Battery separator and applications

Abstract

The application provides a battery separator and application, and relates to the technical field of secondary batteries. Specifically, the battery separator comprises a base film and a coating layer loaded on at least one side of the base film; the coating layer comprises a first ceramic and a second ceramic, the first ceramic is filled in the coating layer, the second ceramic comprises first particles filled in the coating layer and second particles inlaid in the coating layer, and the quantity ratio of the second particles to the first particles is 1.2-4.0. The coated separator of the application can be used in cylindrical battery cells, has good heat resistance, thickness consistency, electrical conductivity and electrolyte wettability, can make the separator and the electrode sheet have larger pores after winding, improve the electrolyte injection efficiency, provide space to release the stress of the electrode sheet, is not prone to lithium precipitation, and can significantly improve the cycle performance and capacity retention of the secondary battery.

Description

Technical Field

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

[0002] As a core safety component of a battery, the separator's main functions include physically isolating the positive and negative electrodes to prevent short circuits, allowing lithium ions to freely transport and form a current loop through its porous structure, and maintaining a uniform electrolyte distribution to preserve battery performance. Currently, common separators are classified by material into polyolefin separators, polyimide separators, and composite separators with an inorganic coating added to a polyolefin base membrane. This is because polyolefin base membranes have relatively poor heat resistance and wettability when used independently; therefore, a coating, such as a ceramic layer to improve heat resistance, is usually applied to its surface to improve the separator's heat resistance and electrolyte wettability.

[0003] Cylindrical batteries are lithium-ion batteries characterized by their cylindrical shape, unique structure, diverse specifications, significant advantages, and wide range of applications. Key processes in assembling cylindrical batteries include: winding pre-treated positive electrode sheets, separators, and negative electrode sheets into a cylindrical cell in a specific order, then placing it in a casing, and finally injecting electrolyte. During this process, the tightly bonded separation membrane and electrode sheets after winding significantly increases the electrolyte injection time. As the electrolyte is injected, the positive and negative electrode sheets absorb it, causing volume expansion. The electrodes and separator then compress each other, potentially squeezing out the electrolyte, leading to decreased cycle performance. Furthermore, the high volume expansion of the electrode sheets can result in a lack of effective stress release space, causing electrode wrinkles and lithium plating defects.

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

[0005] The primary objective of this invention is to provide a battery separator that addresses the issues of excessively long electrolyte injection time in cylindrical battery cells, as well as volume expansion and resulting battery electrical performance defects caused by electrolyte injection.

[0006] The second objective of this invention is to provide a secondary battery.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A battery separator includes a base film and a coating loaded on at least one side of the base film; the coating includes a first ceramic and a second ceramic; the first ceramic is filled in the coating, and the second ceramic includes first particles filled in the coating and second particles embedded in the coating; The number of the first particles is K1, the number of the second particles is K2, and the ratio of K2 to K1 is 1.2 to 4.0.

[0008] In one embodiment, the particle sphericity of the first ceramic is 35% to 65%, and the particle sphericity of the second ceramic is >80%.

[0009] In one embodiment, the compression ratio of the battery separator is 10% to 20%.

[0010] In one embodiment, the D of the first particle 50 The particle size is D1, and the second particle's D 50 The particle size is D2, and the difference between D2 and D1 is 2μm~5μm.

[0011] In one embodiment, the particles D of the first ceramic 50 The particle size is 0.4μm~0.6μm, and the particle size D of the second ceramic is... 50 The particle size is 2.5μm~3.0μm.

[0012] In one embodiment, the deposition thickness of the coating is 1.5 μm to 2.5 μm.

[0013] In one embodiment, σ is the standard deviation of the thickness of the battery separator, and D is the particle size D of the second ceramic. 90 With D 10 The difference is D (90-10) σ and D (90-10) The ratio is less than 0.05.

[0014] In one embodiment, the first ceramic comprises at least one of alumina, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, zirconium oxide, LATP, and LLZTO; the second ceramic comprises at least one of alumina, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, and zirconium oxide.

