Secondary battery composite diaphragm based on compounding of aramid nanofiber and silicon dioxide, preparation method of secondary battery composite diaphragm and secondary battery

Abstract

The invention relates to the technical field of secondary battery diaphragms, in particular to a secondary battery composite diaphragm based on compounding of aramid nanofibers and silicon dioxide, a preparation method of the secondary battery composite diaphragm and a secondary battery. The secondary battery composite diaphragm is a composite material of an aramid nanofiber three-dimensional network structure with silicon dioxide nanoparticles as crosslinking nodes; wherein one or more aramid nanofibers pass through a single silica nanoparticle. When the battery diaphragm provided by the invention is applied to a battery, compared with a commercial polypropylene diaphragm, the battery diaphragm can effectively inhibit the growth of Li dendrites on the surface of an anode, and the battery shows excellent rate capability and stable cycle performance, and has relatively high specific discharge capacity and excellent cycle stability; as a diaphragm material in the field of high-performance lithium ion batteries, the composite material has great application potential, and can effectively prolong the cycle life of the battery and improve the overall performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary battery separators, and particularly to a secondary battery composite separator based on the composite of aramid nanofibers and silica, a preparation method thereof, and a secondary battery. Background Art

[0002] As a core component of modern energy technologies, lithium-ion batteries (LIBs) have demonstrated great application potential and value in various fields such as portable electronic devices, electric vehicles, and large-scale energy storage systems, and have always been the focus of attention in scientific research and industry. In the complex structure of lithium-ion batteries, the separator, as a crucial component, plays a dual role of separating the positive and negative electrodes to prevent physical contact while ensuring the smooth transmission of lithium ions within the battery. Currently, the separator materials widely used commercially are mainly polyolefin materials such as polyethylene (PE) and polypropylene (PP). Polyolefin separator materials dominate the lithium-ion battery field due to their specific pore structure, excellent chemical stability, and relatively low cost-effectiveness.

[0003] However, despite the many advantages of polyolefin separators, they have relatively obvious deficiencies in aspects such as electrolyte wettability and thermal stability. When the battery is subjected to external shocks or is in a high-temperature environment for a long time, insufficient thermal stability may trigger thermal runaway inside the battery, leading to catastrophic safety accidents such as combustion and even explosion. Therefore, in order to improve the overall safety of lithium-ion batteries, it is particularly important to develop new separator materials with intrinsic thermal flame retardancy.

[0004] Aramid nanofibers (ANFs), as a new type of high-performance nanofiber material, have attracted much attention due to their unique inherent properties. ANFs not only have physical properties such as high strength and low density, but also exhibit excellent chemical stability and thermal stability. Among traditional separator materials, ANFs are regarded as one of the potential alternative materials for lithium-ion battery separators.

[0005] Recently, many studies have been devoted to exploring the preparation process of ANF-based lithium-ion battery separators. For example, a pure ANF-based separator is prepared by vacuum-assisted filtration. This separator exhibits high Young's modulus and tensile strength, and also shows excellent thermal stability. Nevertheless, the asymmetry of the chemical structure of ANF itself, that is, the alternating arrangement of benzene rings and amide bonds, results in strong intermolecular interactions, leading to fiber orientation aggregation and high entanglement. This microscopic structural feature directly causes a dense structure in the ANF-based battery system due to limited porosity and low ionic conductivity, ultimately resulting in a significant reduction in electrochemical efficiency and cycle stability. Summary of the Invention

[0006] To solve the deficiencies of the existing technology, the present invention provides a secondary battery composite separator based on the composite of aramid nanofibers and silica, a preparation method thereof, and a secondary battery.

[0007] To achieve the above object, the technical solution of the present invention is: a secondary battery composite separator based on the composite of aramid nanofibers and silica, wherein the secondary battery composite separator is a composite material of a three-dimensional network structure of aramid nanofibers with silica nanoparticles as cross-linking nodes, and one or more aramid nanofibers pass through a single silica nanoparticle.

[0008] Further, the aramid nanofibers include one or more of meta-aramid nanofibers without other comonomers, para-aramid nanofibers without other comonomers, meta-aramid nanofibers with other comonomers, and para-aramid nanofibers with other comonomers.

