CN116547328A - Copolymer, binder for nonaqueous secondary battery electrodes, and slurry for nonaqueous secondary battery electrodes - Google Patents

Abstract

Provided are a copolymer, a binder for a nonaqueous secondary battery electrode, and a slurry for a nonaqueous secondary battery electrode, which are capable of contributing to a reduction in the internal resistance of a battery and excellent cycle characteristics by effectively improving the peel strength of an electrode active material layer to a current collector with less generation of coarse particles. The copolymer of the present invention contains the 1 st to 3 rd structural units derived from the monomers (a 1) to (a 3), respectively. The monomer (a 1) is composed of a nonionic compound having an ethylenically unsaturated bond, the monomer (a 2) has an ethylenically unsaturated bond and an anionic functional group, and the monomer (a 3) has the 31 st to 33 rd structural units represented by the following formulas (1) to (3). The composition comprises 1.0 to 30 parts by mass of the 2 nd structural unit per 100 parts by mass of the 1 st structural unit, and the 3 rd structural unitThe unit is 0.010 mass parts or more and 70 mass parts or less.

Description

Copolymer, binder for non-aqueous secondary battery electrode, and slurry for non-aqueous secondary battery electrode

Technical Field

The present invention relates to a copolymer, a binder for non-aqueous secondary battery electrodes, and a slurry for non-aqueous secondary battery electrodes.

This application claims priority based on Japanese Patent Application No. 2020-184544 filed in Japan on November 4, 2020, the contents of which are incorporated herein by reference.

Background Art

A nonaqueous secondary battery includes, for example, a positive electrode containing a metal oxide or the like as an active material, a negative electrode containing a carbon material such as graphite as an active material, and an electrolyte. A nonaqueous secondary battery is a secondary battery in which ions are transferred between the positive electrode and the negative electrode to charge and discharge the battery.

As a non-aqueous secondary battery, a lithium ion secondary battery can be cited as a representative example. Non-aqueous secondary batteries are used as power sources for notebook personal computers, mobile phones, power tools, and electronic/communication equipment due to their miniaturization and lightweight. In addition, recently, from the perspective of environmentally friendly vehicle applications, they have also been used for electric vehicles and hybrid vehicles. Among them, high output, high capacity, and long life of non-aqueous secondary batteries are strongly required.

The binder used for the positive electrode and the negative electrode has the function of binding the active materials to each other and binding the active materials to the current collector. In order to improve the capacity of non-aqueous secondary batteries and protect the working environment, water-dispersible binders have been developed. For example, a water dispersion of styrene-butadiene rubber (SBR) system using carboxymethyl cellulose (CMC) as a thickener is known.

Patent Document 1 describes a method of polymerizing ethylene oxide, an alkylene oxide other than ethylene oxide, an alkyl glycidyl ether, an allyl glycidyl ether, or a combination thereof, and describes that a composition containing a copolymer obtained by polymerization can be used as a binder material in a battery electrode containing electroactive particles.

Patent document 2 describes a secondary battery negative electrode having an electrode layer, wherein the electrode layer contains a copolymer obtained from a (meth)acrylate and a vinyl monomer having an acid component, and at least one selected from a polyoxyethylene alkyl ether derivative, a polyoxyethylene-polyoxypropylene condensate, and a polyoxyethylene-polyoxypropylene alkyl ether derivative.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application No. 2012-517519

Patent Document 2: Japanese Patent Application Publication No. 2014-239070

Summary of the invention

Problems to be solved by the invention

However, the components described in Patent Documents 1 and 2 have room for improvement in the peel strength of the electrode active material layer to the current collector and room for reduction in the internal resistance of a battery when used as an electrode binder.

The object of the present invention is to provide a copolymer, a binder for non-aqueous secondary battery electrodes, and a slurry for non-aqueous secondary battery electrodes, which can effectively improve the peel strength of the electrode active material layer to the collector in a non-aqueous secondary battery, and can help reduce the internal resistance of the battery and improve the cycle characteristics.

Methods for solving problems

In order to solve the above-mentioned problems, the present invention is as described in the following [1] to [15].

[1] A copolymer characterized in that it is a polymer of a compound having an ethylenically unsaturated bond,

It has a first structural unit derived from a monomer (a1), a second structural unit derived from a monomer (a2), and a third structural unit derived from a monomer (a3); or, it has a first structural unit derived from a monomer (a1), a second structural unit derived from a monomer (a2), a third structural unit derived from a monomer (a3), and a fourth structural unit derived from an internal crosslinking agent (a4),

The monomer (a1) is a nonionic compound having an ethylenically unsaturated bond, having neither a hydroxyl group nor a cyano group, having no polyoxyalkylene structure, and having no independent multiple ethylenically unsaturated bonds.

The monomer (a2) is a compound having an ethylenically unsaturated bond and an anionic functional group and having neither a polyoxyalkylene structure nor a plurality of independent ethylenically unsaturated bonds.

The monomer (a3) has a 31st structural unit represented by the following formula (1), a 32nd structural unit represented by the following formula (2), and a 33rd structural unit represented by the following formula (3),

The copolymer contains 1.0 parts by mass or more and 30 parts by mass or less of the second structural unit relative to 100 parts by mass of the first structural unit.

The third structural unit is contained in an amount of 0.010 parts by mass or more and 70 parts by mass or less relative to 100 parts by mass of the first structural unit.

The content of the fourth structural unit is 0 parts by mass or more and 20 parts by mass or less relative to 100 parts by mass of the first structural unit.

The internal crosslinking agent (a4) is a compound that is not included in the monomer (a3), has multiple independent ethylenically unsaturated bonds, and can form a crosslinked structure in the free radical polymerization of monomers including the monomer (a1), the monomer (a2), and the monomer (a3).

(In formula (2), ^R1 is an optionally branched alkyl group having 1 to 6 carbon atoms.)

(In formula (3), ^R2 is a functional group having an ethylenically unsaturated bond.)

[2] The copolymer according to [1],

The monomer (a3) is

contains 5.0 mol% or more and 98 mol% or less of the 31st structural unit,

contains 0.30 mol% or more and 90 mol% or less of the 32nd structural unit,

The 33rd structural unit is contained in an amount of 0.30 mol% to 10 mol%.

[3] The copolymer according to [1] or [2], wherein the monomer (a3) has a plurality of independent ethylenically unsaturated bonds in one molecule.

[4] The copolymer according to any one of [1] to [3], wherein in the formula (3), R ² has at least one selected from the group consisting of a vinyloxy group, an allyloxy group, a (meth)acryloyl group, a (meth)acryloyloxy group, and -OCH ₂ -CH ₂ -CH ₂ =CH ₂ .

[5] The copolymer according to any one of [1] to [4], wherein in the above formula (3), R ² is represented by the following formula (4).

-R ²¹ -R ²² (4)

(In formula (4), ^R21 is an optionally branched alkylene group having 1 to 5 carbon atoms, and ^R22 is a functional group selected from the group consisting of vinyloxy, allyloxy, (meth)acryloyl, and (meth)acryloyloxy.)

[6] The copolymer according to any one of [1] to [5], wherein the monomer (a3) has a first block comprising the thirty-first structural unit, a second block comprising the thirty-second structural unit, and a third block comprising the thirty-third structural unit.

[7] The copolymer according to any one of [1] to [6], wherein the monomer (a3) contains 90% by mass or more of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit in total in all structural units.

[8] The copolymer according to any one of [1] to [7], wherein the monomer (a1) does not have a polar functional group.

[9] The copolymer according to any one of [1] to [8], wherein the monomer (a2) is a compound having at least one of a carboxyl group and a sulfone group.

[10] The copolymer according to any one of [1] to [9], comprising 80% by mass or more in total of the first structural unit, the second structural unit, and the third structural unit.

[11] The copolymer according to any one of [1] to [10], wherein the content of the fourth structural unit is 0.050 parts by mass or more relative to 100 parts by mass of the first structural unit.

[12] A binder composition for non-aqueous secondary battery electrodes, wherein the copolymer according to any one of [1] to [11] is dispersed in an aqueous medium.

[13] A binder for non-aqueous secondary battery electrodes, comprising the copolymer according to any one of [1] to [11].

[14] A slurry for a non-aqueous secondary battery electrode, comprising the binder described in [13], an electrode active material, and an aqueous medium,

The binder and the electrode active material are dispersed in an aqueous medium.

The aqueous medium is one medium selected from the group consisting of water, a hydrophilic solvent, and a mixture containing water and a hydrophilic solvent.

[15] A non-aqueous secondary battery electrode comprising the binder described in [13].

Effects of the Invention

According to the present invention, a copolymer, a binder for nonaqueous secondary battery electrodes, and a slurry for nonaqueous secondary battery electrodes can be provided, which can effectively improve the peel strength of the electrode active material layer to the collector in a nonaqueous secondary battery, and can help reduce the internal resistance of the battery and improve the cycle characteristics.

DETAILED DESCRIPTION

Hereinafter, as embodiments of the present invention, a copolymer, a binder for nonaqueous secondary battery electrodes, a binder composition for nonaqueous secondary battery electrodes, a slurry for nonaqueous secondary battery electrodes, a nonaqueous secondary battery electrode, and a nonaqueous secondary battery are described.

The term “(meth)acryloyl” is a general term for acryloyl and methacryloyl, and the term “(meth)acrylate” is a general term for acrylate and methacrylate.

