WO2024214639A1

WO2024214639A1 - Binder for electrode of secondary battery provided with secondary battery electrode containing carbon nanotubes, and use thereof

Info

Publication number: WO2024214639A1
Application number: PCT/JP2024/014033
Authority: WO
Inventors: 萌衣内山; 直彦斎藤; 剛史長谷川
Original assignee: 東亞合成株式会社
Priority date: 2023-04-10
Filing date: 2024-04-05
Publication date: 2024-10-17

Abstract

The present invention provides a binder for a secondary battery electrode capable of exhibiting high capacity retention while suppressing expansion and contraction caused by charging and discharging over a longer than period of use than existing binders even when using a silicon-based active material known to undergo significant expansion and contraction caused by charging and discharging. The present invention also provides: a composition for a secondary battery electrode composite layer containing the abovementioned binder; a secondary battery electrode obtained using said composition; and a secondary battery.　This binder for a an electrode of a secondary battery provided with a secondary battery electrode containing carbon nanotubes includes a carboxyl group-containing crosslinked polymer or a salt thereof.

Description

Binder for secondary battery with secondary battery electrode containing carbon nanotubes and use thereof

The present invention relates to an electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes, and its use.

Various types of secondary batteries, such as nickel-hydrogen secondary batteries, lithium-ion secondary batteries, and electric double-layer capacitors, have been put to practical use. The 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, etc., to a current collector and drying it. 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 used in the negative electrode mixture layer composition. In addition, a binder containing an aqueous solution or aqueous dispersion of an acrylic acid-based polymer is known as a binder with excellent dispersibility and binding properties. On the other hand, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used as the binder used in the positive electrode mixture layer.

In recent years, as the applications of various secondary batteries expand, 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, there has been an increase in the use of silicon-based active materials as negative electrode active materials. However, silicon-based active materials are known to have large volume changes during charging and discharging, and repeated use can cause the electrode mixture layer to peel or fall off, resulting in a decrease in battery capacity and a deterioration in cycle characteristics (durability). In order to prevent such problems, it is generally effective to increase the binding properties of the binder, and studies are being conducted to improve the binding properties of binders in order to improve durability.

A binder using the above-mentioned acrylic acid polymer has been proposed as a binder that has good binding properties and is effective in improving durability, and it is known that adding a conductive assistant to the slurry used to manufacture the negative electrode improves electrical contact between the active materials and improves battery performance.

For example, Patent Documents 1 and 2 disclose that an electrode obtained from an electrode mixture layer composition containing a silicon-containing active material, a conductive assistant, and a binder containing a monomer unit derived from acrylic acid and a monomer unit of a specific structure has excellent charge/discharge capacity. It is specifically described that the use of ketjen black or acetylene black as the conductive assistant results in a high capacity retention rate after 10 or 30 charge/discharge cycles.

International Publication No. 2015/163302 JP 2018-029069 A

The binders disclosed in Patent Documents 1 and 2 are both capable of imparting good binding properties, and as described above, can achieve a high capacity retention rate if the number of charge/discharge cycles is as small as 10 to 30. However, with each charge/discharge cycle, the structure of the negative electrode deteriorates significantly due to the expansion and contraction of the silicon-based active material, making it impossible to maintain a conductive path between the active materials, resulting in a problem of capacity deterioration.

The present invention was made in consideration of these circumstances, and aims to provide a binder for secondary battery electrodes that can suppress expansion and contraction due to charging and discharging and exhibit a high capacity retention rate over longer periods of use than before, even when using silicon-based active materials that are considered to expand and contract significantly due to charging and discharging. In addition, the present invention also aims to provide a composition for secondary battery electrode mixture layers that contains the binder, and a secondary battery electrode and secondary battery obtained using the composition.

As a result of intensive research into solving the above problems, the inventors discovered that by using a binder containing a carboxyl group-containing 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 "CNT") as a conductive additive, it is possible to suppress expansion and contraction due to charging and discharging, and to exhibit an excellent charge and discharge capacity retention rate, even when the secondary battery is used at a higher number of cycles than before, and thus completed the present invention.

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 comprising a carboxyl group-containing crosslinked polymer or a salt thereof.
[2] The electrode binder according to [1], wherein the carboxyl group-containing crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer based on the total structural units thereof.
[3] A composition for a secondary battery electrode mixture layer, comprising the electrode binder according to [1] or [2], carbon nanotubes, an active material, and water.
[4] The composition for secondary battery electrode mixture layer according to [3], wherein the content of the silicon-based active material is 5.0 mass% or more based on the total amount of the active material.
[5] The composition for secondary battery electrode mixture layer according to [3] or [4], wherein the content of the electrode binder is 1.0 part by mass or more and 2.5 parts by mass or less with respect to 100 parts by mass of a total amount of the active material.
[6] The composition for secondary battery electrode mixture layer according to any one of [3] to [5], wherein the content of the carbon nanotubes is 0.1 parts by mass or more per 100 parts by mass of a total amount of the active material.
[7] The composition for a secondary battery electrode mixture layer according to any one of [3] to [6], wherein the carbon nanotubes include a single-layer structure.
[8] A secondary electrode comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to any one of [3] to [7].
[9] A secondary battery comprising the secondary electrode according to [8].

