Binder for positive electrode of secondary battery

Abstract

The purpose of the present invention is to provide a binder used for a positive electrode of a secondary battery, wherein coatability of slurry to a current collector, binding properties between the current collector and an active material layer, and charge / discharge characteristics of the secondary battery are good, and swelling in the electrolyte solution is suppressed.　A binder for a positive electrode of a secondary battery includes a core, and core-shell particles including a shell layer positioned outside the core. The core contains a non-diene rubber having a glass transition temperature of -60 to +40° C, and the shell layer contains a shell-forming polymer. The core-shell particles contain a monomer unit having a hydrogen-bonding functional group, and the proportion of the monomer unit having a hydrogen-bonding functional group to the total amount of the core-shell particles is 0.6-20 wt%.

Description

Binder for positive electrodes of secondary batteries

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

[0002] Secondary batteries, especially lithium-ion batteries, are lightweight yet have high energy density and can be repeatedly charged and discharged. They are therefore used in a wide range of applications, including as power sources for personal 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 have good coatability when the slurry is applied to the current collector, and good charge / discharge characteristics when used in a lithium-ion battery.

[0005] Polyvinylidene fluoride (PVDF) is a known material for binders. However, since PVDF is made from chlorofluorocarbon gas and has a significant environmental impact, there is a need to replace it with a material that has a lower 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 described in paragraph

[0018] of a core-shell type rubber particle having a core portion containing acrylonitrile units and a high glass transition temperature, and a flexible shell portion containing acrylate units. In this document, usable electrode active materials are limited to composite oxides containing transition metals (e.g., lithium cobalt oxide).

[0007] Until now, layered rock-salt composite oxides such as lithium cobalt oxide and spinel composite oxides such as lithium manganese oxide have been widely used as positive electrode active materials for lithium-ion batteries. However, in recent years, lithium composite phosphates such as lithium iron phosphate have attracted attention from the viewpoints of availability and thermal stability.

[0008] Japanese Patent Application Laid-Open No. 2002-117834

[0009] When using a lithium composite phosphate such as lithium iron phosphate as a positive electrode active material for a secondary battery, binders other than polyvinylidene fluoride resin have not been fully investigated. Even in the above-mentioned Patent Document 1, lithium composite phosphate is not included in the usable positive electrode active materials.

[0010] The present inventors investigated core-shell rubber particles as a binder for the positive electrode of a secondary battery that could replace polyvinylidene fluoride resin. They found that the coating properties of the slurry on the current collector, the adhesion between the current collector and the active material layer, and the charge / discharge characteristics of the secondary battery may be insufficient. Furthermore, the binder tends to swell in the electrolyte. Binder swelling can increase the volume of the active material layer in the battery and can cause the active material layer to peel off.

[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 secondary battery, which has good slurry coatability onto a current collector, good bonding between the current collector and an active material layer, and good charge / discharge characteristics of the secondary battery, and which is inhibited from swelling in an electrolyte solution.

[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 having a glass transition temperature within a specific range and a shell layer composed of a polymer, and containing a specific ratio of monomer units having hydrogen-bonding functional groups, as a binder in the positive electrode of a secondary battery, thereby completing the present invention.

[0013] Specifically, the present invention relates to a positive electrode binder for a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core comprises a non-diene rubber having a glass transition temperature of −60 to +40°C, and the shell layer comprises a shell-forming polymer, and the core-shell particles comprise a monomer unit having a hydrogen-bonding functional group, and the proportion of the monomer unit having a hydrogen-bonding functional group relative to the total weight of the core-shell particles is 0.6 to 20 wt %. The present invention also relates to a coating dispersion comprising a positive electrode active material, the positive electrode binder, and a solvent. The present invention also relates to a method for producing a positive electrode for a secondary battery, comprising the steps of preparing a dispersion comprising a positive electrode active material, the positive electrode binder, and a solvent, and applying the dispersion onto a current collector and drying it. The present invention also relates to a positive electrode for a secondary battery, comprising a current collector and an active material layer provided on the current collector, the active material layer including a positive electrode active material and the positive electrode binder. The present invention also relates to a secondary battery comprising the positive electrode, a separator, a negative electrode, and an electrolyte.

[0014] According to the present invention, it is possible to provide a binder for use in the positive electrode of a secondary battery, which exhibits good slurry coatability to a current collector, good bonding between the current collector and an active material layer, and good charge / discharge characteristics of the secondary battery, and which exhibits suppressed swelling in an electrolyte. By using the binder according to the present invention, it is possible to suitably manufacture a dispersion for coating a positive electrode current collector, a positive electrode for a secondary battery, and a secondary battery. The binder according to the present invention is a binder for use in the positive electrode of a secondary battery, and is extremely promising as a material to replace polyvinylidene fluoride resin, which has a high environmental impact.

[0015]

[0023] The present invention will be described in detail below with reference to an embodiment thereof. [Binder for Positive Electrode of Secondary Battery] The binder for positive electrode according to this embodiment is a binder used in a positive electrode of a secondary battery, for example, a binder used in a positive electrode containing a positive electrode active material such as an alkali metal composite oxide, particularly a lithium composite oxide. The binder contains at least core-shell particles.

