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 active material, a positive electrode, and a secondary battery.

Background Art

[0002] Patent Document 1 discloses a slurry for a positive electrode active material, which contains a positive electrode active material (A), a binder (B), and an organic solvent (C). The positive electrode active material (A) is represented by formula (A): Li , , l ,

[0003] , , 3 M 1 y M 2 1-y O2 (where 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 is a lithium-containing composite metal oxide. The binder (B) is represented by the compositional formula (B): (VDF) m (TFE) n (HFP) l (where 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. However, m + n + l = 1) and is a fluorine-containing polymer. A slurry for a positive electrode active material of a lithium secondary battery is described.

[0003] Patent Document 2 includes a current collector and a positive electrode active material layer provided on one or both sides of the current collector. The thickness of the positive electrode active material layer is 69 μm or more, and the density of the positive electrode active material layer is 3.0 to 5.0 g / cm 3The present invention describes a positive electrode structure in which the 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 vinylidene fluoride units and fluorinated monomer units (excluding vinylidene fluoride units). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2010 / 092976 [Patent Document 2] International Publication No. 2020 / 071336 [Overview of the project] [Problems that the invention aims to solve]

[0005] The purpose of this disclosure is to provide a positive electrode mixture that is less prone to viscosity increase. [Means for solving the problem]

[0006] According to this disclosure, a positive electrode mixture comprising a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), a non-aqueous solvent (D), and water (E), Fluorine-containing polymer (A) consists of vinylidene fluoride units and 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 Y represents a single bond or a hydrocarbon group with 1 to 8 carbon atoms. 1 represents an inorganic cation and / or an organic cation. It contains monomer units based on monomer (1) represented by ), The fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units, The positive electrode active material (C) is of the general formula (C): Li y Ni 1-x M x O2 The positive electrode active material is represented by the formula (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 water (E) content is 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D). A positive electrode mixture is provided. [Effects of the Invention]

[0007] According to this disclosure, it is possible to provide a cathode mixture that is less prone to viscosity increase. [Modes for carrying out the invention]

[0008] The following describes specific embodiments of this disclosure in detail, but this disclosure is not limited to the embodiments described below.

[0009] Patent Document 1 states that lithium-containing composite oxides containing Ni and Mn are basically basic, and although the reason has not been confirmed, gelation occurs in cathode mixture slurries when they are coexisting with polyvinylidene fluoride (PVdF) or vinylidene fluoride (VdF) copolymers, resulting in a problem of impaired slurry stability. Furthermore, Patent Document 1 states that by using a VdF / TFE copolymer obtained by copolymerizing VdF with tetrafluoroethylene (TFE) in a specific amount, the cathode mixture slurry becomes homogeneous and stable. Patent Document 1 further states that the stability of the slurry is improved by reducing the water content of the organic solvent used in the preparation of the cathode mixture slurry.

[0010] However, it has become clear that when using vinylidene fluoride polymers having specific functional groups and fluorine-containing copolymers containing vinylidene fluoride units and perfluoromonomer units as binders in the cathode mixture, the viscosity of the cathode mixture can increase even when using conventional techniques. Therefore, there is a need for new techniques that can suppress the increase in viscosity of the cathode mixture to an even higher level, even when using specific binders.

[0011] Therefore, after diligently investigating ways to solve the above problem, we unexpectedly discovered that by including water in the cathode mixture within a very limited range of content, the increase in viscosity of the cathode mixture could be suppressed to an even higher level.

[0012] In other words, the present disclosure provides 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), wherein the water (E) content is 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D).

[0013] Since the positive electrode mixture of this disclosure contains water (E) in an amount of 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D), the viscosity of the positive electrode mixture does not easily increase even when the mixture is prepared and left to stand for a while. Therefore, by applying the positive electrode mixture of this disclosure to a current collector, a homogeneous and smooth positive electrode mixture layer can be easily formed on the current collector.

[0014] Next, each component contained in the cathode mixture of this disclosure will be described in detail.

