Positive-electrode binder for secondary battery

Abstract

This positive-electrode binder for a secondary battery comprises core-shell particles each including a core and a shell layer positioned outside the core. The core is composed of at least two layers, including: a core layer 1 containing a non-diene polymer (P1) having a glass transition temperature exceeding 40°C; and a core layer 2 positioned outside the core layer 1 and containing a non-diene rubber (R1) having a glass transition temperature of -60°C to +40°C. The shell layer includes a shell-forming polymer. The core-shell particles each include monomer units having a hydrogen-bonding functional group, the proportion of the monomer units having a hydrogen-bonding functional group relative to the total amount of the core-shell particles being 0.6% to 20% by weight.

Description

Binder for the positive electrode of a secondary battery

[0001] This disclosure relates to a binder used in the positive electrode of a secondary battery.

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

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

[0005] Polyvinylidene fluoride (PVDF) is a well-known material used to make binders. However, since polyvinylidene fluoride is made from chlorofluorocarbons (CFCs) and has a significant environmental impact, there is a desire to replace it with a material that has a lower environmental impact.

[0006] As an example of a positive electrode binder that can replace polyvinylidene fluoride resin, Patent Document 1 discloses the use of a particulate binder made of an organic polymer. As a specific example, a core-shell type rubber particle having a core portion containing acrylonitrile units with a high glass transition temperature and a flexible shell portion containing acrylic acid ester units is described (paragraph

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

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

[0008] Japanese Patent Publication No. 2002-117834

[0009] When using lithium composite phosphoroxides such as lithium iron phosphate as the positive electrode active material for secondary batteries, binders other than polypyrinide fluoride resin have not been sufficiently considered. Even in the aforementioned Patent Document 1, lithium composite phosphoroxides are not included as usable positive electrode active materials.

[0010] The inventors investigated core-shell type rubber particles as a binder for the positive electrode of a secondary battery that could replace polypyrinide fluoride resin. They found that the coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer, and the charge / discharge characteristics of the secondary battery were sometimes insufficient, and that the particles tended to swell easily in the electrolyte. When the binder swells, the volume of the active material layer in the battery may increase, or the active material layer may become more prone to peeling.

[0011] In view of the above situation, this disclosure aims to provide a binder for use in the positive electrode of a secondary battery that has good slurry coating properties on the current collector, good adhesion between the current collector and the active material layer, and good charge / discharge characteristics of the secondary battery, and that suppresses swelling in the electrolyte.

[0012] The inventors of the present invention diligently studied to solve the above problems and discovered that the above objectives can be achieved by using core-shell particles, which consist of at least two layers, a core including a core layer 2 located outside the core layer 1, and a shell layer made of a polymer, and which contain monomer units having hydrogen bonding functional groups in a specific proportion, as a binder in the positive electrode of a secondary battery, leading to this disclosure.

[0013] In other words, the first aspect of this disclosure relates to a binder for the positive electrode of a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is composed of at least two layers, comprising a core layer 1 and a core layer 2 located outside the core layer 1, wherein the core layer 1 comprises a non-diene polymer (P1) having a glass transition temperature of 40°C, the core layer 2 comprises a non-diene rubber (R1) having a glass transition temperature of -60 to +40°C, the shell layer comprises a shell-forming polymer, the core-shell particles comprise monomer units having hydrogen-bonding functional groups, and the ratio of the monomer units having hydrogen-bonding functional groups to the total amount of the core-shell particles is 0.6 to 20% by weight. The second aspect of this disclosure also relates to a coating dispersion comprising a positive electrode active material, the positive electrode binder, and a solvent.

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

[0015] Embodiments of the present disclosure are described in detail below. [Binder for positive electrode of secondary battery] The binder for positive electrode according to this embodiment is a binder used in the positive electrode of a secondary battery, and is used in a positive electrode containing a positive electrode active material such as an alkali metal composite oxide, in particular a lithium composite oxide. The binder includes at least core-shell particles.

[0016] (Core-shell particles) The core-shell particles have a core-shell structure and include a core and a shell layer located outside the core. The core is composed of at least two layers and includes a core layer 2 located outside the core layer 1. The shell layer includes a shell-forming polymer. The core-shell particles include monomer units having hydrogen-bonding functional groups. These monomer units having hydrogen-bonding functional groups can be included in at least one of the core layer 1, the core layer 2, and the polymer constituting the shell. Here, rubber refers to a member having rubber elasticity. Rubber elasticity is the ability to absorb energy from an external force and store it as energy to return to its original state. To exhibit rubber elasticity, the molecules must be sufficiently long, able to move freely, and appropriately bonded to one another. A member having rubber elasticity can easily return to its original shape when the external force is released, even if it is deformed by an external force.

[0017] Core-shell particles with this structure are used as a binder for a secondary battery. By using these core-shell particles, a slurry containing positive electrode active material can be formed. By coating the slurry onto the surface of a current collector and drying it, a positive electrode with an active material layer formed on the current collector can be manufactured. The coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer in the resulting positive electrode, and the charge-discharge characteristics of the secondary battery containing the positive electrode are all good, and the swelling of the core-shell particles in the electrolyte of the secondary battery is suppressed.

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

[0019] (Core) The core is composed of at least two layers, and the core includes a core layer 1 and a core layer 2 located outside the core layer 1, wherein the core layer 1 contains a non-diene polymer (P1) having a glass transition temperature of 40°C or higher, and the core layer 2 contains a non-diene rubber (R1) having a glass transition temperature of -60 to +40°C.

[0020] The non-diene rubber refers to rubbers other than diene rubbers. The diene rubber refers to a rubber containing an aliphatic conjugated diene compound such as 1,3-butadiene as a constituent unit, and specific examples thereof include butadiene rubber and styrene-butadiene rubber (SBR).

[0021] Compared with diene rubbers, non-diene rubbers have the advantage of being resistant to oxidation, so oxidation degradation is less likely to occur during charge and discharge of secondary batteries.

[0022] In the core, the proportion of the core layer 1 is preferably 5 to 70% by weight, more preferably 10 to 65% by weight, still more preferably 10 to 60% by weight, and particularly preferably 35 to 55% by weight. Also, the proportion of the core layer 2 is preferably 30 to 95% by weight, more preferably 35 to 90% by weight, still more preferably 40 to 90% by weight, and particularly preferably 45 to 65% by weight.

[0023] From the viewpoints of improving the coating property of the slurry on the current collector, the binding property between the current collector and the active material layer, and the charge and discharge characteristics of the secondary battery, the proportion of the core in the entire core-shell particles is preferably set within the range of 50 to 90% by weight. The lower limit is preferably 60% by weight or more, preferably 65% by weight or more, more preferably 70% by weight or more, and still more preferably 75% by weight or more from the viewpoint of further improving the charge and discharge characteristics of the secondary battery due to the improvement of the electrolyte resistance. The upper limit is preferably 85% by weight or less from the viewpoint of improving the binding property between the current collector and the active material layer.

[0024] (Core layer 1) As the non-diene polymer contained in the core layer 1, a hard polymer is used from the viewpoints that it is easy to maintain the particle shape, the internal resistance can be reduced, it has excellent binding property between the current collector and the active material layer, and the charge and discharge characteristics of the secondary battery can be improved. Specifically, as the diene polymer (P1) contained in the core layer 1, those having a glass transition temperature (Tg) exceeding 40°C are used.

