Electrode binder, electrode composition, and electrode

Abstract

The present invention provides an electrode binder, an electrode composition, and an electrode that can suppress the increase in DC resistance of a battery during cyclic charging and discharging. [Solution] The electrode binder contains an aliphatic conjugated diene monomer and a copolymer of the aliphatic conjugated diene monomer and another monomer polymerizable with the aliphatic conjugated diene monomer. The decrease in the integral value of the stress-strain curve (S2) of the dry film of the copolymer latex under high humidity conditions is 30% or less compared to the integral value of the stress-strain curve (S1) of the dry film under normal conditions.

Description

Technical Field

[0001] The present invention relates to an electrode binder, an electrode composition, and an electrode.

Background Art

[0002] Conventionally, in the production of electrodes of batteries such as lithium-ion secondary batteries, electrode binders used for binding an active material to a current collector are known (for example, see Patent Document 1 below).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As an electrode binder as described in Patent Document 1, an electrode binder capable of further suppressing an increase in the DC resistance of a battery during cyclic charge and discharge is required.

[0005] The present invention provides an electrode binder, an electrode composition, and an electrode capable of suppressing an increase in the DC resistance of a battery during cyclic charge and discharge.

Means for Solving the Problems

[0006] The present invention [1] includes an electrode binder comprising an aliphatic conjugated diene monomer and a copolymer of the aliphatic conjugated diene monomer and another monomer polymerizable with the aliphatic conjugated diene monomer, wherein the rate of decrease in the integral value (S2) of the stress-strain curve obtained when the dry film of the electrode binder, having a thickness of 0.4 mm ± 0.1 mm and a dumbbell shape of type 3, is left to stand for 36 hours in an environment of 23°C and 50% relative humidity and then subjected to a tensile test at a tensile speed of 500 mm / min, is 30% or less compared to the integral value (S1) of the stress-strain curve obtained when the dry film is left to stand for 36 hours in an environment of 23°C and 50% relative humidity, after being left to stand for 96 hours in an environment of 23°C and 90% relative humidity.

[0007] The present invention [2] includes the electrode binder of [1] above, wherein the other monomer contains a vinyl cyanide monomer, and the proportion of structural units derived from the vinyl cyanide monomer in the copolymer is 20% by mass or less.

[0008] The present invention [3] includes the electrode binder according to [1] or [2] above, wherein the other monomer contains a hydroxyl group-containing ethylenically unsaturated monomer, and the proportion of structural units derived from the hydroxyl group-containing ethylenically unsaturated monomer in the copolymer is 3.0% by mass or less.

[0009] The present invention [4] comprises one of the electrode binders [1] to [3] above, wherein the other monomer contains an ethylenically unsaturated carboxylic acid monomer, and the proportion of structural units derived from the ethylenically unsaturated carboxylic acid monomer in the copolymer is 0.1% by mass to 10% by mass.

[0010] The present invention [5] includes one of the electrode binders [1] to [4] above, which contains a surfactant, wherein the amount of the surfactant is 2.5 parts by mass or less per 100 parts by mass of the copolymer.

[0011] The present invention [6] includes an electrode composition comprising one of the electrode binders [1] to [5] described above and an active material.

[0012] The present invention [7] includes an electrode comprising a current collector and an electrode mixture layer provided on the current collector and formed from the electrode composition described in [6] above. [Effects of the Invention]

[0013] The electrode binder, electrode composition, and electrode of the present invention can suppress the increase in the DC resistance of a battery during cyclic charging and discharging. [Modes for carrying out the invention]

[0014] 1. Electrode binder An electrode binder as one embodiment of the present invention will be described.

[0015] The electrode binder contains a copolymer of an aliphatic conjugated diene monomer and another monomer. The electrode binder may also be an emulsion polymer (polymer latex) of a monomer composition containing an aliphatic conjugated diene monomer and another monomer. The electrode binder may contain a surfactant as needed.

[0016] (1)Copolymer When the electrode binder is a copolymer latex, the copolymer consists of latex particles. The copolymer contains structural units derived from aliphatic conjugated diene monomers and structural units derived from other monomers.

[0017] (1-1) Aliphatic conjugated diene monomers Examples of aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chlor-1,3-butadiene. Preferably, 1,3-butadiene is used as the aliphatic conjugated diene monomer.

