Binder for electrode of secondary battery

Abstract

A binder for an electrode of a secondary battery, including core-shell particles including a core and a shell layer located outside the core, wherein the core includes rubber; the shell layer is composed of a shell-forming polymer; in the binder, tensile strength measured by a specific method has a yield point; and strength at the yield point is 15 MPa or more.

Description

Binder for rechargeable battery electrodes

[0001] This disclosure relates to a binder for electrodes of secondary batteries, and to electrodes for secondary batteries and a method for manufacturing the same.

[0002] Rechargeable batteries, especially lithium-ion batteries, are lightweight, have high energy density, and can be repeatedly charged and discharged. They are used in a wide range of applications, including power supplies for personal computers and smartphones, and as power sources for electric and hybrid vehicles.

[0003] In the negative 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 graphite and a binder such as styrene / butadiene rubber (SBR). Similarly, 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 lithium-containing composite metal oxide and a binder. The electrode active material layer is usually manufactured by mixing the electrode active material, binder, and dispersion medium 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 bonding (adhesion) between the electrode active materials themselves, and the bonding between the electrode active materials and the current collector. Research is underway to improve this binder in order to enhance the characteristics of secondary batteries while ensuring bonding. Good coating properties when applying the slurry to the current collector, and good charge / discharge characteristics when constructing a lithium-ion battery are also required.

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

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

[0007] Incidentally, polyvinylidene fluoride resin (PVDF) is known as a material for forming binders, and N-methyl-2-pyrrolidone (NMP) is known as a dispersion medium used for preparing slurries. However, since the raw material of polyvinylidene fluoride resin is fluorocarbon gas, which imposes a heavy environmental load, and N-methyl-2-pyrrolidone has been reported to have reproductive toxicity, substitution with materials that cause less environmental load and health concern is desired.

[0008] As an example of a positive electrode binder that replaces polyvinylidene fluoride resin, Patent Document 3 discloses the use of a particulate binder made of an organic polymer. As a specific example thereof, core-shell type rubber particles having a core part containing an acrylonitrile unit and having a high glass transition temperature and a flexible shell part containing an acrylate ester unit are described (paragraph

[0018] ). In this document, usable electrode active materials are limited to composite oxides containing a transition metal (for example, lithium cobaltate).

[0009] As positive electrode active materials, layered rock salt-type composite oxides such as lithium cobaltate and spinel-type composite oxides such as lithium manganate have been widely used hitherto. However, in recent years, lithium composite phosphates such as lithium iron phosphate have attracted attention from the viewpoints of availability and thermal stability.

[0010] International Publication No. 2017 / 141791, International Publication No. 2017 / 056466, Japanese Unexamined Patent Publication No. 2002-117834

[0011] The present disclosure relates to a binder used for an electrode of a secondary battery, and an object of the present disclosure is to provide a binder that has excellent binding properties between a current collector and an active material layer, excellent coatability on a current collector, and excellent charge-discharge characteristics of a secondary battery.

[0012] The inventors of the present invention have conducted intensive studies to solve the above problems, and have found that the above object can be achieved by using, as an electrode binder for a secondary battery, a binder that contains core-shell particles having a core containing rubber and a shell layer composed of a shell-forming polymer, and has a yield point in tensile strength measured by a specific method, which has led to the completion of the present disclosure.

[0013] In other words, this disclosure relates to an electrode binder for a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is made of rubber and the shell layer is made of a shell-forming polymer, and the binder has a yield point, the strength of which is 15 MPa or more, as measured by the following method: Tensile strength: Tensile strength measured at 23°C and a tensile speed of 500 mm / min using a dumbbell-shaped test specimen in accordance with JIS K6251.

[0014] According to this disclosure, it is possible to provide a binder for use in the electrodes of a secondary battery that has good adhesion between the current collector and the active material layer, good coating properties on the current collector, and good charge / discharge characteristics of the secondary battery.

[0015] Embodiments of the present disclosure are described in detail below. [Electrode Binder] The electrode binder for the secondary battery according to this embodiment (hereinafter also referred to as "electrode binder") comprises at least core-shell particles. The core-shell particles have a core-shell structure and include a core and a shell layer located outside the core. The core comprises rubber. The shell layer is composed of a shell-forming polymer. The binder has a yield point with a tensile strength measured by a specific method, and the strength at the yield point is also above a specific value.

[0016] By using core-shell particles with this structure as an electrode binder for secondary batteries, the adhesion between the current collector and the electrode active material layer can be improved. This also allows for a reduction in the amount of electrode binder required in the secondary battery electrodes. Furthermore, even when the electrodes repeatedly expand and contract due to charging and discharging of the secondary battery, good adhesion with the electrode active material layer can be maintained, improving the charge-discharge cycle characteristics of the resulting secondary battery. Moreover, a slurry containing core-shell particles with this structure allows for improved coating properties on the current collector, which also contributes to improved charge-discharge cycle characteristics of the resulting secondary battery.

[0017] The binder has a yield point, which is measured by a specific method, and the strength at the yield point is 15 MPa or higher. The tensile strength is measured using a dumbbell-shaped test specimen in accordance with JIS K6251 at 23°C and a tensile speed of 500 mm / min.

[0018] The yield point refers to the point in the stress-strain curve (S-S curve) obtained when a test specimen is deformed at a constant strain rate, where, once the stress acting on the specimen exceeds the elastic limit and reaches a certain value, the strain increases rapidly with almost no further increase in stress. It is the first point in the stress-strain curve when a test specimen is deformed at a constant strain rate where the slope changes abruptly.

[0019] If the current collector does not have the aforementioned yield point, or if the strength of the yield point is less than 15 MPa, the bonding properties between the current collector and the active material layer, the coating properties on the current collector, or the charge / discharge characteristics of the secondary battery may be inferior.

[0020] The yield point may not be present if the proportion of rubber with a low glass transition temperature in the core-shell particles is too high, if the glass transition temperature of the shell-forming polymer becomes too low, and / or if the core-shell structure is not present.

[0021] The yield strength may be 15 MPa or higher, but from the viewpoint of improving bonding performance by improving the peeling strength between the current collector and the active material layer, it is preferable to have a yield strength of 16 MPa or higher, more preferably 20 MPa or higher, and even more preferably 25 MPa or higher. There is no particular upper limit to the yield strength, but for example it may be 100 MPa or less, or 80 MPa or less. It may also be between 15 MPa and 100 MPa.

[0022] The strength of the yield point can be increased, for example, by increasing the proportion of methacrylic monomer units and aromatic vinyl compound units in the core rubber and / or the shell-forming polymer.

[0023] If the proportion of alkyl acrylate monomer units having an alkyl chain with 4 or fewer carbon atoms in the shell-forming polymer becomes too high, the yield strength may fall below 15 MPa or the yield strength may not be present at all. The alkyl acrylates having an alkyl chain with 4 or fewer carbon atoms are methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate.

[0024] Preferably, the core-shell particles contain monomer units having hydrogen-bonding modifying groups as a whole polymer constituting the core-shell particles, and these monomer units having hydrogen-bonding modifying groups can be contained in either the rubber contained in the core or the shell-forming polymer, or both. In particular, it is more preferable that the shell layer contains monomer units having hydrogen-bonding modifying groups. This can further improve the bonding between the current collector and the active material layer. It can also suppress swelling of the core-shell particles in the electrolyte, thereby improving the charge-discharge characteristics of the secondary battery.

[0025] The aforementioned hydrogen-bonding modified group refers to a functional group containing a hydrogen atom that can bond to an atom with high electronegativity, such as an oxygen atom, through hydrogen bonding. Specific examples include hydroxyl groups, carboxyl groups, amide groups, and sulfonic acid groups, and at least one selected from the group consisting of hydroxyl groups, carboxyl groups, amide groups, and sulfonic acid groups can be used. In this disclosure, monomers that are carboxylic acid anhydrides that can generate a carboxyl group by hydrolysis are also included as monomers having a hydrogen-bonding modified group. The amide group may have substituents on the nitrogen atom, but a primary amide without substituents is preferred. Only one type of hydrogen-bonding modified group may be used, or two or more types may be used in combination.

[0026] As monomers having hydrogen-bonding modifying groups, vinyl monomers having hydrogen-bonding modifying groups are preferred, and (meth)acrylic monomers having hydrogen-bonding modifying groups are more preferred. Note that "(meth)acrylic" is a notation used to refer acrylic and methacrylic collectively.

[0027] Among monomers having hydrogen bonding modification groups, examples of monomers having hydroxyl groups include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0028] 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 acrylate-(poly)propylene glycol (e.g., NOF Corporation's Bremmer 50PEP-300), (meth)acrylic acid-(poly)ethylene glycol-(poly)butylene glycol (e.g., NOF Corporation's Bremmer 55PET-800, 50PEP-500D), and (meth)acrylic acid-(poly)propylene glycol-(poly)butylene glycol (e.g., NOF Corporation's Bremmer 10PPB-500B, 10PPB-500BD). Among these, (poly)ethylene glycol methacrylate is preferred, and (poly)ethylene glycol monomethacrylate is more preferred.

