Battery diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention discloses a battery diaphragm, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The battery diaphragm provided by the invention comprises a porous base membrane and a composite ceramic coating arranged on the surface of at least one side of the porous base membrane, the porous base membrane comprises polyolefin and boehmite particles; the composite ceramic coating comprises silicon carbide particles, silicon carbide whiskers and a binder; wherein the mass ratio of the silicon carbide particles to the silicon carbide whiskers is (82-95): (5-18). Through the synergistic effect of the porous base membrane and the composite ceramic coating, the finally obtained battery diaphragm keeps excellent porosity and air permeability while realizing a high in-plane heat conductivity coefficient, so that internal heat can be quickly and uniformly diffused when a lithium ion battery is charged and discharged at a high rate, local overheating and a temperature gradient in the battery are reduced, and the service life of the lithium ion battery is prolonged. And the rapid charging capability, the cycle life and the thermal safety performance of the battery can be improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a battery separator, its preparation method, and a lithium-ion battery. Background Technology

[0002] In the field of lithium-ion batteries, the separator, as a key internal component, directly affects the battery's safety and cycle life. Currently widely used polyolefin separators suffer from insufficient heat resistance. To address this, existing technologies typically coat the base film with a layer of ceramic particles. However, this common improvement method still has significant limitations. First, the ceramic coating offers limited improvement in in-plane thermal conductivity, making it difficult to achieve rapid heat homogenization during high-rate charge and discharge. Localized heat accumulation and temperature gradients still easily occur inside the battery, affecting fast-charging safety and lifespan. Second, a high content of ceramic particles can lead to an overly dense coating, reducing the separator's porosity and ionic conductivity. Third, the mismatch in thermal expansion coefficients between the ceramic layer and the polymer base film can easily lead to interfacial bonding failure during long-term thermal cycling, posing a risk of coating cracking or peeling. Therefore, there is an urgent need to develop a separator material that simultaneously achieves high thermal conductivity, high porosity, and excellent interfacial stability. Summary of the Invention

[0003] In view of this, the present invention provides a battery separator, a method for preparing the same, and a lithium-ion battery. The present invention significantly improves the thermal conductivity, heat resistance, and mechanical strength of the separator by coating a boehmite-modified polyethylene film with a composite coating composed of silicon carbide whiskers and silicon carbide particles in a specific ratio, thereby enhancing the thermal safety and cycle life of the battery.

[0004] In a first aspect, the present invention provides a battery separator, comprising a porous base membrane and a composite ceramic coating disposed on at least one surface of the porous base membrane; The porous base membrane comprises polyolefin and boehmite particles; The composite ceramic coating comprises silicon carbide particles, silicon carbide whiskers and a binder; wherein the mass ratio of the silicon carbide particles to the silicon carbide whiskers is (82~95): (5~18).

[0005] Preferably, the mass ratio of polyolefin to boehmite particles is (20~30): (3~8); the porosity of the porous base membrane is 40~50%.

[0006] Preferably, the thickness of the composite ceramic coating is 1~5μm.

[0007] Preferably, the aspect ratio of the silicon carbide whiskers is (10~50):1, and the D50 particle size of the silicon carbide particles is not greater than 1.0 μm.

[0008] Preferably, the mass ratio of the binder to the total mass of silicon carbide particles and silicon carbide whiskers is 1:(5~10).

[0009] Secondly, the present invention provides a method for preparing the above-mentioned battery separator, comprising the following steps: Polyolefin, boehmite and pore-forming agent are melt-blended and extruded into sheets. The sheets are then subjected to a first biaxial stretching, extraction and a second biaxial stretching to obtain a porous base membrane. A coating slurry containing silicon carbide particles, silicon carbide whiskers, and a binder is applied to at least one side of the porous base membrane and dried to form a composite ceramic coating, thus obtaining the battery separator.

[0010] Preferably, the mass ratio of the polyolefin, boehmite and pore-forming agent is (20~30): (3~8): (65~75).

[0011] Preferably, the coating slurry further comprises a dispersant, a wetting agent, and a stabilizer.

[0012] Furthermore, the preparation process of the coating slurry is as follows: dispersant and wetting agent are added to solvent and mixed, silicon carbide whiskers are added, and after ball milling, silicon carbide particles, binder and stabilizer are added and mixed to obtain the coating slurry.

[0013] Thirdly, the present invention provides a lithium-ion battery comprising the battery separator described above or the battery separator prepared by the above preparation method.

[0014] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) The present invention achieves a significant improvement in the in-plane thermal conductivity of the membrane by setting a composite ceramic coating composed of silicon carbide particles and silicon carbide whiskers in a specific mass ratio on the surface of a polyolefin porous membrane. This specific ratio ensures that the silicon carbide whiskers can effectively "bridge" between the particles to form a three-dimensional continuous thermal conductive network, thereby overcoming the problem of poor thermal conductivity caused by the dense packing of fillers in traditional single ceramic particle coatings. At the same time, this structure is also conducive to maintaining the inherent porosity of the coating.

