groups, and the substituted phenyl, substituted biphenyl, and substituted benzophenone groups include benzene At least four hydrogens on the ring are replaced by carbonyl groups on the imide group, and two carbonyl groups on the same imide group are located adjacent to the benzene ring; _R2 is selected from substituted phenyl, substituted biphenyl One or more of the substituted phenyl group, substituted diphenyl ether group, and at least two hydrogens on the benzene ring in the substituted phenyl group, substituted biphenyl group, and substituted diphenyl ether group are replaced by nitrogen atoms on the imido group.

In addition to being substituted by carbonyl or imino in the above-mentioned benzene ring, other hydrogens on the benzene ring can also be substituted by other various functional groups, and the functional group can be one of halogen, nitro, alkyl, amino, and sulfonic acid groups or several, preferably C ₄ -C ₁₀ straight chain and/or branched chain alkyl.

In the specific embodiment of the present invention, it is preferred that the polyimide is one or more of polypyromellitic imide, polybiphenyl tetracarboxylic imide, and polybenzophenone tetracarboxylic imide. Above-mentioned polyimide can represent with following structural formula (1), (2) and (3) respectively:

In the above formula, the value of n is such that the tensile strength of the battery separator containing the polyimide is 50-100 MPa, the glass transition temperature is 380-420°C, R ₃ , R ₄ , R ₅ , R ₆ , R ₇ and R ₈ are independently selected from one or more of hydrogen, halogen, amino, nitro, alkyl, the halogen can be fluorine, chlorine, bromine, or iodine, and the alkyl is preferably C ₄ -C ₁₀ straight chain alkyl and/or branched chain alkyl.

_R can be a substituted aryl group of the following structural formula:

Wherein R' ₃ , R' ₄ , R' ₅ , R' ₆ , R' ₇ and R' ₈ are each independently selected from one or more of hydrogen, halogen, amino, nitro and alkyl, said The halogen may be fluorine, chlorine, bromine or iodine, and the alkyl group is preferably a C ₄ -C ₁₀ straight chain alkyl group and/or branched chain alkyl group. Preferably, R' ₃ , R' ₄ , R' ₅ , R' ₆ , R' ₇ and R' ₈ are all hydrogen, such R ₂ is relatively cheap and readily available.

In addition, the two imide groups on the repeating unit can also be located at other positions of the benzene ring, for example, for structural formula (2), the imide groups can be located at 2, 3 and/or 2', At the 3' position (according to the IUPCA nomenclature, the position where the two benzene rings are bonded is the 1,1' position). Since the position of the imide group on the benzene ring does not affect the heat resistance, air permeability, puncture strength and thermal shrinkage performance of the battery separator of the present invention, the polyimide of the present invention has an effect on the imide group. The position of is not critical, and may be various positional isomers of the structures described in the above structural formulas (1), (2) and (3) or their mixtures.

Although some polyimides are soluble, because most of the polyimides are insoluble polymers, the degree of polymerization of the polyimides cannot be accurately measured. Therefore, the present invention only uses the batteries concerned by the present invention Separator glass transition temperature and tensile strength to characterize polyimide. In the present invention, as long as the battery separator containing polyimide has a tensile strength of 30-100 MPa, preferably 50-100 MPa, and a glass transition temperature of 150-420°C, preferably 380-420°C, Reach the object of the present invention.

The battery separator of the present invention has no special limitation on the thickness of the base material, the size of the pores distributed on the base material and the density of the pores, as long as it can meet the requirements of the lithium ion secondary battery for the battery separator. Preferably, the average diameter of the pores is preferably 0.01-5 microns, more preferably 0.01-1 microns; the porosity is preferably 20-80% by volume, more preferably 30-60% by volume; the thickness of the membrane is preferably 5-50 microns, more preferably 5-30 microns; the tensile strength is greater than 50 MPa, preferably 50-100 MPa; the glass transition temperature is preferably 380-420°C.

The polyimide of the present invention can be obtained by various methods, for example, it can be obtained commercially, and can also be prepared by various methods. For example, it can be obtained by a condensation reaction between a polyvalent organic carboxylic acid or a derivative thereof and an organic diamine. The polyvalent organic carboxylic acid derivatives are selected from binary organic acid anhydrides, quaternary organic acid chlorides, and quaternary organic acid esters. Because quaternary organic carboxylic acids, quaternary organic acid chlorides, quaternary organic acid esters and organic diamines react to produce small molecular compounds, which bring a lot of inconvenience to the preparation of battery separators, therefore, the present invention prefers said polyimide Preferred are reaction products of binary organic acid anhydrides and organic diamines. The dibasic organic acid anhydrides can be various saturated and/or unsaturated dibasic organic acid anhydrides, and the organic diamines can be various dibasic amines containing two secondary amino groups. In the present invention, it is preferred that the binary organic acid anhydride and/or organic diamine contain a benzene ring structure. One or more of formic dianhydride; the organic diamine can be selected from one or more of diaminodiphenyl ether, p-phenylenediamine, 1,1'-biphenylenediamine or their homologues The homologues are preferably the above-mentioned organic diamines in which hydrogens at other positions on the benzene ring are replaced by C ₄ -C ₁₀ alkyl groups. The molar ratio of the binary organic acid anhydride to the organic diamine is preferably 0.99-1.03, more preferably 1.00-1.02.