[0029] Examples of monomers that are monomers having a carboxyl group or carboxylic acid anhydrides include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, and maleic anhydride (MAH). Among these, at least one selected from the group consisting of acrylic acid, methacrylic acid, and maleic anhydride is preferred.

[0030] 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.

[0031] Examples of monomers having a sulfonic acid group include vinyl monomers having a sulfonic acid group. Examples of sulfonic acid groups include styrene sulfonic acid, methallyl sulfonic acid, allyl sulfonic acid, vinyl sulfonic acid, and isoprene sulfonic acid. Of these, styrene sulfonic acid is preferred, and p-styrene sulfonic acid is more preferred, from the viewpoint of having high surface activity, polymerizability, and polymerization stability, and being able to stably produce core-shell particles.

[0032] As monomers having hydrogen bonding modifiers, at least one selected from the group consisting of hydroxyl groups and carboxyl groups is preferred from the viewpoint of dispersibility of core-shell particles in the slurry. Furthermore, monomers having hydroxyl groups are preferred from the viewpoint of improving bonding by improving the peel strength between the current collector and the active material layer. Monomers having carboxyl groups are preferred from the viewpoint of improving the charge-discharge characteristics of the secondary battery by improving electrolyte resistance. Monomers having amide groups are preferred from the viewpoint of improving charge-discharge characteristics at high temperatures. In addition, monomers having sulfonic acid groups are preferred from the viewpoint of coating properties and the internal resistance of the secondary battery.

[0033] The content of monomer units having hydrogen bonding modifiers in the entire polymer constituting the core-shell particles is preferably 1% by weight or more, more preferably 1.5% by weight or more, even more preferably 2% by weight or more, and particularly preferably 3% by weight or more, from the viewpoint of adhesion between the current collector and the active material layer, electrolyte resistance, and charge / discharge characteristics. The upper limit is preferably 30% by weight or less, 25% by weight or less, 20% by weight or less, 18% by weight or less, 15% by weight or less, and 9% by weight or less, in that order, from the viewpoint of the coating properties of the slurry on the current collector and polymerization stability. It may also be 1 to 30% by weight.

[0034] [Core] The core is a particle containing rubber. The type of rubber is not particularly limited, but from the viewpoint of easily obtaining the yield strength, it is preferable that the rubber contains at least one selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units as constituent monomers. From the viewpoint of polymerization rate, it is preferable that the rubber contains (meth)acrylic monomer units as constituent monomers, and from the viewpoint of polymerization stability, it is preferable that the rubber contains aromatic vinyl compound units as constituent monomers. Furthermore, it is preferable that the rubber contains aliphatic conjugated diene compound units and aromatic vinyl compound units, and more preferably that it is a copolymer of aliphatic conjugated diene compound units and aromatic vinyl compound units, as this can improve the dispersibility of the core-shell particles in the slurry and the compatibility between the core-shell particles and the thickener, and can improve the binding between the current collector and the electrode active material layer.

[0035] Here, "rubber" refers to a material that possesses rubber elasticity. Rubber elasticity is the ability to absorb energy from an external force and store it as energy to return to its original shape. To exhibit rubber elasticity, the molecules must be sufficiently long, able to move freely, and appropriately bonded to one another. A material with rubber elasticity can easily return to its original shape when the external force is released, even if it is deformed by an external force.

[0036] The glass transition temperature of the polymer constituting the core is not particularly limited, but if the core has a single-layer structure, it may be in the range of -50°C to +25°C, for example. From the viewpoint of improving bonding properties and electrode flexibility, 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, and -20°C to +10°C is most preferred.

[0037] The glass transition temperature of the polymer constituting the core can be controlled by changing the type and ratio of monomers that make up the polymer. For example, by using aromatic vinyl compound units and / or methacrylic monomers as the monomers that make up the polymer, it is possible to increase the glass transition temperature of the polymer.

[0038] The glass transition temperature can be measured using a differential scanning calorimeter (DSC). The same applies to the glass transition temperature hereafter.

[0039] The core is composed of a polymer and may or may not contain components other than rubber. Examples of components other than rubber include polymers that do not contain crosslinked structures (polymers that do not contain structural units derived from crosslinkable monomers).

[0040] The proportion of the rubber in the entire core is not particularly limited, but from the viewpoint of bonding properties and electrolyte resistance, 50 to 100% by weight is preferred, 70 to 100% by weight is more preferred, and 90 to 100% by weight is even more preferred.

[0041] The proportion of the core in the core-shell particles is not particularly limited, but from the viewpoint of improving electrolyte resistance, improving binding properties, reducing internal resistance, and improving capacity retention with charge-discharge cycles, it is preferably 45 to 95% by weight, more preferably 50 to 90% by weight, even more preferably 50 to 85% by weight, and still more preferably 55 to 85% by weight. It may be 50 to 80% by weight, 50 to 70% by weight, or 50 to 60% by weight.

[0042] The rubber contained in the core may be a diene-based rubber or a non-diene-based rubber. Compared to non-diene-based rubber, diene-based rubber has the advantage of being easier to bond, which allows for a reduction in the content of electrode binder in the electrodes of secondary batteries (especially lithium-ion batteries). Furthermore, non-diene-based rubber has the advantage of being more resistant to oxidation than diene-based rubber, thus preventing oxidative degradation during charging and discharging of secondary batteries.

[0043] The aforementioned 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).

[0044] (Non-diene rubber) In cases where the rubber contained in the core is a non-diene rubber, a rubber can be used that contains at least one selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units as a constituent monomer.

[0045] The (meth)acrylic monomer is not particularly limited and includes, for example, alkyl (meth)acrylates [alkyl (meth)acrylates having linear or branched aliphatic hydrocarbon groups such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, etc.]; 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; alkoxyalkyl (meth)acrylate; and (meth)acrylamide. In addition, the (meth)acrylic monomers having the aforementioned hydrogen-bonding modifying groups, and monomers having other modifying groups, etc. The (meth)acrylic monomers may be used alone or in combination of two or more types. Other monomers having modifying groups include unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile.

[0046] From the viewpoint of polymerization stability and improved copolymerizability when aromatic vinyl compounds (especially styrene) are used in combination as monomers, it is preferable to use methacrylic monomers.

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

[0048] The proportion of (meth)acrylic monomer units in the total monomer components constituting the non-diene rubber is not particularly limited, but from the viewpoint of polymerization rate, it may be 30 to 100% by weight, 35 to 80% by weight, or 40 to 60% by weight. In this disclosure, "monomer components" refers to monomer components excluding polyfunctional monomers.

[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. Only one aromatic vinyl compound may be used, or two or more may be used in combination.

[0050] The proportion of aromatic vinyl compound units in the total monomer components constituting the non-diene rubber is not particularly limited, but it may be 30 to 80% by weight, 35 to 75% by weight, or 40 to 60% by weight, as this facilitates the production of core-shell particles and makes it easier to adjust the glass transition temperature of the rubber to a suitable range.

[0051] In the non-diene rubber, monomer units other than the (meth)acrylic monomer units and aromatic vinyl compound units may or may not be used. Examples of such other monomers include vinyl cyanide compounds such as acrylonitrile, vinyl halides such as vinyl chloride, vinyl acetate, and alkenes such as ethylene and propylene.

[0052] The total proportion of (meth)acrylic monomer units and aromatic vinyl compound units in the total monomer components constituting the non-diene rubber is not particularly limited, but may be, for example, 80 to 100% by weight. 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.

[0053] The aforementioned non-diene rubber may be obtained by using polyfunctional monomers such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, and 1,3-butylene dimethacrylate during polymerization.

[0054] Furthermore, non-diene rubbers may be polymerized without the use of a chain transfer agent, or they may be polymerized in the presence of a chain transfer agent. The usable chain transfer agents are 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.

[0055] (Diene-based rubber) When the rubber contained in the core is a diene-based rubber, in addition to an aliphatic conjugated diene compound, a rubber can be used that contains at least one selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units as a constituent monomer.

[0056] The rubber is preferably made of aliphatic conjugated diene compound units and aromatic vinyl compound units, and more preferably a copolymer of aliphatic conjugated diene compound units and aromatic vinyl compound units, because it can improve the dispersibility of core-shell particles in the slurry and the compatibility of core-shell particles with thickeners, and can improve the binding between the current collector and the electrode active material (positive electrode active material layer or negative electrode active material). The method for producing the rubber particles is not particularly limited, but it is preferably synthesized by emulsion polymerization.

[0057] Aliphatic conjugated diene compounds are aliphatic compounds that have two carbon-carbon double bonds, which are separated by a single bond and thus conjugated. 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 types may be used in combination. Of these, 1,3-butadiene is preferred.

[0058] The description of the (meth)acrylic monomers that can be used in diene rubbers is the same as that for non-diene rubbers.

[0059] Specific examples of aromatic vinyl compounds used in diene rubbers are the same as those used in non-diene rubbers. 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.

