Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm and a preparation method thereof, and a secondary battery, the diaphragm comprises a base membrane and a coating layer coated on at least one surface of the base membrane, the coating layer is a ceramic coating layer containing viscous microspheres, the contact surface of the ceramic coating layer and the base membrane is used as an inner surface, the opposite surface is used as an outer surface, and the adhesive microspheres are coated on the inner surface of the ceramic coating layer. The vertical distance between the bottoms of the viscous microspheres and the outer surface is the embedding depth h, the thickness of the ceramic coating is H, and for any viscous microsphere, hgt should be met; 1 / 2H. The viscous microspheres are mixed in the inorganic coating to form supporting points of the coating, so that discontinuous gaps exist in the continuous-phase inorganic coating, and the extrusion resistance of the coating is improved. The diaphragm disclosed by the invention has relatively high air permeability, relatively low thermal shrinkage, relatively high wet bonding strength, relatively low powder falling rate, relatively low compression rate and relatively low internal resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary battery separators, and in particular relates to a battery separator and a preparation method thereof, and a secondary battery. Background Art

[0002] The diaphragm is one of the key materials of lithium-ion batteries. It is used to separate the electrolyte of the positive and negative electrodes, while ensuring that the ions involved in the battery reaction can pass freely. The performance of the diaphragm has an important impact on the rate discharge, cycle performance and safety performance of the battery. Many battery safety accidents are caused by the diaphragm being pierced by the electrolyte or melted by high temperature. Therefore, improving the extrusion resistance of the diaphragm is crucial to improving the safety performance of the battery.

[0003] At present, the lithium battery separators on the market are still insufficient in terms of extrusion resistance. During battery assembly, vibration testing and long-term cycling, the separator may be subjected to various extrusion forces from inside and outside the battery, causing the separator to deform, rupture or even fail. This will not only reduce the performance of the battery, but may also cause safety hazards such as battery short circuit and fire.

[0004] In order to improve the extrusion resistance of lithium battery separators, researchers have conducted a lot of exploration and practice. On the one hand, by optimizing the selection of separator materials, such as using high-performance PVDF, PAN and other materials, the high temperature resistance, corrosion resistance and mechanical strength of the separator can be improved. On the other hand, by improving the production process of the separator, such as optimizing the stretching speed and temperature and using porous separator preparation technology, the physical, chemical and electrical properties of the separator can be improved, thereby improving its extrusion resistance.

[0005] However, despite these advances, the existing lithium battery separators still need to be further improved in terms of extrusion resistance. Therefore, developing a lithium battery separator with higher extrusion resistance is of great significance for improving the safety performance and cycle stability of the battery. Summary of the invention

[0006] The purpose of the present invention is to provide a battery separator, which has good extrusion resistance by specially designing the structure of the separator.

[0007] Another object of the present invention is to provide a method for preparing a battery separator.

[0008] Another object of the present invention is to provide a secondary battery.

[0009] In the first aspect, the present invention provides a battery separator, comprising a base film and a coating applied on at least one surface of the base film, wherein the coating is a ceramic coating containing sticky microspheres, the contact surface between the ceramic coating and the base film is the inner surface, and the other opposite surface is the outer surface, the vertical distance between the bottom of the sticky microsphere and the outer surface is the embedding depth h, the thickness of the ceramic coating is H, and for any of the sticky microspheres, h>1 / 2H should be satisfied.

[0010] In a second aspect, the present invention provides a method for preparing a battery separator, comprising the following steps:

[0011] The ceramic particles are mixed with water and dispersed to form an inorganic dispersion, and then the viscous microsphere solution is added and dispersed evenly to obtain a finished slurry;

[0012] The finished slurry is coated on the base film and then dried in a three-stage oven, wherein the drying pressure of the first stage is -40 to -10Pa, and the drying temperature is 65 to 80°C; the drying pressure of the second stage is -80 to -40Pa, and the drying temperature is 65 to 90°C; the drying pressure of the third stage is -80 to -60Pa, and the drying temperature is 65 to 70°C; a chilled roller is arranged in the second roller section, and the chilled roller accounts for 1 / 4 to 1 / 2 of the number of rollers in the second roller section, and the diaphragm is obtained after the drying is completed; in the oven, the length ratio of the first section: the second section: the third section is (2 to 3): 1: (2 to 3); the temperature of the chilled roller is 5 to 15°C.

