Binder composition for lithium composite phosphorus oxide-containing positive electrode

Abstract

This binder composition for a lithium composite phosphorous oxide-containing positive electrode contains an aqueous medium and core-shell particles. The core-shell particles are core-shell particles 1 or core-shell particles 2 described later.　The core-shell particle 1 includes a core (C1) and a shell layer (S1) positioned outside the core (C1). The core (C1) includes a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and lower than 40° C. The shell layer (S1) includes a shell-forming polymer (SP1) having a glass transition temperature of 40-120° C.　The core-shell particle 2 includes a core (C2) and a shell layer (S2) positioned outside the core (C2). The core (C2) is composed of a polymer, the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) contains an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers.

Description

Lithium-compound phosphoroxide-containing binder composition for cathodes

[0001] This disclosure relates to a lithium-compound phosphoroxide-containing binder composition for cathodes and to a cathode for lithium-ion batteries.

[0002] Lithium-ion batteries are lightweight, have high energy density, and can be repeatedly charged and discharged, making them suitable for a wide range of applications, including power sources for personal computers and smartphones, and power sources for electric and hybrid vehicles.

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

[0004] The binder used in such electrode active material layers is required to maintain the binding properties between the electrode active material and the current collector. Furthermore, it is also required to have good coating properties when applying the slurry to the current collector, and good charge-discharge characteristics when forming a lithium-ion battery.

[0005] Polyvinylidene fluoride resin (PVDF) is known as a binder material, and N-methyl-2-pyrrolidone (NMP) is known as a solvent used in slurry preparation. However, polyvinylidene fluoride resin is made from chlorofluorocarbons (CFCs), which have a significant environmental impact, and N-methyl-2-pyrrolidone has been reported to have reproductive toxicity. Therefore, there is a desire to replace these materials with those that have less environmental impact and cause less health concern.

[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. As a specific example, a core-shell type rubber particle having a core portion containing acrylonitrile units with a high glass transition temperature and a flexible shell portion containing acrylic acid ester units is described (paragraph

[0018] ). In this document, the usable electrode active material is limited to a composite oxide containing a transition metal (e.g., lithium cobalt oxide).

[0007] Until now, layered rock salt-type composite oxides such as lithium cobaltate and spinel-type composite oxides such as lithium manganate have been widely used as cathode active materials. However, in recent years, lithium composite phosphorus oxides such as lithium iron phosphate have attracted attention from the standpoint of availability and thermal stability.

[0008] Japanese Patent Publication No. 2002-117834

[0009] When using lithium composite phosphoroxides such as lithium iron phosphate as the active material for the cathode, binders other than polypyrinide fluoride resin have not been sufficiently considered. Even in the aforementioned Patent Document 1, lithium composite phosphoroxides are not included as usable cathode active materials. Furthermore, if water can be used as a solvent for slurry preparation, minimizing environmental impact and health concerns, it would not only reduce environmental impact and health concerns but also lower manufacturing costs.

[0010] In view of the above situation, this disclosure aims to provide a binder composition for use in the positive electrode of a lithium-ion battery containing lithium composite phosphoroxide, which has good coating properties for a slurry using an aqueous solvent on a current collector, good bonding properties between the current collector and the active material layer, and good charge-discharge characteristics of the lithium-ion battery.

[0011] The inventors of the present invention have diligently studied to solve the above problems and have found that the above objective can be achieved by a lithium composite phosphoroxide-containing cathode binder composition comprising an aqueous medium and core-shell particles, wherein the core-shell particles are either core-shell particles 1 or core-shell particles 2 described below, leading to this disclosure. Core-shell particles 1 have a core (C1) containing a non-diene rubber having a glass transition temperature in a specific range, and a shell layer (S1) located outside the core and containing a shell-forming polymer having a glass transition temperature in a specific range. Core-shell particles 2 have a core (C2) composed of a polymer containing an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers, and a shell layer (S2) located outside the core and composed of a shell-forming polymer.

[0012] In other words, this disclosure relates to a lithium composite phosphoroxide-containing cathode binder composition comprising an aqueous medium and core-shell particles, wherein the core-shell particles are either core-shell particle 1 or core-shell particle 2 as described below. Core-shell particle 1: comprises a core (C1) and a shell layer (S1) located outside the core (C1), wherein the core (C1) comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, and the shell layer (S1) comprises a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. Core-shell particle 2: comprises a core (C2) and a shell layer (S2) located outside the core (C2), wherein the core (C2) is composed of a polymer, and the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) comprises an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers.

[0013] According to this disclosure, a binder composition for use in the positive electrode of a lithium-ion battery containing lithium composite phosphoroxide can be provided, which exhibits good coating properties of a slurry using an aqueous solvent onto a current collector, good bonding properties between the current collector and the active material layer, and good charge-discharge characteristics of the lithium-ion battery.

[0014] Embodiments of the present disclosure are described in detail below. [Lithium-compound phosphoroxide-containing binder composition for positive electrode] The lithium-compound phosphoroxide-containing binder composition according to this embodiment is a binder composition used in the positive electrode of a lithium-ion battery, in particular in a positive electrode containing lithium-compound phosphoroxide as an active material. The binder composition comprises at least an aqueous medium and core-shell particles 1 or core-shell particles 2. The binder composition does not contain lithium-compound phosphoroxide, which is the positive electrode active material.

[0015] By using the lithium-compound phosphoroxide-containing positive electrode binder composition, a coating dispersion (hereinafter sometimes referred to as a slurry) containing lithium-compound phosphoroxide can be formed using an aqueous solvent. By coating the slurry onto the surface of a current collector and drying it, a positive electrode with an active material layer formed on the current collector can be manufactured. The coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer in the resulting positive electrode, and the charge-discharge characteristics of the lithium-ion battery containing the positive electrode can all be improved.

[0016] [Aqueous medium] The aqueous medium refers to an aqueous dispersion medium, and any medium mainly composed of water is acceptable. Water is preferred as the aqueous medium to reduce environmental impact, but a mixed solvent of water and an organic solvent can be used as needed. When a mixed solvent of water and an organic solvent is used, the proportion of the organic solvent in the mixed solvent is preferably 5% by weight or less, more preferably 3% by weight or less, and most preferably 0% by weight.

[0017] Examples of organic solvents included in a mixed solvent of water and an organic solvent 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.

[0018] Among the organic solvents contained in the mixed solvent, water-soluble organic solvents are preferred. Furthermore, among water-soluble organic solvents, those that form a homogeneous aqueous solution and can maintain a homogeneous aqueous solution state even when left at the aqueous solution formation temperature are preferred. Examples of water-soluble organic solvents include alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol; alkyl ketones such as acetone; 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.

[0019] [Core-Shell Particle 1] The core-shell particle 1 has a core-shell structure and includes a core (C1) containing a non-diene rubber and a shell layer (S1) located outside the core (C1). The shell layer (S1) includes a shell-forming polymer. The core-shell particle 1 having such a structure contained in the binder composition for the lithium composite phosphate-containing positive electrode functions as a binder used for the positive electrode of a lithium-ion battery.

[0020] The core-shell particle 1 preferably contains a monomer unit having a hydrogen-bonding functional group (FG1). Thereby, the binding property between the current collector and the active material layer can be improved, the swelling of the core-shell particle 1 in the electrolyte can be suppressed, and the charge-discharge characteristics of the lithium-ion battery can be improved.

[0021] As the entire core-shell particle 1, it may contain a monomer unit having a hydrogen-bonding functional group (FG1), and either one of the monomer constituting the non-diene rubber of the core and the monomer constituting the shell-forming polymer, or both the monomer constituting the non-diene rubber of the core and the monomer constituting the shell-forming polymer may contain a monomer unit having a hydrogen-bonding functional group (FG1).

[0022] The hydrogen-bonding functional group refers to a functional group containing a hydrogen atom that can be bonded by hydrogen bonding to an atom with a high electronegativity such as an oxygen atom. Specific examples of the hydrogen-bonding functional group (FG1) include a hydroxyl group, a carboxy group, and an amide group. The amide group may have a substituent on the nitrogen atom, but a primary amide having no substituent is preferred. Only one kind of the hydrogen-bonding functional group (FG1) may be used, or two or more kinds may be used in combination.

[0023] As the monomer having a hydrogen-bonding functional group (FG1), a vinyl monomer having a hydrogen-bonding functional group (FG1) is preferred, and a (meth)acrylic monomer having a hydrogen-bonding functional group (FG1) is more preferred. Note that “(meth)acrylic” is a notation for collectively referring to acrylic and methacrylic.

[0024] Among monomers having a hydrogen-bonding functional group (FG1), examples of monomers having a hydroxyl group include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0025] Furthermore, as monomers having hydroxyl groups, (meth)acrylic acid esters having (poly)alkylene glycol chains with hydroxyl groups at the terminal groups can also be used. Specific examples include (meth)acrylic acid (poly)ethylene glycol (e.g., NOF Corporation's Bremmer PE-90, PE-200, PE-350, AE-90U, AE-200, AE-400), (meth)acrylic acid (poly)propylene glycol (e.g., NOF Corporation's Bremmer PP-500, PP-500D, PP-800, PP-1000, PP-2000D, AP-200, AP-400, AP-400D, AP-550, AP-800, AP-1000D), (Me Examples include (poly)ethylene glycol acrylic acid - (poly)propylene glycol (e.g., NOF Corporation's Bremmer 50PEP-300), (meth)acrylic acid - (poly)ethylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 55PET-800, 50PEP-500D), (meth)acrylic acid - (poly)propylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 10PPB-500B, 10PPB-500BD), etc.

[0026] Examples of monomers having a carboxyl group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, and maleic acid. Among these, acrylic acid and / or methacrylic acid are preferred.

[0027] Examples of monomers 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, and (meth)acryloylmorpholine. Among these, acrylamide and / or methacrylamide are preferred, and methacrylamide is more preferred.

[0028] From the viewpoint of improving bonding properties by improving the peel strength between the current collector and the active material layer, monomers having hydroxyl groups are preferred as monomers having hydrogen bonding functional groups (FG1). From the viewpoint of improving the charge-discharge characteristics of lithium-ion batteries by improving electrolyte resistance, monomers having carboxyl groups are preferred. Furthermore, from the viewpoint of improving charge-discharge characteristics at high temperatures, monomers having amide groups are preferred.

[0029] [Core (C1)] The core (C1) includes a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C. The non-diene rubber refers to rubber other than diene rubber. The diene rubber refers to rubber that contains aliphatic conjugated diene compounds such as 1,3-butadiene as constituent units, and specific examples include butadiene rubber and styrene-butadiene rubber (SBR). Compared to diene rubber, non-diene rubber has the advantage of being resistant to oxidation, so oxidative degradation does not progress easily during charging and discharging of lithium-ion batteries.

[0030] The core (C1) may contain a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, the entire core (C1) may be composed of the non-diene rubber (R1), or a part of the core (C1) may be composed of the non-diene rubber (R1), for example, the core (C1) may be composed of at least two layers, of which at least one layer may be composed of the non-diene rubber (R1).

[0031] If the core (C1) is composed of at least two layers, at least one of them may be made of a non-diene rubber (R1), and the other at least one layer may be made of a non-diene rubber (R1') having a glass transition temperature of 40°C or higher.

[0032] From the viewpoint of particularly excellent bonding properties between the current collector and the active material layer, it is preferable that the core (C1) includes a core layer 1 and a core layer 2 located outside the core layer 1, wherein the core layer 1 includes a non-diene rubber (R1') having a glass transition temperature of 40°C or higher, and the core layer 2 includes a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C.

