Alumina-coated separator and method for preparing the same

Abstract

The application discloses an alumina-coated diaphragm in the field of lithium ion batteries and a preparation method thereof. The diaphragm comprises alumina particles, a fluorine-containing porous organosilicon compound, a phosphorus-heterocyclic covalent organic framework, a binder, a dispersing agent and deionized water. The fluorine-containing porous organosilicon compound is formed by mixing methyltrimethoxysilane and perfluorohexyltrimethoxysilane with water, reacting to form a sol-gel system after adjusting pH with ammonia water, continuously reacting by adding aminosilane, and then performing rotary evaporation, calcination and grinding; the phosphorus-heterocyclic covalent organic framework is formed by dissolving cyanuric chloride and triphenylphosphine oxide in anhydrous tetrahydrofuran, reacting to form an intermediate by adding imidazole bromide, continuously reacting by adding ethylene glycol, and then performing vacuum drying and grinding. The diaphragm synergistically enhances the interface adhesion and electrolyte wettability through the bifunctional compound, and improves the thermal stability and cycle performance.

Description

Technical Field

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

[0002] As the mainstream secondary energy storage device, the safety and cycle life of lithium-ion batteries are key factors restricting their large-scale application. The separator, as one of the core components of the battery, must simultaneously possess high porosity, good electrolyte wettability, excellent thermal shrinkage, and mechanical strength to ensure structural stability and efficient ion transport during charging and discharging. Currently, commercially available lithium-ion battery separators mostly use polyolefin substrates. These materials are widely used due to their low cost and good processing performance, but their heat resistance is insufficient. They are prone to shrinkage and melting at high temperatures, leading to direct contact between the positive and negative electrodes and causing short circuits, seriously threatening battery safety. To improve the thermal stability of polyolefin separators, alumina-coated separators have been extensively studied. Alumina has advantages such as a high melting point, good chemical stability, and strong insulation. It can form a dense ceramic protective layer at high temperatures, inhibiting separator shrinkage and blocking lithium dendrite penetration, thereby improving battery safety performance.

[0003] However, traditional alumina-coated separators still have many drawbacks that limit their further application. First, the surface inertness of alumina particles results in weak interfacial adhesion with polyolefin substrates. Over long-term use or under high-temperature environments, alumina particles easily detach from the substrate surface, leading to coating failure, increased battery internal resistance, and even short circuits. Second, alumina itself is highly hydrophobic, resulting in insufficient wettability with polar carbonate electrolytes. This hinders electrolyte penetration at the interface, leading to high interfacial impedance and reduced battery rate performance and cycle life. Furthermore, coatings formed by simply stacking individual alumina particles have low porosity and uneven distribution, resulting in narrow and tortuous ion transport channels, further affecting battery rate performance. These problems make it difficult to achieve a balance between thermal stability, interfacial adhesion, and ion transport efficiency in traditional alumina-coated separators.

[0004] In existing technologies, research on the modification of alumina-coated separators mainly focuses on adding coupling agents or surfactants. For example, some solutions improve the interfacial adhesion between alumina and the substrate by introducing traditional coupling agents, but these coupling agents lack high-temperature stability and are prone to decomposition during long-term battery use, leading to a decline in interfacial adhesion over time. Other solutions use surfactants to improve electrolyte wettability, but surfactants are prone to volatilization or decomposition at high temperatures, failing to maintain electrolyte adsorption capacity during long-term use and potentially introducing impurities that affect battery performance. Furthermore, the introduction of single-functional modifiers cannot simultaneously solve multiple problems such as interfacial adhesion, electrolyte wettability, and thermal stability, resulting in limited improvement in the overall performance of alumina-coated separators. Therefore, developing a novel modification scheme that combines strong interfacial adhesion, excellent electrolyte wettability, and high thermal stability is crucial for improving the performance of alumina-coated separators. Summary of the Invention

[0005] The purpose of this invention is to provide an alumina-coated diaphragm and its preparation method, which solves the technical problems of poor interfacial adhesion, insufficient electrolyte wettability, and limited thermal stability of existing alumina-coated diaphragms, and the difficulty of solving multiple defects simultaneously by existing modification methods.

[0006] The present invention achieves the above objectives through the following technical solutions: An alumina-coated separator comprises the following raw materials in parts by weight: Alumina granules: 500-800 parts by weight; Fluorine-containing porous organosilicon compounds: 20-80 parts by weight; Phosphorus heterocyclic covalent organic framework: 10-50 parts by weight; Adhesive: 50-150 parts by weight; Dispersant: 10-30 parts by weight; Deionized water: 400-800 parts by weight; The preparation method of the fluorinated porous organosilicon compound includes: A1, adding methyltrimethoxysilane, perfluorohexyltrimethoxysilane and deionized water to a reaction vessel, slowly adding ammonia water dropwise under stirring to adjust the pH to 7-8, and reacting at 60-64℃ to form a sol-gel system; A2, then adding 3-aminopropyltriethoxysilane and continuing the reaction to obtain a fluorinated amino-functionalized silica sol; the sol is rotary evaporated at 100-102℃, then calcined in a muffle furnace at 300-303℃, and after natural cooling, it is ground and sieved.