[0015] In one embodiment, the first ceramic accounts for 67.4% to 82.6% of the mass of the coating, and the second ceramic accounts for 15% to 25% of the mass of the coating.

[0016] In one embodiment, the coating further includes a binder and a silane coupling agent; The adhesive includes at least one of styrene-butadiene rubber, polyacrylate, polyacrylonitrile, epoxy resin, polyacrylamide, and polyacrylic acid; The silane coupling agent includes at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and vinyltrimethoxysilane.

[0017] A secondary battery, comprising the aforementioned battery separator.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a heat-resistant electrolyte storage battery separator suitable for cylindrical battery cells. It ensures that, without compromising the separator's heat resistance, a large porosity is maintained between the separator and the electrode sheets after winding, improving electrolyte injection efficiency. Furthermore, it provides sufficient space for the electrode sheets to release stress when absorbing electrolyte and expanding, thereby enhancing the battery's cycle performance and capacity retention. In addition, the heat-resistant electrolyte storage battery separator provided by this invention exhibits significant performance advantages such as low gas permeability growth rate, high heat resistance, high thickness uniformity, high conductivity, high electrolyte wettability, and resistance to lithium plating. Attached Figure Description

[0019] 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.

[0020] Figure 1 A cross-sectional SEM image of the coating layer in Embodiment 1 of the present invention is provided. Detailed Implementation

[0021] 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. In addition, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0022] A first aspect of the present invention is to provide a battery separator. Specifically, the battery separator includes a base film and a coating loaded on at least one side of the base film; the coating includes a first ceramic and a second ceramic; wherein the first ceramic is filled in the coating, and the second ceramic includes first particles filled in the coating and second particles embedded in the coating; the number of the first particles is K1, the number of the second particles is K2, and the ratio of K2 to K1 is 1.2 to 4.0.

[0023] In this invention, "filling" refers to the particles being entirely embedded inside the coating; "embedding" refers to the particles being partially embedded inside the coating while being partially exposed on the surface of the coating.

[0024] As an optional implementation, the ratio of K2 to K1 includes, but is not limited to, any one or any two of the following: 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4.0.

[0025] In a preferred embodiment, the battery separator includes a base film and a coating loaded on one side of the base film; or, the battery separator includes a base film and a coating loaded on both sides of the base film; further, when the coating exists on both sides, the characteristic parameters such as particle sphericity, composition, and thickness of the coating on both sides do not necessarily have to be exactly the same.

[0026] In a preferred embodiment, the particle sphericity of the first ceramic is 35% to 65%, and the particle sphericity of the second ceramic is >80%.

[0027] It is worth noting that sphericity is a parameter that reflects the regularity of particles. It is a quantitative indicator that measures how close the particle shape is to a perfect sphere, and is directly related to the geometric symmetry and surface uniformity of the particles. Therefore, in this invention, an irregular (or less regular) first ceramic and a highly regular second ceramic are mixed together, and the particles of the second ceramic fill and / or are embedded in the pores of the particles of the first ceramic.

[0028] As an optional implementation, the particle sphericity of the first ceramic includes, but is not limited to, any one or any two of the following values: 35%, 40%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 60%, and 65%; in a more preferred implementation, the particle sphericity of the first ceramic is 45% to 55%.

[0029] As an optional implementation, the particle sphericity of the second ceramic includes, but is not limited to, any one or any two of the following values: 80.1%, 80.5%, 82%, 85%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, and 99.5%; in a more preferred implementation, the particle sphericity of the second ceramic is >90%.

[0030] In a preferred embodiment, the compression ratio of the battery separator is 10% to 20%, including but not limited to any one or any two of the following values: 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0031] In a preferred embodiment, the first ceramic is composed of nanoscale irregular particles, and the particle density (D) 50 The particle size is 0.4μm to 0.6μm, including but not limited to any one or any two of the following: 0.4, 0.42, 0.44, 0.45, 0.46, 0.48, 0.5, 0.52, 0.54, 0.55, 0.56, 0.58, and 0.6 (μm).