[0060] A typical example of the aforementioned diene-based rubber is styrene-butadiene rubber. Styrene-butadiene rubber is a copolymer of 1,3-butadiene and styrene, and is also called styrene rubber or SBR.

[0061] The diene rubber may not contain any 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 alkyl (meth)acrylates such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, and butyl methacrylate, as well as epoxy group-containing (meth)acrylates such as glycidyl methacrylate; monomers having hydrogen bonding modifiers; and monomers having other modifiers. Examples of monomers having other modifiers include unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile.

[0062] The proportion of aromatic vinyl compound units in the total diene rubber is not particularly limited, but may be 30 to 80% by weight, or 35 to 75% by weight.

[0063] The total proportion of aliphatic conjugated diene compound units and aromatic vinyl compound units in the entire diene rubber is not particularly limited, but may be, for example, 80 to 100% by weight. 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.

[0064] Of the polymer constituting the core, the proportion of monomer units having hydrogen-bonding modified groups is preferably 20% by weight or less, more preferably 15% by weight or less, and even more preferably 10% by weight or less, from the viewpoint of polymerization stability. Furthermore, this proportion may be 0% by weight or more.

[0065] The aforementioned diene rubber may be obtained by using a polyfunctional monomer during its polymerization.

[0066] Furthermore, the diene rubber 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. Specific examples of the polyfunctional monomer and chain transfer agent used in the polymerization of the diene rubber are the same as those used for the non-diene rubber.

[0067] In the diene rubber described above, the content ratio (by weight) of the aliphatic conjugated diene compound to the aromatic vinyl compound is not particularly limited and may be in the range of, for example, 10:90 to 10:90. From the viewpoint of improving bonding properties, reducing internal resistance, and maintaining capacity with charge-discharge cycles, a ratio of 20:80 to 80:20 is preferred, 30:70 to 70:30 is more preferred, and 40:60 to 60:40 is even more preferred.

[0068] (Core having a multilayer structure) The core may consist of one layer (also called a single-layer core), or it may have a multilayer structure consisting of at least two layers (hereinafter sometimes referred to as a multilayer core). The multilayer core as a whole may contain rubber, for example, it may contain one or more layers made of rubber, and the other layers may consist of polymer layers that do not contain structural units derived from crosslinkable monomers. It is preferable that each layer of the multilayer core is graft-bonded to the adjacent layer. The rubber contained in the multilayer core may be the non-diene rubber or the diene rubber.

[0069] It is preferable to use a hard polymer as the polymer constituting the innermost layer of the multilayer core and a soft polymer as the polymer constituting the outermost layer. Specifically, it is preferable that the glass transition temperatures of the polymers constituting each layer of the multilayer core decrease in the order of the polymer constituting the innermost layer of the core (innermost core polymer) and the polymer constituting the outermost layer (outermost core polymer). Furthermore, if the multilayer core has an intermediate layer between the innermost and outermost layers, it is preferable that the glass transition temperatures decrease in the order of the polymer constituting the innermost layer of the core, the polymer constituting the intermediate layer, and the polymer constituting the outermost layer.

[0070] (Innermost layer) The glass transition temperature of the innermost core polymer is not particularly limited, but from the viewpoint of yield strength, it may be, for example, 40°C or higher, 80°C or higher, or exceed 85°C. The upper limit is also not particularly limited, but may be 120°C or lower, 110°C or lower, or 40 to 120°C.

[0071] Of the core innermost polymer, the proportion of monomers having hydrogen bonding modifiers is preferably 30% by weight or less, more preferably 25% by weight or less, and even more preferably 20% by weight or less, from the viewpoint of yield strength. Furthermore, this proportion may be 0% by weight or more.

[0072] (Outermost layer) The glass transition temperature of the polymer constituting the outermost layer of the multilayer core is not particularly limited, but from the viewpoint of improving bonding properties and electrode flexibility, it may be, for example, less than 40°C, 10°C or less, 0°C or less, or -10°C or less. Furthermore, the lower limit may be -85°C or higher, -60°C or higher, -40°C or higher, or -40°C or higher and less than 40°C.

[0073] When the rubber contained in the core is a non-diene rubber, from the viewpoint of improving bonding properties and electrode flexibility, the polymer constituting the outermost layer of the core preferably contains acrylic monomers as constituent monomers, and more preferably is acrylic rubber.

[0074] Furthermore, in particular, when the rubber contained in the core is a diene-based rubber, from the viewpoint of ease of adjustment to the binding properties, electrolyte resistance, and glass transition temperature range, it is preferable that the polymer constituting the outermost layer of the core contains an aliphatic conjugated diene compound as a constituent monomer, and more preferably contains aliphatic conjugated diene compound units and aromatic vinyl compound units as constituent monomers.

[0075] 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 modified groups as described above are also examples. Acrylic monomers may be used alone or in combination of two or more types. Among the aforementioned acrylic monomers, alkyl acrylates are preferred, and butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.

[0076] 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 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 secondary battery. The upper limit is 100% by weight or less.

[0077] 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. It is preferable to use aromatic vinyl compounds such as styrene because it facilitates the production of core-shell particles and makes it easier to adjust the glass transition temperature of the rubber to a suitable range. From the viewpoint of polymerization rate, it is preferable to use a methacrylic monomer in the acrylic rubber, and more preferable to use an alkyl methacrylate. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but it is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0078] Of the core outermost polymer, the proportion of monomers having hydrogen bonding modifiers is preferably 20% by weight or less, more preferably 15% by weight or less, and even more preferably 10% by weight or less, from the viewpoint of polymerization stability. Furthermore, this proportion may be 0% by weight or more.

[0079] Of the polymers constituting the multilayer core, the proportion of polymers having a glass transition temperature of 40°C to 120°C, including the polymer constituting the innermost layer, is preferably 35 to 80% by weight, more preferably 35 to 75% by weight, and even more preferably 40 to 70% by weight, from the viewpoint of yield strength.

[0080] Furthermore, for polymers with a glass transition temperature of 40°C to 120°C, the lower limit of the glass transition temperature may be 80°C or higher, and may exceed 85°C. The upper limit may be 110°C or lower.

[0081] Of the polymers constituting the multilayer core, the proportion of polymers with a glass transition temperature of -85°C or higher and less than 40°C, including the polymer constituting the outermost layer, is preferably 25 to 70% by weight, more preferably 25 to 65% by weight, and even more preferably 30 to 60% by weight, from the viewpoint of improving binding properties and electrode flexibility.

[0082] Furthermore, for polymers with a glass transition temperature of -85°C or higher and less than 40°C, the lower limit of the glass transition temperature may be -60°C or higher, or -40°C or higher. The upper limit may be 10°C or lower, 0°C or lower, or -10°C or lower.

[0083] [Shell layer] The shell layer is composed of a polymer that forms the shell layer (hereinafter also referred to as the shell-forming polymer).

[0084] The aforementioned shell layer refers to a polymer layer located on the surface side of the core-shell particles, and is also called a graft layer. Preferably, the shell layer is graft-bonded to the core. However, the polymer forming the shell layer also includes polymers that are not graft-bonded to the core. The shell layer covers the surface of the core, but is not limited to covering the entire surface of the core; it is sufficient if it covers at least a part of the surface of the core.

[0085] By providing the aforementioned shell layer, the dispersibility of core-shell particles and electrode active material in the slurry can be improved, and the coating properties of the slurry onto the current collector can be improved. Furthermore, the charge-discharge characteristics of the secondary battery can also be improved.

[0086] The shell-forming polymer is preferably a vinyl polymer. A vinyl polymer is obtained by homopolymerizing or copolymerizing vinyl monomers. The vinyl monomers constituting the shell-forming polymer are not limited, but from the viewpoint of electrolyte resistance, it is preferable that the shell-forming polymer contains aromatic vinyl compound units as monomer units. Furthermore, from the viewpoint of polymerization rate, dispersibility of core-shell particles in slurry, and binding properties, it is preferable that it contains (meth)acrylic monomer units as monomer units. It may also contain both aromatic vinyl compound units and (meth)acrylic monomer units as monomer units.

[0087] The proportion of aromatic vinyl compound units in the total monomer components constituting the shell-forming polymer is preferably 30 to 100% by weight, more preferably 40 to 100% by weight, and even more preferably 50 to 100% by weight, from the viewpoint of having good electrolyte resistance and being able to improve the charge-discharge characteristics of secondary batteries.

[0088] The description of the aromatic vinyl compound that can be used in the shell-forming polymer is the same as that of the aromatic vinyl compound for the core.

[0089] The proportion of (meth)acrylic monomer units in the total monomer components constituting the shell-forming polymer is preferably 30 to 100% by weight, more preferably 40 to 100% by weight, and even more preferably 50 to 100% by weight, from the viewpoint of polymerization rate, dispersibility of core-shell particles in slurry, and binding properties.

[0090] The (meth)acrylic monomer is not particularly limited. Specific examples of the (meth)acrylic monomer used in the shell-forming polymer are the same as those used in the core. The (meth)acrylic monomer may be used alone or in combination of two or more. Other monomers having a modifying group include unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile.