[0007] In this invention, the preparation mechanism of the fluorinated porous organosilicon compound originates from the hydrolysis-condensation reaction and functionalization modification process of organosilicones. In the initial stage, methyltrimethoxysilane and perfluorohexyltrimethoxysilane are contacted with ammonia in deionized water. Ammonia acts as a catalyst to adjust the pH of the system to near neutral, promoting the hydrolysis of the methoxy groups in the silane molecules to generate reactive silanol groups. These silanol groups have strong reactivity and will further undergo dehydration condensation with other silanol groups or incompletely hydrolyzed silane molecules, forming a three-dimensional network structure dominated by silicon-oxygen bonds. Gradually, the dispersed molecules aggregate into a sol state, ultimately forming a gel system with a certain mechanical strength. During this process, the introduction of the perfluorohexyl group endows the silica sol surface with low surface energy, while the methyl group provides a hydrophobic organic phase. Subsequently, 3-aminopropyltriethoxysilane further participates in the reaction; the silanol groups generated by the hydrolysis of its ethoxy group condense with the existing silanol groups in the silica sol, introducing amino functional groups into the organosilicon network to form a fluorinated amino-functionalized silica sol. Finally, the solvent ethanol is removed by rotary evaporation, and the residual small molecule organic matter and unreacted precursors are removed by high-temperature calcination in a muffle furnace. At the same time, the silicon-oxygen network is further densified and a porous structure is formed. After grinding and sieving, a fluorinated porous organosilicon compound with controllable porosity and surface functional groups is obtained.

[0008] According to a preferred embodiment of the present invention, the alumina particles were purchased from Jiangsu Hehai Nanotechnology Co., Ltd., and the model was HH-Al2O3-1 (average particle size 3μm, purity ≥99.5%).

[0009] According to a preferred embodiment of the present invention, the methyltrimethoxysilane was purchased from Hubei Xingfa Chemical Group Co., Ltd., and the model is XH-01 (purity ≥98%, color ≤10).

[0010] According to a preferred embodiment of the present invention, the perfluorohexyltrimethoxysilane was purchased from Zhejiang Xin'an Chemical Group Co., Ltd., and the model is XJ-02 (perfluorohexyl content ≥95%, boiling point 198℃).

[0011] According to a preferred embodiment of the present invention, the deionized water was purchased from Jiangsu Jiangyin Runma Electronic Materials Co., Ltd., model JR-001 (resistivity ≥18.2MΩ·cm, used as a battery-grade dispersion medium).

[0012] According to a preferred embodiment of the present invention, the reactor was purchased from Shanghai Kexing Instrument Co., Ltd., model SK-200L (316L stainless steel, 200L volume, with temperature control and stirring system).

[0013] According to a preferred embodiment of the present invention, the ammonia water is purchased from Jiangsu Feixiang Chemical Co., Ltd., and the product is AF-25% (analytical grade, ammonia content 25%-28%).

[0014] According to a preferred embodiment of the present invention, the 3-aminopropyltriethoxysilane was purchased from Nanjing Shuguang Silane Chemical Co., Ltd., and the product name was G-01 (purity ≥97%, ammonia value 0.5 mmol / g).

[0015] According to a preferred embodiment of the present invention, the muffle furnace was purchased from Zhengzhou Great Wall Science & Industry Trade Co., Ltd., and the model is SXL-300 (box structure, maximum temperature 300℃, temperature control accuracy ±2℃).

[0016] According to a preferred embodiment of the present invention, the adhesive is purchased from Shenzhen Xinzhoubang Technology Co., Ltd., and the model is PVDF-HFP-1 (molecular weight 800,000, swelling rate ≤5%).

[0017] According to a preferred embodiment of the present invention, the dispersant is purchased from Shandong Fufeng Fermentation Co., Ltd., and the model is FM-01 (food grade CMC-Na, degree of substitution 0.8-1.2, viscosity ≥800mPa·s).

[0018] According to a preferred embodiment of the present invention, in step A1, the reaction time at 60-64°C is 4-6 hours.

[0019] According to a preferred embodiment of the present invention, in step A2, the reaction continues for 2-4 hours; the calcination time at 300-303°C is 2-3 hours; and the material is ground through a 200-mesh sieve.

[0020] According to a preferred embodiment of the present invention, the preparation method of the phosphorus heterocyclic covalent organic framework includes: B1, adding cyanuric chloride, triphenylphosphine oxide and anhydrous tetrahydrofuran to a round-bottom flask and stirring under nitrogen protection until completely dissolved; heating the flask to 80-82°C and slowly adding 1-vinyl-3-ethylimidazolium bromide to react and form an intermediate solution; B2, subsequently adding ethylene glycol and continuing the reaction to obtain a viscous liquid; pouring the liquid into a petri dish and vacuum drying at 60-62°C to obtain a block; finally, grinding and sieving the block.

