Lithium ion battery diaphragm suitable for cold pressing and preparation method thereof

Abstract

The invention discloses a lithium ion battery diaphragm suitable for cold pressing and a preparation method thereof, and relates to the technical field of lithium ion batteries. The diaphragm comprises a base membrane and a coating layer, wherein the coating layer is composed of polyvinylidene fluoride, a polyacrylonitrile adhesive, an aliphatic amide dispersant, polyvinyl alcohol, an acrylic monomer, an emulsifier, an initiator and the like; the preparation method comprises the steps of polyvinyl alcohol modified acrylic emulsion preparation, coating layer slurry preparation, coating diaphragm preparation and the like. The lithium ion battery diaphragm developed by the invention not only has excellent adhesive property and puncture strength, but also can reduce the energy consumption of a battery end and improve the production efficiency, is suitable for a cold pressing process, and solves the problems of low efficiency and high energy consumption of a traditional hot pressing process.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium ion battery separator suitable for cold pressing and a preparation method thereof. Background Art

[0002] The battery separator is an important component of lithium-ion batteries. Structurally, it directly separates the positive and negative electrodes and prevents battery short circuits. At the same time, it allows ions to migrate between the positive and negative electrodes. The performance of the separator directly determines the battery's interface performance, cycle performance, and safety performance.

[0003] At present, the traditional diaphragm composed of polyolefin base film and ceramic coating can no longer meet the ever-increasing battery technology requirements. Although the functional coatings that have emerged in the market can improve the performance of the diaphragm to a certain extent, the adhesive coating with polyvinylidene fluoride as the main component has obvious limitations: its adhesion strength needs to be enhanced, and it requires the help of hot pressing process to achieve adhesion with the electrode.

[0004] As lithium-ion batteries develop towards larger sizes, the thickness of battery cells continues to increase, and the hot pressing process has exposed a series of disadvantages: the upper and lower heating plates have difficulty effectively transferring heat to the center of the battery cell thickness. This not only leads to a significant increase in hot pressing time and a significant reduction in production efficiency, but also fails to guarantee the uniformity of the hot pressing interface, while also causing an increase in energy consumption of the production line.

[0005] Therefore, developing a lithium-ion battery separator suitable for cold pressing process has become an important research direction in this field. Summary of the Invention

[0006] The object of the present invention is to provide a lithium ion battery separator suitable for cold pressing and a preparation method thereof, so as to solve the problems raised in the prior art.

[0007] To achieve the above object, the present invention provides the following technical solutions: A lithium ion battery separator suitable for cold pressing comprises a base film and a coating layer arranged on the surface of the base film.

[0008] Furthermore, the base film is a polyolefin separator.

[0009] Furthermore, the components for preparing the coating layer include: polyvinylidene fluoride, an adhesive, a dispersant, polyvinyl alcohol, an acrylic monomer, an emulsifier, and an initiator.

[0010] Furthermore, the polyvinylidene fluoride is PVDF-HFP powder, with a grafting rate of 2%-8%, a molecular weight of 500,000-1,000,000, and a particle size of 4-9µm.

[0011] Furthermore, the thickness of the separator is 5-20µm.

[0012] Furthermore, the adhesive is a polyacrylonitrile polymer, and the dispersant is an aliphatic amide dispersant.

[0013] Furthermore, the acrylic monomer is a mixture of one or more of acrylic acid, methyl acrylate, ethyl acrylate, and methyl methacrylate; The emulsifier is a mixture of one or more of sodium lauryl sulfate, sodium dodecylbenzene sulfonate, fatty alcohol polyoxyethylene ether, and sodium allyloxy hydroxypropyl sulfonate; The initiator is a mixture of one or more of ammonium persulfate, potassium persulfate and sodium persulfate.

[0014] A method for preparing a lithium-ion battery separator suitable for cold pressing comprises the following steps: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: Mixing acrylic monomer, emulsifier and deionized water, stirring to prepare a pre-emulsion; heating the polyvinyl alcohol solution to 70-80°C, adding the pre-emulsion and initiator solution dropwise, and keeping the temperature to react for 1-2 hours to obtain a polyvinyl alcohol-modified acrylic emulsion; S2: Preparation of coating slurry: Stirring a dispersant, an adhesive and ultrapure water to form a dispersion; adding polyvinylidene fluoride and polyvinyl alcohol modified acrylic emulsion in sequence and stirring to obtain a coating layer slurry; S3: Preparation of coated membrane: The coating layer slurry is coated on the surface of the base film and dried to form a coating layer to prepare a coated diaphragm.

