Electrode binder for secondary battery comprising secondary-battery electrode containing carbon nanotubes, and use of electrode binder

Abstract

The present invention provides a binder for a secondary battery electrode, the binder being capable of reducing the viscosity of an electrode slurry, and being, even in a case where a silicon-based active material exhibiting large expansion and contraction from charging and discharging is used, capable of exhibiting a high capacity retention rate while suppressing expansion and contraction from charging and discharging throughout a longer-than-average service period. The present invention also provides a secondary battery electrode mixture layer composition containing the binder, and a secondary battery electrode and a secondary battery that are obtained using the composition.　The binder for an electrode of a secondary battery comprising a secondary battery electrode containing carbon nanotubes contains: a carboxyl group-containing crosslinked polymer or a salt thereof; and a carboxyl group-containing non-crosslinked polymer or a salt thereof.

Description

Electrode binder for secondary battery having secondary battery electrode containing carbon nanotubes and use thereof

[0001] The present invention relates to an electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes, and to the use thereof.

[0002] Various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors, have been put to practical use. Electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder to a current collector, followed by drying. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder for the negative electrode mixture layer composition. Furthermore, binders containing aqueous solutions or dispersions of acrylic acid-based polymers are known as binders with excellent dispersibility and binding properties. Meanwhile, a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) is widely used as the binder for the positive electrode mixture layer.

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials have become more common. However, silicon-based active materials are known to undergo large volume changes during charge and discharge, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). In order to prevent such problems, it is generally effective to increase the binding properties of the binder, and studies on improving the binding properties of binders have been conducted with the aim of improving durability.

[0004] Binders using the above-mentioned acrylic acid polymers have been proposed as binders that have good binding properties and are effective in improving durability. It is also known that adding a conductive additive to a slurry for producing a negative electrode improves electrical contact between active materials and improves battery performance. For example, Patent Documents 1 and 2 disclose that electrodes obtained from electrode mixture layer compositions containing a silicon-containing active material, a conductive additive, and a binder containing acrylic acid-derived monomer units and monomer units with a specific structure have excellent charge / discharge capacity. They specifically describe that the use of ketjen black or acetylene black as the conductive additive results in a high capacity retention rate after 10 or 30 charge / discharge cycles.

[0005] International Publication No. 2015 / 163302 Japanese Patent Application Laid-Open No. 2018-029069

[0006] The binders disclosed in Patent Documents 1 and 2 are both capable of imparting good binding properties and can achieve high capacity retention over a short charge-discharge cycle of around 10 to 30, as described above. However, with repeated charge-discharge cycles, the structural deterioration of the negative electrode increases due to the expansion and contraction of the silicon-based active material, making it impossible to maintain conductive paths between the active materials, resulting in capacity degradation. Furthermore, there is room for improvement in the coatability of electrode slurries containing these binders.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a binder for secondary battery electrodes that can reduce the viscosity of electrode slurry and that can suppress expansion and contraction due to charge and discharge over a longer period of time than conventional binders, even when using a silicon-based active material that experiences significant expansion and contraction due to charge and discharge, while exhibiting a high capacity retention rate. Another object of the present invention is to provide a composition for secondary battery electrode mixture layers that includes the binder, and a secondary battery electrode and secondary battery obtained using the composition.

[0008] As a result of intensive research to solve the above problems, the present inventors have found that by using a binder containing a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer or a salt thereof as an electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes (hereinafter also referred to as "CNTs") as a conductive additive, it is possible to reduce the viscosity of the electrode slurry, and to exhibit an excellent charge / discharge capacity retention rate while suppressing expansion and contraction due to charge / discharge, even when the secondary battery is used at a higher number of cycles than conventionally, and have completed the present invention.

[0009] The present invention is as follows: [1] An electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes, the electrode binder containing a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer or a salt thereof. [2] The electrode binder according to [1], in which the active material of the secondary battery electrode contains a silicon-based active material. [3] The electrode binder according to [1] or [2], in which the carboxyl group-containing crosslinked polymer contains structural units derived from ethylenically unsaturated carboxylic acid monomers in an amount of 50% by mass to 100% by mass of all structural units thereof, and the carboxyl group-containing non-crosslinked polymer contains structural units derived from ethylenically unsaturated carboxylic acid monomers in an amount of 50% by mass to 100% by mass of all structural units thereof. [4] The electrode binder according to any one of [1] to [3], wherein the content of the carboxyl group-containing crosslinked polymer or salt thereof is 10% by mass or more and 90% by mass or less, based on the total amount of the carboxyl group-containing crosslinked polymer or salt thereof and the carboxyl group-containing non-crosslinked polymer or salt thereof. [5] A composition for a secondary battery electrode mixture layer, comprising the electrode binder according to any one of [1] to [4], carbon nanotubes, an active material, and water. [6] The composition for a secondary battery electrode mixture layer according to [5], wherein the content of the silicon-based active material is 5.0% by mass or more, based on the total amount of the active material. [7] The composition for a secondary battery electrode mixture layer according to [5] or [6], wherein the content of the electrode binder is 1.0 part by mass or more and 2.5 parts by mass or less, based on 100 parts by mass of the total amount of the active material. [8] The composition for a secondary battery electrode mixture layer according to any one of [5] to [7], wherein the content of the carbon nanotubes is 0.1 parts by mass or more relative to 100 parts by mass of the total amount of the active material. [9] The composition for a secondary battery electrode mixture layer according to any one of [5] to [8], wherein the carbon nanotubes have a single-layer structure.

[10] A secondary battery electrode comprising, on a current collector surface, a mixture layer formed from the composition for a secondary battery electrode mixture layer according to any one of [5] to [9].

[11] A secondary battery comprising the secondary battery electrode according to

[10] .

[0010] The electrode binder of the present invention can reduce the viscosity of the electrode slurry, and can provide a secondary battery that exhibits an excellent charge / discharge capacity retention rate while suppressing expansion and contraction due to charge / discharge during long-term use compared to conventional batteries.

[0011] The electrode binder of the present invention (hereinafter also referred to as "the binder") comprises a carboxyl group-containing crosslinked polymer (hereinafter also referred to as "the crosslinked polymer") or a salt thereof (hereinafter also referred to as "the crosslinked polymer salt"), and a carboxyl group-containing non-crosslinked polymer (hereinafter also referred to as "the non-crosslinked polymer") or a salt thereof (hereinafter also referred to as "the non-crosslinked polymer salt"), and can be mixed with carbon nanotubes (hereinafter also referred to as "CNTs"), an active material, and water to form a secondary battery electrode mixture layer composition (hereinafter also referred to as "the composition"). The above composition is preferably in the form of an electrode slurry that can be applied to a current collector in order to achieve the effects of the present invention, but it may also be prepared in a wet powder state to accommodate press processing onto the current collector surface. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as copper foil or aluminum foil. Here, the present binder is preferred in that the effects of the present invention are particularly large when used in a secondary battery electrode mixture layer composition containing a silicon-based active material described below as the active material.