[0015] <Fluorine-containing polymer (A)> The cathode mixture of this disclosure consists of vinylidene fluoride (VdF) units and 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 Y represents a single bond or a hydrocarbon group with 1 to 8 carbon atoms. 1 The compound contains a fluorine-containing polymer (A) which contains monomer units based on monomer (1) represented by ). By containing the fluorine-containing polymer (A) in the positive electrode mixture, excellent adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0016] In general formula (1), Y 1 '' represents inorganic cations and / or organic cations. Examples of inorganic cations include H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4 and NH3R. 5 NH2R 5 2. NHR 5 3. NR 5 4(R 5 Each independently represents an alkyl group having 1 to 4 carbon atoms. Examples of cations include Y. 1 Preferred cations are H, Li, Na, K, Mg, Ca, Al, and NH4; more preferred are H, Li, Na, K, Mg, Al, and NH4; even more preferred are H, Li, Al, and NH4; and particularly preferred is H. For convenience, specific examples of inorganic and organic cations are listed without symbols or valencies.

[0017] In general formula (1), R 1 ~R 3 R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a monovalent hydrocarbon group. The number of carbon atoms in the hydrocarbon group is preferably 4 or less. Examples of the hydrocarbon group include alkyl groups, alkenyl groups, alkynyl groups, etc., with methyl or ethyl groups being preferred. 1 and R 2 R is preferably independently a hydrogen atom, a methyl group, or an ethyl group. 3 It is preferable that this is a hydrogen atom or a methyl group.

[0018] In general formula (1), R 4The symbol represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a divalent hydrocarbon group. The number of carbon atoms in the hydrocarbon group is preferably 4 or less. Examples of the hydrocarbon group include alkylene groups and alkenylene groups with the above number of carbon atoms, and among these, at least one selected from the group consisting of methylene, ethylene, ethylidene, propyridene, and isopropylidene groups is preferred, with methylene being more preferred.

[0019] The monomer (1) is preferably at least one selected from the group consisting of (meth)acrylic acid and its salts, vinylacetic acid (3-butenoic acid) and its salts, 3-pentenoic acid and its salts, 4-pentenoic acid and its salts, 3-hexenoic acid and its salts, 4-heptenoic acid and its salts, and 5-hexenoic acid and its salts, and more preferably at least one selected from the group consisting of (meth)acrylic acid and its salts, 3-butenoic acid and its salts, and 4-pentenoic acid and its salts.

[0020] The fluorine-containing polymer (A) may contain monomer units based on monomers copolymerizable with VdF (excluding monomer units based on monomer (1)). Examples of monomers copolymerizable with VdF include fluorinated monomers and non-fluorinated monomers (excluding monomer (1)), with fluorinated monomers being preferred. Examples of fluorinated monomers include vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, hexafluoroisobutene, 2,3,3,3-tetrafluoropropene, and trans-1,3,3,3-tetrafluoropropene. Examples of non-fluorinated monomers include ethylene, propylene, and acryloyloxyethyl succinic acid.

[0021] In the fluorine-containing polymer (A), at least one fluorinated monomer selected from the group consisting of CTFE, fluoroalkyl vinyl ether, and HFP is preferred as the monomer copolymerizable with VdF.

[0022] The VdF unit content of 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%, even 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, in order to obtain even better adhesion between the positive electrode mixture layer and the current collector.

[0023] The monomer unit content of the fluorine-containing polymer (A) based on monomer (1) 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, in order to obtain even better adhesion between the positive electrode mixture layer and the current collector.

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

[0025] In this disclosure, the compositions of the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) are, for example, 19 It can be measured by F-NMR measurement. Furthermore, in this disclosure, the content of monomer units based on monomer (1) and monomer units having polar groups in the fluorine-containing polymer (A) and fluorine-containing copolymer (B) can be measured by acid-base titration of the acid group.

[0026] The weight-average molecular weight (in polystyrene equivalent) of the fluorine-containing polymer (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, and particularly preferably 2,000,000 or less. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as the solvent.

[0027] The number-average molecular weight (polystyrene equivalent) of the fluorine-containing polymer (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, and particularly preferably 1,100,000 or less. The number-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as the solvent.

[0028] The melting point of the fluorine-containing polymer (A) is preferably 100 to 240°C. This melting point can be determined using a differential scanning calorimetry (DSC) device as the temperature relative to the maximum value in the heat of fusion curve when the temperature is increased at a rate of 10°C / min.

[0029] The storage modulus of the fluorine-containing polymer (A) at 30°C is preferably 2000 MPa or less, and more preferably 1800 MPa or less. The storage modulus of the fluorine-containing polymer (A) at 60°C is preferably 1500 MPa or less, and more preferably 1300 MPa or less. The storage modulus of the fluorine-containing polymer (A) at 30°C is preferably 1000 MPa or more, and more preferably 1100 MPa or more. The storage modulus of the fluorine-containing polymer (A) at 60°C is preferably 600 MPa or higher, and more preferably 700 MPa or higher. If the storage modulus of the fluorine-containing polymer (A) at 30°C or 60°C falls within the above range, the flexibility of the fluorine-containing polymer (A) is improved, making it easy to form electrodes that are less prone to cracking when used as a binder.