[0091] From the viewpoint of polymerization stability and improved copolymerizability when aromatic vinyl compounds (especially styrene) are used in combination as monomers, it is preferable to use methacrylic monomers.

[0092] In order to easily maintain the particle shape of the core-shell particles in the formed electrode active material layer and to lower the internal resistance of the secondary battery, the shell-forming polymer preferably has a glass transition temperature of 40°C or higher, more preferably 60°C or higher, even more preferably 80°C or higher, particularly preferably 90°C or higher, and most preferably 100°C or higher. The upper limit is not particularly limited, but for example, it may be 120°C or lower. It may also be 40°C or higher and 120°C or lower.

[0093] The glass transition temperature of the shell-forming polymer can be controlled by changing the type and ratio of monomers that make up the polymer. For example, by using methacrylic monomers and / or aromatic vinyl compound units as the monomers that make up the shell-forming polymer, it is possible to increase the glass transition temperature of the polymer.

[0094] The monomer units of the shell-forming polymer used together with the aromatic vinyl compound units may include other vinyl monomers other than the aromatic vinyl compound units and (meth)acrylic monomer units.

[0095] The shell-forming polymer may be obtained by using polyfunctional monomers such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, and 1,3-butylene dimethacrylate during polymerization.

[0096] The shell layer may be composed of a crosslinked polymer or a non-crosslinked polymer. A non-crosslinked polymer refers to a polymer that does not contain a crosslinked structure and does not contain structural units derived from crosslinkable monomers, and refers to a polymer that does not fall under the category of rubber elastic materials (for example, butadiene rubber, styrene-butadiene rubber, acrylic rubber, etc.).

[0097] The shell may consist of a single layer, or it may have a multilayer structure consisting of at least two layers (hereinafter sometimes referred to as a multilayer shell). The description of the shell can be applied to the description of the polymers that constitute each layer of the shell. Preferably, each layer of the multilayer shell is graft-bonded to an adjacent layer.

[0098] The ratio of the shell-forming polymer 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 secondary battery, it is preferably 5 to 55% by weight, more preferably 10 to 50% by weight, and even more preferably 15 to 45% by weight.

[0099] When the shell-forming polymer contains monomer units having hydrogen-bonding modified groups, the content of monomers having hydrogen-bonding modified groups in the total monomer components constituting the shell-forming polymer is preferably 1 to 60% by weight, from the viewpoint of the coating properties of the slurry onto the current collector, the binding properties between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2% by weight or more, even more preferably 3.8% by weight or more, and particularly preferably 5% by weight or more, from the viewpoint of charge-discharge characteristics and swelling suppression. 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, from the viewpoint of binding properties.

[0100] The core-shell particles may consist only of a core and a shell layer, but may also have an intermediate layer between the core and the shell layer, as long as the effects of the invention are achieved. The intermediate layer is preferably a layer 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 covers at least a portion of the surface of the intermediate layer.

[0101] (Volume-average particle diameter of core-shell particles) The particle diameter of the core-shell particles is not particularly limited, and for example, the volume-average particle diameter may be about 10 to 1000 nm, about 50 to 1000 nm, or about 100 to 1000 nm. However, from the viewpoint of the dispersibility of the core-shell particles in the slurry, the miscibility of the core-shell particles with the thickener, and the reduction of the internal resistance of the secondary battery, the volume-average particle diameter is preferably 50 to 500 nm, more preferably 100 to 300 nm, and even more preferably 150 to 250 nm. The volume-average particle diameter of the core-shell particles is measured using a particle diameter measuring device in the latex state of the core-shell particles. The particle diameter of the core-shell particles can be controlled by the amount of each monomer constituting the core-forming polymer charged, the amount of core-forming polymer charged when forming the shell portion, as well as the type and amount of polymerization initiator, chain transfer agent, oxidation-reduction agent, emulsifier, etc. used during polymerization, polymerization temperature, polymerization time, etc.

[0102] (Method for producing core-shell particles) The method for producing the core-shell particles is not particularly limited, but for example, emulsion polymerization, miniemulsion polymerization, microemulsion polymerization, and soap-free emulsion polymerization can be used.

[0103] 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.

[0104] 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.

[0105] The nonionic surfactants among the emulsifiers mentioned above are not particularly limited, but examples include the following compounds: polyoxyethylene alkylaryl 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.

[0106] 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.

[0107] 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.

[0108] 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 from the viewpoint of obtaining good fluidity of the resulting latex, and sodium polyoxyethylene lauryl ether phosphate is particularly preferred.

[0109] 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.

[0110] 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.

[0111] 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 known ranges. In addition, when polymerizing polyfunctional monomers, known chain transfer agents can be used within known ranges. Surfactants can also be used, but these are also within known ranges.

[0112] Any solvent that allows emulsion polymerization to proceed stably is acceptable as the solvent used during emulsion polymerization; for example, water can be suitably used.

[0113] 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.

[0114] If the core-shell particles are produced by emulsion polymerization, for example, the latex of the core-shell particles may be dried to obtain a powder that can be redispersed in water, and this powder may be used as an electrode binder (powdered electrode binder).

[0115] The latex can be coagulated by adding one or more coagulants selected from the group consisting of acids and salts, and then dehydrated, washed, dried, etc. to obtain powdered core-shell particles. Furthermore, the particles may be separated by sieving with a predetermined size. Examples of the coagulants include calcium chloride and hydrochloric acid, and salts of these may be used as aqueous solutions.

[0116] Examples of the drying methods include spray drying, drying by standing, and vacuum drying.

[0117] The electrode binder according to this embodiment may be in the form of latex of the core-shell particles, or in the form of powder of the core-shell particles. From the viewpoint of excellent dispersibility in slurry, the electrode binder is preferably in the form of latex of the core-shell particles, and from the viewpoint of ease of handling, it is preferably in the form of powder, but the form of the core-shell particles can be appropriately selected depending on the type of active material, etc.

[0118] <Other Components> In addition to the core-shell particles, the electrode binder according to this embodiment may also contain 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 components, such as those described in International Publication No. 2012 / 115096, can be used. Furthermore, only one type of these component may be used, or two or more types may be used in combination.

[0119] [Slurry] The slurry according to this embodiment includes an electrode active material and the electrode binder. The slurry according to this embodiment has good coating properties on the surface of the current collector, meaning it can be applied uniformly without uneven coating or bumps, and has appropriate thixotropy (viscosity) so that it does not drip excessively, thus making it possible to obtain an electrode active material layer with a highly smooth surface. The slurry can be prepared by mixing the electrode binder according to this embodiment, the electrode active material, and the dispersion medium. By applying the slurry to the surface of the current collector and drying it, an electrode active material layer can be formed on the current collector.

[0120] <Amount of core-shell particles in the slurry> The amount of core-shell particles in the slurry can be appropriately set by those skilled in the art, but from the viewpoint of improving the coating properties of the slurry on the current collector and the bonding properties between the current collector and the electrode active material layer, and reducing internal resistance while improving the charge-discharge characteristics of the secondary battery, it is preferably 0.1 to 5 parts by weight, and more preferably 0.3 to 3 parts by weight, per 100 parts by weight of electrode active material. In particular, in the case of a slurry for the positive electrode, from the viewpoint of the coating properties of the slurry on the current collector, the bonding properties 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, per 100 parts by weight of positive electrode active material.

[0121] (Negative electrode active material) Examples of electrode active materials that can be used as negative electrodes include carbon-based negative electrode active materials, metallic negative electrode active materials, and negative electrode active materials that combine these. Examples of carbon-based negative electrode active materials include carbonaceous materials and graphite materials.

[0122] 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.

[0123] Examples of easily graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMBs), mesophase pitch-based carbon fibers, and pyrolysis vapor-grown carbon fibers.

[0124] 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.

[0125] Furthermore, examples of graphite materials include graphite such as natural graphite and artificial graphite.

[0126] As a metallic anode active material, for example, in the case of an alkali metal ion battery described later, the same alkali metal as the alkali metal is used. In the case of a lithium-ion battery, examples 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, phosphides, etc. Among these, active materials containing silicon (silicon-based anode active materials) are preferred. By using silicon-based anode active materials, the capacity of lithium-ion batteries can be increased.

[0127] 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.

[0128] Examples of silicon-containing alloys include alloy compositions that contain silicon, aluminum, transition metals such as iron, and further rare earth elements such as tin and yttrium.

[0129] 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.

[0130] Examples of the composite of a Si-containing material and conductive carbon include a compound obtained by heat-treating a pulverized mixture of SiO, a polymer such as polyvinyl alcohol, and optionally a carbon material, for example, in an atmosphere containing an organic gas and / or vapor. The composite can also be obtained by known methods such as a method of coating the surface of SiO particles by chemical vapor deposition using an organic gas or the like, and a method of forming composite particles (granulating) from SiO particles and graphite or artificial graphite by a mechanochemical method.

