Positive electrode mixture, positive electrode, and secondary battery

Abstract

To provide a positive electrode mixture whose viscosity hardly increases.SOLUTION: The positive electrode mixture comprises a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E). The fluorine-containing polymer (A) includes a vinylidene fluoride unit and a monomer unit based on monomer (1): CR1R2=CR3-R4CO2Y1. The fluorine-containing copolymer (B) includes a vinylidene fluoride unit and a perfluoro monomer unit. The positive electrode active material (C) is represented by general formula (C): LiyNi1-xMxO2. The content of the water (E) is 200-10000 mass ppm with respect to the mass of the non-aqueous solvent (D).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to 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 battery including a current collector and a positive electrode mixture layer provided on one or both surfaces of the current collector, the thickness of the positive electrode mixture layer being 69 μm or more, and the density of the positive electrode mixture layer being 3.0 to 5.0 g / cm 3The positive electrode mixture layer contains a positive electrode active material and a binder, the positive electrode active material contains a lithium-nickel composite oxide, the binder contains a fluorine-containing copolymer, and the fluorine-containing copolymer contains a vinylidene fluoride unit and a fluorinated monomer unit (excluding the vinylidene fluoride unit). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2010 / 092976 [Patent Document 2] International Publication No. 2020 / 071336 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to provide a positive electrode mixture that is less likely to increase in viscosity. [Means for solving the problem]

[0006] According to the present disclosure, there is provided a positive electrode mixture containing a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E), The fluorine-containing polymer (A) contains vinylidene fluoride units and a fluorine-containing polymer 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; the fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units, The positive electrode active material (C) is represented by the general formula (C): Li y Ni 1-x M x O2 (wherein x is 0.01≦x≦0.7, y is 0.9≦y≦2.0, and M represents a metal atom (excluding Li and Ni), The content of water (E) is 200 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D). A positive electrode mixture is provided. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide a positive electrode mixture whose viscosity is less likely to increase. 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 Document 1 describes that lithium-containing composite oxides containing Ni and Mn are essentially basic, and although the reason for this is not identified, gelation occurs in positive electrode mixture slurries containing polyvinylidene fluoride (PVdF) or vinylidene fluoride (VdF) copolymers, resulting in a problem of impaired slurry stability. Patent Document 1 also describes that the use of a VdF / TFE copolymer in which a specific amount of tetrafluoroethylene (TFE) is copolymerized with VdF makes the positive electrode mixture slurry homogeneous and stable. Patent Document 1 further describes that the stability of the slurry is improved by reducing the water content of the organic solvent used to prepare the positive electrode mixture slurry.

[0010] However, it has become clear that when a vinylidene fluoride polymer having a specific functional group and a fluorine-containing copolymer containing vinylidene fluoride units and perfluoromonomer units are used as a binder for a positive electrode mixture, the viscosity of the positive electrode mixture may increase even when conventional techniques are used. Therefore, a new technique is needed that can suppress the increase in viscosity of the positive electrode mixture to an even greater extent, even when a specific binder is used.

[0011] Therefore, the inventors have intensively investigated means for solving the above problems, and have unexpectedly found that the increase in viscosity of the positive electrode mixture can be suppressed to an even higher level by adding water in an extremely limited range of content in the positive electrode mixture.

[0012] That is, according to the present disclosure, there is provided a positive electrode mixture containing a fluorinated polymer (A), a fluorinated copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E), wherein the content of water (E) is 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D).

[0013] The positive electrode mixture of the present disclosure contains water (E) in a content of 200 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D). Therefore, even when the positive electrode mixture is prepared and left to stand for a while, the viscosity of the positive electrode mixture is unlikely to increase. Therefore, by applying the positive electrode mixture of the present disclosure to a current collector, a uniform and smooth positive electrode mixture layer can be easily formed on the current collector.

[0014] Next, each component contained in the positive electrode mixture of the present disclosure will be described in detail.

[0015] <Fluorine-containing polymer (A)> The positive electrode mixture of the present disclosure comprises a vinylidene fluoride (VdF) unit and a compound represented by the general formula (1): [ka] (In the formula, R 1 ~R 3R 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. The positive electrode mixture contains a fluorine-containing polymer (A) containing a monomer unit based on a monomer (1) represented by the formula: When the positive electrode mixture contains the fluorine-containing polymer (A), excellent adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0016] In general formula (1), Y 1 represents 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.

[0017] 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.

[0018] In general formula (1), R 4represents 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.

[0019] 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 (meth)acrylic acid and salts thereof, 3-butenoic acid and salts thereof, and 4-pentenoic acid and salts thereof.

[0020] The fluorine-containing polymer (A) may contain a monomer unit derived from a monomer copolymerizable with VdF (excluding monomer units derived from monomer (1)). Examples of the monomer copolymerizable with VdF include fluorinated monomers and non-fluorinated monomers (excluding monomer (1)), with fluorinated monomers being preferred. Examples of the fluorinated monomers include vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ethers, hexafluoropropylene (HFP), (perfluoroalkyl)ethylenes, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, and trans-1,3,3,3-tetrafluoropropene. Examples of the non-fluorinated monomers include ethylene, propylene, and acryloyloxyethyl succinate.

