Binder for lithium-ion battery, aqueous binder solution for lithium-ion battery, electrode slurry for lithium-ion battery, electrode for lithium-ion battery, and lithium-ion battery

Abstract

This binder for a lithium-ion battery contains a water-soluble polymer (A). The water-soluble polymer (A) contains, relative to all structural units in the water-soluble polymer (A), 5-80 mol% of a structural unit derived from a hydroxyl group-containing vinyl ether (a). The viscosity of an aqueous solution containing 13 mass% of the water-soluble polymer (A) as measured by a B-type viscometer at 23°C is 3,000 mPa·s or more. Said binder is used for forming an electrode that includes an active material exhibiting a pH of 10 or more in an active-material aqueous dispersion as measured by a prescribed method.

Description

Binder for lithium ion battery, aqueous binder solution for lithium ion battery, electrode slurry for lithium ion battery, electrode for lithium ion battery, and lithium ion battery

[0001] The present invention relates to a binder for a lithium ion battery, an aqueous binder solution for a lithium ion battery, an electrode slurry for a lithium ion battery, an electrode for a lithium ion battery, and a lithium ion battery.

[0002] Lithium-ion batteries are used in electronic devices such as mobile phones and laptops, and in recent years, they have also been increasingly used in automotive and home energy storage applications. Lithium-ion batteries consist of components such as a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes are manufactured by applying electrode slurry, a mixture of an active material capable of inserting and extracting lithium ions, a conductive additive, a binder, a solvent, and other materials, onto a current collector and then drying the mixture.

[0003] Polyvinylidene fluoride (hereinafter also referred to as "PVDF") and the like have been widely used as binders for forming electrodes. However, PVDF needs to be dissolved in an organic solvent such as N-methylpyrrolidone, and an organic solvent has also been used as the liquid medium for electrode slurries. However, in recent years, from the viewpoints of environmental impact and cost, there has been an increasing demand for processes that use electrode slurries that use water as the liquid medium (hereinafter also referred to as "aqueous slurries"). For this reason, water-soluble binders that can be used in aqueous slurries have been developed.

[0004] Patent Document 1 describes an electrode binder containing a polymer containing a structural unit derived from a (meth)acrylic acid alkyl ester monomer and a structural unit derived from an ester monomer having a specific aromatic group.

[0005] Japanese Patent Application Laid-Open No. 2023-121816

[0006] With the recent expansion of applications for lithium-ion batteries, various active materials with functions suited to specific applications have been developed and put into practical use. Some of these active materials exhibit high pH when dispersed in water. However, the inventors' research has revealed that when an aqueous slurry is produced using an active material with a high pH and an acrylic polymer, the electrode cracks due to degradation of the acrylic polymer. Furthermore, binders for aqueous slurries are required to have dispersion stability that suppresses the settling of solids, such as the active material, in the slurry state. Suppressing the settling of solids in the slurry state is desirable because it can cause variations in the thickness of the active material layer formed from the slurry and deteriorate the coatability.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a lithium ion battery binder that can form an electrode even when an aqueous slurry is produced together with an active material that exhibits a high pH, ​​and that has excellent dispersion stability of the solid content in the slurry state; an aqueous lithium ion battery binder solution that uses the lithium ion battery binder, a lithium ion battery electrode slurry, a lithium ion battery electrode, and a lithium ion battery.

[0008] As a result of extensive investigations, the present inventors have found that the above-mentioned problems can be solved by a water-soluble polymer containing a predetermined amount of structural units derived from a specific monomer and having a specific viscosity, and have thus completed the present invention as described below. That is, the present invention relates to the following items [1] to

[10] . [1] A binder for a lithium ion battery containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains 5 to 80 mol % of structural units derived from a hydroxyl group-containing vinyl ether (a) based on all structural units of the water-soluble polymer (A), the viscosity of a 13% by mass aqueous solution of the water-soluble polymer (A) measured at 23°C with a Brookfield viscometer is 3,000 mPa s or more, and the binder is used to form an electrode containing an active material whose aqueous dispersion of the active material exhibits a pH of 10 or more as measured by the following method. <Method for Measuring pH of Aqueous Active Material Dispersion> 10 g of active material is added to 90 g of purified water, stirred at 23°C for 30 minutes, and then allowed to stand for 10 minutes. The pH of the supernatant of the aqueous active material dispersion is measured at 23°C. [2] The binder for lithium ion batteries according to [1] above, wherein the water-soluble polymer (A) further contains a structural unit derived from a (meth)acrylamide group-containing compound (b). [3] The binder for lithium ion batteries according to [1] or [2] above, wherein the hydroxyl group-containing vinyl ether (a) has a hydroxyl value of 400 mgKOH / g or more as measured in accordance with JIS K 0070:1992. [4] The binder for lithium ion batteries according to any one of [1] to [3] above, wherein the active material is lithium-doped SiO or a ternary cathode material. [5] The binder for a lithium ion battery according to any one of [1] to [4] above, wherein the viscosity change rate of the water-soluble polymer (A), as measured by the following method, is −15 to +100%. <Method for Measuring Viscosity Change Rate> 20 g of purified water was added to 80 g of a 13% by mass aqueous solution of the water-soluble polymer (A), and the mixture was stirred for 24 hours to obtain a viscosity measurement liquid A. 20 g of a 10% by mass aqueous solution of sodium hydroxide was added to 80 g of the 13% by mass aqueous solution of the water-soluble polymer (A), and the mixture was stirred for 24 hours to obtain a viscosity measurement liquid B. The viscosities of the viscosity measurement liquids A and B were measured using an E-type viscometer at 23° C. and 50 rpm, and the viscosity of the viscosity measurement liquid A was designated V1, and the viscosity of the viscosity measurement liquid B was designated V2.The viscosity change rate is calculated from V1 and V2 using the following formula (1): Viscosity change rate (%) = (V2 - V1) × 100 / V1 (1) [6] An aqueous binder solution for lithium ion batteries, containing the lithium ion battery binder according to any one of [1] to [5] above and water. [7] The aqueous binder solution for lithium ion batteries according to [6] above, wherein the total content of the water-soluble polymer (A) and water is 80 to 100 mass% of the total amount of the aqueous binder solution for lithium ion batteries. [8] An electrode slurry for lithium ion batteries, containing the lithium ion battery binder according to any one of [1] to [5] above, the active material, and water. [9] An electrode for lithium ion batteries, containing the lithium ion battery binder according to any one of [1] to [5] above and the active material.

