Composite separator, method for manufacturing the same, and secondary battery

Abstract

The application relates to the technical field of batteries, in particular to a composite diaphragm, a preparation method thereof and a secondary battery. The composite diaphragm comprises a base diaphragm, a first coating layer arranged on at least one surface of the base diaphragm and a second coating layer arranged on at least part of the surface of the first coating layer; the first coating layer comprises filler particles and composite particles; the filler particles are stacked to form a filler layer; the composite particles are embedded in the filler layer and form protrusions on the surface of the filler layer; the composite particles comprise secondary particles formed by the first polymer primary particles and ceramic coagglomeration; the second coating layer comprises polymer particles; the polymer particles comprise second polymer primary particles; and the height h of the second coating layer protruding on the filler layer of the composite particles satisfies 1 mu.m <= h <= 4.9 mu.m. The diaphragm has good adhesion with electrode sheets, has sufficient space for storing electrolyte, does not affect the conduction of ions, is not prone to lithium precipitation, and improves the cycle performance and energy density of the battery.

Description

Technical Field

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

[0002] Batteries, as portable chemical energy sources, are widely used in mobile phones, laptops, home energy storage, energy storage power stations, and new energy vehicles due to their advantages such as high energy density, high operating voltage platform, low self-discharge, long service life, and environmental friendliness.

[0003] The separator is a crucial component of a battery. Due to its insulating properties, it blocks electron conduction and is designed between the positive and negative electrodes. The performance of the separator affects the overall battery performance. For example, the heat resistance of the separator affects the overall safety of the battery, and the adhesion between the separator and the electrode plates affects the battery's cycle performance and energy density. For instance, Chinese patent application CN119009359A describes a coated separator with an adhesive layer on an inorganic ceramic substrate, which improves the adhesion between the separator and the electrode plates. However, as the adhesion increases, the liquid storage capacity between the separator and the electrode plates decreases, thus affecting the overall cycle performance and energy density of the battery. (Patent CN1154835...) In the Chinese patent application 00A, a mixed coating of primary PMMA particles and ceramic is used to obtain a mixed layer of PMMA and ceramic. When the coated separator and electrode are hot-pressed, although the primary PMMA particles can provide storage space for the separator and electrode, effectively improving the battery's liquid storage performance and capacity retention, the bonding sites of the primary PMMA particles decrease with the increase of particle size, and the adhesion between the separator and electrode weakens. At the same time, the ion conductivity of the primary PMMA particles becomes worse with the increase of particle size, which can easily lead to lithium plating in the cell. Therefore, it is necessary to develop a composite separator with a coating to improve the cycle performance and energy density of the battery while ensuring high adhesion between the separator and the electrode.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a composite separator, its preparation method, and a secondary battery. The composite separator provided by this invention can improve the adhesion performance between the separator and the electrode sheet, while having enough space for electrolyte storage, without affecting ion conduction, making the battery less prone to lithium plating, and ultimately improving the battery's cycle performance and energy density.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0007] A composite membrane includes a base membrane and a coating disposed on at least one surface of the base membrane; the coating includes a first coating disposed on the surface of the base membrane and a second coating disposed on at least a portion of the surface of the first coating;

[0008] The first coating includes filler particles and composite particles. The filler particles are stacked to form a filler layer, and the composite particles are embedded in the filler layer and form protrusions on the surface of the filler layer. The composite particles include primary polymer particles and secondary particles formed by the co-aggregation of ceramics.

[0009] The second coating comprises polymer particles; the polymer particles comprise second polymer primary particles;

[0010] The height h of the composite particles protruding from the second coating satisfies 1μm≤h≤4.9μm.

[0011] The method for preparing the composite membrane as described above includes the following steps:

[0012] The first raw material is mixed to obtain a first mixed slurry, wherein the first raw material includes filler particles and an emulsion containing composite particles;

[0013] The second raw material is mixed to obtain a second mixed slurry, wherein the second raw material includes an emulsion containing polymer particles;

[0014] The first mixed slurry is coated onto at least one side surface of the base membrane and dried to obtain a first coating; then the second mixed slurry is coated onto the first coating and dried to obtain the composite membrane.

[0015] A secondary battery comprising the composite separator as described above.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) The composite diaphragm provided by the present invention has good air permeability, low internal resistance, and is not easy to shed powder. The diaphragm and the electrode sheet have good adhesion. The diaphragm has high ionic conductivity and is not easy to deposit lithium. The battery cell prepared using the composite diaphragm has a small bulging expansion coefficient, high capacity retention rate, and good cycle performance.

[0018] (2) The composite particles of the present invention include primary polymer particles and secondary particles formed by co-aggregation of ceramic particles. The introduction of ceramic particles during the primary particle aggregation process increases the phase interface between the primary particles in the composite particles, thereby providing more channels for ions and effectively solving the problem of lithium plating caused by the blockage of ions by large polymer particles in the coating, thereby improving the cycle performance. In addition, the addition of ceramic to the composite particles increases the affinity between the composite particles and inorganic fillers during coating slurry preparation, improves the dispersion of the slurry system, and makes it less likely for the composite particles to agglomerate in the composite membrane. The addition of ceramic can also effectively increase the relative density of the composite particles. When mixed with filler particles for coating, the composite particles can be effectively embedded in the pores of the filler layer, increasing the adhesion between the composite particles and the coating, thereby solving the problem of powder shedding when large-diameter polymers and inorganic fillers are mixed for coating.

[0019] (3) The composite particles and polymer particles of the present invention are used in combination. The polymer particles are coated on the surface of the first coating containing the composite particle protrusions, which realizes the bidirectional improvement of electrolyte storage performance and adhesion performance, and also has good heat resistance. The composite particles are embedded in the filler layer and form protrusions on the surface of the filler layer. When the diaphragm and the electrode sheet are hot-pressed, in addition to providing adhesion, they can also provide space points to improve the electrolyte storage performance of the battery cell, thereby improving the cycle performance of the battery cell. The polymer particles are located on the surface of the filler layer and are not embedded in the filler layer, which can effectively improve the adhesion performance of the composite diaphragm. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the structure of the composite diaphragm provided in an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the structure of the first polymer primary particle provided in an embodiment of the present invention;

[0023] Figure 3 This is a surface SEM image of the composite membrane in Embodiment 10 of the present invention;

[0024] Figure 4 This is the Tg diagram of the first polymer primary particles in Example 10 of the present invention;

[0025] Figure 5 The Tg diagram of the polymer particles in Example 10 of this invention;

[0026] Figure 6This is an EDS image of the composite particles in Example 10 of the present invention.

[0027] Figure label:

[0028] 1-Base film; 2-Coating; 21-First coating; 211-Composite particles; 212-Filler layer; 22-Second coating; 221-Polymer particles; 3-Core; 4-Outer shell. Detailed Implementation

[0029] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0030] like Figure 1 As shown, a first aspect of the present invention provides a composite membrane, including a base membrane 1 and a coating 2 disposed on at least one surface of the base membrane 1; the coating 2 includes a first coating 21 disposed on the surface of the base membrane 1 and a second coating 22 disposed on at least a portion of the surface of the first coating 21.

[0031] The first coating 21 includes filler particles and composite particles 211. The filler particles are stacked to form a filler layer 212, and the composite particles 211 are embedded in the filler layer 212 and form protrusions on the surface of the filler layer 212. The composite particles 211 include primary polymer particles and secondary particles formed by co-aggregation of ceramics.

[0032] The second coating 22 includes polymer particles 221; the polymer particles 221 include second polymer primary particles;

[0033] The height h of the composite particles protruding from the second coating satisfies 1μm≤h≤4.9μm. For example, h can be any point value or a range of any two point values ​​among 1μm, 2μm, 3μm, 4μm, and 4.9μm.

[0034] This invention utilizes a combination of composite particles and polymer particles. The composite particles are embedded in the filler layer and form protrusions on the surface of the filler layer. During the hot pressing of the separator and electrode sheet, in addition to providing adhesion, they also provide spatial points, improving the electrolyte storage performance of the battery cell and thus enhancing its cycle performance. The polymer particles are located on the surface of the filler layer and are not embedded in the filler layer, effectively improving the bonding performance of the composite separator. This achieves a two-way improvement in both the electrolyte storage performance and bonding performance of the battery cell. In addition, the separator provided by this invention also has good heat resistance.

