Battery diaphragm, preparation method thereof and battery

Abstract

The invention provides a battery diaphragm, a preparation method thereof and a battery. The battery diaphragm comprises a base material layer, a crosslinking layer crosslinked on two surfaces of the base material layer and a heat-resistant coating coated on at least one surface of the crosslinking layer, the cross-linked layer comprises a cross-linked polymer, the cross-linked polymer is obtained by polymerizing monomers, and the monomers at least comprise unsaturated bonds, phenyl groups, amino groups or imino groups and hydroxyl groups. The battery diaphragm disclosed by the invention has proper hole closing temperature, good puncture strength, heat resistance and wettability, and the peel strength of the heat-resistant coating is high, so that the battery comprising the battery diaphragm disclosed by the invention has good cycle performance and safety performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a battery separator, its preparation method, and a battery. Background Technology

[0002] In batteries, the separator plays a crucial role in separating the positive and negative electrodes to prevent short circuits and ensuring smooth ion migration. Its performance directly affects battery safety and cycle stability. However, traditional separators still have significant shortcomings in practical applications, such as insufficient puncture resistance, low peel strength between the coating and the base film, poor heat resistance, and poor wettability to the electrolyte. To improve these properties, existing technologies have employed irradiation grafting-blending extrusion methods to prepare separators. While this improves these properties to some extent, it increases the pore-closing temperature, leading to a decrease in battery safety. Therefore, improving the puncture strength, heat resistance, wettability, and peel strength between the coating and the base film of the separator without affecting its pore-closing temperature has become an urgent technical problem to be solved. Summary of the Invention

[0003] The purpose of this application is to provide a battery separator, its preparation method, and a battery, which improves the puncture strength, heat resistance, wettability, and peel strength of the heat-resistant coating of the battery separator without affecting the pore-closing temperature, thereby improving the cycle performance and safety performance of the battery. The specific technical solution is as follows:

[0004] The first aspect of this application provides a battery separator comprising a substrate layer, a crosslinked layer crosslinked on two surfaces of the substrate layer, and a heat-resistant coating applied to at least one surface of the crosslinked layer; the crosslinked layer comprises a crosslinked polymer obtained by monomer polymerization, wherein the monomer contains at least four groups: unsaturated bonds, phenyl, amino or imine, and hydroxyl groups.

[0005] In some embodiments of this application, the monomer is selected from at least one of the following compounds: , , , , , , .

[0006] In some embodiments of this application, the sum of the masses of the crosslinking layer and the substrate layer is increased by 0.1% to 15% relative to the mass of the substrate layer.

[0007] In some embodiments of this application, the heat-resistant coating comprises, by weight, 90-120 parts ceramic particles, 0.1-10 parts binder, 0-10 parts thickener, and 0.1-10 parts dispersant. The ceramic particles are selected from at least one of silica, alumina, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, boehmite, aluminum nitride, boron nitride, barium sulfate, barium titanate, calcium fluoride, and barium fluoride. The binder is selected from at least one of carboxymethyl cellulose, polyacrylates, polyurethane, polyacrylamide, polyacrylic acid, and their derivatives. The thickener is selected from at least one of sodium alginate, cellulose, natural gum, and starch. The dispersant is selected from at least one of polyvinyl alcohol, acrylate copolymers, polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, polyacrylic acid, and cellulose.

[0008] The second aspect of this application provides a method for preparing the battery separator provided in the first aspect of this application, which includes the following steps:

[0009] The monomer is dissolved in a solvent to obtain a monomer solution; the initiator is mixed with the monomer solution to obtain a precursor solution.

[0010] The substrate layer is irradiated, then immersed in the precursor liquid, removed by rollers, and cured, cleaned, dried and heat-set to obtain the modified substrate layer.

[0011] The ceramic particles, the binder, the thickener, and the dispersant are mixed evenly with water to obtain a heat-resistant coating slurry;

[0012] The heat-resistant coating slurry is applied to at least one surface of the modified substrate layer and dried to obtain the battery separator.