[0013] In a third aspect, the present invention provides a secondary battery comprising the battery separator described above.

[0014] Beneficial effects of the present invention:

[0015] The present invention mixes the viscous microspheres in the inorganic coating to form the supporting points of the coating, so that the inorganic coating of the continuous phase has discontinuous gaps, thereby improving the coating's extrusion resistance. The diaphragm of the present invention has high air permeability, low thermal shrinkage, high wet bonding strength, low powder loss rate, low compression rate and low internal resistance, so that the lithium battery containing the diaphragm has high safety, good electrical cycle performance and long service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the structure of the viscous microspheres embedded in the ceramic coating in the battery separator of the present invention.

[0017] Wherein, R represents the radius of the polymer microsphere, a represents the polymer microsphere, c represents the base film, Δh represents the height of the polymer microsphere protruding from the upper surface, h represents the embedding depth of the polymer microsphere, and H represents the coating thickness. DETAILED DESCRIPTION

[0018] The present invention will be specifically described below through embodiments. It is necessary to point out here that the following embodiments are only used to further illustrate the present invention and cannot be construed as limiting the protection scope 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 content of the present invention.

[0019] The present invention provides a battery separator, which includes a base film and a coating applied on at least one surface of the base film. The coating is a ceramic coating containing a plurality of viscous microspheres. Taking the contact surface between the ceramic coating and the base film as the inner surface and the opposite side as the outer surface, the vertical distance from the bottom of the viscous microsphere to the outer surface is the embedding depth h, and the thickness of the ceramic coating is H. For any one of the viscous microspheres, h > 1 / 2H should be satisfied, that is, the minimum embedding depth of all viscous microspheres is greater than 1 / 2H. For example, h can be 2 / 3H, 3 / 4H, 4 / 5H, H.

[0020] For the battery separator of the present invention, the embedding depth h of the viscous microspheres can be measured by the following method:

[0021] Cut 10 coated separator specimens of 5 cm × 5 cm, 5 of which are vertically cut along the TD direction, and the other 5 are vertically cut along the MD direction. Use a Zeiss field emission scanning electron microscope (under a 500-fold electron microscope field of view, ZEISS SIGMA 300) to measure the vertical distance from the bottom of the viscous microsphere to the outer surface of the coating within the field of view as the embedding depth h.

[0022] In the present invention, for any one of the viscous microspheres, 1 / 2H < h < H should be satisfied. For example, h can be 0.55H, 0.6H, 0.65H, 0.7H or H, or a range value between any intermediate values. For any one of the viscous microspheres, a large embedding depth makes the separator have better air permeability, smaller powder shedding rate and smaller compression rate. However, if the embedding is too deep, it will affect the wet bonding strength of the separator; if the embedding depth is shallow, the separator has better thermal shrinkage and better internal resistance value, but if the embedding is too small, it will seriously affect the air permeability, wet bonding strength and powder shedding rate of the separator.

[0023] The battery separator described in the present invention, wherein 20% to 70% of the viscous microspheres protrude from the outer surface of the ceramic coating, and the remaining viscous microspheres are buried below the outer surface, such as 20% of the viscous microspheres protrude from the outer surface and 80% are buried below the outer surface, or 30% of the viscous microspheres protrude from the outer surface and 70% are buried below the outer surface, or 40% of the viscous microspheres protrude from the outer surface and 60% are buried below the outer surface, or 50% of the viscous microspheres protrude from the outer surface and 50% are buried below the outer surface, or 60% of the viscous microspheres protrude from the outer surface and 40% are buried below the outer surface, or 70% of the viscous microspheres protrude from the outer surface and 30% are buried below the outer surface. In the present invention, "quantity" refers to the number of viscous microspheres. By dispersing multiple sticky microspheres in the separator in the ceramic coating, some of the sticky microspheres protrude from the outer surface of the ceramic coating, and the protruding sticky microspheres leak out of the ceramic coating, forming channels between each other for storing electrolyte and circulating electrolyte; the sticky microspheres buried in the ceramic coating play a supporting role for the ceramic coating. During the research and development process, the inventor found that by controlling 20% ​​to 70% of the sticky microspheres in the battery separator to protrude from the outer surface of the ceramic coating, the battery separator can have good air permeability, thermal stability, adhesion and liquid absorption rate.

