Electrode binder for lithium-ion battery

Abstract

The present invention provides a binder for an electrode of a lithium-ion battery capable of improving the binding property between a current collector and an electrode active material layer while keeping the internal resistance of the lithium-ion battery low.　The present invention provides a binder for an electrode of a lithium-ion battery, which has good binding property between a current collector and an electrode active material layer, has good electrolyte resistance, and has a high capacity retention rate associated with a charge / discharge cycle.　This binder for an electrode of a lithium-ion battery comprises: a core; and core-shell particles including a shell layer positioned outside the core, wherein the core is composed of rubber. The shell layer is composed of a specific shell-forming polymer.

Description

Binder for lithium-ion battery electrodes

[0001] The present invention relates to an electrode binder for a lithium ion battery, an electrode for a lithium ion battery and a method for manufacturing the same, and a lithium ion battery.

[0002] Lithium-ion batteries are lightweight, have high energy density, and can be repeatedly charged and discharged, making them used in a wide range of applications, such as powering computers and smartphones, and as a power source for electric and hybrid vehicles.

[0003] In the negative electrode of a lithium-ion battery, an electrode active material layer made of an electrode active material such as graphite and a binder such as styrene / butadiene rubber (SBR) is formed on a current collector such as a metal foil. The electrode active material layer is usually produced by coating a current collector with a slurry prepared by mixing the electrode active material, the binder, and a solvent, and then drying the slurry.

[0004] The binder used in such an electrode active material layer is required to maintain the binding (adhesion) between the electrode active materials and between the electrode active material and the current collector. Studies are being conducted to improve the properties of secondary batteries while ensuring the binding by improving this binder.

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

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

[0007] International Publication No. 2017 / 141791 International Publication No. 2017 / 056466

[0008] First, a low internal resistance is desirable in lithium-ion batteries so that they can maintain a high operating voltage.

[0009] In view of the above-described current situation, a first object of the present invention is to provide an electrode binder for a lithium ion battery that can improve the binding strength between a current collector and an electrode active material layer while maintaining a low internal resistance of the lithium ion battery.

[0010] Second, in lithium-ion batteries, a high capacity retention rate during charge-discharge cycling is desirable. Even when the binder was improved to obtain good adhesion between the current collector and the electrode active material layer in order to improve the charge-discharge cycle characteristics, the electrolyte resistance was reduced and sufficient charge-discharge cycle characteristics could not be achieved.

[0011] In view of the above-described current situation, a second object of the present invention is to provide a binder for electrodes of lithium ion batteries that has good binding properties between a current collector and an electrode active material layer, good electrolyte resistance, and a high capacity retention rate during charge-discharge cycles.

[0012] The present inventors conducted extensive research to solve the above-mentioned problems and found that the first object can be achieved by using, as a binder for electrodes of lithium ion batteries, core-shell particles having a multilayer structure composed of at least two layers, the innermost and outermost layers of which have a core composed of a specific rubber and a shell layer composed of a shell-forming polymer containing specific monomer units, and thereby arrived at the present invention.

[0013] That is, a first aspect of the present invention relates to an electrode binder for a lithium ion battery, comprising core-shell particles including a core and a shell layer located on the outside of the core, the core having a multilayer structure composed of at least two layers, at least the innermost and outermost layers of the layers constituting the multilayer structure of the core are composed of a rubber containing aliphatic conjugated diene compound units, the glass transition temperature of the rubber constituting the innermost layer of the core is 25°C or higher and 85°C or lower, the glass transition temperature of the rubber constituting the outermost layer of the core is lower than the glass transition temperature of the rubber constituting the innermost layer of the core, and the shell layer is composed of a shell-forming polymer containing (meth)acrylic acid ester compound units.

[0014] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they discovered that the second object can be achieved by using core-shell particles having a core made of rubber and a shell layer made of a polymer containing specific monomer units as a binder for electrodes of lithium ion batteries, and thus arrived at the present invention.

[0015] That is, a second aspect of the present invention relates to an electrode binder for a lithium ion battery, which comprises core-shell particles including a core and a shell layer located on the outside of the core, the core being made of rubber, the shell layer being made of a shell-forming polymer containing a non-crosslinkable monomer and a crosslinkable monomer as constituent monomers, the non-crosslinkable monomer including a (meth)acrylic acid ester-based monomer, and the content of the crosslinkable monomer copolymerizable with the non-crosslinkable monomer in the shell-forming polymer being 0.4% by weight or more and 7% by weight or less.

[0016] According to the first aspect of the present invention, it is possible to provide a binder for electrodes of lithium ion batteries that can improve the binding strength between a current collector and an electrode active material layer while maintaining the internal resistance of the lithium ion battery low.

[0017] According to the second invention, it is possible to provide a binder for electrodes of lithium ion batteries which has good binding properties between a current collector and an electrode active material layer, good electrolyte resistance, and a high capacity retention rate during charge-discharge cycles.

[0018] An embodiment of the present invention will be described in detail below. [First Electrode Binder] The electrode binder for a lithium ion battery according to this first embodiment (hereinafter also referred to as the "first electrode binder") will be described in detail below. The first electrode binder contains at least first core-shell particles. The first core-shell particles have a core-shell structure, including a core and a shell layer located outside the core. The core has a multilayer structure composed of at least two layers, and at least the innermost and outermost layers of the layers constituting the multilayer structure are composed of a specific rubber. Furthermore, the shell layer is composed of a shell-forming polymer containing a (meth)acrylic acid ester compound unit.

[0019] By using core-shell particles having such a structure as an electrode binder for a lithium ion battery, it is possible to improve the binding strength between the current collector and the electrode active material layer while maintaining a low internal resistance of the lithium ion battery. This also allows the content of the electrode binder in the lithium ion battery electrode to be reduced. Furthermore, a slurry containing core-shell particles having such a structure has good coatability onto the current collector. It is also possible to improve the charge-discharge cycle characteristics of the resulting lithium ion battery.

[0020] (Core) The core has a multilayer structure composed of at least two layers, at least the innermost and outermost layers of which are composed of a rubber containing an aliphatic conjugated diene compound unit.

[0021] In this disclosure, rubber refers to a material that has rubber elasticity. Rubber elasticity is the ability to absorb energy from an external force and store it as energy for returning to its original shape. To exhibit rubber elasticity, the molecules must be sufficiently long, able to move freely, and appropriately bonded to each other. A material with rubber elasticity can easily return to its original shape when the external force is released, even if it is deformed by an external force.

[0022] The type of rubber constituting each layer of the core is not particularly limited as long as it contains an aliphatic conjugated diene compound unit. However, since this can improve the dispersibility of the core-shell particles in the slurry and the compatibility between the core-shell particles and the thickener, and can improve the bonding between the current collector and the electrode active material layer, it is preferable that at least the rubber constituting the innermost layer is a rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit, and more preferably a rubber consisting of a copolymer of an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit. Furthermore, it is preferable that both the rubber constituting the innermost layer and the rubber constituting the outermost layer are rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit, and more preferably a rubber consisting of a copolymer of an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit. In particular, it is preferable that the entire core is rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit, and more preferably a rubber consisting of a copolymer of an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit. The method for producing the core is not particularly limited, but it is preferably synthesized by emulsion polymerization.

[0023] An aliphatic conjugated diene compound is an aliphatic compound having two carbon-carbon double bonds separated by one single bond, resulting in a conjugated diene compound. Specific examples of aliphatic conjugated diene compounds include isoprene, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Only one type of aliphatic conjugated diene compound may be used, or two or more types may be used in combination. Of these, 1,3-butadiene is preferred.

[0024] The aromatic vinyl compound that can be used to form each layer of the core is not particularly limited, and examples thereof include unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; ring-alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; ring-alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; ring-halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; ring-ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and ring-hydroxylated vinyl aromatic compounds such as 4-hydroxystyrene. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred. The aromatic vinyl compounds may be used alone or in combination of two or more.

[0025] A typical example of the rubber that constitutes each layer of the core is styrene-butadiene rubber, which is a copolymer of 1,3-butadiene and styrene, and is also called styrene rubber or SBR.

[0026] The rubber constituting each layer of the core may contain no vinyl monomer units other than aliphatic conjugated diene compound units and aromatic vinyl compound units, or may contain such other vinyl monomer units. Examples of such other vinyl monomers include (meth)acrylic acid and (meth)acrylic acid alkyl esters such as acrylic acid, methacrylic acid, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-hydroxyethyl methacrylate, and glycidyl methacrylate; and unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile.

[0027] The proportion of the aliphatic conjugated diene compound units in the entire rubber constituting the innermost layer of the core is not particularly limited as long as the glass transition temperature of the rubber constituting the innermost layer of the core is in the range of 25°C or higher and 85°C or lower. From the viewpoint of binding to the current collector, the proportion is preferably 10 to 35% by weight, more preferably 15 to 30% by weight, and even more preferably 20 to 25% by weight.

[0028] The proportion of aliphatic conjugated diene compound units in the entire rubber constituting the outermost layer of the core is not particularly limited as long as the glass transition temperature of the rubber constituting the outermost layer of the core is lower than the glass transition temperature of the rubber constituting the innermost layer of the core. From the viewpoint of appropriately softening the outermost layer of the core and improving the binding between electrode active materials or between the electrode active material and the current collector, the proportion is preferably 65 to 90% by weight, more preferably 70 to 85% by weight, and even more preferably 75 to 80% by weight.

[0029] The total proportion of the aliphatic conjugated diene compound units and aromatic vinyl compound units in the entire rubber constituting the innermost layer of the core, and the total proportion of the aliphatic conjugated diene compound units and aromatic vinyl compound units in the entire rubber constituting the outermost layer of the core are not particularly limited, and may be, for example, 80 to 100% by weight, 90% by weight or more, 95% by weight or more, 98% by weight or more, or 99% by weight or more.

[0030] The rubber constituting each layer of the core may be one which uses a polyfunctional monomer such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, or 1,3-butylene dimethacrylate during polymerization.