[10] A lithium ion battery, having the lithium ion battery electrode according to [9] above.

[0009] According to the present invention, it is possible to provide a lithium ion battery binder that can form an electrode even when an aqueous slurry is produced together with an active material that exhibits a high pH, ​​and that has excellent dispersion stability of the solid content in the slurry state, as well as an aqueous lithium ion battery binder solution, a lithium ion battery electrode slurry, a lithium ion battery electrode, and a lithium ion battery that use the lithium ion battery binder.

[0010] In this specification, the lower and upper limits of preferred numerical ranges described in stages can be independently combined. For example, the description "preferably 10 to 90, more preferably 30 to 60" can be combined with the "preferable lower limit (10)" and the "more preferable upper limit (60)" to form "10 to 60."

[0011] In this specification, for example, "(meth)acrylic acid" refers to both "acrylic acid" and "methacrylic acid," and the same applies to other similar terms.

[0012] In this specification, the term "structural unit" refers to a structural unit based on one monomer molecule, which is formed by polymerizing one monomer molecule.

[0013] [Binder for Lithium-ion Batteries] The binder for lithium-ion batteries of this embodiment (hereinafter also simply referred to as "the binder of this embodiment") is a binder for lithium-ion batteries containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains structural units derived from a hydroxyl group-containing vinyl ether (a) in an amount of 5 to 80 mol % of all structural units of the water-soluble polymer (A), wherein a 13 mass % aqueous solution of the water-soluble polymer (A) has a viscosity of 3,000 mPa·s or more as measured at 23°C with a Brookfield viscometer, and wherein the binder is used to form an electrode containing an active material whose aqueous dispersion exhibits a pH of 10 or more as measured by the following method. <Method for Measuring the pH of an Active Material Aqueous Dispersion> 10 g of an active material is added to 90 g of purified water, and the mixture is stirred at 23°C for 30 minutes. After allowing to stand for 10 minutes, the pH of the supernatant of the aqueous dispersion of the active material is measured at 23°C.

[0014] In the binder of this embodiment, the water-soluble polymer (A) contains structural units derived from the hydroxyl group-containing vinyl ether (a) in the above-mentioned predetermined range, so that an electrode can be formed even when an aqueous slurry is produced together with an active material that exhibits a pH of 10 or higher in the above-mentioned pH measurement method (hereinafter simply referred to as an "active material exhibiting a pH of 10 or higher"). While the details of the cause are unknown, it is presumed that one of the reasons is that the structural units derived from the hydroxyl group-containing vinyl ether (a) have excellent hydrolysis resistance in an alkaline environment, making the polymer less susceptible to hydrolysis even when present in water together with the active material exhibiting a pH of 10 or higher. Furthermore, it is presumed that the viscosity of a 13% by mass aqueous solution of the water-soluble polymer (A) measured with a Brookfield viscometer at 23°C (hereinafter simply referred to as the "23°C viscosity of the aqueous solution of the water-soluble polymer (A)") is 3,000 mPa·s or higher, thereby improving the dispersion stability of the solids in the slurry state.

[0015] <Water-soluble polymer (A)> The water-soluble polymer (A) contains a structural unit derived from a hydroxyl group-containing vinyl ether (a). In this specification, "vinyl ether" refers to a compound having a vinyl group and an etheric oxygen atom bonded to the vinyl group. Furthermore, "water-soluble" in the water-soluble polymer (A) means that the solubility in 100 parts by mass of water at 23°C is 1 part by mass or more.

[0016] (Structural Units Derived from Hydroxyl-Containing Vinyl Ether (a)) The structural units derived from the hydroxyl-containing vinyl ether (a) are structural units formed by an addition reaction of the vinyl groups possessed by the hydroxyl-containing vinyl ether (a). The structural units derived from the hydroxyl-containing vinyl ether (a) contained in the water-soluble polymer (A) may be of one type alone or may be of two or more types.

[0017] [Hydroxyl group-containing vinyl ether (a)] The hydroxyl group-containing vinyl ether (a) is not particularly limited as long as it is a compound having a hydroxyl group, a vinyl group, and an etheric oxygen atom bonded to the vinyl group. The number of vinyl groups that the hydroxyl group-containing vinyl ether (a) has in one molecule is one or more, and from the viewpoint of suppressing gelation during synthesis, it is preferably three or less, more preferably two or less, and even more preferably one.

[0018] From the viewpoints of reactivity, flexibility, etc., the hydroxyl group-containing vinyl ether (a) is preferably a vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms or a vinyl ether having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms.

[0019] Examples of the hydroxyalkyl group having 1 to 3 carbon atoms contained in the vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms include a hydroxymethyl group, a 2-hydroxyethyl group, and a 3-hydroxypropyl group. Examples of the vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms include hydroxyalkyl vinyl ethers such as hydroxymethyl vinyl ether, 2-hydroxyethyl vinyl ether, and 3-hydroxypropyl vinyl ether. Of these, 2-hydroxyethyl vinyl ether is preferred.

[0020] Examples of the oxyalkylene unit having 1 to 3 carbon atoms contained in the hydroxyl group-containing vinyl ether (a) include an oxymethylene unit, an oxyethylene unit, and an oxypropylene unit. The number of oxyalkylene units in the hydroxypolyoxyalkylene chain is 2 or more, preferably 4 or less, more preferably 3 or less, and even more preferably 2. Examples of the hydroxyl group-containing vinyl ether (a) include diethylene glycol monovinyl ether, triethylene glycol monovinyl ether, dipropylene glycol monovinyl ether, and tripropylene glycol monovinyl ether. Among these, diethylene glycol monovinyl ether is preferred.

