Diaphragm and method for producing same

Abstract

The application mainly provides a composite diaphragm for secondary batteries and a preparation method thereof, the composite diaphragm comprises: a substrate layer, and a modified layer formed on at least one surface of the substrate layer; wherein the substrate layer comprises a polymer material; the modified layer comprises a modified covalent organic framework material and inorganic particles, the modified covalent organic framework material has a group containing active hydrogen in a molecular structure, hydrogen ions in the group containing active hydrogen can be replaced by lithium ions, and at least part of the hydrogen ions in the group containing active hydrogen are replaced by lithium ions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of new energy, and particularly relates to a diaphragm for lithium secondary batteries, in particular, a composite battery diaphragm based on an organic-inorganic hybrid system and a preparation method thereof. BACKGROUND

[0002] Covalent Organic Frameworks (COFs) compounds are a kind of two-dimensional or three-dimensional structural materials formed by the reaction between organic precursors, which have very strong covalent bonds, thereby providing special porosity, excellent electrical conductivity, high stability and good crystallinity and many other excellent physical and chemical properties.

[0003] As a new type of crystalline porous polymer, covalent organic framework materials have broad application prospects in the fields of adsorption, catalysis, environment and energy. In the past decade, with the continuous deepening of research, COFs materials with different topological structures have been gradually developed. Most of the reported COFs materials have two-dimensional (2D) expansion structures, for example, these 2D-COFs materials are basically expanded from triangular, quadrilateral and hexagonal structures.

[0004] For covalent organic frameworks, the application in lithium secondary batteries, especially in solid-state batteries, has also been developed to a certain extent, and it has been applied to a certain extent in electrode materials, solid electrolyte materials and diaphragms.

[0005] Further, the current lithium battery diaphragm generally uses micro-gravure, spraying, dot coating and other coating technologies to improve the performance of lithium batteries and improve the battery interface problem.

[0006] Some studies have reported the modification of COFs materials by, for example, epoxy, PEG, etc., and then mixed with lithium salt to form a solid-state electrolyte film, while such electrolyte films often have insufficient self-supporting and mechanical properties. The existing COFs-based diaphragm coating is mainly obtained by vacuum self-assembly to obtain a flexible self-supporting diaphragm. For example, some studies have reported a flexible self-supporting diaphragm composed of covalent organic framework material and bacterial cellulose. After mixing the covalent organic framework material and bacterial cellulose in an aqueous solution, it is filtered onto a microporous filter membrane, dried and then peeled off to obtain a covalent organic framework self-supporting diaphragm.

[0007] In addition, in other occasions, it is also proposed to use modified COFs materials mixed with polar adhesive components in a strong polar solvent, and then self-assembled on the surface of an organic film by filtration to obtain a composite diaphragm. SUMMARY

[0008] Problem to be solved by the invention

[0009] As mentioned above, the current COF-based separators are used in the form of electrolyte membranes, which have certain limitations in mechanical properties and use scenarios. In addition, the processing method of forming a self-supporting or organic film layer using COFs and other organic components mainly relies on self-assembly methods such as filtration, which is mainly due to the dispersion of COFs in the mixed system. However, this self-assembly method mainly relies on laboratory-level control methods, which is not as convenient and easy to operate as the conventional coating method for preparing separators.

[0010] In addition, in the current preparation of composite separators with a coating, a layer containing inorganic particles is usually formed on one side or both sides of the polyolefin substrate film layer. However, this structure, although it can improve the overall thermal stability of the membrane, cannot essentially provide faster or higher ion conduction efficiency. In addition, the polyolefin substrate film layer in these composite separators has low polarity and is not completely ideal in terms of electrolyte wettability.

[0011] In view of the above problems, the present application mainly provides a composite separator for a lithium secondary battery, which comprises a substrate film layer and a modified layer formed on at least one surface of the substrate, the modified layer comprising a modified COFs material and inorganic particles.

[0012] In this structure, the modified layer can improve the overall wettability of the separator to the electrolyte, because the inorganic material (such as ceramic material, etc.) has a liquid affinity, the modified (two-dimensional) COFs material has a large specific surface area and can store a large amount of electrolyte, and the ceramic and COFs materials work together to improve the overall wettability of the separator. In addition, the modified COFs material is endowed with lithium ions, which will be dissociated from the COFs framework to play a role in transporting lithium ions, thereby improving the ionic conductivity of the separator; at the same time, during the operation of the high-rate battery, the dissociated lithium ions in the modified layer of the separator can regulate the lithium ion concentration in the electrolyte, uniformize the electric field, regulate the lithium ion flux, and reduce the polarization of the battery as much as possible.

[0013] In addition, it is also found that when the modified COFs material is mixed with inorganic particles, the (two-dimensional) modified COFs material can be well dispersed in the water system with the help of inorganic particles, and then the modified layer can be efficiently formed by conventional coating means without the need for self-assembly as described above.

