Boehmite coated diaphragm and preparation process thereof

Abstract

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

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery separator, in particular to a boehmite coated separator and a preparation process thereof. BACKGROUND

[0002] Lithium ion battery, as the most potential secondary energy storage device, its core component separator performance directly determines the safety, cycle life and energy density of the battery. The separator plays a dual role of "ion channel" and "physical barrier" in the battery, which not only allows lithium ions to pass through efficiently to realize the charging and discharging process, but also strictly separates the positive and negative electrodes to prevent short circuit. However, the traditional commercialized separator is mostly made of polyolefin materials, although this kind of material has good chemical stability and low cost advantage, but in practical application, it has exposed many key defects: first, it is easy to shrink and deform in high temperature environment, when the battery is in high temperature working condition, the thermal shrinkage of the separator will cause the direct contact of the positive and negative electrodes, which will cause the risk of thermal runaway, seriously affecting the safety of the battery; second, the electrolyte absorption capacity is limited, it is difficult to fully soak the electrode and the internal pores of the separator, which leads to the increase of ion transmission resistance and the decrease of battery charge-discharge rate performance; third, the polyolefin material itself is non-polar material, the interface compatibility with the polar ceramic coating is poor, the coating particles are easy to agglomerate and fall off due to the lack of interfacial force, which not only reduces the functionality of the coating, but also increases the interface impedance, and accelerates the capacity decay in the battery cycle process. These problems greatly limit the application of lithium ion battery in the field of high safety and reliability requirements such as electric vehicles and energy storage systems.

[0003] In view of the shortcomings of traditional polyolefin separators, the prior art mainly modifies by coating ceramic functional materials or adding polymer binders. Among them, the ceramic coating has the advantages of high heat resistance, chemical inertness and low cost, and has become the most popular improvement scheme at present. However, the application of single ceramic coating still has obvious shortcomings: on the one hand, the surface of the ceramic particles is mostly polar hydroxyl structure, and the interfacial bonding force with the non-polar polyolefin base film is weak, which is easy to cause the coating to peel off due to the difference in thermal expansion coefficient at high temperature, thereby reducing the long-term stability of the separator; on the other hand, the electrical conductivity of the ceramic particles is limited, and excessive addition will increase the internal resistance of the separator and affect the ion transmission efficiency. In order to improve the above problems, some studies try to add a polymer binder in the slurry, hoping to enhance the bonding of the ceramic particles and the base film through the "bridging" effect of the binder. However, traditional binders are mostly single type of high molecular polymer, and the function is relatively single: part of the binder can improve the dispersibility of the ceramic particles, but due to the strong rigidity and insufficient polarity of the molecular chain, it cannot form effective adhesion with the base film; another part of the binder can improve the interfacial bonding force, but due to the too soft molecular chain or strong lipophilicity, the slurry rheological property is poor, and problems such as sagging, uneven thickness and the like are easy to occur during the coating process. More importantly, the traditional modification method often only focuses on the improvement of a single performance, and it is difficult to optimize multiple core indicators of the separator at the same time, resulting in limited improvement effect of the comprehensive performance.

[0004] With the rapid development of lithium ion batteries towards high energy density and high safety, more stringent requirements are put forward for the performance of the separator: it needs to have low high-temperature shrinkage to ensure safety under extreme working conditions, and also needs to have high electrolyte absorption rate to improve ion transmission efficiency; it needs to form strong interfacial bonding with the ceramic coating to avoid peeling off, and also needs to maintain good mechanical strength to withstand stress changes during the cycle process. The traditional single component or simple blending modification scheme cannot meet the above requirements, and the development of new type of modified material with multiple functions becomes the key to break through the current technical bottleneck. Based on this, the present application provides a boehmite coated separator based on two newly designed modified copolymers and a preparation process thereof, which aims to systematically solve the problems of poor thermal stability, poor electrolyte wettability and weak interfacial bonding force of traditional separators, and provides a new technical path for the development of high-performance lithium ion battery separators. SUMMARY

[0005] The present application aims to provide a boehmite coated separator and a preparation process thereof, which solves the technical problems of high high-temperature shrinkage, poor electrolyte wettability and weak interfacial bonding force of the existing boehmite coated separator.

[0006] The present application realizes the above-mentioned purpose through the following technical scheme: A boehmite coated separator comprises the following raw materials by weight: Super pure water: 50-100 parts by weight; Dispersant: 1-3 parts by weight; Boehmite: 10-30 parts by weight; Thickening agent: 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; Fluorine-siloxane-sulfonic acid group-containing copolymer: 1-3 parts by weight; Five-arm polyamide-polyphosphonate star copolymer: 0.5-2 parts by weight; The preparation method of the fluorine-siloxane-sulfonic acid group-containing copolymer comprises: A1, methyl acrylate trifluoroethyl ester, gamma-(methacryloyloxy) propyl trimethoxysilane and 2-acrylamide-2-methylpropane sulfonic acid are added to a round-bottom flask, N,N-dimethylformamide is added, nitrogen is introduced to remove oxygen, and azobisisobutyronitrile is added, and then the reaction is stirred at 60-80 DEG C under magnetic force; A2, after the reaction is completed, the product is poured into anhydrous ethanol for precipitation, the precipitate is collected by filtration, and then repeatedly dialyzed with deionized water, and finally dried in a vacuum drying oven at 60-64 DEG C.