[0021] The hydroxyl value of the hydroxyl-containing vinyl ether (a), measured in accordance with JIS K 0070: 1992, is preferably 400 mgKOH / g or more, more preferably 500 mgKOH / g or more, and even more preferably 600 mgKOH / g or more, from the viewpoint of easily adjusting the 23°C viscosity of the water-soluble polymer (A) within a preferred range. The hydroxyl value of the hydroxyl-containing vinyl ether (a) may be 1,500 mgKOH / g or less, or may be 1,000 mgKOH / g or less.

[0022] [Content of structural units derived from hydroxyl group-containing vinyl ether (a)] The content of structural units derived from hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is 5 to 80 mol% of the total structural units of the water-soluble polymer (A). When the content of structural units derived from hydroxyl group-containing vinyl ether (a) is 5 mol% or more, the polymer becomes flexible, and when used as a binder, electrode cracking can be suppressed. Furthermore, when the content of structural units derived from hydroxyl group-containing vinyl ether (a) is 80 mol% or less, the generation of residual monomers during the synthesis of the water-soluble polymer (A) can be suppressed. From the same viewpoint as above, the content of structural units derived from hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is preferably 10 to 70 mol%, more preferably 20 to 60 mol%, and even more preferably 30 to 50 mol% of the total structural units of the water-soluble polymer (A).

[0023] (Structural Units Derived from (Meth)acrylamide Group-Containing Compound (b)) The water-soluble polymer (A) preferably contains structural units derived from the (meth)acrylamide group-containing compound (b) in addition to structural units derived from the hydroxyl group-containing vinyl ether (a). The structural units derived from the (meth)acrylamide group-containing compound (b) are structural units formed by an addition reaction of the carbon-carbon double bonds contained in the (meth)acryloyl group of the (meth)acrylamide group-containing compound (b). When the water-soluble polymer (A) contains structural units derived from the (meth)acrylamide group-containing compound (b), the reactivity between monomers during the synthesis of the water-soluble polymer (A) tends to be improved, and the generation of residual monomers can be further suppressed. In this specification, the term "(meth)acrylamide group" refers to a group represented by the following general formula (b-1):

[0024] (In the formula, R ｂ１ indicates a hydrogen atom or a methyl group. * indicates a bonding site.)

[0025] The structural unit derived from the (meth)acrylamide group-containing compound (b) contained in the water-soluble polymer (A) may be of one type alone or may be of two or more types.

[0026] [(Meth)acrylamide group-containing compound (b)] The (meth)acrylamide group-containing compound (b) is not particularly limited as long as it is a compound containing the above-mentioned (meth)acrylamide group, and examples thereof include compounds represented by the following general formula (b-2):

[0027] (In the formula, R ｂ１ represents a hydrogen atom or a methyl group. ｂ２ and R ｂ３ each independently represents a hydrogen atom or a substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms; R ｂ２ and R ｂ３ may be bonded to form a ring.)

[0028] In the above general formula (b-2), R ｂ２ and R ｂ３ Examples of the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms represented by R include a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted alkenyl group having 2 to 20 carbon atoms, a substituted or unsubstituted alkynyl group having 2 to 20 carbon atoms, and a substituted or unsubstituted aromatic hydrocarbon group having 6 to 20 carbon atoms. ｂ２ and R ｂ３ The ring formed by bonding with R ｂ２ and R ｂ３ a heterocyclic ring formed from a nitrogen atom to which R is bonded and 2 to 20 carbon atoms; ｂ２ and R ｂ３ and heterocycles formed from a nitrogen atom to which the group bonded, one or more oxygen atoms, and 2 to 20 carbon atoms. The number of carbon atoms in the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms may be 1 to 10 or 1 to 5. Note that the number of carbon atoms does not include the number of carbon atoms in the substituent. Examples of substituents that may be contained in the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms include a hydroxy group, a halogen atom, a cyano group, a nitro group, a carboxy group, an amino group, an alkoxy group, an aryloxy group, and an acyloxy group. The number of carbon atoms in the substituent may be 1 to 5 or 1 to 3.

[0029] Among the above options, R ｂ２ and R ｂ３is preferably a hydrogen atom from the viewpoint of reactivity.

[0030] Examples of the (meth)acrylamide group-containing compound (b) include (meth)acrylamide; N-alkyl(meth)acrylamides such as N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-n-butyl(meth)acrylamide, N-t-butyl(meth)acrylamide, and N-hexyl(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide; (meth)acrylamides having a hydroxyalkyl group such as N-hydroxymethyl(meth)acrylamide, N-(2-hydroxyethyl)(meth)acrylamide, and N-(2-hydroxypropyl)(meth)acrylamide; and (meth)acrylamides having a ring structure such as N-acryloylpyrrolidine, 3-acryloyl-2-oxazolidinone, 4-acryloylmorpholine, N-acryloylpiperidine, and N-methacryloylpiperidine. Among these, (meth)acrylamide is preferred from the viewpoint of reactivity.

[0031] [Content of Structural Units Derived from (Meth)acrylamide Group-Containing Compound (b)] When the water-soluble polymer (A) of this embodiment contains structural units derived from the (meth)acrylamide group-containing compound (b), the content of the structural units derived from the (meth)acrylamide group-containing compound (b) in the water-soluble polymer (A) is preferably 20 to 95 mol%, more preferably 30 to 90 mol%, even more preferably 40 to 80 mol%, and even more preferably 50 to 70 mol%, based on the total structural units of the water-soluble polymer (A). When the content of the structural units derived from the (meth)acrylamide group-containing compound (b) is equal to or greater than the above-mentioned lower limit, the generation of residual monomers during the synthesis of the water-soluble polymer (A) tends to be more effectively suppressed. Furthermore, when the content of the structural units derived from the (meth)acrylamide group-containing compound (b) is equal to or less than the above-mentioned upper limit, curling during electrode drying tends to be more effectively suppressed.