The electrode binder of the present invention makes it possible to obtain a secondary battery that exhibits an excellent charge/discharge capacity retention rate while suppressing expansion and contraction due to charging and discharging during longer-term use than conventional batteries.

The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") contains 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 can be mixed with carbon nanotubes (hereinafter also referred to as "CNT"), an active material, and water to form a composition for secondary battery electrode mixture layer (hereinafter also referred to as "the composition"). The above composition is preferably an electrode slurry in a slurry state that can be applied to a current collector in terms of achieving the effects of the present invention, but it may be prepared in a wet powder state so that it can be applied to the current collector surface. The secondary battery electrode of the present invention is 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 preferable in that when it is used in a secondary battery electrode mixture layer composition containing a silicon-based active material described below as an active material, the effects of the present invention are particularly large.

The carbon nanotubes, the present crosslinked polymer and its production method, the secondary battery electrode mixture layer composition obtained using the present binder, the secondary battery electrode, and the secondary battery will each be described in detail below.
In this specification, "(meth)acrylic" means acrylic and/or methacrylic, "(meth)acrylate" means acrylate and/or methacrylate, and "(meth)acryloyl group" means acryloyl group and/or methacryloyl group.
In the numerical ranges described in stages in this specification, the upper or lower limit value described in one numerical range may be replaced by the upper or lower limit value of another numerical range described in stages, and the upper or lower limit value of that numerical range may be replaced by a value shown in the examples.

1. Carbon nanotubes The carbon nanotubes (CNTs) used in the composition have high electrical conductivity and chemical stability, and are used as a conductive assistant to ensure electrical contact between active materials in the electrode mixture layer. CNTs are flat graphite rolled into a cylindrical shape, and the type of CNT is not particularly limited, and single-layer CNTs (hereinafter also referred to as "single-layer CNTs") or multi-layer CNTs (hereinafter also referred to as "multi-layer CNTs") can be used. These can be used alone or in combination of two or more types.
As the CNT, it is preferable that the CNT contains a single-walled CNT, since the effect of the present invention is particularly large.
A single-walled CNT has a structure in which one layer of graphite is rolled up, whereas a multi-walled CNT has a structure in which two or more layers of graphite are rolled up.
Here, the sidewalls of the CNTs do not have to have a graphite structure, and CNTs having sidewalls with an amorphous structure are also included in the CNTs of this specification.

The shape of the CNT is not limited. Examples of such shapes include needle-like, cylindrical tube-like, fishbone-like (fishbone or cup stack type), trump-like (platelet) and coil-like. Of these, the shape of the CNT is preferably needle-like or cylindrical tube-like. The CNT may be a single shape or a combination of two or more shapes.

CNTs may have the following forms: graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. CNTs may have any of these forms alone or in combination of two or more of them.

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, then randomly selecting 300 CNTs from the photograph and measuring the outer diameter of each.

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.

The aspect ratio is the fiber length of a CNT divided by its outer diameter. A representative aspect ratio can be calculated using the average fiber length and the average outer diameter.
The higher the aspect ratio of the conductive additive, the higher the conductivity that can be obtained when an electrode is formed, and the aspect ratio of the CNT is preferably 30 or more, more preferably 50 or more, and even more preferably 80 or more. Also, it is preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less.

The specific surface area of the CNT is preferably 100 m ² /g or more, more preferably 150 m ² /g or more, and even more preferably 200 m ² /g or more. Also, it is preferably 1200 m ² /g or less, and more preferably 1000 m ² /g or less. The specific surface area of the CNT can be calculated by the BET method using nitrogen adsorption measurement.
When the average outer diameter, average fiber length, aspect ratio and specific surface area of the CNTs are within the above ranges, a well-developed conductive path is easily formed in the electrode.

The carbon purity of CNT is expressed as the content (mass%) of carbon atoms in the CNT. For 100 mass% 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. By keeping the carbon purity within the above range, it is possible to prevent problems such as short circuits caused by the formation of dendrites due to impurities such as metal catalysts.

The CNT may be surface-treated CNT. The CNT may be a CNT derivative to which a functional group, such as a carboxyl group, has been added. CNT that encapsulates organic compounds, metal atoms, or substances, such as fullerenes, may also be used.

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 contacting 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 a hydrocarbon and/or an alcohol.

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 (a)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing component (a).

<Structural Unit Derived from Ethylenically Unsaturated Carboxylic Acid Monomer>
The crosslinked polymer having a carboxyl group contained in the binder (hereinafter also referred to as the "crosslinked polymer") may have a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as the "component (a)"). When the crosslinked polymer has a carboxyl group due to the structural unit, the adhesion to the current collector is improved, and the desolvation effect of lithium ions and ion conductivity are excellent, so that an electrode having low resistance and excellent high-rate characteristics can be obtained. In addition, water swelling is imparted, so that the dispersion stability of the active material in the composition can be improved.
The component (a) can be introduced into the polymer by, for example, 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. In addition, it can be obtained by polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., and then treating them with a strong alkali, or by reacting a polymer having a hydroxyl group with an acid anhydride.