The reaction conditions and operation methods of the binary organic acid anhydride and the organic diamine are well known to those skilled in the art, and will not be repeated here.

The preparation method of the battery separator provided by the present invention includes forming a film from a solution containing a substrate, a pore-forming substance and a solvent, and removing the pore-forming substance at a temperature lower than the glass transition temperature of the substrate, wherein the substrate contains polyimide amine.

Preferably, the polyimide is polypyromellitic imide, polybiphenyl tetracarboxylic imide, polybenzophenone tetramethylimide having the structure described in the above structural formula (1), (2) or (3). One or more of imides. Since general polyimide is an insoluble substance, in order to use polyimide to obtain a battery separator with uniform quality, it is necessary to mix polyimide and pore-forming substances uniformly, so polyimide is used as a raw material to prepare When the battery separator, the present invention preferably said polypyromellitic imide is selected from polyN-alkylphenylpyromellitic imide, polyN-alkylbiphenylpyromellitic imide, One or more of poly-N-alkyldiphenyl ether-based pyromellitic imides; , one or more of poly-N-alkyl biphenyl biphenyl tetracarboximide, poly-N-alkyl diphenyl ether-based biphenyl tetracarboximide; the polybenzophenone tetracarboximide The imine is selected from polyN-alkylphenylbenzophenone tetracarboximide, polyN-alkylbiphenylbenzophenone tetracarboximide, polyN-alkyldiphenyl ether base benzophenone tetracarboximide One or more of formimides, the alkyl group is preferably a C ₄ -C ₁₀ straight chain and/or branched chain alkyl group.

The pore-forming substance can be a variety of substances that have excellent compatibility with the substrate and can be removed below the glass transition temperature of the substrate. The pore-forming substance can be a conventional pore-forming substance, such as a non-volatile organic Solvents such as nonane, decane, undecane, dodecane, liquid paraffin or mineral oil.

Since it is difficult to control the size and distribution of the pores of the battery separator with the above-mentioned conventional pore-forming substances, the inventors have invented a new pore-forming substance. The novel pore-forming substance, which is a solid having a decomposition temperature higher than the boiling point of the solvent and lower than the glass transition temperature of the substrate, is preferred in the present invention.

Preferably, the new pore-forming substance is selected from polycaprolactone, polypropylene oxide, polymethylstyrene and polystyrene with a weight average molecular weight of 1000-50000, more preferably 10000-20000. The above-mentioned pore-forming substances can be obtained by various methods, for example, they can be purchased commercially, and can also be prepared by various methods.

In order to increase the compatibility of the pore-forming substance with the substrate or the material forming the substrate, the pore-forming substance is more preferably the above-mentioned polycaprolactone, polypropylene oxide, polyformaldehyde, at least one end of the polymer chain is functionalized by an amino group. One or more of base styrene and polystyrene. The above-mentioned polymers in which one end of the polymer chain is functionalized with an amino group are commercially available, and can also be prepared by various methods. The specific operation method of the functionalization of the amino group is well known to those skilled in the art and will not be repeated here.

The solvent can be various solvents that can simultaneously dissolve the substrate and the pore-forming substance and are easy to volatilize and remove. In the present invention, the solvent can be various strong polar nonionic solvents commonly used, preferably N-2-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N -one or more of dimethylformamide (DMF), dimethyl sulfoxide (DMSO), m-cresol, tetrahydrofuran, methanol, more preferably one or more of NMP, DMAC, DMF.

In the present invention, there is no special limitation on the amount of the base material and the pore-forming substance, which is specifically determined according to the required pore density and pore size of the battery separator. Preferably, the weight ratio of the substrate to the pore-forming substance is 0.25-3, more preferably 0.8-2. In the present invention, there is no particular limitation on the amount of the solvent added, as long as the substrate and the pore-forming substance can be completely dissolved to form a uniform solution. Preferably, the weight of the solvent and the substrate is 4.5-10.

In the present invention, the polyimide can be obtained by various methods, for example, it can be obtained commercially, and it can also be prepared by various methods, for example, it can be obtained between a tetravalent organic carboxylic acid or its derivatives and an organic diamine. The condensation reaction is obtained. In the specific embodiment of the present invention, it is preferred to adopt binary organic acid anhydrides such as biphenyltetraic dianhydride, benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride and organic diamines such as diaminodiphenyl ether, p-phenylenediamine Prepared by condensation reaction.

