Binder for positive electrode mixture, positive electrode mixture, positive electrode, and secondary battery

Abstract

To provide a binder for a positive electrode mixture that can form a positive electrode mixture layer with small surface roughness and can form a secondary battery with excellent performance.SOLUTION: A binder for a positive electrode mixture contains a fluoropolymer (A). The fluoropolymer (A) contains a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units, where the content of the hexafluoropropylene units is 0.001 to 1.000 mol% when the total monomer units in all the fluoropolymers contained in the binder for a positive electrode mixture as the fluoropolymer (A) is taken as 100 mol%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a binder for a positive electrode mixture, a positive electrode mixture, a positive electrode, and a secondary battery. [Background technology]

[0002] Patent Document 1 discloses a slurry for a positive electrode mixture containing a positive electrode active material (A), a binder (B), and an organic solvent (C), in which the positive electrode active material (A) is represented by the formula (A): Li x M 1 y M 2 1-y O2 (In the formula, 0.4≦x≦1;0.3≦y≦1;M 1 is at least one selected from the group consisting of Ni and Mn; M 2 is at least one selected from the group consisting of Co, Al, and Fe), and the binder (B) is a lithium-containing composite metal oxide represented by the composition formula (B): (VDF) m (TFE) n (HFP) l (wherein VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45≦m≦1; 0≦n≦0.5; 0≦l≦0.1, provided that m+n+l=1) is disclosed.

[0003] Patent Document 2 describes a slurry for an electrode mixture of a lithium secondary battery containing an electrode active material (A), a binder (B), and an organic solvent (C), in which the binder (B) is (B1) Composition formula (B1): (VDF) m (TFE) n (HFP) l (wherein VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45≦m≦1; 0.05≦n≦0.5; 0≦l≦0.1, provided that m+n+l=1), and a fluorine-containing polymer represented by the formula: (B2) Solvent-soluble thermoplastic resin other than the fluorine-containing polymer (B1) The present invention describes a slurry for an electrode mixture for a lithium secondary battery, which comprises: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2010 / 092976 [Patent Document 2] International Publication No. 2010 / 092977 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to provide a binder for a positive electrode mixture that can form a positive electrode mixture layer with small surface roughness and can form a secondary battery with excellent performance. [Means for solving the problem]

[0006] According to the present disclosure, there is provided a binder for a positive electrode mixture containing a fluoropolymer (A), wherein the fluoropolymer (A) is a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units, and the content of the hexafluoropropylene units is 0.001 to 1.000 mol % when the total monomer units in all the fluoropolymers contained in the binder for a positive electrode mixture as the fluoropolymer (A) is taken as 100 mol %. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide a binder for a positive electrode mixture that can form a positive electrode mixture layer with small surface roughness and that can form a secondary battery with excellent performance. DETAILED DESCRIPTION OF THE INVENTION

[0008] Specific embodiments of the present disclosure will be described in detail below, but the present disclosure is not limited to the following embodiments.

[0009] Patent Documents 1 and 2 propose using the above-mentioned fluorine-containing polymers as binders for use in slurries for electrode mixtures of lithium secondary batteries. By using these fluorine-containing polymers, a highly flexible positive electrode can be obtained, and a lithium secondary battery with excellent battery characteristics can be obtained. However, it has been found that there is room for improvement in the cycle characteristics of the secondary battery.

[0010] As a result of intensive research into means for solving the above problems, it was found that by using hexafluoropropylene units as only a small portion of all the monomer units constituting all the polymers constituting the binder, it is possible to form a positive electrode mixture layer with small surface roughness, and to obtain a secondary battery with excellent cycle characteristics.

[0011] That is, the binder for a positive electrode mixture of the present disclosure contains a fluoropolymer (A), and as the fluoropolymer (A), it contains a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units, and the content of the hexafluoropropylene units is 0.001 to 1.000 mol % when all monomer units in all fluoropolymers contained in the binder for a positive electrode mixture as the fluoropolymer (A) are taken as 100 mol %.

[0012] The positive electrode of the secondary battery can be formed by applying a positive electrode mixture to a positive electrode current collector and pressing the applied film.

[0013] It has been found that a coating film formed using a positive electrode mixture containing a binder with a hexafluoropropylene unit content of less than 0.001 mol% has the problem of high surface roughness. Even when the surface roughness of the coating film is high, a positive electrode mixture layer with a smooth surface can be formed by pressing the coating film, but it is difficult to obtain a secondary battery with excellent cycle characteristics. The reason for this is presumed to be that the components are not sufficiently dispersed in a coating film (positive electrode mixture layer) with high surface roughness before pressing. In contrast, a coating film formed by applying a positive electrode mixture containing the binder for a positive electrode mixture of the present disclosure to a positive electrode current collector has low surface roughness even before pressing, thereby allowing a secondary battery with excellent cycle characteristics to be obtained.

[0014] Furthermore, it has been found that a coating film formed using a positive electrode mixture containing a binder with a hexafluoropropylene unit content of more than 1.000 mol% has a small surface roughness, but when the coating film is pressed, the surface roughness increases. In contrast, a coating film formed by applying a positive electrode mixture containing the binder for a positive electrode mixture of the present disclosure to a positive electrode current collector maintains a small surface roughness even when pressed. Therefore, a positive electrode mixture layer with a small surface roughness can be obtained, thereby obtaining a secondary battery with excellent cycle characteristics.

[0015] Next, each component contained in the binder for a positive electrode mixture of the present disclosure will be described in detail.

[0016] <Fluoropolymer (A)> The binder for a positive electrode mixture of the present disclosure contains a fluoropolymer (A), and as the fluoropolymer (A), it contains at least a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units. The binder for a positive electrode mixture may contain only the fluoropolymer (a1) as the fluoropolymer (A), or may contain a fluoropolymer other than the fluoropolymer (a1).

[0017] Whether the binder for a positive electrode mixture contains only the fluoropolymer (a1) as the fluoropolymer (A) or contains a fluoropolymer other than the fluoropolymer (a1), the content of hexafluoropropylene units in the binder for a positive electrode mixture is adjusted to be within the range of 0.001 to 1.000 mol %, where the total monomer units in all the fluoropolymers contained in the binder for a positive electrode mixture as the fluoropolymer (A) is taken as 100 mol %. By using a binder for a positive electrode mixture whose hexafluoropropylene unit content is adjusted to be within the above range, a positive electrode mixture layer with small surface roughness can be formed, and a secondary battery with excellent cycle characteristics can be formed.

[0018] The content of the hexafluoropropylene unit in the binder for the positive electrode mixture is, for example, 19 It can be measured by F-NMR measurement.

[0019] The content of hexafluoropropylene units in the binder for a positive electrode mixture is preferably 0.005 mol% or more, more preferably 0.008 mol% or more, more preferably 0.010 mol% or more, more preferably 0.050 mol% or more, even more preferably 0.100 mol% or more, particularly preferably 0.120 mol% or more, and most preferably 0.140 mol% or more, and preferably 0.900 mol% or less, and more preferably 0.850 mol% or less, because this allows the formation of a positive electrode mixture layer with even smaller surface roughness.