[0024] The percentage of sticky microspheres protruding from the outer surface of the ceramic coating is calculated by the following method:

[0025] Ten 5 cm × 5 cm coated diaphragm samples were cut, of which five were cut vertically along the TD direction. A Zeiss field emission scanning electron microscope (ZEISS SIGMA 300, 500 times the field of view) was used to observe the range of the microscope. The number of sticky microspheres was recorded as n1, the number of sticky microspheres protruding from the outer surface of the ceramic coating was recorded as n2, and the proportion of sticky microspheres protruding from the outer surface of the ceramic coating in the TD direction was n. TD =n2 / n1, repeat the test for 5 groups of samples and take the average value; another 5 samples were cut vertically along the MD direction and observed using a Zeiss field emission scanning electron microscope (ZEISS SIGMA 300 under a 500x electron microscope field of view). The number of sticky microspheres within the viewing range was recorded as n3, the number of sticky microspheres protruding from the outer surface of the ceramic coating was recorded as n4, and the proportion of sticky microspheres protruding from the outer surface of the ceramic coating in the MD direction was n. MD = n4 / n3, repeat the test for 5 groups of samples and take the average value, then the proportion of microspheres protruding from the outer surface of the ceramic coating = (n TD +n MD ) / 2.

[0026] In the battery separator of the present invention, preferably 50-60% of the adhesive microspheres protrude from the outer surface of the ceramic coating.

[0027] The battery separator described in the present invention, the ceramic particles in the ceramic coating include one or more of α-alumina, γ-alumina, boehmite, calcium carbonate, barium sulfate, barium titanate, hydrotalcite, montmorillonite, spinel, titanium dioxide, silicon dioxide, zirconium dioxide, magnesium oxide, calcium oxide, beryllium oxide, magnesium hydroxide, calcium hydroxide, and silicon carbide.

[0028] The battery separator of the present invention has an average particle size D V50 It can be 0.05 to 1.5 μm, such as 0.05 μm, 0.1 μm, 0.5 μm, 0.7 μm, 0.9 μm, 1.0 μm, 1.2 μm, 1.5 μm, preferably 0.1 to 1.0 μm.

[0029] In the battery separator of the present invention, the thickness of the ceramic coating is 1-5 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm.

[0030] The battery separator described in the present invention, the viscous microspheres include one or more of polyimide water-soluble microspheres, styrene-butadiene rubber microspheres, polyacrylate modified microspheres, polyetherimide, aramid, aromatic sulfone, polyvinylidene fluoride, polymethyl methacrylate, thermoplastic polyurethane elastomer rubber, polyethylene terephthalate, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride and polyvinylidene fluoride-hexafluoropropylene, preferably polymethyl methacrylate.

[0031] In the battery separator of the present invention, the glass transition temperature of the polymethyl methacrylate is 30 to 100° C., such as 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C., 100° C., preferably 50 to 100° C., and more preferably 60 to 100° C. If the glass transition temperature is too low, the embedding depth of the sticky microspheres is affected, thereby affecting the heat shrinkage and adhesion of the separator, and further affecting the compression rate and powder loss rate of the separator.

[0032] The battery separator of the present invention has an average particle size Dv 50 It is 1 to 10 μm, such as 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, preferably 2 to 8 μm, and more preferably 2 to 6 μm.

