A ceramic-free primerless composite heat-resistant polymer coated separator and a method of making the same

Abstract

This invention discloses a ceramic-free composite heat-resistant polymer-coated separator and its preparation method, relating to the field of heat-resistant coated separator technology for lithium-ion batteries. The separator comprises the following components by weight: 5-25 parts modified heat-resistant polymer A, 2-15 parts heat-resistant polymer B, 0.5-5 parts crosslinking agent, 0.1-3 parts curing accelerator, 1-10 parts phase separation pore-forming agent, 0.5-8 parts plasticizer, 0.1-2 parts leveling agent, and 60-90 parts organic solvent. This invention eliminates the need for a ceramic base coating. Through the synergistic effect of siloxane grafting-silane end-capped modification of polyimide, crosslinking curing, and phase separation pore formation, the heat resistance dimensional stability, interfacial bonding strength, and pore structure retention of the coating are improved, giving the separator high heat resistance, strong adhesion, flexibility, and good air permeability.

Description

Technical Field

[0001] This invention relates to the field of heat-resistant coated separator technology for lithium-ion batteries, specifically to a ceramic-free composite heat-resistant polymer coated separator and its preparation method. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, low self-discharge rate, and good environmental adaptability, have been widely used in consumer electronics, new energy vehicles, and energy storage systems. As the requirements for energy density and safety performance of power batteries and high-rate energy storage batteries continue to increase, the thermal stability, mechanical strength, and interface stability of key internal materials have become important factors affecting the overall performance of the battery. Among these, the separator, as a crucial component in lithium-ion batteries that isolates the positive and negative electrodes, prevents short circuits, and allows lithium-ion transport, directly affects the battery's safety under conditions of high temperature, overcharge, nail penetration, or thermal runaway.

[0003] Commercial lithium-ion battery separators mostly use polyethylene, polypropylene, or polyethylene / polypropylene composite microporous membranes. These polyolefin separators have good pore structure and low cost, but their melting point and heat distortion temperature are relatively low. Under high-temperature environments, they are prone to thermal shrinkage, pore blockage, and melting rupture, leading to direct contact between the positive and negative electrodes, causing internal short circuits and posing certain safety hazards. To improve the heat resistance of polyolefin separators, existing technologies typically coat the separator surface with inorganic ceramic particles such as alumina, boehmite, and silica to form ceramic-coated separators, thereby improving the separator's thermal dimensional stability and puncture resistance.

[0004] Traditional ceramic-coated separators still have some shortcomings in practical applications. The interfacial compatibility between inorganic ceramic particles and polyolefin-based membranes is poor, generally requiring binders to form a stable coating. If the binder dosage is insufficient, the coating is prone to powdering, cracking, or peeling; if the binder dosage is excessive, it may clog the membrane micropores, thus affecting electrolyte wettability and ion transport efficiency. Simultaneously, the high density of ceramic particles increases the overall weight of the separator with increased coating thickness and coverage, hindering further improvements in battery energy density. Furthermore, the dispersion stability, coating uniformity, and subsequent processing adaptability of the ceramic coating place higher demands on the production process. To alleviate these problems, using heat-resistant polymers to replace or partially replace ceramic particles to construct heat-resistant coatings has gradually become a research direction in separator modification. Heat-resistant polymers such as polyimides and polyetherimides have high glass transition temperatures and good thermal and chemical stability, which can be used to improve the dimensional retention of separators under high-temperature conditions. However, single heat-resistant polymer coatings usually have problems such as high film-forming stress, insufficient flexibility, and limited adhesion to polyolefin-based films. During drying or battery assembly, defects such as coating brittleness, delamination, or uneven pore structure are prone to occur.

[0005] Some existing polymer-coated membranes still require a ceramic undercoat or transitional adhesive layer to improve the bonding strength between the heat-resistant coating and the base membrane. While these multilayer composite structures can improve interfacial bonding performance to some extent, they also increase the number of preparation steps, raise production costs, and may cause problems such as increased membrane thickness, decreased porosity, and reduced electrolyte wetting performance. Therefore, how to directly and firmly laminate a heat-resistant polymer coating onto the surface of a heat-resistant porous base membrane without a ceramic undercoat, while simultaneously possessing high heat resistance, good flexibility, suitable pore structure, and high peel strength, has become a key technical problem that needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to address the problems in existing heat-resistant polymer-coated separators, such as insufficient bonding between the coating and the base film interface, complex ceramic primer process, poor flexibility of a single heat-resistant polymer coating, and difficulty in controlling the pore structure, by providing a composite heat-resistant polymer-coated separator without a ceramic primer.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: a ceramic-free composite heat-resistant polymer-coated diaphragm, wherein the diaphragm comprises a heat-resistant porous base film and a composite heat-resistant polymer coating directly bonded to at least one surface of the heat-resistant porous base film.

[0008] Furthermore, the composite heat-resistant polymer coating is formed by coating, phase separation and pore formation, drying and curing of a coating liquid containing the following components in parts by mass.

[0009] Furthermore, the coating liquid is prepared from the following raw materials in parts by weight: 5-25 parts of modified heat-resistant polymer A, 2-15 parts of heat-resistant polymer B, 0.5-5 parts of crosslinking agent, 0.1-3 parts of curing accelerator, 1-10 parts of phase separation pore-forming agent, 0.5-8 parts of plasticizer, 0.1-2 parts of leveling agent, and 60-90 parts of organic solvent.

[0010] Furthermore, the heat-resistant porous base membrane is selected from one of polyimide porous membrane, aramid porous membrane, polyetherimide porous membrane, polyphenylene sulfide porous membrane or composite porous membrane.

[0011] Furthermore, the modified heat-resistant polymer A is prepared from the following raw materials in parts by weight: 14-18 parts of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4-8 parts of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 7-11 parts of 4,4'-oxodiphenylamine, 1-3 parts of p-phenylenediamine, 110-140 parts of N-methylpyrrolidone, 3-6 parts of diaminopropyl-terminated polydimethylsiloxane, 0.8-1.8 parts of 3-aminopropyltriethoxysilane, 2-4 parts of γ-glycidyl etheroxypropyltrimethoxysilane, 8-12 parts of anhydrous ethanol, 4-6 parts of deionized water, 0.1-0.5 parts of glacial acetic acid, and 0.2-0.6 parts of triethanolamine.

[0012] Furthermore, the preparation method of the modified heat-resistant polymer A is as follows: Step 1, Monomer Dehydration and Solution Pretreatment: Based on a molar ratio of dianhydride to diamine of 0.98-1.02:1, the dianhydride is composed of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride in a molar ratio of 65:35-80:20, and the diamine is composed of 4,4'-oxodiphenylamine and p-phenylenediamine in a molar ratio of 75:25-90:10.

