WO2012119361A1

WO2012119361A1 - Co-extruded composite membrane comprising nano-sized pre-crosslinked rubber micropowder and lithium-ion battery using same

Info

Publication number: WO2012119361A1
Application number: PCT/CN2011/077074
Authority: WO
Inventors: 李鑫; 李建华; 陈卫; 焦永军; 李龙
Original assignee: 天津东皋膜技术有限公司
Priority date: 2011-03-09
Filing date: 2011-07-12
Publication date: 2012-09-13
Also published as: CN102683628A; CN102683628B

Abstract

A co-extruded composite membrane comprising nano-sized pre- crosslinked rubber micropowder and a lithium-ion battery using same. The membrane has good elasticity, mechanical strength and high temperature resistance properties, can effectively improve the cycling and safety properties of the cell; the pre-crosslinked rubber micropowder has a particle size of 25 to 300 nanometers and a gel content of greater than 80%; the lithium-ion cell comprising the nano-sized pre-crosslinked rubber micropowder comprises a positive pole piece, a negative pole piece, an electrolyte and a co-extruded composite membrane modified by nano-sized pre-crosslinked rubber micropowder.

Description

Co-extruded composite membrane containing nano-pre-crosslinked rubber micropowder and lithium ion battery using the same

The present invention relates to a composite separator for a lithium ion battery and a method for producing the same, and particularly to a lithium ion power battery or a long life energy storage battery which is required to have high safety, cycle life and the like.

Background technique

Since the polyolefin microporous membrane has penetrating network-like submicron micropores, is resistant to high voltage oxidation, and is stable to organic electrolytes of lithium ion batteries, polyolefin microporous membranes have been widely used as separator materials in mobile phones and notebook computers. Battery, typical polyolefin microporous membrane is "dry"

PP/PE/PP three-layer composite separator, single-layer "wet method" high molecular weight PE separator.

The existing polyolefin microporous membranes currently fail to meet the high-end requirements of power batteries in terms of safety and cycle life of batteries. The main technical analysis is as follows:

Existing polyolefin microporous membranes generally employ the following two manufacturing processes:

One is the "dry method" process, and the uniaxial stretching process is mainly used in the "dry process" process. First, a highly crystalline polypropylene or polyethylene film having a low crystallinity is prepared, and a high crystallinity orientation is obtained after high temperature annealing. The film, the film mainly forms micropores by cold stretching to form micro-cracks and continues to undergo hot tensile strengthening. The process is relatively mature. The main advantage of the existing "dry" PP/PE/PP three-layer separator is the manufacturing cost. Low, the main disadvantages are:

1. The diaphragm has insufficient toughness and is easy to tear in the transverse direction;

2. Although the intermediate microporous layer adopts PE which is turned off at a high temperature of 135-145 ° C, the PP microporous layer with limited melting point and hot tensile strengthening still has the disadvantages of large heat shrinkage and high temperature rupture at high temperature. ; 3. Lack of stress absorption in the thickness direction.

The other is the "wet process", the "wet process", also known as the thermal phase separation method, which combines high molecular weight polyolefin resins with "high temperature compatibilizers" (high boiling hydrocarbons such as paraffin oil, or Other plasticizers, the solvent and the polyolefin are mutually dissolved in a thermodynamically high temperature, and can achieve molecular level mixing. The invention defines "high temperature compatibilizer", which is actually a process solvent for pore formation. The high-temperature melt which is uniformly heated and kneaded is rapidly solidified on the surface of the chill roll, phase separation occurs during the cooling process, and the sheet is stretch-strengthened by stepwise biaxial stretching or simultaneous biaxial stretching, and then the volatile cleaning solvent is used. To extract the "high-temperature compatibilizer" in the semi-finished film, and further heat-stretch, heat-set and cool to prepare a microporous membrane material which is interpenetrated inside. The common method is a single-layer PE film, and dry Compared with the diaphragm, the two-way tensile strengthening, the viscosity average molecular weight of the raw material is generally more than 500,000, and the wet film has improved tensile strength and elongation at break, and the existing "wet method" is separated. The main disadvantages of the membrane include: 1. The extraction process must be used, and the production cost is slightly higher;

2. The heat shrinkage is too large at temperatures above 130 °C;

3. Insufficient ability to withstand high temperature and breakage at temperatures above 130 °C;

4. The thickness direction also lacks the stress absorption capability, which can not meet the high-end requirements of the power battery in terms of safety and battery cycle life.

In addition to the polyolefin microporous membrane, there is also a physical gel membrane. The typical PVDF-HFP copolymer porous gel membrane manufactured by the Bel lcore process and the pole piece can be bonded to the pole piece by a hot pressing process. Strength, even as a whole, battery cycle life and safety are high, but PVDF-HFP copolymer gel membrane (which belongs to physical gel, which is dissolved by heating or immersing in acetone), has poor mechanical strength. The pore size of the internal micropores is slightly larger, on the order of 1-2 microns, and if the thickness is controlled to be similar to the thickness of conventional commercial polyolefin membranes, such as 16-25 microns, a large number will occur. The waste of internal short circuit of the battery, therefore, it is required to increase the thickness of the separator (usually designed to have a thickness of 40-50 micrometers) to compensate for the lack of strength, which is disadvantageous to the rate characteristic and the energy density characteristic of the battery, and the gel membrane is usually dissolved. Produced by phase separation, the cost is high.

In terms of the safety performance and life of the power battery, the diaphragm is required to have the following characteristics:

1. In terms of mechanical properties, it is required to have high tensile strength in the longitudinal direction and high toughness in the transverse direction;

2. Suitable surface pore size and gas permeability;

3. 130-20CTC has fused shutdown characteristics at high temperatures and low heat shrinkage;

4. It has the ability of stress absorption, that is, it has the appropriate elastic deformation ability under compressive stress to adapt to the expansion/contraction cycle of the negative electrode, without affecting the charge-discharge cycle ability of the battery, and the elastic recovery ability after pressure release or reduction;

5. High temperature resistant membrane breakage, high melting even at 130-20CTC;

6. The separator and the pole piece have good bonding strength. When the battery is overcharged and internal heat is generated, the local current density is prevented from rising sharply, resulting in thermal runaway.

There is almost no adhesion between the "dry" or "wet" polyolefin separator and the pole piece, even after pressurization at 90 ° C / 1.5 MPa.