[0131] (Positive Electrode Active Material) Examples of electrode active materials that can be used in the positive electrode include transition metal-containing compounds, such as transition metal oxides, transition metal sulfides, and composite metal oxides of an alkali metal and a transition metal. Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

[0132] Here, examples of the transition metal oxide include MnO, MnO ２ , V ２ O ５ , V ６ O １３ , TiO ２ , Cu ２ V ２ O ３ , amorphous V ２ O-P ２ O ５ , amorphous MoO ３ , amorphous V ２ O ５ , amorphous V ６ O １３ , and the like.

[0133] Examples of the transition metal sulfide include TiS ２ , TiS ３ , amorphous MoS ２ , and FeS.

[0134] Examples of the composite metal oxide of an alkali metal and a transition metal include lithium-containing composite metal oxides having a layered structure, lithium-containing composite metal oxides having a spinel structure, lithium-containing composite metal oxides having an olivine structure, and sodium-containing composite metal oxides.

[0135] Examples of lithium-containing composite metal oxides having a layered structure include lithium-containing cobalt oxide (LiCoO2). ２ ), lithium-containing nickel oxide (LiNiO ２ ), Co-Ni-Mn lithium-containing composite oxide, Ni-Mn-Al lithium-containing composite oxide, Ni-Co-Al lithium-containing composite oxide, LiMaO ２ and Li ２ MbO ３ Examples include solid solutions with [these substances].

[0136] Examples of lithium-containing composite metal oxides having a spinel-type structure include lithium manganese (LiMn). ２ O ４ ) and lithium manganese (LiMn ２ O ４ Examples include compounds in which part of the Mn in ) is replaced with other transition metals.

[0137] Examples of lithium-containing composite metal oxides having an olivine-type structure include olivine-type lithium iron phosphate (LiFePO4). ４ : LFP), olivine-type lithium manganese phosphate (LiMnPO) ４ ), olivine-type lithium iron phosphate (LiFePO ４ Examples include olivine-type lithium iron manganese phosphate (LMFP), in which part of the iron is replaced with manganese.

[0138] Examples of sodium-containing composite metal oxides include sodium manganate, sodium nickelate, sodium chromate, sodium nickelmanganate, sodium nickelmanganese cobaltate, sodium ferrite, and sodium ironmanganate.

[0139] The proportion of the positive electrode active material to the total solid content of the slurry may be about 50 to 99% by weight, preferably about 80 to 99% by weight, and more preferably about 90 to 99% by weight.

[0140] The proportion of the negative electrode active material to the total solid content of the slurry may be about 80 to 99% by weight, preferably about 90 to 98% by weight, and more preferably about 93 to 97% by weight.

[0141] (Conductive additives) Conductive additives may be optionally added to the slurry. The conductive additives are 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.

[0142] 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 positive electrode active material.

[0143] <Other Polymers> The slurry may optionally contain polymers other than the core-shell particles mentioned above. Examples of such polymers include thickeners, fluorine-containing polymers, and acrylonitrile polymers.

[0144] (Thickener) The thickener is a component that can improve the dispersion stability of the electrode active material in the slurry and improve the coating properties of the slurry. Water-soluble polymers can be used as thickeners, and specifically, carboxymethylcellulose, methylcellulose, 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.

[0145] The amount of thickener added to the slurry can be set as appropriate, but for example, 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 per 100 parts by weight of electrode active material.

[0146] (Dispersion medium) As the dispersion medium for the slurry, an aqueous medium or an organic medium may be used. An aqueous medium alone may be used, a mixed medium of an aqueous medium and an organic medium may be used, or an organic medium alone or in combination of several types may be used. The dispersion medium for the slurry should be appropriately selected according to the above components, etc.

[0147] Examples of organic media include alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol; alkyl ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, dioxane, and diglyme; amides such as diethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and dimethylimidazolidinone; and sulfur-based solvents such as dimethyl sulfoxide and sulfolane. Among these, amides are preferred, and N-methyl-2-pyrrolidone is particularly preferred, due to their excellent dispersibility and coating properties when using the core-shell particles according to this embodiment.

[0148] As the aqueous medium, water is usually used, but aqueous solutions of any compound or mixed solutions of a small amount of organic medium and water may also be used. Furthermore, the water contained in the slurry may include water that was present in the latex when the electrode binder is a core-shell particle latex, or water that was present in other components such as thickeners.

[0149] The solid content concentration of the slurry is not particularly limited, but may be, for example, about 10 to 80% by weight, and preferably 30 to 70% by weight.

[0150] <Preparation of Slurry> The slurry according to this embodiment can be prepared, for example, by dispersing each of the above components in a dispersion medium. Specifically, the 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.

[0151] [Electrodes of the secondary battery] The electrodes of the secondary battery according to this embodiment include a current collector and an electrode active material layer formed on the current collector. The electrode active material layer includes at least an electrode active material and an electrode binder containing core-shell particles according to this embodiment. The electrodes of the secondary battery according to this embodiment can be obtained by applying the above slurry onto the current collector and drying it.

[0152] Known metal foils may be used as current collectors, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0153] The core-shell particles according to this embodiment tend to maintain their particle shape relatively well within the electrode active material layer after drying. As a result, bonding between electrode active materials, or bonding between electrode active materials and current collectors, can be achieved by point adhesion. Consequently, the movement of ions such as lithium ions is less likely to be hindered by the presence of a binder, and it is presumed that this will lead to a reduction in internal resistance.

[0154] (Coating Process) The method for coating the slurry according to this embodiment 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, brush coating method, etc. In this case, the slurry may 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 after coating can be appropriately set according to the thickness of the electrode active material layer obtained after drying.

[0155] (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.

[0156] After the drying process, the electrode active material layer may be subjected to pressure treatment using a die press or roll press. This improves the adhesion between the electrode active material layer and the current collector, and also reduces the porosity of the electrode active material layer.

[0157] Furthermore, the electrodes of the secondary battery according to this embodiment can also be manufactured by powder molding. In the powder molding method, first, the slurry described above is prepared, composite particles are prepared from the slurry, the composite particles are supplied onto a current collector, and if desired, they are roll-pressed to form an electrode active material layer on the current collector.

[0158] [Secondary Battery] The secondary battery according to this embodiment comprises a positive electrode, a negative electrode, an electrolyte, and a separator, wherein at least one of the positive electrode and the negative electrode uses the electrode of the secondary battery according to this embodiment. When the negative electrode is the electrode for the secondary battery according to this embodiment, the positive electrode is not particularly limited and may be a known positive electrode. When the positive electrode is the electrode for the secondary battery according to this embodiment, the negative electrode is not particularly limited and may be a known negative electrode. Examples of the secondary battery include lithium-ion batteries and alkali metal ion batteries such as sodium-ion batteries. From the viewpoint of battery characteristics considering battery capacity and life characteristics, lithium-ion batteries are preferred.

[0159] <Electrolyte> As the electrolyte, for example, a non-aqueous electrolyte can be used, which is obtained by dissolving a supporting electrolyte in a non-aqueous solvent. As the supporting electrolyte, for example, in the case of an alkali metal ion battery, a salt composed of the same alkali metal as the alkali metal is used. In the case of a lithium-ion battery, a lithium salt is usually used, and in the case of a sodium-ion battery, a sodium salt is usually used. As a lithium salt, for example, LiPF ６ LiAsF ６ LiBF ４ LiSbF ６ LiAlCl ４LiClO 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 Examples include NLi. Among them, LiPF ６ LiClO ４ CF ３ SO ３ Li is preferred. These may be used individually or in combination of two or more. As for sodium salts, for example, NaPF ６ NaClO ４ NaBCl ４ NaSO ３ CF ３ and Na(CH ３ ) C ６ H ４ SO ３ These are some examples. These can be used individually or in combination of two or more types.

[0160] 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; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Among these, carbonates are preferred because they have a high dielectric constant and a wide stable potential range. The non-aqueous solvent may be used alone or two or more may be used in any ratio. Additives may be included in the electrolyte. Examples of additives include carbonate-based compounds such as vinylene carbonate (VC).

[0161] In addition, other electrolytes include, for example, polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel-like polymer electrolytes impregnated with the electrolyte; and LiI, Li ３ Inorganic solid electrolytes such as N may also be used.

[0162] <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.

[0163] <Method for Manufacturing a Secondary Battery> A specific method for manufacturing a secondary 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 electrolyte into the battery container and seal it. Furthermore, if necessary, expanded metal, overcurrent prevention elements such as fuses and PTC elements, and lead plates may be added to prevent pressure rise inside the battery and overcharging / discharging. The shape of the secondary battery may be coin-shaped, button-shaped, sheet-shaped, cylindrical, rectangular, flat, or any other shape.

