Modified polyamide-imide for coating lithium battery diaphragm as well as preparation method and application of modified polyamide-imide

Abstract

The invention discloses modified polyamide-imide resin for coating a lithium battery diaphragm as well as a preparation method and application thereof.According to the modified polyamide-imide resin for coating the lithium battery diaphragm and the preparation method and application of the modified polyamide-imide resin, by introducing PI resin prepared in advance, the thermal shrinkage rate of the battery diaphragm can be obviously improved, the thermal shrinkage rate is maintained to be 10% or below, and the modified polyamide-imide resin can form a modified polyamide-imide resin with proper viscosity and high thermal conductivity. The polyamide-imide resin is suitable for being coated on the lithium battery diaphragm. And the air permeability of the battery diaphragm is not influenced while the heat shrinkage resistance of the battery diaphragm is improved. And the lithium ion battery diaphragm has excellent adhesiveness and can be stably applied to a lithium battery.

Description

Technical Field

[0001] This invention relates to the field of battery separators, specifically to a modified polyamide-imide for coating lithium battery separators, its preparation method, and its application. Background Technology

[0002] The separator is a crucial component of lithium-ion batteries, directly impacting their lifespan and safety. A separator is a battery assembly with a porous structure that separates the positive and negative electrodes; the porosity of the separator directly affects the battery's chemical performance. Separators experience thermal shrinkage at high temperatures, affecting battery lifespan. Larger separator pores allow for greater electron flux, but these pores also reduce the separator's thermal shrinkage rate, impacting battery safety. Therefore, developing a method to suppress separator thermal shrinkage without reducing its ion permeability is essential.

[0003] Chinese invention patent application CN117543161A discloses a high flame-retardant separator for lithium-ion batteries and its preparation method. It employs an alkaline etching-acidification method to carboxylate-modify the surface of polyimide microspheres, resulting in excellent lithium affinity. This induces uniform nucleation of lithium ions on the separator surface, improving lithium ion transport rate and stability, thereby reducing the internal resistance of the separator. However, it does not solve the problem of thermal shrinkage of the separator. CN115411451B discloses an impact-resistant lithium battery composite membrane and its preparation method. A reinforcing filler is prepared by forming boehmite on the surface of epoxy nano-silica. The reinforcing filler reacts with amino groups in the reinforcing spinning solution, causing the polyamic acid structure in the molecular chain to form polyimide, which has excellent mechanical properties. This protects the separator from lithium dendrite puncture after multiple charge-discharge cycles and ensures that the separator will not break when the battery is subjected to external forces. However, the problem of thermal shrinkage of the separator is still not solved. Summary of the Invention

[0004] In order to develop a method for suppressing thermal shrinkage of the separator without reducing the ion permeability of the battery separator, the first aspect of the present invention provides a modified polyamide-imide resin for coating lithium battery separators, the raw materials for preparation including a polyimide precursor, a polymer PI resin and ceramic powder; the raw materials for preparing the polyimide precursor include diisocyanate, benzoic anhydride and solvent 1, wherein the molar ratio of diisocyanate and benzoic anhydride is (1-1.15):1.

[0005] In one embodiment, the raw materials for preparing the polyimide precursor include diisocyanate and benzoic anhydride, with a molar ratio of (1.025-1.03):1.

[0006] In one embodiment, the raw materials for preparing the polyimide precursor include diisocyanate and benzoic anhydride in a molar ratio of 1.03:1.

[0007] In one embodiment, the benzoic anhydride includes at least one of trimellitic anhydride, biphenyltetracarboxylic dianhydride, or pyromellitic dianhydride.

[0008] In one embodiment, the raw materials for preparing the polymer PI resin include benzoic anhydride, an amino and phenyl compound, and solvent 2, wherein the molar ratio of benzoic anhydride to the amino and phenyl compound is (0.95-1.05):1.

[0009] In one embodiment, the molar ratio of the benzoic anhydride to the compound containing amino and phenyl groups is 0.955:1.