[0020] In one embodiment, the fluoropolymer (A) contains, in addition to the fluoropolymer (a1), at least one selected from the group consisting of a fluoropolymer (a2) and a fluoropolymer (a3). The structures of the fluoropolymer (a1), the fluoropolymer (a2), and the fluoropolymer (a3) ​​will be described later.

[0021] In one embodiment, the content ratio of the fluoropolymer (a1) to the fluoropolymer (a2) is, in mass ratio ((a1) / (a2)), 100 / 0 to 0.1 / 99.9. By adjusting the content ratio of the fluoropolymer (a1) to the fluoropolymer (a2) within the above range, a positive electrode mixture layer with even smaller surface roughness can be formed.

[0022] In one embodiment, the binder for the positive electrode mixture contains, as the fluoropolymer (A), a fluoropolymer (a1) and a fluoropolymer (a3), and optionally further contains a fluoropolymer (a2). In this case, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 0.01 to 99.00% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0 to 98.99% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a3) ​​is preferably 1.00 to 40.00% by mass relative to the mass of the fluoropolymer (A).

[0023] In the above embodiment, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 2.00 to 90.00% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0 to 96.00% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a3) ​​is preferably 2.00 to 40.00 mass % based on the mass of the fluoropolymer (A).

[0024] In one embodiment, the binder for the positive electrode mixture contains, as the fluoropolymer (A), a fluoropolymer (a1), a fluoropolymer (a2), and a fluoropolymer (a3). In this case, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a3) ​​is preferably 1.00 to 40.00% by mass relative to the mass of the fluoropolymer (A).

[0025] In the above embodiment, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 2.00 to 90.00% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 30.0 to 96.00% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a3) ​​is preferably 2.00 to 40.00 mass % based on the mass of the fluoropolymer (A).

[0026] In one embodiment, the binder for the positive electrode mixture contains, as the fluoropolymer (A), the fluoropolymer (a1) and the fluoropolymer (a3), but does not contain the fluoropolymer (a2). In this case, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 60.00 to 99.00% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a2) is 0% by mass relative to the mass of the fluoropolymer (A), The content of the fluoropolymer (a3) ​​is preferably 1.00 to 40.00% by mass relative to the mass of the fluoropolymer (A).

[0027] In the above embodiment, the content ratio of each fluoropolymer is as follows: the content of the fluoropolymer (a1) is 60.00 to 90.00% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a2) is 0% by mass relative to the mass of the fluoropolymer (A), The content of the fluoropolymer (a3) ​​is preferably 10.00 to 40.00% by mass relative to the mass of the fluoropolymer (A).

[0028] Next, the structures of the fluoropolymer (a1), the fluoropolymer (a2) and the fluoropolymer (a3) ​​will be described.

[0029] (Fluoropolymer (a1)) The fluoropolymer (a1) contains vinylidene fluoride (VdF) units and hexafluoropropylene (HFP) units.

[0030] The fluoropolymer (a1) may contain other monomer units in addition to VdF units and HFP units. Examples of other monomers include tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, (perfluoroalkyl)ethylene, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, ethylene, propylene, and monomers having a polar group.

[0031] The monomer having a polar group includes a monomer having the general formula (1): [ka] (In the formula, R 1 ~R 3 R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. 1 represents an inorganic cation and / or an organic cation. When the fluoropolymer (a1) contains a monomer unit having a polar group, the adhesion between the positive electrode mixture layer and the current collector is improved.

[0032] In general formula (1), Y 1represents an inorganic cation and / or an organic cation. Examples of inorganic cations include cations such as H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4, NHR, and the like. 5 , NH2R 5 2. NHR 5 3.NR 5 4(R 5 and each independently represent an alkyl group having 1 to 4 carbon atoms. 1 As the cation, H, Li, Na, K, Mg, Ca, Al, and NH4 are preferred, H, Li, Na, K, Mg, Al, and NH4 are more preferred, H, Li, Al, and NH4 are still more preferred, and H is particularly preferred. For convenience, the symbols and valences of specific examples of inorganic cations and organic cations are omitted.

[0033] In general formula (1), R 1 ~R 3 R independently represent a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a monovalent hydrocarbon group. The hydrocarbon group preferably has 4 or less carbon atoms. Examples of the hydrocarbon group include alkyl groups, alkenyl groups, alkynyl groups, etc. having the above carbon numbers, with a methyl group or an ethyl group being preferred. 1 and R 2 are preferably independently a hydrogen atom, a methyl group, or an ethyl group, and R 3 is preferably a hydrogen atom or a methyl group.

[0034] In general formula (1), R 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a divalent hydrocarbon group. The hydrocarbon group preferably has 4 or less carbon atoms. Examples of the hydrocarbon group include alkylene groups and alkenylene groups having the above carbon numbers, and among these, at least one selected from the group consisting of a methylene group, an ethylene group, an ethylidene group, a propylidene group, and an isopropylidene group is preferred, with a methylene group being more preferred.

[0035] Monomer (1) is preferably at least one selected from the group consisting of (meth)acrylic acid and salts thereof, vinylacetic acid (3-butenoic acid) and salts thereof, 3-pentenoic acid and salts thereof, 4-pentenoic acid and salts thereof, 3-hexenoic acid and salts thereof, 4-heptenoic acid and salts thereof, and 5-hexenoic acid and salts thereof, and more preferably at least one selected from the group consisting of 3-butenoic acid and salts thereof, and 4-pentenoic acid and salts thereof.

[0036] Examples of the fluoropolymer (a1) include VdF / HFP copolymer, VdF / TFE / HFP copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / TFE / HFP / (meth)acrylic acid copolymer, VdF / TFE / HFP / 4-pentenoic acid copolymer, VdF / TFE / HFP / 3-butenoic acid copolymer, VdF / TFE / HFP / 2-carboxyethyl acrylate copolymer, and VdF / TFE / HFP / acryloyloxyethyl succinic acid copolymer.

[0037] The content of VdF units in the fluoropolymer (a1) is preferably 84.0 to 99.999 mol%, more preferably 92.0 mol% or more, even more preferably more than 95.0 mol%, still more preferably 97.0 mol% or more, particularly preferably 98.5 mol% or more, more preferably 99.99 mol% or less, and even more preferably 99.9 mol% or less, based on the total monomer units.

[0038] The content of HFP units in the fluoropolymer (a1) is preferably 0.001 to 16.0 mol%, more preferably 0.01 mol% or more, even more preferably 0.1 mol% or more, more preferably 8.0 mol% or less, even more preferably less than 5.0 mol%, still more preferably 3.0 mol% or less, and particularly preferably 1.5 mol% or less, based on all monomer units.

[0039] The content of other monomer units in the fluoropolymer (a1) is preferably 0 to 8.0 mol % based on the total monomer units, more preferably 5.0 mol % or less, even more preferably 3.0 mol % or less, and even more preferably 1.5 mol % or less. When the fluoropolymer (a1) contains other monomer units such as monomer units having a polar group, the content of other monomer units is preferably 0.1 mol % or more based on the total monomer units.