[0013] 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenone tetracarboxylic dianhydride were dried at 110-130℃ under a vacuum of -0.08MPa to -0.095MPa for 2-4 hours. 4,4'-oxodiphenylamine and p-phenylenediamine were dried at 60-80℃ under a vacuum of -0.08MPa to -0.095MPa for 2-4 hours. Subsequently, under nitrogen protection, the dried 4,4'-oxodiphenylamine and p-phenylenediamine were added to N-methylpyrrolidone, controlling the mass ratio of diamine to N-methylpyrrolidone to be 1:6-10. The mixture was stirred at 5-15℃ and 300-600rpm for 30-60 minutes to obtain a diamine mixed solution.

[0014] Step 2, Construction of aromatic polyamic acid hard segments: Under nitrogen protection, the dried 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride from Step 1 are added to the diamine mixed solution in 4-6 batches, with an interval of 10-20 minutes between each batch. During the addition process, the system temperature is controlled at 10-25℃ and the stirring speed is 400-800 rpm. After the addition is completed, the reaction continues for 2-4 hours to obtain an aromatic polyamic acid precursor solution.

[0015] Step 3, siloxane segment grafting and silane end-group blocking: Diaminopropyl-terminated polydimethylsiloxane is diluted with N-methylpyrrolidone to a siloxane solution with a mass concentration of 10-30%, and the amount of diaminopropyl-terminated polydimethylsiloxane added is 5-15% of the solid mass of the aromatic polyamic acid precursor; it is added to the aromatic polyamic acid precursor solution obtained in Step 2 at a dropping rate of 0.5-2.0 mL / min at 20-30℃, and the stirring speed is controlled at 500-900 rpm during the dropping. After the dropping is completed, the reaction continues for 1-3 h; then 3-aminopropyltriethoxysilane is added to the system at a dropping rate of 0.2-1.0 mL / min, and the reaction continues at 25-35℃ for 1-2 h to obtain a modified polyamic acid solution.

[0016] Step 4, Epoxysilane Side Group Grafting: γ-glycidoxypropyltrimethoxysilane is added to an ethanol / deionized water mixture at 5-12% of the solid mass of the aromatic polyamic acid precursor, wherein the mass ratio of ethanol to deionized water in the ethanol / deionized water mixture is 1:0.2-0.8. The pH is adjusted to 4.5-5.5 with acetic acid, and pre-hydrolyzed for 30-60 min at 25-35℃ and 300-500 rpm to obtain an epoxysilane pre-hydrolyzed solution. The epoxysilane pre-hydrolyzed solution is added to the modified polyamic acid solution obtained in Step 3 at a dropping rate of 0.3-1.0 mL / min, and triethanolamine is added to adjust the pH of the system to 6.5-7.5. The reaction is carried out at 50-65℃ and 500-900 rpm for 2-4 h to obtain a siloxane-grafted-silane-terminated polyamic acid solution.

[0017] Step 5, Precipitation, Gradient Imidification, and Powder Control: The siloxane-grafted-silane-terminated polyamic acid solution obtained in Step 4 is added dropwise to an ethanol / water precipitation solution at a rate of 5-15 mL / min. The mass ratio of ethanol to water in the ethanol / water precipitation solution is 8:2-6:4. During precipitation, the stirring speed is controlled at 1000-2000 rpm and the temperature at 20-30℃. After the addition is complete, stirring is continued for 30-60 min. The precipitate is then collected by filtration. The precipitate is then heated at 60-80℃ under a vacuum of -0.08... After drying at MPa to -0.095MPa for 8-12 hours, a gradient thermal imidization treatment was carried out under nitrogen protection. First, the temperature was raised to 100-120℃ and held for 1-2 hours, then raised to 160-180℃ and held for 1-2 hours, then raised to 220-240℃ and held for 1-2 hours, and finally raised to 260-300℃ and held for 0.5-1.5 hours. The heating rate was controlled at 1-3℃ / min. After cooling to room temperature, the modified heat-resistant polymer A was obtained by air jet milling, classification and wet sand milling.

[0018] Furthermore, the heat-resistant polymer B is a soluble polyetherimide or a soluble aromatic polyimide; and the heat-resistant polymer B can form a homogeneous solution with a mass concentration of 8-15% in N,N-dimethylacetamide or N-methylpyrrolidone at 60-80°C.

[0019] Furthermore, the crosslinking agent is selected from one or more combinations of triglycidyl isocyanurate, trimethylolpropane triglycidyl ether, polycarbodiimide, and isophorone diisocyanate trimer.

[0020] Furthermore, the curing accelerator is selected from one or more combinations of 2-methylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, and triethanolamine.

[0021] Furthermore, the phase separation pore-forming agent is selected from one or more combinations of polyethylene glycol, polyvinylpyrrolidone, polyethylene glycol-polypropylene glycol block copolymer, and polyvinyl alcohol.

[0022] Furthermore, the plasticizer is tributyl phosphate or polytetrahydrofuran with a number average molecular weight of 1000-3000.

[0023] Furthermore, the leveling agent is selected from any one of dimethyl silicone oil, polyether-modified silicone oil, or perfluorooctyl polyether surfactant.

[0024] Furthermore, the organic solvent is selected from N,N-dimethylacetamide or N-methylpyrrolidone.

[0025] The method for preparing a ceramic-free, heat-resistant polymer-coated separator is characterized by comprising the following steps: S1. Preparation of coating solution: Dissolve heat-resistant polymer B in 50-70 wt% of the total organic solvent and stir until homogeneous to obtain a heat-resistant polymer B solution with a mass concentration of 8-15%; pre-disperse the modified heat-resistant polymer A in 30-50 wt% of the total organic solvent, with the mass ratio of the modified heat-resistant polymer A to the organic solvent being 1:1.2-6.0, to obtain a modified heat-resistant polymer A dispersion; then add the modified heat-resistant polymer A dispersion to the heat-resistant polymer B solution, and then add the crosslinking agent, curing accelerator, phase separation pore-forming agent, plasticizer, and leveling agent, disperse at 800-1500 rpm for 20-60 min, and then ultrasonically treat with 200-400W for 3-10 min to obtain the composite coating solution.

[0026] S2. Filtration and viscosity adjustment: The composite coating liquid is filtered through a filter membrane or metal filter with a pore size of 5-15μm to remove agglomerates and impurities larger than 10μm, and the viscosity is adjusted to 500-3000mPa·s at 40-70℃ to obtain a refined coating liquid.

[0027] S3. Base film surface activation and coating: At least one surface of the polyimide porous base film is subjected to air plasma treatment with a treatment power of 50-150W and a treatment time of 30-120s. Within 10 minutes after treatment, the refined coating solution is coated onto the activated polyimide porous base film surface, controlling the wet film thickness to be 15-30μm, the dry film thickness to be 2-6μm, and the coating amount to be 0.5-3.0g / m². 2 This forms a wet coating.

[0028] S4. Phase separation and pore formation: The polyimide porous base film forming the wet coating is placed in a mixture of ethanol / water with a mass ratio of 9:1-6:4, or placed in a water / alcohol mixed vapor environment for phase separation and pore formation treatment for 0.5-5 min at a temperature of 20-30℃, so that the coating forms a connected pore structure.