[0018] The copolymer contains structural units derived from at least one aliphatic conjugated diene monomer. The copolymer may contain structural units derived from two or more aliphatic conjugated diene monomers.

[0019] The proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer may be 15% by mass to 60% by mass or less. When the proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer is within the above range, the electrode strength can be improved. When the proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer is within the above range, the contamination of the roll can also be suppressed. The proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer may be 55% by mass or less, 50% by mass or less, or 45% by mass or less. Also, the proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer may be 20% by mass or more, or 25% by mass or more. The proportion of the structural units derived from the aliphatic conjugated diene monomer in the copolymer may be 20% by mass to 55% by mass, 25% by mass to 50% by mass, or 25% by mass to 45% by mass.

[0020] (1-2) Other monomers The other monomers are polymerizable with the aliphatic conjugated diene monomer. Examples of the other monomers include vinyl cyanide monomers, aromatic vinyl monomers, ethylenically unsaturated carboxylic acid ester monomers (excluding hydroxy group-containing ethylenically unsaturated monomers), hydroxy group-containing ethylenically unsaturated monomers, ethylenically unsaturated carboxylic acid amide monomers, and ethylenically unsaturated carboxylic acid monomers.

[0021] That is, the other monomers may contain at least one selected from vinyl cyanide monomers, hydroxy group-containing ethylenically unsaturated monomers, and ethylenically unsaturated carboxylic acid monomers.

[0022] Examples of vinyl cyanide monomers include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile. Preferably, acrylonitrile is mentioned as the vinyl cyanide monomer.

[0023] The copolymer may contain structural units derived from at least one vinyl cyanide monomer. The copolymer may contain structural units derived from two or more vinyl cyanide monomers.

[0024] The proportion of the structural units derived from the vinyl cyanide monomer in the copolymer may be 20% by mass or less, 15% by mass or less, or 10% by mass or less. Also, the proportion of the structural units derived from the vinyl cyanide monomer in the copolymer may be 0.5% by mass or more, 1% by mass or more, or 3% by mass or more. The proportion of the structural units derived from the vinyl cyanide monomer in the copolymer may be 0.5% by mass to 20% by mass, 1% by mass to 15% by mass, or 3% by mass to 10% by mass.

[0025] The aromatic vinyl monomer is an aromatic hydrocarbon having one vinyl group. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, methyl-α-methylstyrene, vinyltoluene, and monochlorostyrene. Preferably, styrene is mentioned as the aromatic vinyl monomer.

[0026] The copolymer may contain structural units derived from at least one aromatic vinyl monomer. The copolymer may contain structural units derived from two or more aromatic vinyl monomers.

[0027] The proportion of the structural units derived from the aromatic vinyl monomer in the copolymer may be 5% by mass to 80% by mass, 10% by mass to 70% by mass, or 15% by mass to 65% by mass.

[0028] Ethylene-unsaturated carboxylic acid monomers are carboxylic acid esters having one ethylenically unsaturated bond. Examples of ethylenically unsaturated carboxylic acid monomers include acrylic acid esters, methacrylic acid esters, fumarate esters, maleate esters, and itaconic acid esters. Examples of acrylic acid esters include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and glycidyl acrylate. Examples of methacrylic acid esters include methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, and glycidyl methacrylate. Examples of fumarate esters include monomethyl fumarate, monoethyl fumarate, dimethyl fumarate, and diletyl fumarate. Examples of maleate esters include monomethyl maleate, monoethyl maleate, dimethyl maleate, and diethyl maleate. Examples of itaconic acid esters include monomethyl itaconate, monoethyl itaconate, dimethyl itaconate, and diethyl itaconate. Preferably, examples of ethylenically unsaturated carboxylic acid monomers include acrylic acid esters and methacrylic acid esters, and more preferably methyl methacrylate.

[0029] The copolymer may contain structural units derived from at least one ethylenically unsaturated carboxylic acid ester monomer. The copolymer may also contain structural units derived from two or more ethylenically unsaturated carboxylic acid ester monomers.

