A boehmite-coated separator and a process for preparing the same

Abstract

The application discloses a boehmite-coated diaphragm and a preparation process thereof in the field of lithium ion battery diaphragms, and the diaphragm raw materials comprise ultrapure water, a dispersing agent, boehmite, a thickening agent, butadiene styrene rubber, lithium acrylate, a wetting agent, a fluorine-siloxane-sulfonic acid group copolymer and a five-arm polyamide-polyphosphate star copolymer. The fluorine-siloxane-sulfonic acid group copolymer is prepared by polymerization of multi-monomers such as trifluoroethyl methacrylate, gamma-(methacryloyloxy) propyl trimethoxysilane and 2-acrylamido-2-methylpropane sulfonic acid. During preparation, the boehmite is mixed and ground with the dispersing agent, the thickening agent and the like to form a slurry, two kinds of modified copolymers and other functional components are added to adjust the performance, and after coating, drying and rolling, the coated diaphragm is formed. The synergistic effect of the two kinds of copolymers significantly improves the thermal stability, electrolyte wettability and interface bonding force of the diaphragm with the base film, effectively solves the comprehensive performance bottleneck of the traditional diaphragm, and is suitable for the field of high-performance lithium ion batteries.

Description

A boehmite-coated diaphragm and its preparation process Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, specifically to a boehmite-coated separator and its preparation process. Background Technology

[0002] As the most promising secondary energy storage device currently available, the performance of the separator, one of the core components of lithium-ion batteries, directly determines the battery's safety, cycle life, and energy density. The separator plays a dual role in the battery as both an "ion channel" and a "physical barrier," allowing lithium ions to pass through efficiently for the charging and discharging process, while strictly separating the positive and negative electrodes to prevent short circuits. However, traditional commercially available separators are mostly made of polyolefin materials. Although these materials have good chemical stability and low cost advantages, they have revealed many key defects in practical applications: First, they are prone to shrinkage and deformation under high-temperature environments. When the battery is under high-temperature conditions, the thermal shrinkage of the separator can lead to direct contact between the positive and negative electrodes, causing the risk of thermal runaway and seriously affecting battery safety. Second, the electrolyte has limited absorption capacity, making it difficult to fully wet the electrodes and the internal pores of the separator, resulting in increased ion transport resistance and decreased battery charge-discharge rate performance. Third, polyolefin materials are non-polar materials, resulting in poor interfacial compatibility with polar ceramic coatings. Coating particles are prone to agglomeration and detachment due to insufficient interfacial forces, which not only reduces the functionality of the coating but also increases interfacial impedance and accelerates capacity decay during battery cycling. These problems greatly limit the widespread application of lithium-ion batteries in fields with extremely high safety and reliability requirements, such as electric vehicles and energy storage systems.

[0003] To address the shortcomings of traditional polyolefin membranes, existing technologies primarily modify them by coating them with ceramic functional materials or adding polymer binders. Among these, ceramic coatings have become the most mainstream improvement method due to their high heat resistance, chemical inertness, and low cost. However, the application of a single ceramic coating still has significant drawbacks: firstly, the surface of ceramic particles is mostly composed of polar hydroxyl structures, resulting in weak interfacial bonding with the non-polar polyolefin base membrane. At high temperatures, the coating is prone to peeling due to differences in thermal expansion coefficients, reducing the long-term stability of the membrane; secondly, the conductivity of ceramic particles themselves is limited, and excessive addition increases the internal resistance of the membrane, affecting ion transport efficiency. To improve these issues, some studies have attempted to add polymer binders to the slurry, hoping to enhance the bonding between ceramic particles and the base membrane through the "bridging" effect of the binder. However, traditional binders are mostly single-type polymers with limited functionality: while some binders can improve the dispersibility of ceramic particles, their rigid molecular chains and insufficient polarity prevent them from forming an effective bond with the base membrane; others, while improving interfacial adhesion, suffer from poor slurry rheology due to overly soft molecular chains or strong oleophilicity, leading to problems such as sagging and uneven thickness during coating. More importantly, traditional modification methods often focus only on improving a single performance characteristic, making it difficult to simultaneously optimize multiple core indicators of the membrane, resulting in limited overall performance improvement.

[0004] With the rapid development of lithium-ion batteries towards higher energy density and higher safety, more stringent requirements have been placed on the performance of separators: they need to possess low high-temperature shrinkage to ensure safety under extreme operating conditions, and high electrolyte absorption rate to improve ion transport efficiency; they need to form a strong interfacial bond with the ceramic coating to prevent detachment, and maintain good mechanical strength to withstand stress changes during cycling. Traditional single-component or simple blending modification schemes can no longer meet these requirements, and developing novel multifunctional synergistic modifying materials has become the key to breaking through current technological bottlenecks. Based on this, this invention proposes a boehmite-coated separator based on two newly designed modified copolymers and its preparation process. Through the synergistic effect of two functionally complementary modifying materials, it aims to systematically solve the problems of insufficient thermal stability, poor electrolyte wettability, and weak interfacial bonding of traditional separators, providing a new technical path for the development of high-performance lithium-ion battery separators. Summary of the Invention

[0005] The purpose of this invention is to provide a boehmite-coated diaphragm and its preparation process, which solves the technical problems of existing boehmite-coated diaphragms, such as high high-temperature shrinkage, poor electrolyte wettability, and weak interfacial adhesion with the base film.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A boehmite-coated diaphragm comprises the following raw materials in parts by weight:

[0008] Ultrapure water: 50-100 parts by weight;

[0009] Dispersant: 1-3 parts by weight;

[0010] Boehmite: 10-30 parts by weight;

[0011] Thickener: 0.5-2 parts by weight;

[0012] Styrene-butadiene rubber: 2-5 parts by weight;

[0013] Lithium acrylate: 1-3 parts by weight;

[0014] Wetting agent: 0.1-0.5 parts by weight;

[0015] Copolymers containing fluorinated siloxane-sulfonic acid groups: 1-3 parts by weight;

[0016] Five-arm polyamide-polyphosphate star copolymer: 0.5-2 parts by weight;

[0017] The preparation method of the copolymer containing fluorine-siloxane-sulfonic acid groups includes: A1, adding trifluoroethyl methacrylate, γ-(methacryloyloxy)propyltrimethoxysilane, and 2-acrylamido-2-methylpropanesulfonic acid to a round-bottom flask, adding N,N-dimethylformamide, purging with nitrogen to remove oxygen, adding azobisisobutyronitrile, and reacting with magnetic stirring at 60-80℃; A2, after the reaction is completed, pouring the product into anhydrous ethanol to precipitate, filtering and collecting the precipitate, repeatedly dialyzing with deionized water, and finally drying in a vacuum drying oven at 60-64℃.