[0029] S5. Staged drying and curing: The pore-forming coating is initially dried at 60-100℃ for 10-30 min, then dried at 140-180℃ for 15-45 min at a medium temperature, and then heated to 200-240℃ under nitrogen protection and held at that temperature for 20-60 min, so that the silane end groups and epoxy silane side groups in the modified heat-resistant polymer A react further with the crosslinking agent to form a composite heat-resistant polymer coating; after cooling to room temperature, the ceramic-free primer composite heat-resistant polymer coated diaphragm is obtained.

[0030] Furthermore, the composite heat-resistant polymer coating has a thermal shrinkage rate of 0.87-2.48% at 200℃, a coating peel strength of 70.3-92.4 N / m, and an electrolyte contact angle of 18.7-27.5°.

[0031] The ceramic-free, heat-resistant polymer-coated separator can be used as a structural component or manufacturing process for inactive parts of electrochemical batteries, excluding fuel cells.

[0032] The ceramic-free, heat-resistant polymer-coated diaphragm can be used in electric vehicles and other motor vehicles, excluding fuel cell vehicles.

[0033] In this invention, modified heat-resistant polymer A, heat-resistant polymer B, crosslinking agent, curing accelerator, phase separation pore-forming agent, plasticizer, and leveling agent work together during coating formation. The aromatic polyimide hard segments in modified heat-resistant polymer A provide the main heat-resistant support, while the flexible segments of polydimethylsiloxane reduce the internal stress of the coating film. During curing, the silane reactive end groups and epoxy silane grafted side groups undergo hydrolysis condensation, epoxy ring-opening, and interfacial coupling reactions, thereby enhancing the degree of internal crosslinking in the coating and the interfacial bonding between the coating and the heat-resistant porous base film. The crosslinking agent further reacts with epoxy, carboxyl, hydroxyl, or silanol groups to form a stable three-dimensional crosslinked network. The curing accelerator promotes epoxy ring-opening and silane condensation reactions, improving curing efficiency and interfacial bonding strength. The phase separation pore-forming agent migrates or separates during pore-forming treatment and drying, creating a connected pore structure in the coating and preventing the heat-resistant polymer coating from becoming excessively dense, which would affect air permeability and electrolyte wettability.

[0034] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention eliminates the ceramic base coating, allowing the composite heat-resistant polymer coating to be directly bonded to the surface of the heat-resistant porous base membrane. Compared with traditional ceramic base-coated diaphragms, this invention reduces the base coating process, avoids problems such as ceramic particle powdering, cracking, peeling, and the base coating increasing the thickness and weight of the diaphragm, and is conducive to obtaining a heat-resistant coated diaphragm with a simpler structure and a more stable interface.

[0035] 2. This invention constructs a composite heat-resistant framework by modifying heat-resistant polymer A and heat-resistant polymer B. The aromatic polyimide hard segment provides good high heat-resistant support, while the heat-resistant polymer B helps to enhance the continuous film-forming ability and structural stability of the coating, so that the diaphragm can maintain a low thermal shrinkage rate and a high glass transition temperature under high temperature conditions, thereby further improving the high-temperature dimensional stability and thermal safety performance of the diaphragm.

[0036] 3. This invention forms an interfacial cross-linking structure through silane reactive end groups, epoxy silane grafted side groups, and cross-linking agents; during the staged drying and high-temperature curing process, silane end groups, epoxy groups, and cross-linking agents undergo hydrolysis condensation, epoxy ring opening, and cross-linking reactions, which enhance the internal cross-linking density of the coating and its bonding force with the heat-resistant porous base film, so that the coating still has high peel strength under the condition of no ceramic primer.

[0037] 4. This invention introduces polydimethylsiloxane flexible segments into the polyimide heat-resistant structure; these flexible segments can reduce the internal stress of the coating film, improve the problems of single heat-resistant polymer coatings being too hard, brittle, and prone to delamination, and enable the separator to have better flexibility and processing adaptability during winding, cutting, and battery assembly.

[0038] 5. This invention uses phase separation pore-forming agents, plasticizers, and leveling agents to synergistically regulate the pore structure of the coating; the phase separation pore-forming treatment enables the coating to form a more uniform interconnected pore structure, while the plasticizers and leveling agents improve the uniformity of the coating film formation, avoiding excessive density of the heat-resistant polymer coating that blocks the pores of the diaphragm, so that the diaphragm can maintain good air permeability, electrolyte wettability, and ion transport adaptability while improving heat resistance and adhesion. Attached Figure Description

[0039] Figure 1 The TGA / DTG thermogravimetric curves of the modified heat-resistant polymer A prepared in Example 1 of this invention are shown.

[0040] Figure 2 The image shows the FTIR infrared spectrum of the modified heat-resistant polymer A prepared in Example 1 of this invention. Detailed Implementation

[0041] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Preparation Example 1 Preparation of modified heat-resistant polymer A: 1. Raw material composition, by mass, includes the following raw materials: 3,3',4,4'-Biphenyltetracarboxylic dianhydride, BPDA, 16.0 parts.

[0043] 3,3',4,4'-benzophenone tetracarboxylic dianhydride, BTDA, 6.0 parts.

[0044] 4,4'-Oxydiphenylamine, ODA, 11.0 parts.

[0045] p-Phenylenediamine, 1.95 parts.

[0046] N-methylpyrrolidone, 124.4 parts.

[0047] Diaminopropyl-terminated polydimethylsiloxane, 4.5 parts, number average molecular weight approximately 2500 g / mol, amine value 0.6-0.8 meq / g, supplier: Merck AG, Darmstadt, Germany.

[0048] 3-Aminopropyltriethoxysilane, 1.2 parts.

[0049] γ-glycidoxypropyltrimethoxysilane, 3.0 parts.

[0050] Anhydrous ethanol, 10.0 parts.

[0051] Deionized water, 5.0 parts.

[0052] Glacial acetic acid, 0.25 parts.

[0053] Triethanolamine, 0.35 parts.

[0054] Where: Total moles of dianhydride : Total moles of diamine = 0.07300 : 0.07296 ≈ 1.0005 : 1.

[0055] The internal molar ratio of dianhydrides is approximately 74.5:25.5:BPDA:BTDA = 0.05438:0.01862.

[0056] Internal molar ratio of diamine: ODA: p-phenylenediamine = 0.05493: 0.01803 ≈ 75.3: 24.7.

