Positive electrode binder, positive electrode mixture, positive electrode, and non-aqueous electrolyte secondary battery

Abstract

The present invention addresses the problem of providing a binder for positive electrodes, the binder exhibiting high adhesion to an active material and a collector even in a small amount. A binder for positive electrodes of nonaqueous electrolyte secondary batteries according to the present invention, which solves the above-described problem, contains a vinylidene fluoride polymer and an epoxy compound that has two or more epoxy groups. With respect to this binder for positive electrodes, the epoxy equivalent of the epoxy compound is 500 g / eq or less; and the total amount of the vinylidene fluoride polymer relative to the total number of moles of the epoxy groups in this binder for positive electrodes is 600 g / eq to 3,000 g / eq.

Description

[Technical field]

[0001] The present invention relates to a positive electrode binder, a positive electrode mixture, a positive electrode, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In the electrodes of non-aqueous electrolyte secondary batteries, vinylidene fluoride polymers are widely used as binders for bonding active materials and current collectors. From the viewpoint of improving the safety and performance of batteries, the binder is required to have excellent adhesive properties with the active materials and current collectors, and vinylidene fluoride polymers with various structures have been proposed. For example, Patent Document 1 describes the use of a polymer of vinylidene fluoride and 2-carboxyethyl acrylate as a binder.

[0003] On the other hand, from the viewpoint of increasing the capacity of a battery, it is desired to reduce the amount of insulating components contained in the mixture layer, that is, components derived from the binder, and to increase the amount of active material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5797206 Summary of the Invention [Problem to be solved by the invention]

[0005] However, if the amount of components derived from the binder is reduced, it becomes difficult to sufficiently adhere the active material, the current collector, etc., and there is a possibility that the battery performance may be deteriorated. Therefore, it is desired to provide a binder that shows high adhesion to the active material and the current collector even in a small amount.

[0006] The present invention has been made in view of the above problems, and aims to provide a positive electrode binder that exhibits high adhesion to an active material and a current collector even in small amounts, and a positive electrode mixture, a positive electrode, and a nonaqueous electrolyte secondary battery that contain the same. [Means for solving the problem]

[0007] The present invention provides a binder for a positive electrode of a non-aqueous electrolyte secondary battery, the binder comprising a vinylidene fluoride polymer and an epoxy compound having two or more epoxy groups, wherein the epoxy compound has an epoxy equivalent of 500 g / eq or less, and the total amount of the vinylidene fluoride polymer relative to the total molar number of the epoxy groups in the binder for the positive electrode is 600 g / eq or more and 3000 g / eq or less.

[0008] The present invention provides a positive electrode mixture containing the above-mentioned positive electrode binder and a positive electrode active material.

[0009] The present invention provides a positive electrode having a current collector and a positive electrode mixture layer disposed on the current collector, the positive electrode mixture layer including a cured product of the positive electrode mixture.

[0010] The present invention provides a nonaqueous electrolyte secondary battery having the above positive electrode. Effect of the Invention

[0011] According to the present invention, it is possible to obtain a positive electrode binder that exhibits high adhesion to an active material or a current collector even in a small amount, a positive electrode mixture containing the same, and further a positive electrode and a nonaqueous electrolyte secondary battery that contain a cured product of the positive electrode binder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] 1. Positive electrode binder The positive electrode binder of the present invention is a binder used for forming a positive electrode of a non-aqueous electrolyte secondary battery. The positive electrode binder exhibits very high adhesion to a positive electrode active material and a current collector, and is therefore suitable for use in forming a positive electrode mixture layer. However, the use of the positive electrode binder of the present invention is not limited thereto.

[0013] Here, the positive electrode binder of the present invention contains a vinylidene fluoride polymer and an epoxy compound having two or more epoxy groups. In the positive electrode binder, the epoxy equivalent of the epoxy compound is 500 g / eq or less. That is, the ratio of epoxy groups in the epoxy compound is relatively high. Furthermore, the total amount of vinylidene fluoride polymer relative to the total moles of epoxy groups in the positive electrode binder is 600 g / eq or more and 3000 g / eq or less. That is, the amount of vinylidene fluoride polymer per mole of epoxy group is relatively small. According to such a positive electrode binder, when forming a positive electrode mixture layer, the epoxy group can be firmly bonded to the metal atom or hydroxyl group on the collector or the surface of the positive electrode active material. As a result, even if the amount of the positive electrode binder used is small, the positive electrode active material, the current collector, etc. are firmly bound by the cured product of the positive electrode binder. Hereinafter, each component of the positive electrode binder will be described.

