Binder for positive electrode of lithium-ion battery

Abstract

To provide a binder for a positive electrode of a lithium-ion battery, the binder having good binding property between a current collector and an active material layer, and good charge and discharge characteristics of the lithium-ion battery.SOLUTION: A binder for a positive electrode of a lithium-ion battery includes: core-shell particles including a core; and a shell layer located outside the core. The core includes a non-diene rubber, the shell layer includes a shell-forming polymer, and the core-shell particles include a monomer unit containing a fluorine atom. The positive electrode active material may include a lithium-containing composite oxide including nickel-manganese-cobalt or nickel-cobalt-aluminum.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a binder used in a positive electrode of a lithium ion battery, a positive electrode of a lithium ion battery and a method for producing the same, and a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries are lightweight, have high energy density, and can be repeatedly charged and discharged, making them used in a wide range of applications, such as powering computers and smartphones, and as a power source for electric and hybrid vehicles.

[0003] In the positive electrode of a lithium-ion battery, an electrode active material layer composed of an electrode active material such as a lithium-containing composite metal oxide and a binder is formed on a current collector such as a metal foil. The electrode active material layer is usually produced by coating a current collector with a slurry prepared by mixing the electrode active material, the binder, and a solvent, and drying the slurry.

[0004] The binder used in such an electrode active material layer is required to maintain the binding strength between the electrode active material and the current collector, and also to provide good charge / discharge characteristics when used in a lithium ion battery.

[0005] Polyvinylidene fluoride (PVDF) is known as a material that makes up the binder. However, since PVDF is made from chlorofluorocarbon gas and has a large environmental impact, there is a need to replace it with a material that has a lesser environmental impact.

[0006] As an example of a positive electrode binder that can replace polyvinylidene fluoride resin, Patent Document 1 discloses the use of a particulate binder made of an organic polymer. A specific example is core-shell rubber particles having a core containing acrylonitrile units and a flexible shell containing acrylic ester units (paragraph

[0018] ). In this document, lithium cobalt oxide is used as the positive electrode active material.

[0007] Known active materials for positive electrodes include lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate. However, ternary composite oxides such as NMC (nickel-manganese-cobalt) have attracted attention as active materials for the positive electrodes of lithium-ion batteries for electric vehicles because of their high capacity, high withstand voltage, and excellent thermal stability. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-117834 Summary of the Invention [Problem to be solved by the invention]

[0009] When using ternary composite oxides such as NMC (nickel-manganese-cobalt) as the active material for the positive electrode, the binder must be alkali-resistant, but binders other than polyvinylidene fluoride resin have not been fully investigated.

[0010] The core-shell particles disclosed in the aforementioned Patent Document 1 do not have alkali resistance. It has been found that when the core-shell particles are used as a binder for a ternary composite oxide, the binding strength between the current collector and the active material layer or the charge / discharge characteristics of the lithium ion battery tend to be insufficient.

[0011] In view of the above-described current situation, an object of the present invention is to provide a binder to be used in the positive electrode of a lithium ion battery, which binder has good bonding properties between a current collector and an active material layer and good charge / discharge characteristics of the lithium ion battery. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the above-mentioned object can be achieved by using core-shell particles having a core containing a non-diene rubber and a shell layer composed of a polymer, and containing fluorine atom-containing monomer units, as a binder in the positive electrode of a lithium ion battery, thereby completing the present invention.

[0013] That is, the present invention provides a binder for a positive electrode of a lithium ion battery, The core-shell particle includes a core and a shell layer located outside the core, the core comprises a non-diene rubber, the shell layer comprises a shell-forming polymer; The present invention relates to a binder for a positive electrode, wherein the core-shell particles contain fluorine atom-containing monomer units. The present invention also relates to a coating dispersion containing a positive electrode active material, the positive electrode binder, and a dispersion medium. The present invention further provides a method for producing a cathode active material, a cathode binder, and a dispersion medium, comprising: The present invention also relates to a method for producing a positive electrode of a lithium ion battery, which comprises a step of applying the dispersion onto a current collector and drying the applied dispersion. Furthermore, the present invention provides a positive electrode for a lithium ion battery, comprising a current collector and an active material layer provided on the current collector, The present invention also relates to a positive electrode, in which the active material layer contains a positive electrode active material and the positive electrode binder. The present invention also relates to a lithium ion battery comprising the positive electrode, a separator, a negative electrode, and an electrolyte solution. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a binder for use in the positive electrode of a lithium ion battery, which binder has good bonding properties between a current collector and an active material layer, and good charge / discharge characteristics of the lithium ion battery. By using the binder according to the present invention, it is possible to suitably produce a dispersion for coating a positive electrode current collector, a positive electrode for a lithium ion battery, and a lithium ion battery.

[0015] The binder according to the present invention can be particularly suitably used in a positive electrode containing a ternary composite oxide such as NMC (nickel-manganese-cobalt) as an active material. This binder is highly promising as a material to replace polyvinylidene fluoride resin. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described in detail. The positive electrode binder according to this embodiment is a binder used in the positive electrode of a lithium ion battery, and includes at least core-shell particles. The core-shell particles have a core-shell structure and include a core containing a non-diene rubber and a shell layer located outside the core. The shell layer contains a shell-forming polymer. The core-shell particles contain fluorine atom-containing monomer units, which may be contained in the non-diene rubber and / or the shell-forming polymer.

[0017] The core-shell particles having such a structure are used as a binder for the positive electrode active material of a lithium-ion battery. The use of the core-shell particles allows for the formation of a slurry containing the positive electrode active material. The slurry is then applied to a current collector surface and dried to produce a positive electrode in which an active material layer is formed on the current collector. The resulting positive electrode can exhibit good adhesion between the current collector and the active material layer, and the charge / discharge characteristics of a lithium-ion battery including the positive electrode can be improved.

[0018] (core) The core is a particle containing a non-diene rubber. The non-diene rubber refers to a rubber other than a diene rubber. The diene rubber refers to a rubber containing an aliphatic conjugated diene compound such as 1,3-butadiene as a structural unit, and specific examples thereof include butadiene rubber and styrene-butadiene rubber (SBR).

[0019] Non-diene rubbers have the advantage of being more resistant to oxidation than diene rubbers, and therefore less susceptible to oxidative degradation during charging and discharging of lithium ion batteries. Examples of the non-diene rubber include acrylic rubber and polyorganosiloxane rubber, with acrylic rubber being particularly preferred from the viewpoint of the adhesiveness between the current collector and the active material layer and the charge / discharge characteristics of the lithium ion battery.

