Secondary battery binder, slurry, electrode, method for manufacturing secondary battery, and secondary battery

Abstract

Provided is a novel secondary battery binder that contains an aqueous polymer, wherein the pH (25°C) of an aqueous solution that is obtained when the aqueous polymer is dissolved in water at a concentration of 2 mass% is 5.0–6.5, and the light absorbance (400 nm) of the aqueous solution is 0.05 or less. The secondary battery binder can suitably contribute to the manufacturing and / or the battery characteristics of a secondary battery. Also provided is a slurry that contains the secondary battery binder.

Description

Secondary battery binder, slurry, electrode, method for manufacturing secondary battery, and secondary battery

[0001] The present disclosure relates to a secondary battery binder, a slurry, an electrode, a method for manufacturing a secondary battery, and a secondary battery.

[0002] In the manufacturing process of secondary batteries, particularly in the manufacturing process of electrodes, a slurry containing a binder is used. In recent years, due to concerns about the impact on the environment, interest has been increasing in shifting from slurries using organic solvents to aqueous slurries using water as the solvent. Patent Document 1 discloses the use of an aqueous slurry containing a latex-based binder.

[0003] JP 2012-49061 A

[0004] The types and properties of the binder are not sufficiently considered in Patent Document 1. An object of the present disclosure is to provide a novel binder that can favorably contribute to battery production and / or battery properties, or a slurry containing the binder.

[0005] The present disclosure includes the following aspects: [Item 1] A secondary battery binder comprising an aqueous polymer, wherein the aqueous polymer is dissolved in water at a concentration of 2% by mass to give an aqueous solution having a pH (25°C) of 5.0 or more and 6.5 or less, and an absorbance (400 nm) of the aqueous solution of 0.05 or less. [Item 2] The secondary battery binder according to Item 1, wherein the aqueous polymer is a vinyl polymer having an acidic functional group-containing repeating unit. [Item 3] The aqueous polymer is a compound represented by the formula: -[CH ２ -C(R １ ) (C(=O)R ２ )] - [wherein, R 1 is a hydrogen atom or CH ３ and R 2 is NH ２ , OM (M is a hydrogen atom or a counter cation), O(CH ２ ) ｎ OH, NH(CH ２ ) ｎ OH (wherein n is independently 1 or more and 6 or less).] and a repeating unit (1) represented by the formula: -[CH ２Item 4. The secondary battery binder according to Item 1 or 2, comprising a repeating unit selected from the repeating unit (2) represented by the formula: —CH(OH)]—. [Item 4] The secondary battery binder according to Item 3, wherein in the aqueous polymer, the amount of the repeating unit selected from the repeating unit (1) and the repeating unit (2) is 40 mol % or more, the amount of the acidic functional group-containing repeating unit is 0.8 mol % or more, and the amount of the nonionic repeating unit is 40 mol % or more. [Item 5] The secondary battery binder according to any one of Items 1 to 4, comprising SBR. [Item 6] A slurry comprising the secondary battery binder according to any one of Items 1 to 5 and water. [Item 7] The slurry according to Item 6, further comprising an electrode active material. [Item 8] A method for producing a secondary battery, comprising a step of applying the slurry according to Item 6 or 7. [Item 9] An electrode comprising the secondary battery binder according to any one of Items 1 to 5 or a component derived from the secondary battery binder. [Item 10] An electrode comprising a heat-dried product of the slurry according to Item 6 or 7. [Item 11] A secondary battery comprising the electrode according to Item 9 or 10.

[0006] The secondary battery binder according to the present disclosure or a slurry containing the secondary battery binder has one or more properties suitable for batteries or their manufacture. In particular, the use of the secondary battery binder according to the present disclosure or a slurry containing the secondary battery binder provides suitable viscosity stability of the slurry used in the battery manufacturing process, improving workability.

[0007] <Definitions of Terms, etc.> In this specification, regardless of whether "each independently" or a similar expression is explicitly stated, unless otherwise stated, when a term (symbol) that may appear multiple times in a chemical structure is defined, the definition applies independently to each occurrence.

[0008] In this specification, "(meth)acrylic" means "acrylic or methacrylic", and "(meth)allyl" means "allyl or methallyl".

[0009] In this specification, when multiple lower limit values ​​and multiple upper limit values ​​are separately described, any lower limit value and any upper limit value can be selected, and a numerical range that is a combination of the lower limit value and the upper limit value can be selected.

[0010] <Secondary Battery Binder> The secondary battery binder in the present disclosure is used to enhance the binding strength between particles inside an electrode in a secondary battery and the binding strength between an active material layer and a current collector.

[0011] [Water-based polymer] The secondary battery binder in the present disclosure includes a water-based polymer. The water-based polymer in the present disclosure is a polymer that can be dispersed in water by itself, and is particularly water-soluble.

[0012] [Characteristics of Water-Based Polymer] The characteristics of the water-based polymer are shown below.

[0013] (Water Solubility) When 2 g of the aqueous polymer is dissolved in 100 g of water at 25°C, the insoluble content may be 20% by mass or less, or 15% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less, relative to the aqueous polymer.

[0014] (Molecular Weight, etc.) The Mw (weight average molecular weight) of the aqueous polymer measured by GPC-RI may be 100,000 or more, 300,000 or more, 500,000 or more, 750,000 or more, or 1,500,000 or more, preferably 100,000 or more, more preferably 200,000 or more, and may be 3,000,000 or less, 1,000,000 or less, 500,000 or less, 250,000 or less, or 100,000 or less, preferably 2,000,000 or less, more preferably 1,000,000 or less. This value is determined by the method described in the examples.

[0015] (pH) The pH (25°C) of the aqueous solution obtained when the aqueous polymer is dissolved in water at a concentration of 2% by mass is 5.0 or more and 6.5 or less. A preferred pH (25°C) is 5.2 or more, more preferably 5.5 or more, and preferably 6.0 or less. This value is determined by the method described in the Examples. Ion-exchanged water is usually used as the water. When the pH is equal to or greater than the lower limit, the solubility in water is thought to be increased, and viscosity stability is improved. Furthermore, when the pH is equal to or less than the upper limit, the hydrogen bonding interaction of the acidic groups is thought to be strengthened, and viscosity stability is thought to be improved. In other words, when the pH is within the above range, the viscosity stability of the polymer aqueous solution is good, which is preferable from the viewpoint of productivity in battery production.