The "non-volatile component" is the component remaining after weighing 1 g of the composition in an aluminum dish with a diameter of 5 cm and drying it at 105°C for 1 hour in a dryer under 1 atmosphere (1013 hPa) while circulating air. The form of the composition may be a solution, a dispersion, or a slurry, but is not limited to these. The so-called "non-volatile component concentration" is the ratio (mass %) of the mass of the non-volatile component after drying under the above conditions to the mass (1 g) of the composition before drying.

The "ethylenically unsaturated bond" means an ethylenically unsaturated bond having free radical polymerizability unless otherwise specified.

In a polymer of a compound having an ethylenically unsaturated bond, a structural unit derived from a compound having an ethylenically unsaturated bond means that the chemical structure of the portion of the compound other than the ethylenically unsaturated bond is the same as the chemical structure of the portion of the structural unit in the polymer other than the portion corresponding to the ethylenically unsaturated bond. For example, a structural unit derived from sodium acrylate has a structure of _-CH2CH (COONa)- in the polymer.

In addition, about the compound with independent multiple ethylenically unsaturated bonds, as the structural unit of polymer, ethylenically unsaturated bonds can remain. So-called independent multiple ethylenically unsaturated bonds refer to multiple ethylenically unsaturated bonds that do not form conjugated dienes with each other. For example, the structural unit derived from divinylbenzene can be a structure without ethylenically unsaturated bonds (all parts corresponding to ethylenically unsaturated bonds are introduced into the form of the chain of polymer), or a structure with 1 ethylenically unsaturated bond (only the part corresponding to one ethylenically unsaturated bond is introduced into the form of the chain of polymer).

Furthermore, when the chemical structure of the monomer does not correspond to the chemical structure of the polymer due to chemical reaction of the portion other than the chain corresponding to the ethylenically unsaturated bond after polymerization, the chemical structure after polymerization is used as a reference. For example, when vinyl acetate is polymerized and then saponified, it is considered that the chemical structure of the polymer used as a reference is not the structural unit derived from vinyl acetate but the structural unit derived from vinyl alcohol.

<1. Copolymer (P)>

In the present embodiment, copolymer (P) is a copolymer for a non-aqueous secondary battery electrode binder. Copolymer (P) is a polymer of a compound having an ethylenically unsaturated bond. Copolymer (P) has a first structural unit derived from monomer (a1), a second structural unit derived from monomer (a2), and a third structural unit derived from monomer (a3). Copolymer (P) may further have a fourth structural unit derived from an internal crosslinking agent (a4). Copolymer (P) may include structural units derived from other monomers (a5) that do not belong to any one of monomers (a1), monomers (a2), monomers (a3) and internal crosslinking agents (a4). The details of each monomer and internal crosslinking agent are described below.

<1-1. Monomer (a1)>

Monomer (a1) is a nonionic compound (having neither anionic functional groups nor cationic functional groups) having an ethylenically unsaturated bond, having no polyoxyalkylene structure, and having no independent multiple ethylenically unsaturated bonds. The term "having no polyoxyalkylene structure" means having no alkylene group sandwiched between two ether bonds (a bond represented by -COCx _- OC- (x is an integer greater than or equal to 1)).

Monomer (a1) has neither hydroxyl nor cyano group. Monomer (a1) preferably has no polar functional group. Monomer (a1) may contain only one compound or more than two compounds. Monomer (a1) is preferably at least one of (meth)acrylate and aromatic vinyl compound without polar functional group, and more preferably contains both. (Meth)acrylate without polar functional group further preferably contains alkyl (meth)acrylate. The total content of alkyl (meth)acrylate and aromatic vinyl compound in monomer (a1) is further preferably 80% by mass or more, further preferably 90% by mass or more, and most preferably 100% by mass.

In addition, regarding the composition of the monomer (a1), in order to adjust the glass transition temperature of the copolymer (P) or to adjust the polymerization rate according to the molecular design, it is preferred to appropriately adjust the preferred compounds and their amounts within the range specified in the present invention.

Examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isobornyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate.

Here, the aromatic vinyl compound does not contain a (meth)acryloyl group. Examples of the aromatic vinyl compound include styrene, tert-butylstyrene, α-methylstyrene, p-methylstyrene, 1,1-diphenylethylene, etc. When the monomer (a1) contains an aromatic vinyl compound, the monomer (a1) more preferably contains at least one of styrene and α-methylstyrene, and further preferably contains styrene.

The monomer (a1) may contain a plurality of ethylenically unsaturated bonds that form a conjugated diene. Examples of the compound having a plurality of ethylenically unsaturated bonds that form a conjugated diene include 1,3-butadiene and 1,3-pentadiene.

[1-2. Monomer (a2)]

Monomer (a2) is a compound having an ethylenically unsaturated bond and an anionic functional group. Monomer (a2) has neither a polyoxyalkylene structure nor multiple independent ethylenically unsaturated bonds. As anionic functional groups, for example, carboxyl, sulfonic group, and phosphoric acid group can be cited. Monomer (a2) preferably contains a compound having at least one of a carboxyl group and a sulfonic group, and more preferably contains a compound having a carboxyl group.

Monomer (a2) may contain only one compound or more than two compounds. Monomer (a2) may contain a compound having multiple anionic functional groups of the same kind in one molecule. That is, copolymer (P) may contain multiple anionic functional groups of the same kind in one structural unit. Monomer (a2) may contain a compound having two or more different anionic functional groups in one molecule. That is, copolymer (P) may contain two or more different anionic functional groups in one structural unit. In addition, monomer (a2) may contain two or more compounds containing different anionic functional groups. That is, copolymer (P) may contain two or more structural units containing different anionic functional groups.

Examples of the monomer (a2) include unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid; unsaturated dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid; half esters of unsaturated dicarboxylic acids; p-styrenesulfonic acid, etc. Among them, the monomer (a2) preferably contains at least one of (meth)acrylic acid and itaconic acid.

At least a part of the structural unit derived from the monomer (a2) may form a salt with a basic substance. Examples of the monomer (a2) that forms a salt include sodium (meth)acrylate and sodium p-styrenesulfonate.

The monomer (a2) preferably contains at least one of a sulfonic acid having an ethylenically unsaturated bond and a salt thereof, and more preferably contains a sulfonate having an ethylenically unsaturated bond. As the sulfonic acid, it is preferably a sulfonic aromatic vinyl compound having a sulfonic group, and more preferably p-styrenesulfonic acid. As the sulfonate, it is preferably a salt of an aromatic vinyl compound having a sulfonic group, and more preferably p-styrenesulfonate, and further preferably sodium p-styrenesulfonate. This is because the generation of coarse particles can be suppressed in the adhesive composition.

[1-3. Monomer (a3)]

Monomer (a3) is a compound comprising a 31st structural unit represented by the following formula (1), a 32nd structural unit represented by the following formula (2), and a 33rd structural unit represented by the following formula (3). Monomer (a3) preferably has a plurality of ethylenically unsaturated bonds in one molecule. Monomer (a3) may comprise only one compound or may comprise two or more compounds.

In addition, in monomer (a3), when describing the composition of the structural unit, the terminal structure is not considered unless otherwise specified. For example, regarding the content of a certain structural unit in monomer (a3), unless otherwise specified, it is the content of the structural unit in the structure other than the terminal structure. In addition, when it is considered that monomer (a3) is composed of a certain structural unit, it may also contain a terminal structure in addition to the structural unit. The so-called terminal structure in monomer (a3) here is a structure that is located more on the molecular terminal side than the ether bond closest to the molecular terminal and is not contained in any of the structures of the following formulas (1) to (3). Furthermore, the structure shown in the following formulas (1) to (3) is not contained in the terminal structure.

In formula (2), ^R1 is an optionally branched alkyl group having 1 to 6 carbon atoms. ^R1 preferably has 4 or less carbon atoms, more preferably 2 or less carbon atoms, and further preferably is a methyl group.

In formula (3), ^R2 is a group having an ethylenically unsaturated bond. ^R2 preferably has at least one selected from the group consisting of a vinyloxy group ( _-OCH2 = _CH2 ), an allyloxy group ( _-OCH2 - _CH2 = _CH2 ), a (meth)acryloyl group, a (meth)acryloyloxy group, and _-OCH2 - _CH2 - _CH2 = _CH2 , more preferably has at least one selected from the group consisting of an allyloxy group, a (meth)acryloyl group, and a (meth)acryloyloxy group, and further preferably has an allyloxy group. The number of ethylenically unsaturated bonds contained in one 33rd structural unit is preferably one.

The structure of R ² is preferably represented by the following formula (4).

-R ²¹ -R ²² (4)

In formula (4), ^R21 is an optionally branched alkylene group having 1 to 5 carbon atoms, and ^R22 is any one of vinyloxy, allyloxy, (meth)acryloyl, (meth)acryloyloxy, and _-OCH2 - _CH2 - _CH2 = _CH2 .

In formula (4), ^R21 is preferably an alkylene group having 1 or 2 carbon atoms, and more preferably a methylene group. In formula (4), ^R22 is more preferably any one of an allyloxy group, a (meth)acryloyl group, and a (meth)acryloyloxy group, and further preferably an allyloxy group.

By adjusting the contents of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit in the monomer (a3), the hydrophilicity of the monomer (a3) is adjusted to an appropriate range, and the hydrophilicity of the copolymer (P) can be controlled. For example, if the content of the 31st structural unit in the monomer (a3) is increased, the hydrophilicity of the copolymer (P) is improved, and if the content of the 31st structural unit is reduced, the hydrophilicity of the copolymer (P) is reduced.

