Aramid polymer coated lithium ion battery diaphragm and preparation method thereof

Abstract

The invention belongs to the field of lithium ion batteries, and particularly relates to an aramid polymer coated lithium ion battery diaphragm and a preparation method thereof, the diaphragm takes a polyethylene porous base membrane as a support body, and the surface of the polyethylene porous base membrane is coated with a composite coating containing a para-aramid polymer, two specific modified compounds and aluminum oxide. The two key modified compounds are perfluorophenylimidazole lithium salt and tetracyanobenzene dicarboximide dimethyl ammonium respectively, and the preparation method is realized through step-by-step reaction of commercially available raw materials. The preparation method of the diaphragm comprises the following steps: carrying out corona treatment on a base membrane, synthesizing an aramid fiber glue solution at a low temperature, adding two modified compounds, dispersing the two modified compounds and aluminum oxide to form a coating solution, and carrying out coating, coagulating bath phase inversion, water washing and vacuum drying to obtain a finished product. The two modified compounds have a synergistic effect, so that the thermal stability, ionic conductivity and electrode interface compatibility of the diaphragm are remarkably improved, and the finally prepared diaphragm has excellent thermal dimensional stability and electrochemical performance and is suitable for high-safety and long-service-life lithium ion batteries.

Description

Technical Field

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

[0002] As a core component of modern electrochemical energy storage, lithium-ion batteries directly affect the safety and cycle life of electric vehicles, portable electronic devices, and energy storage systems. In the structure of a lithium-ion battery, the separator serves as a crucial component, isolating the positive and negative electrodes to prevent short circuits, while also playing a vital role in conducting lithium ions and ensuring the internal ion pathways of the battery. Currently, commercially available separators are mainly represented by polyolefin materials. While these materials possess good mechanical strength and electrochemical stability, their inherent material properties lead to a series of application drawbacks. Polyolefin separators have poor heat resistance, exhibiting significant melting and shrinkage at certain temperatures. When the battery generates heat due to overcharging or short circuits, this significant shrinkage of the separator can directly cause a short circuit between the positive and negative electrodes, triggering serious safety accidents such as thermal runaway. Furthermore, polyolefin materials have low surface energy and poor compatibility with polar electrolytes, resulting in poor electrolyte wetting of the separator. This not only increases the battery's internal resistance but also affects ion transport efficiency, ultimately limiting the battery's rate performance. Although surface modification or coating techniques can improve the wettability of polyolefin membranes to some extent, these improvements often come at the cost of porosity or mechanical strength, and are unlikely to fundamentally solve the thermal stability defects of the material itself.

[0003] To overcome the insufficient thermal stability of polyolefin separators, researchers have turned their attention to high-temperature resistant polymer materials, among which aramid polymers have attracted much attention due to their excellent heat resistance, inherent self-flame retardant properties, and outstanding mechanical strength. Aramid materials can maintain structural integrity at high temperatures; their thermal decomposition temperature is much higher than the melting point of polyolefins, meaning that aramid-coated separators undergo almost no dimensional changes under high temperatures, greatly improving the safety margin of the battery. However, pure aramid-coated separators also exhibit significant limitations in practical applications. Their ionic conductivity is relatively low, failing to meet the requirements of high-power batteries for rapid ion transport. Simultaneously, the rigid molecular structure of aramid materials leads to poor interfacial compatibility with electrode materials, potentially causing increased interfacial impedance and cycle life degradation during long-term cycling. Existing technologies have improved ionic conductivity by adding inorganic nanoparticles, such as alumina, to the aramid coating to improve the pore structure; however, the interfacial bonding between the inorganic particles and the polymer matrix is ​​weak, making particle shedding easy during battery cycling, thus affecting the long-term stability of the separator. In addition, some inorganic fillers may catalyze the decomposition of electrolyte at high potentials, producing gaseous byproducts and accelerating battery performance degradation.

[0004] To address the aforementioned technical challenges, the industry has attempted to introduce various functional additives to optimize the overall performance of aramid-coated separators. These attempts include adding lithium salt compounds that promote lithium-ion transport to the coating, or introducing interface modifiers that can form a protective layer at the electrode interface. However, single-component additives often only address one aspect of performance defects, making it difficult to achieve synergistic improvements in multiple performance aspects. For example, while some lithium salts improve ionic conductivity, they may sacrifice the thermal stability of the separator; and while some interface modifiers improve interfacial compatibility, they may hinder ion migration. Therefore, developing a multifunctional composite separator that can simultaneously satisfy high thermal stability, high ionic conductivity, and excellent interfacial compatibility has become an urgent need for the development of lithium-ion battery separator technology. This invention, against this technical background, develops two novel modifying compounds with synergistic effects through molecular structure design, fundamentally solving the performance bottlenecks faced by traditional aramid-coated separators and providing an ideal separator solution for next-generation high-safety, high-performance lithium-ion batteries. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an aramid polymer-coated lithium-ion battery separator and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing a lithium-ion battery separator coated with aramid polymer, comprising the steps of: S1. First, the porous polyethylene base membrane is subjected to corona treatment; under a nitrogen atmosphere, p-phenylenediamine, terephthaloyl chloride, N-methylpyrrolidone and calcium chloride are reacted at -4~-6℃ to obtain a para-aramid polymer solution. Then, perfluorophenyl imidazole lithium salt, tetracyanophthalimide dimethylammonium and alumina are added to the para-aramid polymer solution and dispersed in a high-speed disperser to obtain an aramid coating solution. S2. The aramid coating solution is coated onto the corona-treated porous polyethylene base membrane to obtain the coated diaphragm; the coated diaphragm is immersed in a deionized water coagulation bath, then washed with deionized water, and vacuum dried at 78-82℃.