[0033] If the core (C1) includes core layer 1 and core layer 2, the proportion of core layer 1 in the core (C1) is preferably 5 to 70% by weight, more preferably 10 to 65% by weight, even more preferably 10 to 60% by weight, and particularly preferably 35 to 55% by weight. The proportion of core layer 2 is preferably 30 to 95% by weight, more preferably 35 to 90% by weight, even more preferably 40 to 90% by weight, and particularly preferably 45 to 65% by weight.

[0034] The ratio of the core (C1) to the total core-shell particles 1 is preferably set within the range of 50 to 90% by weight, from the viewpoint of improving the coating properties of the slurry on the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery. The lower limit is preferably 60% by weight or more, more preferably 65% ​​by weight or more, more preferably 70% by weight or more, and even more preferably 75% by weight or more, from the viewpoint of improving the electrolyte resistance and thereby improving the charge-discharge characteristics of the lithium-ion battery. The upper limit is preferably 85% by weight or less, from the viewpoint of improving the bonding properties between the current collector and the active material layer.

[0035] The non-diene rubber constituting the core (C1) has a crosslinked structure. To introduce a crosslinked structure, for example, when polymerizing monomer components to synthesize the non-diene rubber, a crosslinkable component such as a polyfunctional monomer can be used. In this disclosure, "monomer component" refers to monomer components excluding polyfunctional monomers.

[0036] Examples of the polyfunctional monomers include allyl (meth)acrylate; allylalkyl (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. Preferably, allyl methacrylate, triallyl isocyanurate, butanediol di(meth)acrylate, and divinylbenzene are used, and particularly preferably allyl methacrylate.

[0037] The amount of the polyfunctional monomer used may be within a known range and is not particularly limited, but for example, it may be 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, more preferably 0.1 to 3 parts by weight, and even more preferably 0.2 to 1.6 parts by weight, based on 100 parts by weight of the total monomer components constituting the non-diene rubber.

[0038] The non-diene rubber may consist of a single rubber composition, or it may consist of multiple types of rubber with different types or amounts of monomers and / or polyfunctional monomers.

[0039] (Non-diene rubber (R1) with a glass transition temperature of -60°C or higher and less than 40°C) The glass transition temperature (Tg) of the non-diene rubber (R1) with a glass transition temperature of -60°C or higher and less than 40°C may be -60°C or higher and less than 25°C, -58°C or higher and less than 10°C, -58°C or higher and less than 0°C, -58°C or higher and less than -10°C, -58°C or higher and less than -20°C, or -58°C or higher and less than -30°C. This is to improve the coating properties of the slurry to the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery. If the Tg of the non-diene rubber (R1) is 40°C or higher, the bending strength of the electrode and the bonding properties between the current collector and the active material layer tend to decrease in particular. Furthermore, if the Tg of the non-diene rubber (R1) is lower than -60°C, the polymerization stability and the bonding between the current collector and the active material layer tend to decrease.

[0040] 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, increasing the proportion of aromatic vinyl compounds in the non-diene rubber can raise the glass transition temperature of the non-diene rubber. Furthermore, the glass transition temperature can be measured using a differential scanning calorimeter (DSC). The glass transition temperature described herein is common to all disclosures.

[0041] Examples of the non-diene rubber (R1) include acrylic rubber and polyorganosiloxane rubber. In particular, acrylic rubber is preferred from the viewpoint of bonding properties between the current collector and the active material layer, and the charge / discharge characteristics of the lithium-ion battery.

[0042] The aforementioned acrylic rubber refers to rubber that contains acrylic monomer units as its main constituent units. The aforementioned acrylic monomer is not particularly limited, but examples 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. Acrylic monomers having hydrogen-bonding functional groups (FG1) as described above are also examples. Acrylic monomers may be used individually or in combination of two or more types.

[0043] The acrylic monomer is preferably an alkyl acrylate, more preferably butyl acrylate and 2-ethylhexyl acrylate, and particularly preferably 2-ethylhexyl acrylate.

[0044] The proportion of the alkyl acrylate in the total monomer components 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 viewpoint of the coating properties of the slurry on the current collector, the bonding properties between the current collector and the active material layer, and the charge / discharge characteristics of the lithium-ion battery. The upper limit is 100% by weight or less.

[0045] In the acrylic rubber described above, monomers other than the acrylic monomer are not required, but their use is preferable. Examples of such other monomers include methacrylic monomers, aromatic vinyl compounds such as styrene, vinyl cyanide compounds such as acrylonitrile, vinyl halides such as vinyl chloride; vinyl acetate; alkenes such as ethylene and propylene. From the viewpoint of polymerization stability and improved copolymerizability when aromatic vinyl compounds (especially styrene) are also used as monomers, it is preferable to use methacrylic monomers. It is preferable to use aromatic vinyl compounds such as styrene because it facilitates the production of core-shell particles 1 and makes it easier to adjust the glass transition temperature of the rubber to a suitable range.

[0046] The methacrylic monomer is not particularly limited, but examples include alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, dodecyl methacrylate, stearyl methacrylate, and behenyl methacrylate; aromatic ring-containing methacrylates such as phenoxyethyl methacrylate and benzyl methacrylate; glycidyl methacrylates such as glycidyl methacrylate and glycidyl alkyl methacrylate; and alkoxyalkyl methacrylates. Methacrylic monomers having the hydrogen-bonding functional group (FG1) described above are also examples. The methacrylic monomer may be used alone or in combination of two or more types.

[0047] As the methacrylic monomer, alkyl methacrylate is preferred. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0048] From the viewpoint of maintaining the particle shape of the core-shell particles 1 even after the formation of the slurry on the current collector (especially the pressurized active material layer) and ensuring excellent adhesion between the current collector and the active material layer, it is preferable that the proportion of the alkyl methacrylate in the total monomer components constituting the acrylic rubber is 25 to 40% by weight.

[0049] The aromatic vinyl compound is not particularly limited and includes, for example, unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; cyclic alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; cyclic alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; cyclic halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; cyclic ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and cyclic 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 types.

[0050] From the viewpoint of the applicability of the slurry to the current collector and the charge-discharge characteristics of the lithium-ion battery, the proportion of the aromatic vinyl compound (especially styrene) in the total 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. Furthermore, from the viewpoint of the applicability of the slurry to the current collector, the bonding between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery, the proportion of the aromatic vinyl compound (especially styrene) is preferably 30% by weight or less, more preferably 20% by weight or less, and even more preferably 15% by weight or less. The proportion of the aromatic vinyl compound (especially styrene) in the total monomer components constituting the acrylic rubber may be 1 to 30% by weight.

[0051] The non-diene rubber (R1) preferably contains monomer units having a hydrogen-bonding functional group (FG1). The hydrogen-bonding functional group (FG1) is as described above. Furthermore, the monomer having the hydrogen-bonding functional group (FG1) exemplified above can also be used as the monomer having the hydrogen-bonding functional group (FG1) in the non-diene rubber (R1).

[0052] When the non-diene rubber (R1) contains monomer units having hydrogen bonding functional groups (FG1), the proportion of monomers having hydrogen bonding functional groups (FG1) among the total monomer components constituting the non-diene rubber (R1) is preferably 1 to 30% by weight, from the viewpoint of the coatability of the slurry on the current collector, the binding between the current collector and the active material layer, the charge-discharge characteristics of the lithium-ion battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2% by weight or more, and even more preferably 5% by weight or more, particularly from the viewpoint of the coatability of the slurry on the current collector and polymerization stability. The upper limit is more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less, particularly from the viewpoint of binding.

[0053] (Non-diene rubber (R1') with a glass transition temperature of 40°C or higher) The lower limit of the glass transition temperature (Tg) of the non-diene rubber (R1') with a glass transition temperature of 40°C or higher may be 50°C or higher, 70°C or higher, 80°C or higher, or 90°C or higher. The upper limit may be 150°C or lower, 125°C or lower, or 110°C or lower. This is to improve the bonding between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery. The glass transition temperature (Tg) of the non-diene rubber (R1') may be between 40 and 150°C.

[0054] The non-diene rubber (R1') is preferably composed of a polymer containing at least one selected from the group consisting of (meth)acrylic monomers and aromatic vinyl compounds as a constituent monomer. From the viewpoint of polymerization stability, it is preferable that the polymer is composed of a polymer containing aromatic vinyl compounds as a constituent monomer.

[0055] The (meth)acrylic monomer constituting the non-diene rubber (R1') is not particularly limited and includes, for example, alkyl (meth)acrylates 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. In addition, (meth)acrylic monomers having the hydrogen-bonding functional group (FG1) as described above can also be mentioned. The (meth)acrylic monomers may be used alone or in combination of two or more types.

[0056] The (meth)acrylic monomer constituting the non-diene rubber (R1') is preferably an alkyl (meth)acrylate, and particularly preferably an alkyl methacrylate. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0057] The aromatic vinyl compound constituting the non-diene rubber (R1') is not particularly limited, and examples of aromatic vinyl compounds for the non-diene rubber (R1) can be used, among which substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred.

[0058] Furthermore, examples of the non-diene rubber (R1') include methacrylic rubber and styrene rubber. From the viewpoint of bonding between the current collector and the active material layer, methacrylic rubber is preferred, while from the viewpoint of improving the charge-discharge characteristics of the lithium-ion battery by improving electrolyte resistance, styrene rubber is preferred.

[0059] The term "methacrylic rubber" refers to rubber that contains methacrylic monomer units as its main constituent units, and the term "styrene rubber" refers to rubber that contains styrene monomer units as its main constituent units.

[0060] The methacrylic monomers that can be used and are suitable as the methacrylic monomers constituting the non-diene rubber (R1') are the same as the methacrylic monomers constituting the non-diene rubber (R1).

[0061] The proportion of the alkyl methacrylate in the total monomer components constituting the methacrylic 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 viewpoint of the applicability of the slurry to the current collector, the bonding between the current collector and the active material layer, and the charge / discharge characteristics of the lithium-ion battery. The upper limit is 100% by weight or less.

[0062] The styrene monomer is not particularly limited, but examples include styrene and styrene derivatives such as α-methylstyrene, paramethylstyrene, t-butylstyrene, and chlorostyrene. The styrene monomer may be used alone or in combination of two or more types.

[0063] The proportion of the styrene monomer (especially styrene) in the total monomer components constituting the styrene-based 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 viewpoint of improving the charge and discharge characteristics of the lithium-ion battery by improving the electrolyte resistance. The upper limit is 100% by weight or less.

[0064] In the methacrylic rubber and the styrene rubber, monomers other than the methacrylic monomer and the styrene monomer may or may not be used. Examples of such other monomers include acrylic monomers and vinyl cyanide compounds such as acrylonitrile and methacrylonitrile. From the viewpoint of polymerization stability, alkyl acrylates are preferred.

[0065] Examples of alkyl acrylates include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, dodecyl acrylate, stearyl acrylate, and behenyl acrylate, with butyl acrylate or 2-ethylhexyl acrylate being preferred.

[0066] The non-diene rubber (R1') preferably contains monomer units having a hydrogen-bonding functional group (FG1). The hydrogen-bonding functional group (FG1) is as described above. Furthermore, the monomer having the hydrogen-bonding functional group (FG1) exemplified above can also be used as the monomer having the hydrogen-bonding functional group (FG1) in the non-diene rubber (R1').