[0033] The battery separator of the present invention, the base film is made of polyethylene and / or polypropylene, and the thickness of the base film is 5 to 15 μm, such as 5 μm, 7 μm, 10 μm, 13 μm, 15 μm.

[0034] In the battery separator of the present invention, the ceramic coating is coated on one or both surfaces of the base film.

[0035] In the battery separator of the present invention, the solid mass ratio of the ceramic particles to the sticky microspheres is (20-40):(1-3), such as 20:3, 20:2, 20:1, 25:1, 30:1, 30:2, 35:1, 40:1, preferably (25-35):(1-2).

[0036] The method for preparing the battery separator of the present invention comprises the following steps:

[0037] The ceramic particles are mixed with water and dispersed to form an inorganic dispersion, and then the viscous microsphere solution is added and dispersed evenly to obtain a finished slurry;

[0038] The finished slurry is coated on the base film and then dried in a three-stage oven. The first stage drying pressure is -40 to -10Pa, such as -40Pa, -30Pa, -20Pa, -10Pa, and the drying temperature is 65 to 80°C, such as 65°C, 70°C, 75°C, 80°C. The second stage drying pressure is -80 to -40Pa, such as -80Pa, -70Pa, -60Pa, -50Pa, -40Pa, and the drying temperature is 6 5~90℃, such as 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, the third stage drying pressure is -80~-60Pa, such as -80Pa, -70Pa, -60Pa, the drying temperature is 65~70℃, such as 65℃, 70℃, a chilled roller is set in the second stage of rollers, and the chilled roller accounts for 1 / 4~1 / 2 of the number of the second stage of rollers, such as the chilled roller ratio can be 1 / 4, 1 / 3, 1 / 2, and the diaphragm is obtained after drying.

[0039] The method of the present invention performs drying in a three-stage drying manner. The drying process is performed under negative pressure conditions. At the same time, a chilling roller is added in the second drying process. The Brownian motion of the particles can be reduced under the chilling conditions. Drying is performed after cooling, which helps the sticky microspheres to settle downward, so that the embedding depth of the sticky microspheres in the ceramic coating satisfies h>1 / 2H.

[0040] In the method for preparing the battery separator described in the present invention, in the oven, the length ratio of the first section: the second section: the third section is (2-3): 1: (2-3), and can further be 2: 1: 2. The total length of the three-section oven is ≥ 15m, for example, it can be 15m, 16m, 17m, 18m, 19m, 20m, 21m, 22m. Preferably, the total length of the oven is 18-20m, and more preferably 18m.

[0041] In the method for preparing the battery separator of the present invention, the solid content of the viscous microsphere solution is 10-40wt%, such as 10wt%, 20wt%, 30wt%, 40wt%, preferably 20-35wt%, and more preferably 25-30wt%.

[0042] The preparation method of the battery separator described in the present invention, the solid content of the finished slurry is 30-40wt%, such as 30wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 40wt%, preferably 30-38wt%, the solid content of the finished slurry will directly affect the floating degree of the sticky microspheres, the higher the solid content, the smaller the floating, the lower the solid content, the higher the floating. The present invention uses a finished slurry with a solid content of 30-40%, so that the sticky microspheres are solidified inside the coating to avoid floating and sinking.

[0043] The preparation method of the battery separator described in the present invention, the finished slurry contains 0.1 to 30 parts by weight of sticky microspheres, such as 0.1 parts by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, and 15 to 40 parts by weight of ceramic particles, such as 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight.

[0044] The preparation method of the battery separator of the present invention adopts a coating machine to coat the finished slurry on the base film, the depth of the gravure roller is 20 to 100 μm, such as 20 μm, 30 μm, 50 μm, 70 μm, 90 μm, 100 μm, preferably 20 to 50 μm, and the coating speed is 50 to 250 m / min, such as 50 m / min, 100 m / min, 150 m / min, 200 m / min, 250 m / min, preferably 100 to 150 m / min. The deeper the gravure depth, the lower the position of the sticky microspheres in the coating. The structural stability separator of the present invention selects a gravure roller with a deeper depth for coating while meeting the coating amount requirements.