[0025] The lower limit of the glass transition temperature of the non-diene polymer (P1) contained in the core layer 1 is preferably 50 °C or higher, more preferably 60 °C or higher, still more preferably 70 °C or higher, even more preferably 80 °C or higher, and particularly preferably 90 °C or higher. The upper limit is not particularly limited, and may be, for example, 150 °C or lower, 130 °C or lower, or 120 °C or lower.

[0026] The glass transition temperature of the non-diene polymer (P1) contained in the core layer 1 can be controlled by changing the types and ratios of the monomers in the polymer. For example, by increasing the proportion of the aromatic vinyl compound or methacrylic monomer in the non-diene polymer (P1), the glass transition temperature of the non-diene polymer (P1) can be increased.

[0027] The glass transition temperature can be measured using a differential scanning calorimeter (DSC). The same applies hereinafter.

[0028] The non-diene polymer (P1) contained in the core layer 1 is not particularly limited as long as its glass transition temperature (Tg) exceeds 40 °C, but it is preferably a polymer containing (meth)acrylic monomer or aromatic vinyl compound units. From the viewpoint of polymerization stability, it is preferably composed of a polymer containing an aromatic vinyl compound as a constituent monomer.

[0029] The (meth)acrylic monomer constituting the non-diene polymer (P1) 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, etc. Also, (meth)acrylic monomers having hydrogen-bonding functional groups as described later can be used. The (meth)acrylic monomers may be used alone or in combination of two or more types. Furthermore, "(meth)acrylic" is a term used to refer acrylic and methacrylic collectively.

[0030] The (meth)acrylic monomer constituting the non-diene polymer (P1) is preferably an alkyl (meth)acrylate, and particularly preferably an alkyl methacrylate. 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.

[0031] The aromatic vinyl compound constituting the non-diene polymer (P1) 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, 2,4,6-trimethylstyrene, and t-butylstyrene; cyclic alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; cyclic halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; cyclic ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and cyclic hydroxylated vinyl aromatic compounds such as 4-hydroxystyrene. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred. The aromatic vinyl compound may be used alone or in combination of two or more types.

[0032] Furthermore, examples of the non-diene polymer (P1) include methacrylic polymers and styrene polymers. From the viewpoint of bonding between the current collector and the active material layer, methacrylic polymers are preferred, while from the viewpoint of improving electrolyte resistance and thereby improving the charge-discharge characteristics of the secondary battery, styrene polymers are preferred.

[0033] The methacrylic polymer refers to a polymer that contains methacrylic monomer units as its main constituent units, and the styrene polymer refers to a polymer that contains styrene monomer units as its main constituent units.

[0034] The proportion of the alkyl methacrylate among the monomer components constituting the methacrylic polymer is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, from the viewpoint of the coating properties of the slurry to the current collector, the binding properties 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. In this disclosure, "monomer component" refers to monomer components excluding polyfunctional monomers.

[0035] Of the monomer components constituting the styrene-based polymer, the proportion of the styrene-based monomer (especially styrene) is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, from the viewpoint of improving the charge-discharge characteristics of the secondary battery by improving electrolyte resistance. The upper limit is 100% by weight or less.

[0036] In the methacrylic polymer and the styrene polymer, monomers other than the methacrylic monomer and the styrene monomer may or may not be used. Examples of such other monomers include acrylic monomers and vinyl cyanide compounds such as acrylonitrile and methacrylonitrile. From the viewpoint of polymerization stability, alkyl acrylates are preferred.

[0037] Examples of alkyl acrylates (particularly alkyl acrylates having linear or branched aliphatic hydrocarbon groups) include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, dodecyl acrylate, stearyl acrylate, and behenyl acrylate, with butyl acrylate or 2-ethylhexyl acrylate being preferred.

[0038] The non-diene polymer (P1) preferably contains monomer units having hydrogen-bonding functional groups. This suppresses swelling of core-shell particles in the electrolyte and improves the charge-discharge characteristics of the secondary battery. If at least one of the non-diene rubber and shell-forming polymer of the core layer 2, described later, contains monomer units having hydrogen-bonding functional groups, the non-diene polymer (P1) does not need to contain such units.

[0039] The aforementioned hydrogen-bonding functional 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. The amide group may have substituents on the nitrogen atom, but a primary amide without substituents is preferred. Only one hydrogen-bonding functional group may be used, or two or more may be used in combination.

[0040] As monomers having hydrogen bonding functional groups, vinyl monomers having hydrogen bonding functional groups are preferred, and (meth)acrylic monomers having hydrogen bonding functional groups are more preferred.

[0041] Among monomers having hydrogen-bonding functional groups, examples of monomers having hydroxyl groups include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0042] Furthermore, as monomers having hydroxyl groups, (meth)acrylic acid esters having (poly)alkylene glycol chains with hydroxyl groups at the terminal groups can also be used. Specific examples include (meth)acrylic acid (poly)ethylene glycol (e.g., NOF Corporation's Bremmer PE-90, PE-200, PE-350, AE-90U, AE-200, AE-400), (meth)acrylic acid (poly)propylene glycol (e.g., NOF Corporation's Bremmer PP-500, PP-500D, PP-800, PP-1000, PP-2000D, AP-200, AP-400, AP-400D, AP-550, AP-800, AP-1000D), (Me Examples include (poly)ethylene glycol acrylic acid - (poly)propylene glycol (e.g., NOF Corporation's Bremmer 50PEP-300), (meth)acrylic acid - (poly)ethylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 55PET-800, 50PEP-500D), (meth)acrylic acid - (poly)propylene glycol meth)acrylic acid - (poly)butylene glycol (e.g., NOF Corporation's Bremmer 10PPB-500B, 10PPB-500BD), etc.

[0043] Examples of monomers having a carboxyl group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, and maleic acid. Among these, acrylic acid and / or methacrylic acid are preferred.

[0044] 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.

[0045] Examples of monomers having a sulfonic acid group include vinyl monomer units having a sulfonic acid group. Examples of sulfonic acid groups include styrene sulfonic acid, 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.

[0046] Of the monomers having hydrogen bonding functional groups contained in the non-diene polymer (P1), monomers having hydroxyl groups and / or amide groups are preferred from the viewpoint of binding between the current collector and the active material layer. From the viewpoint of the charge-discharge characteristics of the secondary battery, monomers having carboxyl groups are preferred. Furthermore, from the viewpoint of coating properties and the internal resistance of the secondary battery, monomers having sulfonic acid groups are preferred.

[0047] When the non-diene polymer (P1) contains monomer units having hydrogen bonding functional groups, the proportion of monomers having hydrogen bonding functional groups among the monomer components constituting the non-diene polymer (P1) is preferably 1 to 30% 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, and even more preferably 5% by weight or more, particularly from the viewpoint of charge-discharge characteristics and swelling suppression. The upper limit is more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less, particularly from the viewpoint of binding properties.

[0048] The non-diene polymer (P1) preferably has a crosslinked structure. To introduce a crosslinked structure, for example, when synthesizing the non-diene polymer (P1), a crosslinkable component such as a polyfunctional monomer may be used together with the monomer component.

