Current collector, production method for same, electrode, and battery

Abstract

The present invention addresses the problem of providing a current collector in which cracking etc. of a mixture layer is unlikely to occur during production of an electrode. This current collector solves the aforementioned problem and has a metal-containing base material and a coat layer disposed on at least one surface of the base materia. The coat layer contains an electroconductive auxiliary agent, a vinylidene fluoride copolymer, and a dispersant. The vinylidene fluoride copolymer contains: a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a carboxy group. The amount of the electroconductive auxiliary agent is 45 mass% to 80 mass% with respect to the total mass of the coat layer.

Description

Current collector and manufacturing method thereof, electrode, and battery

[0001] The present invention relates to a current collector, a method for producing the same, an electrode, and a battery.

[0002] Lithium ion secondary batteries are widely used in various electronic devices, electric vehicles, etc. An electrode of a lithium ion secondary battery generally has a current collector and a composite layer containing an active material and a binder disposed on the current collector.

[0003] In recent years, there has been a demand for higher capacity lithium-ion secondary batteries, and attempts have been made to increase the capacity of lithium-ion secondary batteries, for example, by charging at a high voltage. Therefore, studies have been conducted to reduce the electrical resistance at the interface between the current collector and the composite layer by configuring the current collector with a substrate and a conductive layer (hereinafter also referred to as a "coating layer") disposed on the substrate. Patent Document 1 describes a current collector composed of a laminate of aluminum foil and a coating layer containing acrylic acid-modified polyvinylidene fluoride and a conductor.

[0004] Chinese Patent Application Publication No. 113725398

[0005] However, through intensive studies by the present inventors, it has become clear that when a composite layer is formed on the coating layer of the current collector described in Patent Document 1, cracks are likely to occur in the composite layer.

[0006] An object of the present invention is to provide a current collector in which cracks are unlikely to occur in the composite layer during electrode production, a method for producing the same, an electrode using the same, and a battery.

[0007] [1] The present invention provides a current collector having a metal-containing substrate and a coating layer disposed on at least one surface of the substrate, the coating layer comprising a conductive additive, a vinylidene fluoride copolymer, and a dispersant, the vinylidene fluoride copolymer comprising a vinylidene fluoride-derived structural unit and a carboxyl group-containing compound, the amount of the conductive additive being 45 mass % to 80 mass % of the total mass of the coating layer. [2] The present invention provides the current collector according to [1], wherein the carboxyl group-containing compound is a compound represented by the following general formula (1): (In general formula (1), R １ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less), R ２ and R ３ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; X １ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less.) [3] The present invention provides a current collector according to [1] or [2], wherein the vinylidene fluoride copolymer of the coating layer contains structural units derived from vinylidene fluoride and structural units derived from acrylic acid and / or monomethyl maleate. [4] The present invention provides a current collector according to any one of [1] to [3], wherein the dispersant is at least one compound selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, hydroxypropyl methyl cellulose, and N-methylethanolamine. [5] The present invention provides a current collector according to any one of [1] to [4], wherein the amount of the dispersant is 0.2 mass% or more relative to the total mass of the coating layer.

[0008] [6] The present invention provides an electrode having the current collector according to any one of [1] to [5] and a composite layer disposed on the coating layer of the current collector, the composite layer including an active material and a binder.

[0009] [7] The present invention provides the electrode according to [6], wherein the binder contains a structural unit derived from vinylidene fluoride. [8] The present invention provides a battery including the electrode according to [6]. The present invention also provides an electrode having the current collector described above and a composite layer disposed on a coating layer of the current collector, the composite layer containing an active material and a binder.

[0010] [9] The present invention provides a method for producing a current collector, comprising the step of applying a coating liquid containing a conductive additive, a vinylidene fluoride copolymer, a dispersant, and a solvent to at least one surface of a metal-containing substrate, the vinylidene fluoride copolymer containing a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a carboxy group, and the amount of the conductive additive is 45% by mass to 80% by mass of the total solid content of the coating liquid.

[10] The present invention provides a method for producing a current collector according to [9], wherein the compound having a carboxy group is a compound represented by the following general formula (1): (In the general formula (1), R １ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less), R ２ and R ３ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; １ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less.

[0011] The current collector of the present invention is less likely to crack even when a composite layer is formed thereon, and therefore the current collector and electrodes using the same are suitable as components for various lithium-ion secondary batteries.

[0012] The current collector of the present invention is a current collector for use in lithium-ion secondary batteries and the like, and includes a metal-containing substrate and a coating layer of a predetermined composition disposed on at least one surface of the substrate. Typically, a composite layer is formed on the coating layer of the current collector before use.

[0013] As described above, in the current collector of Patent Document 1, that is, a current collector having a substrate and a coating layer disposed on the substrate, the coating layer containing acrylic acid-modified polyvinylidene fluoride (a vinylidene fluoride copolymer obtained by copolymerizing acrylic acid and vinylidene fluoride) and a small amount of conductor, when a composite layer is formed on the coating layer, cracks tend to occur on the surface of the composite layer. The reason for this is unclear, but is thought to be as follows.

[0014] The vinylidene fluoride copolymer used in the coating layer, which is a copolymer of acrylic acid and vinylidene fluoride, is prone to agglomeration when mixed with a conductor, making it difficult to form a uniform coating layer. This agglomeration is thought to be due to the close proximity of the vinylidene fluoride-derived structural units and the acrylic acid-derived carboxyl groups.

