A method for preparing a polyimide-based composite diaphragm for lithium batteries

Abstract

The present invention discloses a method for preparing a polyimide-based composite separator for lithium batteries, relating to the technical field of lithium-ion batteries. The method comprises preparing a polyimide matrix film, coating the polyimide matrix film with an inorganic nanoparticle solution to form a composite separator, and heat-treating the composite separator to improve its thermal stability and electrolyte wettability. This method is simple and easy to implement, and the resulting composite separator exhibits excellent mechanical strength, thermal stability, and electrolyte wettability, making it suitable for high-performance lithium batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing a polyimide-based composite diaphragm for lithium batteries. Background Art

[0002] Lithium-ion batteries are widely used in electronic devices, electric vehicles, and other fields due to their high energy density and long cycle life. As a crucial component of lithium-ion batteries, the separator primarily serves to separate the positive and negative electrodes and prevent short circuits. Traditional polyolefin separators, due to their low thermal stability and electrolyte wettability, have limited the performance of lithium batteries. In recent years, polyimide separators have become a research hotspot due to their excellent thermal stability and mechanical properties.

[0003] A method for preparing a hierarchically porous polyimide lithium battery separator (Chinese patent application number: CN201910509691.0) utilizes electrospinning and template methods to prepare a porogen / polyimide composite film, followed by removal of the porogen to yield a pure polyimide porous membrane. The electrospinning process utilizes controllable parameters, enabling the production of a suitable film. The resulting polyimide battery separator exhibits a hierarchical porous structure (mesopores and macropores), with the mesopores being generated by template etching and the macropores by electrospinning.

[0004] A high-temperature-resistant lithium battery separator and its production method (Chinese patent application number: CN201410128928.8) is based on a polyimide film with uniformly distributed micropores. The production method involves irradiating the polyimide film with heavy ions and etching the irradiated polyimide film with a sodium hypochlorite solution.

[0005] Polyimide lithium battery separator and preparation method thereof (Chinese patent application number: CN201711443141.0) The materials for preparing the polyimide lithium battery separator include a low-boiling point solvent, a polyimide resin, and a pore-forming agent whose reaction products at high temperature are all gas. The polyimide resin is synthesized from dianhydride and diamine, and the molar ratio of dianhydride to diamine is 0.98:1 to 1.2:1; based on the total mass of the polyimide resin and the low-boiling point solvent, the content of the low-boiling point solvent is 75% to 92%, and the content of the polyimide resin is 8% to 25%; based on the total mass of the polyimide resin, the content of the pore-forming agent is 30% to 60%. Compared with the existing polyimide separator and its preparation method, no extra steps are required to remove the pore-forming agent.

[0006] However, some performance improvements of single polyimide separators remain. For example, the separator's electrolyte wettability is crucial for ion conduction during battery charge and discharge. Existing technologies have shown relatively poor electrolyte wettability for single polyimide separators, which compromises overall battery performance. Therefore, a method is urgently needed to further improve the electrolyte wettability of polyimide separators while maintaining their inherent excellent performance. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the present invention proposes a method for preparing a polyimide-based composite membrane for lithium batteries, which comprises preparing a polyimide matrix film, coating an inorganic nanoparticle solution on the polyimide matrix film to form a composite membrane, and heat-treating the composite membrane to improve its thermal stability and electrolyte wettability.

[0008] In order to achieve the aforementioned invention objectives, this method proposes the following solutions:

[0009] A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized by comprising the following steps:

[0010] Step 1: Preparation of polyimide matrix film

[0011] Pyromellitic anhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine, CAS 2073-31-6, and octa(aminophenyl)-T8-silsesquioxane, CAS 518359-82-5, are added in a molar ratio to 200-400 parts of N-methylpyrrolidone (NMP) according to parts by mass, and stirred uniformly to form a uniform slurry; then the temperature is increased to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film;

[0012] Step 2: Prepare coating slurry

[0013] Weigh 100-160 parts by mass of epoxy nano-boron nitride and disperse them in 1050-1450 parts of organic solvent. Add 4-8 parts of polyvinyl pyrrolidone (PVP) and 1-3 parts of sodium ethoxide, and ultrasonically disperse for 20-40 minutes to obtain a uniform coating slurry.

[0014] Step 3: Preparation of composite diaphragm

[0015] The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature is controlled at 60-80°C during the coating process. The coated film is placed in an oven to dry for 0.5-1h, then heat treated for 1-3h, and cooled to room temperature to obtain a polyimide-based composite diaphragm.

