Binder for electrode of lithium ion battery

Abstract

To provide a binder for an electrode of a lithium ion battery, which has a good binding property between a current collector and an electrode active material layer, has good electrolyte resistance, and has a high capacity retention rate accompanying charge / discharge cycles.SOLUTION: A binder for an electrode of a lithium ion battery, comprising a core-shell particle including a core and a shell layer located outside the core, wherein the core is composed of a rubber, the shell layer is composed of a shell-forming polymer, the rubber contains an aliphatic conjugated diene compound as a constituent monomer, and a content ratio of the aliphatic conjugated diene compound in the entire rubber constituting the core is more than 80% by weight.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrode binder for a lithium ion battery, an electrode for a lithium ion battery and a method for manufacturing 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 negative electrode of a lithium-ion battery, an electrode active material layer consisting of an electrode active material such as graphite and a binder such as styrene / butadiene rubber (SBR) 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, binder, and solvent, and then drying the slurry.

[0004] The binder used in such an electrode active material layer is required to maintain the binding (adhesion) between the electrode active materials and between the electrode active material and the current collector. Studies are being conducted to improve the properties of secondary batteries while maintaining the binding by improving this binder.

[0005] For example, Patent Document 1 describes the use of a particulate polymer containing a specific ratio of aliphatic conjugated diene monomer units such as butadiene and carboxylic acid group-containing monomer units or (meth)acrylic acid ester monomer units as a binder for secondary battery electrodes.

[0006] Patent Document 2 describes a particulate polymer having a core-shell structure, in which the core portion is composed of a polymer containing an aliphatic conjugated diene monomer and an aromatic vinyl monomer unit, and the shell portion is composed of a polymer containing 40% by weight or more of a (meth)acrylic acid ester monomer unit, as a binder for secondary battery electrodes. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 141791 [Patent Document 2] International Publication No. 2017 / 056466 Summary of the Invention [Problem to be solved by the invention]

[0008] In lithium-ion batteries, a high capacity retention rate during charge-discharge cycling is desirable. Even when the binder was improved to obtain good adhesion between the current collector and the electrode active material layer in order to improve charge-discharge cycle characteristics, the electrolyte resistance decreased, and sufficient charge-discharge cycle characteristics could not be achieved.

[0009] In view of the above-described current situation, an object of the present invention is to provide a binder for electrodes of lithium ion batteries that has good binding properties between a current collector and an electrode active material layer, good electrolyte resistance, and a high capacity retention rate during charge-discharge cycles. [Means for solving the problem]

[0010] The present inventors conducted extensive research to solve the above-mentioned problems and discovered that the above-mentioned object can be achieved by using core-shell particles having a core composed of a specific rubber and a shell layer composed of a shell-forming polymer as a binder for electrodes of lithium ion batteries, thereby completing the present invention.

[0011] That is, the present invention relates to an electrode binder for a lithium ion battery, which comprises core-shell particles including a core and a shell layer located on the outside of the core, wherein the core is made of rubber, the shell layer is made of a shell-forming polymer, the rubber contains an aliphatic conjugated diene compound as a constituent monomer, and the content of the aliphatic conjugated diene compound in the entire rubber constituting the core exceeds 80% by weight. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a binder for electrodes of lithium ion batteries that has good binding properties between a current collector and an electrode active material layer, good electrolyte resistance, and a high capacity retention rate during charge-discharge cycles. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail. [Electrode binder] The electrode binder of the lithium ion battery according to this embodiment (hereinafter also referred to as "electrode binder") contains at least core-shell particles. The core-shell particles have a core-shell structure and include a core and a shell layer located outside the core. The core is made of a specific rubber.

[0014] The use of such core-shell particles as a binder for lithium-ion battery electrodes can achieve both good adhesion between the current collector and the electrode active material layer and good electrolyte resistance, thereby improving the capacity retention rate during charge-discharge cycles. Furthermore, the improved adhesion between the current collector and the electrode active material layer can lead to a reduction in the binder content in the lithium-ion battery electrodes.

[0015] (core) The core is a rubber particle made of rubber, and the rubber contains an aliphatic conjugated diene compound as a constituent monomer.