[0009] "Para-aramid fiber", that is, poly(p-phenylene terephthalamide) (PPTA), is formed by the polycondensation reaction of terephthaloyl chloride and p-phenylenediamine. Its molecular chain has a rigid benzene ring and strong polar amide bonds, and there is a strong hydrogen bond interaction between molecules. Its structure shows the characteristics of simple symmetry and regular arrangement, and has excellent properties such as high strength, high modulus, and high temperature resistance, while the density is relatively low.

[0010] "Meta-aramid fiber" (PMTA) is formed by the polycondensation of isophthaloyl chloride and m-phenylenediamine. Its molecular chain has good flexibility, the chain segment is zigzag, and the crystallinity and molecular chain orientation degree are relatively low. Therefore, its strength and modulus are lower than those of para-aramid, but it has characteristics such as high temperature resistance, high elongation rate, excellent wear resistance, and compression fatigue resistance.

[0011] Further, the thickness of the secondary battery composite separator is 5-200 μm. More preferably, it is any value between 10-60 μm.

[0012] Further, the diameter of the aramid nanofibers is 10-100 nm.

[0013] In the present invention, the diameter of the aramid nanofibers is in the range of 10-100 nm, and there is no specific requirement for the fiber length. Short fibers or long fibers can be used, specifically depending on the ease of forming the aramid nanofiber suspension.

[0014] A preparation method of a secondary battery composite separator based on the composite of aramid nanofibers and silica includes the following steps:

[0015] S1. Add a water-soluble solvent to the aramid nanofiber hydrogel and stir to make an aramid nanofiber suspension;

[0016] S2. Add alkoxysilane compounds and alkaline regulators to the aramid nanofiber suspension to adjust the pH of the aramid nanofiber suspension to be alkaline.

[0017] S3. After stirring and reacting for a period of time, an aramid nanofiber slurry is formed. The reacted aramid nanofiber slurry is subjected to vacuum filtration and washed with deionized water until neutral to obtain a modified aramid nanofiber hydrogel.

[0018] S4. Redisperse the modified aramid nanofiber hydrogel in deionized water and obtain a modified aramid nanofiber gel film through filtration.

[0019] S5. Dry the modified aramid nanofiber gel film to remove the moisture therein to obtain the secondary battery composite separator based on the composite of aramid nanofibers and silica.

[0020] "Aramid nanofiber hydrogel" refers to a hydrogel structure composed of aramid nanofibers, where the content of aramid nanofibers is 2-5 wt%. The "aramid nanofiber hydrogel" structure may be directly formed during the in-situ synthesis of aramid nanofibers or may be reformed from dried aramid nanofibers through technical means.

[0021] Preferably, the diameter of the aramid nanofibers is 10-100 nm, and the aspect ratio is 1000-5000; and the aramid nanofibers are at least one of 1414 para-aramid nanofibers synthesized with terephthaloyl chloride and p-phenylenediamine as monomers and 1313 meta-aramid nanofibers synthesized with isophthaloyl chloride and m-phenylenediamine as monomers. The aramid nanofiber hydrogel exists in the form of water-containing nano-aramid fibers, where the specific gravity of aramid nanofibers to water is 1.5-4.0:96.0-98.5.

[0022] In step S1, the concentration of aramid nanofibers in the aramid nanofiber suspension ranges from 0.1 to 1.0 wt%, more preferably any concentration value between 0.1 and 0.6 wt%.

[0023] In step S2, the alkoxysilane compounds and alkaline regulators can be directly added to the suspension or added after being formulated into a solution. The water-soluble solvents used can be added separately or mixed into the aramid nanofiber suspension.

[0024] Furthermore, the water-soluble solvents include one or more of water, methanol, ethanol, isopropanol, n-butanol, acetone, butanone, tetrahydrofuran, dimethyl sulfoxide, and N,N-dimethylformamide. The water-soluble solvents can be added separately or mixed into the aramid nanofiber hydrogel.

[0025] Further, the alkoxysilane compound includes one or more of tetraethoxysilane, trimethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane.

[0026] Further, the basic regulator includes one or more of aqueous ammonia solution, sodium hydroxide, potassium hydroxide, and triethylamine.