[0010] In one embodiment, the benzoic anhydride includes at least one of pyromellitic dianhydride or biphenyl dianhydride.

[0011] As one embodiment, the amino and phenyl-containing compound includes at least one of diaminodiphenyl ether or p-phenylenediamine.

[0012] In one embodiment, solvent 1 and solvent 2 each include at least one of N-methylpyrrolidone, dimethyl sulfoxide, or dimethylacetamide.

[0013] In one embodiment, both solvent 1 and solvent 2 are N-methylpyrrolidone.

[0014] In one embodiment, the weight ratio of the polyimide precursor, polymer PI resin, and ceramic powder is 100:(10-30):(40-80).

[0015] In one embodiment, the weight ratio of the polyimide precursor, polymeric PI resin, and ceramic powder is 100:20:60.

[0016] A second aspect of the present invention provides a method for preparing a modified polyamide-imide resin for coating lithium battery separators, comprising the following steps: Preparation of polyimide precursors; Preparation of high molecular weight PI resin; The prepared polymer PI resin was slowly added to the polyimide precursor under stirring, and the mixture was stirred for 1-2 hours to obtain the modified polyamide-imide resin. Ceramic powder is added to the modified polyamide-imide resin under stirring, and the mixture is stirred for 1-2 hours to obtain the modified polyamide-imide resin for lithium battery separator coating.

[0017] As one embodiment, the method for preparing the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride are mixed with 40 wt% solvent 1, heated to 70-90℃, and reacted for 1-3 hours; Continue heating to 90-110℃ and react for 1-3 hours. Continue heating to 120-180℃ and react for 2-5 hours. Then add 20wt% of solvent 1. After reacting for 2-5 hours, add 20 wt% of solvent 1 and continue reacting for 7-24 hours. Then add the remaining solvent 1 and react for 20-25 hours to obtain the polyimide precursor.

[0018] As one embodiment, the preparation method of the polymer PI resin includes the following steps: The amino and phenyl compounds were mixed with 75 wt% solvent 2 and stirred for 10-20 min. Then, benzoic anhydride was added in four portions, with an interval of 20-40 min between each addition. After the addition was complete, the remaining solvent 2 was added and stirred until homogeneous to obtain the high molecular weight PI resin.

[0019] A third aspect of the present invention provides an application of a modified polyamide-imide resin for coating lithium battery separators, applied to lithium battery separators.

[0020] As one implementation method, the application method includes the following steps: The modified polyamide-imide resin for lithium battery separator coating according to any one of claims 2-4 is uniformly coated on one side of the lithium battery separator, placed in an aqueous solution of solvent 1 and left to stand for 1-3 minutes, transferred to pure water and left to stand for 5-15 minutes, and then baked at 70-90°C for 10-20 minutes to complete the process.

[0021] In one embodiment, the aqueous solution of solvent 1 has a mass concentration of 20-40%.

[0022] In one embodiment, the aqueous solution of solvent 1 has a mass concentration of 30%.

[0023] As one embodiment, the coating thickness of the modified polyamide-imide resin on the lithium battery separator is 1-3 μm.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) The modified polyamide-imide resin for lithium battery separator coating described in this invention can significantly improve the thermal shrinkage rate of the battery separator by introducing a pre-prepared PI resin, so that the thermal shrinkage rate is kept below 10%.

[0025] (2) The modified polyamide-imide resin for lithium battery separator coating described in this invention can form a modified polyamide-imide resin with suitable viscosity and suitable for coating onto lithium battery separator by controlling the molar ratio of diisocyanate and benzoic anhydride in the polyimide precursor to (1-1.15):1.

[0026] (3) The modified polyamide-imide resin for lithium battery separator coating described in this invention, by controlling the molar ratio of benzoic anhydride to amino and phenyl compounds in the PI resin to be (0.90-1.10):1, forms a PI resin with a suitable molecular weight, does not form micelles, and is easy to coat and use.

