A battery separator and method of making the same

Abstract

The application discloses a battery separator, which comprises a base film and a functional coating arranged on at least one side of the base film, and raw materials of the functional coating comprise a sulfide graft modified polyimide, a binder, inorganic particles and a surfactant, and the ratio of the sulfide graft modified polyimide, the binder, the inorganic particles and the surfactant is (1 part-3 parts):(50 parts-80 parts):(20 parts-50 parts):(0.01 part-0.1 part) in terms of mass. The separator has the characteristics of high thermal stability, low size change, high ion conduction and good adhesive force. The separator is used in a battery, which is not only beneficial to improving the cycle performance of the battery, but also beneficial to improving the safety performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and specifically to a battery separator and its preparation method. Background Technology

[0002] With the development of batteries, they have a significant impact on people's lifestyles and travel methods. In batteries, the battery separator, as a crucial component of the battery structure, serves two main purposes: firstly, it provides electronic barrier, physically separating the positive and negative electrodes to prevent short circuits and ensure battery safety; secondly, it facilitates ion conduction, providing a high lithium-ion conductivity rate to ensure normal battery operation.

[0003] In existing technologies, battery separators still face several technical challenges. First, during battery cycling, traditional battery separators often shrink due to poor thermal stability, leading to direct contact between the positive and negative electrodes and causing a short circuit. Second, balancing the adhesion and ion conductivity between the separator and the positive and negative electrodes remains a challenge. To prevent detachment, more adhesive is added to improve adhesion, but this hinders lithium-ion migration within the separator, affecting battery performance. Conversely, insufficient adhesive can cause detachment, compromising battery safety. Third, the mechanical properties of the adhesive layer between the separator and the electrodes significantly decrease due to temperature increases during battery cycling, further impacting adhesion. Summary of the Invention

[0004] This invention addresses the problems in the prior art by disclosing a battery separator and its preparation method. The battery separator of this invention has high thermal stability, low dimensional change, and balances ion conduction and adhesion, while also possessing excellent ion conductivity and adhesion.

[0005] Polyimide possesses excellent heat resistance and adhesion properties, but its high glass transition temperature can affect its morphology and structural transformation. While polyimide is commonly used as a binder in battery separators, improving thermal stability and adhesion to some extent, the improvement is not significant, and it can also hinder ion conduction within the separator. Given these factors, how can the performance of polyimide be improved for its application in battery separators to better enhance thermal stability and adhesion, and ultimately improve battery cycle performance?

[0006] This invention is achieved through the following technical solution:

[0007] The present invention provides a battery separator, the battery separator comprising a base film and a functional coating disposed on at least one side of the base film, wherein the raw materials of the functional coating include sulfide-grafted modified polyimide, binder, inorganic particles and surfactant, and the ratio of the sulfide-grafted modified polyimide, binder, inorganic particles and surfactant by mass is (1 part to 3 parts): (50 parts to 80 parts): (20 parts to 50 parts): (0.01 parts to 0.1 parts).

[0008] In the above-described design of this invention, a three-dimensional cross-linked network is formed by the combination of sulfide-grafted modified polyimide and a binder. On the one hand, the three-dimensional cross-linked network structure facilitates ion migration; on the other hand, the formed three-dimensional cross-linked network structure not only improves the membrane's adhesion but also provides good flexibility and high-temperature resistance. Inorganic particles are embedded in the three-dimensional cross-linked network structure, forming at least one layer. This improves the stability of the three-dimensional cross-linked structure, acting as a framework and further enhancing the thermal and dimensional stability of the membrane during battery cycling. Furthermore, the three-dimensional cross-linked network structure forms stable ion transport channels, allowing ions to migrate through the pores between the inorganic particles during battery cycling. The surfactant improves the wettability of the inorganic particles, thereby facilitating the coordination between substances in the slurry and resulting in a battery membrane with high thermal stability, low dimensional change, high ion conductivity, and strong adhesion.

[0009] As a further embodiment, the modifying group of the thioether-grafted polyimide includes a thioether group; the modified group of the thioether-grafted polyimide includes an imide group; and the mass ratio of the thioether group to the imide group is 1:(5-20). By grafting thioether groups onto the polyimide, the glass transition temperature of the polyimide is improved. Furthermore, thioether grafting modification not only reduces the generation of byproducts during the grafting process but also, in conjunction with the CN structure in the polyimide, enhances the electrochemical activity of the polyimide. The introduction of thioether also lowers the energy barrier for the formation of a three-dimensional cross-linked network in the grafted polyimide, thereby promoting the formation of a three-dimensional network structure between the thioether-grafted polyimide and the binder. Within the mass ratio of thioether groups and imide groups in this invention, it is advantageous to match a suitable grafting amount on the imide group. When the grafting amount is too large, repeated grafting may occur on the same imide group. When the grafting amount is insufficient, it may be difficult to improve the performance of polyimide, and in battery applications, the improvement on the battery's electrical performance and safety performance is not significant.

[0010] As a further option, the thioether group is derived from a sulfur-containing salt. Sulfur-containing salts are more conducive to reducing byproducts generated during grafting modification.