[0018] In this invention, the preparation mechanism of the fluorinated-siloxane-sulfonic acid group copolymer is based on the principle of polymer polymerization, formed through a multi-monomer copolymerization process initiated by active free radicals. At the start of preparation, three monomers with different functional groups (containing fluorinated groups, siloxane groups, and sulfonic acid groups, respectively) are mixed with a polar solvent. Under inert gas protection, active oxygen in the system is eliminated (to avoid hindering the polymerization reaction), and then a free radical initiator is added. The initiator decomposes at a specific temperature to generate active free radicals, which attack the unsaturated double bonds of the monomers (located in the molecular chains of each monomer), forming dynamically growing active centers. These active centers continuously combine with unreacted monomer double bonds, gradually extending the molecular chain through a chain-growth reaction. Due to the differences in the reactivity of the three monomers (i.e., the different degrees of ease with which they combine with free radicals), the structural units of the three monomers in the final copolymer molecular chain are arranged randomly, forming a ternary random copolymer structure. In this process, monomers containing fluorinated groups impart lower surface energy to the molecular chains; monomers containing siloxane groups can undergo condensation reactions with hydroxyl groups on the surface of inorganic particles during subsequent processing to form stable chemical bonds; and monomers containing sulfonic acid groups provide ion conductivity through dissociation. After the reaction is complete, the products are subjected to precipitation with polar solvents (to separate unreacted monomers based on solubility differences), dialysis with deionized water (to remove small molecule impurities), and vacuum drying (to remove moisture), ultimately yielding a copolymer with synergistic effects of multiple functional groups.

[0019] According to a preferred embodiment of the present invention, in step A1, the time for introducing nitrogen gas to remove oxygen is 30-40 min; the time for magnetic stirring reaction is 8-12 h.

[0020] According to a preferred embodiment of the present invention, in step A2, the drying time in a vacuum drying oven at 60-64°C is 12-14 hours.

[0021] According to a preferred embodiment of the present invention, the preparation method of the five-armed polyamide-polyphosphate star copolymer includes: B1, adding adipic acid and 1,6-hexanediamine to a three-necked flask, adding deionized water, and reacting under reduced pressure at 120-124°C to generate a polyamide prepolymer; adding ethylene glycol and sodium hydroxide to the system, and continuing the reaction to obtain a hydroxyl-terminated polyamide; dissolving the hydroxyl-terminated polyamide in tetrahydrofuran, adding stannous octoate, and reacting with diisocyanate under nitrogen protection to obtain an isocyanate-terminated polyamide; finally reacting with ethylenediamine to end-cap the five-armed polyamide initiator; B2, adding the five-armed polyamide initiator and poly(ethylene glycol) phosphate to tetrahydrofuran, purging with nitrogen to remove oxygen, adding cuprous bromide and 2,2'-bipyridine, and reacting with magnetic stirring at 80-84°C; after the reaction is completed, adding methanol to precipitate the product, filtering and collecting the precipitate, washing repeatedly with tetrahydrofuran / methanol, and finally drying in a vacuum drying oven at 40-42°C.

[0022] In this invention, the preparation reaction mechanism of the five-armed polyamide-polyphosphate star copolymer involves two steps: first, the multi-step synthesis of a multi-armed polyamide initiator, which constructs a branched structure through condensation polymerization and functional group transformation; second, atom transfer radical polymerization grafting of polyphosphate segments to form a star-shaped topology. The first step, the synthesis of the multi-armed polyamide initiator, begins with the condensation polymerization of two dicarboxylic acids and a diamine. Both monomers contain two reactive polar groups (carboxyl and amino). Under high temperature and low pressure conditions (promoting water evaporation), the carboxyl and amino groups undergo a condensation reaction, removing small molecules to form a linear polyamide prepolymer. Subsequently, by introducing a hydroxyl-containing small molecule compound to react with the terminal carboxyl groups of the prepolymer, the terminal groups of the linear molecule are converted to hydroxyl groups, yielding a hydroxyl-terminated polyamide. Next, the hydroxyl-terminated polyamide reacts with a compound containing isocyanate groups, converting the terminal hydroxyl groups to isocyanate groups, forming an isocyanate-terminated polyamide. Finally, the terminal isocyanate groups react with amino-containing compounds to extend the ends of the linear polyamide molecule into multiple branched arms (forming a five-arm structure) through functional group crosslinking, yielding a multi-arm polyamide initiator. In the second step, polyphosphate segments are grafted onto the multi-arm polyamide initiator via atom transfer radical polymerization. The multi-arm polyamide initiator dissolves in a polar aprotic solvent, and a transition metal catalyst and ligand are added to form a catalytic system. At a specific temperature, the catalyst decomposes to generate active free radicals, which attack the molecular chains of the polyphosphate monomers (containing reactive double bonds) to form primary free radicals. Simultaneously, the terminal isocyanate groups (or activated active sites) of the multi-arm polyamide initiator act as initiation centers, reacting with the active free radicals to initiate chain growth. The polyphosphate monomers are then grafted onto each branched arm of the multi-arm polyamide initiator via a free radical grafting reaction, forming a star-shaped topology with the five-arm polyamide as the core and the polyphosphate segments as branches. After the reaction is complete, a precipitant is added to disrupt the catalytic system and precipitate the product. Unreacted monomers and catalysts are removed by repeated washing with solvent, and finally, the product is obtained by vacuum drying to obtain a copolymer with branched structure and multi-segment synergistic effect.

