Adhesive for the positive electrode of a secondary battery

Abstract

This invention provides an adhesive for use in the positive electrode of a secondary battery. The slurry exhibits good coatability to the current collector, good adhesion between the current collector and the active material layer, and good charge / discharge characteristics of the secondary battery, while also suppressing swelling in the electrolyte. The adhesive for the positive electrode of the secondary battery comprises core-shell particles, each containing a core and a shell layer surrounding the core. The core comprises a non-diene rubber with a glass transition temperature of -60 to +40°C, and the shell layer comprises a shell-forming polymer. The core-shell particles contain monomer units with hydrogen-bonded functional groups, and the proportion of monomer units with hydrogen-bonded functional groups relative to the total amount of core-shell particles is 0.6 to 20% by weight.

Description

Technical Field

[0001] This invention relates to an adhesive used in the positive electrode of a secondary battery, the positive electrode of a secondary battery and its manufacturing method, and the secondary battery itself. Background Technology

[0002] Secondary batteries, especially lithium-ion batteries, are lightweight, have high energy density, and can be repeatedly charged and discharged, making them widely used in personal computers, smartphones, electric vehicles, 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 a metal foil. This layer consists of electrode active materials such as lithium-containing composite metal oxides and a binder. The electrode active material layer is typically manufactured by coating a slurry prepared by mixing the electrode active materials, binder, and solvent onto the current collector and then drying it.

[0004] The binder used in such an electrode active material layer must maintain the adhesion between the electrode active material and the current collector. Furthermore, it must also have good coatability when the slurry is applied to the current collector and good charge / discharge characteristics when forming a lithium-ion battery.

[0005] Polyvinylidene fluoride (PVDF) resin is known as a material that constitutes an adhesive. However, the raw material for PVDF resin is Freon gas, which has a high environmental impact. Therefore, it is desirable to replace it with a material that has a lower environmental impact.

[0006] As an example of a positive electrode binder that replaces polyvinylidene fluoride resin, Patent Document 1 discloses a particulate binder composed of an organic polymer. As a specific example, a core-shell type rubber particle is described (paragraph

[0018] ), which has a core with a high glass transition temperature containing acrylonitrile units and a soft shell containing acrylate units. In this document, the electrode active material that can be used is limited to a composite oxide containing a transition metal (e.g., lithium cobalt oxide).

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

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2002-117834 Summary of the Invention

[0011] When lithium complex phosphates such as lithium iron phosphate are used as the active material for the positive electrode of secondary batteries, binders other than polyvinylidene fluoride resin have not been sufficiently studied. In the aforementioned Patent Document 1, the active material for the positive electrode that can be used does not include lithium complex phosphates.

[0012] The inventors have studied core-shell type rubber particles as a binder for the positive electrode of a secondary battery that can replace polyvinylidene fluoride resin. The results showed that sometimes the slurry's coating properties on the current collector, the adhesion between the current collector and the active material layer, and the charge / discharge characteristics of the secondary battery are insufficient. Furthermore, there is a tendency for the binder to swell easily in the electrolyte. If the binder swells, there are concerns that the volume of the active material layer increases within the battery, or that the active material layer is easily peeled off.

[0013] In view of the above situation, the purpose of the present invention is to provide a binder for the positive electrode of a secondary battery, which has good coating properties for the current collector, good adhesion between the current collector and the active material layer, and good charge and discharge characteristics of the secondary battery, and suppresses swelling in the electrolyte.

[0014] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that by using core-shell particles as a binder in the positive electrode of a secondary battery, the above-mentioned objective can be achieved, thereby completing the present invention. The core-shell particles have a core comprising a non-diene rubber with a glass transition temperature in a specific range and a shell composed of a polymer, and contain monomer units with hydrogen-bonded functional groups in a specific proportion.

[0015] That is, the present invention relates to a binder for the positive electrode, specifically a binder for the positive electrode of a secondary battery.

[0016] It includes core-shell particles, which consist of a nucleus and a shell located outside the nucleus.

[0017] The aforementioned core contains non-diene rubber with a glass transition temperature of -60 to +40°C.

[0018] The aforementioned shell layer comprises a shell-forming polymer.

[0019] The aforementioned core-shell particles contain monomeric units with hydrogen-bonded functional groups.

[0020] The proportion of the monomeric units with hydrogen-bonded functional groups is 0.6 to 20% by weight relative to the total amount of the core-shell particles.

[0021] In addition, the present invention also relates to a coating dispersion comprising a positive electrode active material, the above-mentioned positive electrode binder, and a solvent.

[0022] Furthermore, the present invention also relates to a method for manufacturing a positive electrode of a secondary battery, comprising: a step of preparing a dispersion containing a positive electrode active material, the aforementioned positive electrode binder and solvent, and a step of coating the aforementioned dispersion onto a current collector and drying it.

[0023] Furthermore, the present invention also relates to a positive electrode, which is the positive electrode of a secondary battery comprising a current collector and an active material layer disposed on the current collector, wherein the active material layer comprises a positive electrode active material and a positive electrode binder.

[0024] Furthermore, the present invention also relates to a secondary battery comprising the aforementioned positive electrode, separator, negative electrode, and electrolyte.

[0025] According to the present invention, an adhesive can be provided for the positive electrode of a secondary battery, which has good coating properties for the current collector, good adhesion between the current collector and the active material layer, and good charge and discharge characteristics of the secondary battery, and suppresses swelling in the electrolyte.

[0026] By using the binder of the present invention, it is possible to suitably manufacture dispersions for coating current collectors on positive electrodes, positive electrodes of secondary batteries, and secondary batteries.

[0027] The binder of the present invention is a binder used in the positive electrode of a secondary battery, and it is very promising as a material to replace polyvinylidene fluoride resin, which has a high environmental impact. Detailed Implementation

[0028] The embodiments of the present invention are described in detail below.

[0029] [Adhesive for the positive electrode of a secondary battery]

[0030] The binder for the positive electrode in this embodiment is a binder used in the positive electrode of a secondary battery, for example, a binder used in a positive electrode containing a positive electrode active material such as an alkali metal complex oxide, particularly a lithium complex oxide. This binder contains at least core-shell particles.

[0031] (Nuclear-shell particle)

[0032] The aforementioned core-shell particles have a core-shell structure comprising a core containing a non-diene rubber and a shell layer located outside the core. The shell layer comprises a shell-forming polymer.

[0033] The core-shell particles described above contain monomer units with hydrogen-bonded functional groups. These monomer units with hydrogen-bonded functional groups may be contained in either or both of the aforementioned non-diene rubbers or the aforementioned shell-forming polymers.

[0034] Here, "rubber" refers to a component possessing rubber elasticity. Rubber elasticity refers to the ability to absorb energy generated by external forces and store it as energy for restoring its original shape. To exhibit rubber elasticity, the molecules need to be sufficiently long, capable of free movement, and appropriately bound together. Even if a component with rubber elasticity is deformed by external forces, it can easily return to its original shape by releasing the external forces.

[0035] Core-shell particles with this structure are used as binders in secondary batteries. Using these particles, a slurry containing positive electrode active material can be formed. This slurry is then coated onto the surface of a current collector and dried, thereby creating a positive electrode with an active material layer formed on the current collector. The slurry exhibits good coatability to the current collector, excellent adhesion between the current collector and the active material layer in the resulting positive electrode, and good charge / discharge characteristics of the secondary battery containing this positive electrode. Furthermore, the swelling of the core-shell particles in the electrolyte of the secondary battery is suppressed.