[0067] When the non-diene rubber (R1') contains monomer units having a hydrogen-bonding functional group (FG1), the proportion of monomers having a hydrogen-bonding functional group (FG1) among the total monomer components constituting the non-diene rubber (R1') is preferably 1 to 30% by weight, from the viewpoint of the coatability of the slurry on the current collector, the binding between the current collector and the active material layer, the charge-discharge characteristics of the lithium-ion battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2% by weight or more, and even more preferably 5% by weight or more, particularly from the viewpoint of the coatability of the slurry on the current collector and polymerization stability. The upper limit is more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less, particularly from the viewpoint of binding.

[0068] [Shell layer (S1)] The shell layer (S1) refers to a polymer layer located on the surface side of the core-shell particle 1, and is also called a graft layer. Preferably, the shell layer (S1) is graft-bonded to the core (C1). However, the polymer forming the shell layer (S1) also includes polymers that are not graft-bonded to the core. The shell layer (S1) covers the surface of the core (C1), but is not limited to covering the entire surface of the core (C1); it is sufficient if it covers at least a part of the surface of the core (C1).

[0069] The shell layer (S1) contains a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. This makes it easier to maintain the shape of the core-shell particles 1 and improves the dispersibility of the core-shell particles and lithium composite phosphoroxide in the slurry. The lower limit of the glass transition temperature (Tg) of the shell-forming polymer (SP1) may be 50°C or higher, 60°C or higher, or 70°C or higher. The upper limit may be 110°C or lower, 100°C or lower, or 90°C or lower. If the glass transition temperature (Tg) of the shell-forming polymer (SP1) exceeds 120°C or is below 40°C, the bonding between the current collector and the active material layer tends to decrease.

[0070] The glass transition temperature of the shell-forming polymer (SP1) can be controlled by changing the type and ratio of monomers that make up the polymer. For example, by using methacrylic monomers and aromatic vinyl compound units as monomers that make up the shell-forming polymer (SP1), it is possible to increase the glass transition temperature of the polymer.

[0071] By providing the aforementioned shell layer (S1), the dispersibility of the core-shell particles 1 and lithium composite phosphoroxide in the slurry can be improved, thereby improving the coating properties of the slurry onto the current collector. Furthermore, the bonding properties between the current collector and the active material layer, as well as the charge-discharge characteristics of the lithium-ion battery, can also be improved.

[0072] The shell layer (S1) is preferably composed of a non-crosslinked polymer, from the viewpoint of dispersibility of core-shell particles 1 and lithium composite phosphoroxide in the slurry and the coatability of the slurry on the current collector. A non-crosslinked polymer refers to a polymer that does not contain a crosslinked structure and does not contain structural units derived from polyfunctional monomers, and refers to a polymer that does not fall under rubber elastic materials such as acrylic rubber (hereinafter, this applies to all "non-crosslinked polymers").

[0073] The shell-forming polymer (SP1) is preferably a vinyl polymer. The monomers constituting the shell-forming polymer (SP1) are not particularly limited as long as they are vinyl monomers, but it is preferable that they include at least one selected from the group consisting of (meth)acrylic monomers and aromatic vinyl compounds.

[0074] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer (SP1) and improving the dispersibility of the core-shell particles 1 and lithium composite phosphoroxide in the slurry, the shell-forming polymer (SP1) preferably contains at least methacrylic monomer units, and is particularly preferably contains methacrylic monomer units and acrylic monomer units and / or aromatic vinyl compound units.

[0075] The (meth)acrylic monomer constituting the shell-forming polymer (SP1) is not particularly limited and includes, for example, alkyl (meth)acrylates 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. In addition, (meth)acrylic monomers having the hydrogen-bonding functional group (FG1) as described above can also be used. The (meth)acrylic monomers may be used individually or in combination of two or more types.

[0076] The (meth)acrylic monomer constituting the shell-forming polymer (SP1) is preferably an alkyl (meth)acrylate, and particularly preferably an alkyl methacrylate. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0077] Of the total monomer components constituting the shell-forming polymer (SP1), the proportion of the methacrylic monomer (particularly alkyl methacrylate) units is preferably 40 to 100% by weight, from the viewpoint of increasing the glass transition temperature of the shell-forming polymer (SP1). The lower limit may be 50% by weight or more, and may be 70% by weight or more. The upper limit may be 95% by weight or less, and may be 90% by weight or less, from the viewpoint of bonding between the current collector and the active material layer.

[0078] The aromatic vinyl compound constituting the shell-forming polymer (SP1) is not particularly limited, and examples of aromatic vinyl compounds for the non-diene rubber (R1) can be used. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred.

[0079] Of the total monomer components constituting the shell-forming polymer (SP1), the proportion of the aromatic vinyl compound (especially styrene) units is preferably 40 to 100% by weight, from the viewpoint of improving the charge-discharge characteristics of the lithium-ion battery by improving electrolyte resistance. The lower limit may be 50% by weight or more, and may be 70% by weight or more. The upper limit may be 95% by weight or less, and may be 90% by weight or less, from the viewpoint of bonding between the current collector and the active material layer.

[0080] Other vinyl monomers may be used as monomers constituting the shell-forming polymer (SP1). 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; heterocyclic vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole; and vinyl cyanide compounds such as acrylonitrile and methacrylonitrile.

[0081] It is preferable that the monomer constituting the shell-forming polymer (SP1) includes monomer units having a hydrogen-bonding functional group (FG1). In particular, compared to including monomer units having a hydrogen-bonding functional group (FG1) as monomer units constituting the non-diene rubber of the core, this further improves the bonding between the current collector and the active material layer, suppresses swelling of the core-shell particles 1 in the electrolyte, and improves the charge-discharge characteristics of the lithium-ion battery.

[0082] The hydrogen bonding functional group (FG1) is as described above. Furthermore, the monomer having the hydrogen bonding functional group (FG1) as exemplified above can also be used as the monomer having the hydrogen bonding functional group (FG1) in the shell-forming polymer (SP1).

[0083] When the monomer constituting the shell-forming polymer (SP1) includes monomer units having a hydrogen-bonding functional group (FG1), the proportion of monomers having a hydrogen-bonding functional group (FG1) among the total monomer components constituting the shell-forming polymer (SP1) is preferably 1 to 45% by weight, or 1 to 30% by weight, from the viewpoint of the coating properties of the slurry to the current collector, the binding properties between the current collector and the active material layer, the charge-discharge characteristics of the lithium-ion battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2% by weight or more, and even more preferably 5% by weight or more, particularly from the viewpoint of charge-discharge characteristics and swelling suppression. The upper limit is more preferably 20% by weight or less, even more preferably 18% by weight or less, and particularly preferably 15% by weight or less, particularly from the viewpoint of the coating properties of the slurry to the current collector and polymerization stability.

[0084] The proportion of the shell-forming polymer (SP1) to the total core-shell particles 1 is preferably 10 to 40% by weight, more preferably 10 to 30% by weight, and even more preferably 15 to 25% by weight, from the viewpoint of the coating properties of the slurry to the current collector, the binding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery.

[0085] The core-shell particle 1 may consist only of a core (C1) and a shell layer (S1), but may also have an intermediate layer between the core (C1) and the shell layer (S1) as long as the effects of the invention are achieved. The intermediate layer is preferably a layer made of a polymer and is graft-bonded to the core layer. When such an intermediate layer is present, the intermediate layer covers at least a portion of the surface of the core layer, and the shell layer (S1) covers at least a portion of the surface of the intermediate layer.

[0086] (Volume-average particle diameter of core-shell particles 1) The volume-average particle diameter of the core-shell particles 1 is preferably set within the range of 100 to 400 nm from the viewpoint of improving the coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery. The lower limit is preferably 150 nm or more, and more preferably 200 nm or more. The upper limit is preferably 350 nm or less, and even more preferably 300 nm or less.

[0087] Furthermore, the volume-average particle diameter of the core-shell particles 1 is measured using a particle diameter measuring device while the core-shell particles 1 are in their latex state. The particle diameter of the core-shell particles 1 can be controlled by the type and amount of polymerization initiators, chain transfer agents, redox agents, emulsifiers, etc. used during polymerization, as well as the polymerization temperature and polymerization time.

[0088] The proportion of core-shell particles 1 in the lithium-compound phosphoroxide-containing cathode binder composition 1 according to this embodiment is not particularly limited. From the viewpoint of increasing the slurry concentration, electrode productivity, and storage stability of the binder composition, 30 to 60% by weight is preferred, and 40 to 50% by weight is more preferred.

[0089] [Core-shell particles 2] The core-shell particles 2 have a core-shell structure and consist of a core (C2) made of a polymer containing an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers, and a shell layer (S2) located outside the core. The shell layer (S2) contains a shell-forming polymer. Core-shell particles 2 with such a structure contained in the lithium-composite phosphoroxide-containing positive electrode binder composition function as a binder used in the positive electrode of a lithium-ion battery.

[0090] [Core (C2)] The core (C2) is composed of a polymer containing an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers. It is preferable to include a copolymer (rubber) of aliphatic conjugated diene compound units and aromatic vinyl compound units, as this can improve the dispersibility of the core-shell particles 2 in the slurry and the compatibility between the core-shell particles 2 and the thickener, and can improve the binding between the current collector and the electrode active material layer.

[0091] The aliphatic conjugated diene compound refers to an aliphatic compound having two carbon-carbon double bonds, these double bonds being conjugated by a single bond. 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. One type of aliphatic conjugated diene compound may be used, or two or more may be used in combination. Of these, 1,3-butadiene is preferred.

[0092] Examples of the aromatic vinyl compounds include unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; cyclic alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; cyclic alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; cyclic halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; cyclic ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and cyclic 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. Only one aromatic vinyl compound may be used, or two or more may be used in combination.

[0093] The polymer constituting the core (C2) may include styrene-butadiene rubber. Styrene-butadiene rubber is a copolymer of 1,3-butadiene and styrene, and is also called styrene rubber or SBR.

[0094] The polymer constituting the core (C2) may not contain vinyl monomer units other than aliphatic conjugated diene compound units and aromatic vinyl compound units, or it may contain such other vinyl monomer units. Examples of such other vinyl monomers include (meth)acrylic acids and alkyl (meth)acrylate 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 polymer constituting the core (C2) does not contain vinyl monomer units having carboxyl groups, such as acrylic acid, methacrylic acid, and itaconic acid.

[0095] The proportion of aliphatic conjugated diene compound units in the entire polymer constituting the core (C2) is not particularly limited, but from the viewpoint of adhesion to the current collector, it is preferably 10 to 80% by weight, more preferably 20 to 75% by weight, and even more preferably 30 to 75% by weight.

[0096] The ratio of aliphatic conjugated diene compound to aromatic vinyl compound in the total polymer constituting the core (C2) is not particularly limited, but from the viewpoint of improving binding properties, reducing internal resistance, and maintaining capacity after charging and discharging (cycle characteristics), 10:90 to 80:20 is preferred, 20:80 to 75:25 is more preferred, and 30:70 to 75:25 is even more preferred.

[0097] The total proportion of aliphatic conjugated diene compound units and aromatic vinyl compound units in the entire polymer constituting the core (C2) is not particularly limited, but may be 80 to 100% by weight from the viewpoint of polymerization stability and binding properties. It may also be 90% or more by weight, 95% or more by weight, 98% or more by weight, or 99% or more by weight.

[0098] The polymer constituting the core (C2) may be obtained by using polyfunctional monomers such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, and 1,3-butylene dimethacrylate during polymerization.