[0012] The following describes in detail the carbon nanotubes, the crosslinked polymer and its manufacturing method, the non-crosslinked polymer and its manufacturing method, the binder, the composition for a secondary battery electrode mixture layer obtained using the binder, the secondary battery electrode, and the secondary battery. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Furthermore, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described in this specification in stages, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages, and the upper or lower limit of that numerical range may be replaced with a value shown in the examples.

[0013] 1. Carbon Nanotubes The carbon nanotubes (CNTs) used in the present composition have high electrical conductivity and chemical stability and are used as a conductive additive to ensure electrical contact between active materials in the electrode mixture layer. CNTs have a cylindrical shape formed by rolling planar graphite. The type of CNT is not particularly limited, and single-walled CNTs (hereinafter also referred to as "single-walled CNTs") or multi-walled CNTs (hereinafter also referred to as "multi-walled CNTs") can be used. These can be used alone or in combination of two or more types. Single-walled CNTs are preferred as CNTs, as they provide particularly significant benefits. Single-walled CNTs have a structure in which one layer of graphite is rolled, while multi-walled CNTs have a structure in which two or more layers of graphite are rolled. The sidewalls of the CNTs do not have to have a graphite structure; CNTs with sidewalls having an amorphous structure are also considered CNTs in this specification.

[0014] The shape of the CNTs is not limited. Examples of such shapes include needle-like, cylindrical tubular, fishbone-like (fishbone or cup stacked), trump-like (platelet), and coil-like. Of these, needle-like or cylindrical tubular shapes are preferred. The CNTs may have a single shape or a combination of two or more shapes.

[0015] Examples of the form of CNT include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. CNT may have any one of these forms or a combination of two or more of these forms.

[0016] The average outer diameter of the CNTs is preferably 1 nm or more, more preferably 5 nm or less, and even more preferably 3 nm or less. The average outer diameter of the CNTs can be calculated by first observing and photographing the CNTs using a transmission electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the outer diameter of each.

[0017] The average fiber length of the CNTs is preferably 0.5 μm or more, more preferably 0.8 μm or more, more preferably 1.0 μm or more, and even more preferably 5.0 μm or more. It is also preferably 20 μm or less, and more preferably 10 μm or less. The average fiber length of the CNTs can be calculated by first observing and photographing the CNTs using a scanning electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the fiber length of each.

[0018] The aspect ratio is the value obtained by dividing the fiber length of CNT by the outer diameter. A typical aspect ratio can be calculated using the average fiber length and the average outer diameter. The higher the aspect ratio of a conductive additive, the higher the conductivity that can be obtained when an electrode is formed. The aspect ratio of CNT is preferably 30 or more, more preferably 50 or more, and even more preferably 80 or more. In addition, it is preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less.

[0019] The specific surface area of ​​CNT is 100m ２ / g or more, and ２ / g or more is more preferable, and 200m ２ / g or more is more preferable. ２ / g or less, and ２ / g or less is more preferable. The specific surface area of ​​the CNTs can be calculated by the BET method using nitrogen adsorption measurements. When the average outer diameter, average fiber length, aspect ratio, and specific surface area of ​​the CNTs are within the above ranges, they are likely to form well-developed conductive paths in the electrode.

[0020] The carbon purity of CNT is expressed as the content (mass%) of carbon atoms in the CNT. The carbon purity is preferably 80 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 98 mass% or more, relative to 100 mass% of CNT. By keeping the carbon purity within the above range, problems such as short circuits caused by the formation of dendrites due to impurities such as metal catalysts can be prevented.

[0021] The CNT may be surface-treated CNT, or a CNT derivative to which a functional group such as a carboxyl group has been added. Alternatively, CNT containing an organic compound, a metal atom, or a substance such as fullerene may also be used.

[0022] CNTs can be produced by, but are not limited to, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion. For example, CNTs can be produced by catalytically reacting a carbon source with a catalyst at 500 to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. The carbon source may be at least one of a hydrocarbon and an alcohol.

[0023] 2. The present crosslinked polymer The present crosslinked polymer can have a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a1)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing component (a1).

[0024] <Structural Units Derived from Ethylenically Unsaturated Carboxylic Acid Monomers> The carboxyl group-containing crosslinked polymer (present crosslinked polymer) contained in the present binder may have a structural unit (component (a1)) derived from an ethylenically unsaturated carboxylic acid monomer. When the present crosslinked polymer has such a structural unit and thus a carboxyl group, adhesion to the current collector is improved, and the lithium ion desolvation effect and ionic conductivity are excellent, resulting in an electrode with low resistance and excellent high-rate characteristics. In addition, water swelling properties are imparted, thereby improving the dispersion stability of active materials and the like in the present composition. The above-mentioned component (a1) can be introduced into the polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, (meth)acrylamide, (meth)acrylonitrile, or the like may be polymerized and then treated with a strong alkali, or a method in which a polymer having a hydroxyl group is reacted with an acid anhydride may be used.

[0025] Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and carboxyl group-containing ethylenically unsaturated monomers such as monohydroxyethyl succinate (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate, or their (partially) alkali-neutralized products. One of these may be used alone, or two or more may be used in combination. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, with acrylic acid being particularly preferred, because they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0026] The content of component (a1) in the crosslinked polymer can be 50% by mass or more and 100% by mass or less, based on the total structural units of the crosslinked polymer. By including component (a1) in this range, excellent adhesion to the current collector can be easily ensured, and a high capacity retention rate can be achieved while suppressing electrode expansion and contraction due to charging and discharging of the secondary battery. A lower limit of 50% by mass or more is preferable because it improves the dispersion stability of the composition and provides a higher binding strength. It may be 60% by mass or more, 70% by mass or more, or even 80% by mass or more. The upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, for example, 95% by mass or less, for example, 90% by mass or less, or for example, 80% by mass or less.

[0027] <Other Structural Units> In addition to the component (a1), the present crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (b1)"). Examples of the component (b1) include structural units derived from hydroxyl group-containing ethylenically unsaturated monomers (monomers represented by the following formula (1) and formula (2)), ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing a monomer containing a hydroxyl group-containing ethylenically unsaturated monomer, an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a nonionic ethylenically unsaturated monomer. CH ２ = C(R １ ) COOR ２ (1) [wherein, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Note that R ３represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.] CH ２ = C(R ５ ) CONR ６ R ７ (2) [wherein, R ５ represents a hydrogen atom or a methyl group, R ６ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, R ７ represents a hydrogen atom or a monovalent organic group.

