A lithium ion battery composite separator and a method for manufacturing the same, a secondary battery

Abstract

This invention discloses a lithium-ion battery composite separator and its preparation method, as well as a secondary battery. The composite separator includes a base film and a composite coating located on at least one side of the base film surface. The composite coating includes a mixed coating layer and a ceramic layer. The mixed coating layer is coated on the base film and includes inorganic ceramic particles and organic polymer particles in a mass ratio of (8-20):1. The ceramic layer is coated on the mixed coating layer and is composed of inorganic ceramic particles. The particle size of the organic polymer particles is greater than or equal to the thickness of the mixed coating layer and less than or equal to the thickness of the composite coating layer. The inorganic ceramic particles in the mixed coating layer include large particles and small particles. The particle size range of the large particles is 0.6-1.5 μm, and the particle size range of the small particles is 0.1-0.5 μm. The composite separator of this invention enables the polymer particles to be uniformly distributed vertically in the coating, ensuring uniform suspension of large particles. The composite coating structure is stable and has good adhesion and heat resistance.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery separator technology, specifically relating to a lithium-ion battery composite separator and its preparation method, and a secondary battery. Background Technology

[0002] Lithium-ion batteries, as a widely used type of rechargeable battery, have always had their safety and cycle life as key industry concerns. The separator, as a crucial component of lithium-ion batteries, directly affects the battery's safety and lifespan. Currently, commercially available lithium-ion battery separators mainly use polyolefin porous membranes, but these membranes suffer from poor thermal stability, low electrolyte absorption, and slow electrolyte absorption speed. To improve the separator's thermal stability, electrolyte absorption capacity, and resistance to lithium dendrite formation, a high-temperature resistant coating, such as a ceramic coating, is typically laminated onto the separator surface. However, existing ceramic-coated separators suffer from poor coating uniformity, low adhesion strength, insufficient electrolyte storage space, and poor high-temperature resistance, all of which affect battery safety and lifespan.

[0003] Furthermore, in the lithium-ion battery production process, to improve production efficiency, fully automated high-speed winding machines are used to manufacture battery cores. However, high-speed operation can cause localized compression between the cell electrodes and the separator, resulting in localized deformation of the battery. This prevents lithium ions from embedding into the negative electrode, leading to lithium plating. At the same time, the negative electrode expands, squeezing the separator and the positive and negative electrode sheets, posing a significant safety hazard.

[0004] Therefore, developing a composite coating that is structurally stable, possesses both adhesion and heat resistance, does not undergo excessive expansion after immersion in electrolyte, and does not show an abnormal increase in air permeability, while ensuring that large particles are uniformly suspended in the coating, thereby uniformly distributing the electrolyte inside the separator and improving the wettability and liquid retention of the electrolyte, is of great significance for improving the safety and service life of lithium-ion batteries.

[0005] CN116315457A discloses a composite separator, comprising a base membrane, a gap coating disposed on one side of the base membrane, and a modified coating disposed on the other side of the base membrane. The gap coating of this patent includes a gap coating mixed coating and a base coating, with the base coating disposed between the gap coating mixed coating and the base membrane. While this patent improves the electrolyte wettability and electrolyte retention of the separator, the issue remains of optimizing the formulations of the gap coating mixed coating and the modified coating to further improve the electrolyte wettability, electrolyte retention, and cycle life of the separator.

[0006] CN116454542A discloses a battery separator, including a base film and a coating applied to one or both sides of the base film. The coating contains fluorine-free polymeric resin particles and ceramic particles. While this patent provides a coating with good adhesion, high consistency, strong stability, and good heat resistance, there are still issues requiring further optimization, such as how to improve the separator structure, use more efficient coating techniques, and develop better coating formulations, to further improve the coating's wettability and electrolyte retention, as well as its density and consistency. Summary of the Invention

[0007] The purpose of this invention is to provide a composite separator for lithium-ion batteries to solve the problems of uneven distribution of polymer particles in the vertical direction of the coating, uneven local distribution of electrolyte, and insufficient electrolyte wettability in the separators of the prior art.