[0016] (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 includes a shell-forming polymer. The core-shell particles include a monomer unit having a hydrogen-bonding functional group. The monomer unit having a hydrogen-bonding functional group can be contained in either the non-diene rubber or the shell-forming polymer, or both. Here, rubber refers to a material having rubber elasticity. Rubber elasticity is the elasticity that can absorb energy from an external force and store it as energy for returning to its original state. To exhibit rubber elasticity, the molecules must be sufficiently long, able to move freely, and appropriately bonded to each other. A material having rubber elasticity can easily return to its original shape even if it is deformed by an external force when the external force is released.

[0017] Core-shell particles having such a structure are used as a binder for secondary batteries. Using the core-shell particles, a slurry containing a positive electrode active material can be formed. The slurry is 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 coating properties of the slurry on the current collector, the bonding strength between the current collector and the active material layer in the resulting positive electrode, and the charge / discharge characteristics of a secondary battery including the positive electrode are all excellent, and the core-shell particles are suppressed from swelling in the electrolyte of the secondary battery.

[0018] It is presumed that the presence of hydrogen-bonding functional groups in the core-shell particles changes the intermolecular interaction between the solvent contained in the electrolyte and the core-shell particles, thereby suppressing swelling of the core-shell particles.

[0019] (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 constituent unit, and specific examples thereof include butadiene rubber and styrene-butadiene rubber (SBR).

[0020] Non-diene rubbers have the advantage of being more resistant to oxidation than diene rubbers, and therefore less susceptible to oxidative degradation during charge and discharge of the secondary battery. Examples of the non-diene rubber include acrylic rubbers and polyorganosiloxane rubbers. In particular, acrylic rubbers are preferred from the viewpoints of the adhesiveness between the current collector and the active material layer and the charge and discharge characteristics of the secondary battery.

[0021] Conventional binders using acrylic rubber have a tendency to swell significantly in an electrolyte solution. However, in the present embodiment, such swelling can be suppressed by incorporating a specific proportion of a monomer having a hydrogen-bonding functional group into the core-shell particles. Therefore, in the present embodiment, core-shell particles containing acrylic rubber can be preferably used.

[0022] The acrylic rubber refers to a rubber containing an acrylic monomer unit as a main structural 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; glycidyl acrylates such as glycidyl acrylate and glycidyl alkyl acrylate; and alkoxyalkyl acrylates. Further examples include acrylic monomers having hydrogen-bonding functional groups, as described below. The acrylic monomers may be used alone or in combination of two or more. As the acrylic monomer, alkyl acrylates are preferred, and butyl acrylate is particularly preferred.

[0023] 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 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 secondary battery. The upper limit may be 100% by weight or less.

[0024] The acrylic rubber does not necessarily need to use monomers other than the acrylic monomer, 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, and alkenes such as ethylene and propylene. 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. From the viewpoint of polymerization rate, the acrylic rubber preferably uses a methacrylic monomer, and more preferably uses an alkyl methacrylate ester. The number of carbon atoms in the alkyl methacrylate ester is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0025] From the viewpoints of the coatability of the slurry on the current collector and the charge / discharge characteristics of the secondary battery, the proportion of the aromatic vinyl compound in 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 coatability of the slurry on the current collector, the binding strength between the current collector and the active material layer, and the charge / discharge characteristics of the secondary 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.

[0026] The non-diene rubber preferably contains a monomer unit having a hydrogen-bonding functional group. This can suppress swelling of the core-shell particles in the electrolyte and improve the charge-discharge characteristics of the secondary battery. However, when the shell-forming polymer described below contains such a unit, the non-diene rubber does not need to contain such a unit.

[0027] The hydrogen-bonding functional group refers to a functional group containing a hydrogen atom that can bond with a highly electronegative atom such as an oxygen atom through a hydrogen bond. Specific examples include a hydroxyl group, a carboxyl group, an amide group, a sulfonic acid group, etc., and at least one selected from the group consisting of a hydroxyl group, a carboxyl group, an amide group, and a sulfonic acid group can be used. The amide group may have a substituent on the nitrogen atom, but a primary amide group without a substituent is preferred. Only one type of hydrogen-bonding functional group may be used, or two or more types may be used in combination.

[0028] As the monomer having a hydrogen-bonding functional group, a vinyl-based monomer having a hydrogen-bonding functional group is preferred, and a (meth)acrylic-based monomer having a hydrogen-bonding functional group is more preferred.