[0032] (Other Structural Units) The water-soluble polymer (A) of this embodiment may or may not further contain structural units derived from other monomers other than the hydroxyl group-containing vinyl ether (a) and the (meth)acrylamide group-containing compound (b) (hereinafter also referred to as "other structural units"). Examples of the other monomers include α-olefin compounds such as ethylene, propylene, n-butene, and isobutylene; (meth)acrylic acid ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and n-butyl (meth)acrylate; unsaturated carboxylic acid compounds or salts thereof such as acrylic acid, methacrylic acid, and sodium acrylate; and vinyl cyanide compounds such as acrylonitrile and methacrylonitrile. The other structural units contained in the water-soluble polymer (A) may be of one type alone or two or more types.

[0033] When the water-soluble polymer (A) of the present embodiment contains other structural units, the content of the other structural units in the water-soluble polymer (A) is preferably 60 mol % or less, more preferably 40 mol % or less, and even more preferably 20 mol % or less, of the total structural units of the water-soluble polymer (A).

[0034] (Total Content of Structural Units Derived from Hydroxyl Group-Containing Vinyl Ether (a) and Structural Units Derived from (Meth)acrylamide Group-Containing Compound (b)) The total content of the structural units derived from the hydroxyl group-containing vinyl ether (a) and the structural units derived from the (meth)acrylamide group-containing compound (b) in the water-soluble polymer (A) of the present embodiment is preferably 40 to 100 mol %, more preferably 60 to 100 mol %, and even more preferably 80 to 100 mol %, of all structural units in the water-soluble polymer (A).

[0035] (23°C Viscosity of Aqueous Solution of Water-Soluble Polymer (A)) As described above, the viscosity of a 13% by mass aqueous solution of the water-soluble polymer (A) at 23°C measured with a Brookfield viscometer is 3,000 mPa·s or more. Furthermore, the viscosity of a 13% by mass aqueous solution of the water-soluble polymer (A) at 23°C is preferably 4,000 to 100,000 mPa·s, more preferably 4,500 to 50,000 mPa·s, and even more preferably 5,000 to 20,000 mPa·s. When the 23°C viscosity of the aqueous solution of the water-soluble polymer (A) is equal to or greater than the above-mentioned lower limit, the dispersion stability of the solids in the aqueous slurry state tends to be improved, binder migration is suppressed, and better binding properties tend to be obtained. Furthermore, when the 23°C viscosity of the aqueous solution of the water-soluble polymer (A) is equal to or less than the above-mentioned upper limit, the coatability of the slurry in the slurry state tends to be improved. The viscosity of the aqueous solution of the water-soluble polymer (A) at 23°C can be measured by the method described in the examples.

[0036] <Viscosity Change Rate of Water-Soluble Polymer (A) Aqueous Solution> The viscosity change rate of the water-soluble polymer (A), as measured by the following method, is preferably −15 to +100%. (Method for Measuring Viscosity Change Rate) 20 g of purified water was added to 80 g of a 13% by mass aqueous solution of the water-soluble polymer (A), and the mixture was stirred for 24 hours to obtain viscosity measurement liquid A. 20 g of a 10% by mass aqueous solution of sodium hydroxide was added to 80 g of the 13% by mass aqueous solution of the water-soluble polymer (A), and the mixture was stirred for 24 hours to obtain viscosity measurement liquid B. The viscosity of viscosity measurement liquid A and viscosity measurement liquid B were measured using an E-type viscometer at 23° C. and 50 rpm, and V1 was defined as the viscosity of viscosity measurement liquid A, and V2 was defined as the viscosity of viscosity measurement liquid B. The viscosity change rate was calculated from V1 and V2 using the following formula (1): Viscosity change rate (%)=(V2-V1)×100 / V1 (1) A more specific method for measuring the viscosity change rate is as described in the Examples.

[0037] When the viscosity change rate of the aqueous solution of the water-soluble polymer (A) is within the above range, a deterioration in battery performance due to polymer decomposition and cracking during drying of the electrode do not occur, and stable electrode production becomes possible. From the same viewpoint, the viscosity change rate of the aqueous solution of the water-soluble polymer (A) is preferably −15 to +90%, more preferably −10 to +80%, even more preferably −5 to +60%, and still more preferably 0 to +55%.

[0038] (Method for producing water-soluble polymer (A)) The water-soluble polymer (A) can be produced by polymerizing raw material monomers containing a hydroxyl group-containing vinyl ether (a), a (meth)acrylamide group-containing compound (b) used as needed, other monomers, etc. in a solvent. The polymerization method is not particularly limited, and may be, for example, any method such as solution polymerization or bulk polymerization. From the viewpoint of productivity, however, solution polymerization is preferred, and solution polymerization using water as a solvent is more preferred.

[0039] When carrying out the polymerization reaction, it is preferable to use a polymerization initiator. Examples of the polymerization initiator include azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2-methylpropionamidine) dihydrochloride, and 2,2'-azobis(2-hydroxymethylpropionitrile); organic peroxides such as lauryl peroxide, tert-butyl hydroperoxide, benzoyl peroxide, tert-butyl peroxybenzoate, and (3,5,5-trimethylhexanoyl)peroxide; and inorganic peroxides such as potassium persulfate, ammonium persulfate, and hydrogen peroxide. Among these, azo compounds are preferred, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride is more preferred. One type of polymerization initiator may be used alone, or two or more types may be used in combination.

[0040] The amount of the polymerization initiator used is preferably 0.01 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, and even more preferably 0.5 to 3 parts by mass, based on the total amount of monomers (100 parts by mass).

[0041] The polymerization reaction is preferably carried out under heating. The reaction temperature is, for example, 50 to 95°C, and the reaction time is, for example, 2 to 8 hours. The monomers may be added all at once or may be added gradually. The polymerization reaction is preferably carried out under an inert atmosphere such as nitrogen with stirring.

[0042] <Components Other than Water-Soluble Polymer (A)> The binder for lithium ion batteries of this embodiment may or may not contain components other than the water-soluble polymer (A) as necessary. Examples of components other than the water-soluble polymer (A) include polymers other than the water-soluble polymer (A). The content of the water-soluble polymer (A) in the binder for lithium ion batteries of this embodiment is preferably 80 to 100 mass%, more preferably 90 to 100 mass%, and even more preferably 95 to 100 mass%. The binder for lithium ion batteries of this embodiment may be the water-soluble polymer (A) itself.