[0040] In the present disclosure, the composition of the fluoropolymer may be, for example, 19 It can be measured by F-NMR measurement. In the present disclosure, the content of the monomer unit based on the monomer (1) and the monomer unit having a polar group in the fluoropolymer can be measured by acid-base titration of the acid group.

[0041] The weight average molecular weight (polystyrene equivalent) of the fluoropolymer (a1) is preferably 50,000 to 3,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, particularly preferably 2,000,000 or less. The weight average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0042] The number average molecular weight (polystyrene equivalent) of the fluoropolymer (a1) is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0043] The melting point of the fluoropolymer (a1) is preferably 100 to 240° C. The melting point can be determined as the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0044] (Fluoropolymer (a2)) The fluoropolymer (a2) contains VdF units and does not contain HFP units, and the content of VdF units in the fluoropolymer (a2) is 98.0 mol % or more based on the total monomer units of the fluoropolymer (a2).

[0045] The fluoropolymer (a2) may be a VdF homopolymer consisting of only VdF units, or may be a polymer containing other monomer units other than VdF units and HFP units.

[0046] Examples of other monomers include tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, (perfluoroalkyl)ethylene, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, ethylene, propylene, and monomers having a polar group.

[0047] The monomer having a polar group may be the monomer (1) represented by the above-mentioned general formula (1). When the fluoropolymer (a2) contains a monomer unit having a polar group, the adhesion between the positive electrode mixture layer and the current collector is improved.

[0048] Examples of the fluoropolymer (a2) include VdF homopolymer, VdF / TFE copolymer, VdF / TFE / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / TFE / 4-pentenoic acid copolymer, VdF / TFE / 3-butenoic acid copolymer, VdF / TFE / 2-carboxyethyl acrylate copolymer, VdF / TFE / acryloyloxyethyl succinic acid copolymer, etc. In one embodiment, the fluoropolymer (a2) does not contain a TFE unit as a monomer unit.

[0049] The content of VdF units in the fluoropolymer (a2) is 98.0 mol % or more, more preferably 98.5 mol % or more, and preferably 100 mol % or less, based on all monomer units.

[0050] The content of other monomer units in the fluoropolymer (a2) is preferably 0 to 2.0 mol %, more preferably 1.5 mol % or less, based on the total monomer units. When the fluoropolymer (a2) contains other monomer units such as monomer units having a polar group, the content of other monomer units is preferably 0.1 mol % or more, based on the total monomer units.

[0051] The weight average molecular weight (polystyrene equivalent) of the fluoropolymer (a2) is preferably 50,000 to 3,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, particularly preferably 2,000,000 or less. The weight average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0052] The number average molecular weight (polystyrene equivalent) of the fluoropolymer (a2) is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0053] The melting point of the fluoropolymer (a2) is preferably 100 to 240° C. The melting point can be determined as the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0054] (Fluoropolymer (a3)) The fluoropolymer (a3) ​​contains VdF units and fluoromonomer units (excluding VdF units and HFP units), and the content of VdF units in the fluoropolymer (a3) ​​is less than 98.0 mol% based on the total monomer units of the fluoropolymer (a3).

[0055] Examples of the fluoromonomer include TFE, vinyl fluoride, trifluoroethylene, CTFE, fluoroalkyl vinyl ether, (perfluoroalkyl)ethylene, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, etc. As the fluoromonomer, TFE is preferred because it can form a positive electrode mixture layer with even smaller surface roughness.

[0056] The fluoropolymer (a3) ​​may contain a non-fluorinated monomer unit. Examples of the non-fluorinated monomer include ethylene and propylene. The fluoropolymer (a3) ​​may or may not contain a non-fluorinated monomer unit.

[0057] The fluoropolymer (a3) ​​may contain a monomer having a polar group. Examples of the monomer having a polar group include the monomer (1) represented by the general formula (1) described above. When the fluoropolymer (a3) ​​contains a monomer unit having a polar group, the adhesion between the positive electrode mixture layer and the current collector is improved. The fluoropolymer (a3) ​​may or may not contain a monomer unit based on the monomer (1).

[0058] Examples of the fluoropolymer (a3) ​​include VdF / TFE copolymer, VdF / TFE / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / TFE / 4-pentenoic acid copolymer, VdF / TFE / 3-butenoic acid copolymer, VdF / TFE / 2-carboxyethyl acrylate copolymer, and VdF / TFE / acryloyloxyethyl succinic acid copolymer. In one embodiment, the fluoropolymer (a3) ​​does not contain HFP units as monomer units. In one embodiment, the fluoropolymer (a3) ​​contains only VdF units and TFE units as monomer units.

[0059] The content of VdF units in the fluoropolymer (a3) ​​is less than 98.0 mol%, preferably 95.0 mol% or less, more preferably 89.0 mol% or less, even more preferably 85.0 mol% or less, preferably 57.0 mol% or more, more preferably 60.0 mol% or more, even more preferably 63.0 mol% or more, and still more preferably 64.0 mol% or more, based on all monomer units, so that a positive electrode having excellent flexibility can be formed.

[0060] The content of the fluoromonomer units in the fluoropolymer (a3) ​​is preferably more than 2.0 mol%, more preferably 5.0 mol% or more, even more preferably 8.0 mol% or more, still more preferably 11.0 mol% or more, particularly preferably 15.0 mol% or more, and preferably 43.0 mol% or less, more preferably 40.0 mol% or less, still more preferably 38.0 mol% or less, and particularly preferably 37.0 mol% or less, based on all monomer units, in order to form a positive electrode having excellent flexibility.

[0061] The content of non-fluorinated monomer units and monomer units having a polar group in the fluoropolymer (a3) ​​is preferably 0 to 3.0 mol %, more preferably 1.5 mol % or less, based on the total monomer units. The content of non-fluorinated monomer units and monomer units having a polar group in the fluoropolymer (a3) ​​is, in one embodiment, the fluoropolymer (a3) ​​does not contain any non-fluorinated monomer units. In one embodiment, the fluoropolymer (a3) ​​does not contain any monomer units having a polar group.

[0062] The weight average molecular weight (polystyrene equivalent) of the fluoropolymer (a3) ​​is preferably 50,000 to 3,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, particularly preferably 2,000,000 or less. The weight average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0063] The number average molecular weight (polystyrene equivalent) of the fluoropolymer (a3) ​​is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0064] The melting point of the fluoropolymer (a3) ​​is preferably 100 to 240° C. The melting point can be determined as the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0065] The binder for a positive electrode mixture of the present disclosure may contain other polymers in addition to the fluoropolymer, such as acrylic resins such as polyacrylic acid, polymethacrylate, and polymethyl methacrylate, polyimide, polyamide, and polyamide-imide resins, styrene rubber, butadiene rubber, and styrene-butadiene rubber.

[0066] The binder for a positive electrode mixture of the present disclosure is suitable as a binder used in the positive electrodes of batteries such as secondary batteries and capacitors. The binder for a positive electrode mixture of the present disclosure is particularly suitable for use in the positive electrodes of lithium ion secondary batteries.

