Vinylidene fluoride copolymer for reversible crosslinking, and preparation method and application thereof

Abstract

The invention discloses a vinylidene fluoride copolymer for reversible crosslinking as well as a preparation method and application thereof. The vinylidene fluoride copolymer for reversible crosslinking comprises a vinylidene fluoride structural unit and a structural unit of at least one compound shown in a structural formula (1), a structural formula (2) or a structural formula (3). The invention finds that a copolymer obtained by polymerizing VDF and an olefinic unsaturated polyol monomer can generate dynamic crosslinking with a crosslinking agent of a compound as shown in a structural formula (4) to form a reversible crosslinked network structure, and the copolymer can be repeatedly processed and recycled while the mechanical property is improved. When the metal current collector is used as an electrode material of a lithium battery, the metal current collector not only has relatively strong adhesive property, but also can prevent an electrode active material and a conductive substance from falling off from the metal current collector due to the network-shaped structure. When the composite material is used as a lithium battery diaphragm coating material, the swelling resistance and cohesiveness of a coating can be effectively improved, and the solubility of the coating in an electrolyte is reduced, so that the safety and stability of a lithium ion battery are improved.

Description

Technical Field

[0001] The invention relates to the field of polymers, and in particular to a reversibly cross-linked vinylidene fluoride copolymer, a preparation method and application thereof. Background Art

[0002] Fluorine-containing polymer materials have attracted much attention due to their unique chemical stability and mechanical properties. Among them, polyvinylidene fluoride is a partially fluorinated, semi-crystalline polymer with excellent chemical resistance, heat resistance, weather resistance and dielectric properties, as well as good molding and processing properties. Therefore, it is widely used in coatings, film materials, pipes and cables, and secondary battery adhesives. However, like general thermoplastic resins, the properties of some fluoropolymer resins change with temperature. For example, the modulus and tensile strength decrease with increasing temperature. People use various technical means to modify fluoropolymers, among which some fluoropolymers with unique physical properties are prepared by cross-linking reactions.

[0003] Radiation is a common means of cross-linking polymers. For example, patent CN114276576A discloses a method for cross-linking and modifying polyvinylidene fluoride-chlorotrifluoroethylene membranes, which irradiates the fluorinated copolymer with gamma rays, so that it has photothermal dual shape memory properties. Patent CN106450327B discloses a method for improving the adhesive properties of polyvinylidene fluoride in lithium batteries by irradiation. By irradiating the entire lithium-ion battery, the polyvinylidene fluoride is partially cross-linked, thereby changing its adhesive properties, reducing the battery thickness expansion rate, and improving the battery capacity retention rate, thereby improving battery stability and extending the battery life. Patent CN111094368B discloses a method for preparing a cross-linked fluorinated polymer film, which is formed by depositing a fluorinated polymer, a non-nucleophilic base and a cross-linking agent on a substrate, and then exposing it to ultraviolet radiation. Since the base can cause the fluorinated polymer to dehydrohalogenate to form double bonds, the cross-linking of the polymer film can be achieved under the action of the cross-linking agent.

[0004] In addition, cross-linking of fluorinated polymers can also be achieved by introducing double bonds or reactive functional groups. For example, patent CN103588921B discloses a method for preparing a high-viscosity self-crosslinked vinylidene fluoride copolymer, which copolymerizes vinylidene fluoride with an allenyl ether compound having a unique continuous double bond structure. The resulting polymer product contains double bonds that can continue to react. After further double bond free radical polymerization is initiated by ultraviolet light, cross-linking between copolymer molecules is achieved to form a cross-linked body with high bonding strength. Patent CN108395658B uses atom transfer radical polymerization and single electron transfer radical polymerization to graft active monomers on polyvinylidene fluoride to obtain a modified copolymer containing a reactive functional group side chain, and then achieves cross-linking modification of fluorinated polymers through chemical reactions between the functional group and other groups.

[0005] However, there are some problems in the preparation process of the above cross-linked fluorinated polymers. For example, the radiation method can cross-link the polymer, but it also causes partial degradation of the polymer, produces some side reactions, introduces impurities, and there are also some uncontrollable factors. In addition, these cross-linking processes are often irreversible. The thermal movement of the polymer molecular chain is restricted by its own cross-linked network structure, and it cannot move freely, cannot be dissolved by organic solvents, and even will not melt when decomposed by heat. It is difficult to perform secondary processing after solidification, which has an important impact on the later processing and application of the polymer.