[0015] Furthermore, the S1 includes the following steps: Mix the acrylic monomer, emulsifier and deionized water, and stir at a speed of 200-500 r / min for pre-emulsification for 30-60 minutes to prepare a pre-emulsion liquid; Add polyvinyl alcohol to deionized water, heat to 80-95°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; Take the initiator, add deionized water to dissolve it, and obtain an initiator solution; The polyvinyl alcohol solution was heated to 70-80° C., the pre-emulsion and the initiator solution were added dropwise, stirred at a speed of 500-600 r / min, kept warm for reaction for 1-2 hours, and cooled to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion.

[0016] Furthermore, the S2 includes the following steps: Mix the dispersant, adhesive and ultrapure water, and stir at a speed of 20-40 r / min for 10-30 minutes to prepare a dispersion; Add polyvinylidene fluoride and stir to form a solution; add polyvinyl alcohol modified acrylic emulsion and stir at a speed of 20-40 r / min for 10-30 minutes to obtain a coating layer slurry.

[0017] Furthermore, the S3 includes the following steps: The coating layer slurry is applied on the surface of the base film by spraying or roller coating, and dried at 40-80° C. to form a coating layer to obtain a coated diaphragm.

[0018] Furthermore, in step S1, the pre-emulsion comprises the following components: in terms of mass percentage, 10-12 wt% acrylic monomer, 1-2 wt% emulsifier, and the balance being deionized water; The concentration of the polyvinyl alcohol solution is 1-5%; The concentration of the initiator solution is 0.1-1%; The polyvinyl alcohol-modified acrylic emulsion includes the following components: by mass percentage, 4-5 wt% of acrylic monomer, 0.8-1.0 wt% of emulsifier, 2-3 wt% of polyvinyl alcohol, 0.2-0.3 wt% of initiator, and the balance is deionized water.

[0019] Furthermore, the solid content of the concentrated polyvinyl alcohol-modified acrylic emulsion is controlled at 5-10%.

[0020] Furthermore, in step S2, the dispersion includes the following components: in mass percentage, 0.2-0.6 wt% of a dispersant, 10-20 wt% of a binder, and the balance being ultrapure water.

[0021] Furthermore, in step S2, polyvinylidene fluoride is added in the form of powder, and the particle size of the polyvinylidene fluoride powder is 1-5µm; The stirring process is: first stir at 20-40r / min for 60-80min, then disperse at 1000-2000r / min for 30-40min; The coating layer slurry comprises the following components: by mass percentage, 0.2-0.6 wt% of a dispersant, 10-20 wt% of an adhesive, 10-20 wt% of polyvinylidene fluoride, 5-10 wt% of a polyvinyl alcohol-modified acrylic emulsion, and the balance being ultrapure water.

[0022] Furthermore, in step S3, the thickness of the coating layer is 1-5 μm, and the surface density of the coating layer is 0.2-1.0 g / m 2 , coating speed is 10-150m / min.

[0023] Furthermore, the base film is surface modified, and the surface modification process is as follows: (1) Base film pretreatment: plasma treatment of base film; (2) Preparation of composite coating slurry: Modified montmorillonite and α-alumina are added to ultrapure water, stirred and dispersed to obtain an inorganic filler dispersion; polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), graphene nanosheets, and silane coupling agent are added to the inorganic filler dispersion in sequence, stirred to obtain a composite coating slurry; (3) Coating and drying: The composite coating slurry is coated on the base film by a doctor blade coating method, pre-dried at 80-100°C for 8-10 minutes, and cross-linked and cured at 120-130°C for 12-15 minutes to form a composite coating and obtain a modified base film.

[0024] Furthermore, the plasma treatment process is as follows: the gas is selected as argon / tetrafluoroethylene mixed gas with a volume ratio of (3-4):1, and fluorine-containing groups are introduced on the surface of the base film; the radio frequency power is 50-100W, the treatment time is 10-60s, and the working gas pressure is 10-100Pa.

[0025] Furthermore, the composite coating slurry includes the following components by mass: 47.5-55 parts of modified montmorillonite, 5-8 parts of α-alumina, 38.5-42 parts of PVDF-HFP, 1-1.5 parts of graphene nanosheets, 0.5-1 parts of silane coupling agent, and 80-100 parts of ultrapure water.