[0031] Furthermore, the rubber constituting each layer of the core may be polymerized without using a chain transfer agent, or may be polymerized in the presence of a chain transfer agent. Usable chain transfer agents are not particularly limited, but examples thereof include alkyl mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, t-decyl mercaptan, n-decyl mercaptan, and n-octyl mercaptan; and alkyl ester mercaptans such as 2-ethylhexyl thioglycolate.

[0032] When the innermost layer of the core is composed of a rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit, the content ratio of the aliphatic conjugated diene compound to the aromatic vinyl compound in the rubber constituting the innermost layer is not particularly limited as long as the glass transition temperature of the rubber constituting the innermost layer of the core is in the range of 25°C or higher and 85°C or lower. From the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity after charge and discharge (cycle characteristics), the content ratio is preferably 10:90 to 35:65, more preferably 15:85 to 30:70, and even more preferably 20:80 to 25:75.

[0033] When the outermost layer of the core is composed of a rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit, the content ratio of the aliphatic conjugated diene compound to the aromatic vinyl compound in the rubber constituting the outermost layer is not particularly limited as long as the glass transition temperature of the rubber constituting the outermost layer of the core is lower than the glass transition temperature of the rubber constituting the innermost layer of the core. From the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity after charge and discharge (cycle characteristics), the ratio is preferably 60:40 to 100:0, more preferably 65:35 to 95:5, and even more preferably 70:30 to 90:10.

[0034] The glass transition temperature of the rubber constituting the innermost layer of the core may be in the range of 25° C. to 85° C., preferably 30° C. to 80° C., more preferably 35° C. to 75° C., and even more preferably 40° C. to 70° C. This is because the particle shape of the core-shell particles can be easily maintained, and bonding between electrode active materials or bonding between the electrode active materials and the current collector can be achieved by point bonding, which makes it less likely that the movement of lithium ions will be inhibited by the presence of a binder, thereby enabling the internal resistance of the lithium ion battery to be lowered.

[0035] The glass transition temperature of the rubber constituting the outermost layer of the core may be lower than the glass transition temperature of the rubber constituting the innermost layer of the core, but from the viewpoint of softening the outermost layer of the core-shell particle and further improving the binding strength between the electrode active material and the current collector, the glass transition temperature is preferably −10° C. or lower, more preferably −15° C. or lower, and even more preferably −20° C. or lower. Furthermore, from the viewpoint of the binding strength to the current collector, the glass transition temperature is preferably −85° C. or higher, more preferably −40° C. or higher.

[0036] The glass transition temperature of the rubber can be controlled by changing the ratio of the aliphatic conjugated diene compound units to the aromatic vinyl compound units in the rubber, or the types and ratios of other monomer units that may be contained in the rubber. For example, the glass transition temperature of the rubber can be increased by increasing the proportion of the aromatic vinyl compound in the rubber.

[0037] The glass transition temperature can be measured using a differential scanning calorimeter (DSC). The same applies to the glass transition temperature hereinafter.

[0038] The proportion of the core in the entire core-shell particle is not particularly limited, but from the viewpoints of improving binding property, reducing internal resistance, and maintaining capacity after charge and discharge (cycle characteristics), it is preferably 50 to 95% by weight, more preferably 60 to 90% by weight, and even more preferably 65 to 80% by weight.

[0039] The proportion of the innermost layer of the core in the entire core-shell particle is not particularly limited, but from the viewpoint of binding to the current collector, it is preferably 10 to 40% by weight, more preferably 15 to 35% by weight, and even more preferably 20 to 30% by weight.

[0040] The proportion of the outermost layer of the core in the entire core-shell particle is not particularly limited, but from the viewpoint of binding to the current collector, it is preferably 10 to 40% by weight, more preferably 15 to 35% by weight, and even more preferably 20 to 30% by weight.

[0041] The core may consist of only an innermost layer and an outermost layer, in which case the outermost layer is preferably graft-bonded to the innermost layer. Furthermore, as long as the effects of the invention are achieved, an intermediate layer may be further provided between the innermost and outermost layers. The intermediate layer is preferably a layer composed of a polymer and is graft-bonded to an adjacent layer. The intermediate layer may be composed of a rubber containing an aliphatic conjugated diene compound unit, or may be composed of a rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit.

[0042] (Shell Layer) The shell layer is composed of a polymer that forms the shell layer (hereinafter also referred to as a shell-forming polymer), and the shell-forming polymer contains a (meth)acrylic acid ester compound as a monomer unit.

[0043] The shell layer refers to a polymer layer located on the surface side of the core-shell particle, and is also called a graft layer. The shell layer is preferably graft-bonded to the core. However, the polymer forming the shell layer also includes polymers that are not graft-bonded to the core. The shell layer covers the surface of the core, but is not limited to covering the entire surface of the core, as long as it covers at least a portion of the surface of the core.

[0044] The shell-forming polymer is preferably a vinyl polymer, and in this case, the monomer constituting the shell-forming polymer is not particularly limited as long as it is a vinyl monomer, but it contains at least a (meth)acrylic acid ester compound as a monomer unit. When the shell layer contains a (meth)acrylic acid ester compound as a monomer unit, the glass transition temperature of the shell-forming polymer can be increased, and the internal resistance of the lithium-ion battery can be maintained low.

[0045] The (meth)acrylic acid ester compound unit is not particularly limited, and examples thereof include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate; aromatic ring-containing (meth)acrylates such as phenoxyethyl (meth)acrylate and benzyl (meth)acrylate; hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate; glycidyl (meth)acrylates such as glycidyl (meth)acrylate and glycidyl alkyl (meth)acrylate; and alkoxy (meth)alkyl acrylates. Among these, (meth)acrylic acid alkyl esters are preferred, and acrylic acid alkyl esters are more preferred. The (meth)acrylic acid ester compounds may be used alone or in combination of two or more.

[0046] The number of carbon atoms in the alkyl acrylate unit is not particularly limited, but is preferably 1 to 17, more preferably 1 to 14, and particularly preferably 1 to 11.

[0047] The proportion of the (meth)acrylic acid ester compound units in the entire shell-forming polymer is not particularly limited, but is preferably 1 to 30% by weight, more preferably 5 to 25% by weight, and even more preferably 10 to 20% by weight, from the viewpoint of promoting the polymerization reaction of the shell-forming polymer, dispersibility of the core-shell particles in the slurry, and coatability of the slurry on a current collector.

[0048] The monomer constituting the shell-forming polymer other than the (meth)acrylic acid ester compound unit is not particularly limited, and examples thereof include a (meth)acrylic acid unit, an aromatic vinyl compound unit, etc. In the present application, the term "(meth)acrylic" is used to collectively refer to acrylic and methacrylic.

[0049] From the viewpoint of increasing the glass transition temperature of the shell-forming polymer and reducing the internal resistance of the lithium ion battery, the shell-forming polymer preferably contains, in addition to (meth)acrylic acid ester compound units, (meth)acrylic acid units and aromatic vinyl compound units.

[0050] The proportion of the (meth)acrylic acid units in the entire shell-forming polymer is not particularly limited, but is preferably 40% by weight or less in terms of polymerization stability during production of core-shell particles. From the viewpoint of reducing the internal resistance of lithium ion batteries, it is preferably 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, particularly preferably 15% by weight or less, and most preferably 10% by weight or less. It may also be 9% by weight or less, or 5% by weight or more.

[0051] The proportion of the aromatic vinyl compound units in the entire shell-forming polymer is not particularly limited, but from the viewpoint of achieving good electrolyte resistance and improving the charge / discharge characteristics of a lithium ion battery, the proportion is preferably 30 to 95% by weight, more preferably 50 to 90% by weight, and even more preferably 60 to 85% by weight.

[0052] The aromatic vinyl compound constituting the shell layer is not particularly limited, and examples thereof include unsubstituted vinyl aromatic compounds such as styrene and 2-vinylnaphthalene; substituted vinyl aromatic compounds such as α-methylstyrene; ring-alkylated vinyl aromatic compounds such as 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,5-dimethylstyrene, and 2,4,6-trimethylstyrene; ring-alkoxylated vinyl aromatic compounds such as 4-methoxystyrene and 4-ethoxystyrene; ring-halogenated vinyl aromatic compounds such as 2-chlorostyrene and 3-chlorostyrene; ring-ester-substituted vinyl aromatic compounds such as 4-acetoxystyrene; and ring-hydroxylated vinyl aromatic compounds such as 4-hydroxystyrene. Among these, substituted or unsubstituted styrene is preferred, styrene and / or α-methylstyrene is more preferred, and styrene is particularly preferred. The aromatic vinyl compounds may be used alone or in combination of two or more.

[0053] From the viewpoints of dispersibility of the core-shell particles in the slurry and applicability of the slurry to the current collector, the shell layer is preferably composed of a non-crosslinked polymer. The non-crosslinked polymer refers to a polymer that does not contain a crosslinked structure or a structural unit derived from a crosslinkable monomer, and does not fall under the category of a rubber elastic material (e.g., butadiene rubber, styrene-butadiene rubber, acrylic rubber, etc.).

[0054] The shell-forming polymer preferably has a glass transition temperature (Tg) of 60° C. or higher, more preferably 80° C. or higher, even more preferably 90° C. or higher, and particularly preferably 100° C. or higher, in order to facilitate the retention of the particle shape of the core-shell particles in the formed electrode active material layer, enable bonding of the electrode active materials to each other or between the electrode active material and the current collector by point bonding, enhance binding properties, and further reduce the internal resistance of the lithium ion battery. The upper limit is not particularly limited, but may be, for example, 150° C. or lower, 120° C. or lower, or 110° C. or lower.

[0055] The glass transition temperature of the shell-forming polymer can be controlled by changing the type and ratio of the monomers constituting the polymer. For example, by using a methacrylic monomer and an aromatic vinyl compound unit as the monomers constituting the shell-forming polymer, the glass transition temperature of the polymer can be increased.