[0016] Furthermore, in the step 1, the molar ratio of pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (molecular weight 182) and octa(aminophenyl)-T8-silsesquioxane (molecular weight 1153) is 1:1.3-1.5:0.01-0.05:0.0003-0.002.

[0017] Furthermore, in the step 1, the polymerization reaction temperature is 65-75° C., and the reaction time is 16-24 h.

[0018] Furthermore, the film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 5-10 cm / min, and the cast film is thermally imidized at 280-320° C. for 1-3 hours.

[0019] Furthermore, the preparation method of epoxy nano boron nitride in step 2 is:

[0020] Ratio: 3-glycidyloxypropyltrimethoxysilane (GPTMS): nano-boron nitride: acetic acid: ethanol, the mass ratio is 4-8:100-130:2-5:1000-1500;

[0021] Temperature / time: The reaction is carried out at 25-40°C and stirred continuously for 20-30 hours;

[0022] Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent;

[0023] Post-treatment: filtering, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 60-70° C. for 10-15 hours to obtain epoxy nano-boron nitride.

[0024] Furthermore, the organic solvent in step 2 is selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc).

[0025] Furthermore, in the step 2, the amount of polyvinylpyrrolidone (PVP) used is 1-2 wt% of the nanoparticles.

[0026] Furthermore, the coating thickness in step three is 5-10 μm.

[0027] Furthermore, in step three, the oven drying temperature is 60-100°C, and the heat treatment temperature is 200-350°C.

[0028] Technical effects:

[0029] In the initial stage of the reaction, PMDA and ODA are prepolymerized in a polar solvent to form a soluble high molecular weight polyamic acid. Controlling the molar ratio and reaction conditions is crucial to the molecular weight and physical properties of the final product.

[0030] As the temperature rises, the final curing process takes place, and the unreacted carboxyl and amino groups continue to react, completing the imidization process and forming a polyimide. 2,4-Diamino-6-diethylamino-1,3,5-triazine, as a multifunctional monomer, can further react with the end groups in the prepolymer, increasing the degree of crosslinking and improving the thermal stability and mechanical strength of the material. The introduction of octa(aminophenyl)-T8-silsesquioxane, through the reaction of its active amino groups with the anhydride, embeds the siloxane structure into the polyimide matrix, thereby improving the polymer's heat resistance and reducing the dielectric constant.

[0031] The introduction of triazine rings can improve the thermal stability and mechanical strength of polymers, making them more suitable for applications in high-temperature environments. The rigid structure and high symmetry of the triazine rings help improve the regularity and tight arrangement of polymer chains, thereby enhancing the mechanical properties of the material. The rich nitrogen content of the triazine rings may affect the electronic properties of the material, opening up the possibility of developing functional materials.

[0032] The introduction of nano-boron nitride improves the surface properties of the separator, making it more wettable with the electrolyte, which is beneficial to the battery's charge and discharge performance. Furthermore, an epoxy-amino addition reaction can occur between the epoxy-based nano-boron nitride coating and the polyimide matrix film, making the composite separator more firmly bonded.

[0033] In summary, this PMDA- and ODA-based polyimide containing triazine and silsesquioxane not only possesses the excellent properties of traditional polyimides, but also achieves even better performance through structural modification and technical optimization. This new polyimide material, with its excellent electrical properties, high-temperature resistance, and good mechanical properties, is particularly suitable for applications in the aerospace, electronics, and automotive industries. DETAILED DESCRIPTION

[0034] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0035] Example 1

[0036] A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized by comprising the following steps:

[0037] Step 1: Preparation of polyimide matrix film

[0038] Pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (CAS 2073-31-6), and octa(aminophenyl)-T8-silsesquioxane (CAS 518359-82-5) are added in a molar ratio to 200 g of N-methylpyrrolidone (NMP), wherein the mass of pyromellitic anhydride (PMDA) is 5 g, and stirred to form a uniform slurry; then the temperature is raised to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film;

[0039] Step 2: Prepare coating slurry

[0040] Weigh 100g of epoxy nano-boron nitride and disperse it in 1050g of organic solvent. Add 4g of polyvinyl pyrrolidone (PVP) and 1g of sodium ethoxide, and ultrasonically disperse for 20 minutes to obtain a uniform coating slurry.

