Composite diaphragm, preparation method thereof and secondary battery

Abstract

The invention provides a composite diaphragm, a preparation method thereof and a secondary battery. The composite diaphragm comprises a base membrane and a polyion liquid copolymer coating layer arranged on at least one side of the base membrane, a precursor material of the polyionic liquid copolymer coating layer comprises an ionic liquid monomer and an optional polymerizable monomer. According to the preparation method, the polyion liquid copolymer coating layer is constructed in an in-situ copolymerization manner, so that not only can a compact coating layer with a self-supporting independent structure be formed, but also the interface characteristics of the diaphragm can be effectively improved, and the mechanical strength of the diaphragm can be improved.

Description

Technical Field

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

[0002] Metal ion secondary batteries (such as Li, Na, K, Mg, Al or Ca, etc.) are generally composed of four parts: positive electrode, negative electrode, separator and electrolyte. Although the separator does not participate in the electrochemical reaction process, it greatly affects the ion transport and safety characteristics of metal ion batteries, and it is directly related to key performances such as battery capacity, cycle performance, current density and high and low temperature performance.

[0003] At present, most of the ion battery separators used in large-scale commercialization in the existing technology are polyolefin separators, including PP / PE / PP multi-layer composite separators, PP or PE single-layer microporous membranes and coated membranes. Such separators cannot maintain their size at high temperatures, especially when the temperature exceeds 130°C, which leads to extremely high safety risks inside lithium-ion batteries. As the requirements for the performance of metal-ion secondary batteries become higher and higher, the performance of separators made of pure polyolefin materials is difficult to meet the requirements.

[0004] In order to solve the above problems, researchers modified the diaphragm by adjusting the pore size and thickness of the base film and introducing modifying materials to improve the comprehensive performance of the diaphragm. The technologies for modifying the diaphragm disclosed in the prior art include the following aspects: First, the multi-layer composite diaphragm technology is used. This type of composite diaphragm can well combine the characteristics of polyethylene and polypropylene, and use the synergistic effect to make the produced composite diaphragm have a lower closed-cell temperature and a higher melting temperature, thereby enhancing the safety performance of the battery; second, the composite diaphragm is doped with inorganic nanoparticles to form a rigid skeleton, which can effectively improve the heat resistance of the diaphragm; third, the surface of the polyolefin diaphragm is coated with inorganic ceramic particles or organic matter, such as silicon dioxide, aluminum oxide, titanium dioxide, PVDF or PMMA, etc. The above-mentioned coated diaphragm can improve the high temperature resistance of the diaphragm.

[0005] The performance of the diaphragm mainly includes physical and chemical properties (such as surface density, air permeability, etc.), mechanical properties (such as tensile strength, puncture strength and peel strength), thermal properties (such as heat shrinkage) and electrochemical properties (such as cycle performance, rate performance, internal resistance and self-discharge rate). Among them, the mechanical properties of the diaphragm are generally related to the thickness of the base film, while other properties are closely related to the coating layer on the surface of the diaphragm. By introducing the coating layer, a functional coating can be added to the original base film to increase and improve the comprehensive performance of the diaphragm.

[0006] However, a conventional diaphragm surface is coated to form an inert oxide coating or a solid electrolyte coating. The above diaphragm does not significantly improve the electrical properties of the diaphragm except for reducing the dimensional change at high temperature. The prior art also discloses that the coated diaphragm can inhibit the shuttle effect of polysulfide, thereby reducing self-discharge, but because its polymer layer is a porous structure and the surface coating also has a porous structure, the effect of inhibiting the shuttle of polysulfide in actual use is almost non-existent.

[0007] Therefore, in the art, there is an urgent need to develop a composite diaphragm to solve the above problems. Summary of the invention

[0008] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a composite diaphragm and a preparation method thereof and a secondary battery. The present invention uses an in-situ copolymerization method to construct a polyionic liquid copolymer coating layer, which can not only form a dense self-supporting independent structure coating layer, but also effectively improve the interface characteristics of the diaphragm and increase the mechanical strength of the diaphragm.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a composite membrane, comprising a base membrane and a polyionic liquid copolymer coating layer disposed on at least one side of the base membrane;

[0011] The precursor material of the polyionic liquid copolymer coating layer includes an ionic liquid monomer and an optional polymerizable monomer.