[0045] In the method for preparing the battery separator of the present invention, the temperature of the chilled roller is 5-15°C, such as 5°C, 10°C, or 15°C.

[0046] The preparation method of the battery separator described in the present invention, the finished slurry also includes one or more of a thickener, a binder, a wetting agent and a dispersant, the amount of the thickener is ≤15 parts by weight, the amount of the binder is ≤15 parts by weight, the amount of the wetting agent is ≤3 parts by weight, and the amount of the dispersant is ≤3 parts by weight; the thickener, binder, wetting agent and dispersant are all commonly used substances in the field, wherein the thickener can be one, two or more of sodium carboxymethyl cellulose, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium alginate, and polyacrylamide; the binder can be one or more of styrene-butadiene rubber, acrylate, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, and polytetrafluoroethylene; the wetting agent can be one or more of anionic surfactants, polyethylene surfactants, and polyol surfactants, for example, it can be polyether-modified siloxane; the dispersant can be one or more of polyvinyl pyrrolidone, polyacrylamide, and polyvinyl alcohol.

[0047] A secondary battery comprises the battery separator mentioned above.

[0048] The present invention is described below by means of specific examples.

[0049] Test Examples 1 to 10

[0050] Boehmite and polyvinyl pyrrolidone (solid content 50%) are dispersed in water, and dispersed by sand mill (rotation speed is 1000 rpm, 30 minutes) to form a dispersion liquid, polymethyl methacrylate is added, and after uniform dispersion, acrylate (solid content is 45%), carboxymethyl cellulose (solid content is 4%), and alkylphenol polyoxyethylene ether (solid content is 100%) are added in sequence, dispersed evenly, and sieved to form a finished slurry.

[0051] The finished slurry is coated on the base film by a coater, the depth of the gravure roller is 30 μm, the coating speed is 150 m / min, and the traction roller pulls the slurry into the oven for drying. The oven is divided into three sections, and a chilled roller is arranged in the second section. The chilled roller accounts for 1 / 3 of the number of rollers in the second section. The length ratio of the first section: the second section: the third section is 2 / 5: 1 / 5: 2 / 5, and the total length of the oven is 18 m.

[0052] The amount of each raw material used and the preparation process conditions in Experimental Examples 1 to 10 are shown in Table 1.

[0053] Comparative Example 1

[0054] The only difference from Test Example 2 is that no chilling roller is provided during the drying process.

[0055] The battery separators prepared in the test examples and comparative examples were subjected to performance tests, and the test results are shown in Table 1.

[0056] The performance test method is:

[0057] Air permeability (s / (100cc)):

[0058] The test was conducted in accordance with the requirements of GB / T36363-2018. A 600mm×100mm membrane sample was cut and a Wangyan air permeability meter (ASAHI Co., Ltd., EG01-55-1MR) was used for a test time of 3s. The air permeability of the membrane was measured at any position with an interval of 100mm along the 600mm TD direction. The average value of the above five test points was recorded as the air permeability of the membrane.

[0059] Thermal shrinkage (%):

[0060] Test according to the requirements of GB / T36363-2018. Cut a sample of 10cm×10cm, mark the transverse TD and longitudinal MD on the sample, measure the transverse and longitudinal widths with a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C), clamp the sample between two sealed A4 papers, place the sample in a 150℃ oven for 1h, and measure the transverse and longitudinal widths with a fully automatic image measuring projector after the sample returns to room temperature. Measure 3 times and take the average value:

[0061] MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0062] TD heat shrinkage rate (%) = (TD length before heating - TD length after heating) / TD length before heating × 100.