[0035] Since the second coating is mostly located on the surface of the filler layer, when limiting the height of the composite particles protruding from the second coating, the second coating refers to the second coating located on the surface of the filler layer, that is, the height of the composite particles protruding from the second coating on the filler layer is 1-4.9 μm. If the protrusion height is too low, the gap site capability (height of the spatial sites) provided by the composite particles decreases, the cell's liquid storage capacity decreases, the cell's expansion coefficient increases, and the cell's performance decreases. If the protrusion height is too high, a larger composite particle size or a smaller embedding depth is required. If the composite particle size is too large, the composite particle coverage decreases at the same basis weight, the number of effective gap sites (spatial sites) provided decreases, and when the composite separator and electrode sheet are hot-pressed, an effective gap space cannot be provided, which easily leads to an increase in the cell's expansion coefficient. If the composite particle size is too large or the embedding depth is too small, the composite particles are also prone to falling off, resulting in a decrease in the adhesion between the composite separator and the electrode sheet, thereby affecting the overall cell performance. In some specific embodiments, the test method for h includes: taking a cross-section of the composite separator, measuring the height of multiple composite particles protruding from the second coating by scanning electron microscopy, and taking the average value.

[0036] The composite particles used in this invention comprise primary polymer particles and secondary particles formed by the co-aggregation of ceramic particles. The introduction of ceramic particles increases the phase interface between the primary particles within the composite particles, thereby providing more channels for ions. This effectively solves the problem of lithium plating caused by the blockage of ions by large polymer particles during coating, thus improving cycle performance. In addition, the addition of ceramic to the composite particles can also increase the affinity between the composite particles and inorganic fillers during coating slurry preparation, improving the dispersibility of the slurry system and preventing the composite particles from agglomerating in the composite membrane. The addition of ceramic can also effectively increase the relative density of the composite particles. When mixed with filler particles for coating, the composite particles can be effectively embedded in the pores of the filler layer, increasing the adhesion between the composite particles and the coating. This solves the problem of powder shedding during the mixed coating of large-particle polymers and inorganic fillers, and improves the adhesion between the composite membrane and the electrode.

[0037] The composite separator provided by this invention has good air permeability, low internal resistance, and is not prone to powder shedding. It also has good adhesion to the electrode sheet, with a dry pressure adhesion force greater than 12 N / m. The ionic conductivity of the composite separator is greater than 0.9 mS / cm, making it less prone to lithium plating. The battery cell prepared using this composite separator has a small bulging expansion coefficient, high capacity retention, and excellent cycle stability.

[0038] In some specific embodiments of the present invention, both the first polymer and the second polymer are non-fluorinated polymers.

[0039] In some specific embodiments of the present invention, the hardness of the composite particles is greater than that of the polymer particles. The polymer particles have lower hardness, which can effectively improve the adhesion performance of the separator to the electrode sheet. The composite particles have relatively higher hardness, which can improve the overall hardness of the battery cell.

[0040] In some specific embodiments of the present invention, the thickness of the filler layer is H, and the depth of the composite particles embedded in the filler layer is d, satisfying: 1μm≤H≤2μm, H / 2≤d≤H. For example, H can be any single value or a range of any two values ​​among 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, and 2μm; d can be any single value or a range of any two values ​​among H / 2, 0.6H, 0.7H, 0.8H, 0.9H, and H. Controlling the embedding depth of the composite particles within the above range ensures the adhesion between the composite particles and the coating, avoiding powder shedding problems caused by the mixing and coating of large-particle polymers and fillers. If the filler layer is too thin or the embedding depth is too shallow, the adhesion between the composite particles and the filler layer decreases, resulting in severe powder shedding from the composite diaphragm. The detachment of composite particles affects the adhesion between the composite diaphragm and the electrode sheet, leading to a decrease in cell performance. However, if the filler layer is too thick and the embedding depth is too deep, it will reduce the protrusion height of the composite particles, affecting the electrolyte storage performance of the cell.

[0041] In some specific embodiments of the present invention, the test method for d includes: taking a cross-section of the composite diaphragm, testing the depth of multiple composite particles embedded in the filler layer by taking a scanning electron microscope, and taking the average value.

[0042] In some specific embodiments of the present invention, the coverage rate η1 of the composite particles relative to the base film is 4.2% to 18.8%, for example, it can be any one value or a range of any two values ​​from 4.2%, 5.2%, 5.7%, 6.5%, 7.6%, 8.7%, 9.5%, 11.8%, 12.3%, 14%, 15.6%, 17.3%, and 18.8%; the coverage rate η2 of the polymer particles relative to the base film is 12.5% ​​to 31.7%, for example, it can be any one value or a range of any two values ​​from 12.5%, 13.3%, 15%, 18%, 20%, 25%, 30%, and 31.7%; the total coverage rate η (η=η1+η2) of the first polymer and the second polymer relative to the base film is 25.0% to 40.0%, for example, it can be any one value or a range of any two values ​​from 25%, 30%, 35%, and 40%. Both excessively high and low η1 will lead to a greater increase in the gas permeability growth rate of the composite membrane. Simultaneously, excessively high η1 can cause the composite particles to accumulate, leading to lithium plating. Insufficient η1 reduces the effective space available, resulting in inadequate liquid storage space during hot pressing with the electrode, potentially increasing the cell expansion coefficient. Excessively high η2 worsens the membrane's gas permeability, while excessively low η2 reduces adhesion. If both η1 and η2 are high, the total polymer particle coverage η will be too large, worsening the membrane's gas permeability. Conversely, if both η1 and η2 are low, the total polymer particle coverage η will be too small, significantly reducing adhesion. Therefore, it is necessary to rationally control the ranges of η1, η2, and η. This invention limits the coverage of composite particles and polymer particles, as well as their total coverage, to provide sufficient space without deteriorating the coating's gas permeability, thus maintaining ion conduction and achieving high adhesion.

[0043] In some specific embodiments of the present invention, the testing method for η1 includes: taking a cross-section of the composite diaphragm, measuring the maximum length of multiple composite particles along the coating direction using a scanning electron microscope, taking the average value to obtain the composite particle size L, and calculating the coverage area of ​​a single composite particle according to S=3.14×L×L / 4; taking the composite diaphragm, testing the number of composite particles per unit area of ​​the diaphragm by taking a scanning electron microscope, and calculating η1 according to S×the number of composite particles per unit area.

[0044] In some specific embodiments of the present invention, the test method for η2 includes: taking a composite diaphragm, testing the number of polymer particles per unit area of ​​the diaphragm by taking a scanning electron microscope, and calculating it according to η2=(3.14×D2×D2) / 4×number of polymer particles per unit area, where D2 is the particle size of the polymer particles.

[0045] In some specific embodiments of the present invention, the areal density of the composite particles is X, and the areal density of the polymer particles is Y, both satisfying: 0.20 g / m³.2 ≤X≤0.60g / m 2 Y = -0.125X + a, 0.15 ≤ a ≤ 0.2, for example, X can be 0.20 g / m³. 2 0.3g / m 2 0.4g / m 2 0.5g / m 2 0.6g / m 2 'a' can be any one value or any two values ​​in the range 0.15, 0.16, 0.175, 0.19, 0.2.

[0046] The areal density (coating basis weight) of polymer particles decreases as the areal density (coating basis weight) of composite particles increases. This ensures that the adhesion performance of the composite separator does not deteriorate, and the total coverage of composite and polymer particles in the coating does not increase dramatically, thus not affecting ion conduction and ultimately the cycle performance of the battery cell. When the areal density of composite particles increases, the areal density of polymer particles also increases, leading to an increase in the overall coverage of the composite separator. This significantly increases the separator's permeability, affecting the overall internal resistance of the battery cell. Simultaneously, the increased basis weight of both composite and polymer particles can easily lead to lithium plating in the battery cell, thus affecting its cycle performance. Conversely, when the areal density of composite particles decreases, the areal density of polymer particles also decreases, resulting in a severe decrease in the adhesion between the composite separator and the electrode sheet, an increase in the cell's expansion rate, an increase in the cell's internal resistance, and a decrease in the cell's cycle performance.

[0047] In some specific embodiments of the present invention, the ratio K of the number of polymer particles to the number of composite particles in the total coating satisfies: 11 < K < 750. For example, K can be any one value or any two values ​​from 15, 40, 90, 180, 280, 330, 400, 530, 600, 680, 716, 745 to form a range value.

[0048] In some specific embodiments of the present invention, the testing method for K includes: taking a composite membrane, testing the number of polymer particles and composite particles per unit area of ​​the membrane by taking a scanning electron microscope, and calculating the number of polymer particles per unit area / the number of composite particles per unit area.

[0049] When the total amount of composite particles and polymer particles is appropriate, if the proportion of composite particles in the composite separator is too high, it will lead to a reduction in polymer particles. Since polymer particles are fully coated with primary particles, they provide far more bonding sites than the secondary particles in the composite. Therefore, when bonding with the electrode sheet, polymer particles contribute more to the adhesion than composite particles. A reduction in polymer particles can easily lead to a decrease in the adhesion between the composite separator and the electrode sheet. At the same time, an excessively high proportion of composite particles can also easily lead to a larger gas permeability growth rate of the composite separator, and the composite particles are prone to accumulation, resulting in lithium plating. If the proportion of composite particles is too low, the coverage of polymer particles increases, the gas permeability growth rate of the composite separator increases, affecting the internal resistance of the cell. At the same time, when the proportion of composite particles is too low, the number of gap sites (space sites) provided is reduced, the cell expansion coefficient increases, and the cycle performance decreases.