[0023] According to a preferred embodiment of the present invention, in step B1, the reaction time under reduced pressure at 120-124°C is 6-8 hours; the reaction time continues for 2-4 hours; the reaction time with diisocyanate under nitrogen protection is 4-6 hours; and the final reaction time with ethylenediamine is 2-4 hours.

[0024] According to a preferred embodiment of the present invention, in step B2, the magnetic stirring reaction time is 12-14 h; the drying time in a vacuum drying oven at 40-42°C is 24-26 h.

[0025] The present invention also provides a process for preparing the boehmite-coated diaphragm, comprising the following steps:

[0026] S1. Add boehmite powder to ultrapure water, stir in a high-speed disperser, add dispersant, thickener and wetting agent, and continue stirring until the slurry is free of obvious particles;

[0027] S2. Subsequently, the copolymer containing fluorine-siloxane-sulfonic acid groups, the five-armed polyamide-polyphosphate star copolymer, styrene-butadiene rubber and lithium acrylate are added in sequence and stirred. The pH is adjusted to 6-7 to obtain the coating slurry.

[0028] S3. Apply the slurry to the surface of the PE base film through a comma roller, control the coating thickness, pre-dry at 80-84℃, then vacuum dry at 120-124℃, and finally compact it with a roller press.

[0029] This invention describes the reaction mechanism for preparing boehmite-coated diaphragms. The preparation of boehmite-coated diaphragms is a multi-component synergistic process involving physicochemical changes in stages such as dispersion, mixing, coating, and curing. First, boehmite powder (mainly composed of layered hydroxyl oxides with a surface rich in polar hydroxyl groups) achieves stable dispersion in water through the action of a dispersant: the polar groups (such as carboxyl groups) of the dispersant form hydrogen bonds with the hydroxyl groups on the boehmite surface, while simultaneously creating steric hindrance between particles to prevent agglomeration. A thickener increases the viscosity of the slurry through molecular chain entanglement, regulating rheological properties and preventing uncontrolled slurry flow during coating. A wetting agent reduces the surface tension of the slurry, enhancing its wettability to the base membrane (polyolefin material), allowing the slurry to spread uniformly on the base membrane surface. Subsequently, the fluorinated-siloxane-sulfonic acid group copolymer and the five-armed polyamide-polyphosphate star copolymer were added to the slurry, where they interacted with the boehmite particles in multiple ways: the siloxane structure of the fluorinated-siloxane-sulfonic acid group copolymer condensed with the hydroxyl groups on the boehmite surface to form stable chemical bonds, enhancing the interfacial adhesion between the copolymer and the boehmite; its fluorinated structure reduced the surface energy of the coating, decreasing the tendency to shrink during the coating process; the sulfonic acid groups dissociated into ions, which synergistically reduced the interfacial energy with the polar groups of the wetting agent, improving the wettability of the electrolyte. The multi-armed structure of the five-armed polyamide-polyphosphate star copolymer entangled the boehmite particles, forming a three-dimensional network that enhanced the cohesiveness of the coating; the polar groups of its polyphosphate segments formed hydrogen bonds with the residual polar sites on the base film surface (such as hydroxyl or oxidation sites that were not fully reacted during polymerization), while the amide structure of the multi-armed polyamide core interacted weakly with the carbon-hydrogen bonds on the base film surface, enhancing the interfacial adhesion between the coating and the base film. Styrene-butadiene rubber (SBR) acts as an interparticle binder, physically adsorbing and encapsulating boehmite particles through molecular chains to fill interparticle gaps and prevent coating cracking. The polar groups of lithium acrylate form ion-dipole interactions with polar sites on the base film surface, and the dissociated ions migrate at the interface, reducing interfacial impedance. Finally, the slurry is coated onto the base film surface using a comma roller to control the coating thickness. During the pre-drying stage, some moisture evaporates, and the slurry initially solidifies. Vacuum drying further removes residual moisture, promoting cross-linking of copolymer molecular chains (such as hydrogen bonding of sulfonic acid groups and condensation reactions of siloxanes) and chemical bonding with the base film, forming a stable coating structure. Roller compaction enhances the interfacial contact between the coating and the base film through mechanical pressure, reducing porosity and improving overall density. The final boehmite-coated separator achieves a comprehensive improvement in thermal stability, electrolyte wettability, and interfacial adhesion through the synergistic effect of multiple components.

[0030] According to a preferred embodiment of the present invention, in step S1, the rotation speed of the high-speed disperser is 3000-3400 rpm, the stirring time is 30-40 min, and the stirring time is 60-80 min.

[0031] According to a preferred embodiment of the present invention, in step S2, the stirring speed is 200-300 rpm and the stirring time is 90-100 min.

[0032] According to a preferred embodiment of the present invention, in step S3, the coating thickness is 1-3 μm; the pre-drying time is 3-6 min; the vacuum drying time is 10-20 min; and the pressure of the roller press is 0.5-1 MPa.