[0030] The proportion of structural units derived from ethylenically unsaturated carboxylic acid monomers in the copolymer may be 0.01% to 70% by mass, 0.05% to 30% by mass, or 0.1% to 5% by mass.

[0031] Examples of hydroxyl group-containing ethylenically unsaturated monomers include hydroxyl group-containing acrylic acid esters, hydroxyl group-containing methacrylic acid esters, hydroxyl group-containing fumarate esters, hydroxyl group-containing maleate esters, and hydroxyl group-containing itaconic acid esters. Examples of hydroxyl group-containing acrylic acid esters include hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate. Examples of hydroxyl group-containing methacrylic acid esters include hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, and 3-chloro-2-hydroxypropyl methacrylate. An example of a hydroxyl group-containing fumarate ester is 2-hydroxyethyl methyl fumarate. Examples of hydroxyl group-containing maleate esters include 2-hydroxyethyl maleate, bis(2-hydroxyethyl) maleate, and di(ethylene glycol) maleate. An example of a hydroxyl group-containing itaconic acid ester is di(ethylene glycol) itaconate. Preferably, the hydroxyl group-containing ethylenically unsaturated monomers include hydroxyl group-containing acrylic acid esters and hydroxyl group-containing methacrylic acid esters, and more preferably, hydroxyethyl acrylate.

[0032] The copolymer may contain structural units derived from at least one hydroxyl group-containing ethylenically unsaturated monomer. The copolymer may also contain structural units derived from two or more hydroxyl group-containing ethylenically unsaturated monomers.

[0033] The proportion of structural units derived from hydroxyl group-containing ethylenically unsaturated monomers in the copolymer may be 3% by mass or less, 2% by mass or less, or 1.5% by mass or less. Alternatively, the proportion of structural units derived from hydroxyl group-containing ethylenically unsaturated monomers in the copolymer may be 0.1% by mass or more, 0.5% by mass or more, or 1% by mass or more. The proportion of structural units derived from hydroxyl group-containing ethylenically unsaturated monomers in the copolymer may be 0.1% by mass to 3% by mass, 0.5% by mass to 2% by mass, or 1% by mass to 1.5% by mass.

[0034] Examples of ethylenically unsaturated carboxylic acid amide monomers include acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, and N,N-dimethylacrylamide. Preferably, acrylamide is used as the ethylenically unsaturated carboxylic acid amide monomer.

[0035] The copolymer may contain structural units derived from at least one ethylenically unsaturated carboxylic acid amide monomer. The copolymer may also contain structural units derived from two or more ethylenically unsaturated carboxylic acid amide monomers.

[0036] The copolymer does not need to contain structural units derived from ethylenically unsaturated carboxylic acid amide monomers. The proportion of structural units derived from ethylenically unsaturated carboxylic acid amide monomers in the copolymer may be 0.1% to 5% by mass, 0.2% to 3% by mass, or 0.5% to 2% by mass.

[0037] Examples of ethylenically unsaturated carboxylic acid monomers include monocarboxylic acids and dicarboxylic acids. Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid. Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Preferably, examples of ethylenically unsaturated carboxylic acid monomers include acrylic acid, methacrylic acid, fumaric acid, and itaconic acid.

[0038] The copolymer may contain structural units derived from at least one ethylenically unsaturated carboxylic acid monomer. The copolymer may also contain structural units derived from two or more ethylenically unsaturated carboxylic acid monomers.

[0039] The proportion of structural units derived from ethylenically unsaturated carboxylic acid monomers in the copolymer may be 0.1% by mass or more, 0.2% by mass or more, or 0.5% by mass or more. The proportion of structural units derived from ethylenically unsaturated carboxylic acid monomers in the copolymer may be 10% by mass or less, 9% by mass or less, or 5% by mass or less. The proportion of structural units derived from ethylenically unsaturated carboxylic acid monomers in the copolymer may be 0.1% by mass to 10% by mass, 0.2% by mass to 9% by mass, or 0.5% by mass to 5% by mass.

[0040] Furthermore, the other monomers may contain polyfunctional ethylenically unsaturated monomers having two or more ethylenically unsaturated groups.

[0041] Examples of polyfunctional ethylenically unsaturated monomers include allyl (meth)acrylates, di(meth)acrylates, and divinyl compounds. Examples of di(meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and triethylene glycol di(meth)acrylate. An example of a divinyl compound is divinylbenzene.