[0050] In some specific embodiments of the present invention, the particle size of the composite particles 211 is 4~6μm. For example, it can be any single value or a range of any two values ​​among 4μm, 4.5μm, 5μm, 5.5μm, and 6μm. The particle size in the present invention refers to the D50 particle size. By controlling the particle size of the composite particles to 4~6μm, the present invention can effectively improve the electrolyte storage performance of the battery cell, reduce the battery cell expansion coefficient, and ensure the adhesion between the separator and the electrode sheet. If the particle size of the composite particles is too small, the coverage rate of the composite particles increases under the same weight, the gas permeability growth rate of the composite separator increases, and the internal resistance increases. After hot pressing the composite separator and electrode sheet, the gap site capability (height of the spatial sites) provided by the composite particles decreases, the cell's liquid storage capacity decreases, the cell's expansion coefficient increases, and the cell's performance deteriorates. If the composite particle size is too large, the composite particle coverage decreases under the same weight, and the number of effective gap sites (spatial sites) provided decreases. When the composite separator and electrode sheet are hot pressed, they cannot provide effective gap space, which easily leads to an increase in the cell's expansion coefficient. At the same time, if the composite particle size is too large, it is also easy for the composite particles to fall off, resulting in a decrease in the adhesion between the composite separator and the electrode sheet, thereby affecting the overall cell performance.

[0051] In some specific embodiments of the present invention, the particle size of the first polymer primary particles is 0.4~0.6 μm, for example, it can be any single value or a range of any two values ​​among 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, and 0.6 μm; and / or, the observable primary particle size of the ceramic is 0.4~0.6 μm, for example, it can be any single value or a range of any two values ​​among 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, and 0.6 μm.

[0052] In some specific embodiments of the present invention, the mass percentage of ceramic in the composite particles is 5% to 12%, for example, it can be any one value or a range of any two values ​​from 5%, 6%, 7%, 8%, 9%, 10%, 11%, to 12%.

[0053] If the ceramic content in the composite particles is too low or absent, the dispersibility and affinity between the composite particles and inorganic fillers (such as inorganic oxides and hydroxides) will weaken when the composite particles are mixed with inorganic fillers to form the first slurry. This will lead to easy accumulation of composite particles in the composite membrane, and a decrease in the ion conduction channels of the composite particles, which can easily cause lithium plating in the cell. At the same time, the depth of the composite particles embedded in the filler layer is insufficient, the adhesion between the composite particles and the filler layer is reduced, the composite membrane will shed a lot of powder, and the composite particles will fall off, affecting the adhesion between the composite membrane and the electrode, and the cell performance will decline. If the ceramic content in the composite particles is too high, the adhesion between the composite particles and the electrode will decrease under the same basis weight, resulting in an increase in the interfacial resistance between the membrane and the electrode, a decrease in cycle performance, low adhesion, reduced cell hardness, and a cell that is more prone to expansion.

[0054] In some specific embodiments of the present invention, the ceramic used to form the composite particles includes at least one of boehmite, alumina, silicon dioxide, zirconium oxide, LATP, and LLZO.

[0055] In some specific embodiments of the present invention, the ceramic forming the composite particles is a coupling agent modified ceramic, the purpose of which is to improve the co-agglomeration effect of the ceramic and the primary particles of the first polymer.

[0056] In some specific embodiments of the present invention, the glass transition temperature Tg of the first polymer primary particles is -5 to 5°C. For example, it can be any one value or a range of any two values ​​among -5°C, -3°C, -1°C, 0°C, 2°C, and 5°C.

[0057] In some specific embodiments of the present invention, such as Figure 2 As shown, the first polymer primary particle includes a core 3 and a shell 4 covering the surface of the core 3; the core monomer forming the core 3 and the shell monomer forming the shell 4 are each independently selected from at least one of methacrylonitrile, styrene, acrylonitrile, ethyl acrylate, 1-butene, 2-ethylhexyl acrylate, isooctyl acrylate, and n-butyl acrylate; and the shell monomer contains at least one monomer different from the core monomer.

[0058] In some preferred embodiments of the present invention, the molecular chain of the core includes a first structural unit, and the molecular chain of the shell includes a first structural unit and a second structural unit; the monomer forming the first structural unit includes at least one of methacrylonitrile, styrene, and acrylonitrile, and the monomer forming the second structural unit includes at least one of ethyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, and n-butyl acrylate. By controlling the types of monomers constituting the core-shell structure, the core-shell structure has the characteristics of being soft on the outside and hard on the inside, which not only improves the adhesion performance to the electrode sheets but also enhances the overall hardness of the battery cell.

[0059] In some specific embodiments of the present invention, the particle size of polymer particles 221 is 0.6~1.0μm, for example, it can be any one value or a range of any two values ​​among 0.6μm, 0.7μm, 0.8μm, 0.9μm, and 1.0μm.

[0060] In some specific embodiments of the present invention, the glass transition temperature Tg of the polymer particles is 45~55℃, for example, it can be any one value or a range of any two values ​​among 45℃, 48℃, 50℃, 52℃, and 55℃.

[0061] In some specific embodiments of the present invention, the monomers forming the second polymer include any two of methyl methacrylate, butyl methacrylate, ethyl acrylate, isooctyl acrylate, and n-butyl acrylate.

[0062] In some specific embodiments of the present invention, the mass percentage of composite particles in the total coating is 11.5% to 18.5%, for example, it can be any one value or a range of any two values ​​from 11.5%, 13%, 14%, 15%, 16%, 17%, 18.5%.

[0063] In some specific embodiments of the present invention, the polymer particles account for 3.0% to 9.0% of the total coating by mass. For example, it can be any one value or a range of any two values ​​from 3.0%, 5.0%, 7.0%, to 9.0%.

[0064] In some specific embodiments of the present invention, the mass percentage of filler particles in the total coating is 66% to 86%, for example, it can be any one value or a range of any two values ​​among 66%, 70%, 75%, 80%, and 86%.

[0065] In some specific embodiments of the present invention, the filler particles comprise one or more of inorganic oxides, inorganic hydroxides, metal salts, or organic non-polymer particles. As an example, the filler particles may include at least one of boehmite, alumina, magnesium hydroxide, tin dioxide, silicon dioxide, barium sulfate, LLZO, LATP, and melamine cyanurate.

[0066] In some specific embodiments of the present invention, the particle size of the filler particles is 0.5~0.8μm, for example, it can be any one value or a range of any two values ​​among 0.5μm, 0.6μm, 0.7μm, and 0.8μm.

[0067] In some specific embodiments of the present invention, more than 95% of the second coating is located on the surface of the filler layer. This is because the coverage of the composite particles is lower than that of the filler layer, and since the composite particles protrude from the surface of the filler layer, the slurry coated on the surface of the composite particles is prone to slipping off due to gravity. Therefore, the vast majority of the second coating is located on the surface of the filler layer.

[0068] In some specific embodiments of the present invention, the first coating further contains a first adhesive, the first adhesive accounting for 2% to 8% of the total coating by mass, for example, any one value or a range of any two values ​​from 2%, 4%, 6%, and 8%; and / or, the second coating further contains a second adhesive, the second adhesive accounting for 0.05% to 0.50% of the total coating by mass, for example, any one value or a range of any two values ​​from 0.05%, 0.10%, 0.30%, and 0.50%.

[0069] In some specific embodiments of the present invention, the first adhesive and the second adhesive are each independently selected from at least one of epoxy resin, styrene-butadiene rubber, polyacrylate, polyamide, polystyrene, and polyvinyl alcohol.

[0070] In some specific embodiments of the present invention, the first coating further contains a first wetting agent, the first wetting agent accounting for 0.1% to 1.0% of the total coating by mass, for example, it can be any one value or a range of any two values ​​from 0.1%, 0.3%, 0.5%, 0.8%, 1.0%; and / or, the second coating further contains a second wetting agent, the second wetting agent accounting for 0.02% to 0.2% of the total coating by mass, for example, it can be any one value or a range of any two values ​​from 0.02%, 0.05%, 0.1%, 0.15%, 0.2%.

[0071] In some specific embodiments of the present invention, the first wetting agent and the second wetting agent are each independently selected from at least one of alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty acid polyoxyethylene ether, polyoxyethylene fatty alcohol ether, polyoxyethylene polyoxypropylene block copolymer, and polyoxyethylene alkylphenol ether.

[0072] In some specific embodiments of the present invention, the base film includes any one of a single-layer PP porous membrane, a single-layer PE porous membrane, a double-layer PE / PP composite membrane, and a double-layer PP / PP composite membrane.