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

The content of component (a) in the crosslinked polymer may 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 (a) in this range, excellent adhesion to the current collector can be easily ensured. When the lower limit is 50% by mass or more, the dispersion stability of the composition is good and a higher binding strength can be obtained, which is preferable, and 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.

<Other structural units>
In addition to the component (a), the crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (b)"). Examples of the component (b) include structural units derived from hydroxyl-containing ethylenically unsaturated monomers (monomers represented by the following formula (1) and (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-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)
[In the formula, 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. In addition, R ³ represents 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.]
_CH2 =C( ^R5 ) ^CONR6R7 ( ² )
[In the formula, ^R5 represents a hydrogen atom or a methyl group, ^R6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and ^R7 represents a hydrogen atom or a monovalent organic group.]

The proportion of the (b) component can be 0% by mass or more and 50% by mass or less based on all structural units of the non-crosslinked polymer. The proportion of the (b) 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. In addition, when the (b) component is contained at 0.1% by mass or more based on all structural units of the crosslinked polymer, the affinity to the electrolyte is improved, and therefore the effect of improving lithium ion conductivity can also be expected.

Of the above-mentioned components, the hydroxyl-containing ethylenically unsaturated monomer is preferred as component (b) in that it provides excellent binding properties to the binder containing the present crosslinked polymer salt.
Moreover, from the viewpoint of obtaining an electrode having good resistance to bending, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred.
Examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, and alicyclic structure-containing ethylenically unsaturated monomers.

The monomer represented by the formula (1) is a (meth)acrylate compound having a hydroxyl group. When R ² is a monovalent organic group having 1 to 8 carbon atoms having a hydroxyl group, the number of the hydroxyl groups may be only one or may be two or more. The monovalent organic group is not particularly limited, and examples thereof include an alkyl group that may have a linear, branched or cyclic structure, an aryl group and an alkoxyalkyl group. When R ² is (R ³ O) _m H or R ⁴ O[CO(CH ₂ ) ₅ O] _n H, the alkylene group represented by R ³ or R ⁴ may be linear or branched.

Examples of monomers represented by the above 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, product names "Placcel FM1", "Placcel FM5", etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, product names "Placcel FA1", "Placcel FA10L", etc.), etc. The monomer represented by the above formula (1) may be used alone or in combination of two or more.

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. In formula (2), R ⁷ represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include an alkyl group that may have a linear, branched, or cyclic structure, as well as an aryl group and an alkoxyalkyl group, and is preferably an organic group having 1 to 8 carbon atoms. Alternatively, R ⁷ may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

Examples of the monomer represented by the above 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 the above formula (2) may be used alone or in combination of two or more.

Examples of (meth)acrylamide derivatives 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. One of these may be used alone, or two or more may be used in combination.

Examples of nitrile group-containing ethylenically unsaturated monomers 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. Among the above, acrylonitrile is preferred because of its high nitrile group content.

Examples of alicyclic structure-containing ethylenically unsaturated monomers 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. One of these may be used alone, or two or more may be used in combination.

In view of the excellent binding property 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 the component (b), structural units derived from the monomer represented by the above formula (1) and the monomer represented by the above formula (2) are more preferable in view of the excellent effect of improving the binding property of the present binder.
Among the monomers represented by the above formula (1), hydroxyalkyl (meth)acrylates having a hydroxyalkyl group with 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. Also, among the monomers represented by the above formula (2), (meth)acrylamide derivatives having a hydroxyalkyl group with 1 to 8 carbon atoms are more preferred, with N-hydroxyethyl (meth)acrylamide, N-(2-hydroxypropyl) (meth)acrylamide and N-hydroxybutyl (meth)acrylamide being even more preferred.

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 component (b), it is possible to achieve a strong interaction with the electrode material and to exhibit good binding properties to the active material. This makes it possible to obtain a robust electrode mixture layer with good integrity, and therefore, as the above-mentioned "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.

As other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. Examples of the (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;
Examples of the alkoxyalkyl ester compounds include (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more of them may be used in combination.

From the viewpoint of binding to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. From the viewpoint 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.

Among non-ionic 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 because they provide good binding strength for the binder. In addition, as non-ionic ethylenically unsaturated monomers, compounds whose homopolymer glass transition temperature (Tg) is 0°C or less are preferred because they provide good bending resistance for the resulting electrode.

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 Since 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 using copolymerization of a crosslinkable monomer is preferred from the viewpoints of simple operation and easy control of the degree of crosslinking.

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

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 easily produce a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

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; tri(meth)acrylates of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of trimethylolpropane ethylene oxide modified product, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and poly(meth)acrylates such as tetra(meth)acrylate; and bisamides such as methylene bisacrylamide and hydroxyethylene bisacrylamide.