[0029] Among the monomers having a hydrogen-bonding functional group, examples of the monomer having a hydroxyl group include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0030] Furthermore, as the monomer having a hydroxyl group, a (meth)acrylic acid ester having a (poly)alkylene glycol chain with a hydroxyl group as a terminal group can also be used. Specific examples include (poly)ethylene glycol (meth)acrylate (Blemmer PE-90, PE-200, PE-350, AE-90U, AE-200, AE-400, etc., manufactured by NOF Corp.), (poly)propylene glycol (meth)acrylate (Blemmer PP-500, PP-500D, PP-800, PP-1000, PP-2000D, AP-200, AP-400, AP-400D, AP-550, AP-800, AP-1000D, etc., manufactured by NOF Corp.), (meth)acrylic acid esters having a (poly)alkylene glycol chain with a hydroxyl group as a terminal group can also be used. p) (poly)ethylene glycol acrylate-(poly)propylene glycol (e.g., Blemmer 50PEP-300, manufactured by NOF Corp.), (poly)ethylene glycol (meth)acrylate-(poly)butylene glycol (e.g., Blemmer 55PET-800, 50PEP-500D, manufactured by NOF Corp.), (poly)propylene glycol (meth)acrylate-(poly)butylene glycol (e.g., Blemmer 10PPB-500B, 10PPB-500BD, manufactured by NOF Corp.), and the like.

[0031] Examples of the monomer having a carboxy group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, etc. Among these, acrylic acid and / or methacrylic acid are preferred.

[0032] Examples of the monomer having an amide group include (meth)acrylamide, α-ethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dimethylaminopropylacrylamide, N-methyl(meth)acrylamide, (meth)acryloylmorpholine, etc. Among these, acrylamide and / or methacrylamide are preferred.

[0033] Examples of the monomer having a sulfonic acid group include vinyl-based monomer units having a sulfonic acid group. Examples of the sulfonic acid group include styrene sulfonic acid, methallyl sulfonic acid, allyl sulfonic acid, vinyl sulfonic acid, and isoprene sulfonic acid. Among these, styrene sulfonic acid is preferred, and p-styrene sulfonic acid is more preferred, from the viewpoint of high surface activity, polymerizability, and polymerization stability, and enabling stable production of core-shell particles.

[0034] The monomer having a hydrogen-bonding functional group contained in the non-diene rubber is preferably a monomer having a hydroxyl group and / or an amide group from the viewpoint of the binding strength between the current collector and the active material layer. From the viewpoint of the charge / discharge characteristics of the secondary battery, a monomer having a carboxyl group is preferred. Furthermore, from the viewpoint of coatability and the internal resistance of the secondary battery, a monomer having a sulfonic acid group is preferred.

[0035] When the non-diene rubber contains a monomer unit having a hydrogen-bonding functional group, the proportion of the monomer having a hydrogen-bonding functional group among the monomer components constituting the non-diene rubber is preferably 1 to 30 wt % 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, the charge / discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2 wt % or more, and even more preferably 5 wt % or more, particularly from the viewpoint of the charge / discharge characteristics and the suppression of swelling. The upper limit is more preferably 20 wt % or less, even more preferably 15 wt % or less, and particularly preferably 10 wt % or less, particularly from the viewpoint of the binding property.

[0036] 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.

[0037] 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.

[0038] The amount of the polyfunctional monomer used is not particularly limited and may be within a known range, 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 2 parts by weight, relative to 100 parts by weight of the total of the monomer components constituting the acrylic rubber.

[0039] The acrylic rubber may be composed of a single rubber composition, or may be composed of a plurality of rubbers each containing different types or amounts of monomers and / or polyfunctional monomers.

[0040] As the non-diene rubber, a soft rubber is used from the viewpoints of improving 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 secondary battery. Specifically, the non-diene rubber has a glass transition temperature (Tg) in the range of -60°C to +40°C. When the Tg of the non-diene rubber is higher than 40°C, the coating properties and binding properties in particular tend to decrease. Furthermore, when the Tg of the non-diene rubber is lower than -60°C, deformation of the particle shape tends to occur, and the resistance value tends to increase.

[0041] The lower limit of the glass transition temperature of the non-diene rubber is more preferably −55° C. or higher, even more preferably −50° C. or higher, even more preferably −40° C. or higher, still more preferably −20° C. or higher, and particularly preferably 0° C. or higher. The upper limit is more preferably +35° C. or lower, even more preferably +30° C. or lower, and particularly preferably +25° C. or lower.

[0042] The glass transition temperature of the non-diene rubber can be controlled by changing the type and ratio of 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.

[0043] The glass transition temperature can be measured using a differential scanning calorimeter (DSC).

[0044] The proportion of the non-diene rubber in the entire core-shell particles is preferably 35 to 90% by weight from the viewpoints of coating the slurry onto the current collector, binding between the current collector and the active material layer, charge / discharge characteristics of the secondary battery, and suppression of swelling in the electrolyte. The lower limit is more preferably 50% by weight or more, even more preferably 60% by weight or more, and even more preferably 70% by weight or more, particularly from the viewpoint of binding. The upper limit is more preferably 85% by weight or less, even more preferably 80% by weight or less, particularly from the viewpoint of coating.

[0045] (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.

[0046] 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 secondary battery.

[0047] 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.

[0048] The polymer forming 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 monomer, but 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.

[0049] 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; glycidyl (meth)acrylates such as glycidyl (meth)acrylate and glycidyl alkyl (meth)acrylate; alkoxy(meth)alkyl acrylates; and (meth)acrylamides. Other examples include (meth)acrylic monomers having the aforementioned hydrogen-bonding functional group. The (meth)acrylic monomers may be used alone or in combination of two or more. As the (meth)acrylic monomer, (meth)acrylic acid alkyl esters are preferred, and methacrylic acid alkyl esters are particularly preferred.

