Fireproof and flame-retardant battery separator, preparation method thereof and fireproof and flame-retardant battery

Abstract

This invention discloses a fire-retardant battery separator, its preparation method, and a fire-retardant battery, relating to the field of battery separator technology. In preparing the fire-retardant battery separator, this invention involves reacting a mixture of diglyceride borate and trimellitic anhydride chloride to obtain a flame-retardant monomer; polymerizing 4,4'-diaminodiphenyl ether and the flame-retardant monomer, followed by electrostatic spraying to obtain polyamic acid microspheres; dehydrating and drying the polyamic acid microspheres to obtain polyimide microspheres; etching and acidifying the polyimide microspheres to obtain carboxylated polyimide microspheres; grafting propylene-based phosphoric acid and acrylic acid copolymers onto the surface of a polyethylene film to obtain a modified polyethylene film; and coating the carboxylated polyimide microspheres onto the surface of the modified polyethylene film to obtain the fire-retardant battery separator. The fire-retardant battery separator prepared by this invention exhibits excellent flame retardancy, wetting, and thermal stability.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, specifically to a fire-retardant battery separator, its preparation method, and a fire-retardant battery. Background Technology

[0002] Lithium-ion batteries, as the core power source for new energy vehicles and energy storage systems, are experiencing explosive market demand. As a core component of lithium-ion batteries, the battery separator is a microporous insulating film located between the positive and negative electrodes. Its performance directly determines the battery's safety, cycle life, and rate performance. While traditional polyolefin separators offer good insulation and cost advantages, their insufficient heat resistance makes them prone to severe shrinkage under abnormal conditions such as overcharging and short circuits. This can lead to direct contact between the positive and negative electrodes, causing thermal runaway, and the flammability of the electrolyte further exacerbates the risk of fire spread. Furthermore, the new national standard GB38031-2025, "Safety Requirements for Power Batteries for Electric Vehicles," clearly stipulates that all newly marketed batteries from July 2026 onwards must achieve "no fire or explosion within 2 hours after thermal runaway," imposing unprecedentedly stringent requirements on battery safety protection systems. Against this backdrop, developing new battery separators with both high heat resistance and excellent flame retardant properties has become a core demand for overcoming the technological bottlenecks in fire-resistant and flame-retardant batteries. Summary of the Invention

[0003] The purpose of this invention is to provide a fire-retardant battery separator, its preparation method, and a fire-retardant battery, so as to solve the problems existing in the prior art.

[0004] To solve the above-mentioned technical problems, the present invention provides the following solution:

[0005] A fire-retardant battery separator is provided, comprising: reacting a mixture of borate diglyceride and trimellitic anhydride chloride to obtain a flame-retardant monomer; polymerizing 4,4'-diaminodiphenyl ether and the flame-retardant monomer, followed by electrostatic spraying to obtain polyamic acid microspheres; dehydrating and drying the polyamic acid microspheres to obtain polyimide microspheres; etching and acidifying the polyimide microspheres to obtain carboxylated polyimide microspheres; grafting propylene-based phosphoric acid and acrylic acid copolymer onto the surface of a polyethylene film to obtain a modified polyethylene film; and coating the carboxylated polyimide microspheres onto the surface of the modified polyethylene film.

[0006] The borate diglyceride is prepared by esterification of boric acid and glycerol.

[0007] The mixture of trimellitic anhydride chloride is prepared by reacting boric acid and glycerol to produce borate diglyceride; or by dissolving trimellitic anhydride chloride and triethylamine in N,N-dimethylformamide.

[0008] A method for preparing a fire-retardant battery separator, the method comprising the following preparation steps:

[0009] (1) Mix borate diglyceride and N,N-dimethylformamide at a mass ratio of 1:(6~8), stir at room temperature for 5~15 min under nitrogen protection, add trimellitic anhydride chloride mixture at a uniform rate over 20~30 min, continue stirring for 40~60 min, heat to 55~65℃ and stir for 2~4 h, cool to room temperature, filter, and vacuum dry at 65~75℃ for 8~10 h to obtain flame retardant monomer;

[0010] (2) Dry polyamic acid microspheres at 250~350℃ for 1~2h to obtain polyimide microspheres; immerse the polyimide microspheres in 0.15~0.25mol / L sodium hydroxide solution at 10~20 times their weight, etch for 10~20min, wash with deionized water 2~4 times, immerse in 3wt% acetic acid solution at 8~12 times their weight for 15~25min, wash with deionized water 2~4 times, and vacuum dry at 50~60℃ for 20~24h to obtain carboxylated polyimide microspheres;

[0011] (3) Irradiate the polyethylene film under an electron accelerator for 5-15 min with an irradiation dose of 30-50 kGy to obtain a pre-irradiated polyethylene film; immerse the pre-irradiated polyethylene film in 10-12 times its mass of grafting solution, heat it to 50-60℃ under nitrogen protection, reflux and stir for 2-3 h, take it out, wash it 3-5 times with anhydrous acetone, and vacuum dry it at 55-65℃ for 3-5 h to obtain a modified polyethylene film;

[0012] (4) Mix carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution in a mass ratio of 1:(0.07~0.08):(0.07~0.08):(10~12), ultrasonically disperse at room temperature for 5~15 min, stir for 3~4 h, let stand for 5~15 min, pour into a coating machine, and coat evenly on the surface of the modified polyethylene film. The coating thickness is 1~3 μm, the coating speed is 0.4~0.6 m / min, and vacuum dry at 45~55℃ for 7~9 h to obtain a fire-retardant battery separator.

