Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention provides a diaphragm and a preparation method thereof, and a lithium ion battery, the diaphragm comprises a base membrane, a composite ceramic coating and an interface layer, the composite ceramic coating contains inorganic ceramic particles and organic microcapsule particles, the organic microcapsule particles comprise a core material with a melting point range of 55-65 DEG C and a polymer shell coating the surface of the core material, and the polymer shell is coated on the surface of the core material. The polymer shell is selected from at least one of polyurethane, polyurea, polyamide and polycarbonate, the interface layer is formed by depositing an interface modification polymer on the surface of the composite ceramic coating, and the interface modification polymer is selected from at least one of polycatechol derivatives, polyaniline, polyvinyl pyridine, polyacrylic acid and polyacrylamide. The core material in the organic microcapsule particles is melted when heated to trigger hole closing at the initial stage of thermal runaway at 60-80 DEG C, the hole closing rate can reach 92%, the hole closing response time is shortened by 30%, and the porous structure of the interface layer can enhance the liquid absorption rate.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a diaphragm and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] The separator is a key internal component of lithium-ion batteries, located between the positive and negative electrodes. It is a polymer functional material with a microporous structure. Its main functions include physical isolation: preventing direct contact between the positive and negative electrodes and causing short circuits; ion conduction: allowing lithium ions to pass freely through microporous channels during charging and discharging while blocking electron conduction; safety protection: under high temperature or abnormal conditions (such as overheating), ion transmission is blocked through a closed-pore mechanism to prevent thermal runaway.

[0003] Common separators include polyolefin-based films and ceramic coatings. Polyolefin-based films, such as PE films and PP films, are prone to melting and shrinking at high temperatures, have poor thermal stability, and can easily cause battery short circuits. Ceramic coatings, such as alumina and boehmite, have weak surface polarity, a contact angle >60°, and a low liquid absorption rate of only 120-150%.

[0004] To address these technical issues, existing solutions often employ modified polymer coatings. For example, polymer coatings are combined with ceramic coatings to form multilayer composite coatings. The polymers contain polar groups, which enhance the wettability of the electrolyte to the separator. However, commonly used polymer materials typically have a melting point above 90°C, making them unable to close pores in time at the initial stage of thermal runaway, when the temperature within the battery system is between 60°C and 80°C. This blocks ion transport and results in a poor self-closing pore effect. Furthermore, the low polarity of the ceramic coating surface also results in poor bonding with the polymer coating, resulting in poor structural stability. Summary of the Invention

[0005] In order to solve the above problems, timely pore closure is achieved at 60-80°C in the early stage of battery thermal runaway to block ion transmission, thereby enhancing the safety, liquid absorption performance and structural stability of the diaphragm. The first aspect of the present application provides a diaphragm, comprising a base film, a composite ceramic coating coated on at least one side of the base film, and an interface layer covered on the surface of the composite ceramic coating. The composite ceramic coating contains inorganic ceramic particles and organic microcapsule particles. The organic microcapsule particles include a core material with a melting point range of 55-65°C and a polymer shell coated on the surface of the core material. The polymer shell is selected from: at least one of polyurethane, polyurea, polyamide and polycarbonate. The interface layer is formed by depositing an interface-modified polymer on the surface of the composite ceramic coating. The interface-modified polymer is selected from: at least one of polycatechol derivatives, polyaniline, polyvinylpyridine, polyacrylic acid and polyacrylamide.

[0006] In some optional embodiments, the core material is a paraffin material, and the paraffin material comprises any one or more alkane compounds with a main chain carbon atom number of 16-30.

[0007] In some optional embodiments, the particle size Dv50 of the inorganic ceramic particles is 0.2-0.8 μm, the particle size of the organic microcapsule particles is 2-5 μm, wherein the particle size of the core material is 1.80-4.95 μm, and the radial thickness of the polymer shell is 0.05-0.20 μm.

[0008] In some optional embodiments, the base film is a polyethylene film or a polypropylene film, the thickness of the base film is: 4-14 μm, the thickness D of the composite ceramic coating is 3-5 μm, and the embedding depth d of the organic microcapsule particles from the outer surface of the composite ceramic coating to the internal doping thereof satisfies: 1 / 3D≤d≤D.

[0009] In some optional embodiments, the doping density of the organic microcapsule particles in the thickness direction of the composite ceramic coating varies gradiently, wherein the doping density of the organic microcapsule particles on the side close to the base film is less than the doping density of the organic microcapsule particles on the side away from the base film.

[0010] In some optional embodiments, the embedding depth d of the organic microcapsule particles is divided into d inner close to the base film and d outer away from the base film, and the doping density ρ of the organic microcapsule particles in the embedding depth direction satisfies: 1.2≤ρ d表 / ρ d内 ≤1.4, the range of the d table is: 1 / 4d-1 / 3d.

[0011] In some optional embodiments, the composite ceramic coating further comprises a binder, wherein the mass percentage of the inorganic ceramic particles is 70-90%, the mass percentage of the binder is 1-15%, and the mass percentage of the organic microcapsule particles is 5%-15%.

[0012] In some optional embodiments, the inorganic ceramic particles are selected from at least one of alumina, boehmite and titanium dioxide, the binder is polyacrylate and / or polyvinylidene fluoride; the organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane, and the interface modification polymer is polydopamine.

[0013] In some optional embodiments, the interface layer formed by the deposition of polydopamine on the surface of the composite ceramic coating has a porous structure with a pore size of 5-20 nm, and the specific surface area of the organic microcapsule particles is greater than 200 m 2 / g, and the adhesion between the interface layer and the ceramic coating is ≥2.5N / m.

[0014] A second aspect of the present application provides a method for preparing a diaphragm, wherein the diaphragm is the diaphragm according to any one of the above items, and the preparation method comprises the following steps:

[0015] S1: using interfacial polymerization to polymerize on the surface of the core material to form the polymer shell, thereby preparing the organic microcapsule particles;

[0016] S2: mixing the inorganic ceramic particles, the organic microcapsule particles, the binder, and the solvent in a predetermined ratio, adjusting the solid content to 5-40%, and preparing a ceramic coating slurry;

[0017] S3: Take the base film and spray the ceramic coating slurry onto the surface of the base film using a step-by-step coating process, wherein the electrostatic spraying parameters are set to: voltage 10-15 kV, needle inner diameter 0.3-0.5 mm, spraying distance 20-30 cm; and

[0018] S4: placing the base membrane sprayed with the ceramic coating slurry in a reaction container with the interface modification polymer monomer, depositing the interface modification polymer on the surface of the ceramic coating slurry, forming the interface layer after the deposition is completed, and obtaining the diaphragm after drying and curing.

[0019] In some optional preparation methods, the distributed coating process controls the doping density of the organic microcapsule particles in the composite ceramic coating by adjusting the spraying angle, and the spraying angle range is: 30°-45°, and the spraying angle changes from large to small during the spraying process.