[0033] The beneficial effects of this invention are as follows:

[0034] This invention demonstrates significant advantages in improving the overall performance of lithium-ion battery separators through the synergistic effect of two newly designed modified copolymers and boehmite coatings. The specific technical effects are reflected in the following aspects:

[0035] First, thermal stability is significantly improved, effectively ensuring the battery's safety performance under high-temperature conditions. Traditional polyolefin separators, due to their inherent insufficient heat resistance, are prone to shrinkage and deformation at high temperatures, leading to short circuits at the positive and negative electrodes and posing a risk of thermal runaway. In this invention, the five-armed polyamide-polyphosphate star copolymer, through its unique branched star structure and phosphate groups on the molecular chain, can decompose at high temperatures to form a dense phosphate protective layer covering the base film surface, inhibiting the thermal shrinkage behavior of the substrate. Simultaneously, the siloxane groups in the copolymer molecular chain containing fluorine-siloxane-sulfonic acid groups can undergo a condensation reaction with the hydroxyl groups on the surface of boehmite particles to form stable siloxane bonds, tightly bonding the ceramic coating to the base film and preventing peeling of the coating due to differences in thermal expansion coefficients at high temperatures. The synergistic effect of the two copolymers significantly enhances the dimensional stability of the separator in high-temperature environments, effectively reducing the risk of thermal shrinkage and greatly improving the safety of the battery under extreme conditions such as overcharging, short circuits, or external heating.

[0036] Secondly, the electrolyte wettability and ion transport efficiency are significantly optimized, improving the battery's rate performance and cycle life. Traditional boehmite coatings suffer from insufficient surface polarity matching with the electrolyte, leading to low electrolyte uptake and high ion transport resistance. The fluorinated groups in the copolymer molecular chains containing fluorinated-siloxane-sulfonic acid groups reduce the coating's surface energy, making it easier for the electrolyte to spread and wet; the sulfonic acid groups, by dissociating ions, form ion transport channels at the interface, improving ion conduction efficiency. The multi-arm branched structure of the five-arm polyamide-polyphosphate star copolymer increases the contact area between the molecular chain and the electrolyte, and its hydrophilic polyphosphate segments further enhance the coating's adsorption capacity for the electrolyte. The combination of these two copolymers significantly improves the electrolyte uptake of the separator, reduces ion transport resistance, improves the battery's charge / discharge rate performance, and effectively alleviates capacity decay caused by poor ion transport during cycling, extending the battery's lifespan.

[0037] Finally, the significantly enhanced interfacial adhesion and mechanical strength ensured the structural stability of the separator during long-term cycling. Traditional boehmite coatings exhibit weak interfacial adhesion to the base membrane, making them prone to detachment due to stress changes during cycling, thus affecting battery performance. The copolymer containing fluorinated-siloxane-sulfonic acid groups strengthens the interfacial bond between the coating and the base membrane through chemical bonding between the siloxane groups and boehmite, and through the modification of the base membrane surface by fluorinated groups. The multi-arm structure of the five-arm polyamide-polyphosphate star copolymer can entangle boehmite particles, forming a three-dimensional network structure and enhancing the coating's cohesiveness. Furthermore, styrene-butadiene rubber and lithium acrylate, as auxiliary binders, further solidified the "particle-coating-base membrane" interfacial bonding system through physical adsorption with boehmite particles and chemical interaction with the base membrane, respectively. The synergistic effect of these multiple forces significantly improved the separator's peel strength and mechanical strength, effectively avoiding the short-circuit risk caused by coating detachment during cycling, and ensuring the structural stability and reliability of the battery during long-term use.

[0038] In summary, this invention systematically solves the problems of insufficient thermal stability, poor electrolyte wettability, and weak interfacial bonding of traditional boehmite-coated separators through the innovative design and synergistic effect of two novel modified copolymers. It achieves a breakthrough in comprehensive performance in terms of safety, rate performance, and cycle life, providing a new technical path for the development of high-performance lithium-ion battery separators. Detailed Implementation

[0039] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0040] The following is information on domestic suppliers of the relevant equipment and materials:

[0041] The ultrapure water was purchased from Shanghai Lefeng Biotechnology Co., Ltd.

[0042] The dispersant was purchased from Jiangsu Hehai Nanotechnology Co., Ltd.

[0043] The boehmite was purchased from Anhui Yishitong Materials Technology Co., Ltd.

[0044] The thickener was purchased from Shandong Heda Group Co., Ltd.

[0045] The styrene-butadiene rubber was purchased from Lanzhou Petrochemical Branch of China National Petroleum Corporation.

[0046] The lithium acrylate was purchased from Jiangsu Yulong Chemical Co., Ltd.

[0047] The wetting agent was purchased from Shanghai Huazhirun Chemical Co., Ltd.

[0048] The trifluoroethyl methacrylate was purchased from Zhejiang Juhua Co., Ltd.

[0049] The γ-(methacryloyloxy)propyltrimethoxysilane was purchased from Hubei Xingfa Chemical Group Co., Ltd.

[0050] The 2-acrylamido-2-methylpropanesulfonic acid was purchased from Shandong Baomo Biochemical Co., Ltd.

[0051] The round-bottom flask was purchased from Sichuan Shubo (Group) Co., Ltd.

[0052] The N,N-dimethylformamide was purchased from Jiangsu Yida Chemical Co., Ltd.

[0053] The azobisisobutyronitrile was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0054] The anhydrous ethanol was purchased from Anhui Ante Food Co., Ltd.

[0055] The vacuum drying oven was purchased from Shanghai Yiheng Technology Co., Ltd.

[0056] The adipic acid was purchased from Shandong Haili Chemical Co., Ltd.

[0057] The 1,6-hexanediamine was purchased from Jiangsu Xinhai Petrochemical Co., Ltd.

[0058] The three-necked flask was purchased from Sichuan Shubo (Group) Co., Ltd.

[0059] The deionized water was purchased from Hangzhou Wahaha Group Co., Ltd.

[0060] The ethylene glycol was purchased from Jiangsu Huachang Chemical Co., Ltd.

[0061] The sodium hydroxide was purchased from Shandong Haihua Co., Ltd.

[0062] The tetrahydrofuran was purchased from Jiangsu Yida Chemical Co., Ltd.

[0063] The stannous octoate was purchased from Jiangsu Hehai Nanotechnology Co., Ltd.

[0064] The diisocyanate was purchased from Wanhua Chemical Group Co., Ltd.

[0065] The ethylenediamine was purchased from Jiangsu Xinhai Petrochemical Co., Ltd.