[0049] Examples of the polyfunctional monomers include allyl (meth)acrylate; allylalkyl (meth)acrylate; allyloxyalkyl (meth)acrylates; polyfunctional (meth)acrylates having two or more (meth)acrylic groups, such as polyethylene glycol di(meth)acrylate, butanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate; as well as diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, and divinylbenzene. Preferably, allyl methacrylate, triallyl isocyanurate, butanediol di(meth)acrylate, and divinylbenzene are used, and particularly preferably allyl methacrylate.

[0050] The amount of the polyfunctional monomer used is not particularly limited, but for example, it may be 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, more preferably 0.1 to 3 parts by weight, and even more preferably 0.2 to 2 parts by weight, based on 100 parts by weight of the total monomer components constituting the non-diene polymer (P1).

[0051] The non-diene polymer (P1) may consist of a single-composition polymer, or it may consist of multiple polymers that differ in the type or amount of monomers and / or polyfunctional monomers from one another.

[0052] The ratio of the non-diene polymer (P1) to the total core-shell particles is preferably 10 to 75% by weight, from the viewpoint of coating properties of the slurry to the current collector, binding properties between the current collector and the active material layer, charge-discharge characteristics of the secondary battery, and suppression of swelling in the electrolyte. The lower limit is more preferably 15% by weight or more, and even more preferably 20% by weight or more, particularly from the viewpoint of binding properties. The upper limit is more preferably 60% by weight or less, and even more preferably 45% by weight or less, particularly from the viewpoint of coating properties.

[0053] (Core layer 2) The non-diene rubber (R1) included in core layer 2 is a soft rubber, which is desirable from the viewpoint of improving the coating properties of the slurry to the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery. Specifically, the non-diene rubber (R1) used has a glass transition temperature (Tg) in the range of -60°C to +40°C. When the Tg of the non-diene rubber is higher than 40°C, coating properties and bonding properties tend to decrease, in particular. Also, when the Tg of the non-diene rubber (R1) is lower than -60°C, deformation of the particle shape is more likely to occur, which tends to increase the resistance value. Furthermore, it is preferable that core layer 2 is graft-bonded to adjacent layers.

[0054] The lower limit of the glass transition temperature of the non-diene rubber (R1) contained in the core layer 2 is more preferably -55°C or higher, even more preferably -50°C or higher, even more preferably -40°C or higher, even more preferably -20°C or higher, and particularly preferably 0°C or higher. The upper limit is more preferably +35°C or lower, even more preferably +30°C or lower, and particularly preferably +25°C or lower.

[0055] The method for controlling the glass transition temperature of the non-diene rubber (R1) contained in core layer 2 is the same as that for the non-diene polymer contained in core layer 1.

[0056] The non-diene rubber (R1) contained in the core layer 2 is not particularly limited as long as its glass transition temperature (Tg) is in the range of -60 to +40°C. Examples of non-diene rubbers with a glass transition temperature of -60 to +40°C include acrylic rubber and polyorganosiloxane rubber. In particular, acrylic rubber is preferred from the viewpoint of bonding between the current collector and the active material layer and the charge / discharge characteristics of the secondary battery.

[0057] Conventional binders using acrylic rubber tended to swell significantly in the electrolyte. However, the core-shell particles of this embodiment can suppress such swelling. Therefore, this embodiment can suitably use core-shell particles containing acrylic rubber.

[0058] The acrylic rubber mentioned above refers to rubber that contains acrylic monomer units as its main constituent units. The acrylic monomer is not particularly limited, but examples include alkyl acrylates [methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, dodecyl acrylate, stearyl acrylate, behenyl acrylate, and other alkyl acrylates having linear or branched aliphatic hydrocarbon groups]; aromatic ring-containing acrylates such as phenoxyethyl acrylate and benzyl acrylate; glycidyl acrylates such as glycidyl acrylate and glycidyl alkyl acrylate; alkoxyalkyl acrylates, etc. Also, acrylic monomers having hydrogen-bonding functional groups, as described later, can be mentioned. The acrylic monomer may be used alone or in combination of two or more types. Among the acrylic monomers, alkyl acrylates are preferred, and butyl acrylate or 2-ethylhexyl acrylate are particularly preferred.

[0059] The proportion of the alkyl acrylate among the monomer components constituting the acrylic rubber is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, from the viewpoint of the coating properties of the slurry on the current collector, the binding properties 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.

[0060] 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. Examples of such aromatic vinyl compounds include the aromatic vinyl compounds listed as examples of aromatic vinyl compounds constituting the non-diene polymer (P1).

[0061] From the viewpoint of the coating properties of the slurry onto the current collector and the charge-discharge characteristics of the secondary battery, the proportion of the aromatic vinyl compound among the monomer components constituting the acrylic rubber is preferably 1% by weight or more, more preferably 5% by weight or more, and even more preferably 10% by weight or more. Furthermore, from the viewpoint of the coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery, the proportion of the aromatic vinyl compound is preferably 30% by weight or less, more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less.

[0062] The non-diene rubber (R1) preferably contains monomer units having hydrogen-bonding functional groups. This suppresses swelling of core-shell particles in the electrolyte and improves the charge-discharge characteristics of the secondary battery. If at least one of the non-diene rubber of the core layer 1 and the shell-forming polymer described later contains such units, the non-diene rubber (R1) does not need to contain such units.

[0063] The hydrogen bonding functional groups are as described above. Furthermore, the monomers having hydrogen bonding functional groups exemplified above can also be used as monomers having hydrogen bonding functional groups in the non-diene rubber (R1).

[0064] Of the monomers having hydrogen bonding functional groups contained in the non-diene rubber (R1), monomers having hydroxyl groups and / or amide groups are preferred from the viewpoint of bonding between the current collector and the active material layer. From the viewpoint of the charge-discharge characteristics of the secondary battery, monomers having carboxyl groups are preferred. Furthermore, from the viewpoint of coating properties and the internal resistance of the secondary battery, monomers having sulfonic acid groups are preferred.

[0065] When the non-diene rubber (R1) contains monomer units having hydrogen bonding functional groups, the proportion of monomers having hydrogen bonding functional groups among the monomer components constituting the non-diene rubber (R1) is preferably 1 to 30% by weight, from the viewpoint of the applicability of the slurry to the current collector, the binding properties between the current collector and the active material layer, the charge-discharge characteristics of the secondary battery, and the suppression of swelling in the electrolyte. The lower limit is more preferably 2% by weight or more, and even more preferably 5% by weight or more, particularly from the viewpoint of charge-discharge characteristics and swelling suppression. The upper limit is more preferably 20% by weight or less, even more preferably 15% by weight or less, and particularly preferably 10% by weight or less, particularly from the viewpoint of binding properties.

[0066] The non-diene rubber (R1) preferably has a crosslinked structure. Similar to the non-diene polymer (P1), to introduce a crosslinked structure, for example, when synthesizing the non-diene rubber (R1), a crosslinkable component such as a polyfunctional monomer can be used together with the monomer component, and specific examples of polyfunctional monomers are the same as those for the non-diene polymer (P1).

