Lithium battery composite diaphragm and preparation method thereof

Abstract

The invention discloses a lithium battery composite membrane and a preparation method thereof, and belongs to the technical field of lithium batteries, the composite membrane comprises a base membrane and a coating layer coated on one side or two sides of the base membrane; slurry of the coating layer is obtained through graft copolymerization of EVA, polyamic acid and aluminum oxide, the EVA is hydrolyzed to generate hydroxyl, the aluminum oxide is modified by amino silane to introduce amino, and then the aluminum oxide is subjected to graft copolymerization with the polyamic acid. According to the invention, combination of PI, EVA and alumina is realized through graft copolymerization, multi-component molecular-level compounding is realized, the cost is reduced, and the EVA has low density, so that the mass of a coating layer can be reduced, and the influence on the energy density of a battery is reduced. Based on the combination of a rigid material of PI and aluminum oxide and an EVA flexible material, the provided diaphragm is more tightly attached to a positive electrode material and a negative electrode material, the thermal stability and the mechanical property of the diaphragm are enhanced, and the electrolyte wettability of the diaphragm is good.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a lithium battery composite separator and its preparation method. Background Technology

[0002] Lithium-ion battery separators are functional membrane materials with microporous structures, typically 8–40 μm thick. In the battery system, they separate the positive and negative electrodes, block electrons from passing through the circuit during charging and discharging, and allow lithium ions in the electrolyte to pass freely. They can selectively close the micropores when the battery is charging, discharging, or when the temperature rises to limit excessive current and prevent short circuits. The quality of their performance directly determines the overall performance of the battery.

[0003] Separator products have high performance requirements and high technical barriers. The performance of lithium-ion battery separators determines the key characteristics of lithium-ion batteries, such as capacity, cycle performance, and charge / discharge current density. Separators need to have appropriate thickness, ion permeability, pore size and porosity, as well as sufficient chemical stability, thermal stability, mechanical stability and safety.

[0004] Currently, the main materials used in the large-scale commercial production of lithium-ion battery separators are polyolefins, primarily including polypropylene (PP), polyethylene (PE), and composite materials of polypropylene (PP) and polyethylene (PE). Thinner and lighter separators are a significant trend in the industry. Thinner separators can increase the energy density of lithium batteries by accommodating more electrode material per unit volume or weight, thereby improving the battery's range. However, thinner separators suffer from reduced thermal stability, mechanical properties, and liquid absorption capacity, directly impacting battery safety and lifespan. Summary of the Invention

[0005] The purpose of this invention is to provide a lithium battery composite separator and its preparation method, which improves the overall performance of the separator and extends its service life.

[0006] The objective of this invention can be achieved through the following technical solutions: This invention provides a composite separator for lithium batteries. The composite membrane includes a base membrane and a coating layer coated on one or both sides of the base membrane; The coating slurry is obtained by graft copolymerization of EVA, polyamic acid and alumina. First, EVA is hydrolyzed to generate hydroxyl groups, and alumina is modified with aminosilane to introduce amino groups before graft copolymerization with polyamic acid.

[0007] EVA is an ethylene-vinyl acetate copolymer with excellent softness and toughness. It can maintain good performance at extremely low temperatures (-70℃). EVA material is not easily cracked or damaged by external environmental factors. It is an environmentally friendly material that is biodegradable and does not contain toxic substances.

[0008] Polyamic acid is a precursor for the preparation of polyimide. In the above slurry, the hydroxyl groups in hydrolyzed EVA and the carboxyl groups in polyamic acid undergo a condensation reaction. The amino groups in the aminosilane-modified alumina react with the anhydride groups in the polyamic acid to form amide bonds, thus forming a graft copolymer network.

[0009] Furthermore, the base film is one of PP film, PE film, or PP / PE composite film.

[0010] Furthermore, the thickness of the base film is 10-20 μm, and the porosity is 40-60%.

[0011] Furthermore, the thickness of the coating layer is 2-10 μm.