[0066] The poly(ethylene glycol) phosphate was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0067] The cuprous bromide was purchased from Jiangxi Copper Corporation.

[0068] The 2,2'-bipyridine was purchased from Jiangsu Hehai Nanotechnology Co., Ltd.

[0069] The methanol was purchased from Shaanxi Yulin Energy Group Co., Ltd.

[0070] The high-speed disperser was purchased from Shanghai Rute Electromechanical Equipment Co., Ltd.

[0071] The comma roller was purchased from Shanghai Kaiqian Roller Manufacturing Co., Ltd.

[0072] The PE base film was purchased from Xiamen Changsu Industrial Co., Ltd.

[0073] The roller press was purchased from Beijing North China Microelectronics Equipment Co., Ltd.

[0074] Example 1

[0075] Preparation of fluorinated-siloxane-sulfonic acid group copolymer: 45g of trifluoroethyl methacrylate, 50g of γ-(methacryloyloxy)propyltrimethoxysilane, and 44g of 2-acrylamido-2-methylpropanesulfonic acid were added to a round-bottom flask. 1390mL of N,N-dimethylformamide (10 times the total mass of the monomers) was added, and nitrogen gas was purged for oxygen removal for 35min. Then, 1.4g of azobisisobutyronitrile (1% of the total mass of the monomers) was added, and the mixture was magnetically stirred at 70℃ for 10h. After the reaction was completed, the product was poured into anhydrous ethanol (3 times the volume of the reaction liquid) to precipitate. The precipitate was collected by filtration and repeatedly dialyzed with deionized water until the conductivity of the filtrate was <10μS / cm. Finally, the precipitate was dried in a vacuum drying oven at 62℃ for 13h to obtain the fluorinated-siloxane-sulfonic acid group copolymer.

[0076] Preparation of a five-armed polyamide-polyphosphate star copolymer: 146g of adipic acid and 116g of 1,6-hexanediamine were added to a three-necked flask, and deionized water (5 times the total mass of the monomers) was added. The mixture was reacted under reduced pressure at 122℃ for 7h to generate a polyamide prepolymer. 6.2g of ethylene glycol and 8g of sodium hydroxide were added to the system, and the reaction was continued for 3h to obtain a hydroxyl-terminated polyamide. The hydroxyl-terminated polyamide was dissolved in tetrahydrofuran (10 times the mass of the polyamide), and 1.2g of stannous octoate was added. The mixture was reacted with 174g of diisocyanate under nitrogen protection for 5h to obtain an isocyanate-terminated polyamide. Finally, it was reacted with 3.6g of ethylenediamine for 3h to end-cap the five-armed polyamide initiator. 120g of five-arm polyamide initiator and 150g of poly(ethylene glycol) phosphate were added to tetrahydrofuran (8 times the total mass of the monomers). After purging with nitrogen to remove oxygen, 1.2g of cuprous bromide and 3.8g of 2,2'-bipyridine were added. The mixture was magnetically stirred at 82℃ for 13h. After the reaction was completed, methanol (3 times the volume of the reaction liquid) was added to precipitate the product. The precipitate was collected by filtration and washed three times repeatedly with tetrahydrofuran / methanol (volume ratio 1:1). Finally, the product was dried in a vacuum drying oven at 41℃ for 25h to obtain a five-arm polyamide-polyphosphate star copolymer.

[0077] Preparation of boehmite-coated diaphragm: 20g of boehmite was added to 70g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. 2g of dispersant, 1g of thickener, and 0.3g of wetting agent were added, and stirring was continued for 70min until the slurry was free of obvious particles. Subsequently, 2g of fluorinated-siloxane-sulfonic acid group copolymer, 1g of five-arm polyamide-polyphosphate star copolymer, 3g of styrene-butadiene rubber, and 2g of lithium acrylate were added sequentially and stirred at 250rpm for 95min. The pH was adjusted to 6.5 to obtain a coating slurry with a solid content of 35%. The slurry was coated onto the surface of a PE base membrane (10μm thick, 45% porosity) using a comma roller, with the coating thickness controlled at 2μm. The membrane was pre-dried at 82℃ for 4min, then vacuum-dried at 122℃ for 15min, and finally compacted using a roller press (pressure 0.8MPa) to obtain the boehmite-coated diaphragm.

[0078] Example 2

[0079] The specific implementation method is the same as in Example 1, except that the preparation of the fluorinated-siloxane-sulfonic acid group copolymer is as follows: 40g of trifluoroethyl methacrylate, 45g of γ-(methacryloyloxy)propyltrimethoxysilane, and 38g of 2-acrylamido-2-methylpropanesulfonic acid are added to a round-bottom flask, followed by 1200mL of N,N-dimethylformamide (10 times the total mass of the monomers). After purging with nitrogen for 35 minutes, 1.2g of azobisisobutyronitrile (1% of the total mass of the monomers) is added, and the mixture is magnetically stirred at 70°C for 10 hours. After the reaction is complete, the product is poured into anhydrous ethanol to precipitate, the precipitate is collected by filtration, repeatedly dialyzed with deionized water, and finally dried in a vacuum drying oven at 62°C for 13 hours. Preparation of a five-armed polyamide-polyphosphate star copolymer: 130g of adipic acid and 105g of 1,6-hexanediamine were added to a three-necked flask, and deionized water (5 times the total mass of the monomers) was added. The mixture was reacted under reduced pressure at 122℃ for 7h to generate a polyamide prepolymer. 5.5g of ethylene glycol and 7g of sodium hydroxide were added to the system, and the reaction was continued for 3h to obtain a hydroxyl-terminated polyamide. The hydroxyl-terminated polyamide was dissolved in tetrahydrofuran, and 1.0g of stannous octoate was added. The mixture was reacted with 150g of diisocyanate under nitrogen protection for 5h to obtain an isocyanate-terminated polyamide. Finally, it was reacted with 3.0g of ethylenediamine for 3h to end-cap the five-armed polyamide initiator. 100g of five-arm polyamide initiator and 130g of poly(ethylene glycol) phosphate were added to tetrahydrofuran. After purging with nitrogen to remove oxygen, 1.0g of cuprous bromide and 3.2g of 2,2'-bipyridine were added. The mixture was magnetically stirred at 82℃ for 13h. After the reaction was completed, methanol was added to precipitate the product. The precipitate was collected by filtration, washed repeatedly with tetrahydrofuran / methanol, and finally dried in a vacuum drying oven at 41℃ for 25h. Preparation of boehmite-coated diaphragm: 18g of boehmite was added to 65g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. 1.8g of dispersant, 0.9g of thickener, and 0.25g of wetting agent were added and stirred for another 70min. Subsequently, 1.8g of fluorinated-siloxane-sulfonic acid group copolymer, 0.9g of five-arm polyamide-polyphosphate star copolymer, 2.8g of styrene-butadiene rubber, and 1.8g of lithium acrylate were added sequentially and stirred at 250rpm for 95min. The pH was adjusted to 6.5 to obtain the coating slurry. The slurry was coated onto the surface of a PE base film using a comma roller, with the coating thickness controlled at 2μm. The film was pre-dried at 82℃ for 4min, then vacuum-dried at 122℃ for 15min, and finally compacted using a roller press (pressure 0.8MPa).