U.S. Patent Nos. 4,650,730 and 4, 431,304, 5, 691, 077, etc. all refer to multilayer battery separator structures, some adopt tubular polypropylene film self-capping process, and some use multilayer film. Composite process. The structure of PP/PE/PP is formed, wherein the middle PE layer can function as a high temperature shutdown. The above patents only provide the manufacturing technology of the thermal shutdown diaphragm, and the cycle life of the diaphragm to the lithium ion power battery. Technical solutions were not provided for improvement in reliability.

In order to improve and compensate for the high temperature shrinkage and high temperature film rupture resistance of the existing polyolefin microporous membrane, the Chinese invention patent application 200880003493. 7, 200880000072. 9 reported on the polyolefin microporous membrane A composite membrane technology in which a ceramic micropowder such as alumina is bonded to a porous coating by a bonding agent; Chinese Patent Application No. 200510086061. 5 reports that a polyamide having a high melting point of 180 ° C or higher is used on the surface of the polyolefin microporous membrane. A technical solution for forming a porous coating layer such as polyamideimide, polyimide, etc.; Chinese Patent Application No. 200480034190. 3 proposes a technical solution for coating a surface of a polyolefin microporous membrane with a fluororesin which can be gelled; All of the above are coating methods on the surface of the polyolefin microporous membrane by coating method. The main disadvantages are:

1. Since the existing polyolefin separator is basically an inert material, the adhesion between the coating and the coating is insufficient, the coating is thick and easy to peel off, and the heat shrinkage effect of the polyolefin membrane is not obvious;

2. Due to the capillary action of the micropores of the polyolefin membrane, the colloid and its slurry in the slurry will enter the micropores of the polyolefin membrane during the implementation of the above coating scheme, which may affect the membrane after the solvent is evaporated and dried to form a membrane. The pore size distribution and gas permeability, the consistency of the coating method for mass production is difficult to control, and the coating method composite membrane is expensive to manufacture.

In order to improve the bonding strength between the separator and the positive electrode tab, and to improve the safety of the lithium battery against overcharging, etc., Chinese Patent Application No. 01 112218.8 proposes mixing and adding a monomer which can be thermally crosslinked to form a gel in the electrolyte. The gel is used to increase the bonding strength between the separator and the positive electrode tab. Similarly, the gel forms a gel in the micropores of the separator during the thermal crosslinking formation, thereby affecting the permeability of the separator. The monomer with incomplete reaction may also oxidize, gas, etc. on the positive electrode side, and may even affect the cycle performance of the battery.

In order to improve the mechanical strength such as tear resistance of the "dry method" diaphragm, Chinese invention patent application 02152444. 0 proposes blending less than 10% of thermoplastic olefin elastomer (diethylene propylene rubber, ternary B in polyolefin matrix). Propylene rubber), the thermoplastic polyolefin elastomer used in this method has good compatibility with polyolefin substrates such as PP and PE, but the proportion of thermoplastic olefin elastomer mixed therein is too Low and no cross-linking treatment, so the elastic properties of the separator are limited. If the proportion of thermoplastic polyolefin elastomer is higher than 10%, it will affect the ability of the polyolefin matrix to "dry" into pores. To the appropriate porosity.

In order to improve the compression resistance of the "wet method" diaphragm, Chinese invention patent applications 200680010010. 7, 200680010890. 8, 200680010912. 0, 200680031471. 2 reported the use of a modified thermal stretching process, which improves the elastic properties of the diaphragm. The aspect is still not enough. It must have a high compressive stress of 2. 2MPa/90°C, and the diaphragm has a certain film thickness change rate. There is still a gap between the actual application requirements of the battery and the battery. The compressive stress between the two does not exceed 50PSi (0. 35MPa), otherwise the battery will bulge, and if the internal pressure of the battery is higher than 0. 7MPa, the safety is easy to fail; usually the battery is used before the injection is 85-90 °C In addition to water at high temperature, the normal use temperature is between _1 (T+60 °C), so the diaphragm is required to adapt to the elastic properties under normal charge and discharge conditions in this temperature range.

The "wet method" polyolefin composite membrane is also reported by the co-extrusion process. The Chinese invention patent application 200680035668. 3, 200780005795. 3 , 200510029794. 5 etc. mainly adopts the adjustment of the solid content of the inter-layer polyolefin raw material. The co-extrusion composite membrane is improved by different methods such as different ratios of polyethylene/polypropylene materials and controlling the molecular weight of different membrane materials to obtain different porosity and pore size distribution between layers and different melting points between membrane layers. The high temperature membrane rupture temperature and compression resistance of the separator and the elastic properties are still insufficient.

The current multilayer polyolefin microporous membrane or battery separator, whether produced by "wet method" or "dry method", has the above characteristics and can improve the safety performance and cycle performance of the lithium ion battery. Layer or multilayer polyolefin microporous separators, the disclosed technical solutions have not yet been seen; the present invention has been proposed based on various deficiencies of existing diaphragm and lithium ion battery designs.

Summary of the invention After extensive and in-depth research on the relationship between the safety and service life of lithium-ion batteries and battery materials, the inventors have found that if the separator and the battery pole piece are made of nano-pre-crosslinked rubber micropowder, separator material, formulation and manufacturing method, The new design and the adjustment of the pole piece formulation can achieve the purpose of improving battery safety and service life. The object of the present invention is to provide a co-extruded composite diaphragm having the following comprehensive characteristics and a lithium ion battery using the same. The specific invention is as follows:

(1) A co-extruded composite membrane comprising nano-pre-crosslinked rubber micropowder, characterized in that the co-extruded composite membrane material comprises pre-crosslinked rubber micropowder having a particle size of 25-300 nm and a gel content of more than 80%, co-extruded composite The separator comprises at least two layers of microporous membranes A and B, wherein the layer A microporous membrane is mainly composed of polyethylene having a melting point of 118-145 ° C and nanometer pre-crosslinked rubber micropowder having a weight percentage of 30% or less, wherein the layer B microporous membrane It is mainly composed of polyolefin and nano pre-crosslinked rubber micropowder. The nano-pre-crosslinked rubber micropowder accounts for 30-75% by weight of the B-layer material, and the Gurley value of the co-extruded composite membrane at room temperature is 30-400S/100CC, A. The peel strength between the two layers is greater than 10gf/cm, and the co-extruded composite diaphragm has the following characteristics:

1. In the temperature range of -10 to +60 ° C, after the co-extruded composite diaphragm is applied with a static compressive stress of 0.35 MPa in the thickness direction and held for 5 minutes, the compressive deformation of the coextruded composite diaphragm in the thickness direction It is greater than 5% of the thickness before compression and less than 25%. After 5 minutes of pressure release, the compression set of the co-extruded composite diaphragm is less than 10%. After the compression/release cycle 2000 times, the diaphragm still maintains compressive elasticity. The shrinkage permanent deformation is not more than 10% of the initial thickness, and the Gurley value is still less than 500S/100CC;