[0014] Vinylidene fluoride polymer The vinylidene fluoride polymer is a polymer whose main component is a structural unit derived from vinylidene fluoride. The positive electrode binder may contain only one type of vinylidene fluoride polymer, or may contain two or more types. The vinylidene fluoride polymer may be a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride and other compounds, and the proportion of the structural unit derived from vinylidene fluoride in the vinylidene fluoride polymer is preferably 50% by mass or more and 100% by mass or less, more preferably 80% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less. When the amount of the structural unit derived from vinylidene fluoride in the vinylidene fluoride polymer is 50% by mass or more, the characteristics derived from vinylidene fluoride are easily obtained, and the adhesion between the vinylidene fluoride polymer and the positive electrode active material, the current collector, etc. is improved. The amount of structural units derived from vinylidene fluoride in the vinylidene fluoride polymer is 19 This can be identified by F-NMR analysis, etc.

[0015] When the vinylidene fluoride polymer is a copolymer of vinylidene fluoride and another compound, the type of the other compound is not particularly limited, and examples thereof include compounds represented by the following general formulas (1) to (3), unsaturated basic acids, unsaturated basic acid monoesters, halogenated alkyl vinyl compounds, and the like.

[0016] [ka] R in the above general formula (1) 1 , R 2 , and R 3 Each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. However, it is preferable that there is little steric hindrance during polymerization with vinylidene fluoride, and R 1 and R 2 is preferably hydrogen or an alkyl group having 1 to 3 carbon atoms, more preferably hydrogen or a methyl group. 1 and R 2 Both are hydrogen, or R 1 and R 2 It is preferable that only one of the above is a methyl group.

[0017] In addition, X in the above general formula (1) 1 represents an atomic group having a main chain with 1 to 19 atoms and a molecular weight of 472 or less. The molecular weight of the atomic group is preferably 14 to 172. 1 The number of atoms in the main chain of X is preferably 1 or more and 14 or less, and more preferably 1 or more and 9 or less. 1 The number of atoms in the main chain of X refers to the number of atoms in the longest chain connecting the acryloyl group and the carboxyl group. 1 The main chain may be a hydrocarbon chain, but may also contain nitrogen atoms, sulfur atoms, oxygen atoms, etc.

[0018] Examples of the compound represented by the above general formula (1) include 2-carboxyethyl (meth)acrylate, (meth)acryloyloxyethyl succinate, (meth)acryloyloxypropyl succinate, (meth)acryloyloxyethyl phthalate, (meth)acrylamide-based compounds such as N-carboxyethyl (meth)acrylamide, and thio(meth)acrylate compounds such as carboxyethyl thio(meth)acrylate. In this specification, (meth)acrylate refers to methacrylate, acrylate, or a mixture thereof, (meth)acryl refers to methacryl, acrylic, or a mixture thereof, and (meth)acryloyl refers to methacryloyl, acryloyl, or a mixture thereof.

[0019] [ka] R in the above general formula (2) 4 , R 5 , and R 6 Each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. However, it is preferable that there is little steric hindrance during polymerization with vinylidene fluoride, and R 4 and R 5 is preferably hydrogen or an alkyl group having 1 to 3 carbon atoms, more preferably hydrogen or a methyl group. 4 and R 5 Both are hydrogen, or R 4 and R 5 It is preferable that only one of the above is a methyl group.

[0020] X in the above general formula (2) 2 represents an atomic group having a main chain with 1 to 19 atoms and a molecular weight of 484 or less. The molecular weight of the atomic group is preferably 14 to 184. X 2 The number of atoms in the main chain of X is preferably 1 or more and 14 or less, and more preferably 1 or more and 9 or less. 2 The number of atoms in the main chain refers to the number of atoms in the longest chain connecting the oxygen bonded to the carbon-carbon double bond and the carboxyl group. 2The main chain may be a hydrocarbon chain, but may also contain nitrogen atoms, sulfur atoms, oxygen atoms, etc.

[0021] Examples of the compound represented by the above general formula (2) include vinyl carboxyalkyl ethers such as vinyl carboxymethyl ether and vinyl carboxyethyl ether.

[0022] [ka] R in the above general formula (3) 7 , R 8 , and R 9 Each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. However, it is preferable that there is little steric hindrance during polymerization with vinylidene fluoride, and R 7 and R 8 is preferably hydrogen or an alkyl group having 1 to 3 carbon atoms, more preferably hydrogen or a methyl group. 7 and R 8 Both are hydrogen, or R 7 and R 8 It is preferable that only one of the above is a methyl group.

[0023] X in the above general formula (3) 3 represents a hydrogen atom or a hydrocarbon group containing at least one hydroxy group and having 1 to 5 carbon atoms. Examples of the hydrocarbon group containing a hydroxy group include a hydroxyethyl group and a hydroxypropyl group.

