Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm, a preparation method thereof and a battery, the battery diaphragm comprises a microporous membrane and a coating, and the microporous membrane is obtained by surface modification of an aromatic compound containing alkenyl and ester groups. The aromatic compound containing alkenyl and ester groups is grafted to the surface of the microporous membrane, so that the side reaction between the electrolyte solvent and the positive and negative pole pieces is inhibited, and the cycle stability of the battery is improved. Moreover, through the preparation method of performing surface irradiation on the microporous membrane and then performing crosslinking modification, the puncture resistance, the membrane rupture temperature, the tensile strength, the conductivity and the like of the microporous membrane are improved on the premise that the hole closing temperature of the microporous membrane is not influenced basically.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery separator, and particularly relates to a battery separator and a preparation method thereof and a secondary battery. BACKGROUND

[0002] The battery separator is a thin film with a microporous structure between the positive electrode and the negative electrode, and is one of the key materials with the most technical barriers in the secondary battery industry chain. The battery separator mainly plays two roles of separating the positive and negative electrodes to prevent short circuit and providing micro-channels to support ion migration, and directly affects the safety performance, cycle life and other characteristics of the battery.

[0003] The polyolefin microporous membrane has good stability and low cost, and is the most mature commercialized separator product at present. The main substrates include polyethylene (PE) and polypropylene (PP). The mainstream products are single-layer PP microporous membrane, single-layer PE microporous membrane and three-layer PP / PE / PP microporous composite membrane. However, the polyolefin microporous membrane still has many defects such as low porosity and poor affinity to electrolyte.

[0004] Patent CN 114094284A discloses a new type of cross-linked separator and a preparation method thereof. Acrylate containing multiple branched chains is used as a monomer to cross-link PE, thereby improving the film breaking temperature, tensile strength and puncture strength of the separator. However, the overall closed pore temperature of the separator will also be significantly improved, which affects the safety of the battery in use. At present, how to obtain a separator that does not sacrifice the closed pore temperature while improving the film breaking temperature, tensile strength and puncture strength is a problem to be solved in the field. SUMMARY

[0005] The purpose of the present application is to provide a battery separator to solve the problem that the existing separator is difficult to improve the film breaking temperature without changing or even reducing the closed pore temperature.

[0006] Another purpose of the present application is to provide a preparation method of a battery separator.

[0007] Another purpose of the present application is a secondary battery.

[0008] In a first aspect, the present application provides a battery separator, comprising a microporous membrane and a coating layer, wherein the microporous membrane is surface modified by an aromatic compound containing an alkenyl group and an ester group.

[0009] In a second aspect, the present application provides a preparation method of a battery separator, comprising the following steps:

[0010] I. Preparation of modified microporous membrane:

[0011] S1. Dissolve the aromatic compound containing an alkenyl group and an ester group in a solvent, add an initiator, and prepare a modified solution;

[0012] S2, surface treatment is performed on the microporous membrane material after melt casting, biaxial stretching, extraction and drying of the microporous membrane, so that free radicals are generated on the surface of the microporous membrane, the modified solution is contacted with the microporous membrane after the surface treatment, and then ultraviolet irradiation is performed, and after curing, cleaning, drying and secondary cross stretching, a modified microporous membrane is obtained.

[0013] II, preparation of the coating layer:

[0014] Step A, preparation of the coating layer slurry;

[0015] Step B, coating the coating layer slurry on at least one side of the modified microporous membrane.

[0016] The present application also provides another method for preparing a battery separator, comprising the following steps:

[0017] Step A', preparation of the coating layer slurry;

[0018] Step B', coating the coating layer slurry on at least one side of the finished microporous membrane and drying;

[0019] Step C', dissolving the aromatic compound containing an alkenyl group and an ester group in a solvent, and then adding an initiator to prepare a modified solution;

[0020] Step D', surface treatment is performed on the microporous membrane after coating in step B', so that free radicals are generated on the surface of the microporous membrane, the modified solution is contacted with the microporous membrane after the surface treatment, and then ultraviolet irradiation is performed, and after curing, cleaning, drying and secondary cross stretching, a modified microporous membrane is obtained.

[0021] In a third aspect, the present application provides a secondary battery comprising the above-mentioned battery separator.