[0056] The proportion of the shell layer in the entire core-shell particle is not particularly limited, but from the viewpoints of improving binding property, reducing internal resistance, and maintaining capacity after charge and discharge (cycle characteristics), it is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, and even more preferably 20 to 35% by weight.

[0057] The first core-shell particle and the second core-shell particle described below may consist of only 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 composed 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.

[0058] (Volume Average Particle Diameter of First Core-Shell Particles) The particle diameter of the first core-shell particles is not particularly limited and may be, for example, approximately 10 to 1,000 nm. However, from the viewpoints of the dispersibility of the core-shell particles in the slurry, the mixability of the core-shell particles with the thickener, and the reduction of internal resistance of the lithium-ion battery, the volume average particle diameter is preferably 100 to 300 nm, more preferably 120 to 250 nm, and even more preferably 150 to 200 nm. The volume average particle diameters of the first core-shell particles and the second core-shell particles described below are measured in the form of a latex of the core-shell particles using a particle diameter measuring device. The particle diameter of the core-shell particles can be controlled by the amount of each monomer constituting the core-forming polymer, the amount of the core-forming polymer charged during shell formation, the type and amount of polymerization initiator, chain transfer agent, redox agent, emulsifier, etc. used during polymerization, the polymerization temperature, the polymerization time, etc.

[0059] [Second Electrode Binder] The electrode binder (hereinafter also referred to as "second electrode binder") of the lithium ion battery according to this second embodiment will be described in detail below. The second electrode binder contains at least second core-shell particles. The second core-shell particles have a core-shell structure and include a core and a shell layer located outside the core. The core is made of rubber, and the shell layer is made of a shell-forming polymer containing, as constituent monomers, a non-crosslinkable monomer including a (meth)acrylic acid ester-based monomer and a specific amount of a crosslinkable monomer.

[0060] The use of such core-shell particles as a binder for lithium-ion battery electrodes can achieve both good adhesion between the current collector and the electrode active material layer and good electrolyte resistance, thereby improving the capacity retention rate during charge-discharge cycles. Furthermore, the improved adhesion between the current collector and the electrode active material layer can lead to a reduction in the binder content in the lithium-ion battery electrodes.

[0061] (Core) The core is a rubber particle composed of rubber. The type of rubber is not particularly limited, but a rubber containing an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit is preferred, as this can improve the dispersibility of the core-shell particles in the slurry and the compatibility between the core-shell particles and the thickener, and can improve the binding strength between the current collector and the electrode active material layer. The method for producing the rubber particles is not particularly limited, but it is preferred that the rubber particles are synthesized by emulsion polymerization.

[0062] The definition and specific examples of the aliphatic conjugated diene compound that can be used in the core of the second core-shell particles, as well as the description of suitable aliphatic conjugated diene compounds, are the same as those for the first core-shell particles, and therefore will be omitted here.

[0063] Specific examples of aromatic vinyl compounds that can be used in the core of the second core-shell particles and a description of suitable aromatic vinyl compounds are the same as those for the first core-shell particles, and therefore will be omitted here.

[0064] A typical example of the rubber constituting the core is styrene-butadiene rubber.

[0065] The rubber constituting the core may contain no vinyl-based monomer units other than aliphatic conjugated diene compound units and aromatic vinyl compound units, or may contain such other vinyl-based monomer units. Specific examples of such other vinyl-based monomers are omitted here because they are the same as those of the rubber constituting each layer of the core of the first core-shell particle.

[0066] The total proportion of the aliphatic conjugated diene compound units and aromatic vinyl compound units in the entire rubber constituting the core is not particularly limited, but may be, for example, 80 to 100% by weight, 90% by weight or more, 95% by weight or more, 98% by weight or more, or 99% by weight or more.

[0067] The rubber constituting the core may be one which uses a polyfunctional monomer such as divinylbenzene, allyl methacrylate, ethylene glycol dimethacrylate, or 1,3-butylene dimethacrylate during polymerization.

[0068] Furthermore, the rubber constituting the core may be polymerized without using a chain transfer agent, or may be polymerized in the presence of a chain transfer agent. Specific examples of usable chain transfer agents are the same as those of the rubber constituting each layer of the core of the first core-shell particle, and therefore will not be described here.

[0069] In the rubber constituting the core, the content ratio of the aliphatic conjugated diene compound to the aromatic vinyl compound is not particularly limited and may be, for example, within a range of 30:70 to 80:20. From the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity during charge / discharge cycles, the content ratio is preferably 40:60 to 70:30, more preferably 45:55 to 65:35, and even more preferably 50:50 to 60:40.

[0070] The glass transition temperature (Tg) of the rubber is not particularly limited, but may be, for example, within the range of −50° C. to +25° C. From the viewpoints of improving binding properties, reducing internal resistance, and maintaining capacity during charge / discharge cycles, the temperature is preferably −50° C. to +20° C., more preferably −40° C. to +20° C., even more preferably −30° C. to +15° C., particularly preferably −20° C. to +10° C., and most preferably −15° C. to 0° C.

[0071] The method for controlling the glass transition temperature of the rubber in the second core-shell particles is the same as that in the first core-shell particles, and therefore will not be described here.

[0072] The proportion of the core in the entire core-shell particle is not particularly limited, but from the viewpoints of improving electrolyte resistance, improving binding properties, reducing internal resistance, and improving capacity retention rate with charge-discharge cycles, the proportion of the core in the entire core-shell particle is preferably 50 to 95% by weight, more preferably 60 to 90% by weight, and even more preferably 65 to 80% by weight.

[0073] (Shell Layer) The shell layer refers to a polymer layer located on the surface side of the core-shell particle, and is also called a graft layer. The shell layer is preferably graft-bonded to the core. However, the polymer forming the shell layer also includes polymers that are not graft-bonded to the core. The shell layer covers the surface of the core, but is not limited to covering the entire surface of the core, as long as it covers at least a portion of the surface of the core.

[0074] The shell layer is composed of a polymer that forms the shell layer (hereinafter also referred to as a shell-forming polymer), and contains, as constituent monomers, a non-crosslinkable monomer including a (meth)acrylic acid ester-based monomer and a specific amount of a crosslinkable monomer. This suppresses swelling due to the electrolyte solution, thereby obtaining good electrolyte resistance, and also obtains good bonding between the current collector and the electrode active material layer, thereby improving the capacity retention rate during charge-discharge cycles.

[0075] The shell-forming polymer is not particularly limited as long as it contains a non-crosslinkable monomer including a (meth)acrylic acid ester monomer and a specific amount of a crosslinkable monomer as constituent monomers, but the shell-forming polymer is preferably a vinyl polymer obtained by homopolymerizing or copolymerizing vinyl monomers.

[0076] The (meth)acrylic acid ester monomer, which is a non-crosslinkable monomer constituting the shell-forming polymer, is not particularly limited and examples thereof include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate; aromatic ring-containing (meth)acrylates such as phenoxyethyl (meth)acrylate and benzyl (meth)acrylate; hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate; glycidyl (meth)acrylates such as glycidyl (meth)acrylate and glycidyl alkyl (meth)acrylate; and alkoxy (meth)alkyl acrylates. Among these, (meth)acrylic acid alkyl esters are preferred, and methacrylic acid alkyl esters are particularly preferred. The (meth)acrylic acid ester-based monomers may be used alone or in combination of two or more kinds.

[0077] From the viewpoint of promoting the polymerization reaction of the shell-forming polymer, dispersibility of the core-shell particles in the slurry, and coatability of the slurry onto a current collector, the proportion of the non-crosslinkable (meth)acrylic acid ester monomer in the entire shell-forming polymer is preferably 5 to 60% by weight, more preferably 8 to 50% by weight, and even more preferably 8 to 45% by weight.

[0078] The non-crosslinkable monomer constituting the shell-forming polymer is not particularly limited as long as it contains a (meth)acrylic acid ester monomer, but it is preferable that it further contains an aromatic vinyl compound unit.

[0079] In the second core-shell particle, specific examples of aromatic vinyl compound units that can be used in the non-crosslinkable monomer constituting the shell-forming polymer, and a description of suitable aromatic vinyl compound units are the same as those in the first core-shell particle, and therefore will be omitted here.

[0080] Increasing the glass transition temperature of the shell-forming polymer and hardening the shell layer can improve the dispersibility of the core-shell particles in the slurry while maintaining their particle shape, thereby reducing the internal resistance of the lithium-ion battery and improving the bonding strength between the current collector and the electrode active material layer. From this perspective, the non-crosslinkable monomer of the shell-forming polymer preferably contains a methacrylic acid alkyl ester unit and an aromatic vinyl compound unit. The number of carbon atoms in the alkyl of the methacrylic acid alkyl ester unit is not particularly limited, but is preferably 1 to 6, more preferably 1 to 3, and particularly preferably 1 or 2.

[0081] The proportion of aromatic vinyl compound units, which are non-crosslinkable monomers, in the entire shell-forming polymer is preferably 30 to 95% by weight, more preferably 50 to 85% by weight, and even more preferably 60 to 75% by weight, because this provides better electrolyte resistance and can improve the charge-discharge cycle characteristics of lithium ion batteries.

[0082] Furthermore, from the viewpoint of promoting the polymerization reaction of the shell-forming polymer, the dispersibility of the core-shell particles in the slurry, and the coatability of the slurry onto a current collector, the proportion of the methacrylic acid alkyl ester units in the entire shell-forming polymer is preferably 5 to 60% by weight, more preferably 8 to 50% by weight, and even more preferably 8 to 45% by weight.

[0083] Furthermore, from the viewpoint of better bonding between the current collector and the electrode active material layer and electrolyte resistance, the non-crosslinkable monomer of the shell-forming polymer preferably contains a methacrylic acid unit in addition to the (meth)acrylic acid ester monomer and the aromatic vinyl compound unit, and more preferably contains a methacrylic acid unit in addition to the methacrylic acid alkyl ester unit and the aromatic vinyl compound unit.