[0028] The proportion of the (b1) component can be 0% by mass or more and 50% by mass or less relative to the total structural units of the non-crosslinked polymer. The proportion of the (b1) component may be 0.1% by mass or more and 40% by mass or less, 0.5% by mass or more and 30% by mass or less, 1.0% by mass or more and 20% by mass or less, 2% by mass or more and 12.5% ​​by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when the (b1) component is contained in an amount of 0.1% by mass or more relative to the total structural units of the crosslinked polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0029] Among the above-mentioned components, the (b1) component is preferably a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, in view of the excellent binding properties of the binder containing the crosslinked polymer salt. Furthermore, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred, in view of the obtainment of an electrode with good flex resistance. Examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, and alicyclic structure-containing ethylenically unsaturated monomers.

[0030] The monomer represented by the above formula (1) is a (meth)acrylate compound having a hydroxyl group. ２When R is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of the hydroxyl groups may be one or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. ２ (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ If H, then R ３ or R ４ The alkylene group represented by may be linear or branched.

[0031] Examples of the monomer represented by the formula (1) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl (meth)acrylate; polyalkylene glycol mono(meth)acrylates, such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, and polyethylene glycol-polypropylene glycol mono(meth)acrylate; dihydroxyalkyl (meth)acrylates, such as glycerin mono(meth)acrylate; caprolactone-modified hydroxymethacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FM1," "PLACCEL FM5," etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1," "PLACCEL FA10L," etc.). The monomer represented by the formula (1) may be used singly or in combination of two or more.

[0032] The monomer represented by the above formula (2) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and is preferably an organic group having 1 to 8 carbon atoms. In addition, R ７ may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

[0033] Examples of the monomer represented by formula (2) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, N-hydroxybutyl(meth)acrylamide, N-hydroxyhexyl(meth)acrylamide, and N-hydroxyoctyl(meth)acrylamide, N-methylhydroxyethyl(meth)acrylamide, and N-ethylhydroxyethyl(meth)acrylamide; and N,N-dihydroxyalkyl(meth)acrylamides, such as N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide. The monomer represented by formula (2) may be used singly or in combination of two or more.

[0034] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide. These may be used alone or in combination of two or more.

[0035] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination. Of the above, acrylonitrile is preferred because of its high nitrile group content.

[0036] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, cyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate; and the like. These may be used alone or in combination of two or more.

[0037] In view of the excellent binding properties of the binder, the present crosslinked polymer preferably contains structural units derived from the monomer represented by the above formula (1), the monomer represented by the above formula (2), (meth)acrylamide and its derivatives, as well as nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. Among these, as component (b1), structural units derived from the monomer represented by the above formula (1) and the monomer represented by the above formula (2) are more preferred in view of the excellent effect of improving the binding properties of the present binder. Among the monomers represented by the above formula (1), hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, with 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate being even more preferred. Furthermore, among the monomers represented by the above formula (2), (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, and N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, and N-hydroxybutyl(meth)acrylamide are even more preferred.

[0038] Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (b1), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, as the "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less," an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

[0039] Other nonionic ethylenically unsaturated monomers that may be used include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0040] From the viewpoints of binding strength with the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferably used. From the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0041] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and provide good binder binding strength.Furthermore, as the nonionic ethylenically unsaturated monomer, compounds whose homopolymer has a glass transition temperature (Tg) of 0°C or less are preferred because the resulting electrode has good flex resistance.

[0042] The present crosslinked polymer is a crosslinked polymer having a crosslinked structure. The crosslinking method for the present crosslinked polymer is not particularly limited, and examples thereof include the following methods: 1) Copolymerization of a crosslinkable monomer 2) Utilization of chain transfer to a polymer chain during radical polymerization Because the present crosslinked polymer has a crosslinked structure, a binder containing the crosslinked polymer or a salt thereof can have excellent binding strength. Among the above, the method of copolymerizing a crosslinkable monomer is preferred because of its simple operation and ease of controlling the degree of crosslinking.

[0043] <Crosslinkable Monomer> Examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

[0044] The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in the molecule, and examples thereof include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred in that they are easy to obtain a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0045] Examples of polyfunctional (meth)acryloyl compounds include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; poly(meth)acrylates such as tri(meth)acrylate and tetra(meth)acrylate of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of ethylene oxide-modified trimethylolpropane, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0046] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallylsucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0047] Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate.

[0048] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, N-methoxyalkyl(meth)acrylamide, etc. These compounds can be used alone or in combination of two or more.

[0049] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group.For example, it can be mentioned vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, etc.; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether, etc.; silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc.

[0050] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 to 5.0 parts by mass, more preferably 0.05 to 3.0 parts by mass, even more preferably 0.1 to 2.0 parts by mass, even more preferably 0.1 to 1.7 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). When the amount of the crosslinkable monomer used is 0.01 parts by mass or more, the conductive path between the active materials is well maintained while suppressing expansion and contraction due to charge and discharge during long-term use, which is preferable in that excellent charge and discharge capacity retention can be achieved. When the amount is 5.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be improved. In particular, if the amount is 1.0 part by mass or less, the water-swelling particle size in the electrode slurry becomes suitable, and the area of ​​binding to the active material becomes large, which is preferable in that excellent battery performance can be maintained even during long-term use.

[0051] For the same reason, the amount of the crosslinkable monomer used is preferably 0.001 mol % or more and 2.5 mol % or less, more preferably 0.01 mol % or more and 2.0 mol % or less, even more preferably 0.05 mol % or more and 1.75 mol % or less, still more preferably 0.05 mol % or more and 1.5 mol % or less, and even more preferably 0.1 mol % or more and 1.0 mol % or less, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

[0052] The crosslinked polymer salt is in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred, and alkali metal salts are more preferred, as they are less likely to adversely affect battery characteristics.

[0053] Regarding the characteristics of the crosslinked polymer salt, the crosslinked polymer is preferably used in the form of a salt in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization in the composition is 20 mol% or more. A degree of neutralization of 20 mol% or more is preferable in that it improves water swelling properties and makes it easier to achieve a dispersion stabilization effect. The degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, even more preferably 85 mol% or more, and particularly preferably 90 mol% or more, in terms of excellent electrode slurry coatability and superior charge / discharge capacity retention over longer periods of use than conventional methods. The reason why a higher degree of neutralization improves coatability is presumably because the electrostatic repulsion within the particles is reduced, thereby suppressing the spread of side chains when the polymer swells in water and reducing entanglement in the electrode slurry. The reason why a higher degree of neutralization results in a better capacity retention rate over long periods of use is presumably because a higher neutralization rate of the polymer increases the glass transition temperature, suppressing fusion of the polymer during the heat drying step in the electrode preparation process and enabling the production of a uniform electrode. The upper limit of the degree of neutralization is 100 mol%, but it may also be 98 mol% or 95 mol%. In this specification, the degree of neutralization can be calculated from the amounts of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization can be confirmed by IR analysis of the powder obtained after drying the crosslinked polymer salt at 80°C under reduced pressure for 3 hours, and then determining the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0054] <Particle size of the present crosslinked polymer salt> In the present composition, it is preferable that the present crosslinked polymer salt is not present as large particle size agglomerates (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, because this allows a binder containing the crosslinked polymer salt to exhibit good binding performance.