1. At 130 ° C, the co-extruded composite membrane is applied with a static compressive stress of 0.35 MPa in the thickness direction and kept for 60 minutes, then cooled to room temperature, the membrane remains intact, and its thermal shrinkage in both the longitudinal and transverse directions is less than 10%. 1. 3. Apply a static compressive stress of 0.35 MPa in the thickness direction, and heat the co-extruded composite diaphragm at a rate of 1 °C/min from 100_200 °C. The thermal shutdown temperature of the diaphragm is not higher than 150 °C to 200 °. C and keep it for 5 minutes After cooling to room temperature, the separator remained intact with a thermal shrinkage of less than 15% in both the machine and cross directions and a Gurley value greater than 2000 S/100 cc.

(2) The co-extruded composite separator according to the above (1), wherein the nano-pre-crosslinked rubber fine powder preferably has a particle diameter of 50 to 150 nm, and preferably 50 to 55% by weight of the B-layer material, and constitutes the A layer. The weight percentage of the material is preferably from 10 to 30%.

(3) The coextruded composite separator according to (1) above, wherein the total thickness of the coextruded composite separator is in the range of 10 to 50 μm, preferably 20 to 40 μm, wherein the thickness of the layer A accounts for coextrusion compounding. 30-60% of the total thickness of the diaphragm.

(4) The coextruded composite separator according to (1) above, wherein the material of the nano-pre-crosslinked rubber fine powder is selected from the group consisting of styrene-butadiene rubber SBR (ie, styrene-butadiene rubber) and butyl rubber IIR ( Isobutylene-isoprene rubber, isoprene rubber IR (isoprene-butadiene rubber), styrene-butadiene rubber PSBR

(ie vinyl pyridine-styrene-butadiene rubber), PBR vinyl pyridine-butadiene rubber, SIBR ie styrene-isoprene-butadiene rubber, nitrile rubber NBR (ie acrylonitrile-butyl) Diene rubber), butadiene rubber BR, acrylate rubber ABR (ie acrylate-butadiene rubber), carboxylated styrene butadiene rubber XSBR (ie carboxyl-styrene-butadiene rubber), carboxylated nitrile rubber XNBR ( That is, carboxyl-acrylonitrile-butadiene rubber), carboxyl polybutadiene rubber XBR, ethylene propylene diene monomer EPDM (ie ethylene-propylene-diene terpolymer), SEBS (ie styrene-ethyl butene) - styrene block copolymer), SIS (i.e., styrene-isoprene-styrene block copolymer), silicone rubber, fluororubber or a combination thereof, preferably tetrabutylpyrene having a strong polarity Rubber PSBR (ie vinyl pyridine-styrene-butadiene rubber), PBR vinyl pyridine-butadiene rubber, carboxyl styrene butadiene rubber XSBR (ie carboxyl-styrene-butadiene rubber).

(5) The co-extruded composite separator according to (1) above, wherein the coextruded composite separator has a longitudinal tensile strength of 50 to 150 MPa, a longitudinal elongation at break of more than 50%, and a transverse tensile strength of between 20_75MPa, transverse elongation at break is greater than 100%, and needle punch strength is greater than 300gf/20 microns.

(6) The co-extruded composite separator according to the above (1), wherein the A-layer microporous membrane is mainly composed of a high-density polyethylene material having a weight average molecular weight of 3 to 3,000,000, preferably a weight average molecular weight of 60-300. Ten thousand high density polyethylene.

(7) The coextruded composite separator according to any one of the above (1), wherein the polyethylene or polyolefin material in the A and B microporous films contains 10% by weight. The above maleic anhydride grafted polyethylene MAH-PE.

(8) A method for manufacturing a co-extruded composite membrane comprising nano-pre-crosslinked rubber micropowder, characterized in that it mainly comprises the following steps: 1. Ingredients: Firstly mechanically mixing raw materials of layer A and layer B, respectively, including polyethylene or poly Olefin composition, nano pre-crosslinked rubber micropowder, high temperature compatibilizer; 2. Co-extruded cast, hot drawing: Premixed slurry of layer A and layer B are separately metered into two or three twin-screw extrusions Machine, co-extruded and quenched at a high temperature, and the cooled composite sheet is preheated at 100-118 ° C for the first step of hot stretching; Third, extraction: under normal pressure or high pressure The high temperature compatibilizer in the semi-finished product is extracted from the solvent; 4. The second hot drawing and heat setting treatment, the semi-finished film after the above extraction is preheated at 100-118 ° C, and the second step is hot stretching, heat The stretched film is heat set at 100-118 ° C for 5-30 seconds. 5. Cooling and winding, the above heat-set film is cooled, trimmed, wound, and cut to obtain a co-extruded composite membrane. .

(9) A lithium ion battery comprising a nano-pre-crosslinked rubber micropowder, comprising: a positive electrode tab, a negative electrode tab, an electrolyte, and the nanopre-crosslinked rubber micropowder according to claims 1-8 Co-extruded composite membrane.

(10) The lithium ion battery according to (9) above, wherein the positive and negative electrode sheets contain 0-8% of nano-pre-crosslinked rubber fine powder, preferably 3-8% of positive and negative pole pieces Nano pre-crosslinked rubber Glue powder

(11) The lithium ion battery according to (9) above, wherein the co-extruded composite separator has an A/B double-layer structure, wherein the A layer is in contact with the positive electrode tab, and the B layer is in contact with the negative electrode tab.

(12) The lithium ion battery according to (9) above, wherein the coextruded composite separator is a three-layer coextrusion structure of A/B/A or B/A/B.

(13) The lithium ion battery according to any one of (9) to (12) above, wherein the co-extruded composite separator is preferably a B/A/B three-layer co-extruded structure, wherein the nano-pre-crosslinked rubber fine powder Preferably, styrene-butadiene rubber PSBR or vinyl pyridine-butadiene rubber PBR rubber is used, and the co-extruded composite separator and the positive and negative pole pieces are subjected to 85-100 ° C /0. 7 MPa hot pressing for 5 minutes and stripping between the pole pieces. The strength is greater than 3gf/cm.