[0084] The upper limit of the proportion of methacrylic acid units in the entire shell-forming polymer is not particularly limited, but from the viewpoint of polymerization stability during production of core-shell particles, it can usually be 40% by weight or less. From the viewpoint of reducing the internal resistance of lithium ion batteries, the content is preferably 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, particularly preferably 15% by weight or less, and most preferably 10% by weight or less. It may also be 9% by weight or less.

[0085] The shell-forming polymer contains a specific amount of a crosslinkable monomer as a constituent monomer. The crosslinkable monomer is a compound having two or more unsaturated bonds in one molecule that are copolymerizable with the non-crosslinkable monomer. The copolymerizable unsaturated bond is a carbon-carbon bond, and the bond is a double bond or a triple bond. A carbon-carbon double bond is preferred because the compound has a relatively low reactivity and is easy to handle.

[0086] The crosslinkable monomer in the shell-forming polymer is not particularly limited, and examples thereof include vinyl group-containing ester monomers, divinylbenzene monomers, vinyl group-containing cyanurate monomers, and vinyl group-containing phthalate monomers. At least one selected from the group consisting of vinyl group-containing ester monomers, divinylbenzene monomers, and vinyl group-containing cyanurate monomers is preferred. Only one type of crosslinkable monomer may be used, or two or more types may be used in combination.

[0087] The vinyl group-containing ester monomer, which can be cited as a crosslinkable monomer constituting the shell-forming polymer, is a monomer having one or more vinyl groups (including allyl groups) bonded to an ester bond in one molecule as a functional group having an unsaturated bond copolymerizable with the non-crosslinkable monomer. Examples of the vinyl group (including allyl groups) bonded to an ester bond include, for example, an acryloyl group (H ２ C=CH-C(=O)-O-) and methacryloyl groups (H ２ C=C(CH ３)-C(═O)-O-). Specific examples of the vinyl group-containing ester monomer include (meth)acrylates having an allyl group, such as allyl (meth)acrylate, allyl alkyl (meth)acrylate, and allyloxy alkyl (meth)acrylate; and polyfunctional (meth)acrylates having two or more (meth)acrylic groups, such as (poly)ethylene glycol di(meth)acrylate, butanediol di(meth)acrylate (also known as 1,3-butylene dimethacrylate), ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate.

[0088] The divinylbenzene-based monomer, which can be mentioned as a crosslinkable monomer constituting the shell-forming polymer, is a monomer having two vinyl groups bonded to a benzene ring in one molecule as a functional group having an unsaturated bond copolymerizable with the non-crosslinkable monomer, and specific examples thereof include divinylbenzenes such as o-divinylbenzene, m-divinylbenzene, and p-divinylbenzene.

[0089] The vinyl group-containing cyanurate monomer, which can be mentioned as an example of a crosslinkable monomer constituting the shell-forming polymer, is a monomer having, in one molecule, one or more vinyl groups (including allyl groups) bonded to cyanuric acid or isocyanuric acid as a functional group having an unsaturated bond copolymerizable with the non-crosslinkable monomer, and specific examples thereof include triallyl cyanurate and triallyl isocyanurate.

[0090] The vinyl group-containing phthalate monomer, which can be cited as an example of a crosslinkable monomer constituting the shell-forming polymer, is a monomer having one or more vinyl groups (including allyl groups) bonded to a phthalate ester per molecule as a functional group having an unsaturated bond copolymerizable with the non-crosslinkable monomer, and specific examples thereof include diallyl phthalate, diallyl isophthalate, diallyl terephthalate, and diallyl orthophthalate.

[0091] As the crosslinkable monomer, from the viewpoint of polymerizability in emulsion polymerization, at least one selected from the group consisting of allyl methacrylate, divinylbenzene, butanediol di(meth)acrylate (also known as 1,3-butylene dimethacrylate), and triallyl isocyanurate is preferred, and allyl methacrylate is more preferred.

[0092] The content of the crosslinkable monomer copolymerizable with the non-crosslinkable monomer in the shell-forming polymer is preferably 0.4% by weight to 7% by weight, more preferably 0.4% by weight to 6% by weight, and even more preferably 0.8% by weight to 4% by weight. If the content is less than 0.4% by weight, the battery is prone to swelling with an electrolyte, resulting in poor electrolyte resistance and poor capacity retention during charge-discharge cycling. If the content is more than 7% by weight, the battery is prone to swelling with an electrolyte, resulting in poor electrolyte resistance and poor capacity retention during charge-discharge cycling.

[0093] The preferred range of the glass transition temperature of the shell-forming polymer in the second core-shell particles and the reason therefor will be omitted here because they are the same as those in the first core-shell particles.

[0094] The glass transition temperature of the shell-forming polymer can be controlled by changing the type and ratio of the monomers constituting the polymer. The method for controlling the glass transition temperature of the shell-forming polymer in the second core-shell particles is the same as that in the first core-shell particles, and therefore will not be described here.

[0095] The proportion of the shell layer in the entire core-shell particle is not particularly limited, but from the viewpoints of improving binding property, reducing internal resistance, and maintaining capacity during charge-discharge cycles, the proportion is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, and even more preferably 20 to 35% by weight.

[0096] (Volume Average Particle Diameter of Second Core-Shell Particles) The particle diameter of the second core-shell particles is not particularly limited, and may be, for example, about 10 to 1,000 nm. However, from the viewpoints of dispersibility of the core-shell particles in the slurry, mixability of the core-shell particles with a thickener, reduction of internal resistance of a lithium ion battery, etc., the volume average particle diameter is preferably 100 nm or more, more preferably 120 to 500 nm, even more preferably 150 to 400 nm, and particularly preferably 170 to 300 nm.

[0097] (Method for producing first and second core-shell particles) The method for producing the first and second core-shell particles is not particularly limited, but for example, emulsion polymerization, mini-emulsion polymerization, micro-emulsion polymerization, and emulsifier-free (soap-free) emulsion polymerization can be used.

[0098] The emulsifier that can be used in emulsion polymerization is not particularly limited, and examples thereof include anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc. Dispersants such as polyvinyl alcohol, alkyl-substituted cellulose, polyvinylpyrrolidone, and polyacrylic acid derivatives may also be used in combination.

[0099] Among the above-mentioned emulsifiers, the anionic surfactant is not particularly limited, and examples thereof include the following compounds: fatty acid soaps such as potassium laurate, potassium coconut fatty acid, potassium myristate, potassium oleate, potassium oleate diethanolamine salt, sodium oleate, potassium palmitate, potassium stearate, sodium stearate, mixed fatty acid soda soap, semi-hardened beef tallow fatty acid soda soap, and castor oil potassium soap; alkoxylated surfactants such as sodium dodecyl sulfate, higher alcohol sodium sulfate, triethanolamine dodecyl sulfate, ammonium dodecyl sulfate, sodium polyoxyethylene alkyl ether sulfate, triethanolamine polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl phenyl ether sulfate, and sodium 2-ethylhexyl sulfate; alkyl sulfate salts; sodium alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate; sodium dialkylsulfosuccinates such as sodium di-2-ethylhexyl sulfosuccinate; sodium alkylnaphthalenesulfonates; sodium alkyldiphenyletherdisulfonates; potassium alkylphosphate salts; phosphate ester salts such as sodium polyoxyethylene lauryl ether phosphate; sodium salts of naphthalenesulfonate formalin condensates; polycarboxylic acid type polymeric anions; sodium acyl (beef tallow) methyl taurate; sodium acyl (coconut) methyl taurate; sodium cocoyl isethionate; sodium α-sulfofatty acid ester salts; sodium amidoethersulfonate; oleyl sarcosine; sodium lauroyl sarcosine; rosin acid soap, etc.

[0100] Among the above-mentioned emulsifiers, the nonionic surfactant is not particularly limited, and examples thereof include the following compounds: polyoxyethylene alkyl allyl ethers or polyoxyethylene alkyl ethers such as polyoxyethylene nonylphenyl ether, polyoxyethylene oleyl ether, and polyoxyethylene lauryl ether; polyoxyethylene sorbitan esters such as polyoxyethylene sorbitan monolaurate and polyoxyethylene sorbitan monostearate; polyoxyethylene fatty acid esters such as polyethylene glycol monolaurate, polyethylene glycol monostearate, and polyethylene glycol monooleate; and oxyethylene / oxypropylene block copolymers.

[0101] Of the above emulsifiers, the cationic surfactant is not particularly limited, and examples thereof include the following compounds: alkylamine salts such as coconut amine acetate, stearyl amine acetate, octadecyl amine acetate, and tetradecyl amine acetate; and quaternary ammonium salts such as lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, cetyl trimethyl ammonium chloride, distearyl dimethyl ammonium chloride, alkyl benzyl dimethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, and behenyl trimethyl ammonium chloride.

[0102] Among the above emulsifiers, the amphoteric surfactant is not particularly limited, but examples thereof include the following compounds: alkyl betaines such as lauryl betaine, stearyl betaine, and dimethyl lauryl betaine; sodium lauryl diaminoethyl glycine; amido betaine; imidazoline; lauryl carboxymethyl hydroxyethyl imidazolinium betaine, etc.

[0103] These emulsifiers may be used alone or in combination of two or more. Among the emulsifiers, from the viewpoint of improving the fluidity of the obtained latex, sodium dialkyl sulfosuccinate or a surfactant having an oxyethylene structure is preferred, and sodium polyoxyethylene lauryl ether phosphate is particularly preferred.

[0104] When emulsion polymerization is employed, known polymerization initiators, such as 2,2'-azobisisobutyronitrile, hydrogen peroxide, potassium persulfate, and ammonium persulfate, can be used as thermal decomposition initiators.

[0105] Alternatively, a redox initiator can be used in which a peroxide, such as an organic peroxide such as t-butylperoxyisopropyl carbonate, paramenthane hydroperoxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, or t-hexyl peroxide; or an inorganic peroxide such as hydrogen peroxide, potassium persulfate, or ammonium persulfate, is used in combination with at least one selected from the group consisting of a reducing agent such as sodium formaldehyde sulfoxylate or glucose; a transition metal salt such as iron (II) sulfate; a chelating agent such as ethylenediaminetetraacetic acid disodium; and a phosphorus-containing compound such as sodium pyrophosphate.