[0022] Advantages of the present application:

[0023] The present application realizes inhibition of the side reaction between the electrolyte solvent and the positive and negative electrode sheets by grafting the aromatic compound containing an alkenyl group and an ester group to the surface of the microporous membrane, and improves the cycle stability of the battery. Moreover, by the preparation method of surface irradiation and then crosslinking modification of the microporous membrane, the puncture resistance, membrane breaking temperature, tensile strength, electrical conductivity and the like are improved without substantially affecting the closed pore temperature. DETAILED DESCRIPTION

[0024] The present application will be specifically described below by examples. It is necessary to point out here that the following examples are only used to further illustrate the present application and cannot be understood as limiting the protection scope of the present application, and those skilled in the art can make some non-essential improvements and adjustments to the present application according to the above content of the present application.

[0025] The present application provides a battery separator, comprising a microporous membrane and a coating, wherein the microporous membrane is surface modified by an aromatic compound containing alkenyl and ester groups.

[0026] In the battery separator of the present application, the aromatic compound containing alkenyl and ester groups is one or more of the compounds represented by formula I-1 to formula I-6,

[0027] formula I-1;

[0028] formula I-2;

[0029] formula I-3;

[0030] formula I-4;

[0031] formula I-5;

[0032] formula I-6.

[0033] In the battery separator of the present application, the microporous membrane can be a polyethylene membrane, a polypropylene membrane, a polyimide membrane, an aramid membrane or an arsfurane membrane surface modified by an aromatic compound containing alkenyl and ester groups.

[0034] The present application also provides a preparation method of the battery separator, comprising the following steps:

[0035] I. Preparation of modified microporous membrane:

[0036] S1. Dissolve the aromatic compound containing alkenyl and ester groups in a solvent, add an initiator to prepare a modified solution;

[0037] S2. Surface treat the microporous membrane material obtained by melt casting, biaxial stretching, extraction and drying to generate free radicals on the surface of the microporous membrane, then contact the modified solution with the surface treated microporous membrane, and then perform ultraviolet irradiation, followed by cleaning, drying and secondary horizontal stretching to obtain a modified microporous membrane.

[0038] II. Preparation of coating:

[0039] Step A. Preparation of coating slurry;

[0040] Step B. Coating the coating slurry on at least one side of the modified microporous membrane.

[0041] Step S2 of this preparation method involves the conventional preparation methods in the art, including melt casting, biaxial stretching, extraction and drying, and secondary transverse stretching of the microporous membrane. Specific preparation conditions are not limited. The characteristic of this invention is that after extraction and drying and before secondary transverse stretching, it further includes surface treatment of the microporous membrane, contact with a modification solution, and UV curing to complete the modification of the microporous membrane. The biaxial stretching can be synchronous or asynchronous.

[0042] The method for preparing the battery separator of the present invention uses an aromatic compound containing olefinic and ester groups, which accounts for 0.2% to 20% of the mass of the modified microporous membrane. Specifically, the amount can be 0.2%, 0.5%, 1%, %, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%. This value is calculated using the formula: (mass of the modified microporous membrane - mass of the unmodified microporous membrane) / mass of the modified microporous membrane.

[0043] In the preparation method of the battery separator described in this invention, surface treatment is performed in step S2 by surface irradiation or photodegradation. In this invention, the purpose of surface irradiation and photodegradation is to generate free radicals on the surface of the microporous membrane. The specific methods and conditions for surface irradiation and photodegradation are not specifically limited in this invention, as long as they can generate free radicals on the microporous membrane surface. Preferably, surface irradiation involves irradiating the separator surface with a cobalt source or an electron accelerator at an irradiation dose of 0.1–1 mgy, specifically 0.1 mgy, 0.2 mgy, 0.3 mgy, 0.4 mgy, 0.5 mgy, 0.6 mgy, 0.7 mgy, 0.8 mgy, 0.9 mgy, or 1 mgy, where free radicals are generated on the surface under the action of high-energy rays. Photodegradation refers to irradiating the surface with ultraviolet light (wavelength below 450 nm) to generate free radicals.

[0044] In the preparation method of the battery separator of the present invention, during the curing in step S2, the ultraviolet irradiation conditions are as follows: the ultraviolet wavelength is 200~450nm, specifically 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, and the time is ≤60s, specifically 5s, 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s, 50s, 55s, 60s.