[0067] The amount of the polyfunctional monomer used is within a known range and is not particularly limited, but for example, it may be 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, more preferably 0.1 to 3 parts by weight, and even more preferably 0.2 to 2 parts by weight, based on 100 parts by weight of the total monomer components constituting the non-diene rubber (R1).

[0068] The non-diene rubber (R1) may consist of a single rubber composition, or it may consist of multiple types of rubber with different types or amounts of monomers and / or polyfunctional monomers.

[0069] The ratio of the non-diene rubber (R1) to the total core-shell particles is preferably 20 to 80% by weight, from the viewpoint of bonding between the current collector and the active material layer and polymerization stability. The lower limit is more preferably 30% by weight or more, and even more preferably 40% by weight or more, and the upper limit is more preferably 75% by weight or less, and even more preferably 70% by weight or less.

[0070] The core may consist only of core layer 1 and core layer 2, but may also have layers other than core layer 1 and core layer 2, as long as the effects of the invention are achieved. The layers other than core layer 1 and core layer 2 are preferably layers made of polymers and are graft-bonded to adjacent layers.

[0071] (Shell layer) The 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.

[0072] By providing the aforementioned shell layer, the dispersibility of core-shell particles and positive 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 bonding properties between the current collector and the active material layer, as well as the charge-discharge characteristics of the secondary battery, can also be improved.

[0073] The shell layer is preferably composed of a non-crosslinked polymer, from the viewpoint of dispersibility of core-shell particles and positive electrode active material in the slurry, and coating properties of the slurry onto the current collector. A non-crosslinked polymer refers to a polymer that does not contain crosslinked structures and does not contain structural units derived from polyfunctional monomers, and refers to a polymer that does not fall under the category of rubber elastic materials such as acrylic rubber.

[0074] The polymer forming the shell layer (hereinafter also referred to as the shell-forming polymer) is preferably a vinyl polymer. The monomers constituting the shell-forming polymer are not particularly limited as long as they are vinyl monomers, but it is preferable that they include at least one selected from the group consisting of (meth)acrylic monomers, aromatic vinyl compounds, and vinyl cyanide compounds.

[0075] The (meth)acrylic monomers constituting the shell-forming polymer are not particularly limited and include, 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)acrylates, etc. Also, the (meth)acrylic monomers having the hydrogen-bonding functional groups mentioned above can be used. The (meth)acrylic monomers may be used alone or in combination of two or more types.

[0076] The (meth)acrylic monomer constituting the shell-forming polymer is preferably an alkyl (meth)acrylate, and particularly preferably an alkyl methacrylate. 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.

[0077] The aromatic vinyl compound constituting the shell-forming polymer is not particularly limited, and examples of aromatic vinyl compounds constituting the non-diene polymer (P1) include those listed above. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred. The aromatic vinyl compound may be used alone or in combination of two or more.

[0078] The vinyl cyanide compound is not particularly limited, but examples include acrylonitrile and methacrylonitrile. Of these, acrylonitrile is preferred.

[0079] Other vinyl monomers may be used as monomers constituting the shell-forming polymer. Examples of such monomers include alkenes such as ethylene and propylene; vinyl halides such as vinyl chloride and vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; and heterocyclic vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole.

[0080] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer and improving the dispersibility of core-shell particles and positive electrode active material in the slurry, the shell-forming polymer preferably contains at least methacrylic monomer units or aromatic vinyl compounds, and is particularly preferably composed of methacrylic monomer units, acrylic monomer units and / or aromatic vinyl compound units.

[0081] As the methacrylic monomer, alkyl methacrylate is preferred. The number of carbon atoms in the alkyl group of the alkyl methacrylate is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0082] Of the total monomer components constituting the shell-forming polymer, the proportion of the methacrylic monomer (particularly alkyl methacrylate) units is preferably 60 to 100% by weight, from the viewpoint of increasing the glass transition temperature of the shell-forming polymer. The lower limit is more preferably 70% by weight or more, and even more preferably 80% by weight or more. The upper limit is more preferably 95% by weight or less, and even more preferably 90% by weight or less, from the viewpoint of bonding between the current collector and the active material layer.

[0083] The proportion of the vinyl cyanide compound in the total monomer components constituting the shell-forming polymer may be about 0 to 30% by weight, about 0 to 20% by weight, or about 0 to 10% by weight. The vinyl cyanide compound may not be included in the shell-forming polymer.

[0084] The shell-forming polymer preferably contains monomer units having hydrogen-bonding functional groups. This suppresses swelling of core-shell particles in the electrolyte and improves the charge-discharge characteristics of the secondary battery. If at least one of the non-diene polymer (P1) of the core layer 1 and the non-diene rubber (R1) of the core layer 2 contains such units, the shell-forming polymer does not need to contain such units.

[0085] The hydrogen bonding functional groups are as described above. Furthermore, each of the compounds mentioned above can also be used as monomers possessing hydrogen bonding functional groups.

[0086] From the viewpoint of bonding between the current collector and the active material layer, monomers having hydroxyl groups and / or amide groups are preferred as monomers having hydrogen bonding functional groups in the shell-forming polymer. From the viewpoint of charge-discharge characteristics of the secondary battery, monomers having carboxyl groups are preferred. Furthermore, from the viewpoint of coating properties and internal resistance of the secondary battery, monomers having sulfonic acid groups are preferred.

[0087] When the shell-forming polymer contains monomer units having hydrogen-bonding functional groups, the proportion of monomers having hydrogen-bonding functional groups among the 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, and even more preferably 5% by weight or more, particularly from the viewpoint of charge-discharge characteristics and swelling suppression. The upper limit is more preferably 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, particularly from the viewpoint of binding properties.

[0088] From the viewpoint of making it easier to maintain the shape of the core-shell particles and improving the dispersibility of the core-shell particles and positive electrode active material in the slurry, the shell-forming polymer is preferably rigid. Specifically, the glass transition temperature (Tg) of the shell-forming polymer is preferably 40°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of bonding between the current collector and the active material layer, it is preferably 120°C or lower, more preferably 115°C or lower, and even more preferably 110°C or lower.

[0089] 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 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.

[0090] The proportion of the shell-forming polymer to the total core-shell particles is preferably 10 to 65% by weight, from the viewpoint of coating properties of the slurry onto the current collector, binding properties between the current collector and the active material layer, charge-discharge characteristics of the secondary battery, and suppression of swelling in the electrolyte. The upper limit is more preferably 50% by weight or less, even more preferably 40% by weight or less, and even more preferably 30% by weight or less, particularly from the viewpoint of binding properties. The lower limit is more preferably 15% by weight or more, and even more preferably 20% by weight or more, particularly from the viewpoint of coating properties.

[0091] Monomer units having hydrogen-bonding functional groups may be included in at least one of the polymers constituting the core layer 1, the core layer 2, and the shell. Regardless of the location in which they are included, the total content of monomer units having hydrogen-bonding functional groups is set to a range of 0.6 to 20% by weight relative to the total amount of core-shell particles. If the content is less than 0.6% by weight, the swelling of the core-shell particles in the electrolyte cannot be sufficiently suppressed, resulting in insufficient bonding between the current collector and the active material layer, and insufficient charge-discharge characteristics of the secondary battery. If the content is more than 20% by weight, the coating properties of the slurry on the current collector and the bonding between the current collector and the active material layer will be insufficient. The lower limit is preferably 1% by weight or more, and more preferably 1.5% by weight or more. The upper limit is preferably 15% by weight or less, and more preferably 10% by weight or less.