[0024] Examples of the compound represented by the above general formula (3) include acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methyl acrylate, and 2-hydroxypropyl methacrylate.

[0025] The unsaturated basic acid may be an unsaturated carboxylic acid or a derivative thereof, and examples thereof include compounds in which one or more carboxyl groups are bonded via a linear or branched unsaturated alkylene group having from 1 to 6 carbon atoms. More specific examples of the unsaturated basic acid include crotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, etc.

[0026] The unsaturated basic acid ester is an ester compound derived from the unsaturated basic acid, and specific examples thereof include monomethyl maleate, monoethyl maleate, dimethyl maleate, monomethyl citraconic acid, and monoethyl citraconic acid.

[0027] The above-mentioned halogenated alkyl vinyl compound may be a compound having one vinyl group and one or more halogenated alkyl groups, or a compound having one vinyl group and a halogen atom bonded to the vinyl group (excluding vinylidene fluoride).Specific examples thereof include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, fluoroalkyl vinyl ether, perfluoromethyl vinyl ether, etc.

[0028] The vinylidene fluoride polymer may contain only one type of structure derived from another compound, or may contain two or more types. Among these, it is particularly preferable that the vinylidene fluoride polymer contains one or more structural units derived from the compound represented by any one of the above general formulas (1) to (3). When the vinylidene fluoride polymer contains these structural units, the carboxyl group or hydroxyl group of the vinylidene fluoride polymer reacts with the epoxy group of the epoxy compound, and the adhesive strength of the positive electrode binder to the positive electrode active material, current collector, etc. tends to be stronger. Among the above, it is particularly preferable that the vinylidene fluoride polymer contains a structural unit derived from a carboxyl group.

[0029] The proportion of structural units derived from other compounds in the vinylidene fluoride polymer (particularly, the proportion of structural units derived from compounds represented by any one of the above general formulas (1) to (3)) is preferably 0.1% by mass to 50% by mass, more preferably 0.1% by mass to 20% by mass, and even more preferably 0.1% by mass to 10% by mass. That is, when the vinylidene fluoride polymer contains structural units derived from other compounds, the vinylidene fluoride polymer preferably contains structural units derived from vinylidene fluoride in an amount of 50% by mass to 99.9% by mass, more preferably 80% by mass to 99.9% by mass, and even more preferably 90% by mass to 99.9% by mass. When the amount of structural units derived from other compounds in the vinylidene fluoride polymer is within the above range, the adhesion between the vinylidene fluoride polymer and the positive electrode active material, the current collector, and the like is improved. The amount of structural units derived from other compounds is, for example, a vinylidene fluoride copolymer. 19 It can be identified by analysis using F-NMR or FT-IR.

[0030] As described above, the positive electrode binder may contain multiple types of vinylidene fluoride polymers, but from the viewpoint of strongly bonding the positive electrode active material and the current collector with a small amount of the positive electrode binder, it is preferable to contain a large amount of vinylidene fluoride polymers that are difficult to swell with an electrolyte solution, etc. (for example, vinylidene fluoride homopolymers, copolymers containing structural units derived from compounds represented by any of the above general formulas (1) to (3) and structural units derived from vinylidene fluoride, etc.). In other words, it is preferable that the amount of vinylidene fluoride polymers that are easy to swell with an electrolyte solution is small, and more specifically, it is preferable that the amount is less than 20 mass% with respect to the solid content of the positive electrode binder (total amount excluding components that volatilize during curing). Examples of vinylidene fluoride polymers that are easy to swell with an electrolyte solution, etc. include vinylidene fluoride polymers that mainly contain structural units derived from halogenated alkyl vinyl compounds and structural units derived from vinylidene fluoride.

[0031] Here, the weight average molecular weight of the vinylidene fluoride polymer is preferably 100,000 to 10 million, more preferably 200,000 to 5 million, and even more preferably 300,000 to 2 million. When the weight average molecular weight of the vinylidene fluoride polymer is within this range, the physical properties of the vinylidene fluoride polymer tend to fall within the desired range, and the vinylidene fluoride polymer tends to bind to the active material, the current collector, and the conductive assistant. The weight average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0032] The total amount of vinylidene fluoride polymer in the positive electrode binder is preferably 80% by mass or more and 99.9% by mass or less, more preferably 90% by mass or more and 99% by mass or less, based on the solid content of the positive electrode binder (total amount excluding components that volatilize during curing). When the amount of vinylidene fluoride polymer is within this range, the positive electrode binder is more likely to exhibit physical properties derived from the vinylidene fluoride polymer.

