Separator for secondary battery, preparation method thereof and lithium secondary battery

Abstract

The present invention provides a separator for a secondary battery having an improved coating layer, a method for preparing the same, and a lithium secondary battery. The diaphragm comprises a porous membrane and a coating layer formed on at least one surface of the porous membrane, the coating layer comprises a fiber component, an inorganic filler component and a binder component, the fiber component comprises one or both of a cellulose gel component and a polyimide short fiber component, and the inorganic filler component comprises a binder component and a cellulose gel component. The cellulose group gel component includes a gel derived from microbial fermented cellulose, and the surface of the polyimide staple fiber at least partially has a carboxyl group. The diaphragm provided by the invention has the advantages of high temperature resistance, strengthened structure, better wettability and better ion permeability, realizes the light weight of the coating, improves the ion transmission efficiency and safety of the battery, and prolongs the cycle life of the battery.

Description

Technical Field

[0001] This invention belongs to the field of new energy, specifically to the field of lithium batteries, and more specifically, relates to a separator for secondary batteries with an improved coating, a method for preparing the same, and a lithium secondary battery. Background Technology

[0002] The separator is an indispensable component of a lithium-ion battery system. It prevents short circuits between the positive and negative electrodes, ensuring battery safety, while also providing an effective channel for the rapid migration of lithium ions. As a core component of new energy technologies, the performance and safety of lithium batteries highly depend on innovation in separator materials.

[0003] Typical membrane types include resins (such as polyolefins) that undergo film formation and pore formation processes to obtain membrane materials with a certain porosity.

[0004] Furthermore, fibrous membranes with a certain degree of porosity can also be prepared through the processing of fibers (short or continuous fibers). For example, using wet nonwoven (papermaking) processes to manufacture nanofiber nonwoven membranes can simultaneously achieve good wettability, mechanical strength, heat resistance, and electrochemical properties; or, continuous fibers can be prepared to form fibrous membranes via resin electrospinning. However, the above-mentioned fibrous membranes still have shortcomings in terms of heat resistance. In addition, processes such as electrospinning are complex, have low yields, and their efficiency cannot meet the needs of large-scale production.

[0005] To improve heat resistance, membrane coating modification has been extensively studied in recent years. For example, ceramic-coated membranes (such as alumina ceramic layers) have gradually become a key direction for technological upgrading in the industry due to their unique performance advantages, and occupy an important position in the global lithium battery market. However, the use of a large number of ceramic particles is also detrimental to lightweighting and improving energy density.

[0006] Furthermore, to meet the development trend of high-energy-density and high-safety battery cells, it is necessary to develop a coating material for the separator that balances thinness and high heat resistance. Research has found that using lightweight nanoscale materials (such as nanofibers) can reduce the coating thickness, resulting in a lighter separator (1.0 g / m³). 2 It exhibits advantages in terms of ( / μm), heat resistance, and safety.

[0007] Some existing technologies have studied coating materials for porous membrane bases, which use polymer fibers, such as polyimide nanofibers and aramid nanofibers, to be combined with inorganic particles. Although good thermal stability can be obtained, there are also problems with insufficient wettability and liquid retention.

[0008] Therefore, although existing technologies have conducted some research on coating materials, there is still room for further improvement in terms of lightweighting, wettability, and safety. Summary of the Invention

[0009] The problem the invention aims to solve

[0010] Existing nanofiber membranes are produced using either wet or electrospinning processes, which are complex, costly, and result in relatively thick films.

[0011] Furthermore, in existing membrane-based coating reinforcement schemes, the use of ceramic particles must be maintained at a certain amount to meet heat resistance requirements. However, this obviously leads to problems that hinder weight reduction, and also results in poor wettability of the electrolyte to the final membrane.

[0012] Therefore, the main objective of this invention is to provide an improved coated membrane component that, through the combined use of specific cellulose components and / or specific polyamide short fibers, can significantly reduce the use of inorganic components while meeting heat resistance requirements, thereby reducing the weight of the final membrane product. On the other hand, the wettability of the coating is also improved, which can shorten the electrolyte wetting time and optimize ion conduction efficiency.

[0013] Furthermore, the present invention also aims to provide a method for preparing a membrane with a coating (especially a coating containing bacterial cellulose and / or polyimide) and a lithium battery. The preparation method is simple and low in cost, and the resulting membrane is thin enough to meet the needs of ultra-high energy density battery systems. Moreover, the membrane has good thermal stability, air permeability, wettability, and mechanical strength, which can significantly improve the safety and stability of lithium batteries.

[0014] Solution for solving the problem

[0015] The above-mentioned technical problems can be solved by implementing the following technical solutions:

[0016] The present invention first provides a separator for a secondary battery, the separator comprising a porous membrane and a coating layer formed on at least one surface of the porous membrane.

[0017] The porous membrane is a polymer resin-based porous membrane or an inorganic fiber-based porous membrane.

[0018] The coating layer includes a fiber component, an inorganic filler component, and a binder component, wherein the fiber component includes one or both of a cellulose gel component and a polyimide short fiber component.

[0019] The cellulose gel component comprises a gel derived from cellulose produced by microbial fermentation, and the lignin and hemicellulose content in the cellulose gel component is less than 10% by mass.

[0020] The surface of the polyimide short fibers has at least partially carboxyl groups.