[0042] The proportion of structural units derived from polyfunctional ethylenically unsaturated monomers in the copolymer may be 0.01% to 10% by mass, 0.05% to 5% by mass, or 0.1% to 1% by mass.

[0043] (2) Surfactants Examples of surfactants include anionic surfactants and nonionic surfactants. Examples of anionic surfactants include sulfated salts of higher alcohols, alkylbenzene sulfonates (e.g., sodium dodecylbenzenesulfonate), alkyl diphenyl ether disulfonates (e.g., sodium alkyl diphenyl ether disulfonate), aliphatic sulfonates, aliphatic carboxylates, dehydroabiethinates, formalin condensates of naphthalene sulfonic acid, sulfated salts of nonionic surfactants, and polyphosphates (e.g., sodium tripolyphosphate). Examples of nonionic surfactants include polyethylene glycol alkyl ester type nonionic surfactants, alkylphenyl ether type nonionic surfactants, and alkyl ether type (e.g., polyoxyethylene lauryl ether) nonionic surfactants. The copolymer latex may contain one or more surfactants.

[0044] The amount of surfactant blended is, for example, 2.5 parts by mass or less, preferably 2.0 parts by mass or less, for example, 1.5 parts by mass or less, per 100 parts by mass of copolymer. The lower limit of the amount of surfactant blended is not limited. The lower limit of the amount of surfactant blended is, for example, 0.1 parts by mass or more, per 100 parts by mass of copolymer. The amount of surfactant blended may be 0.1 to 2.5 parts by mass, 0.1 to 2.0 parts by mass, or 0.1 to 1.5 parts by mass, per 100 parts by mass of copolymer.

[0045] 2. Breakage characteristics of the dry film of the electrode binder The dried film of the electrode binder described above exhibits a reduction rate A in the integral value of the stress-strain curve (S2) under high humidity conditions compared to the integral value of the stress-strain curve (S1) under normal conditions, which is 30% or less, preferably 28% or less, and more preferably 26% or less.

[0046] If the rate of decrease A is below the above upper limit, the increase in DC resistance of the resulting battery during cycle charging and discharging can be suppressed.

[0047] Furthermore, if the reduction rate A is below the above upper limit, moisture absorption in the resulting electrode layer is suppressed, thereby suppressing deterioration of the electrode layer caused by the reaction between moisture in the electrode layer and the electrolyte. This is thought to suppress the increase in DC resistance during cycle charging and discharging.

[0048] The rate of decrease A is, for example, greater than 0, preferably 3% or more, and more preferably 12% or more.

[0049] The rate of decline A is greater than 0, may be 30% or less, may be between 3% and 28%, or may be between 12% and 26%.

[0050] If the reduction rate A is above the lower limit value, for example, fine cracks and powder fallout in the electrode mixture layer during electrode cutting and electrode pressing can be suppressed, thereby improving handling and battery performance in the battery manufacturing process.

[0051] The rate of decline A is calculated using the following formula.

[0052] Formula: Decrease rate A = (Integral value (S1) - Integral value (S2)) / Integral value (S1) × 100 The stress-strain curve under normal conditions is obtained by leaving a dry film undisturbed for 36 hours at 23°C and 50% relative humidity, followed by a tensile test at a tensile speed of 500 mm / min.

[0053] The tensile test will be conducted in accordance with JIS K6251:2017. The thickness of the dry film will be 0.4 mm ± 0.1 mm, and the shape of the dry film will be dumbbell-shaped, type 3.

[0054] The stress-strain curve under high humidity conditions is obtained by leaving a dry film standing for 36 hours at 23°C and 50% relative humidity, and then leaving it standing for 96 hours at 23°C and 90% relative humidity, followed by a tensile test at a tensile speed of 500 mm / min.