[0011] As a further embodiment, the sulfur-containing salt includes one or more of anhydrous sodium sulfide, anhydrous ammonium sulfide, and anhydrous potassium sulfide.

[0012] As a further option, the imide group is derived from a halopolyimide. This makes it easier to graft with sulfur-containing salts and reduces side reactions during grafting modification.

[0013] As a further embodiment, the halogenated polyimide includes one or more of chlorinated polyimide, bromopolyimide, iodopolyimide, and fluorinated polyimide.

[0014] As a further embodiment, the weight-average molecular weight of the sulfide-grafted polyimide is 15,000-160,000. When the weight-average molecular weight of the sulfide-grafted polyimide is higher than 160,000, the high molecular weight makes it difficult to disperse; while when the weight-average molecular weight is lower than 15,000, the melting point of the sulfide-grafted polyimide is too low, which is not conducive to processing and molding, and the heat resistance of the sulfide-grafted polyimide deteriorates, resulting in insufficient thermal expansion resistance in aqueous coating solutions. Within this weight-average molecular weight range, the sulfide-grafted polyimide achieves more suitable physical properties and is more conducive to forming a three-dimensional cross-linked network with a more suitable pore distribution with the binder.

[0015] As a further embodiment, the inorganic particles include one or more of titanium dioxide, barium sulfate, calcium carbonate, aluminum oxide, silicon dioxide, and tungsten trioxide.

[0016] As a further option, the particle size of the inorganic particles is selected from 50nm-300nm. When adding inorganic particles of different sizes to the separator raw material, the selection of particle size needs to be considered. If the particle size is too small, the particles are prone to agglomeration, which may prevent them from embedding in the three-dimensional network structure, negatively impacting the thermal and dimensional stability of the separator. Furthermore, it may also affect ion migration within the separator, thus negatively impacting battery cycle performance. If the particle size is too large, a good alignment layer cannot be formed in the functional coating, and the particles are not easily tightly encapsulated after coating formation, leading to separator "powder shedding" during later use. Within this range, the goal is to ensure battery safety while enabling the inorganic particles to have both good dispersibility and form a rigid framework with a good alignment layer.

[0017] As a further embodiment, the binder includes one or more of methylated cellulose salt binders, polyacrylic acid binders, and fluoropolymer binders. Methylated cellulose salt binders have good water solubility, making them easier to disperse in the slurry. In polyacrylic acid binders, the carboxylic acid group is an electron-withdrawing group, while the nitrogen in sulfide-grafted polyimide has a lone pair of electrons. This allows the polyacrylic acid binder to better cooperate with the sulfide-grafted polyimide to form a three-dimensional network structure. Furthermore, the hydroxyl oxygen in the carboxylic acid group of the polyacrylic acid binder is easily dissociated, making the polyacrylic acid binder weakly acidic and allowing for better cooperation with surfactants, thus improving the wettability of the polyacrylic acid binder. Fluoropolymer binders are more suitable for use in slurries where the raw material solution for the diaphragm is an organic solvent, as they exhibit better solubility.

[0018] As a further embodiment, the methylated cellulose salt binder includes one or more of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and ammonium carboxymethyl cellulose.

[0019] As a further embodiment, the polyacrylic adhesive includes one or more of polyacrylic acid and polymethacrylic acid.

[0020] As a further embodiment, the fluoropolymer binder includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, and polyvinylidene fluoride-hexafluoropropylene copolymer.

[0021] As a further embodiment, the surfactant includes one or more of the following: organofluorocarbon compounds, pyrrolidone polymers, alkylaryl polyether alcohols, alkyl sulfonates, and polyacrylic acid polymers.

[0022] As a further embodiment, the organofluorocarbon compound includes one or more of the following: perfluoropolyether-based surfactants (Hubei Yunmei Technology Co., Ltd.), carboxylate fluorocarbon surfactants (Wuhan Rongcan Biotechnology Co., Ltd.), and phosphate fluorocarbon surfactants (Wuhan Pushida Biotechnology Co., Ltd.).

[0023] As a further embodiment, the pyrrolidone polymer includes one or more of polyvinylpyrrolidone, N-dodecyl-2-pyrrolidone, N-octyl-2-pyrrolidone, and dodecylpyrrolidone.

[0024] As a further option, the alkylaryl polyether alcohol includes one or more of Triton WR1339, tetrabutylphenol, and Corning PE-100.

[0025] As a further embodiment, the alkyl sulfonate includes one or more of sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium hexadecyl sulfonate, sodium octadecyl sulfonate, sodium hexadecylbenzene sulfonate, and sodium octadecylbenzene sulfonate.

[0026] As a further embodiment, the polyacrylic polymer includes one or more of alkali metal polyacrylic acid and polyacrylic acid with ammonium groups. Alkali metal polyacrylic acid can not only be used as a surfactant in the preparation of battery separators, but also helps to reduce the impedance of the separator, thereby improving the electrical performance of the battery.