[0021] Based on the diaphragm described above, the content of the cellulose gel component is 0.02% to 4% and the content of the polyimide short fiber component is 2% to 20% based on the total dry weight of the coating layer.

[0022] According to the diaphragm described above, the porous membrane is a polymer porous membrane or a glass fiber porous membrane; the porosity of the porous membrane is 20% to 45% by volume.

[0023] According to the diaphragm described above, the coating layer is formed on both surfaces of the porous membrane; the coating layer at least partially extends below the surface of the porous membrane.

[0024] Based on the diaphragm described above, the cellulose produced by microbial fermentation is bacterial cellulose; the cellulose gel component does not substantially contain lignin and hemicellulose.

[0025] According to the diaphragm described above, the cellulose gel component is only a gel derived from cellulose fermented by microorganisms.

[0026] According to the diaphragm described above, the polyimide short fiber is a polyimide short fiber that has been treated with acid.

[0027] According to the diaphragm described above, the polyimide short fibers have a diameter of 0.3~1.0 μm and a length of less than 30 μm.

[0028] According to the diaphragm described above, the coating layer also includes a dispersant.

[0029] According to the diaphragm described above, the total thickness of the diaphragm is 7~20 μm.

[0030] Furthermore, the present invention provides a method for preparing the above-described diaphragm, which includes the following steps:

[0031] S1: Mix the fiber dispersion, inorganic filler component, binder component and water to obtain a diaphragm coating liquid, optionally, the coating liquid further contains a dispersant component; wherein, the fiber dispersion includes one or both of microbially fermented cellulose and polyimide short fibers.

[0032] S2: The membrane coating liquid is coated on one or both sides of the porous membrane, and after drying, the membrane is obtained.

[0033] Furthermore, the present invention provides a lithium secondary battery, the lithium secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator prepared according to the above-described method or the above-described preparation method.

[0034] The lithium secondary batteries described above are lithium secondary batteries for personal consumer electronic devices, lithium secondary batteries for energy storage stations, lithium secondary batteries for transportation vehicles, or lithium secondary batteries for industrial tools.

[0035] The effects of the invention

[0036] By implementing the above technical solution, the present invention has at least the following technical effects:

[0037] The diaphragm with the improved coating of the present invention has the following advantages:

[0038] High temperature resistance: The diaphragm of the present invention has a high rupture temperature and good heat shrinkage resistance.

[0039] Structural reinforcement: A three-dimensional interwoven support skeleton is formed through a coating process, which solves the problem of "easy peeling at high temperatures" of traditional materials; the separator of the present invention has a lower thermal shrinkage rate and higher tensile strength, the structure of the separator is strengthened, the thermal stability is better, the structural stability is better, and the safety and cycle life of the battery are improved.

[0040] Optimized ion transport efficiency: The hydrophilic properties of the separator of this invention can shorten the electrolyte wetting time, optimize ion conduction efficiency, and reduce energy loss; the separator of this invention has a lower contact angle, better wettability, stronger hydrophilicity of the separator surface, lower interfacial impedance, and higher ion conductivity; the separator of this invention has lower air permeability, better ion permeability, and improves the ion transport efficiency of the battery.

[0041] Synergistic effect: Compared with the separator with only cellulose gel component or only polyimide short fiber component, the separator of the present invention with both cellulose gel component and polyimide short fiber component has better thermal stability and structural stability, stronger hydrophilicity, better wetting performance and better ion permeability. The synergistic effect of cellulose gel component and polyimide short fiber component improves battery safety, cycle life and ion transport efficiency.

[0042] The method for preparing the diaphragm described above in this invention has the following advantages:

[0043] The preparation method is simple: the process is simple, no special equipment or special materials are required, the cost is low, and it is suitable for large-scale production.

[0044] Thin film thickness: The membrane is thinner, and the cellulose gel component and polyimide short fiber component have lower densities, which can further reduce the amount of inorganic particles, achieve lightweight coating, and meet the needs of ultra-high energy density battery systems.

[0045] No thickener required: Conventional coating methods require the use of thickeners to increase the viscosity of the coating solution. The bacterial cellulose used in this invention has a high water absorption rate and good water retention performance, which can keep the slurry in stable suspension and increase the resistance to movement of ceramic particles. Therefore, bacterial cellulose acts as a thickener, so no additional thickener needs to be added to the diaphragm coating solution.

[0046] As a lithium secondary battery containing the above-described separator according to the present invention, it has the following advantages:

[0047] Safety Upgrade: The improved coating membrane can effectively delay heat spread and reduce the risk of thermal runaway. It is compatible with high energy density systems such as 800V high-voltage fast charging and high-nickel ternary batteries. The overall weight can be reduced, and the overall energy density of the battery can be improved.

[0048] Performance improvements: Extended battery cycle life and increased energy density help new energy vehicles achieve "longer range and slower degradation". Attached Figure Description

[0049] Figure 1 Photographs showing the thermal shrinkage properties of the diaphragm prepared in Example 1.

[0050] Figure 2 Photographs showing the thermal shrinkage properties of the diaphragm prepared in Example 2.

[0051] Figure 3 Photographs showing the thermal shrinkage properties of the diaphragm prepared in Example 3.

[0052] Figure 4 Photographs showing the thermal shrinkage properties of the diaphragm prepared for Comparative Example 1.

[0053] Figure 5 Photographs showing the thermal shrinkage properties of the diaphragm prepared for Comparative Example 2.