[0007] In the present application, the preparation reaction mechanism of the fluorine-siloxane-sulfonic acid group-containing copolymer is based on the principle of high molecular polymerization reaction, and is formed through the process of multi-monomer copolymerization initiated by active radicals. At the beginning of preparation, three monomers with different functional groups (containing fluorine group, siloxane group and sulfonic acid group respectively) are mixed with polar solvent, and active oxygen in the system is removed under the protection of inert gas (to avoid hindering the polymerization reaction), and then a free radical initiator is added. The initiator decomposes to form active radicals at a certain temperature, which attacks the unsaturated double bond (located in the molecular chain of each monomer) to form a dynamically growing active center. These active centers continuously combine with unreacted monomer double bonds, and gradually extend the molecular chain through chain growth reaction. Due to the difference in reactivity of the three monomers (i.e. the difficulty of combining with free radicals is different), the structural units of the three monomers in the molecular chain of the finally formed copolymer are randomly arranged, forming a ternary random copolymer structure. Among them, the monomer containing fluorine group gives the molecular chain a lower surface energy; the monomer containing siloxane group can undergo condensation reaction with the hydroxyl group on the surface of inorganic particles in subsequent treatment to form stable chemical bond connection; the monomer containing sulfonic acid group provides ion conduction ability through dissociation. After the reaction is completed, the product is precipitated by polar solvent (to separate unreacted monomers by using the difference in solubility), deionized water dialysis (to remove small molecular impurities) and vacuum drying (to remove water), and finally a copolymer with synergistic effect of multiple functional groups is obtained.

[0008] According to the preferred embodiment of the present application, in step A1, the time for oxygen removal by nitrogen purging is 30-40 min; the time for magnetic stirring reaction is 8-12 h.

[0009] According to the preferred embodiment of the present application, in step A2, the drying time in the vacuum drying oven at 60-64℃ is 12-14 h.

[0010] According to the preferred embodiment of the present application, the preparation method of the five-arm polyamide-polyphosphate star copolymer comprises: B1, adding adipic acid and 1,6-hexanediamine into 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 into the system, and continuing to react 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 cap to obtain a five-arm polyamide initiator; B2, adding the five-arm polyamide initiator and polydi(ethylene glycol) phosphate into tetrahydrofuran, purging oxygen with nitrogen, then adding cuprous bromide and 2,2'-bipyridine, and reacting under magnetic stirring at 80-84℃; after the reaction is completed, adding methanol to precipitate the product, collecting the precipitate by filtration, repeatedly washing with tetrahydrofuran / methanol, and finally drying in a vacuum drying oven at 40-42℃.

[0011] In the present application, the reaction mechanism of the preparation of the five-arm polyamide-polyphosphate star copolymer is as follows: the preparation of the copolymer is divided into two steps: firstly, the multi-step synthesis of the multi-arm polyamide initiator, the branched structure is constructed by condensation reaction and functional group transformation; secondly, the polyphosphate segment is grafted by atom transfer radical polymerization to form a star topological structure. In the first step, the synthesis of the multi-arm polyamide initiator starts from the condensation reaction of two kinds of dicarboxylic acids and diamines. Both of the two monomers contain two reactive polar groups (carboxyl and amino), under the conditions of high temperature and low pressure (to promote water evaporation), the condensation reaction of carboxyl and amino occurs, small molecule substances are removed, and linear polyamide prepolymer is formed. Then, by introducing a small molecule compound containing a hydroxyl group to react with the end carboxyl group of the prepolymer, the end group of the linear molecule is converted into a hydroxyl group, and an end hydroxyl polyamide is obtained. Then, the end hydroxyl polyamide is reacted with a compound containing an isocyanate group to convert the end hydroxyl group into an isocyanate group, forming an end isocyanate group polyamide. Finally, the end isocyanate group is reacted with an amino-containing compound, and the end of the linear polyamide molecule is expanded into multiple branched arms (forming a five-arm structure) by functional group crosslinking to obtain a multi-arm polyamide initiator. In the second step, the polyphosphate segment is grafted to the multi-arm polyamide initiator by atom transfer radical polymerization. The multi-arm polyamide initiator is dissolved in a polar aprotic solvent, a transition metal catalyst and a ligand are added to form a catalytic system, at a specific temperature, the catalyst decomposes to form active radicals, which attack the molecular chain of the polyphosphate monomer (containing a reactive double bond) to form primary radicals. At the same time, the end isocyanate group (or the active site after activation) of the multi-arm polyamide initiator serves as an initiation center and reacts with the active radicals to start chain growth. The polyphosphate monomer is connected to each branched arm of the multi-arm polyamide initiator by radical grafting reaction to form a star topological structure with five-arm polyamide as the core and polyphosphate segment as the branch. After the reaction is completed, a precipitant is added to destroy the catalytic system and precipitate the product, and the unreacted monomer and catalyst are removed by repeated washing with solvent, and finally vacuum dried to obtain a copolymer with branched structure and multi-segment synergistic effect.