[0013] In some embodiments of this application, the solvent is selected from polar solvents, and the concentration of the monomer solution is 2wt% to 40wt%.

[0014] In some embodiments of this application, the mass ratio of the initiator to the monomer is 0.001:1 to 0.1:1, and preferably, the mass ratio of the initiator to the monomer is 0.002:1 to 0.02:1.

[0015] In some embodiments of this application, the initiator is selected from at least one of benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acylphosphine oxides, benzophenone, thioxanthones, and anthraquinones.

[0016] In some embodiments of this application, any of the following features are satisfied:

[0017] (1) The dose of surface irradiation is 0.1 mgy to 1 mgy.

[0018] A third aspect of this application provides a battery that includes the battery separator provided in the first aspect of this application.

[0019] The beneficial effects of this application:

[0020] This application provides a battery separator, its preparation method, and a battery. The battery separator includes a substrate layer, a crosslinked layer crosslinked on two surfaces of the substrate layer, and a heat-resistant coating applied to at least one surface of the crosslinked layer. The crosslinked layer includes a crosslinked polymer obtained by monomer polymerization. The monomer contains at least four groups: unsaturated bonds, phenyl, amino or imino, and hydroxyl groups. These four groups in the monomer can each perform their respective functions and form a synergistic effect during the modification of the substrate layer, ultimately improving the puncture strength, heat resistance, wettability, and peel strength of the heat-resistant coating of the battery separator, without affecting the closed-cell temperature of the battery separator.

[0021] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0022] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0023] The first aspect of this application provides a battery separator, which includes a substrate layer, a crosslinked layer crosslinked on two surfaces of the substrate layer, and a heat-resistant coating applied to at least one surface of the crosslinked layer; the crosslinked layer includes a crosslinked polymer, which is obtained by monomer polymerization, and the monomer contains at least four groups: unsaturated bonds, phenyl, amino or imine, and hydroxyl groups.

[0024] The inventors discovered that, on the one hand, the unsaturated bonds in the monomers of this application can undergo addition polymerization with free radicals generated by irradiation on the surface of the substrate layer under conditions such as ultraviolet light irradiation, thereby forming a cross-linked layer on both surfaces of the substrate layer. The cross-linked layer and the substrate layer have stable chemical bonds, which can inhibit the shrinkage of the substrate layer at high temperatures, thus improving the heat resistance of the battery separator. On the other hand, the phenyl groups with rigid structures in the monomers can improve the thermal shrinkage, rupture temperature, and puncture strength of the battery separator. Simultaneously, the polar groups amino or imino and hydroxyl groups in the monomers can, on the one hand, improve the wettability of the cross-linked layer to the electrolyte, which is beneficial for ion conduction; on the other hand, they can improve the wettability to the heat-resistant coating, enhance the interfacial bonding between the cross-linked layer and the heat-resistant coating, and improve the peel strength of the heat-resistant coating. Furthermore, amino or imino and hydroxyl groups can achieve self-crosslinking and dehydration under vacuum heating conditions, which is beneficial for further improving the puncture strength and thermal shrinkage of the battery separator. Moreover, coating at least one surface of the cross-linked layer with a heat-resistant coating is beneficial for further improving the heat resistance, wettability, and puncture strength of the battery separator.

[0025] In this application, unsaturated bonds include at least one of unsaturated double bonds or unsaturated triple bonds.

[0026] In some embodiments of this application, the monomer is selected from at least one of the following compounds: , , , , , , .

[0027] The monomers mentioned above all contain four groups: unsaturated bonds, phenyl, imino, and hydroxyl. These four groups can work synergistically. Unsaturated bonds form a cross-linked layer through free radical polymerization with the substrate layer, inhibiting high-temperature shrinkage of the substrate layer. Phenyl groups enhance the heat resistance and puncture strength of the battery separator. Imino and hydroxyl groups optimize the wettability of the cross-linked layer to the electrolyte and heat-resistant coating, improve the peel strength of the heat-resistant coating, and ultimately comprehensively improve the overall performance of the battery separator.