[0057] 2. Preparation method: 1) Preparation of monomer dehydration and diamine mixed solution: 16.0 parts by weight of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 6.0 parts by weight of 3,3',4,4'-benzophenone tetracarboxylic dianhydride were placed in a vacuum drying environment and dried at 120℃ and -0.090 MPa for 3 h; 11.0 parts by weight of 4,4'-oxodiphenylamine and 1.95 parts by weight of p-phenylenediamine were placed in a vacuum drying environment and dried at 70℃ and -0.090 MPa for 3 h. After drying, the reaction system was purged with nitrogen for 15 min, with the nitrogen flow rate controlled at 0.20 L / min. Subsequently, under nitrogen protection, 11.0 parts by mass of dried 4,4'-oxodiphenylamine and 1.95 parts by mass of p-phenylenediamine were added to 86.4 parts by mass of N-methylpyrrolidone. The system temperature was controlled at 10°C, the stirring speed was 450 rpm, and the mixture was stirred for 45 min to obtain a diamine mixed solution.

[0058] 2) Construction of the aromatic polyamic acid hard segment: Under nitrogen protection, 16.0 parts by weight of dried 3,3',4,4'-biphenyltetracarboxylic dianhydride and 6.0 parts by weight of 3,3',4,4'-benzophenone tetracarboxylic dianhydride were premixed and then added in 5 batches (4.4 parts by weight each) to the diamine mixed solution obtained in step 1). Each batch was added 15 minutes apart. During the addition process, the system temperature was controlled at 15-20°C, and the stirring speed was 650 rpm. After all the dianhydrides were added, 20.0 parts by weight of N-methylpyrrolidone were added to adjust the solid content of the system. The reaction was continued at 20°C for 3 hours to obtain an aromatic polyamic acid precursor solution with a solid content of approximately 24.7%.

[0059] 3) Siloxane segment grafting and silane end-group blocking: 4.5 parts by mass of diaminopropyl-terminated polydimethylsiloxane were added to 18.0 parts by mass of N-methylpyrrolidone and stirred at 25°C and 500 rpm for 30 min to prepare a siloxane solution with a mass concentration of 20.0%. This siloxane solution was added to the aromatic polyamic acid precursor solution obtained in step 2) at a dropping rate of 1.0 mL / min at 25°C, with the stirring speed controlled at 750 rpm during the dropping. After the dropping was completed, the reaction was continued for 2 h. Subsequently, 1.2 parts by mass of 3-aminopropyltriethoxysilane were added to the system at a dropping rate of 0.5 mL / min, and the reaction was continued at 30°C and 700 rpm for 1.5 h to obtain a modified polyamic acid solution containing flexible polydimethylsiloxane segments and triethoxysilane end groups.

[0060] 4) Epoxysilane side group grafting: 3.0 parts by weight of γ-glycidoxypropyltrimethoxysilane were added to an ethanol / water mixture consisting of 10.0 parts by weight of anhydrous ethanol and 5.0 parts by weight of deionized water. The pH was adjusted to 5.0 with 0.25 parts by weight of glacial acetic acid. The mixture was pre-hydrolyzed at 30°C and 400 rpm for 45 min to obtain an epoxysilane pre-hydrolyzed solution. This epoxysilane pre-hydrolyzed solution was added to the modified polyamic acid solution obtained in step 3) at a dropping rate of 0.5 mL / min. After the addition was complete, 0.35 parts by weight of triethanolamine was added to adjust the pH of the system to 7.0. The mixture was then reacted at 60°C and 750 rpm for 3 h to allow the epoxy groups to undergo a ring-opening grafting reaction with the carboxyl groups in the polyamic acid segments, introducing hydrolyzable silane side groups into the polymer segments to obtain a siloxane-grafted-silane-terminated polyamic acid solution.

[0061] 5) Precipitation, washing, gradient imidization, and powder control: Prepare a 600.0 parts by weight ethanol / water precipitation solution, comprising 420.0 parts by weight of anhydrous ethanol and 180.0 parts by weight of deionized water, with an ethanol to water mass ratio of 7:3 and a pH of 6.8-7.0. Add the siloxane-grafted-silane-terminated polyamic acid solution obtained in step 4) dropwise to the precipitation solution at a rate of 10 mL / min. During precipitation, maintain the temperature at 25℃ and the stirring speed at 1500 rpm. After the addition is complete, continue stirring for 45 min, and filter to collect the precipitate. Wash the precipitate twice with an ethanol / water mass ratio of 7:3, using 200.0 parts by weight each time, followed by washing once with 100.0 parts by weight of deionized water. Filter to obtain a wet filter cake. Dry the wet filter cake at 70℃ and a vacuum of -0.090 MPa for 10 h to obtain a pre-dried polymer. The pre-dried polymer was then subjected to gradient thermal imidization in a nitrogen-protected environment. The nitrogen flow rate was controlled at 0.20 L / min, and the heating rate was controlled at 2 °C / min. The temperature was first raised to 110 °C and held for 1.5 h, then raised to 170 °C and held for 1.5 h, then raised to 230 °C and held for 1.5 h, and finally raised to 280 °C and held for 1 h. After imidization, the polymer was cooled to room temperature under nitrogen protection. The resulting solid was then subjected to air jet milling, cyclone classification, and wet sand milling to obtain modified heat-resistant polymer A.

[0062] Depend on Figure 1 The curves show that under nitrogen atmosphere and a heating rate of 10℃ / min, the sample only experienced slight weight loss at low temperatures, mainly attributed to the removal of adsorbed water, residual trace solvents, or low-molecular-weight components. The high mass retention rate before approximately 400℃ indicates that the aromatic polyimide hard segments formed after gradient thermal imidization possess good thermal stability. Subsequently, the main thermal decomposition process occurs in the range of approximately 450-620℃. The DTG curve shows a shoulder peak near approximately 500℃ and a main weight loss peak near approximately 560-580℃, indicating concentrated pyrolysis of the polyimide backbone and silicon-containing organic segments. At 800℃, approximately 50% char residue remains, indicating that the flexible segments of polydimethylsiloxane, silane end groups, and epoxy silane side groups are conducive to forming silicon-containing char residues or Si-O-Si structures during heat treatment, thereby improving the high-temperature residue rate and heat-resistant support capacity of the modified heat-resistant polymer A.

[0063] Depend on Figure 2 The spectrum shows that the distance between approximately 1770-1780 cm⁻¹ is... -1 and 1710-1720cm -1 A distinct absorption peak for both asymmetric and symmetric stretching vibrations of the imide carbonyl group appears nearby, at approximately 1370-1380 cm⁻¹. -1 A CN stretching vibration peak of imide appears nearby, at approximately 720-730 cm⁻¹. -1The presence of characteristic absorption around the imide ring indicates that the aromatic polyamic acid has formed relatively complete aromatic polyimide hard segments after gradient thermal imidization. Simultaneously, approximately 1600 cm⁻¹... -1 and 1500cm -1 The nearby absorption peak corresponds to the vibration of the aromatic ring skeleton, approximately 2960 cm⁻¹. -1 The nearby components can be classified as -CH3 / -CH in polydimethylsiloxane. 2 - stretching vibration, approximately 1260cm -1 The corresponding characteristic absorption of Si-CH3 is around 1120-1020 cm⁻¹. -1 The strong absorption in the region can be attributed to Si-O-Si and COC-related vibrations, indicating that the flexible segments of polydimethylsiloxane, silane end groups, and epoxy silane side groups have been introduced into the structure of the modified heat-resistant polymer A; approximately 3400 cm -1 The nearby weak broad peaks can be attributed to residual silanols, hydroxyl groups, or adsorbed water.