[0013] As an optimization, the preparation steps of borate diglyceride in step (1) are as follows: Boric acid and glycerol in a molar ratio of 1:(2~2.2) are mixed with toluene in a mass of 0.9~1.1 times that of boric acid. Under nitrogen protection, the temperature is raised to 105~115℃ at a constant rate, and the reaction is stirred for 4.5~5.5h. The mixture is then distilled under reduced pressure at 80~90℃ to obtain borate diglyceride. Borate diglyceride and trimellitic anhydride chloride are weighed in a molar ratio of 1:2.

[0014] As an optimization, the preparation steps of the trimellitic anhydride chloride mixture in step (1) are as follows: trimellitic anhydride chloride is mixed evenly with an equimolar amount of triethylamine, dissolved in N,N-dimethylformamide at 4 to 6 times the mass of trimellitic anhydride chloride, and stirred at room temperature for 5 to 15 minutes to obtain the trimellitic anhydride chloride mixture.

[0015] As an optimization, the preparation steps of the polyamic acid microspheres in step (2) are as follows: 4,4'-diaminodiphenyl ether and N,N-dimethylformamide are mixed evenly at a mass ratio of 1:(24~26), stirred at 0~4℃ for 3~7 min under nitrogen protection, flame retardant monomers with a molar number of 1.01~1.03 times that of 4,4'-diaminodiphenyl ether are added, and the reaction is continued to be stirred for 1.5~2.5 h. The mixture is then aged at 55~65℃ until the kinematic viscosity is 100~200 cP, and polyamic acid microspheres are obtained by electrostatic spraying.

[0016] As an optimization, the process parameters for electrostatic spraying are: positive pressure 18~22kV, negative pressure 3~7kV, distance between receiving plate and nozzle 16~20cm, and solution flow rate 0.5~1.5mL / h to prepare polyamic acid microspheres.

[0017] As an optimization, the polyethylene film in step (3) has a specification of 7 μm and was purchased from Yunnan Enjie New Materials Co., Ltd.

[0018] As an optimization, the preparation steps of the grafting solution in step (3) are as follows: propylene phosphoric acid, acrylic acid, benzoyl peroxide and anhydrous acetone are mixed evenly in a mass ratio of 1:(0.6~0.8):(0.01~0.03):(5~7) and stirred for 10~20 min to obtain the grafting solution.

[0019] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0020] In preparing a fire-retardant battery separator, this invention involves esterifying boric acid and glycerol to obtain boric acid diglyceride; dissolving trimellitic anhydride chloride and triethylamine in N,N-dimethylformamide to obtain a trimellitic anhydride chloride mixture; reacting the boric acid diglyceride and trimellitic anhydride chloride mixture to obtain a flame-retardant monomer; polymerizing 4,4'-diaminodiphenyl ether and the flame-retardant monomer, followed by electrostatic spraying to obtain polyamic acid microspheres; dehydrating and drying the polyamic acid microspheres to obtain polyimide microspheres; etching and acidifying the polyimide microspheres to obtain carboxylated polyimide microspheres; copolymerizing propylene-based phosphoric acid and acrylic acid onto the surface of a polyethylene film to obtain a modified polyethylene film; and coating the carboxylated polyimide microspheres onto the surface of the modified polyethylene film to obtain the fire-retardant battery separator.

[0021] First, boric acid and glycerol are esterified to obtain borate diglyceride. Trimeric triglyceride chloride and triethylamine are dissolved in N,N-dimethylformamide to obtain a mixture of trimellitic triglyceride chloride. The mixture of borate diglyceride and trimellitic triglyceride chloride is reacted to obtain a flame-retardant monomer. The structure of borate diglyceride undergoes ester bond cleavage at high temperature, releasing boric acid and glycerol derivatives. Boric acid can act as a char catalyst to catalyze the dehydration and crosslinking of the polyethylene membrane matrix, inhibiting the volatilization of small molecule combustible gases generated by thermal decomposition. As the temperature continues to rise, boric acid is further dehydrated to become boron oxide. At temperatures above 325°C, boron oxide softens into a glassy state, melts and flows on the polymer surface, and coats the surface of the carbon layer to form a dense inorganic-organic composite barrier layer. This layer blocks heat transfer and oxygen diffusion, and prevents internal combustible gases from escaping, achieving "solid-phase flame retardancy," thus endowing the fire-retardant battery separator with excellent flame-retardant properties.