[0080] Example 3

[0081] The specific implementation method is the same as in Example 1, except that the preparation of the fluorinated-siloxane-sulfonic acid group copolymer is as follows: 50g of trifluoroethyl methacrylate, 55g of γ-(methacryloyloxy)propyltrimethoxysilane, and 48g of 2-acrylamido-2-methylpropanesulfonic acid are added to a round-bottom flask, followed by 1400mL of N,N-dimethylformamide (10 times the total mass of the monomers). After purging with nitrogen for 35 minutes, 1.5g of azobisisobutyronitrile (1% of the total mass of the monomers) is added, and the mixture is magnetically stirred at 70°C for 10 hours. After the reaction is complete, the product is poured into anhydrous ethanol to precipitate, the precipitate is collected by filtration, repeatedly dialyzed with deionized water, and finally dried in a vacuum drying oven at 62°C for 13 hours. Preparation of a five-armed polyamide-polyphosphate star copolymer: 150g of adipic acid and 120g of 1,6-hexanediamine were added to a three-necked flask, and deionized water (5 times the total mass of the monomers) was added. The mixture was reacted under reduced pressure at 122℃ for 7h to generate a polyamide prepolymer. 6.8g of ethylene glycol and 8.5g of sodium hydroxide were added to the system, and the reaction was continued for 3h to obtain a hydroxyl-terminated polyamide. The hydroxyl-terminated polyamide was dissolved in tetrahydrofuran, and 1.3g of stannous octoate was added. The mixture was reacted with 180g of diisocyanate under nitrogen protection for 5h to obtain an isocyanate-terminated polyamide. Finally, it was reacted with 3.8g of ethylenediamine for 3h to end-cap the five-armed polyamide initiator. 130g of five-arm polyamide initiator and 160g of poly(ethylene glycol) phosphate were added to tetrahydrofuran. After purging with nitrogen to remove oxygen, 1.3g of cuprous bromide and 4.0g of 2,2'-bipyridine were added. The mixture was magnetically stirred at 82℃ for 13h. After the reaction was completed, methanol was added to precipitate the product. The precipitate was collected by filtration, washed repeatedly with tetrahydrofuran / methanol, and finally dried in a vacuum drying oven at 41℃ for 25h. Preparation of boehmite-coated diaphragm: 22g of boehmite was added to 75g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. 2.2g of dispersant, 1.1g of thickener, and 0.35g of wetting agent were added and stirred for another 70min. Subsequently, 2.2g of fluorinated-siloxane-sulfonic acid group copolymer, 1.1g of five-arm polyamide-polyphosphate star copolymer, 3.2g of styrene-butadiene rubber, and 2.2g of lithium acrylate were added sequentially and stirred at 250rpm for 95min. The pH was adjusted to 6.5 to obtain the coating slurry. The slurry was coated onto the surface of a PE base film using a comma roller, with the coating thickness controlled at 2μm. The film was pre-dried at 82℃ for 4min, then vacuum-dried at 122℃ for 15min, and finally compacted using a roller press (pressure 0.8MPa).

[0082] Comparative Example 1

[0083] The specific implementation method is the same as in Example 1, except that the boehmite-coated diaphragm is prepared as follows: 20g of boehmite is added to 70g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. Then, 2g of dispersant, 1g of thickener and 0.3g of wetting agent are added and stirred for another 70min. Subsequently, 1g of five-arm polyamide-polyphosphate star copolymer, 3g of styrene-butadiene rubber and 2g of lithium acrylate (without fluorine-siloxane-sulfonic acid group copolymer) are added in sequence and stirred at 250rpm for 95min. The pH is adjusted to 6.5 to obtain the coating slurry. The slurry is coated onto the surface of the PE base film through a comma roller, and the coating thickness is controlled to 2μm. It is first pre-dried at 82℃ for 4min, then vacuum dried at 122℃ for 15min, and finally compacted by a roller press (pressure 0.8MPa).

[0084] Comparative Example 2

[0085] The specific implementation method is the same as in Example 1, except that the boehmite-coated diaphragm is prepared as follows: 20g of boehmite is added to 70g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. Then, 2g of dispersant, 1g of thickener and 0.3g of wetting agent are added and stirred for another 70min. Subsequently, 2g of fluorinated-siloxane-sulfonic acid group copolymer, 3g of styrene-butadiene rubber and 2g of lithium acrylate (without adding five-arm polyamide-polyphosphate star copolymer) are added in sequence and stirred at 250rpm for 95min. The pH is adjusted to 6.5 to obtain the coating slurry. The slurry is coated onto the surface of the PE base film through a comma roller, and the coating thickness is controlled to 2μm. It is first pre-dried at 82℃ for 4min, then vacuum dried at 122℃ for 15min, and finally compacted by a roller press (pressure 0.8MPa).