[0164] The following items list preferred embodiments of the present disclosure, but the present invention is not limited to the following items. [Item 1] An electrode binder for a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core comprises rubber, the shell layer is composed of a shell-forming polymer, and the binder has a yield point, the strength of which is 15 MPa or more, as measured by the following method: Tensile strength: Tensile strength measured at 23°C and a tensile speed of 500 mm / min using a dumbbell-shaped test piece conforming to JIS K6251. [Item 2] The electrode binder according to Item 1, wherein the core-shell particles include monomer units having hydrogen-bonding modified groups. [Item 3] The electrode binder according to Item 2, wherein the hydrogen-bonding modified group is at least one selected from the group consisting of hydroxyl groups, carboxyl groups, amide groups, and sulfonic acid groups. [Item 4] The electrode binder according to any one of Items 1 to 3, wherein the core-shell particles have a volume-average particle diameter of 100 to 1000 nm. [Item 5] The electrode binder according to any one of Items 1 to 4, wherein the secondary battery is an alkali metal ion battery. [Item 6] The electrode binder according to any one of Items 1 to 5, wherein the rubber contains at least one selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units as constituent monomer units. [Item 7] The electrode binder according to any one of Items 1 to 6, wherein the core contains aliphatic conjugated diene compound units and aromatic vinyl compound units as constituent monomer units. [Item 8] The electrode binder according to any one of Items 1 to 7, wherein the electrode is a positive electrode. [Item 9] The electrode binder according to Item 8, wherein the rubber is a non-diene rubber. [Item 10] The electrode binder according to Item 8, wherein the core is composed of at least two layers, and the innermost layer of the core contains a polymer having a glass transition temperature of 40°C or higher. [Item 11] The electrode binder according to Item 10, wherein the outermost layer of the core contains a polymer having a glass transition temperature of -85°C or higher and less than 40°C. [Item 12] The electrode binder according to Item 11, wherein the polymer in the outermost layer of the core is an acrylic rubber.[Item 13] An electrode binder according to any one of Items 1 to 12, wherein the electrode is the negative electrode. [Item 14] A method for manufacturing an electrode for a secondary battery, comprising preparing a slurry containing an electrode active material and an electrode binder according to any one of Items 1 to 13, and applying the slurry onto a current collector and drying it. [Item 15] An electrode for a secondary battery, comprising a current collector and an active material layer formed on the current collector, wherein the active material layer contains an electrode active material and an electrode binder according to any one of Items 1 to 14. [Item 16] A secondary battery comprising the electrode according to Item 15.

[0165] The present disclosure will be further described below with reference to examples, but the present invention is not limited to these examples. Hereinafter, unless otherwise specified, "parts" and "%" refer to "parts by weight" and "weight percent," respectively.

[0166] (Example 1-1) (Formation of the core) 104 parts of deionized water and 0.05 parts of disodium hydrogen phosphate were added to a 100 L pressure-resistant autoclave polymerization machine. Further, 0.0118 parts of ferrous sulfate and 0.00197 parts of disodium ethylenediaminetetraacetate were added, and degassing was carried out at -0.01 MPa for 15 minutes. 0.055 parts of sodium dodecylbenzenesulfonate, 45 parts of styrene, 55 parts of butadiene, and 0.15 parts of t-dodecyl mercaptan were added, and the internal temperature was raised to 50°C. 0.035 parts of sodium formaldehyde sulfoxylate and 0.02 parts of paramentane hydroperoxide were added, and polymerization was started. After 17 hours from the start of polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, and then polymerization was terminated by degassing and removing the remaining butadiene that was not used in polymerization under reduced pressure, thereby obtaining a core-forming polymer.

[0167] (Formation of the shell portion) 55 parts of the obtained core-forming polymer were charged into an 8 L polymerizer, and 0.05 parts of sodium formaldehyde sulfoxylate were added. A mixture of 43.5 parts of styrene, 1.5 parts of methacrylic acid, and 0.15 parts of t-butyl hydroperoxide was added to the polymerizer over 120 minutes. Thereafter, sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were added as needed, and polymerization was completed after 80 minutes with a conversion rate of 100%. The pH was adjusted to approximately 7 using an aqueous sodium hydroxide solution to obtain core-shell polymer particles with a volume-average particle diameter of 200 nm. The volume-average particle diameter of the core-shell polymer was measured using a nanoparticle analyzer NANOTRAC WAVE manufactured by Microtrac. Furthermore, the results of the evaluations using the core-shell polymer particles of Example 1-1, Examples 1-2 to 1-17 and Comparative Examples 1-2 to 1-4 described below, and the non-core-shell polymer of Comparative Example 1-1 are shown in Table 1.

[0168] The abbreviations in Tables 1-5 are as follows: ST: Styrene MMA: Methyl methacrylate BD: Butadiene t-DM: t-Dodecyl mercaptan MAA: Methacrylic acid BA: Butyl acrylate 2EHA: 2-Ethylhexyl acrylate HEMA: 2-Hydroxyethyl methacrylate ALMA: Allyl methacrylate Tg: Glass transition temperature

[0169]

[0170] (Examples 1-2 to 1-5) Core-shell polymer particles were obtained in the same manner as in Example 1-1, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0171] (Examples 1-6 to 1-9) (Formation of the core) 104 parts of deionized water and 0.05 parts of disodium hydrogen phosphate were added to a 100 L pressure-resistant autoclave polymerization machine. Further, 0.0118 parts of ferrous sulfate and 0.00197 parts of disodium ethylenediaminetetraacetate were added, and degassing was carried out at -0.01 MPa for 15 minutes. 0.055 parts of sodium dodecylbenzenesulfonate, 55.5 parts of styrene, and 4.5 parts of butadiene were added, and the internal temperature was raised to 50°C. 0.035 parts of sodium formaldehyde sulfoxylate and 0.02 parts of paramentane hydroperoxide were added, and polymerization of the innermost layer (first layer) was started. Subsequently, from 7 to 17 hours after the start of polymerization, 40 parts of butadiene and 0.15 parts of t-dodecyl mercaptan were added, and polymerization of the outermost layer (second layer) was continued. After confirming that the polymerization conversion rate exceeded 97%, the polymerization was terminated by degassing and removing the remaining butadiene that was not used in polymerization under reduced pressure, thereby obtaining a core-forming polymer.

[0172] (Formation of the shell portion) Core-shell polymer particles were obtained in the same manner as in Example 1-1, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0173] (Examples 1-10 to 1-11) Core-shell polymer particles were obtained in the same manner as in Example 1-6, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0174] (Example 1-12) (Formation of the core) 104 parts of deionized water and 0.05 parts of disodium hydrogen phosphate were added to a 100 L pressure-resistant autoclave polymerization machine. Further, 0.0118 parts of ferrous sulfate and 0.00197 parts of disodium ethylenediaminetetraacetate were added, and degassing was carried out at -0.01 MPa for 15 minutes. 0.055 parts of sodium dodecylbenzenesulfonate and 10 parts of styrene were added, and the internal temperature was raised to 50°C. 0.035 parts of sodium formaldehyde sulfoxylate and 0.02 parts of paramenthane hydroperoxide were added, and polymerization of the innermost layer (first layer) was started. Thereafter, from 1 hour to 7 hours after the start of polymerization, 30 parts of styrene and 10 parts of butadiene were added, and polymerization of the intermediate layer (second layer) was started. Subsequently, from 7 to 17 hours after the start of polymerization, 5 parts of styrene, 45 parts of butadiene, and 0.15 parts of t-dodecyl mercaptan were added, and polymerization of the outermost layer (third layer) was continued. After confirming that the polymerization conversion rate exceeded 97%, the polymerization was terminated by degassing and removing the remaining butadiene that was not used in polymerization under reduced pressure, thereby obtaining a core-forming polymer.

[0175] (Formation of the shell portion) Core-shell polymer particles were obtained in the same manner as in Example 1-1, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0176] (Example 1-13) (Formation of core portion) Core-shell polymer particles were obtained in the same manner as in Example 1-12.

[0177] (Formation of the shell portion) 65 parts of the obtained core-forming polymer were charged into an 8 L polymerizer, and 0.05 parts of sodium formaldehyde sulfoxylate were added. 20 parts of styrene were added to the polymerizer over 70 minutes to start the polymerization of the first layer. Then, 10 parts of styrene and 5 parts of methacrylic acid were added to the polymerizer over 50 minutes to start the polymerization of the second layer. Sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were then added as appropriate, and polymerization was completed after 80 minutes with a conversion rate of 100%. The pH was adjusted to approximately 7 using an aqueous sodium hydroxide solution to obtain core-shell polymer particles with a volume-average particle diameter of 180 nm.

[0178] (Example 1-14) Core-shell polymer particles were obtained in the same manner as in Example 1-13, except that the composition of the shell layer was changed according to the description in Table 1.

[0179] (Examples 1-15 to 1-17) Core-shell polymer particles were obtained in the same manner as in Example 1-12, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0180] (Comparative Example 1-1) 104 parts of deionized water and 0.05 parts of disodium hydrogen phosphate were added to a 100 L pressure-resistant autoclave polymerization reactor. Further, 0.0118 parts of ferrous sulfate and 0.00197 parts of disodium ethylenediaminetetraacetate were added, and degassing was carried out at -0.01 MPa for 15 minutes. 0.055 parts of sodium dodecylbenzenesulfonate, 46.75 parts of styrene, 52 parts of butadiene, 1.25 parts of methacrylic acid, and 0.75 parts of t-dodecyl mercaptan were added, and the internal temperature was raised to 50°C. 0.035 parts of sodium formaldehyde sulfoxylate and 0.02 parts of paramentane hydroperoxide were added, and polymerization was started. After 17 hours from the start of polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, and then polymerization was terminated by degassing and removing the remaining butadiene that was not used in polymerization under reduced pressure, thereby obtaining a non-core-shell polymer.