[0015] (2) By incorporating boehmite particles into a porous base film, this invention improves the intrinsic heat resistance and mechanical strength of the base film, while its thermal expansion behavior is more compatible with the composite ceramic coating. This effectively alleviates the interfacial stress between the coating and the base film caused by temperature changes, enhances the coating adhesion and structural integrity under long-term thermal cycling, and prevents the coating from cracking or peeling.

[0016] (3) Through the synergistic effect of the porous base film and composite ceramic coating of the present invention, the battery separator obtained in the end achieves a high in-plane thermal conductivity while maintaining excellent porosity and air permeability. This characteristic enables the internal heat of the lithium-ion battery to be rapidly and uniformly diffused during high-rate charging and discharging, reducing local overheating and internal temperature gradient of the battery, thereby helping to improve the uniformity of electrode reaction, enhance the battery's fast charging capability, cycle life and thermal safety performance. Attached Figure Description

[0017] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0018] Figure 1 This is an electron microscope image of the battery separator surface in Embodiment 1 of the present invention. Detailed Implementation

[0019] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0020] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is expected that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" means that all real numbers between "0~5" have been listed in this article; "0~5" is just a shortened representation of these numerical combinations.

[0021] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0022] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0023] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially.

[0024] In one embodiment of the present invention, a battery separator is provided, which is particularly suitable for lithium-ion batteries. The battery separator includes a porous base film and a composite ceramic coating disposed on at least one surface of the porous base film.

[0025] In this invention, the porous base membrane comprises polyolefin and boehmite particles.

[0026] In this invention, the polyolefin serves as the backbone material of the matrix, providing basic mechanical strength and electrochemical stability. Exemplarily, the polyolefin is selected from one or more of polyethylene, polypropylene, and polybutene, preferably polyethylene, and more preferably polyethylene with a viscosity-average molecular weight in the range of 1 million to 2 million.

[0027] In this invention, boehmite (chemical formula γ-AlOOH) is uniformly dispersed in a polyolefin matrix as an inorganic filler. The introduction of boehmite particles effectively improves the overall heat resistance of the base film and inhibits high-temperature shrinkage due to its high thermal stability (decomposition temperature above 500℃). Furthermore, the interaction between the boehmite particles and the polyolefin molecular chains enhances the tensile strength and puncture resistance of the base film to a certain extent. The average particle size of the boehmite particles can be from 0.7 μm to 1.5 μm, for example, 0.7 μm, 1.0 μm, or 1.2 μm. Within this particle size range, boehmite can achieve good dispersion in the base film and avoids embrittlement or defects caused by excessively large particle sizes.

[0028] In some embodiments, the mass ratio of the polyolefin to boehmite particles is (20-30):(3-8). For example, this ratio can be 20:3, 25:5, 30:8, or any combination of these values, such as 22:4, 28:6.5, etc. When the boehmite content is below the lower limit of this range, its effect on improving heat resistance and reinforcement is limited; when its content is above the upper limit of this range, it may lead to increased brittleness and decreased flexibility of the base film, and make it prone to film breakage during stretching.

[0029] In this invention, the porous base membrane has an interconnected microporous structure with a porosity of 40-50%, more preferably 42-48%. The air permeability (Gurley value) can be from 80s / 100cc to 150s / 100cc. These structural features ensure good wetting of the electrolyte by the membrane and efficient transport of lithium ions.

[0030] In this invention, the composite ceramic coating comprises silicon carbide particles, silicon carbide whiskers, and a binder. This coating is the core functional layer that imparts high thermal conductivity to the diaphragm.

[0031] In the composite ceramic coating of this invention, the mass ratio of silicon carbide particles to silicon carbide whiskers is (82~95):(5~18). This ratio range is key to achieving synergistic effects. Silicon carbide particles, as the main body of the coating and the primary thermally conductive filler, possess high thermal conductivity. Silicon carbide whiskers (SiC...) w As a one-dimensional reinforcing phase, SiC's role extends beyond just toughening. Under this specific ratio, an appropriate amount of SiC... w It can "bridge" between SiC particles, forming a three-dimensional interconnected thermally conductive network, significantly improving the heat transfer efficiency in the planar direction (in-plane) of the coating. If SiC w If the content is too low, it is difficult to form an effective continuous network, the "bridging" effect is weak, and the improvement in thermal conductivity is limited; if SiC w If the content is too high, it will easily lead to a sharp increase in the viscosity of the slurry and fiber agglomeration in the coating, which will hinder the heat conduction path and may block the pores, thus impairing the air permeability.

[0032] In an optional embodiment of the present invention, the aspect ratio (i.e., the ratio of length to diameter) of the silicon carbide whiskers is (10~50):1, for example, 15:1, 30:1, or 45:1. A larger aspect ratio facilitates the whiskers spanning multiple particles in the coating, more effectively constructing a network. The D50 particle size (median particle size) of the silicon carbide particles is not greater than 1.0 μm, preferably 0.5~1.0 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, etc. Smaller particle size helps fill voids, forming a denser composite structure with shorter thermal paths with the whiskers. The excellent thermal conductivity of the particles and whiskers, combined with the above-mentioned optimized morphology and ratio, jointly contribute to the high in-plane thermal conductivity of the coating.