[0015] On the other hand, the composite layer disposed on the coating layer typically contains a vinylidene fluoride polymer. Therefore, the composite slurry for forming the composite layer often contains a good solvent that has a high affinity with the vinylidene fluoride copolymer in the coating layer. When such a composite slurry is applied to the coating layer, the vinylidene fluoride copolymer in the coating layer dissolves in the good solvent in the composite slurry. Then, when the dissolved vinylidene fluoride copolymer hardens again, shrinkage of the coating layer occurs due to crystallization of the vinylidene fluoride copolymer. As a result, distortion is likely to occur within the composite layer disposed on the coating layer, which is thought to cause cracks on the surface of the composite layer.

[0016] In contrast, the coating layer of the present invention contains a vinylidene fluoride copolymer containing structural units derived from vinylidene fluoride and structural units derived from a compound having a carboxy group (e.g., a compound represented by the general formula (1) described below), but the amount of the conductive additive relative to the total mass of the coating layer is 45% by mass or more and 80% by mass or less. Therefore, even if the vinylidene fluoride copolymer dissolves in a good solvent in the composite slurry when the composite slurry is applied to the coating layer, the conductive additive physically suppresses the movement of the vinylidene fluoride copolymer, making it difficult for distortion to occur in the composite layer. Furthermore, the addition of a dispersant can suppress aggregation of the conductive additive and the vinylidene fluoride copolymer. Therefore, it is believed that uneven portions are unlikely to occur in the coating layer formed on the substrate, and distortion of the coating layer itself is also suppressed. As a result, the composite layer formed on the coating layer of the current collector is unlikely to be distorted or cracked. The configuration of the current collector of the present invention will be described in detail below.

[0017] (1) Substrate The substrate is a base component of the current collector, and the substrate only needs to have sufficient conductivity and contain a metal. The substrate may contain only metal, or may contain a material other than metal (for example, resin, ceramics, etc.). It is more preferable that the substrate is composed of only metal. Here, the type of metal contained in the substrate is appropriately selected according to the type, shape, etc. of the current collector (and thus the battery). The substrate may contain only one type of metal, or may contain two or more types of metal. Examples of metals contained in the substrate include aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc.

[0018] The shape of the substrate is appropriately selected depending on the type and size of the current collector (and thus the battery). The substrate may be, for example, a foil or metal mesh of the above metals. The substrate may also be a material in which the above metal foil or metal mesh is laminated on the surface of a material other than metal.

[0019] (2) Coating Layer The coating layer may contain at least a predetermined amount of a conductive additive, a vinylidene fluoride copolymer having a predetermined structure, and a dispersant, and may contain other components as necessary.

[0020] Conductive additive The conductive additive serves to electrically connect the substrate and the active material in the composite layer. Known conductive additives can be used as the conductive additive. Specific examples of conductive additives include acetylene black, ketjen black, carbon black, graphite powder, graphene, carbon nanofibers, carbon nanotubes, carbon fibers, and metal powders. The coating layer may contain only one type of conductive additive, or two or more types.

[0021] The amount of conductive additive in the coating layer may be 45.0% by mass or more and 80.0% by mass or less, preferably 47.0% by mass or more and 75.0% by mass or less, and more preferably 49.5% by mass or more and 65.0% by mass or less, relative to the total mass of the coating layer. As described above, when the amount of conductive additive in the coating layer is 45% by mass or more, cracks and the like are less likely to occur in the composite layer formed on the current collector (coating layer). On the other hand, when the amount of conductive additive in the coating layer is 80% by mass or less, the adhesive strength between the current collector and the composite layer is likely to be increased.

[0022] Vinylidene fluoride copolymer The vinylidene fluoride copolymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a carboxy group. The coating layer may contain only one type of vinylidene fluoride copolymer, or may contain two or more types.

[0023] The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride copolymer is preferably 90.0 mol% or more and 99.99 mol% or less, more preferably 93.0 mol% or more and 99.98 mol% or less, and particularly preferably 96.0 mol% or more and 99.97 mol% or less, relative to the amount (mol) of all structural units constituting the vinylidene fluoride copolymer. When the amount of vinylidene fluoride-derived structural units is 90.0 mol% or more, the coating layer is likely to exhibit properties derived from vinylidene fluoride, such as electrochemical stability. The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride copolymer is １９ It can be identified by F-NMR.

[0024] Furthermore, the compound having a carboxy group is not particularly limited, and examples thereof include compounds represented by the following general formula (1): Hereinafter, the compound represented by general formula (1) will be used as an example for explanation, but the structure of the compound having a carboxy group is not limited thereto: The vinylidene fluoride polymer may contain only one type of structural unit derived from the compound represented by the following general formula (1), or may contain two or more types. In the above general formula (1), R １ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less). １ is a hydrogen atom, a methyl group, or —C(═O)—OCH ３ is preferred.

[0025] R ２ and R ３ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and from the viewpoint of polymerization reactivity, a substituent with little steric hindrance is preferred, hydrogen or an alkyl group having 1 to 3 carbon atoms is preferred, and hydrogen or a methyl group is preferred.

[0026] X １ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less. １ The number of atoms in the main chain is the number of atoms constituting the shortest chain connecting the terminal carboxy group and the carbon-carbon double bond in the general formula (1). The number of atoms in the main chain is more preferably 15 or less.

[0027] Above X １ The atomic group represented by the formula (I) is not particularly limited in structure, so long as it has a molecular weight of 500 or less and satisfies the above-mentioned number of atoms in the main chain, and may be linear, branched, or contain a ring structure. Examples of structures constituting the atomic group include an alkylene group, a carbonyl group (-C(=O)-), an ether bond (-O-), an aromatic ring, and an alicyclic structure.

[0028] Specific examples of the compound represented by the general formula (1) above include monomethyl maleate (MMM), acrylic acid (AA), methacrylic acid, carboxymethyl acrylate, carboxymethyl methacrylate, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, acryloyloxyethyl succinic acid, methacryloyloxyethyl succinic acid, acryloyloxyethyl phthalic acid, methacryloyloxyethyl phthalic acid, acryloyloxypropyl succinic acid, methacryloyloxypropyl succinic acid, etc. Among these, acrylic acid or monomethyl maleate is preferred.