[0086] Comparative Example 3

[0087] The specific implementation method is the same as in Example 1, except that the boehmite-coated diaphragm is prepared as follows: 20g of boehmite is added to 70g of ultrapure water and stirred at 3200rpm for 35min in a high-speed disperser. Then, 2g of dispersant, 1g of thickener and 0.3g of wetting agent are added and stirred for another 70min. Subsequently, 3g of styrene-butadiene rubber and 2g of lithium acrylate (without the two modified copolymers) are added and stirred at 250rpm for 95min. The pH is adjusted to 6.5 to obtain the coating slurry. The slurry is coated onto the surface of the PE base film through a comma roller, and the coating thickness is controlled to 2μm. It is first pre-dried at 82℃ for 4min, then vacuum dried at 122℃ for 15min, and finally compacted by a roller press (pressure 0.8MPa).

[0088] Performance testing

[0089] The boehmite-coated diaphragms prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following methods:

[0090] 1. Heat shrinkage rate test: Refer to GB / T 12027-2004 "Test method for dimensional change rate of plastic film", cut the diaphragm into 100mm×100mm square samples, lay them flat on a clean glass plate, and place them in a 120℃ constant temperature oven for 1 hour. After taking them out, immediately cool them to room temperature (25℃), measure the length of the long side and the short side of the sample with vernier calipers, and calculate the heat shrinkage rate (shrinkage rate = (initial length - length after test) / initial length × 100%). Take the average value of 5 parallel samples.

[0091] 2. Electrolyte Absorption Rate Test: The diaphragm was completely immersed in a simulated electrolyte (1M LiPF6 dissolved in ethylene carbonate-diethyl carbonate, volume ratio 1:1) and allowed to stand at 25℃ for 24 hours to ensure full absorption of the electrolyte. After removal, the surface residual electrolyte was gently wiped off with anhydrous ethanol. The mass after absorption (m1) was immediately weighed using an analytical balance, and the initial dried mass (m0) was recorded. The absorption rate was calculated using the formula: Absorption rate = (m1-m0) / m0 × 100%, and the average value of 3 parallel samples was taken.

[0092] 3. Ionic conductivity testing: Electrochemical impedance spectroscopy (EIS) was used. The membrane was cut into a 10mm diameter circular sheet and sandwiched between two 12mm diameter stainless steel disk electrodes to assemble a symmetrical cell (the membrane being the sole ion-conducting medium). Nyquist plots were obtained using an electrochemical workstation (CHI660E) scanning at a frequency range of 0.1Hz-100kHz and an amplitude of 5mV. The equivalent circuit was fitted (R... s R is the resistance of the solution. n (where R is the charge transfer resistance), the ionic conductivity σ is calculated using the formula: σ = L / (R s ×A), where L is the diaphragm thickness (cm) and A is the effective area of ​​the electrode (cm²), and the average value of 3 parallel samples is taken.

[0093] 4. Peel Strength Test: The boehmite-coated separator and the uncoated PE base film were laminated by a roll forming process (pressure 0.5MPa, temperature 80℃) to form a "base film-coating-base film" sandwich structure, which was then cut into strips of 15mm × 100mm. A 90° peel test was performed using a universal testing machine (Instron 5967) at a constant rate of 10mm / min. The maximum force (N) during the peel process was recorded, and the peel strength per unit width (N / m) was calculated. The average value of 5 parallel samples was taken.

[0094] 5. Performance test results:

[0095] Table 1: Performance test results of each embodiment and comparative example

[0096]

[0097] As shown in Table 1, the boehmite-coated separators prepared in Examples 1-3 significantly solved the technical problems of high high-temperature shrinkage, poor electrolyte wettability, and weak interfacial adhesion with the base film in traditional boehmite-coated separators through the synergistic effect of the two modified copolymers. Regarding the thermal shrinkage rate, the thermal shrinkage rate of Examples 1-3 was only 0.8%-1.0%, far lower than the 3.2%-4.8% of Comparative Examples 1-3. This is because the phosphate groups of the five-armed polyamide-polyphosphate star copolymer can decompose at high temperatures to form a dense phosphate protective layer, inhibiting the thermal shrinkage behavior of the polyolefin base film. Simultaneously, the siloxane groups of the fluorinated-siloxane-sulfonic acid copolymer undergo a condensation reaction with the hydroxyl groups on the boehmite surface (forming stable Si-O-Al bonds), enhancing the interfacial adhesion between the coating and the base film, and preventing peeling of the coating due to differences in thermal expansion coefficients at high temperatures, thereby effectively reducing the thermal shrinkage rate.

[0098] Regarding electrolyte wettability, the electrolyte absorption rate of Examples 1-3 reached 195%-202%, which is about 50%-60% higher than that of Comparative Examples 1-3 (125%-142%). This is mainly due to the fact that the fluorinated groups of the fluorinated-siloxane-sulfonic acid copolymer reduce the surface energy of the coating (making the electrolyte easier to spread and wet), and the sulfonic acid groups form ion transport channels at the interface by dissociating lithium ions, thereby improving the adsorption capacity of the electrolyte. At the same time, the multi-arm branched structure of the five-arm polyamide-polyphosphate star copolymer increases the contact area between the molecular chain and the electrolyte, and its hydrophilic polyphosphate segments further enhance the adsorption of the electrolyte. The synergistic effect of the two significantly improves the electrolyte wettability of the membrane.