[0106] The use of a redox initiator is preferred because polymerization can be carried out at low temperatures where the peroxide does not substantially decompose thermally, allowing the polymerization temperature to be set over a wide range. Among these, organic peroxides such as cumene hydroperoxide, dicumyl peroxide, and t-butyl hydroperoxide are preferably used as the redox initiator. The amounts of the initiator used, and the amounts of the reducing agent, transition metal salt, chelating agent, phosphorus-containing compound, etc. used when a redox initiator is used, can be within known ranges. Furthermore, when polymerizing a polyfunctional monomer, known chain transfer agents can be used within known ranges. A surfactant can also be used, and this is also within known ranges.

[0107] The solvent used during emulsion polymerization may be any solvent that allows the emulsion polymerization to proceed stably, and for example, water can be suitably used.

[0108] The temperature during emulsion polymerization is not particularly limited as long as the emulsifier is dissolved uniformly in the solvent, but is, for example, 40 to 75°C, preferably 45 to 70°C, and more preferably 49 to 65°C.

[0109] When the core-shell particles are produced by emulsion polymerization, for example, the latex of the core-shell particles may be spray-dried to obtain a powder that can be redispersed in water, which may be used as a binder for an electrode.

[0110] The first and second electrode binders according to this embodiment may be in the form of a latex of the core-shell particles or a powder of the core-shell particles, and are preferably in the form of a latex of the core-shell particles because they have excellent dispersibility in a slurry.

[0111] <Other Components> The first and second electrode binders according to this embodiment may contain, in addition to the core-shell particles, components such as a conductive aid, a reinforcing material, a leveling agent, a viscosity modifier, and an electrolyte additive. These components are not particularly limited as long as they do not affect the battery reaction, and known components, such as those described in WO 2012 / 115096, can be used. Furthermore, these components may be used alone or in combination of two or more types.

[0112] <Slurry> The slurry according to this embodiment contains an electrode active material, a thickener, and the first or second electrode binder. The slurry according to this embodiment has good coatability on the current collector surface, i.e., it can be uniformly coated without uneven coating or bumps, and has appropriate thixotropy (viscosity) to prevent excessive dripping, thereby enabling the formation of an electrode active material layer with high surface smoothness.

[0113] <Amount of Core-Shell Particles in Slurry> The amount of each of the first and second core-shell particles in the slurry can be appropriately determined by a person skilled in the art. From the viewpoints of improving the coatability of the slurry on the current collector and the binding strength between the current collector and the electrode active material layer, thereby improving the charge-discharge characteristics of the lithium ion battery, and reducing the internal resistance, the amount 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 2 parts by weight, relative to 100 parts by weight of the electrode active material.

[0114] (Thickener) The thickener is a component that can improve the dispersion stability of the electrode active material in the slurry and improve the coating properties of the slurry. Water-soluble polymers can be used as the thickener, specifically, carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, polyvinyl alcohol, polycarboxylic acids, their salts, poly(meth)acrylamide, etc. can be used. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, and alginic acid. These water-soluble polymers may be used alone or in combination of two or more. Among these, cellulose-based compounds are preferred, and carboxymethyl cellulose or its salts are particularly preferred.

[0115] The amount of thickener in the slurry may be set as appropriate, but is preferably 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, and even more preferably 0.5 to 3 parts by weight, per 100 parts by weight of the electrode active material.

[0116] (Negative electrode active material) Examples of electrode active materials that can be used in the negative electrode include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that are a combination of these. Examples of carbon-based negative electrode active materials include carbonaceous materials and graphite materials.

[0117] Examples of carbonaceous materials include graphitizable carbon, the carbon structure of which can be easily changed by heat treatment temperature, and non-graphitizable carbon, which has a structure similar to an amorphous structure, such as glassy carbon.

[0118] Examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal, such as coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor-grown carbon fiber.

[0119] Examples of non-graphitizable carbon include phenolic resin baked body, polyacrylonitrile carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin baked body (PFA), and hard carbon.

[0120] Examples of graphite materials include natural graphite and artificial graphite.

[0121] Examples of metal-based negative electrode active materials include lithium metal, elemental metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.), alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. of these. Among these, active materials containing silicon (silicon-based negative electrode active materials) are preferred. By using silicon-based negative electrode active materials, the capacity of lithium-ion batteries can be increased.

[0122] 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 obtained by coating or compounding Si-containing materials with conductive carbon. These silicon-based negative electrode active materials may be used alone or in combination of two or more.

[0123] Examples of alloys containing silicon include alloy compositions containing silicon, aluminum, and a transition metal such as iron, and further containing tin and a rare earth element such as yttrium.

[0124] SiOx is SiO and SiO ２ and Si, and x is usually 0.01 or more and less than 2.

[0125] Examples of composites of Si-containing materials and conductive carbon include compounds obtained by heat-treating a pulverized mixture of SiO, a polymer such as polyvinyl alcohol, and optionally a carbon material in an atmosphere containing an organic gas and / or steam. They can also be obtained by known methods, such as coating the surfaces of SiO particles with an organic gas or the like by chemical vapor deposition, or by forming composite particles (granulation) from SiO particles and graphite or artificial graphite by a mechanochemical method.

[0126] (Positive electrode active material) Examples of electrode active materials that can be used in the positive electrode include compounds containing transition metals, such as transition metal oxides, transition metal sulfides, and composite metal oxides of lithium and transition metals. Examples of transition metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

[0127] Examples of transition metal oxides include MnO and MnO ２ , V ２ O ５ , V ６ O １３ , TiO ２ , Cu ２ V ２ O ３ , amorphous V ２ O-P ２ O ５ , amorphous MoO ３ , amorphous V ２ O ５ , amorphous V ６ O １３ etc.

[0128] Examples of transition metal sulfides include TiS ２ , TiS ３ , amorphous MoS ２ , FeS, etc.

[0129] Examples of the composite metal oxide of lithium and a transition metal include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

[0130] Examples of lithium-containing composite metal oxides having a layered structure include lithium-containing cobalt oxide (LiCoO ２ ), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxide of Co—Ni—Mn, lithium-containing composite oxide of Ni—Mn—Al, lithium-containing composite oxide of Ni—Co—Al, LiMaO ２ and Li ２ MbO ３ Examples include solid solutions with

[0131] Examples of lithium-containing composite metal oxides having a spinel structure include lithium manganese oxide (LiMn ２ O ４ ) and lithium manganese oxide (LiMn ２ O4) in which a part of the Mn is substituted with another transition metal.

[0132] Examples of lithium-containing composite metal oxides having an olivine structure include olivine-type lithium iron phosphate (LiFePO ４ ), olivine-type lithium manganese phosphate (LiMnPO ４ ), olivine-type lithium iron phosphate (LiFePO ４ Examples include olivine-type lithium manganese iron phosphate, in which some of the manganese is replaced with manganese.

[0133] (Conductive Aid) The slurry may optionally contain a conductive aid. The conductive aid is not particularly limited, and known conductive aids can be used. Specific examples include conductive carbon materials such as acetylene black, Ketjen Black (registered trademark), carbon black, and graphite; and fibers and foils of various metals.

[0134] <Other Polymers> The slurry may optionally contain polymers other than the core-shell particles and thickeners described above. Examples of such polymers include fluorine-containing polymers and acrylonitrile polymers.

[0135] The solid content of the slurry is not particularly limited, but may be, for example, about 10 to 80% by weight, preferably 30 to 70% by weight.

[0136] <Preparation of Slurry> The slurry according to this embodiment can be prepared, for example, by dispersing the above-described components in an aqueous medium. Specifically, the slurry can be prepared by mixing the above-described components with the aqueous medium using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, or Filmix. The above-described components can be mixed with the aqueous medium typically at a temperature ranging from room temperature to 80°C for 10 minutes to several hours.

[0137] Although water is usually used as the aqueous medium, it is also possible to use an aqueous solution of any compound, a mixed solution of a small amount of an organic medium and water, etc. The water contained in the slurry may include water contained in the latex when the electrode binder is a latex of core-shell particles, water contained in a thickener, etc.

[0138] [Lithium-ion battery electrode] The lithium-ion battery electrode according to this embodiment includes a current collector and an electrode active material layer formed on the current collector. The electrode active material layer includes at least an electrode active material, a thickener, and a first or second electrode binder containing the core-shell particles according to this embodiment. The lithium-ion battery electrode according to this embodiment can be obtained by applying the above-described slurry to the current collector and drying it.

[0139] As the current collector, a known metal foil may be used, for example, copper foil, aluminum foil, nickel foil, highly conductive stainless steel foil, or the like.

[0140] The core-shell particles according to this embodiment tend to maintain their particle shape in the electrode active material layer after drying, which allows the electrode active materials to be bonded to each other or to the current collector by point bonding. As a result, it is believed that the presence of the binder makes it difficult for the movement of lithium ions to be hindered, thereby achieving a reduction in internal resistance.

[0141] (Coating Step) The method for applying the slurry according to this embodiment to the current collector is not particularly limited, and known methods can be used. Specific examples include a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. In this case, the slurry may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set depending on the thickness of the electrode active material layer obtained by drying.

[0142] (Drying step) The method for drying the slurry film on the current collector is not particularly limited, and any known method can be used, such as drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams.

[0143] After the drying step, the electrode active material layer may be subjected to a pressure treatment using a mold press, a roll press, etc. This can improve the bonding strength between the electrode active material layer and the current collector and reduce the porosity of the electrode active material layer.

[0144] The electrode of the lithium ion battery according to this embodiment can also be manufactured by a powder molding method, in which the above-mentioned slurry is first prepared, composite particles are prepared from the slurry, the composite particles are supplied onto a current collector, and optionally, the composite particles are molded by roll pressing to form an electrode active material layer on the current collector.