[0029] The amount of structural units derived from a compound having a carboxy group (e.g., a compound represented by general formula (1)) in the vinylidene fluoride copolymer is preferably 0.01 mol% or more and 10.0 mol% or less, more preferably 0.02 mol% or more and 7.0 mol% or less, and particularly preferably 0.03 mol% or more and 4.0 mol% or less, relative to the amount (mol) of all structural units constituting the vinylidene fluoride copolymer. When the amount of structural units derived from a compound having a carboxy group (e.g., a compound represented by general formula (1)) is 0.01 mol% or more, the adhesion between the coating layer and the composite layer or the substrate is likely to be further improved. On the other hand, when these amounts are 10.0 mol% or less, the amount of structural units derived from vinylidene fluoride becomes relatively large, and the electrochemical stability of the vinylidene fluoride copolymer, and therefore the electrochemical stability of the coating layer, is likely to be improved. The amount of structural units derived from a compound having a carboxy group (e.g., a compound represented by general formula (1)) is １９ F-NMR and １ It can be analyzed by H-NMR or the like.

[0030] Furthermore, the vinylidene fluoride copolymer may contain structural units derived from compounds other than those derived from vinylidene fluoride and compounds having a carboxy group (for example, compounds represented by general formula (1)) (hereinafter also referred to as "other compounds"), as long as the purpose and effects of the present invention are not impaired. However, the amount (total amount) of structural units derived from other compounds relative to all structural units (number of moles) of the vinylidene fluoride copolymer is preferably 10.0 mol% or less, more preferably 4.0 mol% or less. When the amount of structural units derived from other compounds in the vinylidene fluoride copolymer is 10.0 mol% or less, the electrochemical stability of the coating layer is improved, and the adhesive strength between the coating layer and the substrate is likely to increase. The amount of structural units derived from other compounds is １９ F-NMR and １ It can be analyzed by H-NMR or the like.

[0031] Examples of other compounds include fluorine-based compounds other than vinylidene fluoride, such as vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, hexafluoroethylene, fluoroalkyl vinyl ether, and perfluoromethyl vinyl ether.

[0032] Here, the inherent viscosity of the vinylidene fluoride copolymer is preferably 1.0 or more, more preferably 1.3 dL / g or more, and even more preferably 2.0 dL / g or more. When the inherent viscosity is as described above, the adhesive strength between the coating layer and the substrate and the adhesive strength between the coating layer and the composite layer formed on the coating layer tend to be increased. The inherent viscosity can be measured by the following method. First, 80 mg of vinylidene fluoride copolymer is dissolved in 20 mL of N,N-dimethylformamide, and the viscosity is measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, from the obtained value, the inherent viscosity (η) of the vinylidene fluoride copolymer is calculated based on the following formula: ｉ ) is calculated. ｉ =(1 / C)・ln(η / η ０ In the above formula, η is the viscosity of the solution, η ０is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of the vinylidene fluoride copolymer, i.e., 0.4 g / dL.

[0033] The method for preparing the vinylidene fluoride copolymer is not particularly limited, and vinylidene fluoride and a structural unit derived from a compound having a carboxy group (e.g., a compound represented by general formula (1)), and optionally other compounds, may be polymerized by a known method. Examples of the polymerization method include suspension polymerization, emulsion polymerization, solution polymerization, etc.

[0034] The amount of vinylidene fluoride copolymer in the coating layer is preferably 10.0 mass% or more and 54.8 mass% or less, more preferably 15.0 mass% or more and 52.8 mass% or less, and even more preferably 25.0 mass% or more and 50.3 mass% or less, relative to the total mass of the coating layer. When the amount of vinylidene fluoride copolymer is within this range, the electrochemical stability of the coating layer is likely to be improved, and further, the adhesion between the coating layer and the substrate, and the adhesion between the current collector and the composite layer are likely to be improved. Furthermore, the amount of vinylidene fluoride copolymer is preferably 25 mass% or more and 122 mass% or less, more preferably 53.8 mass% or more and 102 mass% or less, relative to 100 mass% of the conductive additive. When the ratio of the amount of conductive additive to the amount of vinylidene fluoride copolymer is within this range, the composite layer formed on the current collector layer is even less likely to crack. Furthermore, when the ratio of the amount of the conductive assistant to the amount of the vinylidene fluoride copolymer is within the above range, the adhesion between the current collector and the composite layer tends to be even better.

[0035] Dispersant: The dispersant in this specification may be any compound capable of suppressing aggregation of the vinylidene fluoride copolymer and the conductive additive. Examples include vinyl copolymers, cellulose compounds, and alcohol amines. As described above, the inclusion of a dispersant in the coating layer makes it less likely for cracks to occur in the composite layer formed on the current collector (coating layer), and also improves adhesion between the coating layer and the composite layer. The coating layer may contain only one type of dispersant, or may contain two or more types.

[0036] Specific examples of the vinyl copolymer include polyvinylpyrrolidone and polyvinyl alcohol. Examples of the cellulose compound include methyl cellulose and hydroxypropyl methyl cellulose. Furthermore, the alcohol amine may be any compound in which one or more alkyl alcohols are bonded to the nitrogen atom of an amine, and examples thereof include N-methylethanolamine. In other words, preferred examples of the dispersant include polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, hydroxypropyl methyl cellulose, and N-methylethanolamine.

[0037] The dispersant may be liquid or solid at room temperature, but it is more preferable that it has good compatibility with N-methyl-2-pyrrolidone.

