Ultrathin fiber-coated diaphragm with high heat resistance and high electrolyte infiltration and preparation method thereof

Abstract

The invention relates to the field of lithium ion batteries, and particularly discloses an ultrathin high-heat-resistance and high-electrolyte-infiltration fiber-coated diaphragm and a preparation method of the ultrathin high-heat-resistance and high-electrolyte-infiltration fiber-coated diaphragm. An ultrathin fiber-coated diaphragm with high heat resistance and high electrolyte infiltration comprises a base membrane and a coating located on one side or two sides of the base membrane, the coating is formed by curing slurry, and the slurry comprises the following raw materials in parts by weight: 5-30 parts of a binder, 10-100 parts of an inorganic ceramic material, 1-100 parts of nanofibers, 0.1-5 parts of an auxiliary agent, 100-300 parts of a solvent and 0.1-5 parts of a dispersant; the glass transition temperature of the binder is higher than 150 DEG C; the binder is selected from at least one of a modified polyacrylic acid binder, a modified acrylamide binder, a modified polyacrylonitrile binder, a modified styrene butadiene rubber binder and a modified polyvinyl alcohol binder. The fiber-coated diaphragm provided by the invention has high heat resistance and high electrolyte wettability.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries. More specifically, it relates to an ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have been widely used in modern society due to their high specific energy, long cycle life, no memory effect, safety and reliability, and fast charging and discharging, especially in the fields of electric vehicles, 3C digital products, etc. However, the safety issue of batteries has always been one of the key factors restricting their development. As a key component to prevent short circuit between the positive and negative electrodes, the performance of the lithium-ion battery separator directly affects the safety and stability of the battery.

[0003] Currently, the commercially used lithium-ion battery separators are generally polyethylene or polypropylene separators, which have good acid and alkali resistance, high tensile strength, high porosity and other properties. However, the current separators have poor thermal stability, with a large shrinkage above 150°C, and are prone to membrane rupture during battery thermal runaway, resulting in internal short circuit and increasing the risk of thermal runaway. At the same time, the electrolyte wettability is poor, the specific surface area is low, and the liquid absorption and liquid retention ability is poor, which affects the charge and discharge performance and cycle life of the battery, etc., limiting its use in high energy density and high power applications.

[0004] To improve the heat resistance of the separator at high temperatures, the mainstream solution on the market is to coat a ceramic heat-resistant coating with a single-layer thickness of 2-4 μm on one or both sides of the separator, which can effectively reduce the thermal shrinkage of the separator and prevent fire and explosion caused by short circuit between the positive and negative electrodes at high battery temperatures. At the same time, the ceramic coating can also improve the wettability of the separator, prevent internal short circuit caused by lithium dendrite piercing the separator, and improve the safety performance of lithium-ion batteries. The current commercially used lithium-ion battery coated separators have a PE film or PP film as the substrate, and the coating material is generally alumina or boehmite. However, the heat-resistant temperature of the coated ceramic separator can generally only reach 130°C, unable to meet higher safety requirements, and it is difficult to meet the application requirements in battery systems with high electrolyte wettability requirements, such as the electrolyte wettability of 4680 large cylindrical batteries.

[0005] In view of the above related technologies, the inventors found that there is an urgent need to develop a separator with high heat-resistant temperature and good electrolyte wettability. Summary of the Invention

[0006] To enhance the heat resistance and electrolyte wettability of the separator, this application provides an ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability and a preparation method thereof.

[0007] In a first aspect, this application provides an ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability, adopting the following technical solution: An ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability, comprising a base film and coatings located on one or both sides of the base film. The coatings are formed by curing a slurry, and the slurry contains the following raw materials in parts by weight: 5-30 parts of a binder, 10-100 parts of an inorganic ceramic material, 1-100 parts of nanofibers, 0.1-5 parts of an auxiliary agent, 100-300 parts of a solvent, and 0.1-5 parts of a dispersant; The glass transition temperature of the binder > 150 °C; The binder is selected from at least one of modified polyacrylic acid binders, modified acrylamide binders, modified polyacrylonitrile binders, modified styrene-butadiene rubber binders, and modified polyvinyl alcohol binders.