[0028] In this application, the monomers shown in I-1 to I-7 are all commercially available conventional substances. This application does not have any particular restrictions on their source, as long as they can achieve the purpose of this application.

[0029] In some embodiments of this application, the sum of the masses of the crosslinking layer and the substrate layer is increased by 0.1% to 15% relative to the mass of the substrate layer. For example, the increase can be 0.1%, 0.5%, 2%, 5%, 8%, 10%, 12%, 15%, or a range of any two of these values. Controlling the increase in the sum of the masses of the crosslinking layer and the substrate layer relative to the mass of the substrate layer within the above range allows the crosslinking layer to possess both suitable mechanical strength and toughness, which is beneficial for balancing the puncture strength and rupture temperature of the battery separator. Simultaneously, it also allows the crosslinking layer to have a reasonable porous structure, which is beneficial for electrolyte wetting and provides channels for ion conduction, thereby improving the wettability of the battery separator.

[0030] In some embodiments of this application, the heat-resistant coating comprises, by weight, 90-120 parts ceramic particles, 0.1-10 parts binder, 0-10 parts thickener, and 0.1-10 parts dispersant. The ceramic particles are selected from at least one of silica, alumina, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, boehmite, aluminum nitride, boron nitride, barium sulfate, barium titanate, calcium fluoride, and barium fluoride. The binder is selected from at least one of carboxymethyl cellulose, polyacrylates, polyurethane, polyacrylamide, polyacrylic acid, and their derivatives. The thickener is selected from at least one of sodium alginate, cellulose, natural gum, and starch. The dispersant is selected from at least one of polyvinyl alcohol, acrylate copolymers, polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, polyacrylic acid, and cellulose. For example, by weight, ceramic particles can be 90 parts, 95 parts, 100 parts, 105 parts, 110 parts, 115 parts, 120 parts, or any two of these values; binder can be 0.1 parts, 1 part, 3 parts, 5 parts, 8 parts, 10 parts, or any two of these values; thickener can be 0 parts, 2 parts, 4 parts, 6 parts, 8 parts, 10 parts, or any two of these values; dispersant can be 0.1 parts, 1 part, 2 parts, 3 parts, 5 parts, 6 parts, 8 parts, 10 parts, or any two of these values. By controlling the types and contents of ceramic particles, binders, thickeners, and dispersants within the aforementioned ranges, ceramic particles can be uniformly dispersed in the heat-resistant coating, resulting in a heat-resistant coating with good uniformity and consistency. At the same time, the binder can effectively bond the ceramic particles to the cross-linked layer, which is beneficial to improving the peel strength of the heat-resistant coating. In addition, ceramic particles also have good hardness, wettability, and heat resistance, which is beneficial to improving the puncture strength, wettability, and heat resistance of the battery separator.

[0031] The second aspect of this application provides a method for preparing the battery separator provided in the first aspect of this application, which includes the following steps:

[0032] The monomer is dissolved in a solvent to obtain a monomer solution; an initiator is mixed with the monomer solution to obtain a precursor solution; the substrate layer is irradiated, then immersed in the precursor solution, rolled out, cured, cleaned, dried, and heat-set to obtain a modified substrate layer; ceramic particles, binder, thickener, and dispersant are mixed uniformly with water to obtain a heat-resistant coating slurry; the heat-resistant coating slurry is coated on at least one surface of the modified substrate layer and dried to obtain a battery separator. The above preparation method does not affect the pore-closing temperature of the battery separator, is simple to operate, and is easily scalable for mass production.

[0033] This application does not impose any particular restriction on the source of the aforementioned substrate layer, as long as it achieves the purpose of this application. For example, it can be prepared by the applicant. This application does not impose any particular restriction on the preparation method of the substrate layer, and it can be synthesized using any method known to those skilled in the art. For example, the preparation method is as follows: the polymer and white oil are melt-mixed and extruded through a twin-screw extruder, then cooled by a chilling roller, and then subjected to biaxial stretching, extraction, and drying to obtain the substrate layer.