[0012] Furthermore, the preparation steps of the slurry for the coating layer are as follows: S1. Dissolve EVA resin in toluene by heating, add sodium hydroxide-ethanol solution, react at 70-80℃ for 5-10 min, cool to room temperature, add dilute hydrochloric acid to adjust to neutral, add anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product. S2. Add aminosilane to water, add acetic acid to adjust the pH to 3-5 to form a hydrolysate, disperse nano-alumina in an ethanol aqueous solution, add the hydrolysate, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3, EVA hydrolysis product (EVA-OH), modified Al2O3 (N -A EVA and polyamic acid (PAA) solution are dissolved in dimethylacetamide (DMAC) to form a reaction solution with a solid content of 10-30%. p-Toluenesulfonic acid is added as a catalyst, and the reaction is carried out at 60-80℃ for 4-7 hours to form EVA-PAA-A. A grafted network is then added, along with acetic anhydride and pyridine, to form a coating slurry.

[0013] In existing lithium-ion battery separator improvement technologies, boehmite and alumina are mainly used as inorganic materials, while polyvinylidene fluoride (PVDF) is mainly used as organic materials. Inorganic materials are primarily used to improve thermal stability, while PVDF mainly improves electrolyte wettability. The combination of inorganic and organic materials is usually achieved through simple physical blending. However, this method results in poor dispersion of the inorganic material, insufficient bonding with the organic material, and the possibility of detachment, leading to an uneven interface structure and affecting battery cycle performance. This invention introduces amino groups into alumina and utilizes graft copolymerization with polyamic acid to promote dispersion and achieve strong bonding.

[0014] To improve the temperature resistance of the diaphragm, existing technologies use polyimide directly as the base membrane. However, polyimide is expensive and not economically viable for practical products. Using polyimide as a coating layer is problematic because polyimide itself is brittle and has weak adhesion to polyolefin base membranes, and a simple polyimide coating layer is at risk of peeling off. In addition, polyimide lacks hydrophilic groups on its surface, resulting in poor liquid absorption. Mixing polyimide with inorganic materials (materials with good wettability) for coating results in poor compatibility, requiring blending with other polymers to improve compatibility. However, blending presents issues with dispersion uniformity.

[0015] This invention utilizes polyamic acid, a precursor of polyimide, and EVA hydrolysis products, along with modified alumina, for grafting. This avoids the difficulty of directly using polyimide as a grafting material, as polyimide has a relatively stable structure and low grafting activity, while polyamic acid has a looser molecular chain structure and higher grafting activity. EVA is hydrolyzed and alumina is modified before being grafted onto polyamic acid. EVA hydrolysis produces hydroxyl groups, and aminosilane-modified alumina introduces amino groups. Both react with carboxyl and anhydride groups respectively, avoiding competitive grafting and improving the grafting rate.

[0016] Furthermore, the VA content in the EVA is 10-20%. VA is a polar group that can improve the compatibility and adhesion of the material.

[0017] Furthermore, the mass-to-volume ratio of EVA resin to toluene is 10-30 g / L, the sodium hydroxide concentration in the sodium hydroxide-ethanol solution is 0.5-1 mol / L, and the mass ratio of sodium hydroxide to EVA is 0.08-0.2:1.

[0018] Furthermore, the concentration of the dilute hydrochloric acid is 0.1-0.3 mol / L.

[0019] Furthermore, the aminosilane is one of KH550 and KH792, and the mass ratio of aminosilane to water is 1-2:10.

[0020] Furthermore, the particle size of the nano-alumina is 10-50 nm, the mass ratio of ethanol to water in the ethanol aqueous solution is 1-2:1, and the mass-volume ratio of nano-alumina to the ethanol aqueous solution is 10-30 g / L.

[0021] Furthermore, the mass ratio of the aminosilane to nano-alumina is 1:3-7.

[0022] Furthermore, the polyamic acid solution has a solid content of 10-20% and is prepared from pyromellitic dianhydride and 4,4'-diaminodiphenyl ether.

[0023] Furthermore, the mass ratio of the EVA hydrolysis product, modified Al2O3, and polyamic acid is (3-5):(1-2):6.

[0024] Furthermore, the mass of the p-toluenesulfonic acid is 0.5-1% of the mass of the reaction solution.

[0025] Furthermore, the molar ratio of acetic anhydride to polyamic acid in the reaction solution is 2-5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution is 5-10:1.

