A lithium battery separator and a method of making the same

Abstract

This invention relates to the field of battery technology, specifically to a lithium battery separator and its preparation method. The method includes: subjecting a polypropylene microporous separator to ultraviolet ozone treatment on the side facing the negative electrode; then fixing initiation sites through a polydopamine layer; subsequently, using atom transfer radical polymerization, grafting a copolymer of lithium sulfonyl imide methacrylate monomer and polyethylene glycol methyl ether methacrylate onto the negative electrode side in stages to form a grafting density gradient; and finally, grafting 2,2,2-trifluoroethyl methacrylate onto the brush layer ends. The separator prepared by this invention can effectively increase the lithium-ion transport number, homogenize the lithium-ion flux on the negative electrode side, significantly extend the short-circuit time of lithium symmetric batteries, improve the full-cell cycle capacity retention rate, and significantly reduce the risk of dendrite puncture after cycling, without significantly sacrificing pore permeability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a lithium battery separator and its preparation method. Background Technology

[0002] Lithium metal is considered an ideal anode material for next-generation high-energy-density batteries due to its high theoretical capacity and low electrochemical potential. However, the commercial application of lithium metal batteries has long been limited by safety hazards and cycle life degradation caused by lithium dendrite growth. Polyolefin microporous membranes are currently the mainstream membrane for lithium batteries due to their good electrochemical stability and mechanical strength, but they are non-selective for lithium-ion migration and have an inert surface, making it difficult to control the uniform deposition of lithium ions.

[0003] To improve membrane performance, existing technologies have developed various functionalization schemes, including ceramic / polymer coatings, lithiophilic coatings, and single-ion conductor interface layers. These methods typically aim to modulate lithium-ion deposition, suppress dendrites, enhance mechanical properties, or improve thermal safety. Among these, constructing a single-ion conductor interface layer on the membrane surface is considered an effective way to increase the lithium-ion transference number, reduce concentration polarization, and thus improve the uniformity of lithium deposition.

[0004] However, existing functionalization strategies still face significant challenges. On the one hand, uniformly coating or grafting functional layers onto the membrane surface or within the pores often partially blocks the pores, significantly increasing ion transport resistance and sacrificing the membrane's inherent advantages of high permeability and low impedance. On the other hand, if modification is only performed on one side of the membrane, conventional methods struggle to create precise compositional and structural gradients at the pore openings and near the pore ends, making it impossible to achieve spatial control of lithium-ion flux on the negative electrode side. Furthermore, a single component cannot simultaneously meet the demands of high lithium-ion mobility and rapid ion transport. For example, high-density fixed anion sites may restrict chain segment movement, while introducing only flexible segments lacks the ability to anchor anions, resulting in limited improvement in ion mobility. Therefore, developing a functional membrane that can balance high ion mobility, excellent pore connectivity, and effective dendrite suppression is a pressing technological challenge. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a lithium battery separator and its preparation method, so as to solve the technical problem that existing lithium battery separator functionalization methods are difficult to improve lithium ion migration number, homogenize ion flux on the negative electrode side and suppress lithium dendrite puncture without significantly sacrificing the pore permeability and air permeability of the separator.

[0006] To achieve the above objectives, the present invention provides a lithium battery separator, comprising a polypropylene microporous separator substrate, wherein the polypropylene microporous separator substrate has a grafted functional layer on the side facing the negative electrode and the proximal end of its pores and channels when the battery is assembled. The grafted functional layer includes a polydopamine interface layer, a copolymer brush layer grafted onto the polydopamine interface layer, and a fluorinated end group grafted onto the end of the copolymer brush layer. The copolymer brush layer is formed by surface-initiated atom transfer radical polymerization of lithium sulfonylimide methacrylate monomer and polyethylene glycol methyl ether methacrylate. Treat 80-120cm 2 The exposed area of ​​the polypropylene microporous membrane meter, the grafting monomer raw materials forming the copolymer brush layer are divided into a first grafting monomer raw material for full immersion grafting and a second grafting monomer raw material for single-sided grafting on the negative electrode side. The first grafting monomer raw material includes 0.55-1 parts of lithium sulfonylimide methacrylate monomer and 0.3-0.6 parts of polyethylene glycol methyl ether methacrylate. The second grafting monomer raw material includes 0.12-0.18 parts of lithium sulfonylimide methacrylate monomer and 0.15-0.35 parts of polyethylene glycol methyl ether methacrylate. The grafting monomer raw material forming the fluorinated end group includes 0.18-0.35 parts of 2,2,2-trifluoroethyl methacrylate.

[0007] Preferably, the polypropylene microporous membrane substrate is a single-layer polypropylene microporous membrane with a thickness of 20-30 μm; the number-average molecular weight of the polyethylene glycol methyl ether methacrylate is 200-400.

[0008] Preferably, the sulfonylimide lithium salt type methacrylate monomer is obtained by acyl chloride of potassium 3-sulfonate methacrylate, reaction with trifluoromethanesulfonamide, and lithiation with lithium hydroxide; the mass ratio of potassium 3-sulfonate methacrylate, oxaloyl chloride, trifluoromethanesulfonamide, triethylamine and lithium hydroxide monohydrate is 8-12:5.5-7.4:5.2-7.6:7.5-10.8:1.45-2.2.

[0009] Preferably, the polydopamine interface layer is formed by depositing dopamine hydrochloride in a tris(hydroxymethyl)aminomethane buffer solution at pH 8, and the deposition includes pre-deposition on the negative electrode side surface and immersion deposition on the polypropylene microporous membrane substrate.

[0010] Preferably, the polydopamine interface layer is fixed with ATRP initiation sites obtained by grafting 2-bromoisobutyryl bromide; the copolymer brush layer and the fluorinated end groups are both formed by surface grafting initiated by the ATRP initiation sites.

[0011] The present invention also provides a method for preparing a lithium battery separator, comprising the following steps: (1) Preparation of lithium sulfonylimide methacrylate monomer; (2) When using polypropylene microporous membranes for battery assembly, the side facing the negative electrode is subjected to ultraviolet ozone treatment. (3) A polydopamine interface layer is formed on the negative electrode side of the polypropylene microporous membrane obtained in step (2); (4) ATRP initiation sites were immobilized on the polydopamine interface layer using 2-bromoisobutyryl bromide; (5) The diaphragm obtained in step (4) is completely immersed in the first-stage reaction solution containing lithium sulfonyl imide methacrylate monomer and polyethylene glycol methyl ether methacrylate to carry out the first-stage grafting reaction; (6) The second-stage reaction solution containing lithium sulfonylimide methacrylate monomer and polyethylene glycol methyl ether methacrylate is added dropwise to the negative electrode side of the membrane obtained in step (5) to carry out the second-stage grafting reaction; (7) The third-stage reaction liquid containing 2,2,2-trifluoroethyl methacrylate is added dropwise to the negative electrode side of the separator obtained in step (6) to carry out the end-group grafting reaction and obtain the lithium battery separator.