[0054] Figure 6 Photographs showing the thermal shrinkage properties of the diaphragm prepared for Reference Example 1.

[0055] Figure 7 Photographs showing the thermal shrinkage properties of the diaphragm prepared for Reference Example 2. Detailed Implementation

[0056] Various exemplary embodiments, features, and aspects of the present invention will be described in detail below. The term "exemplary" as used herein means "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as superior to or better than other embodiments.

[0057] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art should understand that the present invention can be practiced without certain specific details. In other instances, methods, means, apparatus, and steps well known to those skilled in the art have not been described in detail in order to highlight the spirit of the present invention.

[0058] Unless otherwise stated, all units used in this specification are international standard units, and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors that are unavoidable in industrial production.

[0059] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0060] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0061] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0062] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0063] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0064] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" indicates weight or mass percentage.

[0065] In this specification, the terms "substantially" and "essentially" are used to indicate that the standard deviation from the theoretical model, theoretical data, or target data is within a range of 1%, preferably 0.8%, and more preferably 0.7%, depending on the allowable deviation under the corresponding industrial conditions.

[0066] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0067] This invention primarily provides a separator with a coating layer having an improved composition, a method for preparing the separator, and a secondary battery containing or using the separator. This invention is mainly based on the following insights:

[0068] Diaphragms with a ceramic coating typically exhibit good heat resistance, but this also significantly increases their weight, and the wettability of the ceramic layer is problematic. While cellulose materials theoretically offer weight reduction, their heat resistance is insufficient. Therefore, heat-resistant fiber materials can be further used. Although this component improves the heat resistance of the coating and achieves weight reduction, its poor dispersibility prevents these advantages from being fully realized. In continuous improvement, this invention has experimented with partially replacing ceramic materials with one or both of microbially fermented cellulose and modified polyimide fibers. Unexpectedly, it was discovered that the carboxyl functional groups from the modified polyimide fibers significantly improve its dispersibility, and the gel structure formed by microbially fermented cellulose promotes structural stability and increases toughness. Furthermore, the gel of modified polyimide fibers and microbially fermented cellulose also improves the overall wettability and liquid retention of the diaphragm.

[0069] (Diaphragm)

[0070] The diaphragm of the present invention comprises a porous polymer resin membrane as the base membrane. Furthermore, a coating layer is provided on one or both sides of the porous membrane.

[0071] base membrane

[0072] The porous membrane base of this invention can be a polymer resin-based porous membrane or an inorganic fiber-based porous membrane.

[0073] The polymer resin can be, for example, (modified) polyolefin materials (PE, PP, PP / PE composites, PTFE, etc.), polyacrylonitrile (PAN), polyester (PET), etc.

[0074] For inorganic fiber-based porous membranes, such as porous membranes formed from glass fibers.

[0075] Furthermore, the porous membrane of the present invention can be a single-layer membrane structure or a composite structure of multiple porous membranes.

[0076] The thickness of the porous membrane base of this invention is not particularly limited in principle; for example, it can typically be in the range of 5 to 15 μm. The porosity of the porous membrane base is typically 20% to 45% by volume, for example, 25%, 30%, 35%, 40%, 42%, etc.

[0077] There are no particular restrictions on the formation method of porous membrane base films; they can be obtained through textile, non-textile, molding, extrusion, and other methods.

[0078] In some preferred embodiments of the present invention, the porous membrane base is a PP membrane or a PE membrane.

[0079] Coating layer

[0080] The coating of the present invention comprises a fiber component, an inorganic filler component, and a binder component, wherein the fiber component comprises one or both of a cellulose gel component and a polyimide short fiber component, and optionally, the coating may further comprise a dispersant component.

[0081] The cellulose gel component can be formed by the gelation of the corresponding cellulose, including cellulose derived from microbial fermentation. Bacterial cellulose is preferred for this purpose.

[0082] The cellulose obtained by microbial fermentation has a high purity composition. In some specific embodiments, the total content of lignin and hemicellulose in the cellulose obtained by microbial fermentation is less than 10% by mass, for example, less than 8% by mass, less than 5% by mass, less than 3% by mass, etc. In some preferred embodiments, the cellulose obtained by microbial fermentation is substantially free of lignin and hemicellulose. Therefore, compared with general cellulose containing lignin or hemicellulose to a certain extent, the cellulose of the present invention has a better gel thickening effect and can form a less defective network structure in the swollen gel, thereby providing better toughness, support and wettability.

[0083] For the polyimide short fibers of the present invention, one or more of the polyimide short fibers formed using diamine monomers and dianhydride monomers commonly used in the art can be used. Preferably, at least one of such diamine monomers or dianhydride monomers has an aromatic structure. Furthermore, such fibers typically have a diameter of 0.3 to 1.0 μm and a length of less than 30 μm. In some preferred embodiments, the polyimide short fibers have a diameter of 0.5 to 0.8 μm and a fiber length of 15 to 20 μm.