[0027] Further, in steps S1 and S3, the stirring speed is 100 - 1000 rpm. In step S1, the stirring time is 20 - 45 min. In step S3, the stirring time is 2 - 4.5 h.

[0028] During the dispersion and reaction processes involved in the preparation method, stirring is used as an auxiliary operation, and this stirring can be one or several of mechanical stirring or magnetic stirring. The stirring speed is 100 - 1000 rpm, more preferably any value between 300 - 500 rpm; the operating temperature is between 0°C and 120°C, more preferably between room temperature and 60°C. If heating is required during the mixing process, the heating method can be oil bath or water bath heating.

[0029] In step S5, the modified aramid nanofiber hydrogel is subjected to dehydration and drying treatment to obtain the secondary battery composite separator based on aramid nanofiber and silica. The drying method can be one or several of room temperature drying, drying in a blast drying oven at 40 - 60°C, molding drying, vacuum freeze drying, or papermaking method forming drying. Drying to constant weight can obtain the secondary battery composite separator based on aramid nanofiber and silica.

[0030] A secondary battery includes the above-mentioned secondary battery composite separator based on aramid nanofiber and silica or the composite separator prepared by the preparation method of the above-mentioned secondary battery composite separator based on aramid nanofiber and silica. The composite separator for the secondary battery is used to assemble the battery, and the lithium battery is assembled in the order of negative electrode shell, lithium sheet, (the composite separator with electrolyte dropped), electrode sheet, steel sheet, elastic sheet, and positive electrode shell.

[0031] The beneficial effects achieved by the present invention are as follows:

[0032] 1. In the present invention, silica nanoparticles are uniformly in-situ synthesized in the aramid nanofiber suspension, constructing a three-dimensional network structure as cross-linking points, and multiple aramid nanofibers pass through a single silica nanoparticle. This structure effectively solves the dispersion problem of silica in inert aramid nanofibers. At the same time, the cross-linking points inhibit the orientation aggregation of aramid nanofibers, forming a well-developed porous structure with finer fiber diameters and obvious micropores.

[0033] 2. Thermal stability and flame retardancy: Compared with commercial polypropylene (PP) separators, the separator of the present invention has excellent thermal dimensional stability in the temperature range of 25 to 200 °C, and exhibits excellent self-extinguishing characteristics and flame retardancy in combustion tests, which can effectively prevent battery thermal runaway, fire or explosion. It can be known from thermogravimetric analysis (TGA) that its thermal degradation temperature is significantly higher than that of commercial polypropylene (PP) separators, and with the incorporation of silica nanoparticles, the thermal stability is further enhanced.

[0034] 3. Electrochemical performance: The secondary battery composite separator based on the composite of aramid nanofibers and silica has a high porosity, which can provide more transmission channels for lithium ions, improving the electrolyte absorption amount and ionic conductivity. In the battery cycle performance test, the battery with the secondary battery composite separator based on the composite of aramid nanofibers and silica can still maintain a stable and high capacity after 200 cycles at 0.5C, without obvious capacity decay, and its discharge capacity retention rate is better than that of the batteries based on commercial polypropylene separators and pure aramid nanofiber separators. In the rate performance test at different current densities, the battery with the secondary battery composite separator based on the composite of aramid nanofibers and silica shows a smaller charge transfer resistance and lower capacity loss, has good electrolyte wettability and a stable discharge platform, and can effectively inhibit the growth of lithium dendrites. Description of the Drawings

[0035] Figure 1 a is a scanning electron microscope image of the pure aramid nanofiber secondary battery separator in Comparative Example 2, Figure 1 b is a scanning electron microscope image of the secondary battery composite separator based on the composite of aramid nanofibers and silica in Example 1;

[0036] Figure 2 is a thermal stability test chart of the separators in Example 1 (mANFs), Comparative Example 1 (Celgard), and Comparative Example 2 (ANFs) of the present invention;

[0037] Figure 3 is the electrolyte wettability test of the separators in Example 1 (mANFs), Comparative Example 1 (Celgard), and Comparative Example 2 (ANFs) of the present invention;

[0038] Figure 4 is the cycle performance of the batteries assembled with the separators in Example 1 (mANFs), Comparative Example 1 (Celgard 2500), and Comparative Example 2 (ANFs) of the present invention, and the rate performance at five current densities of (0.2C, 0.5C, 1C, 2C, 3C). Detailed Description of the Invention

[0039] To better understand the purpose, structure, and function of the present invention, the following further describes in detail a secondary battery composite separator based on the composite of aramid nanofibers and silica, its preparation method, and the secondary battery in conjunction with the accompanying drawings.