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 polyallyl saccharose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

Examples of compounds 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.

Specific examples of the monomer having a self-crosslinkable crosslinking 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.

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, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyl dimethoxysilane, vinyl dimethyl methoxysilane, etc.; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyl dimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyl dimethoxysilylpropyl methacrylate, dimethyl methoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; silyl group-containing vinyl esters such as vinyl trimethoxysilyl undecanoate, etc. can be mentioned.

When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 parts by mass or more and 5.0 parts by mass or less, more preferably 0.05 parts by mass or more and 3.0 parts by mass or less, even more preferably 0.1 parts by mass or more and 2.0 parts by mass or less, even more preferably 0.1 parts by mass or more and 1.7 parts by mass or less, and even more preferably 0.5 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). If 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 charging and discharging during use for a longer period than before, and as a result, an excellent charge and discharge capacity retention rate can be exhibited, which is preferable. If it is 5.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be high. In particular, if the amount is 1.0 parts by mass or less, the water-swelling particle size in the electrode slurry becomes favorable, and the binding area to the active material becomes large, which is preferable in that excellent battery performance can be maintained even during long-term use.

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, even 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).

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 because they are less likely to adversely affect the battery characteristics, and alkali metal salts are more preferred.

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 ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization is 20 mol% or more in the composition. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilization effect is easily obtained.
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, and particularly preferably 85 mol% or more, in terms of being able to exhibit a superior charge/discharge capacity retention rate in long-term use compared to 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 by calculation from the charge amount 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 measurement of the powder of the crosslinked polymer salt after drying treatment at 80 ° C. for 3 hours under reduced pressure conditions, and the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O group of the carboxylate salt.

<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 lumps (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.

The crosslinked polymer preferably has a particle size (water-swollen particle size) in the volume-based median size range of 0.1 μm or more and 10.0 μm or less when the crosslinked polymer has a degree of neutralization based on the carboxyl groups of 80 to 100 mol% and is dispersed in water. A more preferred range of 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. If the particle size is in the range of 0.30 μm or more and 2.0 μm or less, the particles are present in the composition at a suitable size and uniformly, so that the composition is highly stable and can exhibit excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk that the binding properties will be insufficient as described above. In addition, it is difficult to obtain a smooth coating surface, and there is a risk that the coating properties will be insufficient. On the other hand, if the particle size is less than 0.1 μm, there are concerns from the perspective of stable production.

3. Method for Producing the Crosslinked Polymer The crosslinked polymer can be produced by known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) are preferred from the viewpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred from the viewpoint of obtaining better performance in terms of binding properties, etc., and among these, 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 larger through aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of several tens to hundreds of nm are secondary aggregated to several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

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 used, etc. In order to obtain a polymer with a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

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, and these can be used alone or in combination of two or more. Alternatively, these 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 at 20° C. of more than 10 g/100 ml.
Of the above, methyl ethyl ketone and acetonitrile are preferred because they have good polymerization stability with little generation of coarse particles and little adhesion to the reactor, the precipitated polymer fine particles are not prone to secondary aggregation (or even if secondary aggregation does occur, it is easily disintegrated in an aqueous medium), they give polymers with small chain transfer constants and large degrees of polymerization (primary chain lengths), and they are easy to operate during the neutralization step described below.

The polymerization initiator may be any known polymerization initiator such as an azo compound, organic peroxide, or inorganic peroxide, but is not particularly limited. The conditions of use can be adjusted so that an appropriate amount of radicals is generated using 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 that the amount of radicals generated is as small as possible within the allowable range of production time.

The preferred amount of 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 polymerization initiator used is 0.001 parts by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, a polymer with a long primary chain length is easily obtained.

The polymerization temperature depends on 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.

Here, the crosslinked polymer can contain 50% by mass or more and 100% by mass or less of ethylenically unsaturated carboxylic acid monomer. The type of ethylenically unsaturated carboxylic acid monomer is as described above.

4. Composition for secondary battery electrode mixture layer The composition for secondary battery electrode mixture layer of the present invention contains the present 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 with respect to 100 parts by mass of the total amount of the active material. The amount is, for example, 0.8 parts by mass or more and 3.0 parts by mass or less, for example, 1.0 parts by mass or more and 2.5 parts by mass or less, and for example, 1.2 parts by mass or more and 1.5 parts by mass or less. If the amount of the binder used is 0.5 parts by mass or more, sufficient binding property can be obtained. In addition, the dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. If the amount of the binder used is 1.5 parts by mass or less, the composition does not become highly viscous, and the coatability to the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

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 above content is, for example, 0.05 parts by mass or more and 0.3 parts by mass or less, and for example, 0.1 parts by mass or more and 0.2 parts by mass or less. If the CNT content is 0.01 parts by mass or more, a sufficient conductive path can be formed. If the CNT content is 0.2 parts by mass or less, the composition does not become highly viscous, and coatability onto the current collector can be ensured. Furthermore, no aggregation of CNTs occurs, and as a result, an electrode with a uniform and smooth surface can be formed.