[0084] Furthermore, polyimide short fibers typically possess good imide content, but this also results in poor dispersibility in some aqueous systems. Therefore, the polyimide short fibers used in this invention are fibers with carboxyl functional groups on their surface. Such functional groups not only improve the dispersibility of these short fibers in aqueous systems but also enhance their binding with the aforementioned cellulose gel components, thereby enabling a more uniform bonding between the two in the coating layer. There are no particular limitations on the source of the carboxyl groups; they can be obtained through radiation or chemical treatment, preferably chemical treatment. In some preferred embodiments, the polyimide short fibers are acid-modified polyimide short fibers. Surface modification of the polyimide short fibers is achieved by using inorganic or organic acids, causing the imide groups on their surface to hydrolyze to produce ammonium acids, exposing carboxyl groups and simultaneously increasing the surface roughness of the fibers. Examples of acids include one or more of sulfuric acid, nitric acid, terephthalic acid, hypochlorous acid, nitrous acid, and chlorosulfonic acid. Furthermore, the acid treatment refers to placing the polyimide short fibers in an acid solution, wherein the mass content of the acid in the acid solution can be 50% to 70%, and the acid treatment time can be 0.5 to 1 hour.

[0085] There are no particular limitations on the inorganic filler composition of this invention, and ceramic particle materials with good heat resistance commonly used in the art can be used. In some embodiments, the ceramic is one or more materials derived from alumina, boehmite, BaTiO3, ZnO, AlN, ZrO2, and magnesium hydroxide. In some preferred embodiments, the ceramic is alumina. In some embodiments, the ceramic is a nanoscale ceramic, that is, the particle size of the ceramic is in the nanometer range, for example, 50~500 nm, preferably 100~400 nm.

[0086] There are no particular limitations on the adhesive components of this invention; adhesive materials commonly used in coating materials in the art can be used. Examples of adhesive materials that can be listed in this invention include one or more of PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber). In some preferred embodiments, the adhesive is PMMA (polymethyl methacrylate). Furthermore, the above-mentioned adhesives are all commonly used in batteries and will not have a significant adverse effect on the performance of the electrode or battery.

[0087] In addition, to aid in the dispersion of the components, a dispersant may be used in the coating layer of the present invention in some specific embodiments. The dispersant suitable for the present invention is preferably selected from one or more surfactants such as fatty acid glycerides, sucrose fatty acid esters, fatty acid sorbitan, polysorbates, polyoxyethylene fatty acid esters, polyoxyethylene fatty alcohol ethers, and polyoxyethylene-polyoxypropylene polymers. In some preferred embodiments, the dispersant is a polyoxyethylene fatty alcohol ether. The above-mentioned dispersants are commonly used surfactants in batteries and will not have a significant adverse effect on the performance of the electrode or battery.

[0088] Furthermore, in this invention, the content of the fiber component, based on the total mass (dry mass) of the coating layer, can be 0.02% to 24%, for example 0.05%, 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 5%, 7%, 10%, 12%, 14%, 15%, 16%, 17%, 18%, 20%, 22%, etc., preferably 1% to 18%.

[0089] Furthermore, in this invention, the content of the cellulose gel component can be 0.02% to 4% based on the total mass (dry mass) of the coating layer, for example, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc., preferably 1% to 3%.

[0090] Furthermore, in this invention, the content of the polyimide short fiber component can be 2% to 20% based on the total mass (dry mass) of the coating layer, for example, 2.5%, 3%, 5%, 7%, 10%, 12%, 14%, 15%, 16%, 18%, etc., preferably 10% to 20%.

[0091] Furthermore, in this invention, the content of the inorganic filler component can be 40% to 90% based on the total mass (dry mass) of the coating layer, for example, 45%, 50%, 60%, 70%, 80%, 85%, 90%, etc., preferably 60% to 90%.

[0092] Furthermore, in this invention, the content of the adhesive component can be 4% to 15% based on the total mass (dry mass) of the coating layer, for example, 5%, 6%, 7%, 8%, 10%, 12%, etc., preferably 8% to 12%.

[0093] Furthermore, in this invention, the content of the dispersant component can be 2% to 10% based on the total mass (dry mass) of the coating layer, for example, 3%, 4%, 6%, 8%, etc., preferably 2% to 6%.

[0094] (Diaphragm characteristics)

[0095] Through the optimization of the coating composition described above, the separator of the present invention has a thinner thickness, which can meet the needs of ultra-high energy density battery systems. In some embodiments, the total thickness of the separator can be 7~20 μm, such as 8 μm, 9 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, etc., preferably 8~12 μm.

[0096] Furthermore, the separator of the present invention has a lower thermal shrinkage rate and better thermal stability, thereby improving battery safety and cycle life. In some embodiments, the thermal shrinkage rate of the separator of the present invention after heating at 150°C for 1 hour is less than 4.5%, preferably less than 4%, more preferably less than 3.5%, for example 3.2%, 3%, 2.5%, 2%, 1%, etc.

[0097] The separator of the present invention has lower air permeability and better ion permeability, thereby improving the ion transport efficiency of the battery. In some embodiments, the air permeability of the separator of the present invention does not exceed 210 s / 100cc, preferably not more than 205 s / 100cc, and more preferably not more than 190 s / 100cc, such as 185 s / 100cc, 180 s / 100cc, 170 s / 100cc, 150 s / 100cc, etc.

[0098] Furthermore, the diaphragm of the present invention has a lower contact angle, better wettability, stronger hydrophilicity of the diaphragm surface, lower interfacial impedance, and higher ionic conductivity. In some embodiments, the contact angle of the diaphragm of the present invention does not exceed 40°, preferably not more than 38°, and more preferably not more than 35°, such as 34°, 32°, 30°, 28°, 25°, etc.