[0027] As a further embodiment, the alkali metal polyacrylate includes one or more of sodium polyacrylate, lithium polyacrylate, and potassium polyacrylate.

[0028] As a further embodiment, the ammonium-containing polyacrylic acid includes ammonium polyacrylate.

[0029] As a further embodiment, the raw materials of the functional coating also include a dispersant. The total mass of the sulfide-grafted modified polyimide, binder, inorganic particles, and surfactant is 8%-50% of the total mass of the sulfide-grafted modified polyimide, binder, inorganic particles, surfactant, and dispersant. The dispersant of this invention can be a single solvent or a mixed solvent. The main solvent of the dispersant is selected based on the solubility or dispersibility of the binder in the solvent. The co-solvent plays a role in wetting, leveling, etc. Battery binders are divided into oil-soluble and water-soluble categories. Generally, in water-based binder systems, the main solvent of the dispersant is deionized water or pure water, and a trace amount of organic solvent can be added as a co-solvent according to the actual situation. In oil-based binder systems, the main solvent of the dispersant is generally an organic solvent, and deionized water or pure water is not selected. Furthermore, the amount of dispersant added in this invention can be adjusted by those skilled in the art according to the actual situation.

[0030] As a further embodiment, the raw materials of the functional coating include a polyacrylic binder, a sulfide-grafted polyimide with a weight average molecular weight of 20,000-25,000, inorganic particles with a particle size selected from 50 nm to 80 nm, a pyrrolidone polymer, and a polyacrylic polymer. By mass, the ratio of the polyacrylic binder: the sulfide-grafted polyimide with a weight average molecular weight of 20,000-25,000: the inorganic particles with a particle size selected from 50 nm to 80 nm: the pyrrolidone polymer: the polyacrylic polymer is (50-70 parts): (1-1.2 parts): (20-30 parts): (0.03-0.05 parts): (0.01-0.03 parts). Based on the preceding discussion, both polyacrylic acid polymers and pyrrolidone polymers are soluble in aqueous or organic solutions. In pyrrolidone polymers, the nitrogen atom in the pyrrole molecule donates a pair of electrons to form a five-center, six-electron conjugated π bond, with the nitrogen atom being an electron-donating group. In polyacrylic acid polymers, the carboxylic acid is an electron-withdrawing group. Polyacrylic acid polymers and pyrrolidone polymers can work together better. On the one hand, they can promote the wetting of inorganic particles in the slurry. On the other hand, the lone pair electrons of the nitrogen atom in the thioether-grafted polyimide are beneficial to the stability of the thioether-grafted polyimide in the slurry, while the pyrrolidone polymer is beneficial to the stability of the polyacrylic acid binder in the slurry. This is conducive to the formation of a stable three-dimensional network structure between the thioether-grafted polyimide and the polyacrylic acid binder. The selection of the particle size of the inorganic particles in this invention is beneficial to the embedding of inorganic particles into the three-dimensional network structure, thereby improving the performance of the membrane.

[0031] As a further embodiment, the raw materials of the functional coating include polyacrylic acid, sulfide-grafted modified polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5), titanium dioxide with a particle size selected from 50nm-80nm, polyvinylpyrrolidone, and alkali metal polyacrylic acid. By mass, the ratio of polyacrylic acid: sulfide-grafted modified polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5): titanium dioxide with a particle size selected from 50nm-80nm: polyvinylpyrrolidone: alkali metal polyacrylic acid is (50 parts-70 parts):(1 part-1.2 parts):(20 parts-30 parts):(0.03 parts-0.05 parts):(0.01 parts-0.03 parts). Building upon the above, we further discovered that the Ti=O bonds in TiO2 and the -CO-NR-CO- bonds in the sulfide-grafted modified polyimide can form titanium-oxygen-nitrogen single bonds, which helps stabilize the organic-inorganic interface and further strengthens the bonding strength between different materials in the coating, ensuring the mechanical strength of the entire coating material's three-dimensional network structure. Simultaneously, after film formation, a tight protective layer is formed on the surface of the base film, improving the product's scratch resistance during manufacturing and use. Polyacrylic acid and alkali metal polyacrylic acid have similar structures, making them easier to integrate. Polyvinylpyrrolidone and alkali metal polyacrylic acid adsorb onto the surface of titanium dioxide, reducing the surface activity of titanium dioxide and thus promoting better integration between titanium dioxide and the sulfide-grafted modified polyimide. Polyacrylic acid, polyvinylpyrrolidone, and alkali metal polyacrylic acid have better flowability and can be better and more uniformly distributed in the slurry, which not only benefits the uniformity of the slurry but also the integration between the various substances in the slurry. Within the mass ratio of thioether groups and imide groups in this invention, it is beneficial to match a suitable grafting mass on the imide group. When the grafting mass is too large, repeated grafting may occur on the same imide group, resulting in an unstable three-dimensional cross-linked network formed with polyacrylic acid, which cannot be connected with titanium dioxide, causing titanium dioxide to detach from the structure. When the grafting amount is insufficient, it may be difficult to improve the performance of polyimide and it cannot form a complex with polyacrylic acid. In battery applications, the improvement on the battery's electrical performance and safety performance is not significant.