Therefore, the preparation method of the battery separator provided by the present invention can also be realized in the following manner: polyvalent organic carboxylic acid or its derivatives, organic diamine, pore-forming substance and solvent are mixed and contacted to obtain a uniform solution, and then the solution is formed into film, and remove pore-forming substances below the glass transition temperature of the substrate. The molar ratio of the polyhydric organic carboxylic acid or its derivatives to the organic diamine is preferably 0.99-1.03, more preferably 1.00-1.02, and the addition amount of the pore-forming substance is preferably polyhydric organic carboxylic acid or its derivatives and organic diamine. The weight ratio of the total amount of amines to the pore-forming substance is 0.25-3, more preferably 0.8-2; the feed weight ratio of the solvent to the polyvalent organic carboxylic acid or its derivatives, and organic diamine feed is preferably 4.5-10 . The temperature for mixing and contacting the polyvalent organic carboxylic acid or its derivatives, organic diamine, pore-forming substance and solvent is preferably 20-70° C., and the contact time is preferably 1-5 hours. The polyvalent organic carboxylic acid or derivatives thereof are preferably binary organic acid anhydrides, and the binary organic acid anhydrides can be selected from one of biphenyltetracarboxylic dianhydride, benzophenone tetracarboxylic dianhydride, and pyromellitic dianhydride. One or more kinds; the organic diamine can be selected from one or more of diaminodiphenyl ether, p-phenylenediamine, 1,1'-biphenylenediamine or their homologues.

The film forming method can adopt conventional methods such as dry film forming method and wet film forming method. For example, by spreading the solution to the desired thickness. The application can be achieved by various methods, for example, an applicator can be used. Like the conventional film-forming method, after the solution is applied to a desired thickness, the solvent is removed to obtain a non-porous film, and then the pore-forming substances in the non-porous film are removed to obtain the battery separator of the present invention. The method and specific operations for removing the solvent are well known to those skilled in the art. The solvent is preferably removed by heating, and the heating temperature depends on the boiling point of the solvent. In the present invention, the temperature for removing the solvent is lower than the boiling temperature of the solvent and lower than the removal temperature of the pore-forming substance, and even lower than the glass transition temperature of the substrate.

When using binary organic acid anhydrides and organic diamines as raw materials to prepare polyimide substrates, since the binary organic acid anhydrides and organic diamines mainly generate polyamic acid under the above contact conditions, it is necessary to dehydrate the polyamic acid to generate polyimide. For imines, it is necessary to add a dehydrating agent or increase the reaction temperature. In order to obtain a battery separator with stable quality and excellent performance, in the present invention, the method of increasing the reaction temperature is preferably used to dehydrate the polyamic acid. Therefore, when using binary organic acid anhydride and organic diamine as raw materials to prepare polyimide substrates, the preparation method of the battery separator of the present invention also includes removing the non-porous membrane after removing the solvent before removing the pore-forming substance. An imidization reaction is performed to convert polyamic acid into polyimide. The reaction temperature of the imidization is 270-300° C., and the reaction time is preferably 3-5 hours. The temperature can be raised directly or programmed to increase the temperature to 270-300°C. In the present invention, the temperature is preferably programmed to 270-300°C at a heating rate of 4-8°C/min, so that the polyamic acid is fully imidized. to generate the desired polyimide. The conversion rate of amic acid into imide can be improved by increasing the reaction temperature and prolonging the reaction time. Under the above-mentioned conditions of the present invention, the conversion rate of amic acid into imide is greater than 99%, and because the existence of a small amount of amic acid has no great influence on the heat resistance, gas permeability and thermal shrinkage of the battery separator obtained in the present invention, Therefore, less than 2% of amic acid is allowed in the battery separator. In the specific embodiment of the present invention, there is no special requirement on the amount of amic acid. The imidization temperature is preferably lower than the decomposition or volatilization temperature of the pore-forming substance, so as to obtain a higher-quality battery separator.

Various methods can be used to remove the pore-forming substances. For example, the non-porous film obtained after removing the solvent can be heated to a temperature at which the pore-forming substances decompose or volatilize, so that the pore-forming substances in the non-porous film can be decomposed or volatilized. Since the decomposition or volatilization temperature of the pore-forming substance is lower than the glass transition temperature of the substrate in the non-porous film, corresponding pores can be left on the substrate after the pore-forming substance is decomposed or volatilized. Obviously, the size of the pores corresponds to the volume (or content) of the pore-forming substances in the non-porous membrane. The larger the volume occupied by the pore-forming substances in the non-porous membrane, the larger the pores left after the pore-forming substances leave.

