Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of diaphragms, in particular to a diaphragm, a preparation method thereof and a lithium ion battery. The diaphragm comprises a base film, and a heat-resistant layer and an insulating gluing layer which are arranged on the surface of the base film; the composite inorganic material in the heat-resistant layer comprises LDH (at) ZIFs, and the LDH (at) ZIFs is a ZIFs porous material synthesized on layered double hydroxides in situ; the insulating gluing layer comprises modified PVDF (Polyvinylidene Fluoride), and the modified PVDF is mainly obtained by carrying out copolymerization grafting modification treatment on PVDF by using a large-dielectric-constant monomer. The diaphragm has super-strong insulativity and high heat resistance, can improve short circuit, can shorten the ion diffusion distance, is beneficial to infiltration, and improves the quality of the battery.

Description

Technical Field

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

[0002] As an important component of new energy vehicles, the quality of power batteries directly affects the quality of new energy vehicles. With the improvement of battery performance requirements, the surface density of lithium-ion battery electrode materials is getting higher and higher, the thickness of the diaphragm is getting thinner and thinner, and the probability of battery short circuit is getting higher and higher. Lithium-ion battery diaphragms are electronic insulators that isolate the positive and negative electrodes from contact in the battery, and play a vital role in preventing battery short circuits.

[0003] Lithium-ion battery short circuit is a common link in battery thermal runaway caused by mechanical abuse, thermal abuse and electrical abuse. In order to improve short circuit and enhance battery quality, existing methods mainly include increasing the puncture strength of the base film or increasing the thickness of the diaphragm, thereby increasing the diaphragm's resistance to foreign matter and reducing the risk of the diaphragm being punctured and causing a short circuit between the positive and negative electrodes. The existing methods for improving the puncture strength of the base film mainly increase the PE molecular weight or adjust the process, but high molecular weight PE is difficult to process, has a low process yield and high overall cost. Increasing the thickness of the diaphragm is contrary to increasing the energy density of the battery. Although it can reduce short circuits, it requires sacrificing the battery energy density.

[0004] The current improvement in thermal abuse is mainly to treat the surface of the PP / PE base film, such as coating the surface with a heat-resistant inorganic coating, such as alumina, boehmite, etc. to enhance the heat resistance of the diaphragm. Adding an inorganic ceramic coating to improve heat resistance, the greater the packing density of ceramic particles in the ceramic coating, the better the heat resistance, but the reduced channels for lithium ion migration in the coating are not conducive to the transmission of lithium ions; the smaller the packing density of the ceramic in the coating, the looser it is, the more channels between ceramic particles, the faster the electrolyte absorption speed, and the more conducive to the transmission of lithium ions, but the heat resistance of the coating is reduced, which will affect safety.

[0005] Existing measures can reduce the risk of mechanical and thermal abuse, but they cannot solve the problem of low insulation and poor voltage resistance in local areas of the diaphragm, which may cause electrical breakdown and short circuits.

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

[0007] The first purpose of the present invention is to provide a diaphragm with super strong insulation and high heat resistance, which can improve short circuit, shorten the ion diffusion distance, facilitate wetting, and improve the battery quality. It solves the problem that the ceramic coating used in the prior art is not conducive to the transmission of lithium ions and has low heat resistance.

[0008] The second object of the present invention is to provide a method for preparing a diaphragm.

[0009] A third object of the present invention is to provide a lithium ion battery.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0011] The present invention first provides a diaphragm, which includes a base film and a heat-resistant layer and an insulating coating layer stacked in sequence on the surface of the base film; wherein the composite inorganic material in the heat-resistant layer includes LDH@ZIFs, and the LDH@ZIFs is a ZIFs porous material synthesized in situ on a layered double metal hydroxide; the insulating coating layer includes modified PVDF, and the modified PVDF is mainly obtained by copolymerizing and grafting PVDF with a large dielectric constant monomer.

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

[0013] Furthermore, the structural formula of the layered double metal hydroxide is [M II 1-x M III x (OH) 2 ] x+ [A n- x / n ]·mH 2 O, where M II Including metal elements with a valence of +2, M III Including metal elements with a valence of +3, A n- Including Cl - Br - 、NO 3 - , CO 3 2- and SO 4 2- At least one of 0.2≤x≤0.33.

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

[0015] Furthermore, the M III Contains 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 ligand includes imidazole and its derivatives.

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

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

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

[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 to 3 μm.

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

[0025] Furthermore, the large dielectric constant monomer includes a monomer with a dielectric constant of 2.5-10.

[0026] Furthermore, the large 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 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.