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

[0031] As the perfluoro monomer, 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) is preferred, and at least one selected from the group consisting of TFE and HFP is more preferred. As the perfluoro monomer, TFE is particularly preferred because it allows for the production of a positive electrode mixture with even less viscosity increase.

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

[0033] Examples of monomers having polar groups include monomer (1) represented by the general formula (1) described above. When the fluorine-containing copolymer (B) contains monomer units having polar groups, the adhesion between the positive electrode mixture layer and the current collector is improved. The fluorine-containing copolymer (B) may or may not contain monomer units based on monomer (1).

[0034] Examples of fluorine-containing copolymers (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 VdF unit content of 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, even more preferably 64.0 mol% or more, preferably 99.0 mol% or less, more preferably 95.0 mol% or less, even more preferably 90.0 mol% or less, and even more preferably 85.0 mol% or less, relative to the total monomer units, in order to form a positive electrode with excellent flexibility.

[0036] The perfluoromonomer unit content of 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, preferably 43.0 mol% or less, more preferably 40.0 mol% or less, even more preferably 38.0 mol% or less, and particularly preferably 37.0 mol% or less, in order to form a positive electrode with excellent flexibility.

[0037] The content of monomer units other than vinylidene fluoride and perfluoromonomer in the fluorine-containing copolymer (B) is preferably 0 to 2.0 mol%, and more preferably 1.5 mol% or less, relative to the total 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 monomer units other than vinylidene fluoride and perfluoromonomer.

[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, and particularly preferably 2,000,000 or less. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as the 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 the solvent.

[0040] The melting point of the fluorine-containing copolymer (B) is preferably 100 to 240°C. This melting point can be determined using a differential scanning calorimetry (DSC) device as the temperature relative to the maximum value in the heat of fusion curve when the temperature is increased at a rate of 10°C / min.

[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 fluorine-containing copolymer (B) at 60°C is preferably 500 MPa or less, and more preferably 350 MPa or less. 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 higher, more preferably 80 MPa or higher, and even more preferably 130 MPa or higher. If the storage modulus of the fluorine-containing copolymer (B) at 30°C or 60°C falls within the above range, the flexibility of the fluorine-containing copolymer (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, in order to obtain a positive electrode mixture in which viscosity is less likely to increase and to obtain even better adhesion between the positive electrode mixture layer and the current collector. 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 this 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, preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, relative to the positive electrode mixture. By setting the content of the fluorine-containing polymer (A) within the above range, a positive electrode mixture that is less prone to viscosity increase can be obtained, and even better adhesion between the positive electrode mixture layer and the current collector can be obtained.

[0044] In the positive electrode mixture of this 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 that is less prone to viscosity increase can be obtained, and even 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, preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, relative to 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 that is less prone to viscosity 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 this 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, polyamide-imide, polycarbonate, styrene rubber, butadiene rubber, and styrene-butadiene rubber.

[0047] <Cathode active material (C)> The positive electrode mixture of this disclosure contains a positive electrode active material (C). The positive electrode active material (C) has the general formula (C):Li y Ni 1-x M x O2 The positive electrode active material is represented by the formula (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).). High-capacity secondary batteries can be obtained by including a positive electrode active material with a high Ni content in the positive electrode mixture. It has now become clear that the viscosity of the positive electrode mixture tends to increase when a positive electrode active material with a high Ni content is used, along with fluorine-containing polymers (A) and fluorine-containing copolymers (B) as binders. However, the positive electrode mixture of this disclosure does not easily increase viscosity because water is present in the positive electrode mixture in a very limited amount.

[0048] In general formula (C), examples of metal atoms M include V, Ti, Cr, Mn, Fe, Co, Cu, Al, Zn, Mg, Ga, Zr, Si, etc. Preferably, the metal atoms M are transition metals such as V, Ti, Cr, Mn, Fe, Co, 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, Si, etc.