Among the above active materials, the positive electrode active material can be a lithium salt of a transition metal oxide, for example, a layered rock salt type and a spinel type lithium-containing metal oxide can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCM {Li (Ni _x , Co _y , Mn _z ), x + y + z = 1} and NCA {Li (Ni _1-a-b Co _a Al _b )}. In addition, as a spinel type positive electrode active material, lithium manganate and the like can be mentioned. In addition to oxides, phosphates, silicates, sulfur, and the like can be used, and as a phosphate, olivine type lithium iron phosphate and the like can be mentioned. 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.

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, causing the dispersion to become alkaline. This may cause corrosion of aluminum foil (Al), which is a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali content eluted from the active material by using the unneutralized or partially neutralized present crosslinked polymer as a binder. It is also preferable to use the unneutralized or partially neutralized present crosslinked polymer in such an amount that the amount of unneutralized carboxyl groups in the present crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

Since all positive electrode active materials have low electrical conductivity, a conductive assistant other than carbon nanotubes may be added. Examples of the conductive assistant 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 easy to obtain excellent conductivity. As carbon black, ketjen black and acetylene black are preferred. The conductive assistant may be one of the above alone or two or more may be used in combination. The amount of the conductive assistant other than carbon nanotubes 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 amount of the active material, from the viewpoint of achieving both electrical conductivity and energy density. The positive electrode active material may be surface-coated with a carbon-based material having electrical conductivity.

On the other hand, examples of the negative electrode active material 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, and graphite such as natural graphite and artificial graphite, and hard carbon are more preferred. In the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and the preferred range of the particle size is, for example, 1 to 20 μm, and, for example, 5 to 15 μm.
In order to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as the negative electrode active material. Among them, 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 5.0% by mass or more, for example, 10.0% by mass or more, and can be, for example, 20.0% by mass or more, based on the total amount of active materials, from the viewpoint of improving the electric capacity of the secondary battery.

Because carbon-based active materials themselves have good electrical conductivity, it is not necessary to add conductive additives other than carbon nanotubes. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 parts by mass or less, and for example, 5 parts by mass or less, per 100 parts by mass of the total amount of active material.

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 it is also advantageous in terms of the cost of drying the medium. On the other hand, if it is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

This composition uses water as a medium. In order to adjust the properties and drying properties of this composition, it may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, N-methyl-2-pyrrolidone, and other water-soluble organic solvents. The proportion of water in the mixed medium is, for example, 50% by mass or more, and, for example, 70% by mass or more.

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

The composition may further contain other binder components such as styrene butadiene rubber (SBR) latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount of the binder components used 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 the active material. If the amount of the other binder components used exceeds 5 parts by mass, the resistance increases and the high-rate characteristics may become insufficient. Among the above, SBR latex and CMC are preferred in terms of the excellent balance between binding strength and flex resistance, and it is more preferable to use SBR latex and CMC in combination.

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 α-methylstyrene, vinyltoluene, divinylbenzene, and the like, in addition to styrene, 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, mainly from the viewpoint of binding properties.
Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, and the like in addition to 1,3-butadiene, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70% by mass, or, for example, 40 to 60% by mass, in terms of improving the binding property of the binder and the flexibility of the resulting electrode.
In addition to the above-mentioned monomers, the styrene/butadiene-based latex may use 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, maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding property.
The content of the structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

The above 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.

The composition for secondary battery electrode mixture layer of the present invention is essentially composed of the binder, CNT, active material and water, and is obtained by mixing the components using known means. The method of mixing the components is not particularly limited, and known methods can be used, but a method of dry blending powder components such as the active material, conductive assistant and binder, and then mixing with a dispersion medium such as water and dispersing and kneading is preferred. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As a mixing means, known mixers such as a planetary mixer, a thin film swirling mixer and a self-revolving mixer can be used, but it is preferable to use a thin film swirling mixer in order to obtain a good dispersion state in a short time. In addition, when using a thin film swirling mixer, it is preferable to perform preliminary dispersion in advance with a stirrer such as a disperser. The pH of the above 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 is less than 11.5 in terms of the small concern of hydrolysis, and even more preferable that it is less than 10.5. The viscosity of the slurry is not particularly limited as long as it provides the effects of the present invention, but can 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 the B-type viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

5. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for secondary battery electrode mixture layer of the present invention on the surface of a current collector such as copper or aluminum. The mixture layer is formed by applying the composition to the surface of the current collector and then drying and removing the medium such as water. The method for applying the composition is not particularly limited, and known methods such as doctor blade method, dip method, roll coating method, comma coating method, curtain coating method, gravure coating method and extrusion method can be adopted. In addition, the drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation.
Usually, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, a roll press, or the like. Compression brings the active material and the binder into close contact with each other, and improves the strength of the mixture layer and its adhesion to the current collector. The thickness of the mixture layer can be adjusted by compression to, for example, about 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is generally about 4 to 200 μm.