[0021] In this invention, the preparation of a phosphorus heterocyclic covalent organic framework is based on the self-assembly process of dynamic covalent bonds and cross-linking reactions. The three chlorine atoms in the cyanuric chloride molecule exhibit high reactivity. When mixed with triphenylphosphine oxide in anhydrous tetrahydrofuran solvent, the phosphine oxide group of triphenylphosphine oxide activates the chlorine atoms of cyanuric chloride through coordination or electronic effects, making them more susceptible to nucleophilic attack. When the system is heated to a set temperature and 1-vinyl-3-ethylimidazolium bromide is added, the nitrogen atom on the imidazole ring acts as a nucleophilic center, gradually replacing the chlorine atoms in the cyanuric chloride molecule, forming an intermediate structure with the imidazole ring as the linking group. Simultaneously, the phosphorus-oxygen double bond of triphenylphosphine oxide interacts with the nitrogen atom or vinyl group of the imidazole ring, constructing preliminary intermolecular connections. The subsequently added ethylene glycol molecule contains two hydroxyl groups, which can undergo esterification or etherification reactions with unreacted chlorine atoms or active hydrogen atoms in the intermediate, forming stable carbon-oxygen bonds, further cross-linking the linear or branched molecular chains into a three-dimensional network structure. Under vacuum drying conditions, the solvent gradually evaporates, and the molecular chains are arranged in an orderly manner through the self-assembly of dynamic covalent bonds (such as imine bonds or ether bonds), eventually forming a phosphorus heterocyclic covalent organic framework with high specific surface area and regular pore structure. After grinding and sieving, a powdered product is obtained.

[0022] According to a preferred embodiment of the present invention, the cyanuric chloride was purchased from Jiangsu Runfeng Synthetic Technology Co., Ltd., and the model was RG-01 (industrial grade, purity ≥99%).

[0023] According to a preferred embodiment of the present invention, the triphenylphosphine oxide was purchased from Shandong Weifang Runfeng Chemical Co., Ltd., and the model was RW-TPPO-01 (analytical grade, purity ≥98%).

[0024] According to a preferred embodiment of the present invention, the anhydrous tetrahydrofuran was purchased from Tianjin Bodi Chemical Co., Ltd., and the model was BD-THF-01 (anhydrous grade, moisture ≤0.05%).

[0025] According to a preferred embodiment of the present invention, the round-bottom flask was purchased from Sichuan Shubo (Group) Co., Ltd., and the model is SH-RB-500 (500mL borosilicate glass flask).

[0026] According to a preferred embodiment of the present invention, the nitrogen gas is purchased from Hangzhou Hangyang Co., Ltd., and the model is HZ-IND-N2 (industrial grade, purity ≥99.99%).

[0027] According to a preferred embodiment of the present invention, the 1-vinyl-3-ethylimidazolium bromide was purchased from Jiangsu Feixiang Chemical Co., Ltd., and the product name was FX-VEIMBr-01 (high purity grade, purity ≥98%).

[0028] According to a preferred embodiment of the present invention, the ethylene glycol was purchased from Jiangsu Huachang Chemical Co., Ltd., and the product name was HC-EG-01 (industrial grade, purity ≥99.5%).

[0029] According to a preferred embodiment of the present invention, the petri dish was purchased from Tianjin Tianbo Technology Development Co., Ltd., and the model is TGB-90 (90mm borosilicate glass petri dish).

[0030] According to a preferred embodiment of the present invention, the planetary ball mill was purchased from Changsha Miqi Instrument Equipment Co., Ltd., and the model is MIQ-2L (2L stainless steel ball mill jar, speed 0-800rpm).

[0031] According to a preferred embodiment of the present invention, the N-methylpyrrolidone was purchased from Jiangsu Ruijia Chemical Co., Ltd., and the product name is RJ-NMP-01 (electronic grade, purity ≥99.5%).

[0032] According to a preferred embodiment of the present invention, the high-speed mixer was purchased from Shanghai Specimen Model Factory, model SP-2000 (electric mixer, maximum speed 2000 rpm).

[0033] According to a preferred embodiment of the present invention, the drying oven was purchased from Shanghai Yiheng Scientific Instruments Co., Ltd., model DHG-9070A (electric thermostatic drying oven, temperature range RT+10-300℃).

[0034] According to a preferred embodiment of the present invention, in step B1, the stirring speed is 200-400 rpm; the reaction time is 6-8 h.

[0035] According to a preferred embodiment of the present invention, in step B2, the reaction continues for 4-6 hours; the vacuum drying time at 60-62°C is 12-14 hours; and the material is ground through a 300-mesh sieve.

[0036] The present invention also provides a method for preparing the alumina-coated diaphragm, comprising the following steps: S1. Alumina particles, fluorine-containing porous organosilicon compounds, and phosphorus heterocyclic covalent organic frameworks are added to a planetary ball mill and ball milled with ethanol as the milling medium to obtain a uniformly dispersed ceramic slurry precursor; the binder is dissolved in N-methylpyrrolidone to prepare a PVDF-HFP solution. S2. Add the dispersant to deionized water and stir until completely dissolved to obtain an aqueous dispersant solution; mix the ceramic slurry precursor with the PVDF-HFP solution and the aqueous dispersant solution, and stir under a high-speed mixer to obtain a composite coating slurry; S3. Immerse the polyolefin base film in the slurry, coat one side of the base film with the slurry using a doctor blade coating method, remove it and pre-dry it at 80-84℃, then place it in a vacuum drying oven at 120-124℃ to dry.