[0067] Furthermore, the binder for a positive electrode mixture of the present disclosure can be suitably used as a binder contained in a positive electrode mixture. Next, each component contained in the positive electrode mixture of the present disclosure will be described in detail.

[0068] <Positive electrode mixture> The positive electrode mixture of the present disclosure contains the above-described binder for the positive electrode mixture, a positive electrode active material, and water or a non-aqueous solvent.

[0069] (Cathode active material) As the positive electrode active material, a material capable of electrochemically absorbing and desorbing lithium ions can be suitably used. As the positive electrode active material, a lithium composite oxide is preferred, and a lithium transition metal composite oxide is more preferred. As the positive electrode active material, a lithium-containing transition metal phosphate compound is also preferred. It is also preferred that the positive electrode active material is a material containing lithium and at least one transition metal, such as a lithium transition metal composite oxide or a lithium-containing transition metal phosphate compound.

[0070] The transition metal of the lithium transition metal composite oxide is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. Specific examples of the lithium transition metal composite oxide include lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMnO2, LiMn2O4, and Li2MnO3, and those in which some of the transition metal atoms that make up the main components of these lithium transition metal composite oxides have been substituted with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, and Si. Examples of the substituted oxides include lithium-nickel-manganese composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-cobalt-manganese composite oxide, lithium-manganese-aluminum composite oxide, and lithium-titanium composite oxide. More specifically, LiNi 0.5 Mn 0.5 O2, LiNi 0.85 Co 0.10 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2, LiMn 1.8 Al 0.2 O4, LiMn 1.5 Ni 0.5 O4, Li4Ti5O12 , LiNi 0.82 Co 0.15 Al 0.03 Examples include O2.

[0071] The transition metal of the lithium-containing transition metal phosphate compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc., and specific examples of the lithium-containing transition metal phosphate compound include iron phosphates such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, cobalt phosphates such as LiCoPO4, and lithium transition metal phosphate compounds in which a portion of the transition metal atoms that constitute the main components of these lithium transition metal phosphate compounds has been substituted with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, and Si.

[0072] In particular, from the viewpoint of high voltage, high energy density, charge / discharge cycle characteristics, etc., LiCoO2, LiNiO2, LiMn2O4, LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.33 Mn 0.33 Co 0.33 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2 and LiFePO4 are preferred.

[0073] As the lithium transition metal composite oxide, a lithium-nickel composite oxide is preferred, and it is represented by the general formula (7): General formula (7): Li y Ni 1-x M x O2 (In the formula, x is 0.01≦x≦0.5, y is 0.9≦y≦1.2, and M represents a metal atom (excluding Li and Ni).) Lithium-nickel composite oxides represented by the following formula are more preferable. Lithium transition metal composite oxides with a high nickel content like this are useful for increasing the capacity of secondary batteries.

[0074] In general formula (7), x is a coefficient that satisfies 0.01≦x≦0.5, and is preferably 0.05≦x≦0.4, and more preferably 0.10≦x≦0.3, since this allows a secondary battery with an even higher capacity to be obtained.

[0075] In general formula (7), examples of the metal atom of M include V, Ti, Cr, Mn, Fe, Co, Cu, Al, Zn, Mg, Ga, Zr, Si, etc. Preferred metal atoms of M are transition metals such as V, Ti, Cr, Mn, Fe, Co, and Cu, or combinations of the above transition metals with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mg, Ga, Zr, and Si.

[0076] Lithium transition metal composite oxides with a high nickel content include LiNi 0.80 Co 0.15 Al 0.05 O2, LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.33 Mn 0.33 Co 0.33 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2 and LiNi 0.90 Mn 0.05 Co 0.05 At least one selected from the group consisting of LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2 and LiNi 0.8 Mn0.1 Co 0.1 At least one selected from the group consisting of O2 is more preferred.

[0077] Furthermore, a substance having a different composition from the substance constituting the main positive electrode active material may be attached to the surface of the positive electrode active material. Examples of the surface-attached substance include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide, sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate, and carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate.

[0078] These surface-attaching substances can be attached to the surface of the positive electrode active material by, for example, a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the substance to the positive electrode active material by impregnation, and drying the solvent; a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the substance to the positive electrode active material by impregnation, and then reacting the substance by heating or the like; a method of adding the substance to the positive electrode active material precursor and simultaneously baking the substance; or the like.

[0079] The amount of the surface-attached substance is preferably 0.1 ppm or more, more preferably 1 ppm or more, and even more preferably 10 ppm or more by mass relative to the positive electrode active material, and is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less by mass relative to the positive electrode active material. The surface-attached substance can suppress the oxidation reaction of the nonaqueous electrolyte on the surface of the positive electrode active material and improve the battery life, but if the amount of attachment is too small, the effect will not be fully exerted, and if it is too large, the movement of lithium ions into and out of the positive electrode active material will be hindered, which may increase the resistance.

[0080] The particle shape of the positive electrode active material may be a block, polyhedron, sphere, ellipsoid, plate, needle, column, or the like, as conventionally used. Among these, those formed by aggregation of primary particles to form secondary particles, with the secondary particles being spherical or ellipsoidal, are preferred. Typically, electrochemical devices experience expansion and contraction of the active material in the electrode during charging and discharging, which can lead to deterioration, such as destruction of the active material or disconnection of the conductive path, due to stress. Therefore, a material formed by aggregation of primary particles to form secondary particles is preferred over a single-particle active material consisting of only primary particles, as this relieves the stress of expansion and contraction and prevents deterioration. Furthermore, spherical or ellipsoidal particles are preferred over plate-like equiaxially oriented particles because they are less oriented during electrode molding, resulting in less expansion and contraction of the electrode during charging and discharging, and are also more easily mixed uniformly with the conductive agent during electrode fabrication.

[0081] The tap density of the positive electrode active material is typically 1.3 g / cm 3 or more, preferably 1.5 g / cm 3 More preferably, 1.6 g / cm 3 or more, most preferably 1.7 g / cm 3 That is all. If the tap density of the positive electrode active material is below the above lower limit, the amount of dispersion medium required when forming the positive electrode mixture layer increases, and the amounts of conductive agent and binder required also increase, which may restrict the filling rate of the positive electrode active material in the positive electrode mixture layer and restrict the battery capacity. By using a metal composite oxide powder with a high tap density, a high-density positive electrode mixture layer can be formed. Generally, the higher the tap density, the more preferable it is, and there is no particular upper limit. However, if it is too high, the diffusion of lithium ions in the positive electrode mixture layer using the nonaqueous electrolyte as a medium becomes rate-limiting, which may lead to a decrease in load characteristics. Therefore, the tap density is usually set to 2.5 g / cm. 3 or less, preferably 2.4 g / cm 3 The following is the result.