[0033] In this invention, the binder is used to firmly bond silicon carbide particles and whiskers together and stably adhere them to the surface of the base film. The mass ratio of the binder to the total mass of the silicon carbide particles and whiskers is 1:(5~10), for example, 1:5, 1:8, 1:10, etc. Exemplary binders include polyacrylate adhesives (such as acrylate copolymer emulsions), polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), etc., preferably polyacrylate adhesives, because they have good adhesion and compatibility with ceramic fillers and polyolefin base films. This ratio ensures that the coating has sufficient bonding strength without significantly reducing the overall thermal conductivity due to the introduction of excessive organic matter.

[0034] In this invention, the thickness of the composite ceramic coating is 1~5μm, for example, 1μm, 2μm, 3μm, 4μm, or 5μm. This thickness range balances functionality with the impact on the overall thickness of the membrane and ion transport impedance. If the coating is too thin, it may not form a complete and continuous functional layer; if it is too thick, it will increase the ion transport distance and may affect the flexibility of the membrane.

[0035] In some embodiments, the composite ceramic coating may also contain other auxiliary components, such as dispersants, wetting agents, and stabilizers, to optimize slurry performance and coating quality, which will be detailed in the preparation method section.

[0036] This invention, through the specific design of the base film and coating, significantly improves the in-plane thermal conductivity of the final battery separator, reaching over 20 W / (m·K), while maintaining excellent porosity, air permeability, mechanical strength, and thermal stability. High thermal conductivity helps the battery to rapidly and evenly dissipate internal heat during high-rate charging and discharging, avoiding localized overheating and large temperature gradients, thereby improving the battery's fast-charging performance, cycle life, and safety.

[0037] In another embodiment of this application, a method for preparing the above-mentioned battery separator is provided. The method includes three main steps: preparing a porous base membrane, preparing a coating slurry, and coating the slurry.

[0038] Step 1: Preparation of porous base membrane Raw material mixing and extrusion: Polyolefin (such as polyethylene), boehmite, and a pore-forming agent are melt-blended at a mass ratio of (20~30):(3~8):(65~75). The pore-forming agent is used to form pores in the subsequent extraction step. The preferred pore-forming agent is paraffin oil, dibutyl phthalate, etc., more preferably paraffin oil. Mixing is carried out in a twin-screw extruder, with the mixing temperature controlled at 180℃~220℃, for example, 180℃, 185℃, 190℃, 195℃, 200℃, 205℃, 210℃, 215℃, 220℃, etc., and the stirring speed can be 80~110 rpm. The uniformly mixed melt is extruded through a die to form a sheet.

[0039] First biaxial stretching: The above-mentioned sheet is subjected to a first biaxial stretching at a temperature of 115℃~130℃. The temperature for the first biaxial stretching can be 115℃, 118℃, 120℃, 122℃, 124℃, 126℃, 128℃, 130℃, etc. The stretching ratio (area ratio) can be 7 to 10 times, for example, 7, 8, 9, or 10 times, or it can be a non-integer multiple. This step initially orients the polymer molecular chains and boehmite particles and increases the sheet area.

[0040] Extraction and Drying: The sheet, after its first stretching, is immersed in an extraction solvent to dissolve and remove the pore-forming agent, thereby forming a porous structure within the sheet. Extraction solvents such as dichloromethane or n-hexane can be used, and extraction can be carried out at room temperature. Subsequently, it is dried at a lower temperature (e.g., 35°C~40°C) to remove residual solvent.

[0041] Second biaxial stretching and heat setting: The extracted and dried porous sheet is subjected to a second biaxial stretching and heat setting at a temperature of 125℃~140℃. The stretching temperature is further preferably 126℃~135℃, for example, 128℃, 130℃, 132℃, 134℃, 136℃, etc. The stretching ratio can be 5 to 7 times. This step further optimizes the pore structure, improves crystallinity, and stabilizes the base film size, ultimately obtaining the porous base film. The thickness of the porous base film can be controlled within 3~10μm through process parameters, for example, 4μm, 5μm, 7μm, etc.

[0042] Step 2: Preparation of coating slurry The coating slurry comprises silicon carbide particles, silicon carbide whiskers, binder, dispersant, wetting agent, stabilizer, and solvent. The solvent is preferably water. The dispersant includes, but is not limited to, one or more of the following: high molecular weight polymers such as Solsperse 41000, ammonium polyacrylate (TEGODispers 750W), and fumed silica. The wetting agent can be a nonionic surfactant (such as Triton X-100), acetylenic diols (such as Dynol 607), etc. The stabilizer can be sodium carboxymethyl cellulose (CMC-Na), etc., etc., and the binder can be a polyacrylate adhesive (such as BMS-2621, BM-720B), etc.