[0049] Examples of positive electrode active material (C) include lithium transition metal composite oxides, such as LiNi 0.80 Co 0.15 Al 0.05 O2, LiLiLi 0.82 Co 0.15 Al 0.03 O2, LiLiLi 0.33 Mn 0.33 Co 0.33 O2, LiLiLi 0.5 Mn 0.3 Co 0.2 O2, LiLiLi 0.6 Mn 0.2 Co 0.2 O2, LiLiLi 0.8 Mn 0.1 Co 0.1 O2 and LiNi 0.90 Mn 0.05 Co 0.05 Preferably, at least one selected from the group consisting of O2, and LiNi 0.82 Co 0.15 Al 0.03 O2, LiLiLi 0.6 Mn 0.2Co 0.2 O2, LiLiLi 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 preferable.

[0050] In addition to the positive electrode active material (C), a combination of different positive electrode active materials may be used. Specifically, the different positive electrode active materials may be LiNiO2, LiCoO2, etc. 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, as the positive electrode active material (C), a material with a different composition from the main positive electrode active material (C) can be used, which is attached to the surface of the positive electrode active material (C). Examples of surface-attached materials 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-adhering substances can be attached to the surface of the positive electrode active material (C) by, for example, dissolving or suspending them in a solvent and impregnating them into the positive electrode active material (C), followed by drying; dissolving or suspending a surface-adhering substance precursor in a solvent and impregnating it into the positive electrode active material (C), then reacting it by heating or other means; or adding it to the positive electrode active material precursor and simultaneously firing it.

[0053] The amount of surface-adhered material is preferably 0.1 ppm or more, more preferably 1 ppm or more, even more preferably 10 ppm or more, 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-adhered material can suppress the oxidation reaction of the non-aqueous electrolyte on the surface of the positive electrode active material (C), thereby improving battery life. However, if the amount of adhesion is too small, the effect will not be fully realized, and if it is too large, it may hinder the movement of lithium ions in and out, potentially increasing resistance.

[0054] The positive electrode active material (C) particles can take on various shapes, such as lumpy, polyhedral, spherical, ellipsoidal, plate-like, needle-like, or columnar, as has been done conventionally. However, it is preferable that the particles are formed by the aggregation of primary particles to create secondary particles, and that these secondary particles are spherical or ellipsoidal in shape. Normally, in electrochemical elements, the active material in the electrode expands and contracts with charging and discharging, making it susceptible to degradation such as destruction of the active material or breakage of the conductive path due to this stress. Therefore, it is preferable to have an active material in which primary particles aggregate to form secondary particles rather than a single-particle active material consisting only of primary particles, as this reduces the stress of expansion and contraction and prevents degradation. Furthermore, spherical or ellipsoidal particles are preferable to plate-like equiaxially oriented particles because they require less orientation during electrode molding, resulting in less expansion and contraction of the electrode during charging and discharging, and they are also easier to mix uniformly with the conductive agent when creating the electrode.

[0055] The tap density of the positive electrode active material (C) is typically 1.3 g / cm³. 3 Preferably 1.5 g / cm³ 3 More preferably 1.6 g / cm³ 3 In summary, the most preferred amount is 1.7 g / cm³. 3The above is a summary. If the tap density of the positive electrode active material (C) falls below the above lower limit, the amount of dispersion medium required during the formation of the positive electrode active material (C) layer increases, as does the required amount of conductive agent, PVdF (A), and fluorine-containing copolymer (B). This restricts the packing rate of the positive electrode active material (C) into the positive electrode mixture layer, which may 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, a higher tap density is preferable, and there is no particular upper limit. However, if it is too high, the diffusion of lithium ions using the non-aqueous electrolyte as a medium within the positive electrode mixture layer becomes the rate-limiting factor, and the load characteristics tend to deteriorate. Therefore, it is usually 2.5 g / cm³. 3 Preferably, 2.4 g / cm³ 3 The following applies:

[0056] The tap density of the positive electrode active material (C) was determined by passing it through a sieve with a mesh size of 300 μm over 20 cm². 3 After dropping the sample into the tapping cell to fill the cell volume, a powder density analyzer (e.g., TapDenser manufactured by Seishin Corporation) is used to perform 1000 taps with a stroke length of 10 mm. The density obtained from the volume and weight of the sample at that time is defined as the tap density.