[0073] In some specific embodiments of the present invention, the thickness of the base film is 5~16μm, for example, it can be any one value or a range of any two values ​​among 5μm, 8μm, 10μm, 12μm, and 16μm.

[0074] A second aspect of the present invention provides a method for preparing the composite separator described in any of the foregoing embodiments, comprising the following steps:

[0075] The first raw material is mixed to obtain a first mixed slurry, wherein the first raw material includes filler particles and an emulsion containing composite particles;

[0076] The second raw material is mixed to obtain a second mixed slurry, wherein the second raw material includes an emulsion containing polymer particles;

[0077] A first mixed slurry is coated onto at least one side surface of the base membrane and dried to obtain a first coating; then a second mixed slurry is coated onto the first coating and dried to obtain a composite membrane.

[0078] The method of this invention mixes filler particles and composite particles into a slurry and coats it onto the surface of a base membrane to form a first coating. This forms a filler layer on the base membrane, while simultaneously embedding the composite particles into the filler layer and creating protrusions on its surface. During the hot pressing of the separator and electrode sheet, these protrusions not only provide adhesion but also create spatial points, improving the electrolyte storage performance of the battery cell and thus enhancing its cycle performance. Coating the surface of the first coating with polymer particles effectively improves the adhesion between the composite separator and the electrode sheet. Furthermore, the composite coating obtained by this method exhibits better heat resistance. Moreover, the preparation process of this invention is simple and easy to implement.

[0079] In some specific embodiments of the present invention, the method for preparing an emulsion containing composite particles includes the following steps:

[0080] S1. A first emulsifier, a core monomer, and a first initiator undergo a first polymerization reaction in a first solvent to obtain a core particle emulsion; the core particle emulsion is subjected to UV irradiation, and a shell monomer and a second initiator are added to undergo a second polymerization reaction to obtain an emulsion containing primary particles of the first polymer;

[0081] S2. The ceramic surface is modified by reacting it with a coupling agent to improve the bonding degree and uniformity between the ceramic and the primary particles of the first polymer. Then, an emulsion containing the primary particles of the first polymer and salt are added and stirred to obtain an emulsion containing composite particles.

[0082] This invention employs an emulsion demulsification method to agglomerate primary particles into secondary particles. During the agglomeration process, ceramic particles are added to increase the phase interface between primary particles within the composite particles, thereby providing more channels for ions. This effectively solves the problem of lithium plating caused by large polymer particles blocking ion passage, thus improving cycle performance. In addition, the addition of ceramics can also improve the dispersibility of the slurry system and increase the relative density of the composite particles, allowing them to be effectively embedded in the voids of the filler layer and increasing the adhesion between the composite particles and the coating.

[0083] In some specific embodiments of the present invention, the UV irradiation conditions are: a wavelength of 320nm~400nm and an energy of 200mJ / cm. 2 ~280mJ / cm 2 Irradiation with ultraviolet light for 1.5 to 2.5 hours, for example, the wavelength can be any single value or a range of any two values ​​from 320 nm, 340 nm, 360 nm, 380 nm, and 400 nm; the energy can be 200 mJ / cm². 2 220mJ / cm 2 240mJ / cm 2 260mJ / cm 2 280mJ / cm 2 The value can be any one point or any two points within the range; the irradiation time can be any one point or any two points within the range of 1.5h, 1.8h, 2h, 2.2h, and 2.5h.

[0084] In some specific embodiments of the present invention, in step S2, the mass ratio of ceramic to the first polymer primary particles is 5:95 to 12:88. For example, it can be any one value or a range of any two values ​​among 5:95, 8:92, 10:90, and 12:88.

[0085] In some specific embodiments of the present invention, in step S2, the stirring speed is 100~200 rpm, for example, it can be any one value or a range of any two values ​​among 100 rpm, 120 rpm, 150 rpm, 180 rpm, and 200 rpm; the stirring time is 6~10 h, for example, it can be any one value or a range of any two values ​​among 6 h, 7 h, 8 h, 9 h, and 10 h.

[0086] In some specific embodiments of the present invention, in step S2, the coupling agent includes, but is not limited to, at least one of vinyltriethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-methacryloyloxypropyltrimethoxysilane.

[0087] In some specific embodiments of the present invention, the salt used in step S2 includes, but is not limited to, sodium sulfate. As an example, sodium sulfate may be added in solution form.

[0088] In some specific embodiments of the present invention, the first emulsifier includes, but is not limited to, sodium dodecyl sulfate.

[0089] In some specific embodiments of the present invention, the first initiator and the second initiator are each independently selected from persulfates, such as potassium persulfate or sodium persulfate.

[0090] In some specific embodiments of the present invention, the method for preparing an emulsion containing polymer particles includes the following steps:

[0091] A second emulsifier, a monomer that forms a second polymer, and a third initiator undergo a third polymerization reaction in a second solvent to obtain an emulsion containing polymer particles.

[0092] In some specific embodiments of the present invention, the second emulsifier includes, but is not limited to, sodium dodecyl sulfate.

[0093] In some specific embodiments of the present invention, the third initiator includes, but is not limited to, potassium persulfate.

[0094] In some specific embodiments of the present invention, the first raw material further includes a first binder and / or a first wetting agent.

[0095] In some specific embodiments of the present invention, the second raw material further includes a second binder and / or a second wetting agent.

[0096] A third aspect of the present invention provides a secondary battery comprising the composite separator described in any one of the foregoing embodiments, or the composite separator prepared by the method described in any one of the foregoing embodiments.

[0097] The following detailed description of some embodiments of the present invention is provided in conjunction with specific application examples. Unless otherwise specified, all raw materials used in the embodiments can be obtained commercially available.

[0098] Example 1

[0099] This embodiment provides a composite membrane, comprising a 7μm thick PE base film and a coating disposed on one side surface of the base film. The coating consists of a first coating disposed on the base film and a second coating disposed on at least a portion of the surface of the first coating, with more than 95% of the second coating located on the surface of the filler layer. The first coating consists of composite particles, filler particles (alumina with a particle size of 0.6μm), a first binder, and a first wetting agent. The second coating consists of polymer particles, a second binder, and a second wetting agent. Both the first and second binders are styrene-butadiene rubber, and both the first and second wetting agents are alkylphenol polyoxyethylene ethers. The total coating composition by mass percentage is: composite particles 12.92%, polymer particles 4.14%, filler particles 78.92%, first binder 3.35%, second binder 0.14%, first wetting agent 0.48%, and second wetting agent 0.05%. The coating basis weight (area density) of the composite particles is 0.38 g / m³. 2 The coating basis weight (area density) of the polymer particles is 0.122 g / m³. 2 .

[0100] The preparation method is as follows:

[0101] (1) Preparation of the first polymer primary particle emulsion: 140 parts of deionized water and 0.5 parts of sodium dodecyl sulfate were added to the reactor and stirred evenly. Nitrogen gas was introduced to purge the air from the reactor. 15 parts of acrylonitrile core monomer were added. The pH of the reaction was adjusted to 7.0~8.0 using 0.01 mol / L acetic acid. The mixture was heated to 85°C and pressurized to 2 MPa. 0.3 parts of potassium persulfate were added, and the reaction was carried out for 4 h to obtain the core particle emulsion. The core particle emulsion was then subjected to UV irradiation (wavelength 360 nm, energy 240 mJ / cm). 2 Irradiate with ultraviolet light for 2 hours), then add 12 parts acrylonitrile and 33 parts 2-ethylhexyl acrylate shell monomer and 0.6 parts potassium persulfate, adjust the temperature to 76℃, adjust the pressure to 4MPa, react for 8 hours, and then cool and depressurize to room temperature and pressure to obtain the first polymer primary particle emulsion.

[0102] (2) Preparation of composite particles: 7.0 parts of boehmite with a particle size of 0.5 μm, 0.08 parts of vinyltriethoxysilane and 30 parts of deionized water were added to the reaction vessel, heated to 80°C, and pressurized to 3 MPa to modify the ceramic surface. The temperature and pressure were reduced to room temperature and pressure, 200 parts of the first polymer primary particle emulsion were added, and 0.3 parts of 0.5 mol / L sodium sulfate were added. The mixture was stirred continuously at 150 rpm for 8 hours to obtain composite particles. The mixture was diluted with water to a solid content of 20% to obtain an emulsion containing composite particles.

[0103] (3) Preparation of polymer particles: Add 160 parts of deionized water and 0.4 parts of sodium dodecyl sulfate to the reaction vessel and stir evenly. Purge the air in the reaction vessel with nitrogen gas. Add 19 parts of methyl methacrylate and 21 parts of ethyl acrylate. Adjust the pH of the reaction to 7-8. Heat to 78°C and pressurize to 2.5 MPa. Add 0.6 parts of potassium persulfate. React for 6 hours. Cool down and depressurize to room temperature and pressure. Dilute with water to a solid content of 15% to obtain an emulsion containing polymer particles.