[0014] Therefore, in general, the composite separator provided by the present application can improve the performance of lithium batteries by synergistically using inorganic particles and COFs materials.

[0015] Solution for solving the problem

[0016] Through long-term practice, it is found that the above technical problems can be solved by implementing the following technical solutions:

[0017] The present application first provides a composite separator for secondary batteries, wherein the composite separator comprises:

[0018] a base material layer, and a modified layer formed on at least one surface of the base material layer;

[0019] wherein,

[0020] the base material layer comprises a polymer material;

[0021] the modified layer comprises a modified covalent organic framework material and inorganic particles,

[0022] the modified covalent organic framework material has a group containing active hydrogen in its molecular structure,

[0023] the hydrogen ions in the group containing active hydrogen can be replaced by lithium ions, and at least part of the hydrogen ions in the group containing active hydrogen are replaced by lithium ions.

[0024] According to the composite separator of the present application, both surfaces of the base material layer have the modified layer, and the two modified layers are the same or different.

[0025] According to the composite separator of the present application, the base material layer comprises a polyolefin-based resin; and the thickness of the modified layer is 5 μm or less.

[0026] According to the composite separator of the present application, the base material layer is a polyolefin porous membrane layer.

[0027] According to the composite separator of the present application, in the modified layer, the mass ratio of the modified covalent organic framework material to the inorganic particles is 3:7 to 7:3.

[0028] According to the composite separator of the present application, the group containing active hydrogen comprises one or more of carboxyl, phosphoric acid group, sulfonic acid group, and phenolic hydroxyl group.

[0029] According to the composite separator of the present application, the hydrogen ions in the group containing active hydrogen are substantially all replaced by lithium ions.

[0030] Further, the present application also provides a method for preparing the composite separator described above, wherein the method comprises:

[0031] a step of modifying the covalent organic framework material to impart a group containing active hydrogen to the molecules of the covalent organic framework material, and then allowing the hydrogen ions in the group containing active hydrogen to be at least partially replaced by lithium ions to obtain the modified covalent organic framework material;

[0032] a mixing step of mixing and dispersing the modified covalent organic framework material and the inorganic particles in water to obtain a mixed slurry;

[0033] a modified layer forming step of coating the mixed slurry on at least one surface of the substrate layer.

[0034] According to the method described above, in the mixing step, the mixing and dispersing is performed in the presence of a binder, and optionally, a processing aid can also be used.

[0035] According to the method described above, in the modified layer forming step, the coating includes roller coating, blade coating, dot coating or spray coating.

[0036] Effects of the invention

[0037] Through the implementation of the above technical solutions, the present application can obtain the following technical effects:

[0038] 1) By using the modified COFs material and the inorganic particles (ceramic material), not only the wettability of the whole membrane can be increased, but also the ion conductivity of the whole membrane can be greatly improved;

[0039] 2) In the modified COFs material, the lithium ions in the molecule contain electrostatic interactions, which can regulate the lithium ion concentration in the electrolyte / electrolyte, uniform the electric field and regulate the ion flux when the battery works at a large rate (for example, 5C, 6C or even 10C), so as to reduce the polarization phenomenon near the interface of the battery separator.

[0040] 3) In the modified layer, the modified COFs and the inorganic particles are used together, which not only improves the heat resistance of the whole membrane, but also improves the ion conductivity. At the same time, the above-mentioned composite use can give better processability, that is, the mixed slurry formed by the modified COFs and the inorganic particles has good dispersibility, which can make it possible to obtain the modified layer of the present application by coating process without the help of self-assembly process such as suction filtration. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 : The structure diagram of various modified COFs materials that can be used in the present application, wherein R: acetate, sulfonate, phosphate, phenolic hydroxyl salt. DETAILED DESCRIPTION

[0042] Hereinafter, the content of the present application will be described in detail. The description of the technical features described below is based on the representative embodiments, specific examples of the present application, but the present application is not limited to these embodiments, specific examples. It should be noted that:

[0043] In the present specification, a numerical range indicated using "numerical value A to numerical value B" means a range including the end point values A and B.

[0044] In the present specification, a numerical range indicated using "above" or "below" means a range including the present number.

[0045] In the present specification, the meaning indicated using "may" includes both the meaning of performing a certain process and the meaning of not performing a certain process.

[0046] In the present specification, "optionally" or "optional" indicates the use or non-use of certain substances, components, execution of steps, application of conditions, and the like.

[0047] In the present specification, "ordinary temperature" or "room temperature" used means an indoor environmental temperature of "23 ± 2°C".

[0048] In the present specification, the unit names used are international standard unit names, and if not specifically stated, "%" used means a percentage by weight or mass.

[0049] In the present specification, "substantially" or "essentially" means within a standard deviation of 1%, preferably 0.8%, and more preferably 0.5% from a theoretical model, theoretical data, or target data.

[0050] In the present specification, the term "comprising" and / or "including" means that a feature, step, operation, device, component, and / or combinations thereof is present.