[0007] In this invention, the preparation process of the aramid polymer-coated separator involves several key steps, each involving complex physicochemical changes. First, a low-temperature solution polycondensation reaction of the para-aramid polymer is carried out in a strictly controlled anhydrous and oxygen-free environment. p-Phenylenediamine and terephthaloyl chloride undergo a nucleophilic substitution reaction in a polar aprotic solvent, where the amino nitrogen atom attacks the carbon atom of the acyl chloride, releasing hydrogen chloride and forming an amide bond, gradually building a rigid polymer chain. Low-temperature conditions effectively suppress side reactions, control the polymerization rate, and ensure an appropriate molecular weight distribution. The addition of calcium chloride enhances the polymer's solubility in the solvent through coordination with the amide bond, preventing premature gelation. Next, the multi-component synergistic mechanism in the coating solution preparation process is discussed. Two modified compounds and alumina particles form a multi-layered composite structure within the aramid matrix. Perfluorophenyl imidazole lithium salt is uniformly dispersed in the polymer network through the loose interaction between its perfluorophenyl group and the aramid segments, while lithium ions form a coordination structure with the amide bond, enhancing component compatibility. Tetracyanophthalimide dimethylammonium, through its planar aromatic ring structure, generates π-π stacking interactions with the aramid backbone, while the cyano functional groups form hydrogen bonds with the hydroxyl groups on the alumina surface, constructing a three-dimensional network structure. The phase transformation process after coating involves complex mass transfer and phase separation mechanisms. When the coating is immersed in a water coagulation bath, solvent and non-solvent diffuse bidirectionally, and the solution supersaturation continuously increases, initiating liquid-liquid phase separation and forming a polymer-rich phase and a polymer-poor phase. The polymer-rich phase solidifies to form the membrane framework, while the polymer-poor phase forms an interconnected microporous structure. Alumina particles act as nucleating agents during phase separation, lowering the nucleation barrier and promoting the formation of a uniform porous structure. Both modified compounds undergo surface enrichment during the phase transformation process: perfluorophenylimidazolium lithium salt tends to be positioned on the pore surface, providing ion transport channels; while tetracyanophthalimide dimethylammonium is enriched at the membrane-electrode interface, optimizing interfacial performance. The final drying process removes residual solvent, solidifies the microstructure, and ensures the membrane possesses ideal thermal stability, ionic conductivity, and interfacial compatibility.

[0008] As a preferred embodiment of the present invention, in step S1, the reaction time at -4~-6℃ is 6-8h.

[0009] As a preferred embodiment of the present invention, in step S2, the vacuum drying time at 78-82°C is 8-10 hours.

[0010] As a preferred technical solution of the present invention, the preparation method of the perfluorophenyl imidazolium lithium salt includes: A1, dissolving 4,5-dicyanimidazole and perfluoroiodobenzene in N-methylpyrrolidone, adding copper powder and cesium carbonate under nitrogen protection, reacting at 115-125℃, cooling to room temperature after the reaction, filtering to obtain filtrate, pouring the filtrate into deionized water to precipitate solid, filtering and washing with deionized water, and vacuum drying to obtain intermediate; A2, then dissolving the intermediate in acetonitrile, adding lithium iodide and trimethylchlorosilane, refluxing at 78-82℃, cooling to room temperature after the reaction, evaporating, adding ethyl acetate to precipitate solid, filtering and washing with cold ethyl acetate, and vacuum drying.

[0011] In this invention, the preparation process of perfluorophenyl imidazolium lithium salt comprises two key steps, each involving a sophisticated chemical reaction mechanism. The first step is a Ullmann-type coupling reaction between 4,5-dicyanimidazole and perfluoroiodobenzene under alkaline conditions. In this reaction, copper powder acts as a catalyst, providing an active copper center that undergoes oxidative addition with perfluoroiodobenzene to form an aryl copper intermediate. This intermediate then undergoes metal transfer with the nitrogen atom on the imidazolium ring of 4,5-dicyanimidazole, ultimately resulting in a carbon-nitrogen bond through reductive elimination in the alkaline environment provided by cesium carbonate. The strong electron-withdrawing properties of perfluorophenyl significantly enhance the leaving power of the iodine atom while reducing the electron cloud density of the aromatic ring, making nucleophilic substitution reactions more readily achievable. Perfluoroiodobenzene is chosen over other halogenated compounds because the larger atomic radius of the iodine atom results in a lower carbon-iodine bond energy, facilitating oxidative addition. The second step involves the reductive cyclization and lithiation of the nitrile group, which involves several key transformations. Lithium iodide provides both lithium and acts as a nucleophile in the reaction, while trimethylchlorosilane serves as a highly efficient silanizing agent. It first combines with the nitrogen atom of the nitrile group to form a silimide intermediate, significantly activating the carbon-nitrogen bond and making it more susceptible to nucleophilic attack by iodide ions. Subsequently, intramolecular cyclization forms a five-membered imidazole ring structure, while the silicon group is replaced by lithium ions, ultimately generating a stable lithium-containing imidazole salt. The addition of trimethylchlorosilane is crucial throughout the reaction process; it not only promotes the reaction but also captures byproducts generated during the reaction, shifting the reaction equilibrium towards the forward direction. Controlling the reaction temperature and time has a decisive impact on the purity and yield of the final product; appropriate reflux conditions ensure complete reaction while preventing product decomposition.