[0099] Furthermore, the polymer constituting the core (C2) may be polymerized without the use of a chain transfer agent, or it may be polymerized in the presence of a chain transfer agent. The usable chain transfer agent is not particularly limited, but examples 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.

[0100] The glass transition temperature of the polymer can be controlled by changing the ratio of aliphatic conjugated diene compound units to aromatic vinyl compound units in the polymer, or by changing the types and ratios of other monomer units that may be contained in the polymer. For example, increasing the proportion of aromatic vinyl compounds in the polymer can raise the glass transition temperature of the polymer.

[0101] The ratio of the core (C2) to the total core-shell particles 2 is not particularly limited, but from the viewpoint of improving binding properties, reducing internal resistance, and maintaining capacity after charging and discharging (cycle characteristics), it is preferably 50 to 95% by weight, more preferably 50 to 90% by weight, and even more preferably 55 to 85% by weight.

[0102] The core (C2) may be a single-layer structure consisting of one layer, or it may have a multilayer structure consisting of at least two layers, as long as it is made of a polymer.

[0103] When the core (C2) has a single-layer structure, the glass transition temperature (Tg) of the polymer constituting the core (C2) is not particularly limited, but may be in the range of -50°C to +25°C, for example. From the viewpoint of improving bonding properties, reducing internal resistance, and maintaining capacity with charge-discharge cycles, -50°C to +20°C is preferred, -40°C to +20°C is more preferred, -30°C to +15°C is even more preferred, -20°C to +10°C is particularly preferred, and -15°C to 0°C is most preferred.

[0104] When the core (C2) has a multilayer structure, the lower limit of the glass transition temperature (Tg) of the polymer constituting the innermost layer of the core (C2) is preferably 25°C or higher, more preferably 30°C or higher, even more preferably 35°C or higher, and particularly preferably 40°C or higher. This is because it is easy to maintain the particle shape of the core-shell particles 2, and bonding of the electrode active materials to each other, or bonding of the electrode active materials to the current collector, can be achieved by point bonding. As a result, the movement of lithium ions is less likely to be hindered by the presence of the binder, and the internal resistance of the lithium-ion battery can be lowered. The upper limit is preferably 110°C or lower, more preferably 105°C or lower, and even more preferably 100°C or lower. The glass transition temperature (Tg) of the polymer constituting the innermost layer of the core (C2) may be 25 to 110°C.

[0105] The polymer constituting the innermost layer of the core (C2) preferably contains an aromatic vinyl compound as a constituent monomer, from the viewpoint of being easily adjustable within the glass transition temperature (Tg) range, and may contain an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers. The proportion of the aromatic vinyl compound units in the total polymer constituting the innermost layer of the core (C2) is not particularly limited, but is preferably 50 to 100% by weight, more preferably 60 to 100% by weight, and even more preferably 70 to 100% by weight.

[0106] Preferably, the glass transition temperature (Tg) of the polymer constituting the outermost layer of the core (C2) is lower than the glass transition temperature (Tg) of the polymer constituting the innermost layer of the core (C2). This is because it softens the outermost layer of the core (C2), increases the flexibility of the electrode, and further improves the bonding between the electrode active material and the current collector.

[0107] The glass transition temperature (Tg) of the polymer constituting the outermost layer of the core (C2) is preferably less than 25°C, 10°C or less, 0°C or less, -10°C or less, -15°C or less, and -20°C or less, in that order. Furthermore, from the viewpoint of adhesion to the current collector, -85°C or higher is preferred, and -40°C or higher is more preferred. The glass transition temperature (Tg) of the polymer constituting the outermost layer of the core (C2) may be -85°C or higher and less than 25°C.

[0108] The polymer constituting the outermost layer of the core (C2) preferably contains aliphatic conjugated diene compound units as constituent monomers. It may also contain aliphatic conjugated diene compounds and aromatic vinyl compounds as constituent monomers. The proportion of the aliphatic conjugated diene compound units in the total polymer constituting the outermost layer of the core (C2) is not particularly limited, but from the viewpoint of moderately softening the outermost layer of the core (C2) and improving the flexibility of the electrode, 50 to 100% by weight is preferred, 60 to 100% by weight is more preferred, and 70 to 100% by weight is even more preferred.

[0109] The core (C2) may contain rubber, and if the core (C2) has a multilayer structure, the polymer constituting at least one layer (especially the outermost layer) of the layers constituting the multilayer structure may be rubber, the polymer constituting the innermost layer and the polymer constituting the outermost layer may be rubber, and all of the polymers constituting the core (C2) may be rubber. Here, rubber refers to a member having rubber elasticity. Rubber elasticity is elasticity that can absorb energy due to an external force and store it as energy to return to its original state, and in order to exhibit rubber elasticity, the molecules must be sufficiently long, able to move freely, and appropriately bonded to each other. A member having rubber elasticity can be deformed by an external force, but can easily return to its original shape when the external force is released.

[0110] The proportion of the innermost layer of the core (C2) in the entire multilayered core (C2) is not particularly limited, but in terms of adhesion to the current collector, it is preferably 5 to 75% by weight, more preferably 15 to 75% by weight, and even more preferably 20 to 75% by weight.

[0111] The proportion of the outermost layer of the core (C2) to the entire multilayer core (C2) is not particularly limited, but in terms of adhesion to the current collector, it is preferably 25 to 95% by weight, more preferably 25 to 85% by weight, and even more preferably 25 to 80% by weight.

[0112] The core (C2) having the multilayer structure described above may consist only of an innermost layer and an outermost layer, but may also have an intermediate layer between the innermost layer and the outermost layer, as long as the effects of the invention are achieved. The intermediate layer is a layer made of a polymer and is preferably graft-bonded to the adjacent layer. The intermediate layer may be made of a polymer containing aromatic vinyl compound units, a polymer containing aliphatic conjugated diene compound units, or a polymer containing aliphatic conjugated diene compound units and aromatic vinyl compound units. Furthermore, there may be multiple intermediate layers in the core (C2).

[0113] [Shell layer (S2)] The shell layer (S2) is composed of a shell-forming polymer (SP2).

[0114] The shell-forming polymer (SP2) is preferably a vinyl polymer, and preferably contains an aromatic vinyl compound as a monomer unit. This is to improve electrolyte resistance and cycle performance.

[0115] The aromatic vinyl compound constituting the shell layer (S2) is not particularly limited and includes, for example, unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; cyclic alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; cyclic alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; cyclic halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; cyclic ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and cyclic 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 types.

[0116] The proportion of the aromatic vinyl compound units in the entire shell-forming polymer (SP2) is not particularly limited, but from the viewpoint of having good electrolyte resistance and being able to improve the charge-discharge characteristics of lithium-ion batteries, 30 to 95% by weight is preferred, 40 to 95% by weight is more preferred, 45 to 93% by weight is even more preferred, 50 to 90% by weight is even more preferred, and 60 to 85% by weight is particularly preferred.

[0117] The monomers constituting the shell-forming polymer (SP2), other than the aromatic vinyl compound units, are not particularly limited, but examples include (meth)acrylic monomers and monomer units having hydrogen-bonding functional groups (FG2).

[0118] The shell-forming polymer (SP2) preferably contains a (meth)acrylic monomer, and more preferably contains both a (meth)acrylic monomer and the aromatic vinyl compound unit, from the viewpoint of increasing the glass transition temperature of the shell-forming polymer (SP2) and improving the dispersibility of core-shell particles and lithium composite phosphoroxide in the slurry. In this application, "(meth)acrylic" is a notation used to collectively refer to acrylic and methacrylic.

[0119] The (meth)acrylic monomer constituting the shell-forming polymer (SP2) is not particularly limited and includes, for example, alkyl (meth)acrylates 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. In addition, (meth)acrylic monomers having hydrogen-bonding functional groups (FG2) as described below can also be used. The (meth)acrylic monomers may be used individually or in combination of two or more types.

[0120] The (meth)acrylic monomer constituting the shell-forming polymer (SP2) is preferably an alkyl (meth)acrylate, and particularly preferably an alkyl methacrylate. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0121] The proportion of the (meth)acrylic monomer units in the entire shell-forming polymer (SP2) is not particularly limited, but from the viewpoint of promoting the polymerization reaction, it is preferably 1 to 55% by weight, more preferably 5 to 53% by weight, even more preferably 10 to 45% by weight, and particularly preferably 10 to 40% by weight.

[0122] The proportion of methacrylic monomer units in the entire shell-forming polymer (SP2) is not particularly limited, but from the viewpoint of bonding between the current collector and the active material layer, it is preferably 1 to 50% by weight, more preferably 1 to 45% by weight, even more preferably 5 to 40% by weight, and particularly preferably 5 to 30% by weight.

[0123] The proportion of the alkyl (meth)acrylate compound units in the entire shell-forming polymer (SP2) is not particularly limited, but from the viewpoint of promoting the polymerization reaction of the shell-forming polymer (SP2), and from the viewpoint of the dispersibility of core-shell particles in the slurry and the coating properties of the slurry on the current collector, it is preferably 1 to 45% by weight, more preferably 5 to 30% by weight, even more preferably 10 to 30% by weight, and particularly preferably 20 to 30% by weight.

[0124] The shell-forming polymer (SP2) preferably contains monomer units having hydrogen-bonding functional groups (FG2). This improves the bonding between the current collector and the active material layer, suppresses swelling of core-shell particles in the electrolyte, and improves the charge-discharge characteristics of the lithium-ion battery.

[0125] Specific examples of hydrogen-bonding functional groups (FG2) include hydroxyl groups, carboxyl groups, and amide groups. Amide groups may have substituents on the nitrogen atom, but primary amides without substituents are preferred. Only one type of hydrogen-bonding functional group may be used, or two or more types may be used in combination.

[0126] As monomers having hydrogen bonding functional groups (FG2), vinyl monomers having hydrogen bonding functional groups (FG2) are preferred, and (meth)acrylic monomers having hydrogen bonding functional groups (FG2) are more preferred.

[0127] Among monomers having a hydrogen-bonding functional group (FG2), examples of monomers having a hydroxyl group include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0128] Furthermore, as monomers having hydroxyl groups, (meth)acrylic acid esters having (poly)alkylene glycol chains with hydroxyl groups at the terminal groups can also be used. Specific examples include (meth)acrylic acid (poly)ethylene glycol (e.g., NOF Corporation's Bremmer PE-90, PE-200, PE-350, AE-90U, AE-200, AE-400), (meth)acrylic acid (poly)propylene glycol (e.g., NOF Corporation's Bremmer PP-500, PP-500D, PP-800, PP-1000, PP-2000D, AP-200, AP-400, AP-400D, AP-550, AP-800, AP-1000D), (Me Examples include (poly)ethylene glycol acrylic acid - (poly)propylene glycol (e.g., NOF Corporation's Bremmer 50PEP-300), (meth)acrylic acid - (poly)ethylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 55PET-800, 50PEP-500D), (meth)acrylic acid - (poly)propylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 10PPB-500B, 10PPB-500BD), etc.

[0129] Examples of monomers having a carboxyl group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, and maleic acid. Among these, acrylic acid and / or methacrylic acid are preferred.

[0130] Examples of monomers 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, and (meth)acryloylmorpholine. Among these, acrylamide and / or methacrylamide are preferred, and methacrylamide is more preferred.

[0131] From the viewpoint of improving bonding properties by improving the peel strength between the current collector and the active material layer, monomers having hydroxyl groups are preferred as monomers having hydrogen bonding functional groups (FG2). From the viewpoint of improving the charge-discharge characteristics of lithium-ion batteries by improving electrolyte resistance, monomers having carboxyl groups are preferred. Furthermore, from the viewpoint of improving charge-discharge characteristics at high temperatures, monomers having amide groups are preferred.

