Diaphragm, secondary battery, and method for producing diaphragm

Abstract

The application provides a diaphragm, a secondary battery and a preparation method of the diaphragm, and belongs to the technical field of battery diaphragms.The diaphragm comprises a base film and a coating.The coating comprises microsphere material, the microsphere material comprises a polymer and a compound with a cavity structure, and the microsphere material has a mesoporous structure.The application effectively improves the electrolyte immersion capacity of the diaphragm by introducing the microsphere material into the coating on the surface of the base film, and effectively promotes the desolvation of lithium ions in the process of ion transmission, so that the ion conductivity of the diaphragm is significantly improved, and the cycle and rate performance of the battery are improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and more particularly to a separator, a secondary battery, and a method for preparing the separator. Background Technology

[0002] With the rapid development of new energy vehicles, energy storage systems, and portable electronic devices, lithium-ion batteries, as core energy storage units, face higher requirements in terms of safety, energy density, and cycle life. The battery separator, as one of the key components of lithium-ion batteries, not only serves as a physical barrier to isolate the positive and negative electrodes and prevent short circuits, but also directly affects the battery's ion conduction performance, rate performance, and safety performance.

[0003] Currently, porous polymer membranes (such as polyolefin microporous membranes) widely used in batteries utilize their porous structures to adsorb liquid electrolyte, providing channels for lithium-ion migration within the battery. However, these polymer membranes often have a uniform chemical environment and smooth pore wall structure. On the one hand, this makes it difficult for the membrane channels to effectively adsorb the surrounding electrolyte within the battery, resulting in weak electrolyte adsorption capacity and poor retention, leading to increased interfacial resistance for ion transport within the battery. On the other hand, the pore structure on the membrane can only conduct unsolvated lithium ions or small solvated ions, but cannot assist in the desolvation of solvated lithium ions during ion transport. Solvated lithium ions in the electrolyte need to expend additional energy to desolvate before they can pass through the membrane channels, thus significantly reducing the membrane's lithium-ion transport efficiency.

[0004] Therefore, it is necessary to design a separator, a secondary battery, and a method for preparing the separator in order to improve the above-mentioned problems. Summary of the Invention

[0005] This invention provides a separator, a secondary battery, and a method for preparing the separator, in order to improve the technical problem of poor lithium-ion conductivity of the separator caused by its poor adsorption capacity for electrolyte and its inability to promote lithium-ion desolvation during ion transport.

[0006] In a first aspect, the present invention provides a diaphragm comprising a base membrane and a coating. The coating comprises a microsphere material, the microsphere material comprising a polymer and a compound having a cavity structure; the microsphere material has a mesoporous structure.

[0007] In one example of the present invention, the surface of the microsphere material has wrinkles.

[0008] In one example of the present invention, the depth of the wrinkles on the microsphere material is 100~500nm, and the spacing between the wrinkles is 500~2000nm.

[0009] In one example of the present invention, the aperture of the cavity is 0.5~2nm.

[0010] In one example of the present invention, at least a portion of the compound is located in a mesoporous structure.

[0011] In one example of the present invention, the compound includes at least one of calixarene, cucurbitaurea, columnar aromatics, and cyclodextrin.

[0012] In one example of the present invention, the polymer has a three-dimensional network structure and is hydrophilic.

[0013] In one example of the present invention, the polymer includes at least one crosslinked product of starch, cellulose, pyrrolidone polymers, acrylate polymers and their derivatives.

[0014] In one example of the present invention, the mass ratio of polymer to compound is 1.2 to 3.6.

[0015] In one example of the present invention, the polymer includes a first polymer, which is a repeating unit comprising a first monomer, the first monomer comprising at least one of starch and starch derivatives.

[0016] In one example of the present invention, the mass ratio of the first polymer to the compound is 1 to 3.

[0017] In one example of the present invention, the polymer further includes a second polymer, which includes repeating units derived from a second monomer, the second monomer including at least one of cellulose, pyrrolidone polymers, acrylate polymers and their derivatives; the mass ratio of the second polymer to the sum of the masses of the first polymer and the compound is 0.1 to 0.3.

[0018] In one example of the present invention, the first polymer and the second polymer form an interpenetrating polymer network.

[0019] In one example of the present invention, the particle size of the microsphere material is 1~7μm.

[0020] In one example of the present invention, the electrolyte adsorption rate of the diaphragm is 100%~500%.

[0021] In one example of the present invention, the ionic conductivity of the membrane is 1.0~1.3 mS / cm.

[0022] In one example of the present invention, the thickness of the coating is 1~5μm.

[0023] In one example of the present invention, the coating further includes at least one of a ceramic material, a binder, a dispersant, and a wetting agent; the ceramic material includes at least one of boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, and yttrium oxide; the binder includes at least one of polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, polyvinylidene fluoride, and polymers thereof; the dispersant includes at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate, and methylpentanol; and the wetting agent includes at least one of alcohols, ethers, and silane compounds.

[0024] In one example of the present invention, the mass ratio of microsphere material, ceramic material, binder, dispersant and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1).

[0025] In a second aspect, the present invention provides a method for preparing a diaphragm, the method comprising the following steps:

[0026] Polymer monomers, compounds with cavity structures, and crosslinking agents are mixed in an aqueous solvent according to a predetermined mass ratio to obtain an aqueous solution. An oil phase solution is provided, and the aqueous solution is added to the oil phase solution to obtain a first reverse emulsion. A catalyst is added to the first reverse emulsion, and the first reverse emulsion is heated to induce crosslinking of the polymer monomers to form a polymer, resulting in a second reverse emulsion. An alcohol solvent is added to the second reverse emulsion and mixed to obtain a third reverse emulsion. Microspheres are separated from the third reverse emulsion. The microspheres are prepared into a slurry, the slurry is coated onto a base membrane, and the base membrane coated with the slurry is heated and dried to obtain a separator.

[0027] In one example of the present invention, the mass ratio of polymer monomer to compound is 1.2 to 3.6, and the mass ratio of crosslinking agent to the sum of the masses of polymer monomer and compound is 0.05 to 0.15.

[0028] In one example of the present invention, the polymer monomer includes at least one of starch, cellulose, pyrrolidone polymers, acrylate polymers and their derivatives; the compound includes at least one of calixarene, cucurbita, columnar aromatics and cyclodextrin; and the crosslinking agent includes at least one of sodium trimetaphosphate and citric acid.

[0029] In one example of the present invention, an aqueous solution is added to an oil phase solution to obtain a first reverse emulsion, comprising: uniformly adding the aqueous solution to the oil phase solution and stirring to mix, thereby obtaining the first reverse emulsion; wherein the stirring rate is 400~1500 rpm and the stirring time is 30~60 minutes.

[0030] In one example of the present invention, the heating temperature of the first reverse emulsion is 60°C to 70°C; the catalyst includes at least one of sodium carbonate, potassium carbonate, sodium hydroxide, or potassium hydroxide.