[0021] In the fluoropolymer (A), the monomer copolymerizable with VdF is preferably at least one fluorinated monomer selected from the group consisting of CTFE, fluoroalkyl vinyl ether and HFP.

[0022] The content of VdF units in the fluorine-containing polymer (A) 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 fact that even better adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0023] The content of monomer units based on the monomer (1) of the fluorine-containing polymer (A) 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 which better adhesion can be obtained between the positive electrode mixture layer and the current collector.

[0024] The content of monomer units based on monomers copolymerizable with VdF in the fluoropolymer (A) (excluding monomer units based on monomer (1)) is preferably 0 to 5.0 mol %, more preferably 3.0 mol % or less, and even more preferably less than 1 mol %, based on all monomer units. The content of monomer units based on monomers copolymerizable with VdF in the fluoropolymer (A) may be 0 mol %. That is, in one embodiment, the fluoropolymer (A) does not contain monomer units based on monomers copolymerizable with VdF.

[0025] In the present disclosure, the compositions of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) may be, for example, 19 In the present disclosure, the contents of the monomer units based on the monomer (1) and the monomer units having a polar group in the fluoropolymer (A) and the fluorocopolymer (B) can be measured by acid-base titration of the acid groups.

[0026] The weight average molecular weight (polystyrene equivalent) of the fluoropolymer (A) 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.

[0027] The number average molecular weight (polystyrene equivalent) of the fluoropolymer (A) 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, 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.

[0028] The melting point of the fluoropolymer (A) is preferably from 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 raised at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0029] The storage modulus of the fluoropolymer (A) at 30°C is preferably at most 2000 MPa, more preferably at most 1800 MPa. The storage modulus of the fluoropolymer (A) at 60°C is preferably 1500 MPa or less, more preferably 1300 MPa or less. The storage modulus of the fluoropolymer (A) at 30°C is preferably at least 1000 MPa, more preferably at least 1100 MPa. The storage modulus of the fluoropolymer (A) at 60°C is preferably at least 600 MPa, more preferably at least 700 MPa. When the storage modulus of the fluoropolymer (A) at 30°C or 60°C is within the above range, the flexibility of the fluoropolymer (A) is improved, and when used as a binder, an electrode that is less likely to crack can be easily formed.

[0030] <Fluorine-containing copolymer (B)> The positive electrode mixture of the present disclosure contains a fluorine-containing copolymer (B) containing vinylidene fluoride units and perfluoromonomer units. By containing the fluorine-containing copolymer (B) in the positive electrode mixture, a positive electrode having excellent flexibility can be formed.

[0031] The perfluoromonomer is preferably at least one selected from the group consisting of tetrafluoroethylene (TFE), perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), and hexafluoropropylene (HFP), and more preferably at least one selected from the group consisting of TFE and HFP. As the perfluoromonomer, TFE is particularly preferred, as it allows for the production of a positive electrode mixture whose viscosity is even less likely to increase.

[0032] The fluorine-containing copolymer (B) may contain other monomer units in addition to vinylidene fluoride and perfluoromonomers. Examples of other monomers include vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), (perfluoroalkyl)ethylenes, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, ethylene, propylene, and monomers having a polar group.

[0033] The monomer having a polar group may be the monomer (1) represented by the above-mentioned general formula (1). When the fluorine-containing copolymer (B) contains a monomer unit having a polar group, the adhesion between the positive electrode mixture layer and the current collector is improved. The fluorine-containing copolymer (B) may or may not contain a monomer unit based on the monomer (1).

[0034] Examples of the fluorine-containing copolymer (B) include VdF / TFE copolymer, VdF / HFP copolymer, VdF / TFE / HFP copolymer, VdF / TFE / (meth)acrylic acid copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / TFE / 4-pentenoic acid copolymer, VdF / TFE / 3-butenoic 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 / 2-carboxyethyl acrylate copolymer, VdF / TFE / HFP / 2-carboxyethyl acrylate copolymer, VdF / TFE / acryloyloxyethyl succinic acid copolymer, and VdF / TFE / HFP / acryloyloxyethyl succinic acid copolymer.

[0035] The content of VdF units in the fluorine-containing copolymer (B) is preferably 57.0 mol% or more, more preferably 60.0 mol% or more, even more preferably 63.0 mol% or more, still more preferably 64.0 mol% or more, and is preferably 99.0 mol% or less, more preferably 95.0 mol% or less, still more preferably 90.0 mol% or less, and still more preferably 85.0 mol% or less, based on all monomer units, since a positive electrode having excellent flexibility can be formed.

[0036] The content of perfluoromonomer units in the fluorine-containing copolymer (B) is preferably 1.0 mol% or more, more preferably 5.0 mol% or more, even more preferably 8.0 mol% or more, still more preferably 10.0 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, particularly preferably 37.0 mol% or less, based on all monomer units, since a positive electrode having excellent flexibility can be formed.