By adjusting the contents of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit in the monomer (a3), the crystallinity of the monomer (a3) is adjusted to an appropriate range, and the crystallinity of the copolymer (P) can be controlled. For example, if the content of the 31st structural unit in the monomer (a3) is increased, the crystallinity of the copolymer (P) is improved, and if the content of the 31st structural unit is reduced, the crystallinity of the copolymer (P) is reduced.

By adjusting the content of the 33rd structural unit in the monomer (a3), the crosslinking density of the copolymer (P) can be adjusted, and the flexibility and strength of the electrode active material layer containing the copolymer (P) as an electrode binder can be controlled. For example, if the content of the 33rd structural unit in the monomer (a3) is increased, the crosslinking density of the copolymer (P) rises, and the strength of the electrode active material layer can be further improved. On the other hand, if the content of the 33rd structural unit is reduced, the crosslinking density of the copolymer (P) is reduced, and the flexibility of the electrode active material layer can be further improved.

Hereinafter, the relationship between the contents of these structural units contained in the monomer (a3) will be described.

In the monomer (a3), the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 5.0 mol% or more, more preferably 18 mol% or more, and further preferably 25 mol% or more.

In the monomer (a3), the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 98 mol% or less, more preferably 97 mol% or less.

In the monomer (a3), the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 0.30 mol% or more, more preferably 0.50 mol% or more, and further preferably 0.70 mol% or more.

In the monomer (a3), the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 90 mol% or less, more preferably 80 mol% or less, and further preferably 75 mol% or less.

In the monomer (a3), the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 0.30 mol% or more, more preferably 0.50 mol% or more, and further preferably 0.70 mol% or more.

In the monomer (a3), the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 10 mol% or less, more preferably 6.0 mol% or less, and further preferably 4.5 mol% or less.

Monomer (a3) can include the structural unit that does not belong to any one of the 31st structural unit, the 32nd structural unit and the 33rd structural unit. However, the total content of the 31st structural unit, the 32nd structural unit and the 33rd structural unit in monomer (a3), in all structural units, is preferably more than 90 mass %, more preferably more than 95 mass %, further preferably 98 mass %, most preferably 100 mass %. Here, terminal structure (defined as above) is not included in the structural unit constituting monomer (a3). About this point, about the monomer (a31) related to the 1st mode described below and the monomer (a32) related to the 2nd mode are also the same.

As monomer (a3), the following two preferred embodiments with different hydrophilicity can be mentioned. The following embodiments are described as the monomer (a31) involved in the first embodiment and the monomer (a32) involved in the second embodiment. The monomer (a32) involved in the second embodiment has higher hydrophilicity than the monomer (a31) involved in the first embodiment.

In the monomer (a31) according to the first embodiment, the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 5.0 mol% or more, more preferably 18 mol% or more, and further preferably 25 mol% or more.

In the monomer (a31) according to the first embodiment, the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 50 mol% or less, more preferably 40 mol% or less.

In the monomer (a31) involved in the first embodiment, the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 40 mol% or more, more preferably 50 mol% or more, and further preferably 60 mol% or more.

In the monomer (a31) involved in the first embodiment, the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 90 mol% or less, more preferably 80 mol% or less, and further preferably 75 mol% or less.

In the monomer (a31) according to the first embodiment, the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 0.30 mol% or more, more preferably 0.50 mol% or more, and even more preferably 0.70 mol% or more.

In the monomer (a31) involved in the first embodiment, the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 10 mol% or less, more preferably 6.0 mol% or less, and further preferably 4.5 mol% or less.

In the monomer (a32) involved in the second embodiment, the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 70 mol% or more, more preferably 80 mol% or more, and further preferably 90 mol% or more.

In the monomer (a32) according to the second embodiment, the content of the 31st structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 98 mol% or less, more preferably 97 mol% or less.

In the monomer (a3) according to the second embodiment, the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 0.30 mol% or more, more preferably 0.50 mol% or more, and even more preferably 0.70 mol% or more.

In the monomer (a32) involved in the second embodiment, the content of the 32nd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 20 mol% or less, more preferably 15 mol% or less, and further preferably 10 mol% or less.

In the monomer (a32) involved in the second embodiment, the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 0.30 mol% or more, more preferably 0.50 mol% or more, and further preferably 0.70 mol% or more.

In the monomer (a32) involved in the second embodiment, the content of the 33rd structural unit relative to the total amount of the 31st structural unit, the 32nd structural unit, and the 33rd structural unit is preferably 10 mol% or less, more preferably 6.0 mol% or less, and further preferably 4.5 mol% or less.

Monomer (a3) is preferably a block copolymer having a first block comprising a 31st structural unit, a second block comprising a 32nd structural unit, and a third block comprising a 33rd structural unit. Monomer (a3) is more preferably a 3-block copolymer consisting of a first block, a second block, and a third block (wherein, as defined above, a terminal structure may be included). Monomer (a3) is further preferably a 3-block copolymer in which a first block, a second block, and a third block are arranged in sequence (i.e., a second block exists between the first block and the third block).

The preferred range of the weight average molecular weight of monomer (a3) varies depending on the presence or absence of water solubility of monomer (a3). When monomer (a3) can be dissolved in 0.1M _NaNO3 aqueous solution to prepare an aqueous solution containing 0.1% by mass of monomer (a3), the weight average molecular weight of monomer (a3) is the pullulan conversion value measured by aqueous GPC under the conditions shown below.

[Water-based GPC]

GPC equipment: GPC-101 (manufactured by Showa Denko K.K.)

Solvent: 0.1M NaNO ₃ aqueous solution

Sample column: Shodex Column Ohpak SB-806HQ (8.0mm I.D. x 300mm) × 2

Reference column: Shodex Column Ohpak SB-800RL (8.0mm I.D. x 300mm) × 2

Column temperature: 40°C

Sample concentration: 0.1 mass%

Detector: RI-71S (manufactured by Shimadzu Corporation)

Flow rate: 1ml/min

Molecular weight standard: Pullulan (P-5, P-10, P-20, P-50, P-100, P-200, P-400, P-800, P-1300, P-2500 (manufactured by Showa Denko Co., Ltd.)

In this case, the weight average molecular weight _Mw (pullulan conversion value) of the monomer (a3) is preferably 10000 or more, more preferably 30000 or more, and further preferably 50000 or more. It is to improve the strength of the electrode. In addition, in this case, the weight average molecular weight _Mw (pullulan conversion value) of the monomer (a3) is preferably 300000 or less, more preferably 200000 or less, and further preferably 120000 or less. It is to improve the dispersibility of the solid content in the electrode slurry described later.

When the monomer (a3) cannot be dissolved in a 0.1 M NaNO ₃ aqueous solution and an aqueous solution containing 0.1 mass % of the monomer (a3) is prepared, the weight average molecular weight of the monomer (a3) is a polystyrene conversion value measured by solvent-based GPC under the conditions shown below.

[Solvent-based GPC]

GPC device: Waters GPC System e2695

Solvent: Tetrahydrofuran

Column: SHODEX KF-806L×2, SHODEX KF-G (Showa Denko K.K.) Column temperature: 40°C

Column temperature: 40°C

Sample concentration: 0.2 mass%

Detector: Waters 2414RI

Flow rate: 0.65mL/min

Molecular weight standard: polystyrene (Shodex Polystyrene STANDARD SL-105, SM-105 (manufactured by Showa Denko Co., Ltd.)

In this case, the weight average molecular weight _Mw (polystyrene conversion value) of the monomer (a3) is preferably 10000 or more, more preferably 20000 or more, and further preferably 30000 or more. It is to improve the strength of the electrode. In addition, in this case, the weight average molecular weight _Mw (polystyrene conversion value) of the monomer (a3) is preferably 200000 or less, more preferably 150000 or less, and further preferably 80000 or less. It is to improve the dispersibility of the solid content in the electrode slurry described later.

[Specific examples of monomer (a3)]

The monomer (a3) is preferably a ternary block copolymer represented by the following formula (5), for example.

In formula (5), preferably n:m:l=5.0-98:0.30-90:0.30-4.5, n:m:l=18-97:0.50-80:0.50-6.0, more preferably n:m:l=25-97:0.70-75:0.80-4.5. The preferred range of the weight average molecular weight is as described above.

The first embodiment of the compound represented by the above formula (5) is that in the above formula (5), n:m:l=5.0-50:40-90:0.30-4.5, preferably n:m:l=18-40:50-80:0.50-6, and more preferably n:m:l=25-40:60-75:0.80-4.5. In the case of preparing an aqueous solution containing 0.1% by mass of the compound involved in the first embodiment which is insoluble in a 0.1M NaNO3 _aqueous solution, the weight average molecular weight is 10,000-200,000, preferably 20,000-150,000, and more preferably 30,000-80,000 in terms of polystyrene.

Specific examples of the first embodiment include compounds (a3-1) wherein in formula (5), n:m:l=30:69:1.0 and Mw=50,000.

The second embodiment of the compound represented by the above formula (5) is a compound represented by the above formula (5), and in the formula (5), n:m:l=70-98:0.30-20:0.30-10, preferably n:m:l=80-97:0.50-15:0.50-6.0, more preferably n:m:l=90-97:0.70-10:0.80-4.5, Mw=50000-80000. When the compound can be dissolved in a 0.1M _NaNO3 aqueous solution to prepare an aqueous solution containing 0.1% by mass of the compound involved in the second embodiment, the weight average molecular weight is 10000-300000, preferably 30000-200000, and more preferably 50000-120000 in terms of pullulan.