[0031] In one example of the present invention, the microsphere material is configured into a slurry, comprising: mixing microporous material, ceramic material, binder, dispersant and wetting agent in a solvent according to a preset mass ratio to form a slurry; wherein the mass ratio of microporous material, ceramic material, binder, dispersant and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1), the ceramic material includes at least one of boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide and yttrium oxide; the binder includes at least one of polyvinylidene fluoride, polyvinyl alcohol, styrene-butadiene rubber and sodium carboxymethyl cellulose; the dispersant includes at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate and methylpentanol; and the wetting agent includes at least one of fluoroalkyl ethoxylate, fatty alcohol polyoxyethylene ether, sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate and sodium dodecyl sulfonate.

[0032] In a third aspect, the present invention also provides a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator as described in any of the above examples.

[0033] The separator provided by this invention has a coating on a base membrane containing microspheres. These microspheres comprise polymers and compounds with cavity structures. The microspheres, through their abundant surface wrinkles and internal mesoporous structure formed by polymer mixing, provide strong capillary action and surface energy to the separator surface, thereby significantly enhancing the separator's wetting and adsorption capacity for the electrolyte. This effectively eliminates the high interfacial impedance defects caused by electrolyte loss from the separator surface. Simultaneously, the microspheres in the coating, while fully wetting the electrolyte, utilize the specific-sized cavity structure within the compound to recognize and attract organic solvent molecules in the electrolyte, spatially weakening the binding force between solvent molecules and lithium ions. This allows the separator to effectively promote lithium ion desolvation during ion transport, significantly improving the separator's ionic conductivity and enhancing the battery's cycle and rate performance. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0035] For simplicity, this article only explicitly discloses some numerical ranges, and every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value, or combined with other lower or upper limits to form a range that is not explicitly stated. It should be noted that, unless otherwise specified, "%" in this article refers to mass percentage content.

[0036] Currently, porous polymer membranes made of polyolefins are widely used in batteries due to their good mechanical strength and cost advantages. These polymer membranes utilize the semi-crystalline properties of polyolefin materials to form a microporous network through dry uniaxial stretching or wet (thermal phase separation) processes. The formed micropores adsorb liquid electrolyte, thereby providing a channel for lithium ions to migrate between the positive and negative electrodes in the battery.

[0037] However, due to the non-directional physical structure of the pores in this type of polymer separator, the adsorption rate of the separator pores for polar ester electrolytes is very limited (typically less than 150% of its own weight), resulting in poor wettability of the separator to the electrolyte. Furthermore, under low-temperature environments or high-rate charging conditions, the separator pores are prone to localized desorption from the electrolyte, further reducing the amount of electrolyte retained by the separator. Simultaneously, the pores on the polymer separator surface can only conduct unsolvated lithium ions or small solvated ions, while the inert environment of the separator surface cannot provide effective desolvation sites for solvated lithium ions or apply additional interaction forces (such as steric hindrance or hydrogen bonding). This means that the separator interface cannot promote the desolvation of lithium ions during ion transport, requiring additional energy to complete the desolvation transport of lithium ions within the battery. This significantly increases the interfacial impedance for lithium ion transport by the separator, reducing its lithium ion transport efficiency.

[0038] The aforementioned defects all affect the ionic conductivity of the separator in the battery, increasing the interfacial impedance for ion transport within the battery and hindering the further development of fast-charging performance. To improve these defects, this application provides a separator with a coating containing microspheres on its surface. The microspheres in the coating, based on an internal nanoporous and surface wrinkled structure formed by hydrophilic polymers, can effectively enhance the separator's adsorption capacity for electrolyte and improve its wetting and adsorption performance. Simultaneously, the microspheres utilize a specific-sized cavity structure provided by the internal compounds to recognize and attract electrolyte solvent molecules, thereby spatially weakening the binding force between solvent molecules and lithium ions. This allows the separator to effectively promote the desolvation of lithium ions during electrolyte transport.

[0039] In a first aspect, this application provides a separator comprising a base membrane and a coating. The coating is disposed on the base membrane and includes a microsphere material comprising a polymer and a compound having a cavity structure. The polymer is formed by cross-linking polymerization of polymer monomers, and the cross-linked polymers are mixed within the microsphere material, such that the gaps between the chain polymers form a mesoporous structure penetrating the microsphere material. The mesoporous structure in the microsphere material effectively increases the specific surface area of ​​the separator surface and provides strong capillary action and surface energy to the separator surface, thereby improving the separator's wetting and adsorption capacity for electrolytes. The compound is embedded in the microsphere material between highly cross-linked polymers, and the molecular rings of the compound have nanoscale internal cavities or through-pore structures. When the electrolyte wets the microsphere material on the separator, the compound in the microsphere material can selectively bind organic solvent molecules in the electrolyte using the internal cavities or through-pore structures of specific sizes, thereby recognizing and attracting organic solvent molecules (such as carbonate solvent molecules) in the electrolyte, thus spatially weakening the binding force between solvent molecules and lithium ions. This allows the separator to promote the desolvation of lithium ions during ion transport, thereby significantly improving the ionic conductivity of the separator and enhancing the cycle and rate performance of the battery.

[0040] In some embodiments, at least a portion of the compound is located in a mesoporous structure within the microsphere material. Compounds located in the mesoporous structure are more likely to attract organic solvent molecules to help desolvate solvated lithium ions as the electrolyte passes through the microsphere material in the coating, thereby reducing the interfacial impedance of the membrane for ion transport and improving the ionic conductivity of the membrane.

[0041] In some embodiments, the pore size of the cavity in the compound is any value in the range of 0.5 to 2 nm, for example, it can be 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm or 2 nm.

[0042] In some embodiments, the polymer in the microsphere material is hydrophilic and has a three-dimensional network structure. The polymers with the three-dimensional network structure are mixed and entangled with each other, which can effectively enrich the pore structure in the microsphere material, greatly increase the specific surface area of ​​the membrane surface, and significantly improve the membrane's wetting and adsorption capacity for electrolyte.

[0043] Furthermore, in some embodiments, the surface of the microsphere material also possesses a wrinkled structure. The surface wrinkles and internal mesoporous structure of the microporous material significantly enhance the wetting and adsorption capacity of the membrane for electrolyte by increasing the specific surface area of ​​the membrane surface (especially the pore walls), resulting in an electrolyte adsorption rate more than twice its own weight, thereby effectively eliminating the high interfacial impedance defects caused by electrolyte loss from the membrane surface. In some embodiments, the depth of the wrinkled structure is 100-500 nm, and the spacing between the wrinkles on the microsphere surface is 500-2000 nm. It should be noted that the wrinkled structure on the microsphere material surface can be formed by any conventional surface treatment method, such as surface plasma treatment, surfactant oxidation treatment, solvent evaporation-induced treatment, etc.