[0055] The crosslinked polymer preferably has a volume-based median particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less when dispersed in water and the degree of neutralization based on the carboxyl groups of the crosslinked polymer is 80 to 100 mol%. A more preferred range for the particle size is 0.15 μm or more and 8.0 μm or less, an even more preferred range is 0.20 μm or more and 6.0 μm or less, an even more preferred range is 0.25 μm or more and 4.0 μm or less, and an even more preferred range is 0.30 μm or more and 2.0 μm or less. When the particle size is in the range of 0.30 μm or more and 2.0 μm or less, the particles are uniformly present in the composition at a suitable size, thereby enabling the composition to exhibit high stability and excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk of insufficient binding properties, as described above. Furthermore, it is difficult to obtain a smooth coating surface, which may result in insufficient coatability. On the other hand, if the particle size is less than 0.1 μm, there is a concern in terms of stable production.

[0056] 3. Method for Producing the Crosslinked Polymer: The crosslinked polymer can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization. However, from the perspective of productivity, precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred because they provide better performance in terms of binding properties, among which precipitation polymerization is more preferred. Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomer but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of tens to hundreds of nanometers in size have secondary aggregation to several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Furthermore, the secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. Precipitation polymerization with suppressed secondary aggregation is generally also called dispersion polymerization.

[0057] In the case of precipitation polymerization, the polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer to be used, etc. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

[0058] Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These solvents can be used alone or in combination. Alternatively, they can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water of greater than 10 g / 100 ml at 20°C. Of the above, methyl ethyl ketone and acetonitrile are preferred in terms of the following: they provide good polymerization stability with minimal generation of coarse particles and adhesion to the reactor; the precipitated polymer fine particles are less likely to undergo secondary aggregation (or, even if secondary aggregation does occur, they are easily disintegrated in an aqueous medium); they produce polymers with a small chain transfer constant and a large degree of polymerization (primary chain length); and they are easy to handle during the neutralization step described below.

[0059] The polymerization initiator may be any known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted so as to generate an appropriate amount of radicals by known methods such as thermal initiation, redox initiation using a reducing agent, or UV initiation. To obtain a crosslinked polymer with a long primary chain length, it is preferable to set the conditions so as to generate as little radicals as possible within the allowable production time range.

[0060] A preferred amount of the polymerization initiator used is, for example, 0.001 to 2 parts by mass, or for example, 0.005 to 1 part by mass, or for example, 0.01 to 0.1 parts by mass, when the total amount of the monomer components used is 100 parts by mass. If the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, a polymer having a long primary chain length is likely to be obtained.

[0061] The polymerization temperature varies depending on conditions such as the type and concentration of the monomers used, but is preferably 0 to 100°C, more preferably 20 to 80°C. The polymerization temperature may be constant or may vary over the course of the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

[0062] Here, the present crosslinked polymer may contain 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)) relative to all structural units thereof, and the preferred range of the content of the structural units is as described above. The type of monomer (a) is as described above.

[0063] 4. The Non-Crosslinked Polymer The non-crosslinked polymer may have a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a2)"). A monomer component containing the component (a2) can be introduced into the polymer by a known polymerization method. The component (a2) is the same as that described for the component (a1) of the crosslinked polymer. <Structural Units Derived from Ethylenically Unsaturated Carboxylic Acid Monomer> The content of the component (a2) in the non-crosslinked polymer may be 50% by mass or more and 100% by mass or less, based on the total structural units of the non-crosslinked polymer. By containing the component (a2) in this range, excellent adhesion to the current collector can be easily ensured, and a high capacity retention rate can be achieved while suppressing electrode expansion and contraction due to charging and discharging of the secondary battery. A lower limit of 50% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. The lower limit may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, for example, 95% by mass or less, for example, 90% by mass or less, or for example, 80% by mass or less.

[0064] <Other Structural Units> In addition to the component (a2), the present non-crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (b2)"). The component (b2) and the method for introducing it are the same as those described for the component (b1) of the present crosslinked polymer. The proportion of the component (b2) can be 0% by mass or more and 50% by mass or less, based on the total structural units of the present polymer. The proportion of the component (b2) may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 20% by mass or less.

[0065] Regarding the Characteristics of the Non-Crosslinked Polymer Salt: The non-crosslinked polymer is used in the form of a salt, in which acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization in the composition is 50 mol% or higher. By achieving a degree of neutralization of 50 mol% or higher and increasing the glass transition temperature of the carboxyl group-containing non-crosslinked polymer, fusion and aggregation of the polymer due to heating when applying an electrode mixture layer composition containing the polymer to the current collector surface and drying it can be suppressed. This is thought to maintain a uniform structure in the electrode and further enhance the effect of conductive path formation by the addition of CNTs. The degree of neutralization is more preferably 60 mol% or higher, even more preferably 70 mol% or higher, even more preferably 75 mol% or higher, even more preferably 80 mol% or higher, and particularly preferably 85 mol% or higher, in order to achieve a superior charge / discharge capacity retention rate over longer periods of use than conventional methods. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. In this specification, the degree of neutralization can be calculated from the amounts of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization can be confirmed by measuring the powder of the crosslinked polymer salt after drying it at 80°C under reduced pressure for 3 hours, and then measuring the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0066] The weight-average molecular weight (Mw) of the non-crosslinked polymer or its salt is not particularly limited, but is preferably 5,000 or more, more preferably 10,000 or more, in terms of excellent electrode slurry coatability and superior charge / discharge capacity retention over longer periods of use than conventional polymers. Mw may be 100,000 or more, 500,000 or more, or even 1,000,000 or more. The upper limit of Mw is also not particularly limited, but from the viewpoint of handling during production, it may be, for example, 10,000,000 or less, or 5,000,000 or less. In this specification, Mw can be measured by the GPC method described in the Examples.