[0040] Example 1

[0041] 18.52 g of aramid nanofiber hydrogel (weight fraction 2.7 wt%, 1414 para-aramid nanofibers synthesized from terephthaloyl chloride and p-phenylenediamine monomers; the diameter of aramid nanofibers is 30 - 60 nm, and the aspect ratio is 1500 - 4000; the aramid nanofiber hydrogel exists in the form of water-containing nano-aramid fibers, where the ratio of aramid nanofibers to water is 1.0 - 3.0:97 - 99) was added to a container with 200 ml of ethanol, and stirred evenly with ethanol as the water-soluble solvent. The stirring speed was 300 rpm - 500 rpm, and the stirring time was 30 min to obtain a uniform and stable aramid nanofiber suspension.

[0042] 6 ml of ammonia water and 2 ml of TEOS (tetraethoxysilane) were added to the prepared aramid nanofiber suspension, stirred at room temperature with a stirring speed of 300 rpm - 500 rpm for 3 h, and then the reacted slurry was vacuum filtered and repeatedly washed with deionized water until the solution was neutral to obtain modified aramid nanofiber hydrogel 1.

[0043] An appropriate amount of modified aramid nanofiber hydrogel 1 was redispersed in deionized water, and the dispersion was spread on a petri dish to obtain a gel film. After drying, a secondary battery composite separator 1 based on the composite of aramid nanofibers and silica was finally obtained.

[0044] First, the prepared secondary battery composite separator 1 based on the composite of aramid nanofibers and silica was observed by scanning electron microscopy for the differences before and after modification. The results are as Figure 1 shown in b. The secondary battery composite separator 1 based on the composite of aramid nanofibers and silica exhibits uniform particle dispersion, finer nanofiber diameter, and obvious micropores.

[0045] Secondly, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica was tested for thermal stability. The results are as Figure 2 (mANFs) shown. Due to the inherent heat-resistant properties of aramid nanofibers themselves and the uniform distribution state of heat-resistant SiO2 nanoparticles in the separator, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica exhibits excellent shrinkage resistance and thermal stability.

[0046] Then, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica was tested for electrolyte wettability. The results are asFigure 3 (mANFs) as shown. The contact angles of commercial polypropylene, pristine aramid nanofibers, and the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica are 45.4°, 6.6°, and 8.2°, respectively. Although the SiO2 particles may slightly reduce the electrolyte wettability due to their solid state and the hindrance to electrolyte penetration, compared with commercial polypropylene Celgard, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica significantly improves the electrolyte wettability. Overall, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica is expected to provide excellent electrochemical performance for the battery.

[0047] Finally, the secondary battery composite separator 1 based on the composite of aramid nanofibers and silica was used as the lithium-ion battery separator, and an LiFePO4 / Li battery was assembled. In a glove box filled with argon, the electrolyte-impregnated separator was sandwiched between the LiFePO4 positive electrode and the Li negative electrode, and then electrochemical tests were carried out at 25 °C on a LAND battery test system (CT2001A, China). The test results are as Figure 4 shown. At 0.5C discharge, there is no attenuation after 200 cycles, and the charge-discharge specific capacity is about 10 - 15 mAh / g higher than that of the commercial polypropylene separator. The discharge specific capacity tests at different rates show that after charge-discharge cycling at 0.2C, 0.5C, 1C, 2C, and 3C rates, when cycling at 0.2C again, the discharge specific capacity remains unchanged.

[0048] Example 2

[0049] 20 g of aramid nanofiber hydrogel (weight fraction 3 wt%, meta-aramid nanofibers synthesized from isophthaloyl chloride and m-phenylenediamine; aramid nanofiber diameter 40 - 70 nm, aspect ratio 2000 - 4500; the nanofiber aramid hydrogel exists in the form of water-containing nanofiber aramid, where the ratio of nanofiber aramid to water is 1.2 - 3.5:96.5 - 98.8) was mixed with 250 ml of methanol in a container and made uniform by magnetic stirring at a stirring speed of 400 rpm - 600 rpm for 40 min to obtain an aramid nanofiber suspension.