[0006] Dynamic cross-linked polymers are three-dimensional mesh materials connected by stable covalent bonds. By introducing dynamic chemical bonds into polymers, thermosetting cross-linked polymers can move like the molecular chains of thermoplastic polymers under certain stimuli. They not only have the cross-linked structural characteristics of thermosetting polymers, but also have the advantages of thermoplastic polymers such as being fusible and easy to process. The borate group has good thermal stability, and the boron-oxygen bond has the characteristics of reversible exchange, and the exchange reaction can be carried out directly under heating conditions without a catalyst. Therefore, reversible covalent cross-linked polymers based on borate groups have become one of the research hotspots of polymer materials.

[0007] People have applied borate groups to dynamic cross-linked networks of polyethylene. Patent CN111978338A discloses a method for preparing a polyethylene dynamic cross-linking agent, which is generated by reacting fatty acid alcohol amide with phenylenediboric acid. The polyethylene dynamic cross-linking agent has good dynamic reversibility and can be used as a cross-linking agent in the polyethylene cross-linking process to achieve dynamic cross-linking of polyethylene. Patent CN112358631B prepares a dynamic covalent cross-linking material that enhances the performance of polyethylene by exchanging linear polyethylene containing borate groups with a polybasic borate cross-linking agent. The material is added to commercial thermoplastic polyethylene to synthesize a composite polymer material of polyethylene / dynamically cross-linked polyethylene. The mechanical properties, creep resistance, solvent resistance, heat resistance and other properties of the composite polymer material are significantly enhanced, and can be used for repeated processing and repeated shaping.

[0008] Therefore, it is very necessary to provide a dynamically cross-linkable vinylidene fluoride copolymer. Summary of the invention

[0009] Technical problem to be solved by the invention

[0010] In order to solve the problems of cross-linked vinylidene fluoride copolymers being difficult to operate and control during the cross-linking process, easy to introduce impurities, and difficult to reprocess and utilize after cross-linking and curing, the present invention finds that the copolymer obtained by polymerization of VDF and ethylenically unsaturated polyol monomers can undergo dynamic cross-linking with the cross-linking agent of the compound shown in the structural formula (4) to form a reversible cross-linked network structure, which can improve the mechanical properties while being repeatedly processed and recycled. When used as a lithium battery electrode material, it not only has strong bonding properties, but also the network structure can prevent the electrode active material and the conductive material from falling off the metal current collector. When used as a lithium battery separator coating material, it can effectively improve the swelling resistance and bonding properties of the coating, reduce the solubility of the coating in the electrolyte, thereby improving the safety and stability of lithium-ion batteries.

[0011] In order to significantly improve mechanical properties, bonding properties and other properties, it is necessary to have a certain content of olefinic unsaturated polyol structural units in the vinylidene fluoride copolymer. When the olefinic unsaturated polyol structural units are too few, the various performance improvements are not obvious. In addition, olefinic unsaturated polyols are difficult to copolymerize with VDF. The present invention increases the content of olefinic unsaturated polyol structural units by adding a water-soluble inorganic salt to water during the polymerization reaction. The amount of the water-soluble inorganic salt added is 0.1 to 10 wt% of the weight of the water phase.

[0012] Technical solutions to solve problems

[0013] In a first aspect, the present invention provides a reversibly cross-linkable vinylidene fluoride copolymer, comprising a vinylidene fluoride structural unit and a structural unit selected from at least one compound represented by structural formula (1), structural formula (2) or structural formula (3),

[0014]

[0015] Among them, R 1 , R 2 , R 3 R is independently selected from hydrogen, halogen, aryl, alkyl having 1 to 12 carbon atoms or perfluoroalkyl; 4 is selected from an alkyl group, an aryl group, an ester group, an amide group, or an alkyl group containing any one of N, P, S, and O having 0 to 12 carbon atoms; n is an integer of 1 to 12,

[0016]

[0017] Among them, R 5 is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group, or an alkyl group containing any one of N, P, S, and O; R 6 is selected from a hydrogen atom, a hydroxyl group, an amino group, an aryl group, an alkyl group having 1 to 12 carbon atoms, a perfluoroalkyl group, or an alkyl group containing any one of N, P, S, and O,

[0018]

[0019] Among them, R 8 , R 9 , R 10 R is independently selected from hydrogen, halogen, aryl, alkyl having 1 to 12 carbon atoms or perfluoroalkyl; 7 It is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group, or an alkyl group containing any one of N, P, S, and O; and n is an integer of 0 to 12.

[0020] The content of the vinylidene fluoride structural unit is 70 to 99.99 wt %,

[0021] The total content of the structural units of the compound represented by the structural formula (1), the structural formula (2) or the structural formula (3) is 0.01 to 10 wt %.