[0012] According to the preferred embodiment of the present application, in step B1, the reaction time under reduced pressure at 120-124℃ is 6-8h; the time for continuing the reaction is 2-4h; the reaction time with diisocyanate under nitrogen protection is 4-6h; and the reaction time with ethylenediamine is 2-4h.

[0013] According to the preferred embodiment of the present application, in step B2, the reaction time under magnetic stirring is 12-14h; and the drying time in the vacuum drying oven at 40-42℃ is 24-26h.

[0014] The present application also provides a preparation process of the boehmite-coated separator, and the steps include: S1, add boehmite powder into ultrapure water, stir in a high-speed dispersion machine, add dispersant, thickening agent and wetting agent, continue to stir until the slurry has no obvious particles; S2, then add fluorine-siloxane-sulfonic acid group-containing copolymer, five-arm polyamide-polyphosphate star copolymer, styrene butadiene rubber and lithium acrylate in sequence, stir, adjust pH to 6-7, and obtain a coating slurry; S3, coat the slurry on the surface of the PE base film through a comma roller, control the coating thickness, pre-dry at 80-84℃, vacuum dry at 120-124℃, and finally compact by a roller compactor.

[0015] In the present application, the preparation reaction mechanism of the boehmite-coated separator. The preparation of the boehmite-coated separator is a process of multi-component synergistic effect, involving physical and chemical changes in stages such as dispersion, mixing, coating and curing. First, the boehmite powder (main component is layered hydroxyl oxide, the surface is rich in polar hydroxyl group) is stably dispersed in water by the action of dispersant: the polar group (such as carboxyl) of the dispersant forms hydrogen bond with the hydroxyl group on the surface of boehmite, and at the same time forms steric hindrance between particles to prevent agglomeration. The thickening agent increases the viscosity of the slurry by winding the molecular chain, adjusts the rheological property, and avoids the flow out of control of the slurry in the coating process. The wetting agent reduces the surface tension of the slurry and enhances the wettability of the base film (polyolefin material), so that the slurry spreads uniformly on the surface of the base film. Subsequently, the fluorine-siloxane-sulfonic acid group copolymer and the five-arm polyamide-polyphosphate star copolymer are added to the slurry, and multiple interactions occur between the boehmite particles: the siloxane structure of the fluorine-siloxane-sulfonic acid group copolymer reacts with the hydroxyl group on the surface of boehmite to form a stable chemical bond, which enhances the interfacial adhesion between the copolymer and boehmite; the fluorine structure reduces the surface energy of the coating, reducing the shrinkage tendency during coating; the sulfonic acid group dissociates ions, which synergistically reduce the interfacial energy with the polar group of the wetting agent, and improve the electrolyte wettability. The multi-arm structure of the five-arm polyamide-polyphosphate star copolymer winds around the boehmite particles to form a three-dimensional network, enhancing the cohesion of the coating; the polar group of the polyphosphate segment forms hydrogen bonds with the residual polar sites on the surface of the base film (such as unreacted hydroxyl or oxidation sites during polymerization), and the amide structure of the multi-arm polyamide core weakly interacts with the carbon-hydrogen bond on the surface of the base film, improving the interfacial bonding force between the coating and the base film. Butadiene rubber acts as an inter-particle binder, wrapping boehmite particles through physical adsorption of molecular chains, filling the inter-particle gaps, and preventing coating cracking; the polar group of lithium acrylate forms ion-dipole interaction with the polar sites on the surface of the base film, and the dissociated ions migrate at the interface to reduce the interfacial impedance. Finally, the slurry is coated on the surface of the base film by comma roller to control the thickness of the coating. In the pre-drying stage, part of the water is volatilized, and the slurry is preliminarily solidified; vacuum drying further removes the remaining water, promotes the crosslinking of the copolymer molecular chain (such as hydrogen bonding of sulfonic acid group, condensation reaction of siloxane) and chemical bonding with the base film, and forms a stable coating structure. The roll press compacts the coating and the base film by mechanical pressure to reduce the porosity and improve the overall density. The finally formed boehmite-coated separator realizes the comprehensive improvement of thermal stability, electrolyte wettability and interfacial bonding force through the synergistic effect of multiple components.

[0016] According to the preferred embodiment of the present application, in step S1, the rotation speed of the high-speed disperser is 3000-3400 rpm, and the stirring time is 30-40 min; the continuous stirring time is 60-80 min.

[0017] According to the preferred embodiment of the present application, in step S2, the stirring speed is 200-300 rpm; and the stirring time is 90-100 min.

[0018] According to the preferred embodiment of the present application, 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 roll press is 0.5-1 MPa.