[0022] Secondly, 4,4'-diaminodiphenyl ether and flame-retardant monomers are polymerized and electrostatically sprayed to obtain polyamic acid microspheres; the polyamic acid microspheres are then dehydrated and dried to obtain polyimide microspheres; the polyimide microspheres are then etched and acidified to obtain carboxylated polyimide microspheres, introducing carboxyl groups onto the surface of the polyimide microspheres; the polyimide molecular chain contains a large number of rigid aromatic rings and imide rings, with high chemical bond energy and a thermal decomposition temperature >500℃, far exceeding the melting temperature of polyethylene film. When the battery is under high-temperature conditions such as overcharging or short circuit, the carboxylated polyimide microspheres will not melt and decompose, thus making them suitable for use in fire-retardant batteries. The physical support framework formed on the surface of the separator hinders the thermal movement and slippage of PE molecular chains, effectively reducing the thermal shrinkage rate of the fire-retardant battery separator, thus endowing the fire-retardant battery separator with excellent thermal stability. The carboxyl groups on the surface of the carboxylated polyimide microspheres are strongly polar groups, which can form hydrogen bonds with carbonate molecules in the electrolyte, reducing the contact angle of the electrolyte on the surface of the fire-retardant battery separator. This gives the fire-retardant battery separator excellent wetting properties, ensuring that the "ion channels" for lithium ion transport are unobstructed during charging and discharging, and improving the rate performance and cycle life of the battery.

[0023] Finally, a modified polyethylene membrane was prepared by copolymerizing propylene phosphate and acrylic acid onto the surface of a polyethylene membrane. Phosphorus-containing and carboxyl groups were introduced onto the polyethylene membrane surface through copolymerization grafting. The copolymer chains of propylene phosphate and acrylic acid were covalently anchored to the polyethylene membrane surface via free radical grafting, forming slight crosslinks between adjacent grafted chains. This created a flexible crosslinked network, restricting the thermal motion and slippage of the polyethylene molecular chains at high temperatures, thereby reducing the thermal shrinkage rate of the fire-retardant battery separator and further improving its thermal stability. Both carboxyl and phosphate groups are strongly polar hydrophilic groups, which can form hydrogen bonds with carbonate molecules in the electrolyte, allowing the electrolyte to spread rapidly and penetrate into the microporous channels of the polyethylene membrane, further improving the fire-retardant battery... The wetting properties of the separator: Phosphate groups undergo a dehydration reaction at high temperatures to generate polyphosphoric acid, which promotes the rapid formation of a continuous and dense carbon layer on the surface of the polyethylene film and the grafted layer. The P2O5 generated by the decomposition of polyphosphoric acid is further converted into glassy phosphate esters, which coat the surface of the carbon layer, improve the structural strength of the carbon layer, prevent the carbon layer from collapsing under the impact of flame, and block the heat and oxygen transfer path. At high temperatures, phosphorus decomposes to generate phosphorus-containing free radicals such as PO· and HPO·. These free radicals can capture active free radicals in the flame, interrupt the chain reaction of gas-phase combustion, and the inert gas generated by decomposition will dilute the concentration of combustible gas in the combustion area, reduce the flame propagation speed, and achieve "gas-phase flame retardancy", thereby further improving the flame retardant performance of the fireproof and flame-retardant battery separator. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1:

[0026] A method for preparing a fire-retardant battery separator, the method comprising the following steps:

[0027] (1) Boric acid and glycerol in a molar ratio of 1:2 were mixed with toluene at 0.9 times the mass of boric acid. Under nitrogen protection, the mixture was heated to 105°C at a constant rate and stirred for 5.5 h. The mixture was then distilled under reduced pressure at 80°C to obtain boric acid diglyceride. Boric acid diglyceride and trimellitic anhydride chloride were weighed in a molar ratio of 1:2. Trimellitic anhydride chloride was mixed with an equimolar amount of triethylamine and dissolved in N,N-dimethylformamide at 4 times the mass of trimellitic anhydride chloride. The mixture was stirred at room temperature for 15 min to obtain trimellitic anhydride chloride mixture. Boric acid diglyceride and N,N-dimethylformamide were mixed in a mass ratio of 1:6. Under nitrogen protection, the mixture was stirred at room temperature for 15 min. Trimellitic anhydride chloride mixture was added at a constant rate over 30 min. The mixture was stirred for 60 min and heated to 55°C. The mixture was stirred for 4 h and cooled to room temperature. The mixture was filtered and dried under vacuum at 65°C for 10 h to obtain flame retardant monomer.