[0063] Wet bond strength (N / m):

[0064] Cut samples of 20mm*60mm size, pole pieces of 20mm*60mm size and PET of 20mm*60mm size, stack them according to the diaphragm / pole piece / PET, put them into 80mm*80mm aluminum-plastic bags, and inject 1.5g electrolyte (LiPF 6 1M, EC:EMC=3 / 7, VC=2%), vacuum (-80kPa) edge sealing, ripening for 24h; pad the top and bottom of the sample with silicone pads, use a hot press (Qmesys, QM940AS) for 10min, where the temperature of the upper and lower clamps of the hot press is 60℃, the pressure is 200kgf, cool, take the sample out of the aluminum-plastic bag, manually peel off the PET, clamp the electrode in the lower chuck of the tensile testing machine (Jinan Sike Testing Technology Co., Ltd., TSL-1002), clamp the diaphragm in the upper chuck of the tensile testing machine to ensure that there is no tilt, the stretching speed is 300mm / min, measure three times, and take the average value.

[0065] Powder loss rate (%):

[0066] The battery diaphragm was cut into 6 pieces, 3cm×22cm in size, and placed on an analytical balance for weighing, and the weight was recorded as ml; ensure that the curved table of the friction color fastness tester (Daei Science Seiki Manufacturing Co., Ltd., RT-300S) is clean, and fix the lens paper on the friction head of the friction color fastness tester, where the smooth side of the lens paper faces outward, and then fix the cut battery diaphragm on the curved table of the friction color fastness tester, with one side of the heat-resistant coating facing up, and then clamp it and gently place the friction head on the diaphragm, set the number of experiments to 5 on the operation panel of the color fastness tester, and click to start. After the experiment is completed, take out the battery diaphragm, place it on an analytical balance for weighing, and record the weight as m2. The powder loss amount M (mg) = m1-m2, and take the average value of the six groups of diaphragm test data. If the diaphragm is a double-layer coating on both sides of the base film, the weight of both sides of the diaphragm after friction testing is m2, and the powder loss data is calculated based on this.

[0067] Compression ratio (%):

[0068] The sample film was cut into 50*50mm square samples, and 5 cut samples were stacked together and measured with a Mahr thickness gauge (Mahr model: C1202) for 10 points of thickness, and the average value was recorded as D0; then the PET sheet was cut into 50*50mm square sheets, and the sample film was placed between two cut PET sheets and aligned, and placed in a hot press (Qmesys, QM940AS) for hot pressing; the hot pressing parameters were set to 1000kgf, 70℃, and the holding time was 1s. After taking out, the diaphragm after hot pressing was measured at 10 points, and the average value was recorded as D1. The compression rate

[0069] Internal resistance(Ω):

[0070] Cut a sample of 100mm×100mm size, ensure that there is no foreign matter on the surface of the insulation resistance test bench, place the sample in the specified area of ​​the test bench, start the YD9820A program-controlled insulation resistance tester (Changzhou Yangzi Electronics Co., Ltd.) for testing, with a voltage of 100V, a time of 5s, and a pressure of 0.05MPa. Test 5 groups and take the average value.

[0071] Table 1

[0072]

[0073] It can be seen from Test Examples 1 to 10 that when the minimum embedding depth of the sticky microspheres in the battery separator is greater than 1 / 2H, the separator can take into account thermal stability, powder loss rate, compression rate and internal resistance.

[0074] When the embedding depth satisfies 1 / 2H < h < H, it has good air permeability (164 - 178 s / (100 cc)), thermal stability (MD150℃ < 1.36, TD150℃ < 1.32), wet adhesion bond strength (greater than 1.02 N / m), powder dropping rate (less than 1.9%), compression rate (less than 1.9), and internal resistance (less than 0.78 Ω).

[0075] Certainly, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention. However, these corresponding changes and deformations should all fall within the protection scope of the claims of the present invention.

Claims

1. A battery separator, comprising a base film and a coating applied on at least one surface of the base film, wherein the coating is a ceramic coating containing sticky microspheres, characterized in that: The contact surface between the ceramic coating and the base film is the inner surface, the other opposite surface is the outer surface, the vertical distance between the bottom of the sticky microsphere and the outer surface is the embedding depth h, the thickness of the ceramic coating is H, and for any of the sticky microspheres, h>1 / 2H should be satisfied.