[0019] The present application has the following advantages: The present application has the following advantages: Firstly, the thermal stability is greatly improved, effectively ensuring the safety performance of the battery under high-temperature working conditions. The traditional polyolefin separator is prone to shrinkage and deformation under high-temperature environment due to the insufficient heat resistance of the material itself, which leads to short circuit of the positive and negative electrodes and causes the risk of thermal runaway. In the present application, the five-armed polyamide-polyphosphate star copolymer can form a dense phosphate protective layer on the surface of the base film through its unique branched star structure and phosphate groups on the molecular chain, thereby inhibiting the thermal shrinkage behavior of the base material. Meanwhile, the siloxane groups in the molecular chain of the fluorine-siloxane-sulfonic acid group copolymer can undergo condensation reaction with the hydroxyl groups on the surface of the boehmite particles to form stable silicon-oxygen bonds, thereby tightly bonding the ceramic coating and the base film and avoiding the peeling phenomenon of the coating due to the difference in thermal expansion coefficient under high temperature. The synergistic effect of the two copolymers significantly enhances the dimensional stability of the separator in high-temperature environment, effectively reduces the risk of thermal shrinkage, and greatly improves the safety of the battery under extreme conditions such as overcharging, short circuit, or external heating.

[0020] Secondly, the electrolyte wettability and ion transport efficiency are significantly optimized, which improves the rate performance and cycle life of the battery. The traditional boehmite coating is prone to low electrolyte absorption rate and high ion transport resistance due to the insufficient polarity and electrolyte compatibility of the surface. The fluorine groups in the molecular chain of the fluorine-siloxane-sulfonic acid group copolymer can reduce the surface energy of the coating, making the electrolyte more easily spread and wet. The sulfonic acid groups can dissociate into ions to form ion transport channels at the interface, thereby improving the ion conduction efficiency. The multi-arm branched structure of the five-armed polyamide-polyphosphate star copolymer can increase the contact area between the molecular chain and the electrolyte, and the hydrophilic polyphosphate segment further enhances the adsorption capacity of the coating to the electrolyte. The combination of the two copolymers significantly improves the electrolyte absorption rate of the separator and reduces the ion transport resistance, thereby improving the charge-discharge rate performance of the battery. At the same time, the capacity decay problem caused by poor ion transport during the cycle process is effectively alleviated, thereby prolonging the service life of the battery.

[0021] Finally, the interface bonding force and mechanical strength are significantly enhanced, ensuring the structural stability of the separator in long-term cycling. The interface bonding force between the traditional boehmite coating and the base film is weak, and the coating is prone to fall off during cycling due to stress changes, affecting the performance of the battery. The copolymer containing fluorine-siloxane-sulfonic acid groups strengthens the interface bonding between the coating and the base film through chemical bonding of the siloxane group with boehmite and modification of the base film surface by the fluorinated group; the multi-arm structure of the five-arm polyamide-polyphosphate star copolymer can entangle boehmite particles to form a three-dimensional network structure, enhancing the cohesion of the coating. In addition, the butadiene-styrene rubber and lithium acrylate as auxiliary adhesives, respectively through physical adsorption with boehmite particles and chemical action with the base film, further consolidate the interface bonding system of "particle-coating-base film". The synergistic effect of multiple forces greatly improves the peel strength of the separator and significantly enhances the mechanical strength, effectively avoiding the short circuit risk caused by the coating falling off during cycling, ensuring the structural stability and reliability of the battery in long-term use.

[0022] In summary, the present application solves the problems of poor thermal stability, poor electrolyte wettability and weak interface bonding force of traditional boehmite coated separators through the innovative design and synergistic effect of two new types of modified copolymers, achieving a breakthrough in comprehensive performance in safety, rate performance and cycle life, providing a new technical path for the development of high-performance lithium ion battery separators. DETAILED DESCRIPTION

[0023] The following detailed description is only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0024] The relevant equipment and material suppliers are as follows: The ultrapure water is purchased from Shanghai Leifeng Biological Technology Co., Ltd.

[0025] The dispersing agent is purchased from Jiangsu Hehaimi Nanotechnology Co., Ltd.

[0026] The boehmite is purchased from Anhui Yishitong Material Technology Co., Ltd.

[0027] The thickening agent is purchased from Shandong Heda Group Co., Ltd.

[0028] The butadiene-styrene rubber is purchased from China Petroleum and Natural Gas Corporation Lanzhou Petrochemical Branch.

[0029] The lithium acrylate is purchased from Jiangsu Yulong Chemical Co., Ltd.

[0030] The wetting agent is purchased from Shanghai Huazhiyun Chemical Co., Ltd.

[0031] The methyl acrylate is purchased from Zhejiang Juhua Co., Ltd.

[0032] The γ-(methacryloyloxy)propyltrimethoxysilane is purchased from Hubei Xingfa Chemical Group Co., Ltd.

[0033] The 2-acrylamido-2-methylpropanesulfonic acid is purchased from Shandong Baomo Biochemical Co., Ltd.

[0034] The round-bottom flask is purchased from Sichuan Shubo (Group) Co., Ltd.

[0035] The N,N-dimethylformamide is purchased from Jiangsu Yida Chemical Co., Ltd.

[0036] The azobisisobutyronitrile is purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0037] The anhydrous ethanol is purchased from Anhui Ante Food Co., Ltd.

[0038] The vacuum drying oven is purchased from Shanghai Yiheng Technology Co., Ltd.

[0039] The adipic acid is purchased from Shandong Haili Chemical Co., Ltd.

[0040] The 1,6-hexanediamine is purchased from Jiangsu Xin Hai Petrochemical Co., Ltd.

[0041] The three-necked flask is purchased from Sichuan Shubo (Group) Co., Ltd.

[0042] The deionized water is purchased from Hangzhou Wahaha Group Co., Ltd.