[0012] Preferably, step (2) includes: immersing the polypropylene microporous membrane in ethanol and deionized water for 10 min each, and then drying it at 60°C for 2 h; laying the dried polypropylene microporous membrane flat on a polytetrafluoroethylene plate with the negative electrode side facing up, fixing the four sides of the membrane, and exposing the area to be 80 mm × 80 mm to 120 mm × 120 mm; and subjecting the exposed area to ultraviolet ozone treatment for 5-12 min in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface being 8-12 cm.

[0013] Preferably, step (5) includes: mixing lithium sulfonyl imide methacrylate monomer, polyethylene glycol methyl ether methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, methanol and deionized water to obtain a first-stage reaction solution; completely immersing the diaphragm obtained in step (4) in the first-stage reaction solution, pre-soaking at 35°C for 45-75 min, then adding L-ascorbic acid, continuing the reaction at 35°C for 20-45 min, rinsing with methanol after the reaction, placing the diaphragm between two layers of nonwoven fabric, and pressing it under a 500g load for 30 s.

[0014] Preferably, step (6) includes: mixing lithium sulfonyl imide methacrylate monomer, polyethylene glycol methyl ether methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, L-ascorbic acid, methanol and deionized water to obtain a second-stage reaction solution; laying the wet film obtained in step (5) with the negative electrode side facing up on a polytetrafluoroethylene plate, uniformly adding all of the second-stage reaction solution to the exposed area on the negative electrode side, maintaining at 35°C for 15-35 min, and rinsing.

[0015] Preferably, step (7) includes: mixing 2,2,2-trifluoroethyl methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, L-ascorbic acid and methanol to obtain a third-stage reaction solution; laying the membrane obtained in step (6) with the negative electrode side facing up on a polytetrafluoroethylene plate, adding all of the third-stage reaction solution to the exposed area on the negative electrode side, and reacting at 35°C for 5-18 min; washing and vacuum drying.

[0016] The beneficial effects of this invention are: (1) This invention provides a stable and highly active initiation site for subsequent atom transfer radical polymerization by treating the negative electrode side of the polypropylene membrane with ultraviolet ozone and constructing a polydopamine interface layer, thereby achieving selective and high-density grafting of functional polymers onto the negative electrode side, which is the basis for forming an effective gradient control layer.

[0017] (2) This invention creatively employs a combination of "first-stage full immersion low-concentration grafting" and "second-stage single-sided high-concentration short-time grafting on the negative electrode side," which enables the sulfonylimide lithium salt polymer brush to form an ideal grafting density gradient on the pore openings and near the pores of the separator on the negative electrode side. This structure efficiently controls the lithium-ion flow preferentially in the interface region near the lithium negative electrode without excessively increasing the mass transfer resistance.

[0018] (3) In this invention, lithium sulfonylimide methacrylate monomer and polyethylene glycol methyl ether methacrylate are introduced into the graft layer simultaneously, and the two produce a significant synergistic effect. The sulfonylimide group acts as a fixed anion site, which greatly increases the lithium ion transference number; while the ether oxygen segment of polyethylene glycol methyl ether methacrylate provides a channel to assist lithium ion hopping.

[0019] (4) In this invention, 2,2,2-trifluoroethyl methacrylate is further grafted onto the end of the polymer brush. The fluorinated end groups formed help to reduce the interfacial energy and adjust the solvation structure of the electrolyte, thereby further suppressing dendrite growth and side reactions.

[0020] (5) By precisely controlling the reaction conditions, the present invention constrains the thickness of the swollen brush layer within a reasonable range, ensuring the basic continuity of the channels. Examples 1-5 of the present invention, while maintaining acceptable air permeability, comprehensively achieve high lithium-ion migration number, long short-circuit time, high capacity retention rate and low dendrite penetration, successfully transforming the separator from a passive barrier layer into an active intelligent regulation layer for lithium-ion flux on the negative electrode side. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0022] The main raw materials used are as follows: the polypropylene microporous membrane is Celgard2400 from Celgard LLC, with a thickness of 25μm, and is a single-layer polypropylene microporous membrane; the number average molecular weight of polyethylene glycol methyl ether methacrylate is 300.

[0023] The ultraviolet ozone treatment uses a low-pressure mercury lamp with emission lines of 185nm and 254nm, and the ultraviolet power density of the lamp tube at the diaphragm surface is 8-15mW / cm². 2 Maintain a suitable air atmosphere during the treatment process, with an ambient temperature of 20-30℃ and a relative humidity of 30%-60%.

[0024] Example 1: Step 1: Weigh 10g of potassium propyl 3-sulfonate methacrylate and 0.02g of 4-methoxyphenol. After vacuum drying at 60℃ for 12h, add them to a three-necked flask equipped with a mechanical stirrer, a constant pressure dropping funnel, and a nitrogen port. Then add 80g of anhydrous acetonitrile and 0.08g of N,N-dimethylformamide. Lower the system to 0℃ and add 6.2g of oxaloyl chloride dropwise under nitrogen protection, controlling the addition time to 30min. After the addition is complete, continue stirring at 0℃ for 30min, then raise the temperature to 40℃ and react for 6h. After the reaction is complete, filter under nitrogen protection to remove inorganic salts. Remove volatile components from the filtrate under reduced pressure at 35℃ to obtain a sulfonyl chloride intermediate containing a methacrylate structure. Add 60g of anhydrous acetonitrile, 6.4g of trifluoromethanesulfonamide, 9g of triethylamine, and 0.02g of... 4-Methoxyphenol was stirred at 0°C for 1 hour, then the temperature was increased to 25°C and stirred for 12 hours. 1.8 g of lithium hydroxide monohydrate and 30 g of methanol were added, and the mixture was stirred at 25°C for 4 hours. The reaction solution was filtered and added dropwise to anhydrous diethyl ether. The precipitate was collected, washed with anhydrous diethyl ether, dissolved in methanol, and then added dropwise to anhydrous diethyl ether for reprecipitation. The resulting solid was dried under vacuum at 50°C for 12 hours to obtain the sulfonylimide lithium salt type methacrylate monomer. Step 2: Immerse the polypropylene microporous membrane in 50g of ethanol and 50g of deionized water for 10 minutes each, then dry it at 60℃ for 2 hours. Lay the dried membrane flat on a clean polytetrafluoroethylene plate with the side intended to face the negative electrode facing up, fixing the four sides of the membrane, with an exposed area of ​​100mm×100mm; treat the exposed negative electrode side with ultraviolet ozone for 8 minutes in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface 10cm. Proceed to the next step immediately after treatment. Step 3: Attach the untreated side of the membrane obtained in Step 2 to a polytetrafluoroethylene (PTFE) plate. Add 20g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.04g dopamine hydrochloride, pH 8) evenly to the exposed area on the negative electrode side. Let it stand at 25°C for 30min. Then remove the membrane from the PTFE plate and add it to 140g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.28g dopamine hydrochloride, pH 8). React at 25°C and 100rpm for 60min. After the reaction, wash with deionized water, then with methanol, and dry at 40°C for 4h. Immediately proceed to the next step.