[0057] The median diameter d50 of the positive electrode active material (C) particles (or secondary particle diameter if primary particles aggregate to form secondary particles) is usually 0.1 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, most preferably 3 μm or more, and usually 20 μm or less, preferably 18 μm or less, more preferably 16 μm or less, and most preferably 15 μm or less. If it falls below the lower limit, it may not be possible to obtain a high bulk density product, and if it exceeds the upper limit, the diffusion of lithium within the particles will take time, which may lead to a decrease in battery performance, or problems such as streaking may occur when preparing the positive electrode of the battery, i.e., when the positive electrode active material (C) and conductive agent, PVdF (A), and fluorine-containing copolymer (B), etc. are slurryed with a solvent and applied as a thin film. Here, by mixing two or more types of positive electrode active material (C) with different median diameters d50, the packing performance during positive electrode preparation can be further improved.

[0058] In addition, the median diameter d50 in the present disclosure is measured by a known laser diffraction / scattering particle size distribution measuring device. When using LA-920 manufactured by HORIBA as the particle size distribution meter, as the dispersion medium used in the measurement, an aqueous solution of 0.1 mass% sodium hexametaphosphate is used, and after ultrasonic dispersion for 5 minutes, the refractive index for measurement is set to 1.24 for measurement.

[0059] When primary particles aggregate to form secondary particles, the average primary particle diameter of the positive electrode active material (C) is usually 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 usually 3 μm or less, preferably 2 μm or less, more preferably 1 μm or less, and most preferably 0.6 μm or less. If it exceeds the above upper limit, it is difficult to form spherical secondary particles, which may adversely affect the powder packing property or significantly reduce the specific surface area, resulting in a high possibility of deterioration of battery performance such as output characteristics. Conversely, if it is below the above lower limit, problems such as poor reversibility of charge and discharge often occur because the crystal is usually underdeveloped. The primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, in a photograph at a magnification of 10,000 times, the longest value of the section by the left and right boundary lines of the primary particles with respect to a horizontal straight line is obtained for any 50 primary particles, and the average value is obtained.

[0060] The BET specific surface area of the positive electrode active material (C) is usually 0.2 m 2 / g or more, preferably 0.3 m 2 / g or more, more preferably 0.4 m 2 / g or more, and usually 4.0 m 2 / g or less, preferably 2.5 m 2 / g or less, more preferably 1.5 m 2 / g or less. If the BET specific surface area is smaller than this range, the battery performance is likely to deteriorate. If it is larger, it is difficult to increase the tap density, and problems are likely to occur in the coating property of the positive electrode mixture.

[0061] The BET specific surface area is defined as the value measured by the nitrogen adsorption BET single-point method using the gas flow method, after pre-drying the sample at 150°C for 30 minutes under nitrogen flow using a surface area meter (for example, a fully automatic surface area measuring device manufactured by Okura Riken Co., Ltd.), and then using a nitrogen-helium mixed gas that has been precisely adjusted so that the relative pressure of nitrogen to atmospheric pressure is 0.3.

[0062] For the production of the positive electrode active material (C), general methods for producing inorganic compounds are used. In particular, various methods can be considered for producing spherical or ellipsoidal active materials. For example, a method is to dissolve or grind and disperse transition metal raw materials such as transition metal nitrates and sulfates, and raw materials of other elements as needed, in a solvent such as water, adjust the pH while stirring to create and recover spherical precursors, dry them as needed, and then add a Li source such as LiOH, Li2CO3, or LiNO3 and calcine at a high temperature to obtain the active material. Alternatively, a method is to dissolve or grind transition metal raw materials such as transition metal nitrates, sulfates, hydroxides, and oxides, and raw materials of other elements as needed, in a solvent such as water. One method involves dispersing the material, drying and molding it with a spray dryer or the like to form a spherical or ellipsoidal precursor, adding a Li source such as LiOH, Li2CO3, or LiNO3, and firing it at a high temperature to obtain the active material. Another method involves dissolving or pulverizing and dispersing transition metal raw materials such as transition metal nitrates, sulfates, hydroxides, or oxides, a Li source such as LiOH, Li2CO3, or LiNO3, and raw materials of other elements as needed, in a solvent such as water, drying and molding the mixture with a spray dryer or the like to form a spherical or ellipsoidal precursor, and firing it at a high temperature to obtain the active material.

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

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

[0065] <Non-aqueous solvent (D)> The positive electrode mixture of this disclosure contains a non-aqueous solvent (D). Examples of non-aqueous solvents (D) 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 general-purpose low-boiling point organic solvents such as mixtures thereof.

[0066] As for the non-aqueous 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 yields a cathode mixture with excellent dispersion stability and coating properties.