2. The battery separator according to claim 1, characterized in that: For any of the sticky microspheres, 1 / 2H <h<H。 3. The battery separator according to claim 1, characterized in that: 20 to 70% of the adhesive microspheres protrude from the outer surface of the ceramic coating.

4. The battery separator according to claim 1, characterized in that: The ceramic particles in the ceramic coating include one or more of α-alumina, γ-alumina, boehmite, calcium carbonate, barium sulfate, barium titanate, hydrotalcite, montmorillonite, spinel, titanium dioxide, silicon dioxide, zirconium dioxide, magnesium oxide, calcium oxide, beryllium oxide, magnesium hydroxide, calcium hydroxide, and silicon carbide; The average particle size Dv of the ceramic particles 50 0.05 to 1.5 μm, preferably 0.1 to 1.0 μm; The thickness of the ceramic coating is 1-5 μm.

5. The battery separator according to claim 1, characterized in that: The sticky microspheres include one or more of polyimide water-soluble microspheres, styrene-butadiene rubber microspheres, polyacrylate-modified microspheres, polyetherimide, aramid, aramid fiber, polyvinylidene fluoride, polymethyl methacrylate, thermoplastic polyurethane elastomer rubber, polyethylene terephthalate, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride and polyvinylidene fluoride-hexafluoropropylene, preferably polymethyl methacrylate.

6. The battery separator according to claim 5, characterized in that: The glass transition temperature of the polymethyl methacrylate is 30 to 100°C, preferably 50 to 100°C, and more preferably 60 to 100°C.

7. The battery separator according to claim 1, characterized in that: The average particle size Dv of the sticky microspheres 50 It is 1 to 10 μm, preferably 2 to 8 μm, and more preferably 2 to 6 μm.

8. The battery separator according to claim 1, characterized in that: The base film is made of polyethylene and / or polypropylene, and has a thickness of 5 to 15 μm.

9. The battery separator according to any one of claims 1 to 8, characterized in that: The ceramic coating is coated on one or both surfaces of the base film.

10. The battery separator according to claim 1, characterized in that: The solid mass ratio of the ceramic particles to the sticky microspheres is (20-40):(1-3), preferably (25-35):(1-2).

11. The method for preparing a battery separator according to any one of claims 1 to 10, characterized in that: The following steps are involved: The ceramic particles are mixed with water and dispersed to form an inorganic dispersion, and then the viscous microsphere solution is added and dispersed evenly to obtain a finished slurry; The finished slurry is coated on the base film, and then dried in a three-stage oven, the first stage drying pressure is -40 to -10Pa, the drying temperature is 65 to 80°C, the second stage drying pressure is -80 to -40Pa, the drying temperature is 65 to 90°C, the third stage drying pressure is -80 to -60Pa, the drying temperature is 65 to 70°C, a chilled roller is set in the second stage of rollers, and the chilled roller accounts for 1 / 4 to 1 / 2 of the number of rollers in the second stage, and the diaphragm is obtained after the drying is completed; In the oven, the length ratio of the first section: the second section: the third section is (2-3): 1: (2-3), preferably 2: 1: 2; The temperature of the chilled roller is 5 to 15°C; The solid content of the viscous microsphere solution is 10-40wt%, preferably 20-35wt%, and more preferably 25-30wt%; the solid content of the finished slurry is 30-40wt%, preferably 30-38wt%; The finished slurry contains 0.1 to 30 parts by weight of sticky microspheres and 15 to 40 parts by weight of ceramic particles; The finished slurry is coated on the base film by a coating machine, the depth of the gravure roll is 20-100 μm, preferably 20-50 μm, and the coating speed is 50-250 m / min, preferably 100-150 m / min.

12. The method for preparing a battery separator according to claim 11, characterized in that: The finished slurry also includes one or more of a thickener, a binder, a wetting agent and a dispersant.

13. A secondary battery, characterized in that: A battery separator comprising the battery separator according to any one of claims 1 to 10 or a battery separator prepared by the method according to any one of claims 11 to 12.

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