[0025] Step 4: Add the diaphragm obtained in Step 3 to a mixture of 100g anhydrous dichloromethane and 6g triethylamine. Under nitrogen protection at 0℃, add 2.3g 2-bromoisobutyryl bromide dropwise over 20min. Continue the reaction at 0℃ for 30min, then raise the temperature to 25℃ and react for 3h. After the reaction is complete, wash with 60g dichloromethane, 60g methanol, and 60g deionized water sequentially, and dry under vacuum at 40℃ for 6h. Then immediately proceed to the next step. Step 5: Mix 0.75g of lithium sulfonylimide methacrylate monomer, 0.45g of polyethylene glycol methyl ether methacrylate, 0.022g of copper dibromide, 0.07g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 35g of methanol, and 15g of deionized water to obtain the first-stage reaction solution; completely immerse the membrane obtained in Step 4 in the first-stage reaction solution and pre-soak it at 35°C for 60min, then add 0.035g of L-ascorbic acid and continue the reaction at 35°C for 30min. After the reaction is completed, rinse with methanol, place the membrane between two layers of dust-free polypropylene nonwoven fabric, press it under a 500g load for 30s, and then immediately proceed to the next step; Step 6: Mix 1.5g of lithium sulfonylimide methacrylate monomer, 0.25g of polyethylene glycol methyl ether methacrylate, 0.015g of copper dibromide, 0.055g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.04g of L-ascorbic acid, 20g of methanol, and 6g of deionized water to obtain the second-stage reaction solution; lay the wet film obtained in Step 5 with the negative electrode side facing up on a polytetrafluoroethylene plate, and uniformly drop the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintain at 35℃ for 25min, then immediately rinse with methanol, then rinse with deionized water, and then immediately proceed to the next step; Step 7: Mix 0.26g of 2,2,2-trifluoroethyl methacrylate, 0.008g of copper dibromide, 0.025g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.015g of L-ascorbic acid, and 20g of methanol to obtain the third-stage reaction solution; lay the membrane obtained in Step 6 with the negative electrode side facing up on a polytetrafluoroethylene plate, and add all of the third-stage reaction solution dropwise to the exposed area on the negative electrode side, and react at 35℃ for 10min; after the reaction, wash with methanol, then wash with deionized water, and vacuum dry at 45℃ until the mass difference between two consecutive weighings 30min apart does not exceed 2mg to obtain the lithium battery separator.

[0026] Example 2: Step 1: Weigh 8g of potassium propyl 3-sulfonate methacrylate and 0.015g of 4-methoxyphenol. After vacuum drying at 55℃ for 10h, add them to a three-necked flask equipped with a mechanical stirrer, a constant pressure dropping funnel, and a nitrogen port. Then add 70g of anhydrous acetonitrile and 0.05g of N,N-dimethylformamide. Lower the system to 0℃ and add 5.5g of oxaloyl chloride dropwise under nitrogen protection, controlling the addition time to 25min. After the addition is complete, continue stirring at 0℃ for 25min, then raise the temperature to 38℃ and react for 5h. After the reaction is complete, filter under nitrogen protection to remove inorganic salts. Remove volatile components from the filtrate under reduced pressure at 35℃ to obtain a sulfonyl chloride intermediate containing a methacrylate structure. Add 50g of anhydrous acetonitrile, 5.2g of trifluoromethanesulfonamide, 7.5g of triethylamine, and 0.015g of... 4-Methoxyphenol was stirred at 0°C for 1 hour, then the temperature was increased to 25°C and stirred for 10 hours. 1.45 g of lithium hydroxide monohydrate and 25 g of methanol were added, and the mixture was stirred at 25°C for 4 hours. The reaction solution was filtered and added dropwise to anhydrous diethyl ether. The precipitate was collected, washed with anhydrous diethyl ether, dissolved in methanol, and then added dropwise to anhydrous diethyl ether for reprecipitation. The resulting solid was dried under vacuum at 48°C for 12 hours to obtain the sulfonylimide lithium salt type methacrylate monomer. Step 2: Immerse the polypropylene microporous membrane in 45g of ethanol and 45g of deionized water for 10 minutes each, then dry it at 60℃ for 2 hours. Lay the dried membrane flat on a clean polytetrafluoroethylene plate with the side intended to face the negative electrode facing up, fixing the four sides of the membrane, with an exposed area of ​​80mm×80mm; treat the exposed negative electrode side with ultraviolet ozone for 5 minutes in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface 12cm. Proceed to the next step immediately after treatment. Step 3: Attach the untreated side of the membrane obtained in Step 2 to a polytetrafluoroethylene (PTFE) plate. Add 15g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.03g dopamine hydrochloride, pH 8) evenly to the exposed area on the negative electrode side. Let it stand at 25°C for 20 minutes. Then remove the membrane from the PTFE plate and add it to 120g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.20g dopamine hydrochloride, pH 8). React at 25°C and 90 rpm for 40 minutes. After the reaction, wash with deionized water, then with methanol, and dry at 40°C for 4 hours. Immediately proceed to the next step. Step 4: Add the diaphragm obtained in Step 3 to a mixture of 90g anhydrous dichloromethane and 4.5g triethylamine. Under nitrogen protection at 0℃, add 1.8g 2-bromoisobutyryl bromide dropwise over 20min. Continue the reaction at 0℃ for 25min, then raise the temperature to 25℃ and react for 2h. After the reaction is complete, wash successively with 50000mg dichloromethane, 50000mg methanol, and 50000mg deionized water, and dry under vacuum at 40℃ for 6h. Then immediately proceed to the next step. Step 5: Mix 0.55g of lithium sulfonylimide methacrylate monomer, 0.30g of polyethylene glycol methyl ether methacrylate, 0.018g of copper dibromide, 0.05g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 30g of methanol, and 10g of deionized water to obtain the first-stage reaction solution; completely immerse the membrane obtained in Step 4 in the first-stage reaction solution and pre-soak it at 35°C for 45min, then add 0.025g of L-ascorbic acid and continue the reaction at 35°C for 20min. After the reaction is completed, rinse with methanol, place the membrane between two layers of dust-free polypropylene nonwoven fabric, press it under a 500g load for 30s, and then immediately proceed to the next step; Step 6: Mix 1.2g of lithium sulfonylimide methacrylate monomer, 0.15g of polyethylene glycol methyl ether methacrylate, 0.010g of copper dibromide, 0.040g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.030g of L-ascorbic acid, 16g of methanol, and 4g of deionized water to obtain the second-stage reaction solution; lay the wet film obtained in Step 5 with the negative electrode side facing up on a polytetrafluoroethylene plate, and uniformly drop the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintain at 35℃ for 15min, then immediately rinse with methanol, then rinse with deionized water, and then immediately proceed to the next step; Step 7: Mix 0.18g of 2,2,2-trifluoroethyl methacrylate, 0.005g of copper dibromide, 0.018g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.010g of L-ascorbic acid, and 16g of methanol to obtain the third-stage reaction solution; lay the membrane obtained in Step 6 with the negative electrode side facing up on a polytetrafluoroethylene plate, and add all of the third-stage reaction solution dropwise to the exposed area on the negative electrode side, and react at 35℃ for 5min; after the reaction is completed, wash with methanol, then wash with deionized water, and vacuum dry at 43℃ until the mass difference between two consecutive weighings 30min apart does not exceed 2mg to obtain the lithium battery separator.