[0020] The acrylic rubber refers to a rubber containing an acrylic monomer unit as a main constituent unit. The acrylic monomer is not particularly limited, but examples thereof include alkyl acrylates such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, dodecyl acrylate, stearyl acrylate, and behenyl acrylate; aromatic ring-containing acrylates such as phenoxyethyl acrylate and benzyl acrylate; hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate; glycidyl acrylates such as glycidyl acrylate and glycidyl alkyl acrylate; and alkoxyalkyl acrylates. Further examples include acrylic esters having a fluorine atom-containing hydrocarbon group, as described below. The acrylic monomers may be used alone or in combination of two or more. As the acrylic monomer, alkyl acrylate is preferred, and butyl acrylate is particularly preferred.

[0021] The proportion of the alkyl acrylate among the monomer components (referring to monomer components excluding polyfunctional monomers described later; the same applies hereinafter) constituting the acrylic rubber is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, from the viewpoints of the coating properties of the slurry on the current collector, the binding properties between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery. The upper limit may be 100% by weight or less.

[0022] The acrylic rubber does not necessarily need to use any other monomers than the acrylic monomers, but it is preferable to use them. Examples of such other monomers include methacrylic monomers, aromatic vinyl compounds such as styrene, vinyl cyanide compounds such as acrylonitrile, vinyl halides such as vinyl chloride, vinyl acetate, alkenes such as ethylene and propylene, etc. The use of aromatic vinyl compounds such as styrene is preferable because it facilitates the production of core-shell particles and makes it easy to adjust the glass transition temperature of the rubber to a suitable range.

[0023] From the viewpoints of the applicability of the slurry to the current collector and the charge / discharge characteristics of the lithium ion battery, the proportion of the aromatic vinyl compound among the monomer components constituting the acrylic rubber is preferably 1% by weight or more, more preferably 5% by weight or more, and even more preferably 10% by weight or more. Also, from the viewpoints of the applicability of the slurry to the current collector, the binding strength between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery, the proportion of the aromatic vinyl compound is preferably 30% by weight or less, more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less.

[0024] The non-diene rubber may contain a fluorine atom-containing monomer unit. Details of the fluorine atom-containing monomer will be described later. However, when the shell-forming polymer contains a fluorine atom-containing monomer unit, the non-diene rubber does not need to contain a fluorine atom-containing monomer unit.

[0025] The acrylic rubber has a crosslinked structure. To introduce the crosslinked structure, for example, a crosslinkable component such as a polyfunctional monomer may be used when synthesizing the acrylic rubber by polymerizing the monomer components.

[0026] Examples of the polyfunctional monomer include allyl (meth)acrylate, allyl alkyl (meth)acrylate, allyloxyalkyl (meth)acrylates, polyfunctional (meth)acrylates having two or more (meth)acrylic groups such as polyethylene glycol di(meth)acrylate, butanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate, diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, and divinylbenzene. Preferred are allyl methacrylate, triallyl isocyanurate, butanediol di(meth)acrylate, and divinylbenzene, and particularly preferred is allyl methacrylate.

[0027] The amount of the polyfunctional monomer used may be within a known range and is not particularly limited, but may be, for example, 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, more preferably 0.1 to 3 parts by weight, and even more preferably 0.2 to 1 part by weight, relative to 100 parts by weight of the total of the monomer components constituting the acrylic rubber.

[0028] The acrylic rubber may be composed of a rubber of a single composition, or may be composed of a plurality of types of rubber that differ from one another in the types or amounts of monomers and / or polyfunctional monomers.

[0029] The non-diene rubber is preferably a soft rubber from the viewpoints of the coating properties of the slurry on the current collector, the bonding properties between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery. Specifically, the glass transition temperature (Tg) of the rubber is preferably within the range of -60°C to 0°C. The lower limit is more preferably -50°C or higher, and even more preferably -40°C or higher. The upper limit is more preferably -10°C or lower, even more preferably -15°C or lower, even more preferably -20°C or lower, particularly preferably -25°C or lower, and most preferably -30°C or lower.

[0030] The glass transition temperature of the rubber can be controlled by changing the type and ratio of the monomers in the rubber. For example, the glass transition temperature of the non-diene rubber can be increased by increasing the proportion of aromatic vinyl compounds in the non-diene rubber.

[0031] The glass transition temperature can be measured using a differential scanning calorimeter (DSC).

[0032] The proportion of the non-diene rubber in the entire core-shell particles is preferably 60 to 90% by weight, from the viewpoints of the coating property of the slurry on the current collector, the binding property between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery. The lower limit is more preferably 65% ​​by weight or more, even more preferably 70% by weight or more, and particularly preferably 75% by weight or more. The upper limit is more preferably 85% by weight or less.

[0033] (shell layer) The shell layer refers to a polymer layer located on the surface side of the core-shell particle, and is also called a graft layer. The shell layer is preferably graft-bonded to the core. However, the polymer forming the shell layer also includes polymers that are not graft-bonded to the core. The shell layer covers the surface of the core, but is not limited to covering the entire surface of the core, as long as it covers at least a part of the surface of the core.

[0034] The provision of the shell layer can improve the dispersibility of the core-shell particles and the positive electrode active material in the slurry, improve the coating properties of the slurry on the current collector, and also improve the adhesion between the current collector and the active material layer, as well as the charge / discharge characteristics of the lithium-ion battery.

[0035] From the viewpoints of dispersibility of the core-shell particles and the positive electrode active material in the slurry and applicability of the slurry to the current collector, the shell layer is preferably composed of a non-crosslinked polymer. The non-crosslinked polymer refers to a polymer that does not contain a crosslinked structure or a structural unit derived from a polyfunctional monomer, and does not fall under the category of a rubber elastomer such as an acrylic rubber.

[0036] The polymer that forms the shell layer (hereinafter also referred to as the shell-forming polymer) is preferably a vinyl polymer. The monomer constituting the shell-forming polymer is not particularly limited as long as it is a vinyl-based monomer, but it preferably contains at least one selected from the group consisting of (meth)acrylic monomers, aromatic vinyl compounds, and vinyl cyanide compounds. Note that "(meth)acrylic" is a term used to collectively refer to acrylic and methacrylic.