[0099] Regarding the interfacial adhesion with the base film, the peel strength of Examples 1-3 (3.2-3.4 N / m) is about 2-3 times higher than that of Comparative Examples 1-3 (0.9-1.1 N / m). This is because the siloxane groups of the fluorinated-siloxane-sulfonic acid copolymer form chemical bonds with the hydroxyl groups on the surface of boehmite, while the modification of the base film surface by the fluorinated groups enhances the interfacial adhesion between the coating and the base film. The multi-arm structure of the five-arm polyamide-polyphosphate star copolymer can wrap around the boehmite particles to form a three-dimensional network structure, which enhances the cohesive force of the coating. The polar groups of its polyphosphate segments form hydrogen bonds with the residual polar sites (such as residual hydroxyl or oxidation sites from polymerization) on the surface of the base film, and the amide groups of the five-arm polyamide core interact weakly with the CH bonds on the surface of the base film, further consolidating the interfacial adhesion system of "particle-coating-base film". In addition, styrene-butadiene rubber and lithium acrylate, as auxiliary binders, synergistically enhance the interfacial adhesion through physical adsorption with boehmite particles and chemical interaction with the base film, respectively.

[0100] In summary, Examples 1-3 systematically solved the problems of high thermal shrinkage, poor electrolyte wettability, and weak interfacial bonding of traditional boehmite-coated diaphragms through the synergistic effect of two novel modified copolymers, achieving a significant improvement in the overall performance of the diaphragm.

[0101] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A boehmite-coated diaphragm, characterized in that, The raw materials include the following parts by weight: ultrapure water: 50-100 parts by weight; dispersant: 1-3 parts by weight; boehmite: 10-30 parts by weight; thickener: 0.5-2 parts by weight; styrene-butadiene rubber: 2-5 parts by weight; lithium acrylate: 1-3 parts by weight; wetting agent: 0.1-0.5 parts by weight; fluorinated-siloxane-sulfonic acid group copolymer: 1-3 parts by weight; five-arm polyamide-polyphosphate star copolymer: 0.5-2 parts by weight; wherein, the fluorinated-siloxane group copolymer... The preparation method of the copolymer of alkyl-sulfonic acid groups includes: A1, adding trifluoroethyl methacrylate, γ-(methacryloyloxy)propyltrimethoxysilane, and 2-acrylamido-2-methylpropanesulfonic acid to a round-bottom flask, adding N,N-dimethylformamide, purging with nitrogen to remove oxygen, adding azobisisobutyronitrile, and reacting with magnetic stirring at 60-80℃; A2, after the reaction is complete, pouring the product into anhydrous ethanol to precipitate, filtering and collecting the precipitate, and repeatedly dialyzing with deionized water. Finally, the mixture is dried in a vacuum drying oven at 60-64℃. The preparation method of the five-armed polyamide-polyphosphate star copolymer includes: B1, adding adipic acid and 1,6-hexanediamine to a three-necked flask, adding deionized water, and reacting under reduced pressure at 120-124℃ to generate a polyamide prepolymer; adding ethylene glycol and sodium hydroxide to the system, and continuing the reaction to obtain a hydroxyl-terminated polyamide; dissolving the hydroxyl-terminated polyamide in tetrahydrofuran, adding stannous octoate, and reacting with diisocyanate under nitrogen protection. The reaction yields isocyanate-terminated polyamides; finally, it reacts with ethylenediamine to cap the ends and obtain a five-armed polyamide initiator; B2, the five-armed polyamide initiator and poly(ethylene glycol) phosphate are added to tetrahydrofuran, and after purging with nitrogen to remove oxygen, cuprous bromide and 2,2'-bipyridine are added, and the reaction is carried out under magnetic stirring at 80-84℃; after the reaction is completed, methanol is added to precipitate the product, the precipitate is collected by filtration, washed repeatedly with tetrahydrofuran / methanol, and finally dried in a vacuum drying oven at 40-42℃.

2. The boehmite-coated diaphragm according to claim 1, characterized in that, In step A1, the time for introducing nitrogen to purge oxygen is 30-40 minutes; the reaction time for magnetic stirring is 8-12 hours.

3. The boehmite-coated diaphragm according to claim 1, characterized in that, In step A2, the drying time in a vacuum drying oven at 60-64℃ is 12-14 hours.

4. The boehmite-coated diaphragm according to claim 1, characterized in that, In step B1, the reaction time under reduced pressure at 120-124℃ is 6-8 hours; the reaction time continues for 2-4 hours; the reaction time with diisocyanate under nitrogen protection is 4-6 hours; and finally, the reaction time with ethylenediamine is 2-4 hours.

5. The boehmite-coated diaphragm according to claim 1, characterized in that, In step B2, the magnetic stirring reaction time is 12-14 hours; the drying time in a vacuum drying oven at 40-42℃ is 24-26 hours.

6. A process for preparing a boehmite-coated diaphragm according to any one of claims 1-5, characterized in that, The steps include: S1. Add boehmite powder to ultrapure water, stir in a high-speed disperser, add dispersant, thickener and wetting agent, and continue stirring until the slurry is free of obvious particles; S2. Subsequently, the copolymer containing fluorine-siloxane-sulfonic acid groups, the five-arm polyamide-polyphosphate star copolymer, styrene-butadiene rubber and lithium acrylate are added in sequence and stirred. The pH is adjusted to 6-7 to obtain the coating slurry. S3. The slurry is coated onto the surface of the PE base film through a comma roller. The coating thickness is controlled. It is first pre-dried at 80-84℃, then vacuum dried at 120-124℃, and finally compacted by a roller press.

7. The preparation process according to claim 6, characterized in that, In step S1, the speed of the high-speed disperser is 3000-3400 rpm, the stirring time is 30-40 min, and the stirring time continues for 60-80 min.

8. The preparation process according to claim 6, characterized in that, In step S2, the stirring speed is 200-300 rpm; the stirring time is 90-100 min.

9. The preparation process according to claim 6, characterized in that, In step S3, the coating thickness is 1-3 μm; the pre-drying time is 3-6 min; the vacuum drying time is 10-20 min; and the pressure of the roller press is 0.5-1 MPa.

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