In order to ensure that the pores left by the pore-forming substance can penetrate the thickness direction of the film, preferably, the method for preparing a battery separator according to the present invention further includes the step of axially stretching the film obtained by removing the pore-forming substance . The axial stretching may be uniaxial stretching or biaxial stretching. The extent of axial stretching is such that the pores on the substrate can all penetrate the thickness direction of the film. Under the conditions of the present invention, the axial stretching is preferably biaxial (transverse and longitudinal) stretching, and the conditions of the biaxial stretching are: stretching temperature 100-140°C, stretching speed 5 -15 mm/min, the stretch ratio is 2 times in the machine direction (MD) and 2 times in the transverse direction (TD). The specific operation of the axial stretching is well known to those skilled in the art and will not be repeated here.

In order to verify that the base material of the battery separator prepared by the above method of the present invention contains polyimide, qualitative characterization can be carried out according to infrared spectroscopy. In the infrared spectrum, the strong peak around 1720cm ^-1 is the C=O stretching vibration absorption peak, and the moderately strong peak around 1380cm ^-1 is the CN stretching vibration absorption peak. The infrared spectrogram is measured by NEXUS470 Fourier Transform Infrared Spectrometer from American Nicolet Company and potassium bromide coating method. The above-mentioned assay methods are well known to those skilled in the art and will not be repeated here.

The above method can be used to make a battery separator of any thickness as required. The specific thickness of the battery separator depends on the coating amount of the film-forming solution. Preferably, the thickness of the battery separator used as the battery separator is 5-50 microns, more preferably 5-30 microns. The size of the pores of the battery separator mainly depends on the axial stretching conditions. Under the axial stretching conditions of the present invention, the diameter of the pores on the obtained battery separator can be 0.01-5 microns, more preferably 0.01-1 microns. The pore density mainly depends on the ratio of the pore-forming substance to the substrate, and the greater the amount of the pore-forming substance, the greater the pore density. In the present invention, the porosity is used to represent the pore density, and the porosity refers to the percentage of the total volume of the pores in the volume of the battery separator. Under the conditions described in the present invention, the porosity of the obtained battery separator can be 20-80% by volume, more preferably 30-60% by volume. The air permeability of the battery separator provided by the present invention can reach 20-1000 seconds/100cc, more preferably 20-500 seconds/100cc; the piercing strength can reach 400gf/18 microns or more; the lateral heat shrinkage at 150°C is less than 1%, Preferably less than 0.5%; transverse thermal shrinkage at 400°C is less than 3%.

The invention provides a lithium ion secondary battery, the battery includes a pole core and an electrolyte, the pole core includes a positive pole, a negative pole and a battery diaphragm separating the positive and negative poles, wherein the battery diaphragm is provided by the present invention battery separator.

Since the present invention only relates to the improvement of the battery diaphragm, other structures of the battery such as electrolyte, positive pole and negative pole are not particularly limited, conventional lithium ion secondary battery electrolyte, positive pole and negative pole are sufficient. As described electrolyte solution is preferably non-aqueous electrolyte solution, comprises electrolyte and solvent, and described electrolyte can be lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate (LiSbF ₆ ), perchloric acid Lithium (LiClO ₄ ), lithium perchlorate, lithium fluorocarbon sulfonate (LiCF ₃ SO ₃ ), Li(CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO ₄ , lithium chloroaluminate (LiAlCl ₄ ), LiN(C _x F _2x+1 SO ₂ )(C _y F _2y+1 SO ₂ ) (where x and y are natural numbers from 1 to 10), lithium chloride (LiCl) and lithium iodide (LiI) one or more of them. The solvent can be γ-butyrolactone, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), propylene carbonate (PC), vinylene carbonate (VC), γ-butyrolactone (γ-BL), sultone, dipropyl carbonate and other fluorine-containing , various organic anhydrides, N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile, N,N-dimethylformamide (DMF), sulfolane, dimethylsulfoxide, sulfur Or at least one of cyclic organic esters containing unsaturated bonds. In order to increase the solubility of the lithium salt in the solvent, the present invention preferably uses a mixed solvent of two or three of the above-mentioned solvents, for example, dimethyl carbonate and diethyl carbonate are usually mixed at 1:0.9-1:3.0 The ratio of dimethyl carbonate, diethyl carbonate and propylene carbonate is prepared as a mixed solvent, or 1:1:0.2-1:0.5:1.5 is prepared as a mixed solvent. The solvent is added in an amount such that the lithium salt concentration is 0.1-2.0 mol/liter, preferably 0.7-1.6 mol/liter.