[0050] 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 compounds may be used alone or in combination of two or more.

[0051] The vinyl cyanide compound is not particularly limited, but examples thereof include acrylonitrile, methacrylonitrile, etc. Among these, acrylonitrile is preferred.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] The proportion of the methacrylic monomer (particularly methacrylic acid alkyl ester) units in the entire monomer components constituting the shell-forming polymer is preferably 40 to 100% by weight from the viewpoint of increasing the glass transition temperature of the shell-forming polymer. The lower limit is more preferably 60% by weight or more, 70% by weight or more, 75% by weight or more, and 80% by weight or more, in that order. 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.

[0056] The proportion of the vinyl cyanide compound in the total monomer components 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 have to be contained in the shell-forming polymer.

[0057] The shell-forming polymer preferably contains a monomer unit having a hydrogen-bonding functional group. This suppresses swelling of the core-shell particles in the electrolyte and improves the charge-discharge characteristics of the secondary battery. However, if the non-diene rubber contains such a unit, the shell-forming polymer does not necessarily need to contain such a unit.

[0058] The hydrogen-bonding functional group is as described above. The above-mentioned compounds can also be used as the monomer having the hydrogen-bonding functional group.

[0059] The monomer having a hydrogen-bonding functional group contained in the shell-forming polymer is preferably a monomer having a hydroxyl group and / or an amide group from the viewpoint of the binding property between the current collector and the active material layer. From the viewpoint of the charge / discharge characteristics of the secondary battery, a monomer having a carboxyl group is preferred. Furthermore, from the viewpoint of coatability and the internal resistance of the secondary battery, a monomer having a sulfonic acid group is preferred.

[0060] When the shell-forming polymer contains a monomer unit having a hydrogen-bonding functional group, the proportion of the monomer having a hydrogen-bonding functional group among the monomer components constituting the shell-forming polymer is preferably 1 to 60 wt % 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, the charge / discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2 wt % or more, and even more preferably 5 wt % or more, particularly from the viewpoint of the charge / discharge characteristics and the suppression of swelling. The upper limit is more preferably 45 wt % or less, even more preferably 30 wt % or less, even more preferably 15 wt % or less, and particularly preferably 10 wt % or less, particularly from the viewpoint of the binding property.

[0061] From the viewpoint of easily maintaining the shape of the core-shell particles and improving the dispersibility of the core-shell particles and the positive electrode active material in the slurry, the shell-forming polymer is preferably hard. Specifically, the glass transition temperature (Tg) of the shell-forming polymer is preferably 40° C. or higher, more preferably 60° C. or higher, even more preferably 70° C. or higher, and particularly preferably 80° C. or higher. From the viewpoint of the binding between the current collector and the active material layer, the upper limit is 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.

[0062] 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.

[0063] The proportion of the shell-forming polymer in the entire core-shell particles is preferably 10 to 65% by weight from the viewpoints of coating the slurry onto the current collector, binding between the current collector and the active material layer, charge / discharge characteristics of the secondary battery, and suppression of swelling in the electrolyte. The upper limit is more preferably 50% by weight or less, even more preferably 40% by weight or less, and even more preferably 30% by weight or less, particularly from the viewpoint of binding. The lower limit is more preferably 15% by weight or more, even more preferably 20% by weight or more, particularly from the viewpoint of coating.

[0064] The monomer units having hydrogen-bonding functional groups may be contained in either or both of the non-diene rubber and the shell-forming polymer. Regardless of the location of the monomer units, the total content of the monomer units having hydrogen-bonding functional groups is set to a range of 0.6 to 20 wt % relative to the total amount of the core-shell particles. If the content is less than 0.6 wt %, swelling of the core-shell particles in the electrolyte solution may not be sufficiently suppressed, and the charge / discharge characteristics of the secondary battery may be insufficient. If the content is more than 20 wt %, the coating properties of the slurry on the current collector and the bonding properties between the current collector and the active material layer may be insufficient. The lower limit is preferably 1 wt % or more, more preferably 1.5 wt % or more. The upper limit is preferably 15 wt % or less, more preferably 10 wt % or less.

[0065] 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.

[0066] (Volume Average Particle Diameter 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 strength between the current collector and the active material layer, and the charge / discharge characteristics of the secondary 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.

[0067] 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.

[0068] (Method for Producing 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.

[0069] 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.

[0070] 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 naphthalenesulfonate 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.

[0071] 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.

[0072] Of 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.

[0073] 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, etc.

[0074] These emulsifiers may be used alone or in combination of two or more. Among the emulsifiers, from the viewpoint of improving the fluidity of the obtained latex, sodium dialkyl sulfosuccinate or a surfactant having an oxyethylene structure is preferred, and sodium polyoxyethylene lauryl ether phosphate is particularly preferred.

[0075] 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.

[0076] Alternatively, a redox initiator can be used in which a peroxide such as an organic peroxide, such as t-butylperoxyisopropyl 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, is used in combination 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.

[0077] 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, and this is also within known ranges.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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. Furthermore, these components may be used alone or in combination of two or more types.