[0036] It is speculated that the presence of hydrogen-bonded functional groups in the core-shell particles alters the intermolecular interactions between the solvent in the electrolyte and the core-shell particles, thereby suppressing the swelling of the core-shell particles.

[0037] (nuclear)

[0038] The aforementioned core consists of particles containing non-diene rubber. Non-diene rubber refers to rubber other than diene rubber. Diene rubber refers to rubber containing aliphatic conjugated diene compounds such as 1,3-butadiene as constituent units; specific examples include butadiene rubber and styrene-butadiene rubber (SBR).

[0039] Compared to diene rubbers, non-diene rubbers are more resistant to oxidation, thus having the advantage that oxidation degradation is less likely to occur during the charging and discharging of secondary batteries.

[0040] Examples of non-diene-based rubbers include acrylic rubbers and polyorganosiloxane rubbers. In particular, from the viewpoint of the adhesion between the current collector and the active material layer, and the charge / discharge characteristics of the secondary battery, acrylic rubbers are preferred.

[0041] Conventional adhesives using acrylic rubbers tend to swell significantly in electrolytes. However, in this embodiment, by including monomers with hydrogen-bonding functional groups in the core-shell particles at a specific ratio, such swelling can be suppressed. Therefore, this embodiment allows for the appropriate use of core-shell particles containing acrylic rubbers.

[0042] The aforementioned acrylic rubber refers to rubber that contains acrylic monomer units as its main constituent units.

[0043] The acrylic monomers mentioned above are not particularly limited, and examples include alkyl acrylates such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, dodecyl acrylate, stearyl acrylate, and behenyl acrylate; acrylates containing aromatic rings such as phenoxyethyl acrylate and benzyl acrylate; glycidyl acrylate and glycidyl acrylate; and alkoxyalkyl acrylates. Additionally, acrylic monomers with hydrogen-bonding functional groups, as described below, can also be included. Acrylic monomers can be used alone or in combination of two or more.

[0044] As the aforementioned acrylic monomers, alkyl acrylates are preferred, and butyl acrylates are particularly preferred.

[0045] From the viewpoints of the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery, the proportion of the alkyl acrylate in the monomer components constituting the above-mentioned acrylic rubber (referring to monomer components excluding the polyfunctional monomers described below; the same applies hereinafter) 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. An upper limit of 100% by weight or less is acceptable.

[0046] In the aforementioned acrylic rubbers, monomers other than those based on acrylic monomers may not be used, but their use is preferred. Examples of such other monomers include methacrylic acid monomers, aromatic vinyl compounds such as styrene, cyanide compounds such as acrylonitrile, halogenated vinyl compounds such as vinyl chloride, vinyl acetate, and olefins such as ethylene and propylene. Aromatic vinyl compounds such as styrene are preferred from the perspective of ease of producing core-shell particles and ease of adjusting the glass transition temperature of the rubber to an appropriate range.

[0047] From the viewpoint of polymerization rate, the aforementioned acrylic rubber preferably uses methacrylate monomers, and more preferably uses alkyl methacrylates. 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 the coating properties of the slurry to the current collector and the charge-discharge characteristics of the secondary battery, the proportion of the aromatic vinyl compound in the monomer component constituting the above-mentioned 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 coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery, the proportion of the aromatic vinyl compound is preferably 30% by weight or less, more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less.

[0049] The aforementioned non-diene rubber preferably contains monomer units with hydrogen-bonded functional groups. This suppresses the swelling of core-shell particles in the electrolyte, resulting in good charge-discharge characteristics of the secondary battery. However, if the shell-forming polymer described later includes this unit, the non-diene rubber may not contain it.

[0050] The aforementioned hydrogen-bonding functional groups refer to functional groups containing hydrogen atoms that can bond with highly electronegative atoms such as oxygen atoms through hydrogen bonds.

[0051] Specific examples include hydroxyl, carboxyl, amide, and sulfonic acid groups, and at least one group selected from hydroxyl, carboxyl, amide, and sulfonic acid groups can be used. The amide group may have substituents on the nitrogen atom, but a primary amide without substituents is preferred. As a hydrogen-bonding functional group, only one type can be used, or two or more types can be used in combination.

[0052] As a monomer with hydrogen-bonding functional groups, vinyl monomers with hydrogen-bonding functional groups are preferred, and (meth)acrylic monomers with hydrogen-bonding functional groups are more preferred.

[0053] Among monomers with hydrogen-bonded functional groups, examples of monomers with hydroxyl groups include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0054] Alternatively, as a monomer containing hydroxyl groups, (meth)acrylates with (poly)alkylene glycol chains having hydroxyl terminal groups can also be used. Specific examples include (meth)acrylate (poly)ethylene glycol esters (Nippon Oil BLEMMER PE-90, PE-200, PE-350, AE-90U, AE-200, AE-400, etc.), (meth)acrylate (poly)propylene glycol esters (Nippon Oil BLEMMER PP-500, PP-500D, PP-800, PP-1000, PP-2000D, AP-200, AP-400, AP-400D, AP-550, AP-800, AP-1000D, etc.), and (meth)acrylate (poly)ethylene glycol-(poly)propylene glycol esters (Nippon Oil BLEMMER). 50PEP-300, etc.), (meth)acrylate (poly)ethylene glycol-(poly)butylene glycol ester (Nippon Oil BLEMMER 55PET-800, 50PEP-500D, etc.), (meth)acrylate (poly)propylene glycol-(poly)butylene glycol ester (Nippon Oil BLEMMER 10PPB-500B, 10PPB-500BD, etc.), etc.

[0055] Examples of monomers containing 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.

[0056] Examples of monomers containing 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.

[0057] Examples of monomers having a sulfonic acid group include vinyl monomer units having a sulfonic acid group. Examples of sulfonic acid groups include styrene sulfonic acid, methyl allyl sulfonic acid, allyl sulfonic acid, vinyl sulfonic acid, and isoprene sulfonic acid. Among these, styrene sulfonic acid is preferred from the viewpoint of having high surface activity, polymerizability, and polymerization stability, and being able to stably manufacture core-shell particles. P-Styrene sulfonic acid is more preferred.

[0058] From the viewpoint of adhesion between the current collector and the active material layer, monomers containing hydrogen-bonded functional groups in the aforementioned non-diene rubbers are preferred as monomers. From the viewpoint of charge-discharge characteristics of secondary batteries, monomers containing carboxyl groups are preferred. Furthermore, from the viewpoint of coating properties and internal resistance of secondary batteries, monomers containing sulfonic acid groups are preferred.

[0059] When the aforementioned non-diene rubber contains monomer units with hydrogen-bonded functional groups, from the viewpoints of the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte, the proportion of monomers with hydrogen-bonded functional groups in the monomer component constituting the non-diene rubber is preferably 1 to 30% by weight. Particularly from the viewpoints of charge-discharge characteristics and suppression of swelling, 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 adhesion, 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.

[0060] The aforementioned acrylic rubbers have a cross-linked structure. To introduce this cross-linked structure, for example, when synthesizing acrylic rubbers by polymerizing monomer components, it is sufficient to use cross-linking components such as multifunctional monomers.

[0061] Examples of the aforementioned multifunctional monomers include allyl methacrylate; allyl alkyl methacrylate; allyloxyalkyl methacrylate; polyfunctional (meth)acrylates having two or more (meth)acrylate 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. Allyl methacrylate, triallyl isocyanurate, butanediol di(meth)acrylate, and divinylbenzene are preferred, and allyl methacrylate is particularly preferred.

[0062] The amount of the aforementioned multifunctional monomers used can be within a known range and is not particularly limited. For example, relative to the total 100 parts by weight of the monomer components constituting the aforementioned acrylic rubber, it can 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 2 parts by weight.