[0067] 5. Method for Producing the Non-Crosslinked Polymer: The method for producing the non-crosslinked polymer can be any known polymerization method (e.g., solution polymerization, precipitation polymerization, suspension polymerization, emulsion polymerization, etc.), and can be appropriately selected based on the molecular weight, composition, etc. The polymerization initiator can be, but is not limited to, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides. The conditions for use can be adjusted to generate an appropriate amount of radicals using known methods such as thermal initiation, redox initiation using a reducing agent, and UV initiation. Furthermore, known chain transfer agents may be used as needed for purposes such as molecular weight adjustment. The non-crosslinked polymer can contain 50% by mass or more and 100% by mass or less of an ethylenically unsaturated carboxylic acid monomer (monomer (a2)) relative to its total structural units, and the preferred range for the content of this structural unit is as described above. The type of monomer (a2) is as described above.

[0068] 6. The Binder The binder contains the crosslinked polymer or a salt thereof and the non-crosslinked polymer or a salt thereof. The content of the crosslinked polymer or a salt thereof is preferably 10% by mass or more and 90% by mass or less, and more preferably 20% by mass or more and 80% by mass or less, relative to the total amount of the crosslinked polymer or a salt thereof and the non-crosslinked polymer or a salt thereof, from the viewpoint of suppressing electrode expansion and contraction due to charging and discharging of the secondary battery while exhibiting a high capacity retention rate. In particular, the content of the crosslinked polymer or a salt thereof is more preferably 30% by mass or more and 70% by mass or less, even more preferably 40% by mass or more and 60% by mass or less, and even more preferably 45% by mass or more and 50% by mass or less, relative to the total amount of the crosslinked polymer or a salt thereof and the non-crosslinked polymer or a salt thereof.

[0069] 7. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention contains the binder, carbon nanotubes (CNTs), an active material, and water. The amount of the binder used in the composition is preferably 0.5 parts by mass or more and 7.0 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The amount used is, for example, 0.8 parts by mass or more and 3.0 parts by mass or less, such as 1.0 parts by mass or more and 2.5 parts by mass or less, or for example, 1.2 parts by mass or more and 1.5 parts by mass or less. When the amount of the binder used is 0.5 parts by mass or more, sufficient binding strength can be obtained. Furthermore, dispersion stability of the active material and the like can be ensured, allowing the formation of a uniform mixture layer. When the amount of the binder used is 1.5 parts by mass or less, the composition does not become highly viscous, ensuring coatability to the current collector. As a result, a mixture layer with a uniform and smooth surface can be formed. This is presumably due to reduced entanglement of the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer salt in the electrode slurry. Furthermore, while suppressing electrode expansion and contraction due to charge and discharge of the secondary battery, superior charge and discharge capacity retention can be achieved over longer periods of use than in the past. This is presumably due to the fact that the brittleness of the binder has a smaller effect on electrode properties, making it less likely for deterioration to occur due to stress from swelling and contraction of the active material during repeated charge and discharge.

[0070] The amount of CNT used in this composition is, for example, 0.01 parts by mass or more and 0.5 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The content is, for example, 0.05 parts by mass or more and 0.3 parts by mass or less, and also, for example, 0.1 parts by mass or more and 0.2 parts by mass or less. When the CNT content is 0.01 parts by mass or more, sufficient conductive paths are formed in many places, and capacity is less likely to deteriorate. When the CNT content is 0.2 parts by mass or less, entanglement between the binder and conductive additive in the electrode slurry is reduced, the composition does not become highly viscous, and coatability to the current collector can be ensured. Furthermore, aggregation of CNTs does not occur, and as a result, an electrode with a uniform and smooth surface can be formed.

[0071] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCMs {Li(Ni ｘ , Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b )}, etc. Examples of spinel-type positive electrode active materials include lithium manganate, etc. In addition to oxides, phosphates, silicates, sulfur, etc. are also used, and examples of phosphates include olivine-type lithium iron phosphate, etc. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0072] When a positive electrode active material containing a layered rock salt-type lithium-containing metal oxide is dispersed in water, the lithium ions on the active material surface are exchanged with hydrogen ions in the water, resulting in an alkaline dispersion. This can potentially corrode aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali elution from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Furthermore, it is preferable to use an amount of the unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

[0073] Because all positive electrode active materials have low electrical conductivity, a conductive additive other than CNT may be added. Examples of such conductive additives include carbon-based materials such as carbon black, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive other than CNT used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total active material, from the viewpoint of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0074] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the standpoint of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, and from 5 to 15 μm. In addition, to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. The amount of silicon-based active material used is preferably 5.0 mass% or more of the total amount of active material, from the viewpoint of improving the electrical capacity of the secondary battery, and can be, for example, 10.0 mass% or more, or can be, for example, 20.0 mass% or more.

[0075] Since the carbon-based active material itself has good electrical conductivity, it is not necessary to add a conductive additive other than CNT. When a conductive additive is added for the purpose of further reducing resistance, etc., from the viewpoint of energy density, the amount used is, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less, relative to 100 parts by mass of the total amount of the active material.

[0076] When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass %, or, for example, in the range of 30 to 65 mass %, based on the total amount of the composition. If the amount of active material used is 10 mass % or more, migration of binders and the like is suppressed, and this is also advantageous in terms of the cost of drying the medium. On the other hand, if the amount is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0077] The present composition uses water as a medium. Furthermore, for the purpose of adjusting the properties and drying properties of the present composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0078] When the present composition is made into a coatable slurry state, the content of the water-containing medium in the entire present composition can be, for example, in the range of 25 to 60 mass %, and can also be, for example, 35 to 60 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0079] The present composition may further contain other binder components, such as styrene butadiene rubber (SBR)-based latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount thereof may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, resistance increases, and high-rate characteristics may become insufficient. Among the above, SBR-based latex and CMC are preferred in terms of their excellent balance of binding strength and flex resistance, and a combination of SBR-based latex and CMC is more preferred.

[0080] The SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, primarily from the viewpoint of binding properties. Examples of the aliphatic conjugated diene monomer include, for example, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70% by mass, or, for example, in the range of 40 to 60% by mass, in order to improve the binding properties of the binder and the flexibility of the resulting electrode. In addition to the above-mentioned monomers, the styrene / butadiene latex may also contain other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding properties. The structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, or, for example, in the range of 0 to 20% by mass.