[0012] The present invention is designed to composite a polyionic liquid-based composite solid electrolyte with a diaphragm base film, and attach a "functional layer" to the surface of the diaphragm base film, which effectively improves the comprehensive performance of the diaphragm. Among them, the polyionic liquid copolymer coating layer provided by the present invention has the advantages of uniform distribution, stable structure and dense coating, and its comprehensive performance is much higher than the ceramic inorganic coating layer disclosed in the prior art.

[0013] In addition, the present invention achieves the purpose of directional modification of the performance of the diaphragm by regulating the different components in the polyionic liquid copolymer coating layer, and has good optimization effects on the interfacial resistance, liquid absorption, thermal stability and volume expansion inhibition of the diaphragm, thereby effectively inhibiting the increase in self-discharge caused by lithium dendrite damage to the diaphragm and reducing the potential short circuit risk. First, the composite diaphragm provided by the present invention can better regulate the lithium ion deposition behavior, inhibit the formation of lithium dendrites, and comprehensively improve the utilization efficiency of lithium ions. Secondly, the composite diaphragm provided by the present invention can inhibit the dissolution of transition metal ions, and in sulfur-based batteries, it can also inhibit the shuttle effect of polysulfide and inhibit the migration of transition metal ions to the negative electrode side. In addition, the composite diaphragm provided by the present invention can improve the fit between the pole piece and the diaphragm during hot pressing of battery packaging, increase the characteristics of the diaphragm, and thereby improve the electrical performance of the battery. Finally, the present invention uses in-situ copolymerization to construct a polyionic liquid copolymer coating layer, thereby forming a coating layer with a dense self-supporting independent structure, effectively improving the interface characteristics of the diaphragm, reducing the interface impedance and the occurrence of side reactions, and also improving the mechanical strength of the diaphragm.

[0014] Preferably, the cationic group in the ionic liquid monomer structure includes any one or a combination of at least two of an imidazolium cation, a pyrrolidine cation, a pyridinium cation, a morpholinium cation, a piperidinium cation, a quaternary ammonium cation, a quaternary phosphonium cation or a guanidinium cation, preferably [EMim] + 、[Py 13 ] + or [PP 13 ] + At least one of .

[0015] Preferably, the anionic groups in the ionic liquid monomer structure include [BF 4 ] - 、[N(CN) 2 ] 2 、[CH 3 COO] - , [TfO] - , [FSI] - or [TFSI] - Any one or a combination of at least two of the following, preferably [FSI] - , [TFSI] - or [TfO] - At least one of .

[0016] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the ionic liquid monomer is 10-100%, preferably 30-90%, for example, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, etc.

[0017] In the present invention, the ionic conductivity of the composite membrane is improved and the homogeneous phase is balanced by adjusting the mass percentage of the ionic liquid monomer. If the content is too low, the ionic conductivity of the membrane will be reduced, and vice versa, heterogeneity or even separation will occur.

[0018] Preferably, the polymerizable monomers include alkylene oxide monomers and / or monomers containing polymerizable double bonds.

[0019] Preferably, the alkylene oxide monomer includes any one of ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5-trioxane, 1,3-dioxane or 1,3-dioxolane, or a combination of at least two thereof.

[0020] Preferably, the monomers containing polymerizable double bonds include monomers containing alkenyl groups.

[0021] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the polymerizable monomer is 0-90%, preferably 4-50%, for example, it can be 0%, 4%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 65%, 70%, 75%, 80%, 85%, 90% and the like.

[0022] In the present invention, the polymerization effect of the monomers is optimized by adjusting the mass percentage of the polymerizable monomers. If the content is too low, the mechanical properties of the membrane will deteriorate, and vice versa, the ionic conductivity of the membrane will decrease.

[0023] Preferably, the precursor material of the polyionic liquid copolymer coating layer also includes an inorganic solid electrolyte.

[0024] Preferably, the inorganic solid electrolyte includes an oxide inorganic solid electrolyte and / or a sulfide solid electrolyte.

[0025] In the present invention, the oxide electrolyte refers to any one of a NASICON-type oxide solid electrolyte, a garnet-type oxide solid electrolyte or a perovskite-type oxide solid electrolyte, or a combination of at least two thereof.

[0026] Preferably, the NASICON-type oxide solid electrolyte includes any one or a combination of at least two of lithium aluminum titanium phosphate, lithium titanium phosphate, lithium germanium aluminum phosphate, lithium zirconium silicon phosphate, lithium germanium phosphate, or lithium zirconium phosphate.