[0022] From the perspective of copolymerization reactivity, the structural formula (1) preferably has R 1 , R 2 , R 3 R is independently selected from hydrogen, fluorine, trifluoromethyl; 4 is selected from an alkyl group and an ester group having 0 to 12 carbon atoms. More preferably, R 1 , R 2 , R 3are hydrogen or fluorine, R 4 It is selected from ester groups having 0 to 6 carbon atoms.

[0023] From the perspective of copolymerization reactivity, the structural formula (2) preferably has R 5 is selected from an alkyl group having 1 to 12 carbon atoms or an alkyl group containing any one of N, S, and O, R 6 is selected from hydrogen, hydroxyl, and an alkyl group having 1 to 6 carbon atoms; more preferably, R 5 is selected from an alkyl group having 1 to 6 carbon atoms and containing N or O atoms, R 6 It is hydroxyl.

[0024] From the perspective of copolymerization reactivity, the structural formula (3) preferably has R 8 , R 9 , R 10 are independently selected from hydrogen, fluorine, trifluoromethyl, R 7 is selected from an alkyl group having 1 to 12 carbon atoms or an alkyl group containing any one of N, S, and O; n is an integer of 0 to 12; more preferably, R 8 , R 9 , R 10 are hydrogen or fluorine, R 7 The group is selected from an alkyl group having 1 to 6 carbon atoms and containing N or O atoms; n is an integer of 1 to 6.

[0025] Furthermore, the compound represented by the structural formula (1) is selected from 3-(allyloxy)-1,2-propanediol.

[0026] Furthermore, the compound represented by the structural formula (2) is selected from at least one of 1-(2,3-dihydroxypropyl)(2Z)-2-butenedioic acid ester and 4-[(2,3-dihydroxypropyl)amino]-4-oxo-2-butenoic acid.

[0027] 1-(2,3-dihydroxypropyl)(2Z)-2-butenedioate, the structural formula of which is:

[0028]

[0029] 4-[(2,3-dihydroxypropyl)amino]-4-oxo-2-butenoic acid, the structural formula of which is:

[0030]

[0031] Furthermore, the compound represented by the structural formula (3) is at least one selected from 2,3-dihydroxypropyl acrylate and 2-methyl-2-acrylate-2,3-dihydroxypropyl ester.

[0032] Preferably, the reversibly cross-linkable vinylidene fluoride copolymer comprises a vinylidene fluoride structural unit, a structural unit of a compound represented by structural formula (1), or a structural unit of a compound represented by structural formula (3).

[0033] The content of the vinylidene fluoride structural unit in the reversibly cross-linkable vinylidene fluoride copolymer of the present invention is 70 to 99.99 wt %, preferably 85 to 95 wt %.

[0034] The total content of the structural units of the compounds represented by the structural formula (1), structural formula (2) or structural formula (3) of the present invention is 0.01 to 10 wt %, preferably 0.1 to 5 wt %.

[0035] Furthermore, the reversibly cross-linkable vinylidene fluoride copolymer also includes other comonomer structural units, and the other comonomers are selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluorovinyl ether, vinyl fluoride, methyl acrylate, methyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, maleate, citraconic acid, vinyl acetate, vinyl carbonate or acrylonitrile, and the content of the other comonomer structural units is 0 to 20wt%, preferably 0.1 to 10wt%.

[0036] In a second aspect, the present invention provides a method for preparing a reversibly cross-linked vinylidene fluoride copolymer, comprising the following steps:

[0037] A1: Add deionized water and polymerization stabilizer into the polymerization reaction device, stir and mix evenly, and evacuate and replace with nitrogen to remove oxygen in the reactor;

[0038] A2: Adding vinylidene fluoride, ethylenically unsaturated polyols and / or other comonomers and additives to a polymerization reaction device, and adding part or all of a chain transfer agent and part of an initiator to start a polymerization reaction;

[0039] A3: Continue to add the remaining polymerization monomers and initiators as needed, the polymerization temperature is 40-120°C, and the polymerization pressure is 2.0-10.0MPa;

[0040] A4: When the polymerization pressure is less than the set value, the polymerization reaction is completed, and the vinylidene fluoride copolymer is obtained after degassing, washing, filtering and drying.

[0041] The initiator is a commonly used polymerization initiator in the art, preferably, the initiator is selected from one or more of di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, diisobutyl peroxydicarbonate, diisohexyl peroxydicarbonate, di-tert-butyl peroxide, tert-butyl perbenzoate, and lauroyl peroxide. The amount of the initiator added is 0.01-2wt% of the total amount of vinylidene fluoride added, preferably, 0.1-1wt%.