[0016] An aliphatic conjugated diene compound is an aliphatic compound having two carbon-carbon double bonds separated by a single bond, resulting in a conjugated diene compound. Specific examples of aliphatic conjugated diene compounds include isoprene, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Only one type of aliphatic conjugated diene compound may be used, or two or more types may be used in combination. Of these, 1,3-butadiene is preferred.

[0017] The content of the aliphatic conjugated diene compound in the entire rubber constituting the core is preferably more than 80% by weight, more preferably 90% by weight or more, more preferably 95% by weight or more, more preferably 98% by weight or more, more preferably 99% by weight or more, and most preferably 100% by weight. If it is 80% by weight or less, it is likely to swell with the electrolyte and has poor electrolyte resistance, resulting in poor capacity retention during charge-discharge cycles.

[0018] When the rubber constituting the core is a rubber containing a monomer unit other than the aliphatic conjugated diene compound, the monomer unit other than the aliphatic conjugated diene compound is not particularly limited, and examples thereof include aromatic vinyl compound units, etc. A rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit is preferred because it can improve the dispersibility of the core-shell particles in the slurry and the compatibility between the core-shell particles and the thickener, thereby improving the binding strength between the current collector and the electrode active material layer.

[0019] 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. The aromatic vinyl compound may be used alone or in combination of two or more.

[0020] Representative examples of rubbers that make up the core are butylene rubber and styrene-butadiene rubber. Butylene rubber is a polymer of butadiene and is also called BR. Styrene-butadiene rubber is a copolymer of 1,3-butadiene and styrene and is also called styrene rubber or SBR.

[0021] The rubber constituting the core may contain no vinyl monomer units other than aliphatic conjugated diene compound units, or may contain such other vinyl monomer units. Examples of such other vinyl monomers include, in addition to the aromatic vinyl compound units, (meth)acrylic acid and (meth)acrylic acid alkyl esters such as acrylic acid, methacrylic acid, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-hydroxyethyl methacrylate, and glycidyl methacrylate; and unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile. However, it is preferable that the rubber constituting the core does not contain any vinyl monomer units having a carboxyl group, such as acrylic acid, methacrylic acid, or itaconic acid.

[0022] Although the method for producing the rubber particles is not particularly limited, they are preferably synthesized by emulsion polymerization. The rubber constituting the core may be polymerized using a polyfunctional monomer such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, or 1,3-butylene dimethacrylate.

[0023] Furthermore, the rubber constituting the core may be polymerized without using a chain transfer agent, or may be polymerized in the presence of a chain transfer agent. 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.

[0024] The glass transition temperature (Tg) of the rubber is not particularly limited, but may be, for example, within the range of from −90° C. to less than −50° C. From the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity during charge / discharge cycles, the temperature is preferably −90° C. to −60° C., more preferably −90° C. to −70° C., even more preferably −90° C. to −75° C., and most preferably −90° C. to −80° C.

[0025] The glass transition temperature of the rubber can be controlled by changing the type and ratio of monomer units other than the aliphatic conjugated diene compound that can be contained in the rubber. For example, the glass transition temperature of the rubber can be increased by increasing the proportion of aromatic vinyl compounds in the rubber.

[0026] The glass transition temperature can be measured using a differential scanning calorimeter (DSC).

[0027] The content of the core in the core-shell particles is not particularly limited, but from the viewpoints of improving electrolyte resistance, improving binding properties, reducing internal resistance, and improving capacity retention rate with charge-discharge cycles, it is preferably 50 to 95% by weight, more preferably 60 to 90% by weight, and even more preferably 65 to 80% by weight.

[0028] (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 portion of the surface of the core.

[0029] The shell layer is composed of a polymer that forms the shell layer (hereinafter also referred to as a shell-forming polymer). The monomer that constitutes the shell-forming polymer is not particularly limited, but the shell-forming polymer is preferably a vinyl polymer. A vinyl polymer is obtained by homopolymerizing or copolymerizing a vinyl monomer.

[0030] The shell-forming polymer preferably contains one or more units selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units, and more preferably contains both (meth)acrylic monomer units and aromatic vinyl compound units. In the present application, the term "(meth)acrylic" refers collectively to acrylic and methacrylic.

[0031] 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; (meth)acrylonitrile, substituted (meth)acrylonitrile, and (meth)acrylamide. Among these, (meth)acrylic acid alkyl esters are preferred, and methacrylic acid alkyl esters are particularly preferred. The (meth)acrylic monomers may be used alone or in combination of two or more kinds.