[0027] Example 3: Step 1: Weigh 12g of potassium propyl 3-sulfonate methacrylate and 0.03g of 4-methoxyphenol. After vacuum drying at 65℃ for 14h, add them to a three-necked flask equipped with a mechanical stirrer, a constant pressure dropping funnel, and a nitrogen port. Then add 90g of anhydrous acetonitrile and 0.12g of N,N-dimethylformamide. Lower the system to 0℃ and add 7.4g of oxaloyl chloride dropwise under nitrogen protection, controlling the addition time to 40min. After the addition is complete, continue stirring at 0℃ for 40min, then raise the temperature to 45℃ and react for 8h. After the reaction is complete, filter under nitrogen protection to remove inorganic salts. Remove volatile components from the filtrate under reduced pressure at 35℃ to obtain a sulfonyl chloride intermediate containing a methacrylate structure. Add 72g of anhydrous acetonitrile, 7.6g of trifluoromethanesulfonamide, 10.8g of triethylamine, and 0.03g of [unclear text - possibly a chemical compound] to this intermediate. 4-Methoxyphenol was stirred at 0°C for 1 hour, then the temperature was increased to 25°C and stirred for 14 hours. 2.2 g of lithium hydroxide monohydrate and 35 g of methanol were added, and the mixture was stirred at 25°C for 4 hours. The reaction solution was filtered and added dropwise to anhydrous diethyl ether. The precipitate was collected, washed with anhydrous diethyl ether, dissolved in methanol, and then added dropwise to anhydrous diethyl ether for reprecipitation. The resulting solid was dried under vacuum at 52°C for 12 hours to obtain the sulfonylimide lithium salt type methacrylate monomer. Step 2: Immerse the polypropylene microporous membrane in 55g of ethanol and 55g of deionized water for 10 minutes each, then dry it at 60℃ for 2 hours. Lay the dried membrane flat on a clean polytetrafluoroethylene plate with the side intended to face the negative electrode upwards, fixing the four sides of the membrane, with an exposed area of ​​120mm×120mm; treat the exposed negative electrode side with ultraviolet ozone for 12 minutes in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface being 8cm. Proceed to the next step immediately after treatment. Step 3: Attach the untreated side of the membrane obtained in Step 2 to a polytetrafluoroethylene (PTFE) plate. Add 25g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.06g dopamine hydrochloride, pH 8) evenly to the exposed area on the negative electrode side. Let it stand at 25°C for 40 min. Then remove the membrane from the PTFE plate and add it to 160g of a tris(hydroxymethyl)aminomethane buffer containing dopamine hydrochloride (0.35g dopamine hydrochloride, pH 8). React at 25°C and 120 rpm for 80 min. After the reaction, wash with deionized water, then with methanol, and dry at 40°C for 4 h. Then immediately proceed to the next step. Step 4: Add the diaphragm obtained in Step 3 to a mixture of 120g anhydrous dichloromethane and 8g triethylamine. Add 2.8g 2-bromoisobutyryl bromide dropwise over 25 minutes at 0°C under nitrogen protection. Continue the reaction at 0°C for 40 minutes, then raise the temperature to 25°C and react for 4 hours. After the reaction is complete, wash successively with 70000mg dichloromethane, 70000mg methanol, and 70000mg deionized water. Dry under vacuum at 40°C for 6 hours, then immediately proceed to the next step. Step 5: Mix 1.0g of lithium sulfonylimide methacrylate monomer, 0.60g of polyethylene glycol methyl ether methacrylate, 0.028g of copper dibromide, 0.09g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 40g of methanol and 20g of deionized water to obtain the first-stage reaction solution; completely immerse the membrane obtained in Step 4 in the first-stage reaction solution and pre-soak it at 35℃ for 75min, then add 0.045g of L-ascorbic acid and continue the reaction at 35℃ for 45min. After the reaction is completed, rinse with methanol, place the membrane between two layers of dust-free polypropylene nonwoven fabric, press it under a 500g load for 30s, and then immediately proceed to the next step; Step 6: Mix 1.8g of lithium sulfonylimide methacrylate monomer, 0.35g of polyethylene glycol methyl ether methacrylate, 0.020g of copper dibromide, 0.070g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.050g of L-ascorbic acid, 24g of methanol, and 8g of deionized water to obtain the second-stage reaction solution; lay the wet film obtained in Step 5 with the negative electrode side facing up on a polytetrafluoroethylene plate, and uniformly drop the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintain at 35℃ for 35min, then immediately rinse with methanol, then rinse with deionized water, and then immediately proceed to the next step; Step 7: Mix 0.35g of 2,2,2-trifluoroethyl methacrylate, 0.012g of copper dibromide, 0.035g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.025g of L-ascorbic acid, and 24g of methanol to obtain the third-stage reaction solution; lay the membrane obtained in Step 6 with the negative electrode side facing up on a polytetrafluoroethylene plate, and add all of the third-stage reaction solution dropwise to the exposed area on the negative electrode side, and react at 35℃ for 18min; after the reaction, wash with methanol, then wash with deionized water, and vacuum dry at 47℃ until the mass difference between two consecutive weighings 30min apart does not exceed 2mg to obtain the lithium battery separator.