[0020] In some optional preparation methods, the base film is a polyethylene film or a polypropylene film with a porosity of 30-45%, the inorganic ceramic particles are alumina, the binder is polyacrylate, the organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane, and the interface-modifying polymer is polydopamine. The preparation method of the organic microcapsule particles comprises the following steps:

[0021] S11: Mix molten paraffin at 65° C. and isocyanate prepolymer in a mass ratio of 1:(0.3-0.5), add 3 wt% of the oil phase reactant emulsifier Span 80, set the stirring speed to 800-1200 rpm, and stir and emulsify for 30 minutes to prepare an emulsion;

[0022] S12: preparing an aqueous solution of chain extender ethylenediamine with a concentration of 2-5 wt%, adding NaOH to adjust the pH value to 9-10, and adding the aqueous solution of ethylenediamine to the emulsion; and

[0023] S13: Control the reaction temperature to 50-60°C, the stirring speed to 300-500 rpm, and the reaction time to 2-4 hours. The size of the prepared organic microcapsule particles is 2-5 μm, the particle size of the paraffin is 1.80-4.95 μm, and the radial thickness of the polyurethane is 0.05-0.20 μm.

[0024] In some optional preparation methods, the ceramic coating slurry is prepared and sprayed as follows:

[0025] S21: adding 75-90 wt% of aluminum oxide, 7-12 wt% of the organic microcapsule particles, 2-8 wt% of polyacrylate, and 1-5 wt% of sodium carboxymethyl cellulose in a mass ratio, adding solvent water, and stirring at 25-35° C. and a speed of 800-1200 rpm to prepare the ceramic coating slurry with a solid content of 20%; and

[0026] S22: Take the base film and use a step-by-step coating process to spray the ceramic coating slurry onto the surface of the base film, wherein the electrostatic spraying parameters are set to: voltage 10-15kV, needle inner diameter 0.3-0.5mm, spraying distance 20-30cm, injection speed 1-3mL / min, and the spraying angle is reduced from 45° to 30° during the spraying process.

[0027] In some optional preparation methods, the deposition of the interface-modifying polymer further comprises the following steps:

[0028] S41: Dissolve dopamine in Tris buffer and adjust the pH to 8.5;

[0029] S42: immersing the base film coated with the ceramic coating slurry in the Tris buffer, controlling the reaction temperature to 25-35° C. and the immersion time to 30-60 seconds, to deposit the interface layer having a thickness of 0.05-0.20 μm; and

[0030] S43: placing the base film after the interface modification polymer deposition at 40° C. and drying it by hot air circulation for 5-10 minutes to obtain the separator.

[0031] A third aspect of the present application provides a lithium-ion battery, comprising a negative electrode sheet, a separator, a positive electrode sheet, and an electrolyte, wherein the separator is the separator according to any one of the above items.

[0032] This application has at least the following technical effects:

[0033] 1) The first aspect of the present application provides a diaphragm, comprising a base film and a composite ceramic coating coated on at least one side of the base film, wherein the composite ceramic coating is composed of a mixture of inorganic ceramic particles and organic microcapsule particles, and the organic microcapsule particles are a core-shell structure in which a polymer shell coats a core material, wherein the melting point range of the core material is 55-65°C, and the polymer shell is selected from at least one of polyurethane, polyurea, polyamide and polycarbonate. The above-mentioned highly elastic and tough polymer shell is used to coat the core material to improve the impact resistance of the organic microcapsule particles and reduce the breakage rate during processing. At the initial stage of thermal runaway at 60-80°C, the core material melts and seeps out of the polymer shell, thereby sealing the gaps between the particles in the composite ceramic coating. It has been calculated that the closed-cell efficiency of the organic microcapsule particles can reach 92%. An interface layer composed of an interface-modified polymer is also deposited on the surface of the composite ceramic coating. The interface-modified polymer is selected from at least one of: polycatechol derivatives, polyaniline, polyvinylpyridine, polyacrylic acid and polyacrylamide. The porous structure formed by the interface-modified polymer and the organic microcapsule particles synergistically form a two-stage effect, amplifying the capillary effect, greatly improving the liquid absorption rate of the diaphragm, and optimizing the electrolyte wettability; in addition, the above-mentioned interface-modified polymer can also form coordination bonds with the metal particles in the inorganic ceramic particles, thereby enhancing the bonding force between the interface layer and the composite ceramic coating through strong intermolecular interactions, improving the peeling strength between the interface layer and the composite ceramic coating as a whole, and improving the structural stability of the diaphragm.

[0034] 2) The second aspect of the present application provides a method for preparing a diaphragm, in which a polymer shell is formed on the surface of a core material by interfacial polymerization to obtain organic microcapsule particles, inorganic ceramic particles and organic microcapsule particles are mixed in proportion to form a ceramic coating slurry, the electrostatic spraying parameters are controlled, the ceramic coating slurry is coated on the surface of the base membrane, and then in situ polymerization modification is performed, and the interfacial modified polymer is deposited on the surface of the ceramic coating slurry to form an interface layer to avoid interface defects caused by synchronous coating.

[0035] 3) The third aspect of the present application provides a lithium-ion battery comprising a negative electrode sheet, a diaphragm, a positive electrode sheet and an electrolyte. The diaphragm is any of the diaphragms described above, which can actively close the pores in the early stage of thermal runaway to ensure safety performance. The multi-layer structure of the diaphragm has strong bonding ability, high mechanical strength and good structural stability. DETAILED DESCRIPTION

[0036] The following describes in detail embodiments of this embodiment. In this description, "several" means one or more, "many" means more than two, "greater than," "less than," and "exceed" are understood to exclude the number itself, while "above," "below," and "within" are understood to include the number itself. The terms "first" and "second" are used solely to distinguish technical features and are not to be construed as indicating or implying relative importance, or as implicitly specifying the number or order of the technical features.

[0037] In the description of this embodiment, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in this embodiment based on the specific content of the technical solution.

[0038] The common structure of existing lithium-ion batteries includes a casing with an opening at one end, a winding core assembled into the casing through the opening, an electrolyte injected into the casing, and a cap positioned over the casing opening. It is understood that the lithium-ion battery can be a cylindrical lithium-ion battery or a prismatic lithium-ion battery.

[0039] The above-mentioned core is formed by winding a stacked positive electrode sheet, a separator and a negative electrode sheet. The positive electrode sheet includes a positive electrode collector and a positive electrode active material layer coated on both sides of the positive electrode collector. The negative electrode sheet includes a negative electrode collector and a negative electrode active material layer coated on both sides of the negative electrode collector. The thickness of the active material coating is greater than the thickness of the current collector. Lithium ions complete the conversion between chemical energy and electrical energy by performing insertion or deinsertion reactions on the surfaces of the positive electrode active material layer and the negative electrode active material layer.