[0038] The amount of the dispersant is preferably 0.2% by mass or more and 10.0% by mass or less, and more preferably 0.4% by mass or more and 10.0% by mass or less, relative to the total mass of the coating layer. When the amount of the dispersant is within this range, cracks are even less likely to occur in the composite layer formed on the current collector layer (coating layer).

[0039] Others In addition to the conductive additive, vinylidene fluoride copolymer, and dispersant described above, the coating layer may contain other components within a range that does not impair the objects and effects of the present invention, examples of which include nitrogen compounds such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, and ammonium compounds; organic esters, silane-based, titanium-based, and aluminum-based coupling agents; and resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN). However, the amount of these other components is preferably 10% by mass or less, and more preferably 5% by mass or less, of the total mass of the coating layer.

[0040] Physical Properties of Coating Layer The coating layer may be disposed on one surface of the substrate, or may be formed on both surfaces. The coating layer may be disposed in a pattern on one surface of the substrate, but is preferably disposed so as to entirely cover one surface or both surfaces of the substrate, from the viewpoint of easily suppressing cracking of the composite layer and reducing electrical resistance between the composite layer and the substrate.

[0041] The thickness of the coating layer on at least one surface of the substrate is preferably 0.5 μm to 10 μm, more preferably 0.8 μm to 5 μm. When the thickness of the coating layer is 0.5 μm or more, the adhesive strength between the coating layer and the composite layer tends to be good. On the other hand, when the thickness of the coating layer is 10 μm or less, the energy density of a battery using the current collector can be increased.

[0042] (3) Manufacturing Method of Current Collector The manufacturing method of the current collector of the present invention is not particularly limited as long as it is a method capable of forming a coating layer on the above-mentioned substrate, but a method including a step of applying a coating liquid to at least one surface of the above-mentioned substrate (hereinafter also referred to as an "applying step") and a step of drying the coating liquid (hereinafter also referred to as a "drying step") is preferred. Each step will be described below.

[0043] The coating liquid applied in the coating step should contain at least the conductive additive, the vinylidene fluoride copolymer, the dispersant, and the organic solvent, and may contain other components as needed. The conductive additive, the vinylidene fluoride copolymer, the dispersant, and the other components are as described above.

[0044] On the other hand, the organic solvent contained in the coating liquid may be any organic solvent capable of uniformly dispersing or dissolving the conductive additive, vinylidene fluoride copolymer, and dispersant. Examples of such organic solvents include amide compounds such as dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohols such as methanol, ethanol, isopropyl alcohol, 2-ethyl-1-hexanol, 1-nonanol, lauryl alcohol, and tripropylene glycol; amine compounds such as o-toluidine, m-toluidine, and p-toluidine; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactones such as γ-butyrolactone and δ-butyrolactone; and sulfoxide / sulfone compounds such as dimethyl sulfoxide and sulfolane. The coating liquid may contain only one organic solvent or two or more organic solvents. Among these, N-methylpyrrolidone is preferred because it can easily dissolve the vinylidene fluoride copolymer.

[0045] The amount of the organic solvent is appropriately selected depending on the type of organic solvent, the desired viscosity of the coating liquid, etc., but is usually preferably 1,000 parts by mass or more and 10,000 parts by mass or less, and more preferably 3,000 parts by mass or more and 7,000 parts by mass or less, per 100 parts by mass of the total amount of the conductive additive and vinylidene fluoride copolymer.

[0046] The method for applying the coating liquid to the substrate is not particularly limited, and methods such as doctor blade method, reverse roll method, comma bar method, gravure method, air knife method, die coating method, and dip coating method can be used.

[0047] Drying step: After application of the coating liquid, the coating liquid is heated at a desired temperature to volatilize the organic solvent and obtain a coating layer. The heating temperature is not particularly limited, and in one example, it is preferably 60°C or higher, more preferably 80°C or higher. The upper limit is appropriately selected depending on the type of coating liquid. Heating may be performed multiple times at different temperatures. The drying step may be performed under atmospheric pressure, increased pressure, or reduced pressure.

[0048] 2. Electrode The electrode of the present invention has the current collector described above and a composite layer containing an active material and a binder disposed on the current collector. The composite layer may contain an active material and a binder, and the type of binder is not particularly limited. However, it is preferable that the binder contains a vinylidene fluoride homopolymer or a vinylidene fluoride-based polymer containing a vinylidene fluoride-derived structural unit and a hexafluoropropylene-derived structural unit and / or a structural unit derived from a compound represented by general formula (2) described below. Each component of the electrode will be described below.

[0049] Active Material The active material contained in the composite layer may be a positive electrode active material or a negative electrode active material, and is appropriately selected depending on the application.

[0050] Examples of negative electrode active materials include carbon materials such as artificial graphite, natural graphite, non-graphitizable carbon, easily graphitizable carbon, activated carbon, or phenolic resin and pitch that have been fired and carbonized; metal and alloy materials such as Cu, Li, Mg, B, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Cd, Ag, Zn, Hf, Zr, and Y; and SiO, SiO ２ , GeO, GeO ２ , SnO, SnO ２ , PbO, PbO ２ , Li ４ Ti ５ O １２ The negative electrode active material may be a metal oxide such as tantalum tin oxide (LTO), etc. The negative electrode active material may be a material having a surface coated thereon, and may contain one type alone or two or more types. The negative electrode active material may be a commercially available product.

[0051] On the other hand, examples of the positive electrode active material include lithium-based positive electrode active materials, such as lithium metal oxides represented by the following general formula (a) and those coated on the surface thereof: LiM ｘ O ２...(a) In general formula (a), M represents at least one metal element including Ni, and the metal element other than Ni is preferably selected from the group consisting of Co, Al, Fe, Mn, Cr, and V. It is more preferable that in addition to Ni, one or more metals selected from the group consisting of Co, Mn, and Al are further contained. Furthermore, in the lithium metal oxide represented by the above formula (a), when the total of the metal elements constituting M is taken as 100 mol %, it is preferable that it contains 55 mol % or more of Ni, and more preferably 70 mol % or more of Ni. In the above general formula (a), 0.5≦x≦1.5, and more preferably 0.7≦x≦1.3.