[0027] Preferably, the garnet-type oxide solid electrolyte includes lanthanum lithium zirconium oxide.

[0028] Preferably, the perovskite-type oxide solid electrolyte includes lanthanum lithium titanium oxide.

[0029] In the present invention, the sulfide electrolyte refers to a sulfide solid electrolyte, including any one or a combination of at least two of Li-P-S type solid electrolytes, Li 11-n M 2-n P 1+n S 12 type solid electrolytes, or Li 6 PS 5 X type solid electrolytes, where 0 < n ≤ 1, M is selected from Ge, Sn, or Si, and X is selected from Cl, Br, or I.

[0030] Preferably, the Li-P-S type solid electrolyte includes Li 3 PS 4 and / or Li 7 P 3 S 11 。

[0031] Preferably, the Li 11-n M 2-n P 1+n S 12 type solid electrolyte includes Li 2 S-GeS 2 -P 2 S 5 。

[0032] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage of the inorganic solid electrolyte is 10-60%, preferably 10-30%, and can be, for example, 10%, 15%, 20%, 25%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 55%, 60%, etc.

[0033] Preferably, the precursor material of the polyionic liquid copolymer coating layer further includes a crosslinking agent.

[0034] Preferably, the crosslinking agent includes any one or a combination of at least two of divinylbenzene or N,N-methylenebisacrylamide.

[0035] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the cross-linking agent is 0.05-20%, preferably 0.1-5%, for example, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 15%, 18%, 20%.

[0036] Preferably, the precursor material of the polyionic liquid copolymer coating layer may further include a binder and an inorganic inert filler.

[0037] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the binder is 1-20%, preferably 2-10%, for example, 1%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 15%, 18%, 20%.

[0038] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the inorganic inert filler is 1-10%, preferably 2-8%, for example, 1%, 2%, 3%, 4%, 5%, 8%, 10%.

[0039] Preferably, the precursor material of the polyionic liquid copolymer coating layer may further include a metal salt.

[0040] Preferably, the metal salt includes any one of lithium salt, sodium salt or potassium salt.

[0041] Preferably, the lithium salt comprises LiTFSI, LiFSI or LiPF 6 Any one or a combination of at least two of the following.

[0042] Preferably, the sodium salt comprises NaTFSI, NaFSI or NaPF 6 Any one or a combination of at least two of the following.

[0043] Preferably, the potassium salt comprises KTFSI, KFSI or KPF 6 Any one or a combination of at least two of the following.

[0044] Preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer as 100%, the mass percentage of the metal salt is 0 to 30wt%, for example, it can be 0%, 0.5%, 1%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30% and the like.

[0045] Preferably, the thickness of the polyionic liquid copolymer coating layer is 0.2 to 10 microns, preferably 0.5 to 5 microns, for example, it can be 0.2 microns, 0.5 microns, 0.8 microns, 1 micron, 1.5 microns, 2 microns, 2.5 microns, 3 microns, 3.5 microns, 4 microns, 4.5 microns, 5 microns, 6 microns, 7 microns, 8 microns, 9 microns, 10 microns, etc.

[0046] Preferably, the composite membrane has a Gurley number greater than 1000 s under 10 mL of argon gas and 6.65 kPa pressure.

[0047] In a second aspect, the present invention provides a method for preparing the composite diaphragm according to the first aspect, the method comprising the following steps:

[0048] The ionic liquid monomer is polymerized to obtain a polyionic liquid-based solid electrolyte, and then a solution containing the polyionic liquid-based solid electrolyte is mixed with an optional polymerizable monomer to obtain a slurry, and then the slurry is coated on at least one side of a base film, and the composite diaphragm is obtained after a cross-linking reaction;

[0049] or,

[0050] The ionic liquid monomer is polymerized to obtain a polyionic liquid-based solid electrolyte, and then the solution containing the polyionic liquid-based solid electrolyte is mixed with an optional polymerizable monomer to obtain a slurry, and then the base membrane is immersed in the slurry to obtain the composite diaphragm after a cross-linking reaction.

[0051] Preferably, the method for preparing the solution containing the polyionic liquid-based solid electrolyte comprises the following process:

[0052] The polyionic liquid-based solid electrolyte, the solvent and the co-solvent are mixed, and then the dispersant is slowly added, and the mixture is heated and dissolved to obtain a solution containing the polyionic liquid-based solid electrolyte.