[0040] The present application provides a diaphragm, which includes a base film, a composite ceramic coating coated on at least one side of the base film, and an interface layer covered on the surface of the composite ceramic coating. The composite ceramic coating contains inorganic ceramic particles and organic microcapsule particles. The organic microcapsule particles include a core material with a melting point range of 55-65°C and a polymer shell coated on the surface of the core material. The polymer shell is selected from: at least one of polyurethane, polyurea, polyamide and polycarbonate. The interface layer is formed by depositing an interface-modified polymer on the surface of the composite ceramic coating. The interface-modified polymer is selected from: at least one of polycatechol derivatives, polyaniline, polyvinylpyridine, polyacrylic acid and polyacrylamide.

[0041] The core material is coated with the highly elastic and tough polymer shell, improving the impact resistance of the organic microcapsule particles and reducing breakage during processing. At the initial stage of thermal runaway at 60-80°C, the core material melts and seeps out of the polymer shell, sealing the interparticle gaps in the composite ceramic coating. Measurements show that the organic microcapsule particles have a closed-cell efficiency of up to 92%. The composite ceramic coating also has an interfacial layer deposited on its surface composed of an interface-modifying polymer selected from at least one of polycatechol derivatives, polyaniline, polyvinylpyridine, polyacrylic acid, and polyacrylamide. The porous structure formed by the interface-modifying polymer synergizes with the organic microcapsule particles to form a two-stage adsorption effect, amplifying capillary action, significantly increasing the liquid absorption rate of the diaphragm, and optimizing electrolyte wettability. Furthermore, the interface-modifying polymer can form coordination bonds with the metal particles in the inorganic ceramic particles, enhancing the bonding between the interface layer and the composite ceramic coating through strong intermolecular interactions, improving the peel strength between the interface layer and the composite ceramic coating as a whole, and enhancing the structural stability of the diaphragm.

[0042] Furthermore, the core material is a paraffin material, and the paraffin material comprises any one or a combination of multiple alkane compounds with a main chain carbon number of 16-30. Specifically, the main chain formula of the alkane compound is C n H 2n+2 , 16≤n≤30, it can be understood that the alkane compound may further contain a branched chain.

[0043] For example, polyurethane can be prepared by stirring molten paraffin and isocyanate prepolymer under the action of an emulsifier to polymerize the polyurethane interface on the surface of the paraffin molecule; polyurea can be prepared by heating paraffin to melt and mixing it with toluene-2,4-diisocyanate (TDI), dissolving it in cyclohexane, adding a non-ionic emulsifier (such as OP-10) to form a stable oil-in-water (O / W) emulsion, adding an amine monomer (such as ethylenediamine, EDA), adjusting the pH to 8.5-9.0, and allowing TDI (oil phase) and EDA (water phase) to undergo a condensation reaction at the oil-water interface to form a polyurea shell layer covering the paraffin core material; polyamide can be prepared by mixing paraffin to melt and mixing it with toluene-2,4-diisocyanate (TDI), dissolving it in cyclohexane, adding a non-ionic emulsifier (such as OP-10), forming a stable oil-in-water (O / W) emulsion, adding an amine monomer (such as ethylenediamine, EDA), adjusting the pH to 8.5-9.0, and allowing TDI (oil phase) and EDA (water phase) to undergo a condensation reaction at the oil-water interface to form a polyurea shell layer covering the paraffin core material. After the wax is heated to melt, it is mixed with an acyl chloride monomer (such as trimesoyl chloride or terephthaloyl chloride), dissolved in an organic solvent such as cyclohexane or toluene, and a non-ionic emulsifier (such as OP-10 or Span-80) and an amine monomer (such as ethylenediamine or piperazine) are added, and dissolved in a weak alkaline buffer solution with a pH of 8.5-9.0. The acyl chloride monomer (oil phase) and the amine monomer (water phase) undergo a condensation reaction at the oil-water interface to form a polyamide shell layer covering the paraffin core material; and polycarbonate can be prepared by heating paraffin wax to melt and mixing it with diphenyl carbonate as an organic phase, adding a non-ionic surfactant and bisphenol A (BPA) and dissolving it in a weak alkaline buffer solution with a pH of 8.5-9.0. In a weak alkaline buffer solution of pH 8.5-9.0, diphenyl carbonate and bisphenol A undergo ester exchange and polycondensation on the surface of molten paraffin to generate polycarbonate coated with a paraffin core material; polycatechol derivatives can be prepared by dissolving catechol monomers in a Tris buffer solution, immersing the base film coated with the ceramic coating slurry in the Tris buffer solution, and depositing an interface-modified polymer on the surface of the ceramic coating to form an interface layer; polyaniline can be prepared by dissolving aniline in an acidic solution, immersing the base film coated with the ceramic coating slurry in an aniline monomer solution, and slowly adding an oxidant (such as ammonium persulfate APS, hydrogen peroxide) at a low temperature, stirring the reaction, and forming an interface layer; polyvinylpyridine can be prepared by dissolving 4-vinylpyridine (4-VP) in a pH 8.5-9.0 solution. 2-3 acidic aqueous solution, protonate pyridine groups, immerse the base membrane coated with the ceramic coating slurry in a 4-VP monomer solution, add an oxidant (such as ammonium persulfate APS) dropwise at low temperature, stir and react to form an interface layer; polyacrylic acid and polyacrylamide can be easily reacted with acrylic acid monomer or acrylamide monomer in water, add ammonium persulfate APS and tetramethylethylenediamine to form a redox system, evenly apply the monomer solution to the surface of the ceramic coating, heat to 60-80°C, react for 30-60 minutes, and thermally decompose APS to generate free radicals to initiate polymerization to form an interface layer, wherein the composite ceramic coating is bonded to the carboxyl group of the polyacrylic acid through hydrogen bonds or amide bonds, and the amino group of the polyacrylamide forms hydrogen bonds with the hydroxyl group on the surface of the composite ceramic coating. It can be understood that the polymer shell also provides active sites for the polymerization reaction of the interface modification polymer, and the synergistic effect of the two further enhances the stability of the multilayer structure of the diaphragm.

[0044] In a specific implementation process, the core material may also be a mixture of paraffin and graphite / graphene, polyethylene glycol or fatty acid eutectic with a melting point in the range of 55-65°C.

[0045] In some optional embodiments, the inorganic ceramic particles have a particle size (Dv50) of 0.2-0.8 μm, the organic microcapsule particles have a particle size of 2-5 μm, the core material has a particle size of 1.80-4.95 μm, and the polymer shell has a radial thickness of 0.05-0.20 μm. With these structural dimensions, the organic microcapsule particles have an optimal surface area / volume ratio, shortening the release time after paraffin wax melts; the shell also has a certain thickness to prevent the inorganic ceramic particles from being crushed.