[0044] In some embodiments, the polymer includes at least one crosslinked product of starch, cellulose, pyrrolidone polymers, acrylate polymers, and their derivatives. That is, the polymer can be any of the types listed above, such as crosslinked starch, crosslinked cellulose, crosslinked pyrrolidone polymers, or crosslinked acrylate polymers; the polymer can also be any combination of two or more types listed above, for example, a combination of starch and cellulose crosslinks, or a combination of cellulose and pyrrolidone polymer crosslinks, or a combination of starch, cellulose, and pyrrolidone polymer crosslinks, etc., which will not be listed here. When the polymer is a combination of two or more types, the proportion of each type within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the polymer can also be of types not listed above.

[0045] In some embodiments, the compound can be any one or more macrocyclic compounds with nanoscale cavity structures, such as at least one of calixarenes, cucurbitacins, columnar aromatics, and cyclodextrins. That is, the compound can be any one of the materials listed above, such as calixarenes, cucurbitacins, columnar aromatics, or cyclodextrins; the compound can also be any combination of two or more of the materials listed above, for example, a combination of calixarenes and cucurbitacins, or a combination of columnar aromatics and cyclodextrins, or a combination of calixarenes, cucurbitacins, and columnar aromatics, or a combination of calixarenes, cucurbitacins, columnar aromatics, and cyclodextrins, etc., and so on. When the compound is a combination of two or more materials, the proportion of each material within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the compound can also be materials not listed above.

[0046] Optionally, in some embodiments, the compound includes cyclodextrins. Cyclodextrins have a hydrophobic cavity structure. Utilizing the specific size of their internal cavities and their hydrophobic structure, cyclodextrins can effectively bind organic molecules while repelling other electrolyte molecules, thereby further enhancing the desolvation function of the separator coating for lithium ions. The cyclodextrins include at least one of α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin. That is, the cyclodextrin can be any of the types listed above, such as α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin; the cyclodextrin can also be any combination of two or more types listed above, for example, a combination of α-cyclodextrin and β-cyclodextrin, or a combination of β-cyclodextrin and γ-cyclodextrin, or a combination of α-cyclodextrin and γ-cyclodextrin, or a combination of α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin, etc., which will not be listed here. When the cyclodextrin is a combination of two or more types, the proportion of each type within the combination is not limited and can be mixed in any proportion. In other embodiments, the cyclodextrin may also be of a type not listed above. Optionally, in one embodiment, the cyclodextrin includes γ-cyclodextrin, which has a relatively large cavity size.

[0047] In some embodiments, the mass ratio of polymer to compound (i.e., polymer mass / compound mass) is any value in the range of 1.2 to 3.6, for example, 1.2, 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.3, or 3.6. Optionally, the mass ratio of polymer to compound is any value in the range of 1.2 to 2, for example, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.

[0048] In some embodiments, the polymer includes a first polymer comprising repeating units derived from a first monomer, the first monomer comprising at least one of starch and starch derivatives, such as amylopectin. The first polymer is a hydrophilic chain polymer with a three-dimensional network structure, which, when mixed in the microsphere material, can increase the specific surface area of ​​the membrane and improve the wetting and adsorption capacity of the membrane surface for the electrolyte.

[0049] Amylopectin includes at least one of corn amylopectin, waxy corn starch, and potato amylopectin. That is, amylopectin can be any of the types listed above, such as corn amylopectin, waxy corn starch, or potato amylopectin; amylopectin can also be any combination of two or more types listed above, for example, a combination of corn amylopectin and waxy corn starch, or a combination of waxy corn starch and potato amylopectin, or a combination of corn amylopectin, waxy corn starch, and potato amylopectin, etc., and not all will be listed here. When amylopectin is a combination of two or more types, the proportion of each type within the combination is not limited, and they can be mixed in any proportion. In other embodiments, amylopectin can also be of types not listed above. Optionally, in one embodiment, the amylopectin includes at least waxy corn starch, and the amylopectin content in the waxy corn starch is higher than 95%.

[0050] In some embodiments, the mass ratio of the first polymer to the compound is any value in the range of 1 to 3, for example, it can be 1, 1.5, 2, 2.5 or 3.

[0051] In some embodiments, the polymer further includes a second polymer comprising repeating units derived from a second monomer, the second monomer comprising at least one of cellulose, pyrrolidone polymers, acrylate polymers, and their derivatives. The first and second polymers are mixed to form an interpenetrating polymer network. The gaps between the first and second polymers in the interpenetrating polymer network constitute a high-density pore structure within the microsphere material, further increasing the specific surface area of ​​the membrane surface and providing strong capillary action and surface energy to the membrane surface, thereby significantly enhancing the membrane's wetting and adsorption capacity for the electrolyte. Furthermore, the second polymer swells and diffuses during the microsphere material molding process to act as a porosimeter, thereby introducing more pore structures and expanding the pore size within the interpenetrating polymer network structure.

[0052] In some embodiments, the mass ratio of the second polymer to the sum of the masses of the first polymer and the compound (i.e., the mass of the second polymer / the sum of the masses of the first polymer and the compound) is any value in the range of 0.1 to 0.3, for example, it can be 0.1, 0.15, 0.2, 0.25 or 0.3.

[0053] In some embodiments, the particle size of the microsphere material is any value in the range of 1 to 7 μm, for example, the particle size of the microsphere material can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm or 7 μm.

[0054] In some embodiments, the thickness of the base membrane is any value within the range of 3 to 30 μm, for example, the thickness of the base membrane can be 3 μm, 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, 23 μm, 25 μm, 27 μm or 30 μm. Furthermore, it should be noted that the base membrane can be a conventional porous polymer membrane in the art, and the diaphragm material can be one or a combination of polyvinylidene fluoride, polystyrene, polyarylethersulfone, polyvinyl chloride, polypropylene, polyethylene, polyamide, polyimide, polyacrylic acid, polyacetal, polycarbonate, polyester, polyetherimide, polyimide, polyketone, polyphenylene ether, polyphenylene sulfide, polymethylpentene, polysulfone nonwoven glass, glass fiber materials, and nonwoven fabric materials. For example, in one instance, the diaphragm is a porous polyethylene (PE) membrane with a thickness of 9 μm to 18 μm, such as 9 μm, 12 μm, 16 μm, or 18 μm; an air permeability of 180 s / 100 mL to 380 s / 100 mL, such as 180 s / 100 mL, 280 s / 100 mL, or 380 s / 100 mL; and a porosity of 30% to 50%, such as 30%, 40%, or 50%.

[0055] In some embodiments, the coating thickness is any value in the range of 1 to 5 μm, for example, the coating thickness can be 1 μm, 2 μm, 3 μm, 4 μm or 5 μm.

[0056] In some embodiments, the coating further includes at least one of ceramic materials, binders, dispersants, and wetting agents. For example, the coating may include a combination of microsphere materials and binders, or a combination of microsphere materials, ceramic materials, and binders, or a combination of microsphere materials, dispersants, and binders, or a combination of microsphere materials, ceramic materials, binders, dispersants, and wetting agents. The ceramic materials in the coating can improve the thermal stability and puncture resistance of the diaphragm, prevent thermal shrinkage of the base membrane at high temperatures, and reduce the risk of short circuits caused by external forces or lithium dendrite punctures. The dispersants in the coating can promote the agglomeration and sedimentation of other material particles within the coating, thereby ensuring uniform mixing of various material particles in the coating. The wetting agents in the coating are used to improve the wetting ability of the coating slurry on the base membrane surface, enabling the coating slurry to spread quickly and uniformly on the base membrane surface, thereby improving the uniformity of the coating thickness on the base membrane.