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

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

[0030] Furthermore, the coating weight m of the insulating rubber layer is 0.1 to 0.3 g / m 2 .

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

[0032] Furthermore, the mass fraction of the modified PVDF in the insulating coating layer is 82% to 86%.

[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 a composite inorganic material on a base film, and drying the slurry to form a heat-resistant layer.

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

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

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The diaphragm provided by the present invention has a heat-resistant layer containing LDH@ZIFs. The thermal stability and chemical stability of ZIFs and the unique pore structure give it good ion conductivity and heat resistance. The layered double hydroxide nanosheets (LDH) are interlaced to form a large number of pores, which can increase the infiltration of lithium ions in the electrolyte and shorten the ion diffusion distance. LDH@ZIFs takes into account the advantages of both ZIFs and double 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 providing an insulating coating layer containing modified PVDF.

[0039] (3) 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 the thermal shrinkage by more than 80%. Therefore, the diaphragm provided by the present invention has super strong insulation and high heat resistance to improve short circuit, and can shorten the ion diffusion distance, which is conducive to infiltration and improves the battery quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0041] Figure 1 This is a reaction principle diagram of LDH@ZIFs provided by the present invention. DETAILED DESCRIPTION

[0042] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0043] If there is no special explanation, in the present invention, "the first aspect", "the second aspect", "the third aspect", "the fourth aspect", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "the first", "the second", "the third", "the fourth", etc. only serve the purpose of non-exhaustive enumeration and description, and it should be understood that they do not constitute a closed limitation on quantity.

[0044] If there is no special explanation, the "include" and "comprising" mentioned in the present invention represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0045] If there is no special explanation, in the present invention, "one or more" or "at least one" refers to any one, any two or more of the listed items. Among them, "several" refers to any two or more of the listed items.

[0046] In a first aspect, the present invention provides a safe battery separator with super strong insulation, which includes a base film and a heat-resistant layer and an insulating coating layer sequentially stacked on the surface of the base film. It is understood that the heat-resistant layer can be arranged on one side of the base film, or on both sides of the base film. When there is a heat-resistant layer on any surface of the base film, there is also an insulating coating layer on the surface of the heat-resistant layer.

[0047] Wherein, the composite inorganic material in the heat-resistant layer includes LDH@ZIFs, and the LDH@ZIFs is a ZIFs porous material synthesized in situ on a layered double hydroxide (LDH).

[0048] The insulating coating layer includes modified PVDF, and the modified PVDF is mainly obtained by copolymerizing and grafting PVDF with a large dielectric constant monomer.

[0049] The diaphragm provided by the present invention has a heat-resistant layer containing LDH@ZIFs, and the thermal stability and chemical stability of ZIFs and the unique pore structure give it good ion conductivity and heat resistance. The layered double hydroxide nanosheets (LDH) are interlaced to form a large number of pores, which can increase the infiltration of lithium ions in the electrolyte and shorten the ion diffusion distance. LDH@ZIFs takes into account the advantages of both ZIFs and double metal hydroxides, and can effectively improve heat resistance and reduce thermal breakdown.

[0050] At the same time, by setting an insulating coating layer containing modified PVDF, it is not only beneficial to improve the insulation performance of the diaphragm, but also the interface contact between the modified PVDF and the heat-resistant layer is more uniform, which is beneficial to reduce the interface impedance. Among them, a monomer with a large dielectric constant is selected to copolymerize and graft the PVDF to modify it, giving the material a high dielectric constant, thereby improving the polarization degree of the material, that is, the electrons inside the material are more likely to rearrange under the action of the electric field to form induced charges. This polarization behavior helps to weaken the influence of the external electric field and improve the insulation performance of the material.

[0051] Therefore, the heat-resistant layer and the insulating coating layer work together to make the diaphragm provided by the present invention have super strong insulation and high heat resistance, which can improve short circuits, and at the same time shorten the ion diffusion distance, which is beneficial to infiltration and improves the battery quality.

[0052] In some specific implementations, 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 the thermal shrinkage by more than 80%.

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

[0054] Among them, the interlayer metal ions of layered double hydroxides (LDH) can be used as metal salts for the synthesis of ZIFs. Organic ligands are added to the organic solvent of the double hydroxide, and after a sealed constant temperature reaction, in-situ grown ZIFs materials can be obtained, such as Figure 1 shown. Figure 1 In the figure, the pentagons represent organic ligands.