[0043] The ethylene glycol is purchased from Jiangsu Huachang Chemical Co., Ltd.

[0044] The sodium hydroxide is purchased from Shandong Haihua Co., Ltd.

[0045] The tetrahydrofuran is purchased from Jiangsu Yida Chemical Co., Ltd.

[0046] The stannous octoate is purchased from Jiangsu Hahai Nanotechnology Co., Ltd.

[0047] The diisocyanate is purchased from Wanhua Chemical Group Co., Ltd.

[0048] The ethylenediamine is purchased from Jiangsu Xin Hai Petrochemical Co., Ltd.

[0049] The poly(di(ethylene glycol))phosphate is purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0050] The cuprous bromide is purchased from Jiangxi Copper Co., Ltd.

[0051] The 2,2'-bipyridine was purchased from Jiangsu Huaian Nanotechnology Co., Ltd.

[0052] The methanol was purchased from Shaanxi Yulin Energy Group Co., Ltd.

[0053] The high-speed dispersion machine was purchased from Shanghai Rute Electrical Equipment Co., Ltd.

[0054] The comma roller was purchased from Shanghai Kaikan Roller Co., Ltd.

[0055] The PE base film was purchased from Xiamen Changshu Industrial Co., Ltd.

[0056] The roller press was purchased from Beijing North Huawu Microelectronics Equipment Co., Ltd.

[0057] Example 1 Preparation of fluorine-siloxane-sulfonic acid group copolymer: 45 g of trifluoroethyl methacrylate, 50 g of γ-(methacryloyloxy)propyl trimethoxysilane, and 44 g of 2-acrylamido-2-methylpropane sulfonic acid were added to a round-bottom flask, 1390 mL of N,N-dimethylformamide (10 times the total mass of the monomers) was added, nitrogen was introduced to remove oxygen for 35 min, 1.4 g of azobisisobutyronitrile (1% of the total mass of the monomers) was added, and the reaction was stirred at 70°C for 10 h. After the reaction was completed, the product was precipitated in anhydrous ethanol (3 times the volume of the reaction liquid), the precipitate was collected by filtration, and repeatedly dialyzed with deionized water until the conductivity of the filtrate was <10 μS / cm. Finally, it was dried in a vacuum drying oven at 62°C for 13 h to obtain a fluorine-siloxane-sulfonic acid group copolymer.

[0058] Preparation of the five-armed polyamide-polyphosphoester star copolymer: adipic acid 146 g, 1,6-hexanediamine 116 g were added into a three-necked flask, deionized water (5 times the total mass of monomers by volume) was added, and the reaction was carried out at 122 °C under reduced pressure for 7 h to produce a polyamide prepolymer; ethylene glycol 6.2 g and sodium hydroxide 8 g were added to the system, and the reaction was continued for 3 h to obtain a hydroxyl-terminated polyamide; the hydroxyl-terminated polyamide was dissolved in tetrahydrofuran (10 times the mass of the polyamide by volume), stannous octoate 1.2 g was added, and the reaction was carried out with diisocyanate 174 g under nitrogen protection for 5 h to obtain an isocyanate-terminated polyamide; finally, the reaction was carried out with ethylenediamine 3.6 g for 3 h to obtain a five-armed polyamide initiator. The five-armed polyamide initiator 120 g, polydi(ethylene glycol) phosphate 150 g were added into tetrahydrofuran (8 times the total mass of monomers by volume), and after oxygen was removed by nitrogen, cuprous bromide 1.2 g and 2,2'-bipyridine 3.8 g were added, and the reaction was carried out under magnetic stirring at 82 °C for 13 h; after the reaction was completed, methanol (3 times the volume of the reaction solution by volume) was added to precipitate the product, the precipitate was collected by filtration, and was repeatedly washed with tetrahydrofuran / methanol (1:1 by volume) for 3 times, and finally dried in a vacuum drying oven at 41 °C for 25 h to obtain the five-armed polyamide-polyphosphoester star copolymer.

[0059] Preparation of the boehmite-coated separator: boehmite 20 g was added into ultrapure water 70 g, and stirred at 3200 rpm for 35 min in a high-speed disperser, dispersant 2 g, thickening agent 1 g and wetting agent 0.3 g were added, and the stirring was continued for 70 min until the slurry was free of obvious particles; then, fluorine-siloxane-sulfonic group copolymer 2 g, five-armed polyamide-polyphosphoester star copolymer 1 g, butadiene-styrene rubber 3 g and lithium acrylate 2 g were added in sequence, and the stirring was carried out at 250 rpm for 95 min, and the pH was adjusted to 6.5 to obtain a coating slurry with a solid content of 35%; the slurry was coated on the surface of a PE-based film (thickness 10 μm, porosity 45%) by comma coating, the coating thickness was controlled to be 2 μm, pre-drying was carried out at 82 °C for 4 min, vacuum drying was carried out at 122 °C for 15 min, and finally compaction was carried out by a roll press (pressure 0.8 MPa) to obtain a boehmite-coated separator.