[0032] As a further option, the base membrane includes one of a polyolefin microporous membrane base membrane and a polyester microporous membrane base membrane.

[0033] As a further embodiment, the thickness of the base film of the separator is 4μm-12μm, and the thickness of the functional coating of the separator is 0.1μm-2.0μm. In this invention, the thickness of the base film and the thickness of the functional coating refer to the thickness of the base film and the functional coating in the final separator product.

[0034] The present invention also provides an electrochemical device having the diaphragm, which can be used in end consumer products, including but not limited to mobile phones, laptops, pen input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, and portable printers.

[0035] This invention also provides an electrochemical device having the aforementioned diaphragm, which can be used in electrical appliances, including large and small electrical appliances. Small electrical appliances include consumer products, wearable electronic devices, or portable electronic devices; large electrical appliances include transportation equipment. Transportation equipment includes, but is not limited to, vehicles such as automobiles, motorcycles, electric bicycles, buses, subways, high-speed trains, airplanes, and ships; wearable electronic devices or portable electronic devices include, but are not limited to, headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, drones, motors, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors. The negative electrode of the battery of this invention is used in an electrochemical device, which can be housed in an electrical appliance in the form of an electrochemical device. Typically, the electrochemical device includes a battery pack or / and multiple battery modules or / and a single battery module or / and a management system for controlling them.

[0036] The present invention also provides a method for preparing the diaphragm, the method comprising:

[0037] S1: Weigh out the sulfide-grafted modified polyimide, binder, inorganic particles and surfactant according to the mass ratio, then add dispersant and mix well to obtain a functional coating slurry with a solid content of 8wt%-50wt%.

[0038] S2: Coat at least one side of the base film after it has been stretched once with the slurry obtained in S1;

[0039] S3: The base film coated with S2 slurry is stretched a second time at the required process temperature to obtain a diaphragm.

[0040] In this invention, during the secondary stretching process, the slurry of the functional coating extends synchronously with the base film. This allows it to easily penetrate the surface layer of the base film, forming good adhesion. This ensures that the functional coating maintains a strong grip on the base film during heating, preventing deformation and improving the battery's safety and stability during cycle time. Furthermore, the simultaneous thermal stretching of the functional coating during the secondary stretching process facilitates the interaction between the inorganic particles and the sulfide-grafted modified polyimide, thereby increasing the mechanical strength of the functional coating. After two simultaneous stretching processes, the final thickness of the base film is 4μm-12μm, and the thickness of the functional coating is 0.1μm-2.0μm.

[0041] As a further embodiment, the mixing method in S1 includes stirring, and the mixing conditions are 30℃-60℃; the process temperature in S3 is 100℃-150℃.

[0042] The present invention also provides a method for preparing the sulfide-grafted modified polyimide, the method comprising: dissolving a sulfur-containing salt in an organic solvent 1, and adding a halogenated polyimide and sodium acetate to react; after the reaction is completed, filtering, and washing several times with hot deionized water and organic solvent 2 until there are no chloride ions in the system, to obtain a white powder product, and drying the product.

[0043] As a further embodiment, the ratio of the sulfur-containing salt: halopolyimide: sodium acetate, by mass, is 1:(5-40):(20-50). The preparation method of the sulfide-grafted modified polyimide in this invention can control the grafting quality of the sulfide groups onto the imide groups by adjusting the amount of sulfur-containing salt and halopolyimide added.

[0044] As a further embodiment, the organic solvent 1 includes an NMP (N-methylpyrrolidone) solution, wherein the amount of NMP added is twice the total mass of the sulfur-containing salt, the halogenated polyimide, and the sodium acetate; and the organic solvent 2 includes acetone.

[0045] As a further refinement, the reaction conditions are: maintaining the temperature at 150℃-220℃ for 3-8 hours, followed by natural cooling to room temperature to complete the reaction; the drying temperature is 25℃-30℃. In this invention, excessively low reaction temperatures may significantly increase the reaction time, while excessively high reaction temperatures may lead to numerous side reactions and uncontrollable product development.

[0046] The features and beneficial effects of this invention are as follows:

[0047] (1) The present invention not only improves the glass transition temperature of polyimide by grafting thioether groups, but also facilitates the commercialization of polyimide.

[0048] (2) The diaphragm of the present invention has the characteristics of high thermal stability, low dimensional change, high ion conductivity and good adhesion.

[0049] (3) The separator of the present invention is used in batteries, which not only helps to improve the cycle performance of batteries, but also helps to improve the safety performance of batteries.

[0050] (4) The raw material dispersant of the membrane in the method of the present invention can be deionized water or pure water, which is conducive to simplifying the production process, reducing manufacturing costs, and improving the overall market competitiveness of the product. In addition, the film-forming temperature is not restricted by the process temperature and has good compatibility with the traditional dry film polyolefin microporous membrane preparation process.

[0051] (5) The battery separator prepared by the technical route of the present invention has a thinner coating and a controllable overall thickness, which greatly saves raw material costs and improves the market competitiveness of the product.