[0132] The content of monomer units having the hydrogen bonding functional group (FG2) in the shell-forming polymer (SP2) is preferably 3% by weight or more, more preferably 4% by weight or more, and even more preferably 6% by weight or more, particularly from the viewpoint of adhesion between the current collector and the active material layer. The upper limit is preferably 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, even more preferably 18% by weight or less, and particularly preferably 15% by weight or less, particularly from the viewpoint of the coating properties of the slurry to the current collector and polymerization stability. The content of monomer units having the hydrogen bonding functional group (FG2) in the shell-forming polymer (SP2) may be 3 to 30% by weight.

[0133] The glass transition temperature of the shell-forming polymer (SP2) is preferably 60°C or higher, more preferably 70°C or higher, even more preferably 80°C or higher, and particularly preferably 90°C or higher, from the viewpoint of making it easier to maintain the shape of the core-shell particles and improving the dispersibility of the core-shell particles and lithium composite phosphoroxide in the slurry. The upper limit may be 110°C or lower, or 100°C or lower. The glass transition temperature of the shell-forming polymer (SP2) may be between 60°C and 110°C.

[0134] The glass transition temperature of the shell-forming polymer (SP2) can be controlled by changing the type and ratio of monomers that make up the polymer. For example, by using methacrylic monomers and aromatic vinyl compounds as monomers that make up the shell-forming polymer (SP2), it is possible to increase the glass transition temperature of the polymer.

[0135] By providing the aforementioned shell layer (S2), the dispersibility of core-shell particles and lithium composite phosphoroxide in the slurry can be improved, and the coating properties of the slurry onto the current collector can be improved. Furthermore, the bonding properties between the current collector and the active material layer, as well as the charge-discharge characteristics of the lithium-ion battery, can also be improved.

[0136] The shell layer (S2) is preferably composed of a non-crosslinked polymer, from the viewpoint of dispersibility of core-shell particles and lithium composite phosphoroxide in the slurry, and the coatability of the slurry on the current collector.

[0137] The ratio of the shell-forming polymer (SP2) to the total core-shell particles is not particularly limited, but from the viewpoint of the coating properties of the slurry on the current collector, the binding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery, 5 to 50% by weight is preferred, 10 to 50% by weight is more preferred, and 15 to 45% by weight is even more preferred.

[0138] The core-shell particles may consist only of a core and a shell layer (S2), but may also have an intermediate layer between the core and the shell layer (S2) as long as the effects of the invention are achieved. The intermediate layer is preferably a layer made of a polymer and is graft-bonded to the core layer. When such an intermediate layer is present, the intermediate layer covers at least a portion of the surface of the core layer, and the shell layer (S2) covers at least a portion of the surface of the intermediate layer.

[0139] (Volume-average particle diameter of core-shell particles 2) The volume-average particle diameter of the core-shell particles 2 is preferably set within the range of 100 to 400 nm from the viewpoint of improving the coating properties of the slurry onto the current collector, the binding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery. The lower limit is preferably 150 nm or more, and more preferably 200 nm or more. The upper limit is preferably 350 nm or less, and even more preferably 300 nm or less.

[0140] Furthermore, the method for measuring and controlling the volume-average particle diameter of core-shell particle 2 is the same as that for core-shell particle 1.

[0141] The proportion of core-shell particles 2 in the lithium-compound phosphoroxide-containing cathode binder composition 2 according to this embodiment is not particularly limited. From the viewpoint of increasing the slurry concentration, electrode productivity, and storage stability of the binder composition, 30 to 60% by weight is preferred, and 40 to 50% by weight is more preferred.

[0142] (Method for producing lithium-compound phosphoroxide-containing cathode binder compositions 1 and 2) The lithium-compound phosphoroxide-containing cathode binder compositions are obtained by producing core-shell particles 1 or 2 in an aqueous medium.

[0143] The method for producing the core-shell particles 1 or 2 is not particularly limited, but for example, emulsion polymerization, miniemulsion polymerization, microemulsion polymerization, and soap-free emulsion polymerization can be used.

[0144] The emulsifier that can be used in emulsion polymerization is not particularly limited, and anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc., can be used. Dispersants such as polyvinyl alcohol, alkyl-substituted cellulose, polyvinylpyrrolidone, and polyacrylic acid derivatives may also be used in combination.

[0145] The anionic surfactants among the emulsifiers mentioned above are not particularly limited, but examples 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 sodium soap, semi-hardened beef tallow fatty acid sodium soap, castor oil potassium soap; and sodium dodecyl sulfate, higher alcohol sodium sulfate, dodecyl sulfate triethanolamine, dodecyl sulfate ammonium, polyoxyethylene alkyl ether sulfate sodium, polyoxyethylene alkyl ether sulfate triethanolamine, polyoxyethylene alkylphenyl ether sulfate sodium, 2-ethylhexyl sulfate sodium. Sodium methyl sulfate; sodium alkylbenzene sulfonates such as sodium dodecylbenzenesulfonate; sodium dialkyl sulfosuccinates such as sodium di-2-ethylhexyl sulfosuccinate; sodium alkylnaphthalene sulfonate; sodium alkyldiphenyl ether disulfonate; potassium alkyl phosphate; phosphate ester salts such as sodium polyoxyethylene lauryl ether phosphate; sodium salts of naphthalene sulfonic acid formalin condensate; polycarboxylic acid type polymer anions; sodium acyl (beef tallow) methyl taurate; sodium acyl (coconut) methyl taurate; sodium cocoyl isethionate; sodium α-sulfo fatty acid esters; sodium amide ether sulfonate; oleyl sarcosine; sodium lauroyl sarcosinate; rosinic acid soap, etc.

[0146] The nonionic surfactants among the emulsifiers mentioned above are not particularly limited, but examples 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.

[0147] The cationic surfactants among the emulsifiers mentioned above are not particularly limited, but examples include the following compounds: alkylamine salts such as coconutamine acetate, stearylamine acetate, octadecylamine acetate, and tetradecylamine acetate; quaternary ammonium salts such as lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, distearyldimethylammonium chloride, alkylbenzyldimethylammonium chloride, hexadecyltrimethylammonium chloride, and behenyltrimethylammonium chloride.

[0148] The amphoteric surfactants among the emulsifiers mentioned above are not particularly limited, but examples include the following compounds: alkyl betaines such as lauryl betaine, stearyl betaine, and dimethyl lauryl betaine; sodium lauryl diaminoethylglycine; amide betaine; imidazoline; lauryl carboxymethyl hydroxyethyl imidazolinium betaine, etc.

[0149] These emulsifiers may be used individually or in combination of two or more. Among the emulsifiers, sodium dialkyl sulfosuccinate or surfactants having an oxyethylene structure are preferred, and sodium polyoxyethylene lauryl ether phosphate is particularly preferred, from the viewpoint of obtaining good fluidity in the resulting lithium-compound phosphoric acid-containing cathode binder composition.

[0150] When employing emulsion polymerization, known polymerization initiators, namely 2,2'-azobisisobutyronitrile, hydrogen peroxide, potassium persulfate, and ammonium persulfate, can be used as thermal decomposition initiators.

[0151] In addition, a redox-type initiator can be used that combines an organic peroxide such as t-butyl peroxyisopropyl carbonate, paramenthane hydroperoxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, and t-hexyl peroxide; an inorganic peroxide such as hydrogen peroxide, potassium persulfate, and ammonium persulfate; and at least one selected from the group consisting of reducing agents such as sodium formaldehyde sulfoxylate and glucose; transition metal salts such as iron(II) sulfate; chelating agents such as ethylenediaminetetraacetate disodium; and phosphorus-containing compounds such as sodium pyrophosphate.

[0152] When a redox-type initiator is used, polymerization can be carried out even at low temperatures in which the peroxide does not substantially decompose thermally, and the polymerization temperature can be set over a wide range, which is preferable. In particular, it is preferable to use organic peroxides such as cumene hydroperoxide, dicumyl peroxide, and t-butyl hydroperoxide as redox-type initiators. The amount of the initiator used, and when a redox-type initiator is used, the amounts of the reducing agent, transition metal salt, chelating agent, phosphorus-containing compound, etc., can be used within a known range. Surfactants can also be used in addition, but this is also within a known range.

[0153] Furthermore, chain transfer agents can be used when polymerizing polyfunctional monomers. While there are no particular limitations on the types of chain transfer agents that can be used, examples include alkyl mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, t-decyl mercaptan, n-decyl mercaptan, and n-octyl mercaptan, as well as alkyl ester mercaptans such as 2-ethylhexyl thioglycolate.

[0154] Any solvent that allows emulsion polymerization to proceed stably can be used as the solvent; for example, water can be suitably used.

[0155] The temperature during emulsion polymerization is not particularly limited as long as the emulsifier is uniformly dissolved in the solvent, but for example, it is 40 to 75°C, preferably 45 to 70°C, and more preferably 49 to 65°C.

[0156] When the core-shell particles 1 or 2 are produced by emulsion polymerization, the resulting latex becomes a lithium-compound phosphoroxide-containing binder composition for the positive electrode. Furthermore, the concentration of the core-shell particles 1 or 2 may be adjusted as needed. Alternatively, the resulting latex can be spray-dried to obtain a powder that can be redispersed in water, which can then be used as the binder for the positive electrode.

[0157] (Optional Components) The lithium composite phosphoroxide-containing cathode binder composition according to this embodiment may contain, in addition to the aqueous medium and the core-shell particles 1 and / or 2, components such as conductive additives, reinforcing materials, leveling agents, viscosity modifiers, and electrolyte additives. These components are not particularly limited as long as they do not affect the battery reaction, and known materials or the materials described later can be used. Furthermore, only one type of these components may be used, or two or more types may be used in combination.

[0158] The proportion of the core-shell particles 1 and / or 2 in the solid content of the lithium-composite phosphoroxide-containing cathode binder composition according to this embodiment is not particularly limited and may be, for example, about 10 to 100% by weight. It may also be about 50 to 100% by weight, about 80 to 100% by weight, or about 90 to 100% by weight.

[0159] [Dispersion for coating] The dispersion for coating according to this embodiment comprises a lithium composite phosphorus oxide and a lithium composite phosphorus oxide-containing binder composition for cathodes.

[0160] The amount of the lithium composite phosphoroxide-containing positive electrode binder composition added to the coating dispersion can be appropriately determined by those skilled in the art. However, from the viewpoint of the coating properties of the slurry onto the current collector, the binding properties between the current collector and the positive electrode active material layer, and the charge-discharge characteristics of the lithium-ion battery, the amount of core-shell particles 1 and / or 2 contained in the lithium composite phosphoroxide-containing positive electrode binder composition is preferably 0.1 to 20 parts by weight, more preferably 0.5 to 15 parts by weight, and even more preferably 1 to 10 parts by weight, per 100 parts by weight of lithium composite phosphoroxide.

[0161] <Positive Electrode Active Material> In this embodiment, lithium composite phosphoroxide is used as the positive electrode active material. By using core-shell particles 1 or 2 according to this embodiment as a binder for lithium composite phosphoroxide, the coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the lithium-ion battery can be improved.

[0162] A specific example of lithium complex phosphoroxide is lithium iron phosphate (LiFePO4). ４ : LFP), Lithium manganese phosphate (LiMnPO) ４ Examples include lithium cobalt phosphate and lithium iron manganese phosphate (LMFP). Among these, at least one selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, and lithium iron manganese phosphate is preferred, and lithium iron phosphate is particularly preferred.