[0055] In some specific embodiments, the structural formula of the layered double metal hydroxide is [M II 1-x M III x (OH) 2 ] x+ [A n-x / n ]·mH 2 O. Among them, M II Including metal elements with a valence of +2 (divalent metal ions), M III Including metal elements with a valence of +3 (trivalent metal ions), A n- Including Cl - Br - 、NO 3 - , CO 3 2- and SO 4 2- At least one of; 0.2≤x≤0.33, wherein 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 embodiments, the M II Includes at least one of Mg, Ni and Zn elements, specifically Mg 2+ 、Ni 2+ and Zn 2+ At least one of the following, preferably Zn element (Zn 2+ ). Among them, Zn 2+ The corresponding metal salt may be zinc nitrate, zinc acetate, etc., but is not limited thereto.

[0057] In some specific embodiments, the M III Includes at least one of Al, Fe and Cr elements, specifically Al 3+ , Fe 3+ and Cr 3+ At least one of the following, preferably Al 3+ ). Among them, Al 3+ The corresponding metal salt may be aluminum nitrate, aluminum acetate, etc., but is not limited thereto.

[0058] In some specific embodiments, the M II With the M III The molar ratio of the metal ions is 2 to 4 (i.e., 2 to 4:1), for example, 2, 2.3, 2.5, 2.8, 3.0, 3.3, 3.5, 3.8 or 4. II / M III The layered hydroxides affect the morphology and structure of ZIFs grown in situ on the surface. II / M III Must be between 2 and 4. M II With M IIIThe molar ratio in this range is more conducive to the in situ crystallization synthesis of ZIFs on the layered hydroxide surface, 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-methylimidazole, hexaphenylimidazole, imidazole and 5,6-dimethylbenzimidazole.

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

[0062] In some specific embodiments, the molar ratio of the layered double hydroxide to the organic ligand is 1:2 to 4, such as 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; the reaction time is 2-4h, for example 2h, 3h or 4h.

[0064] In some specific embodiments, the particle size D50 of the LDH@ZIFs is 0.5-1 μm, such as 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 of thin coating and heat resistance can be met.

[0065] In some specific embodiments, the thickness of the heat-resistant layer is 1-3 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm or 3 μm. The coating thickness within this range can meet the requirements of wettability, liquid retention and heat resistance.

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

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

[0068] In some specific embodiments, the large 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 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. This can improve the insulation resistance and adhesion of the coating. Among them, (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 15-25, 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 D50 median particle size d of the modified PVDF is 0.1 to 0.7 μm, such as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm or 0.7 μm. The particle size range is easy to process and coat, and the range of coverage and coating weight can meet the bonding and air permeability performance.

[0073] In some specific embodiments, the coating weight m of the insulating rubber layer is 0.1 to 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 can meet the requirements of bonding and improving insulation. The coating weight means that the weight of the insulating coating layer after coating and drying is 0.1 to 0.3 g per square meter.

[0074] In some specific embodiments, the coverage f of the insulating coating layer is 30% to 70%, such as 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%. This can meet the bonding and improve insulation properties. The coverage refers to the area ratio of the insulating coating layer on the surface of the coated substrate (i.e., the heat-resistant layer).

[0075] In some specific implementations, the mass fraction of the modified PVDF in the insulating coating layer is 82% to 86%, such as 82%, 83%, 84%, 85% or 86%, which is conducive to further improving the insulation performance of the diaphragm.

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

[0077] In some specific implementations, the heat-resistant layer and / or the insulating adhesive layer further includes a dispersant, a thickener and an aqueous binder, wherein the dispersant, the thickener and the aqueous binder may be any material commonly used in the art.

[0078] As an example, the dispersant can be a carboxylate compound, such as sodium carboxylate, carboxylic acid amine, etc.; 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 this.

[0079] In a second aspect, the present invention provides a method for preparing the above-mentioned diaphragm, comprising the following steps: coating a heat-resistant layer slurry containing a composite inorganic material on a base film, and drying the slurry to form a heat-resistant layer.

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

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

[0082] Furthermore, an insulating coating layer slurry containing modified PVDF is coated on the heat-resistant layer and then dried.

[0083] In some specific implementations, coating is performed on the heat-resistant layer side or the side away from the heat-resistant layer by gravure transfer coating to form the insulating adhesive layer.

[0084] In a third aspect, the present invention provides a lithium-ion battery comprising the above-mentioned separator.

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

[0086] In some specific implementations, the lithium-ion battery further includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, which is not limited in the present invention.

[0087] The embodiments of the present invention will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. If no specific conditions are specified in the examples, the conditions are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially.