[0008] By adopting the above technical solution, coatings are formed on at least one side of the base film, which can improve the heat resistance of the base film. Moreover, by using a binder with a glass transition temperature greater than 150 °C, its structural stability can be better maintained at high temperatures, it is not prone to deformation or degradation, and its dimensional stability at high temperatures is better. Moreover, the binder with a high glass transition temperature can form a denser and more stable three-dimensional network structure heat-resistant framework, which can improve the safety and stability of the battery; adding an inorganic ceramic material and nanofibers to the slurry, due to the high aspect ratio characteristics of the nanofibers, a three-dimensional network of the coating is constructed, enhancing the structural strength of the separator, making it less likely to deform or break at high temperatures, preventing the separator from collapsing at high temperatures, and the nanofibers can provide more active sites for the movement of lithium ions in the electrolyte, helping to improve the wettability of the electrolyte and the ion conduction efficiency; the inorganic ceramic material has high thermal stability, which can effectively prevent the separator from shrinking and melting at high temperatures, and these inorganic ceramic materials are distributed in the three-dimensional structure formed by the binder and nanofibers, forming a specific rigid skeleton, enhancing the thermal stability of the separator. Moreover, the thermal conductivity of the ceramic material is relatively low, which can prevent the expansion of some thermal runaway points in the battery to form an overall thermal runaway, improving the safety of the battery. Moreover, the hydrophilic groups on the surface of the inorganic ceramic material, such as -OH, can improve the wettability of the separator to the electrolyte, helping to improve the ion conduction efficiency of the battery and the service performance of the battery.

[0009] Optionally, the mass ratio of the inorganic ceramic material to the nanofibers is (1-10):(1-10).

[0010] By adopting the above technical solution, precisely controlling the weight ratio of the inorganic ceramic material to the nanofibers can further optimize the thermal stability of the separator, enabling it to still maintain good performance in a high-temperature environment, effectively preventing safety problems such as shrinkage, melting, or explosion of the battery under thermal runaway conditions, enhancing the safety of the separator, improving the wettability of the separator to the electrolyte, increasing the ion conduction efficiency of the battery, enhancing the energy density of the battery, improving the chemical resistance and mechanical strength of the separator, and extending the service life.

[0011] Optionally, the dosage of the binder is 10-40% of the total mass of the inorganic ceramic material and the nanofibers.

[0012] By adopting the above technical solution, controlling the ratio of the dosage of the binder to the total amount of the inorganic ceramic material and the nanofibers, adjusting the viscosity and fluidity of the slurry, enabling the slurry to be evenly coated on the surface of the separator, avoiding uneven thickness or defects, and with an appropriate dosage of the binder, the adhesion between the base film and the coating can be enhanced, preventing the coating from peeling off or shifting during the charge and discharge process of the battery, prolonging the cycle life of the battery, improving the stability and reliability of the battery. In addition, it can also improve the mechanical properties of the coating to a certain extent, avoid unnecessary material waste, reduce the coating thickness of the separator, contribute to enhancing the energy density of the battery, and make the battery more efficient.

[0013] Optionally, the preparation method of the inorganic ceramic material is as follows: at least one of α-aluminum oxide, boehmite, silicon dioxide, cerium dioxide, spinel, zirconium dioxide, magnesium dioxide, barium oxide, titanium dioxide and lithium titanium aluminum phosphate is subjected to calcination - grinding - purification to obtain an inorganic ceramic material with a specific surface area of 5-20 m 2 / g and a D50 particle size of 50-2000 nm.

[0014] By adopting the above technical solution, each of the above raw materials has a higher hardness after calcination, stronger high-temperature resistance performance, and excellent chemical stability, and can improve the high-temperature stability, wear resistance and mechanical strength of the coating, etc.

[0015] Optionally, the inorganic ceramic material is pretreated as follows: The inorganic ceramic material and halloysite nanotubes are mixed and stirred according to a mass ratio of 7-8:2-3, added to the hydrochloric acid dopamine solution, stirred evenly, then cycled to pump vacuum and left to stand at atmospheric pressure, centrifuged, washed and dried.

[0016] By adopting the above technical solutions, halloysite nanotubes have a hollow tubular structure. After being mixed with inorganic ceramic materials, the inorganic ceramic materials can fill into the framework gaps of the halloysite nanotubes, improving the dispersibility of the inorganic ceramic materials to ensure that the coating has better heat resistance. The hollow nanotube framework of the halloysite nanotubes helps to construct a good gas transmission channel, improving the air permeability of the separator. Moreover, the positively charged outer surface and negatively charged inner surface of the halloysite nanotubes are conducive to promoting the electrolysis of lithium salts, thereby increasing the ion transference number, making the battery have good cycle stability and high-temperature safety. The in-situ polymerization of dopamine functionalizes the halloysite nanotubes and inorganic ceramic materials. Due to the coexistence of catechol and amine functional groups, dopamine can be tightly adsorbed on the inner and outer surfaces of the halloysite nanotubes and inorganic ceramic materials through hydrogen bonding, and form a dense polydopamine layer through its own polymerization. The polar functional groups such as NH2, C=O, -OH, etc. in the polydopamine layer have polar interactions with lithium ions, which can promote the rapid diffusion of lithium ions and uniform lithium ion flux, improving the ionic conductivity and ion transference number of the separator. Moreover, polydopamine has strong adhesiveness and hydrophilicity, which can help the inorganic ceramic materials better bind to the base film. In addition, its adhesiveness and hydrophilicity can reduce the mutual attraction between inorganic ceramic filler particles, reducing the agglomeration of inorganic ceramic materials and making them easier to disperse in the slurry. Polydopamine interacts with the binder, such as hydrogen bonding, electrostatic interaction, etc., thereby enhancing the binding force between the inorganic ceramic materials and the binder, improving the dispersion ability, improving the compatibility, enhancing the adhesion between the inorganic ceramic materials and the base film, and reducing the powder falling phenomenon of the ceramic materials.