The above technical solutions and their design ideas are further explained as follows:

The co-extruded composite diaphragm has thermal shutdown capability at high temperatures. After heating, the micropores in the diaphragm are closed, and at least one layer can be closed to prevent the internal electrochemical reaction of the lithium ion battery from continuing under the condition of overheating, and the lithium ion battery is internally There is also thermal inertia when overheating. It is also expected that after the separator is turned off, even if the inside of the battery continues to heat up, the diaphragm does not exhibit high heat shrinkage. The electronic insulation of the diaphragm to the positive and negative electrodes should be maintained. This is to prevent the battery from being out of control early. It is very important that the high-temperature-resistant, high-content nano-pre-crosslinked rubber micropowder can hinder the heat shrinkage of polyethylene or polyolefin materials and release and transfer heat shrinkage stress. The above comprehensive purpose can be achieved, so the co-extruded composite diaphragm is designed. At least one microporous membrane contains a relatively high content of nano-pre-crosslinked rubber micropowder, and at least one microporous membrane contains relatively few nano-pre-crosslinked rubber micropowders, and contains relatively high poly The ethylene content microporous layer provides thermal shutdown capability at elevated temperatures.

The main function of the A-layer microporous membrane design of the co-extruded composite membrane is to achieve high temperature shutdown and high machine Mechanical strength; effective closing of the membrane micropores at high temperatures is very important to prevent thermal runaway ignition and explosion in the case of internal heating of lithium ion batteries. The A-layer microporous membrane material of the co-extruded composite membrane is mainly made of polyethylene, polyethylene material. With a suitable melting point in the range of 118-145 ° C, the co-extruded composite separator can be provided with thermal shutdown performance at high temperature; wherein the nano-pre-crosslinked rubber micropowder is less than 30% by weight, in the A-layer microporous membrane material Preferably, the nano-pre-crosslinked rubber micropowder having a content of 10-30% can appropriately increase the elasticity of the co-extruded composite separator without affecting the thermal shutdown capability, and can also appropriately increase the pore size and permeability of the A-layer microporous membrane, especially It is possible to properly coordinate the elastic deformation of the A/B layer under pressure. Otherwise, the deformation uncoordinated diaphragm is prone to wrinkle and wavy, and the pore size distribution may be inconsistent and affect the uniform distribution of charge and discharge of the battery; nano-pre-crosslinking in the A layer material The proportion of rubber micropowder should not be too high, otherwise it will affect its melting and shutting off performance; the polyethylene in the A layer material can be provided by co-extrusion composite diaphragm after hot tensile strengthening. The tensile strength, preferably the coextruded composite separator has a longitudinal tensile strength of 50-150 MPa, a longitudinal elongation at break of more than 50%, and a needling strength of more than 300 gf/20 μm; the high longitudinal strength can satisfy the tension of the battery winding Dimensional stability and micropore stability under action and prevention of foreign particles puncturing the separator. The A-layer microporous membrane main material is preferably composed of a high-density polyethylene material having a weight average molecular weight of 3 to 3 million, especially using a weight average molecular weight of 60. -3 million high-density polyethylene, the higher the molecular weight, the higher the molecular entanglement density, the same porosity, the double-drawn microporous film can obtain higher elongation at break in the longitudinal direction and the transverse direction, The short circuit of the battery is advantageous. Therefore, in the present invention, it is preferred that the coextruded composite separator has a transverse tensile strength of 20 to 75 MPa and a transverse elongation at break of more than 100%; and the excessively high molecular weight of the polyethylene is disadvantageous in melt viscosity during extrusion processing. Excessively affecting production efficiency; polyethylene includes high density polyethylene, linear low density polyethylene, low density polyethylene, maleic anhydride grafted polyethylene or combinations thereof.

The core idea of the B-layer microporous membrane design is to use a high proportion of rubber micropowder at the expense of the microporous membrane layer. Under the condition of mechanical strength, the co-extruded composite membrane mainly contributes to elastic properties and high temperature resistance. The B-layer microporous membrane is mainly composed of polyolefin and nano-pre-crosslinked rubber micropowder blended, in which high content of nano-pre-crosslinked rubber micropowder is dispersed. Distributed in a continuous polyolefin matrix, the polyolefin matrix serves as a skeleton and a retaining action, and fixes the nano-pre-crosslinked rubber micropowder. The polyolefin comprises a copolymer of polyethylene, polypropylene, ethylene-α-olefin or a combination thereof, preferably weight average High molecular weight high density polyethylene having a molecular weight of 6 to 3 million.

It is important that the co-extruded composite diaphragm is properly elastically deformed in the thickness direction and the diaphragm has an appropriate thickness. When the lithium battery is fully charged, the graphite active material of the negative electrode or the silicon negative electrode and the alloy negative electrode have a volume expansion ratio. Above 5%, the thickness direction of the negative electrode tab is expanded by 3-10%, and if the coextrusion composite diaphragm has a compressive elastic deformation amount of less than 5% or a too thin thickness in the thickness direction, it is disadvantageous for releasing the compressive stress of the negative electrode tab. If the thickness deformation of more than 25% causes the pores in the membrane to be compressed, the pore size becomes smaller, the porosity is reduced, or even some of the pores are closed, which affects the normal charge and discharge capacity of the lithium ion battery; the thickness of the coextruded composite membrane is preferably At 20-40 microns, the proper thickness can provide the necessary elastic deformation ability under the condition of ensuring the strength. When the negative electrode is charged and expanded, it can absorb and reduce the compressive stress. After the lithium ion battery discharges, the negative electrode shrinks and the diaphragm has a suitable elastic recovery. The ability to ensure a tight fit between the positive and negative sheets, uniform current density distribution, too thin co-extruded composite diaphragm is not enough to provide suitable compression The amount of deformation and strength are not enough; too thick affects the internal resistance of the battery and the power characteristics of the battery.