[0016] (APHA) The APHA of the aqueous solution obtained when a water-based polymer is dissolved in water at a concentration of 2% by mass may be 0 or more, 5 or more, 10 or more, or 20 or more, and may be 200 or less, 100 or less, 50 or less, 25 or less, 5 or less, or 1 or less, preferably 50 or less, more preferably 20 or less. This value is determined by the method described in the Examples. Ion-exchanged water is usually used as the water.

[0017] (Absorbance (400 nm)) The absorbance (400 nm) at 25°C of an aqueous solution obtained when a water-based polymer is dissolved in water at a concentration of 2% by mass is 0.05 or less. It is preferably 0.0001 or more, and also preferably 0.03 or less, more preferably 0.015 or less. This value is determined by the method described in the Examples. Ion-exchanged water is usually used as the water. When the absorbance is within the above range, the solubility of the water-based polymer is sufficient, and it is thought that viscosity changes over time are suppressed. As a result, the viscosity stability of the slurry used in the battery manufacturing process is good, which can be advantageous from the viewpoint of productivity in battery manufacturing.

[0018] (0.1 mol / L NaOH Neutralization Titer) The neutralization titer of 70 g of an aqueous solution obtained by dissolving a water-based polymer in water at a concentration of 2% by mass with a 0.1 mol / L aqueous sodium hydroxide solution may be 0.1 mL or more, 0.2 mL or more, 0.4 mL or more, 1.0 mL or more, or 2.0 mL or more, preferably 0.5 mL or more, more preferably 1.0 mL or more, and may be 15 mL or less, 10 mL or less, 8.0 mL or less, 6.0 mL or less, 4.0 mL or less, 2.0 mL or less, 1.0 mL or less, or 0.6 mL or less, preferably 12 mL or less, more preferably 5.0 mL or less. This value is determined by the method described in the Examples. Ion-exchanged water is usually used as the water. By having the neutralization titer within the above range (for example, 0.5 mL or more and 5.0 mL or less, particularly 1.0 mL or more and 3.5 mL or less), the slurry dispersion stability can be improved. When the neutralization titer is equal to or greater than the lower limit (e.g., 0.5 mL or greater), the hydrogen bonding interaction between the acidic groups and the active material can improve the slurry dispersion stability. When the neutralization titer is equal to or less than the upper limit (e.g., 5.0 mL or less), the hydrogen bonding interaction between the acidic groups of the polymers increases the interaction between the active material and the polymer, improving the slurry dispersion stability. The neutralization titer is an indicator of the proportion of free acid, and when this amount is relatively small, it is thought to favorably contribute to the conductivity and the binding between the active material and the current collector (e.g., copper foil) due to ionic interactions, resulting in a high capacity retention rate.

[0019] (Weight Loss at 170 to 290°C) The weight loss of the aqueous polymer at 170 to 290°C may be 3.0% by mass or more, 5.0% by mass or more, 7.5% by mass or more, 10% by mass or more, 12.5% ​​by mass or more, or 15.0% by mass or more, preferably 3.0% by mass or more, more preferably 5.0% by mass or more, and may be 40% by mass or less, 30% by mass or less, 25% by mass or less, 15% by mass or less, 10% by mass or less, or 5% by mass or less, preferably 20% by mass or less, more preferably 15% by mass or less. This value is determined by the method described in the Examples.

[0020] [Structure of aqueous polymer, etc.] The aqueous polymer can be obtained by polymerizing one or more types of monomers. The aqueous polymer may be a vinyl polymer. The vinyl polymer is a polymer obtained by polymerizing a vinyl monomer. Here, the vinyl monomer may be a compound having a polymerizable carbon-carbon double bond (ethylenically unsaturated double bond) (>C=C<), and may be a monomer containing a vinyl group, a vinylene group, a vinylidene group, an acryloyl group, a methacryloyl group, or a derivative group thereof. The aqueous polymer may be a (meth)acrylic polymer containing a repeating unit derived from a monomer having an acryloyl group or a methacryloyl group.

[0021] The water-based polymer may be a random polymer or a block polymer, for example a random polymer.

[0022] (Hydrophilic Repeating Unit) The aqueous polymer has a hydrophilic repeating unit. The hydrophilic repeating unit contains a hydrophilic group. Examples of the hydrophilic group include anionic groups such as a carboxy group, a sulfonic acid group, a phosphate group, and a nitrate group, cationic groups such as an amino group, a hydroxy group, a polyoxyethylene group (e.g., having a repeating number of 2 or more, 5 or more, or 10 or more), and nonionic hydrophilic groups such as an amide group. The anionic group and the cationic group may be in a free acid / base state, or a part or all of them may be in the form of a salt. The hydrophilic repeating unit may not contain a sulfonic acid group-containing repeating unit, and the aqueous polymer may not contain a sulfonic acid group-containing repeating unit.

[0023] In this specification, when an anionic group or a cationic group or a structure containing such a group (for example, a repeating unit containing an acidic functional group) is mentioned, it is intended to encompass not only the anionic group and the cationic group but also salts thereof, unless explicitly stated otherwise.

[0024] Examples of counter cations of anionic groups include metal ions, preferably light metal ions, more preferably lithium ions, sodium ions, or potassium ions, and particularly lithium ions or sodium ions. The counter cations may be monovalent to trivalent, monovalent to divalent, or monovalent, preferably monovalent. Examples of counter anions of cationic groups include inorganic acid ions such as phosphate ions, nitrate ions, and sulfate ions, and halide ions. The aqueous polymer may be anionic, or may not have a cationic group.

[0025] The aqueous polymer in the present disclosure preferably has a repeating unit having an acidic functional group as a hydrophilic repeating unit. The acidic functional group may be an anionic group such as a carboxy group, a sulfonic acid group, a phosphate group, or a nitrate group, preferably a carboxy group or a sulfonic acid group, and more preferably a carboxy group. These groups may exist in the form of a salt, and in this case, the above-mentioned metal ions are suitable as counter cations. Examples of repeating units having an acidic functional group include repeating units derived from (meth)acrylic acid, maleic acid, vinylsulfonic acid, or (meth)allylsulfonic acid.

[0026] The aqueous polymer of the present disclosure may have a nonionic hydrophilic repeating unit, such as a repeating unit derived from (meth)acrylamide, hydroxyalkyl (meth)acrylate, hydroxyalkyl (meth)acrylamide, polyoxyalkylene (meth)acrylate, polyoxyalkylene (meth)acrylamide, or vinyl alcohol.