[0053] Preferably, the solvent comprises any one of ethanol, ether, chloroform, acetone or acetonitrile, or a combination of at least two thereof.

[0054] Preferably, the co-solvent includes any one of diacetyl, dimethyl sulfoxide, ethylene glycol dimethyl ether or methanol, or a combination of at least two thereof.

[0055] Preferably, the dispersant comprises polyvinyl pyrrolidone.

[0056] Preferably, the heating temperature is 50-90°C and the heating time is 10-20h.

[0057] Preferably, the solid content of the solution containing the polyionic liquid-based solid electrolyte is 10-50%, for example, it can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, or 50%.

[0058] Preferably, the mass ratio of the polyionic liquid-based solid electrolyte to the polymerizable monomer is (0.2-10):1, preferably (0.5-5):1, for example, it can be 0.2:1, 0.5:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1, 4:1, 4.2:1, 4.5:1, 4.8:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1 and the like.

[0059] In the present invention, the mass ratio of the polyionic liquid-based solid electrolyte to the polymerizable monomer is regulated so that the ionic conductivity and phase components of the separator are uniform, thereby achieving a dynamic balance.

[0060] Preferably, the slurry further comprises an inorganic solid electrolyte and a cross-linking agent.

[0061] Preferably, the slurry may further include a binder, an inorganic inert filler and a metal salt.

[0062] Preferably, the cross-linking reaction comprises thermal initiation and photoinitiation.

[0063] Preferably, the photo-initiation includes ultraviolet light initiation or infrared light initiation.

[0064] Preferably, the wavelength range of the light source in the photo-initiation is 200-400nm, for example, it can be 200nm, 250nm, 300nm, 350nm, 400nm, etc.; the time is 1s-10min, for example, it can be 1s, 10s, 20s, 50s, 1min, 2min, 5min, 8min, 10min, etc.

[0065] In a third aspect, the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the separator comprises the composite separator according to the first aspect.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] The present invention provides a composite diaphragm, which is designed to composite a polyionic liquid-based composite solid electrolyte and a diaphragm base film, and to attach a "functional layer" to the surface of the diaphragm base film, effectively improving the comprehensive performance of the diaphragm. Among them, the polyionic liquid copolymer coating layer provided by the present invention has the advantages of uniform distribution, stable structure and dense coating, and its comprehensive performance is much higher than that of the ceramic inorganic coating layer disclosed in the prior art.

[0068] In addition, the present invention achieves the purpose of directional modification of the performance of the diaphragm by regulating the different components in the polyionic liquid copolymer coating layer, and has good optimization effects on the interfacial resistance, liquid absorption, thermal stability and volume expansion inhibition of the diaphragm, thereby effectively inhibiting the increase in self-discharge caused by lithium dendrite damage to the diaphragm and reducing the potential short circuit risk. First, the composite diaphragm provided by the present invention can better regulate the lithium ion deposition behavior, inhibit the formation of lithium dendrites, and comprehensively improve the utilization efficiency of lithium ions. Secondly, the composite diaphragm provided by the present invention can inhibit the dissolution of transition metal ions, and in sulfur-based batteries, it can also inhibit the shuttle effect of polysulfide and inhibit the migration of transition metal ions to the negative electrode side. In addition, the composite diaphragm provided by the present invention can improve the fit between the pole piece and the diaphragm during hot pressing of battery packaging, increase the characteristics of the diaphragm, and thereby improve the electrical performance of the battery. Finally, the present invention uses in-situ copolymerization to construct a polyionic liquid copolymer coating layer, thereby forming a coating layer with a dense self-supporting independent structure, effectively improving the interface characteristics of the diaphragm, reducing the interface impedance and the occurrence of side reactions, and also improving the mechanical strength of the diaphragm. DETAILED DESCRIPTION

[0069] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0070] Example 1

[0071] This embodiment provides a composite diaphragm, which includes a polypropylene base film and a polyionic liquid copolymer coating layer arranged on both sides of the polypropylene base film, wherein the thickness of the polyionic liquid copolymer coating layer is 3 microns, and the Gurley number of the composite diaphragm under 10 mL of argon gas and 6.65 kPa pressure is greater than 1000 s.