[0037] The content of other monomer units other than vinylidene fluoride and perfluoromonomers in the fluorine-containing copolymer (B) is preferably 0 to 2.0 mol %, more preferably 1.5 mol % or less, based on all monomer units. The content of other monomer units in the fluorine-containing copolymer (B) may be 0 mol %. That is, in one embodiment, the fluorine-containing copolymer (B) does not contain other monomer units other than vinylidene fluoride and perfluoromonomers.

[0038] The weight average molecular weight (polystyrene equivalent) of the fluorine-containing copolymer (B) 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.

[0039] The number average molecular weight (polystyrene equivalent) of the fluorine-containing copolymer (B) 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.

[0040] The melting point of the fluorine-containing copolymer (B) 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.

[0041] The storage modulus of the fluorine-containing copolymer (B) at 30°C is preferably 1100 MPa or less, more preferably 800 MPa or less, and even more preferably 600 MPa or less. The storage modulus of the fluorocopolymer (B) at 60°C is preferably at most 500 MPa, more preferably at most 350 MPa. The storage modulus of the fluorine-containing copolymer (B) at 30°C is preferably 100 MPa or more, more preferably 150 MPa or more, and even more preferably 200 MPa or more. The storage modulus of the fluorine-containing copolymer (B) at 60°C is preferably 50 MPa or more, more preferably 80 MPa or more, and even more preferably 130 MPa or more. When the storage modulus of the fluorocopolymer (B) at 30°C or 60°C is within the above range, the flexibility of the fluorocopolymer (B) is further improved.

[0042] In the positive electrode mixture of the present disclosure, the mass ratio (A) / (B) of the fluorine-containing polymer (A) to the fluorine-containing copolymer (B) is preferably 99 / 1 to 1 / 99, more preferably 97 / 3 to 3 / 97, even more preferably 95 / 5 to 5 / 95, particularly preferably 90 / 10 to 10 / 90, and most preferably 85 / 15 to 15 / 85, because this allows for a positive electrode mixture that is less likely to increase in viscosity and for better adhesion between the positive electrode mixture layer and the current collector to be obtained. The mass ratio (A) / (B) may be 95 / 5 to 40 / 60, or 90 / 10 to 50 / 50.

[0043] In the positive electrode mixture of the present disclosure, the content of the fluorine-containing polymer (A) is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2.0% by mass or less, particularly preferably 1.6% by mass or less, and preferably 0.1% by mass or more, more preferably 0.5% by mass or more, based on the positive electrode mixture. By setting the content of the fluorine-containing polymer (A) within the above range, a positive electrode mixture whose viscosity is less likely to increase can be obtained, and better adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0044] In the positive electrode mixture of the present disclosure, the content of the fluorine-containing copolymer (B) is preferably 5% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, particularly preferably 0.5% by mass or less, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more, relative to the positive electrode mixture. By setting the content of the fluorine-containing copolymer (B) within the above range, a positive electrode mixture whose viscosity is less likely to increase can be obtained, and better adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0045] In the positive electrode mixture of the present disclosure, the total content of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and preferably 0.1% by mass or more, more preferably 0.5% by mass or more, based on the positive electrode mixture. By setting the total content of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) within the above range, a positive electrode mixture whose viscosity is even less likely to increase can be obtained, and even better adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0046] The positive electrode mixture of the present disclosure may contain other polymers in addition to the fluorine-containing polymer (A) and the fluorine-containing copolymer (B). Examples of other polymers include polyacrylic acid, polymethacrylate, polymethyl methacrylate, polyacrylonitrile, polyimide, polyamide, polyamideimide, polycarbonate, styrene rubber, butadiene rubber, and styrene-butadiene rubber.

[0047] <Cathode active material (C)> The positive electrode mixture of the present disclosure contains a positive electrode active material (C). The positive electrode active material (C) is represented by the general formula (C): Li y Ni 1-x M x O2 (wherein x is 0.01≦x≦0.7, y is 0.9≦y≦2.0, and M represents a metal atom (excluding Li and Ni)). When the positive electrode mixture contains a positive electrode active material rich in Ni, a high-capacity secondary battery can be obtained. It has now become clear that the viscosity of the positive electrode mixture is likely to increase when a positive electrode active material rich in Ni is used and a fluorine-containing polymer (A) and a fluorine-containing copolymer (B) are used as binders. However, the positive electrode mixture of the present disclosure is less likely to increase in viscosity because water is present in the positive electrode mixture within an extremely limited range.

[0048] In general formula (C), 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.

[0049] The positive electrode active material (C) may be a lithium transition metal composite oxide, such as 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 0.2Co 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 O2 is more preferred.

[0050] In addition to the positive electrode active material (C), a different positive electrode active material may be used in combination. Specific examples of the different positive electrode active material include LiNiO2, LiCoO 2、 LiMnO2, LiMn2O4, Li2MnO 3、 LiMn 1.8 Al 0.2 O 4、 Li4Ti5O 12、 LiFePO4, Li3Fe2(PO4)3, LiFeP2O 7、 LiCoPO4, Li 1.2 Fe 0.4 Mn 0.4 Examples include O2.