[0064] Comparative Preparation Example 1 The modified heat-resistant polymer A was prepared according to the preparation method in Preparation Example 1, except that 4.5 parts by mass of diaminopropyl-terminated polydimethylsiloxane was replaced with 4.5 parts by mass of polyetheramine, which was a commercially available diamino-terminated polyether with a number average molecular weight of 2000 g / mol; the other raw material types, amounts, dropping rates, reaction temperatures, stirring speeds, precipitation, washing, gradient imidization, and powder control conditions were kept the same as in Preparation Example 1.

[0065] Comparative Preparation Example 2 The modified heat-resistant polymer A was prepared according to the preparation method in Preparation Example 1, except that 1.2 parts by mass of 3-aminopropyltriethoxysilane were replaced with 0.50 parts by mass of aniline in equal molar amounts of amino groups; the other raw material types, amounts, reaction temperatures, dropping speeds, stirring speeds, precipitation, washing, gradient imidization, and powder control conditions were kept the same as in Preparation Example 1.

[0066] Comparative preparation example 3 The modified heat-resistant polymer A was prepared according to the preparation method in Preparation Example 1, except that 3.0 parts by mass of γ-glycidyl etheroxypropyltrimethoxysilane were replaced with 1.80 parts by mass of glycidyl methacrylate according to the molar number of epoxy groups; the other raw material types, amounts, pre-hydrolysis conditions, pH adjustment conditions, reaction temperature, stirring speed, precipitation, washing, gradient imidization and powder control conditions were kept the same as in Preparation Example 1.

[0067] Comparative preparation example 4 The modified heat-resistant polymer A was prepared according to the preparation method in Preparation Example 1, except that 16.0 parts by mass of 3,3',4,4'-biphenyltetracarboxylic dianhydride was replaced with 16.87 parts by mass of 4,4'-oxydiphthalic anhydride according to the molar number of anhydride functional groups; the other dianhydrides, diamines, N-methylpyrrolidone, diaminopropyl-terminated polydimethylsiloxane, 3-aminopropyltriethoxysilane, γ-glycidyl etheroxypropyltrimethoxysilane and subsequent preparation steps were the same as in Preparation Example 1.

[0068] Example 1 Preparation of a ceramic-free, heat-resistant polymer-coated diaphragm: 1. Raw material composition By weight, it includes the following raw materials: 15.0 parts of the modified heat-resistant polymer A obtained in Preparation Example 1.

[0069] Heat-resistant polymer B: polyetherimide, 8.0 parts, supplier: SABIC.

[0070] Crosslinking agent: Triglycidyl isocyanurate, 2.0 parts.

[0071] Curing accelerator: 2-ethyl-4-methylimidazole, 0.5 parts.

[0072] Phase separation pore-forming agent: polyethylene glycol 2000, 4.0 parts.

[0073] Plasticizer: Tributyl phosphate, 3.0 parts.

[0074] Leveling agent: Dimethyl silicone oil, 0.5 parts.

[0075] Organic solvent: N,N-dimethylacetamide.

[0076] Heat-resistant porous base film: Polyimide porous base film with a thickness of 25μm.

[0077] 2. Preparation method S1. Preparation of the composite coating solution: 8.0 parts by weight of polyetherimide were added to 45.0 parts by weight of N,N-dimethylacetamide, and the mixture was heated to 65°C under nitrogen protection, with the stirring speed controlled at 600 rpm for 3 hours to obtain a heat-resistant polymer B solution; 15.0 parts by weight of the modified heat-resistant polymer A obtained in Preparation Example 1 were added to 30.0 parts by weight of N,N-dimethylacetamide, and dispersed at 50°C and 800 rpm for 60 minutes, followed by wet milling with 0.5 mm zirconia beads for 30 minutes to obtain a dispersion of modified heat-resistant polymer A; subsequently, the modified heat-resistant polymer A was... The dispersion was added to the heat-resistant polymer B solution, and the system temperature was controlled at 55℃. The stirring speed was 1000 rpm, and the mixture was stirred for 20 min. Then, 2.0 parts by weight of triglycidyl isocyanurate, 0.5 parts by weight of 2-ethyl-4-methylimidazole, 4.0 parts by weight of polyethylene glycol 2000, 3.0 parts by weight of tributyl phosphate, and 0.5 parts by weight of dimethyl silicone oil were added sequentially. The mixture was stirred at 55℃ and 1200 rpm for 25 min to obtain the initial composite coating solution. The initial composite coating solution was ultrasonically treated at 300W for 4 min to remove bubbles and obtain a uniform composite coating solution.

[0078] S2. Filtration and viscosity adjustment: The composite coating solution obtained in step S1 is filtered through a 10μm stainless steel metal filter to remove agglomerates and impurities larger than 10μm; after filtration, the composite coating solution is kept at 60℃, the stirring speed is 300rpm, and the temperature is maintained for 10min. The viscosity is stabilized between 1400-1600mPa·s to obtain a refined composite coating solution.

[0079] S3. Pretreatment and coating of heat-resistant porous base film: A 25μm thick polyimide porous base film was used as the heat-resistant porous base film and cut into 100mm×150mm specifications. After wiping the base film surface with anhydrous ethanol, it was dried at 60℃ for 10min. Then, the surface of the polyimide porous base film to be coated was subjected to air plasma treatment with a treatment power of 100W and a treatment time of 60s. Coating was carried out within 5min after the treatment. The refined composite coating liquid obtained in step S2 was uniformly coated onto the surface of the plasma-treated polyimide porous base film by spraying. The nozzle diameter was controlled at 0.3mm, the spraying speed was 10cm / s, the wet film thickness was controlled at 22μm, and the dry film thickness after curing was 4.5μm.

[0080] S4. Phase separation and pore formation treatment: The wet coating after spraying is immediately placed in an ethanol / deionized water mixture for phase separation and pore formation treatment. The mass ratio of ethanol to deionized water in the ethanol / deionized water mixture is 7:3. The treatment temperature is 25℃ and the treatment time is 60s. During the treatment, polyethylene glycol 2000 migrates in the coating and induces polymer phase separation. After the treatment, the film is removed and the surface is gently blown with clean nitrogen to remove residual liquid.

[0081] S5. Staged drying and curing: The membrane material after phase separation and pore formation treatment is placed in a forced-air drying oven and initially dried at 80℃ for 20 minutes to form a preliminary continuous film on the coating surface; then the temperature is raised to 165℃ and held for 30 minutes to further remove the solvent inside the coating and begin to form a partial cross-linked structure; then the temperature is raised to 220℃ at a heating rate of 3℃ / min and held for 45 minutes under nitrogen protection. After curing, it is cooled to room temperature under nitrogen protection to obtain a ceramic-free composite heat-resistant polymer coated membrane.