[0104] (4) Preparation of the first mixed slurry: Take 82.5 parts of alumina with a particle size of 0.6 μm, 67.5 parts of the emulsion containing composite particles prepared in step (2), 3.5 parts of styrene-butadiene rubber, 0.5 parts of alkylphenol polyoxyethylene ether and 300 parts of deionized water, mix them, stir evenly, and obtain the first mixed slurry;

[0105] (5) Preparation of the second mixed slurry: Take 60 parts of the emulsion containing polymer particles prepared in step (3), 0.3 parts of styrene-butadiene rubber, 0.1 parts of alkylphenol polyoxyethylene ether and 140 parts of deionized water, stir and mix evenly to obtain the second mixed slurry;

[0106] (6) Preparation of composite diaphragm: Take the first mixed slurry, use microgravure coating technology, adjust the microgravure coating speed ratio to 1.20, coat the first mixed slurry onto the base film, dry, and obtain a surface density of 2.82 g / m. 2 The first coating layer is applied; then, a second mixed slurry is applied, and the same microgravure coating technique is used, adjusting the microgravure coating speed ratio to 1.15. The second mixed slurry is then applied onto the first coating layer, dried, and the surface density of the second coating layer is controlled to be 0.13 g / m². 2 A composite diaphragm was obtained.

[0107] Example 2

[0108] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0109] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 14.32%, polymer particles 4.35%, filler particles 77.30%, first binder 3.35%, second binder 0.15%, first wetting agent 0.48%, and second wetting agent 0.05%; wherein the basis weight of the composite particles is 0.40 g / m³. 2 The coating weight of polymer particles is 0.120 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0110] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0111] In step (1), the core reaction temperature is 85.5℃, the pressure is 2.2MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 10 parts and 35 parts respectively, the reaction temperature is 76℃, and the pressure is 4.3MPa;

[0112] In step (2), the amount of boehmite added is 7.6 parts, and the stirring speed is 180 rpm;

[0113] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 78.5℃, and the pressure is 2.6MPa;

[0114] In step (4), the amount of alumina added is 81 parts, and the amount of emulsion containing composite particles added is 75 parts;

[0115] In step (6), the areal density of the first coating is 2.64 g / m³. 2 The areal density of the second coating is 0.13 g / m³. 2 .

[0116] Example 3

[0117] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0118] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 15.33%, polymer particles 6.82%, filler particles 73.81%, first binder 3.26%, second binder 0.23%, first wetting agent 0.47%, and second wetting agent 0.08%; wherein the coating basis weight of the composite particles is 0.30 g / m³. 2 The coating weight of polymer particles is 0.132 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0119] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0120] In step (1), the core reaction temperature is 84.5℃, the pressure is 2.1MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 11 parts and 34 parts respectively, the reaction temperature is 76.5℃, and the pressure is 4.2MPa.

[0121] In step (2), the amount of boehmite added is 3.5 parts, and the stirring speed is 145 rpm;

[0122] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 22 parts and 18 parts, respectively, the reaction temperature is 79.0℃, and the pressure is 2.4MPa;

[0123] In step (4), the amount of alumina added is 79.5 parts, and the amount of emulsion containing composite particles added is 82.5 parts;

[0124] In step (6), the areal density of the first coating is 1.80 g / m³. 2 The areal density of the second coating is 0.14 g / m³. 2 .

[0125] Example 4

[0126] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0127] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 17.43%, polymer particles 3.05%, filler particles 75.49%, first binder 3.39%, second binder 0.11%, first wetting agent 0.49%, and second wetting agent 0.04%; wherein the basis weight of the composite particles is 0.57 g / m³. 2 The coating weight of the polymer particles is 0.099 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0128] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0129] In step (1), the core reaction temperature is 85.8℃, the pressure is 2.4MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 8 parts and 37 parts respectively, the reaction temperature is 76.6℃, and the pressure is 4.2MPa;

[0130] In step (2), the amount of boehmite added is 6.9 parts, and the stirring speed is 140 rpm;

[0131] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 21 parts and 19 parts, respectively, the reaction temperature is 77.5℃, and the pressure is 2.4MPa;

[0132] In step (4), the amount of alumina added is 78 parts, and the amount of emulsion containing composite particles added is 90 parts;

[0133] In step (6), the areal density of the first coating is 3.16 g / m³. 2 The areal density of the second coating is 0.10 g / m³. 2 .

[0134] Example 5

[0135] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0136] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 11.98%, polymer particles 7.55%, filler particles 76.42%, first binder 3.23%, second binder 0.26%, first wetting agent 0.47%, and second wetting agent 0.09%; wherein the basis weight of the composite particles is 0.22 g / m³. 2 The coating weight of the polymer particles is 0.141 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0137] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0138] In step (1), the core reaction temperature is 86.2℃, the pressure is 2.1MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 11 parts and 34 parts respectively, the reaction temperature is 75.8℃, and the pressure is 3.9MPa;

[0139] In step (2), the amount of boehmite added is 7.7 parts, and the stirring speed is 170 rpm;

[0140] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 19.5 parts and 20.5 parts, respectively, the reaction temperature is 78.0℃, and the pressure is 2.3MPa;

[0141] In step (4), the amount of alumina added is 83 parts, and the amount of emulsion containing composite particles added is 65 parts;

[0142] In step (6), the areal density of the first coating is 1.72 g / m³. 2 The areal density of the second coating is 0.15 g / m³. 2 .

[0143] Example 6

[0144] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0145] The total coating, consisting of the first and second coatings, comprises the following composition by mass percentage: composite particles 12.47%, polymer particles 7.35%, filler particles 76.13%, first binder 3.24%, second binder 0.25%, first wetting agent 0.47%, and second wetting agent 0.09%; wherein the basis weight of the composite particles is 0.24 g / m³. 2 The coating weight of polymer particles is 0.139 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0146] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0147] In step (1), the core reaction temperature is 84.8℃, the pressure is 1.9MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 12 parts and 33 parts respectively, the reaction temperature is 74.8℃, and the pressure is 4.1MPa;

[0148] In step (2), the amount of boehmite added is 6.4 parts, and the stirring speed is 140 rpm;

[0149] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 18.5 parts and 21.5 parts, respectively, the reaction temperature is 78.6℃, and the pressure is 2.4MPa;

[0150] In step (4), the amount of alumina added is 82.5 parts, and the amount of emulsion containing composite particles added is 67.5 parts;

[0151] In step (6), the areal density of the first coating is 1.75 g / m³. 2 The areal density of the second coating is 0.15 g / m³. 2 .

[0152] Example 7

[0153] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0154] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 13.76%, polymer particles 4.93%, filler particles 77.27%, first binder 3.33%, second binder 0.17%, first wetting agent 0.48%, and second wetting agent 0.06%; wherein the basis weight of the composite particles is 0.35 g / m³. 2 The coating weight of polymer particles is 0.126 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0155] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0156] In step (1), the core reaction temperature is 83.8℃, the pressure is 1.7MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 12 parts and 33 parts respectively, the reaction temperature is 75.8℃, and the pressure is 3.9MPa;

[0157] In step (2), the amount of boehmite added is 5.8 parts, and the stirring speed is 142 rpm;

[0158] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20.0 parts and 20.0 parts, respectively, the reaction temperature is 76.6℃, and the pressure is 2.8MPa;

[0159] In step (4), the amount of alumina added is 81.5 parts, and the amount of emulsion containing composite particles added is 72.5 parts;

[0160] In step (6), the areal density of the first coating is 2.42 g / m³. 2 The areal density of the second coating is 0.13 g / m³. 2 .

[0161] Example 8

[0162] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0163] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 11.90%, polymer particles 8.16%, filler particles 75.89%, first binder 3.21%, second binder 0.28%, first wetting agent 0.46%, and second wetting agent 0.10%; wherein the basis weight of the composite particles is 0.21 g / m³. 2 The coating weight of the polymer particles is 0.143 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0164] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0165] In step (1), the core reaction temperature is 86.8℃, the pressure is 2.1MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 10 parts and 35 parts respectively, the reaction temperature is 76.8℃, and the pressure is 4.3MPa;

[0166] In step (2), the amount of boehmite added is 4.9 parts, and the stirring speed is 135 rpm;

[0167] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20.0 parts and 20.0 parts, respectively, the reaction temperature is 79.6℃, and the pressure is 2.6MPa;

[0168] In step (4), the amount of alumina added is 83 parts, and the amount of emulsion containing composite particles added is 65 parts;

[0169] In step (6), the areal density of the first coating is 1.60 g / m³. 2 The areal density of the second coating is 0.15 g / m³. 2 .