[0026] This invention also provides a method for preparing a lithium battery composite separator, comprising the following steps: Step 1: After degassing, the slurry for the coating layer is applied to one or both sides of the base film; Step 2: After coating, treat at 40-60℃ for 12-24 hours to achieve imidization, converting polyamic acid into polyimide; Step 3: After step 2 is completed, place the membrane in a coagulation bath and let it stand. After coagulation into a film, wash it with deionized water to remove the solvent, and dry it at 50-60℃ to form a coating layer, thus obtaining the diaphragm.

[0027] Furthermore, the coagulation bath is prepared by mixing dimethylacetamide and water, with the mass fraction of dimethylacetamide being 20-40%.

[0028] The beneficial effects of this invention are: (1) The membrane coating of the present invention is obtained by graft copolymerization of EVA, polyamic acid and alumina. After chemical imidization treatment of polyamic acid, polyimide is formed, realizing multi-component molecular-level composite, avoiding the phase separation problem of traditional coatings, and improving the mechanical properties and thermal stability of the membrane.

[0029] (2) In the coating layer of the present invention, alumina and polyimide form a synergistic heat-resistant system. The high strength of polyimide and the high toughness of EVA combine to improve the mechanical properties of the separator. The polar groups in alumina and EVA can improve the electrolyte wettability of the separator, and the high viscoelasticity of EVA enhances the adhesion between the coating layer and the base film, which is beneficial to lithium ion migration.

[0030] (3) This invention achieves the combination of three materials, PI, EVA and alumina, through graft copolymerization, which solves the problem of high cost when using only PI coating. In addition, the low density of EVA can reduce the quality of the coating layer and reduce the impact on the energy density of the battery. Based on the combination of rigid materials such as PI and alumina and flexible materials such as EVA, the provided separator adheres more tightly to the positive and negative electrode materials, further ensuring battery life and safety. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] Example 1

[0033] Slurry for preparing the coating layer: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 5:2:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0034] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 10 μm, thus obtaining a diaphragm with a thickness of 30 μm.

[0035] Example 2

[0036] The only difference from Example 1 is that, in preparing the slurry for the coating layer, the mass ratio of EVA hydrolysis products, modified Al2O3, and polyamic acid in S3 is adjusted to 4:2:6. The steps for preparing the slurry for the coating layer are as follows: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:2:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0037] Example 3

[0038] The only difference from Example 1 is that, in preparing the slurry for the coating layer, the mass ratio of EVA hydrolysis products, modified Al2O3, and polyamic acid in S3 is adjusted to 3:2:6. The steps for preparing the slurry for the coating layer are as follows: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 3:2:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0039] Example 4

[0040] The only difference from Example 2 is that, in preparing the slurry for the coating layer, the mass ratio of EVA hydrolysis products, modified Al2O3, and polyamic acid in S3 is adjusted to 4:1.5:6. The steps for preparing the slurry for the coating layer are as follows: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:1.5:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0041] Example 5

[0042] The only difference from Example 2 is that, in preparing the slurry for the coating layer, the mass ratio of EVA hydrolysis products, modified Al2O3, and polyamic acid in S3 is adjusted to 4:1:6. The steps for preparing the slurry for the coating layer are as follows: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:1:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0043] Example 6

[0044] The only difference from Example 4 is that the thickness of the coating layer is adjusted to 4 μm.

[0045] Slurry for preparing the coating layer: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:1.5:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0046] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 4 μm, thus obtaining a diaphragm with a thickness of 24 μm.

[0047] Example 7

[0048] The only difference from Example 4 is that the thickness of the coating layer is adjusted to 6 μm.

[0049] Slurry for preparing the coating layer: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:1.5:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0050] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 6 μm, thus obtaining a diaphragm with a thickness of 26 μm.

[0051] Example 8

[0052] The only difference from Example 7 is that the base film material is changed to PE.

[0053] Slurry for preparing the coating layer: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA hydrolysis products, modified Al2O3 and polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20%. The mass ratio of EVA hydrolysis products, modified Al2O3 and polyamic acid was 4:1.5:6. p-Toluenesulfonic acid was added as a catalyst, with the mass of p-toluenesulfonic acid being 0.5% of the mass of the reaction solution. The reaction was carried out at 70℃ for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0054] Preparation of composite separators for lithium batteries: Step 1: Select a PE membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PE membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 6 μm, thus obtaining a diaphragm with a thickness of 26 μm.

[0055] Comparative Example 1

[0056] The only difference from Example 1 is that the three materials in the coating slurry are directly mixed.