[0051] In the present specification, "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", and the like mean that the specific elements (for example, features, structures, properties, and / or characteristics) described in relation to the embodiments are included in at least one embodiment described herein, and can be present in other embodiments or can not be present in other embodiments. In addition, it should be understood that the elements can be combined in various embodiments in any suitable manner.

[0052] The present application mainly provides a separator for a secondary battery having a composite structure, the composite separator having a base material layer and a modification layer formed on at least one surface of the base material layer. The present application is mainly based on the following insight:

[0053] Although COFs materials have been used for the preparation of secondary battery separators due to their excellent ion conductivity and other properties, there are problems in modifying them by themselves or mixing them with other components due to their (super) hydrophobic surface properties, for example, the current self-supporting or supported films based on COFs materials generally need to be assisted by self-assembly processes such as suction filtration. The present application found that when modified COFs are mixed with inorganic particles, good dispersibility can be obtained even in a water dispersion system, and as a result, composite separators can be obtained by conventional coating means, and due to the synergistic effect of the modified COFs and inorganic particles, not only the overall wettability of the separator to the electrolyte / liquid of the secondary battery is improved, but also the thermal stability is improved and the ion transport capacity is improved.

[0054] <First aspect>

[0055] In the first aspect of the present application, a composite separator for secondary batteries is provided. The separator includes a substrate layer and a modified layer formed on at least one surface of the substrate layer.

[0056] (Covalent organic framework material - COFs material)

[0057] Covalent organic framework compounds are a class of two-dimensional or three-dimensional structural materials formed by the reaction between organic precursors, which are more commonly used in the form of two-dimensional in the art, therefore, the present application preferably uses two-dimensional COFs materials.

[0058] For the COFs material that can be used in the present application, it can have a structural morphology extended from a triangular, quadrilateral, hexagonal structure.

[0059] Further, for the synthesis method of the COFs material of the present application, there is no particular restriction in principle, and it can be obtained according to the general synthesis method in the art.

[0060] In some specific embodiments, the above synthesis method includes coupling reaction, nucleophilic addition-elimination reaction, condensation reaction and the like. In some preferred embodiments of the present application, the synthesis method can be coupling reaction or nucleophilic addition-elimination reaction, more preferably, it can be based on nucleophilic addition-elimination reaction.

[0061] For the reactants based on nucleophilic addition-elimination reaction, there is no particular restriction in principle, for example, polyamines and polyaldehydes commonly used in the art can be used for reaction.

[0062] In some specific embodiments, the polyamines and polyaldehydes are both compounds with a functionality of 2 or more, and at least one of them has a functionality greater than 2. In addition, in some preferred embodiments, at least two of the amines in the polyamines are primary amines, or all of them are primary amines.

[0063] Further, in some specific embodiments, the polyamine can be a 2-5 membered amine, for example, it can be a di-amine, tri-amine, tetra-amine or penta-amine; and the polyaldehyde can be a 2-5 membered aldehyde, for example, it can be a di-aldehyde, tri-aldehyde, tetra-aldehyde or penta-aldehyde.

[0064] In some preferred embodiments, the polyamine is a 3-5 membered amine, or the polyaldehyde is a 3-5 membered aldehyde.

[0065] For the polyamine and the polyaldehyde, preferably, at least one of them has an aromatic structure, and more preferably, both of them have an aromatic structure.

[0066] For the aromatic structure as described above, in some specific embodiments, it can be an aromatic ring having 4-16 ring atoms, and preferably, it can be an aromatic ring having 5-10 ring atoms, for example, a benzene ring. For such a ring, it can include a single ring or a fused ring.

[0067] Further, for the aromatic ring as described above, it can be a carbon aromatic ring or a heteroaromatic ring containing heteroatoms, and for such heteroatoms, there is no particular limitation, for example, they can be O, N or S atoms, etc.

[0068] In addition, for the polyamine or the polyaldehyde, its structure can have one or more aromatic rings as described above, and preferably, these aromatic rings can be connected via a single bond or an optional polyvalent organic group.

[0069] For the polyamine that can be exemplified in the present application, in some specific embodiments, it can include:

[0070]

[0071] wherein A represents a -NH2 or -CO-NH-NH2 group.

[0072] For n, it represents an integer of 2-4, and for m, it can generally be 1 or 2, and preferably, it can be 1.

[0073] For the polyaldehyde that can be exemplified in the present application, in some specific embodiments, it can include:

[0074]

[0075] wherein B represents a -CHO group.

[0076] For p, it represents an integer of 2-4; and for q, it can generally be 1 or 2, and preferably, it can be 1.

[0077] Further, for the above-mentioned aromatic structure (aromatic ring) of the present application, in addition to the group such as A or B, optionally, other groups can be further attached.

[0078] For such other groups, in principle, there is no particular limitation, for example, including: halogen groups, halogenated hydrocarbon groups, saturated or unsaturated aliphatic hydrocarbon groups, (poly)ether-containing groups, and the like.