[0027] (4) The application method of the modified polyamide-imide resin for lithium battery separator coating described in this invention, by controlling the mass concentration of the aqueous solution of solvent 1 to 30% during the coating process, can improve the air permeability of the battery separator without affecting the thermal shrinkage rate of the battery separator.

[0028] (5) The method for preparing the modified polyamide-imide resin for lithium battery separator coating of the present invention involves first heating to 80°C through a specific heating program, and finally reacting at 130°C. The resulting modified polyamide-imide resin has a peel strength greater than 0.45 N / mm on the battery separator and excellent adhesion to the battery separator.

[0029] (6) The method for preparing modified polyamide-imide resin for lithium battery separator coating described in this invention improves the adhesion between polyamide-imide resin and lithium battery separator by adding solvent in steps, and can be stably used in lithium batteries.

[0030] (7) The modified polyamide-imide resin for lithium battery separator coating described in this invention can be coated on the lithium battery separator to enhance the puncture resistance of the lithium battery separator and reduce the production cost of the lithium battery separator. Detailed Implementation

[0031] Example 1 A modified polyamide-imide resin for coating lithium battery separators is prepared from polyimide precursors, high molecular weight PI resins, and ceramic powders. The polyimide precursors are prepared from diisocyanate, benzoic anhydride, and solvent 1, wherein the molar ratio of diisocyanate to benzoic anhydride is 1.03:1.

[0032] The benzoic anhydride is trimellitic anhydride, and the diisocyanate is diphenylmethane diisocyanate.

[0033] The raw materials for preparing the polymer PI resin include benzoic anhydride, a compound containing amino and phenyl groups, and solvent 2, wherein the molar ratio of benzoic anhydride to the compound containing amino and phenyl groups is 0.955:1.

[0034] The benzoic anhydride is biphenyltetracarboxylic dianhydride, and the compound containing amino and phenyl groups is diaminodiphenyl ether.

[0035] Both solvent 1 and solvent 2 are N-methylpyrrolidone. The mass fraction of solvent 1 in the polyimide precursor is 80%, and the mass fraction of solvent 2 in the polymer PI resin is 80%.

[0036] The polyimide precursor, polymer PI resin, and ceramic powder are in a weight ratio of 100:20:60.

[0037] The ceramic powder was purchased from Yishitong New Energy Technology Co., Ltd.

[0038] A method for preparing a modified polyamide-imide resin for lithium battery separator coating includes the following steps: Preparation of polyimide precursors; Preparation of high molecular weight PI resin; The prepared polymer PI resin was slowly added to the polyimide precursor under stirring, and the mixture was stirred for 1 hour to obtain the modified polyamide-imide resin. Ceramic powder was added to the modified polyamide-imide resin under stirring, and the mixture was stirred for 1 hour to obtain the modified polyamide-imide resin for lithium battery separator coating.

[0039] The method for preparing the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 80 °C, and reacted for 2 h. Continue heating to 100℃ and react for 2 hours. Continue heating to 130℃ and react for 4 hours. Then add 20 wt% of solvent 1. After reacting for another 4 hours, 20 wt% of solvent 1 was added, and after reacting for another 8 hours, the remaining solvent 1 was added. The reaction was continued for 24 hours to obtain the polyimide precursor.

[0040] The preparation method of the polymer PI resin includes the following steps: The amino and phenyl compounds were mixed with 75 wt% solvent 2 and stirred for 15 min. Then, benzoic anhydride was added in four portions, with an interval of 30 min between each addition. After the addition was completed, the remaining solvent 2 was added and stirred until homogeneous to obtain the high molecular weight PI resin.

[0041] An application of a modified polyamide-imide resin for coating lithium battery separators, the application method comprising the following steps: The modified polyamide-imide resin for lithium battery separator coating is uniformly coated on one side of the lithium battery separator, placed in an aqueous solution of N-methylpyrrolidone and left to stand for 2 minutes, then transferred to pure water and left to stand for 10 minutes, and finally baked at 80°C for 15 minutes to complete the process.

