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

Abstract

The present application relates to the technical field of diaphragm, in particular to a diaphragm, a preparation method thereof and a lithium ion battery. The diaphragm comprises a base film, a heat-resistant layer and an insulating adhesive layer arranged on the surface of the base film; the composite inorganic material in the heat-resistant layer comprises LDH@ZIFs, and the LDH@ZIFs are ZIFs porous materials synthesized in situ on layered double hydroxides; the insulating adhesive layer comprises modified PVDF, and the modified PVDF is mainly obtained by copolymerization grafting modification treatment of a large dielectric constant monomer on PVDF. The diaphragm has super insulation and high heat resistance, can improve short circuit, shorten ion diffusion distance, be beneficial to wetting and improve battery quality.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, and more specifically, to a membrane, a method for preparing the membrane, and a lithium-ion battery. Background Technology

[0002] As a crucial component of new energy vehicles, the quality of power batteries directly impacts the overall quality of these vehicles. With increasing performance requirements, lithium-ion battery electrode materials have higher areal densities, and separators are thinner, leading to a higher probability of short circuits. The lithium-ion battery separator, acting as an electronic insulator separating the positive and negative electrodes, plays a vital role in preventing short circuits.

[0003] Short circuits in lithium-ion batteries are a common factor leading to thermal runaway caused by mechanical abuse, thermal abuse, and electrical abuse. To improve short circuit performance and battery quality, existing methods primarily involve increasing the puncture strength of the base film or increasing the thickness of the separator, thereby increasing the separator's resistance to foreign objects and reducing the risk of short circuits caused by separator puncture. Current methods for improving the puncture strength of the base film mainly involve increasing the molecular weight of PE or adjusting the manufacturing process. However, high molecular weight PE is difficult to process, has low yield, and high overall cost. Increasing the separator thickness contradicts increasing battery energy density; while it can reduce short circuits, it requires sacrificing battery energy density.

[0004] Current improvements in heat abuse prevention mainly involve surface treatment of the PP / PE base membrane, such as coating it with a heat-resistant inorganic coating, like alumina or boehmite, to enhance the membrane's heat resistance. Adding an inorganic ceramic coating further improves heat resistance; the higher the ceramic particle density in the coating, the better the heat resistance, but the reduced lithium-ion migration channels hinder lithium-ion transport. Conversely, a lower ceramic particle density (more porous) increases the porosity between ceramic particles, leading to faster electrolyte absorption and thus better lithium-ion transport, but reduces the coating's heat resistance, potentially impacting safety.

[0005] Existing measures can reduce the risks of mechanical and thermal abuse, but they cannot solve the problems of low insulation and poor voltage withstand in local areas of the diaphragm, which can lead to electrical breakdown and short circuits.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The primary objective of this invention is to provide a separator with superior insulation and high heat resistance, which improves short-circuit performance, shortens ion diffusion distance, facilitates wetting, and enhances battery quality. This solves the problems of ceramic coatings in existing technologies, which hinder lithium-ion transport and exhibit low heat resistance.

[0008] A second objective of this invention is to provide a method for preparing a diaphragm.

[0009] A third objective of this invention is to provide a lithium-ion battery.

[0010] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0011] This invention first provides a diaphragm, which includes a base membrane and a heat-resistant layer and an insulating coating layer sequentially stacked on the surface of the base membrane; wherein, the composite inorganic material in the heat-resistant layer includes LDH@ZIFs, and the LDH@ZIFs is a porous ZIFs material synthesized in situ on a layered bimetallic hydroxide; the insulating coating layer includes modified PVDF, and the modified PVDF is mainly obtained by copolymer grafting modification of PVDF with a high dielectric constant monomer.

[0012] Furthermore, the preparation method of the LDH@ZIFs includes: mixing and reacting the layered bimetallic hydroxide, an organic solvent and an organic ligand, so that the interlayer metal ions in the layered bimetallic hydroxide serve as the metal salts for ZIFs synthesis, and obtaining the LDH@ZIFs after the reaction is completed.