[0043] <Use of the Lithium-ion Battery Binder of the Present Embodiment> The lithium-ion battery binder of the present embodiment is used to form an electrode containing the active material exhibiting a pH of 10 or more. The electrode formed using the binder of the present embodiment may be a positive electrode or a negative electrode. The active material exhibiting a pH of 10 or more is not particularly limited, but lithium-doped SiO is preferred as a negative electrode material, and a ternary positive electrode material is preferred as a positive electrode material.

[0044] Average particle diameter of lithium-doped SiO (D ５０ ) may be 0.01 to 30 μm, may be 0.1 to 20 μm, or may be 1 to 10 μm.

[0045] Ternary positive electrode materials include, for example, LiNi １／３ Co １／３ Mn １／３ O ２ , LiNi ０．５ Co ０．３ Mn ０．２ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O２ , LiNi ０．６ Co ０．２ Mn ０．２ O ２ , LiNi ０．８ Co ０．１ Mn ０．１ O ２ LiNi etc. ｘ Co ｙ Mn ｚ O ２ Nickel-cobalt-manganese ternary cathode material (NCM) represented by (0<x<1, 0<y<1, 0<z<1, and x+y+z=1), LiNi ０．８ Co ０．１５ Al ０．０５ O ２ LiNi etc. ｘ Co ｙ Al ｚ O ２ (0<x<1, 0<y<1, 0<z<1, and x+y+z=1) and nickel-cobalt-aluminum ternary cathode materials (NCA). ５０ ) may be 0.1 to 50 μm, may be 0.5 to 30 μm, or may be 1 to 15 μm.

[0046] The shape of the lithium-doped SiO or ternary positive electrode material is not particularly limited, and may be, for example, crushed, spherical, flat, fibrous, or the like.

[0047] The active material exhibiting a pH of 10 or more is not limited to lithium-doped SiO or a ternary positive electrode material, but may be, for example, a lithium pre-doped negative electrode material other than SiO; a lithium composite oxide other than a ternary positive electrode material, such as a quaternary positive electrode material; or the like.

[0048] [Aqueous binder solution for lithium ion batteries] The aqueous binder solution for lithium ion batteries of the present embodiment (hereinafter also simply referred to as "aqueous binder solution of the present embodiment") is an aqueous binder solution for lithium ion batteries containing the binder for lithium ion batteries of the present embodiment and water. An electrode formed using the aqueous binder solution of the present embodiment may be a positive electrode or a negative electrode.

[0049] The content of the water-soluble polymer (A) in the aqueous binder solution of the present embodiment is preferably 1 to 40% by mass, more preferably 5 to 30% by mass, and even more preferably 10 to 20% by mass, from the viewpoint of ease of handling.

[0050] The binder aqueous solution of this embodiment may or may not contain other components (hereinafter also referred to as "other aqueous solution components") other than the binder of this embodiment and water. Examples of other aqueous solution components include solvents other than water, thickeners, humectants, pH adjusters, stabilizers, surfactants, antioxidants, preservatives, etc. Each of these may be used alone, or two or more may be used in combination.

[0051] From the viewpoint of more fully exhibiting the effects of the present invention, the total content of the water-soluble polymer (A) and water is preferably 80 to 100 mass %, more preferably 90 to 100 mass %, and even more preferably 95 to 100 mass %, of the total amount of the aqueous binder solution of the present embodiment.

[0052] The method for producing the aqueous binder solution of this embodiment is not particularly limited. For example, when the water-soluble polymer (A) is synthesized using water as a solvent to obtain the aqueous solution, the aqueous solution of the water-soluble polymer (A) may be used as is as the aqueous binder solution of this embodiment. Furthermore, if necessary, the aqueous binder solution may be produced by isolating the water-soluble polymer (A) synthesized in a solvent and then dissolving it in water at a desired time. Other aqueous solution components may be added at any time.

[0053] [Electrode Slurry for Lithium-Ion Batteries] The electrode slurry for lithium-ion batteries of this embodiment (hereinafter also simply referred to as "electrode slurry of this embodiment") is an electrode slurry for lithium-ion batteries containing the binder for lithium-ion batteries of this embodiment, the active material exhibiting a pH of 10 or higher, and water. The electrode slurry of this embodiment is applied to a current collector and then dried, thereby being used to form an active material layer containing the active material and the binder of this embodiment on the current collector. The electrode formed using the electrode slurry of this embodiment may be a positive electrode or a negative electrode.

[0054] In the electrode slurry of this embodiment, the content of the binder of this embodiment relative to 100 parts by mass of the active material is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 8 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the binder of this embodiment is equal to or greater than the above-mentioned lower limit, the strength of the active material layer can be further improved. On the other hand, when the content of the binder of this embodiment is equal to or less than the above-mentioned upper limit, an increase in the resistance of the electrode tends to be suppressed.

[0055] The electrode slurry of this embodiment may contain a conductive aid as needed. Examples of conductive aids include various graphites; carbon blacks such as acetylene black, ketjen black, furnace black, and channel black; carbon fibers such as carbon nanofibers and carbon nanotubes; and metal materials such as nickel, aluminum, and stainless steel. The shape of the conductive aid is not particularly limited and may be particulate or fibrous. One type of conductive aid may be used alone, or two or more types may be used in combination. When the electrode slurry of this embodiment contains a conductive aid, the content of the conductive aid relative to 100 parts by mass of the active material is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 8 parts by mass, and even more preferably 0.1 to 5 parts by mass.

[0056] From the viewpoint of coatability, the solid content concentration in the electrode slurry of this embodiment is preferably 20 to 90 mass %, more preferably 25 to 85 mass %, and even more preferably 30 to 80 mass %. In this specification, the solid content of the electrode slurry means components other than the liquid medium.

[0057] The electrode slurry of this embodiment may or may not contain components other than the binder of this embodiment, the active material and water exhibiting a pH of 10 or more, and the conductive additive used as needed (hereinafter also referred to as "other slurry components"). Examples of the other slurry components include the same as those of the other aqueous solution components described above.