[0037] The (meth)acrylic monomer is not particularly limited, and examples thereof include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate; aromatic ring-containing (meth)acrylates such as phenoxyethyl (meth)acrylate and benzyl (meth)acrylate; hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate; glycidyl (meth)acrylates such as glycidyl (meth)acrylate and glycidyl alkyl (meth)acrylate; alkoxy(meth)alkyl acrylates; and (meth)acrylamide. Other examples include (meth)acrylic acid esters having a fluorine atom-containing hydrocarbon group, as described below. The (meth)acrylic monomers may be used alone or in combination of two or more. As the (meth)acrylic monomer, a (meth)acrylic acid alkyl ester is preferred, and a methacrylic acid alkyl ester is particularly preferred.

[0038] The aromatic vinyl compound is not particularly limited, and examples thereof include unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; ring-alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; ring-alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; ring-halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; ring-ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and ring-hydroxylated vinyl aromatic compounds such as 4-hydroxystyrene. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred. Aromatic vinyl compounds may be used alone or in combination of two or more.

[0039] The vinyl cyanide compound is not particularly limited, but examples thereof include acrylonitrile, methacrylonitrile, etc. Among these, acrylonitrile is preferred.

[0040] The monomer constituting the shell-forming polymer may be a vinyl-based monomer other than the above-mentioned monomers. Examples of such monomers include alkenes such as ethylene and propylene; vinyl halides such as vinyl chloride and vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; and heterocycle-containing vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole.

[0041] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer and improving the dispersibility of the core-shell particles and the positive electrode active material in the slurry, the shell-forming polymer preferably contains at least a methacrylic monomer unit, and particularly preferably contains a methacrylic monomer unit and an acrylic monomer unit and / or an aromatic vinyl compound unit.

[0042] The methacrylic monomer is preferably a methacrylic acid alkyl ester. The number of carbon atoms in the alkyl of the methacrylic acid alkyl ester is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0043] The proportion of the methacrylic monomer (particularly methacrylic acid alkyl ester) units in all the monomer components (excluding the fluorine atom-containing monomer described below) constituting the shell-forming polymer is preferably 60 to 100% by weight, from the viewpoint of increasing the glass transition temperature of the shell-forming polymer. The lower limit is more preferably 70% by weight or more, and even more preferably 80% by weight or more. The upper limit is more preferably 95% by weight or less, and even more preferably 90% by weight or less, from the viewpoint of the binding strength between the current collector and the active material layer.

[0044] The proportion of the vinyl cyanide compound in the total monomer components (excluding fluorine atom-containing monomers described below) constituting the shell-forming polymer may be about 0 to 30% by weight, about 0 to 20% by weight, or about 0 to 10% by weight. The vinyl cyanide compound does not necessarily have to be contained in the shell-forming polymer.

[0045] The shell-forming polymer preferably contains a fluorine atom-containing monomer unit. The use of fluorine atom-containing monomer units can improve the alkali resistance of the core-shell particles, and can improve the adhesion between the current collector and the active material layer and the charge-discharge characteristics of lithium-ion batteries, even when a ternary composite oxide such as NMC (nickel-manganese-cobalt) is used as the positive electrode active material. The effects achieved by the fluorine atom-containing monomer units are particularly remarkable when the units are contained in the shell-forming polymer.

[0046] The fluorine atom-containing monomer is not particularly limited as long as it contains a fluorine atom and can be copolymerized with other monomers to form a shell-forming polymer, and specific examples include (meth)acrylic acid esters having a fluorine atom-containing hydrocarbon group, vinyl ethers having a fluorine atom-containing hydrocarbon group, and aromatic diene monomers having a fluorine atom.

[0047] Among these, (meth)acrylic acid esters having a fluorine atom-containing hydrocarbon group are preferred because they have good productivity for core-shell particles and are also excellent in improving binding properties and charge-discharge characteristics.

[0048] The (meth)acrylic acid ester having a fluorine atom-containing hydrocarbon group has the formula: CH2=CR 1 COOR 2 (In the formula, R 1 represents a hydrogen atom or a methyl group, and R 2 represents a hydrocarbon group containing a fluorine atom. 2 The hydrocarbon group has, for example, about 1 to 18 carbon atoms, preferably 3 to 12 carbon atoms, and more preferably 5 to 10 carbon atoms.

[0049] Examples of the (meth)acrylic acid ester having a fluorine atom-containing hydrocarbon group include fluorinated alkyl (meth)acrylates, fluorinated aryl (meth)acrylates, and fluorinated aralkyl (meth)acrylates. Specific examples of such monomers include 2,2,2-trifluoroethyl (meth)acrylate, β-(perfluorooctyl)ethyl (meth)acrylate, 2,2,3,3-tetrafluoropropyl (meth)acrylate, 2,2,3,4,4,4-hexafluorobutyl (meth)acrylate, 1H,1H,9H-perfluoro-1-nonyl (meth)acrylate, 1H,1H,11H-perfluoroundecyl (meth)acrylate, perfluorooctyl (meth)acrylate, perfluoroethyl (meth)acrylate, trifluoromethyl (meth)acrylate, 3[4[1-trifluoromethyl-2,2-bis[bis(trifluoromethyl)fluoromethyl]ethynyloxy]benzooxy]-2-hydroxypropyl (meth)acrylate, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl (meth)acrylate (CF 13 C2H4OCOCH=CH2 or C6F 13 C2H4OCOC(CH3)=CH2), 3,3,4,4,5,5,6,6,6-nonafluorohexyl (meth)acrylate (C4F9C2H4OCOCH=CH2 or C4F9C2H4OCOC(CH3)=CH2), 2-chloro-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate (C6F 13 C2H4OCOC(Cl)=CH2), 2-chloro-3,3,4,4,5,5,6,6,6-nonafluorohexyl acrylate (C4F9C2H4OCOC(Cl)=CH2), and the like.

[0050] Examples of vinyl ethers having a fluorine atom-containing hydrocarbon group include perfluoroalkylmethyl vinyl ether, 2-perfluoroalkylethyl vinyl ether, 3-perfluoropropyl vinyl ether, 3-perfluoroalkyl-1-methylpropyl vinyl ether, and 3-perfluoroalkyl-2-propenyl vinyl ether.