[0013] Furthermore, the structural formula of the layered bimetallic hydroxide is [M II 1-x M III x (OH)2] x+ [A n- x / n ]·mH2O, where M II Including metallic elements with a valence of +2, M III Including metallic elements with a valence of +3, A n- Including Cl - ,Br - NO3 - CO3 2- and SO4 2- At least one of them, 0.2≤x≤0.33.

[0014] Furthermore, the M II It includes at least one of the elements Mg, Ni and Zn.

[0015] Furthermore, the M III It includes at least one of the elements Al, Fe, and Cr.

[0016] Furthermore, the M II With the M III The molar ratio is 2 to 4.

[0017] Furthermore, the organic ligands include imidazole and its derivatives.

[0018] Furthermore, the imidazole and its derivatives include at least one of 2-methylimidazolium, hexaphenylimidazolium, imidazole, and 5,6-dimethylbenzimidazole.

[0019] Furthermore, the organic solvent includes at least one selected from methanol, butanol, hexanol, and cyclohexane.

[0020] Furthermore, the molar ratio of the layered bimetallic hydroxide to the organic ligand is 1:2 to 4.

[0021] Furthermore, the reaction temperature is 80–100°C, and the reaction time is 2–4 hours.

[0022] Furthermore, the particle size D50 of the LDH@ZIFs is 0.5–1 μm.

[0023] Furthermore, the thickness of the heat-resistant layer is 1–3 μm.

[0024] Furthermore, the LDH@ZIFs in the heat-resistant layer account for 90% to 95% of the mass fraction.

[0025] Furthermore, the high dielectric constant monomer includes monomers with a dielectric constant of 2.5 to 10.

[0026] Furthermore, the high dielectric constant monomer includes at least one of polyphenylene ether, polyacrylonitrile, polycarbonate, epoxy resin, vinyl chloride, and vinyl ether.

[0027] Furthermore, the dielectric constant ε of the modified PVDF, the median particle size d of the modified PVDF, the coating weight m of the insulating adhesive layer, and the coverage f of the insulating adhesive layer satisfy the following relationship: 0.13 < ε / 15m < 0.45, and 0.25f < m + (0.1~3)d < 6f.

[0028] Furthermore, the dielectric constant ε of the modified PVDF is 15 to 25.

[0029] Furthermore, the median particle size d of the modified PVDF is 0.1–0.7 μm.

[0030] Furthermore, the basis weight m of the insulating adhesive layer is 0.1–0.3 g / m. 2 .

[0031] Furthermore, the coverage f of the insulating coating layer is 30% to 70%.

[0032] Furthermore, the modified PVDF in the insulating coating layer accounts for 82% to 86% by mass.

[0033] The present invention further provides a method for preparing the above-mentioned diaphragm, comprising the following steps: coating a heat-resistant layer slurry containing composite inorganic materials onto a base membrane and drying it to form a heat-resistant layer.

[0034] Furthermore, an insulating adhesive slurry containing modified PVDF is coated onto the heat-resistant layer.

[0035] The present invention also provides a lithium-ion battery comprising the above-described separator.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) The separator provided by this invention, by setting a heat-resistant layer containing LDH@ZIFs, possesses good ion conductivity and heat resistance due to the thermal and chemical stability and unique pore structure of ZIFs. The interlaced layered bimetallic hydroxide nanosheets (LDH) form a large number of pores, which can increase the wetting of lithium ions in the electrolyte and shorten the ion diffusion distance. LDH@ZIFs combines the advantages of both ZIFs and bimetallic hydroxides, which can effectively improve heat resistance and reduce thermal breakdown.

[0038] (2) The diaphragm provided by the present invention can improve the insulation performance of the diaphragm by setting an insulating coating layer containing modified PVDF.