[0045] In the preparation method of the battery separator of the present invention, the content of aromatic compounds containing alkenyl and ester groups in the modified solution of step S1 is 2~40 wt%, specifically 2 wt%, 3 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, and 40 wt%.

[0046] The battery separator preparation method of the present invention, wherein the modified solution in step S1 contains 3-25 wt% aromatic compounds with alkenyl and ester groups, and the separator has high tensile strength, with a transverse tensile strength reaching 2076-2302 kgf / cm². 2 The longitudinal tensile strength can reach 2263~2513 kgf / cm. 2 It exhibits low thermal shrinkage, with transverse thermal shrinkage reaching 0.4~1.6% and longitudinal thermal shrinkage reaching 0.8~2.7%; electrical conductivity reaching 6.9~8.0 mS / cm; puncture strength reaching 351~675gf; pore-closing temperature as low as 136.7~142.6℃; and membrane rupture temperature as high as 172.2~223.9℃.

[0047] The method for preparing the battery separator according to the present invention uses an initiator that is at least one selected from benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acylphosphine oxides, benzophenone, thioxanthones, and anthraquinones, preferably acylphosphine oxides.

[0048] In the preparation method of the battery separator of the present invention, the acylphosphine oxide can be TPO (2,4,6-trimethylbenzoyl diphenylphosphine oxide), TMO (2,4,6-trimethylbenzoyl di(p-tolyl)phosphine oxide), BAPO (phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide), or TPO-L (ethyl(2,4,6-trimethylbenzoyl)phenylphosphine ester).

[0049] In the method for preparing the modified microporous membrane of the present invention, the amount of initiator added is 1 to 15 wt% of the amount of the aromatic compound containing alkenyl and ester groups added (for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 10 wt%, 12 wt%, 14 wt%, or 15 wt%).

[0050] The method for preparing the modified microporous membrane according to the present invention uses a solvent that is at least one of ethers, esters, and haloalkanes.

[0051] The method for preparing the modified microporous membrane of the present invention uses a solvent that is at least one selected from methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, acetone, and DMAc.

[0052] This invention also provides another method for preparing a battery separator, comprising the following steps:

[0053] Step A': Prepare the coating slurry;

[0054] Step B': Apply the coating slurry to at least one side of the finished microporous membrane and dry it;

[0055] Step C' involves dissolving an aromatic compound containing alkenyl and ester groups in a solvent, then adding an initiator to prepare a modified solution;

[0056] Step D' involves surface-treating the membrane coated in step B' to generate free radicals on the surface of the microporous membrane. The modified solution is then brought into contact with the surface-treated microporous membrane, followed by ultraviolet irradiation. After curing, the membrane is cleaned and dried to obtain the modified diaphragm.

[0057] In this preparation method, the finished microporous membrane is obtained by purchasing or by conventional methods in the art. The difference between this and the aforementioned modified microporous membrane is that the finished microporous membrane is a microporous membrane that has undergone secondary transverse stretching.

[0058] In the above two methods for preparing battery separators, the specific preparation method and raw material composition of the coating slurry are not specifically limited by this invention, as they are conventional techniques in the field. Those skilled in the art can adjust and select according to actual needs. The coating slurry may contain inorganic substances and / or high-temperature resistant organic polymers, water or organic solvents, and may also contain binders, wetting agents, etc. When modifying the surface of the microporous membrane, modification can be performed on one side or both sides.

[0059] The modified solution used in step C' can be the same as the modified solution used in step S1 of the aforementioned method, that is, the content of aromatic compounds containing alkenyl and ester groups in the modified solution is 2 to 40 wt%, specifically 2 wt%, 3 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, or 40 wt%. The amount and type of initiator can also be the same as the initiator used in step S1 of the aforementioned method, that is, the initiator can be at least one of benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acylphosphine oxides, benzophenone, thioxanthones, and anthraquinones, preferably acylphosphine oxides, and the amount added is 1 to 15 wt% of the amount of the aromatic compounds containing alkenyl and ester groups added (for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 10 wt%, 12 wt%, 14 wt%, or 15 wt%).