[0057] In some embodiments, the ceramic material includes at least one of boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, and yttrium oxide. That is, the ceramic material can be any one of the materials listed above, such as boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, or yttrium oxide; the ceramic material can also be any combination of two or more of the materials listed above, for example, a combination of boehmite and aluminum oxide, or a combination of aluminum oxide and aluminum nitride, or a combination of aluminum oxide and zinc oxide, or a combination of calcium oxide and magnesium oxide, or a combination of silicon oxide and zirconium dioxide, or a combination of boehmite, silicon oxide, and aluminum oxide, or a combination of calcium oxide, zirconium dioxide, and yttrium oxide, etc., and not all will be listed here. When the ceramic material is a combination of two or more materials, the proportion of each material within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the ceramic material can also be a material not listed above.

[0058] In some embodiments, the adhesive includes at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC). That is, the adhesive can be any one of the materials listed above, such as PVDF, PVA, SBR, or CMC; the adhesive can also be any combination of two or more of the materials listed above, for example, a combination of PVDF and PVA, or a combination of SBR and CMC, or a combination of PVDF and SBR, or a combination of PVA, CMC, and PVDF, etc., and so on. When the adhesive is a combination of two or more materials, the proportion of each material within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the adhesive can also be a material not listed above.

[0059] In some embodiments, the dispersant includes at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate, and methylpentanol. That is, the dispersant can be any one of the materials listed above, such as polyacrylamide, polyacrylate, sodium hexametaphosphate, or methylpentanol; the dispersant can also be any combination of two or more of the materials listed above, for example, a combination of polyacrylamide and polyacrylate, or a combination of sodium hexametaphosphate and methylpentanol, or a combination of polyacrylamide and sodium hexametaphosphate, or a combination of polyacrylate, methylpentanol, and polyacrylamide, etc., and so on. When the dispersant is a combination of two or more materials, the proportion of each material within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the dispersant can also be a material not listed above.

[0060] In some embodiments, the wetting agent includes at least one selected from fluoroalkyl ethoxy alcohol ethers, fatty alcohol polyoxyethylene ethers, sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate, and sodium dodecyl sulfonate. That is, the wetting agent can be any one of the materials listed above, such as fluoroalkyl ethoxy alcohol ethers, fatty alcohol polyoxyethylene ethers, sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate, or sodium dodecyl sulfonate; the wetting agent can also be any combination of two or more of the materials listed above, for example, a combination of fluoroalkyl ethoxy alcohol ethers and fatty alcohol polyoxyethylene ethers, or a combination of sodium butylnaphthalene sulfonate and sodium hydroxyethyl sulfonate, or a combination of sodium hydroxyethyl sulfonate and sodium dodecyl sulfonate, or a combination of fluoroalkyl ethoxy alcohol ethers, fatty alcohol polyoxyethylene ethers, and sodium butylnaphthalene sulfonate, or a combination of sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate, and sodium dodecyl sulfonate, etc., and so on. When the wetting agent is a combination of two or more materials, the proportion of each material within the combination is not limited, and they can be mixed in any proportion. In other embodiments, the wetting agent may also be a material not listed above.

[0061] In some embodiments, when the coating comprises microsphere material, ceramic material, binder, dispersant and wetting agent, the mass ratio of microsphere material, ceramic material, binder, dispersant and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1).

[0062] The diaphragm in this application possesses excellent electrolyte adsorption capacity. In some embodiments, the microsphere material exhibits an adsorption rate of 800% to 1300% for electrolytes containing carbonate solvents. Utilizing the high adsorption capacity of the microspheres for carbonate solvents, the electrolyte adsorption rate of the diaphragm can reach any value within the range of 100% to 500%, for example, 100%, 150%, 200%, 250%, 300%, 350%, 400%, 450%, or 500%. Optionally, in one embodiment, the electrolyte adsorption rate of the diaphragm can reach any value within the range of 150% to 500%, for example, 150%, 200%, 250%, 300%, 350%, 400%, 450%, or 500%; further optionally, in some examples, the electrolyte adsorption rate of the diaphragm is greater than or equal to 200%.

[0063] It should be noted that the electrolyte adsorption rate of the diaphragm is the ratio of the mass of electrolyte absorbed by the diaphragm after immersion in electrolyte to the mass of the diaphragm itself. It is used to reflect the wetting and adsorption performance of the diaphragm for electrolyte.

[0064] Meanwhile, the ionic conductivity of the membrane in this application is significantly better than that of the modified membrane. In some embodiments, the ionic conductivity is any value in the range of 1.0 to 1.3 mS / cm, for example, it can be 1.0 mS / cm, 1.1 mS / cm, 1.2 mS / cm or 1.3 mS / cm.

[0065] In a second aspect, this application also provides a method for preparing a diaphragm, the method comprising the following steps: S1. Mix the polymer monomer, the compound with cavity structure and the crosslinking agent in an aqueous solvent according to a preset mass ratio to obtain an aqueous solution. S2. Provide an oil phase solution, and add an aqueous phase solution to the oil phase solution to obtain a first reverse emulsion; S3. Add a catalyst to the first reverse emulsion and heat the first reverse emulsion to induce cross-linking of polymer monomers to form a polymer, thereby obtaining the second reverse emulsion. S4. The alcohol solvent is placed into the second reverse emulsion and mixed to obtain the third reverse emulsion; the microsphere material is obtained by separating from the third reverse emulsion. S5. The microsphere material is prepared into a slurry, the slurry is coated on the base membrane, and the base membrane coated with slurry is heated and dried to obtain a diaphragm.

[0066] In some embodiments, step S1 includes dispersing polymer monomers in deionized water and stirring and gelatinizing them in a water bath at 70°C to 80°C to obtain a homogeneous first aqueous solution, and cooling the first aqueous solution to 50°C for later use, wherein the first aqueous solution is a branched starch colloidal solution; dissolving cyclodextrin and a pore-forming agent fully in deionized water to obtain a second aqueous solution; mixing the first aqueous solution and the second aqueous solution, and adding a crosslinking agent to the mixed solution and stirring thoroughly to prepare an aqueous solution.

[0067] In some embodiments, in the prepared aqueous solution, the mass ratio of polymer monomer to compound is 1.2 to 3.6; the mass ratio of crosslinking agent to the sum of the masses of polymer monomer and compound is 0.05 to 0.15.

[0068] In some embodiments, the polymer monomer includes at least one of starch, cellulose, pyrrolidone polymers, acrylate polymers and their derivatives.

[0069] In some embodiments, the compound includes at least one of calixarene, cucurbitaurea, columnar aromatics, and cyclodextrin.

[0070] In some embodiments, the crosslinking agent includes at least one of sodium trimetaphosphate and citric acid.