[0058] The method for producing the electrode slurry of this embodiment is not particularly limited, and the electrode slurry can be produced, for example, by mixing, by a known method, the aqueous binder solution of this embodiment, the active material having a pH of 10 or higher, a conductive aid, an additional solvent such as water, other slurry components, etc., which are used as needed. As the mixing device, for example, a kneading machine such as a ball mill, a sand mill, a planetary mixer, a paint shaker, a homomixer, a homodisper, or a homogenizer can be used.

[0059] [Lithium-ion battery electrode] The lithium-ion battery electrode of this embodiment (hereinafter also simply referred to as "electrode of this embodiment") is a lithium-ion battery electrode containing the lithium-ion battery binder of this embodiment and the active material exhibiting a pH of 10 or higher. The electrode of this embodiment may be a positive electrode or a negative electrode. The electrode of this embodiment preferably has an active material layer on a current collector, the active material exhibiting a pH of 10 or higher and the binder of this embodiment. In the electrode of this embodiment, the thickness of the active material layer is not particularly limited and may be, for example, 25 to 500 μm, 50 to 300 μm, or 100 to 200 μm.

[0060] In the electrode of this embodiment, the content of the binder of this embodiment relative to 100 parts by mass of the active material is the same as the preferred range described in the description of the electrode slurry of this embodiment. The electrode of this embodiment may also contain the conductive additive described above. In the electrode of this embodiment, the content of the conductive additive relative to 100 parts by mass of the active material is the same as the preferred range described in the description of the electrode slurry of this embodiment. The electrode of this embodiment may also contain the components exemplified above as other slurry components and other aqueous solution components.

[0061] The current collector is preferably made of a sheet-like metal. Examples of metals used for the current collector include iron, copper, aluminum, nickel, and stainless steel. Among these, aluminum foil is preferred as the current collector for the positive electrode, and copper foil is preferred as the current collector for the negative electrode. The thickness of the current collector is, for example, 5 to 20 μm.

[0062] The electrode of this embodiment can be produced by applying the electrode slurry of this embodiment onto a current collector and then drying it. Examples of methods for applying the electrode slurry of this embodiment onto a current collector include a reverse roll method, a doctor blade method, a direct roll method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a dip method, and a squeeze method.

[0063] The electrode slurry of this embodiment may be applied to only one surface of the current collector, or may be applied to both surfaces. When the electrode slurry is applied to both surfaces of the current collector, it may be applied to one surface at a time, or to both surfaces simultaneously. Furthermore, the electrode slurry may be applied continuously to the entire surface of the current collector, or may be applied intermittently. The amount and application range of the electrode slurry may be determined appropriately depending on the size of the battery, etc.

[0064] Examples of methods for drying the electrode slurry applied to the current collector include a method of applying hot air, a vacuum drying method, a method using electromagnetic waves such as an infrared, far-infrared, or near-infrared heater, etc. The drying temperature for the electrode slurry is, for example, 80 to 150°C, and the drying time is, for example, 1 to 30 minutes.

[0065] The obtained electrode may be subjected to shaping such as cutting, if necessary.

[0066] [Lithium-ion battery] The lithium-ion battery of this embodiment is a lithium-ion battery having the lithium-ion battery electrode of this embodiment. Examples of the lithium-ion battery of this embodiment include a lithium-ion secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator. In the lithium-ion battery of this embodiment, at least one of the positive electrode and the negative electrode includes the electrode of this embodiment, and so long as this is the case, the battery may include an electrode other than the electrode of this embodiment. The electrode other than the electrode of this embodiment, the electrolyte, and the separator may be appropriately selected from known types depending on the application. The shape of the lithium-ion battery of this embodiment is not particularly limited, and may be any shape, such as a coin shape, a button shape, a sheet shape, a cylindrical shape, a prismatic shape, or a flat shape.

[0067] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0068] [Preparation of Aqueous Binder Solution for Lithium-Ion Batteries] Preparation Example 1: 2013.60 g of purified water, 200.00 g of 2-hydroxyethyl vinyl ether, and 400.00 g of a 50% by weight aqueous acrylamide solution were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube. The temperature was raised to 55°C, and oxygen was removed from the reaction system by introducing nitrogen gas. A solution containing 4.00 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 48.00 g of purified water was added, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 442.09 g of purified water was added to obtain an aqueous solution of water-soluble polymer (A). This aqueous solution of water-soluble polymer (A) was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer (A) content: 13% by weight).

[0069] Comparative Production Example 1: 2334.20 g of purified water, 147.35 g of 2-hydroxyethyl acrylate, 219.10 g of a 50% by weight aqueous acrylamide solution, 91.70 g of acrylic acid, and 1.40 g of sodium methallylsulfonate were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube. The temperature was raised to 55°C, and oxygen was removed from the reaction system by passing nitrogen gas through the reactor. A solution containing 2.81 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 28.00 g of purified water was added, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 133.23 g of a 50% (w / v) aqueous sodium hydroxide solution was added for neutralization, yielding an aqueous solution of water-soluble polymer. This aqueous solution of water-soluble polymer was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer content: 13% by weight).

[0070] Comparative Production Example 2 A reaction apparatus equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube was charged with 1,450 g of ion-exchanged water, 137.3 g of 2-hydroxyethyl vinyl ether, 188.6 g of a 50% by mass aqueous acrylamide solution, and 0.09 g of sodium methallylsulfonate, and oxygen was removed from the reaction system by passing nitrogen gas through it, followed by heating to 50° C. 2.3 g of 2,2′-azobis-2-amidinopropane dihydrochloride and 50 g of ion-exchanged water were then added, the temperature was raised to 80° C., and the reaction was carried out for 3 hours, yielding an aqueous solution containing a water-soluble polymer (water-soluble polymer content: 13% by mass).

[0071] Comparative Production Example 3: 665.26 g of purified water and 200.00 g of 2-hydroxyethyl vinyl ether were added to a reaction apparatus equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, and the temperature was raised to 55°C. Oxygen was removed from the reaction system by passing nitrogen gas through the system. A solution containing 2.00 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 24.00 g of purified water was added to the reaction system, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Thereafter, 662.58 g of purified water was added, and the reaction solution was １ When H-NMR measurement was carried out, only the peak of the monomer was confirmed, and it was therefore determined that polymerization had not progressed.