[0037] In this invention, the preparation of the alumina-coated diaphragm involves a multi-component synergistic dispersion and film-forming process. First, alumina particles, a fluorinated porous organosilicon compound, and a phosphorus heterocyclic covalent organic framework are thoroughly mixed with ethanol in a planetary ball mill. The mechanical impact of the milling media disrupts particle aggregation, while the surface tension of ethanol reduces the interfacial energy between the inorganic particles and the organic modifier, promoting uniform coating of the alumina particle surface with the siloxane groups of the fluorinated porous organosilicon compound and the polar groups of the phosphorus heterocyclic covalent organic framework, forming a stable ceramic slurry precursor. On the other hand, the binder PVDF-HFP is dissolved in N-methylpyrrolidone to form a homogeneous solution. The fluorine atoms on its molecular chain have good compatibility with carbonate electrolytes and can serve as a film-forming substance to immobilize the alumina particles. The dispersant CMC-Na, after dissolving in deionized water, forms a micelle structure that adsorbs onto the alumina particle surface through electrostatic interaction or steric hindrance, preventing the particles from re-aggregating during subsequent mixing. When the ceramic slurry precursor is mixed with the PVDF-HFP solution and the dispersant aqueous solution, high-speed stirring promotes the full integration of the three components. The PVDF-HFP molecular chains encapsulate the alumina particles, and the CMC-Na micelles further stabilize the dispersion of the slurry, forming a composite coating slurry with suitable solid content and moderate viscosity. Finally, the slurry is coated onto the surface of a polyolefin-based film using a doctor blade. The pre-drying process removes most of the solvent ethanol and water, allowing the coating to initially cure. Vacuum drying further reduces residual moisture inside the coating, promotes the crystallization and cross-linking of the PVDF-HFP molecular chains, and ultimately forms a continuous, uniform, and firmly bonded alumina-coated membrane to the substrate.

[0038] In this invention, the steps of the doctor blade coating method include: first, placing the polyolefin base film (such as a PE / PP composite film with a thickness of 12μm) flat on the coating machine, ensuring that the base film is under tension to avoid wrinkles; then, pouring the prepared composite coating slurry into the slurry tank of the coating machine, allowing the base film to pass through the slurry tank at a constant speed, so that the slurry evenly immerses one side surface of the base film; next, starting the doctor blade device, adjusting the gap between the doctor blade and the base film to the target coating thickness (such as 3-5μm), and having the doctor blade contact the surface of the base film with appropriate pressure to scrape off excess slurry, so that the slurry forms a thick coating on the surface of the base film. A uniform coating is formed. After coating, the base film with the wet coating is transferred to a pre-drying device and kept at 80-84℃ for 10-14 minutes to remove most of the solvent (such as ethanol and water) in the coating, so that the coating is initially cured and the adhesion to the base film is enhanced. The pre-dried base film is then transferred to a vacuum drying oven and kept in a vacuum environment at 120-124℃ for 30-50 minutes to further remove the trace amounts of residual solvent in the coating, while promoting the crystallization and cross-linking of the binder molecular chains, and finally forming a continuous, uniform alumina coated membrane that is firmly bonded to the substrate.

[0039] According to a preferred embodiment of the present invention, in step S1, the ball milling speed is 300-400 rpm and the ball milling time is 6-8 h.

[0040] According to a preferred embodiment of the present invention, in step S2, the stirring speed is 500-600 rpm; the volume ratio of ceramic slurry precursor to PVDF-HFP solution and dispersant aqueous solution is 5:3:2; the speed of the high-speed mixer is 1500-1800 rpm; and the stirring time is 4-6 h.

[0041] According to a preferred embodiment of the present invention, in step S3, the pre-drying time at 80-84°C is 10-14 min; the drying time in a vacuum drying oven at 120-124°C is 30-50 min.

[0042] The beneficial effects of this invention are as follows: This invention achieves significant improvements in the thermal stability, interfacial adhesion, and electrolyte wettability of alumina-coated diaphragms through the synergistic effect of bifunctional modified compounds and alumina particles.

[0043] First, the interfacial adhesion performance is significantly enhanced. The silane coupling groups in the fluorinated porous organosilicon compound molecules can undergo condensation reactions with the hydroxyl groups on the surface of alumina particles to form stable covalent bonds. Simultaneously, the fluorinated alkyl groups on its molecular chains possess low surface energy, allowing them to penetrate into the non-polar intersegmental spaces within the polyolefin substrate. Through van der Waals forces, they form a physical anchor with the substrate, constructing a three-dimensional interfacial bonding structure of "alumina-modified compound-substrate." The epoxy groups in the phosphorus heterocyclic covalent organic framework molecules can undergo ring-opening reactions with the active sites on the substrate surface to form chemical bonds. Furthermore, the cross-linked network formed between alumina particles further fills the voids within the coating, reducing weak bonding areas between particles. The synergistic effect of the two compounds significantly improves the peel strength between the coating and the substrate, making it less prone to detachment under long-term cycling or high-temperature environments, effectively preventing increased battery internal resistance or short circuits caused by coating failure.