[0043] The solid content of the coating slurry of the present invention can be 30-50 wt%, for example, 30 wt%, 32 wt%, 35 wt%, 38 wt%, 40 wt%, 42 wt%, 45 wt%, 48 wt%, 50 wt%, etc. In the coating slurry of the present invention, the concentration of silicon carbide particles is 30-40 wt%, for example, 30 wt%, 32 wt%, 34 wt%, 35 wt%, 36 wt%, 38 wt%, 40 wt%, etc. The concentration of silicon carbide whiskers is 1-7 wt%, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, etc. The concentration of binder is 3-8 wt%, for example, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, etc. The concentration of dispersant is 0.4-1 wt%, for example, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, etc. The concentration of the wetting agent is 0.08~0.2wt%, for example, 0.08wt%, 0.10wt%, 0.12wt%, 0.15wt%, 0.18wt%, 0.20wt%, etc. The concentration of the stabilizer is 0.3~0.8wt%, for example, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, etc.

[0044] The preferred preparation process for the coating slurry is as follows: First, dissolve the dispersant and wetting agent in water, then slowly add silicon carbide whisker powder. Add zirconia beads and ball mill to ensure that the D50 particle size of the solid particles in the slurry is no greater than 0.5 μm, thus achieving good dispersion and deagglomeration of the whiskers. After ball milling, add silicon carbide powder, binder, and stabilizer, and stir until homogeneous. A pH adjuster (such as ammonia) can be used to adjust the pH of the slurry to 7-8 to maintain slurry stability. Finally, sieve the slurry (e.g., 200-400 mesh) to remove large particles and grinding media, obtaining a uniform and stable coating slurry. The solid content of the slurry can be 30%-50%.

[0045] Step 3: Slurry coating and drying Coating and Drying: The above-mentioned slurry is uniformly coated onto at least one side of the porous base membrane using a gravure roller, micro-gravure roller, or spraying equipment. The slurry coating thickness is controlled so that the thickness of the dry coating layer is in the range of 1~5μm. The coated membrane is then dried in a drying oven. A gradient temperature process is preferably used for drying: pre-drying is performed at a lower temperature (e.g., 80℃~100℃) to initially set the surface and prevent sagging; then final drying is performed at a slightly higher temperature (e.g., 90℃~110℃) to completely remove the solvent and allow the binder to form a film, resulting in a strong composite ceramic coating. The dried membrane is then wound up to obtain the finished battery separator.

[0046] In another embodiment of the present invention, a lithium-ion battery is provided, which includes the battery separator described above, or the battery separator prepared by the above preparation method.

[0047] The lithium-ion battery also includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive active material, a conductive agent, and a binder, coated onto an aluminum foil current collector. The positive active material includes lithium transition metal oxides such as lithium iron phosphate and lithium nickel cobalt manganese oxide. The negative electrode comprises a negative active material (such as graphite or silicon carbide), a conductive agent, and a binder, coated onto a copper foil current collector. The electrolyte comprises a lithium salt (such as lithium hexafluorophosphate) and an organic solvent (such as a mixture of ethylene carbonate and dimethyl carbonate). The battery separator is placed between the positive and negative electrodes, playing a crucial role in electronic insulation, ion conduction, and thermal management.

[0048] Thanks to the use of the high thermal conductivity battery separator of this invention, the Joule heat generated by the electrode reaction can be rapidly diffused laterally through the separator when the lithium-ion battery is charged at high rates (such as 3C or even higher), which effectively reduces the temperature gradient inside the cell and mitigates the side reactions caused by local overheating, thereby significantly improving the battery's fast charging capability, cycle life and thermal safety performance.

[0049] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0050] In the following examples, silicon carbide whiskers (SiC) w The aspect ratio of the silicon carbide particles is 30:1, and the D50 particle size is 0.8μm.

[0051] Example 1 This embodiment provides a battery separator, the preparation method of which is as follows: (1) Sheet preparation: 25% by mass of ultra-high molecular weight polyethylene (viscosity average molecular weight of 1.5 million), 5% of boehmite (average particle size of 1.0 μm) and 70% of paraffin oil are put into a twin-screw extruder and melt-blended at 200°C and 95 rpm. The mixture is then extruded through a die to form a sheet.

[0052] (2) First stretching: The sheet obtained in step (1) is subjected to biaxial stretching at 124°C, with a stretching ratio of 9 times (area ratio).

[0053] (3) Extraction and pore-forming: The sheet stretched in step (2) is immersed in dichloromethane for room temperature extraction to completely remove the paraffin oil, and then dried at 38°C.

[0054] (4) Second stretching and shaping: The porous sheet obtained in step (3) is subjected to a second biaxial stretching (stretching ratio of 6 times) at 135°C and shaped at 128°C to obtain a boehmite / polyethylene porous base film with a thickness of 4μm and a porosity of about 45%.