[0072] This embodiment also provides a method for preparing the composite membrane, which comprises the following steps:

[0073] (1) preparing a polyionic liquid-based solid electrolyte slurry: thermally polymerizing 1-vinyl-3-butylimidazole trifluoromethanesulfonyl imide salt ionic liquid monomer at 80° C. to obtain a polyionic liquid-based solid electrolyte, and then stirring and dissolving the polyionic liquid-based solid electrolyte with an ethanol solvent and a diacetyl cosolvent, slowly and evenly adding a proportion of a polyvinyl pyrrolidone dispersant while stirring, and after the polyvinyl pyrrolidone dispersant is completely dissolved, heating in a boiling water bath and stirring until it is completely dissolved, to obtain a solution containing a polyionic liquid-based solid electrolyte with a solid content of 30%;

[0074] (2) Weighing a polyionic liquid-based solid electrolyte and ethylene oxide in a mass ratio of 1:2, taking the total mass of the ionic liquid monomer and the ethylene oxide as 100%, and mixing the above components with 1% by mass of a divinylbenzene crosslinker, 5% by mass of a polyvinylidene fluoride binder, and 50% by mass of an inorganic solid electrolyte (LATP), dispersing them uniformly, and uniformly mixing them with the solution containing the polyionic liquid-based solid electrolyte in step (1) to serve as a coating slurry, wherein the mass percentage of the ionic liquid monomer is 80% based on the total mass of the ionic liquid monomer and the ethylene oxide as 100%;

[0075] (3) The slurry is coated on both sides of a polypropylene base film having a porosity greater than 60%, and then subjected to a cross-linking reaction at 80° C. to obtain the composite diaphragm.

[0076] Example 2

[0077] This embodiment provides a composite diaphragm, which includes a polypropylene base film and a polyionic liquid copolymer coating layer arranged on both sides of the polypropylene base film, wherein the thickness of the polyionic liquid copolymer coating layer is 3 microns, and the Gurley number of the composite diaphragm under 10 mL of argon gas and 6.65 kPa pressure is greater than 1000 s.

[0078] This embodiment also provides a method for preparing the composite membrane, which comprises the following steps:

[0079] (1) preparing a polyionic liquid-based solid electrolyte slurry: thermally polymerizing 1-vinyl-3-butylimidazole trifluoromethanesulfonyl imide salt ionic liquid monomer at 80° C. to obtain a polyionic liquid-based solid electrolyte, and then stirring and dissolving the polyionic liquid-based solid electrolyte with an ethanol solvent and a diacetyl cosolvent, slowly and evenly adding a proportion of a polyvinyl pyrrolidone dispersant while stirring, and after the polyvinyl pyrrolidone dispersant is completely dissolved, heating in a boiling water bath and stirring until it is completely dissolved, to obtain a solution containing a polyionic liquid-based solid electrolyte with a solid content of 35%;

[0080] (2) Weighing a polyionic liquid-based solid electrolyte and ethylene oxide in a ratio of 5:1, taking the total mass of the ionic liquid monomer and the ethylene oxide as 100%, and mixing the above components with 5% by mass of a divinylbenzene crosslinker, 10% by mass of a polyvinylidene fluoride binder, and 8% by mass of an inorganic solid electrolyte (LATP), dispersing them uniformly, and uniformly mixing them with the solution containing the polyionic liquid-based solid electrolyte in step (1) to serve as a coating slurry, wherein the mass percentage of the ionic liquid monomer is 90% based on the total mass of the ionic liquid monomer and the ethylene oxide as 100%;

[0081] (3) The slurry is coated on both sides of a polypropylene base film having a porosity greater than 60%, and then subjected to a cross-linking reaction at 80° C. to obtain the composite diaphragm.

[0082] Example 3

[0083] This embodiment provides a composite diaphragm, which includes a polypropylene base film and a polyionic liquid copolymer coating layer arranged on both sides of the polypropylene base film, wherein the thickness of the polyionic liquid copolymer coating layer is 3 microns, and the Gurley number of the composite diaphragm under 10 mL of argon gas and 6.65 kPa pressure is greater than 1000 s.