[0032] In this preferred embodiment, the separator will have better performance when the particle size of the first ceramic is within a suitable range. When the particle size of the first ceramic is too small, the density of the first ceramic stack is higher, which easily leads to increased air permeability, thereby affecting ion conduction, reducing cycle performance, or possibly causing lithium plating. Conversely, when the particle size of the first ceramic is too large, the density of the first ceramic stack is reduced, affecting the overall heat resistance of the coated separator. After the coated separator is made into a battery, there are safety performance issues. The reduced density of the coating makes the compression ratio of the coated separator more likely to increase, affecting the electrolyte storage performance of the separator, thereby affecting cycle performance. However, the better performance does not mean that the particle size of the first ceramic must be within the above-mentioned range.

[0033] In a preferred embodiment, the second ceramic is a micron-sized spherical particle, and the particle's D... 50 The particle size is 2.5μm to 3.0μm, including but not limited to any one or any two of the following: 2.5, 2.55, 2.6, 2.65, 2.7, 2.75, 2.8, 2.85, 2.9, 2.95, and 3.0 (μm).

[0034] In this preferred embodiment, the particle size of the second ceramic should be within a suitable range for the diaphragm to exhibit better performance; when the particle size of the second ceramic D... 50 When the temperature is too low, the thermal shrinkage performance of the coated separator deteriorates due to the combined influence of the parameters of the two ceramic materials, affecting battery safety performance. Simultaneously, the second inorganic ceramic reduces the gap space, leading to decreased battery electrolyte storage performance and worsened cycle performance. When the second ceramic D... 50 When the temperature is too high, under the combined influence of the parameters of the two ceramic materials, the density of the first ceramic deposit increases, the air permeability of the coated diaphragm increases, the ionic conductivity decreases, and the cycle performance deteriorates; however, the better performance mentioned above does not mean that the particle size of the second ceramic must be within the above range.

[0035] As a preferred embodiment, the D of the first particle 50 The particle size is D1, and the second particle's D 50 The particle size is D2, and the difference between D2 and D1 is 2μm to 5μm, including but not limited to any one or any two of the following: 2, 2.5, 3, 3.5, 4, 4.5, and 5 (μm).

[0036] In a preferred embodiment, the deposition thickness of the coating is 1.5 μm to 2.5 μm. In this invention, the deposition thickness of the coating refers to the vertical distance from the interface between the coating and the base film to a reference plane on the coating surface; the reference plane refers to the surface jointly formed by the continuous matrix formed by the first ceramic deposition, the embedded portions of the first particle and the second particle filling the matrix, excluding the portion of the second particle protruding from the surface.

[0037] Therefore, in conjunction with the above-mentioned preferred embodiments, it can be seen that two different ceramic materials are used in this invention, and their spatial distribution and particle sphericity are limited. Specifically, based on the above-mentioned property limitations, it can be seen that the irregular first ceramic has a higher stacking density than the regular second ceramic. At the same thickness, the former exhibits stronger heat resistance than the latter. The first ceramic forms a denser stacked layer, which improves the heat resistance of the separator. However, due to its dense stacking, it affects the passage of ions, increases the internal resistance of the battery, and thus affects the cycle performance of the battery. Therefore, it is necessary to introduce some regular inorganic ceramics, i.e., the second ceramic, into its pores to reduce the density of the coating, thereby solving the ion conduction problem and reducing the internal resistance of the battery. However, the introduction of too much second ceramic will inevitably affect the heat resistance of the coated separator. Therefore, after balancing the overall heat resistance of the separator and the internal resistance of the battery, this invention restricts the spatial distribution of the two ceramic materials, that is, there are two forms: first particles and second particles. On the one hand, the small particle size of the second ceramic acts as the first particle, filling the pores of the first ceramic coating, increasing the overall porosity of the coating, thereby reducing the internal resistance of the battery. On the other hand, the large particle size of the second ceramic acts as the second particle, embedded in the pores of the first ceramic coating, reducing the density of the coating. At the same time, the second ceramic forms multiple protrusions on the coating surface, so that when the separator and the electrode sheet are wound together, a larger pore space can be formed between the separator and the electrode sheet. This provides an effective gap during electrolyte injection, shortens the electrolyte injection time, and stores a larger amount of electrolyte, increasing the capacity. In addition, after the electrode sheet absorbs the electrolyte, it expands, providing a larger space for it to expand and release stress, effectively avoiding electrode wrinkling and lithium deposition. In summary, the design also gives the battery separator of this invention a certain compression ratio, providing a reliable barrier for the electrode sheet to release stress.