[0032] 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. The aromatic vinyl compound may be used alone or in combination of two or more.

[0033] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer and reducing the internal resistance of the lithium-ion battery, the shell-forming polymer preferably contains a methacrylic acid alkyl ester unit and an aromatic vinyl compound unit. The number of carbon atoms in the alkyl of the methacrylic acid alkyl ester unit is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0034] The proportion of aromatic vinyl compound units in the entire shell-forming polymer is preferably 30 to 95% by weight, more preferably 50 to 85% by weight, and even more preferably 60 to 75% by weight, because this provides better electrolyte resistance and can improve the charge-discharge cycle characteristics of lithium ion batteries.

[0035] Furthermore, from the viewpoint of promoting the polymerization reaction of the shell-forming polymer, dispersibility of the core-shell particles in the slurry, and applicability of the slurry to a current collector, the proportion of methacrylic acid alkyl ester units in the entire shell-forming polymer is preferably 5 to 60% by weight, more preferably 8 to 50% by weight, and even more preferably 8 to 45% by weight.

[0036] Furthermore, from the viewpoint of better bonding between the current collector and the electrode active material layer and electrolyte resistance, it is preferable that the shell-forming polymer contains methacrylic acid units in addition to methacrylic acid alkyl ester units and aromatic vinyl compound units.

[0037] The upper limit of the proportion of methacrylic acid units in the entire shell-forming polymer is not particularly limited, but from the viewpoint of polymerization stability during the production of core-shell particles, it can usually be 40% by weight or less. From the viewpoint of reducing the internal resistance of lithium ion batteries, the content is preferably 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, particularly preferably 15% by weight or less, and most preferably 10% by weight or less. It may also be 9% by weight or less.

[0038] From the viewpoints of dispersibility of the core-shell particles 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 crosslinkable monomer, and does not fall under the category of a rubber elastic material (e.g., butadiene rubber, styrene-butadiene rubber, acrylic rubber, etc.).

[0039] From the viewpoint of making it easier to maintain the particle shape of the core-shell particles in the formed electrode active material layer, enabling bonding of the electrode active materials to each other or to the current collector by point bonding, improving the bonding strength, and reducing the internal resistance of the lithium ion battery, the shell-forming polymer preferably has a glass transition temperature (Tg) of 60° C. or higher, more preferably 80° C. or higher, even more preferably 90° C. or higher, and particularly preferably 100° C. or higher. The upper limit is not particularly limited, but may be, for example, 150° C. or lower, or 120° C. or lower.

[0040] 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 compound unit as the monomers constituting the shell-forming polymer, the glass transition temperature of the polymer can be increased.

[0041] The glass transition temperature can be measured using a differential scanning calorimeter (DSC).

[0042] The content of the shell layer in the core-shell particles is not particularly limited, but from the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity during charge-discharge cycles, it is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, and even more preferably 20 to 35% by weight.

[0043] 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.

[0044] (Volume average particle size of core-shell particles) The particle size of the core-shell particles is not particularly limited and may be, for example, a volume-average particle size of approximately 10 to 1,000 nm. However, from the viewpoints of dispersibility of the core-shell particles in the slurry, mixability of the core-shell particles with a thickener, and reduction of internal resistance of lithium-ion batteries, the volume-average particle size is preferably 100 nm or more, more preferably 120 to 500 nm, even more preferably 150 to 400 nm, and particularly preferably 170 to 300 nm. The volume-average particle size of the core-shell particles is measured in the state of a latex of the core-shell particles using a particle size measuring device. The particle size 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.

[0045] (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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] Alternatively, a redox initiator may be used which is a combination of a peroxide, such as an organic peroxide (e.g., 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 (e.g., hydrogen peroxide, potassium persulfate, or ammonium persulfate), with at least one selected from the group consisting of a reducing agent (e.g., sodium formaldehyde sulfoxylate, glucose), or a transition metal salt (e.g., iron(II) sulfate); a chelating agent (e.g., disodium ethylenediaminetetraacetic acid); and a phosphorus-containing compound (e.g., sodium pyrophosphate).