[0042] The polymer stabilizer is a commonly used stabilizer in the art. When emulsion polymerization is used, the polymer stabilizer includes but is not limited to perfluorocarboxylic acid, perfluorocarboxylate, perfluoropolyether, preferably perfluoropolyether carboxylate; when suspension polymerization is used, the polymer stabilizer includes but is not limited to methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, polyethylene oxide, maleic acid modified polyethylene, etc. The addition amount of the polymer stabilizer is 0.01-2wt% of the total amount of vinylidene fluoride added, preferably 0.05-1wt%.

[0043] The chain transfer agent can be any compound that can adjust the molecular weight of the fluorine-containing polymer, including but not limited to alcohols, such as methanol, isopropanol, and n-butanol; ketones, such as acetone and butanone; esters, such as ethyl acetate, diethyl carbonate, dimethyl carbonate, and diethyl malonate; halogenated hydrocarbons, such as chloroform and dichloromethane; aliphatic alkanes, such as n-hexane, n-heptane, and n-octane. The amount of the chain transfer agent added is 0.01 to 0.5 wt% of the amount of vinylidene fluoride added.

[0044] As a preferred embodiment, the preparation method comprises the step of adding a water-soluble inorganic salt to water in step A1, wherein the amount of the water-soluble inorganic salt added is 0.1 to 10 wt % of the weight of the water phase.

[0045] The water-soluble inorganic salts include, but are not limited to, alkali metal or alkaline earth metal halides, such as sodium chloride, sodium bromide, potassium chloride, potassium bromide, magnesium chloride or calcium chloride; alkali metal or alkaline earth metal sulfates, such as sodium sulfate, potassium sulfate or magnesium sulfate; alkali metal or alkaline earth metal nitrates, such as sodium nitrate or potassium nitrate.

[0046] The amount of the water-soluble inorganic salt varies according to the ratio of the aqueous phase to the monomer, the type and concentration of the stabilizer, the stirring rate and other reaction parameters. Preferably, the amount of the water-soluble inorganic salt added is 1 to 2 wt % of the weight of the aqueous phase. Too high a concentration will interfere with the effectiveness of the suspending agent, and too low a concentration will not significantly improve the stability of the dispersion and reduce the viscosity. The water-soluble inorganic salt in water can increase the content of the ethylenically unsaturated polyol in the vinylidene fluoride copolymer because the addition of the water-soluble inorganic salt can improve the stability of the dispersed phase.

[0047] In a third aspect, the present invention provides an application of a reversibly cross-linked vinylidene fluoride copolymer, wherein a mixture of the reversibly cross-linked vinylidene fluoride copolymer and a cross-linking agent is applied as a component to an electrode material, wherein the cross-linking agent is a compound represented by structural formula (4),

[0048]

[0049] Among them, R 11 It is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group or an alkyl group containing any one of N, P, S and O; the boron oxygen atom and the cyclic dotted line portion represent a boronic acid or boric acid ester functional group.

[0050] The crosslinking agent of the present invention is a polyboric acid, or a boric acid ester obtained by a dehydration condensation reaction between a polyboric acid and a polyol compound, preferably, a boric acid ester obtained by a dehydration condensation reaction between an alkyl boronic acid, an aryl boronic acid, an alkyl boronic acid or an aryl boronic acid and a 1,2-diol compound.

[0051] The alkylboric acid is selected from at least one of propyldiboric acid, butyldiboric acid, pentyldiboric acid, hexyldiboric acid, heptyldiboric acid, octyldiboric acid, nonyldiboric acid, decyldiboric acid, undecyldiboric acid or dodecyldiboric acid.

[0052] The aryl boronic acid is an aromatic compound containing two or more boronic acid groups, and is preferably selected from at least one of 1,2-phenylenediboronic acid, 1,3-phenylenediboronic acid, and 1,4-phenylenediboronic acid.

[0053] The 1,2-diol compound is selected from at least one of ethylene glycol, 1,2-propylene glycol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol and pinacol.

[0054] Furthermore, the structural formula (4) is an alkylboronic acid or 1,4-phenylenediboronic acid having 1 to 6 carbon atoms, which can undergo a cross-linking reaction with a polyol quickly.

[0055] Furthermore, the content of the crosslinking agent in the mixture of the reversibly crosslinkable vinylidene fluoride copolymer and the crosslinking agent accounts for 0.01 to 10 wt %, preferably 0.1 to 5 wt %, and more preferably 0.25 to 4 wt % of the content of the reversibly crosslinkable vinylidene fluoride copolymer.

[0056] The method for preparing the mixture of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent comprises dissolving the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent in a diluent according to a mass ratio and stirring the mixture evenly.

[0057] The diluent of the present invention is any solvent that can completely dissolve the vinylidene fluoride copolymer, preferably, at least one selected from N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, more preferably, selected from N-methylpyrrolidone.