[0163] <Conductive additive> A conductive additive may be optionally added to the coating dispersion. The conductive additive is not particularly limited, and known conductive additives can be used. Specifically, examples include carbon black such as acetylene black, furnace black, and Ketjenblack (registered trademark); graphite such as natural graphite and artificial graphite; carbon fibers such as polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, vapor-phase carbon fibers, carbon nanotubes, and carbon nanofibers; and various metal fibers and foils.

[0164] The amount of conductive additive can be set as appropriate, but it 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 lithium composite phosphoroxide.

[0165] <Thickening Agent> A thickening agent may be optionally added to the coating dispersion. The thickening agent is a component that can improve the dispersion stability of the positive electrode active material in the slurry and improve the coating properties of the slurry. A water-soluble polymer can be used as the thickening agent, specifically carboxymethylcellulose, methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, polyvinyl alcohol, polycarboxylic acid, salts thereof, poly(meth)acrylamide, etc. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, and alginic acid. Only one type of these water-soluble polymer may be used, or two or more types may be used in combination. Of these, cellulosic compounds are preferred, and carboxymethylcellulose or its salts are particularly preferred.

[0166] The amount of thickener can be set as appropriate, but for example, 0.1 to 5 parts by weight is preferred, 0.3 to 3 parts by weight is more preferred, and 0.5 to 3 parts by weight is even more preferred, per 100 parts by weight of positive electrode active material. However, the thickener may be omitted.

[0167] <Other Polymers> The coating dispersion may optionally contain polymers other than the core-shell particles 1, the core-shell particles 2, and the thickener. Examples of such polymers include fluorine-containing polymers and acrylonitrile polymers.

[0168] <Dispersion Medium> In the coating dispersion liquid, the aqueous medium contained in the lithium composite phosphoroxide-containing cathode binder composition can be used as the dispersion medium. In addition to the aqueous medium contained in the lithium composite phosphoroxide-containing cathode binder composition, the dispersion medium may also contain further aqueous medium, or it may contain water introduced by the addition of optional components such as a thickener.

[0169] The solid content concentration of the coating dispersion is not particularly limited, but may be, for example, about 10 to 80% by weight, and preferably about 30 to 70% by weight. Furthermore, the proportion of lithium composite phosphorus oxide in the total solid content of the coating dispersion may be about 50 to 99% by weight, preferably about 80 to 99% by weight, and more preferably about 90 to 99% by weight.

[0170] [Preparation of Dispersion and Method for Manufacturing the Positive Electrode of a Lithium-Ion Battery] The coating dispersion according to this embodiment can be prepared by mixing the lithium composite phosphoroxide-containing positive electrode binder composition according to this embodiment with lithium composite phosphoroxide, which is the positive electrode active material. Furthermore, by including the step of applying the dispersion onto a current collector (the surface of the current collector) and drying it, a positive electrode active material layer can be formed on the current collector.

[0171] Dispersions can be prepared by dispersing each of the above components in a dispersion medium. Specifically, a slurry can be prepared by mixing each of the above components with the dispersion medium using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, lye crusher, ultrasonic disperser, homogenizer, planetary mixer, or film mixer. The mixing of each of the above components with the dispersion medium can usually be carried out at a temperature range of room temperature to 80°C for 10 minutes to several hours.

[0172] [Positive electrode of lithium-ion battery] The positive electrode for a lithium-ion battery according to this embodiment includes a current collector and an active material layer provided on the current collector. The active material layer includes at least lithium composite phosphoroxide and the core-shell particles 1 or 2. The positive electrode for a lithium-ion battery according to this embodiment can be obtained by a manufacturing method that includes the step of coating the dispersion onto the current collector and drying it.

[0173] Known metal foils may be used as current collectors, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0174] The lithium-composite phosphoroxide-containing binder composition for positive electrodes according to this embodiment contains core-shell particles 1 or 2. Both core-shell particles 1 and 2 tend to maintain their particle shape relatively strongly within the positive electrode active material layer after drying. As a result, bonding between positive electrode active materials, or bonding between positive electrode active materials and the current collector, can be achieved by point adhesion. Consequently, the movement of lithium ions is less likely to be hindered, and it is presumed that a reduction in internal resistance can be achieved.

[0175] (Coating Process) The method for coating the dispersion onto the current collector is not particularly limited, and known methods can be used. Specifically, examples include the doctor blade method, dip method, reverse roll method, direct roll method, gravure method, extrusion method, and brush coating method. In this case, the slurry may be applied to only one side of the current collector or to both sides. The thickness of the slurry film on the current collector before drying after coating can be appropriately set according to the thickness of the positive electrode active material layer obtained after drying.

[0176] (Drying process) The method for drying the slurry film on the current collector is not particularly limited, and known methods can be used, such as drying with hot air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams.

[0177] After the drying process, the positive electrode active material layer may be subjected to pressure treatment using a die press or roll press. This improves the adhesion between the positive electrode active material layer and the current collector, and also reduces the porosity of the positive electrode active material layer.

[0178] Furthermore, the positive electrode for lithium-ion batteries according to this embodiment can also be manufactured by powder molding. In the powder molding method, first, the above-mentioned coating dispersion is prepared, composite particles are prepared from the coating dispersion, the composite particles are supplied onto a current collector, and a positive electrode active material layer can be formed on the current collector by rolling press molding as desired.

[0179] [Lithium-ion battery] The lithium-ion battery according to this embodiment comprises a positive electrode, a separator, a negative electrode, and an electrolyte.

[0180] <Negative Electrode> The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. Known metal foils may be used as the current collector, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0181] Examples of electrode active materials that can be used as negative electrodes include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that combine these.

[0182] Examples of carbon-based anode active materials include carbonaceous materials and graphitic materials. Examples of carbonaceous materials include easily graphitizable carbon, which readily changes its structure depending on the heat treatment temperature, and non-graphitizable carbon, which has a structure close to an amorphous structure, such as glassy carbon. Examples of easily graphitizable carbon include carbonaceous materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolysis vapor-grown carbon fibers. Examples of non-graphitizable carbon include phenolic resin calcined bodies, polyacrylonitrile-based carbon fibers, pseudoisotropic carbon, furfuryl alcohol resin calcined bodies (PFA), and hard carbon. Examples of graphitic materials include graphite, such as natural graphite and artificial graphite.

[0183] Examples of metallic anode active materials include lithium metal, elemental metals that can form 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 their oxides, sulfides, nitrides, silicides, carbides, and phosphides. Among these, silicon-containing active materials (silicon-based anode active materials) are preferred. By using silicon-based anode active materials, the capacity of lithium-ion batteries can be increased.

[0184] 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 individually or in combination of two or more types.

[0185] Examples of silicon-containing alloys include alloy compositions containing silicon, aluminum, transition metals such as iron, and further containing rare earth elements such as tin and yttrium. SiOx is composed of SiO and SiO ２ The compound contains at least one of the two and Si, where x is usually 0.01 or more and less than 2.

[0186] Examples of composite materials 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 carbonaceous material in an atmosphere containing, for example, organic gas and / or vapor. Alternatively, known methods such as coating the surface of SiO particles by chemical vapor deposition using organic gas, or creating composite particles (granulation) of SiO particles and graphite or artificial graphite by mechanochemical methods, can also be used.

[0187] <Electrolyte> As the electrolyte, a non-aqueous electrolyte can be used, which is obtained by dissolving a supporting electrolyte in a non-aqueous solvent. As the supporting electrolyte, lithium salts are usually used. For example, LiPF ６ LiAsF ６ LiBF ４ LiSbF６ , LiAlCl ４ , LiClO ４ , CF 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 and the like can be mentioned. Among them, LiPF ６ , LiClO ４ , CF ３ SO ３ Li is preferable. These may be used alone or in combination of two or more.

[0188] 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 methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; and the like. Among them, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. The non-aqueous solvent may be used alone or in combination of two or more in any ratio. The electrolyte solution may contain an additive. Examples of additives include carbonate-based compounds such as vinylene carbonate (VC).

[0189] Further, as the electrolyte solution other than the above, for example, polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel-like polymer electrolytes in which the polymer electrolyte is impregnated with an electrolyte solution; inorganic solid electrolytes such as LiI and Li ３ N; and the like may be used.

[0190] <Separator> The separator is not particularly limited, but a microporous membrane made of polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride), which is an insulating material, can be used.

[0191] <Method for Manufacturing a Lithium-Ion Battery> A specific method for manufacturing a lithium-ion battery according to this embodiment is, for example, to overlap a positive electrode and a negative electrode with a separator in between, roll or fold them according to the battery shape and place them in a battery container, inject an electrolyte into the battery container and seal it. Furthermore, if necessary, expanded metal, fuses, overcurrent prevention elements such as PTC elements, lead plates, etc., may be added to prevent pressure rise inside the battery and overcharging / discharging. The shape of the lithium-ion battery may be coin-shaped, button-shaped, sheet-shaped, cylindrical, rectangular, flat, or any other shape.