[0050] 8 ml of sodium hydroxide solution and 3 ml of tetraethoxysilane were added to the prepared aramid nanofiber suspension, and stirring was carried out under water bath heating at 40 °C at a stirring speed of 400 rpm - 600 rpm for 4 h. Then the reacted slurry was vacuum filtered and repeatedly washed with deionized water until the solution was neutral to obtain the modified aramid nanofiber hydrogel 2.

[0051] An appropriate amount of the modified aramid nanofiber hydrogel 2 was redispersed in deionized water. The dispersion was laid flat in a vacuum filtration device to filter out the moisture. After further drying, the secondary battery composite separator 2 based on the composite of aramid nanofibers and silica was finally obtained.

[0052] The secondary battery composite separator 2 based on the composite of aramid nanofibers and silica was applied to a lithium battery, and its electrochemical performance was tested in the same way as in Example 1. Under 0.5C discharge, there was no attenuation after 200 cycles, and the charge-discharge specific capacity was about 8 - 12 mAh / g higher than that of a commercial polypropylene separator. The test of the discharge specific capacity at different rates showed that after charge-discharge cycling at rates of 0.2C, 0.5C, 1C, 2C, and 3C, when cycling at 0.2C again, the discharge specific capacity remained unchanged.

[0053] Example 3:

[0054] 15 g of aramid nanofiber hydrogel (with a weight fraction of 2.5 wt%, which is meta-aramid nanofibers synthesized from isophthaloyl chloride and metaphenylenediamine; the diameter of the aramid nanofibers is 25 - 50 nm, and the aspect ratio is 1200 - 3500; the nanofiber aramid hydrogel exists in the form of water-containing nanofiber aramids, and the proportion of aramid nanofibers to water is 0.8 - 2.5:97.5 - 99.2) was added to a container with 180 ml of a mixed solution of ethanol and isopropanol, where the ratio of ethanol to isopropanol is 1:5. It was made uniform by mechanical stirring at a stirring speed of 200 rpm - 400 rpm for 25 min to obtain a uniform and stable aramid nanofiber suspension.

[0055] 5 ml of potassium hydroxide solution and 1.5 ml of methyltrimethoxysilane were added to the prepared aramid nanofiber suspension, and it was stirred at room temperature at a stirring speed of 200 rpm - 400 rpm for 2.5 h. Subsequently, the reacted slurry was vacuum filtered and repeatedly washed with deionized water until the solution was neutral to obtain the modified aramid nanofiber hydrogel 3.

[0056] An appropriate amount of the modified aramid nanofiber hydrogel 3 was redispersed in deionized water, and the dispersion was laid flat in a petri dish to obtain a modified aramid nanofiber gel film. First, the modified aramid nanofiber gel film was dried in a forced-air oven at 60°C for 2.5 hours to remove physically adsorbed water, and then freeze-dried at low temperature for 5 hours to finally obtain the secondary battery composite separator 3 based on the composite of aramid nanofibers and silica.

[0057] The secondary battery composite separator 3 based on the composite of aramid nanofibers and silica was applied to a lithium battery, and its electrochemical performance was tested in the same manner as in Example 1. At a discharge rate of 0.5C, there was no attenuation after 200 cycles, and the charge-discharge specific capacity was about 6-10 mAh / g higher than that of a commercial polypropylene separator. The discharge specific capacity tests at different rates showed that after charge-discharge cycling at rates of 0.2C, 0.5C, 1C, 2C, and 3C, when cycling again at 0.2C, the discharge specific capacity remained unchanged.

[0058] Example 4:

[0059] 25 g of aramid nanofiber hydrogel (with a weight fraction of 3.5 wt%, which is 1313 meta-aramid nanofibers synthesized from isophthaloyl chloride and m-phenylenediamine; the diameter of the aramid nanofibers is 45-75 nm, and the aspect ratio is 2200-4800; the nano-aramid fiber hydrogel exists in the form of water-containing nano-aramid fibers, where the proportion of aramid nanofibers to water is 1.3-3.8:96.2-98.7) was mixed with 300 ml of a mixed solvent of n-butanol and dimethyl sulfoxide in a container, where the ratio of n-butanol to dimethyl sulfoxide is 1:1, and it was made uniform by magnetic stirring at a stirring speed of 450-650 rpm for 45 min to obtain an aramid nanofiber suspension.