[0052] (6) The diaphragm of the present invention can be used in applications such as lithium batteries, sodium batteries, potassium batteries, capacitors, and fuel cells, where it serves as electronic insulation and ion conduction. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 The graph shows the capacity retention results of the embodiments and comparative examples of the present invention. Detailed Implementation

[0055] To facilitate understanding of the battery separator of the present invention, a more comprehensive description of the battery separator of the present invention will be given below, and embodiments of the present invention will be provided, but this does not limit the scope of the present invention.

[0056] The battery separator of this invention is not limited to lithium-ion batteries, but can also be used in sodium batteries and potassium batteries. Taking lithium batteries as an example, this invention further studies the effect of the separator of this invention on improving battery performance.

[0057] We obtained different types of diaphragms using the preparation method of this invention. The preparation method of the diaphragms is as follows:

[0058] S1: Weigh out the sulfide-grafted modified polyimide, binder, inorganic particles and surfactant according to the mass ratio, then add dispersant and stir at 30℃-60℃ to obtain a functional coating slurry with a solid content of 8wt%-50wt%.

[0059] S2: Coat at least one side of the base film after it has been stretched once with the slurry obtained in S1;

[0060] S3: The base film coated with S2 slurry is stretched a second time at the required process temperature of 100℃-150℃ to obtain a diaphragm.

[0061] (1) The mass ratio of the sulfide-grafted modified polyimide, binder, inorganic particles, and surfactant is (1-3 parts):(50-80 parts):(20-50 parts):(0.01-0.1 parts). The total mass of the sulfide-grafted modified polyimide, binder, inorganic particles, and surfactant is 8%-50% of the total mass of the sulfide-grafted modified polyimide, binder, inorganic particles, surfactant, and dispersant.

[0062] (2) The adhesive includes one or more of the following: methylated cellulose salt adhesive, polyacrylic adhesive, and fluoropolymer adhesive.

[0063] Methylated cellulose salt binders include one or more of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and ammonium carboxymethyl cellulose.

[0064] Polyacrylic adhesives include one or more of polyacrylic acid and polymethacrylic acid.

[0065] Fluoropolymer binders include one or more of polyvinylidene fluoride, polytetrafluoroethylene, and polyvinylidene fluoride-hexafluoropropylene copolymer.

[0066] (3) Surfactants include one or more of the following: organofluorocarbon compounds, pyrrolidone polymers, alkylaryl polyether alcohols, alkyl sulfonates, and polyacrylic acid polymers.

[0067] The organofluorocarbon compounds include one or more of the following: perfluoropolyether-based surfactants (Hubei Yunmei Technology Co., Ltd.), carboxylate fluorocarbon surfactants (Wuhan Rongcan Biotechnology Co., Ltd.), and phosphate fluorocarbon surfactants (Wuhan Pushida Biotechnology Co., Ltd.).

[0068] The pyrrolidone polymer includes one or more of polyvinylpyrrolidone, N-dodecyl-2-pyrrolidone, N-octyl-2-pyrrolidone, and dodecylpyrrolidone.

[0069] The alkylaryl polyether alcohols include one or more of Triton WR1339, tetrabutylphenol, and Corning PE-100.

[0070] The alkyl sulfonate includes one or more of sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium hexadecyl sulfonate, sodium octadecyl sulfonate, sodium hexadecylbenzene sulfonate, and sodium octadecylbenzene sulfonate.

[0071] The polyacrylate polymers include one or more of sodium polyacrylate, lithium polyacrylate, ammonium polyacrylate, and potassium polyacrylate.

[0072] (5) Inorganic particles include one or more of titanium dioxide, barium sulfate, calcium carbonate, aluminum oxide, silicon dioxide, and tungsten trioxide.

[0073] The particle size of the inorganic particles is selected from 50nm-300nm.

[0074] (6) The weight-average molecular weight of the thioether-grafted polyimide is 15,000-160,000, wherein the modifying group of the thioether-grafted polyimide includes a thioether group; the modified group of the thioether-grafted polyimide includes an imide group; and the mass ratio of the thioether group to the imide group is 1:(5-20).

[0075] The method for preparing the sulfide-grafted modified polyimide includes: dissolving a sulfur-containing salt in an NMP (N-methylpyrrolidone) solution, adding a halopolyimide and sodium acetate to react, and holding the reaction at 150℃-220℃ for 3-8 hours, then naturally cooling to room temperature. After the reaction is complete, the mixture is filtered and washed several times with hot deionized water and acetone until no chloride ions are present in the system, resulting in a white powder product. The product is then dried at 25℃-30℃.

[0076] The ratio of sulfur-containing salt: halopolyimide: sodium acetate by mass is 1:(5-40):(20-50).

[0077] The amount of NMP added is twice the total mass of the sulfur-containing salt, halogenated polyimide, and sodium acetate.

[0078] Sulfur-containing salts include one or more of anhydrous sodium sulfide, anhydrous ammonium sulfide, and anhydrous potassium sulfide.