[0046] In some optional embodiments, the base film is a polyethylene film or a polypropylene film, and has a thickness of 4-14 μm. The thickness D of the composite ceramic coating is 3-5 μm, and the embedding depth d of the organic microcapsule particles from the outer surface of the composite ceramic coating to the interior thereof satisfies the following condition: 1 / 3D ≤ d ≤ D. Using the aforementioned structural dimensions can reduce the shell thickness, minimize space usage, and improve the utilization rate of the number of organic microcapsule particle layers.

[0047] Furthermore, the doping density of the organic microcapsules varies in a gradient across the thickness of the composite ceramic coating, with the doping density of the organic microcapsules closer to the base membrane being lower than that of the organic microcapsules farther from the base membrane. This gradient in doping density creates a surface-enriched distribution, facilitating a rapid thermal response to temperature changes within the battery system, effectively curbing thermal runaway and reducing thermal shrinkage of the separator.

[0048] Furthermore, the embedding depth d of the organic microcapsule particles is divided into d near the base film and d 内 and away from the basement membrane 表 The doping density ρ of the organic microcapsule particles in the embedding depth direction satisfies: 1.2≤ρ d表 / ρ d内 ≤1.4, the d 表 The range is: 1 / 4d-1 / 3d. The doping density gradient of the organic microcapsules is designed to vary by 20-40%. The surface-enriched organic microcapsules preferentially respond to thermal changes within the battery system, shortening the time it takes to trigger pore closure. At the same time, the gradient distribution creates a capillary effect, which helps improve the liquid absorption of the separator and reduce the internal resistance of the composite ceramic coating.

[0049] In some optional embodiments, the composite ceramic coating further comprises a binder, wherein the mass percentage of the inorganic ceramic particles is 70-90%, the mass percentage of the binder is 1-15%, and the mass percentage of the organic microcapsule particles is 5-15%. The addition of the binder helps to enhance the bonding strength between the base film and the composite ceramic coating, ensuring the stability of the multi-layer structure of the separator.

[0050] In some optional embodiments, the inorganic ceramic particles are selected from at least one of alumina, boehmite, and titanium dioxide, the binder is polyacrylate and / or polyvinylidene fluoride; the organic microcapsule particles include a core material composed of paraffin wax and a polymer shell composed of polyurethane, and the interface modification polymer is polydopamine. The organic microcapsule particles composed of a polyurethane-coated paraffin wax core have good impact resistance and are not easily broken during the slurry preparation and spraying process, thus avoiding premature leakage of paraffin wax; the -NH2 in polydopamine can react with Al3+ or Ti in the inorganic ceramic particles. 4+ A coordination effect is formed to enhance the bonding force between the interface layer and the composite ceramic coating.

[0051] Furthermore, the interface layer formed by the deposition of polydopamine on the surface of the composite ceramic coating has a porous structure with a pore size of 5-20 nm, and the specific surface area of the organic microcapsule particles is greater than 200 m 2 / g, the adhesion between the interface layer and the ceramic coating is ≥2.5N / m. In the specific implementation process, the adhesion test is carried out by fixing the two sides of the composite diaphragm on a fixed fixture and a movable fixture respectively with adhesive tape, initially stretching it in the reverse direction at 1N / m force at 180°C, and increasing it by 0.1N / m each time until the interface layer, the composite ceramic coating and the base film are delaminated and peeled off, and the force applied during the peeling is measured as the adhesion between the corresponding two separated structures. It can be understood that in actual tests, the composite ceramic coating and the base film will be separated before the interface layer and the composite ceramic coating. When the adhesive tape is not detached, it can be equated that the adhesion between the interface layer composed of the interface modified polymer and the composite ceramic coating will be greater than or equal to the adhesion between the composite ceramic coating and the base film, and the stronger the bonding between the interface layer and the composite ceramic coating, the more beneficial it is to the puncture resistance of the diaphragm, and the further improvement of the safety of the battery.

[0052] A second aspect of the present application provides a method for preparing a diaphragm, wherein the diaphragm is the diaphragm according to any one of the above items, and the preparation method comprises the following steps:

[0053] S1: using interfacial polymerization to polymerize on the surface of the core material to form the polymer shell, thereby preparing microcapsule particles;

[0054] S2: mixing the inorganic ceramic particles, the organic microcapsule particles, the binder, and the solvent in a predetermined ratio, adjusting the solid content to 5-40%, and preparing a ceramic coating slurry;

[0055] S3: Take the base film and spray the ceramic coating slurry onto the surface of the base film using a step-by-step coating process, wherein the electrostatic spraying parameters are set to: voltage 10-15 kV, needle inner diameter 0.3-0.5 mm, spraying distance 20-30 cm; and

[0056] S4: placing the base film sprayed with the ceramic coating slurry in a reaction container fed with the interface modification polymer monomer, depositing the interface modification polymer on the surface of the ceramic coating slurry, and drying and curing the membrane after the deposition is completed to obtain the membrane.

[0057] Furthermore, the distributed coating process controls the doping density of the organic microcapsule particles in the composite ceramic coating by adjusting the spraying angle. The spraying angle range is: 30°-45°, and the spraying angle changes from large to small during the spraying process. Specifically, during the spraying process, different spraying angles can be set in stages according to different thickness ranges, or a gradual change angle can be set, gradually decreasing from 45° to 30°. It can be understood that when the spraying angle is large, the horizontal velocity component of the organic microcapsule particles increases, and some particles fail to turn in time due to inertia, and the deposition density is small. At this time, the coating particles will be more evenly distributed, the contact surface with the base film will be larger, and the bonding force will be stronger. When the spraying angle is small, the electric field lines are more concentrated, and the charged organic microcapsule particles are accelerated by the electric field force to move perpendicular to the base film surface, and the deposition density is significantly improved, completing the surface enrichment distribution design of the organic microcapsule particles.

[0058] Furthermore, the base film is a polyethylene film or a polypropylene film with a porosity of 30-45%, the inorganic ceramic particles are alumina, the binder is polyacrylate, the organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane, the interface-modifying polymer is polydopamine, and the preparation method of the organic microcapsule particles comprises the following steps:

[0059] S11: Mix molten paraffin at 65° C. and isocyanate prepolymer (HDI / IPDI) in a mass ratio of 1:(0.3-0.5), add 3 wt% of the oil phase reactant emulsifier Span 80, set the stirring rate to 800-1200 rpm, and stir and emulsify for 30 minutes to prepare an emulsion;

[0060] S12: preparing an aqueous solution of chain extender ethylenediamine (EDA) with a concentration of 2-5 wt%, adding NaOH to adjust the pH value to 9-10, and adding the EDA aqueous solution to the emulsion; and

[0061] S13: Control the reaction temperature to 50-60°C, the stirring speed to 300-500 rpm, and the reaction time to 2-4 hours. The size of the prepared organic microcapsule particles is 2-5 μm, the particle size of the paraffin is 1.80-4.95 μm, and the radial thickness of the polyurethane is 0.05-0.20 μm.