[0067] In the positive electrode mixture of this disclosure, the content of the non-aqueous solvent (D) is determined considering the applicability to the current collector, the thin film formation after drying, etc. In the positive electrode mixture of this 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% by mass, more preferably 60 to 85% by mass, even more preferably 65 to 80% by mass, and still more preferably 65 to 75% by mass.

[0068] <Water(E)> The positive electrode mixture of this disclosure contains water (E). The water (E) content in the positive electrode mixture is 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D). By including a very small amount of water in the positive electrode mixture in this way, a positive electrode mixture that does not easily increase in viscosity can be obtained. Furthermore, by using a positive electrode mixture containing a very 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 water (E) content in the positive electrode mixture is 200 to 10,000 ppm by mass, preferably 1,000 ppm or more, more preferably 2,000 ppm or more, even more preferably 4,000 ppm or more, and still more preferably 6,000 ppm or more, relative to the mass of the non-aqueous solvent (D). A water (E) content above the lower limit sufficiently suppresses the increase in viscosity of the positive electrode mixture. A water (E) content below the upper limit shortens the drying time after coating the positive electrode mixture, thereby improving the productivity of the positive electrode.

[0070] The method for adjusting the water (E) content in the positive electrode mixture is not particularly limited, but it can be adjusted, for example, by adding water to achieve the above-mentioned content when preparing the positive electrode mixture.

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

[0072] <Other ingredients> The positive electrode mixture may further contain a conductive agent. Examples of conductive agents include carbon blacks such as acetylene black and Ketjenblack, carbon materials such as graphite, carbon fibers, multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon nanohorns, and graphene.

[0073] In the positive electrode mixture of this 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 by mass ratio.

[0074] The cathode mixture may contain dispersants such as resins with surfactant properties, cationic surfactants, or nonionic surfactants to improve dispersion stability.

[0075] A method for preparing the positive electrode mixture according to this disclosure includes, for example, dispersing and mixing a positive electrode active material (C) and optionally a conductive agent 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 first mixing the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) with the positive electrode active material (C), and then adding the non-aqueous solvent (D) and optionally a conductive agent. Or, the positive electrode mixture may be prepared by adding a conductive agent to a solution obtained by dissolving the fluorine-containing polymer (A) and the fluorine-containing copolymer (B) in a non-aqueous solvent (D), mixing it, and then further adding and mixing the positive electrode active material (C).

[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 a fluorine-containing polymer (A) and a fluorine-containing copolymer (B) in a non-aqueous solvent (D), or water (E) may be added to a mixture containing a fluorine-containing polymer (A), a fluorine-containing copolymer (B), a positive electrode active material (C), and a non-aqueous solvent (D).

[0077] The viscosity of the positive electrode mixture of this disclosure is preferably 1000 mPa·s or more, more preferably 3000 mPa·s or more, even more preferably 5000 mPa·s or more, even 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, since it is easy to apply and easy to obtain a positive electrode mixture layer of the desired thickness. The viscosity can be measured at 25°C using a B-type viscometer. The positive electrode mixture of this disclosure has a viscosity that allows for easy application and can maintain a viscosity that allows for easy adjustment of the thickness of the positive electrode mixture layer for a long period of time, as its viscosity does not easily increase.

[0078] <Positive electrode> The positive electrode of this disclosure is formed from the positive electrode mixture described above. One method for forming a positive electrode using the positive electrode mixture described above is to apply the positive electrode mixture to a current collector, dry it, and press it to form a thin layer of the positive electrode mixture on the current collector, thereby forming a thin-film electrode. In other words, one preferred embodiment of the positive electrode of this disclosure comprises a current collector and a positive electrode mixture layer formed on the current collector from the positive electrode mixture described above.

[0079] Examples of the current collectors mentioned above include metal foils or metal meshes made of iron, stainless steel, copper, aluminum, nickel, titanium, etc., with aluminum foil being particularly preferred.

[0080] <Secondary battery> The secondary battery of this disclosure comprises the positive electrode described above. Preferably, the secondary battery of this disclosure further comprises a negative electrode and a non-aqueous electrolyte in addition to the positive electrode described above.

[0081] The above non-aqueous electrolyte is not particularly limited, and one or more known solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc. can be used. Any of the conventionally known electrolytes can also be used, and LiClO4, LiAsF6, LiPF6, LiBF4, LiCl, LiBr, CH3SO3Li, CF3SO3Li, cesium carbonate, etc. can be used.