[0008] Another objective of this invention is to provide a method for preparing a composite separator for lithium-ion batteries.

[0009] Another object of the present invention is to provide a secondary battery.

[0010] In a first aspect, the present invention provides a lithium-ion battery composite separator, comprising a base film and a composite coating located on at least one side of the surface of the base film. The composite coating comprises a mixed coating layer and a ceramic layer. The mixed coating layer is coated on the base film and comprises inorganic ceramic particles and organic polymer particles in a mass ratio of (8-20):1. The ceramic layer is coated on the mixed coating layer and is composed of inorganic ceramic particles. The particle size of the organic polymer particles is greater than or equal to the thickness of the mixed coating layer and less than or equal to the thickness of the composite coating layer. The inorganic ceramic particles in the mixed coating layer include large particles and small particles. The particle size range of the large particles is 0.6-1.5 μm, and the particle size range of the small particles is 0.1-0.5 μm.

[0011] Secondly, the present invention provides a method for preparing a composite separator for lithium-ion batteries, comprising the following steps:

[0012] S1, Inorganic ceramic particles, organic polymer particles and solvent are mixed to form a mixed coating slurry, which is then coated on one or both sides of the base film and dried to form a mixed coating layer.

[0013] S2, inorganic ceramic particles and solvent are mixed to form a ceramic slurry, which is then coated onto a mixed coating layer and dried to form a ceramic layer, thus obtaining a composite diaphragm.

[0014] Thirdly, the present invention provides a secondary battery comprising the lithium-ion battery composite separator described above.

[0015] Beneficial effects of this invention:

[0016] This invention first forms a mixed coating with polymer particles on a base membrane, and then forms a pure ceramic layer on top of the mixed coating. By limiting the particle size of the polymer particles, the polymer particles are uniformly distributed vertically within the coating, ensuring that large particles are uniformly suspended within the coating. This creates a structure at the organic-inorganic interface of the separator similar to an electrolyte cell, allowing the electrolyte to be uniformly distributed within the separator. This improves the wettability and retention of the electrolyte, preventing localized concentration or loss of electrolyte, and enhancing the battery's cycle life and safety. The composite coating structure of this invention is stable. The core-shell structure of the organic polymer particles ensures good adhesion and heat resistance of the battery separator, reducing the likelihood of localized deformation during high-speed winding and minimizing battery safety hazards. Furthermore, by optimizing the coating formulation and preparation process, the density, consistency, and stability of the composite coating are improved. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the composite coating separator for lithium-ion batteries of the present invention.

[0018] Where Ln represents the distance from the lowest point of the organic polymer particle to the surface of the base film, Lm represents the distance from the highest point of the organic polymer particle to the surface of the base film, and h represents the thickness of the composite coating. Detailed Implementation

[0019] The present invention will now be described in detail through embodiments. It should be noted that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0020] The lithium-ion battery composite separator of the present invention includes a base film and a composite coating located on one side of the surface of the base film. The composite coating includes a mixed coating layer and a ceramic layer. The mixed coating layer is coated on the base film and includes inorganic ceramic particles and organic polymer particles in a mass ratio of (8-20):1. The specific mass ratio of inorganic ceramic particles to organic polymer particles in the mixed coating layer can be 8:1, 10:1, 12:1, 14:1, 15:1, 16:1, 18:1, 20:1, and more preferably (10-19):1. The ceramic layer is coated on the mixed coating layer and is composed of inorganic ceramic particles. The organic polymer particles have a particle size greater than or equal to the thickness of the mixed coating and less than or equal to the thickness of the composite coating; the inorganic ceramic particles in the mixed coating include large particles and small particles, wherein the particle size of the large particles ranges from 0.6 to 1.5 μm (e.g., 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm), and the particle size of the small particles ranges from 0.1 to 0.5 μm (e.g., 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm).

[0021] The lithium-ion battery composite separator of the present invention has an organic polymer particle suspension height L in the composite coating that satisfies h / 3≤L≤2h / 3, where h is the thickness of the composite coating, L=(Lm+Ln) / 2, Lm is the distance from the highest point of the organic polymer particle to the surface of the base film, and Ln is the distance from the lowest point of the organic polymer particle to the surface of the base film.