6. 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 in a liquid or gel form.
The separator is disposed between the positive and negative electrodes of the battery, and serves to prevent short circuits caused by contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. Specific examples of the material that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

The electrolyte may be a known one that is generally used depending on the type of active material. In the lithium ion secondary battery, specific solvents include cyclic carbonates with high dielectric constant 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, which may be used alone or as a mixed solvent. The electrolyte is used by dissolving lithium salts such as LiPF ₆ , LiSbF ₆ , LiBF ₄ , LiClO ₄ and LiAlO ₄ in these solvents. In the nickel-hydrogen secondary battery, an aqueous potassium hydroxide solution may be used as the electrolyte. The secondary battery is obtained by storing a positive electrode plate and a negative electrode plate separated by a separator in a spiral or stacked structure in a case or the like.

The binder for secondary battery electrodes disclosed in this specification provides excellent toughness of the binder coating film after immersion in electrolyte, and the electrode mixture layer of a secondary battery obtained using an electrode slurry containing the binder exhibits electrolyte resistance. Furthermore, a secondary battery equipped with an electrode obtained using the binder can ensure good integrity and exhibits good durability (cycle characteristics) even after repeated charging and discharging, making it suitable for use as a secondary battery for vehicles, etc.

The present invention will be specifically explained below based on examples. Note that the present invention is not limited to these examples. Note that in the following, "parts" and "%" refer to parts by mass and % by mass unless otherwise specified.

<Production of the present crosslinked polymer salt>
(Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
Into a reactor, 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 were charged. After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 55°C by heating. After confirming that the inside temperature was stable at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator, and the reaction solution became cloudy, so this point was determined 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), and cooling of the reaction solution was started 24 hours after the polymerization initiation point, and after the internal temperature had dropped to 25° C., 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH.H ₂ O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the carboxyl group-containing crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

The resulting polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. The precipitate was then redispersed in acetonitrile of the same weight as the polymerization reaction solution, and the polymer particles were centrifuged to settle and the supernatant was removed. This washing procedure 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. Since crosslinked polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. The powder of crosslinked polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of the Li carboxylate, which was 90 mol%, equal to the calculated value from the charge.

(Production Examples 2 to 6: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-6)
The same operation as in Production Example 1 was carried out except that the amounts of the 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-6.
Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdered crosslinked polymer salts R-2 to R-6. Each carboxyl group-containing crosslinked polymer salt was stored in a sealed container having water vapor barrier properties.

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., product name "Neoallyl T-20")
TEA: triethylamine AcN: acetonitrile V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "V-65")
LiOH.H ₂ O: Lithium hydroxide monohydrate NaOH: Sodium hydroxide

Example 1
(Preparation of electrode mixture layer composition (electrode slurry))
The active materials used were artificial graphite (manufactured by Showa Denko K.K., product name "SCMG-CF") and SiO (manufactured by Osaka Titanium Technologies Co., Ltd., 5 μm). The binder used was a mixture of crosslinked polymer R-1, styrene/butadiene-based latex (SBR), and sodium carboxymethyl cellulose (CMC). The conductive assistant used was single-walled CNT (manufactured by OCSiAl K.K., product name "TuballBATT H ₂ O" (solvent: water, single-walled CNT content: 0.4% by mass).
Water was used as a dilution solvent to add artificial graphite:SiO:crosslinked polymer salt R-1:SBR:CMC:single-walled CNT in a mass ratio of 77.6:19.4:1.0:2.0:1.0:0.1 (solids) to a planetary mixer (Hibismix 2P-03 model, manufactured by Primix Corporation) so that the solids concentration of the electrode mixture layer composition was 53 mass%, and the mixture was mixed for 1 hour and 30 minutes to prepare a slurry-state electrode mixture layer composition (electrode slurry).

(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 ventilated dryer at 80° C. for 15 minutes to form a mixture layer. After that, the mixture layer was rolled to a thickness of 50±5 μm and a mixture density of 1.60±0.10 g/cm ³ , and then punched out into a 3 cm square to obtain a negative electrode plate for battery evaluation.

(Preparation of Positive Electrode Plate)
In N-methylpyrrolidone (NMP) solvent, 100 parts of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) as a positive electrode active material, 2 parts of acetylene black, were mixed and added, and 4 parts of polyvinylidene fluoride (PVDF) were mixed as a positive electrode binder to prepare a positive electrode mixture layer composition. The positive electrode mixture layer composition was applied to an aluminum current collector (thickness: 20 μm) and dried to form a mixture layer. Thereafter, the mixture layer was rolled to a thickness of 125 μm and a mixture density of 3.0 g/cm ³ , and then punched into a 3 cm square to obtain a positive electrode plate for battery evaluation.

(Preparation of Electrolyte)
Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7), and fluoroethylene carbonate (FEC) was added to a concentration of 1 mass%. _LiPF6 was dissolved at 1.2 mol/L to prepare a nonaqueous electrolyte.