[0051] Furthermore, the positive electrode active material (C) may have a substance of a different composition from the substance constituting the main positive electrode active material (C) attached to its surface. 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.

[0052] These surface-attaching substances can be attached to the surface of the positive electrode active material (C) by, for example, a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the surface-attaching substance precursor to the positive electrode active material (C) by impregnation, and then drying the resulting solution; a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the surface-attaching substance precursor to the positive electrode active material (C) by impregnation, and then reacting the surface-attaching substance precursor by heating or the like; or a method of adding the surface-attaching substance precursor to the positive electrode active material and simultaneously baking the surface-attaching substance precursor.

[0053] The amount of the surface-attached substance is preferably 0.1 ppm or more, more preferably 1 ppm or more, even more preferably 10 ppm or more, and 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 (C). The surface-attached substance can suppress the oxidation reaction of the nonaqueous electrolyte on the surface of the positive electrode active material (C) and improve the battery life, but if the amount attached is too small, the effect will not be fully exerted, and if it is too large, the movement of lithium ions will be hindered, which may increase the resistance.

[0054] The shape of the positive electrode active material (C) particles can be any of the conventional shapes, such as block, polyhedron, sphere, ellipsoid, plate, needle, or columnar. 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 stress-induced expansion and contraction of the active material in the electrode, leading to degradation such as destruction of the active material and disconnection of the conductive path. 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 degradation. 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 charge and discharge, and are also more easily mixed uniformly with the conductive agent when preparing the electrode.

[0055] The tap density of the positive electrode active material (C) is usually 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 3That is all. If the tap density of the positive electrode active material (C) is below the above lower limit, the amount of dispersion medium required when forming the positive electrode active material (C) layer increases, and the amounts of the conductive agent, PVdF (A), and fluorine-containing copolymer (B) required also increase, which may restrict the filling rate of the positive electrode active material (C) in the positive electrode mixture layer and limit the battery capacity. By using a positive electrode active material (C) with a high tap density, a high-density positive electrode mixture layer can be formed. Generally, the higher the tap density, the better, and there is no particular upper limit. However, if the tap density 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.

[0056] The tap density of the positive electrode active material (C) is measured by passing it through a sieve with a mesh size of 300 μm and measuring 20 cm 3 After dropping the sample into the tapping cell to fill the cell volume, tapping is performed 1,000 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.

[0057] The median particle diameter d50 of the positive electrode active material (C) (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 too long, resulting in reduced battery performance. Furthermore, problems such as streaking may occur when preparing a battery positive electrode, i.e., when the positive electrode active material (C) is slurried with the conductive agent, PVdF (A), and fluorine-containing copolymer (B) in a solvent and applied as a thin film. Mixing two or more positive electrode active materials (C) with different median diameters d50 can further improve packing during positive electrode preparation.

[0058] 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.

[0059] When primary particles aggregate to form secondary particles, the average primary particle diameter of the positive electrode active material (C) 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 longest intercepts of the horizontal line at the left and right boundaries of the primary particles are determined for any 50 primary particles in a 10,000x magnification photograph, and the average value is calculated.

[0060] The BET specific surface area of ​​the positive electrode active material (C) is usually 0.2 m 2 / g or more, preferably 0.3m 2 / g or more, more preferably 0.4m 2 / g or more, usually 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 application of the positive electrode mixture.

[0061] The BET specific surface area is defined as the value measured by a surface area meter (for example, a fully automatic surface area measuring device manufactured by Okura Riken Co., Ltd.) 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.

[0062] The positive electrode active material (C) can be 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. For example, a method in which a transition metal raw material such as a transition metal nitrate or sulfate, and optionally raw materials of other elements, are dissolved or crushed and dispersed in a solvent such as water, and the pH is adjusted while stirring to produce and recover a spherical precursor, which is then dried as needed, and then a Li source such as LiOH, Li2CO3, or LiNO3 is added and calcined at a high temperature to obtain an active material; a method in which a transition metal raw material such as a transition metal nitrate, sulfate, hydroxide, or oxide, and optionally raw materials of other elements, are dissolved or crushed 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.

[0063] The positive electrode active material (C) may be used alone or in any combination and ratio of two or more different materials having different compositions or different powder properties.

[0064] In the positive electrode mixture of the present disclosure, the ratio of the total mass of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) to the mass of the positive electrode active material (C) 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, because a battery with an even higher capacity can be obtained. In one embodiment, the content of the positive electrode active material (C) in the positive electrode mixture is selected so that the total content of the positive electrode active material (C) together with the contents of other components such as the fluorine-containing polymer (A), the fluorine-containing copolymer (B), the non-aqueous solvent (D), water (E), and the conductive agent is 100 mass%.

[0065] <Non-aqueous solvent (D)> The positive electrode mixture of the present disclosure contains a nonaqueous solvent (D). Examples of the nonaqueous solvent (D) include nitrogen-containing organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and dimethylformamide; ketone-based solvents such as acetone, methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone; ester-based solvents such as ethyl acetate and butyl acetate; ether-based solvents such as tetrahydrofuran and dioxane; and mixed solvents thereof, as well as other low-boiling general-purpose organic solvents.