Specific examples of the second embodiment include compounds (a3-2) in which n:m:l=93:6.0:1.0 and Mw=80,000, and compounds (a3-3) in which n:m:l=96:1.0:3.0 and Mw=80,000 in formula (5).

[Method for synthesizing monomer (a3)]

The method for synthesizing the monomer (a3) is not particularly limited, and the monomer (a3) can be obtained, for example, by ring-opening polymerization of an epoxide using an acid catalyst.

[1-4. Internal crosslinking agent (a4)]

The internal crosslinking agent (a4) is a compound that is not included in the monomer (a3), has multiple independent ethylenically unsaturated bonds, and can form a crosslinked structure in the free radical polymerization of monomers including monomers (a1), (a2), and (a3). Examples of such compounds include divinylbenzene, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and 2-hydroxy-3-acryloyloxypropyl methacrylate.

[1-5. Other monomers (a5)]

Other monomers (a5) do not belong to any of monomers (a1) to (a4). Other monomers (a5) include, but are not limited to, compounds having an ethylenically unsaturated bond and a polar functional group, surfactants having an ethylenically unsaturated bond (hereinafter sometimes referred to as "polymerizable surfactants"), compounds having an ethylenically unsaturated bond and functioning as a silane coupling agent, and the like.

As the polar functional group, it is preferred to contain at least one of a hydroxyl group and a cyano group. As the monomer having an ethylenically unsaturated bond and a polar functional group, for example, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl acrylate, (meth)acrylonitrile, etc. can be cited. The copolymer (P) preferably contains a structural unit derived from a compound containing an ethylenically unsaturated bond and a hydroxyl group, more preferably contains a structural unit derived from a (meth)acrylate having a hydroxyl group, and further preferably contains a structural unit derived from 2-hydroxyethyl (meth)acrylate.

The polymerizable surfactant is a compound having an ethylenically unsaturated bond and having a function as a surfactant, and examples thereof include compounds represented by the following chemical formulas (6) to (9).

In formula (6), ^R3 is preferably an alkyl group, and p is preferably an integer of 10 to 40. The number of carbon atoms of ^R3 is more preferably 10 to 40, and ^R3 is further preferably a linear unsubstituted alkyl group having 10 to 40 carbon atoms.

In formula (7), ^R4 is preferably an alkyl group, and q is preferably an integer of 10 to 12. The number of carbon atoms of ^R4 is more preferably 10 to 40, and ^R4 is further preferably a linear unsubstituted alkyl group having 10 to 40 carbon atoms.

In formula (8), R ⁵ is preferably an alkyl group, and M ¹ is preferably NH ₄ or Na. The number of carbon atoms of R ⁵ is more preferably 10 to 40, and R ⁵ is further preferably a linear unsubstituted alkyl group having 10 to 40 carbon atoms.

In formula (9), ^R6 is preferably an alkyl group, and ^M2 is preferably _NH4 or Na. The number of carbon atoms of ^R6 is more preferably 10 to 40, and ^R6 is further preferably a linear unsubstituted alkyl group having 10 to 40 carbon atoms.

Examples of the compound having an ethylenically unsaturated bond and functioning as a silane coupling agent include vinyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, vinyltriethoxysilane, and γ-methacryloxypropyltriethoxysilane.

[1-6. Content of each structural unit in the copolymer (P)]

In the copolymer (P), the content of the second structural unit derived from the monomer (a2) relative to 100 parts by mass of the first structural unit derived from the monomer (a1) is 1.0 parts by mass or more, preferably 1.5 parts by mass or more, and more preferably 3.0 parts by mass or more. It is to improve the mechanical stability of the copolymer (P). In addition, it is to improve the peel strength of the electrode active material layer containing the copolymer (P) as an electrode binder.

In the copolymer (P), the content of the second structural unit derived from the monomer (a2) is 30 parts by mass or less, preferably 15 parts by mass or less, and more preferably 7.5 parts by mass or less relative to 100 parts by mass of the first structural unit derived from the monomer (a1). This is to suppress the gelation of the copolymer (P). In addition, this is to improve the mechanical stability of the copolymer (P).

In the copolymer (P), the content of the third structural unit derived from the monomer (a3) relative to 100 parts by mass of the first structural unit derived from the monomer (a1) is 0.010 parts by mass or more, preferably 0.030 parts by mass or more, more preferably 0.15 parts by mass or more, and further preferably 0.65 parts by mass or more. It is to improve the peel strength of the electrode active material layer containing the copolymer (P) as an electrode binder. In addition, it is to be able to reduce the DCR of a battery using an electrode containing the copolymer (P) as an electrode binder in an electrode active material layer.

In the copolymer (P), the content of the third structural unit derived from the monomer (a3) relative to 100 parts by mass of the first structural unit derived from the monomer (a1) is 70 parts by mass or less, preferably 40 parts by mass or less, and more preferably 35 parts by mass or less. This is to improve the polymerization stability in free radical polymerization and suppress the generation of coarse particles of the copolymer (P). In addition, this is to suppress the gelation of the copolymer (P).

The mass ratio of the first structural unit, the second structural unit, and the third structural unit in the copolymer (P) is preferably 80 mass % or more, more preferably 85 mass % or more, and further preferably 90 mass % or more. This is because by increasing the content of these structural units, the effect obtained by the present invention becomes better.

In the case where the copolymer (P) contains the fourth structural unit derived from the internal crosslinking agent (a4), the content of the fourth structural unit derived from the internal crosslinking agent (a4) relative to 100 parts by mass of the first structural unit derived from the monomer (a1) is preferably 0.050 parts by mass or more, more preferably 0.075 parts by mass or more, and further preferably 0.50 parts by mass or more. This is to suppress the deterioration of the copolymer (P) and to improve the discharge capacity maintenance rate of a battery using an electrode containing the copolymer (P) as an electrode binder in an electrode active material layer.

When the copolymer (P) contains a structural unit derived from the internal crosslinking agent (a4), the content of the fourth structural unit derived from the internal crosslinking agent (a4) is 20 parts by mass or less, preferably 7.5 parts by mass or less, and more preferably 2.5 parts by mass or less, relative to 100 parts by mass of the first structural unit derived from the monomer (a1) in order to suppress the gelation of the copolymer (P).

[1-7. Glass transition temperature of copolymer (P)]

The glass transition temperature Tg of the copolymer (P) is the peak top temperature of a DDSC chart obtained by DSC measurement using EXSTARDSC/SS7020 manufactured by Hitachi High-Tech Science Corporation at a heating rate of 10°C/min in a nitrogen atmosphere and obtained as the temperature differential of DSC.

The glass transition temperature Tg of the copolymer (P) is preferably -30°C or higher, more preferably -10°C or higher, and further preferably 0°C or higher. This is to improve the cycle characteristics of a nonaqueous secondary battery using the copolymer (P) as an electrode binder.

The glass transition temperature Tg of the copolymer (P) is preferably 100° C. or lower, more preferably 50° C. or lower, and further preferably 30° C. or lower. This is to improve the adhesion of the electrode active material layer containing the copolymer (P) as an electrode binder to the current collector foil.

<2. Synthesis method of copolymer (P)>

The copolymer (P) is obtained by copolymerizing monomers including monomers (a1) to (a3). As monomers, internal crosslinking agents (a4) and other monomers (a5) may also be copolymerized as needed. Here, the monomers used to synthesize the copolymer (P) are sometimes collectively referred to as monomers (a). As a polymerization method, for example, emulsion polymerization of monomers (a) in an aqueous medium (b) can be mentioned. As other components used in the synthesis of the copolymer (P) using emulsion polymerization, for example, non-polymerizable surfactants (c), alkaline substances (d), free radical polymerization initiators (e), chain transfer agents (f), etc. can be mentioned. Below, these components that are required for the synthesis of the copolymer (P) or can be used as needed, and the emulsion polymerization method are explained, but as for the monomer (a), as described above, it is not explained below.

[2-1. Aqueous medium (b)]

The aqueous medium (b) is water, a hydrophilic solvent, or a mixture thereof. As the hydrophilic solvent, methanol, ethanol, isopropanol, and N-methylpyrrolidone etc. can be mentioned. From the viewpoint of polymerization stability, the aqueous medium (b) is preferably water. In addition, as long as the polymerization stability is not impaired, a substance to which a hydrophilic solvent has been added in water can be used as the aqueous medium (b).

[2-2. Non-polymerizable surfactant (c)]

In the emulsion polymerization of the monomer (a), a non-polymerizable surfactant (c) may be used. The surfactant (c) may improve the dispersion stability of the dispersion (emulsion) during and/or after the polymerization. As the surfactant (c), an anionic surfactant or a nonionic surfactant is preferably used.

Examples of the anionic surfactant include alkylbenzene sulfonates, alkyl sulfate ester salts, polyoxyethylene alkyl ether sulfate ester salts, and fatty acid salts.

Examples of the nonionic surfactant include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polycyclic phenyl ethers, polyoxyalkylene alkyl ethers, sorbitan fatty acid esters, and polyoxyethylene sorbitan fatty acid esters.

The above-mentioned surfactants may be used alone or in combination of two or more.