[0060] The surface treatment and curing methods and conditions for the coated film in step D' are the same as those for the aforementioned preparation method, namely, surface treatment is performed by surface irradiation or photodegradation. The purpose of surface irradiation and photodegradation is to generate free radicals on the surface of the microporous membrane. The specific methods and conditions for surface irradiation and photodegradation are not specifically limited in this invention, as long as they can generate free radicals on the microporous membrane surface. Preferably, surface irradiation involves irradiating the membrane surface with a cobalt source or an electron accelerator at a dose of 0.1–1 mgy, specifically 0.1 mgy, 0.2 mgy, 0.3 mgy, 0.4 mgy, 0.5 mgy, 0.6 mgy, 0.7 mgy, 0.8 mgy, 0.9 mgy, or 1 mgy. Free radicals are generated on the microporous membrane surface under the action of high-energy rays. Photodegradation refers to irradiating the surface with ultraviolet light (wavelength below 450 nm) to generate free radicals. During curing, the ultraviolet irradiation conditions are: ultraviolet wavelength of 200–450 nm, specifically 200 nm, 250 nm, 300 nm, 350 nm, or 400 nm. nm, 450nm, time ≤60s, specifically can be 5s, 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s, 50s, 55s, 60s.

[0061] When coating a microporous membrane, the coating can be applied to one side of the microporous membrane or to both sides of the microporous membrane. The coating method is a conventional technique in the field, and the present invention does not impose specific limitations. Those skilled in the art can choose according to actual needs.

[0062] In the two battery separator preparation methods mentioned above, the coating thickness on the microporous membrane surface, that is, the coating thickness on the modified microporous membrane surface or the coating thickness on the finished microporous membrane surface, can be 0.5μm~6μm, specifically 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, and 6μm.

[0063] The method of the present invention will be described in detail below through specific embodiments.

[0064] Example 1

[0065] (1) The compound shown in Formula I-2 was dissolved in ethyl methyl carbonate, and then an acylphosphine oxide (TPO) initiator was added to prepare a modified solution with a compound content of 3 wt% and the amount of initiator was 5 wt% of the compound shown in Formula I-2;

[0066] (2) Polyethylene with a viscosity-average molecular weight of 1 million and white oil were melted, mixed and extruded to form a casting sheet, and then subjected to simultaneous biaxial stretching (the stretching ratio of longitudinal stretching and transverse stretching was 6 times, and the temperature was 120℃), extraction and drying to obtain a PE microporous membrane. The membrane was then irradiated on both sides with a cobalt source (irradiation dose 1 MGy).

[0067] (3) The above-mentioned surface-treated membrane was immersed in the modified solution, and then irradiated with ultraviolet light with a wavelength of 380 nm for 60 s. After cleaning, drying and secondary stretching (stretch ratio 1.1 times, 130℃), a modified microporous membrane with a thickness of 9 μm was obtained.

[0068] (4) Alumina (D50 is 500nm), binder (carboxymethyl cellulose and polyacrylate 1:2) and dispersant (polyacrylic acid) with a mass ratio of 94:6:0.5 are dissolved in water to prepare a coating slurry with a solid content of 36%. The coating slurry is coated on both sides of the modified microporous membrane in step (3) with a coating thickness of 2μm to obtain the battery separator.

[0069] Examples 2-4

[0070] The preparation method is the same as in Example 1, except that the mass content of the compound shown in Formula I-2 in the modified solution in step (1) is different. Please see Table 1 for details.

[0071] Examples 5-8

[0072] The preparation method is the same as in Example 3, except that the aromatic compounds in step (1) are different, as detailed in Table 1.

[0073] Example 9

[0074] (1) The compound shown in Formula I-2 was dissolved in methyl ethyl carbonate, and then an acylphosphine oxide (TPO) initiator was added to prepare a modified solution with a compound content of 15 wt%, wherein the amount of initiator was 5 wt% of the compound shown in Formula I-2;

[0075] (2) Alumina (D50 is 500nm), binder (carboxymethyl cellulose and polyacrylate 1:2) in a mass ratio of 94:6:0.5, and dispersant (polyacrylic acid) are dissolved in water to prepare a coating slurry with a solid content of 36%. The coating slurry is coated on both sides of a finished PE microporous membrane (Sinoma Lithium Membrane Co., Ltd., SNP9) with a thickness of 9μm. The coating thickness is 2μm to obtain a membrane with coating.