[0034] This application does not impose any particular limitation on the ratio of the polymer to the white oil, as long as the purpose of this application is achieved. For example, the mass ratio of the polymer to the white oil can be 30:70 to 10:90. This application does not impose any particular limitation on the type of polymer, as long as the purpose of this application is achieved. For example, polyolefins can be used, and polyolefins can include, but are not limited to, at least one of polyethylene, polypropylene, polybutene, and mixtures or copolymers thereof, and the molecular weight of the polyolefin can be from 100,000 to 7,000,000.

[0035] This application does not impose any particular restrictions on the extrusion temperature, biaxial stretching conditions, extractant used for extraction, extraction temperature, and extraction time, as long as the purpose of this application can be achieved. For example, the extrusion temperature can be 180℃~260℃; the temperature for biaxial stretching can be 110℃~130℃, and the longitudinal and transverse stretching length ratios can be 4:1~15:1 independently; the extractant can include, but is not limited to, dichloromethane, etc.

[0036] In some embodiments of this application, the solvent is selected from polar solvents, and the concentration of the monomer solution is 2wt% to 40wt%. For example, the concentration of the monomer solution can be 2wt%, 5wt%, 10wt%, 15wt%, 18wt%, 25wt%, 30wt%, 35wt%, 40wt%, or a range of any two of these values, more preferably 10wt% to 40wt%. The solvent includes, but is not limited to, polar solvents such as water, dichloromethane, and dimethyl sulfoxide. Polar solvents facilitate the dissolution of monomers, providing a basis for subsequent polymerization. By controlling the concentration of the monomer solution within the above range, the monomer has a suitable reaction rate during polymerization, allowing the polymerization reaction to proceed more fully and uniformly. This results in a well-structured crosslinked polymer with a moderate degree of polymerization, which can improve the heat resistance and puncture strength of the substrate layer and provide a good interface for the subsequent coating of the heat-resistant coating. This, in turn, is beneficial for improving the heat resistance and puncture strength of the battery separator and the peel strength of the heat-resistant coating.

[0037] In some embodiments of this application, the mass ratio of initiator to monomer is 0.001:1 to 0.1:1, preferably 0.002:1 to 0.02:1. For example, the mass ratio of initiator to monomer can be 0.001:1, 0.002:1, 0.01:1, 0.02:1, 0.04:1, 0.06:1, 0.08:1, 0.1:1, or a range consisting of any two of these values. In some embodiments of this application, the initiator is selected from at least one of benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acylphosphine oxides, benzophenone, thioxanthones, and anthraquinones. Controlling the mass ratio of initiator to monomer and the type of initiator within the above range is also beneficial for controlling the reaction rate during monomer polymerization. This results in the formation of a cross-linked polymer with a moderate degree of polymerization and uniform distribution on the surface of the substrate layer. This not only improves the heat resistance and puncture strength of the substrate layer but also provides a good interface for the subsequent coating of the heat-resistant coating. Consequently, it is beneficial for improving the heat resistance and puncture strength of the battery separator and the peel strength of the heat-resistant coating.

[0038] In some embodiments of this application, the surface irradiation dose is 0.1 mgy to 2 mgy. For example, the surface irradiation dose can be 0.1 mGy, 0.3 mGy, 0.5 mGy, 0.8 mGy, 1 mGy, 1.2 mGy, 1.3 mGy, 1.5 mGy, 1.8 mGy, 2 mGy, or a range of any two of these values.

[0039] This application does not specifically limit the method of surface irradiation, as long as it achieves the purpose of this application. For example, irradiation can be carried out using a cobalt source or an electron accelerator.

[0040] In some implementations, curing includes ultraviolet curing, with the wavelength of the ultraviolet light being 200nm~400nm, the irradiation time being 1s~60s, and the ultraviolet radiation power being 1kW~4kW.

[0041] In some implementations, the heat setting temperature is 125℃-135℃, and the time is 0.5s-10s.