[0026] Furthermore, the particle size of α-alumina is 0.1-5µm; The particle size of PVDF-HFP is 1-10µm; The thickness of graphene nanosheets is 4-20nm and the particle size is 5-10µm.

[0027] Furthermore, the stirring and dispersing process is: first stirring at a speed of 200-400 r / min for 30-45 minutes, and then dispersing at a speed of 1500-2000 r / min for 60-80 minutes.

[0028] Furthermore, the process conditions of the blade coating are: a blade gap of 30-50 μm, a coating speed of 10-150 m / min, and a composite coating thickness of 2-3 μm.

[0029] Furthermore, the modified montmorillonite is prepared by the following process: The montmorillonite was added to a 5-7% hydrochloric acid solution, stirred at 60-70°C for 18-24 hours to remove interlayer metal ions, washed with deionized water until neutral (pH 6-7), and then dried at 110-120°C for 8-12 hours. The dried montmorillonite was added to an ethanol / water mixed solution, a silane coupling agent was added, and the mixture was stirred at 70-80°C for 2-3 hours. After filtering and drying, the modified montmorillonite was obtained.

[0030] Furthermore, the mass ratio of the hydrochloric acid solution to the montmorillonite is (3-4):1; The mass ratio of montmorillonite to ethanol / water mixed solution is 1:(5-8); Ethanol / water mixed solution: the volume ratio of ethanol to water is (3-4):1; The particle size of montmorillonite is 1-10µm.

[0031] Furthermore, the silane coupling agent is a mixture of one or more of KH-550, KH-551, A-151 and A171; the added amount is 1.5-2 wt% of the montmorillonite. Compared with the prior art, the present invention has the following beneficial effects: 1. The polyvinyl alcohol and acrylic acid in the polyvinyl alcohol-modified acrylic emulsion of the present invention both have good adhesion. The coated diaphragm prepared by the adhesive has high adhesion to the electrode, thereby ensuring that a good bonding effect between the coated diaphragm and the electrode can be achieved through the cold pressing process.

[0032] 2. The synergistic effect of the polyvinyl alcohol-modified acrylic emulsion and the polyacrylonitrile adhesive in the coating layer of the present invention enables the coated diaphragm to achieve high peel strength with the electrode by cold pressing without the need for hot pressing and heating; the preparation process omits the hot pressing step, reduces the production process and time, and improves production efficiency. Especially for the production of large batteries or thick battery cells, the advantages are more obvious, which can effectively shorten the production cycle and reduce production costs.

[0033] 3. The polyvinyl alcohol-modified acrylic emulsion in the coating layer of the present invention enhances interfacial bonding: through the emulsion polymerization process of pre-emulsion preparation, polyvinyl alcohol solution dissolution and gradient addition of initiator solution, polyvinyl alcohol and acrylic monomers form an interpenetrating network structure, thereby improving the interfacial adhesion between the coating layer and the base film, thereby improving the puncture resistance of the coated diaphragm, helping to prevent the coated diaphragm from rupture or damage during battery assembly and use, and improving the safety and reliability of the battery.

[0034] 4. The present invention performs surface modification and plasma treatment on the base film: introduces fluorine-containing groups on the surface of the base film, enhances the chemical bonding between the base film and the coating layer, and reduces the thermal shrinkage rate; and applies a composite coating to the base film: forms a composite structure of modified montmorillonite rigid support and PVDF-HFP flexible network, thereby improving the mechanical strength and thermal stability of the base film.

[0035] In summary, the lithium-ion battery separator prepared by the present invention has excellent adhesion performance and puncture strength, while also reducing battery end energy consumption and improving production efficiency. It is suitable for cold pressing process, solving the problems of low efficiency and high energy consumption of traditional hot pressing process. DETAILED DESCRIPTION

[0036] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0037] In the following specific embodiments: The “parts” mentioned below are all parts by mass unless otherwise specified.

[0038] Base film: polyolefin separator, thickness 12µm; The adhesive used is polyacrylonitrile water-based adhesive, model LA133; The dispersant used is erucamide, model ER-CH; Polyvinylidene fluoride powder: PVDF-HFP, grafting rate of 6%, molecular weight of 800,000, average particle size of 3µm; Montmorillonite: average particle size is 2µm; α-alumina: average particle size is 1µm; Graphene nanosheets: average thickness is 10nm and average particle size is 5µm.