[0082] The tap density of the positive electrode active material is measured by passing it through a sieve with a mesh size of 300 μm and measuring it in 20 cm 3After dropping the sample into the tapping cell to fill the cell volume, tapping is performed 1000 times with a stroke length of 10 mm using a powder density measuring device (for example, Tap Denser manufactured by Seishin Enterprise Co., Ltd.), and the density calculated from the volume and weight of the sample at that time is defined as the tap density.

[0083] The median particle diameter d50 of the positive electrode active material (the secondary particle diameter when primary particles aggregate to form secondary particles) is typically 0.1 μm or larger, preferably 0.5 μm or larger, more preferably 1 μm or larger, and most preferably 3 μm or larger, and typically 20 μm or smaller, preferably 18 μm or smaller, more preferably 16 μm or smaller, and most preferably 15 μm or smaller. Below the lower limit, high bulk density products may not be obtained. Above the upper limit, lithium diffusion within the particles takes time, resulting in reduced battery performance and problems such as streaking during battery positive electrode fabrication, i.e., when the active material, conductive agent, binder, etc. are slurried with a solvent and applied as a thin film. Mixing two or more positive electrode active materials with different median diameters d50 can further improve the packing properties during positive electrode fabrication.

[0084] The median diameter d50 in this disclosure is measured using a known laser diffraction / scattering particle size distribution analyzer. When using a HORIBA LA-920 as the particle size distribution analyzer, the measurement is performed using a 0.1% by mass aqueous solution of sodium hexametaphosphate as the dispersion medium, and after ultrasonic dispersion for 5 minutes, the measurement is performed with a refractive index set to 1.24.

[0085] When primary particles aggregate to form secondary particles, the average primary particle diameter of the positive electrode active material is typically 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.08 μm or more, and most preferably 0.1 μm or more, and typically 3 μm or less, preferably 2 μm or less, more preferably 1 μm or less, and most preferably 0.6 μm or less. If the diameter exceeds the upper limit, it becomes difficult to form spherical secondary particles, adversely affecting powder packing, and the specific surface area is significantly reduced, potentially resulting in a decrease in battery performance, such as output characteristics. Conversely, if the diameter is below the lower limit, problems such as poor charge / discharge reversibility due to underdeveloped crystals may occur. The primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, the diameter is determined by taking a 10,000x magnification photograph of 50 primary particles and averaging the longest intercepts of a horizontal line at the left and right boundaries of the primary particles.

[0086] The BET specific surface area of ​​the positive electrode active material is 0.2 m 2 / g or more, preferably 0.3m 2 / g or more, more preferably 0.4m 2 / g or more, 4.0m 2 / g or less, preferably 2.5m 2 / g or less, more preferably 1.5m 2 If the BET specific surface area is smaller than this range, the battery performance is likely to decrease, whereas if it is larger, it becomes difficult to increase the tap density, which may easily cause problems with the coating properties when forming the positive electrode mixture layer.

[0087] The BET specific surface area is defined as the value measured by a surface area meter (for example, an automatic surface area measuring device manufactured by Okura Riken) using a nitrogen-helium mixed gas precisely adjusted so that the relative pressure of nitrogen to atmospheric pressure is 0.3, after which the sample is pre-dried at 150°C for 30 minutes under a nitrogen flow, by the nitrogen adsorption BET single-point method using a gas flow method.

[0088] The positive electrode active material is produced by a method generally used for producing inorganic compounds. In particular, various methods can be considered for producing spherical or oval-spherical active materials, such as a method of dissolving or pulverizing and dispersing transition metal raw materials such as transition metal nitrates and sulfates, and if necessary, raw materials of other elements, in a solvent such as water, adjusting the pH while stirring to produce and recover spherical precursors, which are then dried as necessary, and then adding a Li source such as LiOH, Li2CO3, or LiNO3, and calcining at a high temperature to obtain an active material; a method of dissolving or pulverizing transition metal raw materials such as transition metal nitrates, sulfates, hydroxides, or oxides, and if necessary, raw materials of other elements, in a solvent such as water, Examples of such methods include dispersing a precursor in the form of a sphere or ellipsoid, drying and molding it using a spray dryer or the like to form a spherical or ellipsoidal precursor, adding a Li source such as LiOH, Li2CO3, or LiNO3 to the precursor, and firing it at a high temperature to obtain an active material; and dissolving or pulverizing and dispersing a transition metal raw material such as a transition metal nitrate, sulfate, hydroxide, or oxide, a Li source such as LiOH, Li2CO3, or LiNO3, and, if necessary, raw materials of other elements, in a solvent such as water, drying and molding it using a spray dryer or the like to form a spherical or ellipsoidal precursor, and firing this at a high temperature to obtain an active material.

[0089] In the present disclosure, one type of positive electrode active material powder may be used alone, or two or more types having different compositions or different powder properties may be used in any combination and ratio.

[0090] In the positive electrode mixture of the present disclosure, the mass ratio of the binder for the positive electrode mixture to the positive electrode active material is preferably 0.01 / 99.99 to 10 / 90, more preferably 0.5 / 99.5 to 4 / 96, and even more preferably 1 / 99 to 3 / 97, since a battery with a higher capacity can be obtained.

[0091] (Water or non-aqueous solvent) The positive electrode mixture of the present disclosure contains water or a nonaqueous solvent. Examples of the nonaqueous solvent include nitrogen-containing organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and dimethylformamide; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone; ester solvents such as ethyl acetate and butyl acetate; ether solvents such as tetrahydrofuran and dioxane; and mixtures thereof.

[0092] In particular, the positive electrode mixture of the present disclosure preferably contains a nonaqueous solvent, preferably at least one selected from the group consisting of N-methyl-2-pyrrolidone and N,N-dimethylacetamide, and more preferably N-methyl-2-pyrrolidone, in terms of excellent stability and coatability of the positive electrode mixture.

[0093] In the positive electrode mixture of the present disclosure, the content of water or nonaqueous solvent is determined in consideration of the coatability onto a current collector, the formability of a thin film after drying, etc. In the positive electrode mixture of the present disclosure, the total content of the binder for the positive electrode mixture and the positive electrode active material is preferably 50 to 90 mass%, more preferably 60 to 85 mass%, and even more preferably 65 to 80 mass%.

[0094] (Other ingredients) The positive electrode mixture may further contain a conductive agent. Examples of the conductive agent include carbon blacks such as acetylene black and ketjen black, carbon materials such as graphite, carbon fibers, multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon nanohorns, and graphene. As the conductive agent, it is also preferable to use carbon blacks and carbon nanotubes in combination, or it is also preferable to use carbon blacks and single-walled carbon nanotubes in combination.

[0095] In the positive electrode mixture of the present disclosure, the content ratio of the binder for the positive electrode mixture to the conductive agent is preferably 5 / 95 to 90 / 10 by mass. The content ratio of the binder for the positive electrode mixture to the conductive agent in the positive electrode mixture is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, particularly preferably 0.8 or more, more preferably 5 or less, even more preferably 3 or less, and particularly preferably 1.5 or less.

[0096] The positive electrode mixture may contain a dispersant such as a resin having a surface active effect, a cationic surfactant, or a nonionic surfactant in order to improve dispersion stability.