The positive electrode includes a current collector and a positive electrode material coated and/or filled on the current collector. The current collector can be various collectors known to those skilled in the art, such as aluminum foil, copper foil, Nickel-plated steel strip, the present invention selects aluminum foil for use as current collector. The positive electrode material can be various positive electrode materials known to those skilled in the art, and generally includes a positive electrode active material, a binding agent and a conductive agent selectively contained, and the positive electrode active material can be selected from the conventional positive electrode active materials of lithium-ion batteries. Substances, such as Li _x Ni _1-y CoO ₂ (where, 0.9≤x≤1.1, 0≤y≤1.0), Li _m Mn _2-n B _n O ₂ (where, B is a transition metal, 0.9≤m≤1.1 , 0≤n≤1.0), Li _1+a M _b Mn _2-b O ₄ (wherein, -0.1≤a≤0.2, 0≤b≤1.0, M is lithium, boron, magnesium, aluminum, titanium, chromium, One or more of iron, cobalt, nickel, copper, zinc, gallium, yttrium, fluorine, iodine, sulfur).

The negative electrode includes a conductive base and a negative electrode material coated and/or filled on the conductive base. The conductive substrate is well known to those skilled in the art, for example, it can be selected from one or more of aluminum foil, copper foil, nickel-plated steel strip, and punched steel strip. Described negative electrode active material is well known to those skilled in the art, and it comprises negative electrode active material and various additives, and described negative electrode active material can be selected from the conventional negative electrode active material of lithium ion battery, as natural graphite, artificial graphite, petroleum coke, One or more of organic pyrolysis carbon, mesocarbon microspheres, carbon fibers, tin alloys, and silicon alloys. The binder may be selected from conventional binders for lithium ion batteries, such as one or more of polyvinyl alcohol, polytetrafluoroethylene, hydroxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR). Generally, the content of the binder is 0.5-8% by weight of the negative active material, preferably 2-5% by weight.

The preparation methods of the positive electrode and the negative electrode and the battery assembly method are well known to those skilled in the art, and will not be repeated here.

The following examples will further describe the present invention.

Example 1

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

5.214 grams of pyromellitic dianhydride, 4.786 grams of diaminodiphenyl ether, 10.015 grams of polystyrene containing amino groups at one end of the polymer chain (the weight-average molecular weight is 15000, purchased from Aldrich Company) and 100 grams of N-2-methyl Pyrrolidone (NMP) was stirred and mixed uniformly at room temperature to obtain a uniform solution, and then coated with an I-shaped film applicator to obtain a film with a thickness of 0.5 mm. The film was heated at 120°C for 2 hours to remove the solvent, then the temperature was programmed to 300°C at 5°C/min for imidization for 3 hours, and then the temperature was raised to 350°C for 1.5 hours, and finally cooled to 120°C while performing biaxial For stretching treatment, the stretching speed is 10 mm/min, and the stretching ratio is 2 times in the longitudinal direction (MD) and 2 times in the transverse direction (TD), thereby obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The thickness of the battery separator is 18 microns measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator is 380° C. measured by DSC.

Example 2

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

8.864 gram of biphenyltetracarboxylic dianhydride, 3.224 gram of p-phenylenediamine, 8.032 gram of polypropylene oxide (weight average molecular weight is 18000, purchased from Aldrich Company) and 150 gram of DMSO are stirred and mixed at room temperature Uniform, after obtaining a uniform solution, use an I-shaped film coating applicator to coat a film to obtain a film with a thickness of 0.4 mm. The film was heated at 160°C for 2 hours to remove the solvent, then programmed at 4°C/min to 270°C for imidization for 5 hours, then raised to 350°C for 1.5 hours, and finally cooled to 120°C for biaxial stretching Stretching treatment, the stretching speed is 10 mm/min, the stretching ratio is 2 times in the longitudinal direction (MD), and 2 times in the transverse direction (TD), thus obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The battery separator has a thickness of 18 micrometers measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator measured by a DSC method is 420° C.

Example 3

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

With 10.693 grams of benzophenone tetra-acid dianhydride, 3.516 grams of p-phenylenediamine, 5.982 grams of polymethylstyrene (weight average molecular weight 10000, purchased from Aldrich company) and 150 grams of N, N-dimethylformamide at room temperature Stir and mix evenly to obtain a uniform solution, and then use an I-shaped film coating applicator to coat the film to obtain a film with a thickness of 0.5 mm. The film was heated at 160°C for 2 hours to remove the solvent, then the temperature was programmed at 6°C/min to 285°C for imidization for 4 hours, and then the temperature was raised to 350°C for 1.5 hours, and finally cooled to 120°C for double imidization. Axial stretching treatment, the stretching speed is 10 mm/min, the stretching ratio is 2 times in the longitudinal direction (MD) and 2 times in the transverse direction (TD), thus obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The battery separator has a thickness of 18 microns measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator measured by a DSC method is 400° C.