[0017] Optionally, the nanofibers are selected from at least one of polyamide fibers, cellulose acetate, aramid fibers, polyimide fibers, and inorganic nanofibrous materials.

[0018] By adopting the above technical solutions, polyamide fibers have high tensile strength and elasticity, can withstand large external forces, improving the tensile resistance of the separator. Moreover, they have good heat resistance and can maintain the stability of the separator at high temperatures. Cellulose acetate has good film-forming properties and can form a dense and flat film layer in the coating, improving the flexibility and crack resistance of the coating. Aramid fibers have extremely high strength, excellent wear resistance, can maintain stability at high temperatures, are not easy to deform, and have better wettability with the electrolyte, which can accelerate the battery charging speed. Polyimide fibers have high thermal stability and high mechanical strength, and are easy to form a uniform coating. Inorganic nanofibers have high strength and stiffness, strong high-temperature stability, and good heat insulation, which can reduce heat conduction.

[0019] Optionally, the nanofibers are pretreated as follows: The nanofibers are immersed in a titanyl sulfate solution for 20 - 30 min, filtered and then immersed in deionized water for 20 - 30 min, and heated at 350 - 380 °C for 1.5 - 2 h to obtain modified nanofibers; The modified nanofibers are placed in a polyimide sol, aged, subjected to solvent exchange with absolute ethanol, and then supercritically dried with carbon dioxide.

[0020] By adopting the above technical solution, after the nanofibers are impregnated with the titanyl sulfate solution and then with deionized water, the titanyl sulfate undergoes self-hydrolysis. After heating, the titanyl sulfate is transformed into titanium dioxide. During the hydrolysis process, due to the large polarity of the Ti - O bond, the hydroxyl groups formed by the polarization of water molecules during the immersion process will coat the surface of the nanofibers. And the hydroxyl groups contained in the nanofibers can form hydrogen bonds with the hydroxyl groups on TiO(OH)₂, increasing the active sites on the surface of the nanofibers, thereby increasing the interfacial adhesion performance between TiO(OH)₂ and TiO₂. The titanium dioxide containing high-polarity Ti - O bonds can increase the surface polarity of the separator, improve the wettability of the separator, reduce the interfacial resistance, enhance the enrichment of lithium ions, and thus promote the passage of lithium ions through the separator, improve the interfacial compatibility between the separator and the electrode material, and improve the electrochemical stability of the separator. Depositing titanium dioxide on the surface of the nanofibers can increase the surface roughness of the nanofibers and the interfacial adhesiveness between the nanofibers and the PI sol. After the PI sol is supercritically dried, a PI aerogel is formed. The porous structure of the PI aerogel enables it to tightly coat the surface of the nanofibers, forming an effective reinforcing layer, improving the rigidity and strength of the nanofibers, being able to better resist deformation and fracture. The PI molecules have good thermal stability and heat insulation performance, can form a heat insulation barrier, reduce the heat transfer, improve the heat resistance stability of the nanofibers, reduce the high-temperature shrinkage, and moreover, the PI aerogel can increase the surface porosity of the nanofibers and improve the liquid absorption rate of the electrolyte.

[0021] Optionally, the dosage of the auxiliary agent is 0.06 - 3% of the mass of the solvent.

[0022] By adopting the above technical solution, by controlling the dosage of the auxiliary agent, the viscosity of the slurry can be adjusted, its bonding effect can be improved, and the quality of the slurry can be enhanced.

[0023] Optionally, the thickness of the single-sided coating is 0.5 - 2 μm.

[0024] By adopting the above technical solution, forming a coating with a moderate thickness on one or both sides of the base film is beneficial to the conduction of lithium ions, improves the lithium ion conduction efficiency, and at the same time improves the mechanical strength of the separator, enabling it to better withstand the volume change during the charge and discharge process of the battery, being beneficial to improving the cycle stability and safety of the battery. In addition, an appropriate thickness can also promote the uniform distribution and penetration of the electrolyte in the separator, improving the voltage consistency and cycle stability.

[0025] Optionally, the thickness of the base film is 3-20 μm.

[0026] By adopting the above technical solution, an appropriate thickness of the base film is not likely to cause thermal runaway at high temperatures. Meanwhile, it has a short heat conduction path, maintaining the stability of the internal structure and performance of the battery. Moreover, it has a better ability to absorb and retain the electrolyte, facilitating the infiltration and diffusion of the electrolyte, shortening the path of ion migration, improving the ion transport efficiency, and enhancing the charge and discharge performance of the battery.