[0062] Furthermore, the preparation and spraying method of the ceramic coating slurry is as follows:

[0063] S21: adding 75-90 wt% of aluminum oxide, 7-12 wt% of the organic microcapsule particles, 2-8 wt% of polyacrylate, and 1-5 wt% of sodium carboxymethyl cellulose (CMC) in a mass ratio, adding solvent water, and stirring at 25-35° C. and a speed of 800-1200 rpm to prepare the ceramic coating slurry with a solid content of 20%; and

[0064] S22: Take the base film and use a step-by-step coating process to spray the ceramic coating slurry onto the surface of the base film, wherein the electrostatic spraying parameters are set to: voltage 10-15kV, needle inner diameter 0.3-0.5mm, spraying distance 20-30cm, injection speed 1-3mL / min, and the spraying angle is reduced from 45° to 30° during the spraying process.

[0065] Furthermore, the deposition of the interface modification polymer further comprises the following steps:

[0066] S41: Dissolve dopamine in Tris buffer and adjust the pH to 8.5;

[0067] S42: Immerse the base membrane coated with the ceramic coating slurry in the Tris buffer, control the reaction temperature to 25-35°C, and the immersion time to 30-60s, and deposit the interface layer with a thickness of 0.05-0.20μm; and; S43: Place the base membrane with the completed interface modification polymer deposition at 40°C and dry it with hot air circulation for 5-10 minutes to obtain the diaphragm.

[0068] In the specific implementation process, the number of organic microcapsule particles in the thickness direction of the composite ceramic coating was statistically obtained by SEM-EDS analysis of the diaphragm cross-section; the interface modified polymer was characterized by infrared spectroscopy (FTIR). Taking polydopamine as an example, the infrared spectrum of polydopamine peak had characteristic absorption peaks at 1600cm-1 (C=C aromatic ring vibration) and 3200cm-1 (NH stretching vibration).

[0069] The third aspect of the present application provides a lithium-ion battery, comprising a negative electrode sheet, a separator, a positive electrode sheet and an electrolyte. The separator is a separator according to any of the above items, which can actively close the pores in the early stage of thermal runaway to ensure safety performance. The multi-layer structure of the separator has strong bonding ability, high mechanical strength and good structural stability.

[0070] The technical solution of the present application is described below with reference to Examples 1-13 and Comparative Examples 1-9.

[0071] Example 1

[0072] Embodiment 1 provides a lithium-ion battery, comprising:

[0073] Electrolyte, including: lithium salt, solvent and additives;

[0074] A winding core is formed by winding a stacked positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer formed on at least one side of the positive electrode collector; and the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer formed on at least one side of the negative electrode collector.

[0075] The diaphragm includes a base film, a composite ceramic coating, and an interface layer. The base film is a PE film or PP film with a porosity of 30-45%. The inorganic ceramic particles are alumina. The binder is polyacrylate. The organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane. The interface modification polymer is polydopamine. Specifically, the preparation method of the organic microcapsule particles is as follows:

[0076] S11: Mixing molten paraffin at 65° C. with isocyanate prepolymer (HDI / IPDI) in a mass ratio of 1:0.4, adding 3% of emulsifier Span 80, setting the stirring speed to 800-1200 rpm, and stirring and emulsifying for 30 minutes to prepare an emulsion;

[0077] S12: preparing a 3 wt % aqueous solution of chain extender ethylenediamine (EDA), adding NaOH to adjust the pH value to 10, and adding the EDA aqueous solution to the emulsion; and

[0078] S13: The reaction temperature is controlled to be 65° C., the stirring speed is 500 rpm, and the reaction time is 3 h. The size of the prepared organic microcapsule particles is: particle size 3 μm, radial thickness of polyurethane is 0.10 μm, and particle size of paraffin is 2.90 μm.

[0079] The preparation and spraying method of the ceramic coating slurry is as follows:

[0080] S21: adding 85 wt% of aluminum oxide, 10 wt% of the organic microcapsule particles, 3.5 wt% of polyacrylate, and 1.5 wt% of sodium carboxymethyl cellulose in a mass ratio, adding solvent water, and stirring at 25° C. at a speed of 1200 rpm to prepare the ceramic coating slurry with a solid content of 20%; and

[0081] S22: Take the base film, fill the ceramic coating slurry in an electrostatic spraying device, and set the electrostatic spraying parameters: voltage 15 kV, needle inner diameter 0.4 mm, spraying distance 30 cm, injection speed 1.5 mL / min; and

[0082] S23: The ceramic coating slurry is sprayed onto the surface of the base film using a step-by-step coating process, wherein the spraying thickness d is set to 4 μm, and the 3 μm thick composite ceramic coating 3 / 4d is first sprayed at a 45° spraying angle, and then the 1 μm thick 1 / 4d close to the surface is sprayed at a 30° spraying angle, so that the organic microcapsule particle doping density ρ in the area away from the base film is d表 The doping density of organic microcapsules near the base film area ρ d内 Satisfy: ρ d表 / ρ d内 ≈1.3.

[0083] The method for preparing the interface layer further comprises the following steps:

[0084] S41: Dissolve dopamine in Tris buffer and adjust the pH to 8.5;

[0085] S42: immersing the base film coated with the ceramic coating slurry in the Tris buffer, controlling the reaction temperature to 25° C. and the immersion time to 60 seconds, to deposit the interface layer with a thickness of 0.10 μm; and

[0086] S43: placing the base film after the interface modification polymer deposition at 40° C. and drying it by hot air circulation for 5-10 minutes to obtain the separator.

[0087] Production of positive electrode:

[0088] The positive electrode active material, conductive carbon black, dispersant and polyvinylidene fluoride (PVDF) were mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96.5:1.5:0.3:1.7 to obtain a positive electrode slurry with a solid content of 65%. The positive electrode slurry was then coated on a 12.0 μm thick aluminum foil (positive electrode current collector). After drying and cold pressing, a positive electrode sheet was obtained. The single-side surface density of the electrode sheet was 15 mg / cm 2 , the pole piece compaction is 2.75g / cm 3 ;

[0089] Production of negative electrode:

[0090] The negative electrode sheet includes a negative electrode current collector copper foil and a negative electrode slurry coated on both sides of the copper foil. Calculated by mass percentage, the negative electrode slurry includes 96.0% silicon-carbon material (Si content 10.0wt%), 1.5% carbon nanotubes, 1.0% thickener sodium carboxymethyl cellulose (CMC), and 1.5% negative electrode binder polyacrylic acid (PAA). The above substances are added to deionized water and stirred to form a negative electrode slurry with a solid content of 40%. The negative electrode slurry is then coated on both sides of the copper foil, dried, and cold pressed to form a negative electrode sheet with a compaction density of 1.5g / cm 3 ;

[0091] Preparation of electrolyte:

[0092] Lithium hexafluorophosphate is mixed with an organic solvent to obtain an electrolyte, wherein the concentration of lithium hexafluorophosphate in the electrolyte is 1.0 mol / L, and the organic solvent includes components in the following ratio: ethylene carbonate (EC): fluoroethylene carbonate (FEC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 15:15:20:50.