[0039] (3) The separator provided by the present invention can increase the breakdown voltage by 47%, increase the short-circuit internal resistance by more than 20%, and reduce thermal shrinkage by more than 80%. Therefore, the separator provided by the present invention has super insulation and high heat resistance, which improves short circuits, and at the same time can shorten the ion diffusion distance, which is beneficial to wetting and improves battery quality. Attached Figure Description

[0040] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0041] Figure 1 The reaction principle diagram of LDH@ZIFs provided by this invention. Detailed Implementation

[0042] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0043] Unless otherwise specified, in this invention, terms such as "first aspect," "second aspect," "third aspect," and "fourth aspect" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, terms such as "first," "second," "third," and "fourth" serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0044] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0045] Unless otherwise specified, in this invention, "one or more" or "at least one" refers to any one, any two, or any two or more of the listed items. "Several" refers to any two or more.

[0046] In a first aspect, the present invention provides a high-insulation safety battery separator, comprising a base film and a heat-resistant layer and an insulating adhesive layer sequentially stacked on the surface of the base film. It is understood that the heat-resistant layer may be disposed on one side of the base film or on both sides of the base film. When a heat-resistant layer is present on either surface of the base film, the surface of the heat-resistant layer also has an insulating adhesive layer.

[0047] The composite inorganic material in the heat-resistant layer includes LDH@ZIFs, which are porous ZIFs materials synthesized in situ on layered bimetallic hydroxide (LDH).

[0048] The insulating coating layer includes modified PVDF, which is mainly obtained by copolymer grafting modification of PVDF with a high dielectric constant monomer.

[0049] The membrane provided by this invention, by incorporating a heat-resistant layer containing LDH@ZIFs, leverages the thermal and chemical stability of ZIFs, along with their unique pore structure, to achieve excellent ion conductivity and heat resistance. The interwoven layered bimetallic hydroxide nanosheets (LDH) form numerous pores, increasing lithium ion wetting in the electrolyte and shortening the ion diffusion distance. LDH@ZIFs combines the advantages of both ZIFs and bimetallic hydroxides, effectively improving heat resistance and reducing thermal breakdown.

[0050] Meanwhile, by incorporating an insulating coating layer containing modified PVDF, not only is the insulation performance of the diaphragm improved, but the more uniform interface contact between the modified PVDF and the heat-resistant layer also helps reduce interfacial impedance. Specifically, selecting monomers with high dielectric constants for copolymer grafting modification of PVDF imparts a high dielectric constant to the material, thereby enhancing its polarization. This means that electrons within the material are more easily rearranged under the influence of an electric field to form induced charges. This polarization behavior helps to weaken the influence of external electric fields and improve the material's insulation performance.

[0051] Therefore, the heat-resistant layer and the insulating adhesive layer work together to give the separator provided by the present invention super insulation and high heat resistance, which can improve short circuits and shorten the ion diffusion distance, which is beneficial to wetting and improves battery quality.

[0052] In some specific embodiments, the diaphragm provided by the present invention can increase the breakdown voltage by 47%, increase the short-circuit internal resistance by more than 20%, and reduce thermal shrinkage by more than 80%.

[0053] In some specific embodiments, the preparation method of the LDH@ZIFs includes: mixing and reacting the layered bimetallic hydroxide, an organic solvent and an organic ligand, so that the interlayer metal ions in the layered bimetallic hydroxide serve as the metal salt for ZIFs synthesis, and synthesizing ZIFs material in situ on its surface, and obtaining the LDH@ZIFs after the reaction is completed.

[0054] Among them, the interlayer metal ions of layered bimetallic hydroxides (LDHs) can be used as metal salts for the synthesis of ZIFs. By adding organic ligands to the organic solvent of the LDHs and performing a sealed, isothermal reaction, in-situ grown ZIFs materials can be obtained, such as... Figure 1 As shown. Figure 1 In the diagram, the pentagon represents an organic ligand.

[0055] In some specific embodiments, the structural formula of the layered bimetallic hydroxide is [M II 1-x M III x (OH)2] x+ [A n-x / n ]·mH2O. Wherein, M II Including metallic elements with a oxidation state of +2 (divalent metal ions), M III Including metallic elements with a +3 valence (trivalent metal ions), A n- Including Cl - ,Br - NO3 - CO3 2- and SO4 2- At least one of the following; 0.2 ≤ x ≤ 0.33, where x can be, for example, 0.2, 0.22, 0.23, 0.25, 0.28, 0.30, 0.31 or 0.33.