When the lithium battery is charged and discharged, the cycle between the negative electrode and the diaphragm is accompanied by compression and stress release. Therefore, it is desirable that the separator has good elastic recovery ability, and the conventional commercial polyolefin separator has poor elastic deformation ability in the thickness direction, after the negative electrode is expanded. During the discharge process, due to the uneven fluctuation of the distance between the positive and negative electrodes, the local large gap existing after the deformation of the pole piece often cannot be restored to the original uniform small gap state, which may cause a large gap between the pole pieces. It may cause local internal resistance to increase or even local poor liquid state, which leads to battery capacity attenuation and uniformity, and poor cycle performance. which is After 2000 cycles of battery recycling, the coextruded composite membrane of the present invention still has suitable elasticity, which is controlled to be no more than 10% of the initial thickness on the compression set, and the Gurley value is still less than 500S/100CC; The uniform spacing between the pole pieces can accommodate and maintain a sufficient electrolyte, and it is necessary to ensure that the different parts of the pole piece can perform electrochemical reaction in a balanced manner. Therefore, the present invention particularly pays attention to the realization of the function, and the co-extruded composite diaphragm of the present invention is The optimum design of the compression characteristics in the normal operating temperature range of T+60°C is as follows: After the first 0. 35MPa static compressive stress is applied to the coextruded composite diaphragm in the thickness direction and held for 5 minutes, the coextruded composite diaphragm is in the thickness direction. The amount of compressive deformation is greater than 5% of the thickness value before compression, less than 25%, more preferably less than 20%, and the compression set of the coextruded composite membrane is less than 10% after the pressure is released for 5 minutes, and the membrane is still after the compression/release cycle 2000 times. Maintaining compressive elasticity, the compression set is not more than 10% of the initial thickness, and the Gurley value is still less than 500S/100CC.

As a composite material, the coextrusion composite separator is mainly provided by a highly elastic nano-pre-crosslinked rubber micropowder. Therefore, when the material system of the B layer is designed, the proportion of the nano-pre-crosslinked rubber micropowder is 30% or more and 75% or less. It should be high enough, preferably 50-65%; but the proportion of too high above 75% is not conducive to co-extrusion, the surface quality of the membrane is deteriorated, the pore size distribution is easy to be uneven; the proportion of high rubber micropowder and rubber micropowder are high. The degree of bonding is the requirement to ensure the elasticity of the co-extruded composite membrane. The nano-pre-crosslinked rubber micropowder can maintain its physical stability at a high temperature of 100-20 CTC. Dispersion distribution on a continuous polyolefin matrix can hinder the aggregation in the A and B layers. The heat shrinkage of ethylene and polyolefin melts is advantageous for improving the high temperature rupture of the coextruded composite separator and preventing the high temperature short circuit of the positive/negative electrode sheets.

The pre-crosslinked rubber micropowder has a gel content of 80% or more, preferably 90% or more, and a high gel content indicates that the degree of crosslinking (or degree of vulcanization) of the rubber micropowder is high, and the degree of crosslinking directly affects the elasticity of the rubber micropowder. Resilience; compared with raw rubber powder that has not been cross-linked, the cross-linked rubber powder does not It is easily dissolved in the electrolyte and has good compatibility with the electrolyte; the cross-linking treatment is completed in advance and the independent nano-powder can prevent the agglomeration phenomenon of the raw rubber powder, which is convenient for the polyethylene or polyolefin. Mix evenly.

The particle size of the pre-crosslinked rubber micropowder is controlled at the nanometer scale. The micropores of the co-extruded composite membrane are directly related in the submicron order. The particle size of the nano rubber micropowder is too large. The particle size is not conducive to providing elasticity, and thus the nano-pre-crosslinked rubber fine powder has a particle diameter of 25 to 300 nm, preferably 50 to 150 nm.

Maleic anhydride grafted polyethylene is also known as a viscous resin. Polyethylene itself is inert and chemically grafted with polar maleic anhydride groups. It has a certain polarity and adhesion at high temperatures. It has been widely used in the field of membranes and the like, and an appropriate amount of the raw material in the A/B layer of the co-extruded composite separator of the present invention can improve the interface between the polyethylene or polyolefin matrix and the nano-pre-crosslinked rubber micropowder and the electrode sheet. The force is more conducive to the elastic recovery of the co-extruded composite membrane. The maleic anhydride grafted polyethylene and the pre-crosslinked rubber micropowder are preferably selected from the strong polar styrene-butadiene rubber PSBR (ie vinylpyridine-styrene-butadiene rubber). ), PBR vinyl pyridine-butadiene rubber, carboxylated styrene-butadiene rubber XSBR (ie carboxyl-styrene-butadiene rubber) and other joint effects can also improve the adhesion between the separator and the pole piece, the corresponding battery manufacturing process Make appropriate changes: In addition to the use of highly polar nano-pre-crosslinked rubber micropowder in the co-extruded composite diaphragm, the positive and negative pole pieces of the battery can also be blended into 0-8% polar nano-pre-crosslinked rubber. Micro powder, preferably positive, The negative electrode tab is mixed with 3-8% of the highly polar nano-pre-crosslinked rubber micropowder; the co-extruded composite separator of the present invention and the positive and negative pole pieces are subjected to 85-100 ° C / 0. 7MP hot pressing for 5 minutes. The peel strength between the pole pieces can be made 3gf/cm or more, which is advantageous for the battery to prevent the pole pieces from being misaligned and to keep the insulation when overheated. The composite membrane is manufactured by a co-extrusion processing method, and the co-extruded composite membrane has a simple production process and a low production cost compared with the various coating methods described above, and polyethylene and polyolefin in the A/B two-layer semi-finished product during co-extrusion processing. The high temperature melt has a certain interdiffusion entanglement at the interface, which can ensure high bond strength/peel strength between the two layers. The peel strength of lOgf/cm or more between A/B layers can be utilized with high content of nano pre-crosslinked rubber. The B layer of the fine powder effectively suppresses the heat shrinkage of the polyethylene-based A layer at a high temperature.

The appropriate pore size and porosity of the co-extruded composite membrane are integrated in the Gurley value of the gas permeability index. The excessive Gurley value means that the membrane pore resistance is large, the membrane permeability is poor, especially the permeability after compression is worse, the battery capacity. The initial low Gurley value means that the diaphragm has large pores, high porosity, easy battery short-circuit, and poor safety. Therefore, it is preferred in the present invention that the co-extruded composite membrane has a Gurley value of 30-400 S/100 cc at room temperature.

detailed description

Hereinafter, a specific embodiment (hereinafter referred to as "the embodiment") of the present invention will be described in detail. Further, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the invention.

Diaphragm characteristics and evaluation method of nano pre-crosslinked rubber micropowder

(1) Gel content of nano-pre-crosslinked rubber micropowder

The weighed nano pre-crosslinked rubber micropowder W1 is boiled in dilute toluene for more than 8 hours. After filtration through a membrane, the residue W2 is weighed and compared with the initial n. The toluene insoluble matter is defined as a gel, which also refers to the chemical gel content. .

(2) Nano pre-crosslinked rubber micropowder particle size

Surface morphology and testing were viewed using a scanning electron microscope (SEM).