[0084] This embodiment also provides a method for preparing the composite membrane, which comprises the following steps:

[0085] (1) preparing a polyionic liquid-based solid electrolyte slurry: thermally polymerizing 1-vinyl-3-butylimidazole trifluoromethanesulfonyl imide salt ionic liquid monomer at 80° C. to obtain a polyionic liquid-based solid electrolyte, and then stirring and dissolving the polyionic liquid-based solid electrolyte with an ethanol solvent and a diacetyl cosolvent, slowly and evenly adding a proportion of a polyvinyl pyrrolidone dispersant while stirring, and after the polyvinyl pyrrolidone dispersant is completely dissolved, heating in a boiling water bath and stirring until it is completely dissolved, to obtain a solution containing a polyionic liquid-based solid electrolyte with a solid content of 40%;

[0086] (2) Weighing a polyionic liquid-based solid electrolyte and ethylene oxide in a ratio of 0.5:1, taking the total mass of the ionic liquid monomer and the ethylene oxide as 100%, and mixing the above components with 0.5% by mass of a divinylbenzene crosslinker, 2% by mass of a polyvinylidene fluoride binder and 2% by mass of an inorganic solid electrolyte (LATP), dispersing them uniformly, and uniformly mixing them with the polyionic liquid-based solid electrolyte slurry in step (1) to serve as a coating slurry, wherein the mass percentage of the ionic liquid monomer is 40% based on the total mass of the ionic liquid monomer and the ethylene oxide as 100%;

[0087] (3) The slurry is coated on both sides of a polypropylene base film having a porosity greater than 60%, and then subjected to a cross-linking reaction at 80° C. to obtain the composite diaphragm.

[0088] Example 4

[0089] The difference between this embodiment and embodiment 1 is that, based on the total mass of the ionic liquid monomer and ethylene oxide being 100%, 5% by mass of LiTFSI is added in step (2), and the rest is the same as embodiment 1.

[0090] Example 5

[0091] The difference between this embodiment and embodiment 1 is that in step (3), the composite diaphragm is obtained after a cross-linking reaction is carried out under irradiation with an ultraviolet lamp with a wavelength of 400 nm.

[0092] Example 6

[0093] The difference between this embodiment and embodiment 1 is that ethylene oxide is replaced by an equal content of polyionic liquid-based solid electrolyte in the polyionic liquid copolymer coating layer, and the rest is the same as embodiment 1.

[0094] Example 7

[0095] The difference between this embodiment and embodiment 1 is that the mass ratio of the polyionic liquid-based solid electrolyte to ethylene oxide is 5:95, and the rest is the same as embodiment 1.

[0096] Example 8

[0097] The difference between this embodiment and embodiment 1 is that the mass ratio of the polyionic liquid-based solid electrolyte to ethylene oxide is 95:5, and the rest is the same as embodiment 1.

[0098] Example 9

[0099] The difference between this embodiment and embodiment 1 is that the cross-linking temperature is 50°C.

[0100] Example 10

[0101] The difference between this embodiment and embodiment 1 is that the cross-linking temperature is 90°C.

[0102] Comparative Example 1

[0103] The difference between this comparative example and Example 1 is that the ionic liquid monomer is replaced with an equal content of ethylene oxide, and the rest is the same as Example 1.

[0104] Application Examples 1 to 10 and Comparative Application Example 1

[0105] The composite separators provided in Examples 1 to 10 and Comparative Example 1 were assembled into different secondary batteries, and the preparation method was as follows:

[0106] A high specific energy and long cycle ternary NCM811 battery, the preparation process of which comprises the following steps:

[0107] (1) The positive electrode slurry (NCM811:SP:PVDF=97.5:1:1.5, dissolved in an appropriate amount of NMP) is subjected to coating, baking, and cutting steps to obtain a positive electrode sheet;

[0108] (2) The negative electrode slurry (graphite: SP: CMC: SBR = 95: 1: 1.5: 2.5, dissolved in an appropriate amount of deionized water) is subjected to coating, baking, and cutting steps to obtain a negative electrode sheet;

[0109] (3) stacking the composite diaphragm / negative electrode sheet / composite diaphragm / positive electrode sheet in the order of making a battery coil core, welding the tabs, the positive tab is aluminum, and the negative tab is nickel / copper-plated nickel;

[0110] (4) Encapsulate the core in an aluminum-plastic film, leaving an opening on the side for subsequent electrolyte injection, encapsulate, and bake the encapsulated battery cell in a 110° C. forced air oven for 48 h;

[0111] (5) The baked battery cell is transferred to a liquid injection room with a dew point environment of -40°C under a low dew point environment, and an appropriate amount of the above-prepared electrolyte is injected, and the battery cell is sealed, followed by aging, high voltage formation, and capacity separation to obtain the high-performance NCM811 battery.