[0081] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0082] The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned binder, CNT, active material, and water as essential components, and is obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and known methods can be used. However, a preferred method is to dry-blend powder components such as the active material, conductive additive, and binder, then mix them with a dispersion medium such as water and disperse and knead them. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, a thin film gyratory mixer is preferred because it can achieve a good dispersion state in a short time. Furthermore, when using a thin film gyratory mixer, it is preferable to perform pre-dispersion beforehand using a stirrer such as a disperser. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5. For example, when CMC is added, it is more preferable that it be less than 11.5, and even more preferable that it be less than 10.5, in order to reduce the risk of hydrolysis. The viscosity of the slurry is not particularly limited as long as it exhibits the effects of the present invention, but the viscosity may be, for example, in the range of 100 to 30,000 mPa s, or, for example, 500 to 20,000 mPa s, or, for example, 1,000 to 10,000 mPa s, as Brookfield viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0083] 8. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for a secondary battery electrode mixture layer of the present invention on the surface of a current collector made of copper, aluminum, or the like. The mixture layer is formed by applying the composition to the surface of the current collector and then drying to remove the medium, such as water. The method for applying the composition is not particularly limited, and known methods such as doctor blade coating, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. The drying can also be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. The mixture layer obtained after drying is usually subjected to a compression treatment using a mold press, roll press, or the like. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

[0084] 9. Secondary Battery A secondary battery can be produced by providing the secondary battery electrode of the present invention with a separator and an electrolyte. The electrolyte may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0085] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0086] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified.

[0087] <<Production of the Present Crosslinked Polymer Salt>> (Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. The reactor was charged with 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.90 parts of trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% relative to the AA. After thoroughly replacing the atmosphere inside the reactor with nitrogen, the reactor was heated to raise the internal temperature to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. The polymerization reaction was continued while maintaining the internal temperature at 55°C by adjusting the external temperature (water bath temperature). 24 hours after the start of polymerization, cooling of the reaction solution was started. After the internal temperature had dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H ２ After the addition, stirring was continued at room temperature for 12 hours to obtain a polymerization reaction solution in the form of a slurry in which particles of the carboxyl group-containing crosslinked polymer salt R-1 (Li salt, degree of neutralization 90 mol%) were dispersed in the medium.

[0088] The resulting polymerization reaction solution was centrifuged to precipitate the polymer particles, and the supernatant was then removed. The precipitate was then redispersed in acetonitrile of the same weight as the polymerization reaction solution, followed by a washing procedure of centrifuging to precipitate the polymer particles and removing the supernatant, which was repeated twice. The precipitate was collected and dried at 80°C for 3 hours under reduced pressure to remove the volatiles, yielding a powder of carboxyl group-containing polymer salt R-1. Because crosslinked polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The powder of crosslinked polymer salt R-1 was subjected to IR analysis, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O of the carboxylic acid Li, resulting in a value of 90 mol%, equal to the calculated value from the charge.

[0089] (Production Examples 2 to 5: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-5) The same operation as in Production Example 1 was carried out, except that the amounts of monomer, crosslinkable monomer, and neutralizing agent charged were as shown in Table 1, to obtain polymerization reaction solutions containing carboxyl group-containing crosslinked polymer salts R-2 to R-5. Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution, to obtain powdery crosslinked polymer salts R-2 to R-5. Each carboxyl group-containing crosslinked polymer salt was stored in a sealed container with water vapor barrier properties.

[0090]

[0091] Details of the compounds used in Table 1 are shown below: AA: acrylic acid HEA: 2-hydroxyethyl acrylate T-20: trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20") TEA: triethylamine AcN: acetonitrile V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") LiOH.H ２ O: Lithium hydroxide monohydrate

[0092] <<Production of the Present Non-Crosslinked Polymer Salt>> <Molecular Weight Measurement of Carboxyl Group-Containing Non-Crosslinked Polymer> 0.1 g of an aqueous solution containing a carboxyl group-containing non-crosslinked polymer (0.01 g as polymer solids) was collected and diluted with 40 g of 0.1 M sodium nitrate aqueous solution to obtain a measurement sample. The measurement sample was subjected to measurement by gel permeation chromatography (GPC) under the conditions described below to obtain the weight average molecular weight (Mw) calculated as sodium polyacrylate. (GPC Measurement Conditions) Column: 2 TSKgel GMPW columns manufactured by Tosoh Corporation Solvent: 0.1 M sodium nitrate aqueous solution Temperature: 40°C Detector: RI Flow rate: 0.5 mL / min

[0093] (Production Example 6: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salt R-6) Lithium hydroxide monohydrate in an amount equivalent to 90 mol % of the carboxylic acid and ion-exchanged water were added to a 40 mass % aqueous solution of polyacrylic acid (manufactured by Toagosei Co., Ltd., product name "Aron A-10SL", Mw 6,000) to obtain a 15 mass % aqueous solution of lithium polyacrylate containing carboxyl group-containing non-crosslinked polymer salt R-6. "Aron" is a registered trademark of Toagosei Co., Ltd.

[0094] (Production Example 7: Production of carboxyl group-containing non-crosslinked polymer salt R-7) The same procedure as in Production Example 6 was carried out, except that the polyacrylic acid was changed (manufactured by Toagosei Co., Ltd., product name "Jurymer AC-10H", Mw 800,000), to obtain a 15% by mass aqueous solution containing carboxyl group-containing non-crosslinked polymer salt R-7. "Jurymer" is a registered trademark of Toagosei Co., Ltd.

[0095]

[0096] Details of the compounds used in Table 2 are as follows: A-10SL: Manufactured by Toagosei Co., Ltd., product name "Aron A-10SL" AC-10H: Manufactured by Toagosei Co., Ltd., product name "Jurimer AC-10H" LiOH·H ２ O: Lithium hydroxide monohydrate

[0097] <Evaluation of Coatability of Electrode Slurry> The electrode slurries obtained in the following examples and comparative examples were adjusted to 25°C ± 1°C, and then the viscosity was measured at 20 rpm using a Brookfield viscometer, and the coatability was evaluated based on the following criteria: (Criteria for Coatability) A: Less than 15,000 mPa s B: 15,000 or more and less than 20,000 mPa s C: 20,000 or more and less than 25,000 mPa s D: 25,000 mPa s or more

[0098] Example 1 (Preparation of electrode mixture layer composition (electrode slurry)) Artificial graphite (manufactured by Showa Denko K.K., trade name "SCMG-CF") and SiO (manufactured by Osaka Titanium Technologies Co., Ltd., 5 μm) were used as the active material. A mixture of cross-linked polymer salt R-1, non-cross-linked polymer salt R-6, styrene / butadiene latex (SBR), and sodium carboxymethyl cellulose (CMC) was used as the binder. Single-walled CNT (manufactured by OCSiAl, trade name "TuballBATT H") was used as the conductive additive. ２ O" (solvent: water, single-walled CNT content: 0.4% by mass) was used. A planetary mixer (Hibismix 2P-03 manufactured by Primix Corporation) was used to dilute the electrode mixture layer composition so that the solids concentration of the composition was 50% by mass. Water was used as a dilution solvent, and the weight ratio of artificial graphite: SiO: cross-linked polymer salt R-1: non-cross-linked polymer salt R-6: SBR: CMC: single-walled CNT was added at a mass ratio of 77.6: 19.4: 0.5: 0.5: 2.0: 1.0: 0.1 (solids), and the mixture was mixed for 1 hour and 30 minutes to prepare a slurry-state electrode mixture layer composition (electrode slurry). The viscosity of the electrode slurry was 14,500 mPa s, and the coatability based on the above criteria was evaluated as "A".