[0063] The aforementioned acrylic rubbers can be composed of a single type of rubber, or they can be composed of multiple types of rubbers with different types or amounts of monomers and / or polyfunctional monomers.

[0064] As for the aforementioned non-diene rubber, a soft rubber is used from the viewpoint of improving the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery. Specifically, the aforementioned non-diene rubber uses a rubber with a glass transition temperature (Tg) in the range of -60°C to +40°C. If the Tg of the aforementioned non-diene rubber is higher than 40°C, the coating and adhesion tend to decrease, particularly. Furthermore, if the Tg of the aforementioned non-diene rubber is lower than -60°C, particle shape deformation easily occurs, thus the resistivity tends to increase.

[0065] The lower limit of the glass transition temperature of the aforementioned non-diene rubber is more preferably -55°C or higher, further preferably -50°C or higher, even more preferably -40°C or higher, still more preferably -20°C or higher, and particularly preferably 0°C or higher. The upper limit is more preferably +35°C or lower, further preferably +30°C or lower, and particularly preferably +25°C or lower.

[0066] The glass transition temperature of the aforementioned non-diene rubbers can be controlled by changing the types and ratios of monomers in the rubber. For example, increasing the proportion of aromatic vinyl compounds in the non-diene rubber can increase its glass transition temperature.

[0067] The glass transition temperatures mentioned above can be determined using differential scanning calorimetry (DSC). The same applies below.

[0068] From the viewpoints of the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte, the proportion of the aforementioned non-diene rubber in the total core-shell particles is preferably 35 to 90% by weight. Particularly from the viewpoint of adhesion, the lower limit is more preferably 50% by weight or more, further preferably 60% by weight or more, and even more preferably 70% by weight or more. Particularly from the viewpoint of coating properties, the upper limit is more preferably 85% by weight or less, and even more preferably 80% by weight or less.

[0069] (shell)

[0070] The aforementioned shell refers to the polymer layer located on the surface side of the core-shell particle, also known as the graft layer. The shell is preferably grafted to the core. However, the polymers forming the shell may also include polymers that are not grafted to the core.

[0071] The shell covers the surface of the core, but is not limited to the entire surface of the core-covered surface, but only at least a portion of the surface of the core-covered surface.

[0072] By setting the aforementioned shell layer, the dispersibility of core-shell particles and positive electrode active materials in the slurry can be improved, thus enhancing the slurry's coating properties on the current collector. Consequently, the adhesion between the current collector and the active material layer, as well as the charge-discharge characteristics of the secondary battery, can also be improved.

[0073] From the viewpoints of the dispersibility of core-shell particles and positive electrode active material in the slurry, and the coating properties of the slurry on the current collector, the aforementioned shell layer is preferably composed of a non-crosslinked polymer. This non-crosslinked polymer refers to a polymer that does not contain crosslinked structures and does not contain constituent units derived from multifunctional monomers, and is a polymer that does not belong to rubber elastomers such as acrylic rubbers.

[0074] The polymer that forms the shell (hereinafter also referred to as the shell-forming polymer) is preferably a vinyl polymer.

[0075] The monomers constituting the shell-forming polymer described above are not particularly limited to vinyl monomers, but preferably include at least one selected from (meth)acrylic acid monomers, aromatic vinyl compounds, and vinyl cyanide compounds. It should be noted that "(meth)acrylic acid" is used to refer to both acrylic acid and methacrylic acid.

[0076] The monomers described above (meth)acrylate are not particularly limited, and 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 meth acrylates such as phenoxyethyl methacrylate and benzyl methacrylate; glycidyl methacrylate and glycidyl methacrylate; alkoxyalkyl methacrylate; and methacrylamide. Additionally, monomers with hydrogen-bonding functional groups as described above can also be included. Methacrylate monomers can be used alone or in combination of two or more.

[0077] As the above-mentioned (meth)acrylic monomers, alkyl (meth)acrylates are preferred, and alkyl methacrylates are particularly preferred.

[0078] The aromatic vinyl compounds mentioned above are not particularly limited, and examples include unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; cycloalkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; cycloalkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; cyclohalogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; cycloesterified vinyl aromatic compounds such as 4-acetoxystyrene; and cyclohydroxylated vinyl aromatic compounds such as 4-hydroxystyrene. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene are more preferred, and styrene is particularly preferred. Aromatic vinyl compounds can be used alone or in combination of two or more.

[0079] The cyanide compound mentioned above is not particularly limited, and examples include acrylonitrile and methacrylonitrile. Acrylonitrile is preferred.

[0080] As monomers constituting the aforementioned shell-forming polymers, vinyl monomers other than those described above may also be used. Examples of such monomers include olefins such as ethylene and propylene; halogenated vinyl groups 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 isopropylene vinyl ketone; and heterocyclic vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole.

[0081] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer and improving the dispersibility of core-shell particles and positive electrode active materials in the slurry, the shell-forming polymer preferably contains at least methacrylic monomer units, and particularly preferably contains methacrylic monomer units, acrylic monomer units and / or aromatic vinyl compound units.

[0082] As the aforementioned methacrylate 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.

[0083] From the viewpoint of increasing the glass transition temperature of the aforementioned shell-forming polymer, the proportion of the aforementioned methacrylate monomers (especially alkyl methacrylates) units in the total monomer component constituting the aforementioned shell-forming polymer is preferably 40 to 100% by weight. The lower limits are more preferably 60% by weight or more, 70% by weight or more, 75% by weight or more, and 80% by weight or more, in that order. From the viewpoint of the adhesion between the current collector and the active material layer, the upper limit is more preferably 95% by weight or less, and even more preferably 90% by weight or less.

[0084] The proportion of the cyanide compound in the monomer components constituting the shell-forming polymer can be approximately 0-30% by weight, 0-20% by weight, or 0-10% by weight. The shell-forming polymer may not contain the cyanide compound.

[0085] The shell-forming polymer described above preferably contains monomer units with hydrogen-bonded functional groups. This suppresses the swelling of core-shell particles in the electrolyte, resulting in good charge-discharge characteristics of the secondary battery. However, if the non-diene rubber described above contains this unit, the shell-forming polymer may not contain it.

[0086] The hydrogen-bonding functional groups are as described above. Additionally, the aforementioned compounds can also be used as monomers possessing hydrogen-bonding functional groups.

[0087] From the viewpoint of adhesion between the current collector and the active material layer, monomers containing hydrogen-bonding functional groups in the aforementioned shell-forming polymer are preferred as monomers. From the viewpoint of charge-discharge characteristics of the secondary battery, monomers containing carboxyl groups are preferred. Furthermore, from the viewpoint of coating properties and internal resistance of the secondary battery, monomers containing sulfonic acid groups are preferred.

[0088] When the shell-forming polymer comprises monomer units having hydrogen-bonded functional groups, from the viewpoints of the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte, the proportion of monomers having hydrogen-bonded functional groups in the monomer components constituting the shell-forming polymer is preferably 1 to 60% by weight. Particularly from the viewpoints of charge-discharge characteristics and suppression of swelling, 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 adhesion, the upper limit is more preferably 45% by weight or less, even more preferably 30% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less.

[0089] From the viewpoint of easily maintaining the shape of the core-shell particles and improving the dispersibility of the core-shell particles and the positive electrode active material in the slurry, the aforementioned shell-forming polymer is preferably rigid. Specifically, the glass transition temperature (Tg) of the aforementioned shell-forming polymer is preferably 40°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of the adhesion between the current collector and the active material layer, the upper limit is preferably 120°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 90°C or lower.