[0066] As the nonaqueous solvent (D), at least one selected from the group consisting of N-methyl-2-pyrrolidone and N,N-dimethylacetamide is preferred, and N-methyl-2-pyrrolidone is more preferred, as it provides a positive electrode mixture with excellent dispersion stability and coatability.

[0067] In the positive electrode mixture of the present disclosure, the content of the nonaqueous solvent (D) 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 fluorine-containing polymer (A), the fluorine-containing copolymer (B) and the positive electrode active material (C) is preferably 50 to 90 mass%, more preferably 60 to 85 mass%, even more preferably 65 to 80 mass%, and still more preferably 65 to 75 mass%.

[0068] <Water(E)> The positive electrode mixture of the present disclosure contains water (E). The content of water (E) in the positive electrode mixture is 200 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D). By containing a small amount of water in this manner, a positive electrode mixture that is less likely to increase in viscosity can be obtained. Furthermore, by using a positive electrode mixture containing a small amount of water, the adhesion of the positive electrode mixture layer to the current collector and the flexibility of the positive electrode mixture layer can be improved, and the resistance (coating resistance) of the positive electrode mixture layer can be reduced.

[0069] The content of water (E) in the positive electrode mixture is 200 to 10,000 ppm by mass, preferably 1,000 ppm by mass or more, more preferably 2,000 ppm by mass or more, even more preferably 4,000 ppm by mass or more, and still more preferably 6,000 ppm by mass or more, relative to the mass of the nonaqueous solvent (D). When the content of water (E) is equal to or greater than the above lower limit, an increase in the viscosity of the positive electrode mixture can be sufficiently suppressed. When the content of water (E) is equal to or less than the above upper limit, the drying time after application of the positive electrode mixture can be shortened, and positive electrode productivity can be improved.

[0070] The method for adjusting the content of water (E) in the positive electrode mixture is not particularly limited, but for example, it can be adjusted by adding water so that the content becomes as described above when preparing the positive electrode mixture.

[0071] When water is added when preparing the positive electrode mixture, the amount of water added may be 1 mg or more and 5000 mg or less, preferably 10 mg or more, more preferably 50 mg or more, and preferably 1000 mg or less, more preferably 500 mg or less, per 100 g of the positive electrode active material.

[0072] <Other ingredients> The positive electrode mixture may further contain a conductive agent, for example, carbon blacks such as acetylene black and ketjen black, carbon materials such as graphite, carbon fiber, multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon nanohorns, graphene, etc.

[0073] In the positive electrode mixture of the present disclosure, the content ratio of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) to the conductive agent is 5 / 95 to 90 / 10 in mass ratio.

[0074] 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.

[0075] Examples of methods for preparing the positive electrode mixture of the present disclosure include a method in which a positive electrode active material (C) and, if desired, a conductive agent are dispersed and mixed in a solution obtained by dissolving a fluorine-containing polymer (A) and a fluorine-containing copolymer (B) in a non-aqueous solvent (D). Alternatively, the positive electrode mixture may be prepared by a method in which the fluorine-containing polymer (A), the fluorine-containing copolymer (B) and the positive electrode active material (C) are first mixed together, and then the non-aqueous solvent (D) and, if desired, the 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 obtained by dissolving a fluorine-containing polymer (A) and a fluorine-containing copolymer (B) in a non-aqueous solvent (D), and then mixed, and then the positive electrode active material (C) is further added and mixed.

[0076] Water (E) may be added when preparing the positive electrode mixture. The timing of adding water is not particularly limited. For example, water (E) may be added to a solution obtained by dissolving the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) in the non-aqueous solvent (D), or water (E) may be added to a mixture containing the fluorine-containing polymer (A), the fluorine-containing copolymer (B), the positive electrode active material (C), and the non-aqueous solvent (D).

[0077] 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, still more preferably 7000 mPa·s or more, 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 the ease of obtaining a positive electrode mixture layer with a desired thickness. The viscosity can be measured at 25°C using a Brookfield viscometer. The positive electrode mixture of the present disclosure is resistant to viscosity increase, and therefore can maintain a viscosity that allows for easy application and easy adjustment of the thickness of the positive electrode mixture layer for a long time.

[0078] <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.

[0079] 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.