[0042] The aqueous solution of N-methylpyrrolidone has a mass concentration of 30%.

[0043] The lithium battery separator is a PP separator, purchased from Housheng New Energy Technology Co., Ltd.

[0044] Example 2 A modified polyamide-imide resin for coating lithium battery separators, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the benzoic anhydride is pyromellitic dianhydride.

[0045] Example 3 A modified polyamide-imide resin for coating lithium battery separators, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the compound containing amino and phenyl groups is p-phenylenediamine.

[0046] Example 4 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of benzoic anhydride to compounds containing amino and phenyl groups in the polymer PI resin is 1.025:1.

[0047] Example 5 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of benzoic anhydride to compounds containing amino and phenyl groups in the polymer PI resin is 1:1.

[0048] Example 6 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of benzoic anhydride to compounds containing amino and phenyl groups in the polymer PI resin is 0.90:1.

[0049] Example 7 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of diisocyanate and benzoic anhydride in the polyimide precursor is 1.025:1.

[0050] Example 8 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference is that the raw materials include polyimide precursor and ceramic powder, with a weight ratio of 100:60.

[0051] A method for preparing polyimide for coating lithium battery separators includes the following steps: Preparation of polyimide precursors; Ceramic powder was added to the polyimide precursor under stirring, and the mixture was stirred for 1 hour to obtain a modified polyamide-imide resin for lithium battery separator coating.

[0052] Example 9 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the modified polyamide-imide for lithium battery separator coating includes the following steps: Preparation of polyimide precursors; The amino and phenyl compounds, benzoic anhydride and solvent 2 were slowly added to the polyimide precursor under stirring, and the mixture was stirred for 1 hour to obtain the modified polyamide-imide resin. Ceramic powder was added to the modified polyamide-imide resin under stirring, and the mixture was stirred for 1 hour to obtain the modified polyamide-imide resin for lithium battery separator coating.

[0053] The method for preparing the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 80 °C, and reacted for 2 h. Continue heating to 100℃ and react for 2 hours. Continue heating to 130℃ and react for 4 hours. Then add 20 wt% of solvent 1. After reacting for another 4 hours, 20 wt% of solvent 1 was added, and after reacting for another 8 hours, the remaining solvent 1 was added. The reaction was continued for 24 hours to obtain the polyimide precursor.

[0054] Example 10 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the modified polyamide-imide resin for lithium battery separator coating includes the following steps: Preparation of high molecular weight PI resin; Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 80 °C, and reacted for 2 h. Continue heating to 100℃, react for 2 hours, then add high molecular weight PI resin, continue heating to 130℃, react for 4 hours, then add 20wt% solvent 1; After reacting for another 4 hours, 20 wt% of solvent 1 was added, and after reacting for another 8 hours, the remaining solvent 1 was added. The reaction was continued for 24 hours to obtain the modified polyamide-imide resin. Ceramic powder was added to the modified polyamide-imide resin under stirring, and the mixture was stirred for 1 hour to obtain modified polyamide-imide for lithium battery separator coating.

[0055] The preparation method of the polymer PI resin includes the following steps: The amino and phenyl compounds were mixed with 75 wt% solvent 2 and stirred for 15 min. Then, benzoic anhydride was added in four portions, with an interval of 30 min between each addition. After the addition was completed, the remaining solvent 2 was added and stirred until homogeneous to obtain the high molecular weight PI resin.

[0056] Example 11 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference is that the weight ratio of the polyimide precursor, the polymer PI resin and the ceramic powder is 100:10:60.

[0057] Example 12 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference is that the weight ratio of the polyimide precursor, the polymer PI resin and the ceramic powder is 100:30:60.