[0099] The separator of the present invention has higher tensile strength, a strengthened structure, and better structural stability, thereby improving battery safety and cycle life. In some embodiments, the tensile strength of the separator of the present invention is 1500 Kgf / cm². 2 The preferred value is 1550 Kgf / cm³. 2 The above is further optimized to 1650 Kgf / cm 2 The above, for example, 1655 Kgf / cm 2 1660Kgf / cm 2 1670 Kgf / cm 2 1700 Kgf / cm 2 1750 Kgf / cm 2 1800 Kgf / cm 2 2000 Kgf / cm 2 wait.

[0100] The separator of the present invention has a higher membrane breakage temperature and better thermal stability, thereby improving battery safety and cycle life. In some embodiments, the membrane breakage temperature of the separator of the present invention is above 165°C, preferably above 170°C, and more preferably above 180°C, such as 175°C, 185°C, 190°C, 200°C, etc.

[0101] (Method for preparing the diaphragm)

[0102] Although the preparation method of the diaphragm described above is not particularly limited in this invention, the diaphragm of this invention can be prepared more efficiently and the desired effect can be obtained more easily by the following method.

[0103] Therefore, the present invention further provides a method for preparing a diaphragm, which includes the following steps:

[0104] S1: A diaphragm coating liquid is obtained by mixing a fiber dispersion, an inorganic filler component, a binder component, and water, and optionally, a dispersant component may be further mixed in; wherein the fiber dispersion comprises one or both of microbially fermented cellulose and polyimide short fibers.

[0105] S2: Coat one or both sides of the porous membrane with the membrane coating liquid, and dry to obtain the membrane.

[0106] Step S1

[0107] In some embodiments, the fiber dispersion is obtained by further mixing two dispersions: a cellulose dispersion produced by microbial fermentation and a polyimide short fiber dispersion. Further, the mass ratio of the cellulose dispersion produced by microbial fermentation to the polyimide short fiber dispersion is 1:3 to 3:1, for example, 1:2, 1:1, 2:1, etc., preferably 1:2 to 2:1.

[0108] In some embodiments, the fiber dispersion is a cellulose dispersion produced by microbial fermentation.

[0109] In other embodiments, the fiber dispersion is a polyimide short fiber dispersion.

[0110] In some embodiments, the mass fraction of cellulose fermented by microorganisms in the cellulose dispersion is 0.3% to 10%, for example, 0.4%, 0.5%, 1%, 2%, 5%, etc.

[0111] In some embodiments, the mass fraction of polyimide short fibers in the polyimide short fiber dispersion is 30% to 50%, for example, 35%, 40%, 45%, etc.

[0112] In some specific embodiments, the mass ratio of the fiber dispersion, the inorganic filler component, the binder component, the dispersant component and the water is (5~15):(15~25):(1~3):(0.5~1.5):(55.5~78.5), preferably (5~10):(15~20):(1~2):(0.5~1):(67~78.5).

[0113] In some embodiments, the adhesive component is used in the form of an aqueous solution, for example, the adhesive component is an aqueous PMMA solution. In some specific embodiments, the solids content of the aqueous adhesive solution is 10% to 30%, for example, 15%, 20%, 25%, etc.

[0114] In some embodiments, the dispersant component is used in the form of an aqueous solution, for example, the dispersant component is an aqueous solution of polyoxyethylene fatty alcohol ether. In some specific embodiments, the solids content of the aqueous dispersant solution is 20% to 40%, for example, 25%, 29%, 30%, 35%, etc.

[0115] In some specific implementations, the mixing step in step S1 is as follows: first, the fiber dispersion is added to the water for dispersion treatment; then, the inorganic filler component is added for dispersion treatment; then, the binder component is added for dispersion treatment; optionally, the dispersant component may be added for dispersion treatment.

[0116] In some embodiments, the step of forming the fiber dispersion may be: first, adding either a microbially fermented cellulose dispersion or a polyimide short fiber dispersion to water for dispersion treatment, and then adding the other one for dispersion treatment.

[0117] In this invention, the dispersion treatment can be ultrasonic dispersion or stirring dispersion, with ultrasonic dispersion being preferred. In some specific embodiments, the conditions for ultrasonic dispersion are: temperature 25~30℃, power 100~300 Hz or 100~500 Hz or 100~1000 Hz, and time 5~10 min or 10~20 min.

[0118] In some embodiments, the polyimide short fiber dispersion is obtained by dispersing polyimide short fibers and water. In some preferred embodiments, the dispersion is carried out in the presence of a nonionic dispersant. Furthermore, the mass ratio of the polyimide short fibers, the nonionic dispersant, and the water can be (30-50):(1-5):(45-69). In some specific embodiments, the nonionic dispersant is ethylene oxide, and the dispersion treatment is ultrasonic dispersion, with the following conditions: temperature 25-55°C, power 100-1000 Hz, and ultrasonic treatment time 60-120 min.

[0119] In some embodiments, the microbially fermented cellulose dispersion is obtained by dispersing microbially fermented cellulose gel and water. In some preferred embodiments, the dispersion is carried out in the presence of a nonionic dispersant. Furthermore, the mass ratio of the microbially fermented cellulose gel, the nonionic dispersant, and the water can be (80~100):(1~5):(80~100), the solids content of the microbially fermented cellulose gel is 0.8%~4%, and the viscosity range is 10000~15000 mPa·s. In some specific embodiments, the nonionic dispersant is ethylene oxide, and the dispersion treatment is ultrasonic dispersion, with the ultrasonic dispersion conditions being: temperature 25~35℃, power 100~1000Hz, and ultrasonic treatment time 0.5~1 h.