[0090] The glass transition temperature of the aforementioned shell-forming polymer can be controlled by changing the types and ratios of the monomers constituting the polymer. For example, by using methacrylic acid monomers and aromatic vinyl compound units as the monomers constituting the aforementioned shell-forming polymer, the glass transition temperature of the polymer can be increased.

[0091] From the viewpoints of the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte, the proportion of the shell-forming polymer in the total core-shell particles is preferably 10 to 65% by weight. Particularly from the viewpoint of adhesion, the upper limit is more preferably 50% by weight or less, further preferably 40% by weight or less, and even more preferably 30% by weight or less. Particularly from the viewpoint of coating properties, the lower limit is more preferably 15% by weight or more, and even more preferably 20% by weight or more.

[0092] Monomer units with hydrogen-bonded functional groups may be included in either or both of non-diene rubbers or shell-forming polymers, but regardless of their location, the total content of monomer units with hydrogen-bonded functional groups relative to the total amount of the aforementioned core-shell particles is set in the range of 0.6 to 20% by weight.

[0093] If the content is less than 0.6% by weight, the swelling of core-shell particles in the electrolyte cannot be adequately suppressed, or the charge-discharge characteristics of the secondary battery become insufficient. Furthermore, if the content is more than 20% by weight, the slurry's coatability to the current collector and the adhesion between the current collector and the active material layer become insufficient.

[0094] The lower limit is preferably 1% by weight or more, more preferably 1.5% by weight or more. The upper limit is preferably 15% by weight or less, more preferably 10% by weight or less.

[0095] The aforementioned core-shell particles may consist of only a core and a shell, but to achieve the desired effect, an intermediate layer may be further provided between the core and the shell. This intermediate layer is a polymer layer, preferably grafted onto the core layer. In the case of such an intermediate layer, at least a portion of the surface of the core layer is covered by the intermediate layer, and at least a portion of the surface of the shell layer is covered by the shell layer.

[0096] (Volume average particle size of core-shell particles)

[0097] The volume average particle size of the core-shell particles is not particularly limited, and can be, for example, around 10 to 1000 nm. However, from the viewpoints of the dispersibility of the core-shell particles in the slurry, the coating properties of the slurry onto the current collector, the adhesion between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery, the volume average particle size of the core-shell particles is preferably 100 to 500 nm. The lower limit is more preferably 150 nm or more, and even more preferably 200 nm or more. The upper limit is more preferably 400 nm or less, even more preferably 350 nm or less, and particularly preferably 300 nm or less.

[0098] It should be noted that the volume average particle size of the core-shell particles is measured in the latex state using a particle size measuring device. The particle size of the core-shell particles can be controlled by the type or amount of polymerization initiators, chain transfer agents, redox agents, emulsifiers, etc., used during polymerization, as well as the polymerization temperature and polymerization time.

[0099] (Methods for manufacturing core-shell particles)

[0100] There are no particular limitations on the manufacturing method of the aforementioned core-shell particles. For example, emulsion polymerization, microemulsion polymerization, emulsion polymerization, and emulsion polymerization without emulsifiers (soap) can be used.

[0101] There are no particular limitations on the emulsifiers that can be used in emulsion polymerization; anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc., can be used. Additionally, dispersants such as polyvinyl alcohol, alkyl-substituted cellulose, polyvinylpyrrolidone, and polyacrylic acid derivatives can also be used in conjunction.

[0102] Among the emulsifiers mentioned above, there are no particular limitations on anionic surfactants. Examples include: potassium laurate, potassium coconut oil fatty acid, potassium myristate, potassium oleate, potassium oleate diethanolamine salt, sodium oleate, potassium palmitate, potassium stearate, sodium stearate, mixed fatty acid sodium soap, semi-hydrogenated tallow fatty acid sodium soap, castor oil potassium soap, and other fatty acid soaps; sodium lauryl sulfate, sodium higher alcohol sulfate, triethanolamine lauryl sulfate, ammonium lauryl sulfate, sodium polyoxyethylene alkyl ether sulfate, triethanolamine polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl phenyl ether sulfate, 2-ethylhexyl sulfate, etc. Alkyl sulfate salts such as sodium hexyl sulfate; sodium alkylbenzene sulfonate such as sodium dodecylbenzene sulfonate; sodium dialkyl sulfosuccinate such as sodium di-2-ethylhexyl sulfosuccinate; sodium alkylnaphthalene sulfonate; sodium alkyl diphenyl ether disulfonate; potassium alkyl phosphate; phosphate salts such as sodium polyoxyethylene lauryl ether phosphate; sodium salts of naphthalene sulfonic acid formaldehyde condensate; polycarboxylic acid type polymer anions; sodium acyl (tallow) methyl taurate; sodium acyl (coconut oil) methyl taurate; sodium cocoyl hydroxyethyl sulfonate; sodium α-sulfonyl fatty acid ester; sodium amide ether sulfonate; oleoyl sarcosine; sodium lauroyl sarcosine; rosin acid soap, etc.

[0103] Among the emulsifiers mentioned above, there are no particular limitations on nonionic surfactants. Examples include the following compounds: polyoxyethylene nonylphenyl ether, polyoxyethylene oil-based ether, polyoxyethylene lauryl ether, and other polyoxyethylene alkyl allyl ethers or polyoxyethylene alkyl ethers; polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monostearate, and other polyoxyethylene sorbitan esters; polyethylene glycol monolaurate, polyethylene glycol monostearate, polyethylene glycol monooleate, and other polyoxyethylene fatty acid esters; and oxyethylene / oxypropylene block copolymers, etc.

[0104] Among the emulsifiers mentioned above, there are no particular limitations on cationic surfactants. Examples include alkylamine salts such as cocoamine acetate, stearamine acetate, octadecylamine acetate, and tetradecylamine acetate; and quaternary ammonium salts such as lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, distearate dimethylammonium chloride, alkylbenzyldimethylammonium chloride, hexadecyltrimethylammonium chloride, and behenyltrimethylammonium chloride.

[0105] Among the emulsifiers mentioned above, there are no particular limitations on the amphoteric surfactants used. Examples of such compounds include: alkyl betaines such as lauryl betaine, stearyl betaine, and dimethyl lauryl betaine; sodium lauryl diaminoethyl glycinate; amide betaine; imidazoline; and lauryl carboxymethyl hydroxyethyl imidazoline. Betaine, etc.

[0106] These emulsifiers can be used alone or in combination of two or more. From the viewpoint of achieving good flowability of the resulting latex, sodium dialkyl sulfosuccinate or surfactants with an oxyethylene structure are preferred, with sodium polyoxyethylene lauryl ether phosphate being particularly preferred.

[0107] When using emulsion polymerization, known polymerization initiators such as 2,2'-azobisisobutyronitrile, hydrogen peroxide, potassium persulfate, and ammonium persulfate can be used as thermal decomposition initiators.

[0108] Alternatively, a redox initiator can be used, which is a combination of at least one of the following: organic peroxides such as isopropyl tert-butyl carbonate peroxide, p-menthol peroxide, cumene peroxide, dicumyl peroxide, tert-butyl hydroperoxide, di-tert-butyl peroxide, and tert-hexyl peroxide; inorganic peroxides such as hydrogen peroxide, potassium persulfate, and ammonium persulfate; reducing agents such as sodium formaldehyde sulfoxylate and glucose; transition metal salts such as ferric sulfate (II); chelating agents such as disodium ethylenediaminetetraacetate; and phosphorus-containing compounds such as sodium pyrophosphate.