[0044] Secondly, the electrolyte wettability and electrolyte retention capacity are optimized. The low surface energy fluorinated alkyl groups in the fluorinated porous organosilicon compound reduce the surface energy of the coating, decreasing the contact angle of the electrolyte on the coating surface and promoting rapid electrolyte spread. The phosphorus heterocycles and imidazole rings in the phosphorus heterocyclic covalent organic framework, with their polar groups, can adsorb electrolyte molecules through hydrogen bonds or dipole interactions, increasing the coating's electrolyte retention capacity. Under the combined effect of these two factors, the electrolyte penetration within the coating is more uniform, forming denser ion transport channels. The ion conduction resistance at the interface is reduced, improving the battery's rate performance and cycle life. Experiments show that the modified separator has a significantly higher electrolyte absorption capacity than the traditional alumina-coated separator, and the electrolyte is less prone to loss during long-term storage, maintaining a stable ion transport environment.

[0045] Finally, a breakthrough improvement in thermal stability was achieved. The siloxane skeleton in the fluorinated porous organosilicon compound and the carbon-oxygen and carbon-nitrogen bonds in the phosphorus heterocyclic covalent organic framework can still maintain structural stability at high temperatures, effectively inhibiting the thermal migration and agglomeration of alumina particles; at the same time, the thermal decomposition temperature of both compounds is much higher than the upper limit of the normal operating temperature of the battery, avoiding coating defects caused by the decomposition of the modifier. Detailed Implementation

[0046] 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.

[0047] Example 1 Preparation of fluorinated porous organosilicon compounds: 120g of methyltrimethoxysilane, 60g of perfluorohexyltrimethoxysilane, and 300g of deionized water were added to a reaction vessel. 5g of 25% ammonia solution was slowly added dropwise under stirring to adjust the pH to 7.5. The reaction was carried out at 62℃ for 5 hours to form a sol-gel system. Subsequently, 30g of 3-aminopropyltriethoxysilane was added, and the reaction was continued for 3 hours to obtain fluorinated amino-functionalized silica sol. The sol was rotary evaporated at 100℃ for 2 hours, then calcined in a muffle furnace at 300℃ for 2.5 hours. After natural cooling, it was ground through a 200-mesh sieve to obtain fluorinated porous organosilicon compounds.

[0048] Preparation of phosphorus heterocyclic covalent organic framework: 100g of cyanuric chloride, 50g of triphenylphosphine oxide and 400g of anhydrous tetrahydrofuran were added to a round-bottom flask and stirred at 300rpm for 7 hours under nitrogen protection until completely dissolved; the flask was heated to 81℃ and 80g of 1-vinyl-3-ethylimidazolium bromide was slowly added dropwise to form an intermediate solution; then 50g of ethylene glycol was added and the reaction was continued for 5 hours to obtain a viscous liquid; the liquid was poured into a petri dish and dried under vacuum at 61℃ for 13 hours to obtain a block; finally, the block was ground and passed through a 300-mesh sieve to obtain the phosphorus heterocyclic covalent organic framework.

[0049] Preparation of alumina-coated diaphragm: 650g of alumina particles, 50g of fluorine-containing porous organosilicon compound, and 30g of phosphorus heterocyclic covalent organic framework were added to a planetary ball mill and ball-milled at 350rpm for 7 hours using 3000g of ethanol as the milling medium to obtain a uniformly dispersed ceramic slurry precursor; 100g of binder PVDF-HFP was dissolved in 1000g of... N-methylpyrrolidone was prepared into a PVDF-HFP solution. 20g of dispersant CMC-Na was added to 600g of deionized water and stirred at 600rpm until completely dissolved to obtain an aqueous dispersant solution. The ceramic slurry precursor was mixed with the PVDF-HFP solution and the aqueous dispersant solution at a volume ratio of 5:3:2 and stirred at 1600rpm for 5 hours to obtain a composite coating slurry with a solid content of about 45wt%. A 1000cm×1000cm polyolefin-based membrane (PE / PP composite membrane, 12μm thick) was immersed in the slurry and coated with a 4μm thick coating using a doctor blade coating method. After removal, it was pre-dried at 82℃ for 12 minutes and then placed in a vacuum drying oven at 122℃ for 40 minutes to obtain an alumina-coated diaphragm.

[0050] Example 2 The specific implementation method is the same as in Example 1, except that the preparation of the fluorinated porous organosilicon compound is as follows: 100g of methyltrimethoxysilane, 50g of perfluorohexyltrimethoxysilane and 250g of deionized water are added to a reaction vessel, and 4g of 25% ammonia water is slowly added dropwise under stirring to adjust the pH to 7.2. The reaction is carried out at 60°C for 4 hours to form a sol-gel system. Then, 25g of 3-aminopropyltriethoxysilane is added and the reaction is continued for 2 hours to obtain a fluorinated amino-functionalized silica sol. The sol is rotary evaporated at 100°C for 2 hours, then calcined in a muffle furnace at 300°C for 2 hours. After natural cooling, it is ground through a 200-mesh sieve to obtain the fluorinated porous organosilicon compound.

[0051] Preparation of phosphorus heterocyclic covalent organic framework: 80g of cyanuric chloride, 40g of triphenylphosphine oxide and 320g of anhydrous tetrahydrofuran were added to a round-bottom flask and stirred at 200rpm for 6 hours under nitrogen protection until completely dissolved; the flask was heated to 80℃ and 60g of 1-vinyl-3-ethylimidazolium bromide was slowly added dropwise to form an intermediate solution; then 40g of ethylene glycol was added and the reaction was continued for 4 hours to obtain a viscous liquid; the liquid was poured into a petri dish and dried under vacuum at 60℃ for 12 hours to obtain a block; finally, the block was ground and passed through a 300-mesh sieve to obtain the phosphorus heterocyclic covalent organic framework.