[0080] <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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] <1> According to a first aspect of the present disclosure, A positive electrode mixture containing a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E), The fluorine-containing polymer (A) contains vinylidene fluoride units and a fluorine-containing polymer 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; the fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units, The positive electrode active material (C) is represented by the general formula (C): Liy Ni 1-x M x O2 (wherein x is 0.01≦x≦0.7, y is 0.9≦y≦2.0, and M represents a metal atom (excluding Li and Ni), The content of water (E) is 200 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D). A positive electrode mixture is provided. <2> According to a second aspect of the present disclosure, According to a first aspect, there is provided a positive electrode mixture, wherein the content of vinylidene fluoride units in the fluoropolymer (A) is 92.0 to 99.999 mol % based on all monomer units. <3> According to a third aspect of the present disclosure, According to the first or second aspect, there is provided a positive electrode mixture, wherein the content of monomer units based on the monomer (1) in the fluoropolymer (A) is 0.001 to 8.0 mol % based on all monomer units. <4> According to a fourth aspect of the present disclosure, There is provided a positive electrode mixture according to any one of the first to third aspects, wherein the content of vinylidene fluoride units in the fluorine-containing copolymer (B) is 60.0 to 99.0 mol % based on all monomer units. <5> According to a fifth aspect of the present disclosure, There is provided a positive electrode mixture according to any one of the first to fourth aspects, wherein the content of perfluoromonomer units in the fluorine-containing copolymer (B) is 1.0 to 40.0 mol % based on all monomer units. <6> According to a sixth aspect of the present disclosure, According to any one of the first to fifth aspects, there is provided a positive electrode mixture, wherein the non-aqueous solvent (D) contains N-methyl-2-pyrrolidone. <7> According to a seventh aspect of the present disclosure, There is provided a positive electrode mixture according to any one of the first to sixth aspects, wherein the content of water (E) is 1000 to 10000 ppm by mass relative to the mass of the non-aqueous solvent (D). <8> According to an eighth aspect of the present disclosure, the fluorine-containing polymer (A) contains vinylidene fluoride units and monomer units based on at least one monomer (1) 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; The fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units based on at least one perfluoromonomer selected from the group consisting of tetrafluoroethylene and hexafluoropropylene. According to any one of the first to seventh aspects, there is provided a positive electrode mixture. <9> According to a ninth aspect of the present disclosure, the mass ratio (A) / (B) of the fluorine-containing polymer (A) to the fluorine-containing copolymer (B) is 95 / 5 to 15 / 85, the ratio of the total mass of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) to the mass of the positive electrode active material (C) is 1 / 99 to 3 / 97; The total content of the fluorine-containing polymer (A), the fluorine-containing copolymer (B) and the positive electrode active material (C) is 50 to 90 mass %. According to any one of the first to eighth aspects, there is provided a positive electrode mixture. <10> According to a tenth aspect of the present disclosure, According to any one of the first to ninth aspects, there is provided a positive electrode comprising a positive electrode mixture layer formed from the positive electrode mixture. <11> According to an eleventh aspect of the present disclosure, According to a tenth aspect, there is provided a secondary battery comprising a positive electrode. [Example]

[0085] Next, embodiments of the present disclosure will be described with reference to examples, but the present disclosure is not limited to these examples.

[0086] The values ​​in the examples were measured by the following methods.

[0087] <Composition of Fluorine-Containing Polymer (A)> Content of polar group-containing monomer units The content of polar group-containing monomer units (acrylic acid units) in the fluoropolymer (A) was measured by acid-base titration of the carboxylic acid group. Specifically, approximately 0.5 g of the fluoropolymer (A) was dissolved in acetone at a temperature of 70 to 80°C. 5 ml of water was added dropwise under vigorous stirring to avoid solidification of the fluoropolymer (A). Titration with a 0.1 N aqueous NaOH solution was carried out until complete neutralization of the acidity at a neutral transition of approximately -270 mV. From the measurement results, the amount of the polar group-containing monomer units contained in 1 g of the fluoropolymer (A) was determined, and the content of the polar group-containing monomer units was calculated.

[0088] <Composition of Fluorine-Containing Copolymer (B)> Ratio of VdF units to TFE units The ratio of VdF units to TFE units in the fluorine-containing copolymer (B) was measured using an NMR analyzer (Agilent Technologies, VNS400MHz). 19 F-NMR measurements were performed on the polymer in DMF-d7 solution.

[0089] 19 In F-NMR measurement, the areas of the following peaks (A, B, C, D) were determined, and the ratio of VdF units to TFE units was calculated. A: Area of ​​the peak between -86 ppm and -98 ppm B: Area of ​​the peak between -105 ppm and -118 ppm C: Area of ​​the peak between -119 ppm and -122 ppm D: Area of ​​the peak between -122 ppm and -126 ppm Ratio of VdF units: (4A + 2B) / (4A + 3B + 2C + 2D) x 100 [mol %] Ratio of TFE units: (B + 2C + 2D) / (4A + 3B + 2C + 2D) × 100 [mol %]

[0090] Ratio of VdF units to HFP units The ratio of VdF units to HFP units in the fluorine-containing copolymer (B) was measured using an NMR analyzer (Agilent Technologies, VNS400MHz). 19 F-NMR measurements were performed on the polymer in DMF-d7 solution.

[0091] Content of polar group-containing monomer units The content of polar group-containing monomer units (4-pentenoic acid units) in the fluorocopolymer (B) was measured by acid-base titration of the carboxylic acid group. Specifically, approximately 0.5 g of the fluorocopolymer (B) was dissolved in acetone at 70 to 80°C. 5 ml of water was added dropwise under vigorous stirring to avoid solidification of the fluorocopolymer (B). At a neutral transition of approximately -270 mV, titration with a 0.1 N NaOH aqueous solution was carried out until complete neutralization of the acidity. From the measurement results, the amount of the polar group-containing monomer units contained in 1 g of the fluorocopolymer (B) was determined, and the content of the polar group-containing monomer units was calculated.