[0052] Examples of the composition of the lithium-based positive electrode active material represented by the general formula (a) and other lithium-based positive electrode active materials include Li １．０ Ni ０．８ Co ０．２ O ２ , Li １．０ Ni ０．５ Mn ０．５ O ２ , Li １．００ Ni ０．３５ Co ０．３４ Mn ０．３４ O ２ (NCM111), Li １．００ Ni ０．５２ Co ０．２０ Mn ０．３０ O ２ (NCM523), Li １．００ Ni ０．５０ Co ０．３０ Mn ０．２０ O ２ (NCM532), Li １．００ Ni ０．６ Co ０．２ Mn ０．２ O ２ (NCM622), Li １．００ Ni ０．８３ Co ０．１２ Mn ０．０５ O ２ (NCM811), Li １．００ Ni ０．８５ Co ０．１５ Al ０．０５ O ２ (NCA811), LiCoO２ (LCO), LiMn ２ O ４ (LMO), LiFePO ４ (LFP), and LiMnPO ４ (LMP), LiMn １－ｘ Fe ｘ P.O. ４ (LFMP), etc. The positive electrode active material may be one whose surface is coated with these, and may contain one type alone or two or more types. The positive electrode active material may be a commercially available product.

[0053] The amount of active material contained in the composite layer is appropriately selected depending on the type, function of the electrode, type of battery, etc., and is not particularly limited, but in one example, it is preferably 50.0 mass % to 99.9 mass % of the total amount of the composite layer. When the amount of active material is within this range, for example, sufficient charge / discharge capacity can be obtained, and battery performance is likely to be good.

[0054] Binder The binder contained in the composite layer is not particularly limited as long as it can bind the active material, but as described above, it preferably contains a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer containing a structural unit derived from vinylidene fluoride and a structural unit derived from hexafluoropropylene and / or a structural unit derived from a compound represented by general formula (2) described below (hereinafter also referred to as "vinylidene fluoride polymer A"). The binder may contain only a vinylidene fluoride homopolymer, only a vinylidene fluoride polymer A, or both. When the binder contains a vinylidene fluoride polymer A, the binder may contain only one type of vinylidene fluoride polymer A, or may contain two or more types of vinylidene fluoride polymer A.

[0055] The vinylidene fluoride homopolymer usable as a binder is the same as a known vinylidene fluoride homopolymer. The method for preparing the vinylidene fluoride homopolymer is not particularly limited, and vinylidene fluoride may be polymerized by a known method. Examples of the polymerization method include suspension polymerization, emulsion polymerization, solution polymerization, etc.

[0056] On the other hand, the vinylidene fluoride polymer A may be any polymer containing a structural unit derived from vinylidene fluoride, a structural unit derived from hexafluoropropylene, and / or a structural unit derived from a compound represented by general formula (2). The vinylidene fluoride polymer A may be, for example, a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and a compound represented by general formula (2), or a copolymer of vinylidene fluoride, hexafluoropropylene, and a compound represented by general formula (2). The vinylidene fluoride polymer A may also contain other copolymerization components as long as the purpose and effects of the present invention are not impaired.

[0057] The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride polymer A is preferably 50 mol% or more, more preferably 60 mol% or more, and particularly preferably 80 mol% or more, relative to the amount (mol) of all structural units constituting the vinylidene fluoride polymer A. When the amount of vinylidene fluoride-derived structural units is 50 mol% or more, properties derived from vinylidene fluoride, such as electrochemical stability, are easily obtained. The amount of vinylidene fluoride-derived structural units in the vinylidene fluoride polymer A is １９ It can be identified by F-NMR.

[0058] On the other hand, the amount of hexafluoropropylene-derived structural units in vinylidene fluoride polymer A is more preferably 10 mol% or less relative to the amount (mol) of all structural units constituting vinylidene fluoride polymer A. When the amount of hexafluoropropylene-derived structural units is within the above range, the melting point of vinylidene fluoride polymer A tends to fall within a desired range, and the adhesive strength between vinylidene fluoride polymer A and the above-mentioned active material and the like is increased. The amount of hexafluoropropylene-derived structural units in vinylidene fluoride polymer A is １９ It can be identified by F-NMR.

[0059] Next, general formula (2) is shown below: The vinylidene fluoride polymer A may contain only one type of structural unit derived from the compound represented by the following general formula (2), or may contain two or more types. In the above general formula (2), R ４represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less). ４ is a hydrogen atom, a methyl group, or —C(═O)—OCH ３ is preferred.

[0060] Also, R ５ and R ６ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and from the viewpoint of polymerization reactivity, a substituent with little steric hindrance is preferred, hydrogen or an alkyl group having 1 to 3 carbon atoms is preferred, and hydrogen or a methyl group is preferred.

[0061] Furthermore, X in the above general formula (2) ２ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less. ２ The number of atoms in the main chain is the number of atoms constituting the shortest chain connecting the terminal carboxy group and the carbon-carbon double bond in the general formula (2). The number of atoms in the main chain is more preferably 15 or less.

[0062] Above X ２ The atomic group represented by the formula (I) is not particularly limited in structure, so long as it has a molecular weight of 500 or less and satisfies the above-mentioned number of atoms in the main chain, and may be linear, branched, or contain a ring structure. Examples of structures constituting the atomic group include an alkylene group, a carbonyl group (-C(=O)-), an ether bond (-O-), an aromatic ring, and an alicyclic structure.