[0092] 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.

[0093] (Volume-average particle diameter of core-shell particles) The volume-average particle diameter of the core-shell particles is not particularly limited and may be, for example, about 10 to 1000 nm. However, from the viewpoint of the dispersibility of the core-shell particles in the slurry, 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, the volume-average particle diameter of the core-shell particles is preferably 100 to 500 nm. The lower limit is more preferably 150 nm or more, and even more preferably 200 nm or more. The upper limit is more preferably 400 nm or less, even more preferably 350 nm or less, and particularly preferably 300 nm or less.

[0094] The volume-average particle diameter of core-shell particles is measured using a particle diameter measuring device while the core-shell particles are in their latex state. The particle diameter of core-shell particles can be controlled by the type and amount of polymerization initiators, chain transfer agents, redox agents, emulsifiers, etc. used during polymerization, as well as the polymerization temperature and polymerization time.

[0095] (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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] In addition, a redox-type initiator can be used that combines organic peroxides such as t-butyl peroxyisopropyl carbonate, paramentane hydroperoxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, and t-hexyl peroxide; inorganic peroxides and inorganic persulfates 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.

[0104] When a redox-type initiator is used, polymerization can be carried out even at low temperatures in which the peroxide or persulfate does not substantially decompose thermally, which is preferable as it allows the polymerization temperature to be set over a wide range. 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. Surfactants can also be used in addition, but this is also within known ranges.

[0105] Furthermore, chain transfer agents can be used when polymerizing polyfunctional monomers. While there are no particular limitations on the types of chain transfer agents that can be used, examples include alkyl mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, t-decyl mercaptan, n-decyl mercaptan, and n-octyl mercaptan, as well as alkyl ester mercaptans such as 2-ethylhexyl thioglycolate.

[0106] Any solvent that allows emulsion polymerization to proceed stably can be used as the solvent; for example, water can be suitably used.

[0107] 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 85°C, preferably 40 to 75°C, more preferably 45 to 70°C, and even more preferably 49 to 65°C.

[0108] When the core-shell particles are produced by emulsion polymerization, the resulting latex can be used as is, or its concentration can be adjusted as needed, as a binder for the positive electrode.

[0109] Furthermore, the obtained latex can be dried to produce a powder, which can then be used as a positive electrode binder (powdered positive electrode binder).

[0110] 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.

[0111] Examples of the drying methods include spray drying, drying by standing, and vacuum drying.

[0112] The positive electrode binder according to this embodiment may contain, in addition to the core-shell particles, components such as conductive additives, reinforcing materials, leveling agents, viscosity modifiers, and electrolyte additives. These components are not particularly limited as long as they do not affect the battery reaction, and known materials or the materials described later can be used. Furthermore, only one type of these component may be used, or two or more types may be used in combination.

[0113] The proportion of the core-shell particles in the solid content of the positive electrode binder according to this embodiment is not particularly limited and may be, for example, about 10 to 100% by weight. It may also be about 50 to 100% by weight, about 80 to 100% by weight, or about 90 to 100% by weight.

[0114] The binder for the positive electrode 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 ease of handling, it is preferable to be in powder form.

[0115] By using the binder according to this disclosure, a dispersion for coating the current collector of a positive electrode, a positive electrode of a secondary battery, and a secondary battery can be suitably manufactured. The binder according to this disclosure is a binder used in the positive electrode of a secondary battery and is extremely promising as a material to replace polypyrinide fluoride resin, which has a large environmental impact.

[0116] (Slurry) A dispersion liquid for the positive electrode (also called a slurry) can be prepared by mixing the binder for the positive electrode according to this embodiment, the positive electrode active material, and the dispersion medium. By applying the slurry to the surface of the current collector and drying it, a layer of positive electrode active material can be formed on the current collector.

[0117] The amount of core-shell particles added to the slurry can be appropriately set by those skilled in the art, but 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.

[0118] <Positive Electrode Active Material> In this embodiment, a positive electrode active material is used as the positive electrode active material. By using the core-shell particles according to this embodiment as the binder for the positive electrode active material, the coating properties of the slurry onto the current collector, the bonding properties between the current collector and the active material layer, and the charge-discharge characteristics of the secondary battery can be improved.

[0119] Examples of positive electrode active materials include lithium composite oxides and alkali metal composite oxides such as sodium composite oxides. A specific example of a lithium composite oxide is lithium iron phosphate (LiFePO4). ４ : LFP), Lithium manganese phosphate (LiMnPO) ４Examples of sodium-based oxides include polyanionic oxides containing lithium-compound phosphoric acid such as lithium cobalt phosphate and lithium iron manganese phosphate (LMFP); layered oxides such as lithium nickel manganese cobaltate and lithium cobaltate; and spinel-type oxides such as lithium manganeseate. Among these, lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium nickel manganese cobaltate, and lithium cobaltate are preferred, lithium iron phosphate, lithium nickel manganese cobaltate, and lithium cobaltate are more preferred, and lithium iron phosphate is particularly preferred in terms of the charge-discharge characteristics of lithium-ion batteries. Specific examples of sodium-compound oxides include sodium manganese, sodium nickelate, sodium chromate, sodium nickel manganese, sodium nickel manganese cobaltate, sodium ferrite, and sodium iron manganese. Among these, sodium iron manganese is preferred in terms of battery characteristics such as capacity and cycle characteristics.

[0120] <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.

[0121] 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.

[0122] <Thickener> A thickener may be optionally added to the slurry. The thickener is a component that can improve the dispersion stability of the positive electrode active material in the slurry and improve the coating properties of the slurry. Water-soluble polymers can be used as thickeners, specifically carboxymethylcellulose, methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, polyvinyl alcohol, and polycarboxylic acids, as well as their salts, and in addition, poly(meth)acrylamide can be used. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, and alginic acid. Only one of these water-soluble polymers may be used, or two or more may be used in combination. Of these, cellulosic compounds are preferred, and carboxymethylcellulose or its salts are particularly preferred.

[0123] The amount of thickener 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 positive electrode active material. However, the thickener may be omitted.

[0124] <Other Polymers> The slurry may optionally contain polymers other than the core-shell particles and thickeners mentioned above. Examples of such polymers include fluorine-containing polymers and acrylonitrile polymers.

[0125] <Dispersion Medium> In the slurry, a dispersion medium such as water or an organic solvent is used. A mixed solvent of water and an organic solvent may be used, or only organic solvents may be used, either alone or in combination of several. Examples of organic solvents include alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol; alkyl ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, dioxane, and diglyme; amides such as diethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and dimethylimidazolidinone; and sulfur-based solvents such as dimethyl sulfoxide and sulfolane. Among these, amides are preferred, 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.

[0126] The proportion of organic solvent in the entire dispersion medium is preferably about 50 to 100% by weight, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more. If the slurry contains water, that water may include water contained in the latex when the positive electrode binder is a core-shell particle latex, or water contained in thickeners, etc.