[0092] <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).

[0093] <Storage modulus (E')> The storage modulus is a value measured at 30°C or 60°C using dynamic viscoelasticity measurement. Test pieces 30 mm long, 5 mm wide, and 50 to 100 μm thick were measured using a dynamic viscoelasticity measurement device DVA220 manufactured by IT Measurement & Control Co., Ltd. under the following conditions: tensile mode, grip width 20 mm, measurement temperature -30°C to 160°C, heating rate 2°C / min, and frequency 1 Hz.

[0094] The test pieces used for the measurements were prepared by dissolving PVdF and the fluorinated copolymer in N-methyl-2-pyrrolidone (NMP) to a concentration of 10 to 20% by mass to obtain a polymer solution, casting it onto a glass plate, drying it at 100°C for 12 hours, and further drying it under vacuum at 100°C for 12 hours, and cutting the resulting film, 50 to 100 μm thick, into a length of 30 mm and a width of 5 mm.

[0095] <Melting point> Using a differential scanning calorimetry (DSC) device, the temperature was raised from 30°C to 220°C at a rate of 10°C / min, then lowered to 30°C at 10°C / min, and then raised again to 220°C at a rate of 10°C / min. The temperature at the maximum value on the heat of fusion curve was determined as the melting point.

[0096] <Measurement of moisture content> Moisture measurements were performed in an environment with a dew point of -50°C using a Karl Fischer moisture meter (Kyoto Electronics Manufacturing Co., Ltd.).

[0097] <Viscosity of positive electrode mixture> Measurements were taken using a Brookfield viscometer (TV-10M, manufactured by Toki Sangyo Co., Ltd.) at 25°C, rotor No. M4, and a rotation speed of 6 rpm, and the viscosity was recorded as the value measured 10 minutes after the start of measurement.

[0098] <Viscosity change rate> The viscosity (η0) at the time of preparation of the mixture and the viscosity (ηn) after n hours had elapsed since the preparation of the mixture were measured, and the viscosity change rate (Xn) was calculated using the following formula. Xn=ηn / η0×100[%] In Table 1, "x" means that the viscosity of the positive electrode mixture was too high to measure.

[0099] <Adhesion between positive electrode mixture layer and current collector (peel strength)> A 1.2 cm x 7.0 cm test piece was prepared by cutting the single-sided coated positive electrode structure obtained by pressing using a roll press. The positive electrode mixture layer side of the test piece was fixed to a movable jig with double-sided tape, and the tape was then applied to the surface of the positive electrode current collector. The stress (N / cm) when the tape was pulled 90 degrees at a speed of 100 mm / min was measured using an autograph. A 1 N load cell was used for the autograph.

[0100] <Coating film resistance> A test piece of 5 cm x 5 cm was prepared by cutting out a film for measuring the coating resistance, and the coating resistance was evaluated by the four-terminal method.

[0101] <Positive electrode flexibility> The double-sided coated positive electrode structure obtained by pressing using a roll press was cut out to prepare 2 cm × 10 cm test pieces, which were then wrapped around round rods with diameters of 3 mm, 2 mm, 1.5 mm, and 1.0 mm, and the flexibility of the positive electrode was visually confirmed and evaluated according to the following criteria. ◯: No cracks were observed. Δ: Cracks were observed, but no breakage of the positive electrode mixture layer or current collector was observed. ×: The positive electrode mixture layer and the current collector were broken.

[0102] In the examples and comparative examples, polymers having the following physical properties were used.

[0103] <Fluoropolymer (A))> AI: PVdF containing acrylic acid units Acrylic acid unit content: 1.0 mol% Weight average molecular weight 1100000 Storage modulus at 30°C: 1280 MPa Storage modulus at 60℃: 720MPa Melting point: 161°C

[0104] <Fluorine-containing copolymer (B)> BI: Fluorine-containing copolymer containing VdF units and TFE units VdF / TFE=83 / 17 (mol %) Weight average molecular weight 1230000 Storage modulus at 30℃: 490MPa Storage modulus at 60℃: 260MPa Melting point: 131°C B-II: Fluorine-containing copolymer containing VdF units and TFE units VdF / TFE=63 / 37 (mol %) Weight average molecular weight 1130000 Storage modulus at 30°C: 440MPa Storage modulus at 60℃: 180MPa Melting point: 160°C B-III: Fluorine-containing copolymer containing VdF units, TFE units and 4-pentenoic acid units VdF / TFE=82 / 18 (mol %) 4-pentenoic acid content 0.5 mol% Weight average molecular weight 820000 Storage modulus at 30°C: 363 MPa Storage modulus at 60℃: 165MPa Melting point: 123°C B-IV: Fluorine-containing copolymer containing VdF units and HFP units VdF / HFP=95 / 5 (mol %) Weight average molecular weight 700000 Storage modulus at 30°C: 310MPa Storage modulus at 60℃: 145MPa Melting point: 135°C