[0039] Example 1: A method for preparing a lithium-ion battery separator suitable for cold pressing, comprising the following steps: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: S1-1: 10 g of acrylic acid, 2 g of sodium lauryl sulfate, and 88 g of deionized water were mixed and stirred at 300 r / min for 30 min to prepare a pre-emulsified liquid; S1-2: Add 5 g of polyvinyl alcohol to 95 g of deionized water, heat to 90°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; S1-3: Add 0.5 g of potassium persulfate to 29.5 g of deionized water and stir at 300 rpm for 30 min to prepare an initiator solution; S1-4: heating the polyvinyl alcohol solution to 80°C, adding the pre-emulsion and the initiator solution dropwise, stirring at 500 r / min, keeping the temperature for reaction for 2 hours, and cooling to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion; the solid content of the polyvinyl alcohol-modified acrylic emulsion is 10%; S2: Preparation of coating slurry: S2-1: Mix the dispersant, binder and ultrapure water, and stir at 30 r / min for 10 min to prepare a dispersion; S2-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, then stir at a speed of 1000 r / min for 30 min, add polyvinyl alcohol-modified acrylic emulsion, stir at a speed of 30 r / min for 20 min, to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% dispersant erucamide, 10wt% polyacrylonitrile water-based adhesive, 15wt% polyvinylidene fluoride, 5wt% polyvinyl alcohol modified acrylic emulsion, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating layer slurry was roller-coated on the surface of the base film with a thickness of 12 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0040] Example 2: A method for preparing a lithium-ion battery separator suitable for cold pressing, comprising the following steps: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: S1-1: 10 g of acrylic acid, 2 g of sodium lauryl sulfate, and 88 g of deionized water were mixed and stirred at 300 r / min for 30 min to prepare a pre-emulsified liquid; S1-2: Add 5 g of polyvinyl alcohol to 95 g of deionized water, heat to 90°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; S1-3: Add 0.5 g of potassium persulfate to 29.5 g of deionized water and stir at 300 rpm for 30 min to prepare an initiator solution; S1-4: heating the polyvinyl alcohol solution to 80°C, adding the pre-emulsion and the initiator solution dropwise, stirring at 500 r / min, keeping the temperature for reaction for 2 hours, and cooling to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion; the solid content of the polyvinyl alcohol-modified acrylic emulsion is 10%; S2: Preparation of coating slurry: S2-1: Mix the dispersant, binder and ultrapure water, and stir at 30 r / min for 10 min to prepare a dispersion; S2-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, then stir at a speed of 1000 r / min for 30 min, add polyvinyl alcohol-modified acrylic emulsion, stir at a speed of 30 r / min for 20 min, to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% dispersant erucamide, 10wt% polyacrylonitrile water-based adhesive, 15wt% polyvinylidene fluoride, 8wt% polyvinyl alcohol modified acrylic emulsion, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating layer slurry was roller-coated on the surface of the base film with a thickness of 12 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0041] Example 3: A method for preparing a lithium-ion battery separator suitable for cold pressing, comprising the following steps: S1-1: 10 g of acrylic acid, 2 g of sodium lauryl sulfate, and 88 g of deionized water were mixed and stirred at 300 r / min for 30 min to prepare a pre-emulsified liquid; S1-2: Add 5 g of polyvinyl alcohol to 95 g of deionized water, heat to 90°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; S1-3: Add 0.5 g of potassium persulfate to 29.5 g of deionized water and stir at 300 rpm for 30 min to prepare an initiator solution; S1-4: heating the polyvinyl alcohol solution to 80°C, adding the pre-emulsion and the initiator solution dropwise, stirring at 500 r / min, keeping the temperature for reaction for 2 hours, and cooling to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion; the solid content of the polyvinyl alcohol-modified acrylic emulsion is 10%; S2: Preparation of coating slurry: S2-1: Mix the dispersant, binder and ultrapure water, and stir at 30 r / min for 10 min to prepare a dispersion; S2-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, then stir at a speed of 1000 r / min for 30 min, add polyvinyl alcohol-modified acrylic emulsion, stir at a speed of 30 r / min for 20 min, to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% dispersant erucamide, 10wt% polyacrylonitrile water-based adhesive, 15wt% polyvinylidene fluoride, 10wt% polyvinyl alcohol modified acrylic emulsion, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating layer slurry was roller-coated on the surface of the base film with a thickness of 12 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0042] Example 4: A method for preparing a lithium-ion battery separator suitable for cold pressing, comprising the following steps: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: S1-1: 10 g of acrylic acid, 2 g of sodium lauryl sulfate, and 88 g of deionized water were mixed and stirred at 300 r / min for 30 min to prepare a pre-emulsified liquid; S1-2: Add 5 g of polyvinyl alcohol to 95 g of deionized water, heat to 90°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; S1-3: Add 0.5 g of potassium persulfate to 29.5 g of deionized water and stir at 300 rpm for 30 min to prepare an initiator solution; S1-4: heating the polyvinyl alcohol solution to 80°C, adding the pre-emulsion and the initiator solution dropwise, stirring at 500 r / min, keeping the temperature for reaction for 2 hours, and cooling to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion; the solid content of the polyvinyl alcohol-modified acrylic emulsion is 10%; S2: Preparation of coating slurry: S2-1: Mix the dispersant, binder and ultrapure water, and stir at 30 r / min for 10 min to prepare a dispersion; S2-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, then stir at a speed of 1000 r / min for 30 min, add polyvinyl alcohol-modified acrylic emulsion, stir at a speed of 30 r / min for 20 min, to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% dispersant erucamide, 10wt% polyacrylonitrile water-based adhesive, 15wt% polyvinylidene fluoride, 5wt% polyvinyl alcohol modified acrylic emulsion, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating slurry was roller-coated on the surface of the modified base film with a thickness of 14 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0043] The modified base film is prepared by the following process: (1) Base film pretreatment: Plasma treatment of the base film; the plasma treatment process is: the gas is argon / tetrafluoroethylene mixed gas, the volume ratio is 3:1; the radio frequency power is 80W, the treatment time is 25s, and the working gas pressure is 50Pa; (2) Preparation of composite coating slurry: Modified montmorillonite and α-alumina were added to ultrapure water, stirred at a speed of 300 r / min for 40 min, and then dispersed at a speed of 2000 r / min for 60 min to obtain an inorganic filler dispersion; PVDF-HFP, graphene nanosheets, and silane coupling agent KH-550 were added to the inorganic filler dispersion in sequence, stirred until uniformly mixed, and a composite coating slurry was obtained; The composite coating slurry includes the following components by mass: 50 parts of modified montmorillonite, 8 parts of α-alumina, 40 parts of PVDF-HFP, 1 part of graphene nanosheets, 1 part of silane coupling agent, and 80 parts of ultrapure water; (3) Coating and drying: The composite coating slurry is coated on the base film by a doctor blade coating method, pre-dried at 80°C for 8 minutes, and cross-linked and cured at 120°C for 12 minutes to form a composite coating and obtain a modified base film; The process conditions of blade coating are as follows: blade gap of 50 μm, coating speed of 50 m / min, and composite coating thickness of 2 μm.