[0051] Examples of the aromatic diene monomer having a fluorine atom include pentafluorostyrene, 2-fluorostyrene, 4-fluorostyrene, 2-trifluoromethylstyrene, 3-trifluoromethylstyrene, 4-trifluoromethylstyrene, and 3,5-bis(trifluoromethyl)styrene.

[0052] Among the fluorine atom-containing monomers, those having a large number of fluorine atoms per molecule are preferred from the viewpoint of binding properties and charge / discharge characteristics. Specifically, a monomer having 3 or more fluorine atoms per molecule is preferred, a monomer having 5 or more is more preferred, a monomer having 8 or more is even more preferred, a monomer having 10 or more is particularly preferred, and a monomer having 13 or more is most preferred. The upper limit is not particularly limited, but may be, for example, 30 or less, or 20 or less.

[0053] From the viewpoint of the balance between the productivity of the core-shell particles and the effect obtained by using the fluorine atom-containing monomer, the amount of the fluorine atom-containing monomer used is preferably such that the ratio of the fluorine atom-containing monomer units to the total amount of the core-shell particles is 0.1 to 30% by weight, more preferably 0.5 to 25% by weight, even more preferably 1 to 20% by weight, and particularly preferably 5 to 15% by weight.

[0054] Furthermore, the proportion of fluorine atom-containing monomers (limited to fluorine atom-containing monomers contained in the monomer components of the shell-forming polymer) in all the monomer components constituting the shell-forming polymer is preferably 1 to 90% by weight, more preferably 10 to 80% by weight, even more preferably 20 to 70% by weight, and particularly preferably 30 to 60% by weight.

[0055] The shell-forming polymer is preferably hard in order to facilitate the retention of the core-shell particle shape and to enhance the dispersibility of the core-shell particles and the positive electrode active material in the slurry. Specifically, the glass transition temperature (Tg) of the shell-forming polymer is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, and particularly preferably 70°C or higher. In terms of the binding strength between the current collector and the active material layer, the upper limit is more preferably 120°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 90°C or lower.

[0056] The glass transition temperature of the shell-forming polymer can be controlled by changing the type and ratio of the monomers constituting the polymer. For example, by using a methacrylic monomer and an aromatic vinyl monomer as the monomers constituting the shell-forming polymer, the glass transition temperature of the polymer can be increased.

[0057] The proportion of the shell-forming polymer in the entire core-shell particle is preferably 10 to 40% by weight, more preferably 10 to 30% by weight, and even more preferably 15 to 25% by weight, from the viewpoints of the coating property of the slurry on the current collector, the binding property between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery.

[0058] The core-shell particle may consist of only a core and a shell layer, but may further include an intermediate layer between the core and the shell layer as long as the effects of the invention are achieved. The intermediate layer is preferably a layer composed of a polymer and is graft-bonded to the core layer. When such an intermediate layer is included, the intermediate layer covers at least a portion of the surface of the core layer, and the shell layer covers at least a portion of the surface of the intermediate layer.

[0059] (Volume average particle size of core-shell particles) The volume average particle diameter of the core-shell particles is not particularly limited and may be, for example, about 10 to 1,000 nm. However, from the viewpoints of the dispersibility of the core-shell particles in the slurry, the applicability of the slurry to the current collector, the binding between the current collector and the active material layer, and the charge / discharge characteristics of the lithium ion battery, the volume average particle diameter of the core-shell particles is preferably 100 to 500 nm. The lower limit is more preferably 150 nm or more, and even more preferably 200 nm or more. The upper limit is more preferably 400 nm or less, even more preferably 350 nm or less, and particularly preferably 300 nm or less.

[0060] The volume average particle diameter of the core-shell particles is measured in the state of a latex of the core-shell particles using a particle diameter measuring device. The particle diameter of the core-shell particles can be controlled by the types and amounts of polymerization initiators, chain transfer agents, redox agents, emulsifiers, etc. used during polymerization, polymerization temperature, polymerization time, etc.

[0061] (Method of manufacturing core-shell particles) The method for producing the core-shell particles is not particularly limited, but for example, emulsion polymerization, mini-emulsion polymerization, micro-emulsion polymerization, and emulsifier-free (soap-free) emulsion polymerization can be used.

[0062] The emulsifier that can be used in emulsion polymerization is not particularly limited, and examples thereof include anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc. Dispersants such as polyvinyl alcohol, alkyl-substituted cellulose, polyvinylpyrrolidone, and polyacrylic acid derivatives may also be used in combination.

[0063] Among the above-mentioned emulsifiers, the anionic surfactant is not particularly limited, and examples thereof include the following compounds: fatty acid soaps such as potassium laurate, potassium coconut fatty acid, potassium myristate, potassium oleate, potassium oleate diethanolamine salt, sodium oleate, potassium palmitate, potassium stearate, sodium stearate, mixed fatty acid soda soap, semi-hardened beef tallow fatty acid soda soap, and castor oil potassium soap; alkoxylated surfactants such as sodium dodecyl sulfate, higher alcohol sodium sulfate, triethanolamine dodecyl sulfate, ammonium dodecyl sulfate, sodium polyoxyethylene alkyl ether sulfate, triethanolamine polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl phenyl ether sulfate, and sodium 2-ethylhexyl sulfate; alkyl sulfate salts; sodium alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate; sodium dialkylsulfosuccinates such as sodium di-2-ethylhexyl sulfosuccinate; sodium alkylnaphthalenesulfonates; sodium alkyldiphenyletherdisulfonates; potassium alkylphosphate salts; phosphate ester salts such as sodium polyoxyethylene lauryl ether phosphate; sodium salts of naphthalenesulfonic acid formalin condensates; polycarboxylic acid type polymeric anions; sodium acyl (beef tallow) methyl taurate; sodium acyl (coconut) methyl taurate; sodium cocoyl isethionate; sodium α-sulfofatty acid ester salts; sodium amidoethersulfonate; oleyl sarcosine; sodium lauroyl sarcosine; rosin acid soap, etc.

[0064] Among the above-mentioned emulsifiers, the nonionic surfactant is not particularly limited, and examples thereof include the following compounds: polyoxyethylene alkyl allyl ethers or polyoxyethylene alkyl ethers such as polyoxyethylene nonylphenyl ether, polyoxyethylene oleyl ether, and polyoxyethylene lauryl ether; polyoxyethylene sorbitan esters such as polyoxyethylene sorbitan monolaurate and polyoxyethylene sorbitan monostearate; polyoxyethylene fatty acid esters such as polyethylene glycol monolaurate, polyethylene glycol monostearate, and polyethylene glycol monooleate; and oxyethylene / oxypropylene block copolymers.