[0055] (5) Preparation of coating slurry: Prepare water-based coating slurry. First, add 0.1 wt% wetting agent (Triton X-100), 0.5 wt% polyvinylpyrrolidone dispersant, and 0.3 wt% fumed silica dispersant to deionized water. Slowly add 2 wt% silicon carbide whiskers (SiC) according to the total mass of the slurry. w Zirconia beads were added and ball-milled to ensure particle size D50 ≤ 0.5 μm. Then, 38 wt% silicon carbide (SiC) particles were added to allow the SiC particles to mature. w The mass ratio of the binder to SiC is 5:95. Finally, 5 wt% of binder BMS-2621 and 0.5 wt% of stabilizer sodium carboxymethyl cellulose are added, the pH of the slurry is adjusted to 7.5 with ammonia, and then filtered through a 200-mesh sieve.

[0056] (6) Coating and drying: The slurry obtained in step (5) is coated on one side of the porous base film obtained in step (4) using a gravure roller. After the wet film is pre-dried at 90°C and finally dried at 100°C, a composite ceramic coating with a single-sided thickness of about 2μm is formed.

[0057] (7) Rewinding: Rewind the finished diaphragm.

[0058] The electron microscope image of the battery separator surface in this embodiment is as follows: Figure 1 As shown, the elongated structures within the yellow area are SiC. w The red area contains silicon carbide particles, which can be seen to represent SiC. w The SiC is uniformly dispersed in the coating, wherein the SiC w Densely bonded to SiC.

[0059] Example 2 This embodiment provides a battery separator, the preparation method of which is as follows: (1) Sheet preparation: 25% by mass of ultra-high molecular weight polyethylene (viscosity average molecular weight of 1.5 million), 5% of boehmite (average particle size of 1.0 μm) and 70% of paraffin oil are put into a twin-screw extruder and melt-blended at 200°C and 95 rpm. The mixture is then extruded through a die to form a sheet.

[0060] (2) First stretching: The sheet obtained in step (1) is subjected to biaxial stretching at 124°C, with a stretching ratio of 9 times (area ratio).

[0061] (3) Extraction and pore-forming: The sheet stretched in step (2) is immersed in dichloromethane for room temperature extraction to completely remove the paraffin oil, and then dried at 38°C.

[0062] (4) Second stretching and shaping: The porous sheet obtained in step (3) is subjected to a second biaxial stretching (stretching ratio of 6 times) at 135°C and shaped at 128°C to obtain a boehmite / polyethylene porous base film with a thickness of 4μm and a porosity of about 45%.

[0063] (5) Preparation of coating slurry: Prepare water-based coating slurry. First, add 0.1 wt% wetting agent (Triton X-100), 0.5 wt% polyvinylpyrrolidone dispersant, and 0.3 wt% fumed silica dispersant to deionized water. Slowly add 4 wt% silicon carbide whiskers (SiC) according to the total mass of the slurry. w Zirconia beads were added and ball-milled to ensure particle D50 ≤ 0.5 μm. Then, 36 wt% silicon carbide (SiC) particles were added to make SiC... w The mass ratio of the binder to SiC is 10:90. Finally, 5 wt% of binder BMS-2621 and 0.5 wt% of stabilizer sodium carboxymethyl cellulose are added, the pH of the slurry is adjusted to 7.5 with ammonia, and then filtered through a 200-mesh sieve.

[0064] (6) Coating and drying: The slurry obtained in step (5) is coated on one side of the porous base film obtained in step (4) using a gravure roller. After the wet film is pre-dried at 90°C and finally dried at 100°C, a composite ceramic coating with a single-sided thickness of about 2μm is formed.

[0065] (7) Rewinding: Rewind the finished diaphragm.

[0066] Example 3 This embodiment provides a battery separator, the preparation method of which is as follows: (1) Sheet preparation: 25% by mass of ultra-high molecular weight polyethylene (viscosity average molecular weight of 1.5 million), 5% of boehmite (average particle size of 1.0 μm) and 70% of paraffin oil are put into a twin-screw extruder and melt-blended at 200°C and 95 rpm. The mixture is then extruded through a die to form a sheet.

[0067] (2) First stretching: The sheet obtained in step (1) is subjected to biaxial stretching at 124°C, with a stretching ratio of 9 times (area ratio).

[0068] (3) Extraction and pore-forming: The sheet stretched in step (2) is immersed in dichloromethane for room temperature extraction to completely remove the paraffin oil, and then dried at 38°C.

[0069] (4) Second stretching and shaping: The porous sheet obtained in step (3) is subjected to a second biaxial stretching (stretching ratio of 6 times) at 135°C and shaped at 128°C to obtain a boehmite / polyethylene porous base film with a thickness of 4μm and a porosity of about 45%.