[0060] Example 2 The specific implementation is the same as Example 1, except that the preparation of the fluorine-siloxane-sulfonic group copolymer: trifluoroethyl methacrylate 40 g, γ-(methacryloyloxy) propyl trimethoxysilane 45 g, 2-acrylamido-2-methylpropanesulfonic acid 38 g are added to a round-bottom flask, N,N-dimethylformamide 1200 mL (10 times the total mass of the monomers) is added, nitrogen is introduced to remove oxygen for 35 min, azobisisobutyronitrile 1.2 g (1% of the total mass of the monomers) is added, and the reaction is stirred magnetically at 70°C for 10 h. After the reaction is completed, the product is precipitated into anhydrous ethanol, the precipitate is collected by filtration, and is repeatedly dialyzed with deionized water, and finally dried in a vacuum drying oven at 62°C for 13 h. Preparation of the five-armed polyamide-polyphosphate star copolymer: adipic acid 130 g, 1,6-hexanediamine 105 g are added to a three-necked flask, deionized water (5 times the total mass of the monomers by volume) is added, and the reaction is carried out at 122°C under reduced pressure for 7 h to obtain a polyamide prepolymer; ethylene glycol 5.5 g and sodium hydroxide 7 g are added to the system, and the reaction is continued for 3 h to obtain a terminal hydroxyl polyamide; the terminal hydroxyl polyamide is dissolved in tetrahydrofuran, stannous octoate 1.0 g is added, and the reaction is carried out with diisocyanate 150 g under nitrogen protection for 5 h to obtain a terminal isocyanate polyamide; finally, the reaction is carried out with ethylenediamine 3.0 g for 3 h to cap to obtain a five-armed polyamide initiator. The five-armed polyamide initiator 100 g, polydi(ethylene glycol) phosphate 130 g are added to tetrahydrofuran, copper bromide 1.0 g and 2,2'-bipyridine 3.2 g are added after nitrogen is introduced to remove oxygen, and the reaction is carried out magnetically at 82°C for 13 h; after the reaction is completed, methanol is added to precipitate the product, the precipitate is collected by filtration, and is repeatedly washed with tetrahydrofuran / methanol, and finally dried in a vacuum drying oven at 41°C for 25 h. Preparation of the boehmite-coated separator: boehmite 18 g is added to ultrapure water 65 g, stirring is carried out at 3200 rpm for 35 min in a high-speed disperser, dispersant 1.8 g, thickening agent 0.9 g, and wetting agent 0.25 g are added, and stirring is continued for 70 min; then fluorine-siloxane-sulfonic group copolymer 1.8 g, five-armed polyamide-polyphosphate star copolymer 0.9 g, butadiene-styrene rubber 2.8 g, and lithium acrylate 1.8 g are added in sequence, stirring is carried out at 250 rpm for 95 min, and the pH is adjusted to 6.5 to obtain a coating slurry; the slurry is coated on the surface of a PE-based film by comma roller coating, the coating thickness is controlled to be 2 μm, pre-drying is carried out at 82°C for 4 min, vacuum drying is carried out at 122°C for 15 min, and finally compaction is carried out by a roller press (pressure 0.8 MPa).

[0061] Example 3 The specific implementation is the same as Example 1, except that the preparation of the fluorine-siloxane-sulfonic group copolymer: trifluoroethyl methacrylate 50 g, γ-(methacryloyloxy) propyl trimethoxysilane 55 g, 2-acrylamido-2-methylpropanesulfonic acid 48 g are added to a round-bottom flask, N,N-dimethylformamide 1400 mL (10 times the total mass of the monomers) is added, nitrogen is introduced to remove oxygen for 35 min, azobisisobutyronitrile 1.5 g (1% of the total mass of the monomers) is added, and the reaction is stirred magnetically at 70°C for 10 h. After the reaction is completed, the product is precipitated into anhydrous ethanol, the precipitate is collected by filtration, and is repeatedly dialyzed with deionized water, and finally dried in a vacuum drying oven at 62°C for 13 h. Preparation of the five-armed polyamide-polyphosphate star copolymer: adipic acid 150 g, 1,6-hexanediamine 120 g are added to a three-necked flask, deionized water (5 times the total volume of the monomers) is added, and the reaction is carried out at 122°C under reduced pressure for 7 h to obtain a polyamide prepolymer; ethylene glycol 6.8 g and sodium hydroxide 8.5 g are added to the system, and the reaction is continued for 3 h to obtain a terminal hydroxyl polyamide; the terminal hydroxyl polyamide is dissolved in tetrahydrofuran, stannous octoate 1.3 g is added, and the reaction is carried out with diisocyanate 180 g under nitrogen protection for 5 h to obtain a terminal isocyanate polyamide; finally, the reaction is carried out with ethylenediamine 3.8 g for 3 h to cap the five-armed polyamide initiator. The five-armed polyamide initiator 130 g, polydi(ethylene glycol) phosphate 160 g are added to tetrahydrofuran, copper bromide 1.3 g and 2,2'-bipyridine 4.0 g are added after nitrogen is introduced to remove oxygen, and the reaction is carried out magnetically at 82°C for 13 h; after the reaction is completed, methanol is added to precipitate the product, the precipitate is collected by filtration, and is repeatedly washed with tetrahydrofuran / methanol, and finally dried in a vacuum drying oven at 41°C for 25 h. Preparation of the boehmite-coated separator: boehmite 22 g is added to ultrapure water 75 g, stirring is carried out at 3200 rpm for 35 min in a high-speed disperser, dispersant 2.2 g, thickening agent 1.1 g, and wetting agent 0.35 g are added, and stirring is continued for 70 min; then fluorine-siloxane-sulfonic group copolymer 2.2 g, five-armed polyamide-polyphosphate star copolymer 1.1 g, butadiene-styrene rubber 3.2 g, and lithium acrylate 2.2 g are added in sequence, stirring is carried out at 250 rpm for 95 min, and the pH is adjusted to 6.5 to obtain a coating slurry; the slurry is coated on the surface of a PE-based film by comma roller coating, the coating thickness is controlled to be 2 μm, pre-drying is carried out at 82°C for 4 min, vacuum drying is carried out at 122°C for 15 min, and finally compaction is carried out by a roller press (pressure 0.8 MPa).