[0058] The reversible cross-linking vinylidene fluoride copolymer of the present invention and the cross-linking agent can undergo a cross-linking reaction at room temperature. This cross-linking reaction belongs to reversible cross-linking, that is, the cross-linking forms a dynamic covalent bond. Due to the reversibility of the dynamic covalent bond in the structure, the bond exchange can be quickly carried out under high temperature conditions, showing the characteristics of self-repair, repeated processing, and recycling. At low temperatures, the bond exchange reaction is very slow, and the cross-linking network rearrangement is difficult, showing certain characteristics of thermosetting materials. The introduction of this dynamic covalent bond in polyvinylidene fluoride can improve the bonding performance and mechanical properties.

[0059] The content of the mixture of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent of the present invention accounts for 1 to 10 wt % of the total amount of the electrode material.

[0060] The active substance of the electrode material of the present invention is of the general formula LiMY 2 A composite metal compound represented by, wherein M is selected from at least one of Co, Ni, Fe, Mn, Cr and V, Y is O or S, and the content of the active material accounts for 80% to 98wt% of the total amount of the electrode material.

[0061] The electrode material of the present invention further comprises a conductive agent carbon black, the amount of which is 1 to 10 wt%.

[0062] In a fourth aspect, the present invention provides an application of a reversibly cross-linked vinylidene fluoride copolymer, wherein a mixture of the reversibly cross-linked vinylidene fluoride copolymer and a cross-linking agent is used as a component in a lithium battery separator coating material, wherein the cross-linking agent is a compound represented by structural formula (4),

[0063]

[0064] Among them, R 11 It is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group or an alkyl group containing any one of N, P, S and O; the boron oxygen atom and the cyclic dotted line portion represent a boronic acid or boric acid ester functional group.

[0065] The cross-linking agent is the same as the third aspect of the present invention, and will not be described in detail here.

[0066] The method for preparing the mixture of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent comprises dissolving the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent in a diluent according to a mass ratio and stirring the mixture evenly.

[0067] The diluent of the present invention is any solvent that can completely dissolve the vinylidene fluoride copolymer, preferably, at least one selected from N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, more preferably, selected from N-methylpyrrolidone.

[0068] The preparation method of the lithium battery separator coating of the present invention comprises adding inorganic nanoparticles to a mixture of the reversibly cross-linkable vinylidene fluoride copolymer and a cross-linking agent, stirring evenly to obtain a coating slurry, coating the coating slurry on the surface of a polymer porous base film, and drying to obtain a lithium battery separator coating.

[0069] The inorganic nanoparticles of the present invention are any one of aluminum oxide, boehmite, silicon dioxide and titanium dioxide, and the particle size of the inorganic nanoparticles is 10 to 2000 nanometers. The mass percentage of the inorganic nanoparticles is 5 to 50 wt%, preferably 15 to 30 wt%.

[0070] The polymer porous base membrane of the present invention is any one of a polyethylene porous membrane, a polypropylene porous membrane, a cellulose non-woven membrane, a polyimide membrane and a PET non-woven membrane.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] (1) The product of the reaction of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent of the present invention has a reversible covalent bond structural unit, so that the material can be repeatedly processed and recycled, and can form a cross-linked network structure, with significantly improved mechanical properties and tensile strength, and also significantly improved solvent resistance and swelling.

[0073] (2) When the product of the reaction of the reversibly cross-linked polyvinylidene fluoride copolymer and the cross-linking agent of the present invention is used as an electrode material, the introduction of the comonomer reduces the crystallinity of polyvinylidene fluoride, improves the flexibility of the polymer chain, and is beneficial to ion conduction. At the same time, the three-dimensional cross-linked network structure improves the bonding performance and mechanical strength of the material, avoiding the phenomenon of cracks in the electrode due to the shrinkage and expansion effects caused by the charge and discharge cycle.

[0074] (3) When the product of the reaction of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent of the present invention is used as a lithium battery separator coating material, it can effectively improve the swelling resistance and adhesion of the coating and reduce the solubility of the coating in the electrolyte, thereby improving the safety and stability of the lithium ion battery. DETAILED DESCRIPTION

[0075] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all possible alternatives, improvements and equivalents within the scope of the claims.