[0022] The lithium-ion battery composite separator of the present invention has a longitudinal uniformity β of organic polymer particles in the composite coating of ≤1.5 (for example, it can be 0.1, 0.5, 1, 1.1, 1.2, 1.3, 1.4, 1.5), where β=(Lm-Ln) / R, Lm is the distance from the highest point of the organic polymer particle to the surface of the base film, Ln is the distance from the lowest point of the organic polymer particle to the surface of the base film, and R is the D50 of the organic polymer particle. The smaller the value of β, the better the longitudinal uniformity of the organic polymer particles.

[0023] The lithium-ion battery composite separator of the present invention has an organic polymer particle support Z > 0.5 in the composite coating (e.g., 0.6, 0.76, 0.8, 1, 1.2, 1.4, 1.49, 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3), where Z = (C0 / h) / (α*β), and the degree of burial of the organic polymer particles is B0 = L + R / 2 - h. When B0 > 0, C0 = h. -(LR / 2), when the burial degree of organic polymer particles B0 < 0, C0 = R; where α = L / h, β = (Lm-Ln) / R, L = (Lm+Ln) / 2, Lm is the distance from the highest point of the organic polymer particle to the surface of the base film, Ln is the distance from the lowest point of the organic polymer particle to the surface of the base film, R is the D50 of the organic polymer particle, and C0 is the burial depth. The larger the Z value, the better the support of the organic polymer particles for the coating structure.

[0024] In this invention, Lm and Ln are measured by the following method: Under a cross-sectional electron microscope, 20 organic polymer particles are randomly selected, and the average distance from the highest point of each selected organic polymer particle to the surface of the base film is calculated as Lm. The average distance from the lowest point of each selected organic polymer particle to the surface of the base film is calculated as Ln.

[0025] The lithium-ion battery composite separator of the present invention comprises, on the opposite side of the base film relative to the composite coating, the mixed coating or ceramic coating; or,

[0026] On the other side of the base film opposite to the composite coating, the mixed coating is provided, and the ceramic coating is applied on the mixed coating.

[0027] In the lithium-ion battery composite separator of the present invention, the particle size of the inorganic ceramic particles in the ceramic layer ranges from 0.1 to 1 μm (for example, it can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1 μm).

[0028] The lithium-ion battery composite separator of the present invention has a mixed coating thickness of 0.5 to 3 μm, specifically 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, and a ceramic layer thickness of 0.5 to 3 μm, specifically 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm.

[0029] The lithium-ion battery composite separator of the present invention comprises one or more of the following organic polymer particles: polyvinylidene fluoride, polyhexafluoropropylene, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylic acid, and polyacrylate resins; and the following inorganic ceramic particles comprise one or more of the following: alumina, silicon dioxide, zirconium oxide, magnesium oxide, cerium dioxide, titanium dioxide, zinc oxide, iron oxide, silicon nitride, titanium nitride, boron nitride, boehmite, aluminum hydroxide, magnesium hydroxide, barium sulfate, calcium carbonate, wollastonite, and silicon carbide.

[0030] The lithium-ion battery composite separator of the present invention has a base film that is one of polyethylene, polypropylene, ceramic-coated polyethylene, ceramic-coated polypropylene, polyimide, aramid, aramid sulfone, nonwoven fabric, and polyethylene terephthalate porous membrane, with a thickness of 0.1 to 15 μm, specifically 0.1 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or 5 μm, preferably 0.1 to 12 μm, and more preferably 4 to 12 μm.

[0031] The method for preparing the lithium-ion battery composite separator of the present invention includes the following steps:

[0032] S1, Inorganic ceramic particles, organic polymer particles and solvent are mixed to form a mixed coating slurry, which is then coated on one or both sides of the base film and dried to form a mixed coating layer.

[0033] S2, inorganic ceramic particles and solvent are mixed to form a ceramic slurry, which is then coated onto the mixed coating layer and dried to form a ceramic layer, thus obtaining a composite mixed coating membrane.