[0062] Comparative Example 1 The specific implementation is the same as that of Example 1, except that the preparation of the boehmite-coated separator: boehmite 20 g is added to ultrapure water 70 g, stirred at 3200 rpm for 35 min in a high-speed disperser, dispersant 2 g, thickener 1 g and wetting agent 0.3 g are added, and stirring is continued for 70 min; then fluorine-siloxane-sulfonic group copolymer 2 g, butadiene-styrene rubber 3 g and lithium acrylate 2 g (without adding five-armed polyamide-polyphosphate star copolymer) are added in turn, stirring at 250 rpm for 95 min, and the pH is adjusted to 6.5 to obtain a coating slurry; the slurry is coated on the surface of the PE base film by comma roller, the coating thickness is controlled to be 2 μm, pre-dried at 82°C for 4 min, vacuum dried at 122°C for 15 min, and finally pressed by a roller press (pressure 0.8 MPa).

[0063] Comparative Example 2 The specific implementation is the same as that of Example 1, except that the preparation of the boehmite-coated separator: boehmite 20 g is added to ultrapure water 70 g, stirred at 3200 rpm for 35 min in a high-speed disperser, dispersant 2 g, thickener 1 g and wetting agent 0.3 g are added, and stirring is continued for 70 min; then fluorine-siloxane-sulfonic group copolymer 2 g, butadiene-styrene rubber 3 g and lithium acrylate 2 g (without adding five-armed polyamide-polyphosphate star copolymer) are added in turn, stirring at 250 rpm for 95 min, and the pH is adjusted to 6.5 to obtain a coating slurry; the slurry is coated on the surface of the PE base film by comma roller, the coating thickness is controlled to be 2 μm, pre-dried at 82°C for 4 min, vacuum dried at 122°C for 15 min, and finally pressed by a roller press (pressure 0.8 MPa).

[0064] Comparative Example 3 The specific implementation is the same as that of Example 1, except that the preparation of the boehmite-coated separator: boehmite 20 g is added to ultrapure water 70 g, stirred at 3200 rpm for 35 min in a high-speed disperser, dispersant 2 g, thickener 1 g and wetting agent 0.3 g are added, and stirring is continued for 70 min; then fluorine-siloxane-sulfonic group copolymer 2 g, butadiene-styrene rubber 3 g and lithium acrylate 2 g (without adding five-armed polyamide-polyphosphate star copolymer) are added in turn, stirring at 250 rpm for 95 min, and the pH is adjusted to 6.5 to obtain a coating slurry; the slurry is coated on the surface of the PE base film by comma roller, the coating thickness is controlled to be 2 μm, pre-dried at 82°C for 4 min, vacuum dried at 122°C for 15 min, and finally pressed by a roller press (pressure 0.8 MPa).

[0065] Performance test The boehmite-coated separators prepared in Examples 1-3 and Comparative Examples 1-3 above are tested for performance according to the following method: 1. Thermal shrinkage test: According to GB / T 12027-2004 "Plastic film dimensional change rate test method", the separator was cut into a square sample of 100 mm x 100 mm, laid flat on a clean glass plate, and placed in a 120°C constant temperature oven for 1 hour. After taking out, it was immediately cooled to room temperature (25°C), and the length of the long side and the short side of the sample was measured with a vernier caliper. The thermal shrinkage rate was calculated (shrinkage rate = (initial length - length after test) / initial length x 100%), and the average value of 5 parallel samples was taken.

[0066] 2. Electrolyte absorption rate test: The separator was completely immersed in simulated electrolyte (1M LiPF6 dissolved in ethylene carbonate-diethyl carbonate, volume ratio 1:1) and soaked at 25°C for 24 hours to ensure that the separator was fully absorbed. After taking out, the surface residual electrolyte was gently wiped with anhydrous ethanol, and the absorbed mass (m1) was immediately weighed with an analytical balance, and the initial dry mass (m0) was recorded. The absorption rate was calculated according to the formula: absorption rate = (m1-m0) / m0 x 100%, and the average value of 3 parallel samples was taken.