[0192] The following items list preferred embodiments of the present disclosure, but the present invention is not limited to the following items. [Item 1] A lithium composite phosphoroxide-containing cathode binder composition comprising an aqueous medium and core-shell particles, wherein the core-shell particles are core-shell particles 1 or core-shell particles 2 described below. Core-shell particles 1: comprising a core (C1) and a shell layer (S1) located outside the core (C1), wherein the core (C1) comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, and the shell layer (S1) comprises a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. Core-shell particles 2: comprising a core (C2) and a shell layer (S2) located outside the core (C2), wherein the core (C2) is composed of a polymer, and the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) comprises an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers. [Item 2] The lithium composite phosphorus-containing cathode binder composition according to Item 1, wherein the non-diene rubber (R1) is an acrylic rubber containing acrylic monomer units. [Item 3] The lithium composite phosphorus-containing cathode binder composition according to Item 2, wherein the proportion of alkyl acrylate in the total monomer components constituting the acrylic rubber is 50% by weight or more. [Item 4] The lithium composite phosphorus-containing cathode binder composition according to Item 2 or 3, wherein the acrylic rubber further comprises aromatic vinyl compound units. [Item 5] The lithium composite phosphorus-containing cathode binder composition according to any one of Items 1 to 4, wherein the core-shell particles 1 include monomer units having a hydrogen-bonding functional group (FG1). [Item 6] The lithium composite phosphorus-containing cathode binder composition according to Item 5, wherein the hydrogen-bonding functional group (FG1) is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, and an amide group. [Item 7] A lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 6, wherein the ratio of the core (C1) to the total core-shell particles 1 is 50 to 90% by weight.[Item 8] A lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 7, wherein the entire core (C1) is composed of the non-diene rubber (R1). [Item 9] A lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 8, wherein the shell-forming polymer (SP1) contains at least one selected from the group consisting of (meth)acrylic monomers and aromatic vinyl compounds as a constituent monomer. [Item 10] A lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 9, wherein the core (C1) is composed of at least two layers, the core (C1) comprises a core layer 1 and a core layer 2 located outside the core layer 1, the core layer 1 comprises a non-diene rubber (R1') having a glass transition temperature of 40°C or higher, and the core layer 2 comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C. [Item 11] The lithium composite phosphorus oxide-containing cathode binder composition according to Item 10, wherein the proportion of the core layer 1 in the core (C1) is 5 to 70% by weight, and the proportion of the core layer 2 is 30 to 95% by weight. [Item 12] The lithium composite phosphorus oxide-containing cathode binder composition according to Item 10 or 11, wherein the non-diene rubber (R1') contained in the core layer 1 is composed of a polymer containing at least one selected from the group consisting of (meth)acrylic monomers and aromatic vinyl compounds as constituent monomers. [Item 13] The lithium composite phosphorus oxide-containing cathode binder composition according to Item 1, wherein the shell-forming polymer (SP2) contains aromatic vinyl compounds as monomer units. [Item 14] The lithium composite phosphorus oxide-containing cathode binder composition according to Item 13, wherein the content ratio of the aromatic vinyl compound units in the shell-forming polymer (SP2) is 30 to 95% by weight. [Item 15] The lithium composite phosphoroxide-containing cathode binder composition according to Item 13 or 14, wherein the shell-forming polymer (SP2) contains (meth)acrylic monomers as monomer units. [Item 16] The lithium composite phosphoroxide-containing cathode binder composition according to Item 15, wherein the content of the (meth)acrylic monomers in the shell-forming polymer (SP2) is 1 to 55% by weight.[Item 17] The lithium composite phosphoroxide-containing cathode binder composition according to any one of Items 1 to 16, wherein the shell-forming polymer (SP2) contains monomer units having a hydrogen-bonding functional group (FG2), and the content ratio of monomer units having the hydrogen-bonding functional group (FG2) in the shell-forming polymer (SP2) is 3% by weight or more. [Item 18] The lithium composite phosphoroxide-containing cathode binder composition according to Item 17, wherein the content ratio of monomer units having the hydrogen-bonding functional group (FG2) in the shell-forming polymer (SP2) is 30% by weight or less. [Item 19] The lithium composite phosphoroxide-containing cathode binder composition according to any one of Items 1 to 18, wherein the shell-forming polymer (SP2) has a glass transition temperature of 60°C or higher. [Item 20] The lithium composite phosphoroxide-containing cathode binder composition according to any one of Items 1 to 19, wherein the shell-forming polymer (SP2) is a non-crosslinked polymer. [Item 21] The lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 20, wherein the ratio of the core (C2) to the total core-shell particles 2 is 50 to 95% by weight. [Item 22] The lithium composite phosphoroxide-containing cathode binder composition according to any one of items 1 to 21, wherein the core (C2) has a multilayer structure consisting of at least two layers, and the glass transition temperature of the polymer constituting the innermost layer of the core (C2) is 25°C or higher. [Item 23] The lithium composite phosphoroxide-containing cathode binder composition according to item 22, wherein the outermost layer of the core (C2) is composed of a polymer containing aliphatic conjugated diene compound units, and the glass transition temperature of the polymer constituting the outermost layer of the core (C2) is lower than the glass transition temperature of the polymer constituting the innermost layer of the core (C2). [Item 24] The lithium composite phosphoroxide-containing cathode binder composition according to Item 22 or 23, wherein the glass transition temperature of the polymer constituting the outermost layer of the core (C2) is -85°C or higher and less than 25°C. [Item 25] The lithium composite phosphoroxide-containing cathode binder composition according to any one of Items 22 to 24, wherein the proportion of the innermost layer of the core (C2) in the entire multilayer structure of the core (C2) is 5 to 75% by weight, and the proportion of the outermost layer of the core (C2) is 25 to 95% by weight.[Item 26] The lithium composite phosphoroxide-containing positive electrode binder composition according to any one of Items 1 to 25, wherein the core-shell particles 1 or core-shell particles 2 have a volume-average particle diameter of 100 to 400 nm. [Item 27] ​​A coating dispersion comprising lithium composite phosphoroxide and the lithium composite phosphoroxide-containing positive electrode binder composition according to any one of Items 1 to 26. [Item 28] The coating dispersion according to Item 27, wherein the lithium composite phosphoroxide comprises at least one selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, and lithium iron manganese phosphate. [Item 29] A method for manufacturing a positive electrode of a lithium-ion battery, comprising the steps of preparing a dispersion comprising lithium composite phosphoroxide and the lithium composite phosphoroxide-containing positive electrode binder composition according to any one of Items 1 to 28, and coating the dispersion onto a current collector and drying it. [Item 30] A positive electrode for a lithium-ion battery, comprising a current collector and an active material layer provided on the current collector, wherein the active material layer comprises a lithium composite phosphoroxide and core-shell particles, and the core-shell particles are core-shell particle 1 or core-shell particle 2 as described below. Core-shell particle 1: comprises a core (C1) and a shell layer (S1) located outside the core (C1), wherein the core (C1) comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, and the shell layer (S1) comprises a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. Core-shell particle 2: comprises a core (C2) and a shell layer (S2) located outside the core (C2), wherein the core (C2) is composed of a polymer, the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) contains an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers. [Item 31] A lithium-ion battery comprising the positive electrode, separator, negative electrode, and electrolyte described in Item 30.

[0193] The present disclosure will be further described below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "%" hereafter refers to "weight percent".

[0194] (Example 1-1) (Formation of core particles) Following the instructions in Table 1, 500 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (2.5% solids), and 0.1 g of polyoxyethylene lauryl ether phosphate were added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２ A solution of 0.012 g of O) and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 262 g of methyl methacrylate, 418 g of butyl acrylate, 90 g of styrene, 1 g of t-dodecyl mercaptan, 11 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (69% solids), and 3.2 g of polyoxyethylene lauryl ether phosphate was added to the polymerizer over 240 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 80 minutes after the end of addition, and core particles were formed.

[0195] (Formation of the shell layer (S1)) Next, a mixture of 184 g of methyl methacrylate, 46 g of butyl acrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-butyl hydroperoxide (69% solids) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids concentration of 45% by weight, and a volume-average particle size of 200 nm. This latex is a lithium-compound phosphoroxide-containing binder composition for cathodes. The volume-average particle size of the latex was measured using a nanoparticle size analyzer NANOTRAC WAVE manufactured by Microtrac.

[0196] (Examples 1-2 to 1-19) Latex of core-shell structured graft copolymer particles was obtained in the same manner as in Example 1-1, except that the volume-average particle size of the core-shell particles, or the type or amount of monomer used for the core particles and shell layer was changed, as described in Table 1. This latex is a lithium-compound phosphoroxide-containing binder composition for cathodes. The volume-average particle size was controlled by adjusting the amount of emulsifier at the start of polymerization.

[0197]

[0198] (Comparative Example 1-1) (Polymerization of non-core-shell structure copolymer) In accordance with the description in Table 1, 500 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 added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２ A solution of 0.012 g of O and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 396 g of methyl methacrylate, 509 g of butyl acrylate, 95 g of styrene, 1 g of t-dodecyl mercaptan, 15 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (69% solids), and 3.2 g of polyoxyethylene lauryl ether phosphate was added to the polymerizer over 360 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 80 minutes after the end of addition, yielding a non-core-shell structure copolymer latex with a conversion rate of 100%, a solids concentration of 45% by weight, and a volume-average particle size of 200 nm.

[0199] (Comparative Examples 1-2 to 1-3) Latex of a non-core-shell structure copolymer was obtained in the same manner as in Comparative Example 1-1, except that the type or amount of monomer used for the non-core-shell particles was changed according to the description in Table 1.

[0200] (Examples 1-20) (Formation of core layer 1 (hard core layer) containing a non-diene rubber (R1') with a glass transition temperature of 40°C or higher) In accordance with the description in Table 2, 500 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 added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２A solution of 0.012 g of 0.012 g and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 100 g of styrene, 1 g of allyl methacrylate, and 0.35 g of t-butyl hydroperoxide (69% solids) was added to the polymerization apparatus over 30 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 60 minutes after the end of addition, forming hard core particles.

[0201] (Formation of core particles including core layer 1 and core layer 2 (soft core layer) containing a non-diene rubber (R1) with a glass transition temperature of -60°C or higher and less than 40°C) After polymerization of the hard core particles was completed, a mixture of 670 g of 2-ethylhexyl acrylate, 1 g of allyl methacrylate, and 0.8 g of t-butyl hydroperoxide (69% solids) was added to the polymerizer over 150 minutes. During polymerization, polyoxyethylene lauryl ether phosphate and sodium hydroxide (2% solids) were added in arbitrary amounts and at arbitrary times to maintain the pH of the system at 5 to 7. Polymerization was completed 60 minutes after the end of the addition, and core particles were formed.

[0202] (Formation of the shell layer (S1)) Next, a mixture of 194 g of methyl methacrylate, 6 g of butyl acrylate, 30 g of 2-hydroxyethyl methacrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-butyl hydroperoxide (69% solids) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids concentration of 45% by weight, and a volume-average particle size of 200 nm. This latex is a lithium-compound phosphoroxide-containing binder composition for cathodes.

[0203] (Examples 1-21 to 1-29) Latex of core-shell structure graft copolymer particles was obtained in the same manner as in Example 1-20, except that the type or amount of monomers used in core layer 1, core layer 2, and shell layer of the core particles was changed according to the description in Table 2. The latex is a lithium composite phosphoroxide-containing binder composition for cathodes.

[0204]

[0205] (Example 2-1) (Formation of core particles) 300 g of disodium hydrogen phosphate (10% solid content) was added to 17,000 g of deionized water, and further, ferrous sulfate (FeSO4) was added. ４ 7H ２ A solution of 0.237 g of O and 0.395 g of ethylenediaminetetraacetate-disodium dissolved in 125.8 g of deionized water was added, and deoxidation was carried out at -0.01 MPa for 15 minutes. 66.7 g of Neoperex G-15 (manufactured by Kao Corporation: sodium dodecylbenzenesulfonate, 15.0% solids), 11,000 g of butadiene, 9,000 g of styrene, 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. 140.0 g of sodium formaldehyde sulfoxylate (5% solids) and 7.7 g of paramenthane hydroperoxide (52% solids) were added, and polymerization was started. During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, and ferrous sulfate (FeSO4) were added. ４ 7H ２ O) and disodium ethylenediaminetetraacetate were added to a 100 L pressure-resistant autoclave in any amount and at any time. After 15 hours from the start of polymerization, the polymerization was terminated by degassing under reduced pressure to remove any remaining butadiene that was not used in polymerization. A latex containing styrene-butadiene rubber particles (solids content 45.0%) was obtained.

[0206] (Formation of the shell layer (S2)) 1555.6 g of the obtained rubber latex (solids content 45.0%) was charged into an 8 L polymerizer, and 10.0 g of sodium formaldehyde sulfoxylate (solids content 5%) was added. A mixture of 200 g of styrene, 75 g of methyl methacrylate, 25 g of methacrylic acid, and 0.7 g of t-butyl hydroperoxide (solids content 69%) was added to the polymerizer over 120 minutes. t-butyl hydroperoxide (solids content 69%) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids content of 48%, and a volume-average particle size of 200 nm.

[0207] (Examples 2-2 to 2-13) Latex of core-shell structured graft copolymer particles was obtained in the same manner as in Example 2-1, except that the volume-average particle size of the core-shell particles, or the type or amount of constituent monomers of the core particles and shell layer were changed according to the description in Table 3. The volume-average particle size was controlled by adjusting the amount of emulsifier at the start of polymerization.