[0181] (Comparative Examples 1-2 to 1-4) Core-shell polymer particles were obtained in the same manner as in Example 1-1, except that the composition or amount of the core layer and shell layer was changed according to the description in Table 1.

[0182] (Examples 2-1 to 2-17, Comparative Examples 2-1 to 2-4) Core-shell polymer particles were obtained in Examples 2-1 to 2-17 and Comparative Examples 2-1 to 2-4 using the same procedure as in Examples 1-1 to 1-17 and Comparative Examples 1-1 to 1-4, respectively. The results of the evaluations using the core-shell polymer particles from Examples 2-1 to 2-17 and Comparative Examples 2-2 to 2-4, as well as the non-core-shell polymer from Comparative Example 2-1, are shown in Tables 2 and 3.

[0183]

[0184]

[0185] (Example 2-18) (Formation of the innermost core layer) 100 parts of deionized water, 0.05 parts of sodium carbonate, and 0.056 parts of sodium dodecylbenzenesulfonate were added to an 8 L polymerizer, heated to 60°C, and nitrogen was flowed through. 0.0118 parts of ferrous sulfate, 0.00197 parts of disodium ethylenediaminetetraacetate, and 0.035 parts of sodium formaldehyde sulfoxylate were added, and then a mixture of 30 parts of styrene, 0.3 parts of allyl methacrylate, and 0.06 parts of t-butyl hydroperoxide was added to the polymerizer over 90 minutes. t-butyl hydroperoxide and sodium formaldehyde sulfoxylate were added as needed, and polymerization was completed 90 minutes after the end of addition to form hard core particles.

[0186] (Formation of the outermost core layer) After the polymerization of the hard core particles was completed, a mixture of 30 parts 2-ethylhexyl acrylate, 0.45 parts allyl methacrylate, and 0.03 parts t-butyl hydroperoxide was added to the polymerization apparatus over 90 minutes. Sodium dodecylbenzenesulfonate was added as needed during polymerization. Polymerization was completed 90 minutes after the end of the addition, forming soft core particles.

[0187] (Formation of the shell layer) Next, a mixture of 37 parts styrene, 3 parts 2-hydroxyethyl methacrylate, and 0.06 parts t-butyl hydroperoxide was added over 120 minutes. t-butyl hydroperoxide and sodium formaldehyde sulfoxylate were added as appropriate to obtain core-shell polymer particles with a conversion rate of 100%, a solid content concentration of 45%, and a volume-average particle diameter of 150 nm. The results of the evaluations using the core-shell polymer particles of Example 2-18, Examples 2-19 to 2-36 and Comparative Examples 2-6 to 2-10, and the non-core-shell polymer of Comparative Example 2-5 are shown in Tables 4 and 5.

[0188]

[0189]

[0190] (Examples 2-19 to 2-36, Comparative Examples 2-7 to 2-10) Core-shell polymer particles were obtained in the same manner as in Example 2-18, except that the type or amount of constituent monomers used was changed according to the descriptions in Tables 4 and 5.

[0191] (Comparative Example 2-5) (Polymerization of Non-Core-Shell Structure Copolymer) 100 parts of deionized water, 0.05 parts of sodium carbonate, and 0.056 parts of sodium dodecylbenzenesulfonate were added to an 8 L polymerizer, the temperature was raised to 60°C, and nitrogen was flowed through. 0.0118 parts of ferrous sulfate, 0.00197 parts of disodium ethylenediaminetetraacetate, and 0.035 parts of sodium formaldehyde sulfoxylate were added, and then a mixture of 67 parts of styrene, 30 parts of 2-ethylhexyl acrylate, 3 parts of 2-hydroxyethyl methacrylate, and 0.1 part of t-butyl hydroperoxide was added to the polymerizer over 300 minutes. t-butyl hydroperoxide and sodium formaldehyde sulfoxylate were added as needed, and polymerization was completed 90 minutes after the end of the additions, yielding non-core-shell polymer particles with a conversion rate of 100%, a solid content concentration of 45%, and a volume-average particle diameter of 150 nm.

[0192] (Comparative Example 2-6) (Formation of Core Layer) 100 parts of deionized water, 0.05 parts of sodium carbonate, and 0.056 parts of sodium dodecylbenzenesulfonate were added to an 8 L polymerizer, the temperature was raised to 60°C, and nitrogen was flowed through. 0.0118 parts of ferrous sulfate, 0.00197 parts of disodium ethylenediaminetetraacetate, and 0.035 parts of sodium formaldehyde sulfoxylate were added, and then a mixture of 60 parts of butyl acrylate, 0.9 parts of allyl methacrylate, and 0.06 parts of t-butyl hydroperoxide was added to the polymerizer over 180 minutes. t-butyl hydroperoxide and sodium formaldehyde sulfoxylate were added as needed, and polymerization was completed 90 minutes after the end of addition to form core particles.

[0193] (Formation of the shell layer) Next, a mixture of 37 parts styrene, 3 parts 2-hydroxyethyl methacrylate, and 0.06 parts t-butyl hydroperoxide was added over 120 minutes. t-butyl hydroperoxide and sodium formaldehyde sulfoxylate were added as appropriate to obtain core-shell polymer particles with a conversion rate of 100%, a solid content concentration of 45%, and a volume-average particle diameter of 150 nm.

[0194] (Granulation of polymer particles) A ​​calcium chloride aqueous solution was added to the polymer particle latex obtained above to form a slurry. Then, it was dehydrated using a centrifugal dehydrator, washed with deionized water, and dried at 50°C for two days to obtain polymer particle powder.

[0195] (Evaluation of Tensile Properties) The obtained polymer was air-dried at 50°C for 24 hours to obtain a binder film. The obtained film was then compression-molded at 190°C for 5 minutes using a Shinto Digital Press to obtain a dumbbell-shaped test specimen in accordance with JIS K6251:2010. The tensile strength of the obtained dumbbell-shaped test specimen was measured at 23°C at a tensile speed of 500 mm / min using a Shimadzu Autograph AG-X. The presence or absence of a tensile yield point was determined based on the following criteria: Present: A tensile yield point is present in the S-S curve. Absent: A tensile yield point is not present in the S-S curve.

[0196] (Preparation of anode slurry - 1) 1.5 parts of a thickener (product name "CMC2200", manufactured by Daicel Corporation), 85.95 parts of graphite as anode active material, 9.55 parts of silicon monoxide, 1.5 parts of acetylene black, and 100 parts of water were added to a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro"), and the mixture was stirred and mixed at 2000 rpm for 5 minutes. Then, 1.5 parts of each polymer synthesized in Examples 1-1 to 1-17 or Comparative Examples 1-1 to 1-4 were added, and the mixture was stirred and mixed at 2000 rpm for another 5 minutes, followed by defoaming and mixing at 2200 rpm for 1 minute to obtain anode slurry with a solid content of 50%.

[0197] (Fabrication of negative electrode - 1) The negative electrode slurry was uniformly applied to the surface of a current collector made of copper foil with a thickness of 17 μm using the doctor blade method so that the film thickness after drying would be 80 μm, and then vacuum dried at 80°C for 6 hours. After that, the density of the negative electrode active material layer was 6.0 g / cm³. ２ By pressing the current collector with a roll press in such a manner, a negative electrode was obtained in which a negative electrode active material layer was formed on the surface of the current collector.

[0198] (Evaluation of Coatability - 1) The surface of the anode active material layer after drying was visually observed, and the coatability was determined based on the following criteria. ○: No streaks or spots are observed on the surface of the anode active material layer. △: No streaks are observed on the surface of the anode active material layer, but spots are observed. ×: Streaks and spots are observed on the surface of the anode active material layer.

[0199] (Evaluation of bonding properties - 1) The negative electrode prepared as described above was cut into a rectangle 70 mm long and 20 mm wide to make a test piece. With the negative electrode active material layer facing upwards, cellophane tape (as specified in JIS Z1522) was attached to the surface of the negative 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. The bonding properties were judged based on the following criteria: ○: Peel strength of 0.75 N / cm or more △: Peel strength of 0.5 N / cm or more and less than 0.75 N / cm ×: Peel strength less than 0.5 N / cm

[0200] (Preparation of cathode slurry - 1) Add 4 parts polyvinylidene fluoride, 4 parts acetylene black, and LiNiCoMnO as the cathode active material to a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro") ２ 92 parts of (Ni:Co:Mn = 5:2:3) were mixed, N-methyl 2-pyrrolidone was added, and the mixture was stirred at 2000 rpm for 10 minutes, followed by degassing and mixing at 2200 rpm for 1 minute to obtain a cathode slurry with a solid content of 57%.