[0067] 3. Ionic conductivity test: AC impedance method (EIS) was used for testing. The separator was cut into a circular sheet with a diameter of 10 mm, and was sandwiched between two stainless steel disc electrodes (diameter 12 mm) to form a symmetrical battery (separator as the only ion conducting medium). An electrochemical workstation (CHI660E) was used to scan at a frequency range of 0.1 Hz-100 kHz and an amplitude of 5 mV to obtain the Nyquist plot. The ionic conductivity σ was calculated by equivalent circuit fitting (R s is the solution resistance, R n is the charge transfer resistance), and the formula is: σ = L / (R s x A), where L is the thickness of the separator (cm), and A is the effective area of the electrode (cm²), and the average value of 3 parallel samples was taken.

[0068] 4. Peel strength test: The boehmite-coated separator and the PE-based film (uncoated) were compounded by roll pressing process (pressure 0.5 MPa, temperature 80°C) to form a "base film-coating-base film" sandwich structure, and cut into a long strip sample of 15 mm x 100 mm. A universal material testing machine (Instron 5967) was used for 90° peel test at a constant rate of 10 mm / min, and the maximum force value (N) during peeling was recorded. The peel strength per unit width (N / m) was calculated, and the average value of 5 parallel samples was taken.

[0069] 5. Performance test results: Table 1: Performance test results of each example and comparative example

[0070] As can be seen from Table 1, the boehmite coated separator prepared in Examples 1-3 significantly solves the technical problems of high high-temperature shrinkage, poor electrolyte wettability and weak interface bonding force with the base film of the conventional boehmite coated separator through the synergistic effect of the two modified copolymers. In terms of thermal shrinkage, the thermal shrinkage of Examples 1-3 is only 0.8%-1.0%, which is far lower than 3.2%-4.8% of Comparative Examples 1-3, because the phosphate groups of the five-armed polyamide-polyphosphate star copolymer can decompose to form a dense phosphate protective layer at high temperature, inhibiting the thermal shrinkage behavior of the polyolefin base film, and the siloxane groups of the fluorine-siloxane-sulfonic group copolymer condense with the hydroxyl groups on the surface of boehmite (forming stable Si-O-Al bonds), enhancing the interface bonding of the coating and the base film, avoiding the peeling of the coating due to the difference in thermal expansion coefficient at high temperature, and thus effectively reducing the thermal shrinkage.

[0071] In terms of electrolyte wettability, the electrolyte absorption rate of Examples 1-3 is as high as 195%-202%, which is about 50%-60% higher than 125%-142% of Comparative Examples 1-3, which is mainly due to the fluorinated groups of the fluorine-siloxane-sulfonic group copolymer reducing the surface energy of the coating (making the electrolyte more easily spread and wet), and the sulfonic acid groups dissociating lithium ions to form ion transport channels at the interface, improving the adsorption capacity of the electrolyte; at the same time, the multi-arm branched structure of the five-armed polyamide-polyphosphate star copolymer increases the contact area of the molecular chain with the electrolyte, and the hydrophilic polyphosphate segment further enhances the adsorption of the electrolyte, and the synergistic effect of the two significantly improves the electrolyte wettability of the separator.

[0072] For the interface bonding force with the base film, the peeling 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), because the siloxane groups of the fluorine-siloxane-sulfonic group copolymer form chemical bonding with the hydroxyl groups on the surface of boehmite, and the modification of the base film surface by fluorinated groups enhances the interface bonding of the coating and the base film; the multi-arm structure of the five-armed polyamide-polyphosphate star copolymer can wrap boehmite particles to form a three-dimensional network structure, enhancing the cohesion of the coating, and the polar groups of the polyphosphate segment form hydrogen bonds with the residual polar sites (such as residual hydroxyl or oxidation sites) on the surface of the base film, and the amide groups of the five-armed polyamide core weakly interact with the C-H bonds on the surface of the base film, further consolidating the interface bonding system of "particle-coating-base film"; in addition, the butadiene-styrene rubber and lithium acrylate as auxiliary adhesives, respectively through physical adsorption with boehmite particles and chemical action with the base film, synergistically enhance the interface bonding force.

[0073] In summary, examples 1-3 solve the problems of high thermal shrinkage, poor electrolyte wettability and weak interface bonding force of traditional boehmite coated separators by the synergistic effect of two new modified copolymers, and realize the significant improvement of the comprehensive performance of the separator.

[0074] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of protection of the present application.

Claims

1. A boehmite-coated diaphragm, characterized in that, Including the following parts by weight of raw materials: 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; Copolymers containing fluorinated siloxane-sulfonic acid groups: 1-3 parts by weight; Five-arm polyamide-polyphosphate star copolymer: 0.5-2 parts by weight; 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℃.

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, The preparation method of the five-arm 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 to obtain an isocyanate-terminated polyamide; finally reacting with ethylenediamine to end-cap the five-arm polyamide initiator; B2, adding the five-arm 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℃; 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℃.

5. The boehmite-coated diaphragm according to claim 4, 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.

6. The boehmite-coated diaphragm according to claim 4, 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.

7. A process for preparing a boehmite-coated diaphragm according to any one of claims 1-6, characterized in that step... 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-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. 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.

8. The preparation process according to claim 7, 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.

9. The preparation process according to claim 7, characterized in that, In step S2, the stirring speed is 200-300 rpm; the stirring time is 90-100 min.

10. The preparation process according to claim 7, 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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