[0071] In some embodiments, the polymer monomer includes a first polymer monomer, which includes at least one selected from starch and starch derivatives. Furthermore, in some embodiments, the polymer monomer also includes a second polymer monomer, which includes at least one selected from cellulose, pyrrolidone polymers, acrylate polymers, and their derivatives. The mass ratio of the second polymer to the sum of the masses of the first polymer and the compound is 0.1 to 0.3.

[0072] In some embodiments, in step S2, the aqueous solution is uniformly added dropwise to the oil phase solution and mixed at high speed. The shear force from the stirring disperses the added aqueous solution into small spherical droplets of a predetermined particle size in the oil phase solution, thereby forming a stable first reverse emulsion. The high-speed stirring speed is 400 rpm to 1500 rpm, for example, 400 rpm, 600 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1400 rpm, or 1500 rpm; the stirring time is 30 minutes to 60 minutes, for example, 30 minutes, 40 minutes, 50 minutes, or 60 minutes. In the first reverse emulsion, the aqueous solution droplets are surrounded and shaped into spheres by the oil phase solution, facilitating the formation of microsphere material particles of a predetermined particle size in subsequent steps.

[0073] In some embodiments, the preparation step of the oil phase solution in step S2 includes mixing an emulsifier with an oil phase solvent to form an oil phase solution. The emulsifier may be selected from materials such as Span20, Span40, Span60, Span80, Tween20, Tween40, Tween60, Tween80, fatty acid esters, and polyethylene glycol ethers. The oil phase solvent may be selected from materials such as liquid paraffin, petrolatum, and mineral oil.

[0074] In step S3, polymers are formed by promoting the polymerization of polymer monomers shaped in the aqueous solution droplets in the first reverse emulsion to form polymers, thereby initially forming microspheres in the prepared second reverse emulsion. In some embodiments, step S3 includes adding a catalyst to the first reverse emulsion, stirring, and heating at 60°C to 70°C for 2 to 8 hours to promote the full crosslinking of the first polymer monomer (e.g., amylopectin) into the first polymer and the second polymer monomer (e.g., PVP) into the second polymer in the spherical aqueous solution droplets.

[0075] In some embodiments, the catalyst may be any alkaline aqueous solution, for example, the catalyst may be selected from one or more aqueous solutions of sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide.

[0076] In step S4, an alcohol solvent is added to the second reverse emulsion and mixed to obtain a third reverse emulsion. In this step, the alcohol solvent induces surface phase separation in the microsphere material initially formed in the second reverse emulsion, promoting the formation of a wrinkled surface structure in the microsphere material. Then, the microsphere material is separated by filtration from the third reverse emulsion, and the separated microsphere material is washed and dried to remove the oily solvent and alcohol solvent from its surface.

[0077] In some embodiments, in step S5, microporous material, ceramic material, binder, dispersant, and wetting agent are mixed in a solvent according to a preset mass ratio to form a slurry. The slurry is then coated onto a base film, and the slurry-coated base film is placed in an oven for drying, allowing the solvent in the slurry to completely evaporate and form a coating on the base film, thereby obtaining a diaphragm. It should be noted that the heating temperature and time for the slurry-coated base film in step S5 can be adaptively adjusted according to the solid content of the coating and the type of solvent to ensure that all organic solvents in the coating slurry evaporate within the drying time.

[0078] In some embodiments, in step S5, the mass ratio of the microporous material, ceramic material, binder, dispersant, and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1).

[0079] In some embodiments, the ceramic material includes at least one selected from boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, and yttrium oxide.

[0080] In some embodiments, the adhesive includes at least one of polyvinylidene fluoride, polyvinyl alcohol, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0081] In some embodiments, the dispersant includes at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate, and methylpentanol.

[0082] In some embodiments, the wetting agent includes at least one of fluoroalkyl ethoxy alcohol ether, fatty alcohol polyoxyethylene ether, sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate, and sodium dodecyl sulfonate.

[0083] In a third aspect, this application also provides a secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator as described in any of the above embodiments. During the charging and discharging process of the battery, lithium ions are inserted and extracted back and forth between the positive and negative electrode. The separator is disposed between the positive and negative electrode to provide isolation; the electrolyte conducts lithium ions between the positive and negative electrode.

[0084] The composition and preparation method of secondary batteries are described in detail below: The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any combination of one or more forms such as film, mesh, porous, foam, or non-woven fabric. The thickness of the positive current collector is, for example, 8 μm to 15 μm. In one embodiment, the positive current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive current collector has two surfaces opposite each other in its thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. No specific limitations are placed on the positive active material, positive conductive agent, and positive binder here; those skilled in the art can select them according to actual needs.

[0085] As an example, the positive electrode active material can be selected from ternary materials, lithium phosphates, and spinel materials. Ternary materials include lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide doped with metal ions, and lithium nickel cobalt aluminum oxide doped with metal ions, etc.; lithium phosphates include lithium manganese iron phosphate, lithium iron phosphate, and lithium manganese phosphate, etc. The positive electrode binder is selected, for example, from polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The positive electrode conductive agent is selected, for example, from one of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of two or more in any proportion.

[0086] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative current collector can be made of a material with good conductivity and mechanical strength, such as copper foil. The negative current collector has two surfaces opposite each other in its own thickness direction, and the negative active material layer is disposed on either or both of the two opposite surfaces of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, a negative binder, and a thickener. No specific limitations are placed on the specific types of negative active material, negative conductive agent, and negative binder; materials known in the art for use in lithium-ion secondary batteries can be used, and those skilled in the art can select them according to actual needs.

[0087] As an example, the negative electrode active material is selected from one or more combinations of carbon materials and silicon materials. Carbon materials include, for example, hard carbon, artificial graphite, and natural graphite. Silicon materials include, for example, elemental silicon, silicon-oxygen materials, and silicon-carbon materials. The negative electrode conductive agent is selected from one or more of carbon black, acetylene black, graphene, carbon nanotubes, and carbon nanofibers, or a mixture of two or more in any proportion. The negative electrode binder is selected from any one or a mixture of several of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR) in any proportion; the thickener is selected from carboxymethyl cellulose (CMC-Na) or CMC-Li.

[0088] The electrolyte can be any conventional type in the art. For example, the electrolyte can be prepared by mixing the solvent and lithium salt in a mass ratio of (8~9):(1~2). The solvent is selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, fluorinated ethylene carbonate, dipropyl carbonate, dimethyl sulfoxide, and N2SO4. methyl 2 pyrrolidone, γ One or more combinations of butyrolactone and polyethylene glycol dimethyl ether; the lithium salt is selected from one or more combinations of LiBF4, LiBF6, LiAsF6, LiPF6, LiClO4, LiFSI, LiTFSI, LiB(C6H5)4, LiAlCl4, LiBr, LiCF3SO3, LiN(CF3SO2)2 and LiC(CF3SOSO2)3.