[0072] Comparative Production Example 4: 1579.40 g of purified water, 15.00 g of 2-hydroxyethyl vinyl ether, 580.32 g of a 50% by weight aqueous acrylamide solution, and 0.34 g of sodium methallylsulfonate were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube. The temperature was raised to 55°C, and oxygen was removed from the reaction system by passing nitrogen gas through the reactor. A solution containing 3.05 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 36.66 g of purified water was added, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 158.71 g of purified water was added to obtain an aqueous solution of water-soluble polymer. This aqueous solution of water-soluble polymer was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer content: 13% by weight).

[0073] [Production of Lithium-ion Battery Electrode Slurry and Lithium-ion Battery Electrode] Example 1 12.1 g of the binder aqueous solution obtained in Production Example 1 and carbon black (manufactured by MTI Corporation, trade name "Super C65", specific surface area by BET method: 62 m) as a conductive aid were placed in a glass container (volume: 140 mL). ２ / g) (referred to as "CB" in Table 2) and 7.0 g of purified water were added, and the mixture was stirred at 2,000 rpm for 30 minutes using a disperser (manufactured by Primix Corporation, trade name "Labo-Lution", stirring part: Homo Disper 2.5 type). Furthermore, as an active material, a ternary positive electrode material (lithium nickel cobalt manganese oxide, LiNi ０．５ Co ０．２ Mn ０．３ O ２ , manufactured by JiangSu East Spring Material Technology Co., Ltd., trade name "ME5E12D", average particle diameter D ５０ 50.0 g of 12 μm (NCM) (referred to as "NCM" in Table 2) was added, and the mixture was stirred at 2,000 rpm for 10 minutes using the disperser. The mixture was then degassed at 0.04 atmospheres for 5 minutes to obtain a lithium-ion battery electrode slurry with a solids concentration of 75.0 mass %. The prepared lithium-ion battery electrode slurry was applied to aluminum foil (20 μm thick aluminum foil manufactured by Hohsen Co., Ltd.) using a 205 μm gap applicator, and then dried in an oven at 120° C. for 10 minutes to obtain a lithium-ion battery electrode (positive electrode) having a 150 μm thick active material layer on a current collector (aluminum foil).

[0074] Example 2 The active material was lithium-doped SiO (manufactured by Hosen Co., Ltd., product name "LCL1485A-1", average particle diameter D ５０A lithium ion battery electrode slurry was obtained in the same manner as in Example 1, except that the thickness of the electrode was changed to 27.5 g (6 μm), the amount of purified water added was changed to 27.5 g, and the solids concentration of the slurry was changed to 58.0 mass %. The prepared lithium ion battery electrode slurry was applied to a copper foil (rolled copper foil manufactured by UACJ Corporation, thickness 10 μm) using an applicator with a gap of 240 μm, and then dried in an oven at 120° C. for 10 minutes to obtain a lithium ion battery electrode (negative electrode) having a 150 μm-thick active material layer on a current collector (copper foil).

[0075] Comparative Examples 1, 4, and 6 Lithium ion battery electrodes (positive electrodes) were obtained in the same manner as in Example 1, except that the type of aqueous binder solution was changed to that shown in Table 2.

[0076] Comparative Examples 2 and 5 Lithium ion battery electrodes (negative electrodes) were obtained in the same manner as in Example 2, except that the type of aqueous binder solution was changed to that shown in Table 2.

[0077] Comparative Example 3 The type of binder solution was changed to that shown in Table 2, and the active material was changed to graphite (artificial graphite G1-C, manufactured by Jiangxi Zishen Technology Co., Ltd., average particle diameter D ５０ A lithium ion battery electrode slurry was obtained in the same manner as in Example 1, except that the thickness of the current collector (copper foil) was changed to 42.0 g, the amount of purified water added was changed to 42.0 g, and the solids concentration of the slurry was changed to 50.0 mass %. The prepared lithium ion battery electrode slurry was applied to a copper foil (rolled copper foil manufactured by UACJ Corporation, thickness 10 μm) using an applicator with a gap of 280 μm, and then dried in an oven at 120° C. for 10 minutes to obtain a lithium ion battery electrode (negative electrode) having a 150 μm-thick active material layer on the current collector (copper foil).

[0078] [Evaluation of electrode formability] The surface of the active material layer of the lithium ion battery electrode prepared by the above method was visually observed, and the active material layer with no cracks was rated "A", and the active material layer with cracks was rated "F".

[0079] [Method for Measuring Slurry Solids Concentration and Solids Content Change Rate After 24-Hour Standing] The lithium-ion battery electrode slurry obtained in each example was placed in a glass container (20 mL capacity) and allowed to stand for 24 hours with the lid closed. After standing, approximately 1 g was sampled from the top of the slurry, placed on an aluminum dish (weight of aluminum dish: W1), and weighed. The weight of the slurry alone placed on the aluminum dish at this time was designated W2. The aluminum dish with the slurry placed on it was dried in an oven at 120°C for 2 hours, and the post-dry weight W3 (weight of the slurry + aluminum dish) was measured. The slurry solids concentration after 24 hours of standing was calculated using the following formula: Slurry solids concentration (%) after 24 hours of standing = (W3 - W1) × 100 / W2. Furthermore, the slurry solids concentration after 24 hours of standing was compared with the initial solids concentration of the slurry, and the solids content change rate was calculated using the following formula. Solid content change rate (%) = [(slurry solid content concentration after standing for 24 hours) - (charged solid content concentration)] x 100 / charged solid content concentration

[0080] [Method for measuring pH of active material] 90 g of purified water and 10 g of active material were placed in a glass container (volume 140 mL), an oval stirrer (diameter x total length: 10 x 20 mm) was placed inside, and the mixture was stirred for 30 minutes using a magnetic stirrer at 23 ° C. and 1,000 rpm to obtain an aqueous active material dispersion. After stirring, the mixture was allowed to stand for 10 minutes, and the pH of the supernatant of the aqueous active material dispersion was measured at 23 ° C. using a pH tester (manufactured by HORIBA Advanced Techno Co., Ltd., product name "F-2000PI-S", electrode: standard Tou pH electrode (9615S-10D)). The obtained pH was taken as the pH of the active material.