[0109] When using a redox initiator, polymerization can proceed even at low temperatures where the peroxides do not substantially undergo thermal decomposition, allowing for a wider range of polymerization temperatures, which is therefore preferable. Specifically, organic peroxides such as cumene hydroperoxide, dicumyl peroxide, and tert-butyl hydroperoxide are preferred as redox initiators. The amounts of the initiator used, as well as the amounts of the reducing agent, transition metal salt, chelating agent, and phosphorus-containing compound used when using a redox initiator, can be used within known ranges. Surfactants can also be added, and these are also within known ranges.

[0110] In addition, chain transfer agents can be used when polymerizing multifunctional monomers. There are no particular limitations on the chain transfer agents that can be used; examples include alkyl thiols such as n-dodecyl thiols, tert-dodecyl thiols, tert-decyl thiols, n-decyl thiols, and n-octyl thiols, and alkyl ester thiols such as 2-ethylhexyl mercaptoacetate.

[0111] As a solvent used in emulsion polymerization, any solvent that allows the emulsion polymerization to proceed stably is acceptable; for example, water is preferred.

[0112] The temperature during emulsion polymerization is not particularly limited as long as the emulsifier is uniformly dissolved in the solvent. For example, it is 40 to 75°C, preferably 45 to 70°C, and more preferably 49 to 65°C.

[0113] When the core-shell particles described above are manufactured via emulsion polymerization, the resulting latex can be used directly, or with its concentration adjusted as needed, as a binder for the positive electrode. Alternatively, the resulting latex can be spray-dried to obtain a powder that can be redispersed in water, which can then be used as a binder for the positive electrode.

[0114] In addition to the core-shell particles mentioned above, the binder for the positive electrode in this embodiment may also contain conductive additives, reinforcing materials, leveling agents, viscosity modifiers, electrolyte additives, and other components. These components are not particularly limited as long as they do not affect the battery reaction; known materials or the materials described later can be used. Furthermore, only one of these components may be used, or two or more may be used in combination.

[0115] The proportion of the core-shell particles in the solid component of the positive electrode binder in this embodiment is not particularly limited, and can be, for example, about 10 to 100% by weight. It can also be about 50 to 100% by weight, about 80 to 100% by weight, or about 90 to 100% by weight.

[0116] The binder for the positive electrode in this embodiment can be in the form of a latex of the core-shell particles or in the form of a powder of the core-shell particles. From an operability point of view, the powder form is preferred.

[0117] (slurry)

[0118] By mixing the positive electrode binder, positive electrode active material, and dispersion medium of this embodiment, a dispersion for the positive electrode (hereinafter referred to as slurry) can be prepared. By coating the slurry onto the surface of the current collector and allowing it to dry, a layer of positive electrode active material can be formed on the current collector.

[0119] The amount of the aforementioned core-shell particles in the slurry can be appropriately set by those skilled in the art. However, from the viewpoint of the slurry's coating properties on the current collector, the adhesion between the current collector and the positive electrode active material layer, and the charge-discharge characteristics of the secondary battery, it is preferably 0.1 to 30 parts by weight, more preferably 0.5 to 20 parts by weight, and even more preferably 1 to 10 parts by weight, relative to 100 parts by weight of the positive electrode active material.

[0120] <Positive Electrode Active Material>

[0121] In this embodiment, a positive electrode active material is used. By using the core-shell particles of this embodiment as a binder for the positive electrode active material, the coating properties of the slurry to the current collector, the adhesion between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery are all improved.

[0122] Examples of alkali metal composite oxides, such as lithium composite oxides and sodium composite oxides, can be cited as positive electrode active materials.

[0123] As a specific example of lithium composite oxides, lithium iron phosphate (LiFePO4) can be cited. 4： Lithium iron phosphate (LFP), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate, lithium iron manganese phosphate (LMFP), and other lithium complex phosphates are polyanionic oxides; lithium nickel manganese cobalt oxide, lithium cobalt oxide, and other layered oxides; and lithium manganese oxide and other spinel-type oxides. Preferably, it includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium nickel manganese cobalt oxide, and lithium cobalt oxide; more preferably, it includes at least one of lithium iron phosphate, lithium nickel manganese cobalt oxide, and lithium cobalt oxide. From the perspective of the charge and discharge characteristics of lithium-ion batteries, it is particularly preferred to include lithium iron phosphate.

[0124] Specific examples of sodium composite oxides include sodium manganate, sodium nickelate, sodium chromate, sodium nickel manganate, sodium nickel manganese cobaltate, sodium ferrite, and sodium ferromanganate. Among these, sodium ferromanganate is preferred from the viewpoint of battery characteristics such as capacity and cycle performance.

[0125] <Conductive additives>

[0126] Conductive additives can be freely incorporated into the slurry. There are no particular limitations on the conductive additives used; any known conductive additives can be used. Specifically, examples include carbon blacks such as acetylene black, furnace black, and Ketjen black (registered trademark); graphite such as natural graphite and artificial graphite; carbon fibers such as polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, fumed carbon fibers, carbon nanotubes, and carbon nanofibers; and fibers and foils of various metals.

[0127] The amount of conductive additive can be set appropriately, usually about 0.1 to 50 parts by weight relative to 100 parts by weight of positive electrode active material, preferably about 0.5 to 15 parts by weight, and more preferably about 1 to 10 parts by weight.

[0128] Thickener

[0129] Thickeners can be added to the slurry in any way. Thickeners are components that improve the dispersion stability of the positive electrode active material in the slurry and enhance its coatability. Water-soluble polymers can be used as thickeners; specifically, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, polyvinyl alcohol, polycarboxylic acids, their salts, poly(meth)acrylamide, etc., can be used. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, and alginate. Only one of these water-soluble polymers can be used, or two or more can be used in combination. Cellulose-based compounds are preferred, and carboxymethyl cellulose or its salts are particularly preferred.

[0130] The amount of thickener can be set appropriately. For example, relative to 100 parts by weight of the positive electrode active material, it is preferably 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, and even more preferably 0.5 to 3 parts by weight. However, it is also possible to omit the thickener.

[0131] <Other polymers>

[0132] The slurry may contain polymers other than the core-shell particles and thickeners mentioned above. Examples of such polymers include fluoropolymers and acrylonitrile polymers.

[0133] <Dispersion Medium>

[0134] The above-mentioned slurry uses water or organic solvents as a dispersion medium. A mixture of water and organic solvents can be used, or multiple organic solvents can be used alone or in combination. Examples of organic solvents include alcohols such as methanol, ethanol, and propanol; alkyl ketones such as acetone and methyl ethyl ketone; and tetrahydrofuran, dimethyl ethyl ketone, dimethyl ethyl ketone, etc. Ethers such as alkanes and diethylene glycol dimethyl ether; amides such as diethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and dimethylimidazolinone; and sulfur-based solvents such as dimethyl sulfoxide and sulfolane. Among these, amides are preferred, and N-methyl-2-pyrrolidone is particularly preferred, considering the excellent dispersibility and coatability when using the core-shell particles of this embodiment.

[0135] The organic solvent accounts for approximately 50 to 100% by weight of the total dispersion medium, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more. It should be noted that if the slurry contains water, this water may also include water contained in the latex used as a core-shell particle binder for the positive electrode, water contained in thickeners, etc.

[0136] There is no particular limitation on the concentration of solid components in the slurry; for example, it can be about 10 to 80% by weight, preferably about 30 to 70% by weight.

[0137] In addition, the proportion of the positive electrode active material in the total solid components of the slurry can be about 50 to 99% by weight, preferably about 80 to 99% by weight, and more preferably about 90 to 99% by weight.