[0058] Example 13 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 80 °C, and reacted for 2 h. Continue heating to 100℃ and react for 2 hours. Continue heating to 130℃ and react for 4 hours. Then, heat to 150℃ and add 20 wt% solvent 1. After reacting for another 4 hours, 20 wt% of solvent 1 was added, and after reacting for another 8 hours, the remaining solvent 1 was added. The reaction was continued for 24 hours to obtain the polyimide precursor.

[0059] Example 14 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 100 °C, and reacted for 4 h. Continue heating to 130℃, and after reacting for 4 hours, add 20wt% of solvent 1; After reacting for another 4 hours, 20 wt% of solvent 1 was added, and after reacting for another 8 hours, the remaining solvent 1 was added. The reaction was continued for 24 hours to obtain the polyimide precursor.

[0060] Example 15 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with solvent 1, and the mixture was heated to 80°C and reacted for 2 hours. The temperature was further increased to 100℃ and reacted for 2 hours. The temperature was then further increased to 130℃ and reacted for 40 hours to obtain the polyimide precursor.

[0061] Example 16 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference is that the weight ratio of the polyimide precursor, the polymer PI resin and the ceramic powder is 100:20:40.

[0062] Example 17 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference is that the weight ratio of the polyimide precursor, the polymer PI resin and the ceramic powder is 100:20:80.

[0063] Example 18 A modified polyamide-imide resin for coating lithium battery separators, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the mass concentration of the aqueous solution of N-methylpyrrolidone is 40%.

[0064] Example 19 A modified polyamide-imide resin for coating lithium battery separators, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the mass concentration of the aqueous solution of N-methylpyrrolidone is 20%.

[0065] Example 20 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with 40 wt% solvent 1, heated to 80 °C, and reacted for 2 h. Then the remaining solvent 1 was added all at once, the temperature was raised to 180℃, and the reaction was carried out for 12 hours to obtain the polyimide precursor.

[0066] Example 21 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with solvent 1, heated to 150°C, and reacted for 24 h to obtain a polyimide precursor.

[0067] Example 22 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with solvent 1, heated to 110°C, and reacted for 48 h to obtain a polyimide precursor.

[0068] Example 23 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of diisocyanate and benzoic anhydride in the polyimide precursor is 1.1:1.

[0069] Example 24 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of diisocyanate and benzoic anhydride in the polyimide precursor is 1.15:1.

[0070] Example 25 A modified polyamide-imide resin for lithium battery separator coating, its preparation method and application, the specific implementation method is the same as in Example 1, the difference being that the molar ratio of benzoic anhydride to compounds containing amino and phenyl groups in the polymer PI resin is 1.1:1.

[0071] Comparative Example 1 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with solvent 1, heated to 80°C, and reacted for 48 h to obtain a polyimide precursor.

[0072] Comparative Example 2 A modified polyamide-imide resin for lithium battery separator coating, its preparation method, and its application are disclosed. The specific implementation method is the same as in Example 1, except that the preparation method of the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride were mixed with solvent 1, heated to 50°C, and reacted for 60 h to obtain a polyimide precursor.

[0073] Performance testing 1. The coating thickness of the modified polyamide-imide resin prepared in the examples and comparative examples on lithium battery separators was tested using a micrometer.

[0074] 2. Viscosity: The viscosity of the modified polyamide-imide resins prepared in the examples and comparative examples was tested according to GB / T 40280-2021.

[0075] 3. Increment in air permeability: The increment in air permeability of the modified polyamide-imide resins prepared in the examples and comparative examples was tested according to GB / T 36363-2018.

[0076] 4. Heat shrinkage rate: The heat shrinkage rate of the modified polyamide-imide resin prepared in the examples and comparative examples was tested at 130℃ for 1h using GB / T 36363-2018.

[0077] 5. Peel strength: The peel strength of the modified polyamide-imide resins prepared in the examples and comparative examples was tested according to GB / T 2792-1998.