[0012] As a preferred embodiment of the present invention, in step A1, the reaction time at 115-125°C is 12-14 hours.

[0013] As a preferred embodiment of the present invention, in step A2, the reaction is carried out under reflux at 78-82°C for 24-30 hours.

[0014] As a preferred technical solution of the present invention, the preparation method of the tetracyanophthalimide dimethylammonium includes: B1, mixing 1,4,5,8-naphthalenetetracarboxylic dianhydride, urea and ammonium chloride, melting and reacting at 255-265°C, cooling to room temperature after the reaction, pulverizing and washing with hot water, and vacuum drying to obtain an intermediate; B2, then dissolving the intermediate and N,N-dimethylformamide dimethyl acetal in dimethyl sulfoxide, adding cuprous cyanide and tetra-n-butylammonium bromide, reacting at 155-165°C under nitrogen protection, cooling to room temperature after the reaction, filtering, pouring the filtrate into diethyl ether to precipitate a solid, filtering the solid and washing with diethyl ether, treating with an ion exchange column, and vacuum drying.

[0015] In this invention, the synthetic route of tetracyanophthalimide dimethylammonium comprises two main stages, each involving a unique chemical transformation process. The first stage is the classic cyclization condensation reaction of tetracarboxylic dianhydride and urea in a high-temperature molten state. During this process, the isocyanate and ammonia produced by the thermal decomposition of urea undergo nucleophilic ring-opening with the anhydride group of the tetracarboxylic dianhydride, followed by intramolecular ring closure to form an aromatic imide structure with an extended conjugated system. The addition of ammonium chloride not only provides a nitrogen source, but its decomposition also catalyzes the cyclization process, accelerating the formation of imine bonds. High-temperature conditions facilitate the evaporation of water in the reaction system, shifting the chemical equilibrium towards the product, while simultaneously promoting the aromatization of intermediate products and enhancing the thermal stability and planarity of the final product. The second stage involves the nucleophilic cyanidation and quaternization of the aromatic imide compound, a step that showcases ingenious molecular design. Nitrogen-dimethylformamide dimethyl acetal decomposes under heating conditions to generate active dimethylamine fragments. These electron-rich groups first undergo nucleophilic substitution with the aromatic imide backbone, activating specific positions on the aromatic ring and creating favorable conditions for subsequent cyanoation. Cuprous cyanide, as the cyano source, effectively transfers the cyano anion to the reaction center under the phase-transfer catalyst tetra-n-butylammonium bromide, where it undergoes a nucleophilic substitution reaction with the activated aromatic ring, introducing four cyano groups stepwise. The high polarity of the dimethyl sulfoxide solvent not only promotes the dissolution of cuprous cyanide but also stabilizes the reaction intermediate through a solvation effect. The final ion exchange step converts the initially formed quaternary ammonium salt into the target cation structure, ensuring that the product possesses ideal solubility and electrochemical stability. Throughout the reaction pathway, the introduction of four cyano groups not only enhances the electron affinity of the molecule but also creates multiple lithium-ion coordination sites, laying the foundation for subsequent battery applications.

[0016] As a preferred embodiment of the present invention, in step B1, the melting reaction time at 255-265°C is 6-8 hours.

[0017] As a preferred embodiment of the present invention, in step B2, the reaction time at 155-165°C under nitrogen protection is 48-50 hours.

[0018] In a second aspect, the present invention provides a method for preparing an aramid polymer-coated lithium-ion battery separator, wherein the aramid polymer-coated lithium-ion battery separator comprises the following raw materials in parts by weight: 80-120 parts of a porous polyethylene base membrane; 8-12 parts of p-phenylenediamine; 12-18 parts of terephthaloyl chloride; 150-250 parts of N-methylpyrrolidone; 5-10 parts of calcium chloride; 1-5 parts of perfluorophenyl imidazole lithium salt; 1-4 parts of tetracyanophthalimide dimethylammonium; and 3-8 parts of alumina nanoparticles.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) The aramid polymer-coated lithium-ion battery separator provided by this invention introduces two newly designed modified compounds, which, while maintaining the excellent heat resistance and mechanical strength of aramid materials, successfully solves the technical problems of low ionic conductivity, poor interfacial compatibility, and insufficient long-term cycle stability of traditional aramid-coated separators. The synergistic effect of perfluorophenyl imidazole lithium salt and tetracyanophthalimide dimethylammonium salt comprehensively improves the overall performance of the separator. These two compounds specifically improve several key performance parameters of the separator from the molecular structure design level. Among them, perfluorophenyl imidazole lithium salt provides an efficient lithium-ion transport channel with its unique molecular structure, while the perfluorophenyl structure endows the separator with higher electrochemical stability; tetracyanophthalimide dimethylammonium salt forms a close contact with the electrode surface through its planar conjugated structure and multiple cyano functional groups, effectively uniformly distributing lithium-ion flow and inhibiting dendrite growth. The synergistic effect of the two compounds enables the separator to maintain dimensional integrity under high temperature conditions, significantly improving the safety performance of the battery, while providing more activation sites for lithium-ion migration and greatly reducing interfacial impedance.