[0027] Examples of suitable hydrophilic repeating units include those of the formula: —[CH ２ -C(R １ ) (C(=O)R ２ )] - [wherein, R 1 is a hydrogen atom or CH ３ and R 2 is NH ２ , OM (M is a hydrogen atom or a counter cation), O(CH ２ ) ｎOH, NH(CH ２ ) ｎ OH (wherein n is independently 1 or more and 6 or less).] and a repeating unit (1) represented by the formula: -[CH ２ -CH(OH)]- (2)

[0028] In the repeating unit (1), R 1 is preferably a hydrogen atom.

[0029] In the repeating unit (1), R ２ M in the formula (I) may be a metal cation, preferably a light metal cation, more preferably a lithium ion, a sodium ion, or a potassium ion, and particularly preferably a lithium ion or a sodium ion. M may be monovalent to trivalent, monovalent to divalent, or monovalent, and is preferably monovalent.

[0030] In the repeating unit (1), R ２ In the formula, n may be 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less, preferably 3 or less, and especially 2 or less.

[0031] The repeating unit (1) is R 2 It is preferable that the repeating unit (1) contains at least one of an NH group, an OH group, an ONa group, and an OLi group. 2 is only NH2 group, R 2 is only OH group, R 2 is only an ONa group, or R 2 The repeating unit (1) may be an OLi group only. 2 may contain ONa groups and OH groups, and R 2 may contain an OLi group and an OH group, may contain an NH group and an OH group, may contain an NH group, an OH group and an ONa group, may contain an NH group, an OH group, an ONa group and an OLi group, or R 2 may contain OH groups, OLi groups and NH2 groups.

[0032] The repeating unit (2) can be introduced, for example, by polymerizing a vinyl ester (vinyl acetate, vinyl propionate, etc., particularly vinyl acetate) followed by saponification, or by reacting a polymer having a vinyl alcohol-based repeating unit (e.g., polyvinyl alcohol).

[0033] The aqueous polymer preferably contains a repeating unit derived from (meth)acrylic acid (particularly a sodium salt or lithium salt).

[0034] (Non-hydrophilic repeating unit) The aqueous polymer may have a non-hydrophilic repeating unit. The non-hydrophilic repeating unit does not have a hydrophilic group (e.g., an ionic group). Examples of the non-hydrophilic repeating unit include repeating units derived from (meth)acrylonitrile, (meth)acrylic acid alkyl ester, vinyl chloride, etc.

[0035] [Composition of aqueous polymer, etc.] The amount of hydrophilic repeating units in the aqueous polymer may be 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less.

[0036] The amount of the acidic functional group-containing repeating unit in the aqueous polymer may be 0.8 mol% or more, 3 mol% or more, 5 mol% or more, 10 mol% or more, 20 mol% or more, 40 mol% or more, or 60 mol% or more, preferably 3 mol% or more, more preferably 5 mol% or more, and may be 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 90 mol% or less, more preferably 70 mol% or less.

[0037] The amount of nonionic hydrophilic repeating units in the aqueous polymer may be 10 mol% or more, 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 10 mol% or more, more preferably 40 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 95 mol% or less.

[0038] The amount of non-hydrophilic repeating units in the aqueous polymer may be 0 mol% or more, 1 mol% or more, 3 mol% or more, 5 mol% or more, 20 mol% or more, 40 mol% or more, or 50 mol% or more, preferably 1 mol% or more, more preferably 5 mol% or more, and may be 70 mol% or less, 50 mol% or less, 40 mol% or less, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 3 mol% or less, preferably 60 mol% or less, more preferably 10 mol% or less. The aqueous polymer may not contain non-hydrophilic repeating units.

[0039] The amount of nonionic repeating units in the aqueous polymer may be 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 40 mol% or more, more preferably 60 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 95 mol% or less.

[0040] The total amount of the repeating unit (1) and the repeating unit (2) in the aqueous polymer may be 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 40 mol% or more, more preferably 50 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less.

[0041] The amount of repeating unit (1) in the aqueous polymer may be 3 mol% or more, 5 mol% or more, 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 3 mol% or more, more preferably 5 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 90 mol% or less, more preferably 80 mol% or less.

[0042] The amount of the repeating unit (2) in the aqueous polymer may be 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 20 mol% or more, more preferably 40 mol% or more, and may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 95 mol% or less. The aqueous polymer may not contain the repeating unit (2).

[0043] A repeating unit derived from (meth)acrylamide (R ２ NH ２ The amount of the hydroxybenzoate (H2O3) in the aqueous polymer may be 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, preferably 10 mol% or more, more preferably 30 mol% or more, and may be 90 mol% or less, 80 mol% or less, or 70 mol% or less, preferably 90 mol% or less, more preferably 70 mol% or less.

[0044] A repeating unit derived from (meth)acrylic acid or a salt thereof (R ２ The amount of the OM) in the aqueous polymer may be 20 mol% or more, 40 mol% or more, 60 mol% or more, 80 mol% or more, or 90 mol% or more, and is preferably 3 mol% or more, and more preferably 5 mol% or more, and may be 90 mol% or less, 80 mol% or less, or 70 mol% or less, and is preferably 90 mol% or less, and more preferably 70 mol% or less.

[0045] [Method for Producing Aqueous Polymers] The method for producing the aqueous polymer is not particularly limited, and can be produced using known copolymer production methods. Preferably, the polymer can be synthesized by aqueous radical polymerization. Specifically, a radical polymerization initiator and, if necessary, a chain transfer agent are added to a monomer mixture, and the polymerization reaction is carried out at a reaction temperature of approximately 50 to 100°C (e.g., 60 to 70°C) while stirring. The reaction time is not particularly limited, and may be approximately 1 to 10 hours. After the reaction time has elapsed, the mixture may be further aged for 15 minutes to 2 hours (e.g., 30 minutes to 1.5 hours) at a temperature of +3°C to +10°C (e.g., +4°C to +8°C) above the reaction temperature. Aging can reduce the amount of unreacted monomer. When the aqueous polymer contains vinyl alcohol units, the saponification conditions may be set with reference to, for example, the method for producing a copolymer of vinyl alcohol and an alkali metal neutralized product of an ethylenically unsaturated carboxylic acid, as described in WO 2017 / 168947.

[0046] The monomer concentration at the start of polymerization may be 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more, and may be 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less, and in one aspect, is 10% by mass or more and 11% by mass or less, 15% by mass or more and 17% by mass or less, or 11% by mass or more and 13% by mass or less.