[0078] 6. Areal density: The areal density of the modified polyamide-imide resin prepared in the test examples and comparative examples on the surface of the battery separator. Coating areal density (g / m²) 2 = Coating weight (g) / Coating area (m²) 2 ).

[0079] The test results are shown in Table 1.

[0080] Table 1

Claims

1. A modified polyamide-imide resin for coating lithium battery separators, characterized in that, The raw materials for preparation include polyimide precursor, high molecular weight PI resin and ceramic powder; the raw materials for preparing the polyimide precursor include diisocyanate, benzoic anhydride and solvent 1, wherein the molar ratio of diisocyanate and benzoic anhydride is (1-1.15):

1.

2. The modified polyamide-imide resin for lithium battery separator coating according to claim 1, characterized in that, The raw materials for preparing the polymer PI resin include benzoic anhydride, a compound containing amino and phenyl groups, and solvent 2, wherein the molar ratio of benzoic anhydride to the compound containing amino and phenyl groups is (0.90-1.10):

1.

3. The modified polyamide-imide resin for lithium battery separator coating according to claim 2, characterized in that, Solvent 1 and solvent 2 are both at least one of N-methylpyrrolidone, dimethyl sulfoxide, or dimethylacetamide.

4. The modified polyamide-imide resin for lithium battery separator coating according to claim 1, characterized in that, The weight ratio of the polyimide precursor, polymer PI resin and ceramic powder is 100:(10-30):(40-80).

5. A method for preparing a modified polyamide-imide resin for coating lithium battery separators according to any one of claims 2-4, characterized in that, Includes the following steps: Preparation of polyimide precursors; Preparation of high molecular weight PI resin; The prepared polymer PI resin was slowly added to the polyimide precursor under stirring, and the mixture was stirred for 1-2 hours to obtain the modified polyamide-imide resin. Ceramic powder is added to the modified polyamide-imide resin under stirring, and the mixture is stirred for 1-2 hours to obtain the modified polyamide-imide resin for lithium battery separator coating.

6. The method for preparing the modified polyamide-imide resin for lithium battery separator coating according to claim 5, characterized in that, The method for preparing the polyimide precursor includes the following steps: Diisocyanate and benzoic anhydride are mixed with 40 wt% solvent 1, heated to 70-90℃, and reacted for 1-3 hours; Continue heating to 90-110℃ and react for 1-3 hours. Continue heating to 120-180℃ and react for 2-5 hours. Then add 20wt% of solvent 1. After reacting for 2-5 hours, add 20 wt% of solvent 1 and continue reacting for 7-24 hours. Then add the remaining solvent 1 and react for 20-25 hours to obtain the polyimide precursor.

7. The method for preparing the modified polyamide-imide resin for lithium battery separator coating according to claim 5, characterized in that, The preparation method of the polymer PI resin includes the following steps: The amino and phenyl compounds were mixed with 75 wt% solvent 2 and stirred for 10-20 min. Then, benzoic anhydride was added in four portions, with an interval of 20-40 min between each addition. After the addition was complete, the remaining solvent 2 was added and stirred until homogeneous to obtain the high molecular weight PI resin.

8. An application of the modified polyamide-imide resin for coating lithium battery separators according to any one of claims 2-4, characterized in that, It is used in lithium battery separators.

9. The application of the modified polyamide-imide resin for lithium battery separator coating according to claim 8, characterized in that, The application method includes the following steps: The modified polyamide-imide resin for lithium battery separator coating according to any one of claims 2-4 is uniformly coated on one side of the lithium battery separator, placed in an aqueous solution of solvent 1 or solvent 2 and left to stand for 1-3 minutes, transferred to pure water and left to stand for 5-15 minutes, and then baked at 70-90°C for 10-20 minutes to complete the process.

10. The application of the modified polyamide-imide resin for lithium battery separator coating according to claim 9, characterized in that, The aqueous solution of solvent 1 or solvent 2 has a mass concentration of 20-40%.

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