(3) Film thickness ( μ πι) The test was carried out using a CHY-C2 thickness gauge manufactured by Jinan Languang Electromechanical Technology Co., Ltd., and a 50 _mm ×50 _mm sample was cut from the porous film, and a 9-point measurement was performed uniformly on the surface of the sample with a thickness gauge, and then the film thickness was measured. The values are averaged.

(4) Air permeability

The microporous membrane was tested for gas permeability in accordance with JIS P8117.

(5) Needle strength

The measuring instrument was tested by MTS's CMT4000 electronic tester, and the maximum load when the needle was inserted into the polyolefin porous film at a speed of 2 mm/s with a 1 直径 diameter needle with a spherical surface (curvature radius R: 0.5 mm) at the front end was measured. .

(6) Tensile strength at break and tensile elongation at break

A long strip of film sample having a width of 20 inches was used and measured using an MTS CMT4000 electronic tester.

(7) Peel strength

After the pole piece and the diaphragm are hot pressed, the strip has a width of 20 匪 and a length of 50 ,. The two clamps of the peel strength tester are clamped to the pole piece and the diaphragm, respectively, and the pole piece and the diaphragm are separated. The maximum force required.

(8) Stress absorption and diaphragm elasticity test

Stack 20 layers of 50mm×50mm diaphragms together, and then sandwich them between the highly smooth stainless steel plates, and pressurize the film with a compressive stress of 0.35MPa at -10°C to +60°C. Minutes, the thickness before and after pressing was measured using a film thickness tester.

Film thickness deformation after pressing (%) = (thickness before pressing (20 layer average) - thickness after pressing (20 layer average)) / thickness before pressing (20 layer average). (9) Thermal shutdown temperature and high temperature breaking film test

a. Production of positive pole piece

92 parts of positive electrode active material LiNiCoMn0 _{2, 2} parts of acetylene black, 2 parts of conductive flake graphite KS15, 4 parts of polyvinylidene fluoride (PVDF), 5 parts of pre-crosslinked nano-styrene-butadiene rubber fine powder, dispersed to N- with high-speed mixer In methylpyrrolidone (Li P), a slurry is prepared. Then, the slurry was applied on one side to an aluminum foil having a thickness of 20 μm by a coater, dried at 130 ° C for 30 minutes, and then compression-molded by a roll press at a pressure of 5 MPa, and the areal density of the positive electrode active material was 200g / m ^2, bulk density of active material is compressed to 2. 5g / cm ^3.

b. Production of negative pole piece

5份, acetylene black 1.5 parts, polyvinylidene fluoride (PVDF) 3 parts, pre-crosslinked styrene-butadiene rubber 6 parts, dispersed in a high-speed mixer to N-methylpyrrolidone In (NMP), a slurry is prepared. Then, the slurry was coated on one side of a negative electrode current collector copper foil having a thickness of 12 μm by a coater, dried at 130 degrees for 30 minutes, and then compression-molded by a roll press under a pressure of 5 MPa, and the negative electrode active material was used. foot amount 110g / m ^2, bulk density of the active material is 1. 3g / cm

c. electrolyte

The concentration of LiFP ₆ is 1.0 mol/L. The concentration of LiFP ₆ is 1.0 mol/L. The concentration of LiFP ₆ is 1.0 mol/L.

d. Evaluation method

A film having a shear size of 55 mm X 55 mm was placed between a positive electrode piece cut into 50 mm×50 _{mm and} a negative electrode piece cut into 52 _mm ×52 _mm , and pressed with a smooth teflon plate, and the periphery was bolted and placed. Bake in the oven for 30 minutes, cool it after baking To normal temperature, immerse it in the above electrolysis, take it out after 5 minutes, measure 1 kHz AC resistance, heat at 1 °C/min, test temperature 100-200 °C, the lowest temperature of AC resistance over 1000 Ω is The thermal shutdown temperature, the temperature at which the AC resistance is again reduced to less than 100 Ω is the high temperature rupture temperature.

(10) Battery evaluation

a. Production of positive pole piece

In the same manner as in the slurry mixing method of (9) a, the slurry was coated on both sides onto a 20 μm aluminum current collector and rolled, and the areal density of the active material was 400 g/m ² . The positive electrode piece was cut into a size of 270 匪 X 100 mm, a total of 20 pieces.

b. Production of negative pole piece

In the slurry mixing method of (9) b, the slurry was coated on both sides of a 12 μm copper current collector and rolled, and the surface density of the active material was 220 g/m ² . The negative electrode tab was cut into a size of 275 mm X 105 mm, a total of 21 pieces.

c electrolyte

The ratio of the electrolyte is the same as described in (9) c.

d. diaphragm

Cut the size of the diaphragm to 280mm x 110mm, a total of 21 pieces.

e. Battery assembly and evaluation

The double-sided coating material of each of the positive pole pieces having a width of 100 mm and a 20-inch wide edge area is cleaned; the same method is to make the negative pole piece width of 105 匪 on one side, and the 20 mm wide edge area on both sides The coating material is cleaned up. The cleaned positive electrode piece, the diaphragm, and the cleaned negative electrode piece are stacked in sequence, wherein the positive electrode piece has a cleaning area on one side and a negative side has a cleaning area in opposite directions, the negative electrode The active material material region of the pole piece is completely covered with the active material portion of the positive electrode tab, and the separator is located between the positive and negative materials, and completely covers the active material region of the negative electrode. According to the above requirements, 20 positive electrode sheets, 21 separators, and 21 negative electrode sheets are laminated in turn, and then all the positive electrode sheets are cleaned on one side and 60 mm X 100 mm positive electrode polymer batteries are used by an ultrasonic welding machine. The tabs are welded together, and all the negative electrode polymer cells on the side of the negative electrode tab are soldered to the poles of 60 mm X 100 mm, and then the battery is sealed with an aluminum plastic film, and a liquid injection hole is left on one side. After 85 ° C vacuum is 0. O lMpa, after baking for 12 hours, take out the electrolyte and seal it.

The battery is charged at a temperature of 8 ° C (about 0.5 ° C) to a voltage of 4. 2 V, and maintained at a voltage of 4. 2 V, when the current is less than 0.8 A, the charging is completed, after being left for 30 minutes, The current of 8A discharges the battery to 3.0V.

Then, the current is discharged to 3. 0V, and the current is discharged to 3. 0V, and the current is discharged to 3. 0V, when the current is less than 1. 6A, the charging is completed, after being left for 30 minutes, discharged with a current of 16A to 3. 0V, The discharge capacity at this time was 1 C discharge capacity (Ah).