[0070] (5) Preparation of coating slurry: Prepare water-based coating slurry. First, add 0.1 wt% wetting agent (Triton X-100), 0.5 wt% polyvinylpyrrolidone dispersant, and 0.3 wt% fumed silica dispersant to deionized water. Slowly add 6 wt% silicon carbide whiskers (SiC) according to the total mass of the slurry. w Zirconia beads were added and ball-milled to ensure particle size D50 ≤ 0.5 μm. Then, 34 wt% silicon carbide (SiC) particles were added to allow the SiC particles to mature. w The mass ratio of the binder to SiC is 15:85. Finally, 5 wt% of binder BMS-2621 and 0.5 wt% of stabilizer sodium carboxymethyl cellulose are added, the pH of the slurry is adjusted to 7.5 with ammonia, and then filtered through a 200-mesh sieve.

[0071] (6) Coating and drying: The slurry obtained in step (5) is coated on one side of the porous base film obtained in step (4) using a gravure roller. After the wet film is pre-dried at 90°C and finally dried at 100°C, a composite ceramic coating with a single-sided thickness of about 2μm is formed.

[0072] (7) Rewinding: Rewind the finished diaphragm.

[0073] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not perform steps (5) and (6), and directly obtains a boehmite / polyethylene porous base membrane with a thickness of 4μm and a porosity of about 45%.

[0074] Comparative Example 2 The difference between this comparative example and Example 1 is that silicon carbide whiskers (SiC) are not added in step (5) of this comparative example. w The mass fraction of silicon carbide (SiC) particles is 40 wt%, which makes SiC... w The mass ratio of SiC to SiC is 0:100.

[0075] Comparative Example 3 The difference between this comparative example and Example 1 is that in step (5) of this comparative example, silicon carbide whiskers (SiC) are used. w The mass fraction of the silicon carbide (SiC) particles is 30 wt%, and the mass fraction of the silicon carbide (SiC) particles is 10 wt%, making SiC w The mass ratio of SiC to SiC is 25:75.

[0076] Comparative Example 4 This comparative example provides the preparation of a traditional ceramic-coated diaphragm, and the specific preparation method is as follows: (1) Sheet preparation: 30% by mass of ultra-high molecular weight polyethylene (viscosity average molecular weight of 1.5 million) and 70% of paraffin oil are put into a twin-screw extruder and melt-blended at 200°C and 95 rpm. The mixture is then extruded through a die to form a sheet.

[0077] (2) First stretching: The sheet obtained in step (1) is subjected to biaxial stretching at 124°C with a stretching ratio of 9 times.

[0078] (3) Extraction and pore-forming: The sheet stretched in step (2) is immersed in dichloromethane for room temperature extraction to completely remove the paraffin oil, and then dried at 38°C.

[0079] (4) Second stretching and shaping: The porous sheet obtained in step (3) is subjected to a second biaxial stretching (stretching ratio of 6 times) at 135°C and shaped at 128°C to obtain a polyethylene porous base film with a thickness of 4μm and a porosity of about 45%.

[0080] (5) Preparation of coating slurry: Prepare a slurry containing 30wt% alumina (Al2O3) particles, 0.2wt% polyacrylamide, 0.2wt% polyvinyl butyral, and the balance being water.

[0081] (6) Coating and drying: The slurry obtained in step (5) is coated on one side of the porous base film obtained in step (4) using a gravure roller. After the wet film is pre-dried at 90°C and finally dried at 100°C, an Al2O3 ceramic coating with a single-sided thickness of about 2μm is formed.

[0082] (7) Rewinding: Rewind the finished diaphragm.

[0083] Test case 1. Diaphragm performance testing The air permeability, mechanical properties, heat resistance, liquid retention, and wetting properties of the diaphragms from Examples 1-3 and Comparative Examples 1-4 were measured. The base film thickness of all samples was controlled at 4 μm, and the coating was double-sided, with a dry coating thickness of approximately 2 μm on each side. The test results are summarized in Tables 1 and 2.

[0084] Brief description of the testing method: Air permeability: The time required for 100cc of air to pass through a diaphragm sample with a diameter of 2.54cm was measured using a Gurley air permeability meter (model 4110N) (unit: s / 100cc).

[0085] Thermal conductivity: The thermal conductivity (λ∥) in the in-plane direction of the diaphragm was measured using a Hot Disk thermal constant analyzer (TPS 2500S) and the transient planar heat source method.

[0086] Liquid retention rate: A known mass of the diaphragm sample (m0) was completely immersed in the electrolyte (EC / DMC / EMC containing 1M LiPF6, volume ratio 1:1:1) for 1 hour. After removal, the residual liquid droplets on the surface were quickly blotted dry with filter paper, and the sample was weighed (m1). Liquid retention rate = (m1 - m0) / m0 × 100%.

[0087] Wetting angle: Using a contact angle meter (DSA100), take a 2μL droplet of electrolyte and measure its static contact angle on the diaphragm surface.

[0088] Tensile strength: The tensile strength of the diaphragm sample in the MD (longitudinal) and TD (transverse) directions was tested using a universal testing machine (Instron 3365) at a tensile speed of 50 mm / min, and the average value was taken.

[0089] Needle penetration strength: The maximum force required to puncture the diaphragm sample was tested using a diaphragm puncture strength tester (KES-G5) at a needle penetration speed of 2 mm / s (unit: gf).