[0020] (2) In terms of thermal stability and safety, the composite separator prepared by this invention exhibits excellent performance. The high-temperature resistance inherent in the aramid polymer, combined with the thermal stabilizing effect of the two modified compounds, ensures that the separator hardly shrinks under high-temperature conditions, and its thermal shrinkage rate is much lower than that of traditional polyolefin separators. The perfluorophenyl structure in the perfluorophenyl imidazole lithium salt can form a stable protective layer at high temperatures, effectively preventing the thermal degradation process of the separator; the rigid planar structure of tetracyanophthalimide dimethylammonium enhances the thermomechanical strength of the entire coating material. When the internal temperature of the battery rises abnormally, the separator can maintain its structural integrity, prevent short circuits between the positive and negative electrodes, and fundamentally avoid the occurrence of thermal runaway. In addition, the inherent flame-retardant properties of the aramid material and the synergistic effect of the two modified compounds further enhance the fire resistance of the separator, providing multiple safety guarantees for the battery system.

[0021] (3) In terms of electrochemical performance, the separator of this invention exhibits significant advantages. The introduction of the two modified compounds significantly improves the ionic conductivity and lithium-ion transference number of the separator, resulting in lower polarization and better rate performance of the battery. The lithium-ion active center in the perfluorophenyl imidazole lithium salt works synergistically with the imidazole ring to form an efficient ion conduction pathway; the multiple cyano functional groups of tetracyanophthalimide dimethylammonium can reversibly coordinate with lithium ions, promoting the uniform deposition and dissolution of lithium ions. This unique ion transport mechanism effectively inhibits the formation and growth of lithium dendrites, extending the cycle life of the battery. At the same time, the functional layer formed by the two compounds at the electrode-separator interface significantly improves the interfacial compatibility and reduces the interfacial impedance, enabling the battery to maintain stable performance under both high and low temperature environments. The assembled battery can still maintain a high capacity after multiple cycles, demonstrating the significant progress of the separator in long-term cycle stability and providing reliable technical support for the development of high-energy-density and high-safety lithium-ion batteries. Detailed Implementation

[0022] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0023] The sources of some components in the examples and comparative examples are as follows: The polyethylene porous base membrane was purchased from Suzhou LiDun Energy Storage Materials Technology Co., Ltd.

[0024] The p-phenylenediamine was purchased from Zhejiang Longsheng Group Co., Ltd.

[0025] The terephthaloyl chloride was purchased from Zhejiang Runtu Co., Ltd.

[0026] The 4,5-dicyanimidazol was purchased from Shanghai Hongbang Pharmaceutical Technology Co., Ltd.

[0027] The perfluoroiodobenzene was purchased from Guangzhou Jiangshun Chemical Technology Co., Ltd.

[0028] The cesium carbonate was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0029] The lithium iodide was purchased from Shanghai McLean Biochemical Technology Co., Ltd.

[0030] The trimethylchlorosilane was purchased from Anaiji Chemical Co., Ltd.

[0031] The 1,4,5,8-naphthalenetetracarboxylic dianhydride was purchased from Henan Guangwo New Materials Co., Ltd.

[0032] The urea was purchased from Hubei Yihua Group Co., Ltd.

[0033] The N,N-dimethylformamide dimethyl acetal was purchased from Wuhan Kemic Biomedical Technology Co., Ltd.

[0034] The cuprous cyanide was purchased from Beijing Huawirui Chemical Co., Ltd.

[0035] The tetrabutylammonium bromide was purchased from Hubei Xinghengye Technology Co., Ltd.

[0036] Example 1 Preparation of lithium perfluorophenyl imidazolium salt: 50.0 g of 4,5-dicyanimidazole and 120.0 g of perfluoroiodobenzene were dissolved in 600.0 mL of N-methylpyrrolidone. Under nitrogen protection, 10.0 g of copper powder and 100.0 g of cesium carbonate were added, and the reaction was carried out at 120.0 °C for 12.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C and filtered through a Buchner funnel to obtain the filtrate. The filtrate was poured into 2000 mL of deionized water to precipitate a white solid. This solid was filtered through a vacuum filtration device and washed with 3 × 500 mL of deionized water. The solid was then dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain intermediate 4. 5-Dicyano-2-perfluorophenylimidazolium; then 40.0 g of this intermediate was dissolved in 400.0 mL of acetonitrile, 30.0 g of lithium iodide and 20.0 mL of trimethylchlorosilane were added, and the mixture was refluxed at 80.0 °C for 24.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, and most of the solvent was removed by evaporation using a rotary evaporator at a water bath temperature of 60.0 °C. 300 mL of ethyl acetate was added to precipitate a light yellow solid, which was filtered through a Buchner funnel and washed with 3 × 100 mL of cold ethyl acetate. The solid was dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, lithium perfluorophenylimidazolium salt.

[0037] Preparation of tetracyanophthalimide dimethylammonium: 60.0 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride was thoroughly mixed with 100.0 g of urea and 30.0 g of ammonium chloride, and the mixture was melt-reacted in an oil bath at 260.0 °C for 6.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, pulverized using a mortar and pestle, washed with 3 × 500 mL of 80.0 °C hot water, and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain intermediate 1,4,5,8-naphthalenetetracarboximide; then 40.0 g of this intermediate was dissolved in 100.0 mL of N,N-dimethylformamide dimethyl acetal in... 50.0 g of cuprous cyanide and 10.0 g of tetrabutylammonium bromide were added to 400.0 mL of dimethyl sulfoxide. The mixture was reacted at 160.0 °C for 48.0 h under nitrogen protection. After the reaction was completed, the mixture was cooled to 25.0 °C. The insoluble matter was removed by filtration through a Buchner funnel. The filtrate was poured into 2000 mL of diethyl ether, and an orange-red solid precipitated. The solid was filtered through a Buchner funnel and washed with 3 × 100 mL of diethyl ether. The solid was treated with an ion exchange column using ammonium formate / methanol solution as the eluent and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, tetracyanophthalimide dimethylammonium.