[0042] This application does not have any particular limitation on the drying method, as long as it can achieve the purpose of this application. For example, the cured and cleaned substrate layer can be dried in a vacuum environment of 80°C to 120°C. Under this environment, the amino or imino and hydroxyl groups in the crosslinked polymer can self-crosslink and dehydrate, which is beneficial to further improve the puncture strength and heat shrinkability of the battery separator.

[0043] In some implementations, the solid content of the heat-resistant coating slurry is 25wt% to 50wt%.

[0044] In some embodiments, the substrate layer is a polyolefin-based film or other polymer-based film. This application does not specifically limit the type of polyolefin, as long as it achieves the purpose of this application. For example, polyolefins may include, but are not limited to, at least one of polyethylene, polypropylene, polybutene, and mixtures or copolymers thereof; other polymers may include, but are not limited to, polyethylene terephthalate (PET), polyimide (PI), polyvinylidene fluoride (PVDF), etc. A third aspect of this application provides a battery that includes the battery separator provided in the first aspect of this application. The battery separator of this application has a suitable pore-closing temperature, good puncture strength, heat resistance, and wettability, and the heat-resistant coating has high peel strength. Therefore, the battery including the battery separator of this application has good cycle performance and safety performance.

[0045] In some implementations, the battery of this application may include, but is not limited to, lithium-ion batteries or sodium-ion batteries.

[0046] Example

[0047] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0048] Test methods and equipment:

[0049] Quality enhancement test of substrate layer before and after crosslinking:

[0050] Weigh the substrate layer and the mass of the substrate layer and crosslinked layer after polymerization, respectively. The difference between the two is multiplied by the ratio of the mass of the substrate layer to 100% to obtain the mass increase of the substrate layer before and after crosslinking, that is, the mass increase of the sum of the masses of the crosslinked layer and the substrate layer relative to the mass of the substrate layer.

[0051] Battery separator performance test:

[0052] Puncture strength test:

[0053] The test was conducted according to the method specified in GB / T 36363-2018. A battery separator with a size of 50×100mm was cut along the TD direction. The battery separator was fixed on the sample fixture of the puncture tester (model: KES-GNDG5, KNC Technology Co., Ltd.). A steel needle with a diameter of 1.0 mm was used to puncture the battery separator at a speed of 0.1cm / sec. The maximum load of the steel needle penetrating the battery separator was read. The test was performed more than 5 times and the arithmetic mean was taken.

[0054] Heat shrinkability test:

[0055] Cut a 150mm×150mm battery separator and mark it with 100mm marks in both the length direction (MD direction) and the width direction (TD direction). Place the battery separator between two 200×200mm, 3mm thick glass plates and then place it in an electric heating constant temperature chamber. Heat it at 110℃ for 1 hour and record the length and width values ​​marked on the battery separator after heating, which are L1 mm and L2 mm, respectively. The heat shrinkage rate in the MD direction is (100-L1) / 100×100%; the heat shrinkage rate in the TD direction is (100-L2) / 100×100%.

[0056] Membrane rupture temperature test:

[0057] After baking the battery separator at 85℃ for 6 hours under vacuum, a 30mm long and 10mm wide section of the separator was taken and tested using a static thermomechanical analyzer under nitrogen atmosphere, with a tensile force of 0.03N, a heating rate of 5℃ / min, and a termination temperature of 400℃. The length of the battery separator as a function of temperature was recorded until the separator broke, at which point the rupture temperature was determined. The temperature at which the separator length increased instantaneously was the rupture temperature.

[0058] The smaller the heat shrinkage rate and the higher the film rupture temperature, the better the heat resistance of the battery separator.

[0059] Wetting test:

[0060] A 5mm wide battery separator sample was suspended vertically with its lower end in contact with the electrolyte surface. The height the electrolyte rose over 1 minute was recorded to obtain the absorption rate. The electrolyte composition was as follows: ethylene carbonate, ethyl methyl carbonate, and ethyl propionate in a mass ratio of 1:3:1; based on the electrolyte mass, fluoroethylene carbonate accounted for 3% of the mass, ethylene sulfate accounted for 0.5% of the mass, and the LiPF6 concentration was 1 mol / L.