[0027] Optionally, the auxiliary agent is selected from at least one of sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, cetyltrimethylammonium bromide, fatty alcohol polyoxyethylene ether, polydimethylsiloxane, polyoxyethylene ether, sorbitan fatty acid ester, sodium diisooctyl sulfosuccinate, sorbitan monooleate, sodium dioctyl sulfosuccinate, sodium lauryl sulfate, and alkylphenol polyoxyethylene ether.

[0028] By adopting the above technical solution, the above-mentioned auxiliary agents can improve the leveling property, smoothness, and uniformity of the coating, increase the viscosity of the slurry, eliminate the air bubbles in the slurry, and make the coating adhere more tightly to the base film.

[0029] Optionally, the solvent is selected from at least one of water, ethanol, isopropanol, acetone, and N-methylpyrrolidone.

[0030] Optionally, the dispersant is selected from at least one of sodium polyacrylate, ammonium polyacrylate, carboxylate, and sulfonate.

[0031] By adopting the above technical solution, dispersants such as sodium polyacrylate can effectively prevent the aggregation of particles such as inorganic ceramic materials and nanofibers, keep these particles evenly distributed in the slurry, and ensure the stability of the slurry over a long period.

[0032] In a second aspect, the present application provides a method for preparing an ultra-thin, highly heat-resistant, and highly electrolyte-infiltrating fiber-coated separator, adopting the following technical solution: A method for preparing an ultra-thin, highly heat-resistant, and highly electrolyte-infiltrating fiber-coated separator, comprising the following steps: Preparing the slurry: adding a dispersant, nanofibers, and inorganic ceramic materials to a solvent in sequence, followed by dispersion and sanding, and then adding a binder and an auxiliary agent. After uniform dispersion, the slurry is obtained. Coating: uniformly coating the slurry on one or both sides of the base film and drying to obtain the fiber-coated separator.

[0033] By adopting the above technical solution, a coated separator is obtained by coating and drying on one or both sides of the base film. The separator is not likely to rupture during battery thermal runaway, improving the safety of battery use. Moreover, it has good wettability to the electrolyte, strong liquid absorption and retention ability, enhancing the charge and discharge performance and cycle service life of the battery.

[0034] Optionally, the coating method is one of manual scraping, gravure roll coating, slot extrusion coating, dip coating, and spraying.

[0035] In summary, the present application has the following beneficial effects: 1. Since the present application uses a binder with a high glass transition temperature and is combined with raw materials such as inorganic ceramic materials and nanofibers to form an ultra-thin coating on the surface of the ultra-thin base film, an ultra-thin separator is thus prepared, which can reduce the weight of the battery, improve the energy density, and has high heat resistance and high electrolyte wettability, ensuring the stability and safety of the battery in a high-temperature environment, helping to reduce the internal resistance of the battery, improve the charge and discharge efficiency, improve the wetting speed after injection, and improve the battery production efficiency.

[0036] 2. In the present application, a base film with a thickness of 3 - 20 μm is preferably used, and the unilateral coating thickness is 0.5 - 2 μm. The ultra-thin base film is paired with an ultra-thin coating to reduce the thickness of the separator and improve the battery energy density.

[0037] 3. In the present application, halloysite nanotubes and dopamine solution are preferably used to pretreat the inorganic ceramic materials, which can improve the dispersibility of the inorganic ceramic materials and the interfacial bonding force with the slurry, improve the bonding force between the coating and the separator, and at the same time enhance the high-temperature stability of the separator and the wettability to the electrolyte; in addition, titanium dioxide and polyimide aerogel are also used to treat the nanofibers, which improves the heat resistance of the nanofibers, increases the surface roughness of the nanofibers at the same time, improves the interfacial bonding strength, and enhances the wettability to the electrolyte. Description of the Drawings

[0038] Figure 1 It is the electron scanning electron microscope image of the coating in the fiber-coated separator prepared in Example 1.

[0039] Figure 2 It is Figure 1 The enlarged view of the electron scanning electron microscope image of the middle coating. Detailed Description of the Embodiments

[0040] The following embodiments further illustrate the present application in detail. Embodiment

[0041] Example 1: A super-thin, highly heat-resistant, and highly electrolyte-wettable fiber-coated separator, comprising a base film and a coating layer on one side of the base film. The base film is a PP base film with a thickness of 7.1 μm, and the coating layer has a thickness of 2 μm. The coating layer is formed by curing a slurry, and the raw material dosage of the slurry is shown in Table 1. Among them, the binder is a modified polyacrylic acid binder, selected from Gaorui GR-506, with a glass transition temperature of 170 °C. The inorganic ceramic material is prepared by mixing α-aluminum oxide and silicon dioxide in a mass ratio of 1:1, heating to 1200 °C at a rate of 5 °C / min, holding for 2 h, and then grinding. The specific surface area of the inorganic ceramic material is 20 m 2 / g, the D50 particle size is 200 nm, the nanofibers are polyamide fibers with a length of 80 nm and a diameter of 12 nm, the auxiliary agent is sodium dodecylbenzenesulfonate, the solvent is water, and the dispersant is sodium polyacrylate.