[0038] In a preferred embodiment, the thickness of the coating is distributed between 2.5 μm and 4.5 μm.

[0039] As a preferred embodiment, σ is the standard deviation of the thickness of the battery separator, and D is the particle size D of the second ceramic. 90 With D 10 The difference is D (90-10) σ and D (90-10) The ratio is less than 0.05. It is worth noting that D 90 With D 10 These represent the particle sizes corresponding to cumulative particle size distributions of 90% and 10%, respectively.

[0040] In a preferred embodiment, the thickness of the base film is 4μm to 16μm, including but not limited to any one or any two of the following values: 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 (μm).

[0041] In a preferred embodiment, the base film includes at least one of PP film, PE film, PP film, and PE film, or a composite film obtained by combining the above films.

[0042] In a preferred embodiment, the mass ratio of the first ceramic to the coating is 67.4% to 82.6%, including but not limited to any one or any two of the following values: 67.4%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, and 82.6%; the mass ratio of the second ceramic to the coating is 15% to 25%, including but not limited to any one or any two of the following values: 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, and 25%.

[0043] In this preferred embodiment, when the amounts of both types of ceramics are within an appropriate range, the separator will exhibit better performance. When the proportion of the second ceramic is too low, on the one hand, the number of gaps created by the second inorganic ceramic decreases, the compression ratio of the coated separator increases, the battery's electrolyte storage performance decreases, and the cycle performance deteriorates. On the other hand, the density of the first ceramic stack increases, the permeability of the coated separator increases, the ionic conductivity decreases, and the battery's cycle performance deteriorates. Conversely, when the proportion of the second ceramic is too high, on the one hand, the number of gaps increases, the compression ratio decreases, the stress space for the coated separator to release the electrode plates decreases, which can easily lead to local lithium plating. On the other hand, the first ceramic stack has too many spherical particles, which worsens the thermal shrinkage of the coated separator and affects the battery's safety performance.

[0044] In a preferred embodiment, the first ceramic comprises at least one of alumina, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, zirconium oxide, LATP, or LLZTO.

[0045] In a preferred embodiment, the second ceramic comprises at least one of alumina, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, or zirconium oxide.

[0046] In a preferred embodiment, the coating further includes a binder and a silane coupling agent.

[0047] In a more preferred embodiment, the adhesive includes at least one of styrene-butadiene rubber, polyacrylate, polyacrylonitrile, epoxy resin, polyacrylamide, or polyacrylic acid; in some optional embodiments, the mass ratio of the adhesive to the coating is 2% to 6%.

[0048] In a more preferred embodiment, the silane coupling agent includes at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and vinyltrimethoxysilane; in some optional embodiments, the mass ratio of the silane coupling agent to the coating is 0.5% to 1.5%.

[0049] In a more preferred embodiment, the coating further includes a wetting agent and a dispersant.

[0050] As a further preferred embodiment, the wetting agent includes at least one of ethylene oxide, nonylphenol polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, fatty alcohol polyoxyethylene ether, and fatty acid polyoxyethylene ether; in some optional embodiments, the mass ratio of the wetting agent to the coating is 0.2% to 0.8%.

[0051] As a further preferred embodiment, the dispersant includes at least one of sodium tripolyphosphate, sodium hexametaphosphate, alkyl aryl phosphate, alkylbenzene sulfonate, or dialkyl sulfosuccinate; in some optional embodiments, the mass ratio of the dispersant to the coating is 0.2% to 0.8%.