[0083] 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.

[0084] 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.

[0085] (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.

[0086] The amount of the core-shell particles to be incorporated into 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 secondary 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, relative to 100 parts by weight of the positive electrode active material.

[0087] <Positive Electrode Active Material> In this embodiment, a positive electrode active material is used. By using the core-shell particles according to this embodiment as a binder for the positive electrode active material, 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 secondary battery can all be improved.

[0088] Examples of the positive electrode active material include lithium composite oxides and alkali metal composite oxides such as sodium composite oxides. Specific examples of lithium composite oxides include lithium iron phosphate (LiFePO ４ : LFP), lithium manganese phosphate (LiMnPO ４ Examples of suitable oxides include polyanion oxides containing lithium composite phosphates such as lithium cobalt phosphate and lithium iron manganese phosphate (LMFP); layered oxides such as lithium nickel manganese cobaltate and lithium cobaltate; and spinel oxides such as lithium manganate. Among these, at least one oxide selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium nickel manganese cobaltate, and lithium cobaltate is preferred, and at least one oxide selected from the group consisting of lithium iron phosphate, lithium nickel manganese cobaltate, and lithium cobaltate is more preferred. From the perspective of the charge / discharge characteristics of lithium-ion batteries, lithium iron phosphate is particularly preferred. Specific examples of sodium composite oxides include sodium manganate, sodium nickelate, sodium chromate, sodium nickel manganate, sodium nickel manganese cobaltate, sodium ferrite, and sodium iron manganate. Among these, sodium iron manganate is preferred from the perspective of battery characteristics such as capacity and cycle characteristics.

[0089] <Conductive Aid> 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.

[0090] The amount of the conductive additive 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.

[0091] <Thickener> The slurry may optionally contain a thickener. 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. As the thickener, a water-soluble polymer can be used, specifically, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, polyvinyl alcohol, polycarboxylic acid, 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.

[0092] 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. However, the thickener does not necessarily have to be added.

[0093] <Other Polymers> The slurry may optionally contain polymers other than the core-shell particles and thickeners described above. Examples of such polymers include fluorine-containing polymers and acrylonitrile polymers.

[0094] <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 they exhibit excellent dispersibility and coatability when the core-shell particles according to this embodiment are used, and N-methyl-2-pyrrolidone is particularly preferred.

[0095] 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 of core-shell particles, water contained in a thickener, etc.

[0096] The solid content of the slurry is not particularly limited, but may be, for example, about 10 to 80 wt %, preferably about 30 to 70 wt %. The proportion of the positive electrode active material in the total solid content of the slurry may be about 50 to 99 wt %, preferably about 80 to 99 wt %, and more preferably about 90 to 99 wt %.

[0097] <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.

[0098] (Positive electrode for secondary battery) The positive electrode for secondary 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 a binder according to this embodiment. The positive electrode for secondary battery according to this embodiment can be obtained by applying the above-described slurry to a current collector and drying it. Known metal foils may be used as the current collector, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0099] The core-shell particles according to this embodiment 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, it is believed that the presence of the binder makes it difficult for the movement of ions such as lithium ions to be hindered, thereby achieving a reduction in internal resistance.

[0100] [Coating Step] The method for applying the slurry 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 positive electrode active material layer obtained by drying.

[0101] [Drying Step] 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.

[0102] 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.

[0103] The positive electrode for a secondary 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.

[0104] (Secondary Battery) The secondary battery according to this embodiment includes the positive electrode, negative electrode, electrolyte, and separator according to this embodiment. Examples of the secondary battery include alkali metal ion batteries such as lithium ion batteries and sodium ion batteries. From the viewpoint of battery characteristics, including battery capacity and life characteristics, lithium ion batteries are preferred.

[0105] The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. The current collector may be a known metal foil, such as copper foil, aluminum foil, nickel foil, highly conductive stainless steel foil, or lithium foil.

[0106] 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.

[0107] Examples of carbon-based negative electrode active materials include carbonaceous materials and graphite materials. Examples of carbonaceous materials include graphitizable carbon, which easily changes its carbon structure depending on the 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. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor-grown carbon fiber. Examples of non-graphitizable carbon include phenolic resin calcined body, polyacrylonitrile-based carbon fiber, pseudoisotropic carbon, furfuryl alcohol resin calcined body (PFA), and hard carbon. Examples of graphitizable materials include graphite, such as natural graphite and artificial graphite.

[0108] As the metal-based negative electrode active material, for example, in the case of an alkali metal ion battery, the same alkali metal as the alkali metal is used. In the case of a lithium ion battery, 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.) and their alloys, as well as oxides, sulfides, nitrides, silicides, carbides, phosphides, etc., thereof can be mentioned. Among them, active materials containing silicon (silicon-based negative electrode active materials) are preferred. By using a silicon-based negative electrode active material, the capacity of the alkali ion battery can be increased.

[0109] 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 silicon-containing alloys include alloy compositions containing silicon, aluminum, and transition metals such as iron, and further containing rare earth elements such as tin and yttrium. SiOx is a composite of SiO and SiO ２ 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 an organic gas and / or steam. Furthermore, composites can also be obtained by known methods, such as a method of coating the surfaces of SiO particles by chemical vapor deposition using an organic gas or the like, or a method of forming composite particles (granulation) from SiO particles and graphite or artificial graphite by a mechanochemical method.