[0145] The lithium ion battery electrode according to a preferred aspect of this embodiment is a negative electrode for a lithium ion battery.

[0146] [Lithium-ion battery] The lithium-ion battery according to the present embodiment includes a positive electrode, a negative electrode, an electrolyte, and a separator, and uses an electrode of the lithium-ion battery according to the present embodiment for at least one of the positive electrode and the negative electrode (particularly the negative electrode). When the negative electrode is the lithium-ion battery electrode according to the present embodiment, the positive electrode is not particularly limited and may be a known positive electrode.

[0147] <Electrolyte> As the electrolyte, for example, a non-aqueous electrolyte solution in which a supporting electrolyte is dissolved in a non-aqueous solvent can be used. As the supporting electrolyte, a lithium salt is usually used. As the lithium salt, for example, LiPF ６ , LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlCl ４ , LiClO ４ , C.F. ３ SO ３ Li, C ４ F ９ SO ３ Li, CF ３ COOLi, (CF ３CO) ２ NLi, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ )NLi, etc. Among them, LiPF ６ , LiClO ４ , C.F. ３ SO ３ Li is preferred. These may be used alone or in combination of two or more.

[0148] The nonaqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of nonaqueous solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and ethyl methyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range. One type of nonaqueous solvent may be used alone, or two or more types may be used in any ratio. The electrolyte may contain an additive. Examples of additives include carbonate-based compounds such as vinylene carbonate (VC).

[0149] Other examples of the electrolyte solution include polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel polymer electrolytes obtained by impregnating the polymer electrolyte with an electrolyte; LiI, Li ３ Inorganic solid electrolytes such as N; and the like may also be used.

[0150] <Separator> The separator is not particularly limited, but a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) which is an insulating material can be used.

[0151] <Method for Manufacturing Lithium-Ion Battery> A specific method for manufacturing the lithium-ion battery according to this embodiment includes, for example, stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the stack according to the battery shape, placing the stack in a battery container, injecting an electrolyte into the battery container, and sealing the battery container. Furthermore, if necessary, an expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate, etc. may be inserted to prevent pressure buildup inside the battery and overcharging and discharging. The shape of the lithium-ion battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, etc.

[0152] The following items list preferred aspects of the present disclosure, but the present invention is not limited to them. [Item 1] An electrode binder for a lithium ion battery, comprising core-shell particles including a core and a shell layer located outside the core, the core having a multilayer structure composed of at least two layers, at least the innermost and outermost layers of the layers constituting the multilayer structure of the core are composed of a rubber containing an aliphatic conjugated diene compound unit, the rubber constituting the innermost layer of the core has a glass transition temperature of 25°C or higher and 85°C or lower, the glass transition temperature of the rubber constituting the outermost layer of the core is lower than the glass transition temperature of the rubber constituting the innermost layer of the core, and the shell layer is composed of a shell-forming polymer containing a (meth)acrylic acid ester compound unit. [Item 2] The electrode binder according to Item 1, wherein the rubber constituting the innermost layer of the core is a copolymer of an aliphatic conjugated diene compound unit and an aromatic vinyl compound unit. [Item 3] The electrode binder according to Item 1 or 2, wherein the rubber constituting the outermost layer of the core has a glass transition temperature of -10°C or lower. [Item 4] The electrode binder according to any one of items 1 to 3, wherein the rubber constituting the outermost layer of the core has a glass transition temperature of -85°C or higher. [Item 5] The electrode binder according to any one of items 1 to 4, wherein the content of the (meth)acrylic acid ester compound unit in the shell-forming polymer is 1 to 30% by weight. [Item 6] The electrode binder according to any one of items 1 to 5, wherein the shell-forming polymer contains a (meth)acrylic acid unit and an aromatic vinyl compound unit. [Item 7] The electrode binder according to item 6, wherein the content of the aromatic vinyl compound unit in the shell-forming polymer is 30 to 95% by weight. [Item 8] The electrode binder according to any one of items 1 to 7, wherein the shell-forming polymer has a glass transition temperature of 60°C or higher. [Item 9] The electrode binder according to any one of items 1 to 8, wherein the shell-forming polymer is a non-crosslinked polymer. [Item 10] The electrode binder according to any one of Items 1 to 9, wherein the ratio of the core to the entire core-shell particles is 50 to 95% by weight.[Item 11] The electrode binder according to any one of items 1 to 10, wherein the ratio of the innermost layer of the core to the entire core-shell particle is 10 to 40 wt %. [Item 12] The electrode binder according to any one of items 1 to 11, wherein the ratio of the outermost layer of the core to the entire core-shell particle is 10 to 40 wt %. [Item 13] The electrode binder according to any one of items 1 to 12, wherein the core-shell particles have a volume average particle diameter of 100 to 300 nm. [Item 14] The electrode binder according to any one of items 1 to 13, wherein the electrode binder is a latex of the core-shell particles. [Item 15] An electrode binder for a lithium ion battery, comprising core-shell particles including a core and a shell layer located outside the core, wherein the core is made of rubber, and the shell layer is made of a shell-forming polymer containing a non-crosslinkable monomer and a crosslinkable monomer as constituent monomers, wherein the non-crosslinkable monomer includes a (meth)acrylic acid ester-based monomer, and the content of the crosslinkable monomer copolymerizable with the non-crosslinkable monomer in the shell-forming polymer is 0.4% by weight or more and 7% by weight or less. [Item 16] The electrode binder according to Item 15, wherein the crosslinkable monomer in the shell-forming polymer is at least one selected from the group consisting of a vinyl group-containing ester-based monomer, a divinylbenzene-based monomer, and a vinyl group-containing cyanurate-based monomer. [Item 17] The electrode binder according to Item 15 or 16, wherein the non-crosslinkable monomer in the shell-forming polymer contains a methacrylic acid alkyl ester unit and an aromatic vinyl compound unit. [Item 18] The electrode binder according to item 17, wherein the content of the aromatic vinyl compound units in the shell-forming polymer is 30 to 95% by weight. [Item 19] The electrode binder according to item 17 or 18, wherein the non-crosslinkable monomer in the shell-forming polymer contains methacrylic acid units. [Item 20] The electrode binder according to any one of items 15 to 19, wherein the shell-forming polymer has a glass transition temperature of 60°C or higher. [Item 21] The electrode binder according to any one of items 15 to 20, wherein the rubber contains aliphatic conjugated diene compound units and aromatic vinyl compound units.[Item 22] The electrode binder according to any one of items 15 to 21, wherein the rubber has a glass transition temperature of -50°C to 20°C. [Item 23] The electrode binder according to any one of items 15 to 22, wherein a ratio of the cores to the entire core-shell particles is 50 to 95% by weight. [Item 24] The electrode binder according to any one of items 15 to 23, wherein the core-shell particles have a volume average particle diameter of 100 nm or more. [Item 25] The electrode binder according to any one of items 15 to 24, wherein the electrode binder is a latex of the core-shell particles. [Item 26] A method for producing an electrode for a lithium ion battery, comprising the steps of: preparing a slurry containing an electrode active material, a thickener, and the electrode binder according to any one of items 1 to 25; and applying the slurry to a current collector and drying the slurry. [Item 27] ​​A lithium ion battery electrode comprising a current collector and an electrode active material layer formed on the current collector, wherein the electrode active material layer comprises an electrode active material, a thickener, and the electrode binder according to any one of items 1 to 25. [Item 28] A lithium ion battery comprising the electrode according to item 27.

[0153] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, unless otherwise specified, "parts" means "parts by weight" and "%" means "% by weight".

[0154] Example 1-1 (Formation of the Innermost Layer of the Core) 10 g of disodium hydrogen phosphate (solid content 10%) was added to 24,400 g of deionized water, and further ferrous sulfate (FeSO ４ ・7H ２ A solution prepared by dissolving 0.237 g of ethylenediaminetetraacetic acid and 0.395 g of disodium ethylenediaminetetraacetic acid in 125.8 g of deionized water was added, and deacidification was carried out at −0.01 MPa for 15 minutes.

[0155] 66.7 g of Neopelex G-15 (Kao Corporation: sodium dodecylbenzenesulfonate, solids content 15.0%), 2,000 g of butadiene, 8,000 g of styrene, and 50 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0156] 70.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 3.85 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0157] During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO ４ ・7H ２ Each of butadiene and disodium ethylenediaminetetraacetic acid was added to a 100 L pressure autoclave in an arbitrary amount and at an arbitrary time. After confirming that the polymerization conversion rate of the butadiene and styrene added to the autoclave was 97% or higher, the autoclave was degassed under reduced pressure to remove the butadiene that was not used in the polymerization, thereby completing the polymerization of the innermost layer of the core (inner core layer).

[0158] (Formation of the Outermost Layer of the Core) After completion of polymerization of the innermost layer of the core (inner core layer), 8,000 g of butadiene and 2,000 g of styrene were placed in a 100 L pressure-resistant autoclave, followed by the addition of 70.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 3.85 g of paramenthane hydroperoxide (solid content 52%) to initiate polymerization of the outermost layer of the core (outer core layer).

[0159] During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO ４ ・7H ２0) and disodium ethylenediaminetetraacetic acid were each added to a 100 L pressure autoclave in arbitrary amounts and at arbitrary times, and after confirming that the polymerization conversion of the butadiene and styrene added to the autoclave was 97% or higher, the autoclave was degassed under reduced pressure to remove the butadiene remaining unused in the polymerization, thereby completing the polymerization of the innermost layer of the core (outer core layer). As a result, a latex (solid content 45.0%) containing particles with a multilayer structure having an innermost layer and an outermost layer composed of styrene-butadiene rubber was obtained.

[0160] (Formation of Shell Layer) 2900.0 g of the obtained rubber latex (solid content 45.0%) was charged into an 8 L polymerizer, and 60.0 g of sodium formaldehyde sulfoxylate (solid content 5%) was added. A mixture of 580 g (29.0 parts) of styrene, 90.0 g (4.5 parts) of butyl acrylate, 30.0 g (1.5 parts) of methacrylic acid, and 0.76 g (0.035 parts) of t-butyl hydroperoxide (solid content 69%) was added to the polymerizer over 120 minutes.