[0099] (Preparation of negative electrode plate) Next, the electrode slurry was applied onto a 16.5 μm thick current collector (copper foil) using a variable applicator, and dried in a forced-air dryer at 80° C. for 15 minutes to form a mixture layer. After that, the thickness of the mixture layer was 50±5 μm and the mixture density was 1.60±0.10 g / cm. ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a negative electrode plate for battery evaluation.

[0100] (Preparation of Positive Electrode Plate) In N-methylpyrrolidone (NMP) solvent, LiNi was used as a positive electrode active material. ０．５ Co ０．２ Mn ０．３ O ２ A mixture of 100 parts of ethylenediamine fluoride (NCM) and 2 parts of acetylene black was added to the mixture, and 4 parts of polyvinylidene fluoride (PVDF) was added as a positive electrode binder to prepare a positive electrode composite layer composition. The positive electrode composite layer composition was applied to an aluminum current collector (thickness: 20 μm) and dried to form a composite layer. Thereafter, the thickness of the composite layer was 125 μm, and the composite density was 3.0 g / cm. ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a positive electrode plate for battery evaluation.

[0101] (Preparation of Electrolyte Solution) Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7) so that the concentration of VC was 1 mass % and fluoroethylene carbonate (FEC) was 2 mass %, and LiPF ６ was dissolved in a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0102] (Preparation of Secondary Battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrodes facing each other via a separator (made of polyethylene: film thickness 16 μm, porosity 47%), placing the electrode assembly in an aluminum laminate battery exterior, injecting the electrolyte, and sealing it to prepare a test battery. The design capacity of this prototype battery was 50 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

[0103] <Evaluation of cycle characteristics> The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge / discharge rate of 0.1 C under conditions of 2.5 to 4.2 V in a CC charge / discharge environment of 45°C. ０ Furthermore, charging and discharging were repeated in an environment of 45°C at a charge / discharge rate of 0.5C under conditions of 2.5 to 4.2V by CC charging and discharging, and the capacity C １００ and the capacity C after 300 cycles ３００The cycle characteristics (ΔC) were calculated using the following formula: Charge / discharge capacity retention rate ΔC after 100 cycles １００ =C １００ / C ０ × 100 (%) Charge / discharge capacity retention rate ΔC after 300 cycles ３００ =C ３００ / C ０ × 100 (%) ΔC calculated by the above formula １００ The cycle characteristic was evaluated as "A" based on the following criteria. ３００ The cycle characteristic was evaluated as "A" based on the following criteria. A higher ΔC value indicates better cycle characteristics. １００ (Criteria for determining the cycle characteristics ΔC) A: Charge / discharge capacity retention rate is 90% or more B: Charge / discharge capacity retention rate is 80% or more and less than 90% C: Charge / discharge capacity retention rate is 70% or more and less than 80% D: Charge / discharge capacity retention rate is less than 70% ３００ Judgment criteria) A: Charge / discharge capacity retention rate is 80% or more B: Charge / discharge capacity retention rate is 70% or more and less than 80% C: Charge / discharge capacity retention rate is 60% or more and less than 70% D: Charge / discharge capacity retention rate is less than 60%

[0104] <Evaluation of Electrode Expansion Rate> The laminated cell lithium-ion secondary battery that had undergone the 300-cycle test described above was disassembled to recover the negative electrodes. Each recovered negative electrode was washed with DMC (dimethyl carbonate) solvent and allowed to air-dry at room temperature for one day, after which its thickness was measured. The measured thickness was then substituted into the following formula to calculate the negative electrode expansion rate: [Electrode Expansion Rate (%)] = 100 × {(Thickness of discharged negative electrode of battery) - (Thickness of copper foil)} / {(Thickness of negative electrode before assembly) - (Thickness of copper foil)} The electrode expansion rate calculated using the above formula was 156%, and this expansion rate was evaluated as "A" based on the following criteria. Note that a lower electrode expansion rate indicates a better electrode expansion suppression effect and better battery performance. (Criteria for determining electrode expansion rate) A: The electrode expansion rate is less than 160%. B: The electrode expansion rate is 160% or more and less than 180%. C: The electrode expansion rate is 180% or more and less than 200%. D: The electrode expansion rate is 200% or more.

[0105] Examples 2 to 11 and Comparative Examples 1 and 2 Electrode slurries were prepared in the same manner as in Example 1, except that the formulations were used as shown in Table 3, and the coating properties were evaluated. In addition, the cycle characteristics (ΔC １００、 ΔC ３００ The results of these evaluations are shown in Table 3.

[0106]

[0107] Details of the compounds used in Table 3 are shown below: SBR: styrene butadiene rubber CMC: sodium carboxymethyl cellulose

[0108] <Evaluation Results> As is clear from the results of Examples 1 to 11, the composition for secondary battery electrode mixture layer (electrode slurry) containing the electrode binder of the present invention has excellent coatability, and secondary batteries equipped with electrodes obtained using the composition not only exhibited excellent capacity retention after 100 cycles, but also showed high capacity retention even after use at a high charge / discharge count of 300 cycles. Furthermore, the effect of suppressing electrode expansion after testing was also observed. Among these, focusing on the content of carboxyl group-containing crosslinked polymer salt in the binder, when the content of carboxyl group-containing crosslinked polymer salt relative to the total amount of carboxyl group-containing crosslinked polymer salt and carboxyl group-containing non-crosslinked polymer salt was 80% by mass (Example 4), the viscosity of the electrode slurry was 21,300 mPa·s, ΔC ３００ When the content of the carboxyl group-containing crosslinked polymer was 20% by mass (Example 5), the electrode slurry viscosity was 10,500 mPa·s, and ΔC ３００ The electrode expansion rate was 66.5%, and the electrode expansion rate was 178%. These results show that when the content of the carboxyl group-containing crosslinked polymer salt relative to the total amount of the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer salt was 50% by mass (Example 1), the coating property was superior to when it was 80% by mass (Example 4), and the capacity retention rate after 300 cycles and the electrode expansion suppression effect were even superior to when it was 20% by mass (Example 5).