[0089] Battery assembly is carried out according to conventional methods. Taking a pouch cell as an example, the prepared negative electrode, separator, and positive electrode are stacked in sequence, placed in an aluminum-plastic film, and baked at 80°C to remove moisture, resulting in a dry cell. Finally, the prepared electrolyte is injected into the dry cell and sealed. After processes such as settling, hot and cold pressing, formation, clamping, and capacity testing, the finished lithium-ion secondary battery is obtained.

[0090] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0091] Example 1 This embodiment provides a diaphragm, which includes a base membrane and a coating on the surface of the base membrane. The base membrane is a polyethylene membrane with a thickness of 9 μm, and the coating has a thickness of 2 μm. The coating contains microsphere material, which includes a first polymer and a second polymer. The first polymer is formed by polymerizing waxy corn starch, and the second polymer is formed by polymerizing polyvinylpyrrolidone (PVP K30). The mass ratio of the first polymer to the second polymer is 4:3.

[0092] The preparation method of this diaphragm includes the following steps: (1) Weigh 2.0g of waxy corn starch and disperse it in 40mL of deionized water. Stir and gelatinize it in a water bath at 85℃ for 30 minutes to obtain a homogeneous first aqueous solution. Cool the first aqueous solution to 50℃ for later use. Weigh 0.5g of PVP K30 and 1.5g of γ-cyclodextrin and dissolve them in 20mL of deionized water. After PVP K30 and γ-cyclodextrin are completely dissolved, a second aqueous solution is obtained. Mix the first aqueous solution and the second aqueous solution, add 0.3g of sodium trimetaphosphate and 0.4g of citric acid catalyst, and stir at a low speed of 200 rpm until homogeneous to prepare an aqueous solution.

[0093] (2) Add 150 mL of liquid paraffin and 4.5 g of SPAN 80 to a 250 mL four-necked flask, and stir at 300 rpm for 20 minutes at 40°C to prepare an oil phase solution. Then, while stirring at 300 rpm, slowly add the aqueous phase solution dropwise to the oil phase solution through a constant pressure dropping funnel. After the addition is complete, increase the stirring speed of the mixture to 1000 rpm and stir at high speed for 40 minutes to obtain the first reverse emulsion.

[0094] (3) Dissolve 0.5g of sodium carbonate catalyst in 5mL of deionized water and slowly add it to the first reverse emulsion. Stir at 400 rpm for 4 hours at a heating temperature of 65℃ to obtain the second reverse emulsion.

[0095] (4) Add 100 mL of anhydrous ethanol to the second reverse emulsion and stir continuously for 30 minutes to obtain the third reverse emulsion; then filter and separate the microsphere material from the third reverse emulsion; wash the microsphere material three times with cyclohexane and anhydrous ethanol alternately to completely remove the residual liquid paraffin and ethanol on the surface of the microsphere material; finally, place the washed microsphere material in a supercritical CO2 dryer for drying.

[0096] (5) Microsphere material, alumina ceramic material, polyacrylate binder, and polyacrylamide dispersant were mixed in N-methylpyrrolidone solvent at a mass ratio of 1:3:0.3:0.05 and stirred at 2000 rpm for 2 hours using a planetary mixer to obtain a slurry with a solid content of 30% and a viscosity of 70 mPa•S. The slurry was coated onto the surface of the base film using a microgravure coating machine and then dried in a 60°C oven to form a 2 μm thick coating on the surface of the base film, thereby obtaining the diaphragm.

[0097] Example 2 This embodiment provides a diaphragm with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (1), 1.75g ​​of waxy corn starch and 1.75g ​​of γ-cyclodextrin are dispersed in the prepared aqueous solution.

[0098] Example 3 This embodiment provides a diaphragm with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (1), 2.625g of waxy corn starch and 0.875g of γ-cyclodextrin are dispersed in the prepared aqueous solution.

[0099] Example 4 This embodiment provides a membrane with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (1), 0.35g of PVP K30 is dispersed in the prepared aqueous solution.

[0100] Example 5 This embodiment provides a membrane with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (1), 1.05g of PVP K30 is dispersed in the prepared aqueous solution.

[0101] Example 6 This embodiment provides a membrane with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (1), in the prepared aqueous solution, the second polymer monomer is replaced by sodium carboxymethyl cellulose (CMC) instead of PVP K30.

[0102] Example 7 This embodiment provides a membrane with the same system as in Example 1. The difference between this embodiment and Example 1 is that, in step (1), the second polymer monomer PVP K30 is not dispersed in the prepared aqueous solution.

[0103] Example 8 This embodiment provides a membrane with the same system as in Example 2. The difference between this embodiment and Example 2 is that, in step (1), the second polymer monomer PVP K30 is not dispersed in the prepared aqueous solution.

[0104] Example 9 This embodiment provides a membrane with the same system as in Example 3. The difference between this embodiment and Example 3 is that, in step (1), the second polymer monomer PVP K30 is not dispersed in the prepared aqueous solution.

[0105] Example 10 This embodiment provides a diaphragm with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (2), after all the aqueous solution is added dropwise to the oil solution, the stirring speed of the mixed liquid is increased to 1500 rpm, and the first reverse emulsion is obtained after high-speed stirring for 60 minutes; finally, the particle size of the separated microsphere material in step (4) is 1 μm.

[0106] Example 11 This embodiment provides a diaphragm with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step (2), after all the aqueous solution is added dropwise to the oil solution, the stirring speed of the mixed liquid is increased to 400 rpm, and the first reverse emulsion is obtained after high-speed stirring for 30 minutes; finally, the particle size of the separated microsphere material in step (4) is 7 μm; in step (5), the amount of coating slurry is adjusted to form a coating with a thickness of 5 μm on the surface of the base film.

[0107] Example 12 This embodiment provides a diaphragm with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that in step (5), the amount of coating slurry is adjusted to form a coating with a thickness of 1.5 μm on the surface of the base film.

[0108] Example 13 This embodiment provides a diaphragm with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that in step (5), the amount of coating slurry is adjusted to form a coating with a thickness of 5 μm on the surface of the base film.

[0109] Comparative Example 1 This comparative example provides a diaphragm comprising a base membrane and a coating on the surface of the base membrane. The base membrane is a 9 μm thick polyethylene membrane, and the coating is 2 μm thick. The coating contains only ceramic materials, a binder, and a dispersant, and does not contain microspheres. The preparation method of this diaphragm includes the following steps: Alumina (ceramic material), polyacrylate (binder), and polyacrylamide (dispersant) were mixed in N-methylpyrrolidone solvent at a mass ratio of 3:0.3:0.05 and stirred at 2000 rpm for 2 hours using a planetary mixer to obtain a slurry with a solid content of 30% and a viscosity of 70 mPa•S. The slurry was then coated onto the surface of a base film using a microgravure coating machine and dried in a 60°C oven to form a 2 μm thick coating on the base film surface, thus producing the diaphragm.

[0110] Comparative Example 2 This comparative example provides a diaphragm with the same system as in Example 1. The difference between this comparative example and Example 1 is that 3.5 g of waxy corn starch was added to the aqueous solution prepared in step (1), instead of γ-cyclodextrin.