[0056] In some specific implementations, the M II Includes at least one of the elements Mg, Ni, and Zn, specifically Mg 2+ Ni 2+ and Zn 2+ At least one of them, preferably Zn element (Zn 2+ ), of which Zn 2+ Corresponding metal salts can be selected from zinc nitrate, zinc acetate, etc., but are not limited to these.

[0057] In some specific implementations, the M III It includes at least one of the elements Al, Fe, and Cr, specifically Al 3+ Fe 3+ and Cr 3+ At least one of them, preferably Al element (Al 3+ ), where Al 3+ The corresponding metal salts can be aluminum nitrate, aluminum acetate, etc., but are not limited to these.

[0058] In some specific implementations, the M II With the M III The molar ratio is 2–4 (i.e., 2–4:1), for example, 2, 2.3, 2.5, 2.8, 3.0, 3.3, 3.5, 3.8, or 4. Different metal ion ratios M II / M III Layered hydroxides affect the morphology and structure of ZIFs grown in situ on the surface. II / M III It must meet a requirement between 2 and 4. M II With M III The molar ratio in this range is more conducive to the in-situ crystallization synthesis of ZIFs on the surface of layered hydroxides, and the specific surface area and pore volume are optimal, which is more conducive to the diffusion and migration of lithium ions in the electrolyte.

[0059] In some specific embodiments, the organic ligand includes imidazole and its derivatives.

[0060] In some specific embodiments, the imidazole and its derivatives include at least one of 2-methylimidazolium, hexaphenylimidazolium, imidazole, and 5,6-dimethylbenzimidazole.

[0061] In some specific embodiments, the organic solvent includes at least one selected from methanol, butanol, hexanol, and cyclohexane.

[0062] In some specific embodiments, the molar ratio of the layered bimetallic hydroxide to the organic ligand is 1:2 to 4, for example, 1:2, 1:3 or 1:4.

[0063] In some specific embodiments, the reaction temperature is 80–100°C, for example 80°C, 85°C, 90°C, 95°C, 98°C, or 100°C; and the reaction time is 2–4 hours, for example 2 hours, 3 hours, or 4 hours.

[0064] In some specific embodiments, the particle size D50 of the LDH@ZIFs is 0.5–1 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1 μm. When the particle size meets this range, the requirements for thin coating and heat resistance can be satisfied.

[0065] In some specific embodiments, the thickness of the heat-resistant layer is 1–3 μm, for example, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm. A coating thickness within this range can satisfy the requirements for wetting and heat resistance.

[0066] In some specific embodiments, the mass fraction of LDH@ZIFs in the heat-resistant layer is 90% to 95%, for example, 90%, 91%, 92%, 93%, 94%, or 95%. This is beneficial for improving heat resistance, safety, and wettability.

[0067] In some specific embodiments, the high dielectric constant monomer includes monomers with a dielectric constant of 2.5 to 10, wherein the dielectric constant is, for example, 2.5, 3, 4, 5, 6, 7, 8, 9 or 10.

[0068] In some specific embodiments, the high dielectric constant monomer includes at least one of polyphenylene ether, polyacrylonitrile, polycarbonate, epoxy resin, vinyl chloride, and vinyl ether.

[0069] In some specific embodiments, the dielectric constant ε of the modified PVDF, the median particle size d of the modified PVDF, the basis weight m of the insulating coating, and the coverage f of the insulating coating satisfy the following relationship: 0.13 < ε / 15m < 0.45, and 0.25f < m + (0.1~3)d < 6f. This can improve the insulation resistance and adhesion of the coating. Wherein, (0.1~3)d can be 0.1d, 0.15d, 0.2d, 0.3d, 0.5d, 0.8d, 1d, 1.5d, 2d, 2.5d, or 3d.

[0070] In some specific embodiments, the dielectric constant ε of the modified PVDF is 15 to 25, for example 15, 18, 20, 22, 23 or 25.

[0071] After modification, the dielectric constant of modified PVDF can be increased by more than 25%.