[0079] Further, in some specific embodiments, the other groups, preferably active hydrogen-containing groups, for such groups, generally can include one or more of carboxyl groups, phosphoric acid groups, sulfonic acid groups, hydroxyl groups, mercapto groups, and the like, preferably one or more of carboxyl groups, phosphoric acid groups, sulfonic acid groups, or phenolic hydroxyl groups.

[0080] Therefore, in some specific embodiments of the present application, the active hydrogen-containing groups in the COFs material can be introduced via the raw material monomers in the synthesis of the COFs material.

[0081] Further, for the above-mentioned synthesis method of the COFs material, in principle, there is no particular limitation, and can be performed according to the methods available in the art.

[0082] (Modified COFs material)

[0083] The modified COFs material of the present application mainly refers to the replacement of the active hydrogen in the active hydrogen-containing groups possessed by the above-mentioned COFs material molecules with lithium ions. As mentioned above, for such active hydrogen-containing groups, generally can include one or more of carboxyl groups, phosphoric acid groups, sulfonic acid groups, hydroxyl groups, mercapto groups, and the like, preferably one or more of carboxyl groups, phosphoric acid groups, sulfonic acid groups, or phenolic hydroxyl groups.

[0084] Further, according to the source of the active hydrogen-containing groups, two cases are included:

[0085] Case 1: As mentioned above, if one or both of the polyamines or polyaldehydes in the synthesis of the COFs material already have active hydrogen-containing groups in their molecules, then the obtained COFs material directly has active hydrogen-containing groups, at this time, the COFs material can be directly subjected to the subsequent replacement of lithium ions;

[0086] Case 2: If neither of the polyamines or polyaldehydes in the synthesis of the COFs material has active hydrogen-containing groups in their molecules, then after the synthesis of the COFs material, further treatment is performed to impart active hydrogen-containing groups to its molecular structure.

[0087] For further processing in case 2, there is no particular restriction in principle, for example, by means of coupling reaction, addition reaction, grafting reaction, etc., the active hydrogen-containing group can be further introduced into the COF molecular structure.

[0088] In the present application, for the above-mentioned case 1 and case 2, both can be referred to as imparting an active hydrogen-containing group to the covalent organic framework material molecule.

[0089] Further, the above-mentioned substitution can be carried out under alkaline conditions. In some specific embodiments, a solution containing lithium salt and / or LiOH can be used to treat COFs.

[0090] A specific modified COF molecular structure can be specifically listed in the present application Figure 1 The "R" group is treated by alkaline conditions to obtain various types of lithium salt-containing groups.

[0091] Through the above-mentioned substitution treatment, the hydrogen ions in the active hydrogen-containing groups in the COF material can be at least partially replaced, and further, these hydrogen ions are substantially all replaced by lithium ions.

[0092] (Composite separator)

[0093] The composite separator of the present application mainly includes a substrate layer and a modified layer containing the above-mentioned modified COF material and inorganic particles.

[0094] Substrate layer

[0095] For the substrate layer of the present application, there is no particular restriction in principle, which can be a porous membrane layer commonly used in secondary batteries, especially lithium secondary batteries.

[0096] For such a porous membrane, preferably, various high molecular material-based porous membranes can be used, which include various artificial polymers, natural polymers. In some specific embodiments, the porous membrane is formed by a composition including polyolefin-based resin, a composition containing cellulose material. More preferably, the porous membrane can be formed by polyolefin-based material.

[0097] For the above-mentioned polyolefin material, which can be listed includes monounsaturated olefin monomer with carbon atom number of 2-8 formed by homopolymerization or copolymerization. In some preferred embodiments, the olefin monomer can be ethylene, propylene, styrene, butene, etc. More preferably, the porous membrane of the present application can be formed by

[0098] formed by polyethylene or polypropylene, or formed by polyethylene or polypropylene.

[0099] The porosity of the substrate layer of the present application is not particularly limited, and can be, for example, 20 to 45 v%, preferably 25 to 42 v%.

[0100] Further, the thickness of the substrate layer described above is not particularly limited in principle, and a thickness of 20 μm or less, for example, 5 to 10 μm, can be cited.

[0101] Modified layer

[0102] The modified layer of the present application is mainly obtained by mixing the modified COFs material described above and inorganic particles to obtain a system suitable for coating.

[0103] The conventional COFs material has a poor dispersibility due to the high hydrophobicity thereof, in addition to the surface morphology thereof.

[0104] The present application has found that a well-dispersible mixed system can be obtained even by using an aqueous solvent or even water as a solvent when the modified COFs material described above and inorganic particles are mixed. It is presumed that this can be related to the lithium ions imparted to the modified COFs material, or to the adsorptivity of the modified COFs material and the interaction thereof with the inorganic particles.

[0105] Further, in some specific embodiments of the present application, the modified COFs material and inorganic particles can be mixed in water. Optionally, a binder and other processing aids can be used as needed.