[2-3. Alkaline substances (d)]

When monomer (a) is subjected to emulsion polymerization in aqueous medium (b), alkaline substance (d) may be added. By adding alkaline substance (d), the acidic components contained in monomer (a) can be neutralized and pH can be adjusted. By adjusting pH, the mechanical stability and chemical stability of the dispersion during and/or after emulsion polymerization can be improved.

The pH of the dispersion at 23°C can be appropriately adjusted according to the electrode specifications, the conditions for preparing the slurry described below, etc., and is not limited, but is preferably 1.5 to 10, more preferably 6.0 to 9.0, and even more preferably 5.0 to 9.0. This is to suppress the precipitation of the active material in the electrode slurry described below.

Examples of the alkaline substance (d) include ammonia, triethylamine, sodium hydroxide, lithium hydroxide, etc. These alkaline substances (d) may be used alone or in combination of two or more.

[2-4. Radical polymerization initiator (e)]

The free radical polymerization initiator (e) used in the emulsion polymerization is not particularly limited, and known substances can be used. As the free radical polymerization initiator, for example, persulfates such as ammonium persulfate and potassium persulfate; hydrogen peroxide; azo compounds; organic peroxides such as tert-butyl hydroperoxide, tert-butyl peroxybenzoate, and isopropylbenzene hydroperoxide. Among them, persulfates and organic peroxides are preferred. In this embodiment, the free radical polymerization initiator can be used together with a reducing agent such as sodium bisulfite, Rongalite, and ascorbic acid during emulsion polymerization to perform redox polymerization.

The amount of the free radical polymerization initiator added is preferably 0.10 parts by mass or more relative to 100 parts by mass of the monomer (a), and more preferably 0.80 parts by mass or more. It is to make the conversion rate of the monomer (a) to the copolymer (P) high during polymerization. The amount of the free radical polymerization initiator added is preferably 3.0 parts by mass or less relative to 100 parts by mass of the monomer (a), and more preferably 2.0 parts by mass or less. It is to increase the molecular weight of the copolymer (P) and reduce the swelling rate of the electrode active material layer in the electrolyte.

[2-5. Chain transfer agent (f)]

The chain transfer agent (f) is used in the emulsion polymerization to adjust the molecular weight of the copolymer (P). Examples of the chain transfer agent (f) include n-dodecyl mercaptan, tert-dodecyl mercaptan, n-butyl mercaptan, 2-ethylhexyl mercaptoacetate, 2-mercaptoethanol, β-mercaptopropionic acid, methanol, n-propanol, isopropanol, tert-butanol, benzyl alcohol, and the like.

[2-6. Emulsion polymerization method]

As the emulsion polymerization method, for example, a method of performing emulsion polymerization while continuously supplying each component used for emulsion polymerization can be mentioned. The temperature of the emulsion polymerization is not particularly limited, and is, for example, 30 to 90° C., preferably 50 to 85° C., and more preferably 55 to 80° C. The emulsion polymerization is preferably performed while stirring. In addition, the monomer (a) and the free radical polymerization initiator are preferably continuously supplied in a uniform manner in the reaction vessel.

<3. Binders for non-aqueous secondary battery electrodes>

The non-aqueous secondary battery electrode binder (or non-aqueous secondary battery electrode binder. Hereinafter, sometimes referred to as "electrode binder") of this embodiment includes the copolymer (P) described above. The electrode binder may include other components, for example, a polymer other than the copolymer (P), a surfactant, etc.

Here, the electrode binder is composed of components that do not volatilize and remain in the heating process in the manufacturing process of the battery described later. Specifically, the components constituting the electrode binder are components that remain after weighing 1 g of a mixture containing the electrode binder in an aluminum dish with a diameter of 5 cm, and drying it at 105° C. for 1 hour in a dryer while circulating air under atmospheric pressure.

The content of the copolymer (P) in the electrode binder is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 95% by mass or more, and further preferably 98% by mass or more. This is to increase the contribution of the copolymer (P) to the effect of the present invention.

<4. Binder composition for non-aqueous secondary battery electrodes>

Regarding the binder composition for non-aqueous secondary battery electrodes of the present embodiment (hereinafter, sometimes referred to as the binder composition), the copolymer (P) is dispersed in an aqueous medium (B). In addition to these components, the binder composition may also include, for example, the above-mentioned components used to synthesize the copolymer (P). The binder composition may be a dispersion obtained by a method for synthesizing the copolymer (P), a dispersion obtained by dispersing a copolymer (P) obtained by a method other than emulsion polymerization in an aqueous medium (B), or a dispersion obtained by other methods.

[4-1. Aqueous medium (B)]

The aqueous medium (B) is water, a hydrophilic solvent, or a mixture thereof. Examples of hydrophilic solvents are as given in the description of the aqueous medium (b) in the synthesis of the copolymer (P). The aqueous medium (B) may be the same as or different from the aqueous medium (b) used to synthesize the copolymer (P).

The aqueous medium (B) may be the aqueous medium (b) used for synthesizing the copolymer (P) as it is, or may be a composition in which an aqueous solvent is added to the aqueous medium (b), or may be a composition in which the aqueous medium (b) is replaced with a new aqueous solvent after the synthesis of the copolymer (P). Here, the added or replaced aqueous solvent may have the same composition as the solvent used for synthesizing the copolymer (P), or may have a different composition.

[4-2. Non-volatile content concentration of adhesive composition]

The non-volatile component concentration of the adhesive composition is preferably 20% by mass or more, more preferably 25% by mass or more, and further preferably 30% by mass or more. This is to increase the amount of active ingredients contained in the adhesive composition. The non-volatile component concentration of the adhesive composition can be adjusted by the amount of the aqueous medium (B).

The nonvolatile content of the adhesive composition is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less. This is to suppress an increase in the viscosity of the adhesive composition and facilitate preparation of the slurry described below.

<5. Slurry for non-aqueous secondary battery electrodes>

Next, the non-aqueous secondary battery electrode slurry (hereinafter, sometimes referred to as "electrode slurry") is described in detail. The electrode slurry has a structure in which the copolymer (P) and the electrode active material are dispersed in an aqueous medium. In addition to these components, the electrode slurry may also contain a thickener, a conductive additive, the above-mentioned components used in the synthesis of the copolymer (P), etc.

[5-1. Content of copolymer (P)]

The content of the copolymer (P) is preferably 0.50 parts by mass or more, more preferably 1.0 parts by mass or more, based on 100 parts by mass of the electrode active material, in order to fully demonstrate the effects of the copolymer (P).

The content of the copolymer (P) is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, and further preferably 3.0 parts by mass or less, relative to 100 parts by mass of the electrode active material, in order to increase the content of the electrode active material in the electrode active material layer produced using the electrode slurry.

〔5-2. Electrode active material〕

The electrode active material is a material that can intercalate/deintercalate ions such as lithium ions that serve as charge carriers. The ions that serve as charge carriers are preferably alkali metal ions, more preferably lithium ions, sodium ions, and potassium ions, and even more preferably lithium ions.

When the electrode is a negative electrode, the electrode active material, i.e., the negative electrode active material, preferably includes at least one of a carbon material, a material including silicon, and a material including titanium. As the carbon material used as the electrode active material, for example, coke such as petroleum coke, asphalt coke, coal coke, carbide of organic polymer, graphite such as artificial graphite, and natural graphite can be cited. As the material including silicon, for example, silicon compounds such as silicon simple substance and silicon oxide can be cited. As the material including titanium, for example, lithium titanate can be cited. These materials can be used alone, or they can be mixed or composited and used.

The negative electrode active material preferably contains at least one of a carbon material and a material containing silicon, and more preferably contains a carbon material, because the copolymer (P) has a very large effect of improving the binding properties between the electrode active materials and between the electrode active materials and the current collector.

When the electrode is a positive electrode, the electrode active material, i.e., the positive electrode active material, uses a material having a higher standard electrode potential than the negative electrode active material. As the positive electrode active material, lithium composite oxides containing nickel such as Ni-Co-Mn lithium composite oxides, Ni-Mn-Al lithium composite oxides, Ni-Co-Al lithium composite oxides, lithium cobalt oxide (LiCoO ₂ ), spinel lithium manganese oxide (LiMn ₂ O ₄ ), olivine lithium iron phosphate, TiS ₂ , MnO ₂ , MoO ₃ , V ₂ O ₅ and other chalcogen compounds can be cited. As the positive electrode active material, these substances can be used alone, or two or more can be used in combination.

〔5-3. Thickener〕

As thickeners, celluloses such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, hydroxypropyl cellulose, ammonium salts of celluloses, alkali metal salts of celluloses, polyvinyl alcohol, polyvinyl pyrrolidone, etc. can be cited. The thickener preferably contains at least one of carboxymethyl cellulose, ammonium salts of carboxymethyl cellulose, and alkali metal salts of carboxymethyl cellulose. This is to facilitate the dispersion of the electrode active material in the electrode slurry.

The content of the thickener in the electrode slurry is preferably 0.50 parts by mass or more, more preferably 0.80 parts by mass or more relative to 100 parts by mass of the electrode active material, in order to improve the binding properties between the electrode active materials and between the electrode active material and the current collector in the electrode active material layer produced using the electrode slurry.

The content of the thickener in the electrode slurry is preferably 3.0 parts by mass or less, more preferably 2.0 parts by mass or less, and further preferably 1.5 parts by mass or less, based on 100 parts by mass of the electrode active material, in order to improve the coating properties of the electrode slurry.