[0079] Halogenated polyimides include one or more of chlorinated polyimides, bromopolyimides, iodopolyimides, and fluorinated polyimides.

[0080] Comparative Example 1: A conventional diaphragm in the prior art was selected. A PE base membrane was selected, and an adhesive layer with PVDF was coated on the PE base membrane. The PVDF was dissolved in NMP solution to form the adhesive layer. The coating was applied to the surface of the PE base membrane and then dried.

[0081] Comparative Example 2: The formulation of the membrane obtained in Comparative Example 2 is the same as that in Example 1, but ungrafted modified polyimide is used in Comparative Example 2.

[0082] We also tested the performance of the obtained separator and used the separator of the present invention in a battery to further test the effect of the separator of the present invention on the electrical performance of the battery.

[0083] (7) Battery fabrication:

[0084] Positive electrode: A positive electrode slurry is prepared according to the mass ratio of NCM (nickel-cobalt-manganese material): conductive agent SP (conductive carbon black): binder PVDF (polyvinylidene fluoride): NMP of 95:2:3:100. The slurry is coated on aluminum foil and dried to prepare a positive electrode sheet. The positive electrode sheet is baked in a vacuum oven at 100°C to remove trace water and obtain the positive electrode sheet.

[0085] Separator: A slurry for the functional coating was prepared according to the slurry formulation in Table 1. The obtained slurry was coated on the surface of the base membrane to obtain the battery separator of the present invention. The thickness of the base membrane of the final battery separator product obtained in the present invention is 4 μm-12 μm, and the thickness of the functional coating is 0.1 μm-2.0 μm. The base membrane used in Examples 1-8 is a polyolefin microporous membrane substrate.

[0086] Electrolyte: The electrolyte is a 1 mol / L LiPF6 solution, and the solvent is composed of EC (ethylene carbonate) and DEC (diethyl carbonate) in a volume ratio of 1:1;

[0087] Negative electrode: A negative electrode slurry was prepared according to the mass ratio of graphite: conductive agent SP: binder SBR (styrene-butadiene rubber): CMC (carboxymethyl cellulose): deionized water of 95:1:3:1:100. The slurry was coated on copper foil and dried to prepare a negative electrode sheet. The negative electrode sheet was baked in a vacuum oven at 100°C to remove trace amounts of water and obtain the negative electrode sheet.

[0088] Assembly: The above-mentioned positive electrode, negative electrode, electrolyte and separator are assembled into a lithium-ion pouch battery according to the lithium-ion battery assembly requirements.

[0089] We also obtained the battery and conducted relevant performance tests:

[0090] (1) Cyclic performance test: Cyclic test was conducted using a charging rate of 0.2C and discharging rate of 0.5C.

[0091] (2) Battery internal resistance: Use an AC internal resistance meter to test the AC internal resistance of the battery.

[0092] We also obtained the diaphragm and conducted relevant performance tests:

[0093] (1) Heat shrinkage rate test: High heat resistance is characterized by heat shrinkage (90℃×1h), and the test is carried out in accordance with the standard GB / T 36363-2018.

[0094] (2) Scratch resistance: It is best if there are no scratches within 100 meters of the entire film roll. It is not good if there are 1-3 scratches. It is poor if there are more than 3 scratches.

[0095] (3) Adhesion: The adhesion was tested using the cross-cut adhesion test. When the adhesion was optimal, it was classified as 5B. As the adhesion decreased, the adhesion level decreased to 4B, 3B, 2B, 1B, and 0B respectively. 0B was the worst adhesion level.

[0096] Analysis of verification results:

[0097] Table 1. Formulation of raw materials for the diaphragm in this embodiment of the invention.

[0098]

[0099] Table 2. Sulfide-grafted modified polyimides in the embodiments and comparative examples of the present invention.

[0100] — Weight-average molecular weight Mass ratio of thioether groups to imide groups Example 1 20000-25000 1:5 Example 2 30000-40000 1:8 Example 3 55000-65000 1:10 Example 4 100000-150000 1:6 Example 5 30000-40000 1:18 Example 6 80000-100000 1:20 Example 7 55000-65000 1:12 Example 8 80000-100000 1:14 Example 9 20000-25000 1:5

[0101] Table 3 Test results provided by the present invention and its embodiments

[0102]