[0120] In some embodiments, the water used in this invention is deionized water or purified water. In some preferred embodiments, the water is deionized water.

[0121] Since the cellulose gel liquid produced by microbial fermentation has a water absorption rate of about 200 times, its gel strength is high and its water retention performance is good, it can keep the slurry in stable suspension and increase the resistance to movement of inorganic filler particles. Therefore, the cellulose produced by microbial fermentation acts as a thickener, so there is no need to add additional thickeners such as CMC to the diaphragm coating liquid.

[0122] Furthermore, in some embodiments, the solid content of the diaphragm coating liquid obtained in step S1 above is 27%~35%, the viscosity is 50~150 mPa·s, and the particle size distribution of the solids is D. 10 : 0.2~0.6 μm, D 50 : 0.4~0.7 μm, D 90 : 5~12 μm.

[0123] Step S2

[0124] In some implementations, in step S2, the diaphragm coating liquid is uniformly coated onto one or both sides of the base membrane using a wire rod, preferably both sides.

[0125] In some implementations, in step S2, drying refers to drying off the moisture; further, the drying temperature is 50~80℃, for example 55℃, 60℃, 65℃, 70℃, etc., and the drying time is 4~8 h, for example 5 h, 6 h, 7 h, etc.

[0126] In some implementations, in step S2, the thickness of the base film is 3~15 μm, such as 5 μm, 6 μm, 8 μm, 9 μm, 12 μm, etc., preferably 5~8 μm.

[0127] In some implementations, the coating thickness in step S2 is 2~3 μm.

[0128] (Lithium battery)

[0129] Furthermore, the present invention provides a lithium secondary battery comprising the separator described in the present invention.

[0130] The lithium secondary battery of the present invention may include a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode.

[0131] In some preferred embodiments, the positive electrode active material in the positive electrode can be an active material with a high nickel content.

[0132] In some specific embodiments, the battery of the present invention appears and is used in a single form; in other specific embodiments, the battery of the present invention can be used in parallel or series in any number of units.

[0133] In some specific implementations, the lithium secondary battery of the present invention can be a lithium secondary battery for personal consumer electronic devices, a lithium secondary battery for energy storage stations, a lithium secondary battery for transportation vehicles, or a lithium secondary battery for industrial tools.

[0134] Example

[0135] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0136] Example 1

[0137] 1. Modification of polyimide (PI) short-cut fibers and preparation of PI dispersion.

[0138] PI chopped fibers were treated with 70% sulfuric acid for 30 min, filtered to remove the sulfuric acid, and then washed three times with deionized water by centrifugation to obtain modified PI chopped fibers. The PI chopped fibers were a yellow powder with a main fiber diameter of 0.5–0.8 μm and a main fiber length of 15–20 μm, and were commercially available.

[0139] Modified PI chopped fibers were dispersed in deionized water, and nonionic dispersant ethylene oxide was added. The mixture was then ultrasonically dispersed to obtain a PI dispersion. The mass ratio of modified PI chopped fibers, dispersant, and water was 30:1:69. The ultrasonic dispersion conditions were: temperature 25℃, power 100 Hz, and ultrasonic treatment time 120 min.

[0140] 2. Preparation of bacterial cellulose dispersion

[0141] Bacterial cellulose gel was added to deionized water, and nonionic dispersant ethylene oxide was added. The mixture was then ultrasonically dispersed to obtain a bacterial cellulose dispersion. The bacterial cellulose gel was a milky white gel-like liquid with a solid content of 2% by mass, a viscosity of 10000 mPa·s, a fiber diameter of 50–100 nm, a fiber length >20 μm, and a cellulose I crystal structure. It was commercially available. The mass ratio of bacterial cellulose gel, dispersant, and water was 80:1:80. The ultrasonic dispersion conditions were: temperature 25℃, power 100 Hz, and ultrasonic treatment time 60 min.

[0142] 3. Preparation of diaphragm coating solution

[0143] Deionized water was added to the bacterial cellulose dispersion from step 2, and ultrasonic dispersion was performed under the following conditions: temperature 25℃, power 100 Hz, and ultrasonic treatment time 20 min. Then, the PI dispersion from step 1 was added, and dispersion continued under the same ultrasonic conditions. Next, alumina with an average particle size of 300 nm was added, and dispersion continued under the same ultrasonic conditions. Then, the binder PMMA (added as an aqueous solution with a solid content of 20%) was added, and ultrasonic dispersion continued under the following conditions: temperature 30℃, power 100 Hz, and ultrasonic treatment time 20 min. Finally, the surfactant polyoxyethylene fatty alcohol ether (added as an aqueous solution with a solid content of 29%) was added, and ultrasonic dispersion continued under the following conditions: temperature 25℃, power 100 Hz, and ultrasonic treatment time 5 min, yielding the slurry, i.e., the diaphragm coating solution.

[0144] The mass ratio of bacterial cellulose dispersion: PI dispersion: alumina: PMMA: polyoxyethylene fatty alcohol ether: deionized water is 5:5:20:2:1:67.