[0052] In a first aspect of the present invention, a feasible method for preparing the battery separator is also provided, which mainly includes the following steps: fully dispersing a first ceramic and a second ceramic to obtain a mixed powder, then preparing a mixed liquid containing the mixed powder, coating the mixed liquid onto the surface of a base film, and drying it to obtain a battery separator.

[0053] In a preferred embodiment, the thorough dispersion is achieved using an air jet mill; the feed air flow rate, feed pressure, and airflow velocity of the air jet mill can be adaptively adjusted.

[0054] In a preferred embodiment, the preparation of the mixture includes: first preparing a first mixture containing the mixed powder and an organic solvent, adjusting the pH to 4.2-5.2 with acetic acid, then adding a silane coupling agent and reacting fully, separating the solid and liquid, washing the precipitate and drying to obtain a solid phase; preparing a second mixture containing the solid phase, a binder, a dispersant, a wetting agent and deionized water, mixing fully to obtain the mixture.

[0055] In a more preferred embodiment, the sufficient reaction and / or the sufficient mixing can be assisted by means of oscillation, stirring, shaking, centrifugation, ultrasound, heating, etc., which helps to accelerate dispersion and obtain a relatively uniform dispersion system or provide relatively suitable reaction conditions; the solid-liquid separation includes, but is not limited to, decantation, filtration, centrifugation, filter screen or membrane separation, etc.

[0056] In a preferred embodiment, the coating is performed using a wet coating process, including but not limited to roller coating, microgravure coating, slot extrusion coating, dip coating, etc., with microgravure coating being more preferred.

[0057] In a preferred embodiment, the drying temperature and time are adaptively adjusted to ensure that the solvent in the mixture is completely removed.

[0058] A second aspect of the present invention is to provide the use of the battery separator as described in the first aspect in secondary batteries. It is understood that the use includes, but is not limited to: composite separators comprising the battery separator and methods for preparing the same, secondary batteries comprising the battery separator and methods for preparing the same, and electrical appliances comprising the battery separator and methods for preparing the same, etc.

[0059] In a preferred embodiment, the secondary battery includes a cylindrical cell; the type or performance parameters of the positive or negative electrode of the cylindrical cell are not limited.

[0060] Examples 1-8, Comparative Examples 1-4 S1. Take the first inorganic ceramic material (alumina) and the second inorganic ceramic material (alumina) respectively. The weight parts, D50 particle size and sphericity values ​​of the two ceramic materials are shown in Table 1. Put them into an air jet mill and adjust the air flow rate to 50L / min. Mix and disperse the two ceramic materials thoroughly to obtain the first mixed powder.

[0061] S2. Take 200 parts of 95% ethanol solution into a reaction vessel, adjust the pH value to 5.0 with 0.5 mol / L acetic acid, add 0.8 parts of γ-methacryloxypropyltrimethoxysilane, stir for 30 min, and then add the first mixed powder obtained in S1; then heat to 45℃, stir for 2 h, let stand to remove the supernatant, then wash the precipitate three times with ethanol, and then vacuum dry at 25℃ to remove moisture to obtain the second mixed powder.

[0062] S3. Take the second mixed powder into a mixing tank, add 200 parts of deionized water, 0.5 parts of dialkyl sulfosuccinate, 5 parts of styrene-butadiene rubber and 0.5 parts of nonylphenol polyoxyethylene ether, adjust the ultrasonic power to 800W, sonicate for 30 minutes, and then stir at 2000rpm for 120 minutes to obtain the first mixed liquid.

[0063] S4. Using microgravure coating technology, with a printing roller line count of 75 LTP and a depth of 55 μm, and adjusting the microgravure speed ratio to 1.62, the first mixture is coated onto the diaphragm surface, dried, and a coating amount of 3.89 g / m² is obtained. 2 The coated diaphragm.