[0052] Preparation of alumina-coated diaphragm: 500g of alumina particles, 20g of fluorine-containing porous organosilicon compound, and 10g of phosphorus heterocyclic covalent organic framework were added to a planetary ball mill and ball-milled at 300rpm for 6 hours using 2000g of ethanol as the milling medium to obtain a uniformly dispersed ceramic slurry precursor; 50g of binder PVDF-HFP was dissolved in 500g of... N-methylpyrrolidone was prepared into a PVDF-HFP solution. 10g of dispersant CMC-Na was added to 400g of deionized water and stirred at 500rpm until completely dissolved to obtain an aqueous dispersant solution. The ceramic slurry precursor was mixed with the PVDF-HFP solution and the aqueous dispersant solution at a volume ratio of 5:3:2 and stirred at 1500rpm for 4 hours to obtain a composite coating slurry with a solid content of about 40wt%. A 1000cm×1000cm polyolefin-based membrane was immersed in the slurry and coated with a 3μm thick coating using a doctor blade coating method. After removal, it was pre-dried at 80℃ for 10 minutes and then placed in a vacuum drying oven at 120℃ for 30 minutes to obtain an alumina-coated diaphragm.

[0053] Example 3 The specific implementation method is the same as in Example 1, except that the preparation of the fluorinated porous organosilicon compound is as follows: 140g of methyltrimethoxysilane, 70g of perfluorohexyltrimethoxysilane and 350g of deionized water are added to a reaction vessel, and 6g of 25% ammonia water is slowly added dropwise under stirring to adjust the pH to 7.8. The reaction is carried out at 64°C for 6 hours to form a sol-gel system. Then, 35g of 3-aminopropyltriethoxysilane is added and the reaction is continued for 4 hours to obtain a fluorinated amino-functionalized silica sol. The sol is rotary evaporated at 100°C for 2 hours, and then calcined in a muffle furnace at 300°C for 3 hours. After natural cooling, it is ground through a 200-mesh sieve to obtain the fluorinated porous organosilicon compound.

[0054] Preparation of phosphorus heterocyclic covalent organic framework: 120g of cyanuric chloride, 60g of triphenylphosphine oxide and 480g of anhydrous tetrahydrofuran were added to a round-bottom flask and stirred at 400rpm for 8 hours under nitrogen protection until completely dissolved; the flask was heated to 82℃ and 100g of 1-vinyl-3-ethylimidazolium bromide was slowly added dropwise to form an intermediate solution; then 60g of ethylene glycol was added and the reaction was continued for 6 hours to obtain a viscous liquid; the liquid was poured into a petri dish and dried under vacuum at 62℃ for 14 hours to obtain a block; finally, the block was ground and passed through a 300-mesh sieve to obtain the phosphorus heterocyclic covalent organic framework.

[0055] Preparation of alumina-coated diaphragm: 800g of alumina particles, 80g of fluorine-containing porous organosilicon compound, and 50g of phosphorus heterocyclic covalent organic framework were added to a planetary ball mill and ball-milled at 400rpm for 8 hours using 4000g of ethanol as the milling medium to obtain a uniformly dispersed ceramic slurry precursor; 150g of binder PVDF-HFP was dissolved in 1500g of... N-methylpyrrolidone was prepared into a PVDF-HFP solution. 30g of dispersant CMC-Na was added to 800g of deionized water and stirred at 600rpm until completely dissolved to obtain an aqueous dispersant solution. The ceramic slurry precursor was mixed with the PVDF-HFP solution and the aqueous dispersant solution at a volume ratio of 5:3:2 and stirred at 1800rpm for 6 hours to obtain a composite coating slurry with a solid content of about 50wt%. A 1000cm×1000cm polyolefin-based membrane was immersed in the slurry and coated with a 5μm thick coating using a doctor blade coating method. After removal, it was pre-dried at 84℃ for 14 minutes and then placed in a vacuum drying oven at 124℃ for 50 minutes to obtain an alumina-coated diaphragm.

[0056] Comparative Example 1 The specific implementation method is the same as in Example 1, except that the preparation of the fluorinated porous organosilicon compound is as follows: methyltrimethoxysilane and perfluorohexyltrimethoxysilane are omitted, and it is prepared directly (i.e., the amount of fluorinated porous organosilicon compound used is 0g), and the remaining steps are the same as in Example 1. The preparation of the alumina-coated diaphragm is as follows: 650g of alumina particles, 0g of the fluorinated porous organosilicon compound, and 30g of phosphorus heterocyclic covalent organic framework are added to a planetary ball mill, and the remaining steps are the same as in Example 1.

[0057] Comparative Example 2 The specific implementation method is the same as in Example 1, except that the preparation of the phosphorus heterocyclic covalent organic framework is as follows: cyanuric chloride and triphenylphosphine oxide are omitted, and the framework is prepared directly (i.e., the amount of phosphorus heterocyclic covalent organic framework is 0g), and the remaining steps are the same as in Example 1. The preparation of the alumina-coated diaphragm is as follows: 650g of alumina particles, 50g of fluorine-containing porous organosilicon compound, and 0g of phosphorus heterocyclic covalent organic framework are added to a planetary ball mill, and the remaining steps are the same as in Example 1.