[0038] Preparation of aramid polymer-coated membrane: 100.0 g of porous polyethylene base membrane was corona treated to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 10.0 g of p-phenylenediamine and 15.0 g of terephthaloyl chloride were dissolved in 200.0 mL of N-methylpyrrolidone, and 8.0 g of calcium chloride was added. The mixture was reacted in a -5.0℃ low-temperature reactor for 6.0 h to obtain a para-aramid polymer solution; then, 3.0 g of perfluorophenylimidazolium lithium salt and 2.0 g of tetracyanophthalimide dimethylammonium were reacted with 5.0 g of alumina nanoparticles. The aramid resin was added to the aramid resin solution and dispersed at 3000 rpm for 12.0 h in a high-speed disperser to obtain a uniform aramid coating solution. The coating solution was uniformly coated onto the pretreated polyethylene substrate using an dip coating method, and the wet film thickness was controlled to be 50.0 μm using a wet film preparation device. The coated membrane was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 500 mL of deionized water and dried in a vacuum drying oven at 80.0 °C for 8.0 h to obtain the final aramid polymer coated membrane.

[0039] Example 2 Preparation of perfluorophenylimidazolium lithium salt: 45.0 g of 4,5-dicyanimidazole and 110.0 g of perfluoroiodobenzene were dissolved in 550.0 mL of N-methylpyrrolidone. Under nitrogen protection, 9.0 g of copper powder and 90.0 g of cesium carbonate were added, and the reaction was carried out at 118.0 °C for 13.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C and filtered through a Buchner funnel to obtain the filtrate. The filtrate was poured into 1800 mL of deionized water to precipitate a white solid. This solid was filtered through a vacuum filtration device and washed with 3 × 450 mL of deionized water. The solid was then dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain intermediate 4,5-dicyanimidazole. -Dicyano-2-perfluorophenylimidazolium; then 35.0 g of this intermediate was dissolved in 350.0 mL of acetonitrile, 25.0 g of lithium iodide and 18.0 mL of trimethylchlorosilane were added, and the mixture was refluxed at 79.0 °C for 26.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, and most of the solvent was removed by evaporation using a rotary evaporator at a water bath temperature of 60.0 °C. 280 mL of ethyl acetate was added to precipitate a light yellow solid, which was filtered through a Buchner funnel and washed with 3 × 90 mL of cold ethyl acetate. The solid was dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, lithium perfluorophenylimidazolium salt.

[0040] Preparation of tetracyanophthalimide dimethylammonium: 55.0 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride was thoroughly mixed with 90.0 g of urea and 25.0 g of ammonium chloride, and the mixture was melt-reacted in an oil bath at 258.0 °C for 7.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, pulverized using a mortar and pestle, washed with 3 × 450 mL of 80.0 °C hot water, and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the intermediate 1,4,5,8-naphthalenetetracarboximide; then 35.0 g of this intermediate was dissolved in 90.0 mL of N,N-dimethylformamide dimethyl acetal... 45.0 g of cuprous cyanide and 9.0 g of tetrabutylammonium bromide were added to 350.0 mL of dimethyl sulfoxide. The mixture was reacted at 158.0 °C for 49.0 h under nitrogen protection. After the reaction was completed, the mixture was cooled to 25.0 °C. The insoluble matter was removed by filtration through a Buchner funnel. The filtrate was poured into 1800 mL of diethyl ether to precipitate an orange-red solid. The solid was filtered through a Buchner funnel and washed with 3 × 90 mL of diethyl ether. The solid was treated with an ion exchange column using ammonium formate / methanol solution as the eluent and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, tetracyanophthalimide dimethylammonium.

[0041] Preparation of aramid polymer-coated membrane: 90.0 g of porous polyethylene base membrane was subjected to corona treatment to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 9.0 g of p-phenylenediamine and 14.0 g of terephthaloyl chloride were dissolved in 180.0 mL of N-methylpyrrolidone, and 7.0 g of calcium chloride was added. The mixture was reacted in a -4.0℃ low-temperature reactor for 7.0 h to obtain a para-aramid polymer solution; then 2.0 g of perfluorophenyl imidazole lithium salt and 1.5 g of tetracyanophthalimide dimethylammonium were added with 4.0 g of alumina nanoparticles. The aramid resin was added to the aramid resin solution and dispersed at 3000 rpm for 12.0 h in a high-speed disperser to obtain a uniform aramid coating solution. The coating solution was uniformly coated onto the pretreated polyethylene substrate using an dip coating method, and the wet film thickness was controlled to be 45.0 μm using a wet film preparation device. The coated membrane was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 450 mL of deionized water and dried in a vacuum drying oven at 79.0 °C for 9.0 h to obtain the final aramid polymer coated membrane.

[0042] Example 3 Preparation of lithium perfluorophenyl imidazolium salt: 55.0 g of 4,5-dicyanimidazole and 130.0 g of perfluoroiodobenzene were dissolved in 650.0 mL of N-methylpyrrolidone. Under nitrogen protection, 11.0 g of copper powder and 110.0 g of cesium carbonate were added. The reaction was carried out at 122.0 °C for 13.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C and filtered through a Buchner funnel to obtain the filtrate. The filtrate was poured into 2200 mL of deionized water to precipitate a white solid. This solid was filtered through a vacuum filtration device and washed with 3 × 550 mL deionized water. The solid was dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain intermediate 4. 5-Dicyano-2-perfluorophenylimidazolium; then 45.0 g of this intermediate was dissolved in 450.0 mL of acetonitrile, 35.0 g of lithium iodide and 22.0 mL of trimethylchlorosilane were added, and the mixture was refluxed at 81.0 °C for 28.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, and most of the solvent was removed by evaporation using a rotary evaporator at a water bath temperature of 60.0 °C. 320 mL of ethyl acetate was added to precipitate a light yellow solid, which was filtered through a Buchner funnel and washed with 3 × 110 mL of cold ethyl acetate. The solid was dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, lithium perfluorophenylimidazolium salt.