(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 through a separator (made of polyethylene: film thickness 16 μm, porosity 47%) in an aluminum laminate battery exterior, injecting electrolyte, and sealing the battery to prepare a test battery. The design capacity of this prototype battery was 50 mAh. The design capacity of the battery was based on the charge cut-off voltage of 4.2 V.

<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 by CC charging/discharging in an environment of 45° C., and the initial capacity C ₀ was measured. Furthermore, charging/discharging was repeated at a charge/discharge rate of 0.5 C under conditions of 2.5 to 4.2 V by CC charging/discharging in an environment of 45° C., and the capacity C ₁₀₀ after 100 cycles and the capacity C ₃₀₀ after 300 cycles were measured.
Here, the cycle characteristic (ΔC) was calculated by the following formula.
Charge/discharge capacity retention rate after 100 cycles ΔC ₁₀₀ =C ₁₀₀ /C ₀ ×100(%)
Charge/discharge capacity retention rate after 300 cycles ΔC ₃₀₀ =C ₁₀₀ /C ₀ ×100(%)
The ΔC ₁₀₀ calculated by the above formula was 91.2%, and the cycle characteristics based on the following criteria were evaluated as "A".
Moreover, ΔC ₃₀₀ calculated by the above formula was 82.6%, and the cycle characteristics based on the following criteria were evaluated as “A”.
A higher ΔC value indicates better cycle characteristics.
(Criteria for Cycle Characteristics ΔC ₁₀₀ )
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% (criteria for determining cycle characteristic ΔC ₃₀₀ ).
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%.

<Evaluation of electrode expansion rate>
The laminated cell lithium ion secondary battery that had been subjected to the 300 cycle test was disassembled to recover the negative electrode. Each recovered negative electrode was washed with a DMC (dimethyl carbonate) solvent, naturally dried at room temperature for one day, and then its thickness was measured. The measured thickness was then substituted into the following equation to calculate the expansion rate of the negative electrode.
[Expansion rate of electrode (%)]=100×{(Thickness of discharged negative electrode of battery)−(Thickness of copper foil)}/{(Thickness of negative electrode before assembly)−(Thickness of copper foil)}
The expansion rate of the electrode calculated by the above formula was 162%, and the expansion rate based on the following criteria was evaluated as "A".
The lower the expansion rate of the electrode, the greater the effect of suppressing the expansion of the electrode, which indicates superior battery performance.
(Criteria for electrode expansion rate)
A: The expansion rate of the electrode is less than 180%. B: The expansion rate of the electrode is 180% or more and less than 200%. C: The expansion rate of the electrode is 200% or more and less than 250%. D: The expansion rate of the electrode is 250% or more.

Examples 2 to 14 and Comparative Examples 1 to 2
Electrode slurries were prepared in the same manner as in Example 1, except that the compositions were as shown in Table 2. The cycle characteristics and electrode expansion rate of the negative electrode plates obtained using each electrode slurry were evaluated, and the results are shown in Table 2.

Details of the compounds used in Table 2 are shown below.
SBR: Styrene butadiene rubber CMC: Sodium carboxymethyl cellulose AB: Acetylene black

<Evaluation Results>
As is clear from the results of Examples 1 to 14, the secondary battery provided with an electrode obtained using the composition for secondary battery electrode mixture layer (electrode slurry) of the present invention not only exhibited an excellent capacity retention rate at 100 cycles, but also showed a high capacity retention rate even when used at a high number of charge/discharge cycles of 300. In addition, the effect of suppressing the expansion of the negative electrode after the test was also observed.
Among these, when comparing the amount of carbon nanotubes used, the results showed that when 0.1 part by mass (Example 1) and 0.2 part by mass (Example 2) were used relative to 100 parts by mass of the total amount of the active material, the capacity retention rate and expansion inhibition effect at 300 cycles were superior to when 0.05 part by mass of single-walled CNTs was used (Example 3).
This is thought to be due to the effect that, in response to the increase in the distance between active materials due to the expansion of the negative electrode caused by repeated charging and discharging, the more carbon nanotubes with a high aspect ratio structure are used, the more locations where conductive paths are formed, making the capacity less likely to deteriorate.

In contrast, when acetylene black (AB) was used as the conductive additive (Comparative Example 1), the capacity retention rate at 100 cycles was slightly lower, and after further repeated charging and discharging, the capacity retention rate at 300 cycles was significantly lower.

Next, in terms of the degree of neutralization of the carboxyl group-containing crosslinked polymer, the results showed that the battery performance was superior when the degree of neutralization of the polymer was 80 mol % (Example 4) compared to when the degree of neutralization of the polymer was 60 mol % (Example 5).
This is believed to be because, as the degree of neutralization of the carboxyl group-containing crosslinked polymer increases, the glass transition point of the polymer increases, suppressing fusion of the polymer during the heating and drying step in the electrode production process, thereby enabling the production of a uniform electrode.