[0076] Example 1

[0077] Add 2500g of deionized water, 25g of sodium sulfate, and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 15g of hexafluoropropylene monomer (HFP), 5g of 3-(allyloxy)-1,2-propylene glycol monomer (GAME) and vinylidene fluoride monomer (VDF) to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0078] Example 2

[0079] Add 2500g of deionized water, 25g of sodium sulfate, and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 15g of hexafluoropropylene monomer, 10g of 3-(allyloxy)-1,2-propylene glycol monomer and vinylidene fluoride monomer to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0080] Example 3

[0081] Add 2500g of deionized water, 25g of sodium sulfate, and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 15g of hexafluoropropylene monomer, 1g of 3-(allyloxy)-1,2-propylene glycol monomer and vinylidene fluoride monomer to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0082] Example 4

[0083] Add 2500g of deionized water, 25g of sodium sulfate, and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 5g of hexafluoropropylene monomer, 1g of 1-(2,3-dihydroxypropyl)(2Z)-2-butenedioate monomer and vinylidene fluoride monomer to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0084] Example 5

[0085] Add 2500g of deionized water, 25g of sodium sulfate, and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 5g of hexafluoropropylene monomer, 1g of 2,3-dihydroxypropyl acrylate monomer and vinylidene fluoride monomer to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0086] Example 6

[0087] Add 2500g of deionized water and 5g of hydroxypropyl methylcellulose to a 5L vertical polymerization reactor, evacuate and replace with nitrogen until the oxygen content in the reactor is less than 10ppm, then add 15g of hexafluoropropylene monomer, 5g of 3-(allyloxy)-1,2-propylene glycol monomer and vinylidene fluoride monomer to make the pressure in the reactor reach 2.0MPa. Then start stirring at a rate of 500rpm, raise the temperature in the reactor to 40°C, continue to add vinylidene fluoride monomer until the pressure in the reactor reaches 6.0MPa, add 1.0g of diisopropyl peroxydicarbonate and 2.0g of diethyl carbonate, and start polymerization. The pressure in the reactor was maintained at 6.0 MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 g / 20 minutes. After the total amount of 1000 g of vinylidene fluoride monomer was added, the reaction temperature was maintained at 40°C. When the reaction pressure in the reactor dropped below 3.0 MPa, the addition of diisopropyl peroxydicarbonate was stopped. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and then the obtained polymer dispersion was degassed, washed with water, filtered, and then dried at 90°C for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0088] Comparative Example 1

[0089] 2500 grams of deionized water, 25 grams of sodium sulfate, and 5 grams of hydroxypropyl methylcellulose were added to a 5-liter vertical polymerization reactor, and vacuumed and replaced with nitrogen until the oxygen content in the reactor was less than 10ppm, and then 15 grams of hexafluoropropylene monomer and vinylidene fluoride monomer were added to make the pressure in the reactor reach 2.0MPa. Then stirring was started at a rate of 500rpm, the temperature in the reactor was raised to 40°C, and vinylidene fluoride monomer was continuously added until the pressure in the reactor reached 6.0MPa, 1.0 grams of diisopropyl peroxydicarbonate and 2.0 grams of diethyl carbonate were added, and polymerization was started. The pressure in the reactor was maintained at 6.0MPa by adding vinylidene fluoride monomer, and diisopropyl peroxydicarbonate was added at a rate of 0.1 grams / 20 minutes. After the total amount of 1000 grams of vinylidene fluoride monomer was added, the reaction temperature was continued to be maintained at 40°C, and the addition of diisopropyl peroxydicarbonate was stopped when the reaction pressure in the reactor dropped below 3.0MPa. The polymerization reaction was completed when the reaction pressure dropped to 2.0 MPa, and the obtained polymer dispersion was then degassed, washed with water, filtered, and dried at 90° C. for 10 hours to obtain a vinylidene fluoride copolymer resin.

[0090] The molecular weight of the vinylidene fluoride copolymer resin prepared in Test Examples 1-6 and Comparative Example 1 was tested using a liquid gel chromatography instrument. The test parameters were: a sample concentration of 1.0 mg / mL, a detector temperature of 50°C, a mobile phase of N,N-dimethylformamide (lithium bromide concentration of 0.02 mol / L), a flow rate of 1.0 mL / min, and calibration with a polystyrene standard.

[0091] The melting points of the vinylidene fluoride copolymer resins prepared in Test Examples 1-6 and Comparative Example 1 were measured according to ASTM D4591. The heating program was as follows: heating from 40°C to 190°C at a heating rate of 10°C / min, keeping at 190°C for 10 minutes, cooling from 190°C to 25°C at a cooling rate of 10°C / min, keeping at 40°C for 2 minutes, and heating from 40°C to 190°C at a heating rate of 10°C / min. The DSC spectrum at the second melting was recorded.