[0028] Example 4: Step 1: Weigh 10g of potassium propyl 3-sulfonate methacrylate and 0.02g of 4-methoxyphenol. After vacuum drying at 60℃ for 12h, add them to a three-necked flask equipped with a mechanical stirrer, a constant pressure dropping funnel, and a nitrogen port. Then add 80g of anhydrous acetonitrile and 0.08g of N,N-dimethylformamide. Lower the system to 0℃ and add 6.2g of oxaloyl chloride dropwise under nitrogen protection, controlling the addition time to 30min. After the addition is complete, continue stirring at 0℃ for 30min, then raise the temperature to 40℃ and react for 6h. After the reaction is complete, filter under nitrogen protection to remove inorganic salts. Remove volatile components from the filtrate under reduced pressure at 35℃ to obtain a sulfonyl chloride intermediate containing a methacrylate structure. Add 60g of anhydrous acetonitrile, 6.4g of trifluoromethanesulfonamide, 9g of triethylamine, and 0.02g of... 4-Methoxyphenol was stirred at 0°C for 1 hour, then the temperature was increased to 25°C and stirred for 12 hours. 1.8 g of lithium hydroxide monohydrate and 30 g of methanol were added, and the mixture was stirred at 25°C for 4 hours. The reaction solution was filtered and added dropwise to anhydrous diethyl ether. The precipitate was collected, washed with anhydrous diethyl ether, dissolved in methanol, and then added dropwise to anhydrous diethyl ether for reprecipitation. The resulting solid was dried under vacuum at 50°C for 12 hours to obtain the sulfonylimide lithium salt type methacrylate monomer. Step 2: Immerse the polypropylene microporous membrane in 50g of ethanol and 50g of deionized water for 10 minutes each, then dry it at 60℃ for 2 hours. Lay the dried membrane flat on a clean polytetrafluoroethylene plate with the side intended to face the negative electrode facing up, fixing the four sides of the membrane, with an exposed area of ​​100mm×100mm; treat the exposed negative electrode side with ultraviolet ozone for 10 minutes in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface 10cm. Proceed to the next step immediately after treatment. Step 3: Attach the untreated side of the membrane obtained in Step 2 to a polytetrafluoroethylene (PTFE) plate. Add 22g of a tris(hydroxymethyl)aminomethane buffer solution containing dopamine hydrochloride (0.05g dopamine hydrochloride, pH 8) evenly to the exposed area on the negative electrode side. Let it stand at 25°C for 35 min. Then remove the membrane from the PTFE plate and add it to 150g of the same buffer solution containing dopamine hydrochloride (0.30g dopamine hydrochloride, pH 8). React at 25°C and 100 rpm for 70 min. After the reaction, wash with deionized water, then with methanol, and dry at 40°C for 4 h. Immediately proceed to the next step. Step 4: Add the diaphragm obtained in Step 3 to a mixture of 105g anhydrous dichloromethane and 6.5g triethylamine. Under nitrogen protection at 0℃, add 2.4g 2-bromoisobutyryl bromide dropwise over 20min. Continue the reaction at 0℃ for 30min, then raise the temperature to 25℃ and react for 3h. After the reaction is complete, wash with 60g dichloromethane, 60g methanol, and 60g deionized water sequentially, and dry under vacuum at 40℃ for 6h. Then immediately proceed to the next step. Step 5: Mix 0.85g of lithium sulfonylimide methacrylate monomer, 0.60g of polyethylene glycol methyl ether methacrylate, 0.024g of copper dibromide, 0.08g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 36g of methanol, and 16g of deionized water to obtain the first-stage reaction solution; completely immerse the membrane obtained in Step 4 in the first-stage reaction solution and pre-soak it at 35°C for 65min, then add 0.038g of L-ascorbic acid and continue the reaction at 35°C for 35min. After the reaction is complete, rinse with methanol, place the membrane between two layers of dust-free polypropylene nonwoven fabric, press it under a 500g load for 30s, and then immediately proceed to the next step; Step 6: Mix 1.4g of lithium sulfonylimide methacrylate monomer, 0.35g of polyethylene glycol methyl ether methacrylate, 0.016g of copper dibromide, 0.058g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.040g of L-ascorbic acid, 20g of methanol, and 6g of deionized water to obtain the second-stage reaction solution; lay the wet film obtained in Step 5 with the negative electrode side facing up on a polytetrafluoroethylene plate, and uniformly drop the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintain at 35℃ for 25min, then immediately rinse with methanol, then rinse with deionized water, and then immediately proceed to the next step; Step 7: Mix 0.24g of 2,2,2-trifluoroethyl methacrylate, 0.008g of copper dibromide, 0.025g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.015g of L-ascorbic acid, and 20g of methanol to obtain the third-stage reaction solution; lay the membrane obtained in Step 6 with the negative electrode side facing up on a polytetrafluoroethylene plate, and add all of the third-stage reaction solution dropwise to the exposed area on the negative electrode side, and react at 35℃ for 10min; after the reaction, wash with methanol, then wash with deionized water, and vacuum dry at 45℃ until the mass difference between two consecutive weighings 30min apart does not exceed 2mg to obtain the lithium battery separator.

[0029] Example 5: Step 1: Weigh 9g of potassium propyl 3-sulfonate methacrylate and 0.018g of 4-methoxyphenol. After vacuum drying at 60℃ for 12h, add them to a three-necked flask equipped with a mechanical stirrer, a constant pressure dropping funnel, and a nitrogen port. Then add 75g of anhydrous acetonitrile and 0.07g of N,N-dimethylformamide. Lower the system to 0℃ and add 5.8g of oxaloyl chloride dropwise under nitrogen protection, controlling the addition time to 30min. After the addition is complete, continue stirring at 0℃ for 30min, then raise the temperature to 40℃ and react for 6h. After the reaction is complete, filter under nitrogen protection to remove inorganic salts. Remove volatile components from the filtrate under reduced pressure at 35℃ to obtain a sulfonyl chloride intermediate containing a methacrylate structure. Add 55g of anhydrous acetonitrile, 5.8g of trifluoromethanesulfonamide, 8.2g of triethylamine, and 0.018g of [unclear text - possibly a chemical compound] to this intermediate. 4-Methoxyphenol was stirred at 0°C for 1 hour, then the temperature was increased to 25°C and stirred for 12 hours. 1.6 g of lithium hydroxide monohydrate and 28 g of methanol were added, and the mixture was stirred at 25°C for 4 hours. The reaction solution was filtered and added dropwise to anhydrous diethyl ether. The precipitate was collected, washed with anhydrous diethyl ether, dissolved in methanol, and then added dropwise to anhydrous diethyl ether for reprecipitation. The obtained solid was dried under vacuum at 50°C for 12 hours to obtain lithium sulfonylimide methacrylate monomer. Step 2: Immerse the polypropylene microporous membrane in 50g of ethanol and 50g of deionized water for 10 minutes each, then dry it at 60℃ for 2 hours. Lay the dried membrane flat on a clean polytetrafluoroethylene plate with the side intended to face the negative electrode upwards, fixing the four sides of the membrane, with an exposed area of ​​100mm×100mm; treat the exposed negative electrode side with ultraviolet ozone for 6 minutes in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface 10cm. Proceed to the next step immediately after treatment. Step 3: Attach the untreated side of the membrane obtained in Step 2 to a polytetrafluoroethylene (PTFE) plate. Add 18g of a tris(hydroxymethyl)aminomethane buffer solution containing dopamine hydrochloride (0.035g dopamine hydrochloride, pH 8) evenly to the exposed area on the negative electrode side. Let it stand at 25°C for 30 min. Then remove the membrane from the PTFE plate and add it to 135g of a tris(hydroxymethyl)aminomethane buffer solution containing dopamine hydrochloride (0.25g dopamine hydrochloride, pH 8). React at 25°C and 100 rpm for 55 min. After the reaction, wash with deionized water, then with methanol, and dry at 40°C for 4 h. Immediately proceed to the next step. Step 4: Add the diaphragm obtained in Step 3 to a mixture of 100g anhydrous dichloromethane and 5.5g triethylamine. Under nitrogen protection at 0℃, add 2.1g 2-bromoisobutyryl bromide dropwise over 20min. Continue the reaction at 0℃ for 30min, then raise the temperature to 25℃ and react for 3h. After the reaction is complete, wash successively with 60g dichloromethane, 60g methanol, and 60g deionized water, and dry under vacuum at 40℃ for 6h. Then immediately proceed to the next step. Step 5: Mix 0.65g of lithium sulfonylimide methacrylate monomer, 0.40g of polyethylene glycol methyl ether methacrylate, 0.020g of copper dibromide, 0.060g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 34g of methanol, and 14g of deionized water to obtain the first-stage reaction solution; completely immerse the membrane obtained in Step 4 in the first-stage reaction solution and pre-soak it at 35℃ for 55min, then add 0.032g of L-ascorbic acid and continue the reaction at 35℃ for 28min. After the reaction is completed, rinse with methanol, place the membrane between two layers of dust-free polypropylene nonwoven fabric, press it under a 500g load for 30s, and then immediately proceed to the next step; Step 6: Mix 1.7g of lithium sulfonylimide methacrylate monomer, 0.20g of polyethylene glycol methyl ether methacrylate, 0.016g of copper dibromide, 0.060g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.045g of L-ascorbic acid, 22g of methanol, and 6g of deionized water to obtain the second-stage reaction solution; lay the wet film obtained in Step 5 with the negative electrode side facing up on a polytetrafluoroethylene plate, and uniformly drop the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintain at 35℃ for 30min, then immediately rinse with methanol, then rinse with deionized water, and then immediately proceed to the next step; Step 7: Mix 0.30g of 2,2,2-trifluoroethyl methacrylate, 0.009g of copper dibromide, 0.028g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 0.018g of L-ascorbic acid, and 22g of methanol to obtain the third-stage reaction solution; lay the membrane obtained in Step 6 with the negative electrode side facing up on a polytetrafluoroethylene plate, and add all of the third-stage reaction solution dropwise to the exposed area on the negative electrode side, and react at 35℃ for 14min; after the reaction, wash with methanol, then wash with deionized water, and vacuum dry at 45℃ until the mass difference between two consecutive weighings 30min apart does not exceed 2mg to obtain the lithium battery separator.