In addition, the results showed that the battery characteristics were better when the content of the structural unit derived from the crosslinkable monomer in the carboxyl group-containing crosslinked polymer was low (the degree of crosslinking was low) (Example 1) than when the content of the structural unit was high (the degree of crosslinking of the crosslinked polymer was increased) (Example 6).
This is believed to be because an increase in the degree of crosslinking of the carboxyl group-containing crosslinked polymer reduces the degree of swelling in water in the electrode slurry, and the number of contact points with the active material decreases, resulting in a slight decrease in adhesion and making the polymer more susceptible to the effects of swelling and shrinkage of the active material due to repeated charging and discharging.

When the neutralization salt was changed to a Na salt (Example 7), the battery performance was equivalent to that when the neutralization salt was changed to a Li salt (Example 1).
This is believed to be because in a 45° C. environment, the neutralized salt of the crosslinked polymer exhibits the same binding properties and rigidity regardless of the type.

Focusing on the content of structural units derived from ethylenically unsaturated carboxylic acid monomers, the results showed that the battery characteristics were superior when the content was 100% by mass (Example 1) compared to when the content was 80% by mass (Example 8).
This is believed to be because the greater the content of carboxyl groups in the crosslinked polymer, the better the binding ability with the active material, and therefore the more likely it is that the destruction of the electrode structure can be suppressed even during long-term battery use.

When comparing the amounts of the carboxyl group-containing crosslinked polymer salt used, the result showed that when the amount of the carboxyl group-containing crosslinked polymer salt used was 1.0 part by mass (Example 1) relative to 100 parts by mass of the total amount of the active material, the capacity retention rate at 300 cycles and the expansion suppression effect were more excellent than when the amount of the carboxyl group-containing crosslinked polymer salt used was 0.5 parts by mass (Example 9), 1.5 parts by mass (Example 10), and 3.0 parts by mass (Example 11).
This is believed to be because the smaller the amount of carboxyl group-containing crosslinked polymer salt used, the fewer the bonding points in the electrode, which makes it easier for the distance between the active materials to increase due to high cycle charging and discharging. Also, the larger the amount of carboxyl group-containing crosslinked polymer salt used, the more the bonding points increase, but the brittleness of the polymer affects the physical properties of the electrode, making it more susceptible to the stress caused by the expansion and contraction of the active materials, which leads to deterioration.

Regarding the amount of silicon-based active material used, even when it was increased from Example 1 to 29.1 parts by mass (Example 12), or when it was decreased from Example 1 to 9.7 parts by mass (Example 13), or to 4.9 parts by mass (Example 14), good capacity retention and expansion suppression effects were observed.
This is believed to be because the combination of this crosslinked polymer salt and single-walled CNTs makes it possible to perform repeated charging and discharging while suppressing structural destruction and conductive path disconnection in the composite layer, regardless of the amount of silicon particles used.

In contrast, when no carboxyl group-containing crosslinked polymer salt was used (Comparative Example 2), a good cycle retention rate was observed at the 100th cycle, but the capacity was significantly deteriorated after 300 cycles of repeated charging and discharging.
This is thought to be because at the 100th cycle, the conductive paths between the active materials were maintained and the capacity was preserved due to the good conductivity of the single-walled CNTs, but when the cross-linked polymer salt was not used, the binding strength was significantly inferior, and therefore the distance between the active materials increased with the number of repeated charge/discharge cycles, causing the conductive paths through the single-walled CNTs to be cut, leading to deterioration of the capacity.

A secondary battery equipped with an electrode obtained using the binder for secondary battery electrodes disclosed in this specification can ensure good integrity, and when used for a longer period of time than before, exhibits good durability (cycle characteristics) even when repeatedly charged and discharged while suppressing expansion and contraction due to charging and discharging, and is therefore expected to contribute to increasing the capacity of vehicle-mounted secondary batteries, etc.
The binder for secondary battery electrodes of the present invention can be suitably used in particular for electrodes of non-aqueous electrolyte secondary batteries, and is particularly useful for non-aqueous electrolyte lithium ion secondary batteries having high energy density.

Claims

An electrode binder for a secondary battery having a secondary battery electrode containing carbon nanotubes,
An electrode binder comprising a carboxyl group-containing crosslinked polymer or a salt thereof.
The electrode binder according to claim 1, 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 the total structural units of the polymer.
A composition for a secondary battery electrode mixture layer, comprising the electrode binder according to claim 1 or 2, carbon nanotubes, an active material, and water.
The composition for secondary battery electrode mixture layer according to claim 3, wherein the content of the silicon-based active material is 5.0 mass% or more based on the total amount of the active material.
The composition for secondary battery electrode mixture layer according to claim 3, wherein the content of the electrode binder is 1.0 parts 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.
The composition for secondary battery electrode mixture layer according to claim 3, 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.
The secondary battery electrode mixture layer composition according to claim 3, wherein the carbon nanotubes include a single-layer structure.
A secondary electrode having a mixture layer formed from the secondary battery electrode mixture layer composition described in claim 3 on a collector surface.
A secondary battery comprising the secondary electrode according to claim 8.