[0082] Example 2 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out according to the preparation method in Example 1, except that 2.0 parts by weight of triglycidyl isocyanurate in Example 1 are replaced with 2.0 parts by weight of trimethylolpropane triglycidyl ether. Except for the type of crosslinking agent, the modified heat-resistant polymer A, heat-resistant polymer B, curing accelerator, phase separation pore-forming agent, plasticizer, leveling agent, organic solvent dosage and process parameters S1-S5 are the same as in Example 1.

[0083] Example 3 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out according to the preparation method in Example 1, except that 0.5 parts by weight of 2-ethyl-4-methylimidazole in Example 1 are replaced with 0.5 parts by weight of 1-cyanoethyl-2-ethyl-4-methylimidazole; except for the type of curing accelerator, the modified heat-resistant polymer A, heat-resistant polymer B, crosslinking agent, phase separation pore-forming agent, plasticizer, leveling agent, organic solvent dosage and process parameters S1-S5 are the same as in Example 1.

[0084] Example 4 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out according to the preparation method in Example 1, except that 15.0 parts by weight of the modified heat-resistant polymer A obtained in Example 1 is replaced with 5.0 parts by weight of the modified heat-resistant polymer A obtained in Example 1; the 5.0 parts by weight of the modified heat-resistant polymer A is still pre-dispersed in 30.0 parts by weight of N,N-dimethylacetamide, and the mass ratio of the modified heat-resistant polymer A to the N,N-dimethylacetamide is 1:6.0; except for the amount of modified heat-resistant polymer A, the other raw material types, raw material amounts and process parameters S1-S5 are the same as in Example 1.

[0085] Example 5 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out according to the preparation method in Example 1, except that 15.0 parts by weight of the modified heat-resistant polymer A obtained in Example 1 is replaced with 25.0 parts by weight of the modified heat-resistant polymer A obtained in Example 1; the 25.0 parts by weight of the modified heat-resistant polymer A is still pre-dispersed in 30.0 parts by weight of N,N-dimethylacetamide, and the mass ratio of the modified heat-resistant polymer A to the N,N-dimethylacetamide is 1:1.2; except for the amount of modified heat-resistant polymer A, the other raw material types, raw material amounts and process parameters S1-S5 are the same as in Example 1.

[0086] Comparative Examples 1-4 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out by referring to the preparation method in Example 1, except that the modified heat-resistant polymer A is replaced with the modified heat-resistant polymer A prepared in Comparative Preparation Examples 1-4, and the rest is the same as in Example 1.

[0087] Comparative Example 5 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out by referring to the preparation method in Example 1, except that the modified heat-resistant polymer A is replaced with unmodified aromatic polyimide powder, and the rest is the same as in Example 1.

[0088] Among them, the unmodified aromatic polyimide powder is P84. ® NT1 05P DF, Supplier: EvonikOperations GmbH.

[0089] Comparative Example 6 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out by referring to the preparation method in Example 1, except that the modified heat-resistant polymer B is replaced with polyvinylidene fluoride, and the rest is the same as in Example 1.

[0090] Comparative Example 7 The preparation of a ceramic-free composite heat-resistant polymer-coated diaphragm is carried out by referring to the preparation method in Example 1, except that the crosslinking agent triglycidyl isocyanurate is replaced with polyvinylpyrrolidone K30, and the rest is the same as in Example 1.

[0091] Performance testing 1. Heat Shrinkage Rate Test: The heat shrinkage rate test was conducted according to the method specified in GB / T 36363-2018, with the test temperature adjusted to 200℃. The test equipment included a blower-type constant temperature chamber, a stainless steel plate, and a length measuring instrument; the temperature accuracy of the blower-type constant temperature chamber was ±1℃, and the resolution of the length measuring instrument was 0.1mm. Three 100mm×100mm square diaphragms were cut from the longitudinal direction of the film roll. When cutting the diaphragms, one edge was made parallel to the longitudinal edge of the diaphragm, with a maximum deviation angle not exceeding 5°. The data are shown in Table 1.

[0092] 2. Coating peel strength test: The coating peel strength test was conducted in accordance with GB / T 2790-1995, and the data are shown in Table 1.

[0093] 3. Air permeability test: The air permeability test was conducted according to the method specified in GB / T 36363-2018, and the data are shown in Table 1.

[0094] 4. Electrolyte wettability test: The electrolyte wettability test was conducted using the contact angle measurement method. The testing equipment was a video optical contact angle tension meter with a resolution of 0.01°. The test temperature was 25℃, and the relative humidity was 50%RH.

[0095] Electrolyte preparation: Ethyl carbonate, diethyl carbonate and dimethyl carbonate were mixed in a mass ratio of 2:5:3 as solvent, and LiPF6 was added to prepare an electrolyte with a concentration of 1 mol / L.

[0096] The ceramic-free, heat-resistant polymer-coated diaphragm of the present invention was cut into 50mm×50mm samples, laid flat on a glass slide and fixed; 2μL of the above electrolyte was drawn up using a microsyringe and dropped vertically onto the diaphragm surface; the high-speed camera system was started to record the droplet spreading process; after the droplet stabilized for 30s, the contact angle formed between the droplet and the diaphragm surface was measured and recorded using the height measurement method, and the data are shown in Table 1.

[0097] 5. Puncture strength test: The puncture strength test was conducted in accordance with the method specified in GB / T 36363-2018, and the data are shown in Table 1.

[0098] Table 1. Performance test data of the examples and comparative examples

[0099] As shown in Table 1, Example 1 exhibits the most balanced overall performance, with a heat shrinkage rate of only 1.08% at 200℃, a coating peel strength of 92.4 N / m, an air permeability of 236 s / 100 mL, an electrolyte contact angle of 18.7°, and a puncture strength of 5.62 N. This indicates that the aromatic polyimide hard segments, polydimethylsiloxane flexible segments, silane end groups, epoxy silane side groups, and crosslinking agents in the modified heat-resistant polymer A obtained in Example 1 form a relatively complete heat-resistant-flexible-interfacial crosslinking network.

[0100] In Example 2, after replacing the crosslinking agent with trimethylolpropane triglycidyl ether, the heat shrinkage rate increased to 1.31%, the peel strength decreased to 86.5 N / m, and the puncture strength decreased to 5.39 N, indicating that the crosslinking agent can still crosslink to form a film, but the rigidity and interface curing efficiency are slightly lower than those of triglycidyl isocyanurate.

[0101] After changing the curing accelerator in Example 3, the heat shrinkage rate was 1.18%, the peel strength was 88.9 N / m, and the air permeability was 239 s / 100 mL, which were close to those in Example 1. This indicates that 1-cyanoethyl-2-ethyl-4-methylimidazole can still promote epoxy ring opening and silane condensation, but the curing activity is slightly weaker.