[0054] The use of a redox initiator is preferred because polymerization can be carried out at a low temperature where the peroxide does not substantially decompose thermally, allowing the polymerization temperature to be set over a wide range. Among these, organic peroxides such as cumene hydroperoxide, dicumyl peroxide, and t-butyl hydroperoxide are preferably used as the redox initiator. The amount of the initiator used, and the amount of the reducing agent, transition metal salt, chelating agent, phosphorus-containing compound, etc. used when a redox initiator is used, can be within known ranges. Furthermore, when polymerizing a polyfunctional monomer, known chain transfer agents can be used within known ranges. A surfactant can also be used, but this is also within known ranges.

[0055] 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.

[0056] 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.

[0057] When the core-shell particles are produced by emulsion polymerization, for example, the latex of the core-shell particles may be spray-dried to obtain a powder that can be redispersed in water, which may be used as a binder for an electrode.

[0058] The electrode binder according to the present embodiment may be in the form of a latex of the core-shell particles or a powder of the core-shell particles, and is preferably in the form of a latex of the core-shell particles because it has excellent dispersibility in a slurry.

[0059] <Other ingredients> In addition to the core-shell particles, the electrode binder according to this embodiment may contain components such as a conductive aid, 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 components, such as those described in WO 2012 / 115096, can be used. Furthermore, these components may be used alone or in combination of two or more.

[0060] <Slurry> The slurry according to this embodiment contains an electrode active material, a thickener, and the electrode binder. The slurry according to this embodiment has good coatability on the current collector surface, i.e., it can be uniformly coated without uneven coating or bumps, and has appropriate thixotropy (viscosity) to prevent excessive dripping, thereby enabling the formation of an electrode active material layer with a high surface smoothness.

[0061] <Amount of core-shell particles in the slurry> The amount of the core-shell particles in the slurry can be appropriately determined by a person skilled in the art, but from the viewpoint of improving the coating properties of the slurry on the current collector and the binding properties between the current collector and the electrode active material layer, thereby improving the charge / discharge characteristics of the lithium ion battery while reducing the internal resistance, the amount 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 2 parts by weight, per 100 parts by weight of the electrode active material.

[0062] (thickener) The thickener is a component that can improve the dispersion stability of the electrode active material in the slurry and improve the coating properties of the slurry. As the thickener, a water-soluble polymer can be used, specifically, carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, polyvinyl alcohol, polycarboxylic acid, salts thereof, poly(meth)acrylamide, etc. can be used. Examples of the polycarboxylic acid 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.

[0063] The amount of thickener in the slurry may be appropriately set, 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 electrode active material.

[0064] (Negative electrode active material) 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. Examples of carbon-based negative electrode active materials include carbonaceous materials and graphite materials.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] Examples of graphite materials include graphite such as natural graphite and artificial graphite.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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 an organic gas and / or steam. Also, composites can 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) of SiO particles with graphite or artificial graphite by a mechanochemical method.

[0074] (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.

[0075] 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.

[0076] Examples of transition metal sulfides include TiS2, TiS3, amorphous MoS2, and FeS.

[0077] 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.

[0078] 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 Co-Ni-Mn, lithium-containing composite oxides of Ni-Mn-Al, lithium-containing composite oxides of Ni-Co-Al, and solid solutions of LiMaO and LiMbO.

[0079] 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.

[0080] Examples of lithium-containing composite metal oxides having an olivine structure include olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), and olivine-type lithium manganese iron phosphate in which part of olivine-type lithium iron phosphate (LiFePO4) is replaced with manganese.

[0081] (Conductive additive) The slurry may optionally contain a conductive additive. The conductive additive is not particularly limited, and known conductive additives can be used. Specific examples include conductive carbon materials such as acetylene black, Ketjen Black (registered trademark), carbon black, and graphite; and various metal fibers and foils.

[0082] <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.

[0083] The solid content concentration of the slurry is not particularly limited, but may be, for example, about 10 to 80% by weight, and preferably 30 to 70% by weight.

[0084] <Preparation of slurry> The slurry according to this embodiment can be prepared, for example, by dispersing the above-described components in an aqueous medium. Specifically, the slurry can be prepared by mixing the above-described components with the aqueous medium using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, or Filmix. The above-described components can be mixed with the aqueous medium typically at room temperature to 80°C for 10 minutes to several hours.