[0063] Specific examples of the compound represented by the general formula (2) above include monomethyl maleate (MMM), acrylic acid (AA), methacrylic acid, carboxymethyl acrylate, carboxymethyl methacrylate, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, acryloyloxyethyl succinic acid, methacryloyloxyethyl succinic acid, acryloyloxyethyl phthalic acid, methacryloyloxyethyl phthalic acid, acryloyloxypropyl succinic acid, methacryloyloxypropyl succinic acid, and the like.

[0064] The amount of the structural units derived from the compound represented by the general formula (2) is preferably 0.01 mol% or more and 10 mol% or less, more preferably 0.02 mol% or more and 7 mol% or less, and particularly preferably 0.03 mol% or more and 4 mol% or less, relative to the amount (mol) of all structural units constituting the vinylidene fluoride polymer A. When the amount of the structural units derived from the compound represented by the general formula (2) is 0.01 mol% or more, the adhesive strength between the binder and the active material and the adhesive strength between the composite layer and the coating layer are increased. On the other hand, when the amount of the structural units derived from the compound represented by the general formula (2) is 10 mol% or less, the amount of the structural units derived from vinylidene fluoride is relatively large, and the electrochemical stability of the composite layer is likely to be improved. The amount of the structural units derived from the compound represented by the general formula (2) in the vinylidene fluoride polymer A is １９ F-NMR and １ It can be analyzed by H-NMR or the like.

[0065] Furthermore, as described above, the vinylidene fluoride polymer A may partially contain structural units derived from compounds other than vinylidene fluoride, hexafluoropropylene, and the compound represented by general formula (2) (hereinafter also referred to as "other compounds"), within a range that does not impair the objects and effects of the present invention. Specific examples of other compounds include the other compounds listed in the description of the vinylidene fluoride copolymer of the coating layer above. The amount of structural units derived from other compounds is preferably 10 mol% or less relative to the amount (mol) of all structural units constituting the vinylidene fluoride polymer A. When the amount of structural units derived from other compounds in the vinylidene fluoride polymer A is 10 mol% or less, the electrochemical stability of the composite layer is likely to be improved, and the adhesive strength between the composite layer and the coating layer is likely to be improved. The amount of structural units derived from other compounds is １９ F-NMR and １ It can be analyzed by H-NMR or the like.

[0066] The method for preparing the vinylidene fluoride polymer A is not particularly limited, and vinylidene fluoride, hexafluoropropylene and / or the compound represented by the general formula (2), and, if necessary, other compounds may be polymerized by a known method. Examples of the polymerization method include suspension polymerization, emulsion polymerization, solution polymerization, etc.

[0067] The amount of binder in the composite layer is appropriately selected depending on the binder composition, electrode function, battery type, etc., and is not particularly limited. However, in one example, it is preferably 0.1% by mass or more and 50% by mass or less of the total amount of the composite layer. When the amount of binder is within this range, the adhesive strength between the composite layer and the coating layer is improved. Furthermore, since the amount of active material is relatively sufficient, sufficient charge / discharge capacity is obtained, and battery performance is likely to be improved.

[0068] Others The composite layer may contain known dispersants, adhesive aids, thickeners, etc., as long as the purpose and effects of the present invention are not impaired. Furthermore, it may further contain nitrogen compounds such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, and ammonium compounds; organic esters, various silane-based, titanium-based, and aluminum-based coupling agents; and resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN). However, the total amount of these is preferably 15% by mass or less of the total amount of the composite layer.

[0069] The composite layer can be formed by applying a composite slurry containing the active material, binder, and optionally other components, and an organic solvent onto the coating layer of the current collector by a known method, and then drying the slurry by a known method. The organic solvent contained in the composite slurry is the same as the organic solvent used in the coating liquid.

[0070] 3. Batteries The current collectors and electrodes described above can be used in various lithium ion secondary batteries, but may also be used for other purposes.

[0071] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0072] 1. Preparation of Materials The following conductive additive, vinylidene fluoride copolymer, and dispersant were prepared.

[0073] (Conductive additive) Super-P manufactured by Timcal Japan Co., Ltd.

[0074] (Vinylidene fluoride copolymers) VDF / AA1: vinylidene fluoride-acrylic acid copolymer (prepared in Preparation Example 1 below) VDF / AA2: vinylidene fluoride-acrylic acid copolymer (prepared in Preparation Example 2 below) VDF / MMM: vinylidene fluoride-monomethyl maleate copolymer (prepared in Preparation Example 3 below) VDF / AES: vinylidene fluoride-acryloyloxyethyl succinate (prepared in Preparation Example 4 below) PVDF: vinylidene fluoride homopolymer (prepared in Preparation Example 5 below)

[0075] Preparation Example 1: A 2-liter autoclave was charged with 900 g of ion-exchanged water, 0.4 g of hydroxypropyl methylcellulose, 3.1 g of t-butyl peroxypivalate, 396 g of vinylidene fluoride, and an initial amount of 0.2 g of acrylic acid, and heated to 50°C. A 0.7 wt% aqueous acrylic acid solution containing acrylic acid was continuously fed into the reaction vessel under conditions of maintaining constant pressure during polymerization. The resulting polymer slurry was dehydrated and dried to obtain a vinylidene fluoride copolymer (VDF / AA1). A total of 3.52 g of acrylic acid, including the amount initially added, was added. The inherent viscosity, measured by the method described below, was 2.5 dL / g.