[0145] The slurry has a solids content of 33% by mass, a viscosity of 106 mPa·s, and a particle size distribution of D. 10 : 0.3 μm, D 50 : 0.5 μm, D 90 : 8 μm.

[0146] 4. Diaphragm coating

[0147] Using a wire rod, the membrane coating solution from step 3 is evenly coated onto both sides of the base membrane, and then baked at 60°C for 5 hours to dry the moisture, thus obtaining the membrane.

[0148] The base membrane is made of PE with a thickness of 7 μm, and the coating thickness on each side is 2 μm. After drying, the total thickness of the resulting membrane is 9 μm. Based on the total dry weight of the coating layer of the membrane, the content of bacterial cellulose is 2%, polyimide chopped fibers is 15%, alumina is 70%, PMMA is 10%, and polyoxyethylene fatty alcohol ether is 3%.

[0149] Example 2

[0150] The difference between Example 2 and Example 1 is that in step 3, only bacterial cellulose dispersion is added, and PI dispersion is not added. In step 3, the mass ratio of bacterial cellulose dispersion: alumina: PMMA: polyoxyethylene fatty alcohol ether: deionized water is 5:25:2:1:67. The slurry obtained in step 3 has a solids content of 33% by mass, a viscosity of 120 mPa·s, and a particle size distribution of D. 10 : 0.5 μm, D 50 : 0.6 μm, D 90 7 μm. All other conditions remained the same as in Example 1.

[0151] The final membrane contains, by weight of the coating layer, 2% bacterial cellulose, 85% alumina, 10% PMMA, and 3% polyoxyethylene fatty alcohol ether.

[0152] Example 3

[0153] The difference between Example 3 and Example 1 is that in step 3, only the PI dispersion is added, and the bacterial cellulose dispersion is not added. In step 3, the mass ratio of PI dispersion: alumina: PMMA: polyoxyethylene fatty alcohol ether: deionized water is 5:25:2:1:67. The slurry obtained in step 3 has a solid content of 33% by mass, a viscosity of 120 mPa·s, and a particle size distribution of D. 10 : 0.4 μm, D 50 : 0.6 μm, D 90 10 μm. All other conditions remained the same as in Example 1.

[0154] The final membrane contains, by weight of the coating layer, 10% polyimide chopped fiber, 77% alumina, 10% PMMA, and 3% polyoxyethylene fatty alcohol ether.

[0155] Comparative Example 1

[0156] A 7 μm PE base membrane was used as the separator without any treatment.

[0157] Comparative Example 2

[0158] The difference from Example 1 is that PI and bacterial cellulose dispersion were not added. In step 3, the mass ratio of alumina ceramic:PMMA:polyoxyethylene fatty alcohol ether:deionized water is 30:2:1:67. The slurry obtained in step 3 has a solid content of 33%, a viscosity of 120 mPa·s, and a particle size distribution of D. 10 : 0.3 μm, D 50 : 0.4 μm, D 90 5 μm. All other conditions remained the same as in Example 1.

[0159] The final membrane contains 87% alumina, 10% PMMA, and 3% polyoxyethylene fatty alcohol ether, based on the total dry mass of the coating layer.

[0160] Reference Example 1

[0161] The difference from Example 1 is that in step 3, the mass ratio of bacterial cellulose dispersion: PI dispersion: alumina: PMMA: polyoxyethylene fatty alcohol ether: deionized water is 7:5:18:2:1:67.

[0162] The final membrane contains, by weight of the coating layer, 5% bacterial cellulose, 15% polyimide chopped fiber, 67% alumina, 10% PMMA, and 3% polyoxyethylene fatty alcohol ether.

[0163] See Example 2

[0164] The difference from Example 1 is that in step 3, the mass ratio of bacterial cellulose dispersion: PI dispersion: alumina: PMMA: polyoxyethylene fatty alcohol ether: deionized water is 5:13:12:2:1:67.

[0165] The final membrane contains, by weight of the coating layer, 2% bacterial cellulose, 30% polyimide chopped fibers, 55% alumina, 10% PMMA, and 3% polyoxyethylene fatty alcohol ether.

[0166] Performance testing:

[0167] The membranes obtained in the examples and comparative examples were subjected to the following performance tests, and the test results are shown in Table 1.

[0168] 1. Diaphragm heat shrinkage test:

[0169] The test was conducted according to GB / T 36363-2018. A 10.0 cm × 10.0 cm line was drawn on the diaphragm along the MD (longitudinal) × TD (transverse) direction. The diaphragm was sandwiched between two sheets of A4 paper and placed in an oven. It was heated at 150°C for 1 hour, and three samples were measured at the same temperature. The average value was taken as the test result, and the thermal shrinkage rate of the diaphragm was measured. The diaphragm photographs obtained after the thermal shrinkage test of Examples 1-3, Comparative Examples 1-2, and Reference Examples 1-2 are shown below. Figures 1-5 As shown.

[0170] 2. Air permeability test:

[0171] In a test environment with normal temperature, humidity, and pressure, the parameters of the air permeability tester were set as follows: starting pressure 0.1 Pa, ending pressure 100.0 Pa, test gas pressure 1.21 kPa, test chamber 123, and rinsing time 2 s. Three consecutive measurements were taken, and the average value was used as the test result.