[0041] Step 3: Preparation of composite diaphragm

[0042] The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature is controlled at 60°C during the coating process. The coated film is placed in an oven for drying for 0.5h, then heat treated for 1h, and cooled to room temperature to obtain a polyimide-based composite diaphragm.

[0043] In the step 1, the molar ratio of pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (molecular weight 182) and octa(aminophenyl)-T8-silsesquioxane (molecular weight 1153) is 1:1.3:0.01:0.0003.

[0044] In the step 1, the polymerization reaction temperature is 65° C. and the reaction time is 16 h.

[0045] The film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 5 cm / min, and the cast film is thermally imidized at 280° C. for 1 hour.

[0046] The preparation method of epoxy nano boron nitride in step 2 is:

[0047] Ratio: The mass ratio of 3-glycidyloxypropyltrimethoxysilane (GPTMS): nano-boron nitride: acetic acid: ethanol is 4:100:2:1000;

[0048] Temperature / time: The reaction was carried out at 25°C and stirred for 20 hours;

[0049] Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent;

[0050] Post-treatment: filtering, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 60° C. for 10 hours to obtain epoxy nano-boron nitride.

[0051] The organic solvent in step 2 is selected from N-methylpyrrolidone (NMP).

[0052] The amount of polyvinylpyrrolidone (PVP) used in step 2 is 1 wt% of the nanoparticles.

[0053] The coating thickness in step three is 5 μm.

[0054] In step 3, the oven drying temperature is 60°C and the heat treatment temperature is 200°C.

[0055] Example 2

[0056] A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized by comprising the following steps:

[0057] Step 1: Preparation of polyimide matrix film

[0058] Pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (CAS 2073-31-6), and octa(aminophenyl)-T8-silsesquioxane (CAS 518359-82-5) are added in a molar ratio to 300 g of N-methylpyrrolidone (NMP), wherein the mass of pyromellitic anhydride (PMDA) is 5 g, and stirred to form a uniform slurry; then the temperature is raised to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film;

[0059] Step 2: Prepare coating slurry

[0060] Weigh 120g of epoxy nano-boron nitride, disperse it in 1150% organic solvent, add 6g of polyvinyl pyrrolidone (PVP) and 2g of sodium ethoxide, and ultrasonically disperse for 30 minutes to obtain a uniform coating slurry;

[0061] Step 3: Preparation of composite diaphragm

[0062] The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature is controlled at 70°C during the coating process. The coated film is placed in an oven to dry for 0.5h, then heat treated for 2h, and cooled to room temperature to obtain a polyimide-based composite diaphragm.

[0063] In the step 1, the molar ratio of pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (molecular weight 182) and octa(aminophenyl)-T8-silsesquioxane (molecular weight 1153) is 1:1.4:0.03:0.0006.

[0064] The polymerization reaction temperature in step 1 is 70° C. and the reaction time is 18 h.

[0065] The film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 5 cm / min, and the cast film is thermally imidized at 300° C. for 2 hours.

[0066] The preparation method of epoxy nano boron nitride in step 2 is:

[0067] Ratio: 3-glycidyloxypropyltrimethoxysilane (GPTMS): nano-boron nitride: acetic acid: ethanol, the mass ratio is 6:110:3:1200;

[0068] Temperature / time: The reaction was carried out at 30°C and stirred for 35 hours;

[0069] Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent;

[0070] Post-treatment: filtering, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 65°C for 12 hours to obtain epoxy nano-boron nitride.

[0071] The organic solvent in step 2 is selected from N-methylpyrrolidone (NMP).

[0072] The amount of polyvinylpyrrolidone (PVP) used in step 2 is 1.5 wt% of the nanoparticles.

[0073] The coating thickness in step three is 5 μm.

[0074] In step 3, the oven drying temperature is 80°C and the heat treatment temperature is 250°C.

[0075] Example 3

[0076] A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized by comprising the following steps:

[0077] Step 1: Preparation of polyimide matrix film

[0078] Pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (CAS 2073-31-6), and octa(aminophenyl)-T8-silsesquioxane (CAS 518359-82-5) are added to N-methylpyrrolidone (NMP) in a molar ratio, wherein the mass of pyromellitic anhydride (PMDA) is 5 g, and stirred to form a uniform slurry; then the temperature is increased to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film;

[0079] Step 2: Prepare coating slurry

[0080] Weigh 140g of epoxy nano-boron nitride and disperse it in 1350g of organic solvent. Add 6g of polyvinyl pyrrolidone (PVP) and 2g of sodium ethoxide and ultrasonically disperse for 30 minutes to obtain a uniform coating slurry.