[0138] <Preparation of Slurry>

[0139] The slurry can be prepared by dispersing the above-mentioned components in a dispersion medium. Specifically, the slurry can be prepared by using mixers such as ball mills, sand mills, bead mills, pigment dispersers, pulverizers, ultrasonic dispersers, homogenizers, planetary mixers, and FILMIX mixers, and mixing the above-mentioned components with the dispersion medium. The mixing of the above-mentioned components with the dispersion medium can usually be carried out within the range of room temperature to 80°C for 10 minutes to several hours.

[0140] (Positive electrode for secondary batteries)

[0141] The positive electrode for a secondary battery according to this embodiment includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes at least a positive electrode active material and a binder according to this embodiment. The positive electrode for a secondary battery according to this embodiment can be obtained by coating the above-described slurry onto the current collector and allowing it to dry.

[0142] As a current collector, known metal foils can be used, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0143] In this embodiment, the core-shell particles exhibit a strong tendency to maintain their particle shape within the dried positive electrode active material layer. Consequently, point connections can be used to bond the positive electrode active materials to each other or to bond the positive electrode active materials to the current collector. As a result, the presence of the binder makes it less likely to hinder the movement of lithium ions, thus it is presumably possible to reduce the internal resistance.

[0144] [Coating process]

[0145] There are no particular limitations on the method for coating the slurry onto the current collector; known methods can be used. Specifically, examples include the doctor blade method, dip method, reverse roller method, direct roller method, gravure method, extrusion method, and brush coating method. The slurry can be coated on only one side of the current collector or on both sides. The thickness of the slurry film on the current collector before drying can be appropriately set according to the thickness of the positive electrode active material layer obtained after drying.

[0146] [Drying process]

[0147] There are no particular limitations on the method for drying the slurry film on the current collector; known methods can be used, such as drying using warm air, hot air, or low-humidity air; vacuum drying; and drying methods using infrared radiation, electron beams, etc.

[0148] After the drying process, the positive electrode active material layer can be pressurized using a molding machine or roller press. This improves the adhesion between the positive electrode active material layer and the current collector, and reduces the porosity of the positive electrode active material layer.

[0149] Alternatively, the positive electrode for the secondary battery in this embodiment can also be manufactured by a powder molding method. In the powder molding method, the above-mentioned slurry is first prepared, composite particles are prepared from the slurry, the composite particles are supplied to the current collector, and rolled as needed to form a positive electrode active material layer on the current collector.

[0150] (Secondary battery)

[0151] The secondary battery of this embodiment includes a positive electrode, a negative electrode, an electrolyte, and a separator. Examples of such secondary batteries include alkali metal ion batteries such as lithium-ion batteries and sodium-ion batteries. From the viewpoint of battery characteristics based on capacity and lifespan, lithium-ion batteries are preferred.

[0152] <Negative electrode>

[0153] The negative electrode comprises a current collector and a layer of negative electrode active material formed on the current collector. Known metal foils can be used as the current collector, such as copper foil, aluminum foil, nickel foil, highly conductive stainless steel foil, lithium foil, etc.

[0154] Examples of electrode active materials that can be used in the negative electrode include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials composed of combinations of these.

[0155] Examples of carbon-based anode active materials include carbonaceous materials and graphitic materials.

[0156] Examples of carbonaceous materials include easily graphitized carbon, whose structure can be easily altered by heat treatment temperature, and difficult-to-graphitized carbon, represented by glassy carbon, which has a near-amorphous structure.

[0157] Examples of graphitizable carbon materials include those made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesophase carbon microspheres (MCMB), mesophase pitch-based carbon fibers, and thermally decomposed vapor-grown carbon fibers.

[0158] Examples of non-graphitized carbons include phenolic resin calcined bodies, polyacrylonitrile-based carbon fibers, quasi-isotropic carbon, furfuryl alcohol resin calcined bodies (PFA), and hard carbon.

[0159] In addition, graphite can be cited as a graphitic material, for example, natural graphite and artificial graphite.

[0160] As a metal-based negative electrode active material, for example, in the case of alkali metal-ion batteries, the same alkali metal is used. In the case of lithium-ion batteries, examples include lithium metal, elemental metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.) and their alloys, as well as their oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. Among these, active materials containing silicon (silicon-based negative electrode active materials) are preferred. By using silicon-based negative electrode active materials, the capacity of alkali-ion batteries can be increased.

[0161] Examples of silicon-based anode active materials include silicon (Si), silicon-containing alloys, SiO, SiOx, and composites of Si-containing materials and conductive carbon formed by coating or compositing Si-containing materials with conductive carbon. It should be noted that these silicon-based anode active materials can be used alone or in combination of two or more.

[0162] Examples of silicon-containing alloys include alloy compositions containing silicon, aluminum, and transition metals such as iron, and further containing rare earth elements such as tin and yttrium.

[0163] SiOx is a compound containing at least one of SiO and SiO2 with Si, where x is typically greater than 0.01 and less than 2.

[0164] Examples of compounds containing Si materials and conductive carbon include those obtained by heat treatment of a pulverized mixture of SiO, polymers such as polyvinyl alcohol, and optionally carbon materials, for instance, under an atmosphere containing organic gases and / or vapors. Alternatively, SiO can be obtained by known methods such as chemical vapor deposition of SiO particles onto a surface using organic gases, or by mechanochemical methods to composite SiO particles with graphite or artificial graphite (granulation).

[0165] Electrolyte

[0166] As an electrolyte, a non-aqueous electrolyte prepared by dissolving the supporting electrolyte in a non-aqueous solvent can be used.

[0167] As a supporting electrolyte, for example in the case of alkali metal ion batteries, a salt composed of the same alkali metal as the alkali metal is used. In the case of lithium ion batteries, lithium salts are typically used; in the case of sodium ion batteries, sodium salts are typically used.

[0168] Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Among these, LiPF6, LiClO4, and CF3SO3Li are preferred. These can be used individually or in combination of two or more.

[0169] Examples of sodium salts include NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3. These can be used individually or in combination of two or more.

[0170] As a non-aqueous solvent, there are no particular limitations 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), butyl carbonate (BC), and methyl ethyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Among these, carbonates are preferred due to their high dielectric constant and wide stable potential range. A single non-aqueous solvent can be used, or two or more can be used in any proportion.

[0171] When using carbonates as a non-aqueous solvent for the electrolyte, the binder of conventional core-shell particles tends to swell significantly in the electrolyte. However, since the core-shell particles of this embodiment can suppress such swelling, secondary batteries such as lithium-ion batteries can be appropriately constructed even when using carbonates as a non-aqueous solvent for the electrolyte.

[0172] The electrolyte may contain additives. Examples of additives include carbonate compounds such as vinylene carbonate (VC).

[0173] In addition to the above, other electrolytes may be used, for example, polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel-like polymer electrolytes impregnated with electrolytes; and inorganic solid electrolytes such as LiI and Li3N.

[0174] <Isolation Component>

[0175] As an insulating component, there are no particular limitations; a microporous membrane made of insulating material, namely polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride), can be used.

[0176] <Manufacturing Method of Secondary Batteries>

[0177] As a specific manufacturing method for the secondary battery of this embodiment, examples include overlapping the positive and negative electrodes through a separator, winding or folding it according to the battery shape, placing it in a battery container, injecting electrolyte into the battery container, and sealing it. Furthermore, expanded metal mesh, overcurrent protection components such as fuses and PTC elements, and lead plates can be added as needed to prevent pressure rise and overcharging / discharging inside the battery. The secondary battery can be any of the following shapes: coin-shaped, button-shaped, sheet-shaped, cylindrical, square, or flat.

[0178] Preferred embodiments of this disclosure are set forth in the following items, but the invention is not limited to these items.