[0076] (3) The coated diaphragm prepared in step (2) was irradiated on both sides with a cobalt source (irradiation dose 1 mgy).

[0077] (4) The surface-treated coated separator is immersed in the modified solution prepared in step (1), and then irradiated with ultraviolet light with a wavelength of 380nm for 60s. After cleaning and drying, the battery separator is obtained.

[0078] Example 10

[0079] The preparation method is the same as in Example 9, except that the aromatic compound in step (1) is the compound shown in Formula I-3, and the microporous membrane is the finished PE microporous membrane (SNP9) from Sinoma Lithium Membrane Co., Ltd.

[0080] Example 11

[0081] The preparation method is the same as in Example 9, except that the aromatic compound in step (1) is the compound shown in Formula I-1.

[0082] Comparative Example 1

[0083] (1) Polyethylene with a viscosity-average molecular weight of 1 million and white oil were melted, mixed and extruded to form a cast sheet, and then subjected to simultaneous biaxial stretching (stretching ratio of 6 times for longitudinal stretching and transverse stretching, temperature of 120℃), extraction, drying and secondary transverse stretching (stretching ratio of 1.1 times, 130℃) to obtain a PE microporous membrane with a thickness of 9μm.

[0084] (2) Alumina (D50 is 500nm), binder (carboxymethyl cellulose and polyacrylate 1:2) and dispersant (polyacrylic acid) with a mass ratio of 94:6:0.5 are dissolved in water to prepare a coating slurry with a solid content of 36%. The coating slurry is coated on both sides of the microporous membrane in step (3) with a coating thickness of 2μm to obtain the battery separator.

[0085] Comparative Example 2

[0086] The preparation method is the same as in Example 3, except that the modified material in step (1) is different. Please see Table 1 for details.

[0087] The battery separators prepared in the examples and comparative examples were subjected to the following performance tests.

[0088] Compound content in modified microporous membrane / modified diaphragm: In Examples 1-8 and Comparative Example 2, the method for testing the compound content in the modified microporous membrane is as follows: cut a 1m*1m microporous membrane sample obtained in step (2), measure its mass and record it as m1, cut a 1m*1m microporous membrane sample after drying in step (3) and before the second horizontal stretching, measure its mass and record it as m2, take 5 groups of samples, take their average value, and then the compound content in the modified microporous membrane is (m2-m1) / m2*100%.

[0089] In Examples 9-11, the method for testing the compound content in the modified diaphragm is as follows: cut a 1m*1m diaphragm sample obtained in step (3), measure its mass and record it as m1, cut a 1m*1m diaphragm sample from step (4), measure its mass and record it as m2, take 5 groups of samples, and take their average value. Then the compound content in the modified diaphragm is (m2-m1) / m2*100%.

[0090] Puncture resistance:

[0091] The test was conducted according to the method specified in GB / T 36363-2018. A diaphragm with a size of 50*100 mm was cut along the TD direction. The diaphragm was fixed on the sample fixture of the puncture test machine (model: KES-GNDG5, KNC Technology Co., Ltd.). A steel needle with a diameter of 1.0 mm was used to puncture the diaphragm at a speed of 0.1 cm / sec. The maximum load of the steel needle penetrating the diaphragm was read. The test was performed more than 5 times and the arithmetic mean was taken.

[0092] Electrical conductivity:

[0093] Assemble an Al|separator|Al symmetric cell, inject electrolyte (EC / DMC = 3:7, LiPF6 1M), and vacuum seal. Connect the test circuit and apply an AC signal using an electrochemical workstation or similar instrument; the frequency range is typically from 10 mHz to 100 kHz. Record impedance data at different frequencies and plot the impedance spectrum. Fit and analyze the impedance spectrum using an equivalent circuit model to obtain the membrane resistance value.

[0094] The conductivity is calculated using the formula σ=L / RS, where σ is the conductivity, L is the membrane thickness, R is the membrane resistance, and S is the contact area between the electrode and the membrane.