[0033] Epoxy compounds An epoxy compound is a compound having two or more epoxy groups in one molecule and an epoxy equivalent of 500 g / eq or less. In this specification, the term "epoxy equivalent" refers to the value obtained by theoretically dividing the molecular weight of an epoxy compound by the number of epoxy groups contained in the epoxy compound. In this specification, the term "epoxy equivalent" also refers to a value calculated by a measurement method standardized in JIS K7236:2001. If the epoxy equivalent of an epoxy compound is 500 g / eq or less, the epoxy compound is likely to interact with an active material, a current collector, etc., when a positive electrode mixture layer is formed using a positive electrode binder. The epoxy equivalent is more preferably 300 g / eq or less, and even more preferably 200 g / eq or less. Usually, the lower limit is 43 g / eq. The epoxy equivalent can theoretically be calculated by specifying the structure of the epoxy compound. It can also be calculated by a measurement method standardized in JIS K7236:2001.

[0034] The structure of the epoxy compound is not particularly limited, and may be, for example, an aliphatic epoxy compound, an aromatic epoxy compound, or an alicyclic epoxy compound.

[0035] Examples of aliphatic epoxy compounds include bifunctional aliphatic epoxy compounds such as neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, and polypropylene glycol diglycidyl ether; trifunctional aliphatic epoxy compounds such as glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, triglycidyl isocyanurate and its derivatives, and pentaerythritol triglycidyl ether; pentaerythritol tetraglycidyl ether, diglycerol tetraglycidyl ether, and ditrimethylol tetrafunctional aliphatic epoxy compounds such as propane tetraglycidyl ether and sorbitol tetraglycidyl ether; pentafunctional aliphatic epoxy compounds such as polyglycerol pentaglycidyl ether, pentaerythritol pentaglycidyl ether and dipentaerythritol pentaglycidyl ether; hexafunctional aliphatic epoxy compounds such as sorbitol hexaglycidyl ether and dipentaerythritol hexaglycidyl ether; and polyfunctional aliphatic epoxy compounds such as trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether and sorbitol polyglycidyl ether.

[0036] Examples of the aromatic epoxy compounds include difunctional aromatic epoxy compounds such as bisphenol A diglycidyl compounds, bisphenol F diglycidyl compounds, and biphenyl diglycidyl compounds, such as diglycidyl terephthalate, diglycidyl-o-phthalate, diglycidyl resorcinol ether, and 2,2-bis(4-glycidyloxyphenyl)propane; and polyfunctional aromatic epoxy compounds such as phenol novolac epoxy compounds and cresol novolac epoxy compounds.

[0037] Examples of the alicyclic epoxy compounds include bifunctional alicyclic epoxy compounds such as hydrogenated bisphenol A diglycidyl compounds, hydrogenated bisphenol F diglycidyl compounds, and hydrogenated biphenyl diglycidyl compounds.

[0038] Furthermore, the epoxy compound may be a glycidylamine compound such as 4,4'-methylenebis(N,N-diglycidylaniline) or N,N-diglycidyl-4-glycidyloxyaniline.

[0039] Among the above, from the viewpoint of suppressing thickening of the positive electrode binder, epoxy compounds that do not contain amines are preferred.

[0040] In addition, the epoxy compound may be a solid compound, but a liquid compound is preferable because it is easy to mix uniformly with the vinylidene fluoride polymer, etc. The viscosity of the liquid epoxy compound measured at 25°C with an E-type viscometer is preferably 100 Pa s or less, more preferably 50 Pa s or less.

[0041] The molecular weight of the epoxy compound is preferably 2000 or less, more preferably 1000 or less. The molecular weight of the epoxy compound is 80 or more. When the molecular weight of the epoxy compound is within this range, it is easy to mix uniformly with the vinylidene fluoride polymer. Furthermore, when the molecular weight of the epoxy compound is small, the epoxy equivalent is likely to be small.

[0042] The amount of the epoxy compound in the positive electrode binder is preferably 0.1% by mass to 20% by mass, more preferably 1% by mass to 10% by mass, based on the solid content of the positive electrode binder (total amount excluding components that volatilize during curing). When the amount of the epoxy compound is within this range, the total amount of vinylidene fluoride polymer relative to the total number of epoxy groups contained in the positive electrode binder can be 600 g / eq to 3000 g / eq.

[0043] Polar solvents The positive electrode binder may further contain a polar solvent. When the positive electrode binder contains a polar solvent, the vinylidene fluoride polymer and the epoxy compound can be dissolved or dispersed therein, and the positive electrode binder can be made liquid.