[0065] Among the above emulsifiers, the cationic surfactant is not particularly limited, and examples thereof include the following compounds: alkylamine salts such as coconut amine acetate, stearyl amine acetate, octadecyl amine acetate, and tetradecyl amine acetate; and quaternary ammonium salts such as lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, cetyl trimethyl ammonium chloride, distearyl dimethyl ammonium chloride, alkyl benzyl dimethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, and behenyl trimethyl ammonium chloride.

[0066] Among the above emulsifiers, the amphoteric surfactant is not particularly limited, but examples thereof include the following compounds: alkyl betaines such as lauryl betaine, stearyl betaine, and dimethyl lauryl betaine; sodium lauryl diaminoethyl glycine; amido betaine; imidazoline; lauryl carboxymethyl hydroxyethyl imidazolinium betaine; and the like.

[0067] These emulsifiers may be used alone or in combination of two or more. Among the emulsifiers, sodium dialkyl sulfosuccinate or a surfactant having an oxyethylene structure is preferred, from the viewpoint of improving the fluidity of the resulting latex, and sodium polyoxyethylene lauryl ether phosphate is particularly preferred.

[0068] When emulsion polymerization is employed, known polymerization initiators, such as 2,2'-azobisisobutyronitrile, hydrogen peroxide, potassium persulfate, and ammonium persulfate, can be used as thermal decomposition initiators.

[0069] Alternatively, a redox initiator can be used which combines a peroxide such as an organic peroxide, such as t-butyl peroxyisopropyl carbonate, paramenthane hydroperoxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, or t-hexyl peroxide; or an inorganic peroxide, such as hydrogen peroxide, potassium persulfate, or ammonium persulfate, with at least one selected from the group consisting of a reducing agent, such as sodium formaldehyde sulfoxylate or glucose; a transition metal salt, such as iron (II) sulfate; a chelating agent, such as disodium ethylenediaminetetraacetate; and a phosphorus-containing compound, such as sodium pyrophosphate.

[0070] When a redox initiator is used, polymerization can be carried out even at a low temperature where the peroxide does not substantially decompose thermally, and the polymerization temperature can be set within a wide range, which is preferable. Among these, organic peroxides such as cumene hydroperoxide, dicumyl peroxide, and t-butyl hydroperoxide are preferably used as the redox initiator. The amounts of the initiator used, and when a redox initiator is used, the amounts of the reducing agent, transition metal salt, chelating agent, phosphorus-containing compound, etc. used can be within known ranges. A surfactant can also be used, which is also within known ranges.

[0071] Furthermore, when polymerizing the polyfunctional monomer, a chain transfer agent can be used. Usable chain transfer agents are not particularly limited, but examples thereof include alkyl mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, t-decyl mercaptan, n-decyl mercaptan, and n-octyl mercaptan, and alkyl ester mercaptans such as 2-ethylhexyl thioglycolate.

[0072] The solvent used during emulsion polymerization may be any solvent that allows the emulsion polymerization to proceed stably, and for example, water can be suitably used.

[0073] The temperature during emulsion polymerization is not particularly limited as long as the emulsifier is dissolved uniformly in the solvent, but is, for example, 40 to 75°C, preferably 45 to 70°C, and more preferably 49 to 65°C.

[0074] When the core-shell particles are produced by emulsion polymerization, the resulting latex can be used as a binder for a positive electrode as is, or after adjusting the concentration as necessary. Alternatively, the resulting latex can be spray-dried to obtain a powder that can be redispersed in water, and this can be used as a binder for a positive electrode.

[0075] In addition to the core-shell particles, the positive electrode binder according to this embodiment may contain components such as a conductive additive, a reinforcing material, a leveling agent, a viscosity modifier, and an electrolyte additive. These components are not particularly limited as long as they do not affect the battery reaction, and known materials or the materials described below can be used. These components may be used alone or in combination of two or more.

[0076] The proportion of the core-shell particles in the solid content of the positive electrode binder according to this embodiment is not particularly limited, and may be, for example, about 10 to 100% by weight, about 50 to 100% by weight, about 80 to 100% by weight, or about 90 to 100% by weight.

[0077] The positive electrode binder according to this embodiment may be in the form of a latex of the core-shell particles or in the form of a powder of the core-shell particles. From the viewpoint of ease of handling, the powder form is preferred.

[0078] (slurry) A dispersion liquid for a positive electrode (hereinafter referred to as a slurry) can be prepared by mixing the positive electrode binder according to this embodiment, a positive electrode active material, and a dispersion medium. The slurry can be applied to the surface of a current collector and dried to form a positive electrode active material layer on the current collector.

[0079] The amount of the core-shell particles to be added to the slurry can be appropriately determined by a person skilled in the art. However, from the viewpoints of the coating property of the slurry on the current collector, the binding property between the current collector and the positive electrode active material layer, and the charge / discharge characteristics of the lithium ion battery, the amount is preferably 0.1 to 30 parts by weight, more preferably 0.5 to 20 parts by weight, and even more preferably 1 to 10 parts by weight, per 100 parts by weight of the positive electrode active material.

[0080] <Cathode active material> Examples of electrode active materials that can be used in the positive electrode include compounds containing transition metals, such as transition metal oxides, transition metal sulfides, and composite metal oxides of lithium and transition metals. Examples of transition metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo. Examples of transition metal oxides include MnO, MnO2, V2O5, and V6O 13 , TiO2, Cu2V2O3, amorphous V2O-P2O5, amorphous MoO3, amorphous V2O5, amorphous V6O 13 etc. Examples of transition metal sulfides include TiS2, TiS3, amorphous MoS2, and FeS.

[0081] Examples of the composite metal oxide of lithium and a transition metal include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure. Examples of lithium-containing composite metal oxides having a layered structure include lithium-containing cobalt oxide (LiCoO), lithium-containing nickel oxide (LiNiO), lithium-containing composite oxides of Ni-Mn-Co, lithium-containing composite oxides of Ni-Mn-Al, lithium-containing composite oxides of Ni-Co-Al, and solid solutions of LiMaO and LiMbO. Examples of lithium-containing composite metal oxides having a spinel structure include lithium manganate (LiMn2O4) and compounds in which part of the Mn in lithium manganate (LiMn2O4) is substituted with another transition metal. Examples of lithium-containing composite metal oxides having an olivine structure include olivine-type lithium iron phosphate (LiFePO4) and olivine-type lithium manganese phosphate (LiMnPO4).