[0095] Tensile strength:

[0096] The test was conducted according to GB / T 1040.3-2006. Under the test requirements of 23±3℃ and ≤60% humidity, a 1.5cm×20cm sample was cut and marked with the MD / TD direction of the diaphragm. The sample was tested using a tensile testing machine (High-Speed ​​Rail Testing Instruments (Dongguan) Co., Ltd., AI-3000-SU). The sample was fixed between the upper and lower clamps of the tensile testing machine (the distance between the clamps was 100±5 mm), ensuring the sample was flat and wrinkle-free, and vertically aligned. The tensile speed was 500mm / min. The tensile testing machine output the tensile strength value based on the sample width and thickness. Three tests were performed, and the average value was taken as the tensile strength (MPa) and elongation at break (%) in the MD / TD direction. Heat shrinkage:

[0097] Closed-cell temperature (air permeability method):

[0098] Cut a circular sample with a diameter of 50 mm and fix it in the sample slot of the hot stage of the gas transmission rate tester, ensuring that the edges are sealed. Start the heating program and record the real-time temperature and gas transmission rate data. When the transmission rate drops sharply to below 5% of the initial value, the corresponding temperature is the closed-cell temperature.

[0099] Membrane rupture temperature:

[0100] A 30mm long and 10mm wide diaphragm was tested using the TMA tensile method. The curve of the diaphragm length changing with temperature was recorded until the diaphragm broke. The temperature at which the diaphragm length increased instantaneously was the rupture temperature.

[0101] Heat shrinkage rate:

[0102] The test was conducted according to the requirements of GB / T36363-2018. A 10cm × 10cm sample was cut, and the transverse (TD) and longitudinal (MD) widths were marked on the sample. The transverse and longitudinal widths were measured using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C). The sample was held between two sealed A4 sheets of paper and placed in a 150℃ oven for 1 hour. After the sample returned to room temperature, the transverse and longitudinal widths were measured again using the fully automatic image measuring projector. Three measurements were taken, and the average value was recorded.

[0103] MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0104] TD heat shrinkage rate (%) = (TD length before heating - TD length after heating) ÷ TD length before heating × 100; Table 1

[0105] As shown in Table 1, modifying the microporous membrane surface with aromatic compounds containing alkenyl and ester groups can increase the membrane rupture temperature with minimal impact on the pore closure temperature, while also improving puncture resistance, tensile strength, and electrical conductivity to some extent. Experimental data from Examples 2 and 3, and Comparative Example 2, demonstrate that this invention can significantly improve membrane performance with relatively small amounts of modifying materials.

[0106] Experimental Example

[0107] The separators prepared in the examples and comparative examples were assembled into batteries, and the battery hit-pot defect rate and rate performance were tested.

[0108] Lithium cobalt oxide (LCO), conductive agent Super P, polyvinylidene fluoride (PVDF), and carbon nanotubes were mixed in a weight ratio of 96:2:2. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The obtained positive electrode slurry was uniformly coated onto a positive electrode current collector aluminum foil. The aluminum foil coated with the positive electrode slurry was dried at 90 °C, and then cold-pressed, cut, and slit to obtain the positive electrode sheet.

[0109] The negative electrode active material graphite, conductive additive Super P, sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed in a weight ratio of 95:1.5:1.5:2. Deionized water was added, and the mixture was stirred in a vacuum mixer to obtain a negative electrode slurry with a solid content of 50 wt%. The negative electrode slurry was uniformly coated onto a negative electrode current collector copper foil. The copper foil was dried at 80 ℃, and then cold-pressed, cut, and slit. Finally, it was dried under vacuum at 110 ℃ for 12 h to obtain the negative electrode sheet.

[0110] Prepare the electrolyte in a dry argon atmosphere glove box: ethylene carbonate, methyl ethyl carbonate, and ethyl propionate are mixed in a ratio of 1:3:1, 3% fluoroethylene carbonate and 0.5% ethylene sulfate are mixed evenly to form the electrolyte, and then 1M LiPF6 is added and fully dissolved.

[0111] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. After welding the tabs, the battery is placed in an outer packaging foil-aluminum-plastic film, dried, and then tested for high potency. The prepared electrolyte is then injected, and the battery undergoes vacuum sealing, formation, capacity testing, degassing, shaping, and capacity testing to obtain a lithium-ion battery.