[0055] Furthermore, the reduction rate A tends to increase as the proportion of vinyl cyanide monomers increases. Also, the reduction rate A tends to increase as the proportion of hydroxyl group-containing ethylenically unsaturated monomers increases. Also, the reduction rate A tends to increase as the proportion of ethylenically unsaturated carboxylic acid amide monomers increases. Also, the reduction rate A tends to increase when the ethylenically unsaturated carboxylic acid monomer is a trans dicarboxylic acid. Also, the reduction rate A tends to increase as the proportion of ethylenically unsaturated carboxylic acid monomers increases. Also, the reduction rate A tends to increase when the copolymer latex is alkaline (pH exceeds 7). Also, the reduction rate A tends to increase as the proportion of surfactants in the copolymer latex increases.

[0056] Based on these trends, the rate of decrease A can be adjusted by adjusting the monomer composition, pH, and the proportion of surfactants used.

[0057] 3. Manufacturing of electrode binders Next, we will explain the manufacturing method for electrode binders.

[0058] To produce an electrode binder, for example, the monomer composition described above is subjected to emulsion polymerization. However, the polymerization method is not limited to emulsion polymerization. Other polymerization methods include suspension polymerization, solution polymerization, and bulk polymerization. When solution polymerization or bulk polymerization is used, the resulting polymer may be dispersed in water using a surfactant if necessary.

[0059] 4. Composition for electrodes Next, an electrode composition as one embodiment of the present invention will be described.

[0060] The electrode composition contains the electrode binder described above, an active material, and optionally a conductive material and an auxiliary agent. To manufacture the electrode composition, the electrode binder, the active material, and optionally a conductive material and an auxiliary agent are mixed together.

[0061] The active material is the positive electrode active material when a positive electrode is being manufactured, and the negative electrode active material when a negative electrode is being manufactured.

[0062] The positive electrode active material is not limited. When fabricating the positive electrode of a lithium-ion secondary battery, examples of positive electrode active materials include transition metal oxides, composite metal oxides, lithium-rich composite oxides, transition metal sulfides, and metal fluorides. Examples of transition metal oxides include MnO2, MoO3, V2O5, V6O 13 Examples include Fe2O3 and Fe3O4. Examples of composite metal oxides include composite metal oxides having a layered structure, composite metal oxides having a spinel structure, and composite metal oxides having an olivine structure. Examples of composite metal oxides having a layered structure include LiCoO2, LiMnO2, LiNiO2, and LMO2 (where M represents two or more metal elements selected from Ni, Mn, Co, and Al). Examples of composite metal oxides having a spinel structure include LiMn2O4 and LiM2O4 (where M represents two or more metal elements selected from Ni, Mn, Co, and Al). An example of a composite metal oxide having an olivine structure is LiFePO4. An example of a lithium-rich composite oxide is Li2MnO3. Examples of transition metal sulfides include TiS2, TiS3, MoS3, and FeS2. Examples of metal fluorides include CuF2 and NiF2. The positive electrode active material can be used alone or in combination of two or more types.

[0063] The negative electrode active material is not limited. When fabricating the negative electrode of a lithium-ion secondary battery, examples of negative electrode active materials include conductive carbon materials, silicon-based active materials, and conductive polymers. Examples of conductive carbon materials include fluorinated carbon, graphite, carbon fiber, resin-fired carbon, linear graphite hybrid, coke, pyrolysis vapor-grown carbon, furfuryl alcohol resin-fired carbon, mesocarbon microbeads, mesophase pitch-based carbon, graphite whiskers, pseudo-isotropic carbon, and sintered natural materials. Examples of conductive polymers include polyacene-based organic semiconductors, polyacetylene, and poly-p-phenylene. Examples of silicon-based active materials include silicon, SiOx (0.01 ≤ x < 2), and alloys of silicon and transition metals. The negative electrode active material can be used alone or in combination of two or more types.

[0064] Examples of conductive materials include carbon black, carbon nanotubes, carbon nanofibers, graphite powder, and carbon fibers. Conductive materials can be used individually or in combination of two or more types.

[0065] Examples of auxiliary agents include water-soluble thickeners, dispersants, and stabilizers. Examples of water-soluble thickeners include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), starch oxide, phosphorylated starch, and casein. Examples of dispersants include sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, and sodium polyacrylate. Examples of stabilizers include nonionic surfactants and anionic surfactants. Auxiliary agents can be used individually or in combination of two or more.

[0066] 5.Electrode Next, we will describe an electrode as an embodiment of one invention.

[0067] The electrode comprises a current collector and an electrode mixture layer. The electrode can be used, for example, as the positive or negative electrode of a lithium-ion secondary battery.