[0110] <Electrolyte> As the electrolyte, a non-aqueous electrolyte in which a supporting electrolyte is dissolved in a non-aqueous solvent can be used. For example, in the case of an alkali metal ion battery, a salt composed of the same alkali metal as the alkali metal is used as the supporting electrolyte. In the case of a lithium ion battery, a lithium salt is usually used, and in the case of a sodium ion battery, a sodium salt is usually used. As the lithium salt, for example, LiPF ６ , LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlCl ４ , LiClO ４ , C.F. ３ SO ３ Li, C ４ F ９ SO ３ Li, CF ３ COOLi, (CF ３ CO) ２ NLi, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ )NLi, etc. Among them, LiPF ６ , LiClO ４ , C.F. ３ SO ３ Li is preferred. These may be used alone or in combination of two or more. Examples of sodium salts include NaPF ６ , NaClO ４ , NaBCl ４ , NaSO ３ CF ３ and Na(CH ３ ) C ６ H ４ SO ３ These may be used alone or in combination of two or more.

[0111] The non-aqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of non-aqueous 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 non-aqueous solvent may be used alone, or two or more types may be used in combination in any ratio.

[0112] When carbonates are used as the non-aqueous solvent of the electrolyte, binders using conventional core-shell particles tend to swell significantly in the electrolyte. However, the core-shell particles according to the present embodiment suppress such swelling, and therefore can be suitably used to form secondary batteries such as lithium-ion batteries even when carbonates are used as the non-aqueous solvent of the electrolyte.

[0113] The electrolyte may contain additives, such as carbonate compounds such as vinylene carbonate (VC).

[0114] Other examples of the electrolyte solution include polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel polymer electrolytes obtained by impregnating the polymer electrolyte with an electrolyte; LiI, Li ３ Inorganic solid electrolytes such as N; and the like may also be used.

[0115] <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.

[0116] <Method for Manufacturing Secondary Battery> A specific method for manufacturing the secondary 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, an expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate, etc. may be inserted to prevent pressure buildup inside the battery and overcharging and discharging. The shape of the secondary battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, etc.

[0117] The following items list preferred aspects of the present disclosure, but the present invention is not limited to them. [Item 1] A binder for a positive electrode of a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, the core comprising a non-diene rubber having a glass transition temperature of -60 to +40°C, the shell layer comprising a shell-forming polymer, the core-shell particles comprising a monomer unit having a hydrogen-bonding functional group, and a ratio of the monomer unit having the hydrogen-bonding functional group to the total weight of the core-shell particles being 0.6 to 20 wt%. [Item 2] The binder for a positive electrode according to Item 1, wherein the hydrogen-bonding functional group is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, an amide group, and a sulfonic acid group. [Item 3] The binder for a positive electrode according to Item 1 or 2, wherein the non-diene rubber is an acrylic rubber containing an acrylic monomer unit. [Item 4] The positive electrode binder according to Item 3, wherein the proportion of alkyl acrylate ester among the monomer components constituting the acrylic rubber is 50% by weight or more. [Item 5] The positive electrode binder according to Item 3 or 4, wherein the acrylic rubber further contains an aromatic vinyl compound unit. [Item 6] The positive electrode binder according to any one of Items 1 to 5, wherein the proportion of the non-diene rubber relative to the total weight of the core-shell particles is 35 to 90% by weight. [Item 7] The positive electrode binder according to any one of Items 1 to 6, wherein the core-shell particles have a volume average particle diameter of 100 to 500 nm. [Item 8] The positive electrode binder according to any one of Items 1 to 7, wherein the shell-forming polymer contains at least one monomer selected from the group consisting of a (meth)acrylic monomer, an aromatic vinyl compound, and a vinyl cyanide compound as a constituent monomer. [Item 9] The positive electrode binder according to any one of Items 1 to 8, wherein the shell-forming polymer has a Tg of 40 to 120°C. [Item 10] The positive electrode binder according to any one of items 1 to 9, wherein the secondary battery is an alkali metal ion battery. [Item 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 solvent.[Item 12] The coating dispersion according to Item 11, wherein the positive electrode active material is an alkali metal composite oxide. [Item 13] A method for manufacturing a positive electrode of a secondary battery, comprising the steps of: preparing a dispersion containing a positive electrode active material, the positive electrode binder according to any one of Items 1 to 10, and a solvent; and applying the dispersion onto a current collector and drying it. [Item 14] A positive electrode of a secondary battery, comprising a current collector and an active material layer provided on the current collector, wherein the active material layer contains a positive electrode active material and the positive electrode binder according to any one of Items 1 to 10. [Item 15] A secondary battery comprising the positive electrode according to Item 14, a separator, a negative electrode, and an electrolyte.

[0118] 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" and "%" refer to "parts by weight" and "% by weight", respectively.