[0028] (2) 4,4'-diaminodiphenyl ether and N,N-dimethylformamide were mixed evenly at a mass ratio of 1:24. Under nitrogen protection, the mixture was stirred at 0°C for 7 min. Flame retardant monomer with a molar ratio of 1.01 times that of 4,4'-diaminodiphenyl ether was added, and the reaction was continued with stirring for 2.5 h. The mixture was then aged at 55°C until the kinematic viscosity reached 100 cP. Polyamic acid microspheres were obtained by electrostatic spraying. The process parameters were: positive pressure 18 kV, negative pressure 3 kV, and distance between the receiving plate and the nozzle 16 cm. m, solution flow rate 0.5 mL / h; polyamic acid microspheres were dried at 250℃ for 2 h to obtain polyimide microspheres; the polyimide microspheres were immersed in 0.15 mol / L sodium hydroxide solution with a mass of 10 times their mass, etched for 20 min, washed twice with deionized water, immersed in 3wt% acetic acid solution with a mass of 8 times their mass of the polyimide microspheres for acidification for 25 min, washed twice with deionized water, and vacuum dried at 50℃ for 24 h to obtain carboxylated polyimide microspheres;

[0029] (3) Mix propylene phosphoric acid, acrylic acid, benzoyl peroxide and anhydrous acetone in a mass ratio of 1:0.6:0.01:5 and stir for 20 min to obtain a grafting solution; irradiate the polyethylene film under an electron accelerator for 15 min with an irradiation dose of 30 kGy to obtain a pre-irradiated polyethylene film; immerse the pre-irradiated polyethylene film in 10 times its mass of the grafting solution, heat it to 50°C under nitrogen protection, reflux and stir for 3 h, take it out, wash it 3 times with anhydrous acetone, and vacuum dry it at 55°C for 5 h to obtain a modified polyethylene film;

[0030] (4) Carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution were mixed evenly at a mass ratio of 1:0.07:0.07:10. The mixture was ultrasonically dispersed at room temperature for 15 min, stirred for 4 h, and allowed to stand for 15 min. The mixture was then poured into a coating machine and evenly coated on the surface of the modified polyethylene film. The coating thickness was 1 μm and the coating speed was 0.4 m / min. The mixture was then vacuum dried at 45 °C for 9 h to obtain a fire-retardant battery separator.

[0031] Example 2:

[0032] A method for preparing a fire-retardant battery separator, the method comprising the following steps:

[0033] (1) Boric acid and glycerol in a molar ratio of 1:2.1 were mixed with toluene in a mass ratio of 1 to 100% of boric acid. Under nitrogen protection, the mixture was heated to 110°C at a constant rate and stirred for 5 hours. The mixture was then distilled under reduced pressure at 85°C to obtain boric acid diglyceride. Boric acid diglyceride and trimellitic anhydride chloride were weighed in a molar ratio of 1:2. Trimericic anhydride chloride was mixed with an equimolar amount of triethylamine and dissolved in N,N-dimethylformamide in a mass ratio of 5 to 5 times that of trimellitic anhydride chloride. The mixture was stirred at room temperature for 10 minutes to obtain trimellitic anhydride chloride mixture. Boric acid diglyceride and N,N-dimethylformamide were mixed in a mass ratio of 1:7. Under nitrogen protection, the mixture was stirred at room temperature for 10 minutes. Trimericic anhydride chloride mixture was added at a constant rate over 25 minutes. The mixture was stirred for 50 minutes, heated to 60°C, stirred for 3 hours, cooled to room temperature, filtered, and dried under vacuum at 70°C for 9 hours to obtain flame retardant monomer.

[0034] (2) Mix 4,4'-diaminodiphenyl ether and N,N-dimethylformamide at a mass ratio of 1:25 until homogeneous. Stir at 2°C for 5 min under nitrogen protection. Add flame retardant monomer at 1.02 times the molar amount of 4,4'-diaminodiphenyl ether and continue stirring for 2 h. Aging at 60°C until the kinematic viscosity is 150 cP. Obtain polyamic acid microspheres by electrostatic spraying. The process parameters are: positive pressure 20 kV, negative pressure 5 kV, and distance between the receiving plate and the nozzle 18 cm. The solution flow rate was 1 mL / h; polyamic acid microspheres were dried at 300℃ for 1.5 h to obtain polyimide microspheres; the polyimide microspheres were immersed in 0.2 mol / L sodium hydroxide solution (15 times their weight) for 15 min, etched, washed three times with deionized water, acidified in 3wt% acetic acid solution (10 times their weight) for 20 min, washed three times with deionized water, and vacuum dried at 55℃ for 22 h to obtain carboxylated polyimide microspheres;

[0035] (3) Mix propylene phosphoric acid, acrylic acid, benzoyl peroxide and anhydrous acetone in a mass ratio of 1:0.7:0.02:6 and stir for 15 min to obtain a grafting solution; irradiate the polyethylene film under an electron accelerator for 10 min with an irradiation dose of 40 kGy to obtain a pre-irradiated polyethylene film; immerse the pre-irradiated polyethylene film in 11 times its mass of the grafting solution, heat it to 55°C under nitrogen protection, reflux and stir for 2.5 h, take it out, wash it 4 times with anhydrous acetone, and vacuum dry it at 60°C for 4 h to obtain a modified polyethylene film;

[0036] (4) Carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution were mixed evenly at a mass ratio of 1:0.075:0.075:11. The mixture was ultrasonically dispersed at room temperature for 10 min, stirred for 3.5 h, and allowed to stand for 10 min. The mixture was then poured into a coating machine and evenly coated on the surface of the modified polyethylene film. The coating thickness was 2 μm and the coating speed was 0.5 m / min. The mixture was then vacuum dried at 50 °C for 8 h to obtain a fire-retardant battery separator.