[0068] Examples of current collectors include metal foil. Examples of current collector materials include copper, aluminum, nickel, and stainless steel.

[0069] The electrode mixture layer is provided on the current collector. The electrode mixture layer is formed from an electrode composition. Specifically, the electrode mixture layer can be formed on the current collector by applying the electrode composition to the current collector and drying it.

[0070] For applying the electrode composition to the current collector, known methods such as the reverse roll method, comma bar method, gravure method, and air knife method can be used, and for drying, methods such as standing drying, forced-air drying, hot-air drying, infrared heating, and far-infrared heating can be used.

[0071] 6. Effects According to the electrode binder, electrode composition, and electrode of the present invention, the rate of decrease A of the integral value (S2) of the stress-strain curve of the dry film of the electrode binder under high humidity conditions compared to the integral value (S1) of the stress-strain curve of the dry film under normal conditions is 30% or less.

[0072] Therefore, the increase in DC resistance during the cycle charging and discharging of the resulting battery can be suppressed. [Examples]

[0073] Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to the following examples. Furthermore, the specific numerical values ​​of the blending ratios or content ratios used in the following description can be replaced with the corresponding upper or lower limits of the blending ratios or content ratios described in the "Modes for Carrying Out the Invention" above.

[0074] 1. Manufacturing of electrode binders <Example 1> In a pressure-resistant polymerization reactor, 90 parts by mass of polymerization water, the first monomer composition, and 0.1 parts by mass of alkyldiphenyl ether disulfonate sodium were charged under a nitrogen atmosphere, and stirring was started.

[0075] The first monomer composition consists of 9.15 parts by mass of styrene, 5.7 parts by mass of 1,3-butadiene, and 1 part by mass of itaconic acid.

[0076] Next, 0.3 parts by mass of ammonium persulfate was added, and the temperature was raised to 65°C.

[0077] Next, a mixture (emulsion) of the second monomer composition, 10 parts by mass of polymerization water, 0.7 parts by mass of alkyldiphenyl ether disulfonate sodium, and 0.3 parts by mass of t-dodecyl mercaptan was continuously added over 540 minutes.

[0078] The second monomer composition consists of 51.85 parts by mass of styrene and 32.3 parts by mass of 1,3-butadiene.

[0079] Subsequently, polymerization was carried out for 480 minutes while maintaining the internal temperature at 65°C.

[0080] Next, the obtained copolymer latex was pH-adjusted to 7 using an aqueous ammonia solution, and unreacted monomers and other low-boiling point compounds were removed by steam distillation to obtain an electrode binder.

[0081] <Examples 2-9> Electrode binders for each example and comparative example were obtained in the same manner as in Example 1, except that emulsion polymerization was carried out using the monomer compositions listed in Table 1.

[0082] <Example 10> In a pressure-resistant polymerization reactor under a nitrogen atmosphere, 90 parts by mass of polymerization water, the first monomer composition, 2 parts by mass of cyclohexene, 0.25 parts by mass of sodium dodecylbenzenesulfonate, and 0.3 parts by mass of sodium bicarbonate were charged, and stirring was started.

[0083] The first monomer composition consists of 7.76 parts by mass of styrene, 7.2 parts by mass of 1,3-butadiene, 0.48 parts by mass of methyl methacrylate, 1 part by mass of hydroxyethyl acrylate, 1.6 parts by mass of fumaric acid, and 0.9 parts by mass of acrylic acid.

[0084] Next, 0.8 parts by mass of ammonium persulfate was added, and the temperature was raised to 70°C.

[0085] Next, a mixture of the second monomer composition, 0.25 parts by mass of sodium dodecylbenzenesulfonate, and 0.4 parts by mass of t-dodecyl mercaptan was continuously added over 420 minutes.

[0086] The second monomer composition consists of 40.74 parts by mass of styrene, 37.8 parts by mass of 1,3-butadiene, and 2.52 parts by mass of methyl methacrylate.

[0087] Subsequently, polymerization was carried out for 150 minutes while maintaining the internal temperature at 70°C. After that, the temperature inside the polymerization reactor was raised to 80°C, and polymerization was terminated when the polymerization conversion rate exceeded 95%.