〔5-4. Aqueous medium〕

The aqueous medium is water, a hydrophilic solvent, or a mixture thereof. Examples of hydrophilic solvents are as given in the description of the aqueous medium (b) in the synthesis of the copolymer (P). The aqueous medium contained in the electrode slurry and the aqueous medium (B) contained in the binder composition or the aqueous medium (b) used in the synthesis of the copolymer (P) may be the same or different.

〔5-5. Conductive additive〕

As the conductive auxiliary agent, carbon black, carbon fiber, etc. are preferably used. As carbon black, furnace black, acetylene black, DENKA BLACK (registered trademark) (made by DENKA Co., Ltd.), KETCHEN BLACK (registered trademark) (made by KETCHEN BLACK Instrumental Co., Ltd.), etc. can be cited. Carbon fiber can include carbon nanotubes, carbon nanofibers, etc. As carbon nanotubes, VGCF (registered trademark, made by Showa Denko Co., Ltd.) as vapor grown carbon fiber can be cited as a preferred example.

〔5-6. Properties of electrode slurry〕

The non-volatile component concentration of the electrode slurry is preferably 20% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass or more. This is to increase the concentration of the active ingredient in the electrode slurry so that a sufficient amount of electrode active material layer can be formed with a small amount of electrode slurry. The non-volatile component concentration of the electrode slurry can be adjusted by the amount of the aqueous medium in the electrode slurry.

The nonvolatile content of the electrode slurry is preferably 85% by mass or less, more preferably 75% by mass or less, and still more preferably 65% by mass or less. This is to maintain good coating properties of the electrode slurry.

The viscosity of the electrode slurry is preferably 20000 mPa·s or less, more preferably 10000 mPa·s or less, and further preferably 5000 mPa·s or less. This is to improve the coating property of the electrode slurry on the current collector and the productivity of the electrode. The viscosity of the electrode slurry is greatly affected by the concentration of the non-volatile component of the electrode slurry and the type and amount of the thickener.

The pH of the electrode slurry at 23° C. is not limited and can be appropriately adjusted according to the electrode specifications and production conditions, but is preferably 2.0 to 10, more preferably 4.0 to 9.0, and even more preferably 6.0 to 9.0. This is to improve the durability of a battery produced using the electrode slurry.

[5-7. Method for producing electrode slurry]

As a method for modulating the electrode slurry in this embodiment, a method of mixing a binder composition, an electrode active material, a thickener as needed, an aqueous medium as needed, a conductive aid as needed, and other components as needed can be cited, but it is not limited thereto. The order of the added components is not particularly limited, as long as it is appropriately determined. As a mixing method, a method using a mixing device such as a stirring type, a rotating type, and an oscillating type can be cited.

＜6. Non-aqueous secondary battery electrodes＞

The non-aqueous secondary battery electrode (hereinafter sometimes referred to as an "electrode") involved in the present embodiment comprises a current collector and an electrode active material layer formed on the current collector. As the shape of the electrode, for example, a laminate and a wound body can be cited, but there is no particular limitation. In addition, the formation range of the electrode active material layer on the current collector is not particularly limited, and it can be formed on the entire surface of the current collector or on a part of the surface of the current collector. When the current collector is in the shape of a plate, foil, etc., the electrode active material layer can be formed on both sides of the current collector or only on one side.

〔6-1. Current Collector〕

The current collector is preferably a metal sheet having a thickness of 0.001 mm to 0.5 mm, and examples of the metal include iron, copper, aluminum, nickel, stainless steel, etc. When the nonaqueous secondary battery electrode is a negative electrode of a lithium ion secondary battery, the current collector is preferably copper foil.

[6-2. Electrode active material layer]

The electrode active material layer according to the present embodiment includes an electrode binder and an electrode active material. The electrode active material layer may include a conductive additive, a thickener, etc. The components listed here are as described above.

[6-3. Method for manufacturing electrode]

The electrode can be produced, for example, by applying an electrode slurry onto a current collector, drying the slurry to form an electrode active material layer, and then cutting the slurry into a suitable size.

The method for coating the electrode slurry on the collector is not particularly limited, and examples thereof include reverse roll method, direct roll method, scraper method, knife method, extrusion method, curtain method, gravure method, rod method, immersion method, extrusion method, etc. Among them, if various physical properties such as viscosity and dryness of the electrode slurry are considered, scraper method, knife method, or extrusion method is preferably used. This is to obtain an electrode active material layer with a smooth surface and small thickness deviation.

The electrode slurry can be applied only on one side of the collector or on both sides. When the electrode slurry is applied on both sides of the collector, it can be applied one side at a time or on both sides at the same time. In addition, the electrode slurry can be applied to the collector continuously or intermittently. The coating amount of the electrode slurry can be appropriately determined according to the design capacity of the battery and the composition of the electrode slurry. Although the coating amount of the electrode slurry is related to the properties of the electrode slurry, it is preferably less than 13 mg/ ^cm2 (the coating amount on each side when applied on both sides). This is to suppress the occurrence of cracks on the electrode surface during the drying process of the electrode slurry.

The electrode slurry coated on the collector is dried to form an electrode active material layer on the collector. The drying method of the electrode slurry is not particularly limited, and for example, hot air, reduced pressure or vacuum environment, (far) infrared, low temperature wind can be used alone or in combination. The drying temperature and drying time of the electrode slurry can be appropriately adjusted by the concentration of non-volatile components in the electrode slurry, the coating amount on the collector, etc. The drying temperature is preferably above 40°C and below 350°C, and from the perspective of productivity, it is more preferably above 60°C and below 100°C. The drying time is preferably above 1 minute and below 30 minutes.

The electrode sheet having an electrode active material layer formed on a current collector can be cut into a suitable size and shape for use as an electrode. The electrode sheet can be cut by any method, such as slit, laser, wire cutting, cutting machine, Thomson, etc.

Before or after the electrode sheet is cut, the electrode sheet can be pressed as needed. The electrode active material is thereby firmly bonded by the current collector, and the miniaturization of the non-aqueous battery brought about by making the electrode thin can be further achieved. As a method of pressing, a general method can be used, and a mold pressing method or a roller press method is particularly preferred. In the case of the mold pressing method, the pressing pressure is not particularly limited, but is preferably 0.5t/cm2 or ^more and 5t/cm2 ^or less. In the case of the roller press method, the line pressure is not particularly limited, but is preferably 0.5t/cm2 or more and 5t/cm2 or less. This is to obtain the above-mentioned effects brought about by pressing, while suppressing the reduction in the insertion and removal capacity of charge carriers such as lithium ions to the electrode active material.

＜7. Non-aqueous secondary batteries＞

As a preferred example of the non-aqueous secondary battery involved in the present embodiment, a lithium-ion secondary battery is described, but the composition of the battery is not limited to that described here. The non-aqueous secondary battery involved in the present embodiment contains a positive electrode, a negative electrode, an electrolyte, and components such as a separator as required in an outer casing, and one or both of the positive electrode and the negative electrode use an electrode made by the above method. In the non-aqueous secondary battery involved in the present embodiment, at least one of the positive electrode and the negative electrode contains a copolymer (P) in the electrode binder, but preferably at least the negative electrode contains a copolymer (P).

〔7-1. Electrolyte〕

As the electrolyte, a non-aqueous liquid having ion conductivity is used. As the electrolyte, a solution in which an electrolyte is dissolved in an organic solvent, an ionic liquid, etc. can be cited, but the former is preferred. This is to reduce the manufacturing cost and obtain a non-aqueous battery with low internal resistance.

As the electrolyte, an alkali metal salt can be used, and it can be appropriately selected according to the type of electrode active material, etc. As the electrolyte, for example, LiClO ₄ , LiBF ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiB ₁₀ Cl ₁₀ , LiAlCl ₄ , LiCl, LiBr, LiB(C ₂ H ₅ ) ₄ , CF ₃ SO ₃ Li, CH ₃ SO ₃ Li, LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , Li(CF ₃ SO ₂ ) ₂ N, aliphatic lithium carboxylate, etc. In addition, other alkali metal salts can also be used as the electrolyte.

As the organic solvent for dissolving the electrolyte, there is no particular limitation, and for example, carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC) can be cited; nitrile compounds such as acetonitrile; carboxylates such as ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. These organic solvents can be used alone or in combination of two or more. Among them, it is preferred to use a substance that combines straight-chain carbonate-based solvents. As straight-chain carbonate-based solvents, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate can be cited.

[7-2. Outer body]

As the outer casing, for example, a laminate of aluminum foil and resin film can be used appropriately, but is not limited thereto. The shape of the battery can be any shape such as coin, button, sheet, cylinder, square, flat, etc.

Example

In the following examples, as an example of the composition of the present invention, a negative electrode of a lithium ion secondary battery and a lithium ion secondary battery are made, and the effect of the present invention is confirmed by comparing the negative electrode of the lithium ion secondary battery and the lithium ion secondary battery involved in the comparative example. In addition, the present invention is not limited thereto. In addition, the water used in the following examples and comparative examples is ion exchange water unless otherwise specified.

<1. Synthesis of copolymer (P) and preparation of adhesive composition>

In each of the examples and comparative examples, the monomer (a), surfactant (c), and chain transfer agent (f) were mixed with water as an aqueous medium (b) in the amounts shown in Tables 1 and 2, and emulsified to prepare a monomer emulsion. In addition, an 80% by mass aqueous solution of acrylic acid was used, and the amount of acrylic acid shown in the table is the amount of acrylic acid contained in the aqueous solution. In addition, the amount of water added here was 360 parts by mass in total with the amount of water contained in the acrylic acid aqueous solution.