[0161] Sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were appropriately added, and polymerization was completed after 80 minutes, yielding a latex of core-shell structure-containing polymer particles (core-shell particles) with a solid content of 48.0% and a volume average particle diameter of 182 nm at a polymerization conversion of 100%. The volume average particle diameter of the latex was measured using a nanoparticle size measuring device NANOTRAC WAVE manufactured by Microtrac Corporation. This measurement method was common to all examples and comparative examples.

[0162] [Examples 1-2 to 1-6 and Comparative Examples 1-1 to 1-3] Latices of core-shell structure-containing polymer particles (core-shell particles) were obtained in the same manner as in Example 1-1, except that the amounts of butadiene and styrene used to form the innermost layer and the outermost layer of the core were changed according to the description in Table 1.

[0163] Comparative Example 1-4 (Formation of Core Portion) 10 g of disodium hydrogen phosphate (solid content 10%) was added to 24,400 g of deionized water, and further ferrous sulfate (FeSO ４ ・7H ２A solution prepared by dissolving 0.237 g of ethylenediaminetetraacetic acid and 0.395 g of disodium ethylenediaminetetraacetic acid in 125.8 g of deionized water was added, and deacidification was carried out at −0.01 MPa for 15 minutes.

[0164] 66.7 g of Neopelex G-15 (Kao Corporation: sodium dodecylbenzenesulfonate, solid content 15.0%), 10,000 g of butadiene, 10,000 g of styrene, and 50 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0165] 140.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 7.7 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0166] Fifteen hours after the start of polymerization, the mixture was degassed under reduced pressure to remove the butadiene remaining without being used in the polymerization, thereby terminating the polymerization. A latex (solid content 45.0%) containing styrene-butadiene rubber particles was obtained. During the polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO ４ ・7H ２ Each of the ethylenediaminetetraacetic acid disodium salt and the ethylenediaminetetraacetic acid disodium salt was added to a 100 L pressure autoclave in an arbitrary amount at an arbitrary time.

[0167] (Formation of Shell Layer) 2900.0 g of the obtained rubber latex (solid content 45.0%) was charged into an 8 L polymerizer, and 60.0 g of sodium formaldehyde sulfoxylate (solid content 5%) was added. A mixture of 580 g (29.0 parts) of styrene, 90.0 g (4.5 parts) of butyl acrylate, 30.0 g (1.5 parts) of methacrylic acid, and 0.76 g (0.035 parts) of t-butyl hydroperoxide (solid content 69%) was added to the polymerizer over 120 minutes.

[0168] Sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were appropriately added, and the polymerization was completed after 80 minutes, yielding a latex of core-shell structure-containing polymer particles (core-shell particles) with a solid content of 48.0% and a volume average particle diameter of 185 nm at a polymerization conversion of 100%.

[0169] [Example 2-1] (Formation of Core Portion) 10 g of disodium hydrogen phosphate (solid content 10%) was added to 24,400 g of deionized water, and further ferrous sulfate (FeSO ４ ・7H ２ A solution prepared by dissolving 0.237 g of ethylenediaminetetraacetic acid and 0.395 g of disodium ethylenediaminetetraacetic acid in 125.8 g of deionized water was added, and deacidification was carried out at −0.01 MPa for 15 minutes.

[0170] 66.7 g of Neopelex G-15 (Kao Corporation: sodium dodecylbenzenesulfonate, solids content 15.0%), 11,000 g of butadiene, 9,000 g of styrene, and 150 g of t-dodecyl mercaptan were placed in a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0171] 140.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 7.7 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0172] After 15 hours from the start of polymerization, the mixture was degassed under reduced pressure to remove the butadiene remaining without being used in the polymerization, thereby terminating the polymerization. A latex (solid content 45.0%) containing styrene-butadiene rubber particles was obtained.

[0173] During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO ４ ・7H ２ Each of the ethylenediaminetetraacetic acid disodium salt and the ethylenediaminetetraacetic acid disodium salt was added to a 100 L pressure autoclave in an arbitrary amount at an arbitrary time.

[0174] (Formation of Shell Layer) 2500.0 g (75 parts of solids) of the obtained rubber latex (solids content 45.0%) was charged into an 8 L polymerizer, and 60.0 g of sodium formaldehyde sulfoxylate (solids content 5%) was added. A mixture of 250.5 g (16.7 parts of styrene), 93.0 g (6.2 parts of methyl methacrylate), 31.5 g (2.1 parts of methacrylic acid), 3.8 g (0.25 parts of allyl methacrylate), and 0.76 g (0.035 parts of t-butyl hydroperoxide (solids content 69%) was added to the polymerizer over 120 minutes.

[0175] Sodium formaldehyde sulfoxylate and t-butyl hydroperoxide were appropriately added, and the polymerization was completed after 80 minutes, yielding a latex of core-shell structure-containing polymer particles (core-shell particles) with a solid content of 49.0% and a volume average particle diameter of 200 nm at a polymerization conversion of 100%.

[0176] Examples 2-2 to 2-16 and Comparative Examples 2-2 to 2-4 Latices of core-shell structure-containing polymer particles (core-shell particles) were obtained in the same manner as in Example 2-1, except that the volume average particle diameter of the core-shell particles or the types or amounts of the monomers for the core and shell layers were changed according to the description in Table 1. The volume average particle diameter was controlled by adjusting the amount of emulsifier at the start of polymerization.

[0177] Comparative Example 2-1 Polymerization of Polymer Particles 10 g of disodium hydrogen phosphate (solid content 10%) was added to 24,400 g of deionized water, and ferrous sulfate (FeSO ４ ・7H ２ A solution prepared by dissolving 0.237 g of ethylenediaminetetraacetic acid and 0.395 g of disodium ethylenediaminetetraacetic acid in 125.8 g of deionized water was added thereto, and deacidification was carried out at −0.01 MPa for 15 minutes.

[0178] 66.7 g of Neopelex G-15 (manufactured by Kao Corporation: sodium dodecylbenzenesulfonate, solids content 15.0%), 9,100 g (45.5 parts) of butadiene, 10,400 g (52 parts) of styrene, 500 g (2.5 parts) of methacrylic acid, and 150 g of t-dodecyl mercaptan were charged into a 100 L pressure-resistant autoclave, and the internal temperature was raised to 50°C.

[0179] 140.0 g of sodium formaldehyde sulfoxylate (solid content 5%) and 7.7 g of paramenthane hydroperoxide (solid content 52%) were added to initiate polymerization.

[0180] After 15 hours from the start of polymerization, the mixture was degassed under reduced pressure to remove the butadiene remaining without being used in the polymerization, thereby terminating the polymerization. A latex (solid content 45.0%) containing non-core-shell styrene-butadiene rubber particles containing methacrylic acid units was obtained.

[0181] During polymerization, sodium formaldehyde sulfoxylate, paramenthane hydroperoxide, ferrous sulfate (FeSO ４ ・7H ２ Each of the ethylenediaminetetraacetic acid disodium salt and the ethylenediaminetetraacetic acid disodium salt was added to a 100 L pressure autoclave in an arbitrary amount at an arbitrary time.

[0182] (Preparation of negative electrode slurry) 1 part by weight (solid content equivalent) of a thickener (product name "CMC2200" manufactured by Daicel Corporation), 100 parts by weight (solid content equivalent) of graphite as a negative electrode active material, and 100 parts by weight of water were added to a stirring and defoaming machine (manufactured by Thinky Corporation, product name "Awatori Rentaro"), and the mixture was stirred and mixed for 5 minutes at 2000 rpm. Thereafter, 1 part by weight (solid content equivalent) of the polymer particles synthesized in each example or comparative example was added, and the mixture was further stirred and mixed at 2000 rpm for 5 minutes, followed by defoaming and mixing at 2200 rpm for 1 minute to obtain a negative electrode slurry with a solid content concentration of 50%.

[0183] (Preparation of Negative Electrode) The negative electrode slurry was uniformly applied to the surface of a current collector made of copper foil with a thickness of 17 μm by a doctor blade method so that the film thickness after drying would be 80 μm, and the negative electrode slurry was subjected to vacuum drying treatment at 80° C. for 6 hours. Thereafter, the density of the negative electrode active material layer was adjusted to 6.0 g / cm ２ The negative electrode was obtained by pressing the current collector with a roll press so that a negative electrode active material layer was formed on the surface of the current collector.

[0184] (Evaluation of Coatability) The surface of the negative electrode active material layer after drying was visually observed, and the coatability was judged based on the following criteria: ◯: Neither streaks nor spots were observed on the surface of the negative electrode active material layer. Δ: No streaks were observed on the surface of the negative electrode active material layer, but spots were observed. ×: Streaks and spots were observed on the surface of the negative electrode active material layer.

[0185] (Preparation of Positive Electrode Slurry) 4 parts by weight (solid content equivalent) of polyvinylidene fluoride, 4 parts by weight (solid content equivalent) of acetylene black, and LiCoO 2 having a volume average particle diameter of 15 μm as a positive electrode active material were placed in a stirring deaerator (manufactured by Thinky Corporation, product name "Awatori Rentaro"). ２ 100 parts by weight (solid content equivalent) of the above and 46 parts by weight of N-methylpyrrolidone were added, and the mixture was stirred and mixed at 2000 rpm for 10 minutes, and then degassed and mixed at 2200 rpm for 1 minute to obtain a positive electrode slurry with a solid content concentration of 70%.

[0186] (Preparation of Positive Electrode) The positive electrode slurry was uniformly applied to the surface of a current collector made of aluminum foil with a thickness of 15 μm by a doctor blade method so that the film thickness after drying would be 80 μm, and the resulting layer was vacuum dried at 80° C. for 6 hours. After that, the density of the positive electrode active material layer was adjusted to 10.0 mg / cm ２ The current collector was pressed with a roll press so that a positive electrode active material layer was formed on the surface of the current collector.