[0112] Test conditions

[0113] The different secondary batteries provided in Application Examples 1 to 10 and Comparative Application Example 1 were tested, and the testing method was as follows:

[0114] Cycle performance test: The constant current charge and discharge cycle test is performed using a battery tester. Install the assembled battery on the battery tester, connect the red to the positive pole and the black to the negative pole. Then set the battery process steps as follows:

[0115] ① Leave for 12 hours;

[0116] ② Constant current charging: current 0.1C cut-off voltage 4.2V;

[0117] ③ Constant current discharge: current 0.2C cut-off voltage 2.7V;

[0118] ④ Constant current charging: current 0.2C cut-off voltage 4.2V;

[0119] ⑤ Cycle 2000 times;

[0120] ⑥End.

[0121] The test results are shown in Table 1:

[0122] Table 1

[0123]

[0124] As can be seen from Table 1, changing the mass ratio of polyionic liquid-based solid electrolyte to ethylene oxide, the content of crosslinking agent, the content of binder and the content of inorganic solid electrolyte in Application Examples 1-3 can optimize the battery performance of the composite diaphragm and obtain the best battery data.

[0125] Application Example 4 shows that the addition of lithium salt can effectively improve the ionic conductivity of the separator; Application Example 5 shows that polyionic liquid-based solid electrolyte and ethylene oxide can also undergo cross-linking reaction by photoinitiation.

[0126] The mass ratio of the polyionic liquid-based solid electrolyte to ethylene oxide in Application Examples 1 and Application Examples 7-8 will also affect the performance of the battery. The polyionic liquid-based solid electrolyte can effectively transmit lithium ions. As its content increases, it can improve the performance of the battery to a certain extent. However, when the content is too high, it will cause difficulty in contacting the polymer formed by polymerization with ethylene oxide, and the internal consistency of the copolymer is poor, and there are certain defects, which makes the battery performance worse. At the same time, it can also be shown that the presence of ethylene oxide affects the consistency of the copolymer system. When the content of ethylene oxide is low, the electrolyte material cannot be completely coated, and the contact with the diaphragm is poor, resulting in poor battery performance. When the content of ethylene oxide is too high, the content of the internal electrolytic components is low, lithium ions cannot be effectively transmitted, and the overall performance of the battery is also poor.

[0127] In application examples 1 and 9-10, as the cross-linking temperature increases, the capacity retention rate of the battery tends to increase first and then decrease. When the temperature is low, the degree of polymerization of the molecules is low, the molecular weight is large, the polymer molecular segments are difficult to move, and the lithium ion transmission is difficult, resulting in poor battery performance; when the temperature is high, the degree of polymerization of the molecules is high, the molecular weight is small, and the coating effect on the diaphragm is poor, resulting in poor battery performance.

[0128] The applicant declares that the present invention illustrates the process method of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned process steps, that is, it does not mean that the present invention must rely on the above-mentioned process steps to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of the raw materials selected by the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A composite separator, characterized in that, the composite separator comprises a base film and a poly(ionic liquid) copolymer coating layer provided on at least one side of the base film; the precursor material of the poly(ionic liquid) copolymer coating layer comprises an ionic liquid monomer and an optional polymerizable monomer.

2. The composite separator according to claim 1, characterized in that, The cationic group in the ionic liquid monomer structure includes any one or a combination of at least two of imidazole cation, pyrrolidine cation, pyridine cation, morpholine cation, piperidine cation, quaternary ammonium cation, quaternary phosphonium cation or guanidine cation, preferably [EMim] + , [Py 13 + or [PP 13 + at least one of;​​ Preferably, the anion group in the ionic liquid monomer structure includes [BF 4 - , [N(CN) 2 2 , [CH 3 COO] - , [TfO] - , [FSI] - or [TFSI] - , and any one or a combination of at least two of them is preferred, preferably at least one of [FSI] - , [TFSI] - or [TfO] - .​​ 3. The composite separator according to claim 1 or 2, characterized in that, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the ionic liquid monomer is 10 - 100%, preferably 30 - 90%; preferably, the polymerizable monomer comprises an alkylene oxide monomer and / or a monomer containing a polymerizable double bond; preferably, the alkylene oxide monomer comprises any one or a combination of at least two of ethylene oxide, 1,2 - epoxypropane, 1,2 - epoxybutane, 1,4 - dioxane, 1,3,5 - trioxane, 1,3 - dioxane or 1,3 - dioxolane; preferably, the monomer containing a polymerizable double bond comprises a monomer containing an alkenyl group; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the polymerizable monomer is 0 - 90%, preferably 4 - 50%.