[0047] Various known initiators can be used without particular limitation. Examples of radical polymerization initiators include persulfates such as potassium persulfate and ammonium persulfate; redox-based polymerization initiators that combine such persulfates with a reducing agent such as sodium hydrogen sulfite; and azo-based initiators such as 2,2'-azobis-2-amidinopropane dihydrochloride (V-50, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (VA-044, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). The amount of radical polymerization initiator used is preferably about 0.05 to 2% by mass, and more preferably about 0.1 to 1.5% by mass, relative to 100% by mass of the monomer group that serves as the raw material for the aqueous polymer of the present disclosure.

[0048] The polymerization conditions can be appropriately set in accordance with the structure of the target compound.

[0049] [Amount of Water-Based Polymer] The amount of the water-based polymer in the secondary battery binder may be 25% by mass or more, 50% by mass or more, 75% by mass or more, 95% by mass or more, or 99% by mass or more, preferably 50% by mass or more, more preferably 75% by mass or more, and may be 99% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less. The secondary battery binder may be the water-based polymer alone.

[0050] [Other Binder Components] The secondary battery binder according to the present disclosure may contain other binder components in addition to the aqueous polymer. Examples of the other binder components include known binder resins, such as styrene-butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), polyimide (PI), polyamide, and ethylene-vinyl acetate copolymer (EVA).

[0051] The amount of other binder components in the secondary battery binder may be 0% by mass or more, 0.5% by mass or more, 5% by mass or more, 10% by mass or more, or 25% by mass or more, and may be 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less, for example, 3% by mass or less.

[0052] <Slurry> The slurry of the present disclosure contains the secondary battery binder and water, and may further contain an electrode active material. The slurry may also contain battery particles such as a conductive additive and other liquid media, and is typically an electrode slurry containing an electrode active material. The slurry may be a positive electrode slurry containing a positive electrode active material or a negative electrode slurry containing a negative electrode active material.

[0053] [Liquid Medium] The slurry contains a liquid medium (aqueous medium) containing water. The liquid medium is preferably water alone from the viewpoint of environmental concerns, but may also contain an organic solvent, such as alcohol (ethanol, methanol, isopropyl alcohol, etc.). The amount of the organic solvent in the liquid medium may be 30% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, or 3% by mass or less, and preferably 10% by mass or less.

[0054] [Amount of Liquid Medium] The amount of the liquid medium is adjusted according to the desired slurry solids concentration. The amount of the liquid medium may be 20% by mass or more, 40% by mass or more, 60% by mass or more, 80% by mass or more, or 100% by mass or more, preferably 30% by mass or more, and may be 200% by mass or less, 100% by mass or less, 80% by mass or less, 60% by mass or less, 50% by mass or less, or 40% by mass or less, preferably 160% by mass or less, more preferably 150% by mass or less, relative to 100% by mass of the slurry solids (total amount of the secondary battery binder, active material, and conductive additive).

[0055] [Secondary Battery Binder] The slurry contains a secondary battery binder, the types of which are as described above.

[0056] [Amount of Secondary Battery Binder] The amount of the secondary battery binder in the slurry solids (total amount of the secondary battery binder, active material, and conductive additive) may be 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 1.0% by mass or more, 1.5% by mass or more, 3.0% by mass or more, or 5.0% by mass or more, preferably 0.3% by mass or more, and 10% by mass or less, 7.5% by mass or less, 5.0% by mass or less, 3.0% by mass or less, 1.0% by mass or less, 0.5% by mass or less, preferably 5.0% by mass or less, more preferably 4.0% by mass or less. Being equal to or greater than the lower limit is preferable from the viewpoint of exhibiting the effect of the secondary battery binder well. Being equal to or less than the upper limit is preferable from the viewpoint of achieving a high capacity battery.

[0057] [Active Material] The active material is an electrode active material, and may be a negative electrode active material or a positive electrode active material. When the active material is a negative electrode active material, it may contain, for example, a carbon material, and may also contain, for example, at least one of silicon and silicon oxide. Specific examples of the negative electrode active material and the positive electrode active material are shown below.

[0058] (Negative Electrode Active Material) The negative electrode active material may be any negative electrode active material used in the art.

[0059] Carbon materials such as crystalline carbon and amorphous carbon may be used as the negative electrode active material. Examples of crystalline carbon include graphite, such as natural graphite or artificial graphite, which may be amorphous, plate-like, flake-like, spherical, or fibrous. Examples of amorphous carbon include soft carbon (easily graphitizable carbon) or hard carbon (non-graphitizable carbon), mesophase pitch carbide, and calcined coke.

[0060] Materials capable of absorbing and releasing large amounts of lithium ions, such as silicon (Si), tin (Sn), and titanium (Ti), may be used as the negative electrode active material. These materials may be used in the form of simple substances, alloys, compounds, solid solutions, or composite active materials containing silicon-containing materials, tin-containing materials, and titanium-containing materials. Examples of silicon-containing materials include Si, Si / C, SiOx (0.05<x<1.95), or alloys, compounds, or solid solutions in which at least one element selected from the group consisting of B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn is substituted for a portion of the Si. The silicon-containing material may be a silicon oxide. Examples of tin-containing materials include Ni2Sn4, Mg2Sn, SnOx (0<x<2), SnO2, SnSiO3, LiSnO, etc. Examples of titanium-containing materials include Li2TiO3 and Li4Ti5O 12 Examples of the material include lithium titanates such as those mentioned above, and titanium-niobium composite compounds. These materials can be used singly or in combination of two or more. Among these, silicon or silicon oxide is preferred, and may be, for example, simple silicon or silicon oxide.

[0061] As the negative electrode active material, it is more preferable to use a composite obtained by mixing silicon or silicon oxide as the first negative electrode active material and a carbon material as the second negative electrode active material. As the carbon material, any carbon material generally used in secondary batteries, particularly non-aqueous electrolyte secondary batteries, can be used, and crystalline carbon, amorphous carbon, or a combination of these may be used. Examples of crystalline carbon include those described above.

[0062] The method for producing the negative electrode active material is not particularly limited. When producing an active material composite by mixing the first negative electrode active material and the second negative electrode active material, a method in which both materials are uniformly dispersed may be employed, such as a method in which the first negative electrode active material and the second negative electrode active material are mixed in a ball mill.

[0063] (Positive Electrode Active Material) The positive electrode active material is not particularly limited, and any positive electrode active material used in the art may be used.