[0097] Examples of methods for preparing the positive electrode mixture of the present disclosure include a method in which a positive electrode active material and, if desired, a conductive agent are dispersed and mixed in a solution in which a binder for the positive electrode mixture is dissolved in water or a non-aqueous solvent. Alternatively, the positive electrode mixture may be prepared by a method in which the binder for the positive electrode mixture and the positive electrode active material are first mixed, and then water or a non-aqueous solvent and, if desired, a conductive agent are added. Alternatively, the positive electrode mixture may be prepared by a method in which the conductive agent is added to a solution in which the binder for the positive electrode mixture is dissolved in water or a non-aqueous solvent, and then the conductive agent is added and mixed, and then the positive electrode active material is further added and mixed.

[0098] The viscosity of the positive electrode mixture of the present disclosure is preferably 1000 mPa·s or more, more preferably 3000 mPa·s or more, even more preferably 5000 mPa·s or more, and preferably 80000 mPa·s or less, more preferably 70000 mPa·s or less, and even more preferably 60000 mPa·s or less, because this facilitates application and also facilitates obtaining a positive electrode mixture layer with a desired thickness. The viscosity can be measured at 25°C using a Brookfield viscometer.

[0099] <Positive electrode> The positive electrode of the present disclosure is formed from the above-described positive electrode mixture. A method for forming a positive electrode using the above-described positive electrode mixture includes, for example, applying the above-described positive electrode mixture to a current collector, drying, and pressing the current collector to form a thin positive electrode mixture layer on the current collector, thereby forming a thin-film electrode. That is, in one preferred embodiment, the positive electrode of the present disclosure includes a current collector and a positive electrode mixture layer formed from the above-described positive electrode mixture on the current collector.

[0100] Examples of the current collector include metal foils or metal meshes made of iron, stainless steel, copper, aluminum, nickel, titanium, etc., and among these, aluminum foil is preferred.

[0101] <Secondary battery> The secondary battery of the present disclosure includes the above-described positive electrode. The secondary battery of the present disclosure preferably further includes, in addition to the above-described positive electrode, a negative electrode and a non-aqueous electrolyte solution.

[0102] The non-aqueous electrolyte is not particularly limited, and one or more of known solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc. Any of the conventionally known electrolytes can be used, such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCl, LiBr, CH3SO3Li, CF3SO3Li, and cesium carbonate.

[0103] The positive electrode mixture of the present disclosure is useful for nonaqueous electrolyte secondary batteries, not only for the lithium ion secondary batteries using the liquid electrolyte described above, but also for polymer electrolyte lithium secondary batteries, and is also useful for electric double layer capacitors.

[0104] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims.

[0105] <1> According to a first aspect of the present disclosure, A binder for a positive electrode mixture containing a fluoropolymer (A), The fluoropolymer (A) contains a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units, The content of hexafluoropropylene units is 0.001 to 1.000 mol % when the total monomer units in all fluoropolymers contained as the fluoropolymer (A) in the binder for the positive electrode mixture are taken as 100 mol %. A binder for a positive electrode mixture is provided. <2> According to a second aspect of the present disclosure, The fluoropolymer (A) further contains at least one selected from the group consisting of fluoropolymer (a2) and fluoropolymer (a3), the fluoropolymer (a2) contains vinylidene fluoride units and does not contain hexafluoropropylene units, and the content of vinylidene fluoride units in the fluoropolymer (a2) is 98.0 mol% or more based on the total monomer units of the fluoropolymer (a2); There is provided a binder for a positive electrode mixture according to a first aspect, wherein the fluoropolymer (a3) ​​contains vinylidene fluoride units and fluoromonomer units (excluding vinylidene fluoride units and hexafluoropropylene units), and the content of the vinylidene fluoride units in the fluoropolymer (a3) ​​is less than 98.0 mol% based on the total monomer units of the fluoropolymer (a3). <3> According to a third aspect of the present disclosure, There is provided a binder for a positive electrode mixture according to a second aspect, wherein the content ratio of the fluoropolymer (a1) to the fluoropolymer (a2) is a mass ratio ((a1) / (a2)) of 100 / 0 to 0.1 / 99.9. <4> According to a fourth aspect of the present disclosure, The fluoropolymer (A) contains fluoropolymer (a1), fluoropolymer (a2) and fluoropolymer (a3), the content of the fluoropolymer (a1) is 0.01 to 99.00% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0 to 98.99% by mass relative to the mass of the fluoropolymer (A); According to a second or third aspect, there is provided a binder for a positive electrode mixture, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass % relative to the mass of the fluoropolymer (A). <5> According to a fifth aspect of the present disclosure, The fluoropolymer (A) contains fluoropolymer (a1), fluoropolymer (a2) and fluoropolymer (a3), the content of the fluoropolymer (a1) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); According to a second or third aspect, there is provided a binder for a positive electrode mixture, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass % relative to the mass of the fluoropolymer (A). <6> According to a sixth aspect of the present disclosure, The fluoropolymer (A) contains a fluoropolymer (a1) and a fluoropolymer (a3), but does not contain a fluoropolymer (a2), the content of the fluoropolymer (a1) is 60.00 to 99.00% by mass relative to the mass of the fluoropolymer (A); The content of the fluoropolymer (a2) is 0% by mass relative to the mass of the fluoropolymer (A), According to a second or third aspect, there is provided a binder for a positive electrode mixture, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass % relative to the mass of the fluoropolymer (A). <7> According to a seventh aspect of the present disclosure, The fluoropolymer (a2) is represented by the general formula (1): [ka] (In the formula, R 1 ~R 3 R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. 1 represents an inorganic cation and / or an organic cation. According to any one of the second to sixth aspects, there is provided a binder for a positive electrode mixture, further comprising a monomer unit based on a monomer (1) represented by: <8> According to an eighth aspect of the present disclosure, According to any one of the second to seventh aspects, there is provided a binder for a positive electrode mixture, wherein the content of vinylidene fluoride units in the fluoropolymer (a3) ​​is 57.0 to 89.0 mol % based on the total monomer units of the fluoropolymer (a3). <9> According to a ninth aspect of the present disclosure, According to any one of the second to eighth aspects, there is provided a binder for a positive electrode mixture, wherein the fluoromonomer unit in the fluoropolymer (a3) ​​is a tetrafluoroethylene unit. <10> According to a tenth aspect of the present disclosure, the content of vinylidene fluoride units in the fluoropolymer (a1) is 84.0 to 99.999 mol % based on the total monomer units of the fluoropolymer (a1); There is provided a binder for a positive electrode mixture according to any one of the first to ninth aspects, wherein the content of hexafluoropropylene units in the fluoropolymer (a1) is 0.001 to 16.0 mol % based on the total monomer units of the fluoropolymer (a1). <11> According to an eleventh aspect of the present disclosure, The fluoropolymer (a1) is represented by the general formula (1): [ka] (In the formula, R 1 ~R 3 R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. 1represents an inorganic cation and / or an organic cation. In accordance with any one of the first to tenth aspects, there is provided a binder for a positive electrode mixture, further comprising a monomer unit based on a monomer (1) represented by: <12> According to a twelfth aspect of the present disclosure, According to any one of the first to eleventh aspects, there is provided a positive electrode mixture containing a binder for a positive electrode mixture, a positive electrode active material, and water or a non-aqueous solvent. <13> According to a thirteenth aspect of the present disclosure, According to a twelfth aspect, there is provided a positive electrode mixture, wherein the positive electrode active material contains a lithium transition metal composite oxide. <14> According to a fourteenth aspect of the present disclosure, According to a twelfth or thirteenth aspect, there is provided a positive electrode mixture containing the non-aqueous solvent, wherein the non-aqueous solvent contains N-methyl-2-pyrrolidone. <15> According to a fifteenth aspect of the present disclosure, According to any one of the twelfth to fourteenth aspects, there is provided a positive electrode including a positive electrode mixture layer formed from the positive electrode mixture. <16> According to a sixteenth aspect of the present disclosure, According to a fifteenth aspect, there is provided a secondary battery comprising a positive electrode. [Example]