[0093] Lithium-ion battery assembly:

[0094] The positive and negative electrode sheets are rolled and slit, respectively, and then wound together with the separator to obtain a 21700 cylindrical battery core. The battery core is then welded to the connecting sheet and loaded into the battery casing. After completing the electrolyte injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The casing of the lithium-ion battery is cylindrical, and its dimensional parameters are: diameter: 16-55 mm, and length: 63-140 mm.

[0095] Example 2

[0096] Example 2 provides a lithium ion battery. The difference between Example 2 and Example 1 is that the particle size of the organic microcapsule particles is 2 μm, and the other conditions are the same as those of Example 1.

[0097] Example 3

[0098] Example 3 provides a lithium-ion battery. The difference between Example 3 and Example 1 is that the particle size of the organic microcapsule particles is 4 μm, and the other conditions are the same as those of Example 1.

[0099] Example 4

[0100] Example 4 provides a lithium ion battery. The difference between Example 4 and Example 1 is that: the organic microcapsule particles have a d表 / ρ d内 ≈1.2, and the other conditions are the same as in Example 1.

[0101] Example 5

[0102] Example 5 provides a lithium ion battery. The difference between Example 5 and Example 1 is that: the organic microcapsule particles have a d表 / ρ d内 ≈1.4, and the other conditions are the same as those in Example 1.

[0103] Example 6

[0104] Example 6 provides a lithium ion battery. The difference between Example 6 and Example 1 is that the thickness of the interface layer is 0.08 μm, and the other conditions are the same as those of Example 1.

[0105] Example 7

[0106] Example 7 provides a lithium ion battery. The difference between Example 7 and Example 1 is that the thickness of the interface layer is 0.15 μm, and the other conditions are the same as those of Example 1.

[0107] Example 8

[0108] Example 8 provides a lithium-ion battery. The difference between Example 8 and Example 1 is that boehmite is used as the inorganic ceramic particles, and the other conditions are the same as those of Example 1.

[0109] Example 9

[0110] Example 9 provides a lithium-ion battery. The difference between Example 9 and Example 1 is that titanium dioxide is used as inorganic ceramic particles, and the other conditions are the same as those of Example 1.

[0111] Example 10

[0112] Example 10 provides a lithium-ion battery. The difference between Example 10 and Example 1 is that the ratio of aluminum oxide: organic microcapsule particles: polyacrylate: CMC is 70:10:11:9, and the other conditions are the same as those in Example 1.

[0113] Example 11

[0114] Example 11 provides a lithium-ion battery. The difference between Example 11 and Example 1 is that the ratio of aluminum oxide: organic microcapsule particles: polyacrylate: CMC is 88:10:1:1, and the other conditions are the same as those in Example 1.

[0115] Example 12

[0116] Example 12 provides a lithium-ion battery. The difference between Example 12 and Example 1 is that the thickness of the composite ceramic coating is 3 μm, and the other conditions are the same as those of Example 1.

[0117] Example 13

[0118] Example 13 provides a lithium-ion battery. The difference between Example 13 and Example 1 is that the thickness of the composite ceramic coating is 6 μm, and the other conditions are the same as those of Example 1.

[0119] Comparative Example 1

[0120] Comparative Example 1 provides a lithium ion battery. The difference between Comparative Example 1 and Example 1 is that the particle size of the organic microcapsule particles is 1 μm, and the other conditions are the same as those of Example 1.

[0121] Comparative Example 2

[0122] Comparative Example 2 provides a lithium ion battery. The difference between Comparative Example 2 and Example 1 is that the particle size of the organic microcapsule particles is 5 μm, and the other conditions are the same as those of Example 1.

[0123] Comparative Example 3

[0124] Comparative Example 3 provides a lithium ion battery. The difference between Comparative Example 3 and Example 1 is that the organic microcapsule particles have a d 表 / d 内 ≈1.1, and the other conditions are the same as in Example 1.

[0125] Comparative Example 4

[0126] Comparative Example 4 provides a lithium ion battery. The difference between Comparative Example 4 and Example 1 is that the thickness of the interface layer is 0.05 μm, and the other conditions are the same as those of Example 1.

[0127] Comparative Example 5

[0128] Comparative Example 5 provides a lithium ion battery. The difference between Comparative Example 5 and Example 1 is that the thickness of the interface layer is 0.20 μm, and the other conditions are the same as those of Example 1.

[0129] Comparative Example 6

[0130] Comparative Example 6 provides a lithium-ion battery. The difference between Comparative Example 6 and Example 1 is that the inorganic ceramic particles are calcium carbonate, and the other conditions are the same as those in Example 1.

[0131] Comparative Example 7

[0132] Comparative Example 7 provides a lithium-ion battery. The difference between Comparative Example 7 and Example 1 is that the ratio of aluminum oxide: organic microcapsule particles: polyacrylate: CMC is 60:10:16:14, and the other conditions are the same as those in Example 1.

[0133] Comparative Example 8

[0134] Comparative Example 8 provides a lithium ion battery. The difference between Comparative Example 8 and Example 1 is that the thickness of the composite ceramic coating is 1 μm, and the other conditions are the same as those of Example 1.

[0135] Comparative Example 9

[0136] Comparative Example 9 provides a lithium ion battery. The difference between Comparative Example 9 and Example 1 is that the thickness of the ceramic layer is 13 μm, and the other conditions are the same as those of Example 1.

[0137] The following tests were performed on Examples 1-13 and Comparative Examples 1-9:

[0138] (1) Diaphragm closed-cell efficiency: Differential scanning calorimetry (DSC), refer to GB / T 19466.3-2004.

[0139] Instrument: Differential Scanning Calorimeter

[0140] Method: Take a 10-15 mg sample of the diaphragm and seal it in an aluminum crucible. Ramp the temperature at a rate of 5-10°C / min under a nitrogen atmosphere over a temperature range of 30-150°C. Record the onset temperature and peak area of the endothermic peak (paraffin melting, approximately 60°C) and the exothermic peak (pore closure, excluding the endothermic peak). Pore closure efficiency (%) = (exothermic peak area / theoretical paraffin melting enthalpy) x 100%, where the theoretical paraffin melting enthalpy is 220 J / g for a C22-C28 alkane mixture.