[0044] Examples of polar solvents include amide compounds such as dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohols such as methanol, ethanol, isopropyl alcohol, 2-ethyl-1-hexanol, 1-nonanol, lauryl alcohol, and tripropylene glycol; amine compounds such as o-toluidine, m-toluidine, and p-toluidine; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactones such as γ-butyrolactone and δ-butyrolactone; and sulfoxide and sulfone compounds such as dimethyl sulfoxide and sulfolane. The positive electrode binder may contain only one polar solvent or two or more polar solvents.

[0045] The amount of the polar solvent is preferably 500 parts by mass or more and 50,000 parts by mass or less, and more preferably 1,000 parts by mass or more and 10,000 parts by mass or less, relative to 100 parts by mass of the total amount of the vinylidene fluoride polymer and the epoxy compound. When the amount of the polar solvent is within this range, the vinylidene fluoride polymer and the epoxy compound can be uniformly dispersed or dissolved in the polar solvent.

[0046] Other Ingredients The positive electrode binder may contain components other than the vinylidene fluoride polymer, the epoxy compound, and the polar solvent, as long as the objects and effects of the present invention are not impaired. For example, it may further contain other resins such as acrylic resins, fillers such as inorganic fillers, and various additives.

[0047] Physical properties The positive electrode binder may be any one that contains a vinylidene fluoride polymer and an epoxy compound, and may be a solid composition, a liquid composition, or a slurry composition.

[0048] The amount of vinylidene fluoride polymer relative to the total number of epoxy groups in the positive electrode binder is 600 g / eq or more and 3000 g / eq or less, preferably 700 g / eq or more and 2500 g / eq or less, and more preferably 800 g / eq or more and 2000 g / eq or less. When the amount of vinylidene fluoride polymer relative to the total number of epoxy groups is within this range, even if the amount of the positive electrode binder is small, the positive electrode active material and the current collector can be firmly bonded. The value can be specified by analyzing the composition and specifying the amount of vinylidene fluoride polymer, the amount of epoxy compound, and the structure of the epoxy compound. It can also be specified by specifying the amount of vinylidene fluoride polymer, the amount of epoxy compound, and the epoxy equivalent of the epoxy compound calculated by a measurement method standardized in JIS K7236:2001.

[0049] ·Preparation method The preparation method of the positive electrode binder is not particularly limited and is appropriately selected according to its composition. For example, when the positive electrode binder contains a powdered vinylidene fluoride polymer and a liquid epoxy compound, the liquid epoxy compound may be mixed with the powdered vinylidene fluoride polymer and the epoxy compound may be adsorbed on the vinylidene fluoride polymer. When the positive electrode binder further contains a polar solvent, the vinylidene fluoride polymer, the epoxy compound, and the polar solvent may be mixed to disperse or dissolve the vinylidene fluoride polymer or the epoxy compound in the polar solvent.

[0050] 2. Cathode active material mixture The above-mentioned binder for the cathode and the cathode active material can be mixed to form a cathode active material mixture. At this time, a conductive assistant, a solvent, other additives, etc. may be further added.

[0051] The amount of the binder for the cathode in the cathode active material mixture is not particularly limited. However, as described above, even a small amount of the binder for the cathode can exhibit sufficient adhesiveness to the cathode active material and the like. Therefore, the ratio of the solid content derived from the binder for the cathode (total amount excluding components volatilized during curing), the cathode active material, and the conductive assistant to the total amount of the solid content derived from the binder for the cathode is preferably 0.2% by mass or more and 20% by mass or less, more preferably 0.4% by mass or more and 10% by mass or less, and even more preferably 0.6% by mass or more and 4% by mass or less.

[0052] As the cathode active material, conventionally known cathode active materials can be used. Examples of the cathode active material include lithium-based cathode active materials containing lithium. Specific examples thereof include composite metal chalcogen compounds represented by the general formula LiMY2 (0 < x ≤ 1) such as LiCoO2, LiNi x Co 1-x O2 (where M represents one or more elements selected from transition metals such as Co, Ni, Fe, Mn, Cr, V, etc., and Y represents a chalcogen element such as O or S); composite metal oxides having a spinel structure such as LiMn2O4; and olivine-type lithium compounds such as LiFePO4; etc. are included. Further, the surface of the cathode active material may be coated with various substances. The cathode active material may be a commercially available product.

[0053] The amount of the positive electrode active material in the positive electrode mixture is appropriately selected according to the type of the positive electrode active material and the desired physical properties of the positive electrode mixture layer, and is not particularly limited, but is usually preferably 50% by mass or more and 99.9% by mass or less with respect to the total amount of the solid content (total amount excluding components that volatilize during curing) derived from the positive electrode binder, the positive electrode active material, and the conductive assistant. When the amount of the positive electrode active material is within this range, for example, sufficient charge / discharge capacity is obtained, and the battery performance is likely to be good. Note that the above-mentioned positive electrode binder exhibits sufficient adhesion to the positive electrode active material, etc. even in a small amount, so the amount of the positive electrode active material with respect to the total amount of the solid content derived from the positive electrode binder, the positive electrode active material, and the conductive assistant may be 90% by mass or more.