[0060] 10 ml of triethylamine and 3.5 ml of ethyltrimethoxysilane were added to the above aramid nanofiber suspension, and it was stirred under heating in an oil bath at 50°C at a stirring speed of 450-650 rpm for 4.5 h. Then, the reacted slurry was vacuum filtered and repeatedly washed with deionized water until the solution was neutral to obtain the modified aramid nanofiber hydrogel 4.

[0061] An appropriate amount of the modified aramid nanofiber hydrogel 4 was redispersed in deionized water and dried by the papermaking method to finally obtain the secondary battery composite separator 4 based on the composite of aramid nanofibers and silica.

[0062] The secondary battery composite separator 4 based on the composite of aramid nanofibers and silica was applied to a lithium battery, and its electrochemical performance was tested in the same manner as in Example 1. At a discharge rate of 0.5C, there was no attenuation after 200 cycles, and the charge-discharge specific capacity was about 6-8 mAh / g higher than that of a commercial polypropylene separator. The discharge specific capacity tests at different rates showed that after charge-discharge cycling at rates of 0.2C, 0.5C, 1C, 2C, and 3C, when cycling again at 0.2C, the discharge specific capacity remained unchanged.

[0063] Example 5:

[0064] 1. Add 12 g of aramid nanofiber hydrogel (weight fraction is 2.2 wt%, which is 1414 para-aramid nanofibers synthesized from terephthaloyl chloride and p-phenylenediamine; the diameter of aramid nanofibers is 20 - 40 nm, and the aspect ratio is 1000 - 3000; the nanofiber aramid hydrogel exists in the form of water-containing nanofiber aramid, where the ratio of aramid nanofibers to water is 0.6 - 2.0:98.0 - 99.4) into a container, then add 150 ml of methyl ethyl ketone, and make it uniform by mechanical stirring. The stirring speed is 150 rpm - 350 rpm, and the stirring time is 20 min to obtain a uniform and stable aramid nanofiber suspension.

[0065] Add 4 ml of ammonia water and 1 ml of trimethoxysilane to the aramid nanofiber suspension, stir at room temperature. The stirring speed is 150 rpm - 350 rpm, and the stirring time is 2 h. Then, perform vacuum filtration on the reacted slurry and wash it repeatedly with deionized water until the solution is neutral to obtain the modified aramid nanofiber hydrogel 5.

[0066] Take an appropriate amount of the modified aramid nanofiber hydrogel 5 and redisperse it in deionized water, then spread the dispersion on a culture dish to obtain the modified aramid nanofiber gel film. First, dry the modified aramid nanofiber gel film in a forced-air oven at 55 °C for 4 hours to finally obtain the secondary battery composite separator 5 based on the composite of aramid nanofibers and silica.

[0067] Apply the secondary battery composite separator 5 based on the composite of aramid nanofibers and silica to a lithium battery and test its electrochemical performance in the same way as in Example 1. Under 0.5C discharge, there is no attenuation after 200 cycles, and the charge-discharge specific capacity is about 3 - 8 mAh / g higher than that of the commercial polypropylene separator. The test of the discharge specific capacity at different rates shows that after charge-discharge cycling at rates of 0.2C, 0.5C, 1C, 2C, and 3C, when cycling at 0.2C again, the discharge specific capacity remains unchanged.

[0068] Comparative Example 1

[0069] This comparative example is a commercial polypropylene separator (Celgard 2500).

[0070] Perform a thermal stability test on the commercial polypropylene separator, and the results are as Figure 2 (Celgard) shown. The Celgard separator starts to curl when heated to 50 °C and maintained for 0.5 h, and when the temperature rises to 120 °C, softening and shrinking phenomena occur.

[0071] Perform an electrolyte wettability test on the commercial polypropylene separator, and the results are as Figure 3As shown, the contact angle of the commercial polypropylene separator (Celgard 2500) is 45.4°, which is higher than that of the original aramid nanofibers and the secondary battery composite separator based on the composite of aramid nanofibers and silica.