[0090] Heat shrinkage rate: Cut the diaphragm sample into a 100mm×100mm square, place it in a forced-air drying oven at a set temperature and heat for 1 hour. After cooling, measure the dimensional changes of each side. Heat shrinkage rate = (length before heating - length after heating) / length before heating × 100%, and take the average value in the MD and TD directions.

[0091] Membrane breakage temperature: Using a thermomechanical analyzer (TMA, model Q400), the diaphragm sample was clamped and a constant load of 0.02 N was applied. The temperature was increased from room temperature to 300 °C at a rate of 5 °C / min in air atmosphere, and the temperature at the moment of sample breakage was recorded.

[0092] Table 1. Physicochemical and thermal conductivity data of the diaphragm serial number Liquid retention rate (%) Wetting angle (°) Breathability (s / 100cc) <![CDATA[In-plane thermal conductivity (W·m -1 ·K -1 )]]> Example 1 70 16.8 105 22 Example 2 71 15.3 101 28 Example 3 72 13.9 98 31 Comparative Example 1 63 65.2 95 0.7 Comparative Example 2 68 27.5 100 18 Comparative Example 3 70 16.6 112 27 Comparative Example 4 70 38.1 114 12 Table 2. Data on the mechanical and heat resistance properties of the diaphragm. serial number Tensile strength (MPa) Needle puncture intensity (gf) Heat shrinkage rate (%) at 150℃ / 1h Heat shrinkage rate (%) at 180℃ / 1h Film rupture temperature (°C) Example 1 213 452 8.2 10.4 143.2 Example 2 220 451 6.2 8.7 144.5 Example 3 230 449 4.1 6.8 145.2 Comparative Example 1 212 451 Rupture of membrane Rupture of membrane 138.3 Comparative Example 2 215 457 9.1 12.3 140.7 Comparative Example 3 221 449 3.5 5.5 146.8 Comparative Example 4 215 450 10 Rupture of membrane 140.3 As can be seen from the data in Tables 1 and 2, Examples 1-3 of the present invention utilize boehmite / polyethylene film and SiC w The combination of / SiC composite coatings achieves a synergistic improvement in performance.

[0093] The in-plane thermal conductivity of Examples 1-3 is significantly higher than that of the traditional Al2O3 coated membrane (Comparative Example 4) and the polyethylene-based membrane (Comparative Example 1). This indicates that SiC w The introduction of SiC particles forms a highly efficient three-dimensional thermally conductive network. Notably, the thermal conductivity of the pure SiC particle coating (Comparative Example 2) is lower than that of Example 1 with a similar SiC content, demonstrating the effectiveness of SiC...w The key role of “bridging” in constructing a continuous heat conduction path.

[0094] The tensile strength and rupture temperature of the boehmite-based films in Examples 1-3 were superior to those in Comparative Example 4, demonstrating the reinforcing effect of boehmite on the substrate. Regarding coatings, with the development of SiC... w Content (relative to SiC) w As the total mass of SiC particles increased from 0% to 15% (Comparative Example 2, Examples 1-3), the thermal shrinkage rate of the diaphragm at 150°C gradually decreased, while the needle-punching strength increased, indicating that an appropriate amount of SiC... w The resulting three-dimensional network effectively suppressed the thermal shrinkage of the base film and enhanced the rigidity of the coating and the overall structure. (SiC) w The permeability of the sample with excessively high content (Comparative Example 3, 25%) deteriorated significantly, indicating that excessive whiskers may clog the pores.

[0095] In all embodiments of the present invention, the wetting angle is less than 20°, and the liquid retention rate is high, demonstrating excellent affinity for the electrolyte. This is attributed to the uniform porous structure of the coating and the addition of hydrophilic components. Although the traditional Al2O3 coating (Comparative Example 4) has a high liquid retention rate, its extremely low thermal conductivity is a drawback.

[0096] 2. Lithium-ion battery performance testing Assembly of lithium-ion batteries: The positive electrode uses lithium iron phosphate (LiFePO4), the negative electrode uses artificial graphite, and the electrolyte is a mixed solution of EC / DMC / EMC (volume ratio 1:1:1) containing 1 mol / L LiPF6. Pouch batteries with a rated capacity of 2 Ah were fabricated using the separators from Examples 1-3 and Comparative Examples 1-4, respectively.

[0097] The testing method is as follows: Cycling performance: At an ambient temperature of 45°C, the battery was charged at a constant current of 1C to 3.65V, then switched to constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 1C to 2.0V. This cycle was repeated 500 times. The discharge capacity retention rate (relative to the capacity of the 3rd cycle) was recorded after the 500th cycle.

[0098] Rate performance: At 25°C, the discharge capacity of the battery at discharge rates of 0.2C, 1C, 2C, and 3C was tested, and the retention rate of the 3C discharge capacity relative to the 0.2C discharge capacity was calculated.