[0111] The separators prepared in Examples 1 to 13 and Comparative Examples 1 to 2 were subjected to parameter testing; and the separators prepared in Examples 1 to 13 and Comparative Examples 1 to 2 were assembled into batteries to conduct battery cycle performance and rate performance tests to verify the improvement effect of the separator of this application on battery cycle performance and rate performance. The test results are shown in Table 1.

[0112] Coating thickness test: Measure the diaphragm thickness and subtract the base film thickness from the measured diaphragm thickness to obtain the coating thickness on the base film.

[0113] Electrolyte adsorption rate test of the diaphragm: The diaphragm sample was cut into 4cm×4cm samples and vacuum dried at 60℃ for 12 hours. The dry weight (M1) was then measured. In a glove box under argon protection, the diaphragm was completely immersed in sufficient electrolyte and allowed to stand at 25℃ for 24 hours. The sample was then removed and suspended vertically for 1 minute until no more liquid dripped. The wet weight (M2) was immediately measured. Finally, the electrolyte adsorption rate of the diaphragm was calculated based on the wet weight (M2) and dry weight (M1). The adsorption rate is ((M2-M1) / M1). 100%, and the average of three parallel tests was taken as the test result. The electrolyte consisted of lithium hexafluorophosphate and an organic solvent, which included EC, DMC and DEC in a volume ratio of 1:1:1. The lithium ion concentration in the electrolyte was 1 mol / L.

[0114] Ionic conductivity test of the diaphragm: Five test samples matching the resistance test mold were cut from the diaphragm. The samples were immersed in an electrolyte solution for 2 hours. The electrolyte solution included lithium hexafluorophosphate and an organic solvent, namely EC, DMC, and DEC in a volume ratio of 1:1:1. The lithium ion concentration in the electrolyte solution was 1 mol / L. Then, the samples immersed in the electrolyte solution were gently picked up with plastic tweezers and placed into the test fixture one by one. The AC impedance of a single-layer diaphragm sample was measured. Then, another diaphragm sample was placed on top of the diaphragm sample and the AC impedance of the double-layer diaphragm sample was measured. After placing five diaphragm samples in the test fixture, the AC impedance resistances R1, R2, R3, R4, and R5 of the 1st to 5th diaphragm samples were measured respectively. Using the number of membrane layers as the abscissa and the AC impedance of the membrane as the ordinate, a curve relating the membrane resistance to its thickness is fitted, and the slope and linearity of the curve are calculated. When the linearity is greater than 0.99, the ionic conductivity of the membrane is calculated according to equations (1) and (2). When the linearity is less than 0.99, the test needs to be repeated. Equations (1) and (2) are shown below: (1) (2) In equation (1), R is the AC impedance resistance of a single-layer membrane sample, and k is the slope of the curve when the goodness of fit is greater than 0.99. In equation (2), σ is the ionic conductivity of the membrane, in Siemens per centimeter (S / cm); d is the membrane thickness, in micrometers (μm); and S is the area of ​​the membrane sample, in square centimeters (cm²). 2 ).

[0115] The process of assembling a battery with a diaphragm is as follows: Preparation of the positive electrode sheet: The positive electrode active material NCM811, the positive electrode conductive agent acetylene black, and the positive electrode binder PVDF were mixed at a mass ratio of 92:4:4. N-methylpyrrolidone solvent was added, and the mixture was stirred under vacuum until it became homogeneous and transparent, obtaining a positive electrode slurry with a solid content of 50 wt%. The positive electrode slurry was uniformly coated onto aluminum foil, which was then air-dried at room temperature and transferred to an oven for further drying. After cold pressing and slitting, the positive electrode sheet was obtained.

[0116] Preparation of negative electrode sheet: The negative electrode active material artificial graphite, the negative electrode conductive agent acetylene black, the negative electrode thickener sodium carboxymethyl cellulose (CMC-Na), and the negative electrode binder styrene-butadiene rubber (SBR) are mixed in deionized water at a mass ratio of 95:2:1.5:1.5. The mixture is stirred thoroughly under vacuum until homogeneous to obtain a negative electrode slurry with a solid content of 40wt%. The negative electrode slurry is uniformly coated onto the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for drying. The negative electrode sheet is obtained through cold pressing, slitting, and other processes.

[0117] Electrolyte preparation: In a glove box containing 99.999% argon, less than 0.1 ppm oxygen, and less than 0.1 ppm moisture, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0118] Battery assembly: The positive electrode, separator and negative electrode are placed in sequence and wrapped with aluminum-plastic film. After being baked at 80°C to remove water, the lithium-ion battery electrolyte prepared above is injected at 3.0 g / Ah and then sealed. After standing, hot and cold pressing, formation, clamping and capacity testing, a soft-pack lithium-ion battery with a capacity of 1 Ah is obtained.

[0119] Battery cycle performance test: The battery was placed in a constant temperature chamber set to 25℃ and charged at a constant current rate of 1C to 4.2V, then charged at a constant voltage of 4.2V to a constant current rate of 0.05C. After resting for 10 minutes, the battery was discharged at a constant current rate of 1C to 3.0V, and the initial cycle discharge capacity was tested. The battery was subjected to 500 charge-discharge cycles under the above conditions, and the discharge capacity in the 500th cycle was recorded. The capacity retention rate after 500 cycles was calculated using the following formula: Capacity retention rate (%) = (Discharge capacity after 500 cycles / Initial cycle discharge capacity) × 100%.

[0120] Battery rate performance test: The prepared battery was placed in a constant temperature testing device at 25℃ and charged to 4.2V using a constant current-constant voltage method at a current rate of 0.33C. After resting for 30 minutes, the battery was discharged to 3.0V using a constant current rate of 0.33C, yielding the discharge capacity C0. After resting for 30 minutes, the battery was charged to 4.2V using a constant current-constant voltage method at a current rate of 2C. After resting for 30 minutes, the battery was discharged to 3.0V using a constant current rate of 0.2C, yielding the discharge capacity C2. The 2C rate charging capacity retention rate is C2 / C0.

[0121] Table 1: Separator parameters and battery performance test results in Examples 1 to 13 and Comparative Examples 1 to 2

[0122] Comparing the test results of Examples 1 to 9 and Comparative Example 1, it can be seen that the membrane in Comparative Example 1, which did not incorporate microspheres in its surface coating, exhibited poor electrolyte wettability and ionic conductivity, further affecting the cycle life and rate performance of the assembled battery. In contrast to Comparative Example 1, the membrane of this application incorporates microspheres in its base membrane surface coating. This membrane utilizes the mesoporous structure formed by the mixing of polymers (such as amylopectin polymers) in the microspheres and the surface-induced wrinkled structure to increase the electrolyte adsorption rate of the membrane to more than twice the original level. Simultaneously, based on the specific-sized cavities and through-pore structures of the compounds in the microspheres, it achieves desolvation of lithium ions in the wetted electrolyte, thereby significantly improving the ionic conductivity of the membrane and enhancing the cycle life and rate performance of the assembled battery.