[0106] For the inorganic particles of the present application, various ceramic particles generally used in battery separators in the art can be used. In some preferred embodiments of the present application, the ceramic can be selected from at least one of boehmite, alumina, magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, and barium sulfate. The particle size of such particles is not particularly limited, and the Dv50 thereof can be generally 0.5 to 2 μm, preferably 0.7 to 1.5 μm.

[0107] The use ratio of the modified COFs material and inorganic particles can be generally 3:7 to 7:3, preferably 5:5 to 7:3, from the viewpoint of dispersibility, heat resistance, and ion conductivity.

[0108] The binder described above is not particularly limited in principle, and examples that can be cited include fluorinated olefin-based binders, such as COPNA resin-based binders, PVDF binders, and the like.

[0109] For the above-mentioned processing aids, there is no particular restriction in principle, and examples that can be cited include dispersants, thickeners, wetting agents, and the like. In some specific embodiments, the dispersants include hydrolyzed polymaleic anhydride-based dispersants; the thickeners include sodium carboxymethylcellulose-based thickeners; and the wetting agents include silanol nonionic surfactant-based wetting agents.

[0110] Further, in the mixed system, the total content of the modified COFs material and the inorganic particles, based on the solid matter or dry matter, can be 94 mass% or more, preferably 94 mass% to 96 mass%.

[0111] By the above mixing, a slurry containing the modified COFs material and the inorganic particles can be obtained. Further, by applying the slurry to the above-mentioned base layer to form a modified layer, a composite separator of the present application can be obtained.

[0112] For the manner of application, there is no particular limitation, and roll coating, blade coating, dot coating, or spray coating can be performed depending on the specific solid content and viscosity of the slurry. More specifically, a slurry liquid is obtained, and then the slurry liquid is applied to the surface of the separator by microgravure / gravure roll coating, spray coating, or dot coating.

[0113] For the thickness of the modified layer in the finally obtained composite separator, there is no particular restriction in principle, and it can generally be 5 μm or less, for example, it can be 0.1 to 4.5 μm, preferably 1.5 to 4 μm, and examples that can be cited include 0.5 μm, 1.0 μm, 1.8 μm, 2.2 μm, 2.5 μm, 3.0 μm, 3.5 μm, and the like.

[0114] In the present application, first, the use of a coating layer containing inorganic particles in the composite separator can increase the thermal stability of the separator; second, the coating layer also improves the wettability of the separator to the electrolyte, since LiOH or a lithium salt is used, the active hydrogen ions in the COFs molecules are replaced by Li through lithium ion exchange, becoming a new type of ion conductor, and at the same time, by virtue of the large specific surface area of the COFs material, a large amount of electrolyte can be stored. The lithium ions in the structures such as lithium sulfonate and lithium carboxylate present on the surface of the covalent organic framework material can be dissociated from the COFs framework, playing a role in transporting lithium ions and improving the ionic conductivity of the separator; at the same time, during the operation of the high-rate battery, the dissociated lithium ions in the modified layer of the separator can regulate the lithium ion concentration in the electrolyte, uniformize the electric field, regulate the lithium ion flux, and as much as possible reduce the polarization of the battery. By exerting the synergistic effect of the inorganic particles and the modified COFs material, the performance of the secondary battery is improved.

[0115] (Secondary battery)

[0116] The secondary battery according to the present application includes various secondary batteries having ion conductivity, and particularly, refers to a lithium secondary battery, including a non-aqueous electrolyte lithium secondary battery, a semi-solid, quasi-solid, etc. lithium secondary battery.

[0117] The secondary battery according to the present application can be a power battery, i.e., a battery for providing power to a transportation or a vehicle, or a secondary battery for a power storage device for wind power, hydroelectric power, solar power, or conventional fossil fuel power.

[0118] In some specific embodiments, the battery according to the present application is used in a single form, and in other specific embodiments, the battery according to the present application can be used in parallel or in series in any number of scales.

[0119] The lithium secondary battery according to the present application can include a cathode, an anode, an electrolyte, and the composite separator described above.

[0120] The cathode includes a current collector and a cathode active material, and optionally, an auxiliary agent, etc. The cathode active material is not particularly limited in principle, and for example, can be a transition metal-doped lithium oxide, and typically, can be a ternary lithium cathode active material doped with nickel, manganese, and cobalt.

[0121] The anode can include a current collector, an anode active material, and optionally, an auxiliary agent, etc. The anode active material can include a carbon-based material and a non-carbon-based material.

[0122] The carbon-based material includes a graphite material (natural graphite, artificial graphite, and meso-carbon microbeads) and other carbon-based materials (hard carbon, soft carbon, and graphene); and the non-carbon-based material can be classified into a titanium-based material, a silicon-based material, a tin-based material, a nitride, and metallic lithium, etc.

[0123] Further, the electrolyte that can be used in the secondary battery according to the present application is not particularly limited in principle.

[0124] In some specific embodiments, the electrolyte includes an electrolyte and a non-aqueous solvent.