[0088] Next, the obtained copolymer latex was pH-adjusted to 7 using an aqueous ammonia solution, and unreacted monomers and other low-boiling point compounds were removed by steam distillation to obtain an electrode binder.

[0089] <Examples 11-13, Comparative Examples 1 and 2> Electrode binders for each example and comparative example were obtained in the same manner as in Example 10, except that emulsion polymerization was carried out using the monomer compositions listed in Table 2.

[0090] [Table 1]

[0091] [Table 2]

[0092] 2. Measurement of the rate of decline A <Preparation of test specimens> To the electrode binder obtained in each example and comparative example, an aqueous solution of sodium polyacrylate (manufactured by Toagosei Co., Ltd., trade name: GLX) was added as a thickening agent at a ratio of 0.5% by mass in terms of solid content relative to the dry weight of the electrode binder to prepare a coating solution.

[0093] The obtained coating solution was thinned using a film applicator, and then left to dry for 48 hours under temperature and humidity conditions corresponding to the minimum film formation temperature of the electrode binder to obtain a dried film of the electrode binder.

[0094] Next, the obtained dried film was heated at 130°C for 15 minutes to obtain a dried film with a thickness of 0.4 mm ± 0.1 mm.

[0095] The obtained dried film was punched out into the shape and dimensions of a dumbbell-shaped No. 3 as described in JIS K6251:2017 to prepare test specimens.

[0096] <Tensile testing of test specimens under normal environmental conditions> After the test specimens were left to stand for 36 hours in an environment of 23°C and 50% relative humidity, they were subjected to a tensile test at a tensile speed of 500 mm / min using a horizontal rubber tensile testing machine (manufactured by Yonekura Seisakusho Co., Ltd.), and stress-strain curves were obtained.

[0097] <Tensile test of specimens in a humid environment> The test specimens were left to stand for 36 hours in an environment of 23°C and 50% relative humidity, and then left for another 96 hours in a constant temperature and humidity chamber at 23°C and 90% relative humidity. Immediately after removing the test specimens from the constant temperature and humidity chamber, a tensile test was performed using a horizontal rubber tensile testing machine (manufactured by Yonekura Seisakusho Co., Ltd.) at a tensile speed of 500 mm / min to obtain a stress-strain curve.

[0098] <Calculation of the decline rate A> By approximating the elongation-stress curves obtained using the Python programming software with a polynomial, the integral values ​​of the stress-strain curve under normal conditions (S1) and the integral value of the stress-strain curve under high humidity conditions (S2) were calculated. The rate of decrease A was calculated using the following formula. The results are shown in Tables 1 and 2.

[0099] Formula: Decrease rate A = (Integral value (S1) - Integral value (S2)) / Integral value (S1) × 100 3. Making a battery (1) Fabrication of the negative electrode Natural graphite was used as the negative electrode active material. To 100 parts by mass of natural graphite, 1 part by mass of aqueous carboxymethylcellulose as a thickening agent and 2 parts by mass of the electrode binders of each example and comparative example as a binder were mixed with an appropriate amount of pure water so that the total solid content was 45% by mass, and the negative electrode composition (electrode composition) was prepared.

[0100] The obtained negative electrode composition was applied to a 20 μm thick copper foil that would serve as a current collector, dried at 80°C for 10 minutes, and then roll-pressed at room temperature to obtain a negative electrode with an electrode mixture layer thickness of 70 μm.

[0101] (2) Preparation of the positive electrode NiCoMnO2 (Ni:Co:Mn=6:2:2) was used as the positive electrode active material. 100 parts by mass of NiCoMnO2 were dry-blended with 3 parts by mass of acetylene black as a conductive material. Then, 3 parts by mass of N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF) was added, and an appropriate amount of NMP solvent was added and kneaded to achieve a total solid content of 65% by mass, thereby preparing the positive electrode composition.

[0102] The positive electrode composition obtained as described above was applied to a 20 μm thick aluminum foil to serve as a current collector, dried at 130°C for 20 minutes, and then roll-pressed at room temperature to obtain a positive electrode with an electrode mixture layer thickness of 70 μm.