[0043] Preparation of tetracyanophthalimide dimethylammonium: 65.0 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride was thoroughly mixed with 110.0 g of urea and 35.0 g of ammonium chloride, and the mixture was melt-reacted in an oil bath at 262.0 °C for 7.0 h. After the reaction was completed, the mixture was cooled to 25.0 °C, pulverized using a mortar and pestle, washed with 3 × 550 mL of hot water at 80.0 °C, and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain intermediate 1,4,5,8-naphthalenetetracarboximide; then 45.0 g of this intermediate was dissolved in 110.0 mL of N,N-dimethylformamide dimethyl acetal in 450.0 mL of dimethyl sulfoxide. In a reaction mixture, 55.0 g of cuprous cyanide and 11.0 g of tetrabutylammonium bromide were added, and the mixture was reacted at 162.0 °C for 49.0 h under nitrogen protection. After the reaction was completed, the mixture was cooled to 25.0 °C, and the insoluble matter was removed by filtration through a Buchner funnel. The filtrate was poured into 2200 mL of diethyl ether, and an orange-red solid precipitated. The solid was then filtered through a Buchner funnel and washed with 3 × 110 mL of diethyl ether. The solid was treated with an ion exchange column using ammonium formate / methanol solution as the eluent, and dried in a vacuum drying oven at 80.0 °C for 12.0 h to obtain the final product, tetracyanophthalimide dimethylammonium.

[0044] Preparation of aramid polymer-coated membrane: 110.0 g of porous polyethylene base membrane was subjected to corona treatment to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 11.0 g of p-phenylenediamine and 16.0 g of terephthaloyl chloride were dissolved in 220.0 mL of N-methylpyrrolidone, and 9.0 g of calcium chloride was added. The mixture was reacted in a -5.0℃ low-temperature reactor for 7.0 h to obtain a para-aramid polymer solution; then 4.0 g of perfluorophenylimidazolium lithium salt and 2.5 g of tetracyanophthalimide dimethylammonium were reacted with 6.0 g of alumina nanoparticles. The aramid resin was added to the aramid resin solution and dispersed at 3000 rpm for 12.0 h in a high-speed disperser to obtain a uniform aramid coating solution. The coating solution was uniformly coated onto the pretreated polyethylene substrate using an dip coating method, and the wet film thickness was controlled to be 55.0 μm using a wet film preparation device. The coated membrane was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 550 mL of deionized water and dried in a vacuum drying oven at 81.0 °C for 9.0 h to obtain the final aramid polymer coated membrane.

[0045] Comparative Example 1 The difference between this comparative example and Example 1 lies in the preparation of the aramid polymer-coated diaphragm: 100.0 g of polyethylene porous base membrane was subjected to corona treatment to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 10.0 g of p-phenylenediamine and 15.0 g of terephthaloyl chloride were dissolved in 200.0 mL of N-methylpyrrolidone, and 8.0 g of calcium chloride was added. The mixture was reacted in a -5.0°C low-temperature reactor for 6.0 h to obtain a para-aramid polymer solution; then 5.0 g of alumina nanoparticles were added to... Aramid resin was dispersed in a high-speed disperser at 3000 rpm for 12.0 h to obtain an aramid coating solution. The coating solution was uniformly coated onto a pretreated polyethylene substrate using an immersion coating method, and the wet film thickness was controlled to be 50.0 μm using a wet film preparation device. The coated diaphragm was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 500 mL of deionized water and dried in a vacuum drying oven at 80.0 °C for 8.0 h to obtain an aramid-coated diaphragm.

[0046] Comparative Example 2 The difference between this comparative example and Example 1 lies in the preparation of the aramid polymer-coated membrane: 100.0 g of polyethylene porous base membrane was subjected to corona treatment to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 10.0 g of p-phenylenediamine and 15.0 g of terephthaloyl chloride were dissolved in 200.0 mL of N-methylpyrrolidone, and 8.0 g of calcium chloride was added. The mixture was reacted in a -5.0°C low-temperature reactor for 6.0 h to obtain a para-aramid polymer solution; then 3.0 g of perfluorophenyl imidazole lithium salt and 5.0 g of alumina were added... Nanoparticles were added to the aramid resin and dispersed in a high-speed disperser at 3000 rpm for 12.0 h to obtain an aramid coating solution. The coating solution was uniformly coated onto a pretreated polyethylene substrate using an dip coating method, and the wet film thickness was controlled to be 50.0 μm using a wet film preparation device. The coated membrane was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 500 mL of deionized water and dried in a vacuum drying oven at 80.0 °C for 8.0 h to obtain an aramid-coated membrane.