[0125] The kind of the non-aqueous solvent is not particularly limited in the present application, as long as it is a non-aqueous solvent generally used for a non-aqueous electrolyte.

[0126] In some specific embodiments, the non-aqueous solvent can be selected from one or more of a cyclic carbonate-based solvent, a linear carbonate-based solvent, an ether-based solvent, an ester-based solvent, and a ketone-based solvent.

[0127] The cyclic carbonate solvent can be selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), etc. The linear carbonate solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC), etc. The ester solvent can be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and methyl pivalate, etc. The ether solvent can be selected from dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), dioxolane (DOL), etc. The ketone solvent can be selected from polymethyl vinyl ketone, etc. These non-aqueous solvents can be used alone or in the form of a mixture of two or more.

[0128] In some preferred embodiments, the non-aqueous solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate, propyl acetate, ethyl propionate, propyl ester, methyl butyrate, and ethyl butyrate.

[0129] For the electrolyte of the present application, generally, various lithium salts can be used, and for the kind of lithium salt, the present application is not particularly limited, and can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt can be selected from one or more of a salt of lithium ion and the following anions: PF6 - , BF4 - , CI - , Br - , I - , CIO4 - , AsF6 - , SiF6 2- , AICI4 - , B(C2O4)2 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , N(FSO2)2 - , C(CF2SO2)3 - , C2BF2O4 - .

[0130] For other functional additives that can be used in the electrolyte of the present application, there is no particular limitation in principle, for example, film formation can be promoted by the use of some additives.

[0131] Among them, the inorganic particle can be selected from lithium tetrafluoroborate (LiBF4), trimethylsilyl phosphate (TMSP), trimethylsilyl borate (TMSB) and the like; the sulfur-containing additive can be selected from 1,3-propane sultone (1,3PS), 1,4-butane sultone (1,4-BS), 2,4-butane sultone (2,4-BS), 1,3-propylene sultone (PST), ethylene sulfate (DTD), methane disulfonate methylene (MMDS), ethylene sulfite (ES) and the like; the oxalate-containing additive can be selected from lithium difluoro oxalate borate (LiDFOB), lithium bisoxalate borate (LiBOB), lithium tetrafluoro oxalate phosphate (LiTFOP), lithium difluoro bisoxalate phosphate (LiDFOP) and the like. These additives can be used alone or in the form of a mixture of two or more.

[0132] In summary, in the secondary battery of the present application, due to the design of the structure of the modified layer formed by the inorganic particles and the modified COFs, the effects of high heat resistance, high wettability, high liquid absorption rate, high mechanical strength and high ionic conductivity of the composite separator are achieved. Mixing the inorganic particles with the modified COF material not only greatly improves the liquid wettability of the inorganic particle / ceramic to the wettability of the electrolyte to the separator, but also has good uniform dispersibility in the slurry obtained by mixing the two, which greatly improves the wettability of the modified COF material to the electrolyte, so that a large amount of electrolyte can be stored by taking advantage of the large specific surface area of the COFs. And the lithium ions endowed in the modified COF material will be dissociated from the COF skeleton, playing a role in transporting lithium ions and improving the ionic conductivity of the separator. In addition, during the operation of the high-rate battery, the dissociated lithium ions in the modified layer of the separator can regulate the lithium ion concentration in the electrolyte, uniformize the electric field, regulate the lithium ion flux, and as much as possible reduce the polarization of the battery.

[0133] Examples

[0134] The embodiments of the present application will be described in detail below with reference to the examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions are not specified in the examples, and the conventional conditions or the conditions recommended by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained by purchase.

[0135] Raw material :

[0136] Modified COFs material: The modified COFs material used in the present embodiment and comparative examples 1-3 is as follows Figure 1 Structure representation, wherein R in comparative example 3 is replaced by a sulfonic acid group.

[0137] Comparative example 4 is a COFs material formed by m-dialdehyde toluene and 4,4'-diaminobiphenyl.

[0138] Thermal shrinkage test method :

[0139] The coated separator was cut into a 10 cm x 10 cm piece, clamped with A4 paper, and placed in a 200°C air oven for 1h. After the test was completed, the length in the TD and MD directions was measured with a steel ruler, and the heat shrinkage rate was calculated by the shrinkage length. The formula for calculating the heat shrinkage rate of the separator is as follows:

[0140] Heat shrinkage rate (%) = (Li-Lf) / Li*100%, Li is the size before testing, and Lf is the size after testing.

[0141] Ionic conductivity test method :

[0142] By assembling 1 layer, 2 layers, 3 layers of symmetrical battery, according to the measured impedance of the separator, a linear curve is fitted, and the slope k is the resistance of the separator. According to the formula, the ionic conductivity σ = 0.1d / (Rs*A) is calculated, where d is the thickness (μm), Rs is the resistance of the separator (Ω), and the value of the ionic conductivity is obtained.