[0072] In some specific embodiments, the median D50 particle size d of the modified PVDF is 0.1–0.7 μm, for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, or 0.7 μm. This particle size range is easy to process and coat, and the range of coverage and coating basis weight can meet the requirements for adhesion and air permeability performance.

[0073] In some specific embodiments, the basis weight m of the insulating adhesive layer is 0.1–0.3 g / m. 2 For example, 0.1g / m 2 0.15g / m 2 0.2g / m 2 0.25g / m 2 or 0.3g / m 2 The coating weight range is sufficient to meet both adhesion and insulation performance requirements. The coating weight refers to the weight of the dried insulating adhesive layer per square meter, ranging from 0.1 to 0.3 g.

[0074] In some specific embodiments, the coverage f of the insulating adhesive layer is 30% to 70%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%. This satisfies both adhesion and improved insulation. The coverage rate refers to the area ratio of the insulating adhesive layer on the surface of the coated substrate (i.e., the heat-resistant layer).

[0075] In some specific embodiments, the modified PVDF in the insulating coating layer accounts for 82% to 86% by mass, for example, 82%, 83%, 84%, 85%, or 86%. This is beneficial for further improving the insulation performance of the diaphragm.

[0076] In some specific embodiments, the base membrane includes any base membrane material commonly used in the art, such as a polyolefin base membrane, the thickness of which may be 5 to 12 μm and the porosity of which may be 40%, but is not limited thereto.

[0077] In some specific embodiments, the heat-resistant layer and / or insulating coating layer further include a dispersant, a thickener, and a water-based binder. The dispersant, thickener, and water-based binder can be any materials commonly used in the art.

[0078] As an example, the dispersant can be a carboxylate compound, such as sodium carboxylate or carboxylate amine; the water-based adhesive can be one or more of polymethyl acrylate, polybutyl methacrylate or styrene-butadiene latex; the thickener can be sodium carboxymethyl cellulose, but is not limited to these.

[0079] Secondly, the present invention provides a method for preparing the above-mentioned diaphragm, comprising the following steps: coating a heat-resistant layer slurry containing composite inorganic materials onto a base membrane and drying it to form a heat-resistant layer.

[0080] This method has the advantages of simple operation, short process, low cost, and the ability to achieve mass production.

[0081] In some specific embodiments, a heat-resistant slurry can be coated on one or both surfaces of the base film. The coating method includes, but is not limited to, gravure transfer coating.

[0082] Further, an insulating adhesive slurry containing modified PVDF is coated onto the heat-resistant layer, and then dried.

[0083] In some specific embodiments, an insulating adhesive layer is formed by applying a coating on the side of the heat-resistant layer or the side away from the heat-resistant layer using a gravure transfer coating method.

[0084] Thirdly, the present invention provides a lithium-ion battery comprising the above-described separator.

[0085] This lithium-ion battery features improved breakdown voltage, increased short-circuit internal resistance, reduced thermal shrinkage, low short-circuit rate, and long cycle life.

[0086] In some specific embodiments, the lithium-ion battery also includes a positive electrode, a negative electrode, and an electrolyte, but this invention does not limit this.

[0087] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0088] Example 1

[0089] The method for preparing the diaphragm provided in this embodiment is as follows:

[0090] (1) LDH@ZIFs, sodium carboxylate dispersant, sodium carboxymethyl cellulose thickener, and polymethyl acrylate water-based adhesive were mixed at a mass percentage of 92.5%, 0.5%, 1%, and 6% respectively. Deionized water was added to make the slurry solid content 35%. After mixing evenly, a heat-resistant layer slurry was obtained. Then, the heat-resistant layer slurry was coated on one side of a PE base film with a thickness of 9 μm by gravure transfer coating and dried to obtain a heat-resistant layer with a thickness of 2 μm.