[0119] (Example 1) (Formation of Core Particles) 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of sodium polyoxyethylene lauryl ether phosphate were added to 1547 g of deionized water in an 8-L polymerizer, heated to 80°C, and nitrogen was flowed through. ４ ・7H ２ A solution of 0.012 g of ethylenediaminetetraacetic acid (O) and 0.058 g of disodium ethylenediaminetetraacetic acid dissolved in 69.3 g of deionized water was added, followed by the addition of a mixture of 211.9 g of methyl methacrylate, 418.1 g of butyl acrylate, 90 g of styrene, 50 g of methacrylic acid, 1 g of t-dodecyl mercaptan, 11 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (solid content 69%), and 3.2 g of sodium polyoxyethylene lauryl ether phosphate to the polymerizer over 240 minutes. During the polymerization, sodium hydroxide (solid content 2%) was added in any amount and at any time to maintain the pH of the system at 5 to 7. The polymerization was completed 80 minutes after the end of the addition, resulting in the formation of core particles.

[0120] (Shell Layer Formation) Next, a mixture of 184.1 g of methyl methacrylate, 45.9 g of butyl acrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-Butyl hydroperoxide (solid content 69%) and sodium formaldehyde sulfoxylate were appropriately added to form a shell layer, yielding a latex of core-shell structured graft copolymer particles with a conversion rate of 100%, a solid content concentration of 34.1%, and a volume average particle diameter of 200 nm. The volume average particle diameter of the latex was measured using a nanoparticle size measurement device, NANOTRAC WAVE, manufactured by Microtrac Corporation.

[0121] (Granulation of Core-Shell Graft Copolymer) A calcium chloride aqueous solution was added to the latex of the core-shell graft copolymer particles obtained above 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.

[0122] (Examples 2 to 22 and Comparative Examples 2 to 4) A latex of core-shell graft copolymer particles was obtained in the same manner as in Example 1, and a powder of core-shell graft copolymer was obtained from the latex, except that the volume average particle diameter of the core-shell particles or the types or amounts of monomers used for the core particles and shell layer were changed according to the descriptions in Table 1 or 2. The volume average particle diameter was controlled by adjusting the amount of emulsifier at the start of polymerization.

[0123] Examples 23 to 31 Latexes of core-shell graft copolymer particles were obtained in the same manner as in Example 1, except that the types or amounts of the monomers for the core particles and shell layer were changed according to the description in Table 3, and powders of the core-shell graft copolymer were obtained from the latexes.

[0124] (Comparative Examples 1 and 5) (Polymerization of Polymer Particles) 1547 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 polymerization reactor, heated to 80° C., and nitrogen was flowed through. ４ ・7H ２A solution of 0.012 g of ethylenediaminetetraacetic acid (O) and 0.058 g of disodium ethylenediaminetetraacetic acid dissolved in 69.3 g of deionized water was added, followed by addition of a mixture of 396 g of methyl methacrylate, 464 g of butyl acrylate, 90 g of styrene, 50 g of methacrylic acid, 1 g of t-dodecyl mercaptan, 15 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (solids content 69%), and 3.2 g of polyoxyethylene lauryl ether phosphate to the polymerizer over 360 minutes. During the polymerization, sodium hydroxide (solids content 2%) was added in any amount and at any time to maintain the pH of the system at 5 to 7. The polymerization was completed 80 minutes after the end of addition, yielding a non-core-shell structured copolymer latex with a conversion of 100%, a solids concentration of 34.1%, and a volume average particle size of 200 nm.

[0125] (Granulation of non-core-shell copolymer) A calcium chloride aqueous solution was added to the non-core-shell copolymer latex obtained above to form a slurry, which was then dehydrated using a centrifugal dehydrator, washed with deionized water, and dried at 50°C for 2 days to obtain a powder of the non-core-shell copolymer.

[0126] (Preparation of Positive Electrode Slurry) For Examples 1 to 22 and Comparative Examples 1 to 4, lithium iron phosphate (LFP) was used as the positive electrode active material, acetylene black was used as the conductive additive, and each of the powders obtained above was mixed in a weight 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%. A positive electrode slurry having a solid content concentration of 57% was prepared in the same manner as in Example 1 except that, for Examples 23 to 29 and Comparative Example 5, lithium nickel manganese cobalt oxide (Ni:Co:Mn=5:2:3) was used instead of lithium iron phosphate (LFP) as the positive electrode active material, for Example 30, lithium nickel manganese cobalt oxide (Ni:Co:Mn=8:1:1) was used instead of lithium iron phosphate (LFP) as the positive electrode active material, and for Example 31, lithium cobalt oxide was used instead of lithium iron phosphate (LFP) as the positive electrode active material.

[0127] (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 shape with a diameter of 14 mm to prepare a positive electrode.

[0128] (Fabrication of Coin-Type Secondary Battery) A coin-type secondary battery was constructed using the above-mentioned positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte was a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7, in which LiPF ６ The battery components were assembled and housed in an atmosphere with a dew point of −50° C. or lower in a conventional manner to obtain a coin-type secondary battery (CR-2032).