[0058] Comparative Example 3 The specific implementation method is the same as in Example 1, except that: In the preparation of the fluorinated porous organosilicon compound, methyltrimethoxysilane and perfluorohexyltrimethoxysilane (0g) are omitted. In the preparation of the phosphorus heterocyclic covalent organic framework, cyanuric chloride and triphenylphosphine oxide (0g) are omitted. In the preparation of the alumina-coated diaphragm, 650g of alumina particles, 0g of the fluorinated porous organosilicon compound, and 0g of the phosphorus heterocyclic covalent organic framework are added to a planetary ball mill, and the remaining steps are the same as in Example 1.

[0059] Performance testing The alumina-coated membranes prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests according to the following methods: 1. Heat shrinkage rate test: Cut the diaphragm to be tested into 100mm×100mm square samples, and treat them at 120℃ and 150℃ ovens for 1 hour respectively. After removing them and cooling them to room temperature, measure the longitudinal and transverse dimensional changes of the samples, and calculate the heat shrinkage rate (heat shrinkage rate = (original size - treated size) / original size × 100%).

[0060] 2. Peel strength test: The sample after the diaphragm and polyolefin substrate are composited is cut into strips of 25mm×150mm. The two ends of the sample are fixed to the upper and lower clamps of the electronic universal testing machine with double-sided tape. The clamp spacing is 100mm. The peel force between the coating and the substrate is tested at a tensile rate of 100mm / min. The average value of 5 tests is taken as the peel strength (unit: N / m).

[0061] 3. Electrolyte absorption rate test: Cut the diaphragm to be tested into a square sample of 50mm×50mm and weigh the initial mass m0 using an analytical balance; immerse the sample completely in a mixed electrolyte of ethylene carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) (volume ratio 1:1:1) and soak for 24 hours. Remove the sample and use filter paper to absorb the excess electrolyte on the surface. Weigh the sample again and calculate the absorption rate (absorption rate = (m1-m0) / m0×100%).

[0062] 4. Ionic conductivity test: The diaphragm to be tested was cut into circular samples with a diameter of 10 mm and immersed in the electrolyte for 24 hours to reach swelling equilibrium; using a CHI 660E electrochemical workstation, with the diaphragm as the working electrode and a platinum sheet as the counter electrode, Ag / Ag + The electrode is a reference electrode, and the test frequency range is 10. -2 ~10 5 The Hz frequency and the amplitude of 5mV are used to obtain the bulk resistance R_b of the diaphragm by fitting the complex impedance spectrum. The ionic conductivity σ is then calculated by combining the diaphragm thickness d (σ=l / (R_b×A), where l is the electrode spacing and A is the electrode area).

[0063] 5. Cyclic performance test: A coin cell (CR2032) was assembled using lithium iron phosphate (LiFePO4) as the positive electrode, graphite as the negative electrode, and the separator under test as the separator. In the Blue Battery test system, charge and discharge cycles were performed at a current density of 0.5C (1C=170mA / g) within a voltage range of 2.5~4.2V. The capacity retention rate after 500 cycles was tested (capacity retention rate = capacity of the 500th cycle / capacity of the first cycle × 100%).

[0064] 6. Performance test results: Table 1: Performance test results of each embodiment and comparative example

[0065] As shown in Table 1, the comparison results of Examples 1-3 with Comparative Examples 1-3 clearly verify that they solve the multiple defects of existing alumina-coated diaphragms. Regarding thermal stability, the thermal shrinkage rates of Examples 1-3 at 120℃ and 150℃ (1.2%-1.5% and 2.5%-2.8%, respectively) are significantly lower than those of Comparative Examples 1 (3.8%, 6.2%), 2 (2.1%, 3.5%), and 3 (4.5%, 7.8%). This indicates that the synergistic effect of the fluorinated porous organosilicon compound (high-temperature stable siloxane framework) and the phosphorus heterocyclic covalent organic framework (high-temperature resistant carbon-oxygen / carbon-nitrogen bonds) effectively inhibits the thermal migration of alumina particles and improves the dimensional stability of the diaphragm at high temperatures. Regarding interfacial adhesion, the peel strength of Examples 1-3 (7.5-8.2 N / m) was much higher than that of Comparative Example 1 (5.1 N / m), Comparative Example 2 (6.8 N / m), and Comparative Example 3 (3.2 N / m). This indicates that the fluorinated porous organosilicon compound forms a three-dimensional anchor by condensing silane coupling groups with hydroxyl groups on the alumina surface and by penetrating fluorinated alkyl groups into the substrate. The phosphorus heterocyclic covalent organic framework enhances the adhesion between the coating and the substrate through the reaction of epoxy groups with the substrate and the interparticle crosslinking network, thus solving the problem of weak interfacial adhesion in traditional diaphragms. Regarding electrolyte wettability, the absorbance rates of Examples 1-3 (198%-215%) were significantly better than those of Comparative Examples 1 (162%), 2 (185%), and 3 (120%). This was attributed to the low surface energy of the fluorinated porous organosilicon, which reduced interfacial energy and promoted electrolyte spreading, and the adsorption of electrolyte molecules by the polar groups (phosphorus heterocycles, imidazole rings) of the phosphorus heterocyclic covalent organic framework through hydrogen bonding / dipole interactions, which jointly improved electrolyte penetration and retention capacity. Ionic conductivity (1.6 × 10⁻⁶) -3 -1.8×10 - 3 The improved performance (S / cm) and cycling properties (capacity retention of 90.1%-92.3% after 500 cycles) further validate the effectiveness of bifunctional modification in reducing interfacial impedance and optimizing ion transport. In contrast, the comparative example, lacking one or both of the fluorinated porous organosilicon or phosphorus heterocyclic covalent organic frameworks, resulted in a significant decrease in corresponding properties (such as heat shrinkage, peel strength, and liquid absorption). This fully demonstrates that the present invention, through synergistic modification with bifunctional compounds, can simultaneously solve multiple technical problems such as poor interfacial adhesion, insufficient electrolyte wettability, and limited thermal stability.