[0064] Table 1 below shows the particle size, amount added, and sphericity characteristics of the first and second inorganic ceramic materials used in each embodiment and comparative example. It is worth noting that the weight parts of the first and second ceramics in Table 1 correspond to the amount of the first and second inorganic ceramic materials added in step S1.

[0065] Example 9: Basically the same as Example 1, except that: the first inorganic ceramic material is silicon dioxide and the second inorganic ceramic material is titanium dioxide.

[0066] Example 10: Basically the same as Example 1, except that: the first inorganic ceramic material is zirconium oxide and the second inorganic ceramic material is LLZTO.

[0067] Example 11: Basically the same as Example 1, except that: the silane coupling agent in step S2 is replaced with vinyltrimethoxysilane; the binder in step S3 is replaced with epoxy resin, the wetting agent is replaced with fatty acid polyoxyethylene ether, and the dispersant is replaced with alkylbenzene sulfonate.

[0068] It should be added that the sphericity testing method in Table 1 includes: taking SEM images of the ceramic material, dispersing the particles as much as possible during sample preparation to avoid particle accumulation; using a scanning electron microscope at a voltage of 5kV and a magnification of 10000x for the first inorganic ceramic material and 5000x for the second inorganic ceramic material, randomly selecting an area containing at least 150 independent particles to ensure clear images and complete particle outlines. The captured images are then imported into ImageJ software, where 50 particles within the field of view are manually selected, and the area (A) and perimeter (P) are automatically obtained. The sphericity value (φ=4πA / P) is then automatically analyzed by the software. 2 Test 10 groups simultaneously using the above method, and calculate the sphericity by taking the average value.

[0069] Test case (a) Diaphragm performance testing (1) Gas permeability growth rate of coated diaphragm: 100 mL of gas passed through 642.2 mm. 2 The required time for the diaphragm is calculated as follows: air permeability growth rate = (air permeability time of coated diaphragm - air permeability time of base membrane) / air permeability time of base membrane × 100%. The results are recorded in Table 2.

[0070] (2) Heat shrinkage of coated diaphragm: Cut the diaphragm sample into 10cm×10cm square pieces, mark the MD and TD directions of the diaphragm, and then place the cut diaphragm between two 20cm×20cm glass plates (each glass plate weighing 1kg). Place the sandwiched diaphragm in a forced-air drying oven, adjust the heating rate of the oven to 5℃ / min until the temperature reaches 150℃, stop heating, and maintain the temperature at 150℃ for 30min. Then remove the coated diaphragm and test its length. Heat shrinkage performance = (length before baking - length after baking) / length before baking × 100%. Calculate the heat shrinkage values ​​in the MD and TD directions of the diaphragm. Record the results in Table 2.

[0071] (3) Electrolyte wettability: Take the coated separator and lithium iron phosphate positive electrode (positive electrode material is 95 parts lithium iron phosphate, 3 parts conductive carbon black and 2 parts PVDF), cut them into 20m×2cm and 20cm×2cm respectively, align the coated surface of the separator with the long side and the long side of the positive electrode, and sandwich the electrode and separator with two 19cm×1.8cm steel plates, with the long side of the steel plate aligned with the long side of the separator and the long side of the electrode, and align the end faces of the separator and the electrode. The edge corresponds to the short side of the diaphragm and electrode, positioned in the middle of the short side, with 0.1 cm protruding from each side of the diaphragm and electrode. A pressure of 1 MPa is applied to the two steel plates. The pressed diaphragm and electrode are then placed vertically with the non-aligned ends facing down in the electrode solution. The diaphragm and electrode are immersed in the electrode solution to a depth of 0.5 cm, allowing the electrolyte to rise for 5 minutes. After 5 minutes, the diaphragm is removed, and the height of the electrolyte rise through the diaphragm is measured. The results are recorded in Table 2.

[0072] (4) Coating buildup thickness: Take a cross section of the coated diaphragm and measure the buildup thickness using a scanning electron microscope at 3000x magnification. Test 5 values. Use the same method to test 6 samples in parallel and take the final average value. Record the results in Table 1.