[0030] Comparative Example 1: The difference from Example 1 is that the UV ozone treatment time on the negative side in step 2 is adjusted from 8 min to 0 min, while the other conditions are the same as in Example 1.

[0031] Comparative Example 2: The difference from Example 1 is that in step 3, instead of adding 20g of dopamine hydrochloride-containing tris(hydroxymethyl)aminomethane buffer to the exposed area on the negative electrode side and letting it stand for 30 minutes, the 20g of dopamine hydrochloride-containing tris(hydroxymethyl)aminomethane buffer is combined with the subsequent 140g of dopamine hydrochloride-containing tris(hydroxymethyl)aminomethane buffer to form 160g of dopamine hydrochloride-containing tris(hydroxymethyl)aminomethane buffer, and the membrane obtained in step 2 is directly and completely immersed in it, and reacted at 25°C and 100 rpm for 90 minutes. The other conditions are the same as in Example 1.

[0032] Comparative Example 3: The difference from Example 1 is that in step 5, 0.75g of lithium sulfonylimide methacrylate monomer is replaced by 0.75g of polyethylene glycol methyl ether methacrylate, and in step 6, 1.5g of lithium sulfonylimide methacrylate monomer is replaced by 1.5g of polyethylene glycol methyl ether methacrylate. The total mass of monomers in steps 5 and 6 remains unchanged, and the other conditions are the same as in Example 1.

[0033] Comparative Example 4: The difference from Example 1 is that in step 5, 0.45g of polyethylene glycol methyl ether methacrylate is replaced by 0.45g of lithium sulfonylimide methacrylate monomer by the same mass, and in step 6, 0.25g of polyethylene glycol methyl ether methacrylate is replaced by 0.25g of lithium sulfonylimide methacrylate monomer by the same mass. The total mass of monomers in steps 5 and 6 remains unchanged, and the other conditions are the same as in Example 1.

[0034] Comparative Example 5: The difference from Example 1 is that steps 5 and 6 do not use the sequential combination of the first stage of full immersion low-concentration grafting and the second stage of single-sided high-concentration short-time grafting on the negative electrode side. Instead, the following ingredients are used in step 5: 0.75g of lithium sulfonylimide methacrylate monomer, 0.45g of polyethylene glycol methyl ether methacrylate, 0.022g of copper dibromide, 0.07g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, 35g of methanol, 15g of deionized water, and 0.035g of L-ascorbic acid, and in step 6: 1.5g of lithium sulfonylimide methacrylate monomer, 0.25g of polyethylene glycol methyl ether methacrylate, 0.015g of copper dibromide, 0.055g of N,N,N′,N″,N″-pentamethyldiethylenetriamine, and 0.04g of... L-ascorbic acid, 20g methanol, and 6g deionized water were combined to obtain a combined reaction solution. The diaphragm obtained in step 4 was completely immersed in the combined reaction solution and reacted at 35°C for 55 minutes. After the reaction was completed, it was rinsed with methanol and then rinsed with deionized water. Then, it was treated according to step 7 of Example 1, with the remaining conditions being the same as in Example 1.

[0035] Comparative Example 6: The difference from Example 1 is that in step 7, 0.26g of 2,2,2-trifluoroethyl methacrylate is replaced by 0.26g of polyethylene glycol methyl ether methacrylate, the total mass of monomers in the reaction solution in the third stage remains unchanged, and the other conditions are the same as in Example 1.

[0036] Comparative Example 7: The difference from Example 1 is that the time for maintaining the reaction solution at 35°C in step 6 is adjusted from 25 min to 80 min, while the other conditions are the same as in Example 1.

[0037] Performance testing: Sample Preparation: The lithium battery separators obtained in Examples 1-5 and Comparative Examples 1-7 were used as test samples. Samples used for air permeability and puncture strength testing were cut into 60mm × 60mm squares and equilibrated for 24 hours at 23℃ and 50% relative humidity. Samples used for electrochemical testing were cut into 19mm diameter discs, vacuum dried at 45℃ for 12 hours, and then transferred to an argon glove box with a moisture content ≤0.1ppm and an oxygen content ≤0.1ppm. During battery assembly, the negative electrode side of the separator was positioned facing the lithium metal negative electrode. The electrolyte was a uniform 1mol / L lithium hexafluorophosphate solution of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:1, with 2wt% vinylene carbonate added. 60μL of electrolyte was added to each coin cell. The coin cell model was CR2032, and the sealing pressure was 800kg. After sealing, the cells were allowed to stand at 25℃ for 12 hours before testing.

[0038] Brush layer thickness characterization: The brush layer thickness was measured using silicon wafer samples that were treated simultaneously with the diaphragm. The silicon wafers underwent surface treatment and grafting reaction according to the same steps as in the examples or comparative examples, and were then immersed in the above electrolyte for 30 min. After removal, they were pressed and released under a 500 g load for 30 s using a dust-free polypropylene nonwoven fabric, and the height was immediately measured using an atomic force microscope. The height orientation calibration of the atomic force microscope was performed according to GB / T 27760-2011, with a scanning range of 5 μm × 5 μm and a scanning rate of 1 Hz. Five positions were tested for each sample, and the average value was taken as the swollen brush layer thickness.