[0034] The method for preparing the lithium-ion battery composite separator of the present invention includes a mixed coating slurry with a solid content of 32-38%, specifically 32%, 33%, 34%, 35%, 36%, 37%, or 38%, and a viscosity of 50-150 mPa·s, specifically 50 mPa·s, 70 mPa·s, 90 mPa·s, 100 mPa·s, 120 mPa·s, 130 mPa·s, or 150 mPa·s; and a ceramic slurry with an inorganic ceramic particle solid content of 27-33%, specifically 27%, 28%, 29%, 30%, 31%, 32%, or 33%, and a viscosity of 5-100 mPa·s, specifically 5 mPa·s, 10 mPa·s, 20 mPa·s, 40 mPa·s, 60 mPa·s, 80 mPa·s, or 100 mPa·s.

[0035] The method for preparing the lithium-ion battery composite separator of the present invention further includes a dispersant, a binder, and a wetting agent in the mixed coating slurry.

[0036] The method for preparing the lithium-ion battery composite separator of the present invention, based on the mass of the mixed coating slurry as 100%, comprises 25-35% (e.g., 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, or any range between the above values) of inorganic ceramic particles, 1.5-15% (e.g., 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any range between the above values) of organic polymer particles, and 0.01-0.5% (e.g., 0.01%, 0.02%, 0.04%, 0.06%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any range between the above values). The composition includes: a dispersant (any range), a binder (1% to 15% (e.g., 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any range between the above values), a wetting agent (0.001% to 0.1% (e.g., 0.001%, 0.002%, 0.004%, 0.006%, 0.008%, 0.01%, 0.02%, 0.04%, 0.06%, 0.08%, 0.1%, or any range between the above values), and a water content of 45% to 65% (e.g., 45%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 65%, or any range between the above values).

[0037] The method for preparing the lithium-ion battery composite separator of the present invention further includes a dispersant, a binder, and a wetting agent in the ceramic slurry.

[0038] The method for preparing the lithium-ion battery composite separator of the present invention, based on the mass of the ceramic slurry as 100%, includes 20-35% (for example, it can be 20%, 22%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, or any range between the above values) of inorganic ceramic particles, and 0.01-0.5% (for example, it can be 0.01%, 0.02%, 0.04%, 0.06%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, 0. The dispersant comprises 5% (or any range between the above values), 1 to 15% (e.g., 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any range between the above values), 0.001 to 0.1% (e.g., 55%, 56%, 58%, 60%, 62%, 64%, 65%, 66%, 68%, 70%, 72%, 74%, 75%, or any range between the above values), water.

[0039] The method for preparing the lithium-ion battery composite separator of the present invention includes the following: the dispersant can be one or more of polyvinylpyrrolidone, polyacrylamide, polyvinyl alcohol, and sodium polyacrylate; the binder can be one or more of styrene-butadiene rubber, acrylate, sodium polyacrylate, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, and polytetrafluoroethylene; and the wetting agent can be one or more of anionic surfactants, polyethylene surfactants, and polyol surfactants, for example, polysiloxane.

[0040] The method for preparing the lithium-ion battery composite separator of the present invention allows the binder to be one or more of acrylate and sodium polyacrylate; further, the binder includes sodium polyacrylate and acrylate, and further, the mass ratio of sodium polyacrylate to acrylate is (1-5):(1-5) (for example, it can be 1:1, 2:1, 1:2, 5:4, 5:3, 5:2, 5:1, 4:3, 4:1, 3:2, 3:1, 4:5, 3:5, 2:5, 1:5, 3:4, 1:4, 2:3, 1:3, or any range between the above ratios).

[0041] In this invention, when the content of acrylate is greater than the content of sodium polyacrylate, it is more helpful to improve the thermal stability of the lithium-ion battery composite separator of this invention.

[0042] The method for preparing a lithium-ion battery composite separator of the present invention involves organic polymer particles with a core-shell structure. The glass transition temperature (Tg) of the shell layer is 10–50°C, specifically 10°C, 20°C, 30°C, 40°C, or 50°C, and the glass transition temperature (Tg) of the core layer is greater than 50°C, specifically 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, or 120°C. The D50 diameter of the organic polymer particles is 1–5 μm, specifically 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm. When the surface temperature of the separator reaches 10–50°C, the shell layer of the organic polymer particles softens, acting as an adhesive, but the core layer of the organic polymer particles does not soften. This ensures that the separator maintains good adhesion at high temperatures and is less prone to local deformation during high-speed winding.