[0057] Slurry for preparing the coating layer: A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. EVA resin (VA content of 18%), nano-alumina (particle size of 30nm) and the polyamic acid solution were added to dimethylacetamide solvent and mixed to form a mixture with a solid content of 20%. Acetic anhydride and pyridine were then added. The molar ratio of acetic anhydride to polyamic acid in the mixture was 5:1, and the molar ratio of pyridine to polyamic acid in the mixture was 10:1 to form a slurry for coating.

[0058] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55℃ to form a coating layer with a total thickness of 10 μm, thus obtaining a diaphragm with a thickness of 30 μm.

[0059] Comparative Example 2

[0060] The only difference from Example 1 is that no EVA hydrolysis products are added to the slurry of the coating layer.

[0061] Slurry for preparing the coating layer: S1. Add 10g of KH550 to 100mL of water, add acetic acid to adjust the pH to 3 to form a hydrolysate, prepare nano-alumina with a particle size of 30nm, prepare an ethanol-water solution with a mass ratio of 1:1, add 50g of nano-alumina to 5L of ethanol-water solution, ultrasonically disperse for 30min, add the above hydrolysate after dispersion, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S2. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. Modified Al2O3 and the polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20% and a mass ratio of modified Al2O3 to polyamic acid of 2:6. p-Toluenesulfonic acid was added as a catalyst, with a mass of 0.5% of the mass of the reaction solution. The reaction was carried out at 70°C for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0062] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 10 μm, thus obtaining a diaphragm with a thickness of 30 μm.

[0063] Comparative Example 3

[0064] The only difference from Example 1 is that no modified Al2O3 is added to the slurry of the coating layer.

[0065] Slurry for preparing the coating layer: S1. Dissolve 25g of EVA resin (VA content 18%) in 1L of toluene at a constant temperature of 80℃, add 1.25L of sodium hydroxide-ethanol solution (0.5mol / L), react at 70℃ for 10min, cool to room temperature, add 0.1mol / L dilute hydrochloric acid to adjust to neutral to pH 7.0, add 1L of anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product; S2. A polyamic acid solution with a solid content of 20% was prepared by polycondensation reaction using pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as raw materials. The EVA hydrolysis product and the polyamic acid solution were added to dimethylacetamide solvent to form a reaction solution with a solid content of 20% and a mass ratio of EVA hydrolysis product to polyamic acid of 5:6. p-Toluenesulfonic acid was added as a catalyst, with a mass of 0.5% of the mass of the reaction solution. The reaction was carried out at 70°C for 6 hours. After the reaction was completed, acetic anhydride and pyridine were added. The molar ratio of acetic anhydride to polyamic acid in the reaction solution was 5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution was 10:1 to form a slurry for coating.

[0066] Preparation of composite separators for lithium batteries: Step 1: Select a PP membrane with a thickness of 20μm and a porosity of 42%. Degas the slurry of the coating layer prepared above and coat it on both sides of the PP membrane. Step 2: After coating, treat at 50℃ for 24 hours; Step 3: After step 2 is completed, place the membrane in a coagulation bath (dimethylacetamide mass: water mass = 3:7) and let it stand for 12 hours. After coagulation and film formation, wash with deionized water to remove the solvent, and dry at 55°C to form a coating layer with a total thickness of 10 μm, thus obtaining a diaphragm with a thickness of 30 μm.

[0067] Comparative Example 4

[0068] The only difference from Example 1 is that this comparative example provides a lithium battery separator, a PP film with a thickness of 20 μm and a porosity of 42%, without any coating treatment.

[0069] Performance tests were conducted on Examples 1-8 and Comparative Examples 1-4, and the results are shown in Table 1: The heat shrinkage rate and puncture strength are tested in accordance with the standard GB / T36363-2018 Polyolefin separator for lithium-ion batteries, and the peel strength of the coating and base film is tested using a mechanical testing machine.

[0070] Liquid absorption rate test: The prepared diaphragm was cut into 2cm round pieces, dried in a vacuum drying oven and weighed, recorded as M0. The round pieces were immersed in electrolyte (LiPF6 solute, mass fraction of solute is 20%, solvent is EC+DMC+DEC, mass ratio is 4:2:4) for 12h. The residual liquid on the surface was absorbed by filter paper and weighed, recorded as M1. Liquid absorption rate = (M1-M0) / M0×100%.