[0123] Comparing the test results of Example 1 and Comparative Example 2, it can be seen that in Comparative Example 2, only branched starch was introduced into the microsphere material of the membrane surface coating, without the introduction of cyclodextrin. This resulted in the membrane in Comparative Example 2 being able to improve the electrolyte adsorption rate by virtue of the large specific surface area provided by the physical structure of the microsphere material, but failing to promote the desolvation of lithium ions under the premise that the membrane is fully wetted by the electrolyte. Consequently, the ionic conductivity of the membrane in Comparative Example 2 was relatively low compared to other examples, further limiting the membrane's effect on improving battery rate performance. In contrast to Comparative Example 2, the microsphere material in the membrane surface coating provided in the embodiments of this application all incorporates cyclodextrin polymers. The microsphere material can utilize the cavity structure provided by the cyclodextrin polymer to assist in the desolvation of lithium ions in the electrolyte, thereby significantly improving the ionic conductivity of the membrane and effectively improving the rate performance of the assembled battery.

[0124] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A diaphragm, characterized in that, include: Base film; A coating comprising a microsphere material, the microsphere material comprising a polymer and a compound having a cavity structure; The microsphere material has a mesoporous structure.

2. The diaphragm according to claim 1, characterized in that, The surface of the microsphere material has wrinkles.

3. The diaphragm according to claim 1, characterized in that, On the microsphere material, the depth of the wrinkles is 100~500nm, and the spacing of the wrinkles is 500~2000nm.

4. The diaphragm according to claim 1, characterized in that, The aperture of the cavity is 0.5~2nm.

5. The diaphragm according to claim 1, characterized in that, At least a portion of the compound is located within the mesoporous structure.

6. The diaphragm according to claim 1, characterized in that, The compound includes at least one of calixarene, cucurbitaurea, columnar aromatics, and cyclodextrin.

7. The diaphragm according to claim 1, characterized in that, The polymer has a three-dimensional network structure and is hydrophilic.

8. The diaphragm according to claim 1 or 7, characterized in that, The polymer includes at least one crosslinked from starch, cellulose, pyrrolidone polymers, acrylate polymers and their derivatives.

9. The diaphragm according to claim 1, characterized in that, The mass ratio of the polymer to the compound is 1.2 to 3.

6.

10. The diaphragm according to claim 1, characterized in that, The polymer includes a first polymer, which is a repeating unit derived from a first monomer, the first monomer including at least one of starch and starch derivatives.

11. The diaphragm according to claim 10, characterized in that, The mass ratio of the first polymer to the compound is 1 to 3.

12. The diaphragm according to claim 10, characterized in that, The polymer further includes a second polymer, which includes repeating units derived from a second monomer, the second monomer including at least one of cellulose, pyrrolidone polymers, acrylate polymers and their derivatives; the mass ratio of the second polymer to the sum of the masses of the first polymer and the compound is 0.1 to 0.

3.

13. The diaphragm according to claim 12, characterized in that, The first polymer and the second polymer form an interpenetrating polymer network.

14. The diaphragm according to claim 1, characterized in that, The microsphere material has a particle size of 1~7μm.

15. The diaphragm according to claim 1, characterized in that, The electrolyte adsorption rate of the diaphragm is 100%~500%.

16. The diaphragm according to claim 1, characterized in that, The ionic conductivity of the membrane is 1.0~1.3 mS / cm.

17. The diaphragm according to claim 1, characterized in that, The thickness of the coating is 1~5μm.

18. The diaphragm according to claim 1, characterized in that, The coating further includes at least one of ceramic materials, binders, dispersants, and wetting agents; the ceramic materials include at least one of boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, and yttrium oxide; the binders include at least one of polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, polyvinylidene fluoride, and their polymers; the dispersants include at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate, and methylpentanol; and the wetting agents include at least one of alcohols, ethers, and silane compounds.

19. The diaphragm according to claim 18, characterized in that, The mass ratio of the microsphere material, ceramic material, binder, dispersant and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1).

20. A method for preparing a diaphragm, characterized in that, include: The polymer monomer, the compound with a cavity structure, and the crosslinking agent are mixed in an aqueous solvent according to a preset mass ratio to obtain an aqueous solution. An oil phase solution is provided, and an aqueous phase solution is added to the oil phase solution to obtain a first reverse emulsion; A catalyst is added to the first reverse emulsion, and the first reverse emulsion is heated to induce the polymer monomers to crosslink and form a polymer, thereby obtaining a second reverse emulsion. An alcohol solvent is added to the second reverse emulsion and mixed to obtain a third reverse emulsion; microsphere materials are then separated from the third reverse emulsion. The microsphere material is prepared into a slurry, the slurry is coated onto a base membrane, and the base membrane coated with the slurry is heated and dried to obtain a diaphragm.

21. The preparation method according to claim 20, characterized in that, The mass ratio of the polymer monomer to the compound is 1.2 to 3.6, and the mass ratio of the crosslinking agent to the sum of the masses of the polymer monomer and the compound is 0.05 to 0.

15.

22. The preparation method according to claim 20, characterized in that, The polymer monomer includes at least one of starch, cellulose, pyrrolidone polymers, acrylate polymers and their derivatives; the compound includes at least one of calixarene, cucurbituril, columnar aromatics and cyclodextrin; the crosslinking agent includes at least one of sodium trimetaphosphate and citric acid.

23. The preparation method according to claim 20, characterized in that, The step of adding the aqueous phase solution to the oil phase solution to obtain the first reverse emulsion includes: The aqueous phase solution is added dropwise to the oil phase solution at a uniform rate and stirred to obtain a first reverse emulsion; wherein the stirring rate is 400~1500 rpm and the stirring time is 30~60 minutes.

24. The preparation method according to claim 20, characterized in that, The heating temperature for the first reverse emulsion is 60℃~70℃; the catalyst includes at least one of sodium carbonate, potassium carbonate, sodium hydroxide, or potassium hydroxide.

25. The preparation method according to claim 20, characterized in that, The step of preparing the microsphere material into a slurry includes: The microporous material, ceramic material, binder, dispersant, and wetting agent are mixed in a solvent according to a preset mass ratio to form a slurry; wherein the mass ratio of the microporous material, ceramic material, binder, dispersant, and wetting agent is 1:(1~10):(0.1~1):(0.01~0.5):(0.01~0.1), the ceramic material includes at least one of boehmite, silicon dioxide, aluminum oxide, aluminum nitride, magnesium oxide, nickel oxide, calcium oxide, zinc oxide, zirconium dioxide, and yttrium oxide; the binder includes at least one of polyvinylidene fluoride, polyvinyl alcohol, styrene-butadiene rubber, and sodium carboxymethyl cellulose; the dispersant includes at least one of polyacrylamide, polyacrylate, sodium hexametaphosphate, and methylpentanol; and the wetting agent includes at least one of fluoroalkyl ethoxylate, fatty alcohol polyoxyethylene ether, sodium butylnaphthalene sulfonate, sodium hydroxyethyl sulfonate, and sodium dodecyl sulfonate.

26. A secondary battery, characterized in that, The diaphragm includes any one of claims 1 to 19.

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