[0073] (5) Diaphragm K-value test: Take a cross-section of the coated diaphragm and take a cross-sectional electron microscope image at 3000x magnification. Record the number of filling particles with a second ceramic particle size smaller than the thickness of the first ceramic and no protrusion relative to the first inorganic ceramic layer at this magnification as K', and the number of embedded particles with the second inorganic ceramic particles forming protrusions relative to the first inorganic ceramic layer as K”. Then K = K” / K'. In the same way, test the K value of 30 samples in parallel and take the average value. Record the results in Table 1.

[0074] In addition, such as Figure 1 The image shown is a cross-sectional scanning electron microscope image of the coating layer in Example 1.

[0075] (6) Diaphragm M value test: Take three points on average along the TD direction of the diaphragm, and take one point every 20m along the MD direction, for a total of 30 points. Use a micrometer to measure the thickness of these 90 points and calculate the standard deviation σ of these 90 points. Record the results in Table 1.

[0076] A cross-section of the coated diaphragm was taken and imaged using a scanning electron microscope at 3000x magnification. The particle size of all second inorganic ceramic particles at this magnification was recorded. Thirty parallel samples were tested using the same method. Statistical analysis was performed on all particles in the cross-section of these 30 samples at 3000x magnification to determine the D-value of the second inorganic ceramic particles in the diaphragm. 10 and D 90 If the particle sizes correspond to d1 and d2, then D (90-10) =d2-d1, thus yielding the thickness σ / D (90-10) The ratio M is recorded in Table 1.

[0077] (7) Diaphragm compression ratio test: Cut the diaphragm into 8cm×5cm pieces, and randomly test 9 points with a micrometer. Take the average value of the 9 points as h1. Adhere the diaphragm to the PET film, place it in a hot press, adjust the hot press pressure to 5MPa, temperature to 85℃, and hot press for 5min. When the hot pressing time is up, take out the diaphragm and randomly test 9 points with a micrometer. Take the average value of the 9 points as h2. Then the compression ratio of the coated diaphragm = (h1-h2) / h1×100%. Test the compression ratio of 10 groups of samples in parallel using the same test method, and take the average value. Record the results in Table 1.

[0078] (8) Diaphragm N-value test: Take a cross-section of the coated diaphragm and take a cross-sectional electron microscope image at 2000x magnification. Statistically measure the particle size of all second inorganic ceramic particles filling the first inorganic ceramic layer and the particle size of all second inorganic ceramic particles embedded in the first inorganic ceramic layer at this magnification. Then take the average value. Repeat this method to test 10 samples in parallel and take the final average value as D' and D”, respectively. Calculate N = D” - D'. Record the results in Table 1.

[0079] Table 1

[0080] (ii) Electrochemical performance test results are recorded in Table 2.

[0081] (1) Ionic conductivity test: In an argon-filled glove box, a 2016 button cell was made with a diaphragm and an appropriate amount of electrolyte (EC:EMC:DEC=2:4:1, 1.5mol / L LiPF6) was added. The ionic conductivity σ = L / (Rb×A) was obtained by AC impedance testing in an electrochemical workstation, where σ is the ionic conductivity (S / cm). -1 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 ).

[0082] (2) Capacity retention rate test: Each separator, lithium iron phosphate positive electrode, and graphite negative electrode were wound to form a cell. The same electrolyte as in the above ionic conductivity test was used. The battery was charged at a constant current of 0.5C to 3.6V, and then charged at a constant voltage of 3.6V to a current of 0.02C. The charging was stopped and the battery was allowed to stand for 15 minutes. The battery was then discharged at a constant current of 0.5C to 2.0V. The discharge was stopped and the battery was allowed to stand for 30 minutes. The first discharge capacity was recorded. This cycle was repeated 350 times. The discharge capacity of the 350th cycle was recorded. The discharge capacity of the 350th cycle was divided by the discharge capacity of the first cycle to obtain the capacity retention rate of the battery after 350 cycles.