[0072] Apply the commercial polypropylene separator (Celgard 2500) to the lithium battery and test its performance. The results are as Figure 4 shown. The discharge specific capacity tests at different rates show that the discharge specific capacity is low at different rates, and the discharge specific capacity drops severely at 3C, with poor electrochemical stability. Under 0.5C discharge, there is no attenuation after 200 cycles, and the discharge specific capacity is about 140 - 146 mAh / g, which is about 10 - 15 mAh / g lower than that of the secondary battery composite separator of the original aramid nanofibers and the composite of aramid nanofibers and silica.

[0073] Comparative Example 2

[0074] Add 18.52 g of aramid nanofiber hydrogel (weight fraction is 2.7 wt%, which is 1414 para-aramid nanofibers synthesized from terephthaloyl chloride and p-phenylenediamine monomers; the diameter of aramid nanofibers is 30 - 60 nm, and the aspect ratio is 1500 - 4000; the aramid nanofiber hydrogel exists in the form of hydrated nano-aramid fibers, and the proportion of aramid nanofibers to water is 1.0 - 3.0:97 - 99) to a container, add 200 ml of ethanol, stir evenly with ethanol as the dispersant, the stirring speed is 300 rpm - 500 rpm, and the stirring time is 30 min to obtain a uniform and stable aramid nanofiber suspension.

[0075] Spread the aramid nanofiber suspension on a petri dish to obtain an aramid nanofiber gel film. After drying, finally obtain the original aramid nanofiber battery separator 1.

[0076] Observe the differences before and after modification of the original aramid nanofiber battery separator 1 using a scanning electron microscope. The results are as Figure 1 shown in a. The original aramid nanofiber battery separator presents a continuous and intertwined structure, forming a dense and tightly interlaced pore arrangement. This dense structure will hinder ion transport, resulting in a relatively high interfacial resistance.

[0077] Conduct a thermal stability test on the original aramid nanofiber battery separator 1. The results are as Figure 2 (ANFs) shown. Even when the separator is heated to 200 °C, there is no dimensional change.

[0078] Conduct an electrolyte wettability test on the original aramid nanofiber battery separator 1. The results are as Figure 3 shown. The contact angle of the original aramid nanofiber battery separator is 6.6°.

[0079] The original aramid nanofiber separator 1 was applied to a lithium battery and its electrochemical performance was tested in the manner of Example 1. The results are as follows. Figure 4 As shown, at 0.5C discharge, there was no attenuation after 200 cycles, and the discharge specific capacity was about 153 mAh / g, which was between that of commercial polypropylene and the secondary battery composite separator based on the composite of aramid nanofibers and silica. The discharge specific capacity tests at different rates showed that after charge-discharge cycling at rates of 0.2C, 0.5C, 1C, 2C, and 3C, when cycling at 0.2C again, the discharge specific capacity remained unchanged, but the discharge specific capacities at different rates were all lower than those of the secondary battery composite separator based on the composite of aramid nanofibers and silica.

[0080] In summary, multi-faceted performance studies were carried out on the original aramid nanofiber separator and the secondary battery composite separator based on the composite of aramid nanofibers and silica. By characterizing its surface morphology through scanning electron microscopy, the original aramid nanofiber separator showed a continuous entangled and dense pore structure, which hindered ion transport and led to a high interfacial resistance; for the secondary battery composite separator based on the composite of aramid nanofibers and silica, due to the in-situ synthesized SiO2 nanoparticles as cross-linking points, a three-dimensional structure was formed, presenting the characteristics of uniform dispersion of particles, finer fibers, and obvious micropores.

[0081] In terms of thermal stability, the thermal dimensional stability of the separator within the range of 25 to 200°C was evaluated by thermal shrinkage measurement. The Celgard separator curled after heating at 50°C for 0.5 hours and softened and shrank at 120°C. However, for the original aramid nanofiber separator and the secondary battery composite separator based on the composite of aramid nanofibers and silica, due to the heat resistance of aramid fibers and the uniform distribution of SiO2 nanoparticles, there was no dimensional change even when heated to 200°C.