[0081] Step 3: Preparation of composite diaphragm

[0082] The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature is controlled at 70°C during the coating process. The coated film is placed in an oven to dry for 1 hour, then heat-treated for 2 hours, and cooled to room temperature to obtain a polyimide-based composite diaphragm.

[0083] In the step 1, the molar ratio of pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (molecular weight 182) and octa(aminophenyl)-T8-silsesquioxane (molecular weight 1153) is 1:1.4:0.04:0.001.

[0084] In the step 1, the polymerization reaction temperature is 70° C. and the reaction time is 22 h.

[0085] The film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 10 cm / min, and the cast film is thermally imidized at 300° C. for 2 hours.

[0086] The preparation method of epoxy nano boron nitride in step 2 is:

[0087] Ratio: 3-glycidyloxypropyltrimethoxysilane (GPTMS): nano-boron nitride: acetic acid: ethanol, the mass ratio is 6:115:4:1300;

[0088] Temperature / time: The reaction was carried out at 35°C with continuous stirring for 25 hours;

[0089] Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent;

[0090] Post-treatment: filtering, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 65° C. for 13.5 hours to obtain epoxy nano-boron nitride.

[0091] The organic solvent in step 2 is selected from N,N-dimethylformamide (DMF).

[0092] The amount of polyvinylpyrrolidone (PVP) used in step 2 is 1.5 wt% of the nanoparticles.

[0093] The coating thickness in step three is 10 μm.

[0094] In step 3, the oven drying temperature is 90°C and the heat treatment temperature is 300°C.

[0095] Example 4

[0096] A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized by comprising the following steps:

[0097] Step 1: Preparation of polyimide matrix film

[0098] Pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (CAS 2073-31-6), and octa(aminophenyl)-T8-silsesquioxane (CAS 518359-82-5) are added to N-methylpyrrolidone (NMP) in a molar ratio, wherein the mass of pyromellitic anhydride (PMDA) is 5 g, and stirred to form a uniform slurry; then the temperature is increased to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film;

[0099] Step 2: Prepare coating slurry

[0100] Weigh 160g of epoxy nano-boron nitride and disperse it in 1450g of organic solvent. Add 8g of polyvinyl pyrrolidone (PVP) and 3g of sodium ethoxide and ultrasonically disperse for 40 minutes to obtain a uniform coating slurry.

[0101] Step 3: Preparation of composite diaphragm

[0102] The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature is controlled at 80°C during the coating process. The coated film is placed in an oven to dry for 1 hour, then heat-treated for 3 hours, and cooled to room temperature to obtain a polyimide-based composite diaphragm.

[0103] In the step 1, the molar ratio of pyromellitic anhydride (PMDA), 4,4'-diaminodiphenyl ether (ODA), 2,4-diamino-6-diethylamino-1,3,5-triazine (molecular weight 182) and octa(aminophenyl)-T8-silsesquioxane (molecular weight 1153) is 1:1.5:0.05:0.002.

[0104] The polymerization reaction temperature in step 1 is 75° C. and the reaction time is 24 h.

[0105] The film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 10 cm / min, and the cast film is thermally imidized at 320° C. for 3 hours.

[0106] The preparation method of epoxy nano boron nitride in step 2 is:

[0107] Ratio: 3-glycidyloxypropyltrimethoxysilane (GPTMS): nano-boron nitride: acetic acid: ethanol, the mass ratio is 8:130:5:1500;

[0108] Temperature / time: The reaction was carried out at 40°C with continuous stirring for 30 hours;

[0109] Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent;

[0110] Post-treatment: filtering, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 70°C for 15 hours to obtain epoxy nano-boron nitride.

[0111] The organic solvent in step 2 is selected from N,N-dimethylacetamide (DMAc).

[0112] The amount of polyvinylpyrrolidone (PVP) used in step 2 is 2 wt% of the nanoparticles.

[0113] The coating thickness in step three is 10 μm.

[0114] In step 3, the oven drying temperature is 100°C and the heat treatment temperature is 350°C.

[0115] Comparative Example 1

[0116] The only difference between this example and the embodiment is that the epoxy-based nano-boron nitride is replaced by an equal amount of nano-boron nitride, which will not be described in detail here.