[0141] (2) Liquid absorption test method: Cut a 50 mm × 50 mm separator sample and accurately weigh m0; immerse it in an electrolyte (1 M LiPF6, EC:DMC:EMC = 1:1:1) and let it stand at 25°C for 1 hour. After removing it, hang it vertically for 30 seconds to drain it, and weigh the saturated mass m.

[0142] Calculation formula:

[0143] Liquid absorption rate (%) = [(m-m0) / m0] × 100%

[0144] (3) Test method for thermal shrinkage: Cut the diaphragm into 120 mm / 100 mm sizes according to the machine direction / transverse direction (MD / TD), cover the surface of the diaphragm with a piece of A4 paper, and let it stand at 180°C for 1 hour. Measure the shrinkage of the diaphragm in each direction and calculate the shrinkage ratio.

[0145] (4) Peel strength test method: Cut a 40 mm × 100 mm diaphragm, fix the two sides of the composite diaphragm on a fixed fixture and a movable fixture respectively with tape, initially stretch it in the reverse direction at 180°C with a force of 1 N / m, and increase the force by 0.1 N / m each time until the composite ceramic coating and the base film are peeled off, and measure the force applied during peeling.

[0146] (5) Test method for effectiveness of thermal runaway prevention: Charge the battery to 4.3V using a constant current charge rate of 0.33C, then charge to the charge cut-off voltage using a constant voltage charge rate of 0.05C. Fully charge the battery, place it in a 70°C temperature box after leaving it for 30 minutes, and then place it in a 70°C temperature box. If the battery voltage drops (>0.5V) within 24 hours, it is judged that the thermal runaway is invalid and is recorded as N. Otherwise, it is judged to be valid and is recorded as Y.

[0147] The results of closed cell efficiency, liquid absorption rate, thermal shrinkage rate, coating peel strength, and thermal runaway temperature of Examples 1-13 and Comparative Examples 1-9 of the present application are shown in the following table:

[0148]

[0149]

[0150] It can be seen from the results of Examples 1-3 and Comparative Examples 1-2 that Example 1 has the best overall performance. This is because the particle size of the organic microcapsule particles is within a certain range and is more suitable for the thickness of the composite ceramic coating. If the particle size is too small (<2 μm), it will cause insufficient embedding in the composite ceramic coating, and if it is too large (>5 μm), it will destroy the uniformity of the coating. Organic microcapsule particles with a particle size of 2-5 μm have an advantage in surface area to volume ratio, and can be quickly released through the micropores on the polymer shell when the paraffin core material melts. At the same time, considering the mechanical strength dimension, if the particle size of the organic microcapsule particles is too small (<2 μm), it is easy to be squeezed and broken by the inorganic ceramic particles during mixing, and if it is too large (>5 μm), the density of the composite ceramic coating will be reduced.

[0151] The results of Examples 1, 4-5, and Comparative Example 3 demonstrate that a surface-enriched distribution of the doping density of the organic microcapsule particles is beneficial for improving separator performance. Under a specific density gradient design, the separator responds more quickly to thermal runaway events: the surface-enriched organic microcapsule particles preferentially respond to thermal changes in the battery system, shortening the closed-cell triggering time by 30%. Furthermore, the gradient distribution creates a capillary effect, which helps improve the separator's liquid absorption rate.

[0152] From the results of Examples 1, 6-7 and Comparative Examples 4-5, it can be seen that the thickness of the interface layer also has a close influence on the performance of the diaphragm. This is because within a certain thickness range, the thickness of the interface layer can balance the mechanical strength and the paraffin release efficiency. At the same time, at the interface of the composite ceramic coating, the -NH2 groups in the interface layer and the Al2O3 groups in the ceramic particles are closely related. 3+ The formation of coordination bonds can improve the peel strength of the composite ceramic coating.

[0153] The results of Examples 1, 8, and 9, compared to Comparative Example 6, show that the membrane using calcium carbonate as the inorganic ceramic particles exhibits an extremely low liquid absorption rate of only 115%, leading to a direct failure of thermal runaway management. This is because calcium carbonate reacts with the electrolyte and lacks stability. Based on the different material properties, the inorganic ceramic particles used in Examples 1, 8, and 9 exhibit the following characteristics:

[0154] Al2O3: High specific surface area improves liquid absorption rate and can form hydrogen bonds with the hydroxyl groups of polydopamine;

[0155] Boehmite: The flake structure can enhance the puncture resistance of the coating;

[0156] TiO2: Nano-sized particles can fill large pores and reduce the thermal shrinkage of the diaphragm.

[0157] The results of Examples 1, 10-11, and Comparative Example 7 show that the mass percentage of inorganic ceramic particles in the composite ceramic coating significantly affects thermal shrinkage and liquid absorption. This is because a high percentage of inorganic ceramic particles can form a continuous ceramic skeleton, reducing the thermal shrinkage of the separator. However, an excessively high percentage of inorganic ceramic particles can reduce the separator's porosity, resulting in poor electrolyte wetting and a drop in liquid absorption from 160% to 128%.

[0158] From the results of Examples 1, 12-13 and Comparative Examples 8-9, it can be seen that the thickness of the composite ceramic coating has a close influence on the liquid absorption rate, closed-cell efficiency, and thermal shrinkage performance of the diaphragm. This is because: ① When the composite ceramic coating is too thin, the porosity is large, the electrolyte penetrates quickly, but the liquid retention capacity is weak, and the liquid absorption capacity decreases; ② When the composite ceramic coating is too thin, the embedding depth of the organic microcapsule particles is closer to the surface, the closed-cell triggering time is short during thermal response, but the mechanical support is insufficient, and the structure is prone to collapse after the closed-cell, resulting in thermal runaway thermal management failure. When the composite ceramic coating is too thick, the core material release is delayed, reducing safety performance. ③ When the composite ceramic coating is too thin, the thermal shrinkage rate is high due to insufficient filling of ceramic particles, while when the composite ceramic coating is too thick, the internal stress accumulation will cause local cracking, affecting performance. Therefore, it is necessary to select a ceramic coating of appropriate thickness to balance safety performance and electrical performance when used in the battery.

[0159] In summary, the lithium-ion battery provided in this application applies a specially functional modified diaphragm to the lithium-ion battery. The modified diaphragm undergoes multi-level optimization in safety performance such as thermal closure, liquid absorption rate, and mechanical strength, which can enable the battery to internally short-circuit at a lower temperature, prevent thermal runaway caused by short circuit, and effectively improve the safety performance of the battery.

[0160] Throughout this specification, references to "some embodiments," "one example," or similar descriptions indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment or example. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0161] Although examples of the present embodiment have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and intent of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A diaphragm, characterized in that: The diaphragm includes a base film, a composite ceramic coating coated on at least one side of the base film, and an interface layer covered on the surface of the composite ceramic coating. The composite ceramic coating contains inorganic ceramic particles and organic microcapsule particles. The organic microcapsule particles include a core material with a melting point range of 55-65°C and a polymer shell coated on the surface of the core material. The polymer shell is selected from: at least one of polyurethane, polyurea, polyamide and polycarbonate. The interface layer is formed by depositing an interface-modified polymer on the surface of the composite ceramic coating. The interface-modified polymer is selected from: at least one of polycatechol derivatives, polyaniline, polyvinylpyridine, polyacrylic acid and polyacrylamide.