[0092] Electrodes were prepared using the vinylidene fluoride copolymer resin prepared in Examples 1-6 and Comparative Example 1, and the bonding strength to the electrode sheet was tested according to ISO 4624. Electrode preparation process: At room temperature, 2.0 grams of vinylidene fluoride copolymer resin was completely dissolved in 50 grams of N-methylpyrrolidone under mechanical stirring, 2.0 grams of conductive carbon black and 30 grams of lithium iron phosphate were slowly added, and finally 0.05 grams of 1,4-phenylenediboric acid as a cross-linking agent were added, and the viscosity and fineness of the paste electrode adhesive composition meeting the requirements were obtained. The composition was evenly coated on aluminum foil with a coating machine, dried in a vacuum oven at 100°C for 12 hours, and an electrode sheet was prepared. The bonding strength of the electrode sheet was measured using a tensile tester, and each value was averaged from at least 5 electrode sheets.

[0093] The vinylidene fluoride copolymer resin prepared in Examples 1-6 and Comparative Example 1 was used to make a sheet, and the tensile strength was tested by an electronic universal testing machine, specifically: 10 grams of vinylidene fluoride copolymer resin and 190 grams of DMF solvent were accurately weighed, poured into a wide-mouth bottle, heated and magnetically stirred to dissolve, 0.1 grams of 1,4-phenylenediboric acid as a cross-linking agent were added, and fully mixed to ensure uniformity, the reaction solution was poured into methanol to precipitate the polymer precipitate, placed in a 100°C oven, and vacuum dried to constant weight. The reversibly cross-linked vinylidene fluoride copolymer resin was hot-pressed into a sheet with a thickness of 1 mm using a flat vulcanizer, cut into a dumbbell shape with a cutter, the test environment was 25°C, 0.01N preload, 50mm / min tensile rate were used, until the sample broke, 5 parallel samples were measured, and the average value was taken.

[0094] The swelling rate of the mixture of vinylidene fluoride copolymer resin and crosslinking agent prepared in Examples 1-6 and Comparative Example 1 was tested, specifically as follows: 5 g of vinylidene fluoride copolymer resin and 95 g of NMP solvent were accurately weighed, poured into a wide-mouth bottle, heated and magnetically stirred to dissolve, and a 5 wt% NMP solution was prepared, 0.05 g of 1,4-phenylenediboric acid as a crosslinking agent was added, and the mixture was fully mixed to ensure uniformity; an aluminum foil was folded into a box of 50 mm*100 mm in size, and the solution was poured into the box; the aluminum foil box was placed The solvent was heated on a magnetic heating stirrer at a temperature of about 50°C until the solution gelled; the aluminum foil box was placed in a 70°C oven and continued to be dried for 48 hours to obtain a film; an electrolyte was prepared, EC:DMC:DEC=1:1:1; the film was immersed in the electrolyte and placed in a 30°C oil bath for a swelling test. After 24 hours, the film was taken out and weighed. The swelling rate was calculated as follows: [(mass of the film after immersion)-(mass of the film before immersion)] / (mass of the film before immersion)=swelling rate.

[0095] Comparative Example 2

[0096] The vinylidene fluoride copolymer resin and electrode sheet were prepared in the same manner as in Example 1, except that no crosslinking agent was added during the preparation of the electrode sheet, and the bond strength measurements are summarized in Table 1. The only difference is that when preparing the dumbbell-shaped specimens required for the tensile strength test, the fluorine-containing polymer used was a vinylidene fluoride copolymer resin without any crosslinking agent added.

[0097] Table 1 shows the performance test data of Examples 1-6 and Comparative Examples 1-2.

[0098] Table 1 Performance test results

[0099]

[0100] From the data in Table 1, it can be seen that the performance of the reversibly cross-linked vinylidene fluoride copolymer prepared in the embodiment is significantly better than that of the comparative example. The introduction of the polyol structural unit can improve the adhesion of the material. After the dynamic cross-linking network is further introduced into the material, its adhesion, tensile modulus and swelling rate have been greatly improved. The prepared reversibly cross-linked vinylidene fluoride copolymer has relatively good swelling resistance, and the formation of a three-dimensional polymerization network is conducive to reducing the volume expansion rate of the battery.

[0101] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A reversibly cross-linkable vinylidene fluoride copolymer, Features: The reversibly cross-linkable vinylidene fluoride copolymer comprises a vinylidene fluoride structural unit and a structural unit of at least one compound selected from structural formula (1), structural formula (2) or structural formula (3). Among them, R 1 , R 2 , R 3 R is independently selected from hydrogen, halogen, aryl, alkyl having 1 to 12 carbon atoms or perfluoroalkyl; 4 is selected from an alkyl group, an aryl group, an ester group, an amide group or an alkyl group containing any one of N, P, S and O having 0 to 12 carbon atoms; n is an integer of 1 to 12, Among them, R 5 is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group, or an alkyl group containing any one of N, P, S, and O; R 6 is selected from a hydrogen atom, a hydroxyl group, an amino group, an aryl group, an alkyl group having 1 to 12 carbon atoms, a perfluoroalkyl group, or an alkyl group containing any one of N, P, S, and O, Among them, R 8 , R 9 , R 10 R is independently selected from hydrogen, halogen, aryl, alkyl having 1 to 12 carbon atoms or perfluoroalkyl; 7 is selected from an alkyl group, an aryl group, an ester group, an amide group, or an alkyl group containing any one of N, P, S, and O having 1 to 12 carbon atoms; n is an integer of 0 to 12, The content of the vinylidene fluoride structural unit is 70 to 99.99 wt %, The total content of the structural units of the compound represented by the structural formula (1), the structural formula (2) or the structural formula (3) is 0.01 to 10 wt %.