[0117] Comparative Example 2

[0118] The only difference between this example and the embodiment is that octa(aminophenyl)-T8-silsesquioxane is not added, which will not be described in detail here.

[0119] Comparative Example 3

[0120] The only difference between this example and the embodiment is that 2,4-diamino-6-diethylamino-1,3,5-triazine is not added, which will not be described in detail here.

[0121] Example detection method and test results:

[0122] Detection method

[0123] 1. Mechanical strength test: Use a tensile tester to test the tensile strength and elongation at break of the composite diaphragm.

[0124] 2. Thermal stability test: The mass change of the composite diaphragm at different temperatures was tested by thermogravimetric analyzer (TGA).

[0125] 3. Electrolyte wettability test: Immerse the composite diaphragm in the electrolyte and measure the liquid absorption rate of the diaphragm.

[0126] Test results

[0127]

[0128]

[0129] In summary, the polyimide-based composite separator provided by the present invention has excellent mechanical properties, thermal stability and electrolyte wettability, and is suitable for application in high-performance lithium-ion batteries.

[0130] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide-based composite diaphragm for a lithium battery, characterized in that: The following steps are included: Step 1: Preparation of polyimide matrix film According to the mass ratio, pyromellitic anhydride PMDA and 4,4'-diaminodiphenyl ether ODA, 2,4-diamino-6-diethylamino-1,3,5-triazine, and octa(aminophenyl)-T8-silsesquioxane are added in a molar ratio to 200-400 parts of N-methylpyrrolidone NMP, and stirred evenly to form a uniform slurry; then the temperature is raised to carry out a polymerization reaction to obtain a polyimide precursor slurry, and the precursor slurry is formed into a film by a casting method to form a polyimide matrix film; Step 2: Prepare coating slurry Weigh 100-160 parts by mass of epoxy nano-boron nitride and disperse them in 1050-1450 parts of organic solvent. Add 4-8 parts of polyvinylpyrrolidone (PVP) and 1-3 parts of sodium ethoxide, and ultrasonically disperse for 20-40 minutes to obtain a uniform coating slurry. Step 3: Preparation of composite diaphragm The coating slurry is evenly coated on the surface of the polyimide base film by spraying. The temperature during the coating process is controlled at 60-80°C. The coated film is placed in an oven to dry for 0.5-1h, then heat-treated for 1-3h, and cooled to room temperature to obtain a polyimide-based composite membrane. The preparation method of epoxy nano boron nitride in step 2 is: Ratio: The mass ratio of 3-glycidyloxypropyltrimethoxysilane GPTMS: nano-boron nitride: acetic acid: ethanol is 4-8:100-130:2-5:1000-1500; Temperature / time: The reaction is carried out at 25-40°C and stirred continuously for 20-30 hours; Catalyst / Solvent: Acetic acid was used as the catalyst for the hydrolysis of GPTMS and ethanol was used as the solvent; Post-treatment: filtration, washing with ethanol and deionized water to remove unreacted silane coupling agent, and then vacuum drying at 60-70°C for 10-15 hours to obtain epoxy nano-boron nitride.

2. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: In the step 1, the molar ratio of pyromellitic anhydride PMDA, 4,4'-diaminodiphenyl ether ODA, 2,4-diamino-6-diethylamino-1,3,5-triazine and octa(aminophenyl)-T8-silsesquioxane is 1:1.3-1.5:0.01-0.05:0.0003-0.

002.

3. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: In the step 1, the polymerization reaction temperature is 65-75° C., and the reaction time is 16-24 hours.

4. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: The film-making process of the casting method in step 1 is as follows: the casting speed is controlled at 5-10 cm / min, and the cast film is thermally imidized at 280-320° C. for 1-3 hours.

5. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: The organic solvent in step 2 is selected from at least one of N-methylpyrrolidone NMP, N,N-dimethylformamide DMF or N,N-dimethylacetamide DMAc.

6. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: In the step 2, the amount of polyvinyl pyrrolidone (PVP) used is 1-2 wt% of the epoxy nano-boron nitride.

7. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: The coating thickness in step three is 5-10 μm.

8. The method for preparing a polyimide-based composite separator for a lithium battery according to claim 1, wherein: In step 3, the oven drying temperature is 60-100°C, and the heat treatment temperature is 200-350°C.

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