[0103] We successfully prepared a battery separator using the method of this invention, as shown in Tables 1-2. We also conducted performance tests on the obtained battery separator and applied it to a battery to investigate the improvement of battery electrical performance by the battery separator of this invention. The results are shown in Table 3. Figure 1This also verifies that the capacity retention rate of Example 1 is superior to that of the traditional battery separator (Comparative Example 1). We found that the performance of the separator and the electrical performance of the battery obtained in Examples 1-9 of this invention are superior to those in Comparative Examples 1-2. We believe this may be because the sulfide-grafted modified polyimide and the binder form a three-dimensional cross-linked network structure. On the one hand, the three-dimensional network structure is conducive to the migration of lithium ions, thereby improving the cycle performance of the battery. On the other hand, the formed three-dimensional cross-linked network structure not only gives the separator better adhesion, but also good flexibility and high-temperature resistance. Furthermore, during the preparation of the separator, the functional coating slurry and the base film undergo secondary stretching together, which is conducive to stronger adhesion between the three-dimensional cross-linked network structure and the base film, and improves the thermal stability and adhesion of the separator. Inorganic particles are embedded in a three-dimensional cross-linked network structure, forming an arrangement layer with at least one layer. On the one hand, the inorganic particles act as a framework, improving the stability of the three-dimensional cross-linked network structure. On the other hand, the pores formed between the inorganic particles facilitate the migration of lithium ions within the separator. Therefore, the addition of inorganic particles not only enhances the thermal stability of the separator but also improves its mechanical properties and prevents scratches. Surfactants improve the wettability of the inorganic particles and other substances in the slurry, thus facilitating the coordination between the various substances in the slurry. Consequently, the battery separator obtained by this invention exhibits high thermal stability, low dimensional change, high ion conductivity, and strong adhesion.

[0104] In existing technologies, polyimide is used as a binder in separators. Polyimide possesses good heat resistance and adhesion properties, but its high glass transition temperature affects its morphological characteristics and structural transformation. In this invention, to promote the formation of a three-dimensional cross-linked network between the polyimide and the binder, the separator exhibits better thermal stability and adhesion. Furthermore, when used in a battery, this separator forms unobstructed lithium-ion transport channels. We further modified the polyimide. The energy barrier for forming the three-dimensional network structure between the thioether-grafted polyimide and the binder was significantly reduced. We believe this is because the synergy between the thioether groups and the CN structure in the polyimide enhances the electrochemical activity of the polyimide. We verified this through comparisons between Examples 1-9 and Comparative Example 2. It is evident that the synergy between the thioether-grafted polyimide and the binder significantly improves both the separator's performance and the battery's electrical performance.

[0105] Based on this, we further investigated the optimization of membrane performance and battery electrical performance through the selection of various substances in the membrane slurry.

[0106] Building upon this, we further investigated the synergy between other substances in this invention. In Example 1, the carboxylic acid group in the polyacrylic binder is an electron-withdrawing group, while the lone pair electrons on the N atom of the sulfide-grafted polyimide facilitate synergy between the polyacrylic binder and the sulfide-grafted polyimide. Furthermore, we selected sulfide-grafted polyimides with a weight-average molecular weight of 20,000-25,000. Sulfide-modified polyimides within this range exhibit better dispersibility in the slurry, are more easily and uniformly dispersed, and promote the formation of a three-dimensional cross-linked network with the polyacrylic binder. To facilitate the successful embedding of inorganic particles into the three-dimensional network structure, we need to select inorganic particles with a more suitable particle size. The particle size is selected from 50nm-80nm. On the one hand, it can be successfully embedded into the three-dimensional network structure and act as a skeleton structure, which is beneficial to improving the mechanical properties of the separator. On the other hand, it will not cause particle agglomeration, which is conducive to the formation of lithium ion migration channels in the three-dimensional network structure. We also found that inorganic particles with a single particle size range have a more compact arrangement than inorganic particles with multiple particle sizes (such as in Examples 5-6), which is beneficial to improving the scratch resistance and thermal stability of the separator. Based on this, we further considered the wettability of surfactants on substances in the slurry. Comparing Examples 1 with Examples 2-9, we found that both the polyacrylic acid polymer and the pyrrolidone polymer in Example 1 are soluble in aqueous or organic solutions, thus not limiting the type of slurry for the diaphragm. In the pyrrolidone polymer, the nitrogen atom in the pyrrole molecule provides a pair of electrons to form a five-center, six-electron conjugated π bond, which is an electron-donating group. The carboxylic acid in the polyacrylic acid polymer is an electron-withdrawing group. The polyacrylic acid polymer and the pyrrolidone polymer can cooperate better. On the one hand, they can promote the wetting of inorganic particles in the slurry. On the other hand, the lone pair electrons of the nitrogen in the thioether-grafted polyimide are beneficial to the stability of the thioether-grafted polyimide in the slurry. The pyrrolidone polymer is beneficial to the stability of the polyacrylic acid binder in the slurry. Thus, it is beneficial for the thioether-grafted polyimide and the polyacrylic acid binder to form a stable three-dimensional network structure. In summary, the raw materials for the functional coating include a polyacrylic binder, a sulfide-grafted polyimide with a weight average molecular weight of 20,000-25,000, inorganic particles with a particle size selected from 50 nm to 80 nm, a pyrrolidone polymer, and a polyacrylic polymer. By mass, the ratio of the polyacrylic binder: the sulfide-grafted polyimide with a weight average molecular weight of 20,000-25,000: the inorganic particles with a particle size selected from 50 nm to 80 nm: the pyrrolidone polymer: the polyacrylic polymer is (50-70 parts): (1-1.2 parts): (20-30 parts): (0.03-0.05 parts): (0.01-0.03 parts).