[0047] Comparative Example 3 The difference between this comparative example and Example 1 lies in the preparation of the aramid polymer-coated diaphragm: 100.0 g of porous polyethylene base membrane was subjected to corona treatment to achieve a surface tension of 40 dynes / cm; under a nitrogen atmosphere, 10.0 g of p-phenylenediamine and 15.0 g of terephthaloyl chloride were dissolved in 200.0 mL of N-methylpyrrolidone, and 8.0 g of calcium chloride was added. The mixture was reacted in a -5.0°C low-temperature reactor for 6.0 h to obtain a para-aramid polymer solution; then 2.0 g of tetracyanophthalimide dimethylammonium and 5.0 g of oxygen... Aluminum chloride nanoparticles were added to the aramid resin and dispersed in a high-speed disperser at 3000 rpm for 12.0 h to obtain an aramid coating solution. The coating solution was uniformly coated onto a pretreated polyethylene substrate using an dip coating method, and the wet film thickness was controlled to be 50.0 μm using a wet film preparation device. The coated membrane was immediately immersed in a deionized water coagulation bath to form a porous structure through phase inversion. The residual solvent was then washed with 3 × 500 mL of deionized water and dried in a vacuum drying oven at 80.0 °C for 8.0 h to obtain an aramid-coated membrane.

[0048] The performance of the lithium-ion battery separators coated with aramid polymers obtained in Examples 1-3 and Comparative Examples 1-3 was tested according to national and industry standards. Separator thermal shrinkage rate test: Separator samples were cut into 10cm × 10cm square specimens, placed horizontally in a 130℃ constant temperature oven for 1 hour, and then cooled to 25℃. The longitudinal and transverse dimensional changes were measured, and the arithmetic mean of the shrinkage rates in both directions was taken as the final thermal shrinkage rate. Thermal stability test: A thermomechanical analyzer was used. Separator samples were cut into 5mm × 10mm rectangular specimens, and heated from 25℃ to 300℃ at a rate of 5℃ / min under a nitrogen atmosphere. The change curve of the separator dimensions with temperature was recorded, and the temperature at which the dimensions changed by 5% was taken as the heat distortion temperature. Porosity was tested using the n-butanol adsorption method. The membrane sample was cut into 5cm × 5cm square specimens, immersed in n-butanol solution for 30 min, removed, and excess liquid was wiped off. The mass difference before and after n-butanol adsorption was measured, and the porosity was calculated based on the apparent volume of the membrane and the density of n-butanol. The Gurley value was tested using a Gurley permeability meter. The membrane sample was cut into 2.5cm diameter circular specimens, a pressure of 1.23 kPa was applied, and the time required for 100 mL of air to pass through the membrane was measured. The result was expressed in seconds per 100 mL. Ionic conductivity was tested using the AC impedance method. The membrane sample was cut into 1.6cm diameter circular specimens, immersed in a 1 mol / L LiPF6 EC / DMC (volume ratio 1:1) electrolyte for 24 h, assembled into a stainless steel symmetrical cell, and its impedance spectrum was measured using an electrochemical workstation in the frequency range of 100 kHz to 0.1 Hz. The bulk resistance was calculated using the high-frequency region and the real axis intercept, and the ionic conductivity was calculated in conjunction with the membrane thickness and electrode area. The lithium-ion transport number was tested using the DC polarization method. The separator sample was assembled into a lithium-ion symmetric battery, a 10mV DC polarization voltage was applied, and the initial and steady-state currents were recorded. Combined with the initial and steady-state interfacial impedance obtained from AC impedance testing, the lithium-ion transport number was calculated using the Bruce-Vincent formula. The electrolyte contact angle was tested using a contact angle meter. A 1μL electrolyte droplet was added to the separator surface, the droplet morphology was recorded, and the contact angle value was calculated. The average value of the measurements at five different locations was taken. For battery cycle performance testing, the separator, NCM positive electrode material, and graphite negative electrode were assembled into a 2025-type button cell. Charge-discharge cycle tests were conducted at a rate of 0.5C within a voltage range of 2.5-4.2V. The discharge capacity at week 1 and week 100 was recorded, and the capacity retention rate was calculated. The high-temperature needle penetration test involves assembling the separator and electrodes into a pouch battery, charging it to 100% SOC, and then piercing the battery with a 3mm diameter steel needle at a speed of 25mm / s at 150℃. The highest surface temperature of the battery and whether fire or explosion occurs are recorded.

[0049] The performance test data above are shown in Table 1.

[0050] Table 1 Performance Test Results

[0051] The test results in Table 1 clearly show that Examples 1-3 exhibit significant advantages over Comparative Examples 1-3 in all performance indicators, fully demonstrating that the synergistic effect of the two modified compounds, perfluorophenyl imidazole lithium salt and tetracyanophthalimide dimethylammonium, effectively solves key problems in the prior art. Regarding thermal stability, the heat shrinkage rates of Examples 1-3 are all below 1.5%, far lower than the 2.8% of Comparative Example 1, with Example 1 showing a heat shrinkage rate of only 0.8%, indicating that the two modified compounds significantly improve the thermal dimensional stability of the membrane through molecular-level interactions. The heat distortion temperature test results show that Example 1 reaches 257℃, nearly 60℃ higher than Comparative Example 1, proving that the stable structure formed by the modified compounds and the aramid matrix effectively inhibits molecular chain movement at high temperatures. In terms of pore structure, Example 1 achieves a porosity of 58.3%, while the Gurley value decreases to 152 s / 100 mL, achieving a balance between high porosity and good air permeability. This is attributed to the regulatory effect of the two modified compounds on the pore structure during phase separation.