[0085] Although water is usually used as the aqueous medium, it is also possible to use an aqueous solution of any compound, a mixed solution of a small amount of an organic medium and water, etc. The water contained in the slurry may include water contained in the latex when the electrode binder is a latex of core-shell particles, water contained in a thickener, etc.

[0086] [Lithium-ion battery electrodes] The lithium-ion battery electrode according to this embodiment includes a current collector and an electrode active material layer formed on the current collector. The electrode active material layer includes at least an electrode active material, a thickener, and an electrode binder containing the core-shell particles according to this embodiment. The lithium-ion battery electrode according to this embodiment can be obtained by applying the above-described slurry to the current collector and drying it.

[0087] 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.

[0088] The core-shell particles according to this embodiment tend to maintain their particle shape in the electrode active material layer after drying, which allows the electrode active materials to be bonded to each other or to the current collector by point bonding. As a result, it is believed that the presence of the binder makes it difficult for the movement of lithium ions to be hindered, thereby achieving a reduction in internal resistance.

[0089] (Coating process) The method for applying the slurry according to this embodiment to the current collector is not particularly limited, and known methods can be used. Specific examples include a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. 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 electrode active material layer obtained by drying.

[0090] (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.

[0091] After the drying step, the 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 electrode active material layer and the current collector and reduce the porosity of the electrode active material layer.

[0092] The electrode of the lithium ion battery according to this embodiment can also be manufactured by a powder molding method, in which the above-mentioned slurry is first prepared, composite particles are prepared from the slurry, the composite particles are supplied onto a current collector, and optionally, the composite particles are molded by roll pressing to form an electrode active material layer on the current collector.

[0093] The lithium ion battery electrode according to a preferred aspect of this embodiment is a negative electrode for a lithium ion battery.

[0094] [Lithium-ion battery] The lithium ion battery according to the present embodiment includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the electrode of the lithium ion battery according to the present embodiment is used for at least one of the positive electrode and the negative electrode (particularly the negative electrode). When the negative electrode is the lithium ion battery electrode according to the present embodiment, the positive electrode is not particularly limited and may be a known positive electrode.

[0095] <Electrolyte> The electrolyte may be, for example, a non-aqueous electrolyte prepared by dissolving a supporting electrolyte in a non-aqueous solvent. A lithium salt is typically 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.

[0096] 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 ethyl methyl carbonate (EMC); 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 any ratio. The electrolyte may contain an additive. Examples of additives include carbonate-based compounds such as vinylene carbonate (VC).

[0097] 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.

[0098] <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.

[0099] <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, rectangular type, flat type, etc.

[0100] 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 an electrode of a lithium ion battery, The core-shell particle includes a core and a shell layer located outside the core, the core is made of rubber, the shell layer is composed of a shell-forming polymer; The rubber contains an aliphatic conjugated diene compound as a constituent monomer, The binder for electrodes, wherein the content of the aliphatic conjugated diene compound in the entire rubber constituting the core exceeds 80% by weight. [Item 2] 2. The electrode binder according to item 1, wherein the rubber has a glass transition temperature of −90° C. or higher and lower than −50° C. [Item 3] 3. The binder for electrodes according to item 1 or 2, wherein the ratio of the core to the entire core-shell particles is 50 to 95% by weight. [Item 4] 4. The electrode binder according to any one of items 1 to 3, wherein the shell-forming polymer contains a methacrylic acid alkyl ester unit and an aromatic vinyl compound unit. [Item 5] 5. The electrode binder according to item 4, wherein the content of the aromatic vinyl compound unit in the shell-forming polymer is 30 to 95% by weight. [Item 6] 6. The binder for electrodes according to item 4 or 5, wherein the shell-forming polymer comprises methacrylic acid units. [Item 7] 7. The binder for electrodes according to any one of items 1 to 6, wherein the shell-forming polymer has a glass transition temperature of 60° C. or higher. [Item 8] 8. The electrode binder according to any one of items 1 to 7, wherein the shell-forming polymer is a non-crosslinked polymer. [Item 9] 9. The binder for an electrode according to any one of items 1 to 8, wherein the core-shell particles have a volume average particle diameter of 100 nm or more. [Item 10] 10. The electrode binder according to any one of items 1 to 9, wherein the electrode binder is a latex of the core-shell particles. [Item 11] A step of preparing a slurry containing an electrode active material, a thickener, and the electrode binder according to any one of items 1 to 10; and a step of applying the slurry to a current collector and drying the applied slurry; [Item 12] An electrode for a lithium ion battery, comprising a current collector and an electrode active material layer formed on the current collector, 11. An electrode for a lithium ion battery, wherein the electrode active material layer comprises an electrode active material, a thickener, and the electrode binder according to any one of items 1 to 10. [Item 13] Item 13. A lithium ion battery comprising the electrode according to item 12. [Example]