[0179] [Project 1]

[0180] A type of binder for the positive electrode, specifically for use in secondary batteries.

[0181] It contains a core-shell particle, which consists of a nucleus and a shell located outside the nucleus.

[0182] The aforementioned core contains non-diene rubber with a glass transition temperature of -60 to +40°C.

[0183] The aforementioned shell layer comprises a shell-forming polymer.

[0184] The aforementioned core-shell particles contain monomeric units with hydrogen-bonded functional groups.

[0185] The proportion of the monomeric units with hydrogen-bonded functional groups is 0.6 to 20% by weight relative to the total amount of the core-shell particles.

[0186] [Project 2]

[0187] According to the binder for the positive electrode described in Project 1, the hydrogen-bonding functional group is selected from at least one of hydroxyl, carboxyl, amide and sulfonic acid groups.

[0188] [Project 3]

[0189] According to the binder for the positive electrode described in Project 1 or 2, the aforementioned non-diene rubber is an acrylic rubber containing acrylic monomer units.

[0190] [Project 4]

[0191] According to the binder for the positive electrode described in Project 3, the proportion of alkyl acrylate in the monomer components constituting the above-mentioned acrylic rubber is 50% by weight or more.

[0192] [Project 5]

[0193] According to the adhesive for the positive electrode as described in item 3 or 4, the aforementioned acrylic rubber further comprises aromatic vinyl compound units.

[0194] [Project 6]

[0195] According to any one of items 1 to 5, the binder for the positive electrode, wherein the proportion of the non-diene rubber is 35 to 90 by weight relative to the core-shell particles as a whole.

[0196] [Project 7]

[0197] The positive electrode binder according to any one of items 1 to 6, wherein the volume average particle size of the core-shell particles is 100 to 500 nm.

[0198] [Project 8]

[0199] The positive electrode binder according to any one of items 1 to 7, wherein the shell-forming polymer comprises at least one selected from (meth)acrylic monomers, aromatic vinyl compounds and cyanide compounds as constituent monomers.

[0200] [Project 9]

[0201] The positive electrode binder according to any one of items 1 to 8, wherein the Tg of the shell-forming polymer is 40 to 120°C.

[0202] [Project 10]

[0203] The binder for the positive electrode according to any one of items 1 to 9, wherein the secondary battery is an alkali metal ion battery.

[0204] [Project 11]

[0205] A coating dispersion comprising a positive electrode active material, a positive electrode binder as described in any one of items 1 to 10, and a solvent.

[0206] [Project 12]

[0207] According to the coating dispersion described in Project 11, the above-mentioned positive electrode active material is an alkali metal composite oxide.

[0208] [Project 13]

[0209] A method for manufacturing the positive electrode of a secondary battery, comprising:

[0210] The process for preparing a dispersion comprising a positive electrode active material, a positive electrode binder as described in any one of items 1 to 10, and a solvent, and

[0211] The process of coating the above dispersion onto the current collector and drying it.

[0212] [Project 14]

[0213] A positive electrode is the positive electrode of a secondary battery that includes a current collector and an active material layer disposed on the current collector.

[0214] The above-mentioned active material layer includes a positive electrode active material and a positive electrode binder as described in any one of items 1 to 10.

[0215] [Project 15]

[0216] A secondary battery comprising the positive electrode, separator, negative electrode, and electrolyte described in item 14.

[0217] Example

[0218] The present invention will be further illustrated below with examples, but the present invention is not limited to these examples. Unless otherwise specified, “parts” and “%” will refer to “parts by weight” and “% by weight”, respectively.

[0219] (Example 1)

[0220] (The formation of nuclear particles)

[0221] 1547g of deionized water, 4.7g of boric acid, 18.9g of sodium carbonate (2.5% solid content), and 0.1g of sodium lauryl phosphate were added to an 8L polymerizer, heated to 80℃, and nitrogen gas was introduced.

[0222] A solution prepared by dissolving 0.012 g of ferrous sulfate (FeSO4·7H2O) and 0.058 g of disodium ethylenediaminetetraacetate in 69.3 g of deionized water was added. Then, a mixture of 211.9 g of methyl methacrylate, 418.1 g of butyl acrylate, 90 g of styrene, 50 g of methacrylic acid, 1 g of tert-dodecyl mercaptan, 11 g of allyl methacrylate, 2.5 g of tert-butyl hydroperoxide (69% solids) and 3.2 g of sodium lauryl phosphate was added to the polymerizer for 240 minutes.

[0223] During polymerization, sodium hydroxide (2% solids) is added in any amount and at any time to maintain the pH of the system between 5 and 7.

[0224] The polymerization process ends 80 minutes after the addition of the final product, resulting in the formation of nuclear particles.

[0225] (Formation of the shell)

[0226] Next, a mixture of 184.1 g of methyl methacrylate, 45.9 g of butyl acrylate, and 0.3 g of tert-butyl hydroperoxide was added over 70 minutes.

[0227] By appropriately adding tert-butyl hydrogen peroxide (69% solids content) and sodium formaldehyde bisulfite to form a shell, a latex with core-shell graft copolymer particles with a conversion rate of 100%, a solids content concentration of 34.1%, and a volume average particle size of 200 nm is obtained.

[0228] It should be noted that the volume average particle size was measured in the above latex using a nanoparticle size measuring device (NANOTRACWAVE) manufactured by Microtrac.

[0229] (Granulation of core-shell graft copolymers)

[0230] A slurry was prepared by adding an aqueous calcium chloride solution to the latex of the core-shell graft copolymer particles obtained above. The slurry was then dehydrated using a centrifuge, washed with deionized water, and dried at 50°C for 2 days to obtain the core-shell graft copolymer powder.

[0231] (Examples 2-22 and Comparative Examples 2-4)

[0232] According to Table 1 or 2, except for changing the volume average particle size of the core-shell particles, or the type or amount of monomers used in the core particles and shell, a latex of core-shell graft copolymer particles was obtained in the same manner as in Example 1, and a powder of the core-shell graft copolymer was obtained from the latex. It should be noted that the volume average particle size was controlled by adjusting the emulsion dosage at the start of polymerization.

[0233] (Examples 23-31)

[0234] According to Table 3, except for changing the type or amount of monomers used in the core particles and shell, the latex of core-shell graft copolymer particles was obtained in the same manner as in Example 1, and the powder of core-shell graft copolymer was obtained from the latex.

[0235] (Compare Examples 1 and 5)

[0236] (polymerization of polymer particles)

[0237] 1547g of deionized water, 4.7g of boric acid, 18.9g of sodium carbonate (2.5% solid content), and 0.1g of polyoxyethylene lauryl ether phosphoric acid were added to an 8L polymerizer, heated to 80℃, and nitrogen gas was introduced.

[0238] A solution prepared by dissolving 0.012 g of ferrous sulfate (FeSO4·7H2O) and 0.058 g of disodium ethylenediaminetetraacetate in 69.3 g of deionized water was added. Then, a mixture of 396 g of methyl methacrylate, 464 g of butyl acrylate, 90 g of styrene, 50 g of methacrylic acid, 1 g of tert-dodecyl mercaptan, 15 g of allyl methacrylate, 2.5 g of tert-butyl hydroperoxide (69% solids) and 3.2 g of polyoxyethylene lauryl ether phosphoric acid was added to the polymerizer and the reaction was carried out over 360 minutes.

[0239] During polymerization, sodium hydroxide (2% solids) is added in any amount and at any time to maintain the pH of the system between 5 and 7.