[0054] The conductive assistant is not particularly limited as long as it is a compound that can further increase the conductivity between the positive electrode active materials or between the positive electrode active material and the current collector. Examples of the conductive assistant include acetylene black, ketjen black, carbon black, graphite powder, carbon nanofibers, carbon nanotubes, and carbon fibers.

[0055] The amount of the conductive assistant contained in the electrode mixture for the positive electrode is appropriately selected depending on the type, etc. From the viewpoint of improving both the conductivity and the dispersibility of the conductive assistant, the amount is preferably 0.1% by mass to 15% by mass or less, more preferably 0.1% by mass to 7% by mass or less, and even more preferably 0.1% by mass to 5% by mass or less, based on the total amount of the solid content derived from the binder for the positive electrode, the positive electrode active material, and the conductive assistant.

[0056] The positive electrode mixture may contain a solvent different from the polar solvent contained in the positive electrode binder. The solvent can be selected from the polar solvents that can be contained in the positive electrode binder.

[0057] The total amount of the solvent in the positive electrode mixture (including the amount of the polar solvent in the binder) is not particularly limited, but is usually preferably 20 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the above-mentioned positive electrode active material.

[0058] The positive electrode mixture may further contain a dispersant, an adhesive auxiliary, a thickener, etc., and these may be known compounds. The amount of these is not particularly limited as long as it is within a range that does not impair the object and effect of the present invention, but is preferably 15 mass % or less based on the total amount of the solid content derived from the positive electrode binder and the positive electrode active material.

[0059] The positive electrode mixture may further contain additives such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, nitrogen compounds such as ammonium compounds, organic esters, various silane-based, titanium-based and aluminum-based coupling agents, vinylidene fluoride polymers other than the above-mentioned vinylidene fluoride copolymers, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN) and other resins. These are not particularly limited as long as they do not impair the purpose and effect of the present invention, but are preferably 15 mass% or less based on the total amount of the solid content derived from the positive electrode binder and the positive electrode active material.

[0060] The positive electrode mixture may be prepared by mixing all the components at once, or by mixing some of the components first and then mixing the remaining components. The positive electrode mixture and the positive electrode binder may be prepared simultaneously. For example, the above-mentioned vinylidene fluoride polymer, epoxy compound, positive electrode active material, polar solvent, etc. may be mixed to obtain a positive electrode mixture containing the positive electrode binder (vinylidene fluoride polymer and epoxy compound) and the positive electrode active material.

[0061] The viscosity of the positive electrode mixture is not particularly limited as long as it can prevent dripping, uneven coating, and delayed drying after coating when applying the positive electrode mixture to obtain a positive electrode mixture layer, and provides good workability and applicability when preparing the positive electrode mixture. Normally, a viscosity of 0.1 Pa·s or more and 100 Pa·s or less is preferred. The viscosity of the positive electrode mixture is measured using an E-type viscometer or the like.

[0062] 3. Positive electrode The above-mentioned positive electrode mixture can be used to form a positive electrode mixture layer of various non-aqueous electrolyte secondary batteries. The positive electrode of the non-aqueous electrolyte secondary battery includes, for example, a current collector and a positive electrode mixture layer disposed on the current collector. In this case, the above-mentioned positive electrode mixture can be used to form the positive electrode mixture layer.

[0063] Current collector The current collector is a terminal for taking out electricity. The material of the current collector is not particularly limited, and metal foil or metal mesh of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc. can be used. In addition, the current collector may be a medium in which the above-mentioned metal foil or metal mesh is applied to the surface of the other medium.

[0064] Positive electrode mixture layer The positive electrode mixture layer is a layer formed by applying the above-mentioned positive electrode mixture onto a current collector and curing the same. The positive electrode mixture layer may be formed on only one surface of the current collector, or may be disposed on both surfaces of the current collector.

[0065] The components in the positive electrode mixture layer are appropriately selected according to the type of non-aqueous electrolyte secondary battery. The positive electrode mixture layer usually contains the above-mentioned cured product of the positive electrode binder and the positive electrode active material, but may also contain various additives such as a conductive assistant, a dispersant, an adhesive assistant, and a thickener. These can be the same as those described in the positive electrode mixture.