[0172] 3. Surface contact angle:

[0173] Sample preparation: Cut the diaphragm to standard size (e.g., 20 mm × 20 mm), clean the surface impurities with deionized water, and fix it on the test platform after drying.

[0174] Droplet control: Use a microsyringe to drop 0.5~3 μL of electrolyte (1.0 mol / L LiPF6, EC:EMC:DMC volume ratio = 1:1:1) onto the membrane surface to ensure that the droplets are of uniform size.

[0175] Image acquisition: Images of the contact between the droplet and the membrane are captured vertically using a high-speed camera or microscope.

[0176] Data analysis: The contact angle was calculated using the ellipse fitting method. The test results are shown in Table 1.

[0177] 4. Tensile strength:

[0178] Sample preparation: Cut strip specimens with a width of 15±0.1 mm and a length of ≥150 mm using a high-precision cutter. Clamp the specimens vertically between the upper and lower fixtures, avoiding wrinkles or skewing. Test speed: 250 mm / min (lithium-ion battery separator standard). Fixture spacing: 100 mm. Start the testing machine and record the force-displacement curve during the tensile process. Observe the specimen deformation until the specimen breaks. Calculate the tensile strength (maximum force / specimen cross-sectional area) based on the force-displacement curve.

[0179] 5. Membrane rupture temperature test:

[0180] Take a 20~30 mm diaphragm and test it using the TMA tensile method. Record the curve of the diaphragm length changing with temperature until the diaphragm breaks to test the rupture temperature. The test temperature range is 50~300℃. The temperature at which the diaphragm length increases instantaneously is the rupture temperature.

[0181] Table 1: Test Results of Examples, Comparative Examples, and Reference Examples

[0182]

[0183] As shown in Examples 1-3 and Comparative Examples 1-2, compared to the separator obtained by using only the base membrane as the separator without adding PI and bacterial cellulose to the coating solution, the separator obtained by adding PI and / or bacterial cellulose to the coating solution has a lower thermal shrinkage rate and a higher membrane rupture temperature, indicating better thermal stability of the separator and improved battery safety and cycle life; it has lower air permeability, indicating better ion permeability of the separator and improved ion transport efficiency of the battery; it has a lower contact angle, indicating better wettability of the separator, stronger hydrophilicity of the separator surface, lower interfacial impedance, and higher ionic conductivity; and it has higher tensile strength, indicating that the separator structure is strengthened, the structural stability is better, and the battery safety and cycle life are improved.

[0184] As can be seen from Examples 1-3, compared with the membranes obtained by adding only PI or only bacterial cellulose to the coating solution, the membranes obtained by adding both PI and bacterial cellulose to the coating solution have better performance.

[0185] As can be seen from Examples 1 and 2, the content of bacterial cellulose and PI in the coating solution needs to be controlled within a reasonable range.

[0186] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0187] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A separator for a secondary battery, characterized in that, The diaphragm includes a porous membrane and a coating layer formed on at least one surface of the porous membrane. The porous membrane is a polymer resin-based porous membrane or an inorganic fiber-based porous membrane. The coating layer includes a fiber component, an inorganic filler component, and a binder component, wherein the fiber component includes one or both of a cellulose gel component and a polyimide short fiber component. The cellulose gel component comprises a gel derived from cellulose produced by microbial fermentation, and the lignin and hemicellulose content in the cellulose gel component is less than 10% by mass. The surface of the polyimide short fibers has at least partially carboxyl groups.

2. The diaphragm according to claim 1, characterized in that, Based on the total dry weight of the coating layer, the content of the cellulose gel component is 0.02% to 4%, and the content of the polyimide short fiber component is 2% to 20%.

3. The diaphragm according to claim 1 or 2, characterized in that, The porous membrane is a polymer porous membrane or a glass fiber porous membrane; the porosity of the porous membrane is 20% to 45% by volume. The coating is formed on both surfaces of the porous membrane; the coating extends at least partially beneath the surfaces of the porous membrane.

4. The diaphragm according to any one of claims 1 to 3, characterized in that, The cellulose produced by microbial fermentation is bacterial cellulose; the cellulose gel component does not substantially contain lignin or hemicellulose.

5. The diaphragm according to any one of claims 1 to 4, characterized in that, The cellulose gel component is a gel derived solely from cellulose produced by microbial fermentation.

6. The diaphragm according to any one of claims 1 to 5, characterized in that, The polyimide short fiber is a polyimide short fiber that has been treated with acid.

7. The diaphragm according to any one of claims 1 to 6, characterized in that, The total thickness of the diaphragm is 7~20 μm.

8. A method for preparing a diaphragm according to any one of claims 1 to 7, comprising the following steps: S1: Mix the fiber dispersion, inorganic filler component, binder component and water to obtain a diaphragm coating liquid, optionally, the coating liquid further contains a dispersant component; wherein, the fiber dispersion includes one or both of microbially fermented cellulose and polyimide short fibers. S2: The membrane coating liquid is coated on one or both sides of the porous membrane, and after drying, the membrane is obtained.

9. A lithium secondary battery, characterized in that, The lithium secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator according to any one of claims 1 to 7 or a separator prepared by the preparation method according to claim 8.

10. The lithium secondary battery according to claim 9 is a lithium secondary battery for personal consumer electronic devices, a lithium secondary battery for energy storage stations, a lithium secondary battery for transportation vehicles, or a lithium secondary battery for industrial tools.

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