[0129] (Evaluation of Electrolyte Resistance) Each powder obtained above was pressed in a press with a 1 mm thick spacer inserted at 170°C for 10 minutes to obtain a 1 mm thick film. The obtained film was cut into a square with sides of 1 cm, and the weight was precisely weighed. The precisely weighed film was immersed in an electrolyte (ethylene carbonate / dimethyl carbonate = 3 / 7 (volume ratio)) in a test tube. The test tube containing the film and electrolyte was then left to stand at 23°C for 24 hours. The film was then removed from the electrolyte. The electrolyte adhering to the film was wiped off, and the weight of the film was precisely weighed. The electrolyte immersion swelling ratio was then calculated from the weights of the film before and after immersion in the electrolyte using the following formula. The smaller this calculated value, the less swelling of the polymer powder in the electrolyte. Electrolyte immersion swelling ratio (%) = (film weight after immersion / film weight before immersion) × 100

[0130] (Evaluation of Coatability) The surface of the dried positive electrode active material layer was visually observed, and the coatability was judged based on the following criteria: ◯: Neither streaks nor spots were observed on the surface of the positive electrode active material layer. Δ: No streaks were observed on the surface of the positive electrode active material layer, but spots were observed. ×: Streaks and spots were observed on the surface of the positive electrode active material layer.

[0131] (Evaluation of Adhesion) The positive electrode prepared as described above was cut into a rectangular shape having a length of 60 mm and a width of 20 mm 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 side facing up. One end of the current collector was pulled in the vertical direction 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.

[0132] (Evaluation of Charge-Discharge Cycle Characteristics) The produced coin-type secondary battery was charged 100 times in a 30° C. environment at a constant current of 0.3 C until the battery voltage reached 4.2 V, and then discharged at a constant current of 0.3 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.

[0133] The results of the evaluation described above are shown in Tables 1, 2, and 3.

[0134]

[0135]

[0136]

[0137] As shown in Tables 1 and 2, Examples 1 to 22, which used lithium iron phosphate (LFP) as the positive electrode active material, exhibited excellent electrolyte resistance, good coatability to the current collector, sufficient peel strength, and excellent cycle characteristics. Furthermore, as shown in Table 3, Examples 23 to 31, which used lithium nickel manganese cobalt oxide or lithium cobalt oxide as the positive electrode active material, also exhibited excellent electrolyte resistance, good coatability to the current collector, sufficient peel strength, and excellent cycle characteristics.

[0138] On the other hand, Comparative Examples 1 and 5, which used non-core-shell copolymer particles, showed poor coatability and poor charge-discharge cycle performance compared to the other Examples. Comparative Examples 2 and 3, which used core-shell graft copolymer particles that had a core-shell structure but did not contain hydrogen-bonding functional groups or whose content was less than a predetermined value, showed poor electrolyte resistance and poor charge-discharge cycle performance compared to the other Examples. Furthermore, Comparative Example 4, which used core-shell graft copolymer particles whose content of hydrogen-bonding functional groups exceeded a predetermined value, showed a low T-peel strength compared to the other Examples.

Claims

1. A binder for a positive electrode of a secondary battery, comprising core-shell particles including a core and a shell layer located on the outside of the core, wherein the core comprises a non-diene rubber having a glass transition temperature of -60 to +40°C, the shell layer comprises a shell-forming polymer, the core-shell particles comprise a monomer unit having a hydrogen-bonding functional group, and the proportion of the monomer unit having a hydrogen-bonding functional group to the total weight of the core-shell particles is 0.6 to 20 wt%.

2. The positive electrode binder according to claim 1, wherein the hydrogen-bonding functional group is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, an amide group, and a sulfonic acid group.

3. The positive electrode binder according to claim 1 or 2, wherein the non-diene rubber is an acrylic rubber containing an acrylic monomer unit.

4. The positive electrode binder according to claim 3, wherein the proportion of alkyl acrylate in the monomer components constituting the acrylic rubber is 50% by weight or more.

5. The positive electrode binder according to claim 3, wherein the acrylic rubber further contains an aromatic vinyl compound unit.

6. The positive electrode binder according to claim 1 or 2, wherein the proportion of said non-diene rubber relative to the total weight of said core-shell particles is 35 to 90% by weight.

7. The positive electrode binder according to claim 1 or 2, wherein the core-shell particles have a volume average particle diameter of 100 to 500 nm.

8. The positive electrode binder according to claim 1 or 2, wherein the shell-forming polymer contains at least one constituent monomer selected from the group consisting of (meth)acrylic monomers, aromatic vinyl compounds, and vinyl cyanide compounds.

9. The binder for a positive electrode according to claim 1 or 2, wherein the shell-forming polymer has a Tg of 40 to 120°C.

10. The positive electrode binder according to claim 1 or 2, wherein the secondary battery is an alkali metal ion battery.

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

12. The coating dispersion according to claim 11, wherein the positive electrode active material is an alkali metal complex oxide.

13. A method for manufacturing a positive electrode of a secondary battery, comprising the steps of: preparing a dispersion containing a positive electrode active material, the positive electrode binder according to claim 1 or 2, and a solvent; and applying the dispersion onto a current collector and drying it.

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

15. A secondary battery comprising the positive electrode according to claim 14, a separator, a negative electrode, and an electrolyte.