[0082] In particular, ternary lithium-containing composite oxides such as NMC (nickel-manganese-cobalt) and NCA (nickel-cobalt-aluminum) contain alkali components, and therefore, the binder used with these composite oxides is required to be alkali-resistant. The core-shell particles according to this embodiment have alkali resistance, and therefore can be suitably used as a binder used with such composite oxides.

[0083] <Conductive additive> The slurry may optionally contain a conductive aid. The conductive aid is not particularly limited, and known conductive aids can be used. Specific examples include carbon black such as acetylene black, furnace black, and Ketjen Black (registered trademark); graphite such as natural graphite and artificial graphite; carbon fibers such as polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, vapor-grown carbon fiber, carbon nanotube, and carbon nanofiber; and fibers and foils of various metals.

[0084] The amount of the conductive auxiliary agent may be set as appropriate, but is usually about 0.1 to 50 parts by weight, preferably about 0.5 to 15 parts by weight, and more preferably about 1 to 10 parts by weight, per 100 parts by weight of the positive electrode active material.

[0085] <Thickener> A thickener may be optionally blended into the slurry. The thickener is a component that can improve the dispersion stability of the positive electrode active material in the slurry and improve the coating properties of the slurry. A water-soluble polymer can be used as the thickener, and specifically, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, polyvinyl alcohol, polycarboxylic acids, salts thereof, poly(meth)acrylamide, etc. can be used. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, and alginic acid. These water-soluble polymers may be used alone or in combination of two or more. Among these, cellulose-based compounds are preferred, and carboxymethyl cellulose or its salts are particularly preferred.

[0086] The amount of thickener to be added may be set as appropriate, but is preferably 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, and even more preferably 0.5 to 3 parts by weight, per 100 parts by weight of the positive electrode active material, although the thickener may not be added.

[0087] <Other polymers> The slurry may optionally contain a polymer other than the core-shell particles and thickener described above, such as a fluorine-containing polymer or an acrylonitrile polymer.

[0088] <Dispersion medium> The slurry uses a dispersion medium such as water or an organic solvent. A mixed solvent of water and an organic solvent may be used, or a single organic solvent or a combination of several organic solvents may be used. Examples of organic solvents include alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol; alkyl ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, dioxane, and diglyme; amides such as diethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and dimethylimidazolidinone; and sulfur-based solvents such as dimethyl sulfoxide and sulfolane. Among these, amides are preferred because of their excellent dispersibility and coatability when used with the binder according to this embodiment, and N-methyl-2-pyrrolidone is particularly preferred.

[0089] The proportion of the organic solvent in the entire dispersion medium is preferably about 50 to 100% by weight, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more. When the slurry contains water, the water may include water contained in the latex when the positive electrode binder is a latex, water contained in a thickener, etc.

[0090] The solid content concentration of the slurry is not particularly limited, but may be, for example, about 10 to 80% by weight, and preferably about 30 to 70% by weight. The proportion of the positive electrode active material in the total solid content of the slurry may be about 50 to 99% by weight, preferably about 80 to 99% by weight, and more preferably about 90 to 99% by weight.

[0091] <Preparation of slurry> The slurry can be prepared by dispersing the above-mentioned components in a dispersion medium. Specifically, the slurry can be prepared by mixing the above-mentioned components with the dispersion medium using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, or Filmix. The mixing of the above-mentioned components with the dispersion medium can usually be carried out at a temperature ranging from room temperature to 80°C for 10 minutes to several hours.

[0092] (Lithium-ion battery cathode) The positive electrode for a lithium ion battery according to this embodiment includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes at least a positive electrode active material and the binder according to this embodiment. The positive electrode for a lithium ion battery according to this embodiment can be obtained by applying the above-described slurry to the current collector and drying it. As the current collector, a known metal foil may be used, for example, copper foil, aluminum foil, nickel foil, highly conductive stainless steel foil, or the like.

[0093] The core-shell particles tend to maintain their particle shape in the dried cathode active material layer, which allows the cathode active materials to be bonded to each other or to the current collector by point bonding. As a result, the presence of the binder is less likely to inhibit the movement of lithium ions, which is thought to contribute to a reduction in internal resistance.

[0094] [Coating process] The method for applying the slurry to the current collector is not particularly limited, and known methods can be used, specifically, doctor blade method, dipping method, reverse roll method, direct roll method, gravure method, extrusion method, brush coating method, etc. In this case, the slurry may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set depending on the thickness of the positive electrode active material layer obtained by drying.

[0095] [Drying process] The method for drying the slurry film on the current collector is not particularly limited, and any known method can be used, such as drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams.

[0096] After the drying step, the positive electrode active material layer may be subjected to a pressure treatment using a mold press, a roll press, etc. This can improve the adhesion between the positive electrode active material layer and the current collector and reduce the porosity of the positive electrode active material layer.

[0097] The positive electrode for a lithium ion battery according to this embodiment can also be manufactured by a powder molding method, in which the above-described slurry is first prepared, composite particles are prepared from the slurry, the composite particles are supplied onto a current collector, and optionally, roll-pressed to form a positive electrode active material layer on the current collector.

[0098] (Lithium-ion battery) The lithium ion battery according to this embodiment includes the positive electrode, negative electrode, electrolyte, and separator according to this embodiment.

[0099] <Negative electrode> The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. As the current collector, a known metal foil, such as copper foil, aluminum foil, nickel foil, or highly conductive stainless steel foil, may be used.

[0100] Examples of electrode active materials that can be used in the negative electrode include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that are a combination of these.

[0101] Examples of carbon-based negative electrode active materials include carbonaceous materials and graphite materials. Examples of carbonaceous materials include graphitizable carbon, the carbon structure of which can be easily changed by heat treatment temperature, and non-graphitizable carbon, which has a structure similar to an amorphous structure, such as glassy carbon. Examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal, such as coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor-grown carbon fiber. Examples of non-graphitizable carbon include phenolic resin baked body, polyacrylonitrile carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin baked body (PFA), and hard carbon. Examples of graphite materials include graphite such as natural graphite and artificial graphite.