[0081] [Method for measuring viscosity of aqueous water-soluble polymer solution at 23°C] The aqueous binder solution for lithium ion batteries (13 mass% aqueous solution of water-soluble polymer) obtained in each Production Example and Production Comparative Example was used as the measurement object, and the viscosity was measured at 23°C using the following spindles using a Brookfield viscometer (product name "Analog Viscometer RVT") at 50 rpm. When the viscosity was 500 mPa·s or more and less than 1,500 mPa·s: RV-3 spindle When the viscosity was 1,500 mPa·s or more and less than 2,000 mPa·s: RV-4 spindle When the viscosity was 2,000 mPa·s or more and less than 5,500 mPa·s: RV-5 spindle When the viscosity was 5,500 mPa·s or more and less than 15,000 mPa·s: RV-6 spindle When the viscosity was 15,000 mPa·s or more: RV-7 spindle

[0082] [Method for Measuring Viscosity Change Rate of Water-Soluble Polymer Aqueous Solution] 20 g of purified water was added to 80 g of the aqueous binder solution for lithium ion batteries (a 13% by mass aqueous solution of water-soluble polymer) obtained in each Production Example and Production Comparative Example, and the mixture was stirred using a mixer rotor at 23°C and 20 rpm for 24 hours to obtain viscosity measurement solution A. 20 g of a 10% by mass aqueous sodium hydroxide solution was added to 80 g of the obtained binder solution (a 13% by mass aqueous solution of water-soluble polymer), and the mixture was stirred using a mixer rotor at 23°C and 20 rpm for 24 hours to obtain viscosity measurement solution B. The viscosity of viscosity measurement solution A and viscosity measurement solution B were measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd., trade name "RE-85U", cone rotor specifications: 1°34' x R24) at 23°C and 50 rpm, and the viscosity of viscosity measurement solution A was designated V1, and the viscosity of viscosity measurement solution B was designated V2. The viscosity change rate was calculated from V1 and V2 using the following formula (1): Viscosity change rate (%) = (V2 - V1) x 100 / V1 (1)

[0083] Table 1 shows the evaluation results of the water-soluble polymers obtained in each of the Production Examples and Production Comparative Examples.

[0084]

[0085] Details of the monomers listed in Table 1 are as follows: HEVE: 2-hydroxyethyl vinyl ether AM: acrylamide HEA: 2-hydroxyethyl acrylate AA: acrylic acid SMAS: sodium methallyl sulfonate

[0086] Table 2 shows the evaluation results of the electrodes obtained in each of the Examples and Comparative Examples.

[0087]

[0088] Tables 1 and 2 show that the lithium ion battery binder of this embodiment containing the water-soluble polymer (A) obtained in Production Example 1 has high alkali resistance and can form an electrode even when an aqueous slurry is produced together with an active material exhibiting a high pH. It is also shown that the lithium ion battery binder of this embodiment containing the water-soluble polymer (A) obtained in Production Example 1 has excellent dispersion stability of the solid content in the slurry state.

Claims

1. A binder for lithium ion batteries containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains 5 to 80 mol % of structural units derived from a hydroxyl group-containing vinyl ether (a) based on all structural units of the water-soluble polymer (A), wherein a 13% by mass aqueous solution of the water-soluble polymer (A) has a viscosity of 3,000 mPa·s or more as measured with a Brookfield viscometer at 23°C, and wherein the binder is used to form an electrode containing an active material whose aqueous dispersion has a pH of 10 or more as measured by the following method: <Method for measuring the pH of the aqueous active material dispersion> 10 g of active material is added to 90 g of purified water, stirred at 23°C for 30 minutes, and then allowed to stand for 10 minutes, and the pH of the supernatant of the aqueous active material dispersion is measured at 23°C.

2. The binder for lithium ion batteries according to claim 1, wherein the water-soluble polymer (A) further contains a structural unit derived from a (meth)acrylamide group-containing compound (b).

3. The binder for lithium ion batteries according to claim 1 or 2, wherein the hydroxyl group-containing vinyl ether (a) has a hydroxyl value of 400 mgKOH / g or more as measured in accordance with JIS K 0070:1992.

4. The binder for lithium ion batteries according to claim 1 or 2, wherein the active material is lithium-doped SiO or a ternary positive electrode material.

5. The binder for lithium ion batteries according to claim 1 or 2, wherein the viscosity change rate of the water-soluble polymer (A) measured by the following method is −15 to +100%. <Method for Measuring Viscosity Change Rate> Viscosity measurement liquid A is obtained by adding 20 g of purified water to 80 g of a 13% by mass aqueous solution of the water-soluble polymer (A) and stirring for 24 hours, and viscosity measurement liquid B is obtained by adding 20 g of a 10% by mass aqueous solution of sodium hydroxide to 80 g of the 13% by mass aqueous solution of the water-soluble polymer (A) and stirring for 24 hours. The viscosity of viscosity measurement liquid A and viscosity measurement liquid B are measured using an E-type viscometer at 23°C and 50 rpm, and the viscosity of viscosity measurement liquid A is defined as V1, and the viscosity of viscosity measurement liquid B is defined as V2. The viscosity change rate is calculated from V1 and V2 using the following formula (1): Viscosity change rate (%) = (V2 - V1) × 100 / V1 (1) 6. An aqueous binder solution for lithium ion batteries, comprising the binder for lithium ion batteries according to claim 1 or 2 and water.

7. The aqueous binder solution for lithium ion batteries according to claim 6, wherein the total content of the water-soluble polymer (A) and water is 80 to 100 mass % of the total amount of the aqueous binder solution for lithium ion batteries.

8. An electrode slurry for a lithium ion battery, comprising the binder for a lithium ion battery according to claim 1 or 2, the active material, and water.

9. A lithium ion battery electrode comprising the binder for lithium ion batteries according to claim 1 or 2 and the active material.

10. A lithium ion battery comprising the lithium ion battery electrode according to claim 9.

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