[0064] The positive electrode active material may be a lithium-containing composite oxide. Examples of the lithium-containing composite oxide include LiMnO, LiFeO, LiCoO, LiMnO, LiFeSiO, and LiNi. 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi x Co y M z O2 (wherein 0≦x<1, 0≦y<1, 0≦z<1, x+y+z=1, and M is at least one element selected from the group consisting of Mn, V, Mg, Mo, Nb, Fe, Cu, and Al), LiMn （１－ｗ） Fe ｗ PO4 (where 0<w<1), LiFePO4, etc.

[0065] [Amount of active material] The amount of the active material in the slurry solids (total amount of secondary battery binder, active material, and conductive additive) may be 35% by mass or more, 45% by mass or more, 55% by mass or more, 65% by mass or more, 75% by mass or more, or 95% by mass or more, preferably 55% by mass or more, more preferably 75% by mass or more, and may be 99% by mass or less, 95% by mass or less, 90% by mass or less, or 85% by mass or less.

[0066] [Conductive Aid] As the conductive aid, a conductive aid used in the present technical field can be used. The conductive aid is not particularly limited as long as it has conductivity, but carbon powder is preferred. Examples of carbon powder include commonly used carbon materials such as acetylene black (AB), ketjen black (KB), graphite, carbon fiber, carbon tube, graphene, amorphous carbon, hard carbon, soft carbon, glassy carbon, carbon nanofiber, and carbon nanotube. These may be used alone or in combination of two or more.

[0067] [Amount of Conductive Aid] The amount of the conductive aid may be 0.1% by mass or more, 0.5% by mass or more, 1.0% by mass or more, 3.0% by mass or more, or 5.0% by mass or more, and preferably 0.3% by mass or more, and may be 10% by mass or less, 5.0% by mass or less, 3.0% by mass or less, or 1.0% by mass or less, and preferably 3.0% by mass or less, and more preferably 1.0% by mass or less, based on the slurry solids content (total amount of secondary battery binder, active material, and conductive aid).

[0068] [Dispersing Aid] The slurry of the present disclosure may further contain a dispersing aid. Examples of the dispersing aid include an organic acid containing a carboxyl group and at least one substituent selected from the group consisting of a hydroxyl group, an amino group, and an imino group, or humic acid. Examples of organic acids containing a hydroxyl group and a carboxyl group include lactic acid, tartaric acid, citric acid, malic acid, glycolic acid, tartronic acid, glucuronic acid, and humic acid. Examples of organic acids containing an amino group and a carboxyl group include glycine, alanine, phenylalanine, 4-aminobutyric acid, leucine, isoleucine, lysine, glutamic acid, aspartic acid, glutamine, asparagine, histidine, tryptophan, cysteine, and polymers thereof. Examples of organic acids containing an imino group and a carboxyl group include proline, 3-hydroxyproline, 4-hydroxyproline, and pipecolic acid. Among these, glucuronic acid, humic acid, glycine, polyglycine, aspartic acid, and glutamic acid are preferred from the viewpoint of availability.

[0069] [Amount of Dispersion Aid] The amount of the dispersion aid may be 0.1 parts by mass or more, 0.5 parts by mass or more, 1.0 parts by mass or more, 1.5 parts by mass or more, or 3.0 parts by mass or more, and preferably 0.3 parts by mass or more, and may be 5.0 parts by mass or less, 3.0 parts by mass or less, or 1.0 part by mass or less, and preferably 3.0 parts by mass or less, and more preferably 1.0 part by mass or less, relative to 100 parts by mass of the slurry solids (total amount of secondary battery binder, active material, and conductive aid).

[0070] Other Components The slurry of the present disclosure may contain other components, such as conventional additives.

[0071] [Amount of Other Components] The amount of other components (individual amount or total amount) may be 0.1 parts by mass or more, 0.5 parts by mass or more, 1.0 parts by mass or more, 3.0 parts by mass or more, or 5.0 parts by mass or more, and preferably 0.3 parts by mass or more, relative to 100 parts by mass of the slurry solids content (total amount of secondary battery binder, active material, and conductive additive), and may be 10 parts by mass or less, 5.0 parts by mass or less, 3.0 parts by mass or less, 1.0 part by mass or less, or 0.5 parts by mass or less, and preferably 3.0 parts by mass or less, and more preferably 1.0 part by mass or less.

[0072] [Method for Producing Slurry] The method for producing the slurry of the present disclosure is not particularly limited, and the slurry is produced by mixing components. For example, a secondary battery binder, a liquid medium, an active material, and optionally, a conductive additive, a dispersing aid, etc., are mixed to produce a slurry. The timing of adding the liquid medium is not particularly limited. The secondary battery binder of the present disclosure may be dispersed or dissolved in the liquid medium in advance, and then the active material and other components may be mixed to produce a slurry. Alternatively, the active material, the binder of the present disclosure, and optionally, a conductive additive, a dispersing aid, etc. may be mixed in a solid state, and then the liquid medium may be added to produce a paste-like slurry.

[0073] <Electrode> The electrode of the present disclosure is an electrode for a secondary battery, and includes the secondary battery binder (or a component derived from the secondary battery binder) and an active material according to the present disclosure. That is, the electrode of the present disclosure can be produced, for example, by applying the slurry of the present disclosure to a current collector and drying it. Therefore, the electrode may include a heat-dried product of the slurry. The secondary battery binder may be decomposed or reacted by heat drying and converted into a component derived from the secondary battery binder.

[0074] The temperature during heat drying may be 50°C or higher, 70°C or higher, 90°C or higher, 110°C or higher, or 130°C or higher, preferably 70°C or higher, and 300°C or lower, 250°C or lower, 200°C or lower, 175°C or lower, 150°C or lower, or 125°C or lower, preferably 150°C or lower. Heat drying may be carried out under reduced pressure (for example, 0.05 MPa or lower, 0.03 MPa or lower, or 0.01 MPa or lower). The heat drying time may be 1 hour or longer, 3 hours or longer, 6 hours or longer, 10 hours or longer, or 15 hours or longer, preferably 6 hours or longer, and may be 72 hours or shorter, 48 hours or shorter, 36 hours or shorter, 24 hours or shorter, or 18 hours or shorter, preferably 24 hours or shorter.