[0044] Modified montmorillonite is produced by the following process: Montmorillonite was added to a 5% hydrochloric acid solution with a mass ratio of hydrochloric acid solution to montmorillonite of 3:1. The mixture was stirred at 60°C for 24 hours to remove interlayer metal ions. The mixture was washed with deionized water until neutral (pH 7) and then dried at 110°C for 12 hours. The dried montmorillonite was added to an ethanol / water mixed solution (volume ratio of 3:1) with a ratio of montmorillonite to ethanol / water mixed solution of 1:7. 2wt% of silane coupling agent KH-551 was added to the montmorillonite. The mixture was stirred at 80°C for 2 hours and filtered to obtain modified montmorillonite.

[0045] Example 5: A method for preparing a lithium-ion battery separator suitable for cold pressing, comprising the following steps: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: S1-1: 10 g of acrylic acid, 2 g of sodium lauryl sulfate, and 88 g of deionized water were mixed and stirred at 300 r / min for 30 min to prepare a pre-emulsified liquid; S1-2: Add 5 g of polyvinyl alcohol to 95 g of deionized water, heat to 90°C, stir and dissolve, and cool to room temperature after complete dissolution to obtain a polyvinyl alcohol solution; S1-3: Add 0.5 g of potassium persulfate to 29.5 g of deionized water and stir at 300 rpm for 30 min to prepare an initiator solution; S1-4: heating the polyvinyl alcohol solution to 80°C, adding the pre-emulsion and the initiator solution dropwise, stirring at 500 r / min, keeping the temperature for reaction for 2 hours, and cooling to room temperature to obtain a polyvinyl alcohol-modified acrylic emulsion; the solid content of the polyvinyl alcohol-modified acrylic emulsion is 10%; S2: Preparation of coating slurry: S2-1: Mix the dispersant, binder and ultrapure water, and stir at 30 r / min for 10 min to prepare a dispersion; S2-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, then stir at a speed of 1000 r / min for 30 min, add polyvinyl alcohol-modified acrylic emulsion, stir at a speed of 30 r / min for 20 min to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% dispersant erucamide, 10wt% polyacrylonitrile water-based adhesive, 15wt% polyvinylidene fluoride, 5wt% polyvinyl alcohol modified acrylic emulsion, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating layer slurry was roller-coated on the surface of the modified base film with a thickness of 15 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0046] The modified base film is prepared by the following process: (1) Base film pretreatment: Plasma treatment of the base film; the plasma treatment process is: the gas is argon / tetrafluoroethylene mixed gas, the volume ratio is 4:1; the radio frequency power is 100W, the treatment time is 10s, and the working gas pressure is 10Pa; (2) Preparation of composite coating slurry: Modified montmorillonite and α-alumina were added to ultrapure water, stirred at a speed of 400 r / min for 30 min, and then dispersed at a speed of 1500 r / min for 80 min to obtain an inorganic filler dispersion; PVDF-HFP, graphene nanosheets, and silane coupling agent KH-550 were added to the inorganic filler dispersion in sequence, stirred until uniformly mixed, and a composite coating slurry was obtained; The composite coating slurry includes the following components by mass: 53 parts of modified montmorillonite, 5 parts of α-alumina, 40 parts of PVDF-HFP, 1 part of graphene nanosheets, 1 part of silane coupling agent, and 100 parts of ultrapure water; (3) Coating and drying: The composite coating slurry is coated on the base film by a doctor blade coating method, pre-dried at 100°C for 10 minutes, and cross-linked and cured at 130°C for 15 minutes to form a composite coating and obtain a modified base film; The process conditions of blade coating are as follows: blade gap of 30 μm, coating speed of 100 m / min, and composite coating thickness of 3 μm.

[0047] Modified montmorillonite is produced by the following process: Montmorillonite was added to a 6% hydrochloric acid solution with a mass ratio of hydrochloric acid solution to montmorillonite of 4:1. The mixture was stirred at 70°C for 18 hours to remove interlayer metal ions. The mixture was washed with deionized water until neutral (pH 6) and then dried at 120°C for 8 hours. The dried montmorillonite was added to an ethanol / water mixed solution (volume ratio of 4:1) with a ratio of montmorillonite to ethanol / water mixed solution of 1:5. 1.5wt% of silane coupling agent KH-551 was added to the montmorillonite. The mixture was stirred at 70°C for 3 hours and filtered and dried to obtain modified montmorillonite.

[0048] Comparative Example 1: Based on Example 1, the preparation of the coating layer slurry was adjusted without adding polyvinyl alcohol-modified acrylic emulsion, comprising the following steps: S1: Preparation of coating slurry: S1-1: Mix the dispersant, binder and ultrapure water, and stir at a speed of 30 r / min for 10 min to prepare a dispersion; S1-2: Add polyvinylidene fluoride, stir at a speed of 30 r / min for 60 min, and then stir at a speed of 1000 r / min for 30 min to obtain a coating layer slurry; The coating layer slurry includes the following components: in mass percentage, 0.2wt% of dispersant erucamide, 10wt% of polyacrylonitrile water-based adhesive, 15wt% of polyvinylidene fluoride, and the balance is ultrapure water; S3: Preparation of coated membrane: The coating layer slurry was roller-coated on the surface of the base film with a thickness of 12 μm, and dried at 65° C. to form a coating layer, thereby preparing a coated diaphragm; The coating thickness is 2µm and the surface density of the coating is 1.0g / m 2 , coating speed is 100m / min.

[0049] Comparative Example 2: A polyolefin separator was used without coating.

[0050] Comparative Example 3: Based on Example 4, the modified montmorillonite in the composite coating slurry is adjusted to montmorillonite. The only difference from Example 4 is that the composite coating slurry includes the following components by mass: 50 parts of montmorillonite, 8 parts of α-alumina, 40 parts of PVDF-HFP, 1 part of graphene nanosheets, 1 part of silane coupling agent, and 80 parts of ultrapure water.