[0105] In the examples and comparative examples, the following positive electrode active materials and conductive agents were used. Ni90:LiNi 0.9 Mn 0.05 Co 0.05 O2 AB: Acetylene black

[0106] Example 1 (Preparation of Positive Electrode Mixture) A fluorine-containing polymer (AI) serving as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluorine-containing polymer (AI) solution with a concentration of 8% by mass. Similarly, a fluorine-containing copolymer (BI) serving as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluorine-containing copolymer (BI) solution with a concentration of 8% by mass. 20 g of the fluorine-containing polymer (AI) solution, 5 g of the fluorine-containing copolymer (BI) solution, and 2 g of acetylene black as a conductive agent were added and kneaded using a stirrer to obtain a conductive paste. 4 mg of water was added to the conductive paste and further kneaded. 96 g of Ni90 as a positive electrode active material was added to the obtained paste and mixed using a stirrer to obtain a mixed solution. NMP was further added to the obtained mixed solution and mixed to prepare a positive electrode mixture with a solids concentration of 67% by mass.

[0107] (Fabrication of positive electrode structure) The obtained positive electrode mixture was immediately applied uniformly to one side of a positive electrode current collector (aluminum foil with a thickness of 20 μm) after preparation, and after the NMP was completely evaporated, a pressure of 10 t was applied using a roll press to press the positive electrode, thereby producing a positive electrode structure. The resulting positive electrode mixture was immediately applied uniformly to both sides of a positive electrode current collector (aluminum foil with a thickness of 20 μm), and after completely volatilizing the NMP, the positive electrode mixture layer was pressed using a roll press until the density of the positive electrode mixture layer reached 3.6 g / cm. 3 Pressure was applied and the mixture was repeatedly pressed until the temperature reached a certain level, thereby producing a positive electrode structure.

[0108] (Preparation of film for measuring coating resistance) The resulting positive electrode mixture was immediately applied uniformly to one side of a polyethylene terephthalate film (thickness: 100 μm) after preparation, and the NMP was completely volatilized to prepare a film for measuring the coating resistance.

[0109] Examples 2 to 8 and Comparative Examples 1 to 4 A positive electrode mixture was prepared, a positive electrode structure and a film for measuring coating resistance were fabricated, and evaluated in the same manner as in Example 1, except that the type of binder, the amount of water added, and the like were changed as shown in Table 1. The results are shown in Table 1.

[0110]

Table 1

Claims

1. A positive electrode mixture containing a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E), The fluorine-containing polymer (A) contains vinylidene fluoride units and a fluorine-containing polymer having 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 represents an inorganic cation and / or an organic cation; the fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units, The positive electrode active material (C) is represented by the general formula (C): Li ｙ Ni １－ｘ M ｘ O ２ (wherein x is 0.01≦x≦0.7, y is 0.9≦y≦2.0, and M represents a metal atom (excluding Li and Ni)), The content of water (E) is 200 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D). Positive electrode mixture.

2. 2. The positive electrode mixture according to claim 1, wherein the content of vinylidene fluoride units in the fluoropolymer (A) is 92.0 to 99.999 mol % based on all monomer units.

3. 3. The positive electrode mixture according to claim 1, wherein the content of monomer units based on the monomer (1) in the fluoropolymer (A) is 0.001 to 8.0 mol % based on all monomer units.

4. 3. The positive electrode mixture according to claim 1, wherein the content of vinylidene fluoride units in the fluorine-containing copolymer (B) is 60.0 to 99.0 mol % based on all monomer units.

5. 3. The positive electrode mixture according to claim 1, wherein the content of perfluoromonomer units in the fluorine-containing copolymer (B) is 1.0 to 40.0 mol % based on all monomer units.

6. 3. The positive electrode mixture according to claim 1, wherein the non-aqueous solvent (D) contains N-methyl-2-pyrrolidone.

7. 3. The positive electrode mixture according to claim 1, wherein the content of water (E) is 1,000 to 10,000 ppm by mass relative to the mass of the nonaqueous solvent (D).

8. the fluorine-containing polymer (A) contains vinylidene fluoride units and monomer units based on at least one monomer (1) 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; The fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units based on at least one perfluoromonomer selected from the group consisting of tetrafluoroethylene and hexafluoropropylene. The positive electrode mixture according to claim 1 or 2.

9. the mass ratio (A) / (B) of the fluorine-containing polymer (A) to the fluorine-containing copolymer (B) is 95 / 5 to 15 / 85, the ratio of the total mass of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) to the mass of the positive electrode active material (C) is 1 / 99 to 3 / 97; The total content of the fluorine-containing polymer (A), the fluorine-containing copolymer (B) and the positive electrode active material (C) is 50 to 90 mass %. The positive electrode mixture according to claim 1 or 2.

10. A positive electrode comprising a positive electrode mixture layer formed from the positive electrode mixture according to claim 1 or 2.

11. A secondary battery comprising the positive electrode according to claim 10.

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