[0043] The preparation method of the lithium-ion battery composite separator of the present invention includes the following steps: Step S1 involves drying in a hot convection oven at a temperature of 60–90°C, specifically 60°C, 70°C, 80°C, or 90°C; Step S2 involves two stages of drying: first, drying in a hot convection oven, and then, without cooling, drying directly in a hot roller pressing system composed of upper and lower pressure rollers. The temperatures for both stages are 50–80°C, specifically 50°C, 60°C, 70°C, or 80°C. In Step S2, the first stage uses conventional hot convection drying, while the second stage uses heat conduction and hot melt plasticizing processes. That is, after drying, the separator immediately enters the hot roller pressing system without cooling. After this process, the polymer shell melts between the first and second coating layers, forming anchor points and solidifying the stability of the two-layer structure.

[0044] The technical solution of the present invention will be described in detail below through specific embodiments.

[0045] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0046] Examples 1-5

[0047] Step 1: Mix large-particle inorganic ceramics, small-particle inorganic ceramics, organic polymer particles, dispersant (sodium polyacrylate), binder A (polyacrylate), binder B (polyacrylic acid), wetting agent (polysiloxane) and water to prepare a mixed coating slurry. Coat the mixed coating slurry evenly on one side of the diaphragm base membrane with a base membrane thickness of 7μm to form a mixed coating layer, and then dry it.

[0048] Step 2: Mix inorganic ceramic particles, dispersant (sodium polyacrylate), binder A (polyacrylate), binder B (polyacrylic acid), wetting agent (polysiloxane) and water to prepare a ceramic slurry. Coat the ceramic slurry evenly on the mixed coating to form a ceramic layer. Dry the mixture to form a composite mixed coating.

[0049] The preparation methods of Examples 1 to 5 are the same. For specific raw material selection and process condition control, please refer to Tables 1-1 and 1-2.

[0050] Comparative Example 1

[0051] The preparation method is basically the same as in Example 1, except that the mixed coating slurry contains only large inorganic ceramic particles and not small inorganic ceramic particles.

[0052] Comparative Example 2

[0053] The preparation method is basically the same as in Example 1, except that the mixed coating slurry does not contain inorganic ceramic particles, the polymer particles are not core-shell structured, and there is no ceramic layer.

[0054] Comparative Example 3

[0055] The preparation method is basically the same as in Example 1, except that the polymer particles in the mixed coating slurry are not core-shell structured and the particle size is smaller than the thickness of the mixed coating layer.

[0056] Comparative Example 4

[0057] The preparation method is basically the same as in Example 1, except that the proportion of polymer particles in the mixed coating slurry is higher.

[0058] The membranes prepared in the above embodiments and comparative examples were subjected to performance tests. The test methods are as follows, and the test results are shown in Table 2.

[0059] Internal resistance:

[0060] Cut a 100mm×100mm sample, ensuring the surface of the insulation resistance test bench is free of foreign objects, place the sample within the designated area of ​​the test bench, and start the YD9820A programmable insulation resistance tester (Changzhou Yangzi Electronics Co., Ltd.) for testing. The voltage is 100V, the time is 5s, and the pressure is 0.05MPa. Perform 5 tests and take the average value.

[0061] Breathability (s / (100cc)):

[0062] The test was conducted according to the requirements of GB / T36363-2018. A 600mm × 100mm diaphragm sample was cut and tested using a Wang Yan-type air permeability meter (ASAHI Corporation, EG01-55-1MR). The test time was 3 seconds. The air permeability of the diaphragm was measured at any position at 100mm intervals along the 600mm TD direction. The average value of the above 5 test points was recorded as the air permeability of the diaphragm.