The charge and discharge cycle test was carried out in accordance with the above method, and the ratio of the discharge capacity after the predetermined cycle to the discharge capacity at the first cycle was 70% as the end life, thereby judging the battery cycle performance. Example 1

Co-extruded composite membrane material formulation: Formula A: Weight average molecular weight (Mw) 1.5 million ultra-high molecular weight polyethylene (UHMWPE): 10 parts; Maleic anhydride grafted high-density polyethylene: 15 parts; Irradiation cross-linked with particle size of 100-150 nm 5 parts of benzene rubber micropowder, dioctyl sebacate (DOS): 70 parts; antioxidant 1010: 0.3 parts;

Formula B Formula: Ultra-high molecular weight polyethylene (UHMWPE) with a weight average molecular weight (Mw) of 2.5 million: 10 parts; Maleic anhydride grafted high density polyethylene: 10 parts; Irradiation crosslinked with a particle size of 100-150 nm Benzene rubber micropowder PSBR: 28 parts; dioctyl sebacate (DOS): 120 parts; antioxidant 1010: 0.2 parts;

Co-extrusion composite diaphragm processing method:

(1) Co-extruded cast piece: The above A and B raw materials are separately swelled and mixed in a different stainless steel stirred tank at 90 ° C for 24 hours to prepare a uniform slurry; then the slurry is equally equivalent by a metering pump The conveyance amount was melt-kneaded in parallel co-rotating twin-screw extruders A and B having a length to diameter ratio of 1:68. The temperature setting range of the extruder is between 190 °C and 210 °C. The melts of layers A and B pass through the mixer and are extruded into the same co-extrusion flat die and quenched and cast. The thickness of the cast piece is controlled to 1.0 mm and the width is 800 。.

(2) Synchronous stretching: The composite sheet of the above cast piece is preheated by 110-12 CTC and then subjected to simultaneous stretching, the longitudinal stretching ratio is 4 times, the transverse stretching ratio is 1.8 times, and the material is cooled to below 60 ° C. After being combined with PP non-woven fabric, it is wound up, and the winding diameter is 800mm _;

(3) High-pressure extraction cleaning: using pentafluoroacetamidine, hexafluoroacetic acid or its combination for extraction and dissolution. The composite rolled product is placed in a supercritical extraction kettle for cleaning. The cleaning process is: Cleaning temperature: 75 °C, cleaning pressure: 5.5MPa, separation pressure is 1.5MPa, separation temperature is 65 °C, extraction solvent is circulated throughout the system to clean the product. (4) The stepwise hot stretching, the longitudinal heat stretching of the semi-finished film after the extraction is 1.6 times, the transverse heat stretching is 2 times, and the stretching temperature is 120 ° C;

(5) Heat setting treatment, the film after the horizontal drawing is gradually reduced by 8% in the width direction of 120 ° C, and kept for 20 seconds;

(6) Cooling and winding, cooling the above heat-set film to 30 ° C and then winding to obtain the finished co-extruded composite microporous film; product thickness 30 microns; porosity 45%, Gurley value: 60-80S/100CC Tensile strength: MD direction 98MPa, TD direction 45MPa; Elongation at break: 65% in longitudinal direction, 193% in transverse direction; Needle strength 430gf; Peel strength between A/B two layers 35gf/cm, co-extruded composite diaphragm The following features:

At a temperature of -10 °C and 60 °C, after the co-extruded composite diaphragm is applied with a static compressive stress of 0.35 MPa in the thickness direction and held for 5 minutes, the compressive deformation amount of the coextruded composite diaphragm in the thickness direction is 5-value of the thickness before compression. 8%, after 5 minutes of pressure release, the compression set of the co-extruded composite membrane was tested to be less than 5%. After the compression/release cycle 2000 times, the diaphragm still maintains compressive elasticity, the compression set is not more than 12% of the initial thickness, and the Gurley value is finally 95S. /100CC;

The co-extruded composite separator was applied at a temperature of 130 ° C in a thickness direction of 0. 35 MPa static compressive stress and held for 60 minutes and then cooled to room temperature, the separator remained intact, and the heat shrinkage ratio in both the longitudinal and transverse directions was less than 10%; 0. 35MPa static compressive stress, heated from 100-200 °C to the co-extruded composite membrane at a rate of 1 °C / min, the membrane thermal shutdown temperature of 139 ° C, to 200 ° C for 5 minutes and then cooled to room temperature, diaphragm It remains intact with a thermal shrinkage of less than 12% in both the machine and cross directions and a Gurley value greater than 2600 S/100 cc.

The composite separator is used, the A side thereof is in contact with the positive electrode tab, and the B side is in contact with the above negative electrode tab. Before the liquid injection, the pole group is pressurized at 100 ° C / 1.5 MPa for 10 min, and then dried and injected. Electrolysis The liquid was made into a lithium ion battery and tested at 150 ° C for hot box, acupuncture, short circuit, and 1 C cycle at room temperature of 25 ° C. The battery safety test was all qualified, and the cycle life was 2,900 times.

Comparative example 1

The battery manufacturing process is the same as in the first embodiment. The diaphragm is made of a dry PP/PE/PP film from a foreign company, with a thickness of 25 μm, a porosity of 40%, a Gurley value of 600-630 S/100 CC, and a tensile strength of 165 MPa in the MD direction. The TD direction is 13 MPa, and the transverse elongation at break is 12%. Battery safety test 150 ° C hot box, acupuncture, short circuit are unqualified, cycle life: 835 times.

Comparative example 2

The battery manufacturing process is the same as in the first embodiment. The diaphragm is made of a wet single-layer PE separator from a foreign company. The thickness is 23 micrometers, the porosity is 49%, the Gurley value is 95S/100CC, and the tensile strength is 143 MPa in the MD direction and 21 MPa in the TD direction. The longitudinal elongation at break was 42% and the transverse elongation at break was 344%. Battery safety test 150 ° C hot box, acupuncture, short circuit are unqualified, cycle life: 1376 times.