[0083] (3) Lithium plating: The coated separator, lithium iron phosphate positive electrode, and graphite negative electrode are wound to form the cell. The battery is charged at a constant current of 1.5C to 3.6V, then charged at a constant voltage of 3.6V to a current of 0.02C, the charging is stopped, and the battery is left to stand for 20 minutes. Then it is discharged at a constant current of 1.5C to 2.0V, the discharge is stopped, and the battery is left to stand for 20 minutes. This cycle is repeated 500 times. Then the cell is removed and the lithium plating of the cell is checked.

[0084] (4) Safety performance test: The coated separator and lithium iron phosphate positive electrode and graphite negative electrode are wound to form the battery cell. After being fully charged, the battery is placed in an oven. The oven is heated to 110°C at 5°C / min and held for 20 min. Then it is heated to 120°C at 2°C / min and held for 20 min. Similarly, it is heated to 130°C at 2°C / min and held for 20 min. Then it is heated to 140°C at 2°C / min and held for 20 min. Finally, it is heated to 150°C at 2°C / min and held for 20 min. If the battery explodes or catches fire, the battery safety performance is deemed to have failed. If the battery explodes or catches fire as the temperature rises, the experiment is terminated and the battery safety performance is deemed to have failed. The safety performance of 10 batteries is tested in parallel using the same method. Then the failure rate of the battery is calculated as: number of failures / 10 × 100%.

[0085] Table 2

[0086] As can be seen from Table 2, the membranes in each embodiment exhibit good performance in all aspects, with strong air permeability, heat resistance and electrolyte wettability, and show good electrochemical performance when applied to batteries.

[0087] 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 battery separator, characterized in that, The battery separator includes a base film and a coating loaded on at least one side of the base film; The coating comprises a first ceramic and a second ceramic; The first ceramic is filled in the coating, and the second ceramic includes a first particle filled in the coating and a second particle embedded in the coating; The number of the first particles is K1, the number of the second particles is K2, and the ratio of K2 to K1 is 1.2 to 4.0; The particle sphericity of the first ceramic is 35%~65%, and the particle sphericity of the second ceramic is >80%. The first ceramic particle D 50 The particle size is 0.4μm~0.6μm, and the particle size D of the second ceramic is... 50 The particle size is 2.5μm~3.0μm; With the first particle's D 50 The particle size is D1, and the second particle's D 50 The particle size is D2, and the difference between D2 and D1 is 2μm~5μm.

2. The battery separator according to claim 1, characterized in that, The compression ratio of the battery separator is 10%~20%; The compression ratio test conditions include: hot press pressure of 5MPa, temperature of 85℃, and hot pressing for 5min.

3. The battery separator according to claim 1, characterized in that, The coating has a deposition thickness of 1.5 μm to 2.5 μm.

4. The battery separator according to claim 1, characterized in that, Let σ be the standard deviation of the thickness of the battery separator, and D be the particle size D of the second ceramic. 90 With D 10 The difference is D (90-10) σ and D (90-10) The ratio is less than 0.

05.

5. The battery separator according to claim 1, characterized in that, The first ceramic comprises at least one of alumina, boehmite, silica, titanium dioxide, magnesium oxide, zirconium oxide, LATP, and LLZTO; the second ceramic comprises at least one of alumina, boehmite, silica, titanium dioxide, magnesium oxide, and zirconium oxide. And / or, the first ceramic accounts for 67.4% to 82.6% of the mass of the coating, and the second ceramic accounts for 15% to 25% of the mass of the coating.

6. The battery separator according to claim 1, characterized in that, The coating also includes a binder and a silane coupling agent; The adhesive includes at least one of styrene-butadiene rubber, polyacrylate, polyacrylonitrile, epoxy resin, polyacrylamide, and polyacrylic acid; The silane coupling agent includes at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and vinyltrimethoxysilane.

7. A secondary battery, characterized in that, Includes the battery separator as described in any one of claims 1 to 6.

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