[0127] The solid content concentration of the slurry is not particularly limited, but may be, for example, about 10 to 80% by weight, and preferably about 30 to 70% by weight. 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.

[0128] <Preparation of Slurry> Slurry can be prepared by dispersing the above-mentioned components in a dispersion medium. Specifically, slurry can be prepared by mixing the above-mentioned components with the dispersion medium using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, lye crusher, ultrasonic disperser, homogenizer, planetary mixer, or film mixer. The mixing of the above-mentioned 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.

[0129] (Positive electrode for secondary battery) The positive electrode for a secondary battery according to this embodiment includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes at least a positive electrode active material and a binder according to this embodiment. The positive electrode for a secondary battery according to this embodiment can be obtained by applying the above-described slurry onto the current collector and drying it. Known metal foils may be used as the current collector, such as copper foil, aluminum foil, nickel foil, and highly conductive stainless steel foil.

[0130] The core-shell particles according to this embodiment tend to maintain their particle shape relatively well within the positive electrode active material layer after drying. As a result, bonding between positive electrode active materials, or bonding between positive electrode active materials and the current collector, can be achieved by point adhesion. Consequently, the movement of 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.

[0131] [Coating Process] The method for coating the slurry onto the current collector is not particularly limited, and known methods can be used. Specifically, examples include the doctor blade method, dip method, reverse roll method, direct roll method, gravure method, extrusion method, and brush coating method. In this case, the slurry may be 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 positive electrode active material layer obtained after drying.

[0132] [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.

[0133] After the drying process, the positive electrode active material layer may be subjected to pressure treatment using a die press or roll press. This improves the adhesion between the positive electrode active material layer and the current collector, and also reduces the porosity of the positive electrode active material layer.

[0134] Furthermore, the positive electrode for the secondary battery according to this embodiment can also be manufactured by a powder molding method. 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 a positive electrode active material layer can be formed on the current collector by rolling press molding as desired.

[0135] (Secondary Battery) The secondary battery according to this embodiment comprises a positive electrode, a negative electrode, an electrolyte, and a separator according to this embodiment. 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, taking into account battery capacity and life characteristics, lithium-ion batteries are preferred.

[0136] <Negative Electrode> The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. Known metal foils may be used as the current collector, such as copper foil, aluminum foil, nickel foil, highly conductive stainless steel foil, lithium foil, etc.

[0137] Examples of electrode active materials that can be used as negative electrodes include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that combine these.

[0138] Examples of carbon-based anode active materials include carbonaceous materials and graphitic materials. Examples of carbonaceous materials include easily graphitizable carbon, which readily changes its structure depending on the heat treatment temperature, and non-graphitizable carbon, which has a structure close to an amorphous structure, such as glassy carbon. Examples of easily graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolysis vapor-grown carbon fibers. Examples of non-graphitizable carbon include phenolic resin calcined bodies, polyacrylonitrile-based carbon fibers, pseudoisotropic carbon, furfuryl alcohol resin calcined bodies (PFA), and hard carbon. Examples of graphitic materials include graphite, such as natural graphite and artificial graphite.

[0139] As a metallic anode active material, for example, in the case of an alkali metal ion battery, the same alkali metal as the alkali metal is used. In the case of a lithium-ion battery, 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 an alkali-ion battery can be increased.

[0140] 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. Examples of silicon-containing alloys include alloy compositions containing silicon, aluminum, transition metals such as iron, and further containing rare earth elements such as tin and yttrium. SiOx is composed of SiO and SiO ２ The compound contains at least one of the two and Si, where x is usually 0.01 or more and less than 2. Examples of composites of Si-containing material and conductive carbon include compounds obtained by heat-treating a pulverized mixture of SiO, a polymer such as polyvinyl alcohol, and optionally a carbon material in an atmosphere containing organic gas and / or vapor. It can also be obtained by known methods such as coating the surface of SiO particles by a chemical vapor deposition method using organic gas, or by creating composite particles (granulation) of SiO particles and graphite or artificial graphite by a mechanochemical method.

[0141] <Electrolyte> As the electrolyte, a non-aqueous electrolyte in which a supporting electrolyte is dissolved in a non-aqueous solvent can be used. As the supporting electrolyte, for example, in the case of an alkali metal ion battery, a salt composed of the same alkali metal as that alkali metal is used. In the case of a lithium ion battery, usually a lithium salt is used, and in the case of a sodium ion battery, usually a sodium salt is used. ６ For example, as the lithium salt, LiPF ６ , LiAsF ４ , LiBF ６ , LiSbF ４ , LiAlCl ４ , LiClO ３ SO ３ Li, C ４ F ９ SO ３ Li, CF ３ COOLi, (CF ３ CO) ２ NLi, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ )NLi, etc. can be mentioned. Among them, LiPF ６ , LiClO ４ , CF ３ SO ３ Li is preferable. These may be used alone or in combination of two or more. ６ For example, as the sodium salt, NaPF ４ , NaClO ４ , NaBCl ３ , NaSO ３ CF ３ and Na(CH ６ H ４ SO ３ ), etc. can be mentioned. These may be used alone or in combination of two or more.

[0142] The non-aqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of non-aqueous solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and ethyl methyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Among these, carbonates are preferred because 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.

[0143] When carbonates are used as the non-aqueous solvent for the electrolyte, binders using conventional core-shell particles tend to swell significantly in the electrolyte. However, the core-shell particles according to this embodiment suppress such swelling, making it possible to suitably construct secondary batteries such as lithium-ion batteries even when carbonates are used as the non-aqueous solvent for the electrolyte.

[0144] The electrolyte may contain additives. Examples of additives include carbonate-based compounds such as vinylene carbonate (VC).

[0145] 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.

[0146] <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.

[0147] <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 an electrolyte into the battery container and seal it. Furthermore, if necessary, expanded metal, fuses, overcurrent prevention elements such as PTC elements, lead plates, etc., may be added to prevent pressure rise inside the battery and overcharging / discharging. The shape of the secondary battery may be coin-shaped, button-shaped, sheet-shaped, cylindrical, rectangular, flat, etc.