[0066] Here, the thickness of the positive electrode mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. The basis weight of the positive electrode mixture layer formed on one surface of the current collector is not particularly limited, and can be any basis weight, but in one example, it is 50 g / m 2 More than 1000g / m 2 Less than 100 g / m 2 More than 500g / m 2 The following is more preferred:

[0067] · Formation of positive electrode mixture layer The positive electrode mixture layer can be formed by carrying out a step of applying the above-mentioned positive electrode mixture onto a current collector and a step of curing the applied mixture.

[0068] The method for applying the positive electrode mixture is not particularly limited, and a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, a dip coating method, or the like can be applied.

[0069] After the application of the positive electrode mixture, the mixture is heated at an arbitrary temperature to dry the polar solvent and thermally cure the epoxy compound. In one example, the curing temperature is preferably 60°C or higher and 500°C or lower, more preferably 80°C or higher and 200°C or lower. Heating may be performed multiple times at different temperatures. The solvent in the positive electrode mixture may be dried under atmospheric pressure, under pressure, or under reduced pressure. After drying, a heat treatment may be further performed.

[0070] After the electrode mixture is applied and dried, a pressing process may be further performed. The pressing process can improve the electrode density. In one example, the pressing pressure is preferably 1 kPa or more and 10 GPa or less.

[0071] 4.Nonaqueous electrolyte secondary battery The above-mentioned positive electrode binder and positive electrode mixture can be used in the positive electrodes of various nonaqueous electrolyte secondary batteries, as described above, but may also be used to form other layers of the nonaqueous electrolyte secondary battery. EXAMPLES

[0072] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0073] 1. Preparation of Vinylidene Fluoride Polymer Composition Vinylidene fluoride polymers A and B were prepared by the following method.

[0074] (Preparation of vinylidene fluoride polymer A) In a 2-liter autoclave, 1096 g of ion-exchanged water, 0.2 g of Metolose 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of 50 wt% diisopropyl peroxydicarbonate-Flon 225cb solution, 426 g of vinylidene fluoride, and 0.2 g of acryloyloxypropyl succinic acid were charged, and the temperature was raised to 26°C in 1 hour. Thereafter, while maintaining the temperature at 26°C, a 6% by mass aqueous solution of acryloyloxypropyl succinic acid was gradually added at a rate of 0.5 g / min. The obtained polymer slurry was dehydrated and dried to obtain vinylidene fluoride polymer A. A total of 4 g of acryloyloxypropyl succinic acid was added, including the amount added initially.

[0075] (Preparation of vinylidene fluoride polymer B) As a homopolymer of vinylidene fluoride (vinylidene fluoride polymer B), KF#7200 manufactured by Kureha Corporation was used.

[0076] 2. Epoxy Compound Preparation The following epoxy compounds were prepared: [Table 1]

[0077] 3. Preparation of positive electrode binder and / or positive electrode mixture (Reference examples 1~3) The vinylidene fluoride polymer A or B, the positive electrode active material (LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523, average particle size 11 μm) and a conductive additive (Super-P, manufactured by Meris Graphite & Carbon Co., Ltd.) were dispersed in N-methylpyrrolidone to prepare a slurry-like positive electrode mixture (Reference Example 1 and Reference Example 2).

[0078] (Examples 1 to 9 and Comparative Examples 1 to 4) An epoxy compound, vinylidene fluoride polymer A, a positive electrode active material (NCM523), and a conductive assistant (Super-P) having the composition ratios shown in Table 2 were dispersed in N-methylpyrrolidone to prepare a positive electrode mixture.

[0079] (Examples 10 to 13 and Comparative Examples 5 and 6) An epoxy compound, vinylidene fluoride polymer B, a positive electrode active material (NCM523), and a conductive assistant (Super-P) having the composition ratios shown in Table 3 were dispersed in N-methylpyrrolidone to prepare a positive electrode mixture.

[0080] 4. Evaluation The positive electrode mixtures obtained in the above-mentioned Reference Example, Example, and Comparative Example were each applied to an aluminum foil having a thickness of 15 μm using a bar coater, and then cured to obtain a positive electrode. The positive electrode mixtures were cured in a nitrogen circulating thermostatic chamber at 110° C. for 30 minutes. The coating weight on one side was 200±20 g / m 2 The positive electrode was used as the positive electrode for evaluation.

[0081] The obtained positive electrode was cut into a length of 50 mm and a width of 20 mm, and the peel strength of the mixture layer was evaluated by the following method. The results are shown in Tables 2 and 3. First, the surface on which the mixture layer was formed was attached to a thick plastic plate (made of acrylic resin, thickness 5 mm) with double-sided tape, and a 90° peel strength test (measurement of strength required for peeling) was performed in accordance with JISK6854-1. The test speed was 10 mm per minute. It can be said that the higher the value, the higher the adhesive strength.