[0106] Next, embodiments of the present disclosure will be described with reference to examples, but the present disclosure is not limited to these examples.

[0107] The values ​​in the examples were measured by the following methods.

[0108] <Fluoropolymer composition> The composition of the fluoropolymer was determined by solution NMR. Measurement equipment: Varian VNMRS400 Resonance frequency: 376.04 (Sfrq) Pulse width: 30° (pw=6.8)

[0109] <Fluoropolymer composition> The content of acrylic acid units in fluoropolymer (a1-4) and fluoropolymer (a2-3) was measured by acid-base titration of the carboxylic acid group. Specifically, approximately 0.5 g of fluoropolymer was dissolved in acetone at a temperature of 70 to 80°C. 5 ml of water was added dropwise under vigorous stirring to avoid coagulation of the fluoropolymer. At a neutral transition of approximately -270 mV, titration with aqueous NaOH having a concentration of 0.1 N was carried out until complete neutralization of the acidity. Based on the measured acid equivalent, the amount of acrylic acid contained in 1 g of fluoropolymer was determined from the measurement results, and the content of acrylic acid units was calculated.

[0110] <Weight average molecular weight> Measurement was performed by gel permeation chromatography (GPC). Calculations were made using data measured using Tosoh AS-8010, CO-8020, and columns (three GMHHR-H columns connected in series) and Shimadzu RID-10A, with dimethylformamide (DMF) as the solvent at a flow rate of 1.0 ml / min (reference: polystyrene).

[0111] <Surface roughness> Using a shape analysis laser microscope (VK-X1100, Keyence Corporation), the arithmetic mean height (surface roughness) was obtained before and after pressing, and after 96 hours at 25°C.

[0112] <Positive evaluation> (Preparation of Electrolyte) Ethylene carbonate, a high-dielectric constant solvent, and ethyl methyl carbonate, a low-viscosity solvent, were mixed in a volume ratio of 30:70, and LiPF6 was added to this to a concentration of 1.0 mol / L. 2% by mass of vinylene carbonate was then added to this to obtain a nonaqueous electrolyte solution.

[0113] (Fabrication of lithium-ion secondary batteries) The positive electrodes having a positive electrode mixture layer on one side prepared in the examples and comparative examples were cut into a shape having a coated portion (positive electrode mixture layer) 50 mm wide and 30 mm long and an uncoated portion 5 mm wide and 9 mm long.

[0114] To 98 parts by mass of artificial graphite, 1 part by mass of an aqueous dispersion of sodium carboxymethylcellulose (concentration of sodium carboxymethylcellulose: 1% by mass) and 1 part by mass of an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber: 50% by mass) were added as thickeners and binders, and the mixture was mixed in a disperser to form a slurry. The resulting slurry was applied to a 20 μm thick copper foil, dried, and rolled in a press. The foil was then cut into a shape with a coated portion (negative electrode mixture layer) 52 mm wide and 32 mm long, and an uncoated portion 5 mm wide and 9 mm long to form a negative electrode.

[0115] (Fabrication of aluminum laminated cells) The positive electrode and the negative electrode were placed opposite each other with a 20 μm-thick microporous polyethylene film (separator) interposed therebetween, and the nonaqueous electrolyte solution obtained above was poured into the battery. After the nonaqueous electrolyte solution had sufficiently permeated the separator and the like, the battery was sealed, pre-charged, and aged to prepare a lithium ion secondary battery.

[0116] (Cycle capacity retention rate) At 20°C, the secondary battery was charged to 4.2 V at a constant current equivalent to 0.2 C, and then discharged to 3.0 V at a constant current of 0.2 C. This cycle was repeated twice to stabilize the battery. The secondary battery was then charged to 4.2 V at a constant current of 1 C at 20°C, then charged at a constant voltage of 4.2 V until the current reached 0.05 C, and then discharged to 3.0 V at a constant current of 1 C to determine the initial discharge capacity. Charge and discharge were repeated in the same manner, and the discharge capacity after 500 cycles was measured. The ratio of the discharge capacity after 500 cycles to the initial discharge capacity was calculated using the following formula, and this was defined as the cycle capacity retention (%). The results are shown in Table 1. Cycle capacity retention rate (%) = (discharge capacity after 500 cycles) / (initial discharge capacity) × 100

[0117] The following polymers were used in the examples and comparative examples.

[0118] Fluoropolymer (a1) a1-1: PVdF containing HFP units VdF / HFP=95.0 / 5.0 (mol %) Weight average molecular weight 700000 a1-2: PVdF containing HFP units and CTFE units VdF / HFP / CTFE=96.5 / 2.5 / 1.0 (mol %) Weight average molecular weight 1110000 a1-3: PVdF containing HFP units VdF / HFP=90.0 / 10.0 (mol %) Weight average molecular weight 900000 a1-4: PVdF containing HFP units and acrylic acid units VdF / HFP=97.5 / 2.5 (mol %) Acrylic acid unit content: 1.0 mol% Weight average molecular weight 1110000

[0119] Fluoropolymer (a2) a2-1: VdF homopolymer Weight average molecular weight 900000 a2-2: VdF homopolymer Weight average molecular weight 1800000 a2-3: PVdF containing acrylic acid units Acrylic acid unit content: 1.0 mol% Weight average molecular weight 1100000

[0120] Fluoropolymer (a3) a3-1: Fluorine-containing copolymer containing VdF units and TFE units VdF / TFE=80.0 / 20.0 (mol %) Weight average molecular weight 1000000 a3-2: Fluorine-containing copolymer containing VdF units and TFE units VdF / TFE=85.0 / 15.0 (mol %) Weight average molecular weight 1200000 a3-3: Fluorine-containing copolymer containing VdF units and TFE units VdF / TFE=63.0 / 37.0 (mol %) Weight average molecular weight 1130000 a3-4: Fluorine-containing copolymer containing VdF units and CTFE units VdF / CTFE=97.5 / 2.5 (mol%) Weight average molecular weight 800000

[0121] Example 1 Fluoropolymer (a1-1), fluoropolymer (a2-1), and fluoropolymer (a3-1) were each dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluoropolymer solution with a concentration of 8% by mass. 0.0360 g of the fluoropolymer (a1-1) solution, 1.1640 g of the fluoropolymer (a2-1) solution, 0.3000 g of the fluoropolymer (a3-1) solution, and 1.5 g of a conductive agent (Ketjen Black (SuperP Li, manufactured by TIMCAL)) were mixed using a stirrer to obtain a mixed solution. A positive electrode active material (LiNi 0.8 Mn 0.1 Co 0.1 97 g of O2 (NMC811) was added, and NMP was further added so that the solid content concentration became 71 mass %, and the mixture was mixed using a stirrer to obtain a positive electrode mixture.