[0148] The following items list preferred embodiments of the present disclosure, but the present invention is not limited to the following items. [Item 1] A binder for the positive electrode of a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is composed of at least two layers, and the core comprises a core layer 1 and a core layer 2 located outside the core layer 1, the core layer 1 comprises a non-diene polymer (P1) having a glass transition temperature of 40°C, the core layer 2 comprises a non-diene rubber (R1) having a glass transition temperature of -60 to +40°C, the shell layer comprises a shell-forming polymer, the core-shell particles comprise monomer units having hydrogen-bonding functional groups, and the ratio of the monomer units having hydrogen-bonding functional groups to the total amount of the core-shell particles is 0.6 to 20% by weight. [Item 2] The positive electrode binder according to Item 1, wherein the hydrogen bonding functional group is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, an amide group, and a sulfonic acid group. [Item 3] The positive electrode binder according to Item 1 or 2, wherein the non-diene rubber (R1) contained in the core layer 2 is an acrylic rubber containing acrylic monomer units. [Item 4] The positive electrode binder according to Item 3, wherein the proportion of alkyl acrylate among the monomer components constituting the acrylic rubber is 50% by weight or more. [Item 5] The positive electrode binder according to Item 3 or 4, wherein the acrylic rubber further contains aromatic vinyl compound units. [Item 6] The positive electrode binder according to any one of Items 1 to 5, wherein the proportion of the non-diene rubber (R1) to the total core-shell particles is 20 to 80% by weight. [Item 7] The positive electrode binder according to any one of items 1 to 6, wherein the non-diene polymer (P1) contained in the core layer 1 is a rubber containing (meth)acrylic monomers or aromatic vinyl compound units. [Item 8] The positive electrode binder according to any one of items 1 to 7, wherein the proportion of the core layer 1 in the core is 5 to 70% by weight, and the proportion of the core layer 2 is 30 to 95% by weight. [Item 9] The positive electrode binder according to any one of items 1 to 8, wherein the proportion of the core to the total core-shell particles is 50 to 90% by weight.[Item 10] The positive electrode binder according to any one of items 1 to 9, wherein the core-shell particles have a volume-average particle diameter of 100 to 500 nm. [Item 11] The positive electrode binder according to any one of items 1 to 10, wherein the shell-forming polymer contains at least one selected from the group consisting of (meth)acrylic monomers, aromatic vinyl compounds, and vinyl cyanide compounds as a constituent monomer. [Item 12] The positive electrode binder according to any one of items 1 to 11, wherein the shell-forming polymer has a glass transition temperature of 40 to 120°C. [Item 13] The positive electrode binder according to any one of items 1 to 12, which is in powder form. [Item 14] The positive electrode binder according to any one of items 1 to 13, wherein the secondary battery is an alkali metal ion battery. [Item 15] A coating dispersion comprising a positive electrode active material, the positive electrode binder according to any one of items 1 to 14, and a solvent. [Item 16] The coating dispersion according to Item 15, wherein the positive electrode active material is an alkali metal composite oxide. [Item 17] The coating dispersion according to Item 16, wherein the alkali metal composite oxide comprises at least one selected from the group consisting of lithium nickel manganese cobaltate, lithium cobaltate, lithium iron phosphate, lithium manganese phosphate, and lithium iron manganese phosphate. [Item 18] A method for manufacturing a positive electrode of a secondary battery, comprising the steps of preparing a dispersion containing a positive electrode active material, a positive electrode binder according to any one of Items 1 to 17, and a solvent, and coating the dispersion onto a current collector and drying it. [Item 19] A positive electrode of a secondary battery comprising a current collector and an active material layer provided on the current collector, wherein the active material layer comprises a positive electrode active material and a positive electrode binder according to any one of Items 1 to 18. [Item 20] A secondary battery comprising the positive electrode according to Item 19, a separator, a negative electrode, and an electrolyte.

[0149] 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.

[0150] (Example 1) (Formation of core layer 1 (hard core particles)) 500 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of polyoxyethylene lauryl ether phosphate were added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２ A solution of 0.012 g of 0.012 g and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 100 g of styrene, 1 g of allyl methacrylate, and 0.35 g of t-butyl hydroperoxide (69% solids) was added to the polymerization apparatus over 30 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 60 minutes after the end of addition, forming hard core particles.

[0151] (Formation of Core Layer 2 (Soft Core Particles)) After the polymerization of the hard core particles was completed, a mixture of 670 g of 2-ethylhexyl acrylate, 10 g of allyl methacrylate, and 0.8 g of t-butyl hydroperoxide (69% solids) was added to the polymerization apparatus over 150 minutes. During polymerization, polyoxyethylene lauryl ether phosphate and sodium hydroxide (2% solids) were added in arbitrary amounts and at arbitrary times to maintain the pH of the system at 5 to 7. Polymerization was completed 60 minutes after the end of the additions, and soft core particles were formed.

[0152] (Formation of the shell layer) Next, a mixture of 194 g of methyl methacrylate, 6 g of butyl acrylate, 30 g of 2-hydroxyethyl methacrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-butyl hydroperoxide (69% solids content) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids content of 45%, and a volume-average particle size of 200 nm.

[0153] (Examples 2-18 and Comparative Example 4) Latex of core-shell structured graft copolymer particles was obtained in the same manner as in Example 1, except that the type or amount of monomers used for core layer 1, core layer 2, and shell layer was changed according to the description in Table 1.

[0154] (Granulation of core-shell structured graft copolymer) A slurry was prepared by adding an aqueous calcium chloride solution to the latex of the core-shell structured graft copolymer particles obtained above. Then, the slurry was dehydrated using a centrifugal dehydrator, washed with deionized water, and dried at 50°C for two days to obtain a powder of the core-shell structured graft copolymer.

[0155] (Comparative Example 1) (Polymerization of non-core-shell structure copolymer) 500 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of polyoxyethylene lauryl ether phosphate were added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２ A solution of 0.012 g of O and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 194 g of methyl methacrylate, 6 g of butyl acrylate, 400 g of styrene, 370 g of 2-ethylhexyl acrylate, 30 g of methacrylic acid, 1 g of t-dodecyl mercaptan, 15 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (69% solids), and 3.2 g of polyoxyethylene lauryl ether phosphate was added to the polymerizer over 360 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 80 minutes after the end of addition, yielding a latex of a non-core-shell structure copolymer with a conversion rate of 100%, a solids concentration of 45%, and a volume-average particle size of 200 nm.

[0156] (Granulation of non-core-shell copolymer) The non-core-shell copolymer latex obtained above was mixed with an aqueous calcium chloride solution to form a slurry. The slurry was then dehydrated using a centrifugal dehydrator, washed with deionized water, and dried at 50°C for two days to obtain a powder of the non-core-shell copolymer.

[0157] (Comparative Example 2) (Formation of Core Particles) 500 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of polyoxyethylene lauryl ether phosphate were added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２ A solution of 0.012 g of 0) and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then a mixture of 800 g of styrene, 12 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (69% solids), and 3.2 g of polyoxyethylene lauryl ether phosphate was added to the polymerizer over 240 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 80 minutes after the end of addition, and core particles were formed.

[0158] (Formation of the shell layer) Next, a mixture of 194 g of methyl methacrylate, 6 g of butyl acrylate, 30 g of 2-hydroxyethyl methacrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-butyl hydroperoxide (69% solids content) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids content of 45%, and a volume-average particle size of 200 nm.

[0159] (Comparative Example 3) (Formation of Core Particles) 500 g of deionized water, 4.7 g of boric acid, 18.9 g of sodium carbonate (solid content 2.5%), and 0.1 g of polyoxyethylene lauryl ether phosphate were added to an 8 L polymerization reactor, the temperature was raised to 80°C, and nitrogen was flowed through. Ferrous sulfate (FeSO4) ４ 7H ２A solution of 0.012 g of O and 0.058 g of ethylenediaminetetraacetate-disodium dissolved in 69.3 g of deionized water was added, and then an emulsified mixture consisting of 770 g of 2-ethylhexyl acrylate, 11.6 g of allyl methacrylate, 2.5 g of t-butyl hydroperoxide (69% solids), 3.2 g of polyoxyethylene lauryl ether phosphate, and 385 g of water was added to the polymerization apparatus over 240 minutes. During polymerization, sodium hydroxide (2% solids) was added in any amount and at any time to maintain the pH of the system between 5 and 7. Polymerization was completed 80 minutes after the end of addition, and core particles were formed.