[0208]

[0209] (Comparative Example 2-1) (Polymerization of non-core-shell structure copolymer) 300 g of disodium hydrogen phosphate (10% solid content) was added to 17,000 g of deionized water, and further, ferrous sulfate (FeSO4) was added. ４ 7H ２ A solution of 0.237 g of O and 0.395 g of ethylenediaminetetraacetate disodium dissolved in 125.8 g of deionized water was added, and deoxidation was carried out at -0.01 MPa for 15 minutes. 66.7 g of Neoperex G-15 (manufactured by Kao Corporation: sodium dodecylbenzenesulfonate, 15.0% solids), 9,100 g of butadiene, 10,400 g of styrene, 500 g 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. 140.0 g of sodium formaldehyde sulfoxylate (5% solids) and 7.7 g of paramenthane hydroperoxide (52% solids) were added, and polymerization was started. During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, and ferrous sulfate (FeSO4) were added. ４ 7H ２ O) and ethylenediaminetetraacetate disodium were added to a 100 L pressure-resistant autoclave in any amount and at any time. After 15 hours from the start of polymerization, the polymerization was terminated by degassing under reduced pressure to remove any remaining butadiene that was not used in polymerization. A latex (solids content 45.0%) containing styrene-butadiene rubber particles containing methacrylic acid units was obtained.

[0210] (Example 2-14) (Formation of the innermost core (hard core)) 300 g of disodium hydrogen phosphate (10% solid content) was added to 17,000 g of deionized water, and further, ferrous sulfate (FeSO4) was added.４ 7H ２ A solution of 0.237 g of O and 0.395 g of ethylenediaminetetraacetate-disodium dissolved in 125.8 g of deionized water was added, and deoxidation was carried out at -0.01 MPa for 15 minutes. 66.7 g of Neoperex G-15 (manufactured by Kao Corporation: sodium dodecylbenzenesulfonate, 15.0% solids), 1,050 g of butadiene, 5,950 g of styrene, and 50 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave and the internal temperature was raised to 50°C. 70.0 g of sodium formaldehyde sulfoxylate (5% solids) and 3.85 g of paramenthane hydroperoxide (52% solids) were added, and polymerization was started. During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, and ferrous sulfate (FeSO4) were added. ４ 7H ２ O) and ethylenediaminetetraacetate disodium were added to a 100 L pressure-resistant autoclave in arbitrary amounts and at arbitrary times. After confirming that the polymerization conversion rate of butadiene and styrene was 97% or higher, the polymerization of the innermost layer of the core (core layer) was completed by degassing under reduced pressure to remove any remaining butadiene that was not used in polymerization.

[0211] (Formation of the outermost layer of the core (soft core)) After the polymerization of the hard core was completed, 5,600 g of butadiene and 1,400 g of styrene were placed in a 100 L pressure-resistant autoclave. Subsequently, 70.0 g of sodium formaldehyde sulfoxylate (5% solids) and 3.85 g of paramenthane hydroperoxide (52% solids) were added, and polymerization of the outermost layer of the core (outer core layer) was started. During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, and ferrous sulfate (FeSO4) were added. ４ 7H ２O) and ethylenediaminetetraacetate disodium were added to a 100 L pressure-resistant autoclave in any amount and at any time. After confirming that the polymerization conversion rate of butadiene and styrene was 97% or higher, the polymerization of the core was completed by degassing under reduced pressure to remove the remaining butadiene that was not used in polymerization. As a result, a latex (solids content 45.0%) containing styrene-butadiene rubber and multilayered particles having an innermost layer and an outermost layer was obtained.

[0212] (Formation of the shell layer (S2)) 1555.6 g of the obtained rubber latex (solids content 45.0%) was charged into an 8 L polymerizer, and 10.0 g of sodium formaldehyde sulfoxylate (solids content 5%) was added. A mixture of 200 g of styrene, 75 g of methyl methacrylate, 25 g of methacrylic acid, and 0.7 g of t-butyl hydroperoxide (solids content 69%) was added to the polymerizer over 120 minutes. t-butyl hydroperoxide (solids content 69%) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids content of 48%, and a volume-average particle size of 200 nm.

[0213] (Examples 2-15 to 2-24) Latex of core-shell structured graft copolymer particles was obtained in the same manner as in Example 2-1, except that the type or amount of constituent monomers of the core particles and shell layer was changed according to the description in Table 4.

[0214]

[0215] (Preparation of cathode slurry) Lithium iron phosphate (LFP), acetylene black, single-walled carbon nanotubes, carboxymethylcellulose (CMC), and the latex binder obtained above were mixed in a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro") in a weight ratio of 96.95:1:0.05:1:1 (latex binder is the amount of solids) to prepare a cathode slurry with a solid content of 50% by weight.

[0216] (Preparation of positive electrode) The obtained positive electrode slurry was coated to a thickness of 100 μm onto a current collector made of 20 μm thick aluminum foil using a coating machine, and vacuum dried at 80°C for 12 hours. It was then pressed at 80°C and 5 kN using a roll press, and subsequently punched out into a φ14 mm disc shape to form the positive electrode.

[0217] (Fabrication of coin-type secondary battery) A coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of φ15 mm was used as the negative electrode. For the electrolyte, a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was used, and LiPF ６ A solution of [substance name] dissolved at a concentration of 1 mol / L was used. A porous polymer film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using a conventional method to obtain a coin-type secondary battery (CR-2032).

[0218] (Evaluation of the coating properties of the slurry on the current collector) The surface of the positive electrode active material layer after drying was visually observed and the coating properties were determined based on the following criteria. ○: No streaks or spots are observed on the surface of the positive electrode active material layer. △: No streaks are observed on the surface of the positive electrode active material layer, but spots are observed. ×: Streaks and spots are observed on the surface of the positive electrode active material layer.

[0219] (Evaluation of bonding properties) The positive electrode prepared as described above was cut into a rectangle 60 mm long and 20 mm wide to make a test piece. With the positive electrode active material layer facing upwards, cellophane tape (as specified in JIS Z1522) was attached to the surface of the positive electrode active material layer, and the stress was measured when the tape was peeled off by pulling one end of the current collector vertically at a speed of 50 mm / min. Three measurements were taken, and the average value was calculated and defined as the T-shaped peel strength.

[0220] (Evaluation of charge-discharge cycle characteristics) The fabricated coin-type secondary battery was charged at a constant current of 0.3C until the battery voltage reached 3.8V, and then discharged at a constant current of 0.3C until the battery voltage reached 2.5V. This process was repeated 100 times in a 30°C environment. The ratio of the discharge capacity after 100 cycles to the discharge capacity after 1 cycle (charge-discharge capacity retention rate = (discharge capacity after 100 cycles / discharge capacity after 1 cycle) × 100%) was then calculated.

[0221] As shown in Tables 1 and 2, the slurry prepared using the latex of the core-shell structure graft copolymer particles obtained in Examples 1-1 to 1-29 exhibited good coating properties for current collectors. The positive electrode prepared using this slurry showed excellent peel strength between the current collector and the active material, and the battery using this positive electrode exhibited excellent charge-discharge cycle characteristics.

[0222] On the other hand, the slurry prepared using the non-core-shell copolymer latex obtained in Comparative Example 1-1 showed inferior coating properties to the current collector compared to each of the examples 1-1 to 1-29. The positive electrode prepared using this slurry showed inferior peel strength between the current collector and the active material, and the battery using this positive electrode showed inferior charge-discharge cycle characteristics. Furthermore, the positive electrode prepared using the slurry prepared using the non-core-shell copolymer latex obtained in Comparative Examples 1-2 and 1-3 showed inferior peel strength between the current collector and the active material, and the battery using this positive electrode showed inferior charge-discharge cycle characteristics.

[0223] As shown in Tables 3 and 4, Examples 2-1 to 2-24 exhibited good coating properties for current collectors, as well as excellent peel strength and cycle characteristics.

[0224] On the other hand, Comparative Example 1, which used non-core-shell copolymer particles, showed lower peel strength and charge-discharge cycle characteristics compared to each of the examples 2-1 to 2-24.

Claims

1. A lithium composite phosphoroxide-containing cathode binder composition comprising an aqueous medium and core-shell particles, wherein the core-shell particles are either core-shell particle 1 or core-shell particle 2 described below. Core-shell particle 1: comprises a core (C1) and a shell layer (S1) located outside the core (C1), wherein the core (C1) comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, and the shell layer (S1) comprises a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. Core-shell particle 2: comprises a core (C2) and a shell layer (S2) located outside the core (C2), wherein the core (C2) is composed of a polymer, and the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) comprises an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers.

2. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the non-diene rubber (R1) is an acrylic rubber containing acrylic monomer units, and the proportion of alkyl acrylate in the total monomer components constituting the acrylic rubber is 50% by weight or more.

3. The lithium composite phosphoroxide-containing cathode binder composition according to claim 2, wherein the acrylic rubber further comprises aromatic vinyl compound units.

4. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the core-shell particles 1 include monomer units having a hydrogen-bonding functional group (FG1).

5. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the core (C1) is composed of at least two layers, the core (C1) includes a core layer 1 and a core layer 2 located outside the core layer 1, the core layer 1 includes a non-diene rubber (R1') having a glass transition temperature of 40°C or higher, and the core layer 2 includes a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C.

6. The lithium composite phosphoroxide-containing cathode binder composition according to claim 5, wherein the proportion of the core layer 1 in the core (C1) is 5 to 70% by weight, and the proportion of the core layer 2 is 30 to 95% by weight.

7. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the shell-forming polymer (SP2) comprises an aromatic vinyl compound and a (meth)acrylic monomer as monomer units.

8. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the shell-forming polymer (SP2) comprises monomer units having a hydrogen-bonding functional group (FG2), and the content ratio of monomer units having the hydrogen-bonding functional group (FG2) in the shell-forming polymer (SP2) is 3% by weight or more.

9. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the shell-forming polymer (SP2) has a glass transition temperature of 60°C or higher.

10. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the shell-forming polymer (SP2) is a non-crosslinked polymer.

11. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the core (C2) has a multilayer structure consisting of at least two layers, and the glass transition temperature of the polymer constituting the innermost layer of the core (C2) is 25°C or higher.

12. The lithium composite phosphoroxide-containing cathode binder composition according to claim 11, wherein the outermost layer of the core (C2) is composed of a polymer containing aliphatic conjugated diene compound units, and the glass transition temperature of the polymer constituting the outermost layer of the core (C2) is lower than the glass transition temperature of the polymer constituting the innermost layer of the core (C2).

13. The lithium composite phosphoroxide-containing cathode binder composition according to claim 11, wherein the proportion of the innermost layer of the core (C2) in the entire multilayer structure of the core (C2) is 5 to 75% by weight, and the proportion of the outermost layer of the core (C2) is 25 to 95% by weight.

14. The lithium composite phosphoroxide-containing cathode binder composition according to claim 1, wherein the core-shell particles 1 or core-shell particles 2 have a volume-average particle diameter of 100 to 400 nm.

15. A positive electrode for a lithium-ion battery, comprising a current collector and an active material layer provided on the current collector, wherein the active material layer comprises a lithium composite phosphoroxide and core-shell particles, and the core-shell particles are either core-shell particle 1 or core-shell particle 2 as described below. Core-shell particle 1: comprises a core (C1) and a shell layer (S1) located outside the core (C1), wherein the core (C1) comprises a non-diene rubber (R1) having a glass transition temperature of -60°C or higher and less than 40°C, and the shell layer (S1) comprises a shell-forming polymer (SP1) having a glass transition temperature of 40 to 120°C. Core-shell particle 2: comprises a core (C2) and a shell layer (S2) located outside the core (C2), wherein the core (C2) is composed of a polymer, and the shell layer (S2) is composed of a shell-forming polymer (SP2), and the core (C2) contains an aliphatic conjugated diene compound and an aromatic vinyl compound as constituent monomers.

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