[0075] When the electrode of the present disclosure is a negative electrode, the material constituting the current collector may be, for example, a conductive material such as C, Cu, Ni, Fe, V, Nb, Ti, Cr, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, or Al, or an alloy containing two or more of these conductive materials (e.g., stainless steel). The current collector may also be made of Fe plated with Cu. From the viewpoints of high electrical conductivity, excellent stability in the electrolyte, and excellent oxidation resistance, Cu, Ni, stainless steel, etc. are preferred as the material for the negative electrode current collector, and Cu or Ni is more preferred from the viewpoint of material cost.

[0076] When the electrode of the present disclosure is a positive electrode, the material constituting the current collector may be, for example, a conductive substance such as C, Ti, Cr, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, or Al, or an alloy containing two or more of these conductive substances (for example, stainless steel). From the viewpoints of high electrical conductivity, excellent stability in the electrolyte, and excellent oxidation resistance, C, Al, stainless steel, etc. are preferred as the material for the positive electrode current collector, and Al is more preferred from the viewpoint of material cost.

[0077] The shape of the current collector is not particularly limited, and for example, a foil substrate, a three-dimensional substrate, etc. Use of a three-dimensional substrate (foam metal, mesh, woven fabric, nonwoven fabric, expanded fabric, etc.) tends to improve high-rate charge / discharge characteristics.

[0078] <Battery> The secondary battery of the present disclosure includes the secondary battery electrode of the present disclosure. The secondary battery of the present disclosure may include the secondary battery electrode of the present disclosure as either one or both of the positive electrode and the negative electrode. The secondary battery of the present disclosure is produced using the secondary battery electrode of the present disclosure (i.e., using the secondary battery binder of the present disclosure) by a method used in the technical field.

[0079] The secondary battery of the present disclosure is preferably a nonaqueous electrolyte secondary battery, and particularly preferably a lithium ion secondary battery. Because lithium ion secondary batteries must contain lithium ions, a lithium salt is preferred as the electrolyte. Examples of the lithium salt include lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium trifluoromethanesulfonyl imide. The electrolyte may be used alone or in combination of two or more.

[0080] Examples of the electrolyte solution that can be used include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and γ-butyrolactone. The electrolyte solution can be used alone or in combination of two or more. Propylene carbonate alone, a mixture of ethylene carbonate and diethyl carbonate, or γ-butyrolactone alone are particularly preferred. The mixing ratio of the above-mentioned mixture of ethylene carbonate and diethyl carbonate can be adjusted as long as one of the components accounts for 10 to 90% by volume.

[0081] Other configurations of the secondary battery may also be the same as those of known secondary batteries.

[0082] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims.

[0083] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples.

[0084] The abbreviations have the following meanings: AN: Acrylonitrile AA: Acrylic acid AAm: Acrylamide 2-HEA: 2-hydroxyethyl acrylate SMAS: Sodium methallyl sulfonate VA: Vinyl alcohol CMC: Carboxymethyl cellulose SBR: Styrene butadiene rubber

[0085] <Preparation of Polymer> A water-based polymer was prepared according to the procedure described below.

[0086] Reference Example 1 22.3 g (0.31 mol) of acrylic acid, 30.2 g (0.57 mol) of acrylonitrile, 8.5 g (0.12 mol) of acrylamide, 0.405 g of potassium persulfate, and 50 g of ion-exchanged water were added to a sealable vial having an internal volume of 500 ml and mixed to prepare an aqueous monomer solution. ２ A 1 L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 500.0 g of ion-exchanged water, and N ２ After deoxygenating the system by blowing in gas, the internal temperature was raised to 75°C. Subsequently, the prepared monomer aqueous solution was added dropwise to the reaction vessel over 3 hours using a dropping funnel while stirring. After the dropwise addition, the internal temperature was maintained at 80°C for 1 hour. The internal temperature was then cooled to 40°C or below, and 69.5 g (0.31 mol) of a 5 mol / L lithium hydroxide aqueous solution was added to adjust the pH to 7.0, yielding an aqueous solution containing an aqueous polymer. The repeating unit composition of the aqueous polymer was AN / AA(H / Li) / AAm = 57 / 31 / 12 (molar ratio).

[0087] Example 1 A sealable vial having an internal volume of 500 ml was charged with 34.9 g (0.49 mol) of acrylamide, 0.2 g (0.001 mol) of sodium methallylsulfonate, 35.3 g (0.49 mol) of acrylic acid, 48.6 g (0.42 mol) of 2-hydroxyethyl acrylate, 1.36 g of 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (product name "VA-044" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 210.0 g of ion-exchanged water, and the charged solution was mixed to prepare an aqueous monomer solution, which was then deoxygenated. Separately, a stirrer, a thermometer, and N ２A 2 L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 858.3 g of ion-exchanged water, and N ２ After deoxygenating the system by blowing in gas, the internal temperature was raised to 68°C. Subsequently, the deoxygenated aqueous monomer solution was added dropwise to the reaction vessel over 3 hours using a dropping funnel while stirring. After the dropwise addition, the internal temperature was maintained at 68°C for 2 hours. The mixture was further stirred at 73°C for 1 hour, cooled to 30°C, and 30.3 g of a 48% by mass aqueous NaOH solution was added until the pH reached 5.5, yielding an aqueous solution containing an aqueous polymer. The repeating unit composition of the aqueous polymer was AAm / AA(H / Na) / 2-HEA / SMAS = 35 / 35 / 29.9 / 0.1 (molar ratio).

[0088] [Example 2] Stirrer, thermometer, N ２ A 1 L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 485.3 g of ion-exchanged water and 100.0 g of terminal thiol-modified polyvinyl alcohol (degree of polymerization: 375, degree of saponification: 98.7 mol%). ２ After blowing gas into the system to deoxygenate it, the internal temperature was raised to 95°C. After visually confirming that the polyvinyl alcohol had dissolved, the system was cooled to room temperature. Subsequently, the pH was adjusted to 3.0 with 0.5N sulfuric acid. Next, the internal temperature was raised to 62°C, and a deoxygenated mixture of 9.9g (0.14mol) of acrylic acid, 1.70g of ammonium persulfate, and 75.2g of ion-exchanged water was added dropwise over 1.5 hours using a dropping funnel while stirring. After the dropwise addition, the internal temperature was maintained at 62°C for 0.5 hours, and then stirred at 67°C for 1 hour. The internal temperature was then cooled to 30°C, and 5.72g (0.69mol) of a 48% by mass aqueous sodium hydroxide solution was added to obtain an aqueous solution containing a water-based polymer. The repeating unit composition of the water-based polymer was VA / AA (H / Na) = 94 / 6 (molar ratio).