[0076] Preparation Example 2: A 2-liter autoclave was charged with 900 g of ion-exchanged water, 0.4 g of hydroxypropyl methylcellulose, 2.0 g of t-butyl peroxypivalate, 396 g of vinylidene fluoride, and an initial amount of 0.2 g of acrylic acid, and heated to 50°C. A 0.7 wt% aqueous acrylic acid solution containing acrylic acid was continuously fed into the reaction vessel under conditions of maintaining constant pressure during polymerization. The resulting polymer slurry was dehydrated and dried to obtain a vinylidene fluoride copolymer (VDF / AA2). A total of 3.08 g of acrylic acid, including the amount initially added, was added. The inherent viscosity, measured by the method described below, was 3.1 dL / g.

[0077] Preparation Example 3: A 2-liter autoclave was charged with 1,084 g of ion-exchanged water, 0.63 g of Metrose SM-100 (Shin-Etsu Chemical Co., Ltd.), 3.81 g of a 50 wt % diisopropyl peroxydicarbonate-HFE-347pc-f solution, 414 g of vinylidene fluoride (VDF), and 4.2 g of monomethyl maleate (MMM), and the temperature was raised to 29°C over 1 hour. Polymerization was then carried out while maintaining the temperature at 29°C. The resulting polymer slurry was dehydrated and dried to obtain a vinylidene fluoride copolymer (VDF / MMM). The inherent viscosity, measured by the method described below, was 2.1 dl / g.

[0078] [Preparation Example 4] A 2-liter autoclave was charged with 1092 g of ion-exchanged water, 0.2 g of Metrose 90SH-100 (Shin-Etsu Chemical Co., Ltd.), 3.6 g of a 50% by weight diisopropyl peroxydicarbonate-HFE-347pc-f solution, 5.3 g of ethyl acetate, 423 g of vinylidene fluoride, and an initial addition amount of 0.2 g of acryloyloxyethyl succinic acid (AES). The temperature was then raised to 26°C over 1 hour. Thereafter, while maintaining the temperature at 26°C, a 5% by weight aqueous acryloyloxyethyl succinic acid solution was gradually added at a rate of 0.6 g / min. The resulting polymer slurry was dehydrated and dried to obtain a vinylidene fluoride copolymer (VDF / AES). Including the amount initially added, a total of 4.2 g of acryloyloxyethyl succinic acid was added. The inherent viscosity measured by the method described below was 1.8 dl / g.

[0079] Preparation Example 5 A 2-liter autoclave was charged with 1,085 g of ion-exchanged water, 0.2 g of Metrose SM-100 (Shin-Etsu Chemical Co., Ltd.), 1.3 g of a 50 wt % diisopropyl peroxydicarbonate-HFE-347pc-f solution, 2.1 g of ethyl acetate, and 424 g of vinylidene fluoride (VDF), and the temperature was raised to 26°C over 1 hour. Polymerization was then carried out while maintaining the temperature at 26°C. The resulting polymer slurry was dehydrated and dried to obtain a vinylidene fluoride homopolymer (PVDF). The inherent viscosity, measured by the method described below, was 3.1 dl / g.

[0080] (Dispersing agent) PVP: Polyvinylpyrrolidone PVA: Polyvinyl alcohol Methyl cellulose HPMC: Hydroxypropyl methyl cellulose N-methylethanolamine

[0081] 2. Electrode Preparation (Example 1) - Coating Layer Preparation The vinylidene fluoride copolymer (VDF / AA1) prepared in Preparation Example 1 was dissolved in N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") to prepare a polymer solution containing 6 mass% of vinylidene fluoride copolymer (VDF / AA1). Subsequently, a conductive additive (Super-P manufactured by Timcal Japan Co., Ltd.), a dispersant (PVP), and NMP were added to the polymer solution to prepare a coating solution with a solids concentration of 4%. The mass ratio of the conductive additive (Super-P) to the vinylidene fluoride copolymer (VDF / AA1) in the coating solution was 50:50. The amount of dispersant (PVP) was 1 mass% relative to the total amount of the vinylidene fluoride copolymer and the conductive additive. The obtained coating liquid was applied to a 15 μm thick aluminum foil (substrate) as a current collector using a TH-C type coater manufactured by Thank Metal Co., Ltd., and dried to give a coating weight of approximately 0.7 g / m ２ A coating layer of the above was formed.

[0082] Preparation of Composite Layer The vinylidene fluoride homopolymer (PVDF) prepared in Preparation Example 5 was dissolved in NMP to prepare a polymer solution containing 6 mass% of vinylidene fluoride homopolymer. Thereafter, a dispersion of conductive additive CNT (carbon tube) (tube diameter 7 nm and specific surface area 300 m) was added to the polymer solution. ２ An NMP solution containing 4.3 mass% of MWCNT (multi-walled carbon nanotubes) (19.9 m2 / g) and NMP were added and kneaded. ２ / g) was added to adjust the solid content concentration to 55 mass %. This was then kneaded to prepare a composite slurry. The mass ratio of the positive electrode active material (LFP), conductive additive (CNT), and vinylidene fluoride homopolymer in the composite slurry was 100:2:2, in this order. The obtained composite slurry was applied to the coating layer using a TH-C coater manufactured by Thank Metals, and dried to give a coating weight of approximately 150 g / m ２ The weight of the composite layer was approximately 150 g / m ２ It was.

[0083] (Examples 2 to 11 and Comparative Examples 1 to 6) Electrodes were prepared in the same manner as in Example 1, except that the type and amount of vinylidene fluoride (co)polymer and the type and amount of dispersant in the coating solution used to form the coating layer were changed as shown in Table 1.

[0084] 3. Evaluation Measurement and evaluation of various physical properties were carried out as follows.

[0085] -Method for measuring the inherent viscosity of vinylidene fluoride (co)polymer The inherent viscosity of the above-mentioned vinylidene fluoride (co)polymer was measured as follows. First, 80 mg of vinylidene fluoride (co)polymer was dissolved in 20 mL of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a thermostatic bath at 30°C. Then, from the obtained value, the inherent viscosity (η ｉ ) was calculated. ｉ =(1 / C)・ln(η / η ０ In the above formula, η is the viscosity of the solution, η ０ is the viscosity of the solvent N,N-dimethylformamide alone, and C is the concentration of the vinylidene fluoride polymer, ie, 0.4 g / dL.