[0101] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, unless otherwise specified, "parts" means "parts by weight" and "%" means "% by weight."

[0102] Example 1 (Core formation) 10 g of disodium hydrogen phosphate (10% solids) was added to 90 g of deionized water, and a solution of 0.237 g of ferrous sulfate (FeSO4·7H2O) and 0.395 g of disodium ethylenediaminetetraacetic acid dissolved in 125.8 g of deionized water was then added, followed by deoxidation at -0.01 MPa for 15 minutes.

[0103] 66.7 g of Neopelex G-15 (Kao Corporation: sodium dodecylbenzenesulfonate, solid content 15.0%), 20,000 g of butadiene, and 150 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0104] 140.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 7.7 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0105] After 15 hours from the start of polymerization, the mixture was degassed under reduced pressure to remove the butadiene remaining in the mixture, thereby terminating the polymerization. A latex (solid content 45.0%) containing butadiene rubber particles was obtained.

[0106] During the polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO4·7H2O), and disodium ethylenediaminetetraacetic acid were added to a 100 L pressure autoclave in desired amounts and at desired times.

[0107] (Shell layer formation) 2500.0 g (75 parts solids) of the obtained rubber latex (45.0% solids) was charged into an 8 L polymerizer, and 60.0 g of sodium formaldehyde sulfoxylate (5% solids) was added. A mixture of 250.5 g (16.7 parts) of styrene, 93.0 g (6.2 parts) of methyl methacrylate, 31.5 g (2.1 parts) of methacrylic acid, and 0.76 g (0.035 parts) of t-butyl hydroperoxide (69% solids) was added to the polymerizer over 120 minutes.

[0108] Sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were added appropriately, and the polymerization was completed after 80 minutes, yielding a latex of core-shell structure-containing polymer particles (core-shell particles) with a solid content of 49.0% and a volume average particle diameter of 200 nm at a polymerization conversion rate of 100%.

[0109] The volume average particle size of the latex was measured using a nanoparticle size measuring device NANOTRAC WAVE manufactured by Microtrac Corporation.

[0110] (Examples 2 to 7 and Comparative Examples 3 to 4) A latex of core-shell structure-containing polymer particles (core-shell particles) was obtained in the same manner as in Example 1, except that the volume average particle diameter of the core-shell particles or the types or amounts of the monomers for the core and shell layers were changed according to the description in Table 1. The volume average particle diameter was controlled by adjusting the amount of emulsifier at the start of polymerization.

[0111] (Comparative Example 1) (Polymerization of polymer particles) 10 g of disodium hydrogen phosphate (10% solids) was added to 90 g of deionized water, and a solution of 0.237 g of ferrous sulfate (FeSO4·7H2O) and 0.395 g of disodium ethylenediaminetetraacetic acid dissolved in 125.8 g of deionized water was then added, followed by deoxidation at -0.01 MPa for 15 minutes.

[0112] 66.7 g of Neopelex G-15 (Kao Corporation: sodium dodecylbenzenesulfonate, solids content 15.0%), 19,500 g (97.5 parts) of butadiene, 500 g (2.5 parts) of methacrylic acid, and 150 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0113] 140.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 7.7 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0114] After 15 hours from the start of polymerization, the mixture was degassed under reduced pressure to remove the butadiene remaining in the polymerization, thereby terminating the polymerization. A latex (solid content 45.0%) containing non-core-shell butadiene rubber particles containing methacrylic acid units was obtained.

[0115] During the polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO4·7H2O), and disodium ethylenediaminetetraacetic acid were added to a 100 L pressure autoclave in desired amounts and at desired times.