[0089] Reference Example 2: 40.0 g (0.56 mol) of acrylic acid, 60.0 g (0.52 mol) of 2-hydroxyethyl acrylate, 0.367 g of ammonium persulfate, and 50.0 g of ion-exchanged water were added to a sealable vial with an internal volume of 500 ml and mixed to prepare an aqueous monomer solution, which was then deoxygenated. ２A 1 L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 500.0 g of ion-exchanged water, and N ２ After deoxygenating the system by blowing in gas, the internal temperature was raised to 75°C. Subsequently, the prepared aqueous monomer solution was added dropwise to the reaction vessel over a period of 3 hours using a dropping funnel while stirring. After the dropwise addition, the temperature was maintained for 2 hours. The internal temperature was then raised to 80°C and maintained for 1 hour. The internal temperature was then cooled to 40°C, and 124.9 g (0.53 mol) of a 5 mol / L aqueous sodium hydroxide solution was added to adjust the pH to 6.5, yielding an aqueous solution containing an aqueous polymer. The repeating unit composition of the aqueous polymer was 2-HEA / AA (H / Na) = 48 / 52 (molar ratio).

[0090] [Comparative Example 1] Stirrer, thermometer, N ２ A 2 L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 831.0 g of ion-exchanged water, and N ２ After deoxygenating the system by blowing in gas, the internal temperature was raised to 68°C. A previously prepared mixture of 126.6 g (1.78 mol) of acrylamide, 2.9 g (0.02 mol) of sodium methallylsulfonate, 1.45 g of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (Fujifilm Wako Pure Chemical Industries, Ltd. product name "VA-044"), and 250.0 g of ion-exchanged water was added dropwise over 3 hours using a dropping funnel. After the dropwise addition, the mixture was maintained at the same temperature for 1.5 hours. Subsequently, 1.45 g of VA-044 and 20 g of ion-exchanged water were added, and the mixture was stirred at the same temperature for 1 hour, followed by stirring at 73°C for 1 hour, yielding an aqueous solution containing a water-based polymer. The repeating unit composition of the water-based polymer was AAm / SMAS = 99 / 1 (molar ratio).

[0091] Comparative Example 2 46.5 g (0.54 mol) of methyl acrylate and 211.8 g (2.46 mol) of vinyl acetate were added to a sealable vial with an internal volume of 500 ml and mixed to prepare an aqueous monomer solution. ２ A 2-L reaction vessel equipped with a gas inlet tube, a reflux condenser, and a dropping funnel was charged with 768 g of ion-exchanged water and 12 g of anhydrous sodium sulfate, and N ２Gas was blown into the system to deoxygenate it. Subsequently, 1 g of partially saponified polyvinyl alcohol (saponification degree 88%) and 1.2 g of lauryl peroxide were added, and the internal temperature was raised to 60°C. Subsequently, the prepared monomer aqueous solution was added dropwise to the reaction vessel over 4 hours using a dropping funnel while stirring. After the addition, the internal temperature was maintained at 65°C for 2 hours, and the precipitated solid was filtered. The same reaction vessel as above was charged with the obtained solid, 450 g of methanol, 420 g of ion-exchanged water, 140 g of sodium hydroxide, and 0.52 g of hydrazine, and the mixture was stirred at 35°C for 3 hours. Subsequently, the internal temperature was cooled to 30°C, acetic acid was added to adjust the pH to 7.5, and the solid was filtered. The solid was then washed with methanol and dried under reduced pressure at 60°C for 8 hours to obtain an aqueous polymer. The repeating unit composition of the aqueous polymer was VA / AA (H / Na) = 82 / 18 (molar ratio).

[0092] Comparative Example 3 A mixture of carboxymethyl cellulose and styrene-butadiene rubber (CMC / SBR=1 / 2 (solid weight ratio)) was used.

[0093] <Measurement of polymer properties> The properties of the aqueous polymers obtained in the above-mentioned Reference Examples, Examples, and Comparative Examples were measured. The test methods are as follows. Note that all aqueous polymer solutions used in each test method were prepared by dissolving an aqueous polymer in ion-exchanged water.

[0094] [Molecular Weight, etc.] The weight-average molecular weight (Mw) of the aqueous polymers obtained in the above Reference Examples, Examples, and Comparative Examples was measured using GPC-RI in terms of standard polyethylene glycol / polyethylene oxide. Detailed conditions are as follows: [GPC-RI] GPC device: HLC-8320GPC (Tosoh Corporation) Column: TSK GMPW XL (Tosoh Corporation) Carrier: 0.1 M sodium nitrate Column temperature: 40°C Flow rate: 1.0 mL / min Injection volume: 200 μL Concentration: 0.5 mg / mL

[0095] [pH] A polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was kept at 25°C, and then the pH was measured using a pH meter (manufactured by Horiba, Ltd., model "D-71", glass pH electrode: model "9681S-10D"). The pH was calculated by rounding the measured value to one decimal place.

[0096] [APHA] 30 ml of a polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was placed in a screw cap bottle, and the Hazen color number of the measurement sample was measured using a Hazen meter Type: HM-IV (X Electronics Design Co., Ltd.) in an environment of 25°C.

[0097] [Absorbance (400 nm)] A polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was placed in an acrylic resin cell (manufactured by AS ONE, model "ST-MA"), and the absorbance at 400 nm was measured in an environment of 25°C using an ultraviolet-visible-near-infrared spectrophotometer (manufactured by Hitachi High-Tech Science, model "UH4150").

[0098] [0.1 mol / L NaOH Neutralization Titration Amount] 70 g of a polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was placed in a 100 mL beaker, and neutralization titration was carried out with 0.1 mol / L NaOH using an automatic titrator (manufactured by HIRANUMA Corporation, model "COM-1700A"), and the titration amount was calculated using a pH inflection point detection method.

[0099] [Weight Loss Between 170 and 290°C] 20 g of a polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was added to a polypropylene tray (manufactured by AS ONE, model "DT-1") and dried at 60°C for 8 hours in a constant temperature incubator (manufactured by Yamato Scientific, model "DKM300"), followed by additional drying for 24 hours at 60°C and a reduced pressure of 0.1 MPa in a vacuum dryer (manufactured by AS ONE, model "AVO-310N") to produce a film. The film produced by the above method was cut into approximately 2 mm square pieces, and several pieces were used to weigh 10 mg. Measurement was performed using a differential scanning calorimeter (manufactured by Hitachi High-Tech Science, model "STA200RV") under the following conditions. Measurement conditions: The temperature was increased from 30°C to 500°C at a rate of 10°C / min, and the weight change was observed. Sample pan: Aluminum pan. Mass of measurement sample: 4 mg. Calculation of weight loss: The mass of the measurement sample was Wa, and the weight change detected between 170°C and 290°C was Wb. The weight loss rate was calculated using the following formula: Weight loss rate = Wb / Wa × 100 [mass %].