[0082] The electrolyte wettability was measured by the contact angle. The contact angles of commercial polypropylene, the original aramid nanofibers, and the secondary battery composite separator based on the composite of aramid nanofibers and silica were 45.4°, 6.6°, and 8.2° respectively. Although the SiO2 particles slightly reduced the wettability of the modified membrane, the secondary battery composite separator based on the composite of aramid nanofibers and silica still significantly improved the wettability compared to Celgard, and was expected to provide excellent electrochemical performance.

[0083] The secondary battery composite separator based on the composite of aramid nanofibers and silica was used to assemble a LiFePO4 / Li battery for testing. Its cycle stability was excellent, with no attenuation after 200 cycles of 0.5C discharge. The charge-discharge specific capacity was 3 to 8 mAh / g higher than that of the commercial polypropylene separator. When cycling at different rates and then cycling at 0.2C again, the discharge specific capacity remained unchanged, and it had high electrochemical stability, which could cooperate with conventional cathode materials to improve the practicality of the battery. The overall tests verified the effectiveness and superiority of the separator of the present invention in terms of cycle performance, rate performance, and inhibition of lithium dendrite growth.

[0084] It is understood that the present invention is described by way of some embodiments, and those skilled in the art will know that various changes or equivalent replacements can be made to these features and embodiments without departing from the spirit and scope of the present invention. Additionally, under the teaching of the present invention, these features and embodiments can be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application belong to the scope protected by the present invention.

Claims

1. A secondary battery composite separator based on the composite of aramid nanofibers and silica, characterized in that, The secondary battery composite separator is a composite material of a three-dimensional network structure of aramid nanofibers with silica nanoparticles as cross-linking nodes, and one or more aramid nanofibers pass through a single silica nanoparticle.

2. The secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 1, wherein The aramid nanofibers include one or more of meta-aramid nanofibers without other comonomers, para-aramid nanofibers without other comonomers, meta-aramid nanofibers with other comonomers, and para-aramid nanofibers with other comonomers.

3. The secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 2, wherein The thickness of the secondary battery composite separator is 5 to 200 μm.

4. The secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 2, characterized in that, The diameter of the aramid nanofibers is 10 to 100 nm.

5. A method for preparing a secondary battery composite separator based on the composite of aramid nanofibers and silica according to any one of claims 1 to 4, characterized in that, It includes the following steps: S1. Add a water-soluble solvent to the aramid nanofiber hydrogel, stir, and make an aramid nanofiber suspension; S2. Add an alkoxysilane compound and an alkaline regulator to the aramid nanofiber suspension, and adjust the pH of the aramid nanofiber suspension to be alkaline; S3. After stirring and reacting for a period of time, form an aramid nanofiber slurry, perform vacuum filtration on the reacted aramid nanofiber slurry, and wash it with deionized water until neutral to obtain a modified aramid nanofiber hydrogel; S4. Redisperse the modified aramid nanofiber hydrogel in deionized water again, and obtain a modified aramid nanofiber gel film through filtration; S5. Dry the modified aramid nanofiber gel film to remove the moisture therein to obtain the secondary battery composite separator based on the composite of aramid nanofibers and silica.

6. The preparation method of a secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 5, characterized in that, The water-soluble solvent includes one or more of water, methanol, ethanol, isopropanol, n-butanol, acetone, methyl ethyl ketone, tetrahydrofuran, dimethyl sulfoxide, and N, N-dimethylformamide.

7. The preparation method of a secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 5, characterized in that, The alkoxysilane compounds include one or more of tetraethoxysilane, trimethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane.

8. The preparation method of a secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 5, characterized in that, The alkaline regulators include one or more of aqueous ammonia solution, sodium hydroxide, potassium hydroxide, and triethylamine.

9. The preparation method of a secondary battery composite separator based on the composite of aramid nanofibers and silica according to claim 5, characterized in that, In steps S1 and S3, the stirring speed is 100 to 1000 rpm. In step S1, the stirring time is 20 to 45 min. In step S3, the stirring time is 2 to 4.5 h.

10. A secondary battery, characterized in that, It includes the composite separator prepared by the preparation method of the secondary battery composite separator based on the composite of aramid nanofibers and silica described in any one of claims 1 to 4 or the secondary battery composite separator based on the composite of aramid nanofibers and silica described in any one of claims 5 to 9.

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