[0063] Heat shrinkage rate (%):

[0064] The test was conducted according to the requirements of GB / T36363-2018. A 10cm × 10cm sample was cut, and the transverse (TD) and longitudinal (MD) widths were marked on the sample. The transverse and longitudinal widths were measured using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C). The sample was held between two sealed A4 sheets of paper and placed in a 150℃ oven for 0.5 hours. After the sample returned to room temperature, the transverse and longitudinal widths were measured again using the fully automatic image measuring projector. Three measurements were taken, and the average value was recorded.

[0065] MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0066] TD heat shrinkage rate (%) = (TD length before heating - TD length after heating) ÷ TD length before heating × 100.

[0067] Compression ratio (%):

[0068] The sample film was cut into 50*50mm square samples. Five cut samples were stacked together and the thickness was measured at 10 points using a Mahr thickness gauge (Mahr model: C1202). The average value was recorded as D0. Then, PET sheets were cut into 50*50mm square sheets. The sample film was placed between two cut PET sheets, aligned, and placed under a hot press (Qmesys, QM940AS) for hot pressing. The hot pressing parameters were set to 1000kgf, 70℃, and a holding time of 1s. After removal, the diaphragm was measured at 10 points after hot pressing, and the average value was recorded as D1. The compression ratio was then calculated.

[0069] Electrolyte wetting: Cut the sample into two standard specimens of 25mm × 100mm. Place both specimens in a 500ml beaker containing 200ml of electrolyte, ensuring they are completely submerged in the electrolyte (LiPF6 1M, EC: EMC = 3 / 7, VC = 2%). Seal the beaker with plastic wrap to reduce electrolyte evaporation. Immerse for 30 minutes. Lay the two cut specimens flat on a table. Using a disposable dropper, draw a small amount of electrolyte and gently drip it onto the center of the specimen. Use a ruler to measure and record the longest and widest wetting areas.

[0070] Peel strength: Cut the sample using a 2.5cm×30cm mold, stick the special double-sided adhesive tape for peel strength onto the test plate, peel off the surface sticker, leaving about 2cm of sticker, stick the sample flat to the end of the double-sided adhesive tape with the sticker torn off, roll it back and forth three times with a pressure roller, manually peel off 1cm to make the sample strip to be tested, clamp the end with the sticker residue in the lower clamp of the tensile testing machine (Jinan Sike Testing Technology Co., Ltd., TSL-1002), clamp the other end of the sample strip to the upper clamp (the distance between the clamps is (100±5)mm), ensure no tilting, the tensile speed is 50mm / min, and the average value of the three measurements is taken.

[0071] Table 1-1

[0072]

[0073] Table 1-2

[0074]

[0075] Table 2

[0076]

[0077] As can be seen from the above embodiments and comparative examples, the composite separator that meets the requirements of the present invention has significant advantages over the comparative examples in terms of adhesion, heat resistance, electrolyte wettability, deformation resistance, and battery safety.

[0078] The battery separator provided by this invention is not limited to the preparation method provided by this invention. Battery separators that conform to this invention and are obtained by other methods should be protected within the scope of the claims of this invention.

[0079] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A composite separator for lithium-ion batteries, characterized in that, The system comprises a base film and a composite coating located on at least one side of the base film surface. The composite coating includes a mixed coating layer and a ceramic layer. The mixed coating layer is applied onto the base film and comprises inorganic ceramic particles and organic polymer particles in a mass ratio of (8~20):

1. The ceramic layer is applied onto the mixed coating layer and is composed of inorganic ceramic particles. The particle size of the organic polymer particles is greater than or equal to the thickness of the mixed coating layer and less than or equal to the thickness of the composite coating layer. The inorganic ceramic particles in the mixed coating layer include large particles and small particles. The particle size of the large particles ranges from 0.6 to 1.5 μm, and the particle size of the small particles ranges from 0.1 to 0.5 μm. The organic polymer particles have a core-shell structure. The glass transition temperature (Tg) of the shell layer is 10~50℃, the glass transition temperature (Tg) of the core layer is >50℃, and the D50 diameter of the organic polymer particles is 1~5 μm. The organic polymer particles include one or more of polyvinylidene fluoride, polyhexafluoropropylene, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylic acid, and polyacrylate resins.