[0091] The D50 particle size of LDH@ZIFs is 0.8 μm. LDH@ZIFs are porous ZIFs materials synthesized in situ on layered bimetallic hydroxides. The preparation method of LDH@ZIFs is as follows: layered bimetallic hydroxides, organic solvent methanol, and organic ligand 2-methylimidazole are mixed and reacted at 90℃ under sealed conditions for 3 hours to obtain LDH@ZIFs; wherein the molecular structure of layered bimetallic hydroxide (LDH) is [Zn... 2+ 1-x Al 3+ x (OH)2] x+ [A n- x / n ]·mH2O, A n- NO3 - (i.e., n=1), m is 6, x is 0.25, M II With M III The molar ratio is 3; the molar ratio of layered bimetallic hydroxide to organic ligand is 1:2.

[0092] (2) Modified PVDF, dispersant, thickener, and water-based adhesive are mixed at a mass percentage of 85.5%, 6%, 1%, and 7.5% respectively, and deionized water is added to make the slurry solid content 8%. After mixing evenly, an insulating coating slurry is obtained. Then, the insulating coating slurry is coated on the surface of the heat-resistant layer by gravure transfer coating, and then dried to form an insulating coating layer, thus obtaining a diaphragm.

[0093] The modified PVDF was obtained by copolymer grafting modification of PVDF with a high dielectric constant monomer, which is polycarbonate with a dielectric constant of 3. The dielectric constant ε, median particle size d, coating weight m and coverage f of the insulating coating layer of the modified PVDF obtained after modification are shown in Table 1.

[0094] Examples 2-14

[0095] The differences between Examples 2-14 and Example 1 are shown in Table 1.

[0096] In Examples 1 and 4, the difference lies in the dielectric constant of the modified PVDF; the rest are the same. Examples 1, 5, and 6 differ in the median particle size of the modified PVDF; the rest are the same. Examples 1, 7, and 8 differ in the coverage of the insulating coating layer; the rest are the same. Examples 1, 2, and 3 differ in the coating weight of the insulating coating layer; the rest are the same. Examples 1, 9, and 10 are M... II / M III The molar ratios differed (where x was 0.33 in Example 9 and 0.2 in Example 10), but the rest were the same.

[0097] Among them, Examples 1-10 satisfy the relationships 0.13 < ε / 15m < 0.45 and 0.25f < m + (0.1~3)d < 6f. Examples 11 and 12 do not satisfy 0.13 < ε / 15m < 0.45. Examples 13 and 14 do not satisfy 0.25f < m + (0.1~3)d < 6f.

[0098] In Example 4 and Example 12, PVDF was obtained by copolymer grafting modification with polyacrylonitrile with a dielectric constant of 3.26; in Example 11, PVDF was obtained by copolymer grafting modification with polyphenylene ether with a dielectric constant of 2.65.

[0099] Example 15

[0100] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the layered bimetallic hydroxide used in this embodiment has the structural formula [Zn 2+ 1-x Al 3+ x (OH)2] x+ [A n- x / n ]·mH2O, x is 0.3, A n- NO3 - (i.e., n=1), M II With M III The molar ratio is 2.3, and m is 6.

[0101] Example 16

[0102] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the organic ligand used in this embodiment is hexaphenylimidazolium.

[0103] Example 17

[0104] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the organic ligand used in this embodiment is 5,6-dimethylbenzimidazole.

[0105] Example 18

[0106] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the organic solvent used in this embodiment is methanol, and the reaction temperature is 80°C and the reaction time is 4h.

[0107] Example 19

[0108] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the particle size D50 of the LDH@ZIFs prepared in this embodiment is 0.5 μm.

[0109] Example 20

[0110] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that: in step (1), the thickness of the heat-resistant layer obtained in this embodiment is 3μm.

[0111] Example 21

[0112] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that in step (1), the mass percentages of the effective components of LDH@ZIFs, dispersant, thickener and water-based adhesive in this embodiment are 95%, 0.5%, 1% and 3.5%, respectively.

[0113] Example 22

[0114] The preparation method of the diaphragm provided in this embodiment is basically the same as that in Example 1, except that in step (2), the mass percentages of the effective components of modified PVDF, dispersant, thickener and water-based adhesive in this embodiment are 82%, 7%, 1.5% and 9.5%, respectively.

[0115] Comparative Example 1

[0116] The preparation method of the diaphragm provided in this comparative example is basically the same as that in Example 1, except that in step (1), LDH@ZIFs is replaced with alumina of equal mass and equal D50 particle size.