[0102] Examples of metal-based negative electrode active materials include lithium metal, elemental metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.), alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. of these. Among these, active materials containing silicon (silicon-based negative electrode active materials) are preferred. The use of silicon-based negative electrode active materials can increase the capacity of lithium-ion batteries.

[0103] Examples of silicon-based negative electrode active materials include silicon (Si), silicon-containing alloys, SiO, SiOx, and composites of Si-containing materials and conductive carbon, which are obtained by coating or compounding Si-containing materials with conductive carbon. These silicon-based negative electrode active materials may be used alone or in combination of two or more. Examples of alloys containing silicon include alloy compositions containing silicon, aluminum, and a transition metal such as iron, and further containing tin and a rare earth element such as yttrium. SiOx is a compound containing at least one of SiO and SiO2, and Si, where x is usually 0.01 or more and less than 2. Examples of composites of Si-containing materials and conductive carbon include compounds obtained by heat-treating a pulverized mixture of SiO, a polymer such as polyvinyl alcohol, and optionally a carbon material in an atmosphere containing organic gas and / or steam. They can also be obtained by known methods, such as coating the surface of SiO particles with an organic gas or the like by chemical vapor deposition, or by forming composite particles (granulation) from SiO particles and graphite or artificial graphite by a mechanochemical method.

[0104] <Electrolyte> As the electrolyte, a non-aqueous electrolyte in which a supporting electrolyte is dissolved in a non-aqueous solvent can be used. A lithium salt is usually used as the supporting electrolyte. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Of these, LiPF6, LiClO4, and CF3SO3Li are preferred. These may be used alone or in combination of two or more.

[0105] The nonaqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of nonaqueous solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range. One type of nonaqueous solvent may be used alone, or two or more types may be used in combination in any ratio. The electrolyte may contain additives, such as carbonate compounds such as vinylene carbonate (VC).

[0106] Other examples of the electrolyte solution that may be used include polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel polymer electrolytes obtained by impregnating the polymer electrolyte with an electrolyte; and inorganic solid electrolytes such as LiI and LiN.

[0107] <separator> The separator is not particularly limited, but a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride), which is an insulating material, can be used.

[0108] <Lithium-ion battery manufacturing method> A specific method for manufacturing the lithium-ion battery according to this embodiment includes, for example, stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the stack according to the battery shape, placing the stack in a battery container, injecting an electrolyte into the battery container, and sealing the container. Furthermore, if necessary, expanded metal; an overcurrent protection element such as a fuse or a PTC element; or lead plates may be inserted to prevent pressure buildup within the battery and overcharging and overdischarging. The shape of the lithium-ion battery may be any of a coin type, button type, sheet type, cylindrical type, prismatic type, flat type, etc.

[0109] The following items list preferred aspects of the present disclosure, but the present invention is not limited to the following items. [Item 1] A binder for a positive electrode of a lithium ion battery, The core-shell particle includes a core and a shell layer located outside the core, the core comprises a non-diene rubber, the shell layer comprises a shell-forming polymer; The binder for a positive electrode, wherein the core-shell particles contain fluorine atom-containing monomer units. [Item 2] 2. The binder for a positive electrode according to item 1, wherein the shell-forming polymer comprises the fluorine atom-containing monomer unit. [Item 3] 3. The binder for a positive electrode according to item 1 or 2, wherein the proportion of the fluorine atom-containing monomer unit relative to the total amount of the core-shell particles is 1 to 20% by weight. [Item 4] 4. The binder for a positive electrode according to any one of items 1 to 3, wherein the fluorine atom-containing monomer has 8 or more fluorine atoms per molecule. [Item 5] 5. The binder for a positive electrode according to any one of items 1 to 4, wherein the fluorine atom-containing monomer is a (meth)acrylic acid ester having a fluorine atom-containing hydrocarbon group. [Item 6] 6. The binder for a positive electrode according to any one of items 1 to 5, wherein the non-diene rubber is an acrylic rubber containing an acrylic monomer unit. [Item 7] 7. The binder for a positive electrode according to item 6, wherein the proportion of alkyl acrylate in the monomer components constituting the acrylic rubber is 50% by weight or more. [Item 8] 8. The binder for a positive electrode according to any one of items 1 to 7, wherein the proportion of the non-diene rubber relative to the entire core-shell particles is 60 to 90% by weight. [Item 9] 9. The binder for a positive electrode according to any one of items 1 to 8, wherein the core-shell particles have a volume average particle diameter of 100 to 500 nm. [Item 10] 10. The binder for a positive electrode according to any one of items 1 to 9, wherein the shell-forming polymer contains, as a constituent monomer, at least one selected from the group consisting of a (meth)acrylic monomer, an aromatic vinyl compound, and a vinyl cyanide compound. [Item 11] 11. The binder for a positive electrode according to any one of items 1 to 10, which is in a powder form. [Item 12] 11. A coating dispersion comprising a positive electrode active material, the positive electrode binder according to any one of items 1 to 10, and a dispersion medium. [Item 13] Item 13. The coating dispersion according to item 12, wherein the positive electrode active material is a lithium-containing composite oxide containing nickel-manganese-cobalt or nickel-cobalt-aluminum. [Item 14] A step of preparing a dispersion liquid containing a positive electrode active material, the positive electrode binder according to any one of items 1 to 10, and a dispersion medium; and a step of applying the dispersion onto a current collector and drying the dispersion; [Item 15] A positive electrode for a lithium ion battery comprising a current collector and an active material layer provided on the current collector, A positive electrode, wherein the active material layer comprises a positive electrode active material and the positive electrode binder according to any one of items 1 to 10. [Item 16] Item 16. A lithium ion battery comprising the positive electrode according to item 15, a separator, a negative electrode, and an electrolyte. [Example]

[0110] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0111] Example 1 (Core particle formation) 1347 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of polyoxyethylene lauryl ether phosphate were charged into an 8 L polymerizer, heated to 80° C., and nitrogen was flowed through. A solution of 0.012 g of ferrous sulfate (FeSO4·7H2O) and 0.058 g of disodium ethylenediaminetetraacetic acid dissolved in 69.3 g of deionized water was added, and then a mixture of 720 g of butyl acrylate, 80 g of styrene, 0.8 g of t-dodecyl mercaptan, 2.0 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (solid content 69%), and 3.2 g of polyoxyethylene lauryl ether phosphate was added to the polymerizer over 240 minutes. During the polymerization, sodium hydroxide (solid content 2%) was added in an arbitrary amount at an arbitrary time to maintain the pH in the system at 5 to 7. 80 minutes after the addition was completed, polymerization was completed and core particles were formed.