Example 4

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

5.031 grams of poly-N-pentylphenylpyromellitic imide (average degree of polymerization 200, tensile strength 60 MPa, glass transition temperature 390 ° C), 5.893 grams of polystyrene containing amino groups at one end of the polymer chain (weight Average molecular weight 15000, purchased from Aldrich Company) and 100 grams of N-2-methylpyrrolidone (NMP) were stirred and mixed evenly at 70 ° C, and after obtaining a uniform solution, the film was coated with an I-shaped film applicator to obtain a thickness of 0.5 mm membrane. The film was heated at 120°C for 2 hours to remove the solvent, then heated to 350°C and kept for 1.5 hours, and finally cooled to 120°C for biaxial stretching at the same time, the stretching speed was 10 mm/min, and the stretching ratio in the longitudinal direction ( MD) is doubled, and the transverse direction (TD) is doubled, thus obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The thickness of the battery separator is 18 microns measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator is 380° C. measured by DSC.

Example 5

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

The battery separator of the present invention was prepared according to the method of Example 3, except that liquid paraffin was used as the pore-forming substance. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The thickness of the battery separator is 18 microns measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator is 380° C. measured by DSC.

Example 6

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

5.214 grams of pyromellitic dianhydride, 4.786 grams of diaminodiphenyl ether, 10.015 grams of polystyrene containing amino groups at one end of the polymer chain (the weight-average molecular weight is 15000, purchased from Aldrich Company) and 100 grams of N-2-methyl Pyrrolidone (NMP) was stirred and mixed evenly at room temperature to obtain a uniform solution, and then coated with an I-shaped coating film applicator to obtain a film with a thickness of 0.8 mm. The film was heated at 120°C for 2 hours to remove the solvent, then the temperature was programmed to 300°C at 5°C/min for imidization for 3 hours, and then the temperature was raised to 350°C for 1.5 hours, and finally cooled to 120°C while performing biaxial For stretching treatment, the stretching speed is 10 mm/min, and the stretching ratio is 2 times in the longitudinal direction (MD) and 2 times in the transverse direction (TD), thereby obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The thickness of the battery separator is 25 micrometers measured with a 1/10000 double thickness instrument, and the glass transition temperature of the separator is 380° C. measured by DSC. The tensile strength of the separator was 110 MPa.

Example 7

This example is used to illustrate the battery separator provided by the present invention and its preparation method.

8.864 gram of biphenyltetracarboxylic dianhydride, 3.224 gram of p-phenylenediamine, 8.032 gram of polypropylene oxide (weight average molecular weight is 18000, purchased from Aldrich Company) and 150 gram of DMSO are stirred and mixed at room temperature Uniform, after obtaining a uniform solution, use an I-shaped film coating applicator to coat a film to obtain a film with a thickness of 0.3 mm. The film was heated at 160°C for 2 hours to remove the solvent, then programmed at 4°C/min to 270°C for imidization for 5 hours, then raised to 350°C for 1.5 hours, and finally cooled to 120°C for biaxial stretching Stretching treatment, the stretching speed is 10 mm/min, the stretching ratio is 2 times in the longitudinal direction (MD), and 2 times in the transverse direction (TD), thus obtaining the battery separator of the present invention. Among them, the strong peak near 1720cm ^-1 in the Fourier transform infrared spectrum is the C=O stretching vibration absorption peak, and the medium-strong peak near 1380cm ^-1 is the CN stretching vibration absorption peak, indicating that there is an imide group in the product. There are absorption peaks at 1600 cm ^-1 , 1575 cm ^-1 , 1490 cm ^-1 , and 1450 cm ^-1 (characteristic peaks of benzene rings) in the figure, indicating the presence of benzene rings in the product. The battery separator has a thickness of 5 microns measured by a 1/10000 double thickness instrument, and the glass transition temperature of the separator measured by a DSC method is 420° C. The tensile strength of the separator was 50 MPa.

Comparative example 1

This comparative example is used to illustrate the battery separator and its preparation method in the prior art.

The battery separator was prepared according to the method described in CN 1512607A Example 1, except that the stretch ratio was 2 times in the longitudinal direction (MD) and 2 times in the transverse direction (TD), and the thickness of the obtained separator was 18 microns.

Battery separator performance test

Measure the porosity, average diameter of pores, tensile strength, air permeability, puncture strength, heat resistance and heat shrinkage of the battery separators prepared in Examples 1-5 and Comparative Example 1 respectively, and determine the structure As shown in Table 1 below.

Porosity: adopt the routine test method test of this field: cut out the square sample of certain length from microporous film, measure its volume ( ^cm ) and weight (g), calculate by following formula: porosity (%)= 100×(1-weight/(resin density×volume))

Average diameter of pores: Determined by a pore diameter distribution curve obtained by an ammonia adsorption/desorption type specific surface area/pore distribution tester ASAP2010 manufactured by Shimadzu Corp according to the BJH method.

Pore distribution: use the battery separators obtained in Examples 1-5 and Comparative Example 1 as separators to make LP043450 batteries, charge and discharge the above-mentioned batteries at 4.3-2.75 volts cycle, disassemble the batteries after 100 cycles, and observe the battery separators The distribution of white zinc dendrites on the top, if the white zinc dendrites are evenly distributed, it means that the pores of the separator are evenly distributed, and vice versa.