[0122] A coating amount of 22.5 mg / cm was applied to one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm). 2 The positive electrode mixture was uniformly applied so that the NMP was completely evaporated. The surface roughness of the positive electrode mixture layer of the obtained positive electrode was measured using the method described above. Furthermore, a pressure of 10 ton was applied to the obtained positive electrode using a roll press to obtain a positive electrode with a smoothed surface. The arithmetic mean height (surface roughness) was obtained before pressing and after 96 hours at 25°C after pressing.

[0123] Furthermore, a lithium ion secondary battery was fabricated using the obtained positive electrode by the method described above, and the cycle capacity retention rate was evaluated. The results are shown in Table 1.

[0124] Examples 2 to 7, Comparative Examples 1 to 3 A positive electrode mixture, a positive electrode, and a lithium ion secondary battery were obtained in the same manner as in Example 1, except that the types and amounts of the fluoropolymers (a1) to (a3), the positive electrode active material, and the conductive agent were changed to those shown in Table 1. The obtained positive electrode and lithium ion secondary battery were evaluated by the methods described above. The results are shown in Table 1.

[0125] [Table 1]

[0126] In Table 1, "SWCNT" stands for single-walled carbon nanotubes. The following single-walled carbon nanotubes were used in the examples and comparative examples. Single-walled carbon nanotubes (product name "TUBALL BATT SWCNT", manufactured by OCSiAl) Average outer diameter: 1.6±0.4nm Length: 5 μm or more Average G / D ratio: 86.5±7.1

Claims

1. A binder for a positive electrode mixture containing a fluoropolymer (A), The fluoropolymer (A) contains a fluoropolymer (a1) containing vinylidene fluoride units and hexafluoropropylene units, The content of hexafluoropropylene units is 0.001 to 1.000 mol % when the total amount of all monomer units in all fluoropolymers contained in the binder for the positive electrode mixture as the fluoropolymer (A) is taken as 100 mol %. Binder for positive electrode mixture.

2. The fluoropolymer (A) further contains at least one selected from the group consisting of fluoropolymer (a2) and fluoropolymer (a3), the fluoropolymer (a2) contains vinylidene fluoride units and does not contain hexafluoropropylene units, and the content of vinylidene fluoride units in the fluoropolymer (a2) is 98.0 mol% or more based on the total monomer units of the fluoropolymer (a2); 2. The binder for a positive electrode mixture according to claim 1, wherein the fluoropolymer (a3) ​​contains vinylidene fluoride units and fluoromonomer units (excluding vinylidene fluoride units and hexafluoropropylene units), and the content of the vinylidene fluoride units in the fluoropolymer (a3) ​​is less than 98.0 mol% based on all monomer units of the fluoropolymer (a3).

3. The binder for a positive electrode mixture according to claim 2, wherein the content ratio of the fluoropolymer (a1) to the fluoropolymer (a2) is a mass ratio ((a1) / (a2)) of 100 / 0 to 0.1 / 99.

9.

4. The fluoropolymer (A) contains a fluoropolymer (a1), a fluoropolymer (a2) and a fluoropolymer (a3), the content of the fluoropolymer (a1) is 0.01 to 99.00% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0 to 98.99% by mass relative to the mass of the fluoropolymer (A); 4. The binder for a positive electrode mixture according to claim 2, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass% relative to the mass of the fluoropolymer (A).

5. The fluoropolymer (A) contains a fluoropolymer (a1), a fluoropolymer (a2) and a fluoropolymer (a3), the content of the fluoropolymer (a1) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); the content of the fluoropolymer (a2) is 0.01 to 98.99% by mass relative to the mass of the fluoropolymer (A); 4. The binder for a positive electrode mixture according to claim 2, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass% relative to the mass of the fluoropolymer (A).

6. The fluoropolymer (A) contains a fluoropolymer (a1) and a fluoropolymer (a3), but does not contain a fluoropolymer (a2), The content of the fluoropolymer (a1) is 60.00 to 99.00% by mass relative to the mass of the fluoropolymer (A), The content of the fluoropolymer (a2) is 0% by mass relative to the mass of the fluoropolymer (A), 4. The binder for a positive electrode mixture according to claim 2, wherein the content of the fluoropolymer (a3) ​​is 1.00 to 40.00 mass% relative to the mass of the fluoropolymer (A).

7. The fluoropolymer (a2) is represented by the general formula (1): 【Chemistry 4】 (In the formula, R １ ~R ３ R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. ４ represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. １ The binder for a positive electrode mixture according to claim 2 or 3, further comprising a monomer unit based on a monomer (1) represented by the formula:

8. 4. The binder for a positive electrode mixture according to claim 2, wherein the content of vinylidene fluoride units in the fluoropolymer (a3) ​​is 57.0 to 89.0 mol% based on all monomer units of the fluoropolymer (a3).

9. 4. The binder for a positive electrode mixture according to claim 2, wherein the fluoromonomer unit in the fluoropolymer (a3) ​​is a tetrafluoroethylene unit.

10. The vinylidene fluoride units in the fluoropolymer (a1) are 84.0 to 99.999 mol % based on the total monomer units of the fluoropolymer (a1), 3. The binder for a positive electrode mixture according to claim 1, wherein the content of hexafluoropropylene units in the fluoropolymer (a1) is 0.001 to 16.0 mol% based on the total monomer units of the fluoropolymer (a1).

11. The fluoropolymer (a1) is represented by the general formula (1): 【Chemistry 5】 (In the formula, R １ ~R ３ R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. ４ represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. １ The binder for a positive electrode mixture according to claim 1 or 2, further comprising a monomer unit based on a monomer (1) represented by the formula:

12. A positive electrode mixture comprising the binder for a positive electrode mixture according to claim 1 or 2, a positive electrode active material, and water or a non-aqueous solvent.

13. The positive electrode mixture according to claim 12, wherein the positive electrode active material contains a lithium transition metal composite oxide.

14. The positive electrode mixture according to claim 12, wherein the positive electrode mixture contains a non-aqueous solvent, and the non-aqueous solvent contains N-methyl-2-pyrrolidone.

15. A positive electrode comprising a positive electrode mixture layer formed from the positive electrode mixture according to claim 12.

16. A secondary battery comprising the positive electrode according to claim 15.

Images