[0102] In Example 4, after reducing the modified heat-resistant polymer A to 5 parts, the heat shrinkage rate increased significantly to 2.48%, the peel strength decreased to 70.3 N / m, and the puncture strength decreased to 4.72 N. Although the air permeability decreased to 221 s / 100 mL, this was an "apparent improvement in air permeability" caused by insufficient coating skeleton content and low pore resistance, which could not compensate for the decrease in heat resistance support and interfacial bonding.

[0103] In Example 5, when the modified heat-resistant polymer A was increased to 25 parts, the heat shrinkage rate decreased to 0.87% and the puncture strength increased to 5.80 N, indicating that the heat-resistant skeleton was enhanced. However, the air permeability increased to 276 s / 100 mL and the contact angle increased to 22.4°, reflecting that the coating was too dense and the pore transport was restricted. Therefore, the overall effect was not as balanced as in Example 1.

[0104] In Comparative Example 1, after replacing diaminopropyl-terminated polydimethylsiloxane with polyetheramine, the heat shrinkage rate increased to 3.72% and the peel strength decreased to 55.8 N / m, indicating that the lack of heat-resistant siloxane flexible segments resulted in insufficient stress release and interface compatibility in the coating.

[0105] In Comparative Example 2, when aniline was used instead of 3-aminopropyltriethoxysilane, the peel strength was further reduced to 48.7 N / m, indicating that the coupling reaction between the base film and the coating was insufficient when no hydrolyzable silane end groups were available.

[0106] Comparative Example 3, which replaced γ-glycidyl etheroxypropyltrimethoxysilane with glycidyl methacrylate, showed a heat shrinkage rate of 3.08% and a peel strength of 53.2 N / m, indicating that when epoxy groups are retained but silane condensation sites are lacking, internal crosslinking and interfacial bonding are still significantly weakened.

[0107] Comparative Example 4, after replacing BPDA with 4,4'-oxybisphthalic anhydride, showed a heat shrinkage rate of 3.56% and a puncture strength of 4.37 N, indicating that the rigid biphenylimide hard segment was reduced and the heat resistance dimensional stability decreased.

[0108] Comparative Example 5 uses unmodified aromatic polyimide powder, with a peel strength of only 34.5 N / m, an air permeability as high as 306 s / 100 mL, and a contact angle of 32.6°. This indicates that the pure heat-resistant powder lacks flexible segments and reactive silane structures, and is prone to forming weak interfacial bonding and dense pore-blocking coatings.

[0109] In Comparative Example 6, after replacing the heat-resistant polymer B with PVDF, the heat shrinkage rate reached a maximum of 5.90% and the peel strength was only 43.8 N / m, indicating that the heat-resistant skeleton of PVDF was insufficient and its synergy with the modified heat-resistant polymer A was poor.

[0110] In Comparative Example 7, after replacing the crosslinking agent with PVP K30, although the air permeability (209s / 100mL) and contact angle (17.2°) showed improved pore opening and wettability, the heat shrinkage rate (5.36%), peel strength (38.7N / m), and puncture strength (3.32N) all deteriorated significantly. This indicates that PVP K30 mainly plays a pore-forming / liquid-loving role rather than a crosslinking enhancement role, resulting in a loose coating structure and insufficient heat resistance and mechanical integrity.

[0111] 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 ceramic-free composite heat-resistant polymer-coated separator, characterized in that, The diaphragm includes a heat-resistant porous base membrane and a composite heat-resistant polymer coating directly bonded to at least one surface of the heat-resistant porous base membrane. The composite heat-resistant polymer coating is formed by coating an aqueous solution containing the following components in parts by weight through coating, phase separation and pore formation, drying and curing: The coating liquid is prepared from the following raw materials in parts by weight: 5-25 parts modified heat-resistant polymer A, 2-15 parts heat-resistant polymer B, 0.5-5 parts crosslinking agent, 0.1-3 parts curing accelerator, 1-10 parts phase separation pore-forming agent, 0.5-8 parts plasticizer, 0.1-2 parts leveling agent, and 60-90 parts organic solvent; The heat-resistant porous base membrane is selected from one of polyimide porous membrane, aramid porous membrane, polyetherimide porous membrane, polyphenylene sulfide porous membrane or composite porous membrane; The modified heat-resistant polymer A is prepared from the following raw materials in parts by weight: 14-18 parts of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4-8 parts of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 7-11 parts of 4,4'-oxodiphenylamine, 1-3 parts of p-phenylenediamine, 110-140 parts of N-methylpyrrolidone, 3-6 parts of diaminopropyl-terminated polydimethylsiloxane, 0.8-1.8 parts of 3-aminopropyltriethoxysilane, 2-4 parts of γ-glycidyl etheroxypropyltrimethoxysilane, 8-12 parts of anhydrous ethanol, 4-6 parts of deionized water, 0.1-0.5 parts of glacial acetic acid, and 0.2-0.6 parts of triethanolamine.

2. The ceramic-free undercoated composite heat-resistant polymer coated separator of claim 1, wherein, The method for preparing the modified heat-resistant polymer A is as follows: Step 1, Monomer Dehydration and Solution Pretreatment: Based on a total molar ratio of dianhydride to diamine of 0.98-1.02:1, the dianhydride is composed of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenone tetracarboxylic dianhydride in a molar ratio of 65:35-80:20, and the diamine is composed of 4,4'-oxodiphenylamine and p-phenylenediamine in a molar ratio of 75:25-90:

10. 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenone tetracarboxylic dianhydride were dried at 110-130℃ under a vacuum of -0.08MPa to -0.095MPa for 2-4 hours. 4,4'-oxodiphenylamine and p-phenylenediamine were dried at 60-80℃ under a vacuum of -0.08MPa to -0.095MPa for 2-4 hours. Subsequently, under nitrogen protection, the dried 4,4'-oxodiphenylamine and p-phenylenediamine were added to N-methylpyrrolidone, controlling the mass ratio of diamine to N-methylpyrrolidone to be 1:6-10. The mixture was stirred at 5-15℃ and 300-600rpm for 30-60 minutes to obtain a diamine mixed solution. Step 2, Construction of aromatic polyamic acid hard segment: Under nitrogen protection, the dried 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride from Step 1 are added to the diamine mixed solution in 4-6 batches, with an interval of 10-20 minutes between each batch. During the addition process, the system temperature is controlled at 10-25℃ and the stirring speed is 400-800 rpm. After the addition is completed, the reaction continues for 2-4 hours to obtain the aromatic polyamic acid precursor solution. Step 3, siloxane segment grafting and silane end-group blocking: Diaminopropyl-terminated polydimethylsiloxane is diluted with N-methylpyrrolidone to a siloxane solution with a mass concentration of 10-30%, and the amount of diaminopropyl-terminated polydimethylsiloxane added is 5-15% of the solid mass of the aromatic polyamic acid precursor; it is added to the aromatic polyamic acid precursor solution obtained in Step 2 at a dropping rate of 0.5-2.0 mL / min at 20-30℃, and the stirring speed is controlled at 500-900 rpm during the dropping. After the dropping is completed, the reaction continues for 1-3 h; then 3-aminopropyltriethoxysilane is added to the system at a dropping rate of 0.2-1.0 mL / min, and the reaction continues at 25-35℃ for 1-2 h to obtain a modified polyamic acid solution; Step 4, Epoxysilane Side Group Grafting: γ-glycidoxypropyltrimethoxysilane is added to an ethanol / deionized water mixture at 5-12% of the solid mass of the aromatic polyamic acid precursor, wherein the mass ratio of ethanol to deionized water in the ethanol / deionized water mixture is 1:0.2-0.