[0061] A higher liquid absorption rate indicates better wettability of the battery separator.

[0062] Peel strength test:

[0063] The test shall be conducted in accordance with the method specified in GB / T 2792—2014.

[0064] Cut a 2.5cm×20cm sample. Confirm that the peel strength test plate (stainless steel plate, specific model [1Cr18Ni9Ti]) is 100mm long. Manually peel 10mm to a fixed position. Attach the peel strength double-sided adhesive tape (3M adhesive model 4910) to the test plate and peel off the surface sticker, leaving approximately 20mm of the sticker (at the tape position mark). The sample peel length is 70mm. Roll the sample back and forth three times with a pressure roller (1.7kg pressure roller). Manually peel 10mm to a fixed position and fix the prepared sample between the upper and lower clamps of the tensile testing machine (High-speed Rail Testing Instruments (Dongguan) Co., Ltd., AI-3000-SU) (the distance between the clamps is (100±5)mm). Ensure the sample is flat and wrinkle-free, vertical and not skewed. The tensile speed is 50mm / min. Based on the width and thickness of the sample, click the program to measure the peel strength value.

[0065] Battery performance test:

[0066] Cyclic performance test:

[0067] At an ambient temperature of 25°C, the lithium-ion battery was left to stand for 4 hours to reach a constant temperature. It was then charged at a constant current of 1C to 4.4V, followed by constant voltage charging to a cutoff current of 0.05C. After standing for 10 minutes, it was discharged at a constant current of 1C to 3V. The discharge capacity at this point was recorded as the initial capacity C0, constituting one charge-discharge cycle. This charge-discharge cycle was repeated for 500 cycles, and the discharge capacity C1 was recorded. The cycle capacity retention rate of the lithium-ion battery was then calculated.

[0068] Cyclic capacity retention = C1 / C0 100%.

[0069] Hot box performance test:

[0070] At an ambient temperature of 25℃, the lithium-ion battery was left to stand for 4 hours to allow it to reach a constant temperature. The lithium-ion battery was then charged at a constant current of 0.5C to 4.4V, followed by constant voltage charging until the cutoff current was 0.05C. The battery was then placed in an oven, heated to 130℃ and held for 30 minutes, then heated to 140℃ and held for 30 minutes. This process was repeated until the lithium-ion battery caught fire or exploded, or until it was held at 200℃ for 30 minutes.

[0071] Example 1

[0072] <Preparation of Battery Separator>

[0073] The monomers shown in Formula I-6 were dissolved in dimethyl sulfoxide (DMSO) to obtain a monomer solution with a concentration of 18 wt%. Then, acylphosphine oxide 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO) initiator was added to prepare a precursor solution. The amount of initiator was 1 wt% of the substance shown in Formula I-6.

[0074] Polyethylene with a molecular weight of 1 million and white oil were melt-mixed at a mass ratio of 19:81 and extruded through a twin-screw extruder at an extrusion temperature of 230°C. The mixture was then cooled by a chiller roller, followed by biaxial stretching (longitudinal and transverse stretching ratio of 6:1 at a temperature of 120°C), extraction, and drying to obtain a substrate layer. Both sides of the substrate layer were then irradiated with a cobalt source (irradiation dose of 1 mgy).

[0075] (3) The surface-treated substrate layer is immersed in the precursor solution, then irradiated with ultraviolet light at a wavelength of 380 nm for 60 s, and then cleaned, dried and heat-set at 130 °C to obtain a modified substrate layer with a thickness of 9 μm. Alumina (Dv50 of 500 nm), binder (carboxymethyl cellulose and polyacrylate 1:2) and dispersant (polyacrylic acid) in a mass ratio of 94:6:0.5 are dissolved in water to prepare a coating slurry with a solid content of 36 wt%. The coating slurry is coated on both sides of the modified substrate layer in step (3) with a coating thickness of 2 μm to obtain the battery separator.