[0037] Example 3:

[0038] A method for preparing a fire-retardant battery separator, the method comprising the following steps:

[0039] (1) Boric acid and glycerol in a molar ratio of 1:2.2 were mixed with toluene in a mass ratio of 1.1 times that of boric acid. Under nitrogen protection, the mixture was heated to 115°C at a constant rate and stirred for 4.5 h. The mixture was then distilled under reduced pressure at 90°C to obtain boric acid diglyceride. Boric acid diglyceride and trimellitic anhydride chloride were weighed in a molar ratio of 1:2. Trimellitic anhydride chloride was mixed with an equimolar amount of triethylamine and dissolved in N,N-dimethylformamide in a mass ratio of 6 times that of trimellitic anhydride chloride. The mixture was stirred at room temperature for 5 min to obtain a trimellitic anhydride chloride mixture. Boric acid diglyceride and N,N-dimethylformamide were mixed in a mass ratio of 1:8. Under nitrogen protection, the mixture was stirred at room temperature for 5 min. Trimellitic anhydride chloride mixture was added at a constant rate over 20 min. The mixture was stirred for 40 min and heated to 65°C. The mixture was stirred for 2 h and cooled to room temperature. The mixture was filtered and dried under vacuum at 75°C for 8 h to obtain a flame-retardant monomer.

[0040] (2) 4,4'-diaminodiphenyl ether and N,N-dimethylformamide were mixed evenly at a mass ratio of 1:26. Under nitrogen protection, the mixture was stirred at 4°C for 3 min. Flame retardant monomer with a molar ratio of 1.03 times that of 4,4'-diaminodiphenyl ether was added, and the reaction was continued for 1.5 h. The mixture was then aged at 65°C until the kinematic viscosity reached 200 cP. Polyamic acid microspheres were obtained by electrostatic spraying. The process parameters were: positive pressure 22 kV, negative pressure 7 kV, and distance between the receiving plate and the nozzle 20 cm. m, solution flow rate 1.5 mL / h; polyamic acid microspheres were dried at 350℃ for 1 h to obtain polyimide microspheres; the polyimide microspheres were immersed in 0.25 mol / L sodium hydroxide solution with a mass of 20 times their mass, etched for 10 min, washed 4 times with deionized water, acidified in 3wt% acetic acid solution with a mass of 12 times their mass for 15 min, washed 4 times with deionized water, and vacuum dried at 60℃ for 20 h to obtain carboxylated polyimide microspheres;

[0041] (3) Mix propylene phosphoric acid, acrylic acid, benzoyl peroxide and anhydrous acetone in a mass ratio of 1:0.8:0.03:7 and stir for 10 min to obtain a grafting solution; irradiate the polyethylene film under an electron accelerator for 5 min with an irradiation dose of 50 kGy to obtain a pre-irradiated polyethylene film; immerse the pre-irradiated polyethylene film in 12 times its mass of the grafting solution, heat it to 60°C under nitrogen protection, reflux and stir for 2 h, take it out, wash it 5 times with anhydrous acetone, and vacuum dry it at 65°C for 3 h to obtain a modified polyethylene film;

[0042] (4) Carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution were mixed evenly at a mass ratio of 1:0.08:0.08:12. The mixture was ultrasonically dispersed at room temperature for 5 min, stirred for 3 h, and allowed to stand for 5 min. The mixture was then poured into a coating machine and evenly coated on the surface of the modified polyethylene film. The coating thickness was 3 μm and the coating speed was 0.6 m / min. The mixture was then vacuum dried at 55 °C for 7 h to obtain a fire-retardant battery separator.