[0112] Hipot test: Apply 100V to the electrodes of the lithium-ion battery for 1 second. If the maximum leakage current is ≤1mA, the test is considered passed; otherwise, the Hipot is considered defective. 10,000 batteries are tested.

[0113] Cyclic performance test: The battery was charged at a constant current of 1C to 4.4V, then charged at a constant voltage of 0.05C. After resting for 10 minutes, it was discharged at 1C to 3V, and the capacity at this point was recorded as the initial capacity C0. The above charging process was repeated 300 times, and the discharge capacity C1 was recorded at this point. The capacity retention rate after 300 cycles was calculated as C1 / C0 * 100%. The test results are shown in Table 2.

[0114] Table 2

[0115] As can be seen from the data in Table 2, using the separator provided by this invention to prepare lithium batteries significantly reduces the Hipot defect rate and significantly improves the cycle stability of the battery.

[0116] The battery separator provided by this invention is not limited to the preparation method provided by this invention. Battery separators that conform to this invention and are obtained by other methods should be protected within the scope of the claims of this invention.

[0117] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A battery separator comprising a microporous membrane and a coating, characterized in that, The microporous membrane is surface modified by an aromatic compound containing an alkenyl group and an ester group.

2. The battery separator of claim 1, wherein, The aromatic compound containing an alkenyl group and an ester group is one or several of the compounds shown in formula I-1 to formula I-6, Formula I-1; Formula I-2; Formula I-3; Formula I-4; Formula I-5; Formula I-6.

3. The method for producing a battery separator as claimed in claim 1 or 2, characterized by, The method comprises the following steps: I. Preparation of the modified microporous membrane: S1. Dissolving the aromatic compound containing an alkenyl group and an ester group in claim 1 or 2 in a solvent, adding an initiator to prepare a modified solution; S2. Surface treating the microporous membrane material after melt casting, biaxial stretching, extraction and drying to generate free radicals on the surface of the microporous membrane, contacting the modified solution with the microporous membrane after surface treatment, then performing ultraviolet irradiation, cleaning, drying and secondary horizontal stretching to obtain the modified microporous membrane; II. Preparation of the coating layer: Step A. Preparing a coating slurry; Step B. Coating the coating slurry on at least one side of the modified microporous membrane.

4. The method of claim 3, wherein the polymeric material is a polyolefin. The aromatic compound containing an alkenyl group and an ester group is 0.2 to 20% of the mass of the modified microporous membrane.

5. The method of claim 3, wherein the polymeric material is a polyolefin. In step S2, the surface treatment is performed by surface irradiation or photodegradation, and the surface irradiation is performed by a cobalt source or an electron accelerator with a dose of 0.1 to 1 MGy. In step S2, the ultraviolet irradiation is performed at a wavelength of 200 to 450 nm for ≤60 s.

6. The method of claim 3, wherein the polymeric material is a polyolefin. In the modified solution of step S1, the content of the aromatic compound containing an alkenyl group and an ester group is 2 to 40 wt%, and the amount of the initiator is 1 to 15 wt% of the weight of the aromatic compound containing an alkenyl group and an ester group.

7. The method of claim 3, wherein the battery separator is prepared by the steps of: The initiator is at least one of benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acyl phosphine oxides, benzophenone, thioxanthone and anthraquinone. The solvent is at least one of ethers, esters and halogenated alkanes.

8. The method of producing a battery separator according to claim 1 or 2, characterized by, The method comprises the following steps: Step A'. Preparing a coating slurry; Step B'. Coating the coating slurry on at least one side of the finished microporous membrane and drying; Step C'. Dissolving the aromatic compound containing an alkenyl group and an ester group in claim 1 or 2 in a solvent, then adding an initiator to prepare a modified solution; Step D'. Surface treating the membrane after coating in step B' to generate free radicals on the surface of the microporous membrane, contacting the modified solution with the microporous membrane after surface treatment, then performing ultraviolet irradiation, cleaning and drying after curing to obtain the modified separator.

9. The method of claim 8, wherein the battery separator is prepared by a process comprising: The thickness of the coating layer coated in step B' is 0.5 μm to 6 μm.

10. A secondary battery characterized by comprising: The battery separator prepared by the method of any one of claims 1 to 9.

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