[0042] The preparation method of the above-mentioned super-thin, highly heat-resistant, and highly electrolyte-wettable fiber-coated separator includes the following steps: Preparing the slurry: Add the dispersant, nanofibers, and inorganic ceramic materials to the solvent in sequence, then perform dispersion and sanding, and then add the binder and auxiliary agent. After uniform dispersion, the slurry is obtained; Coating: Coat the slurry on the polyethylene base film in a roll-to-roll manner using a gravure roll coater, with single-sided coating. The coating speed is 20 m / min, the oven temperature is 55 °C, and the oven length is 10 min. After drying, the fiber-coated separator is obtained.

[0043] Examples 2 - 6: A super-thin, highly heat-resistant, and highly electrolyte-wettable fiber-coated separator, which is different from Example 1 in that the raw material dosage of the slurry is shown in Table 1.

[0044] Table 1 Raw material dosage of the slurry in Examples 1 - 6 Example 7: A super-thin, highly heat-resistant, and highly electrolyte-wettable fiber-coated separator, which is different from Example 1 in that the slurry is manually scrape-coated on both sides of the polyethylene base film. The polyethylene base film has a thickness of 5.1 μm, and the single-sided coating thickness is 1.25 μm.

[0045] Example 8: A super-thin, highly heat-resistant, and highly electrolyte-wettable fiber-coated separator, which is different from Example 1 in that the slurry is manually scrape-coated on both sides of the polyethylene base film. The thickness of the polyethylene base film is 5.2 μm, and the single-sided coating thickness is 0.65 μm.

[0046] Example 9: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 1 in that the binder is a modified acrylamide binder, selected from Gaorui GR-508A, and its glass transition temperature is 184 °C.

[0047] Example 10: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 1 in that the binder is a modified acrylamide binder, selected from Gaorui GR-508BII, its glass transition temperature is 206 °C, and the solid content is 20%.

[0048] Example 11: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 1 in that the binder is a modified polyacrylonitrile binder, selected from Zhuhai Chenyu CYJ-18.

[0049] Example 12: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 8 in that the inorganic ceramic material is pretreated as follows: Mix and stir the inorganic ceramic material and halloysite nanotubes in a mass ratio of 7:3, add them to a 2 mg / ml dopamine hydrochloride solution (prepared by dissolving dopamine hydrochloride in a 10 mM, pH = 8.5 Tris-HCl buffer solution), stir evenly, evacuate and let stand for 30 min, then return to atmospheric pressure and let stand for 30 min, cycle the evacuation and atmospheric pressure standing 3 times, centrifuge, wash and dry. The total amount of the inorganic ceramic material and halloysite nanotubes is 2.5 wt% of the mass of the dopamine hydrochloride solution.

[0050] Example 13: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 8 in that the inorganic ceramic material is pretreated as follows: Mix and stir the inorganic ceramic material and halloysite nanotubes in a mass ratio of 8:2, add them to a 2 mg / ml dopamine hydrochloride solution (prepared by dissolving dopamine hydrochloride in a 10 mM, pH = 8.5 Tris-HCl buffer solution), stir evenly, evacuate and let stand for 30 min, then return to atmospheric pressure and let stand for 30 min, cycle the evacuation and atmospheric pressure standing 3 times, centrifuge, wash and dry. The total amount of the inorganic ceramic material and halloysite nanotubes is 3 wt% of the mass of the dopamine hydrochloride solution.

[0051] Example 14: A super-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator, which is different from Example 13 in that the inorganic ceramic material is pretreated as follows: Mix and stir the inorganic ceramic material and halloysite nanotubes in a mass ratio of 8:2 evenly.

[0052] Example 15: A fiber-coated separator with ultra-thin, high heat resistance and high electrolyte wettability, which is different from Example 13 in that the inorganic ceramic material is pretreated as follows: The inorganic ceramic material was added to a 2 mg / ml dopamine hydrochloride solution (prepared by dissolving dopamine hydrochloride in a 10 mM, pH = 8.5 Tris-HCl buffer solution). After stirring evenly, it was evacuated and left standing for 30 min, then returned to atmospheric pressure and left standing for 30 min. The evacuation and standing at atmospheric pressure were cycled 3 times, followed by centrifugation, washing and drying. The amount of the inorganic ceramic material was 2.5 wt% of the mass of the dopamine hydrochloride solution.