[0112] (Shell layer formation) Next, 75 g of methyl methacrylate, 25 g of butyl acrylate, 100 g of tridecafluoro-n-octyl methacrylate, 1 g of sodium polyoxyethylene lauryl ether phosphate with a solids concentration of 25%, 200 g of deionized water, and 0.3 g of t-butyl hydroperoxide were stirred and emulsified, and the resulting mixture was added over 70 minutes. A shell layer was formed by adding t-butyl hydroperoxide (solid content 69%) and sodium formaldehyde sulfoxylate appropriately, and a core-shell structure graft copolymer latex was obtained with a conversion rate of 100%, a solid content concentration of 34.1%, and a volume average particle diameter of 250 nm. The volume average particle size of the latex was measured using a nanoparticle size measuring device NANOTRAC WAVE manufactured by Microtrac Corporation.

[0113] (Granulation of core-shell structured graft copolymer) The core-shell graft copolymer latex obtained above was added with an aqueous calcium chloride solution to form a slurry, which was then dehydrated in a centrifugal dehydrator, washed with deionized water, and dried at 50°C for 2 days to obtain a powder of the core-shell graft copolymer.

[0114] (Examples 2 to 6 and Comparative Example 1) A powder of core-shell graft copolymer particles was obtained in the same manner as in Example 1, except that the type or amount of the monomer for the shell layer was changed according to the description in Table 1.

[0115] (Preparation of positive electrode slurry) LiNiCoMnO2 (Ni:Co:Mn=5:2:3), acetylene black, and the powders obtained above were mixed in a mass ratio of 92:4:4, and N-methyl-2-pyrrolidone was added thereto and thoroughly kneaded to prepare a positive electrode slurry with a solid content of 57%.

[0116] (Preparation of positive electrode) The obtained positive electrode slurry was applied to a current collector made of aluminum foil with a thickness of 20 μm using a coater, and then vacuum dried for 12 hours at 80° C. The resultant was pressed using a roll press at 80° C. and 5 kN, and then punched into a disk with a diameter of 14 mm to form a positive electrode.

[0117] (Production of coin-type secondary batteries) A coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte was a 1 mol / L solution of LiPF6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. A polymer porous film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using standard methods to obtain a coin-type secondary battery (CR-2032).

[0118] (Evaluation of adhesiveness) The positive electrode prepared as described above was cut into a rectangular shape 60 mm long and 20 mm wide to prepare a test piece, and cellophane tape (specified in JIS Z1522) was attached to the surface of the positive electrode active material layer with the positive electrode active material layer facing up. One end of the current collector was pulled vertically at a pulling rate of 50 mm / min to measure the stress when peeled off. The measurement was performed three times, and the average value was calculated and used as the T-peel strength.

[0119] (Evaluation of charge / discharge cycle characteristics) The produced coin-type secondary batteries were charged 20 times at a constant current of 0.1 C in a 25°C environment until the battery voltage reached 4.3 V, and then discharged at a constant current of 0.1 C until the battery voltage reached 2 V. The ratio of the 20th discharge capacity to the 1st discharge capacity (charge / discharge capacity retention rate = (20th discharge capacity / 1st discharge capacity) × 100%) was then calculated.

[0120] The results of the evaluation described above are shown in Table 1.

[0121] [Table 1]

[0122] As shown in Table 1, Examples 1 to 6 had sufficient peel strength and exhibited excellent charge-discharge cycle characteristics.

[0123] On the other hand, Comparative Example 1, which used core-shell particles that did not contain fluorine atom-containing monomer units, exhibited poor charge-discharge cycle characteristics compared to the other Examples.

Claims

1. A binder for a positive electrode of a lithium ion battery, The core-shell particle includes a core and a shell layer located outside the core, the core comprises a non-diene rubber, the shell layer comprises a shell-forming polymer; The binder for a positive electrode, wherein the core-shell particles contain fluorine atom-containing monomer units.

2. 2. The binder for a positive electrode according to claim 1, wherein the shell-forming polymer comprises the fluorine atom-containing monomer unit.

3. 3. The positive electrode binder according to claim 1, wherein the proportion of the fluorine atom-containing monomer unit relative to the total amount of the core-shell particles is 1 to 20% by weight.

4. 3. The positive electrode binder according to claim 1, wherein the fluorine atom-containing monomer has eight or more fluorine atoms per molecule.

5. 3. The binder for a positive electrode according to claim 1, wherein the fluorine atom-containing monomer is a (meth)acrylic acid ester having a fluorine atom-containing hydrocarbon group.

6. 3. The positive electrode binder according to claim 1, wherein the non-diene rubber is an acrylic rubber containing an acrylic monomer unit.

7. 7. The binder for a positive electrode according to claim 6, wherein the proportion of alkyl acrylate in the monomer components constituting the acrylic rubber is 50% by weight or more.

8. 3. The positive electrode binder according to claim 1, wherein the proportion of the non-diene rubber relative to the total weight of the core-shell particles is 60 to 90% by weight.

9. 3. The binder for a positive electrode according to claim 1, wherein the core-shell particles have a volume average particle diameter of 100 to 500 nm.

10. 3. The binder for a positive electrode according to claim 1, wherein the shell-forming polymer contains, as a constituent monomer, at least one selected from the group consisting of a (meth)acrylic monomer, an aromatic vinyl compound, and a vinyl cyanide compound.

11. The positive electrode binder according to claim 1 or 2, which is in powder form.

12. A coating dispersion comprising a positive electrode active material, the positive electrode binder according to claim 1 or 2, and a dispersion medium.

13. 13. The coating dispersion according to claim 12, wherein the positive electrode active material is a lithium-containing composite oxide containing nickel-manganese-cobalt or nickel-cobalt-aluminum.

14. A step of preparing a dispersion liquid containing a positive electrode active material, the positive electrode binder according to claim 1 or 2, and a dispersion medium; and a step of applying the dispersion onto a current collector and drying the dispersion;

15. A positive electrode for a lithium ion battery comprising a current collector and an active material layer provided on the current collector, A positive electrode, wherein the active material layer comprises a positive electrode active material and the positive electrode binder according to claim 1 or 2.

16. A lithium ion battery comprising the positive electrode of claim 15, a separator, a negative electrode, and an electrolyte.

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