[0039] Air permeability test: The air permeability test was conducted according to GB / T 36363-2018. Before the test, the sample was equilibrated for 24 hours in an environment of 23℃ and 50% relative humidity. During the test, the sample was held flat in the air permeability tester fixture, and the test area was 6.45 cm². 2 The pressure difference was 1.21 kPa. The time required for 100 mL of air to pass through the diaphragm was recorded, in seconds per 100 mL. Five different locations were tested for each sample, and the average value was taken.

[0040] Puncture strength test: Puncture strength test was conducted according to GB / T 36363-2018 and in conjunction with GB / T 6672-2001. First, the sample thickness was determined using a mechanical measurement method, with 5 points taken for each sample, and the average thickness was recorded. Then, the diaphragm was flattened and clamped in a puncture fixture, and a 1.0 mm diameter steel needle was used to vertically puncture the diaphragm at a speed of 100 mm / min. The maximum load at the moment of penetration was recorded, and the puncture strength was calculated as the ratio of the maximum load to the average thickness of the diaphragm, in N / μm. Five pieces were tested for each sample, and the average value was recorded.

[0041] Lithium-ion transport number test: The electrochemical testing environment was controlled at 25℃ according to the temperature consistency requirements for electrical performance testing specified in GB / T 36276-2023. A CR2032 symmetrical coin cell with lithium metal sheets, a separator, and a lithium metal sheet structure was assembled in an argon glove box. The lithium metal sheet diameter was 15.6 mm, the thickness was 450 μm, the separator diameter was 19 mm, and the electrolyte volume was 60 μL. The negative electrode side of the separator faced one of the lithium metal sheets. After the battery was allowed to stand for 12 hours, an AC impedance test was performed with a frequency range of 100 kHz to 0.1 Hz and a perturbation voltage of 5 mV. The initial interface resistance was recorded. Subsequently, a 10 mV DC polarization voltage was applied, and the initial current and steady-state current were recorded. The steady-state determination time was 3600 s. After polarization, an AC impedance test was performed again, and the steady-state interface resistance was recorded. The lithium-ion transport number was calculated using the Bruce-Vincent formula.

[0042] Lithium-ion symmetric battery deposition and peeling stability test: The battery assembly method is the same as in test item five. The test environment temperature is 25℃. A constant current deposition and peeling test is performed on the battery test system with a current density of 1mA / cm². 2 The single deposition or stripping capacity is 1 mAh / cm³. 2 The duration of a single half-cycle is 1 hour, and the voltage curve is recorded during the cycle. When the battery voltage suddenly drops to 0V and remains there for 10 minutes, or when the polarization voltage rises to 1V and remains there for 10 minutes, the battery is considered to have short-circuited or failed, and the short-circuit time is recorded in hours.

[0043] Cyclic performance and post-cycle dendrite puncture observation of lithium metal full cells: The electrochemical testing environment was controlled at 25℃ according to the temperature consistency requirements for electrical performance testing specified in GB / T 36276-2023. CR2032 coin cells with lithium metal anode, separator, and lithium nickel cobalt manganese oxide cathode were assembled in an argon glove box. The lithium metal anode had a diameter of 15.6 mm and a thickness of 450 μm, and the lithium nickel cobalt manganese oxide cathode had an areal capacity of 2.0 mAh / cm². 2 The separator diameter was 19 mm, and the electrolyte volume was 60 μL. The negative electrode side of the separator faced the lithium metal negative electrode. After the battery was allowed to stand for 12 hours, it was tested within a voltage range of 2.8-4.3V. It was first cycled for 3 weeks at 0.1C, then for 200 cycles at 1C, and the capacity retention rate was recorded at the 200th cycle. After the cycling was completed, the battery was disassembled in an argon glove box, the separator was removed, and gently rinsed three times with dimethyl carbonate for 10 seconds each time. It was then vacuum dried at 25℃ for 2 hours. The negative electrode side surface of the separator was then observed under the scanning electron microscope conditions specified in Test Item 2. Five 1 mm² areas were selected from each sample. 2 The field of view was used to count the number of dendrite puncture points that penetrated or embedded in the orifice, and the result was converted to points per cm. 2 .

[0044]

[0045] As shown in Table 1, in Comparative Example 1, without UV ozone treatment on the negative electrode side, despite subsequent treatments with dopamine hydrochloride, 2-bromoisobutyryl bromide, and grafting reaction, there were insufficient active sites on the negative electrode side for polydopamine attachment and initiator fixation. The swollen brush layer thickness was only 6.2 nm, the lithium-ion transference number was 0.446, the short-circuit time of the lithium symmetric battery was 326 h, the capacity retention rate after 200 cycles was 70.8%, and the number of dendrite puncture points after cycling reached 54.4 per cm. 2 In Comparative Example 2, after removing the pre-deposition of dopamine hydrochloride on the negative electrode side, the brush layer thickness and air permeability were 10.2 nm and 697.25 s / 100 mL, respectively. However, the lithium ion migration number, short-circuit time, and capacity retention rate were still lower than those in Example 1, indicating that simple full immersion dopamine treatment is difficult to form an effective gradient adjustment layer for the negative electrode side orifice.

[0046] In Comparative Example 3, replacing the lithium sulfonyl imide methacrylate monomer with polyethylene glycol methyl ether methacrylate by mass resulted in a decrease in the lithium-ion migration number to 0.352 and an increase in the dendrite penetration number to 68.8 per cm. 2 This indicates that relying solely on the ether oxygen segment to improve wetting and segmental movement is insufficient to fix anions and homogenize lithium-ion flux.

[0047] In Comparative Example 4, after removing polyethylene glycol methyl ether methacrylate, although the lithium sulfonyl imide salt group increased the lithium ion transference number to 0.661, the capacity retention was only 76.2%, indicating that when the fixed anion sites are too dense or the chain segment movement is insufficient, ion migration and interface stability are still limited.

[0048] Comparative Example 5 used a one-time full immersion grafting method, and the air permeability increased to 776.88 s / 100 mL with a short-circuit time of only 508 h, indicating that uniform grafting without spatial gradient is more likely to increase the pore mass transfer resistance. After removing the 2,2,2-trifluoroethyl methacrylate end group in Comparative Example 6, the short-circuit time and capacity retention rate were 782.0 h and 82.9%, respectively, which were lower than those in Example 1, indicating that the fluorinated end group has an auxiliary effect on the inhibition of solvent retention on the negative electrode side and interfacial side reactions. After extending the grafting time on the negative electrode side in Comparative Example 7, the brush layer thickness increased to 22.7 nm and the air permeability increased to 928.51 s / 100 mL. Although the lithium ion transference number was still 0.724, the cycle stability decreased, indicating that excessive grafting amount will lead to pore blockage and increased polarization.