[0053] Example 16: A fiber-coated separator with ultra-thin, high heat resistance and high electrolyte wettability, which is different from Example 12 in that the nanofibers are also pretreated as follows: The nanofibers were placed in a 0.3 mol / l titanyl sulfate solution and soaked for 30 min. Then, the solution escaping from the surface was absorbed with filter paper, and then the nanofibers were soaked in deionized water for 30 min to allow the titanyl sulfate solution on the fiber surface to hydrolyze fully. Subsequently, they were heated at 350 °C for 2 h to obtain modified nanofibers; The modified nanofibers were placed in a polyimide sol twice their mass. After aging for 30 s, they were washed 4 times with absolute ethanol for solvent exchange, with each exchange time being 12 h. Then, supercritical carbon dioxide drying was carried out at a reaction temperature of 55 °C and a pressure of 12 MPa. The preparation method of the polyimide sol: 2 g of 4,4'-diaminodiphenyl ether and 55.8 g of N,N-dimethylacetamide were mixed, and 2.2 g of pyromellitic dianhydride was added in 4 batches. Under ice-water bath conditions and N2 protection, continuous stirring was carried out to allow the polymerization to occur fully, obtaining a PAA solution; 1 g of acetic anhydride and 2 g of triethylamine were added to the PAA solution, and stirring was carried out for 30 min to complete chemical imidization, obtaining the polyimide sol.

[0054] Example 17: A fiber-coated separator with ultra-thin, high heat resistance and high electrolyte wettability, which is different from Example 16 in that the nanofibers are also pretreated as follows: The nanofibers were placed in a 0.3 mol / l titanyl sulfate solution and soaked for 30 min. Then, the solution escaping from the surface was absorbed with filter paper, and then the nanofibers were soaked in deionized water for 20 min to allow the titanyl sulfate solution on the fiber surface to hydrolyze fully. Subsequently, they were heated at 380 °C for 1 h.

[0055] Example 18: A fiber-coated separator with ultra-thin, high heat resistance and high electrolyte wettability, which is different from Example 16 in that the nanofibers are also pretreated as follows: The nanofibers were placed in a polyimide sol that was twice its mass. After aging for 30 s, they were washed 4 times with absolute ethanol for solvent exchange, with each exchange time being 12 h. Then, supercritical carbon dioxide drying was carried out at a reaction temperature of 55 °C and a pressure of 12 MPa. The preparation method of the polyimide sol: 2 g of 4,4-diaminodiphenyl ether and 55.8 g of N,N-dimethylacetamide were mixed, and 2.2 g of pyromellitic dianhydride was added in 4 batches. Under the condition of an ice-water bath and N2 protection, continuous stirring was carried out to fully polymerize to obtain a PAA solution; 1 g of acetic anhydride and 2 g of triethylamine were added to the PAA solution, and stirring was carried out for 30 min to complete chemical imidization to obtain the polyimide sol.

[0056] Comparative example Comparative example 1: An ultra-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator. The difference from Example 1 is that the binder uses an acrylate polymer with a glass transition temperature less than 150 °C, selected from Sichuan Yindi Le Material Technology, model ABE-5. A polyethylene film with a thickness of 9.1 μm was used, and a slurry was coated on one side of the polyethylene film by a gravure roll to form a coating with a thickness of 3 μm.

[0057] Comparative example 2: An ultra-thin, highly heat-resistant and highly electrolyte-wetting fiber-coated separator. The difference from Example 1 is that no fiber material was added to the slurry.

[0058] Performance detection test The fiber-coated separators were prepared according to the methods in the examples and comparative examples, and the performance was detected with reference to the following methods. The test results were recorded in Table 2.

[0059] 1. Thermal shrinkage rate: The coated separator samples of each group were cut into separator specimens of 100*100 mm, and marked by drawing lines in the middle of the samples in the transverse direction (TD) and the longitudinal direction (MD). The separator specimens were placed between two A3 papers and put into a drying oven. The oven temperature was set at 130 °C or 180 °C. After drying for 1 h, the separator specimens were taken out, and the longitudinal and transverse dimensions were measured using an image measuring instrument. The heat resistance of the coated separator was judged by the shrinkage rate. The calculation formula of the shrinkage rate is as follows: shrinkage rate / % = (length before drying - length after drying) / length before drying × 100%. 5 specimens were tested in each group, and the test results were averaged.

[0060] 2. Peel Strength: Cut a specimen with a width of 30 mm and a length of 200 mm along the machine direction (MD) of the diaphragm; adhere a 20-mm-wide 3M tape to a steel plate, tear off the release paper, and then adhere the coated surface of the prepared diaphragm specimen to the 3M tape (parallel and completely covering). Press the bonding surface firmly with a roller under uniform pressure; place the prepared steel plate on the fixture of the testing machine and test the 180° peel force between the coating and the diaphragm. The peel speed is 100 mm / min, and the peel strength = peel force (N) / 0.020 m. Test 5 specimens in each group, and take the average value of the test results.

[0061] 3. Air Permeability Value: The air permeability of the diaphragm refers to the time required for a certain volume of air to pass through the diaphragm placed in an air permeability detector under certain pressure and membrane area. The result of the air permeability performance is usually expressed by the Gurley value. The smaller the Gurley value, the shorter the time for gas to pass through, and the better the air permeability of the diaphragm. During testing, use the Wangyan air permeability meter 0518-P of Kumagai Riki Kogyo Co., Ltd., Japan for detection. Test 5 specimens in each group, and take the average value of the test results.