[0240] The polymerization was terminated 80 minutes after the addition, resulting in a latex of a non-core-shell copolymer with a conversion rate of 100%, a solid content concentration of 34.1%, and a volume average particle size of 200 nm.

[0241] (Granulation of non-core-shell copolymers)

[0242] A slurry was prepared by adding an aqueous solution of calcium chloride to the non-core-shell copolymer latex obtained above. The slurry was then dehydrated using a centrifugal dewatering machine, washed with deionized water, and dried at 50°C for 2 days to obtain the non-core-shell copolymer powder.

[0243] (Preparation of slurry for positive electrode)

[0244] For Examples 1-22 and Comparative Examples 1-4, lithium iron phosphate (LFP) as the positive electrode active material, acetylene black as the conductive additive, and the powders obtained above were mixed in a weight ratio of 92:4:4. N-methyl-2-pyrrolidone was added and the mixture was thoroughly kneaded to prepare a positive electrode slurry with a solid content concentration of 57%.

[0245] For Examples 23-29 and Comparative Example 5, lithium nickel manganese cobalt oxide (Ni:Co:Mn=5:2:3) was used instead of lithium iron phosphate (LFP) as the positive electrode active material; for Example 30, lithium nickel manganese cobalt oxide (Ni:Co:Mn=8:1:1) was used instead of lithium iron phosphate (LFP) as the positive electrode active material; for Example 31, lithium cobalt oxide was used instead of lithium iron phosphate (LFP) as the positive electrode active material. Otherwise, a positive electrode slurry with a solid content concentration of 57% was prepared in the same manner as in Example 1.

[0246] (Fabrication of the positive electrode)

[0247] The obtained positive electrode slurry was coated onto a current collector made of aluminum foil with a thickness of 20 μm using a coating machine, and then vacuum dried at 80°C for 12 hours. The slurry was then pressed using a roller press at 80°C and 5 kN, and then punched into a φ14 mm disc shape to serve as the positive electrode.

[0248] (Making of a coin-shaped secondary battery)

[0249] A coin-shaped secondary battery was constructed using the above-described positive electrode. The negative electrode used lithium foil cut to a diameter of 15 mm. The electrolyte was a solution obtained by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. A porous polymer membrane was used as the separator. These battery components were assembled and stored using conventional methods under an atmosphere with a dew point below -50°C to obtain the coin-shaped secondary battery (CR-2032).

[0250] (Evaluation of electrolyte resistance)

[0251] The powders obtained above were pressed using a press at 170°C for 10 minutes with a 1mm thick spacer in between, to obtain a 1mm thick film. The obtained film was cut into squares with 1cm sides and weighed accurately.

[0252] The precisely weighed membrane was immersed in the electrolyte (ethylene carbonate / dimethyl carbonate = 3 / 7 (volume ratio)) in a test tube. Then, the test tube containing the membrane and electrolyte was left to stand at 23°C for 24 hours. Afterward, the membrane was removed from the electrolyte. The electrolyte adhering to the membrane was wiped off, and the membrane was accurately weighed.

[0253] Then, based on the weight of the membrane before and after immersion in the electrolyte, the electrolyte immersion swelling ratio is calculated using the following formula. The smaller the calculated value, the less the polymer powder swells in the electrolyte.

[0254] Electrolyte impregnation swelling rate (%) = (Weight of membrane after impregnation / Weight of membrane before impregnation) × 100

[0255] (Evaluation of coating properties)

[0256] The coating performance was determined by visually inspecting the surface of the dried positive electrode active material layer based on the following criteria.

[0257] 〇: No streaks or spots were observed on the surface of the positive electrode active material layer.

[0258] △: No streaks were seen on the surface of the positive electrode active material layer, but spots were observed.

[0259] ×: Stripes and spots are seen on the surface of the positive electrode active material layer.

[0260] (Evaluation of adhesion)

[0261] Cut a rectangle 60 mm long and 20 mm wide from the positive electrode prepared as described above to serve as a test piece. With the positive electrode active material layer facing upwards, attach transparent adhesive tape (the tape specified in JIS Z1522) to the surface of the positive electrode active material layer. Measure the stress when one end of the current collector is stretched vertically at a tensile speed of 50 mm / min. Perform three measurements and calculate the average value, which is taken as the T-peel strength.

[0262] (Evaluation of charge-discharge cycle characteristics)

[0263] For the coin-shaped secondary battery, it was charged at a constant current of 0.3C until the battery voltage reached 4.2V at 30℃, and then discharged at a constant current of 0.3C until the battery voltage reached 2V. This operation was repeated 100 times. Then, the ratio of the discharge capacity of the 100th discharge to the discharge capacity of the 1st discharge was calculated (charge-discharge capacity retention rate = (discharge capacity of the 100th discharge / discharge capacity of the 1st discharge) × 100%).

[0264] The evaluation results described above are shown in Table 1, 2 or 3.

[0265] [Table 1]

[0266]

[0267] [Table 2]

[0268]

[0269] [Table 3]

[0270]

[0271] As shown in Tables 1 and 2, Examples 1–22, which used lithium iron phosphate (LFP) as the positive electrode active material, exhibited excellent electrolyte resistance, good coating properties to the current collector, sufficient peel strength, and excellent cycle characteristics. Similarly, as shown in Table 3, Examples 23–31, which used lithium nickel manganese cobalt oxide or lithium cobalt oxide as the positive electrode active material, also exhibited excellent electrolyte resistance, good coating properties to the current collector, sufficient peel strength, and excellent cycle characteristics.

[0272] On the other hand, Comparative Examples 1 and 5, which use non-core-shell copolymer particles, exhibit poor coatability and low charge-discharge cycle characteristics compared to the examples.

[0273] Comparative Examples 2 and 3, which use particles with a core-shell structure but not containing hydrogen-bonded functional groups or containing core-shell graft copolymer particles in proportions less than specified values, exhibited lower electrolyte resistance and lower charge-discharge cycle characteristics compared to the embodiments.

[0274] In addition, Comparative Example 4, which uses core-shell graft copolymer particles with hydrogen-bonded functional groups in proportions exceeding the specified values, showed a lower T-peel strength value compared to the other examples.

Claims

1. A binder for the positive electrode, specifically for the positive electrode of a secondary battery. It includes core-shell particles, which consist of a nucleus and a shell located outside the nucleus. The core comprises a non-diene rubber with a glass transition temperature of -60 to +40°C. The shell layer comprises a shell-forming polymer. The core-shell particles contain monomeric units with hydrogen-bonded functional groups. The proportion of the monomeric units with hydrogen-bonded functional groups is 0.6 to 20% by weight relative to the total amount of the core-shell particles.

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

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

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

5. The binder for the positive electrode according to claim 3, wherein, The acrylic rubber further comprises aromatic vinyl compound units.

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

7. The binder for the positive electrode according to claim 1 or 2, wherein, The volume average particle size of the core-shell particles is 100–500 nm.

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

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

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

11. A coating dispersion comprising a positive electrode active material, a positive electrode binder as described in claim 1 or 2, and a solvent.

12. The dispersion for application according to claim 11, wherein, The positive electrode active material is an alkali metal composite oxide.

13. A method for manufacturing the positive electrode of a secondary battery, comprising: The process of preparing a dispersion comprising a positive electrode active material, a positive electrode binder as described in claim 1 or 2, and a solvent, and The process of coating the dispersion onto the current collector and drying it.

14. A positive electrode, comprising a current collector and an active material layer disposed on the current collector, is the positive electrode of a secondary battery. The active material layer comprises a positive electrode active material and a positive electrode binder as described in claim 1 or 2.

15. A secondary battery comprising the positive electrode, separator, negative electrode, and electrolyte as described in claim 14.

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