[0117] Comparative Example 2

[0118] The preparation method of the diaphragm provided in this comparative example is basically the same as that in Example 1, except that in step (2), the modified PVDF is replaced with unmodified PVDF (Arkema LBG) of equal mass and equal D50 particle size.

[0119] Comparative Example 3

[0120] The preparation method of the diaphragm provided in this comparative example is basically the same as that in Example 1, except that in step (1), LDH@ZIFs are replaced with ZIFs (i.e., 2-methylimidazole) of equal mass and equal D50 particle size.

[0121] Comparative Example 4

[0122] The preparation method of the diaphragm provided in this comparative example is basically the same as that in Example 1, except that in step (1), LDH@ZIFs is replaced with LDH of equal mass and equal D50 particle size (the structure is the same as that in Example 1).

[0123] Table 1 Comparison of parameters for each embodiment and comparative example.

[0124]

[0125]

[0126] Experimental Example

[0127] The separators prepared in the above embodiments and comparative examples were used to prepare lithium batteries: (1) Preparation of positive electrode sheet: Lithium iron phosphate was used as the main material, and the rest were PVDF, conductive agent, etc. to prepare the slurry. The positive electrode accounted for 95.6% of the coating. The prepared slurry was coated onto carbon-coated aluminum foil to obtain the positive electrode sheet. (2) Preparation of negative electrode sheet: Artificial graphite, CMC, conductive agent and SBR were used to prepare the slurry. The slurry was coated onto copper foil and treated to prepare the negative electrode sheet. The negative electrode accounted for 95.5% of the coating. (3) Preparation of electrode assembly: The separator, negative electrode and positive electrode were stacked into an electrode assembly and hot-pressed. (4) Packaging and electrolyte injection: The prepared electrode assembly was packaged in a soft pack and then injected with electrolyte. (5) Pre-charge and formation: The cell was pre-charged and formed to obtain the lithium battery.

[0128] The diaphragms prepared in each embodiment and comparative example were subjected to the following tests: (1) Breakdown voltage test: According to GB / T36363-2018 6.6.1, voltage: 5000V, leakage current: 1mA, voltage rise time 25S, at least 15 data points were used as a group, and the average value was recorded. (2) Electrical weakness test: The test equipment was Beijing Huace HCRD-300 electrical weakness test instrument, the test voltage was 2000V, the test speed was 5m / min, the test diaphragm length was 100m, and the width was 60mm. The number of breakdowns per unit area was calculated as the number of electrical weaknesses. (3) Heat shrinkage: According to GB / T36363-2018, 3 groups of tests were conducted on each sample and the average value was taken. The test results are shown in Table 2.

[0129] The following tests were performed on the batteries assembled with the separators prepared in each embodiment and each comparative example: (4) Short circuit test: The electrode assembly was subjected to a short circuit test with a test voltage of 250V and a test time of 2s. The short circuit resistance was recorded. (5) Cyclic performance test: The batteries were charged at 1C constant current to 3.65V and constant voltage to 0.05C at 25℃ and 45℃, and discharged at 1C constant current to 2.8V. The number of cycles was 1000, and the capacity retention rate was compared. The test results are shown in Table 2.

[0130] Table 2 Performance Test Results

[0131]

[0132]

[0133] As can be seen from Table 1, compared with Comparative Examples 1, 3 and 4, the separators prepared in each embodiment have improved breakdown voltage and short-circuit internal resistance, reduced thermal shrinkage, and better cycle performance, thus improving short circuit and battery quality.

[0134] Meanwhile, compared to Comparative Example 2, the diaphragm of Example 1 has better insulation performance.

[0135] It is evident that the diaphragm provided by this invention has superior insulation and high heat resistance, can shorten the ion diffusion distance, and can significantly improve short circuits.