[0088] Example 1

[0089] The preparation method of the diaphragm provided in this embodiment is as follows:

[0090] (1) LDH@ZIFs, dispersant sodium carboxylate, thickener sodium carboxymethyl cellulose and aqueous adhesive polymethyl acrylate were mixed according to the mass percentage of active ingredients of 92.5%, 0.5%, 1% and 6%, and deionized water was added thereto to make the solid content of the slurry 35%, and the mixture was uniformly mixed to obtain a heat-resistant layer slurry. 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 is a porous ZIF material synthesized in situ on layered double hydroxides. The preparation method of LDH@ZIFs is as follows: layered double hydroxides, organic solvent methanol and organic ligand 2-methylimidazole are mixed and sealed at 90°C for 3 hours to obtain LDH@ZIFs; wherein the molecular structure of layered double hydroxide (LDH) is [Zn 2+ 1-x Al 3+ x (OH) 2 ] x+ [A n- x / n ]·mH 2 O, A n- NO 3 - (i.e. n=1), m is 6, x is 0.25, M II With M III The molar ratio of the layered double hydroxide and the organic ligand is 1:2.

[0092] (2) Modified PVDF, dispersant, thickener and aqueous adhesive are mixed according to the mass percentage of active ingredients of 85.5%, 6%, 1% and 7.5%, and deionized water is added thereto to make the solid content of the slurry 8%, and the mixture is evenly mixed to obtain an insulating coating layer slurry. The insulating coating layer slurry is then coated on the surface of the heat-resistant layer by gravure transfer coating, and then dried to form an insulating coating layer to obtain a diaphragm.

[0093] Among them, the modified PVDF is obtained by copolymerizing and grafting PVDF with a large dielectric constant monomer, and the large dielectric constant monomer is a polycarbonate with a dielectric constant = 3; the dielectric constant ε of the modified PVDF obtained after the modification, 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 are shown in Table 1 respectively.

[0094] Example 2-Example 14

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

[0096] Among them, Example 1 and Example 4 are the difference in the dielectric constant of the modified PVDF, and the rest are the same. Example 1, Example 5 and Example 6 are the difference in the median particle size of the modified PVDF, and the rest are the same. Example 1, Example 7 and Example 8 are the difference in the coverage of the insulating coating layer, and the rest are the same. Example 1, Example 2 and Example 3 are the difference in the coating weight of the insulating coating layer, and the rest are the same. Example 1, Example 9 and Example 10 are M II / M III The molar ratio is different (wherein, x is 0.33 in Example 9 and 0.2 in Example 10), and the rest is the same.

[0097] Among them, Examples 1 to 10 satisfy the relationship 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] Among them, Example 4 and Example 12 are obtained by copolymerizing and grafting PVDF with polyacrylonitrile having a dielectric constant of 3.26; Example 11 is obtained by copolymerizing and grafting PVDF with polyphenylene ether having a dielectric constant of 2.65.

[0099] Embodiment 15

[0100] The preparation method of the diaphragm provided in this embodiment is basically the same as that in embodiment 1, except that: in step (1), the structure of the layered double metal hydroxide used in this embodiment is [Zn 2+ 1-x Al 3+ x (OH) 2 ] x+ [A n- x / n ]·mH 2 O, x is 0.3, A n- NO 3 - (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 Embodiment 1, except that in step (1), the organic ligand used in this embodiment is hexaphenylimidazole.

[0103] Embodiment 17

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

[0105] Embodiment 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 4 h.

[0107] Embodiment 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 LDH@ZIFs prepared in this embodiment is 0.5 μm.

[0109] Embodiment 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 prepared in this embodiment is 3 μm.

[0111] Embodiment 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 ingredients of LDH@ZIFs, dispersant, thickener and aqueous adhesive in this embodiment are 95%, 0.5%, 1% and 3.5% respectively.

[0113] Embodiment 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 ingredients of modified PVDF, dispersant, thickener and aqueous 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 of Example 1, except that in step (1), LDH@ZIFs is replaced by 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 of Example 1, except that in step (2), the modified PVDF is replaced by 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 is replaced by 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 by LDH of equal mass and equal D50 particle size (the structural formula is the same as that in Example 1).

[0123] Table 1 Comparison of parameters of various embodiments and comparative examples

[0124]

[0125]

[0126] Experimental example

[0127] The diaphragms prepared in the above-mentioned embodiments and comparative examples are respectively used to prepare lithium batteries: (1) Preparation of positive electrode sheets: lithium iron phosphate is used as the main material, and the rest are PVDF, conductive agent, etc. to prepare slurry, and the positive electrode accounts for 95.6% of the coating. The prepared slurry is applied to carbon-coated aluminum foil to obtain a positive electrode sheet. (2) Preparation of negative electrode sheets: artificial graphite, CMC, conductive agent and SBR are used to prepare slurry, and the slurry is applied to copper foil to prepare a treated negative electrode sheet, and the negative electrode accounts for 95.5% of the coating. (3) Preparation of electrode groups, stacking the diaphragm, negative electrode and positive electrode into electrode groups in the form of laminates and hot pressing. (4) Packaging and liquid injection: The prepared electrode group is packaged in a soft package form, and then the electrolyte is injected. (5) Pre-charging and formation: The battery cell is pre-charged and formed to obtain a lithium battery.