8. The pH is adjusted to 4.5-5.5 with acetic acid, and pre-hydrolyzed for 30-60 min at 25-35℃ and 300-500 rpm to obtain an epoxysilane pre-hydrolyzed solution. The epoxysilane pre-hydrolyzed solution is added to the modified polyamic acid solution obtained in Step 3 at a dropping rate of 0.3-1.0 mL / min, and triethanolamine is added to adjust the pH of the system to 6.5-7.

5. The reaction is carried out at 50-65℃ and 500-900 rpm for 2-4 h to obtain a siloxane-grafted-silane-terminated polyamic acid solution. Step 5, Precipitation, Gradient Imidification, and Powder Control: The siloxane-grafted-silane-terminated polyamic acid solution obtained in Step 4 is added dropwise to an ethanol / water precipitation solution at a rate of 5-15 mL / min. The mass ratio of ethanol to water in the ethanol / water precipitation solution is 8:2-6:

4. During precipitation, the stirring speed is controlled at 1000-2000 rpm and the temperature at 20-30℃. After the addition is complete, stirring is continued for 30-60 min. The precipitate is then collected by filtration. The precipitate is then heated at 60-80℃ under a vacuum of -0.08... After drying at MPa to -0.095MPa for 8-12 hours, a gradient thermal imidization treatment was carried out under nitrogen protection. First, the temperature was raised to 100-120℃ and held for 1-2 hours, then raised to 160-180℃ and held for 1-2 hours, then raised to 220-240℃ and held for 1-2 hours, and finally raised to 260-300℃ and held for 0.5-1.5 hours. The heating rate was controlled at 1-3℃ / min. After cooling to room temperature, the modified heat-resistant polymer A was obtained by air jet milling, classification and wet sand milling.

3. The ceramic-free undercoated composite heat-resistant polymer coated separator of claim 1, wherein, The heat-resistant polymer B is a soluble polyether imide or a soluble aromatic polyimide; Furthermore, the heat-resistant polymer B can form a homogeneous solution with a mass concentration of 8-15% in N,N-dimethylacetamide or N-methylpyrrolidone at 60-80℃. The crosslinking agent is selected from one or more combinations of triglycidyl isocyanurate, trimethylolpropane triglycidyl ether, polycarbodiimide, and isophorone diisocyanate trimer.

4. The ceramic-free undercoat composite heat-resistant polymer coated separator of claim 1, wherein, The curing accelerator is selected from one or more of 2-methylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, and triethanolamine.

5. The ceramic-free, heat-resistant polymer-coated separator according to claim 1, characterized in that, The phase separation pore-forming agent is selected from one or more combinations of polyethylene glycol, polyvinylpyrrolidone, polyethylene glycol-polypropylene glycol block copolymer, and polyvinyl alcohol.

6. The ceramic-free primerless composite heat-resistant polymer coated separator of claim 1, wherein, The plasticizer is tributyl phosphate or polytetrahydrofuran with a number average molecular weight of 1000-3000.

7. The ceramic-free primerless composite heat-resistant polymer coated separator of claim 1, wherein, The leveling agent is selected from any one of dimethyl silicone oil, polyether-modified silicone oil, or perfluorooctyl polyether surfactant.

8. The ceramic-free primerless composite heat-resistant polymer coated separator of claim 1, wherein, The organic solvent is selected from N,N-dimethylacetamide or N-methylpyrrolidone.

9. A method for preparing a ceramic-free, heat-resistant polymer-coated separator as described in any one of claims 1-8, characterized in that, Includes the following steps: S1. Preparation of coating solution: Dissolve heat-resistant polymer B in 50-70 wt% of the total organic solvent and stir until homogeneous to obtain a heat-resistant polymer B solution with a mass concentration of 8-15%; pre-disperse the modified heat-resistant polymer A in 30-50 wt% of the total organic solvent, with the mass ratio of the modified heat-resistant polymer A to this portion of organic solvent being 1:1.2-6.0, to obtain a modified heat-resistant polymer A dispersion; then add the modified heat-resistant polymer A dispersion to the heat-resistant polymer B solution, and then add the crosslinking agent, curing accelerator, phase separation pore-forming agent, plasticizer, and leveling agent, disperse at 800-1500 rpm for 20-60 min, and then ultrasonically treat with 200-400W for 3-10 min to obtain a composite coating solution; S2. Filtration and viscosity adjustment: The composite coating liquid is filtered through a filter membrane or metal filter with a pore size of 5-15μm to remove agglomerates and impurities larger than 10μm, and the viscosity is adjusted to 500-3000mPa·s at 40-70℃ to obtain a refined coating liquid. S3. Base film surface activation and coating: At least one surface of the polyimide porous base film is subjected to air plasma treatment with a treatment power of 50-150W and a treatment time of 30-120s. Within 10 minutes after treatment, the refined coating solution is coated onto the activated polyimide porous base film surface, controlling the wet film thickness to be 15-30μm, the dry film thickness to be 2-6μm, and the coating amount to be 0.5-3.0g / m². 2 This forms a wet coating. S4. Phase separation and pore formation: The polyimide porous base film forming the wet coating is placed in a mixture of ethanol / water with a mass ratio of 9:1-6:4, or placed in a water / alcohol mixed vapor environment for phase separation and pore formation treatment for 0.5-5 min at a temperature of 20-30℃, so that the coating forms a connected pore structure. S5. Staged drying and curing: The pore-forming coating is initially dried at 60-100℃ for 10-30 min, then dried at 140-180℃ for 15-45 min at a medium temperature, and then heated to 200-240℃ under nitrogen protection and held at that temperature for 20-60 min, so that the silane end groups and epoxy silane side groups in the modified heat-resistant polymer A react further with the crosslinking agent to form a composite heat-resistant polymer coating; after cooling to room temperature, the ceramic-free primer composite heat-resistant polymer coated diaphragm is obtained.

10. A ceramic-free primerless composite heat-resistant polymeric coated separator according to any one of claims 1-8, characterized in that, The composite heat-resistant polymer coating has a thermal shrinkage rate of 0.87-2.48% at 200℃, a peel strength of 70.3-92.4 N / m, and an electrolyte contact angle of 18.7-27.5°.

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