[0201] (Fabrication of positive electrode - 1) The above-mentioned positive electrode slurry was uniformly applied to the surface of a current collector made of 15 μm thick aluminum foil using the doctor blade method so that the film thickness after drying was 80 μm, and then vacuum dried at 80°C for 6 hours. After that, the density of the positive electrode active material layer was 10.0 mg / cm³. ２ By pressing the current collector with a roll press in such a manner, a positive electrode was obtained in which a positive electrode active material layer was formed on the surface of the current collector.

[0202] (Assembly of Lithium-ion Battery - 1) A single-layer polypropylene separator (65 mm wide, 500 mm long, 20 μm thick; manufactured by dry process; 60% porosity) was prepared and cut into a 3 cm x 3 cm square. An aluminum packaging material was also prepared as the battery casing. Then, in a glove box where the dew point was substituted with Ar to be below -80°C, the positive electrode prepared as described above was cut into a 2 cm x 2 cm square and placed so that the surface on the current collector side was in contact with the aluminum packaging material. Next, the square separator was placed on the surface on the positive electrode active material layer side. Furthermore, the negative electrode prepared as described above was cut into a 2.2 cm x 2.2 cm square and placed on the separator so that the surface on the negative electrode active material layer side faced the separator. Subsequently, a 1.0 M LiPF6 solution was filled as the electrolyte (the solvent was a mixed solvent of ethylene carbonate / methyl ethyl carbonate = 3 / 7 (volume ratio), with 2 vol% vinylene carbonate (solvent ratio) added as an additive). Furthermore, to seal the opening of the aluminum packaging, the aluminum packaging was heat-sealed at 150°C, and the laminate cell type lithium-ion secondary battery was manufactured.

[0203] (Evaluation of Charge / Discharge Cycle Characteristics - 1) For the laminate cell type secondary battery prepared as described above, the operation of charging with a constant current of 0.3C until the battery voltage reached 4.3V and discharging with a constant current of 0.3C until the battery voltage reached 2.5V 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. The charge / discharge cycle characteristics were judged based on the following criteria: ○: Capacity retention rate of 80% or more △: Capacity retention rate of 60% or more but less than 80% ×: Capacity retention rate less than 60%

[0204] As shown in Table 1, the core-shell particles (binders) of Examples 1-1 to 1-17 all exhibited high T-shaped peel strength, excellent adhesion between the current collector and the active material layer, and superior charge-discharge characteristics of secondary batteries and coating properties for the current collector.

[0205] On the other hand, the particles (binder) of Comparative Example 1-1, which lacked a core-shell structure and had no yield point in tensile strength, were insufficient in terms of adhesion between the current collector and the active material layer, charge-discharge characteristics of the secondary battery, and coating properties on the current collector.

[0206] Furthermore, the particles (binders) of Comparative Examples 1-2 to 1-4, which either lacked a yield point in tensile strength or had a low yield point, exhibited insufficient bonding between the current collector and the active material layer, as well as poor charge-discharge characteristics of the secondary battery.

[0207] (Preparation of cathode slurry - 2) Lithium iron phosphate, carbon black, and the powders obtained in Examples 2-1 to 2-36 or Comparative Examples 2-1 to 2-10 were mixed in a weight ratio of 92:4:4. N-methyl-2-pyrrolidone was added to this mixture and thoroughly kneaded using a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro") to prepare a cathode slurry with a solid content of 54%.

[0208] (Fabrication of positive electrode - 2) The above-mentioned positive electrode slurry was uniformly applied to the surface of a current collector made of 15 μm thick aluminum foil using the doctor blade method so that the film thickness after drying was 80 μm, and then vacuum dried at 80°C for 6 hours. After that, the density of the positive electrode active material layer was 10.0 mg / cm³.２ By pressing the current collector with a roll press in such a manner, a positive electrode was obtained in which a positive electrode active material layer was formed on the surface of the current collector.

[0209] (Evaluation of Coatability - 2) The surface of the positive electrode active material layer after drying was visually observed, and the coatability was 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.

[0210] (Evaluation of bonding properties - 2) The positive electrode prepared as described above was cut into a rectangle 70 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 one end of the current collector was pulled vertically at a speed of 50 mm / min to peel it off. Three measurements were taken, and the average value was calculated and defined as the T-shaped peel strength. The bonding properties were judged based on the following criteria: ○: Peel strength of 0.75 N / cm or more △: Peel strength of 0.5 N / cm or more and less than 0.75 N / cm ×: Peel strength less than 0.5 N / cm

[0211] (Preparation of anode slurry - 2) 1.5 parts of a thickener (product name "CMC2200", manufactured by Daicel Corporation), 85.95 parts of graphite as anode active material, 9.55 parts of silicon monoxide, 1.5 parts of acetylene black, and 100 parts of water were added to a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro"), and the mixture was stirred and mixed at 2000 rpm for 5 minutes. Then, 1.5 parts of the core-shell polymer particles (SBR binder) obtained in Example 2-1 were added in terms of solid content, and the mixture was stirred and mixed again at 2000 rpm for 5 minutes, followed by defoaming and mixing at 2200 rpm for 1 minute to obtain anode slurry with a solid content of 50%.

[0212] (Fabrication of negative electrode - 2) The negative electrode slurry was uniformly applied to the surface of a current collector made of copper foil with a thickness of 17 μm using the doctor blade method so that the film thickness after drying would be 80 μm, and then vacuum dried at 80°C for 6 hours. After that, the density of the negative electrode active material layer was 6.0 g / cm³. ２By pressing the current collector with a roll press in such a manner, a negative electrode was obtained in which a negative electrode active material layer was formed on the surface of the current collector.

[0213] (Assembly of Lithium-ion Battery - 2) A laminate cell type lithium-ion secondary battery was manufactured in accordance with "Assembly of Lithium-ion Battery - 1" described above.

[0214] (Evaluation of Charge / Discharge Cycle Characteristics - 2) For the laminate cell type secondary battery prepared as described above, the operation of charging with a constant current of 0.3C until the battery voltage reached 3.8V and discharging with a constant current of 0.3C until the battery voltage reached 2.5V 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. The charge / discharge cycle characteristics were judged based on the following criteria: ○: Capacity retention rate of 90% or more △: Capacity retention rate of 80% or more but less than 90% ×: Capacity retention rate less than 80%

[0215] As shown in Tables 2-5, the core-shell particles (binders) of Examples 2-1 to 36 all exhibited high T-shaped peel strength, excellent adhesion between the current collector and the active material layer, and superior charge-discharge characteristics of secondary batteries and coating properties for the current collector.

[0216] On the other hand, as shown in Tables 3 and 5, the particles (binders) of Comparative Examples 2-1 and 2-5, which lacked a core-shell structure and had no yield point in tensile strength, were insufficient in terms of adhesion between the current collector and the active material layer, charge-discharge characteristics of the secondary battery, and coating properties on the current collector.

[0217] Furthermore, the particles (binders) of Comparative Examples 2-2 to 4 and 6 to 10, which either lacked a yield point or had a low yield point, exhibited insufficient bonding between the current collector and the active material layer, as well as poor charge-discharge characteristics of the secondary battery.

Claims

1. A binder for electrodes of a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is made of rubber, the shell layer is made of a shell-forming polymer, and the binder has a yield point with a tensile strength measured by the following method, the strength at the yield point being 15 MPa or more. Tensile strength: Tensile strength measured at 23°C and a tensile speed of 500 mm / min using a dumbbell-shaped test piece conforming to JIS K6251.

2. The electrode binder according to claim 1, wherein the core-shell particles include monomer units having hydrogen-bonding modified groups.

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

4. The electrode binder according to claim 1 or 2, wherein the core-shell particles have a volume-average particle diameter of 100 to 1000 nm.

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

6. The electrode binder according to claim 1 or 2, wherein the rubber comprises at least one selected from the group consisting of (meth)acrylic monomer units and aromatic vinyl compound units as a constituent monomer unit.

7. The electrode binder according to claim 1 or 2, wherein the core comprises aliphatic conjugated diene compound units and aromatic vinyl compound units as constituent monomer units.

8. The electrode binder according to claim 1 or 2, wherein the electrode is the positive electrode.

9. The electrode binder according to claim 8, wherein the rubber is a non-diene rubber.

10. The electrode binder according to claim 8, wherein the core is composed of at least two layers, and the innermost layer of the core contains a polymer having a glass transition temperature of 40°C or higher.

11. The electrode binder according to claim 10, wherein the outermost layer of the core comprises a polymer having a glass transition temperature of -85°C or higher and less than 40°C.

12. The electrode binder according to claim 11, wherein the polymer of the outermost layer of the core is an acrylic rubber.

13. The electrode binder according to claim 1 or 2, wherein the electrode is a negative electrode.

14. A method for manufacturing an electrode for a secondary battery, comprising preparing a slurry containing an electrode active material and an electrode binder according to claim 1 or 2, and applying the slurry onto a current collector and drying it.

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

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