[0187] (Assembly of Lithium-ion Battery) A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 20 μm; manufactured by a dry process; porosity 60%) was prepared and cut into a 3 cm × 3 cm square. An aluminum packaging material was also prepared as the battery exterior.

[0188] Then, in a glove box purged with Ar to maintain a dew point of −80°C or less, the positive electrode prepared as described above was cut into a 2 cm × 2 cm square and placed so that the surface on the current collector side was in contact with an aluminum packaging exterior. Next, a square separator was placed on the surface on the positive electrode active material layer side. Furthermore, the negative electrode prepared as described above was cut into a 2.2 cm × 2.2 cm square and placed on the separator so that the surface on the negative electrode active material layer side faced the separator. Then, a 1.0 M LiPF6 solution (solvent: a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (volume ratio), with 2 vol% vinylene carbonate (solvent ratio) added as an additive) was filled as an electrolyte. Furthermore, the aluminum packaging exterior was closed by heat sealing at 150°C to seal the opening, and a laminate cell-type lithium ion secondary battery was produced.

[0189] (Evaluation of Adhesion: Peel Test of Negative Electrode) The negative electrode prepared as described above was cut into a rectangular shape having a length of 30 mm and a width of 15 mm to prepare a test piece, and cellophane tape (specified in JIS Z1522) was attached to the surface of the negative electrode active material layer with the surface on which the negative electrode active material layer was formed facing up. One end of the current collector was pulled in the vertical direction at a pulling rate of 37.5 mm / min to peel it off, and the stress when peeled off was measured. The measurement was performed three times, and the average value was calculated and used as the T-peel strength (N / cm).

[0190] (Charge-Discharge Cycle Characteristics) The laminate cell type lithium ion secondary batteries prepared by the above method using the polymer particles of Examples 1-1 to 1-6 and Comparative Examples 1-1 and 1-4 were subjected to L ｉ PF ６ After filling with the solution, the battery was left to stand for 12 hours, and then charged to 4.2 V and discharged to 2.5 V by a constant current method of 0.2 C. This charge-discharge cycle was repeated 20 times at 25°C. The capacity at the first cycle, i.e., the initial discharge capacity X1, and the discharge capacity at the 20th cycle, X20, were measured. The capacity retention rate, which is calculated as the charge-discharge cycle characteristic = X20 / X1 × 100 (%), was calculated as the cycle characteristic.

[0191] (Measurement of DC Resistance During Charge and Discharge) The laminated cell type lithium ion secondary battery prepared as described above was charged at a current of 0.2 C for 2.5 hours in an environment of 25°C, and the depth of charge was adjusted (state of charge (SOC) 50%). After a one-hour pause, the open-circuit voltage (OCV) was measured and the DC resistance (charge DCR) was calculated from the voltage change during the pause. The discharge DCR was calculated as follows. After a one-hour pause, the battery was charged at a current of 0.1 C for 10 hours in an environment of 25°C, and then the battery was paused for one hour. The battery was then discharged at a current of 0.1 C for 0.5 hours to adjust the depth of charge (state of charge (SOC) 90%). After a one-hour pause, the open-circuit voltage (OCV) was measured and the DC resistance (discharge DCR) was calculated from the voltage change during the pause.

[0192] (Evaluation of electrolyte resistance) The dried polymer particle latexes synthesized in the above Examples and Comparative Examples were pressed in a press with a 1 mm spacer inserted at 170°C for 10 minutes to obtain a film with a thickness of 1 mm. The obtained film was cut into a square with a length of 1 cm and a width of 1 cm, and the weight was precisely weighed.

[0193] The precisely weighed film was immersed in an electrolyte solution (a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (volume ratio)) in a test tube. The test tube containing the film and the electrolyte solution was left to stand at 23°C for 24 hours. Thereafter, 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 precisely weighed.

[0194] Thereafter, the swelling ratio due to immersion in the electrolyte solution was calculated from the weights of the film before and after immersion in the electrolyte solution using the following formula: Swelling ratio due to immersion in the electrolyte solution (%) = (weight of film after immersion / weight of film before immersion) x 100

[0195] (Charge-Discharge Cycle Characteristics) The laminate cell type lithium ion secondary batteries prepared by the above method using the polymer particles of Examples 2-1 to 2-16 and Comparative Examples 2-1 and 2-4 were charged 100 times in a 30° C. environment at a constant current of 1.0 C until the battery voltage reached 4.2 V, and then discharged 100 times at a constant current of 1.0 C until the battery voltage reached 2 V. Then, the ratio of the 100th discharge capacity to the first discharge capacity (charge-discharge capacity retention rate=(100th discharge capacity / 1st discharge capacity)×100%) was calculated.

[0196] The results of the above-described evaluation are shown in Tables 1 and 2.

[0197]

[0198] As shown in Table 1, Examples 1-1 to 1-6 had high T-peel strength, good adhesion between the current collector and the electrode active material layer, high capacity retention after charge / discharge cycles, and low DC resistance (low internal resistance) during both charge and discharge. Furthermore, the coating properties on the current collector were also good.

[0199] On the other hand, Comparative Examples 1-1 to 1-3, in which the glass transition temperature of the rubber constituting the innermost layer of the core was lower than 25°C, and Comparative Example 1-4, in which the core did not have a multilayer structure, had low internal resistance, but compared with Examples 1-1 to 1-6, had low T-peel strength, poor adhesion between the current collector and the electrode active material layer, and were inferior in capacity retention rate after charge-discharge cycling.

[0200]

[0201] As shown in Table 2, Examples 2-1 to 2-16 had a low electrolyte immersion swelling rate and excellent electrolyte resistance, and had high T-peel strength and good adhesion between the current collector and the electrode active material layer, and accordingly had a high charge / discharge capacity retention rate and excellent charge / discharge cycle characteristics.

[0202] On the other hand, in Comparative Example 2-1, which is a non-core-shell structure copolymer, the electrolyte immersion swelling ratio was high, the T-peel strength was low, and the electrolyte resistance and the binding strength between the current collector and the electrode active material layer were poor compared to Examples 2-1 to 2-16, and therefore the charge-discharge cycle characteristics were poor.

[0203] Comparative Examples 2-2 and 2-3, which used core-shell particles having a core-shell structure but not containing a crosslinkable monomer in the shell layer or whose content was below the specified level, had a higher swelling rate upon immersion in an electrolyte solution and poorer electrolyte resistance than Examples 2-1 to 2-16, and therefore had poor charge-discharge cycle characteristics.

[0204] Comparative Example 2-4, which had a core-shell structure but used core-shell particles in which the content of the crosslinkable monomer in the shell layer was equal to or greater than the specified level, had excellent electrolyte resistance compared to Examples 2-1 to 2-16, but had low T-peel strength and poor adhesion between the current collector and the electrode active material layer, and therefore had poor cycle characteristics.

Claims

1. An electrode binder for a lithium ion battery, comprising core-shell particles including a core and a shell layer located outside the core, the core having a multilayer structure consisting of at least two layers, at least the innermost and outermost layers of the layers constituting the multilayer structure of the core are composed of a rubber containing aliphatic conjugated diene compound units, the glass transition temperature of the rubber constituting the innermost layer of the core is 25°C or higher and 85°C or lower, the glass transition temperature of the rubber constituting the outermost layer of the core is lower than the glass transition temperature of the rubber constituting the innermost layer of the core, and the shell layer is composed of a shell-forming polymer containing (meth)acrylic acid ester compound units.

2. The electrode binder according to claim 1, wherein the rubber constituting the outermost layer of the core has a glass transition temperature of -10°C or lower.

3. The electrode binder according to claim 1 or 2, wherein the rubber constituting the outermost layer of the core has a glass transition temperature of -85°C or higher.

4. The electrode binder according to claim 1 or 2, wherein the shell-forming polymer contains (meth)acrylic acid units and aromatic vinyl compound units.

5. The electrode binder according to claim 1 or 2, wherein the shell-forming polymer has a glass transition temperature of 60°C or higher.

6. The electrode binder according to claim 1 or 2, wherein the shell-forming polymer is a non-crosslinked polymer.

7. The electrode binder according to claim 1 or 2, wherein the ratio of the innermost layer of the core to the entire core-shell particle is 10 to 40% by weight.

8. The electrode binder according to claim 1 or 2, wherein the proportion of the outermost layer of the core in the entire core-shell particle is 10 to 40% by weight.

9. An electrode binder for a lithium ion battery, comprising core-shell particles including a core and a shell layer located on the outside of the core, wherein the core is made of rubber, and the shell layer is made of a shell-forming polymer containing a non-crosslinkable monomer and a crosslinkable monomer as constituent monomers, the non-crosslinkable monomer includes a (meth)acrylic acid ester-based monomer, and the content of the crosslinkable monomer copolymerizable with the non-crosslinkable monomer in the shell-forming polymer is 0.4% by weight or more and 7% by weight or less.

10. The electrode binder according to claim 9, wherein the crosslinkable monomer in the shell-forming polymer is at least one selected from the group consisting of vinyl group-containing ester monomers, divinylbenzene monomers, and vinyl group-containing cyanurate monomers.

11. The electrode binder according to claim 9 or 10, wherein the shell-forming polymer has a glass transition temperature of 60°C or higher.

12. The electrode binder according to claim 9 or 10, wherein the rubber has a glass transition temperature of -50°C to +20°C.

13. A method for manufacturing an electrode for a lithium ion battery, comprising the steps of: preparing a slurry containing an electrode active material, a thickener, and the electrode binder according to any one of claims 1, 2, 9, and 10; and applying the slurry to a current collector and drying the slurry.

14. An electrode for a lithium ion battery comprising a current collector and an electrode active material layer formed on the current collector, wherein the electrode active material layer comprises an electrode active material, a thickener, and the electrode binder according to any one of claims 1, 2, 9, and 10.

15. A lithium ion battery comprising the electrode of claim 14.