[0062] 4. Electrolyte Absorption Rate: Weigh the mass of the non-wetted diaphragm and record it as M1. Immerse the diaphragm completely in the electrolyte (LB-008) for 1 h, gently and quickly wipe off the excess electrolyte on the surface, and weigh the mass of the immersed diaphragm and record it as M2. Calculate the electrolyte absorption rate through the following formula: Electrolyte absorption rate = (M2 - M1) / M1 × 100%. Test 5 specimens in each group, and take the average value of the test results.

[0063] 5. Electrolyte Retention Rate: Weigh the mass of the diaphragm as M1. Immerse the diaphragm completely in the electrolyte for 1 h. After immersion, take it out and gently and quickly wipe off the excess electrolyte on the surface, and weigh the mass of the immersed diaphragm and record it as M2. Place the diaphragm absorbing the electrolyte at room temperature until the weight no longer changes, and weigh the mass of the diaphragm and record it as M3. Calculate the electrolyte retention rate through the following formula: Electrolyte retention rate = [(M2 - M1) - (M3 - M1)] / (M2 - M1) × 100%. Test 5 specimens in each group, and take the average value of the test results.

[0064] 6. Electrolyte Creeping Speed: Hang the diaphragm vertically and mark it with lines at the lower and upper parts. Add the electrolyte (containing an inert color developer) to a beaker, stir evenly to prepare a test solution. Lower the diaphragm vertically into the beaker containing the test solution until the part between the bottom end and the lower mark is immersed in the test solution. Let the diaphragm stand still and record the time required for the test solution to creep up to L2, denoted as T. After the diaphragm specimen absorbs the test solution to the saturated state, measure the length between L1 and L2, denoted as L1. Calculate the creeping speed according to the method of L / T.

[0065] Table 2 Continued Table 2 It can be seen from the data in Table 2 that in Examples 1-6, specific amounts of binder, inorganic ceramic material and nanofibers were used, and raw materials such as additives and dispersants were added. The prepared slurry was gravure-coated on one or both sides of the base film, and the formed separator had high high-temperature stability, small thermal shrinkage rate, excellent wettability to the electrolyte, high electrolyte absorption rate, and could improve the rate performance of the battery.

[0066] Compared with Example 1, in Examples 7 and 8, manual scraping was carried out on both sides of the base film, and the thickness of the base film and the coating was adjusted. It can be seen that the prepared separator had stronger high-temperature resistance, and higher electrolyte absorption rate, liquid retention rate and liquid creeping speed, and the separator had stronger wettability to the electrolyte.

[0067] Compared with Example 1, in Examples 9 and 10, two different modified polyacrylamide binders were used. The data in Table 2 showed that the separator made of the modified acrylamide binder had high-temperature resistance similar to that of Example 1 and good wettability to the electrolyte.

[0068] Compared with Example 1, in Example 11, a polyacrylonitrile binder was used. It can be seen that the separator made therefrom had high-temperature resistance and electrolyte wettability similar to those of Example 1.

[0069] Compared with Example 8, in Examples 12-13, halloysite nanotubes and dopamine solution were also used to pretreat the inorganic ceramic material. It can be seen that the separator made therefrom had a further reduced shrinkage rate at high temperature, further improved high-temperature stability, increased peel strength between the coating and the base film, and increased liquid retention rate and liquid creeping speed of the electrolyte, and could improve the lithium ion transference number, enhance the affinity between the separator and the electrolyte, and the ability of the separator to regulate ion transport.

[0070] Compared with Example 13, in Example 14, only halloysite nanotubes were used to pretreat the inorganic ceramic material, and in Example 15, only dopamine solution was used to treat the inorganic ceramic material. The data in Table 2 showed that compared with Example 13, the separator prepared in Example 14 had a slightly decreased high-temperature resistance, and the peel strength between the coating and the base film decreased significantly. The separator prepared in Example 15 had a significantly decreased high-temperature resistance, but still had a high peel strength, and at the same time, the wettability of the electrolyte became poor.

[0071] Compared with Example 12, in Example 16, the nanofibers were also pretreated with a titanyl sulfate solution and a polyimide sol. As can be seen from the data in Table 2, the separator prepared in Example 16 has better high-temperature stability and electrolyte infiltration ability than that in Example 12.

[0072] Compared with Example 16, in Example 17, only the titanyl sulfate solution was used to pretreat the nanofibers, while in Example 18, the polyimide sol was used for pretreatment. It can be seen that the high-temperature resistance of the separator prepared in Example 17 decreased, and the infiltration effect of the electrolyte decreased slightly. However, for the separator prepared in Example 18, the high-temperature resistance decreased significantly, the peel strength between the coating and the base film decreased, and the electrolyte wettability became poor.