[0116] (Comparative Example 2) A latex of non-core-shell styrene-butadiene rubber particles was obtained in the same manner as in Comparative Example 1, except that the type or amount of the monomer was changed according to the description in Table 1.

[0117] (Preparation of negative electrode slurry) One part by weight (solids equivalent) of a thickener (product name "CMC2200" manufactured by Daicel Corporation), 100 parts by weight (solids equivalent) of graphite as a negative electrode active material, and 100 parts by weight of water were added to a stirring and degassing machine (manufactured by Thinky Corporation, product name "Awatori Rentaro") and stirred and mixed for 5 minutes at 2000 rpm. Then, 1 part by weight (solids equivalent) of the polymer particles synthesized in each Example or Comparative Example was added, and the mixture was further stirred and mixed at 2000 rpm for 5 minutes, followed by degassing and mixing at 2200 rpm for 1 minute to obtain a negative electrode slurry with a solids concentration of 50%.

[0118] (Preparation of negative electrode) The negative electrode slurry was uniformly applied to the surface of a current collector made of copper foil with a thickness of 17 μm by a doctor blade method so that the film thickness after drying would be 80 μm, and the negative electrode slurry was vacuum dried at 80° C. for 6 hours. After that, the density of the negative electrode active material layer was 6.0 g / cm 2 The negative electrode was obtained by pressing the current collector with a roll press so that a negative electrode active material layer was formed on the surface of the current collector.

[0119] (Preparation of positive electrode slurry) Into a stirring / defoaming machine (manufactured by Thinky Corporation, product name "Awatori Rentaro"), 4 parts by weight (solid content equivalent) of polyvinylidene fluoride, 4 parts by weight (solid content equivalent) of acetylene black, 100 parts by weight (solid content equivalent) of LiCoO2 having a volume average particle diameter of 15 μm as a positive electrode active material, and 46 parts by weight of N-methylpyrrolidone were charged, and the mixture was stirred and mixed at 2000 rpm for 10 minutes, and then degassed and mixed at 2200 rpm for 1 minute, to obtain a positive electrode slurry with a solid content concentration of 70%.

[0120] (Preparation of positive electrode) The positive electrode slurry was uniformly applied to the surface of a current collector made of aluminum foil with a thickness of 15 μm by a doctor blade method so that the film thickness after drying would be 80 μm, and the coating was vacuum dried at 80° C. for 6 hours. After that, the density of the positive electrode active material layer was 10.0 mg / cm 2 The current collector was pressed with a roll press so that a positive electrode active material layer was formed on the surface of the current collector.

[0121] (Lithium-ion battery assembly) A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 20 μm; manufactured by a dry method; porosity 60%) was prepared and cut into a 3 cm × 3 cm square. An aluminum packaging material was also prepared as the exterior of the battery.

[0122] Then, in a glove box purged with Ar gas to maintain a dew point of -80°C or less, the positive electrode prepared as described above was cut into a 2 cm x 2 cm square and placed so that the surface on the current collector side was in contact with an aluminum packaging exterior. Next, a square separator was placed on the surface on the positive electrode active material layer side. Furthermore, the negative electrode prepared as described above was cut into a 2.2 cm x 2.2 cm square and placed on the separator so that the surface on the negative electrode active material layer side faced the separator. Then, a 1.0 M LiPF solution (solvent: a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (volume ratio), with 2 vol% vinylene carbonate (solvent ratio) added as an additive) was filled as the electrolyte. Furthermore, the aluminum packaging exterior was heat-sealed at 150°C to seal the opening, and a laminate cell-type lithium-ion secondary battery was produced.

[0123] (Evaluation of electrolyte resistance) The dried polymer particle latex synthesized in each of the above Examples and Comparative Examples was pressed in a press with a 1 mm spacer inserted at 170°C for 10 minutes to obtain a 1 mm thick film. The obtained film was cut into a 1 cm long x 1 cm wide square, and the mass was precisely weighed.

[0124] The precisely weighed film was immersed in an electrolyte solution (a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (volume ratio)) in a test tube. The test tube containing the film and electrolyte solution was left to stand at 23°C for 24 hours. The film was then removed from the electrolyte solution. The electrolyte solution adhering to the film was wiped off, and the mass of the film was precisely weighed.