2. The diaphragm according to claim 1, characterized in that The core material is a paraffin material, and the paraffin material contains any one or more alkane compounds with a main chain carbon atom number of 16-30.

3. The diaphragm according to claim 1, characterized in that The particle size Dv50 of the inorganic ceramic particles is 0.2-0.8 μm, the particle size of the organic microcapsule particles is 2-5 μm, wherein the particle size of the core material is 1.80-4.95 μm, and the radial thickness of the polymer shell is 0.05-0.20 μm.

4. The diaphragm according to claim 1, characterized in that The base film is selected from polyethylene film or polypropylene film, the thickness of the base film is: 4-14 μm, the thickness D of the composite ceramic coating is 3-5 μm, and the embedding depth d of the organic microcapsule particles from the outer surface of the composite ceramic coating to the inner doping thereof satisfies: 1 / 3D≤d≤D.

5. The diaphragm according to claim 4, characterized in that The doping density of the organic microcapsule particles in the thickness direction of the composite ceramic coating varies in a gradient, wherein the doping density of the organic microcapsule particles on the side close to the base film is lower than the doping density of the organic microcapsule particles on the side away from the base film.

6. The diaphragm according to claim 5, characterized in that The embedding depth d of the organic microcapsule particles is divided into d near the base film and d 内 and away from the basement membrane 表 The doping density ρ of the organic microcapsule particles in the embedding depth direction satisfies: 1.2≤ρ d表 / ρ d内 ≤1.4, the d 表 The range is: 1 / 4d-1 / 3d.

7. The diaphragm according to claim 1, characterized in that The composite ceramic coating further comprises a binder, wherein the mass percentage of the inorganic ceramic particles is 70-90%, the mass percentage of the binder is 1-15%, and the mass percentage of the organic microcapsule particles is 5%-15%.

8. The diaphragm according to claim 7, characterized in that The inorganic ceramic particles are selected from at least one of alumina, boehmite and titanium dioxide; the binder is polyacrylate and / or polyvinylidene fluoride; the organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane; and the interface modification polymer is polydopamine.

9. The diaphragm according to claim 8, characterized in that The interface layer formed by the deposition of polydopamine on the surface of the composite ceramic coating has a porous structure with a pore size of 5-20 nm, and the specific surface area of the organic microcapsule particles is greater than 200 m 2 / g, and the adhesion between the interface layer and the ceramic coating is ≥2.5N / m.

10. A method for preparing a diaphragm, characterized in that: The diaphragm is the diaphragm according to any one of claims 1 to 9, and the preparation method comprises the following steps: S1: using interfacial polymerization to polymerize on the surface of the core material to form the polymer shell, thereby preparing the organic microcapsule particles; S2: mixing the inorganic ceramic particles, the organic microcapsule particles, the binder, and the solvent in a predetermined ratio, adjusting the solid content to 5-40%, and preparing a ceramic coating slurry; S3: Take the base film and spray the ceramic coating slurry onto the surface of the base film using a step-by-step coating process, wherein the electrostatic spraying parameters are set to: voltage 10-15 kV, needle inner diameter 0.3-0.5 mm, spraying distance 20-30 cm; and S4: placing the base membrane sprayed with the ceramic coating slurry in a reaction container with the interface modification polymer monomer, depositing the interface modification polymer on the surface of the ceramic coating slurry, forming the interface layer after the deposition is completed, and obtaining the diaphragm after drying and curing.

11. The method for preparing a diaphragm according to claim 10, characterized in that: The distributed coating process controls the doping density of the organic microcapsule particles in the composite ceramic coating by adjusting the spraying angle. The spraying angle range is 30°-45°, and the spraying angle changes from large to small during the spraying process.

12. The method for preparing a diaphragm according to claim 11, characterized in that: The base film is a polyethylene film or a polypropylene film with a porosity of 30-45%, the inorganic ceramic particles are alumina, the binder is polyacrylate, the organic microcapsule particles include a core material composed of paraffin and a polymer shell composed of polyurethane, the interface modification polymer is polydopamine, and the preparation method of the organic microcapsule particles comprises the following steps: S11: Mix molten paraffin at 65° C. and isocyanate prepolymer in a mass ratio of 1:(0.3-0.5), add 3 wt% of the oil phase reactant emulsifier Span 80, set the stirring speed to 800-1200 rpm, and stir and emulsify for 30 minutes to prepare an emulsion; S12: preparing an aqueous solution of chain extender ethylenediamine with a concentration of 2-5 wt%, adding NaOH to adjust the pH value to 9-10, and adding the aqueous solution of ethylenediamine to the emulsion; and S13: Control the reaction temperature to 50-60°C, the stirring speed to 300-500 rpm, and the reaction time to 2-4 hours. The size of the prepared organic microcapsule particles is 2-5 μm, the particle size of the paraffin is 1.80-4.95 μm, and the radial thickness of the polyurethane is 0.05-0.20 μm.

13. The method for preparing a diaphragm according to claim 12, characterized in that: The preparation and spraying method of the ceramic coating slurry is as follows: S21: adding 75-90 wt% of aluminum oxide, 7-12 wt% of the organic microcapsule particles, 2-8 wt% of polyacrylate, and 1-5 wt% of sodium carboxymethyl cellulose in a mass ratio, adding solvent water, and stirring at 25-35° C. and a speed of 800-1200 rpm to prepare the ceramic coating slurry with a solid content of 20%; and S22: Take the base film and use a step-by-step coating process to spray the ceramic coating slurry onto the surface of the base film, wherein the electrostatic spraying parameters are set to: voltage 10-15kV, needle inner diameter 0.3-0.5mm, spraying distance 20-30cm, injection speed 1-3mL / min, and the spraying angle is reduced from 45° to 30° during the spraying process.

14. The method for preparing a diaphragm according to claim 12, wherein: The deposition of the interface modification polymer further comprises the following steps: S41: Dissolve dopamine in Tris buffer and adjust the pH to 8.5; S42: immersing the base film coated with the ceramic coating slurry in the Tris buffer, controlling the reaction temperature to 25-35° C. and the immersion time to 30-60 seconds, to deposit the interface layer having a thickness of 0.05-0.20 μm; and S43: placing the base film after the interface modification polymer deposition at 40° C. and drying it by hot air circulation for 5-10 minutes to obtain the separator.

15. A lithium-ion battery comprising a negative electrode, a separator, a positive electrode and an electrolyte, characterized in that: The diaphragm is the diaphragm according to any one of claims 1 to 9.

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