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

[0129] The following tests were conducted on the batteries assembled with the diaphragms prepared in each embodiment and each comparative example. (4) Short circuit test: The electrode group was subjected to a short circuit test, with a test voltage of 250V and a test time of 2s, and the short circuit resistance was recorded. (5) Cyclic performance test: At 25°C and 45°C, the battery was charged to 3.65V at 1C constant current, charged to 0.05C at constant voltage, and discharged to 2.8V at 1C constant current. 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] It can be seen from Table 1 that, compared with Comparative Example 1, Comparative Example 3 and Comparative Example 4, the diaphragms prepared in each embodiment have improved breakdown voltage and short-circuit internal resistance, reduced thermal shrinkage, better cycle performance, improved short circuit, and improved battery quality.

[0134] At the same time, compared with Comparative Example 2, the insulation performance of the diaphragm of Example 1 is better.

[0135] It can be seen that the diaphragm provided by the present invention has super strong insulation and high heat resistance, can shorten the ion diffusion distance, and can significantly improve the short circuit.

[0136] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A diaphragm, characterized in that: It comprises 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, wherein the LDH@ZIFs is a ZIFs porous material synthesized in situ on a layered double metal hydroxide; The insulating coating layer includes modified PVDF, and the modified PVDF is mainly obtained by copolymerizing and grafting PVDF with a large dielectric constant monomer.

2. The diaphragm according to claim 1, characterized in that: The preparation method of LDH@ZIFs comprises: mixing and reacting a layered double metal hydroxide, an organic solvent and an organic ligand, using the interlayer metal ions in the layered double metal hydroxide as metal salts for synthesizing ZIFs, and obtaining the LDH@ZIFs after the reaction is completed; Preferably, the structural formula of the layered double metal hydroxide is [M II 1-x M III x (OH)2] x+ [A n- x / n ]·mH2O, where M II Including metal elements with a valence of +2, M III Including metal elements with a valence of +3, A n- Including Cl - Br - 、NO3 - 、CO3 2- and SO4 2- At least one of 0.2≤x≤0.33; More preferably, the M II Includes at least one of Mg, Ni and Zn elements; More preferably, the M III Includes at least one of Al, Fe and Cr elements; More preferably, the M II With the M III The molar ratio is 2 to 4; Preferably, the organic ligand includes imidazole and its derivatives; more preferably, the imidazole and its derivatives include at least one of 2-methylimidazole, hexaphenylimidazole, imidazole, and 5,6-dimethylbenzimidazole; Preferably, the organic solvent comprises at least one of methanol, butanol, hexanol and cyclohexane; Preferably, the molar ratio of the layered double metal hydroxide to the organic ligand is 1:2 to 4; Preferably, the reaction temperature is 80-100° C., and the reaction time is 2-4 h.

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

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

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

6. The diaphragm according to any one of claims 1 to 5, characterized in that: The large dielectric constant monomers include monomers with a dielectric constant of 2.5 to 10; Preferably, the large dielectric constant monomer includes at least one of polyphenylene ether, polyacrylonitrile, polycarbonate, epoxy resin, vinyl chloride and vinyl ether.

7. The diaphragm according to any one of claims 1 to 5, 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; Preferably, the dielectric constant ε of the modified PVDF is 15 to 25; Preferably, the median particle size d of the modified PVDF is 0.1 to 0.7 μm; Preferably, the coating weight m of the insulating rubber layer is 0.1 to 0.3 g / m 2 ; Preferably, the coverage f of the insulating coating layer is 30% to 70%.

8. The diaphragm according to any one of claims 1 to 5, characterized in that: The mass fraction of the modified PVDF in the insulating coating layer is 82% to 86%.

9. The method for preparing a diaphragm according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: coating a heat-resistant layer slurry containing a composite inorganic material on a base film, and drying the slurry to form a heat-resistant layer; Preferably, an insulating coating layer slurry containing modified PVDF is coated on the heat-resistant layer.

10. A lithium ion battery, characterized in that: The invention comprises the diaphragm according to any one of claims 1 to 8.

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