[0073] In Comparative Example 1, a base film with a larger thickness was used, and the coating was gravure-coated on one side of the polyethylene base film with a larger thickness. The coating slurry used a polyacrylate binder with a glass transition temperature of less than 150 °C. The separator thus prepared has poor air permeability, a high areal density, an increased thermal shrinkage rate, poor high-temperature stability, and a significant decrease in both the electrolyte absorption rate and the liquid retention rate. The electrolyte wettability also becomes poor.

[0074] In Comparative Example 2, nanofibers were not added to the slurry. Compared with Example 1, the high-temperature resistance of the separator in Comparative Example 2 became worse, and the electrolyte wettability decreased significantly.

[0075] This specific embodiment is only an interpretation of the present application and does not limit the present application. After reading this specification, those skilled in the art can make modifications to this embodiment without creative contributions as needed, but as long as it is within the scope of the claims of the present application, it is protected by the patent law.

Claims

1. An ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability, characterized in that, It includes a base film and coatings on one or both sides of the base film. The coatings are formed by curing a slurry, and the slurry contains the following raw materials in parts by weight: 5 - 30 parts of a binder, 10 - 100 parts of an inorganic ceramic material, 1 - 100 parts of nanofibers, 0.1 - 5 parts of an auxiliary agent, 100 - 300 parts of a solvent, and 0.1 - 5 parts of a dispersant; The glass transition temperature of the binder > 150 °C; The binder is selected from at least one of modified polyacrylic acid binders, modified acrylamide binders, modified polyacrylonitrile binders, modified styrene - butadiene rubber binders, and modified polyvinyl alcohol binders.

2. The ultra-thin, highly heat-resistant and highly electrolyte-wettable fiber-coated separator according to claim 1, wherein: The mass ratio of the inorganic ceramic material to the nanofibers is (1 - 10):(1 - 10).

3. The ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability according to claim 2, characterized in that: The dosage of the binder is 10 - 40% of the total mass of the inorganic ceramic material and the nanofibers.

4. The ultra-thin high heat-resistant and high electrolyte-wetting fiber-coated separator according to claim 1, characterized in that: The preparation method of the inorganic ceramic material is as follows: at least one of α-aluminum oxide, boehmite, silicon dioxide, cerium dioxide, spinel, zirconium dioxide, magnesium dioxide, barium oxide, titanium dioxide and lithium titanium aluminum phosphate is calcined and ground to obtain an inorganic ceramic material with a specific surface area of 5-20 m 2 / g and a D50 particle size of 50-2000 nm.

5. The ultra-thin fiber-coated separator with high heat resistance and high electrolyte wettability according to claim 4, characterized in that: The inorganic ceramic material is pretreated as follows: Mix and stir the inorganic ceramic material and halloysite nanotubes in a mass ratio of 7 - 8:2 - 3, add them to a hydrochloric acid dopamine solution, stir evenly, then cycle through vacuum pumping and atmospheric pressure standing, and centrifuge, wash, and dry.

6. The ultra-thin high heat-resistant and high electrolyte-wetting fiber-coated separator according to claim 1, wherein: The nanofibers are selected from at least one of polyamide fibers, cellulose acetate, aramid fibers, polyimide fibers, and inorganic nanofibrous materials.

7. The ultra-thin, highly heat-resistant and highly electrolyte-wettable fiber-coated separator according to claim 6, characterized in that: The nanofibers are pretreated as follows: Soak the nanofibers in a titanyl sulfate solution, filter and soak them in deionized water, and heat at 350 - 380 °C to obtain modified nanofibers; Place the modified nanofibers in a polyimide sol, age, perform solvent exchange with absolute ethanol, and then carry out supercritical carbon dioxide drying.

8. The ultra-thin, highly heat-resistant and highly electrolyte-wettable fiber-coated separator according to claim 1, characterized in that: The thickness of the single - side coating is 0.5 - 2 μm.

9. The ultra-thin, highly heat-resistant and highly electrolyte-wettable fiber-coated separator according to claim 1, characterized in that: The auxiliary agent is selected from at least one of sodium dodecylbenzenesulfonate, alkylbenzenesulfonate, cetyltrimethylammonium bromide, fatty alcohol polyoxyethylene ether, polydimethylsiloxane, polyoxyethylene ether, sorbitan fatty acid ester, sodium diisooctyl sulfosuccinate, sorbitan monooleate, sodium dioctyl sulfosuccinate, sodium lauryl sulfate, and alkylphenol polyoxyethylene ether.

10. The preparation method of the ultra-thin, highly heat-resistant and highly electrolyte-wettable fiber-coated separator according to any one of claims 1-9, characterized in that: It includes the following steps: Prepare the slurry: Add the dispersant, nanofibers, and inorganic ceramic material to the solvent in sequence, then disperse and sand - grind, and then add the binder and the auxiliary agent, and disperse evenly to obtain the slurry; Coat: Uniformly coat the slurry on one or both sides of the base film, and dry to obtain the fiber - coated separator.

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