[0109] In contrast, when no carboxyl group-containing non-crosslinked polymer salt was included (Comparative Example 1), the viscosity of the electrode slurry was 28,800 mPa·s, and ΔC ３００ The viscosity of the electrode slurry was 8,930 mPa·s and the expansion rate of the electrode was 150%, resulting in poor coating properties. ３００ The capacity retention rate after 300 cycles was 57.2%, and the expansion rate of the electrode was 214%, which resulted in a deterioration in the capacity retention rate after 300 cycles and the expansion suppression effect of the electrode.

[0110] Next, focusing on the carbon nanotube content, when the carbon nanotube content was 0.1 parts by mass per 100 parts by mass of the total active material (Example 1), the capacity retention rate at 300 cycles and the electrode expansion suppression effect were better than when the carbon nanotube content was 0.05 parts by mass (Example 2). This is thought to be due to the effect that the greater the amount of carbon nanotubes with a high aspect ratio structure used, the more conductive paths formed, and the less capacity degradation occurs, in response to the increase in the distance between the active materials due to expansion of the negative electrode caused by repeated charge and discharge. Furthermore, compared to when the carbon nanotubes were 0.2 parts by mass (Example 3), the results showed excellent coatability. This is thought to be due to the effect that the smaller the amount of high aspect ratio carbon nanotubes used, the less entanglement between the binder and conductive additive in the electrode slurry, thereby reducing the viscosity of the electrode slurry.

[0111] Furthermore, when focusing on the content of the binder containing the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer salt, the results showed that the coating properties, the capacity retention rate after 300 cycles, and the effect of suppressing electrode expansion were superior when the content was 1.0 part by mass (Example 1) compared to when the content was 2.1 parts by mass (Example 6) relative to 100 parts by mass of the total amount of active material. This is thought to be because the lower the content of the binder relative to 100 parts by mass of the total amount of active material, the less entanglement between the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer in the electrode slurry, reducing the viscosity of the electrode slurry and, in addition, the effect of the brittleness of the binder on the electrode properties was reduced, making it less likely that deterioration would occur due to the stress of swelling and shrinkage of the active material during repeated charge and discharge.

[0112] Compared with the case where the molecular weight of the carboxyl group-containing non-crosslinked polymer salt was high (Example 7), the case where the molecular weight of the polymer salt was low (Example 1) resulted in superior coatability, which is thought to be due to the fact that the lower the molecular weight of the polymer salt, the less entanglement of binders in the electrode slurry.

[0113] Next, focusing on the degree of neutralization of the carboxyl group-containing crosslinked polymer, the results showed that when the degree of neutralization of the polymer was 60 mol% (Example 9), the degree of neutralization of the polymer was 80 mol% (Example 8) and the capacity retention rate at 300 cycles were superior. Regarding the coatability, the higher the degree of neutralization of the carboxyl group-containing crosslinked polymer, the smaller the electrostatic repulsion within the particles. Therefore, when the polymer swells in water, the spreading of the side chains is suppressed, resulting in less entanglement in the electrode slurry. Regarding the capacity retention rate after 300 cycles, the higher the neutralization rate of the polymer, the higher the glass transition point, which suppresses fusion of the polymer during the heat drying step in the electrode production process, thereby enabling the production of a uniform electrode.

[0114] Furthermore, compared to Example 10, where the carboxyl group-containing crosslinked polymer contained a low content of structural units derived from the crosslinkable monomer (i.e., where the crosslinking degree of the crosslinked polymer was low), the results showed that the coating properties and electrode expansion suppression effect were even better when the content of the structural units was high (i.e., where the crosslinking degree of the crosslinked polymer was high) (Example 1). The coating properties were thought to be due to the fact that an increased crosslinking degree of the carboxyl group-containing crosslinked polymer reduced the degree of water swelling in the electrode slurry, and the reduced volume fraction of the polymer reduced the frequency of entanglement between binders. The electrode expansion rate was also thought to be due to the fact that a decreased water swelling degree of the carboxyl group-containing crosslinked polymer increased the binder density per contact point with the active material, slightly improving adhesion, thereby suppressing deterioration due to swelling and shrinkage of the active material caused by repeated charging and discharging.

[0115] Focusing on the content of structural units derived from ethylenically unsaturated carboxylic acid monomers, the results showed that the battery characteristics (capacity retention rate at 300 cycles and electrode expansion suppression effect) were better when the content in the carboxyl group-containing crosslinked polymer was 100% by mass (Example 1) than when the content was 80% by mass (Example 11). This is thought to be because the higher the content of carboxyl groups in the crosslinked polymer, the better the binding with the active material, and therefore the more likely it is that destruction of the electrode structure can be suppressed even during long-term battery use.

[0116] The electrode binder of the present invention can reduce the viscosity of electrode slurry, and secondary batteries equipped with electrodes obtained using the binder exhibit high capacity retention rates while suppressing expansion and contraction due to charge and discharge over longer periods of use than conventional batteries, even when using silicon-based active materials that undergo large expansion and contraction due to charge and discharge. Therefore, the electrode binder is expected to contribute to increasing the capacity of automotive secondary batteries, etc. The electrode binder of the present invention can be particularly suitably used in non-aqueous electrolyte secondary batteries, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. An electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes, the electrode binder comprising a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer or a salt thereof.

2. The electrode binder according to claim 1, wherein the active material of the secondary battery electrode comprises a silicon-based active material.

3. The electrode binder according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to all of its structural units, and the carboxyl group-containing non-crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to all of its structural units.

4. The electrode binder according to claim 1 or 2, wherein the content of the carboxyl group-containing crosslinked polymer or its salt is 10 mass % or more and 90 mass % or less based on the total amount of the carboxyl group-containing crosslinked polymer or its salt and the carboxyl group-containing non-crosslinked polymer or its salt.

5. A composition for a secondary battery electrode mixture layer, comprising the electrode binder according to claim 1, carbon nanotubes, an active material and water.

6. A composition for a secondary battery electrode mixture layer as described in claim 5, wherein the content of the silicon-based active material is 5.0 mass% or more based on the total amount of the active material.

7. A composition for a secondary battery electrode mixture layer as described in claim 5 or 6, wherein the content of the electrode binder is 1.0 part by mass or more and 2.5 parts by mass or less per 100 parts by mass of the total amount of the active material.

8. The composition for secondary battery electrode mixture layer according to claim 5 or 6, wherein the content of the carbon nanotubes is 0.1 parts by mass or more per 100 parts by mass of the total amount of the active material.

9. The secondary battery electrode mixture layer composition according to claim 5 or 6, wherein the carbon nanotubes include a single-wall structure.

10. A secondary battery electrode comprising a current collector having a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 5 or 6 on a surface of the current collector.

11. A secondary battery comprising the secondary battery electrode according to claim 10.

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