Regarding the monomer (a3), the structures of the compound (a3-1), the compound (a3-2), and the compound (a3-3) in Table 1 and Table 2 are as follows.

Compound (a3-1), compound (a3-2), and compound (a3-3) are all tertiary block copolymers in which a block consisting of a structural unit (10), a block consisting of a structural unit (11), and a block consisting of a structural unit (12) are arranged in sequence. The structures of the structural units (10), (11), and (12) are as shown below. Compound (a3-1) contains 30 mol% of the structural unit (10), 69 mol% of the structural unit (11), and 1.0 mol% of the structural unit (12), and the weight average molecular weight converted to polystyrene is 50,000. Compound (a3-2) contains 93 mol% of the structural unit (10), 6.0 mol% of the structural unit (11), and 1.0 mol% of the structural unit (12), and the weight average molecular weight converted to pullulan is 80,000. The compound (a3-3) contained 96 mol% of the structural unit (10), 1.0 mol% of the structural unit (11), and 3.0 mol% of the structural unit (12), and had a pullulan-equivalent weight average molecular weight of 80,000.

In a separable flask equipped with a cooling tube, a thermometer, a stirrer, and a dropping funnel, 169 parts by mass of water were added, and the temperature was raised to 75° C. In the separable flask, the above-mentioned monomer emulsion and an aqueous solution obtained by dissolving 2.2 parts by mass of potassium persulfate as a polymerization initiator (e1) in 48 parts by mass of water were added dropwise at 80° C. while stirring for 4 hours to carry out emulsion polymerization. After the monomer emulsion and the potassium persulfate aqueous solution were added dropwise, the mixed solution in the separable flask was stirred at 80° C. for 60 minutes.

Next, 0.81 parts by mass of tert-butyl peroxybenzoate (produced by Chemical Aquassor Co., Ltd., Kayabutelu B) and 0.52 parts by mass of ascorbic acid as a polymerization initiator (e2) were dissolved in an aqueous solution of 8.1 parts by mass of water. Next, an aqueous solution of 2.3 parts by mass of tert-butyl hydroperoxide (produced by Chemical Aquassor Co., Ltd., Kayabutelu H-70) as a polymerization initiator (e3) was dissolved in 15 parts by mass of water, and an aqueous solution of 2.3 parts by mass of Rongalite was dissolved in 15 parts by mass of water was added dropwise, and the mixture was stirred for 60 minutes to polymerize. Then, an aqueous emulsion of the copolymer (P) involved in the embodiment and an aqueous emulsion of the polymer (CP) involved in the comparative example were obtained.

The obtained aqueous emulsion was cooled to room temperature. Then, in order to neutralize the aqueous emulsion, 19 parts by mass of 25% ammonia water and 155 parts by mass of water were added to a separable flask to obtain an adhesive composition containing the copolymer (P) involved in the example and an adhesive composition containing the polymer (CP) involved in the example. In addition, the amount of ammonia shown in Table 1 and Table 2 represents the amount of ammonia contained in the ammonia water.

The total amount of water used in the synthesis step of the copolymer (P) and the synthesis step of the polymer (CP) is shown in Tables 1 and 2. The amount of water shown in Tables 1 and 2 includes not only water added independently but also water contained in the above-mentioned acrylic acid aqueous solution and ammonia water, and water added together with the polymerization initiator and the like.

<2. Evaluation of copolymer (P), polymer (CP) and adhesive composition>

The physical properties and performance evaluations of the binder compositions obtained in Examples and Comparative Examples and batteries obtained using the binder compositions were performed by the following methods. The evaluation results are shown in Tables 1 and 2.

[2-1. Glass transition temperature Tg of copolymer (P) and polymer (CP)]

The adhesive composition was cast on a polyethylene sheet, dried at 50° C. for 5 hours, and then vacuum dried at 50° C. and 98 kPa for 1 hour to obtain a film having a thickness of 0.5 mm.

The obtained film was cut into 2 mm × 2 mm, sealed in an aluminum pan, and DSC was measured using EXSTAR DSC/SS7020 manufactured by Hitachi High-Tech Science Co., Ltd. at a heating rate of 10°C/min in a nitrogen atmosphere. The peak temperature of the DDSC graph obtained as the temperature differential of DSC was measured, and the temperature was set as the glass transition temperature Tg (°C) of the copolymer (P) and the polymer (CP). The measurement temperature range was set to -40°C to 200°C.

〔2-2. Non-volatile component concentration〕

1 g of the adhesive composition was weighed in an aluminum dish having a diameter of 5 cm, and dried in a dryer at 105° C. for 1 hour while circulating air under 1 atmosphere (1013 hPa), and the mass of the remaining components was measured. The ratio (mass %) of the mass of the above-mentioned components remaining after drying to the mass (1 g) of the adhesive composition before drying was calculated as the non-volatile component concentration.

〔2-3. Amount of coarse particles〕

1L of the adhesive composition was sieved using a metal mesh with a mesh size of 300 meshes, and then the metal mesh was washed with ion exchange water to remove the liquid residual components of the adhesive composition. The washed metal mesh was dried at 105°C for 1 hour in a dryer under 1 atmosphere (1013hPa) while circulating air, and the mass of the metal mesh was measured. The difference between the mass of the dried metal mesh and the mass of the metal mesh alone was calculated as the coarse particle amount (g/L).

＜3. Evaluation of electrode and battery performance＞

Using the binder composition prepared in each of the Examples and Comparative Examples, a negative electrode and a lithium ion secondary battery were prepared and evaluated.

〔3-1. Battery production〕

[3-1-1. Preparation of positive electrode]

To a mixture of 94 parts by mass of LiNi _0.6 Mn _0.2 Co _0.2 O ₂ as a positive electrode active material, 3 parts by mass of acetylene black as a conductive aid, and 3 parts by mass of polyvinylidene fluoride as a binder, 50 parts by mass of N-methylpyrrolidone was added and further mixed to prepare a positive electrode slurry.

The positive electrode slurry was applied to both sides of a 15 μm thick aluminum foil (positive electrode current collector) by direct roll method. The amount of positive electrode slurry applied to the positive electrode current collector was adjusted so that the thickness after the roll press process described later would be 125 μm per side.

The positive electrode slurry coated on the positive electrode collector was dried at 120°C for 5 minutes and pressed by a roller press (manufactured by Suntech Metal Co., Ltd., with a press load of 5 t and a roller width of 7 cm) to obtain a positive electrode sheet having a positive electrode active material layer formed thereon. The obtained positive electrode sheet was cut into 50 mm × 40 mm, and a conductive tab was installed to produce a positive electrode.

[3-1-2. Preparation of negative electrode]

100 parts by mass of artificial graphite (G49, manufactured by Jiangxi Zichen Technology Co., Ltd.) as a negative electrode active material, 3.9 parts by mass of the binder composition prepared in each embodiment and comparative example (1.5 parts by mass as a non-volatile component), and 62 parts by mass of a 2% aqueous solution of CMC (carboxymethylcellulose-sodium salt/SANLOZ (registered trademark) MAC500LC manufactured by Nippon Paper Chemical Co., Ltd.) were mixed, and 28 parts by mass of water were further added to obtain a negative electrode slurry.

The negative electrode slurry was applied on both sides of a 10 μm thick copper foil (negative electrode current collector) by direct roll method. The amount of negative electrode slurry applied to the negative electrode current collector was adjusted so that the thickness after the roll press process described later would be 170 μm per side.

The negative electrode slurry coated on the negative electrode collector was dried at 90°C for 10 minutes and pressed by a roller press (manufactured by Suntech Metal Co., Ltd., with a press load of 8 tons and a roller width of 7 cm) to obtain a negative electrode sheet having a negative electrode active material layer formed on the collector. The obtained negative electrode sheet was cut into 52 mm × 42 mm, and a conductive connector was installed to produce a negative electrode.

[3-1-3. Battery production]

A separator made of a porous film of a polyolefin system (made of polyethylene, 25 μm) is placed between the positive electrode and the negative electrode, and is housed in an aluminum laminated outer package (battery pack) in a manner that the positive electrode active material layer and the negative electrode active material layer are opposite to each other. The outer package is filled with an electrolyte and vacuum impregnated, and packaged with a vacuum heat sealer to produce a lithium ion secondary battery for evaluation. In 99 parts by mass of a solution obtained by dissolving LiPF ₆ at 1.0 mol/L in a mixed solvent of ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) = 30/50/20 (volume ratio), 1 part by mass of vinylene carbonate is mixed to produce an electrolyte.

[3-2. Evaluation of electrodes and batteries]

[3-2-1. Peel strength of negative electrode active material layer (electrode performance)]

The peel strength of the negative electrode active material layer to the current collector was measured as follows. The negative electrode sheet after pressing in the above-mentioned negative electrode production process was cut into a size of 25mm×100mm to prepare a test piece. The negative electrode active material layer on the test piece was bonded to a SUS plate with a width of 50mm and a length of 200mm using a double-sided tape (NITTOTAPE (registered trademark) No. 5, manufactured by Nitto Denko (Co., Ltd.)) in such a way that the center of the test piece was aligned with the center of the SUS plate. In addition, the double-sided tape was bonded in a way that covered the entire range of the test piece.