[0052] The improvement in electrochemical performance is particularly significant. Example 1 achieved an ionic conductivity of 0.79 mS / cm and a lithium-ion transference number of 0.69, far exceeding the 0.51 mS / cm and 0.31 of Comparative Example 1. This significant improvement stems from the synergistic effect of the lithium-ion transport channels provided by the perfluorophenylimidazolium lithium salt and the lithium-ion coordination of the tetracyanophthalimide dimethylammonium. In the electrolyte wettability test, the contact angle of Example 1 decreased to 8.2°, indicating that the surface-active groups of the two modified compounds effectively improved the interfacial compatibility between the separator and the electrolyte. Regarding battery cycle performance, Example 1 maintained a capacity retention of 85.4% after 100 cycles, while Comparative Example 1 only maintained 61.3%, demonstrating that the stable interfacial layer formed by the modified compounds effectively suppressed interfacial side reactions during cycling. The most convincing evidence comes from the high-temperature needle penetration test results. Examples 1-3 all passed the test, while Comparative Example 1 caught fire, and Comparative Examples 2 and 3 showed signs of smoke. This fully demonstrates that the synergistic effect of the two modified compounds significantly improved the thermal stability and safety performance of the diaphragm.

[0053] Comprehensive analysis of all test data reveals that the superior performance of Examples 1-3 compared to Comparative Examples 1-3 confirms the synergistic mechanism of perfluorophenylimidazolium lithium salt and tetracyanophthalimide dimethylammonium: perfluorophenylimidazolium lithium salt provides an efficient lithium-ion transport pathway and thermal stability through its unique molecular structure, while tetracyanophthalimide dimethylammonium optimizes interfacial performance and lithium-ion flow distribution through its planar conjugated structure and multiple cyano functional groups. The synergistic effect of these two compounds in the aramid matrix achieves a comprehensive improvement in the membrane's thermal stability, ionic conductivity, and interfacial compatibility, effectively solving the technical bottlenecks of traditional aramid-coated membranes in terms of high-temperature safety, electrochemical performance, and long-term cycling stability. In particular, the test results of Comparative Examples 2 and 3, using only a single modified compound, further confirm the necessity of the synergistic effect of the two compounds; no single component could achieve the comprehensive performance improvement demonstrated in Examples 1-3.

Claims

1. A method for preparing a lithium-ion battery separator coated with aramid polymer, characterized in that the steps include... include: S1. First, the porous polyethylene base membrane is subjected to corona treatment; under a nitrogen atmosphere, p-phenylenediamine, terephthaloyl chloride, N-methylpyrrolidone and calcium chloride are reacted at -4~-6℃ to obtain a para-aramid polymer solution. Then, perfluorophenyl imidazole lithium salt, tetracyanophthalimide dimethylammonium and alumina are added to the para-aramid polymer solution and dispersed in a high-speed disperser to obtain an aramid coating solution. S2. The aramid coating solution is coated onto the corona-treated porous polyethylene base membrane to obtain the coated diaphragm; the coated diaphragm is immersed in a deionized water coagulation bath, then washed with deionized water, and vacuum dried at 78-82℃.

2. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 1, characterized in that, In step S1, the reaction time at -4 to -6°C is 6-8 hours.

3. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 1, characterized in that, In step S2, the vacuum drying time at 78-82℃ is 8-10 hours.

4. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 1, characterized in that, The preparation method of the perfluorophenyl imidazolium lithium salt includes: A1, dissolving 4,5-dicyanimidazole and perfluoroiodobenzene in N-methylpyrrolidone, adding copper powder and cesium carbonate under nitrogen protection, reacting at 115-125℃, cooling to room temperature after the reaction, filtering, obtaining filtrate, pouring the filtrate into deionized water to precipitate solid, filtering and washing with deionized water, and vacuum drying to obtain intermediate; A2, then dissolving the intermediate in acetonitrile, adding lithium iodide and trimethylchlorosilane, refluxing at 78-82℃, cooling to room temperature after the reaction, evaporating, adding ethyl acetate to precipitate solid, filtering and washing with cold ethyl acetate, and vacuum drying.

5. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 4, characterized in that, In step A1, the reaction time is 12-14 hours at 115-125℃.

6. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 4, characterized in that, In step A2, the reaction is carried out under reflux at 78-82℃ for 24-30 hours.

7. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 1, characterized in that, The preparation method of the tetracyanophthalimide dimethylammonium includes: B1, mixing 1,4,5,8-naphthalenetetracarboxylic dianhydride, urea and ammonium chloride, melting and reacting at 255-265℃, cooling to room temperature after the reaction, pulverizing and washing with hot water, and vacuum drying to obtain an intermediate; B2, then dissolving the intermediate with N,N-dimethylformamide dimethyl acetal in dimethyl sulfoxide, adding cuprous cyanide and tetra-n-butylammonium bromide, reacting at 155-165℃ under nitrogen protection, cooling to room temperature after the reaction, filtering, pouring the filtrate into diethyl ether to precipitate a solid, filtering the solid and washing with diethyl ether, treating with an ion exchange column, and vacuum drying.

8. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 7, characterized in that, In step B1, the melting reaction time at 255-265℃ is 6-8 hours.

9. The method for preparing the lithium-ion battery separator coated with aramid polymer according to claim 7, characterized in that, In step B2, the reaction time at 155-165℃ under nitrogen protection is 48-50 hours.

10. A lithium-ion battery separator coated with aramid polymer, prepared by the method according to any one of claims 1-9, characterized in that, The raw materials include the following parts by weight: 80-120 parts of porous polyethylene base membrane; 8-12 parts of p-phenylenediamine; 12-18 parts of terephthaloyl chloride; 150-250 parts of N-methylpyrrolidone; 5-10 parts of calcium chloride; 1-5 parts of perfluorophenyl imidazole lithium salt; 1-4 parts of tetracyanophthalimide dimethylammonium; and 3-8 parts of alumina nanoparticles.

Images