2. The reversibly crosslinkable vinylidene fluoride copolymer according to claim 1, Features: The structural formula (1) is selected from 3-(allyloxy)-1,2-propanediol; The structural formula (2) is at least one selected from 1-(2,3-dihydroxypropyl)(2Z)-2-butenedioic acid ester and 4-[(2,3-dihydroxypropyl)amino]-4-oxo-2-butenoic acid; The structural formula (3) is at least one selected from 2,3-dihydroxypropyl acrylate and 2-methyl-2-acrylate-2,3-dihydroxypropyl ester.

3. The reversibly crosslinkable vinylidene fluoride copolymer according to claim 1 or 2, Features: The reversibly cross-linkable vinylidene fluoride copolymer comprises a vinylidene fluoride structural unit, a structural unit of a compound represented by structural formula (1), or a structural unit of a compound represented by structural formula (3).

4. The reversibly crosslinkable vinylidene fluoride copolymer according to claim 1, Features: The reversibly cross-linkable vinylidene fluoride copolymer also includes other comonomer structural units, and the other comonomers are selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluorovinyl ether, vinyl fluoride, methyl acrylate, methyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, maleate, citraconic acid, vinyl acetate, vinyl carbonate or acrylonitrile, and the content of the other comonomer structural units is 0 to 20wt%.

5. A method for preparing the reversibly cross-linkable vinylidene fluoride copolymer according to any one of claims 1 to 4, Features: The preparation method comprises the step of adding a water-soluble inorganic salt into water, wherein the amount of the water-soluble inorganic salt added is 0.1 to 10 wt % of the weight of the water phase.

6. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to any one of claims 1 to 4, Features: The mixture of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent is used as a component in the electrode material, and the cross-linking agent is a compound represented by the structural formula (4). Among them, R 11 It is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group or an alkyl group containing any one of N, P, S and O; the boron oxygen atom and the cyclic dotted line portion represent a boronic acid or boric acid ester functional group.

7. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to claim 6, Features: The cross-linking agent is polyboric acid, or a boric acid ester obtained by dehydration condensation reaction of polyboric acid and polyol compounds.

8. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to claim 7, Features: The polyboric acid is selected from at least one of alkylboric acid and arylboric acid; and the polyol compound is a 1,2-diol compound.

9. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to claim 8, Features: The alkyl boronic acid is selected from at least one of propyl diboric acid, butyl diboric acid, pentyl diboric acid, hexyl diboric acid, heptyl diboric acid, octyl diboric acid, nonyl diboric acid, decyl diboric acid, undecyl diboric acid and dodecyl diboric acid; the aryl boronic acid is selected from at least one of 1,2-phenylenediboric acid, 1,3-phenylenediboric acid and 1,4-phenylenediboric acid; the 1,2-diol compound is selected from at least one of ethylene glycol, 1,2-propylene glycol, 1,2-butylene glycol, 1,2-pentanediol, 1,2-hexanediol and pinacol.

10. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to claim 6, Features: The amount of the cross-linking agent used is 0.01 to 10 wt % of the mass of the reversibly cross-linkable vinylidene fluoride copolymer.

11. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to claim 6, Features: The active material of the electrode material is of the general formula LiMY 2 A composite metal compound represented by, wherein M is selected from at least one of Co, Ni, Fe, Mn, Cr and V, and Y is O or S.

12. Use of the reversibly cross-linkable vinylidene fluoride copolymer according to any one of claims 1 to 4, Features: The mixture of the reversibly cross-linkable vinylidene fluoride copolymer and the cross-linking agent is used as a component in a lithium battery separator coating material, wherein the cross-linking agent is a compound represented by the structural formula (4). Among them, R 11 It is selected from an alkyl group having 1 to 12 carbon atoms, an aryl group, an ester group, an amide group or an alkyl group containing any one of N, P, S and O; the boron oxygen atom and the cyclic dotted line portion represent a boronic acid or boric acid ester functional group.

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