After the test piece was placed in a state where it was bonded to the SUS plate for 10 minutes, the negative electrode active material layer was peeled off 20 mm from one end of the test piece in the length direction, and the test piece on the copper foil side was folded back 180°, and the part (the copper foil side of the part of the test piece where the negative electrode active material layer was peeled off) was clamped with the upper chuck of the testing machine. Further, one end of the SUS plate on the side where the negative electrode active material layer was peeled off was clamped with the lower chuck. In this state, the copper foil was peeled off from the test piece at a speed of 100±10 mm/min, and a graph of peeling length (mm)-peeling force (mN) was obtained. In the obtained graph, the average value (mN) of the peeling force at a peeling length of 10 to 45 mm was calculated, and the value obtained by dividing the average value of the peeling force by the width of the test piece 25 mm was set as the peeling strength (mN/mm) of the negative electrode active material layer. In addition, in any of the embodiments and comparative examples, no peeling between the double-sided tape and the SUS plate, and no interfacial peeling between the double-sided tape and the negative electrode active material layer occurred during the test.

[3-2-2. Cycle capacity maintenance rate at high temperature (battery performance)]

The cycle capacity maintenance rate of the battery at high temperature was measured by repeating the following steps (i) to (iv) at 45° C. Here, one cycle was defined as one operation of the series of steps (i) to (iv).

(i) The battery is charged at a current of 1 C until the voltage reaches 4.2 V (constant current (CC) charging).

(ii) The battery is charged at a voltage of 4.2 V until the current reaches 0.05 C (constant voltage (CV) charging).

(iii) Allow to stand for 30 minutes.

(iv) Discharging at a current of 1 C until the voltage reaches 2.75 V (constant current (CC) discharge).

The time integral value of the current in steps (i) and (ii) is set as the charge capacity, and the time integral value of the current in step (iv) is set as the discharge capacity. The discharge capacity of the first cycle and the discharge capacity of the 100th cycle were measured. 100×(discharge capacity of the 100th cycle)/(discharge capacity of the first cycle)[%] was calculated as the cycle capacity retention rate of the battery at high temperature, and is shown in Tables 1 and 2.

〔3-2-3. Internal resistance (DCR)〕

The internal resistance (DCR (Ω)) test of the battery was conducted at 25°C by the following steps. A constant current charge of 0.2C was performed from the static potential until 3.6V, and the state of charge was made 50% of the initial capacity (SOC50%). Then, a discharge was performed for 60 seconds at each current value of 0.2C, 0.5C, 1C and 2C. The DCR (Ω) at SOC50% was determined from the relationship between these four current values (values for 1 second) and the voltage. The results are shown in Tables 1 and 2.

＜4. Evaluation results＞

If the evaluation results of each embodiment are observed, it can be seen that in any embodiment, the amount of coarse particles in the binder composition is small, so the copolymer (P) involved in these embodiments can suppress the generation of coarse particles. It can be seen that in the evaluation of the electrode, the peel strength of the negative electrode active material layer of the electrode involved in any embodiment is high. In the evaluation of the battery, the battery involved in any embodiment has a low internal resistance and a high discharge capacity retention rate (excellent cycle characteristics).

In Comparative Example 1, a polymer not having the third structural unit derived from the monomer (a3) was synthesized. The electrode produced using the polymer of Comparative Example 1 as an electrode binder had low peel strength of the negative electrode active material layer and a battery produced using the electrode had high internal resistance.

In Comparative Example 2, a polymer containing the third structural unit derived from the monomer (a3) in excess was synthesized, but gelation proceeded, and an electrode and a battery using the polymer as an electrode binder could not be produced.

In Comparative Example 3, a polymer not including the second structural unit derived from the monomer (a2) was synthesized. The electrode produced using the polymer according to Comparative Example 3 as an electrode binder had a low peel strength of the negative electrode active material layer.

In Comparative Example 4, a polymer containing the second structural unit derived from the monomer (a2) in excess was synthesized, but gelation proceeded, and an electrode and a battery using the polymer as an electrode binder could not be produced.

In Comparative Example 5, a polymer containing an excessive amount of the fourth structural unit derived from the internal crosslinking agent (a4) was synthesized, but gelation proceeded, and an electrode and a battery using the polymer as an electrode binder could not be produced.

The above results show that the copolymer (P) having the structure according to the present invention can effectively improve the peel strength of the electrode active material layer to the collector in a non-aqueous secondary battery, and can contribute to the reduction of the internal resistance of the battery and the improvement of the cycle characteristics.

Claims (15)

1. A copolymer characterized by being a polymer of a compound having an ethylenically unsaturated bond, which has a1 st structural unit derived from the monomer (a 1), a2 nd structural unit derived from the monomer (a 2), and a3 rd structural unit derived from the monomer (a 3); alternatively, it has a1 st structural unit derived from the monomer (a 1), a2 nd structural unit derived from the monomer (a 2), a3 rd structural unit derived from the monomer (a 3), and a4 th structural unit derived from the internal crosslinking agent (a 4),

The monomer (a 1) is a nonionic compound having an ethylenically unsaturated bond, neither a hydroxyl group nor a cyano group, having no polyoxyalkylene structure, and having no independent plural ethylenically unsaturated bonds,

the monomer (a 2) is a compound having an ethylenically unsaturated bond and an anionic functional group and having neither a polyoxyalkylene structure nor independent plural ethylenically unsaturated bonds,

the monomer (a 3) has a 31 st structural unit represented by the following formula (1), a 32 nd structural unit represented by the following formula (2), and a 33 rd structural unit represented by the following formula (3),

the copolymer contains 1.0 to 30 parts by mass of the 2 nd structural unit per 100 parts by mass of the 1 st structural unit,

the 3 rd structural unit is contained in an amount of 0.010 to 70 parts by mass based on 100 parts by mass of the 1 st structural unit,

the content of the 4 th structural unit is 0 to 20 parts by mass based on 100 parts by mass of the 1 st structural unit,

the internal crosslinking agent (a 4) is a compound which does not belong to the monomer (a 3), has a plurality of independent ethylenic unsaturated bonds, and is capable of forming a crosslinked structure in radical polymerization of a monomer including the monomer (a 1), the monomer (a 2), and the monomer (a 3),

In formula (2), R ¹ An alkyl group having 1 to 6 carbon atoms which may have a branched chain;

in formula (3), R ² Is a functional group having an ethylenically unsaturated bond.

2. The copolymer according to claim 1,

the monomer (a 3) relative to the total amount of the 31 st structural unit, the 32 nd structural unit, and the 33 rd structural unit

Comprising 5.0 mol% or more and 98 mol% or less of the 31 st structural unit,

comprising 0.30 mol% or more and 90 mol% or less of the 32 nd structural unit,

the 33 th structural unit is contained in an amount of 0.30 mol% or more and 10 mol% or less.

3. The copolymer according to claim 1 or 2, wherein the monomer (a 3) has a plurality of independent ethylenic unsaturated bonds in 1 molecule.

4. The copolymer according to any one of claims 1 to 3, wherein in the formula (3), R ² Having a moiety selected from the group consisting of vinyloxy, allyloxy, (meth) acryl, (meth) acryloyloxy, and-OCH ₂ -CH ₂ -CH ₂ ＝CH ₂ At least any 1 of the above.

5. The copolymer according to any one of claims 1 to 4, in the formula (3)，R ² Represented by the following formula (4),

-R ²¹ -R ²² (4)

in formula (4), R ²¹ Is an alkylene group having 1 to 5 carbon atoms which may have a branched chain, R ²² Is 1 functional group selected from the group consisting of vinyloxy, allyloxy, (meth) acryl, and (meth) acryloyloxy.

6. The copolymer according to any one of claims 1 to 5, the monomer (a 3) having a1 st block comprising the 31 st structural unit, a2 nd block comprising the 32 nd structural unit, and a3 rd block comprising the 33 rd structural unit.

7. The copolymer according to any one of claims 1 to 6, wherein the monomer (a 3) contains the 31 st structural unit, the 32 nd structural unit, and the 33 rd structural unit in a total of 90 mass% or more.

8. The copolymer according to any one of claims 1 to 7, wherein the monomer (a 1) has no polar functional group.

9. The copolymer according to any one of claims 1 to 8, wherein the monomer (a 2) is a compound having at least any one of a carboxyl group and a sulfo group.

10. The copolymer according to any one of claims 1 to 9, which contains the 1 st structural unit, the 2 nd structural unit, and the 3 rd structural unit in a total of 80 mass% or more.

11. The copolymer according to any one of claims 1 to 10, wherein the content of the 4 th structural unit is 0.050 parts by mass or more relative to 100 parts by mass of the 1 st structural unit.

12. A binder composition for nonaqueous secondary battery electrodes, wherein the copolymer according to any one of claims 1 to 11 is dispersed in an aqueous medium.

13. A binder for nonaqueous secondary battery electrodes, comprising the copolymer according to any one of claims 1 to 11.

14. A slurry for a nonaqueous secondary battery electrode, comprising the binder according to claim 13, an electrode active material, and an aqueous medium,

the binder and the electrode active material are dispersed in an aqueous medium,

the aqueous medium is 1 medium selected from the group consisting of water, hydrophilic solvents, and mixtures comprising water and hydrophilic solvents.

15. A nonaqueous secondary battery electrode comprising the binder of claim 13.