[0043] Comparative Example 1:

[0044] The difference between the preparation method of the fire-retardant battery separator in Comparative Example 1 and Example 2 is that step (1) is omitted, and step (2) is changed to: 4,4'-diaminodiphenyl ether and N,N-dimethylformamide are mixed evenly at a mass ratio of 1:25, stirred at 2°C for 5 min under nitrogen protection, 1.02 times the molar amount of 4,4'-diaminodiphenyl ether in pyromellitic anhydride is added, the reaction is continued to be stirred for 2 h, and aged at 60°C until the kinematic viscosity is 150 cP. Polyamic acid microspheres are obtained by electrostatic spraying. The process parameters are positive pressure. Under a voltage of 20 kV, a negative voltage of 5 kV, a distance of 18 cm between the receiving plate and the nozzle, and a solution flow rate of 1 mL / h, polyamic acid microspheres were dried at 300 °C for 1.5 h to obtain polyimide microspheres. The polyimide microspheres were then immersed in a 0.2 mol / L sodium hydroxide solution (15 times their weight) for 15 min of etching, washed three times with deionized water, and then acidified in a 3 wt% acetic acid solution (10 times their weight) for 20 min of acidification. After washing three times with deionized water, the microspheres were vacuum dried at 55 °C for 22 h to obtain carboxylated polyimide microspheres. The remaining steps were the same as in Example 2.

[0045] Comparative Example 2:

[0046] The difference between the preparation method of the fire-retardant battery separator in Comparative Example 2 and Example 2 is that steps (1), (2), and (4) are omitted, and step (3) is changed to: propylene phosphoric acid, acrylic acid, benzoyl peroxide, and anhydrous acetone are mixed evenly in a mass ratio of 1:0.7:0.02:6 and stirred for 15 min to obtain a grafting solution; polyethylene film is irradiated for 10 min under an electron accelerator with an irradiation dose of 40 kGy to obtain a pre-irradiated polyethylene film; the pre-irradiated polyethylene film is immersed in 11 times its mass of the grafting solution, heated to 55°C under nitrogen protection, refluxed and stirred for 2.5 h, taken out, washed 4 times with anhydrous acetone, and vacuum dried at 60°C for 4 h to obtain the fire-retardant battery separator.

[0047] Comparative Example 3:

[0048] The difference between the preparation method of the fire-retardant battery separator in Comparative Example 3 and Example 2 is that step (3) is omitted, and step (4) is changed to: mixing carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution in a mass ratio of 1:0.075:0.075:11, ultrasonically dispersing at room temperature for 10 min, stirring for 3.5 h, standing for 10 min, pouring into a coating machine, uniformly coating on the surface of a polyethylene film with a coating thickness of 2 μm and a coating speed of 0.5 m / min, and vacuum drying at 50 °C for 8 h to obtain the fire-retardant battery separator. The remaining steps are the same as in Example 2.

[0049] Test Example 1

[0050] Flame retardant performance test

[0051] Test method: The fire-retardant battery separators of the examples and comparative examples were cut into standard samples with a size of 2cm × 10cm, and the limiting oxygen index was tested using an HC-2 oxygen index tester. The results are shown in Table 1.

[0052] Table 1

[0053]

[0054] A comparison of the experimental data from Examples 1-3 and Comparative Examples 1-3 in Table 1 reveals that the fire-retardant battery separator prepared by this invention has good flame-retardant properties.

[0055] By comparison, the limiting oxygen index of Examples 1-3 is greater than that of Comparative Examples 1-2, indicating that the esterification reaction of boric acid and glycerol yields boric acid diglyceride; the mixture of trimellitic anhydride chloride and triethylamine is dissolved in N,N-dimethylformamide to obtain trimellitic anhydride chloride solution; the reaction of boric acid diglyceride and trimellitic anhydride chloride solution yields flame-retardant monomer; the structure of boric acid diglyceride undergoes ester bond cleavage at high temperature, releasing boric acid and glycerol derivatives; boric acid can act as a char-forming catalyst to catalyze the dehydration and cross-linking of the polyethylene membrane matrix, inhibiting the volatilization of small molecule combustible gases generated by thermal decomposition; as the temperature continues to rise, boric acid further dehydrates to become boron oxide, and at temperatures above 325°C, boron oxide softens into a glassy state, melts and flows on the polymer surface, coating the surface of the carbon layer to form a dense inorganic-organic composite barrier layer, blocking heat transfer and oxygen diffusion, and preventing internal combustible gases from escaping outward, achieving "solid-phase flame retardancy," thereby endowing the fire-retardant battery separator with excellent flame-retardant properties.

[0056] By comparison, the limiting oxygen index of Examples 1-3 is greater than that of Comparative Example 3, indicating that by copolymerizing propylene-based phosphoric acid and acrylic acid onto the surface of a polyethylene film to prepare a modified polyethylene film, phosphorus-containing groups and carboxyl groups are introduced onto the surface of the polyethylene film through copolymer grafting. The phosphoric acid groups undergo a dehydration reaction at high temperature to generate polyphosphoric acid, which promotes the rapid formation of a continuous and dense carbon layer on the surface of the polyethylene film and the grafted layer. The P2O5 generated by the decomposition of polyphosphoric acid is further converted into glassy phosphate esters, which coat the surface of the carbon layer, improve the structural strength of the carbon layer, prevent the carbon layer from collapsing under the impact of flame, and block the heat and oxygen transfer path. Furthermore, at high temperature, phosphorus decomposes to generate phosphorus-containing free radicals such as PO· and HPO·. These free radicals can capture active free radicals in the flame, interrupt the chain reaction of gas-phase combustion, and the inert gas generated by decomposition will dilute the concentration of combustible gas in the combustion area, reduce the flame propagation speed, and achieve "gas-phase flame retardancy", thereby further improving the flame retardant performance of the fireproof and flame-retardant battery separator.