2. The lithium-ion battery composite separator according to claim 1, characterized in that, The suspension height L of the organic polymer particles in the composite coating satisfies h / 3≤L≤2h / 3, where h is the thickness of the composite coating, L=(Lm+Ln) / 2, Lm is the distance from the highest point of the organic polymer particles to the surface of the base film, and Ln is the distance from the lowest point of the organic polymer particles to the surface of the base film.

3. The lithium-ion battery composite separator according to claim 1, characterized in that, The longitudinal uniformity β of the organic polymer particles in the composite coating is ≤1.5, where β = (Lm - Ln) / R, Lm is the distance from the highest point of the organic polymer particle to the surface of the base film, Ln is the distance from the lowest point of the organic polymer particle to the surface of the base film, and R is the D50 of the organic polymer particle.

4. The lithium-ion battery composite separator according to claim 1, characterized in that, The support of the organic polymer particles in the composite coating is Z > 0.5, Z = (C0 / h) / (α*β), where the burial degree of the organic polymer particles is B0 = L + R / 2 - h. When B0 > 0, C0 = h - (LR / 2), and when the burial degree of the organic polymer particles is B0 < 0, C0 = R; α = L / h, β = (Lm - Ln) / R, L = (Lm + Ln) / 2, where Lm is the distance from the highest point of the organic polymer particle to the surface of the base film, Ln is the distance from the lowest point of the organic polymer particle to the surface of the base film, R is the D50 of the organic polymer particle, and C0 is the burial depth.

5. The lithium-ion battery composite separator according to claim 1, characterized in that, The inorganic ceramic particles in the ceramic layer have a particle size range of 0.1~1μm.

6. The lithium-ion battery composite separator according to claim 1, characterized in that, The thickness of the mixed coating is 0.5~3μm, and the thickness of the ceramic layer is 0.5~3μm.

7. The lithium-ion battery composite separator according to claim 1, characterized in that, The inorganic ceramic particles include one or more of the following: alumina, silicon dioxide, zirconium oxide, magnesium oxide, cerium dioxide, titanium dioxide, zinc oxide, iron oxide, silicon nitride, titanium nitride, boron nitride, boehmite, aluminum hydroxide, magnesium hydroxide, barium sulfate, calcium carbonate, wollastonite, and silicon carbide.

8. The lithium-ion battery composite separator according to claim 1, characterized in that, The base membrane is one of polyethylene, polypropylene, ceramic-coated polyethylene, ceramic-coated polypropylene, polyimide, aramid, aramid sulfone, nonwoven fabric and polyethylene terephthalate porous membrane, with a thickness of 0.1~15μm.

9. The lithium-ion battery composite separator according to claim 8, characterized in that, The thickness of the base film is 0.1~12μm.

10. The lithium-ion battery composite separator according to claim 9, characterized in that, The thickness of the base film is 4~12μm.

11. The method for preparing the lithium-ion battery composite separator according to any one of claims 1 to 10, characterized in that, Includes the following steps: S1, Inorganic ceramic particles, organic polymer particles and solvent are mixed to form a mixed coating slurry, which is then coated on one or both sides of the base film and dried to form a mixed coating layer. S2, Inorganic ceramic particles and solvent are mixed to prepare a ceramic slurry, which is then coated onto a mixed coating layer and dried to form a ceramic layer, thus obtaining a composite diaphragm; The solid content of the mixed coating slurry is 32-38%, and the viscosity is 50-150 mPa·S; in the ceramic slurry, the solid content of the inorganic ceramic particles is 27-33%, and the viscosity is 5-100 mPa·S. The drying in step S1 is carried out in a hot convection oven at a temperature of 60~90℃; the drying in step S2 is a two-stage drying process, first in a hot convection oven, and then without cooling, directly in a hot roller pressing system composed of upper and lower pressure rollers, with a temperature of 50~80℃ for both stages of drying.

12. A secondary battery, characterized in that, The lithium-ion battery composite separator includes any one of claims 1 to 10 or the lithium-ion battery composite separator prepared by the method of claim 11.

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