[0170] Example 9

[0171] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0172] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 17.92%, polymer particles 3.04%, filler particles 75.02%, first binder 3.39%, second binder 0.11%, first wetting agent 0.48%, and second wetting agent 0.04%; wherein the basis weight of the composite particles is 0.58 g / m³. 2 The coating weight of polymer particles is 0.098 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0173] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0174] In step (1), the core reaction temperature is 83.6℃, ​​the pressure is 2.3MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 7 parts and 38 parts respectively, the reaction temperature is 74.8℃, and the pressure is 4.1MPa;

[0175] In step (2), the amount of boehmite added is 3.8 parts, and the stirring speed is 180 rpm;

[0176] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 19 parts and 21 parts, respectively, the reaction temperature is 77.7℃, and the pressure is 2.3MPa;

[0177] In step (4), the amount of alumina added is 77.5 parts, and the amount of emulsion containing composite particles added is 92.5 parts;

[0178] In step (6), the areal density of the first coating is 3.14 g / m³. 2 The areal density of the second coating is 0.10 g / m³. 2 .

[0179] Example 10

[0180] The structure of the composite diaphragm in this embodiment is similar to that in Embodiment 1, except that:

[0181] The total coating, consisting of the first and second coatings, comprises the following components by mass percentage: composite particles 17.10%, polymer particles 4.83%, filler particles 74.03%, first binder 3.33%, second binder 0.17%, first wetting agent 0.48%, and second wetting agent 0.06%; wherein the basis weight of the composite particles is 0.41 g / m³. 2 The coating weight of polymer particles is 0.115 g / m³. 2 The differences in other parameters are shown in Tables 1 and 2.

[0182] The preparation method is the same as that of the composite membrane in Example 1, except that:

[0183] In step (1), the core reaction temperature is 84.6℃, the pressure is 2.2MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 11 parts and 13 parts respectively, the reaction temperature is 75.8℃, and the pressure is 4.0MPa;

[0184] In step (2), the amount of boehmite added is adjusted to 7.9 parts, and the stirring speed is 155 rpm;

[0185] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 78.7℃, and the pressure is 2.4MPa;

[0186] In step (4), the amount of alumina added is 78 parts, and the amount of emulsion containing composite particles added is 90 parts;

[0187] In step (6), the areal density of the first coating is 2.27 g / m². 2 The areal density of the second coating is 0.12 g / m³. 2 .

[0188] Comparative Example 1

[0189] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0190] In step (1), the core reaction temperature is 85.6℃, the pressure is 2.3MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 8 parts and 37 parts respectively, the reaction temperature is 73.3℃, ​​and the pressure is 4.4MPa;

[0191] In step (2), the amount of boehmite added is adjusted to 1.2 parts, and the stirring speed is 170 rpm;

[0192] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 21 parts and 19 parts, respectively, the reaction temperature is 76.7℃, and the pressure is 2.7MPa;

[0193] In step (4), the amount of alumina added is 79.5 parts, and the amount of emulsion containing composite particles added is 82.5 parts;

[0194] In step (6), the areal density of the first coating is 2.68 g / m³. 2 The areal density of the second coating is 0.12 g / m³. 2 .

[0195] Comparative Example 2

[0196] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0197] In step (1), the core reaction temperature is 82.6℃, the pressure is 2.4MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 8 parts and 37 parts respectively, the reaction temperature is 76.2℃, and the pressure is 3.8MPa;

[0198] In step (2), the amount of boehmite added is 10.6 parts, and the stirring speed is 165 rpm;

[0199] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 77.0℃, and the pressure is 2.3MPa;

[0200] In step (4), the amount of alumina added is 78 parts, and the amount of emulsion containing composite particles added is 90 parts;

[0201] In step (6), the areal density of the first coating is 2.84 g / m³. 2 The areal density of the second coating is 0.11 g / m³. 2 .

[0202] Comparative Example 3

[0203] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0204] In step (1), the core reaction temperature is 83.9℃, the pressure is 1.9 MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 10 parts and 35 parts respectively, the reaction temperature is 74.3℃, and the pressure is 4.2 MPa;

[0205] In step (2), the amount of boehmite added is 0 parts, and the stirring speed is 170 rpm;

[0206] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 78.7℃, and the pressure is 2.8MPa;

[0207] In step (4), the amount of alumina added is 79 parts, and the amount of emulsion containing composite particles added is 85 parts;

[0208] In step (6), the areal density of the first coating is 2.60 g / m³. 2 The areal density of the second coating is 0.12 g / m³. 2 .

[0209] Comparative Example 4

[0210] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0211] In step (1), the core reaction temperature is 83.2℃, the pressure is 2.1MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 9 parts and 36 parts respectively, the reaction temperature is 75.3℃, and the pressure is 4.0MPa;

[0212] In step (2), the amount of boehmite added is 7.6 parts, and the stirring speed is 200 rpm;

[0213] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 18 parts and 22 parts, respectively, the reaction temperature is 75.8℃, and the pressure is 2.9MPa;

[0214] In step (4), the amount of alumina added is 78 parts, and the amount of emulsion containing composite particles added is 90 parts;

[0215] In step (6), the areal density of the first coating is 3.16 g / m³. 2 The areal density of the second coating is 0.10 g / m³. 2 .

[0216] Comparative Example 5

[0217] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0218] In step (1), the core reaction temperature is 87.2℃, the pressure is 1.8MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 11 parts and 34 parts respectively, the reaction temperature is 76.3℃, and the pressure is 4.2MPa;

[0219] In step (2), the amount of boehmite added is 5.8 parts, and the stirring speed is 120 rpm;

[0220] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 21.5 parts and 18.5 parts, respectively, the reaction temperature is 76.4℃, and the pressure is 2.2MPa;

[0221] In step (4), the amount of alumina added is 83 parts, and the amount of emulsion containing composite particles added is 65 parts;

[0222] In step (6), the areal density of the first coating is 1.6 g / m³. 2 The areal density of the second coating is 0.15 g / m³. 2 .

[0223] Comparative Example 6

[0224] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0225] In step (1), the core reaction temperature is 84.8℃, the pressure is 2.4MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 13 parts and 32 parts respectively, the reaction temperature is 77.3℃, and the pressure is 3.8MPa;

[0226] In step (2), the amount of boehmite added is 6.3 parts, and the stirring speed is 180 rpm;

[0227] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 22 parts and 18 parts, respectively, the reaction temperature is 76.7℃, and the pressure is 2.3MPa;

[0228] In step (4), the amount of alumina added is 71 parts, and the amount of emulsion containing composite particles added is 125 parts;

[0229] In step (6), the areal density of the first coating is 3.16 g / m³. 2 The areal density of the second coating is 0.08 g / m³. 2 .

[0230] Comparative Example 7

[0231] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0232] In step (1), the core reaction temperature is 86.2℃, the pressure is 2.3MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 12 parts and 33 parts respectively, the reaction temperature is 76.1℃, and the pressure is 4.2MPa;

[0233] In step (2), the amount of boehmite added is 6.8 parts, and the stirring speed is 175 rpm;

[0234] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 75.8℃, and the pressure is 2.9MPa;

[0235] In step (4), the amount of alumina added is 88 parts, and the amount of emulsion containing composite particles added is 40 parts;

[0236] In step (6), the areal density of the first coating is 1.62 g / m³. 2 The areal density of the second coating is 0.16 g / m³. 2 .

[0237] Comparative Example 8

[0238] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0239] In step (1), the core reaction temperature is 87.2℃, the pressure is 1.8MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 12 parts and 33 parts respectively, the reaction temperature is 75.8℃, and the pressure is 4.1MPa;

[0240] In step (2), the amount of boehmite added is 4.2 parts, and the stirring speed is 180 rpm;

[0241] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 20 parts and 20 parts respectively, the reaction temperature is 76.6℃, and the pressure is 2.3MPa;

[0242] In step (4), the amount of alumina added is 78 parts, and the amount of emulsion containing composite particles added is 90 parts;

[0243] In step (6), the areal density of the first coating is 3.02 g / m³. 2 The areal density of the second coating is 0.16 g / m³. 2 .

[0244] Comparative Example 9

[0245] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0246] In step (1), the core reaction temperature is 84.6℃, the pressure is 2.2MPa, the amount of acrylonitrile and 2-ethylhexyl acrylate added to the shell are 11 parts and 34 parts respectively, the reaction temperature is 73.9℃, and the pressure is 4.3MPa;

[0247] In step (2), the amount of boehmite added is 4.7 parts, and the stirring speed is 145 rpm;

[0248] In step (3), the amounts of methyl methacrylate and ethyl acrylate added are 22 parts and 18 parts, respectively, the reaction temperature is 77.3℃, and the pressure is 2.5MPa;

[0249] In step (4), the amount of alumina added is 83 parts, and the amount of emulsion containing composite particles added is 65 parts;

[0250] In step (6), the areal density of the first coating is 1.64 g / m³. 2 The areal density of the second coating is 0.10 g / m³. 2 .