[0057] Test Example 2

[0058] Wetting performance test

[0059] Test method: The fire-retardant battery separators of the examples and comparative examples were cut into standard samples with a size of 3cm × 3cm, placed on a glass slide, and 5μL of electrolyte was added. After standing for 15s, the static contact angle was measured using a fully automatic contact angle measuring instrument. The results are shown in Table 2.

[0060] Table 2

[0061]

[0062] A comparison of the experimental data from Examples 1-3 and Comparative Examples 1-3 in Table 2 reveals that the fire-retardant battery separator prepared by this invention has good wetting properties.

[0063] By comparison, the contact angles of Examples 1-3 are smaller than those of Comparative Example 2, indicating that polyamic acid microspheres are prepared by electrostatic spraying of 4,4'-diaminodiphenyl ether and flame-retardant monomers; polyimide microspheres are prepared by dehydrating and drying the polyamic acid microspheres; and carboxylated polyimide microspheres are prepared by etching and acidifying the polyimide microspheres, thereby introducing carboxyl groups on the surface of the polyimide microspheres. The carboxyl groups on the surface of the carboxylated polyimide microspheres are strong polar groups, which can form hydrogen bonds with carbonate molecules in the electrolyte, thereby reducing the contact angle of the electrolyte on the surface of the fire-retardant battery separator, thus giving the fire-retardant battery separator excellent wetting properties.

[0064] By comparison, the contact angles of Examples 1-3 are smaller than those of Comparative Example 3, indicating that by copolymerizing propylene-based phosphoric acid and acrylic acid onto the surface of a polyethylene film to prepare a modified polyethylene film, phosphorus-containing groups and carboxyl groups are introduced onto the surface of the polyethylene film through copolymer grafting. Both carboxyl and phosphoric acid groups are strongly polar hydrophilic groups, which can form hydrogen bonds with carbonate molecules in the electrolyte, allowing the electrolyte to spread rapidly and penetrate into the microporous channels of the polyethylene film, thereby further improving the wetting performance of the fire-retardant battery separator.

[0065] Test Example 3

[0066] Thermal stability performance test

[0067] Test Method: The fire-retardant battery separators of the examples and comparative examples were cut into standard samples with a size of 3cm × 3cm. Each sample was placed between two sheets of A4 paper and then in an oven. The temperature was increased from room temperature to 180℃ at a rate of 5℃ / min, and held at each temperature setting for 30 minutes. The samples were then removed, and the surface area S (cm²) was measured. 2 Thermal stability efficiency = S / 9 × 100%. The results are shown in Table 3.

[0068] Table 3

[0069]

[0070] A comparison of the experimental data of Examples 1-3 and Comparative Examples 1-3 in Table 3 shows that the fire-retardant battery separator prepared by the present invention has good thermal stability.

[0071] By comparison, the thermal stability efficiency of Examples 1-3 is greater than that of Comparative Example 2, indicating that polyamic acid microspheres are prepared by electrostatic spraying after polymerization of 4,4'-diaminodiphenyl ether and flame retardant monomers; polyimide microspheres are prepared by dehydration and drying of polyamic acid microspheres; and carboxylated polyimide microspheres are prepared by etching and acidification of polyimide microspheres, introducing carboxyl groups on the surface of polyimide microspheres. The polyimide molecular chain contains a large number of rigid aromatic rings and imide rings, with high chemical bond energy and a thermal decomposition temperature >500℃, which is much higher than the melting temperature of polyethylene film. When the battery is under high temperature conditions such as overcharging and short circuit, the carboxylated polyimide microspheres will not melt and decompose, and can form a physical support skeleton on the surface of the fire-retardant battery separator, hindering the thermal movement and slippage of PE molecular chains, effectively reducing the thermal shrinkage rate of the fire-retardant battery separator, thereby giving the fire-retardant battery separator excellent thermal stability performance.

[0072] By comparison, the thermal stability efficiency of Examples 1-3 is greater than that of Comparative Example 3, indicating that by grafting propylene-based phosphoric acid and acrylic acid copolymer onto the surface of a polyethylene film to obtain a modified polyethylene film, phosphorus-containing groups and carboxyl groups are introduced onto the surface of the polyethylene film through copolymer grafting. The copolymer chains of propylene-based phosphoric acid and acrylic acid are covalently anchored to the surface of the polyethylene film through free radical grafting reaction, and slight cross-linking is formed between adjacent grafted chains to construct a flexible cross-linked network, which restricts the thermal movement and slippage of polyethylene molecular chains at high temperatures, thereby reducing the thermal shrinkage rate of the fire-retardant battery separator and further improving the thermal stability performance of the fire-retardant battery separator.