[0071] Table 1

[0072] As can be seen from Table 1, the lithium battery composite separator prepared according to the present invention has high thermal stability, strong mechanical properties, and good electrolyte wettability, and its overall performance is significantly better than that of the comparative separator.

[0073] Comparative Example 1 is a coating slurry formed by directly mixing the coating materials without graft copolymerization. The membrane prepared with this coating slurry has certain improvements in mechanical properties and wettability compared with the conventional base film of Comparative Example 4, based on the characteristics of the coating materials themselves. However, due to the poor bonding force between the materials, there is discontinuity between the materials, and the thermal stability of the EVA material itself is also poor. As a coating layer, the actual thermal stability is not as good as that of the conventional base film.

[0074] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0075] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A lithium battery composite separator, characterized in that, The composite membrane includes a base membrane and a coating layer coated on one or both sides of the base membrane; The coating slurry is obtained by graft copolymerization of EVA, polyamic acid and alumina. First, EVA is hydrolyzed to generate hydroxyl groups, and alumina is modified with aminosilane to introduce amino groups before graft copolymerization with polyamic acid.

2. The lithium battery composite separator according to claim 1, characterized in that, The base film is one of PP film, PE film, or PP / PE composite film.

3. The lithium battery composite separator according to claim 1, characterized in that, The thickness of the base film is 10-20 μm, and the porosity is 40-60%. The thickness of the coating layer is 2-10 μm.

4. The lithium battery composite separator according to claim 1, characterized in that, The preparation steps for the slurry of the coating layer are as follows: S1. Dissolve EVA resin in toluene by heating, add sodium hydroxide-ethanol solution, react at 70-80℃ for 5-10 min, cool to room temperature, add dilute hydrochloric acid to adjust to neutral, add anhydrous ethanol to precipitate, filter under reduced pressure, wash with water, filter and dry to obtain EVA hydrolysis product. S2. Add aminosilane to water, add acetic acid to adjust the pH to 3-5 to form a hydrolysate, disperse nano-alumina in an ethanol aqueous solution, add the hydrolysate, reflux at 80℃, centrifuge and dry to obtain modified Al2O3. S3. Dissolve EVA hydrolysis products, modified Al2O3 and polyamic acid solution in dimethylacetamide to form a reaction solution with a solid content of 10-30%. Add p-toluenesulfonic acid as a catalyst and react at 60-80℃ for 4-7 hours. Then add acetic anhydride and pyridine to form a coating slurry.

5. A lithium battery composite separator according to claim 4, characterized in that, The VA content of the EVA resin is 10-20%; The mass-to-volume ratio of EVA resin to toluene is 10-30 g / L, the sodium hydroxide concentration in the sodium hydroxide-ethanol solution is 0.5-1 mol / L, and the mass ratio of sodium hydroxide to EVA is 0.08-0.2:

1.

6. A lithium battery composite separator according to claim 4, characterized in that, The aminosilane is one of KH550 and KH792, and the mass ratio of aminosilane to water is 1-2:

10.

7. A lithium battery composite separator according to claim 4, characterized in that, The mass ratio of aminosilane to nano-alumina is 1:3-7.

8. A lithium battery composite separator according to claim 4, characterized in that, The mass ratio of the EVA hydrolysis product, modified Al2O3, and polyamic acid is (3-5):(1-2):6; The molar ratio of acetic anhydride to polyamic acid in the reaction solution is 2-5:1, and the molar ratio of pyridine to polyamic acid in the reaction solution is 5-10:

1.

9. A method for preparing a lithium battery composite separator as described in any one of claims 1-8, characterized in that, Includes the following steps: Step 1: After degassing, the slurry for the coating layer is applied to one or both sides of the base film; Step 2: After coating, treat at 40-60℃ for 12-24 hours to achieve imidization, converting polyamic acid into polyimide; Step 3: After step 2 is completed, place the membrane in a coagulation bath and let it stand. After coagulation into a film, wash it with deionized water to remove the solvent, and dry it at 50-60℃ to form a coating layer, thus obtaining the diaphragm.

10. The method for preparing a lithium battery composite separator according to claim 9, characterized in that, The coagulation bath is prepared by mixing dimethylacetamide and water, with the mass fraction of dimethylacetamide being 20-40%.

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