[0143] Example 1 :

[0144] The ceramic particles and the modified COFs material were uniformly dispersed in water, and the mass ratio of ceramic to modified COFs material was 45:55. The solid and auxiliary agent accounted for 95 and 5% respectively (dispersant accounted for 0.4%, binder accounted for 4%, thickener accounted for 0.5%, wetting agent accounted for 0.1%). The mixed slurry was uniformly coated on the PE base film in the form of micro-gravure roller coating, and the coating thickness was 3um. After heating at 200°C for 1h, the heat shrinkage ratio of the separator was TD<2.3, MD<2.5. The lithium ion conductivity was 1.83mS / cm, and the film breaking temperature was 210°C.

[0145] Example 2 :

[0146] Ceramic particles and modified COFs material were uniformly dispersed in water, ceramic, modified COFs material mass ratio was 35%:65%. The solid and auxiliary accounted for 95 and 5% (dispersant accounted for 0.4%, binder accounted for 4%, thickener accounted for 0.5%, wetting agent accounted for 0.1%). The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, the coating thickness was 3um. 200℃ heating for 1h, the heat shrinkage ratio of the separator TD<3.0, MD<3.0. Lithium ion conductivity was 1.24mS / cm, the film breaking temperature was 205℃.

[0147] Example 3 :

[0148] Ceramic particles and modified COFs material were uniformly dispersed in water, ceramic, modified COFs material mass ratio was 65%:35%. The solid and auxiliary accounted for 95 and 5% (dispersant accounted for 0.4%, binder accounted for 4%, thickener accounted for 0.5%, wetting agent accounted for 0.1%). The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, the coating thickness was 3um. 200℃ heating for 1h, the heat shrinkage ratio of the separator TD<2.5, MD<2.8. Lithium ion conductivity was 1.32mS / cm, the film breaking temperature was 208℃.

[0149] Example 4 :

[0150] Except that the lithium sulfonate group in the modified COFs material was replaced by lithium phosphate group (modified COFs-1), the rest was the same as example 1. Ceramic, modified COFs-1 material mass ratio was 45%:55%. The solid and auxiliary accounted for 95 and 5% (dispersant accounted for 0.4%, binder accounted for 4%, thickener accounted for 0.5%, wetting agent accounted for 0.1%).

[0151] The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, the coating thickness was 3um. 200℃ heating for 1h, the heat shrinkage ratio of the separator TD<2.7, MD<2.9. Lithium ion conductivity was 1.25mS / cm, the film breaking temperature was 207℃.

[0152] Example 5 :

[0153] Except that the lithium sulfonate group in the modified COFs material was replaced by lithium phenolic hydroxyl group (modified COFs-2), the rest was the same as example 1. Ceramic, modified COFs-2 material mass ratio was 45%:55%. The solid and auxiliary accounted for 95 and 5% (dispersant accounted for 0.4%, binder accounted for 4%, thickener accounted for 0.5%, wetting agent accounted for 0.1%).

[0154] The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, with a coating thickness of 3 um. After heating at 200°C for 1 h, the membrane heat shrinkage ratio TD < 2.8, MD < 3.0. The lithium ion conductivity was 1.17 mS / cm, and the membrane breaking temperature was 207°C.

[0155] Comparative Example 1 :

[0156] The ceramic particles were uniformly dispersed in water, with a ceramic: modified COFs material mass ratio of 100%:0%, and a solid: auxiliary ratio of 95 and 5% (dispersant 0.4%, binder 4%, thickener 0.5%, wetting agent 0.1%). The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, with a coating thickness of 3 um. After heating at 200°C for 1 h, the membrane broke. The lithium ion conductivity was 0.72 mS / cm, and the membrane breaking temperature was 156°C.

[0157] Comparative Example 2 :

[0158] The modified COFs material was dispersed in water, with a ceramic: modified COF mass ratio of 0%:100%, and a solid: auxiliary ratio of 95 and 5% (dispersant 0.4%, binder 4%, thickener 0.5%, wetting agent 0.1%). The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, with a coating thickness of 3 um. After heating at 200°C for 1 h, the membrane broke. The lithium ion conductivity was 0.059 mS / cm, and the membrane breaking temperature was 152°C.

[0159] Comparative Example 3 :

[0160] Except that the modified COFs material was replaced by a COFs (modified COFs-3) material without sulfonic acid groups but not lithiumated, the rest was the same as Example 1. The ceramic: modified COFs-3 material mass ratio was 45%:55%. The solid: auxiliary ratio was 95 and 5% (dispersant 0.4%, binder 4%, thickener 0.5%, wetting agent 0.1%). The mixed slurry was uniformly coated on the PE base film by micro-gravure roll coating, with a coating thickness of 3 um. After heating at 200°C for 1 h, the membrane heat shrinkage ratio TD < 2.7, MD < 2.9. The lithium ion conductivity was 1.03 mS / cm, and the membrane breaking temperature was 200°C.