4. The composite separator according to any one of claims 1 - 3, characterized in that, the precursor material of the poly(ionic liquid) copolymer coating layer further comprises an inorganic solid electrolyte; preferably, the inorganic solid electrolyte comprises an oxide inorganic solid electrolyte and / or a sulfide solid electrolyte; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the inorganic solid electrolyte is 10 - 60%, preferably 10 - 30%.

5. The composite separator according to any one of claims 1 - 4, characterized in that, the precursor material of the poly(ionic liquid) copolymer coating layer further comprises a cross - linker; preferably, the cross - linker comprises any one or a combination of at least two of divinylbenzene or N,N - methylenebisacrylamide; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the cross - linker is 0.05 - 20%, preferably 0.1 - 5%; preferably, the precursor material of the poly(ionic liquid) copolymer coating layer may further comprise a binder and an inorganic inert filler; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the binder is 1 - 20%, preferably 2 - 10%; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the inorganic inert filler is 1 - 10%, preferably 2 - 8%; preferably, the precursor material of the poly(ionic liquid) copolymer coating layer may further comprise a metal salt; preferably, the metal salt comprises any one of a lithium salt, a sodium salt or a potassium salt; Preferably, the lithium salt includes any one or a combination of at least two of LiTFSI, LiFSI or LiPF 6 in; Preferably, the sodium salt includes any one or a combination of at least two of NaTFSI, NaFSI or NaPF 6 ; Preferably, the potassium salt includes any one or a combination of at least two of KTFSI, KFSI or KPF 6 ; preferably, based on the total mass of the ionic liquid monomer and the polymerizable monomer being 100%, the mass percentage content of the metal salt is 0 - 30wt%. Preferably, the thickness of the polyionic liquid copolymer coating layer is 0.2 to 10 μm, preferably 0.5 to 5 μm; Preferably, the Gurley number of the composite separator under 10 mL of argon gas and a pressure of 6.65 kPa is greater than 1000 s.

6. A method for preparing the composite separator according to any one of claims 1-5, characterized in that the method comprises the following steps: Polymerizing an ionic liquid monomer to obtain a polyionic liquid-based solid electrolyte, then mixing a solution containing the polyionic liquid-based solid electrolyte with an optional polymerizable monomer to obtain a slurry, and then coating the slurry on at least one side of a base film, and obtaining the composite separator after a crosslinking reaction; or, Polymerizing an ionic liquid monomer to obtain a polyionic liquid-based solid electrolyte, then mixing a solution containing the polyionic liquid-based solid electrolyte with an optional polymerizable monomer to obtain a slurry, and then immersing the base film in the slurry, and obtaining the composite separator after a crosslinking reaction.

7. The method according to claim 6, characterized in that the preparation method of the solution containing the polyionic liquid-based solid electrolyte comprises the following process: Mixing the polyionic liquid-based solid electrolyte, a solvent and a co-solvent, and then slowly adding a dispersant, and dissolving by heating to obtain the solution containing the polyionic liquid-based solid electrolyte; Preferably, the solvent comprises any one or a combination of at least two of ethanol, ether, chloroform, acetone or acetonitrile; Preferably, the co-solvent comprises any one or a combination of at least two of butanedione, dimethyl sulfoxide, ethylene glycol dimethyl ether or methanol; Preferably, the dispersant comprises polyvinylpyrrolidone; Preferably, the heating temperature is 50-90 °C and the time is 10-20 h; Preferably, the solid content of the solution containing the polyionic liquid-based solid electrolyte is 10-50%.

8. The method according to claim 6 or 7, characterized in that the mass ratio of the polyionic liquid-based solid electrolyte to the polymerizable monomer is (0.2-10):1, preferably (0.5-5):1; Preferably, the slurry further comprises an inorganic solid electrolyte and a crosslinking agent; Preferably, the slurry may further comprise a binder, an inorganic inert filler and a metal salt.

9. The method according to any one of claims 6-8, characterized in that the crosslinking reaction mode includes thermal initiation and photoinitiation modes; Preferably, the photoinitiation includes ultraviolet light initiation or infrared light initiation; Preferably, the wavelength range of the light source in the photoinitiation is 200-400 nm and the time is 1 s-10 min.

10. A secondary battery, characterized in that the secondary battery comprises a positive electrode, a negative electrode, an electrolyte and a separator, and the separator comprises the composite separator according to any one of claims 1-5.

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