[0251] Comparative Example 10

[0252] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0253] In step (6), the first mixed slurry and the second mixed slurry are mixed together and coated in one step to obtain a composite membrane. The coating weight of the composite particles and polymer particles remains unchanged, and the other conditions are the same as in Example 1.

[0254] Comparative Example 11

[0255] This comparative example refers to the preparation method of the composite membrane in Example 1, the difference being:

[0256] In step (2), no boehmite was added, and all other conditions were the same as in Example 1.

[0257] Experimental Example

[0258] 1. Testing of glass transition temperature (Tg), particle size, and ceramic content in composite particles.

[0259] (1) Tg test: The first polymer primary particle emulsion and polymer particle emulsion in each example and comparative example were dried separately, and 5~6mg of each sample was weighed. The Tg was tested by differential scanning calorimetry. The test equipment was METTLER DSC3, the test temperature range was -80~200℃, and the heating rate was 6℃ / min. After the test, the test curve was integrated to obtain the Tg of the test sample.

[0260] (2) Particle size test: Take the composite particle emulsion and polymer particle emulsion from each example and comparative example and drop them into Malvern 3000 to test the particle size. The test results are: refractive index: 1.52, absorptivity: 0.1, and shading rate: 8%~16%, to obtain the particle sizes D1 and D2 of the composite particles and polymer particles.

[0261] (3) Test of ceramic proportion in composite particles: The composite particle emulsions in each example and comparative example were dried, and the EDS of the dried powder was tested by scanning electron microscopy to obtain the proportion of Al element, and the proportion of ceramic was calculated. Finally, the proportion of ceramic in the first polymer was obtained.

[0262] The test results are shown in Table 1. Figure 4 This is the Tg diagram of the first polymer primary particles in Example 10 of the present invention; Figure 5 The Tg diagram of the polymer particles in Example 10 of this invention; Figure 6 This is an EDS image of the composite particles in Example 10 of the present invention.

[0263] Table 1

[0264]

[0265] 2. Composite membrane performance testing

[0266] (1) Air permeability growth rate test: air permeability growth rate = (composite membrane air permeability - base membrane air permeability) / base membrane air permeability × 100%, where air permeability refers to the time required for 100ml of gas to pass through a fixed area membrane.

[0267] (2) Thickness H of filler layer in the first coating: Take the cross section of the composite diaphragm, test the cross section of the composite diaphragm with a scanning electron microscope, measure the thickness of the filler layer, randomly test 5 points on the same sample, take the average value, test 6 samples in parallel in the same way, and take the final average value, which is the thickness of the filler layer.

[0268] (3) Embedding depth d and protrusion height h of composite particles: Take the cross section of the coated diaphragm and use scanning electron microscope to test the embedding depth of composite particles in the filler layer and the protrusion height of the second coating layer. Randomly test the embedding depth of composite particles in the filler layer and the protrusion height of the second filler layer of 5 samples on the same sample and take the average value. Then test 10 samples in parallel in the same way and take the final average value, which is the embedding depth d of composite particles in the filler layer and the protrusion height h of the second coating layer.

[0269] (4) Polymer particle coverage test:

[0270] Composite particle coverage η1: Take a cross-section of the composite diaphragm, select 10 points, and select the composite particles in the field of view at 2000x magnification. Measure the maximum length of the composite particles along the coating surface direction, and calculate the average of the maximum lengths of all composite particles at these 10 points. Perform parallel testing on 5 samples using the same method, and finally take the average of the 5 samples, L1. Take an electron microscope of the coated diaphragm at 3000x magnification (42.7μm×32.0μm) and take 15 pictures. Count the number of composite particles at each point at this magnification, and take the average of the 15 points, N1. Then, the composite particle coverage η1 = (N1×3.14×L1×...) L1) / (4×42.7×32.0), and test 5 samples in parallel using the same method. Take the average value of the 5 samples as the coverage of the composite particles. (When calculating the particle size of the composite particles, this invention uses a cross-sectional method to measure the maximum length of the composite particles parallel to the coating surface, which can better reflect the true particle size of the composite particles distributed in the first coating. For example, if the particle size of the composite diaphragm is calculated by taking an electron microscope on the surface of the composite diaphragm, the measured particle size will be smaller than the actual size for composite particles that are more than half filled in the filler layer, which will eventually lead to a decrease in the calculated coverage of the composite particles and a large error with the actual situation.)

[0271] Polymer particle coverage η2: Take an electron microscope with the coated diaphragm and take 6 pictures at 5000x (25.6μm×19.2μm). Count the number of polymer particles at each point at this magnification and take the average value as N2. Then the coverage of the second polymer η2 = (N2×3.14×D2×D2) / (4×25.6×19.2). Test 5 samples in parallel in the same way and take the average value of the 5 samples as the coverage of polymer particles.

[0272] (5) Test of the ratio K of polymer particles to composite particles: According to the test results of the above steps, the number of composite particles in each point (42.7μm×32.0μm) at 3000x magnification is N1, and the number of polymer particles in each point (25.6μm×19.2μm) at 5000x magnification is N2. Then K=(42.7×32×N2) / (25.6×19.2×N1)=2.78×N2 / N1. Five samples were tested in parallel using the same method, and the average value was taken as the second ratio K of polymer particles to composite particles.

[0273] (6) Composite diaphragm powder shedding rate test: The test was conducted using a dyeing fastness tester RT-300S. The composite diaphragm was cut into strips of 30cm×2cm, weighed, and the coated side was placed up. The strips were fixed, lens paper was placed on the rubbing device, and the device was placed on the diaphragm. The power was turned on, and the rubbing device was rubbed back and forth on the diaphragm 5 times. The rubbed diaphragm was removed, shaken gently three times, and the mass of the diaphragm was measured again. The difference between the two weights was divided by the total weight of the strip diaphragm coating to obtain the composite diaphragm powder shedding rate.

[0274] (7) Adhesion test with electrode: The composite diaphragm and positive electrode sheet were cut into 4cm×6cm specifications. The coating surface of the electrode sheet (coating composition is 95%LFP+2.5%PVDF+0.8%SP+1.6%CNT+0.1%plasticizer) corresponded to and overlapped with the coating surface of the diaphragm. After adjusting the parameters of the hot press (3MPa, 3mins, 90℃), the dry press was used. The adhesion between the diaphragm and the positive electrode sheet was tested by peeling at 180° with a peeling speed of 60mm / min. The adhesion between the coated diaphragm and the electrode sheet was obtained.

[0275] (8) Ionic conductivity performance: (In an argon-filled glove box, the diaphragm was made into a 2016 button cell, and an appropriate amount of electrolyte (EC:PC:DEC=1:3:5, 1.5 Mol / L LiPF6) was added. The AC impedance was measured using an electrochemical workstation, and the result was σ=L / (Rb×A), where σ is the ionic conductivity (mS / cm); L is the thickness of the diaphragm (cm); Rb is the intrinsic resistance of the diaphragm (Ω); and A is the effective area (cm²). 2 ).

[0276] (9) Heat shrinkage performance: Take the composite diaphragm, cut a 100mm×50mm rectangular sheet along the MD direction of the composite diaphragm and test the lengths M1 and T1 of MD and TD. Clamp the sheet with two A4 sheets of paper and put it into a 130℃ oven and bake for 30min. Test the lengths M2 and T2 of the MD and TD directions of the diaphragm again. The heat shrinkage values ​​of the coated diaphragm in the MD and TD directions are (M1-M2) / M1 and (T1-T2) / T1, respectively.

[0277] The performance test results of the composite diaphragm are shown in Table 2. Figure 3 This is a surface SEM image of the composite membrane in Embodiment 10 of the present invention.

[0278] Table 2

[0279]

[0280] 3. Electrical performance testing

[0281] (1) Capacity retention rate test: The composite separator, lithium iron phosphate positive electrode and graphite negative electrode are stacked to form the cell. An appropriate amount of electrolyte (EC:PC:DEC=1:3:5, 1.5 mol / L LiPF6) is added. The battery is charged at 1C constant current to 3.6V, and then charged at 3.6V constant voltage to 0.02C. The charging is terminated and the battery is left to stand for 30 minutes. The battery is discharged at 1C constant current to 2.0V, the discharge is terminated and the battery is left to stand for 30 minutes. The first discharge capacity is recorded. This cycle is repeated 300 times. The discharge capacity of the 300th cycle is recorded. The discharge capacity of the 300th cycle is divided by the discharge capacity of the first cycle to obtain the capacity retention rate of the battery after 300 cycles.

[0282] (2) Lithium plating in the cell: After 300 cycles of capacity retention testing, the cell was disassembled and the lithium plating status of the cell was recorded.

[0283] (3) Battery swelling condition: The composite separator, lithium iron phosphate positive electrode and graphite negative electrode are stacked to form the cell. An appropriate amount of electrolyte (EC:PC:DEC=1:3:5, 1.5 mol / L LiPF6) is added. The overall thickness of the cell is recorded as h1. The cell is cyclically charged at 0.2C for 200 cycles. The cell thickness after 200 cycles is recorded as h2. The battery expansion coefficient is h2 / h1.