[0076] <Preparation of the positive electrode>

[0077] The positive electrode active material LiCoO2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under vacuum until homogeneous to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated on the surface of the positive electrode current collector aluminum foil, dried at 90°C, and then cold-pressed, cut, and slit to obtain the positive electrode sheet.

[0078] <Preparation of Negative Electrode Sheets>

[0079] The negative electrode active material graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1.5:1.5:2. Deionized water was added as a solvent, and the mixture was stirred under vacuum until homogeneous to obtain a negative electrode slurry with a solid content of 50 wt%. The negative electrode slurry was uniformly coated onto a negative electrode current collector copper foil, dried at 80°C, and then cold-pressed, cut, and slit. Finally, it was dried under vacuum at 110°C for 12 hours to obtain the negative electrode sheet.

[0080] <Preparation of Electrolyte>

[0081] The electrolyte was prepared in a dry argon-atmospheric glove box by mixing ethylene carbonate, ethyl methyl carbonate, and ethyl propionate in a mass ratio of 1:3:1. Then, fluoroethylene carbonate and ethylene sulfate were added and mixed thoroughly. Finally, LiPF6 was added and dissolved completely to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate was 3%, the mass percentage of ethylene sulfate was 0.5%, and the concentration of LiPF6 was 1 mol / L.

[0082] <Preparation of Lithium-ion Batteries>

[0083] The positive electrode, battery separator, and negative electrode prepared above are stacked in sequence, with the battery separator positioned between the positive and negative electrode to act as a separator. After welding tabs onto the positive and negative electrode, the battery is placed in an aluminum-plastic film packaging bag, dried to remove moisture, and then injected with the electrolyte prepared above. The lithium-ion battery is obtained through processes such as vacuum sealing, formation, capacity testing, degassing, shaping, and capacity testing.

[0084] Examples 2 to 10

[0085] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0086] In Example 2, the molar ratio of monomers I-1 and I-5 is 1:1.

[0087] Comparative Example 1

[0088] Except for the preparation of the battery separator, in which a heat-resistant coating is directly applied to both surfaces of the substrate layer, the rest is the same as in Example 1.

[0089] Comparative Example 2

[0090] Except for the preparation of the battery separator, in which a heat-resistant coating is directly applied to both surfaces of the substrate layer, and the heat-resistant coating slurry is made with 0.5 wt% more polyoxyethylene ether wetting agent than the heat-resistant coating slurry in Example 1, the rest is the same as in Example 1.

[0091] Comparative Example 3

[0092] Except for the monomer being ethyl methacrylate in <Preparation of Battery Separator>, the rest is the same as in Example 1.

[0093] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0094] Table 1

[0095] As can be seen from Examples 1 to 10 and Comparative Examples 1 to 3, crosslinking layers are first crosslinked on two surfaces of the substrate layer, and the crosslinking polymer is obtained by polymerization of monomers within the scope of this application. Then, a heat-resistant coating is applied to at least one surface of the crosslinking layer. The battery separator has a low thermal shrinkage rate and high peel strength, puncture strength, membrane breakage temperature, and liquid absorption rate. It is possible to obtain a battery separator with a suitable pore temperature, good puncture strength, heat resistance, and wettability, and a high peel strength of the heat-resistant coating. Therefore, the battery including the battery separator of this application has a high cycle capacity retention rate and hot box pass temperature, indicating that the battery has good cycle performance and safety performance.

[0096] The type of monomer affects the performance of the battery separator. As can be seen from Example 5 and Comparative Example 3, a cross-linked layer is obtained by polymerization of monomers within the scope of this application, and then a battery separator is obtained. The battery separator has a low thermal shrinkage rate and high peel strength, puncture strength, membrane rupture temperature, and liquid absorption rate, indicating that the battery separator has a suitable pore-closing temperature, good puncture strength, heat resistance, and wettability. Moreover, the heat-resistant coating has high peel strength. Therefore, the battery including the battery separator of this application has a high cycle capacity retention rate and hot box pass temperature, indicating that the battery has good cycle performance and safety performance.