[0051] Comparative Example 4: The only difference from Example 4 is that the modified base film is not subjected to plasma treatment.

[0052] Experiment: The membranes prepared in Examples 1-5 and Comparative Examples 1-4 were used to prepare samples, and the performance of the samples was tested. The test results are shown in Table 1: Positive electrode-cold press peel strength: The separator and the positive electrode sheet are bonded together through a cold press process to form a test sample. The separator is peeled off from the positive electrode sheet using a 180° peeling method at a speed of 50 mm / min. The force value during the peeling process is recorded, and the average value is taken as the peel strength; Air permeability: The TAPPI T460 Gurley air permeability tester is used to measure the time required for 100 mL of air to pass through a 100 cm² membrane. The air permeability value reflects the porosity and pore size distribution of the membrane. The shorter the time, the better the air permeability. Puncture strength: Using an electronic universal material testing machine, fix the diaphragm on a circular fixture and use a hemispherical puncture needle (1 mm in diameter) to vertically puncture the diaphragm at a speed of 100 mm / min. Record the maximum force during the puncture process. Thermal shrinkage: Cut a 100mm×100mm membrane sample, place it in a thermal shrinkage test chamber, and heat it at 105°C for 1 hour. After taking it out, measure the dimensional change rate of the sample in the longitudinal direction (MD) and transverse direction (TD). The calculation formula is: Thermal shrinkage (%) = (initial size - size after shrinkage) / initial size.

[0053]

[0054] Conclusion: From the comparison of the data in the table, it can be seen that the performance of the membranes prepared in Comparative Examples 1-4 is significantly worse than that of the membranes prepared in Examples 1-5. Comparative Example 1 is based on Example 1, and the coating slurry does not add polyvinyl alcohol modified acrylic emulsion. The prepared membrane has significantly worse positive electrode-cold pressing peeling strength, thermal shrinkage, and puncture strength performance compared with the membrane prepared in Example, while the air permeability is improved, indicating that polyvinyl alcohol modified acrylic emulsion can enhance interfacial bonding and improve the mechanical strength of the membrane; Comparative Example 2 uses a polyolefin base film as a battery membrane without coating. Compared with the membrane prepared in Example, its positive electrode-cold pressing peeling strength, thermal shrinkage, and puncture strength are significantly worse. The performance is obviously deteriorated, while the air permeability is improved; Comparative Example 3 is based on Example 4, and the modified montmorillonite in the composite coating slurry is adjusted to montmorillonite. Compared with the diaphragm prepared in Example 4, its positive electrode-cold pressing peel strength, thermal shrinkage, and puncture strength performance are obviously deteriorated, while the air permeability does not change much; Comparative Example 4 is based on Example 4, and the modified base film is not plasma treated. Compared with the diaphragm prepared in Example 4, its positive electrode-cold pressing peel strength, thermal shrinkage, and puncture strength performance are obviously deteriorated, while the air permeability does not change much.

[0055] Comparing Examples 1-3 and Comparative Examples 1-2, it can be seen that when the amount of polyvinyl alcohol-modified acrylic emulsion added to the coating layer slurry gradually increases: The positive electrode-cold pressing peeling performance of the corresponding coated diaphragm is getting better and better, that is, the adhesion to the positive electrode sheet is getting better and better (at the same PVDF coating layer density), and is higher than that of the coated diaphragm corresponding to the coating layer slurry without adding polyvinyl alcohol modified acrylic emulsion; The heat shrinkage and puncture resistance of the corresponding coated diaphragms are getting better and better (at the same coating layer density), and are both better than those of the coated diaphragms corresponding to the coating layer slurry without the addition of polyvinyl alcohol-modified acrylic emulsion, confirming the effectiveness of polyvinyl alcohol-modified acrylic emulsion in improving the safety performance of the diaphragm; The air permeability of the corresponding coated diaphragm deteriorates (at the same coating layer density), and is worse than that of the coated diaphragm corresponding to the coating slurry without adding polyvinyl alcohol modified acrylic emulsion. Therefore, in order to balance the various performances of the coated diaphragm, the addition amount of polyvinyl alcohol modified acrylic emulsion should be moderate, not the more the better. Compared with the existing water-based polyvinylidene fluoride coating technology, the coated diaphragm prepared by the present invention also has higher electrode bonding performance and higher safety under cold pressing conditions, and has good application prospects in the diaphragm field.