[0082] The positive electrode mixture of the present disclosure is useful not only for lithium-ion secondary batteries using the liquid electrolyte described above but also for polymer electrolyte lithium secondary batteries as a positive electrode mixture for non-aqueous electrolyte secondary batteries. It is also useful as an electric double layer capacitor.

[0083] As described above, the embodiments have been described, but it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims.

[0084] <1> According to the 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 a vinylidene fluoride unit and a monomer unit based on a monomer (1) represented by the general formula (1):

Chemical formula

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

[0086] Each value in the examples was measured by the following method.

[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 fluorine-containing polymer (A) was measured by acid-base titration of the carboxylic acid group. Specifically, approximately 0.5 g of fluorine-containing polymer (A) was dissolved in acetone at a temperature of 70-80°C. 5 ml of water was added dropwise under vigorous stirring to avoid coagulation of fluorine-containing polymer (A). Titration was performed with a 0.1 N NaOH aqueous solution until the acidity was completely neutralized at a neutralization transition of approximately -270 mV. From the measurement results, the amount of polar group-containing monomer units contained in 1 g of fluorine-containing polymer (A) was determined, and the content of 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 fluorine-containing copolymer (B) was determined using an NMR analyzer (Agilent Technologies, VNS400MHz). 19 The polymer was measured in DMF-d7 solution using F-NMR.

[0089] 19 The areas of the following peaks (A, B, C, D) were determined by 1F-NMR measurement, 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: Peak area between -119 ppm and -122 ppm D: Area of ​​the peak between -122 ppm and -126 ppm Percentage of VdF units: (4A + 2B) / (4A + 3B + 2C + 2D) × 100 [mol%] Percentage of TFE units: (B + 2C + 2D) / (4A + 3B + 2C + 2D) × 100 [mol%]

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

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

[0092] <Weight average molecular weight> Measurements were taken using gel permeation chromatography (GPC). A Tosoh AS-8010, CO-8020, and column (three GMHHR-H columns connected in series) were used, along with a Shimadzu RID-10A. Dimethylformamide (DMF) was used as the solvent, flowing at a rate of 1.0 ml / min. The data (reference: polystyrene) was used for calculation.

[0093] <Storage modulus (E')> The storage modulus is a value measured by dynamic viscoelasticity measurement at 30°C or 60°C. It was measured using an IT Measurement Control Co., Ltd. DVA220 dynamic viscoelasticity instrument on a test specimen measuring 30 mm in length, 5 mm in width, and 50-100 μm in thickness, in tensile mode, with a grip width of 20 mm, a measurement temperature of -30°C to 160°C, a heating rate of 2°C / min, and a frequency of 1 Hz.

[0094] The test specimens used for measurement were prepared by dissolving PVdF and a fluorine-containing copolymer in N-methyl-2-pyrrolidone (NMP) to a concentration of 10-20% by mass, casting the polymer solution onto a glass plate, drying it at 100°C for 12 hours, and then drying it again under vacuum at 100°C for another 12 hours. The resulting film, 50-100 μm thick, was then cut to a length of 30 mm and a width of 5 mm.

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

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

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

[0098] <Viscosity change rate> The viscosity (η0) at the time of compound preparation and the viscosity (ηn) after n hours have elapsed since compound preparation were measured, and the viscosity change rate (Xn) was calculated using the following formula. Xn = ηn / η0 × 100 [%] In Table 1, "×" indicates that the viscosity of the positive electrode mixture was too high, and therefore the viscosity could not be measured.

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

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

[0101] <Positive pole flexibility> By cutting out a double-sided coated cathode structure obtained by pressing with a roll press, 2cm x 10cm test specimens were prepared. These specimens were then wrapped around round rods of various diameters (3mm, 2mm, 1.5mm, and 1.0mm), and the cathode flexibility was visually inspected and evaluated according to the following criteria. ○: No cracks were observed. △: Cracks were observed, but no rupture of the positive electrode mixture layer or current collector was observed. ×: The positive electrode compound layer and current collector were fractured.

[0102] The examples and comparative examples used polymers having the following physical properties.