[0097] The concentration of the monomer solution affects the performance of the battery separator. As can be seen from Examples 1, 5 to 10, by controlling the concentration of the monomer solution within the range specified in this application, the battery separator exhibits a lower thermal shrinkage rate and higher peel strength, puncture strength, rupture temperature, and liquid absorption rate. This indicates that the battery separator has a suitable pore-closing temperature, good puncture strength, heat resistance, and wettability. Furthermore, the heat-resistant coating has high peel strength. Therefore, batteries including the battery separator of this application have higher cycle capacity retention and hot box pass temperature, indicating good cycle performance and safety performance. In Example 10, the monomer content was relatively high. After the monomer solubility reached saturation, the monomer no longer dissolved. The undissolved monomer could not participate in polymerization to form a cross-linked layer, resulting in waste of raw materials and increased production costs. The above descriptions are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A battery separator, comprising a substrate layer, a crosslinked layer crosslinked on two surfaces of the substrate layer, and a heat-resistant coating applied to at least one surface of the crosslinked layer; The cross-linked layer comprises a cross-linked polymer, which is obtained by monomer polymerization, and the monomer contains at least four groups: unsaturated bond, phenyl, amino or imine, and hydroxyl.

2. The battery separator according to claim 1, wherein, The monomer is selected from at least one of the following compounds: 、 、 、 、 、 、 。 3. The battery separator according to claim 1, wherein, The sum of the masses of the crosslinked layer and the substrate layer increases by 0.1% to 15% relative to the mass of the substrate layer.

4. The battery separator according to claim 1, wherein, By weight, the heat-resistant coating comprises 90-120 parts ceramic particles, 0.1-10 parts binder, 0-10 parts thickener, and 0.1-10 parts dispersant. The ceramic particles are selected from at least one of silicon dioxide, alumina, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, boehmite, aluminum nitride, boron nitride, barium sulfate, barium titanate, calcium fluoride, and barium fluoride. The binder is selected from at least one of carboxymethyl cellulose, polyacrylates, polyurethane, polyacrylamide, polyacrylic acid, and their derivatives. The thickener is selected from at least one of sodium alginate, cellulose, natural gum, and starch. The dispersant is selected from at least one of polyvinyl alcohol, acrylate copolymers, polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, polyacrylic acid, and cellulose.

5. A method for preparing a battery separator according to any one of claims 1 to 4, comprising the following steps: The monomer is dissolved in a solvent to obtain a monomer solution; The initiator is mixed with the monomer solution to obtain the precursor solution; The substrate layer is irradiated, then immersed in the precursor liquid, removed by rollers, and cured, cleaned, dried and heat-set to obtain the modified substrate layer. The ceramic particles, the binder, the thickener, and the dispersant are mixed evenly with water to obtain a heat-resistant coating slurry; The heat-resistant coating slurry is applied to at least one surface of the modified substrate layer and dried to obtain the battery separator.

6. The preparation method according to claim 5, wherein, The solvent is selected from polar solvents, and the concentration of the monomer solution is 2wt% to 45wt%, preferably 5wt% to 35wt%.

7. The preparation method according to claim 5, wherein, The mass ratio of the initiator to the monomer is 0.001:1 to 0.1:1, preferably 0.002:1 to 0.02:

1.

8. The preparation method according to any one of claims 5 to 7, wherein, The initiator is selected from at least one of benzoin ethers, α-hydroxy ketones, acetophenone derivatives, acylphosphine oxides, benzophenone, thioxanthones, and anthraquinones.

9. The preparation method according to any one of claims 5 to 7, wherein it satisfies any one of the following characteristics: (1) The dose of surface irradiation is 0.1 MGy to 2 MGy.

10. A battery comprising the battery separator according to any one of claims 1 to 4.

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