[0086] Electrode Condition After the formation of the composite layer, the composite layer was visually observed, and those with cracks were evaluated as C, those with minor cracks as B, and those with no cracks as A. The electrode was pressed with a roll press to reduce the electrode density to 2.3 g / cm. ３When the temperature was adjusted to 100°C, the cracks in the composite layer rated B were repaired and no longer observed. On the other hand, the cracks in the composite layer rated C were not repaired even when the same roll press was performed.

[0087] Peel strength measurement: After roll pressing, electrodes in which no cracks were observed (those rated A or B) were subjected to a peel strength measurement between the composite layer and the coating layer. A sample of 50 mm in length and 20 mm in width was cut out, and a 90-degree peel test was performed at a head speed of 10 mm / min using a tensile tester ("STB-1225S" manufactured by A&D Co., Ltd.) in accordance with JIS K6854-1 to measure the peel strength (gf / mm). When the current collector peeled off from the composite layer before the measurement, this was referred to as "peeling."

[0088]

[0089] As shown in Table 1 above, when the coating layer did not contain a dispersant or when the amount of conductive additive in the coating layer was too small (less than 45 mass % of the total mass of the coating layer), cracks occurred on the surface of the composite layer (Comparative Examples 1, 2, 4, and 5). This is thought to be because, during the formation of the composite layer, part of the coating layer was dissolved by the good solvent in the composite slurry, and the coating layer shrunk when re-hardened.

[0090] On the other hand, when the vinylidene fluoride polymer was a vinylidene fluoride homopolymer, cracks were unlikely to occur on the surface of the composite layer, but peeling was likely to occur at the interface between the current collector and the coating layer (Comparative Example 3). This is thought to be due to weak adhesion between the components in the coating layer and the current collector when the coating layer was a vinylidene fluoride homopolymer. Furthermore, when the amount of conductive additive in the coating layer was too high (exceeding 80% by mass of the total mass of the coating layer), cracks were unlikely to occur on the surface of the composite layer, but the composite layer was likely to peel off (Comparative Example 6). It is thought that the low resin content in the coating layer made it difficult for the coating layer (current collector) and the composite layer to adhere to each other.

[0091] On the other hand, when the coating layer contained a conductive additive, a vinylidene fluoride copolymer having a structural unit derived from the above-mentioned compound having a carboxy group (a compound represented by general formula (1)), and a dispersant, and the amount of the conductive additive was 45% by mass or more and 80% by mass or less relative to the total mass of the coating layer (Examples 1 to 11), cracks were less likely to occur in the composite layer, and adhesion between the composite layer and the coating layer (current collector) was good. It is believed that by including a conductive additive in a predetermined range and further including a dispersant, aggregation of the vinylidene fluoride copolymer described above could be suppressed. It is also believed that by setting the amount of the conductive additive to 80% by mass or less, adhesion between the coating layer (current collector) and the composite layer was easier.

[0092] This application claims priority from Japanese Patent Application No. 2023-055710, filed March 30, 2023, the entire contents of which are incorporated herein by reference.

[0093] When a composite layer is formed on the current collector to form an electrode, the current collector of the present invention is less likely to cause cracks on the surface of the electrode, making it possible to produce high-quality electrodes and batteries, and is therefore very useful for producing lithium-ion secondary batteries and the like.

Claims

1. a substrate including a metal; a coating layer disposed on at least one surface of the substrate; A current collector having the coating layer contains a conductive additive, a vinylidene fluoride copolymer, and a dispersant; The vinylidene fluoride copolymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a carboxy group, The amount of the conductive additive is 45% by mass or more and 80% by mass or less with respect to the total mass of the coating layer. Current collector.

2. The compound having a carboxy group is a compound represented by the following general formula (1): The current collector according to claim 1 . 【Chemistry 1】 (In general formula (1), R １ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less), R ２ and R ３ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, X １ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less.

3. The vinylidene fluoride copolymer of the coating layer contains a structural unit derived from vinylidene fluoride, and a structural unit derived from acrylic acid and / or a structural unit derived from monomethyl maleate. The current collector according to claim 1 .

4. The dispersant is at least one compound selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, hydroxypropyl methyl cellulose, and N-methylethanolamine. The current collector according to any one of claims 1 to 3.

5. The amount of the dispersant is 0.2 mass% or more relative to the total mass of the coating layer. The current collector according to any one of claims 1 to 3.

6. The current collector according to any one of claims 1 to 3, a composite layer disposed on the coating layer of the current collector; and and The composite layer includes an active material and a binder. electrode.

7. The binder contains a structural unit derived from vinylidene fluoride.

7. The electrode of claim 6.

8. The electrode according to claim 6, battery.

9. The method includes applying a coating liquid containing a conductive additive, a vinylidene fluoride copolymer, a dispersant, and a solvent to at least one surface of a substrate containing a metal, The vinylidene fluoride copolymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a carboxy group, The amount of the conductive assistant is 45% by mass or more and 80% by mass or less with respect to the total amount of solids in the coating liquid. A method for manufacturing a current collector.

10. The compound having a carboxy group is a compound represented by the following general formula (1): The method for producing the current collector according to claim 9 . 【Chemistry 2】 (In general formula (1), R １ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or —C(═O)—OC ｎ H ２ｎ＋１ (n is 1 or more and 5 or less), R ２ and R ３ each independently represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, X １ represents a single bond or a divalent atomic group having 1 to 20 atoms in the main chain and a molecular weight of 500 or less.

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