Tensile strength: stretch a battery separator with a width of 5 mm and a thickness of d mm on a tensile machine at 5 mm/s, record the maximum tensile force F _max (N) displayed by the tensile meter before breaking, and calculate according to the following formula: tensile strength (MPa)=F _max /(5×d).

Air permeability: Measured according to JIS P8117.

Puncture strength: fix the battery separator, use a needle with a diameter of 1 mm and a tip bending radius of 0.5 mm to move at a rate of 2 cm/s, and the maximum force during the process of piercing the battery separator is measured as the puncture strength.

Heat shrinkability: Place the battery separator with a transverse length of L ₀ in an oven at 150°C and 400°C for 1 hour in a natural state, and then measure the transverse length L ₁ , then

Heat resistance: The heat resistance of the battery diaphragm is tested by the heat-resistant film rupture method described in CN 1512607A, the difference is that the heating rate of the battery is 10 °C/min, and the temperature is raised to 400 °C.

Table 1

From the results given in Table 1 above, it can be seen that the battery separator obtained in the embodiment of the present invention has excellent high temperature resistance, and does not rupture when heated to a high temperature of 400°C; the thermal shrinkage rate of the battery separator at 150°C is less than 0.5%. The heat shrinkage rate at 400°C is not more than 1.5%, far less than the 3% and 5% heat shrinkage rates in the prior art; the puncture strength is also much greater than the puncture strength of the battery separator in the prior art; the average pore The diameter and porosity both meet the requirements of electrical conductivity, and have suitable and excellent gas permeability, and the heat shrinkage rate is also much smaller than that of the battery separator in the prior art. In addition, it can be seen from the pore distribution experiment that the battery separator formed with the new pore-forming material provided by the present invention has a more uniform pore distribution than the organic solvent used as the pore-forming material in the prior art.

Battery high temperature resistance test

The battery separator made in Examples 1-5 and Comparative Example 1 and a single-layer PE material (tonen, outsourced) and three-layer PP/PE/PP material (celgard, outsourced) with a thickness of 18 microns are made as separators For LP043450 battery, the high-temperature performance test is carried out on the above-mentioned battery. The high-temperature performance test is a 150°C furnace heat test. The temperature was raised from room temperature to 150°C at a rate of 5°C/min, and a battery voltage drop greater than 0.2 volts was considered a short circuit. The test results are shown in Table 2.

Table 2

Diaphragm source short circuit condition thermal runaway Example 1 not short circuited No explosion, smoke, fire, or liquid leakage Example 2 not short circuited No explosion, smoke, fire, or liquid leakage Example 3 not short circuited No explosion, smoke, fire, or liquid leakage Example 4 not short circuited No explosion, smoke, fire, or liquid leakage Example 5 not short circuited No explosion, smoke, fire, or liquid leakage Comparative example 1 short circuit explode single layer PE short circuit explode Three-layer PP/PE/PP short circuit explode

From the results in Table 2 above, it can be seen that the battery made of the battery separator provided by the present invention has excellent high temperature resistance, and no unsafe phenomena such as explosion, smoke, fire, and liquid leakage will not occur even at a high temperature of 150 ° C. .

Claims (17)

1. A battery separator, which includes a substrate and pores distributed on the substrate, characterized in that the substrate contains polyimide, and the polyimide is a polyimide having the following structural formula amine:

Wherein, R ₁ and R ₂ are the same or different, and are various substituted alkyl groups or substituted aryl groups, and the value of the degree of polymerization n makes the battery separator containing the polyimide have a tensile strength of 30-100 MPa, glass The melting temperature is 380-420°C. 2. The battery separator according to claim 1, wherein, said R ₁ and R ₂ are groups containing an aromatic ring, and the carbonyl group is directly bonded to the aromatic ring of R ₁ ; the nitrogen atom on the imide group and The aromatic rings of R ₂ are directly bonded, and the value of n is such that the battery separator containing the polyimide has a tensile strength of 50-100 MPa and a glass transition temperature of 380-420°C. 3. The battery separator according to claim 2, wherein the polyimide is selected from polyimides having the following structural formula (1), (2) or (3):

In the above formula, R ₃ , R ₄ , R ₅ , R ₆ , R ₇ and R ₈ are independently selected from hydrogen, fluorine, chlorine, bromine, iodine, amino, nitro, C ₄ -C ₁₀ linear alkyl and /or one or more of the branched chain alkyl groups; _R is a substituted aryl group of the following structural formula:

Wherein R' ₃ , R' ₄ , R' ₅ , R' ₆ , R' ₇ and R' ₈ are independently selected from hydrogen, fluorine, chlorine, bromine, iodine, amino, nitro, C ₄ -C ₁₀ straight One or more of chain alkyl and/or branched chain alkyl. 4. The battery separator according to claim 1, wherein the diameter of the pores is 0.01-5 microns, the porosity is 20-80% by volume, the thickness of the film is 5-50 microns, the tensile strength of the film is It is 50-100 MPa, and the glass transition temperature is 380-420°C. 5. A method for preparing a battery separator, the method comprising forming a film from a solution containing a substrate, a pore-forming substance and a solvent, and removing the pore-forming substance at a temperature lower than the glass transition temperature of the substrate, characterized in that the substrate The material contains polyimide, and the polyimide is a polyimide having the following structural formula:

Wherein, R ₁ and R ₂ are the same or different, and are various substituted alkyl groups or substituted aryl groups, and the value of the degree of polymerization n makes the battery separator containing the polyimide have a tensile strength of 30-100 MPa, glass The melting temperature is 380-420°C. 6. The method according to claim 5, wherein the weight ratio of the polyimide to the pore-forming substance is 0.25-3, and the weight ratio of the solvent to the polyimide is 4.5-10. 7. The method according to claim 5 or 6, wherein the _R1 and _R2 are groups containing an aromatic ring, and the carbonyl group is directly bonded to the aromatic ring of _R1 ; the nitrogen atom on the imide group It is directly bonded to the aromatic ring of _R2 , and the value of n makes the battery separator containing the polyimide have a tensile strength of 50-100 MPa and a glass transition temperature of 380-420°C. 8. The method according to claim 7, wherein the polyimide is selected from polyimides having the following structural formula (1), (2) or (3):

In the above formula; R ₃ , R ₄ , R ₅ , R ₆ , R ₇ and R ₈ are independently selected from hydrogen, fluorine, chlorine, bromine, iodine, amino, nitro, C ₄ -C ₁₀ linear alkyl and/or one or more of branched chain alkyl groups; _R is a substituted aryl group of the following structural formula:

Wherein R' ₃ , R' ₄ , R' ₅ , R' ₆ , R' ₇ and R' ₈ are independently selected from hydrogen, fluorine, chlorine, bromine, iodine, amino, nitro, C ₄ -C ₁₀ straight One or more of chain alkyl and/or branched chain alkyl. 9. The method according to claim 5, wherein the method comprises mixing and contacting polyvalent organic carboxylic acid or derivatives thereof, organic diamine, and pore-forming substance with a solvent to obtain a uniform solution, and then forming a film from the solution, and Pore-forming substances are removed below the glass transition temperature of the substrate, and the polyvalent organic carboxylic acid or its derivatives are tetravalent organic carboxylic acids, tetravalent organic acid chlorides, tetravalent organic acid esters or divalent organic acid anhydrides. 10. The method according to claim 9, wherein the molar ratio of the polyvalent organic carboxylic acid or its derivatives to the organic diamine is 0.99-1.03, and the total amount of the polyvalent organic carboxylic acid or its derivatives and the organic diamine The weight ratio of the solvent to the pore-forming substance is 0.25-3, and the weight ratio of the solvent to the polyvalent organic carboxylic acid or its derivatives and the organic diamine polyimide is 4.5-10. 11. The method according to claim 9, wherein the temperature of mixing and contacting polyvalent organic carboxylic acid or its derivatives, organic diamine, pore-forming substance and solvent is 20-70°C, and the contacting time is 1-5 hours . 12. The method of claim 9, further comprising heating the solvent-removed membrane at 270-300°C for 3-5 hours before removing the pore-forming substances. 13. The method according to claim 9, 10 or 11, wherein the polyvalent organic carboxylic acid or its derivatives are selected from biphenyltetracarboxylic dianhydride, benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride One or more of them; the organic diamine is selected from one or more of diaminodiphenyl ether, p-phenylenediamine, and 1,1'-biphenylenediamine. 14. The method of claim 5, 6, 9, 10 or 11, wherein the pore forming substance is a solid having a decomposition temperature higher than the boiling point of the solvent and lower than the glass transition temperature of the substrate. 15. The method according to claim 14, wherein the pore-forming substance is selected from the group consisting of polycaprolactone, polypropylene oxide, and polymethylbenzene with a weight average molecular weight of 10,000-20,000 and at least one end of the polymer chain is aminated. Vinyl, polystyrene. 16. The method according to claim 5, 6, 9, 10 or 11, wherein the solvent is selected from N-2-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethyl One or more of methyl formamide, dimethyl sulfoxide, m-cresol, tetrahydrofuran, methanol. 17. A lithium-ion secondary battery, the battery includes a pole core and an electrolyte, the pole core includes a positive electrode, a negative electrode and a battery separator separating the positive and negative electrodes, characterized in that the battery separator is the one according to claim 1- The battery separator described in any one of 4.