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

[0104] <Fluorine-containing copolymer (B)> Fluorine-containing copolymer containing BI:VdF units and TFE units VdF / TFE = 83 / 17 (mol%) Weight average molecular weight 1230000 Storage modulus at 30°C: 490 MPa Storage modulus at 60°C: 260 MPa Melting point 131℃ 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: 440 MPa Storage modulus at 60°C: 180 MPa Melting point 160℃ 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°C: 165 MPa Melting point 123℃ 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: 310 MPa Storage modulus at 60°C: 145 MPa Melting point 135℃

[0105] Furthermore, the following positive electrode active material and conductive agent were used in the examples and comparative examples. Ni90:LiLi 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) as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare an 8% by mass solution of the fluorine-containing polymer (AI). Similarly, a fluorine-containing copolymer (BI) as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare an 8% by mass solution of the fluorine-containing copolymer (BI). 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 agent paste. 4 mg of water was added to the conductive agent paste and kneaded further. 96 g of Ni90 as a positive electrode active material was added to the obtained paste and mixed using a stirrer to obtain a mixture. NMP was further added to the obtained mixture and mixed to prepare a positive electrode mixture with a solid content of 67% by mass.

[0107] (Fabrication of positive electrode structure) The resulting positive electrode mixture was immediately and uniformly applied to one side of a positive electrode current collector (aluminum foil with a thickness of 20 μm). After the NMP was completely evaporated, the positive electrode structure was fabricated by pressing it with a roll press machine under a pressure of 10 tons. Furthermore, the resulting positive electrode mixture was immediately and uniformly applied to both sides of the positive electrode current collector (aluminum foil with a thickness of 20 μm) after preparation. After the NMP was completely evaporated, a roll press was used to obtain a positive electrode mixture layer with a density of 3.6 g / cm³. 3 The positive electrode structure was fabricated by repeatedly applying pressure and pressing until the desired result was reached.

[0108] (Preparation of film for measuring coating resistance) The resulting positive electrode mixture was immediately and uniformly applied to one side of a polyethylene terephthalate film (100 μm thick) to prepare a film for measuring coating resistance by completely volatilizing the NMP.

[0109] Examples 2-8 and Comparative Examples 1-4 Except for changing the type of binder and the amount of water added as shown in Table 1, the positive electrode mixture was prepared in the same manner as in Example 1, and the positive electrode structure and coating resistance measurement film were fabricated and evaluated in the same manner as in Example 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), Fluorine-containing polymer (A) consists of vinylidene fluoride units and 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 This represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. １ This represents an inorganic cation and / or an organic cation. It contains monomer units based on a monomer (1) represented by ), The fluorine-containing copolymer (B) contains vinylidene fluoride units and perfluoromonomer units, The positive electrode active material (C) is of the general formula (C): Li ｙ Ni １－ｘ M ｘ O ２ The positive electrode active material is represented by the formula (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 water (E) content is 200 to 10,000 ppm by mass relative to the mass of the non-aqueous solvent (D). Cathode mixture.

2. The positive electrode mixture according to claim 1, wherein the content of vinylidene fluoride units in the fluorine-containing polymer (A) is 92.0 to 99.999 mol% with respect to the total monomer units.

3. The positive electrode mixture according to claim 1 or 2, wherein the content of monomer units based on monomer (1) in the fluorine-containing polymer (A) is 0.001 to 8.0 mol% relative to the total monomer units.

4. The positive electrode mixture according to claim 1 or 2, wherein the content of vinylidene fluoride units in the fluorine-containing copolymer (B) is 60.0 to 99.0 mol% relative to the total monomer units.

5. The positive electrode mixture according to claim 1 or 2, wherein the content of perfluoromonomer units in the fluorine-containing copolymer (B) is 1.0 to 40.0 mol% relative to the total monomer units.

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

7. The positive electrode mixture according to claim 1 or 2, wherein the water (E) content is 2,000 to 10,000 ppm by mass relative to the mass of the non-aqueous 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 its salts, vinyl acetate (3-butenoic acid) and its salts, 3-pentenoic acid and its salts, 4-pentenoic acid and its salts, 3-hexenoic acid and its salts, 4-heptenoic acid and its salts, and 5-hexenoic acid and its salts. The fluorine-containing copolymer (B) contains perfluoromonomer units based on vinylidene fluoride units and 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 fluorine-containing copolymer (B) to the mass of the positive electrode active material (C) is between 1 / 99 and 3 / 97. The total content of the fluorine-containing polymer (A), fluorine-containing copolymer (B), and positive electrode active material (C) is 50 to 90% by mass. The positive electrode mixture according to claim 1 or 2.

10. A positive electrode comprising a positive electrode mixture layer formed by the positive electrode mixture described in claim 1 or 2.

11. A secondary battery comprising the positive electrode described in claim 10.

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