[0056] By comparing Examples 4-5 and Comparative Examples 3-4, it can be seen that plasma treatment can enhance the chemical bonding between the base film and the coating layer and reduce the thermal shrinkage rate; compared with montmorillonite, the modified montmorillonite can better combine with other components and enhance the mechanical strength and thermal stability of the base film.

[0057] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A lithium-ion battery separator suitable for cold pressing, characterized in that: The invention comprises the following structures: a base film and a coating layer arranged on the surface of the base film; the coating layer comprises the following components: polyvinylidene fluoride, an adhesive, a dispersant, polyvinyl alcohol, an acrylic monomer, an emulsifier and an initiator.

2. A lithium-ion battery separator suitable for cold pressing according to claim 1, characterized in that: The acrylic monomer is a mixture of one or more of acrylic acid, methyl acrylate, ethyl acrylate, and methyl methacrylate.

3. The lithium-ion battery separator suitable for cold pressing according to claim 1, characterized in that: The base film is a polyolefin separator; The adhesive is a polyacrylonitrile polymer, and the dispersant is an aliphatic amide dispersant.

4. The lithium-ion battery separator suitable for cold pressing according to claim 1, characterized in that: The emulsifier is a mixture of one or more of sodium lauryl sulfate, sodium dodecylbenzene sulfonate, fatty alcohol polyoxyethylene ether, and sodium allyloxy hydroxypropyl sulfonate.

5. The lithium-ion battery separator suitable for cold pressing according to claim 1, characterized in that: The initiator is a mixture of one or more of ammonium persulfate, potassium persulfate and sodium persulfate.

6. A method for preparing a lithium-ion battery separator suitable for cold pressing, characterized in that: The following steps are involved: S1: Preparation of polyvinyl alcohol modified acrylic emulsion: Mixing acrylic monomers, emulsifiers and deionized water to prepare a pre-emulsion; heating the polyvinyl alcohol solution to 70-80°C, adding the pre-emulsion and initiator solution dropwise, and keeping the temperature to react to obtain a polyvinyl alcohol-modified acrylic emulsion; S2: Preparation of coating slurry: Stirring a dispersant, a binder and ultrapure water to form a dispersion; adding polyvinylidene fluoride and polyvinyl alcohol modified acrylic emulsion in sequence and stirring to obtain a coating layer slurry; S3: Preparation of coated membrane: The coating layer slurry is coated on the surface of the base film and dried to form a coating layer to prepare a coated diaphragm.

7. The method for preparing a lithium-ion battery separator suitable for cold pressing according to claim 6, characterized in that: The pre-emulsion comprises the following components: in mass percentage, 10-12 wt% acrylic monomer, 1-2 wt% emulsifier, and the balance is deionized water; The polyvinyl alcohol-modified acrylic emulsion includes the following components: by mass percentage, 4-5 wt% of acrylic monomer, 0.8-1.0 wt% of emulsifier, 2-3 wt% of polyvinyl alcohol, 0.2-0.3 wt% of initiator, and the balance is deionized water.

8. The method for preparing a lithium-ion battery separator suitable for cold pressing according to claim 6, characterized in that: The coating layer slurry comprises the following components: by mass percentage, 0.2-0.6 wt% of a dispersant, 10-20 wt% of an adhesive, 10-20 wt% of polyvinylidene fluoride, 5-10 wt% of a polyvinyl alcohol-modified acrylic emulsion, and the balance being ultrapure water.

9. The method for preparing a lithium-ion battery separator suitable for cold pressing according to claim 6, characterized in that: The thickness of the coating layer is 1-5 μm, and the surface density of the coating layer is 0.2-1.0 g / m 2 , coating speed is 10-150m / min.

10. The method for preparing a lithium-ion battery separator suitable for cold pressing according to claim 6, characterized in that: The base film is surface modified, and the surface modification process is as follows: (1) Base film pretreatment: plasma treatment of base film; (2) Preparation of composite coating slurry: adding modified montmorillonite and α-alumina to ultrapure water, stirring and dispersing to obtain an inorganic filler dispersion; adding polyvinylidene fluoride-hexafluoropropylene copolymer, graphene nanosheets, and silane coupling agent to the inorganic filler dispersion in sequence, stirring to obtain a composite coating slurry; (3) Coating and drying: The composite coating slurry is coated on the base film by a doctor blade coating method, and then dried to form a composite coating to obtain a modified base film.

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