Claims

Rights statement

A co-extruded composite membrane comprising nano-pre-crosslinked rubber micropowder, characterized in that the co-extruded composite membrane material comprises pre-crosslinked rubber micropowder having a particle size of 25-300 nm and a gel content of more than 80%, and a co-extruded composite membrane The invention comprises at least two layers of microporous membranes A and B, wherein the layer A microporous membrane is mainly composed of polyethylene having a melting point of 118-145 ° C and nanometer pre-crosslinked rubber micropowder having a weight percentage of 30% or less, wherein the B layer microporous membrane is mainly It consists of polyolefin and 30-75% by weight of nano-pre-crosslinked rubber micropowder. The co-extrusion composite membrane has a Gurley value of 30_400S/100CC at room temperature, and the peel strength between A/B layers is greater than lOgf/cm. The extruded composite membrane has the following characteristics:

1. In the temperature range of _10~ +60 °C, after the co-extruded composite diaphragm is applied with a static compressive stress of 0.35 MPa in the thickness direction and held for 5 minutes, the compressive deformation of the coextruded composite diaphragm in the thickness direction is greater than 5% of the thickness before compression, less than 25%, the compression set of the co-extruded composite diaphragm is less than 10% after 5 minutes of pressure release, so the diaphragm remains compressed and compressed after 2000 compression/release cycles. The permanent deformation is not more than 10% of the initial thickness, and the Gurley value is still less than 500S/100CC;

1. 2 at 130 ° C for the co-extruded composite diaphragm in the thickness direction of 0. 35MPa static compressive stress and maintained for 60 minutes and then cooled to room temperature, the diaphragm remains intact, its thermal shrinkage in the longitudinal and transverse directions are less than 10%;

1. 3 Apply a static compressive stress of 0.35 MPa in the thickness direction, heat the co-extruded composite diaphragm at a rate of rC/min from 100-200 ° C, the thermal shutdown temperature of the diaphragm is not higher than 150 ° C, to 200 ° C and keep 5 After a minute and cooling to room temperature, the membrane remained intact with a thermal shrinkage of less than 15% in both the machine and cross directions and a Gurley value greater than 2000 S/100 cc.

2 . The co-extruded composite separator according to claim 1 , wherein the nano-pre-crosslinked rubber fine powder has a particle diameter of preferably 50-150 nm, preferably 50-65% by weight of the B-layer material, and comprises A layer material. The weight percentage is preferably 10-30%.

3. The coextruded composite membrane of claim 1 wherein the total thickness of the coextruded composite membrane In the range of 10-50 microns, preferably 20-40 microns, wherein the thickness of layer A is from 30 to 60% of the total thickness of the coextruded composite separator.

The co-extruded composite separator according to claim 1, wherein the material of the nano-pre-crosslinked rubber fine powder is selected from the group consisting of styrene-butadiene rubber SBR (ie, styrene-butadiene rubber) and butyl rubber IIR (ie, isobutylene). -isoprene rubber), isoprene rubber IR (isoprene-butadiene rubber), styrene-butadiene rubber PSBR (ie vinylpyridine-styrene-butadiene rubber), PBR vinylpyridine- Butadiene rubber, SIBR, ie styrene-isoprene-butadiene rubber, nitrile rubber NBR (ie acrylonitrile-butadiene rubber), butadiene rubber BR, acrylate rubber ABR (ie acrylate) Butadiene rubber), carboxylated styrene butadiene rubber XSBR (ie carboxyl-styrene-butadiene rubber), carboxylated nitrile rubber XNBR (ie carboxyl-acrylonitrile-butadiene rubber), carboxyl polybutadiene rubber XBR, EPDM EPDM (ie ethylene-propylene-diene terpolymer), SEBS (ie styrene-ethylbutene-styrene block copolymer), SIS (ie styrene-isoprene) One of styrene block copolymer), silicone rubber, fluororubber or a combination thereof .

The co-extruded composite separator according to claim 1 , wherein the coextruded composite separator has a longitudinal tensile strength of 50 to 150 MPa, a longitudinal elongation at break of more than 50%, and a transverse tensile strength of 20 to 75 MPa. The transverse elongation at break is greater than 100% and the needling strength is greater than 300 gf / 20 microns.

The co-extruded composite separator according to claim 1, wherein the A-layer microporous membrane is mainly composed of a high-density polyethylene material having a weight average molecular weight of 3 to 3 million, preferably having a weight average molecular weight of 6 to 3,000,000. High-density polyethylene.

The co-extruded composite separator according to any one of claims 1 to 6, wherein the polyethylene or polyolefin material in the A and B layer microporous membranes contains 10% by weight or more of maleic anhydride grafted. Polyethylene MAH-PE.

8. A method of producing a co-extruded composite membrane comprising nano-pre-crosslinked rubber micropowder, characterized in that The method comprises the following steps: 1. Ingredients: Firstly, mechanically mixing the raw materials of layer A and layer B respectively, including polyethylene or polyolefin composition, nano pre-crosslinked rubber micropowder, high temperature compatibilizer; 2. co-extruded casting, hot drawing Stretch: The premixed slurry of layer A and layer B is separately transferred into the twin-screw extruder, co-extruded and quenched at a high temperature, and the cooled composite sheet is preheated at 100-118 °C. After the first step of hot stretching; Third, extraction: the use of atmospheric pressure or high pressure in the solvent to extract the high temperature compatibilizer in the semi-finished product; Fourth, the second hot stretching, heat setting treatment, after the above extraction The semi-finished film is preheated at 100-118 ° C and then subjected to the second step of hot stretching. The film after hot stretching is heat set at 100-118 ° C for 5-30 seconds. 5. Cooling and winding, the above process The heat-set film is cooled, trimmed, wound, and slit to obtain a coextruded composite separator.

A lithium ion battery comprising a nano-pre-crosslinked rubber micropowder, comprising: a positive electrode tab, a negative electrode tab, an electrolyte, and a nano-pre-crosslinked rubber micropowder according to claims 1-8. Extruded composite diaphragm.

10. The lithium ion battery according to claim 9, wherein the positive and negative electrode sheets contain 0-8% of nano-pre-crosslinked rubber fine powder.

11. The lithium ion battery according to claim 9, wherein the coextruded composite separator is an A/B double layer structure, wherein the layer A is in contact with the positive electrode tab, and the layer B is in contact with the negative electrode tab.

The lithium ion battery according to claim 9, wherein the coextruded composite separator is a three-layer coextrusion structure of A/B/A or B/A/B.

The lithium ion battery according to any one of claims 9 to 12, wherein the co-extruded composite separator is preferably a B/A/B three-layer co-extruded structure, wherein the nano-pre-crosslinked rubber fine powder is preferably a styrene-butadiene rubber PSBR. Or vinyl pyridine-butadiene rubber PBR rubber, preferably 3-8% nano-pre-crosslinked rubber micropowder in positive and negative pole pieces, co-extruded composite separator and positive and negative pole pieces after 85-100 °C/0 The peel strength between the pole piece and the pole piece after 7 minutes of hot pressing at 7 MPa is more than 3 gf/cm.