[0284] The test results are shown in Table 3.

[0285] Table 3

[0286]

[0287] In Examples 1-10, all parameters are within the scope of the present invention, and the resulting composite separators exhibit good performance at both the separator level and the cell level.

[0288] When the ceramic content in the composite particles is too low or there is no ceramic, the dispersibility and affinity between the composite particles and the filler are weakened, the composite particles in the separator are prone to accumulation, and the ion conduction channels of the composite particles decrease, which can easily lead to lithium plating in the cell. At the same time, the depth of the composite particles embedded in the ceramic is not deep enough, the adhesion between the composite particles and the ceramic is reduced, the separator powder is severely shed, the composite particles fall off, affecting the adhesion between the separator and the electrode, and the cell performance is reduced, as shown in Comparative Examples 1, 3 and 11.

[0289] If the composite particle size is too small, the composite particle coverage increases under the same basis weight, the membrane permeability growth rate increases, the internal resistance increases, and after the membrane and electrode sheet are hot-pressed, the gap space provided by the composite particles decreases, the cell's liquid storage capacity decreases, the cell's expansion coefficient increases, and the cell's performance decreases. If the composite particle size is too large, the composite particle coverage decreases under the same basis weight, the number of effective gap sites provided decreases, and when the membrane and electrode sheet are hot-pressed, they cannot provide effective gap space, which easily leads to an increase in the cell's expansion coefficient. At the same time, if the composite particle size is too large, it is easy for the composite particles to fall off, and the adhesion between the membrane and the electrode sheet decreases, thus affecting the overall cell performance, as shown in comparisons 4 and 5.

[0290] If the proportion of composite particles in the separator is too high, the basis weight of the polymer particle coating will decrease, which will easily lead to a decrease in adhesion. At the same time, if the proportion of composite particles is too high, it will also easily lead to a larger rate of increase in the gas permeability of the separator, and the composite particles will easily accumulate, leading to lithium plating. If the proportion of composite particles is too low, the basis weight of the polymer particles will increase, the coverage of the polymer particles will increase, the rate of increase in the gas permeability of the coated separator will increase, affecting the internal resistance of the cell. When the proportion of composite particles is too low, the number of gap points provided will decrease, the cell expansion coefficient will increase, and the cycle performance will decrease, as shown in comparison 6 and 7.

[0291] When the basis weight of the composite particle coating and the basis weight of the polymer particles increase simultaneously, the overall coverage of the separator increases, the rate of increase in air permeability increases significantly, affecting the overall internal resistance of the cell and easily leading to lithium plating in the cell, thus affecting the cell cycle performance. When the basis weight of the composite particle coating and the basis weight of the polymer particles decrease simultaneously, the adhesion between the separator and the electrode sheet decreases significantly, the cell expansion rate increases, the cell internal resistance increases, and the cell cycle performance decreases, as shown in comparisons 8 and 9.

[0292] In Comparative Example 10, since most of the polymer particles were filled in the filler layer, the coverage rate measured by the polymer particle coverage rate test method of the present invention was much lower than the actual situation. Furthermore, since most of the polymer particles were filled in the filler layer, the number of polymer particles on the surface was small, resulting in a significant decrease in both adhesion and heat resistance.

[0293] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A composite diaphragm, characterized in that, It includes a base film and a coating disposed on at least one surface of the base film; the coating includes a first coating disposed on the surface of the base film and a second coating disposed on at least a portion of the surface of the first coating; The first coating comprises filler particles and composite particles. The filler particles are stacked to form a filler layer, and the composite particles are embedded in the filler layer and form protrusions on the surface of the filler layer. The composite particles comprise primary polymer particles and secondary particles formed by the co-aggregation of ceramics. The filler particles comprise one or more of inorganic oxides, inorganic hydroxides, metal salts, or organic non-polymer particles. The second coating comprises polymer particles; the polymer particles comprise second polymer primary particles; The height h of the composite particles protruding from the second coating satisfies 1μm≤h≤4.9μm; The coverage rate η1 of the composite particles relative to the base film is 4.2%~18.8%, the coverage rate η2 of the polymer particles relative to the base film is 12.5%~31.7%, and the total coverage rate η of the composite particles and the polymer particles relative to the base film is 25.0%~40.0%.

2. The composite diaphragm according to claim 1, characterized in that, The thickness of the filler layer is H, and the depth to which the composite particles are embedded in the filler layer is d, satisfying: 1μm≤H≤2μm, H / 2≤d≤H.

3. The composite diaphragm according to claim 1, characterized in that, The areal density of the composite particles is X, and the areal density of the polymer particles is Y, both satisfying: 0.20 g / m³. 2 ≤X≤0.60g / m 2 , Y=-0.125X+a, 0.15≤a≤0.2; And / or, the ratio K of the number of polymer particles to the number of composite particles satisfies: 11 < K < 750.

4. The composite diaphragm according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) The particle size of the composite particles is 4~6μm, and / or the particle size of the first polymer primary particles is 0.4~0.6μm, and / or the particle size of the ceramic is 0.4~0.6μm; (2) In the composite particles, the mass percentage of the ceramic is 5% to 12%; (3) The ceramic includes at least one of boehmite, alumina, silica, zirconium oxide, LATP, and LLZO; (4) The ceramic forming the composite particles is a coupling agent modified ceramic; (5) The glass transition temperature Tg of the first polymer primary particles is -5~5℃; (6) The first polymer primary particle includes a core and a shell covering the surface of the core; the core monomer forming the core and the shell monomer forming the shell are each independently selected from at least one of methacrylonitrile, styrene, acrylonitrile, ethyl acrylate, 1-butene, 2-ethylhexyl acrylate, isooctyl acrylate, and n-butyl acrylate; the shell monomer contains at least one monomer different from the core monomer; (7) The particle size of the polymer particles is 0.6~1.0μm; (8) The glass transition temperature Tg of the polymer particles is 45~55℃; (9) The monomers forming the polymer particles include any two of methyl methacrylate, butyl methacrylate, ethyl acrylate, isooctyl acrylate, and n-butyl acrylate.

5. The composite diaphragm according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) The composite particles account for 11.5% to 18.5% of the total coating by mass; (2) The polymer particles account for 3.0% to 9.0% of the total coating mass; (3) The filler particles account for 66% to 86% of the total coating by mass; (4) The filler particles include at least one of boehmite, alumina, magnesium hydroxide, tin dioxide, silicon dioxide, barium sulfate, LLZO, LATP, and melamine cyanurate; (5) The particle size of the filler particles is 0.5~0.8μm; (6) More than 95% of the second coating is located on the surface of the filler layer; (7) The first coating further contains a first adhesive, the first adhesive accounting for 2% to 8% of the total coating by mass; and / or, the second coating further contains a second adhesive, the second adhesive accounting for 0.05% to 0.50% of the total coating by mass; (8) The first coating further contains a first wetting agent, the first wetting agent accounting for 0.1% to 1.0% of the total coating by mass; and / or, the second coating further contains a second wetting agent, the second wetting agent accounting for 0.02% to 0.2% of the total coating by mass.

6. The method for preparing the composite diaphragm according to any one of claims 1 to 5, characterized in that, Includes the following steps: The first raw material is mixed to obtain a first mixed slurry, wherein the first raw material includes filler particles and an emulsion containing composite particles; The second raw material is mixed to obtain a second mixed slurry, wherein the second raw material includes an emulsion containing polymer particles; The first mixed slurry is coated onto at least one side surface of the base membrane and dried to obtain a first coating; then the second mixed slurry is coated onto the first coating and dried to obtain the composite membrane.

7. The method for preparing the composite diaphragm according to claim 6, characterized in that, The method for preparing the emulsion containing composite particles includes the following steps: S1. A first emulsifier, a core monomer, and a first initiator undergo a first polymerization reaction in a first solvent to obtain a core particle emulsion; the core particle emulsion is subjected to UV irradiation, and a shell monomer and a second initiator are added to undergo a second polymerization reaction to obtain an emulsion containing primary particles of the first polymer; S2. The ceramic surface is modified by reacting it with a coupling agent, and then an emulsion containing primary particles of the first polymer and salt are added and stirred to obtain the emulsion containing composite particles.

8. The method for preparing the composite diaphragm according to claim 6 or 7, characterized in that, The method for preparing the emulsion containing polymer particles includes the following steps: A second emulsifier, a monomer forming a second polymer, and a third initiator undergo a third polymerization reaction in a second solvent to obtain the emulsion containing polymer particles.

9. A secondary battery, characterized in that, This includes the composite membrane according to any one of claims 1 to 5, or the composite membrane prepared by the method according to any one of claims 6 to 8.

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