[0049] Examples 1-5, through a combination of negative electrode-side ultraviolet ozone treatment, a polydopamine interface layer, two-stage grafting of lithium sulfonyl imide methacrylate / polyethylene glycol methyl ether methacrylate, and 2,2,2-trifluoroethyl methacrylate end-group treatment, increased the lithium-ion transference number to 0.641-0.769, extended the short-circuit time of the lithium symmetric battery to 674.0-1186.0 h, and reduced the number of dendrite puncture points after cycling to 4.8-18.4 per cm. 2 In Example 3, under conditions of a swollen brush layer thickness of 14.8 nm and an air permeability of 739.56 s / 100 mL, the lithium-ion transference number reached 0.769, the short-circuit time reached 1186.0 h, the capacity retention rate reached 90.6% after 200 cycles, and the number of dendrite punctures after cycling was only 4.8 per cm. 2 This demonstrates that within the grafting density, fixed anion sites, and brush layer thickness range defined by this invention, the diaphragm can improve lithium-ion guiding migration ability and reduce dendrite puncture risk while maintaining pore connectivity.

[0050] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A lithium battery separator, comprising a polypropylene microporous separator substrate, characterized in that, The polypropylene microporous membrane substrate has a grafted functional layer on the side facing the negative electrode and near the opening and channel of the pores when the battery is assembled. The grafted functional layer includes a polydopamine interface layer, a copolymer brush layer grafted onto the polydopamine interface layer, and a fluorinated end group grafted onto the end of the copolymer brush layer. The copolymer brush layer is formed by surface-initiated atom transfer radical polymerization of lithium sulfonylimide methacrylate monomer and polyethylene glycol methyl ether methacrylate. Treat 80-120cm 2 The exposed area of ​​the polypropylene microporous membrane meter, the grafting monomer raw materials forming the copolymer brush layer are divided into a first grafting monomer raw material for full immersion grafting and a second grafting monomer raw material for single-sided grafting on the negative electrode side. The first grafting monomer raw material includes 0.55-1 parts of lithium sulfonylimide methacrylate monomer and 0.3-0.6 parts of polyethylene glycol methyl ether methacrylate. The second grafting monomer raw material includes 0.12-0.18 parts of lithium sulfonylimide methacrylate monomer and 0.15-0.35 parts of polyethylene glycol methyl ether methacrylate. The grafting monomer raw material forming the fluorinated end group includes 0.18-0.35 parts of 2,2,2-trifluoroethyl methacrylate.

2. The lithium battery separator according to claim 1, characterized in that, The polypropylene microporous membrane substrate is a single-layer polypropylene microporous membrane with a thickness of 20-30 μm; the number average molecular weight of the polyethylene glycol methyl ether methacrylate is 200-400.

3. The lithium battery separator according to claim 1, characterized in that, The sulfonylimide lithium salt type methacrylate monomer is obtained by acyl chloride of potassium 3-sulfonate methacrylate, reaction with trifluoromethanesulfonamide, and lithiation with lithium hydroxide; the mass ratio of potassium 3-sulfonate methacrylate, oxaloyl chloride, trifluoromethanesulfonamide, triethylamine, and lithium hydroxide monohydrate is [missing information]. 8-12:5.5-7.4:5.2-7.6:7.5-10.8:1.45-2.2。 4. The lithium battery separator according to claim 1, characterized in that, The polydopamine interface layer is formed by depositing dopamine hydrochloride in a tris(hydroxymethyl)aminomethane buffer solution at pH 8, and the deposition includes pre-deposition on the negative electrode side surface and immersion deposition on the polypropylene microporous membrane substrate.

5. The lithium battery separator according to claim 1, characterized in that, The polydopamine interface layer is fixed with ATRP initiation sites obtained by grafting 2-bromoisobutyryl bromide; the copolymer brush layer and the fluorinated end groups are both formed by surface grafting initiated by the ATRP initiation sites.

6. A method for preparing a lithium battery separator according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Preparation of lithium sulfonylimide methacrylate monomer; (2) When using polypropylene microporous membranes for battery assembly, the side facing the negative electrode is subjected to ultraviolet ozone treatment. (3) A polydopamine interface layer is formed on the negative electrode side of the polypropylene microporous membrane obtained in step (2); (4) ATRP initiation sites were immobilized on the polydopamine interface layer using 2-bromoisobutyryl bromide; (5) The diaphragm obtained in step (4) is completely immersed in the first-stage reaction solution containing lithium sulfonyl imide methacrylate monomer and polyethylene glycol methyl ether methacrylate to carry out the first-stage grafting reaction; (6) The second-stage reaction solution containing lithium sulfonylimide methacrylate monomer and polyethylene glycol methyl ether methacrylate is added dropwise to the negative electrode side of the membrane obtained in step (5) to carry out the second-stage grafting reaction; (7) The third-stage reaction liquid containing 2,2,2-trifluoroethyl methacrylate is added dropwise to the negative electrode side of the separator obtained in step (6) to carry out the end-group grafting reaction and obtain the lithium battery separator.

7. The method for preparing the lithium battery separator according to claim 6, characterized in that, Step (2) includes: immersing the polypropylene microporous membrane in ethanol and deionized water for 10 min each, and then drying it at 60°C for 2 h; laying the dried polypropylene microporous membrane flat on a polytetrafluoroethylene plate with the negative electrode side facing up, and fixing the four sides of the membrane; and treating the exposed area with ultraviolet ozone for 5-12 min in an air atmosphere, with the distance between the ultraviolet lamp and the membrane surface being 8-12 cm.

8. The method for preparing the lithium battery separator according to claim 6, characterized in that, Step (5) includes: mixing lithium sulfonyl imide methacrylate monomer, polyethylene glycol methyl ether methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, methanol and deionized water to obtain the first stage reaction solution; immersing the diaphragm obtained in step (4) completely into the first stage reaction solution, pre-soaking at 35°C for 45-75 min, then adding L-ascorbic acid, continuing the reaction at 35°C for 20-45 min, rinsing with methanol after the reaction, placing the diaphragm between two layers of nonwoven fabric, and pressing it under a 500g load for 30 s.

9. The method for preparing a lithium battery separator according to claim 6, characterized in that, Step (6) includes: mixing lithium sulfonyl imide methacrylate monomer, polyethylene glycol methyl ether methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, L-ascorbic acid, methanol and deionized water to obtain the second-stage reaction solution; laying the wet film obtained in step (5) with the negative electrode side facing up on a polytetrafluoroethylene plate, uniformly dripping the entire second-stage reaction solution onto the exposed area on the negative electrode side, maintaining it at 35°C for 15-35 min, and rinsing.

10. The method for preparing a lithium battery separator according to claim 6, characterized in that, Step (7) includes: mixing 2,2,2-trifluoroethyl methacrylate, copper dibromide, N,N,N′,N″,N″-pentamethyldiethylenetriamine, L-ascorbic acid and methanol to obtain the third-stage reaction solution; laying the membrane obtained in step (6) with the negative electrode side facing up on a polytetrafluoroethylene plate, adding all of the third-stage reaction solution to the exposed area on the negative electrode side, and reacting at 35°C for 5-18 min; washing and vacuum drying.

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