[0066] 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 scope of protection of the present invention.

Claims

1. An alumina-coated lithium-ion battery separator, characterized in that, The alumina-coated lithium-ion battery separator comprises a substrate and a coating, wherein the coating comprises the following raw materials in parts by weight: Alumina granules: 500-800 parts by weight; Fluorine-containing porous organosilicon compounds: 20-80 parts by weight; Phosphorus heterocyclic covalent organic framework: 10-50 parts by weight; Adhesive: 50-150 parts by weight; Dispersant: 10-30 parts by weight; Deionized water: 400-800 parts by weight; The preparation method of the fluorinated porous organosilicon compound includes: A1, adding methyltrimethoxysilane, perfluorohexyltrimethoxysilane and deionized water to a reaction vessel, slowly adding ammonia water dropwise under stirring to adjust the pH to 7-8, and reacting at 60-64℃ to form a sol-gel system; A2, then adding 3-aminopropyltriethoxysilane and continuing the reaction to obtain a fluorinated amino-functionalized silica sol; the sol is rotary evaporated at 100-102℃, then calcined in a muffle furnace at 300-303℃, and after natural cooling, it is ground and sieved; The preparation method of the phosphorus heterocyclic covalent organic framework includes: B1, adding cyanuric chloride, triphenylphosphine oxide and anhydrous tetrahydrofuran to a round-bottom flask and stirring under nitrogen protection until completely dissolved; heating the flask to 80-82℃ and slowly adding 1-vinyl-3-ethylimidazolium bromide to react and form an intermediate solution; B2, then adding ethylene glycol and continuing the reaction to obtain a viscous liquid; pouring the liquid into a petri dish and vacuum drying at 60-62℃ to obtain a block; finally, grinding and sieving the block.

2. The alumina-coated lithium-ion battery separator according to claim 1, characterized in that, In step A1, the reaction time is 4-6 hours at 60-64℃.

3. The alumina-coated lithium-ion battery separator according to claim 1, characterized in that, In step A2, the reaction continues for 2-4 hours; calcination at 300-303℃ lasts for 2-3 hours; and the mixture is ground through a 200-mesh sieve.

4. The alumina-coated lithium-ion battery separator according to claim 1, characterized in that, In step B1, the stirring speed is 200-400 rpm; the reaction time is 6-8 h.

5. The alumina-coated lithium-ion battery separator according to claim 1, characterized in that, In step B2, the reaction continues for 4-6 hours; vacuum drying at 60-62℃ takes 12-14 hours; and the mixture is ground through a 300-mesh sieve.

6. A method for preparing an alumina-coated lithium-ion battery separator according to any one of claims 1-5, characterized in that, step include: S1. Alumina particles, fluorine-containing porous organosilicon compounds, and phosphorus heterocyclic covalent organic frameworks are added to a planetary ball mill and ball milled with ethanol as the milling medium to obtain a uniformly dispersed ceramic slurry precursor; the binder is dissolved in N-methylpyrrolidone to prepare a PVDF-HFP solution. S2. Add the dispersant to deionized water and stir until completely dissolved to obtain an aqueous dispersant solution; mix the ceramic slurry precursor with the PVDF-HFP solution and the aqueous dispersant solution, and stir under a high-speed mixer to obtain a composite coating slurry; S3. Immerse the polyolefin base film in the slurry, coat one side of the base film with the slurry using a doctor blade coating method, remove it and pre-dry it at 80-84℃, then place it in a vacuum drying oven at 120-124℃ to dry.

7. The preparation method according to claim 6, characterized in that, In step S1, the ball milling speed is 300-400 rpm and the ball milling time is 6-8 hours.

8. The preparation method according to claim 6, characterized in that, In step S2, the stirring speed is 500-600 rpm until completely dissolved; the volume ratio of ceramic slurry precursor to PVDF-HFP solution and dispersant aqueous solution is 5:3:2; the speed of the high-speed mixer is 1500-1800 rpm, and the stirring time is 4-6 hours.

9. The preparation method according to claim 6, characterized in that, In step S3, the pre-drying time at 80-84℃ is 10-14 min; the drying time in the vacuum drying oven at 120-124℃ is 30-50 min.

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