[0160] (Formation of the shell layer) Next, a mixture of 194 g of methyl methacrylate, 6 g of butyl acrylate, 30 g of 2-hydroxyethyl methacrylate, and 0.3 g of t-butyl hydroperoxide was added over 70 minutes. t-butyl hydroperoxide (69% solids content) and sodium formaldehyde sulfoxylate were added as appropriate to obtain a core-shell structure graft copolymer latex with a conversion rate of 100%, a solids content of 45%, and a volume-average particle size of 200 nm. The volume-average particle size of the copolymers obtained in each example and comparative example was measured using a nanoparticle size analyzer NANOTRAC WAVE manufactured by Microtrac Corporation for the latex.

[0161] (Granulation of core-shell structured graft copolymer) A slurry was prepared by adding an aqueous calcium chloride solution to the latex of the core-shell structured graft copolymer particles obtained above. Then, the slurry was dehydrated using a centrifugal dehydrator, washed with deionized water, and dried at 50°C for two days to obtain a powder of the core-shell structured graft copolymer.

[0162] (Preparation of Cathode Slurry) For Examples 1 to 16 and Comparative Examples 1 to 4, lithium iron phosphate (LFP) was used as the positive electrode active material, acetylene black as a conductive additive, and the powders obtained above were mixed in a weight ratio of 92:4:4. N-methyl-2-pyrrolidone was added to this mixture and kneaded thoroughly to prepare a positive electrode slurry with a solid content of 57%. For Example 17, lithium nickel manganese cobaltate (Ni:Co:Mn=5:2:3) was used instead of lithium iron phosphate (LFP) as the positive electrode active material, and for Example 18, lithium cobaltate was used instead of lithium iron phosphate (LFP) as the positive electrode active material. The same procedure as in Example 1 was followed to prepare a positive electrode slurry with a solid content of 57%.

[0163] (Preparation of positive electrode) The obtained positive electrode slurry was applied to a current collector made of 20 μm thick aluminum foil using a coating machine and vacuum dried at 80°C for 12 hours. It was then pressed at 80°C and 5 kN using a roll press machine, and subsequently punched out into a φ14 mm disc shape to form the positive electrode.

[0164] (Fabrication of coin-type secondary battery) A coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of φ15 mm was used as the negative electrode. For the electrolyte, a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was used, and LiPF ６ A solution of [substance name] dissolved at a concentration of 1 mol / L was used. A porous polymer film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using a conventional method to obtain a coin-type secondary battery (CR-2032).

[0165] (Evaluation of Electrolyte Resistance) Each powder obtained above was pressed in a press machine with a 1 mm thick spacer in place at 170°C for 10 minutes to obtain a 1 mm thick film. The obtained film was cut into 1 cm squares and weighed accurately. The weighed film was immersed in an electrolyte solution (ethylene carbonate / dimethyl carbonate = 3 / 7 (volume ratio)) in a test tube. The test tube containing the film and electrolyte solution was left standing at 23°C for 24 hours. After that, the film was removed from the electrolyte solution. The electrolyte solution adhering to the film was wiped off and the weight of the film was weighed accurately. Then, the swelling rate of the film before and after immersion in the electrolyte solution was calculated using the following formula. A smaller calculated value indicates less swelling of the polymer powder in the electrolyte solution. Swelling rate of the film after immersion (%) = (Weight of film after immersion / Weight of film before immersion) × 100

[0166] (Evaluation of Coatability) 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.

[0167] (Evaluation of bonding properties) The positive electrode prepared as described above was cut into a rectangle 60 mm long and 20 mm wide to make a test piece. With the positive electrode active material layer facing upwards, cellophane tape (as specified in JIS Z1522) was attached to the surface of the positive electrode active material layer, and the stress was measured when the tape was peeled off by pulling one end of the current collector vertically at a speed of 50 mm / min. Three measurements were taken, and the average value was calculated and defined as the T-shaped peel strength.

[0168] (Evaluation of charge-discharge cycle characteristics) The fabricated coin-type secondary battery was charged at a constant current of 0.3C until the battery voltage reached 4.2V, and then discharged at a constant current of 0.3C until the battery voltage reached 2V. This process was repeated 100 times in a 30°C environment. The ratio of the discharge capacity after 100 cycles to the discharge capacity after 1 cycle (charge-discharge capacity retention rate = (discharge capacity after 100 cycles / discharge capacity after 1 cycle) × 100%) was then calculated.

[0169] The results of the evaluation described above are shown in Table 1.

[0170]

[0171] As shown in Table 1, Examples 1 to 18 exhibited excellent electrolyte resistance, good coating properties for current collectors, sufficient peel strength, and excellent cycle characteristics.

[0172] On the other hand, Comparative Example 1, which used non-core-shell copolymer particles, was inferior to each of the examples in terms of slurry coating properties onto the current collector, adhesion between the current collector and the active material layer, and charge-discharge cycle characteristics.

[0173] Comparative Example 2, which lacked the core layer 2, was inferior to each of the embodiments in terms of the bonding between the current collector and the active material layer, as well as in the charge-discharge cycle characteristics.

[0174] Comparative Example 3, which lacked the core layer 1, exhibited inferior bonding between the current collector and the active material layer compared to each of the embodiments.

[0175] Comparative Example 4, in which the core-shell particles did not contain monomer units having hydrogen-bonding functional groups, was inferior to each of the examples in terms of electrolyte resistance, bonding between the current collector and the active material layer, and charge-discharge cycle characteristics.

Claims

1. A binder for the positive electrode of a secondary battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is composed of at least two layers, the core comprising a core layer 1 and a core layer 2 located outside the core layer 1, the core layer 1 comprising a non-diene polymer (P1) having a glass transition temperature of 40°C, the core layer 2 comprising a non-diene rubber (R1) having a glass transition temperature of -60 to +40°C, the shell layer comprising a shell-forming polymer, the core-shell particles comprising monomer units having hydrogen-bonding functional groups, and the ratio of the monomer units having hydrogen-bonding functional groups to the total amount of the core-shell particles being 0.6 to 20% by weight.

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

3. The positive electrode binder according to claim 1 or 2, wherein the non-diene rubber (R1) contained in the core layer 2 is an acrylic rubber containing acrylic monomer units.

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

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

6. The positive electrode binder according to claim 1 or 2, wherein the ratio of the non-diene rubber (R1) to the total core-shell particles is 20 to 80% by weight.

7. The cathode binder according to claim 1 or 2, wherein the non-diene polymer (P1) contained in the core layer 1 is a polymer containing (meth)acrylic monomers or aromatic vinyl compound units.

8. The positive electrode binder according to claim 1 or 2, wherein the proportion of the core layer 1 in the core is 5 to 70% by weight, and the proportion of the core layer 2 is 30 to 95% by weight.

9. The positive electrode binder according to claim 1 or 2, wherein the ratio of the core to the total core-shell particles is 50 to 90% by weight.

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

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

12. The cathode binder according to claim 1 or 2, wherein the shell-forming polymer has a glass transition temperature of 40 to 120°C.

13. The positive electrode binder according to claim 1 or 2, which is in powder form.

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

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

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