[0161] Comparative Example 4 :

[0162] The rest is the same as Example 1 except that the modified COFs material is replaced by MOF-5 material; the mass ratio of ceramic to MOF-5 material is 45%:55%. The solid and auxiliary agent account for 95 and 5% respectively (dispersant accounts for 0.4%, binder accounts for 4%, thickener accounts for 0.5%, wetting agent accounts for 0.1%). The mixed slurry is uniformly coated on the PE base film by micro-gravure roll coating, and the coating thickness is 3um. Heat at 200℃ for 1h, the heat shrinkage ratio of the separator TD<2.8, MD<3.0. The lithium ion conductivity is 0.045mS / cm, and the film breaking temperature is 204℃.

[0163] Comparative Example 5 :

[0164] The rest is the same as Example 1 except that the modified COFs material is replaced by ZIF-67 material; the mass ratio of ceramic to ZIF-67 material is 45%:55%. The solid and auxiliary agent account for 95 and 5% respectively (dispersant accounts for 0.4%, binder accounts for 4%, thickener accounts for 0.5%, wetting agent accounts for 0.1%). The mixed slurry is uniformly coated on the PE base film by micro-gravure roll coating, and the coating thickness is 3um. Heat at 200℃ for 1h, the heat shrinkage ratio of the separator TD<2.8, MD<3.0. The lithium ion conductivity is 0.032mS / cm, and the film breaking temperature is 203℃.

[0165] Comparative Example 6 :

[0166] The rest is the same as Example 1 except that the modified COFs material is replaced by ZIF-67 material; the mass ratio of ceramic to ZIF-67 material is 45%:55%. The solid and auxiliary agent account for 95 and 5% respectively (dispersant accounts for 0.4%, binder accounts for 4%, thickener accounts for 0.5%, wetting agent accounts for 0.1%). The mixed slurry is uniformly coated on the PE base film by micro-gravure roll coating, and the coating thickness is 3um. Heat at 200℃ for 1h, the heat shrinkage ratio of the separator TD<2.8, MD<3.0. The lithium ion conductivity is 0.032mS / cm, and the film breaking temperature is 203℃.

[0167] The composition and performance test data of the above examples and comparative examples are shown in Tables 1 and 2 respectively.

[0168] Table 1:

[0169]

[0170] Table 2:

[0171]

[0172] It should be noted that although the technical solutions of the present application are described with specific examples, those skilled in the art can understand that the present disclosure should not be limited thereto.

[0173] The above has described various embodiments of the present disclosure, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles, practical applications, or improvements to the technology in the market of the embodiments, or to enable other ordinary skilled in the art to understand the embodiments disclosed herein.

Claims

1. A composite separator for secondary batteries, characterized in that, The composite diaphragm comprises: A substrate layer, and a modified layer formed on at least one surface of the substrate layer; in, The substrate layer includes a polymer material; The modified layer includes a modified covalent organic framework material and inorganic particles. The modified covalent organic framework material has groups containing active hydrogen in its molecular structure. The hydrogen ions in the group containing active hydrogen can be replaced by lithium ions, and at least a portion of the hydrogen ions in the group containing active hydrogen are replaced by lithium ions.

2. The diaphragm according to claim 1, characterized in that, The modified layer is present on both surfaces of the substrate layer, and the two modified layers may be the same or different.

3. The diaphragm according to claim 1 or 2, characterized in that, The substrate layer comprises a polyolefin resin; the thickness of the modified layer is less than 5 μm.

4. The diaphragm according to claim 1 or 2, characterized in that, The substrate layer is a porous resin film layer.

5. The diaphragm according to claim 1 or 2, characterized in that, In the modified layer, the mass ratio of the modified covalent organic framework material to the inorganic particles is 3:7 to 7:

3.

6. The diaphragm according to claim 1 or 2, characterized in that, The groups containing active hydrogen include one or more of carboxyl, phosphate, sulfonic acid, and phenolic hydroxyl groups.

7. The diaphragm according to claim 1 or 2, characterized in that, The hydrogen ions in the group containing active hydrogen are substantially all replaced by lithium ions, wherein "substantially" means that the standard deviation from the theoretical model, theoretical data or target data is within 1%.

8. A method for preparing a diaphragm according to any one of claims 1 to 7, characterized in that, The method includes: The step of modifying a covalent organic framework material involves giving the covalent organic framework material molecule a group containing active hydrogen, and then at least partially replacing the hydrogen ions in the active hydrogen-containing group with lithium ions to obtain the modified covalent organic framework material. The mixing step involves dispersing and mixing the modified covalent organic framework material with the inorganic particles in water to obtain a mixed slurry; The step of forming the modified layer involves coating the mixed slurry onto at least one surface of the substrate layer.

9. The method according to claim 8, characterized in that, In the mixing step, the dispersion mixing is carried out in the presence of an adhesive.

10. The method according to claim 9, characterized in that, Additives are also added during the dispersion and mixing process.

11. The method according to claim 8 or 9, characterized in that, In the step of forming the modified layer, the coating includes roller coating, blade coating, dot coating, or spray coating.

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