[0107] Based on this, we further discovered that, compared to barium sulfate in Example 9, the titanium dioxide in the inorganic particles in Example 1 and the sulfide-grafted modified polyimide are more compatible. The Ti=O bond in TiO2 and the -CO-NR-CO- in the sulfide-grafted modified polyimide can form titanium-oxygen-nitrogen single bonds, which is beneficial to stabilizing the organic-inorganic interface and further strengthening the bonding strength between different materials in the coating, ensuring the mechanical strength of the three-dimensional network structure of the entire coating material. At the same time, after film formation, a tight protective layer is formed on the surface of the base film, improving the scratch resistance of the product during the manufacturing process and use. To promote the stable embedding of titanium dioxide into the three-dimensional network structure, we further developed a sulfide-grafted polyimide. Based on a weight-average molecular weight of 20,000-25,000 for the sulfide-grafted polyimide, we further designed the mass ratio of sulfide groups to imide groups to be 1:(4-5). This mass ratio prevents, on the one hand, from excessive grafting leading to repeated grafting on the same imide group, which would result in an unstable three-dimensional cross-linked network with polyacrylic acid, preventing the connection with titanium dioxide and causing it to detach from the structure. On the other hand, it avoids insufficient grafting, which might fail to improve the performance of the polyimide, hindering its interaction with polyacrylic acid, causing titanium dioxide to scatter within the structure or even detach from the coating. This further enhances the inter-complementarity and uniformity of the slurry. We also found that the structurally similar polyacrylic acid and alkali metal polyacrylic acid in Example 1 are easier to combine, which is beneficial for achieving both the wettability of polyacrylic acid and the combination between polyacrylic acid and sulfide-grafted modified polyimide. Furthermore, the use of alkali metal polyacrylic acid in the separator can reduce the separator's impedance, thus improving the battery's electrical performance. Meanwhile, polyvinylpyrrolidone and alkali metal polyacrylic acid adsorb onto the surface of titanium dioxide, reducing the surface activity of titanium dioxide and promoting better interaction between titanium dioxide and sulfide-grafted modified polyimide. In addition, polyacrylic acid, polyvinylpyrrolidone, and alkali metal polyacrylic acid have better flowability and can be more uniformly distributed in the slurry, which not only benefits the uniformity of the slurry but also the interaction between the various substances in the slurry.In summary, the raw materials for the functional coating include polyacrylic acid, sulfide-grafted polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5), titanium dioxide with a particle size selected from 50nm-80nm, polyvinylpyrrolidone, and alkali metal polyacrylic acid. By mass, the ratio of polyacrylic acid:sulfide-grafted polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5):titanium dioxide with a particle size selected from 50nm-80nm:polyvinylpyrrolidone:alkali metal polyacrylic acid is (50 parts-70 parts):(1 part-1.2 parts):(20 parts-30 parts):(0.03 parts-0.05 parts):(0.01 parts-0.03 parts).

[0108] In summary, the interaction between the various substances in the slurry of the functional coating of the separator of the present invention results in a separator with high thermal stability, low dimensional change, high ion conductivity and good adhesion. Furthermore, using the separator of the present invention in batteries can also improve the electrical performance of the batteries.

[0109] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A battery separator, characterized in that, The battery separator includes a base film and a functional coating disposed on at least one side of the base film. The raw materials of the functional coating include polyacrylic acid, sulfide-grafted modified polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5), titanium dioxide with a particle size selected from 50nm-80nm, polyvinylpyrrolidone, and alkali metal polyacrylic acid. By mass, the ratio of polyacrylic acid: sulfide-grafted modified polyimide with a mass ratio of sulfide groups to imide groups of 1:(4-5): titanium dioxide with a particle size selected from 50nm-80nm: polyvinylpyrrolidone: alkali metal polyacrylic acid is (50 parts-70 parts):(1 part-1.2 parts):(20 parts-30 parts):(0.03 parts-0.05 parts):(0.01 parts-0.03 parts).

2. The battery separator according to claim 1, characterized in that, The thioether-grafted modified polyimide has a weight-average molecular weight of 15,000-160,000.

3. An electrochemical device, characterized in that, The electrochemical device includes the battery separator as described in any one of claims 1-2.

4. An electrical appliance, characterized in that, The electrical equipment includes the electrochemical device as described in claim 3.

5. The method for preparing the battery separator according to any one of claims 1-2, characterized in that, The preparation method includes: S1: Weigh out polyimide grafted with sulfide in a mass ratio of 1:(4-5), polyacrylic acid, titanium dioxide with a particle size of 50nm-80nm, polyvinylpyrrolidone, and alkali metal polyacrylic acid according to the mass ratio of sulfide group to imide group 1:(4-5), then add dispersant and mix well to obtain a slurry with a solid content of 8wt%-50wt% for the functional coating. S2: Coat at least one side of the base film after it has been stretched once with the slurry obtained in S1; S3: The base film coated with S2 slurry is stretched a second time at the required process temperature to obtain a diaphragm.

6. The preparation method according to claim 5, characterized in that, The mixing method in S1 includes stirring, and the mixing conditions are 30℃-60℃; the process temperature in S3 is 100℃-150℃.

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