[0099] Capacity recovery after high-temperature storage: A fully charged battery (SOC 100%) was stored at 85°C for 8 hours, cooled to room temperature, and then discharged at 0.5C to the cutoff voltage. The recovered capacity was recorded. Capacity recovery rate = (Discharge capacity after storage / Discharge capacity before storage) × 100%.

[0100] The test results are shown in Table 3.

[0101] Table 3 Comparison of Electrochemical Performance of Lithium-ion Batteries serial number Capacity retention rate (%) after 500 cycles at 1C / 1C, 45℃. 3C rate discharge capacity retention rate (%) Capacity recovery rate (%) after storage at 85℃ for 8 hours Example 1 84 92 94 Example 2 88 91 94 Example 3 95 95 95 Comparative Example 1 76 75 90 Comparative Example 2 82 91 92 Comparative Example 3 85 86 94 Comparative Example 4 79 88 94 The separators in Examples 1-3 exhibit significantly improved in-plane thermal conductivity, enabling rapid lateral diffusion of heat generated during battery cycling. This greatly reduces the temperature gradient across the electrode plane, thus avoiding accelerated degradation in localized overheated areas and resulting in more uniform reaction and degradation across the entire electrode. Consequently, they exhibit high cycle retention.

[0102] The embodiments of this invention also lead in rate performance. This is due to the synergistic design of its "high thermal conductivity" and "high porosity ion pathway". High thermal conductivity dissipates heat under high current in a timely manner, avoiding accelerated electrolyte decomposition and increased electrode polarization caused by temperature spikes; while good porosity, liquid retention rate and wettability ensure sufficient electrolyte retention and low-resistance lithium ion transport. Embodiment 3 achieves optimal rate performance with its optimal thermal conductivity and good air permeability.

[0103] The embodiments of this invention exhibit the highest capacity recovery rate after high-temperature storage, which is attributed to the comprehensive stability of the multilayer membrane structure: the boehmite / polyethylene membrane provides a higher membrane breakage temperature and resistance to heat shrinkage; SiC w The SiC ceramic coating itself possesses excellent thermal stability and chemical inertness. During high-temperature storage, the membrane structure remains intact, effectively preventing internal short circuits caused by thermal shrinkage and reducing side reactions caused by material degradation, thereby maximizing the protection of the electrodes and active lithium.

[0104] Based on the above membrane performance and battery test results, it can be concluded that this invention constructs a SiC membrane with a specific ratio on a boehmite-modified polyethylene base membrane. w The SiC composite ceramic coating successfully endows the separator with ultra-high in-plane thermal conductivity, enhanced mechanical properties, and excellent thermal stability. These three factors work synergistically to fundamentally improve the thermal management capabilities of lithium-ion batteries, especially high-energy, fast-charging battery systems, thereby significantly enhancing battery cycle life, rate performance, and high-temperature safety.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A battery separator, characterized in that, It includes a porous base membrane and a composite ceramic coating disposed on at least one surface of the porous base membrane; The porous base membrane comprises polyolefin and boehmite particles; The composite ceramic coating comprises silicon carbide particles, silicon carbide whiskers and a binder; wherein the mass ratio of the silicon carbide particles to the silicon carbide whiskers is (82~95): (5~18).

2. The battery separator as described in claim 1, characterized in that, The mass ratio of polyolefin to boehmite particles is (20~30): (3~8); the porosity of the porous base membrane is 40~50%.

3. The battery separator as described in claim 1, characterized in that, The thickness of the composite ceramic coating is 1~5μm.

4. The battery separator as described in claim 1, characterized in that, The aspect ratio of the silicon carbide whiskers is (10~50):1, and the D50 particle size of the silicon carbide particles is not greater than 1.0μm.

5. The battery separator as described in claim 1, characterized in that, The mass ratio of the binder to the total mass of silicon carbide particles and silicon carbide whiskers is 1:(5~10).

6. The method for preparing the battery separator according to any one of claims 1 to 5, characterized in that, Includes the following steps: Polyolefin, boehmite and pore-forming agent are melt-blended and extruded into sheets. The sheets are then subjected to a first biaxial stretching, extraction and a second biaxial stretching to obtain a porous base membrane. A coating slurry containing silicon carbide particles, silicon carbide whiskers, and a binder is applied to at least one side of the porous base membrane and dried to form a composite ceramic coating, thus obtaining the battery separator.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the polyolefin, boehmite and pore-forming agent is (20~30): (3~8): (65~75).

8. The preparation method according to claim 6, characterized in that, The coating slurry also contains dispersants, wetting agents, and stabilizers.

9. The preparation method according to claim 8, characterized in that, The preparation process of the coating slurry is as follows: add the dispersant and wetting agent to the solvent and mix well, add silicon carbide whiskers, ball mill, add silicon carbide particles, binder and stabilizer, and mix well to obtain the coating slurry.

10. A lithium-ion battery, characterized in that, The battery separator comprises any one of claims 1 to 5 or the battery separator prepared by any one of claims 6 to 9.

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