[0073] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of making a fire resistant, flame retardant battery separator, characterized by, The method for preparing the fire-retardant battery separator includes the following preparation steps: (1) Mix borate diglyceride and N,N-dimethylformamide at a mass ratio of 1:(6~8), stir at room temperature for 5~15 min under nitrogen protection, add trimellitic anhydride chloride mixture at a uniform rate over 20~30 min, continue stirring for 40~60 min, heat to 55~65℃ and stir for 2~4 h, cool to room temperature, filter, and vacuum dry at 65~75℃ for 8~10 h to obtain flame retardant monomer; (2) Mix 4,4'-diaminodiphenyl ether and N,N-dimethylformamide at a mass ratio of 1:(24~26) until homogeneous. Under nitrogen protection, stir at 0~4℃ for 3~7 min. Add flame retardant monomer at 1.01~1.03 times the molar amount of 4,4'-diaminodiphenyl ether and continue stirring for 1.5~2.5 h. Aging at 55~65℃ until the kinematic viscosity is 100~200 cP, and electrostatic spray to obtain polyamic acid microspheres. Then, heat the polyamic acid microspheres at 250℃. Polyimide microspheres were obtained by drying at ~350℃ for 1~2h; the polyimide microspheres were then immersed in 0.15~0.25mol / L sodium hydroxide solution at 10~20 times their weight, etched for 10~20min, washed with deionized water 2~4 times, acidified by immersion in 3wt% acetic acid solution at 8~12 times their weight for 15~25min, washed with deionized water 2~4 times, and vacuum dried at 50~60℃ for 20~24h to obtain carboxylated polyimide microspheres. (3) Irradiate the polyethylene film under an electron accelerator for 5-15 min with an irradiation dose of 30-50 kGy to obtain a pre-irradiated polyethylene film; mix propylene phosphoric acid, acrylic acid, benzoyl peroxide and anhydrous acetone in a mass ratio of 1:(0.6-0.8):(0.01-0.03):(5-7) evenly and stir for 10-20 min to obtain a grafting solution; immerse the pre-irradiated polyethylene film in 10-12 times its mass of the grafting solution, heat it to 50-60℃ under nitrogen protection, reflux and stir for 2-3 h, take it out, wash it with anhydrous acetone 3-5 times, and vacuum dry it at 55-65℃ for 3-5 h to obtain a modified polyethylene film; (4) Mix carboxylated polyimide microspheres, polyvinylpyrrolidone, sodium carboxymethyl cellulose, and 50 vol% ethanol aqueous solution in a mass ratio of 1:(0.07~0.08):(0.07~0.08):(10~12), ultrasonically disperse at room temperature for 5~15 min, stir for 3~4 h, let stand for 5~15 min, pour into a coating machine, and coat evenly on the surface of the modified polyethylene film. The coating thickness is 1~3 μm, the coating speed is 0.4~0.6 m / min, and vacuum dry at 45~55℃ for 7~9 h to obtain a fire-retardant battery separator.

2. The method for preparing a fire-retardant battery separator according to claim 1, characterized in that, The preparation steps of borate diglyceride in step (1) are as follows: Boric acid and glycerol in a molar ratio of 1:(2~2.2) are mixed with toluene in a mass ratio of 0.9~1.1 times that of boric acid. Under nitrogen protection, the mixture is heated at a constant rate to 105~115℃ and stirred for 4.5~5.5h. The mixture is then distilled under reduced pressure at 80~90℃ to obtain borate diglyceride. Borate diglyceride and trimellitic anhydride chloride are weighed in a molar ratio of 1:

2.

3. The method for preparing a fire-retardant battery separator according to claim 1, characterized in that, The preparation steps of the trimellitic anhydride chloride mixture in step (1) are as follows: Trimetriatic anhydride chloride is mixed with an equimolar amount of triethylamine, dissolved in N,N-dimethylformamide at 4 to 6 times the mass of trimellitic anhydride chloride, and stirred at room temperature for 5 to 15 minutes to obtain the trimellitic anhydride chloride mixture.

4. The method for preparing a fire-retardant battery separator according to claim 1, characterized in that, The electrostatic spraying process parameters in step (2) are: positive pressure 18~22kV, negative pressure 3~7kV, distance between receiving plate and nozzle 16~20cm, and solution flow rate 0.5~1.5mL / h to obtain polyamic acid microspheres.

5. The method for preparing a fire-retardant battery separator according to claim 1, characterized in that, The polyethylene film in step (3) has a specification of 7 μm.

6. A fire-retardant battery separator, characterized in that, The fire-retardant battery separator is prepared by the method described in any one of claims 1 to 5.

7. A fire-retardant battery, characterized in that, The battery separator of the fire-retardant battery is prepared by the method described in any one of claims 1 to 5.

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