[0100] <Characteristics of Battery / Slurry / Aqueous Polymer Solution> Battery / slurry / aqueous polymer solution was prepared using the aqueous polymers obtained in the above-mentioned Reference Examples / Examples / Comparative Examples, and the characteristics thereof were measured. The test methods were as follows.

[0101] [Viscosity Change Rate After 1 Week at 20°C] A polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was kept at 25°C, and the viscosity before storage was measured using a Brookfield viscometer (manufactured by Brookfield, model "DV1MLVTJ0"). The 2% by mass polymer aqueous solution was then left to stand at 20°C for 1 week, kept at 25°C, and the viscosity after storage was measured using a Brookfield viscometer (manufactured by Brookfield, model "DV1MLVTJ0"). The viscosity change rate was calculated using the following formula: Viscosity change rate = (viscosity after storage - viscosity before storage) / viscosity before storage × 100 [%]. Note that the spindle and rotation speed corresponding to the viscosity were used as follows. LV-2, 12 rpm: 200 mPa·s or more and 2500 mPa·s or less LV-1, 30 rpm: 20 mPa·s or more and less than 200 mPa·s

[0102] [Viscosity Change Rate After 1 Week at 40°C] A polymer aqueous solution prepared by dissolving a water-based polymer in water to a solids concentration of 2% by mass was kept at 25°C, and the viscosity before storage was measured using a Brookfield viscometer (manufactured by Brookfield, model "DV1MLVTJ0"). The 2% by mass aqueous solution was then allowed to stand at 40°C for 1 week, and the viscosity after storage was measured using a Brookfield viscometer (manufactured by Brookfield, model "DV1MLVTJ0"). The viscosity change rate was calculated using the following formula: Viscosity change rate = (viscosity after storage - viscosity before storage) / viscosity before storage × 100 [%]. Note that the spindle and rotation speed corresponding to the viscosity were used as follows. LV-2, 12 rpm: 200 mPa·s or more and 2500 mPa·s or less LV-1, 30 rpm: 20 mPa·s or more and less than 200 mPa·s

[0103] The test results are summarized in the table below.

[0104] The present disclosure is suitable for use as a power source for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, and the like.

[0105] The present disclosure includes the following aspects: [Item 1] A secondary battery binder comprising an aqueous polymer, wherein the aqueous polymer is dissolved in water at a concentration of 2% by mass to give an aqueous solution having a pH (25°C) of 5.0 or more and 7.5 or less, and an absorbance (400 nm) of the aqueous solution of 0.05 or less. [Item 2] The secondary battery binder according to Item 1, wherein the aqueous polymer is a vinyl polymer having an acidic functional group-containing repeating unit. [Item 3] The aqueous polymer is a compound represented by the formula: -[CH ２ -C(R １ ) (C(=O)R ２ )] - [wherein, R 1 is a hydrogen atom or CH ３ and R 2 is NH ２ , OM (M is a hydrogen atom or a counter cation), O(CH ２ ) ｎ OH, NH(CH ２ ) ｎ OH (wherein n is independently 1 or more and 6 or less).] and a repeating unit (1) represented by the formula: -[CH２ Item 4. The secondary battery binder according to Item 1 or 2, comprising a repeating unit selected from the repeating unit (2) represented by the formula: —CH(OH)]—. [Item 4] The secondary battery binder according to Item 3, wherein in the aqueous polymer, the amount of the repeating unit selected from the repeating unit (1) and the repeating unit (2) is 40 mol % or more, the amount of the acidic functional group-containing repeating unit is 0.8 mol % or more, and the amount of the nonionic repeating unit is 40 mol % or more. [Item 5] The secondary battery binder according to any one of Items 1 to 4, comprising SBR. [Item 6] A slurry comprising the secondary battery binder according to any one of Items 1 to 5 and water. [Item 7] The slurry according to Item 6, further comprising an electrode active material. [Item 8] A method for producing a secondary battery, comprising a step of applying the slurry according to Item 6 or 7. [Item 9] An electrode comprising the secondary battery binder according to any one of Items 1 to 5 or a component derived from the secondary battery binder. [Item 10] An electrode comprising a heat-dried product of the slurry according to Item 6 or 7. [Item 11] A secondary battery comprising the electrode according to Item 9 or 10.

Claims

1. A secondary battery binder comprising a water-based polymer, wherein the pH (25°C) of an aqueous solution obtained when the water-based polymer is dissolved in water at a concentration of 2% by mass is 5.0 or more and 6.5 or less, and the absorbance (400 nm) of the aqueous solution is 0.05 or less.

2. The secondary battery binder according to claim 1, wherein the water-based polymer is a vinyl polymer having a repeating unit containing an acidic functional group.

3. The water-based polymer has the formula: -[CH ２ -C(R １ ) (C(=O)R ２ )] - [wherein, R 1 is a hydrogen atom or CH ３ and R 2 is NH ２ , OM (M is a hydrogen atom or a counter cation), O(CH ２ ) ｎ OH, NH(CH ２ ) ｎ OH (wherein n is independently 1 or more and 6 or less).] and a repeating unit (1) represented by the formula: -[CH ２ 3. The secondary battery binder according to claim 1, comprising a repeating unit selected from the repeating unit (2) represented by the formula:

4. The secondary battery binder according to claim 3, wherein in the aqueous polymer, the amount of the repeating unit selected from the repeating unit (1) and the repeating unit (2) is 40 mol % or more, the amount of the repeating unit containing an acidic functional group is 0.8 mol % or more, and the amount of the nonionic repeating unit is 40 mol % or more.

5. The secondary battery binder according to claim 1 or 2, which contains SBR.

6. A slurry comprising the secondary battery binder according to claim 1 or 2 and water.

7. The slurry of claim 6, further comprising an electrode active material.

8. A method for producing a secondary battery, comprising the step of applying the slurry according to claim 6.

9. An electrode comprising the secondary battery binder according to claim 1 or 2 or a component derived from said secondary battery binder.

10. An electrode comprising the heat-dried slurry of claim 7.

11. A secondary battery comprising the electrode according to claim 9.

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