[0136] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A diaphragm, characterized in that, It includes a base film and a heat-resistant layer and an insulating coating layer sequentially stacked on the surface of the base film; The composite inorganic material in the heat-resistant layer includes LDH@ZIFs, which is a porous ZIFs material synthesized in situ on a layered bimetallic hydroxide. The insulating coating layer includes modified PVDF, which is obtained by copolymer grafting modification of PVDF with a high dielectric constant monomer; the high dielectric constant monomer includes monomers with a dielectric constant of 2.5 to 10; the high dielectric constant monomer includes at least one of polyphenylene ether, polyacrylonitrile, polycarbonate, epoxy resin, vinyl chloride and vinyl ether. The preparation method of the LDH@ZIFs includes: mixing and reacting a layered bimetallic hydroxide, an organic solvent and an organic ligand, wherein the interlayer metal ions in the layered bimetallic hydroxide are used as metal salts for ZIFs synthesis, and the LDH@ZIFs are obtained after the reaction is completed.

2. The diaphragm according to claim 1, characterized in that, The structural formula of the layered bimetallic hydroxide is [M II 1- x M III x (OH)2] x+ [A n- x / n ]·mH2O, where M II Including metallic elements with a valence of +2, M III Including metallic elements with a valence of +3, A n- Including Cl - ,Br - NO3 - CO3 2- and SO4 2- At least one of them, 0.2≤x≤0.

33.

3. The diaphragm according to claim 2, characterized in that, The M II It includes at least one of the elements Mg, Ni and Zn.

4. The diaphragm according to claim 2, characterized in that, The M III It includes at least one of the elements Al, Fe, and Cr.

5. The diaphragm according to claim 2, characterized in that, The M II With the M III The molar ratio is 2~4.

6. The diaphragm according to claim 1, characterized in that, The organic ligands include imidazole and its derivatives.

7. The diaphragm according to claim 6, characterized in that, The imidazole and its derivatives include at least one of 2-methylimidazolium, hexaphenylimidazolium, imidazole, and 5,6-dimethylbenzimidazole.

8. The diaphragm according to claim 1, characterized in that, The organic solvent includes at least one of methanol, butanol, hexanol, and cyclohexane.

9. The diaphragm according to claim 1, characterized in that, The molar ratio of the layered bimetallic hydroxide to the organic ligand is 1:2~4.

10. The diaphragm according to claim 1, characterized in that, The reaction temperature is 80~100℃, and the reaction time is 2~4h.

11. The diaphragm according to claim 1, characterized in that, The particle size D50 of the LDH@ZIFs is 0.5~1μm.

12. The diaphragm according to claim 1, characterized in that, The thickness of the heat-resistant layer is 1~3μm.

13. The diaphragm according to claim 1, characterized in that, The mass fraction of LDH@ZIFs in the heat-resistant layer is 90%~95%.

14. The diaphragm according to any one of claims 1 to 13, characterized in that, The dielectric constant ε of the modified PVDF, the median particle size d of the modified PVDF, the coating weight m of the insulating coating layer, and the coverage f of the insulating coating layer satisfy the following relationship: 0.13 < ε / 15m < 0.45, and 0.25f < m + (0.1~3)d < 6f.

15. The diaphragm according to claim 14, characterized in that, The dielectric constant ε of the modified PVDF is 15~25.

16. The diaphragm according to claim 14, characterized in that, The median particle size d of the modified PVDF is 0.1~0.7μm.

17. The diaphragm according to claim 14, characterized in that, The basis weight (m) of the insulating adhesive layer is 0.1~0.3 g / m. 2 .

18. The diaphragm according to claim 14, characterized in that, The coverage f of the insulating coating layer is 30%~70%.

19. The diaphragm according to any one of claims 1 to 13, characterized in that, The modified PVDF in the insulating coating layer accounts for 82% to 86% by mass.

20. The method for preparing the diaphragm according to any one of claims 1 to 19, characterized in that, The process includes the following steps: coating a heat-resistant layer slurry containing composite inorganic materials onto a base film and drying it to form a heat-resistant layer.

21. The method for preparing the diaphragm according to claim 20, characterized in that, An insulating adhesive slurry containing modified PVDF is coated onto the heat-resistant layer.

22. A lithium-ion battery, characterized in that, Includes the diaphragm as described in any one of claims 1 to 19.

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