[0082] The measured peel strength of each positive electrode mixture of the Examples and Comparative Examples was compared with the peel strength of any of Reference Examples 1 to 3, which used the same type of vinylidene fluoride polymer and the same amount of conductive assistant relative to the amount of positive electrode active material, and was evaluated. The larger the difference or ratio with the Reference Example, the better the result.

[0083] [Table 2]

[0084] [Table 3]

[0085] As shown in Tables 2 and 3 above, when a binder contained an epoxy compound and a vinylidene fluoride polymer, the epoxy equivalent of the epoxy compound was 500 g / eq or less, and the total amount of vinylidene fluoride polymer relative to the total moles of epoxy groups contained in the binder (the above PVDF / epoxy groups) was 600 g / eq or more and 3000 g / eq or less, the peel strength (strength required for peeling) was increased by 1.4 times or more in all cases (Examples 1 to 13).

[0086] In contrast, in Comparative Examples 1 and 6, in which epoxy compound a having only one epoxy group was used, the peel strength hardly changed compared to Reference Examples 1 and 3 in which no epoxy compound was added. In Comparative Example 2, in which epoxy compound b having too high an epoxy equivalent was used, the peel strength decreased due to the addition of epoxy compound b. It is believed that the amount of epoxy groups was insufficient.

[0087] In addition, even when the total amount of vinylidene fluoride polymer (above PVDF / epoxy group) relative to the total mole number of epoxy groups exceeded 3000 g / eq (Comparative Example 3), the peel strength hardly changed. It is considered that the relative number of epoxy groups was small and the effect was not sufficiently exhibited.

[0088] Furthermore, when the total amount of vinylidene fluoride polymer (above PVDF / epoxy group) relative to the total mole number of epoxy groups was less than 600 g / eq (Comparative Examples 4 and 5), the peel strength did not change or was significantly reduced. It is believed that the mechanical strength was reduced because the amount of the epoxy compound, which has a relatively low molecular weight, was excessive.

[0089] This application claims priority based on Japanese Patent Application No. 2021-177878, filed on October 29, 2021. The entire contents of the specification of that application are incorporated herein by reference. [Industrial Applicability]

[0090] According to the present invention, it is possible to obtain a positive electrode binder that exhibits high adhesion to an active material or a current collector even in a small amount, a positive electrode mixture containing the same, and further a positive electrode and a non-aqueous electrolyte secondary battery that contain a cured product of the positive electrode binder. Therefore, it is very useful for the manufacture of non-aqueous electrolyte secondary batteries.

Claims

1. A binder for a positive electrode of a non-aqueous electrolyte secondary battery, comprising: Contains a vinylidene fluoride polymer and an epoxy compound having two or more epoxy groups, The epoxy equivalent of the epoxy compound is 500 g / eq or less, a total amount of the vinylidene fluoride polymer relative to the total number of moles of the epoxy groups in the positive electrode binder is 600 g / eq or more and 3000 g / eq or less; Positive electrode binder.

2. The vinylidene fluoride polymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from at least one compound selected from the group consisting of compounds represented by any one of the following general formulas (1) to (3), unsaturated basic acids, unsaturated basic acid monoesters, and halogenated alkyl vinyl compounds: The proportion of structural units derived from vinylidene fluoride in the vinylidene fluoride polymer is 50% by mass or more and 99.9% by mass or less. The positive electrode binder according to claim 1 . 【Chemistry 1】 (In general formula (1), R １ ~R ３ each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms; X １ represents an atomic group having a main chain with 1 to 19 atoms and a molecular weight of 472 or less. 【Chemistry 2】 (In general formula (2), R ４ ~R ６ each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms; X ２ represents an atomic group having a main chain with 1 to 19 atoms and a molecular weight of 484 or less. 【Chemistry 3】 (In general formula (3), R ７ ~R ９ each independently represents a hydrogen atom, a fluorine atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms; X ３ represents a hydrogen atom or a hydrocarbon group having 1 to 5 carbon atoms and including one or more hydroxyl groups.

3. Further comprising a polar solvent, the epoxy compound and the vinylidene fluoride polymer are dissolved or dispersed in the polar solvent; The positive electrode binder according to claim 1 .

4. Further comprising a polar solvent, the epoxy compound and the vinylidene fluoride polymer are dissolved or dispersed in the polar solvent; The positive electrode binder according to claim 2 .

5. The positive electrode binder according to any one of claims 1 to 4, A positive electrode active material; A positive electrode mixture comprising:

6. A current collector; a positive electrode mixture layer disposed on the current collector; having The positive electrode mixture layer includes a cured product of the positive electrode mixture according to claim 5. Positive electrode.

7. The positive electrode according to claim 6, Nonaqueous electrolyte secondary battery.

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