[0125] Thereafter, the swelling ratio upon immersion in the electrolyte solution was calculated from the masses of the film before and after immersion in the electrolyte solution using the following formula. Swelling rate after immersion in electrolyte (%) = (film mass after immersion / film mass before immersion) x 100

[0126] (Evaluation of adhesion: peeling test of negative electrode) The negative electrode prepared as described above was cut into a rectangular shape 30 mm long and 15 mm wide to prepare a test piece, and cellophane tape (specified in JIS Z1522) was attached to the surface of the negative electrode active material layer with the surface on which the negative electrode active material layer was formed facing up. One end of the current collector was pulled in the vertical direction at a pulling rate of 37.5 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 (N / cm).

[0127] (Charge / discharge cycle characteristic test) The fabricated laminate cell type lithium ion secondary battery was charged 100 times in a 30°C environment at a constant current of 1.0 C until the battery voltage reached 4.2 V, and then discharged at a constant current of 1.0 C until the battery voltage reached 2 V. The ratio of the 100th discharge capacity to the first discharge capacity (charge / discharge capacity retention rate = (100th discharge capacity / 1st discharge capacity) × 100%) was then calculated.

[0128] The results of the evaluation described above are shown in Table 1.

[0129] [Table 1]

[0130] As shown in Table 1, Examples 1 to 7 exhibited low electrolyte immersion swelling rates, excellent electrolyte resistance, sufficient T-peel strength, and maintained good adhesion between the current collector and the electrode active material layer, while also exhibiting high charge / discharge capacity retention rates and excellent cycle characteristics.

[0131] On the other hand, Comparative Example 1, which is a non-core-shell structure copolymer, was superior in electrolyte resistance compared to each of the Examples, but the T-peel strength was very low and the binding strength between the current collector and the electrode active material layer was poor, resulting in poor charge-discharge cycle characteristics.

[0132] In Comparative Example 2, which was a non-core-shell styrene-butadiene copolymer, the electrolyte immersion swelling ratio was high and the electrolyte resistance was poor compared to the other Examples, and the T-peel strength was low, resulting in poor adhesion between the current collector and the electrode active material layer, and therefore the charge-discharge cycle characteristics were poor.

[0133] Comparative Examples 3 and 4, which used core-shell particles in which the butadiene content in the entire rubber constituting the core was outside the specified range, had a higher swelling rate when immersed in electrolyte, poorer electrolyte resistance, and poorer charge-discharge cycle characteristics than the other Examples.

Claims

1. A binder for an electrode of a lithium ion battery, The core-shell particle includes a core and a shell layer located outside the core, the core is made of rubber, the shell layer is composed of a shell-forming polymer; The rubber contains an aliphatic conjugated diene compound as a constituent monomer, The binder for electrodes, wherein the content of the aliphatic conjugated diene compound in the entire rubber constituting the core exceeds 80% by weight.

2. 2. The electrode binder according to claim 1, wherein the rubber has a glass transition temperature of −90° C. or higher and lower than −50° C.

3. 3. The electrode binder according to claim 1, wherein the ratio of the core to the entire core-shell particles is 50 to 95% by weight.

4. 3. The electrode binder according to claim 1, wherein the shell-forming polymer contains methacrylic acid alkyl ester units and aromatic vinyl compound units.

5. 5. The electrode binder according to claim 4, wherein the content of the aromatic vinyl compound unit in the shell-forming polymer is 30 to 95% by weight.

6. 5. The electrode binder according to claim 4, wherein the shell-forming polymer comprises methacrylic acid units.

7. 3. The electrode binder according to claim 1, wherein the shell-forming polymer has a glass transition temperature of 60°C or higher.

8. 3. The electrode binder according to claim 1, wherein the shell-forming polymer is a non-crosslinked polymer.

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

10. The electrode binder according to claim 1 or 2, wherein the electrode binder is a latex of the core-shell particles.

11. A step of preparing a slurry containing an electrode active material, a thickener, and the electrode binder according to claim 1 or 2; and a step of applying the slurry to a current collector and drying the applied slurry;

12. An electrode for a lithium ion battery, comprising a current collector and an electrode active material layer formed on the current collector, An electrode for a lithium ion battery, wherein the electrode active material layer comprises an electrode active material, a thickener, and the electrode binder according to claim 1 or 2.

13. A lithium ion battery comprising the electrode of claim 12.

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