[0103] (3) Fabrication of laminate cells (batteries) The obtained positive electrode was cut so that the electrode portion had a long side of 103 mm, a short side of 60 mm, and a tab weld area of ​​15 mm square, and an aluminum tab was welded to the tab weld area. The obtained negative electrode was cut so that the electrode portion had a long side of 107 mm, a short side of 62 mm, and a tab weld area of ​​15 mm square, and a nickel tab was welded to the tab weld area. In addition, a polypropylene separator was cut into a rectangle with a long side of 117 mm and a short side of 66 mm and placed between the formed positive and negative electrodes. This electrode assembly was embedded in an aluminum laminate sheet, injected with electrolyte, and then sealed to produce a laminate cell. In this case, the electrolyte used was ethylene carbonate (EC):diethyl carbonate (DEC) = 1:1 (volume ratio) containing 1 M LiPF6.

[0104] 4. DC resistance increase rate The obtained laminate cell was charged at 25°C with a constant current of 0.1C to 4.2V, then charged with a constant voltage until the current value was 0.01C, and then discharged with a constant current of 0.1C to 3V. This operation was repeated twice. For the third time, the cell was charged with a constant current of 0.1C to 4.2V, then charged with a constant voltage until the current value was 0.01C, and then discharged with a constant current of 1C for 10 seconds. The DC resistance was measured from the voltage change during the first 10 msec after the start of discharge. Subsequently, it was discharged with a constant current of 0.1C to 3V. For the fourth time, the charge and discharge were performed under the same conditions as the first and second times. For the fifth time, the cell was charged with a constant current of 1C to 4.2V, then charged with a constant voltage until the current value was 0.1C, and then discharged with a constant current of 1C to 3V. The fifth charge and discharge was considered the first cycle, and the same operation was repeated 300 times.

[0105] After 300 charge-discharge cycles, the battery was charged with a constant current of 0.1C up to 4.2V, then charged with a constant voltage until the current value reached 0.01C, and then discharged with a constant current of 0.1C up to 3V. Subsequently, it was charged with a constant current of 0.1C up to 4.2V, then charged with a constant voltage until the current value reached 0.01C, and then discharged with a constant current of 1C for 10 seconds. The DC resistance was measured from the voltage change during the first 10 msec after the start of discharge. The DC resistance increase rate was evaluated by calculating the ratio of the DC resistance after 300 cycles to the DC resistance before 300 cycles as a percentage, and evaluated according to the following criteria. The results are shown in Tables 1 and 2.

[0106] A: The DC resistance increase rate is less than 106%.

[0107] B: The DC resistance increase rate is 106% or more and less than 113%.

[0108] C: The DC resistance increase rate is 113% or more and less than 120%.

[0109] D: The DC resistance increase rate is 120% or more.

Claims

1. A binder for electrodes, It contains an aliphatic conjugated diene monomer and a copolymer of the aliphatic conjugated diene monomer and another monomer polymerizable with the aliphatic conjugated diene monomer, An electrode binder wherein the dry film for the electrode has a thickness of 0.4 mm ± 0.1 mm and a dumbbell shape of type 3, and when the dry film is left to stand for 36 hours in an environment of 23°C and 50% relative humidity and then subjected to a tensile test at a tensile speed of 500 mm / min, the rate of decrease in the integral value of the stress-strain curve (S2) obtained when the dry film is left to stand for 36 hours in an environment of 23°C and 50% relative humidity, and then subjected to a tensile test at a tensile speed of 500 mm / min, is between 6% and 30%.

2. The other monomer contains a vinyl cyanide monomer, The electrode binder according to claim 1, wherein the proportion of structural units derived from the vinyl cyanide monomer in the copolymer is 20% by mass or less.

3. The other monomers mentioned above contain a hydroxyl group-containing ethylenically unsaturated monomer, The electrode binder according to claim 1, wherein the proportion of structural units derived from the hydroxyl group-containing ethylenically unsaturated monomer in the copolymer is 3.0% by mass or less.

4. The other monomers include ethylenically unsaturated carboxylic acid monomers, The electrode binder according to claim 1, wherein the proportion of structural units derived from the ethylenically unsaturated carboxylic acid monomer in the copolymer is 0.1% by mass to 10% by mass.

5. Contains surfactants, The electrode binder according to claim 1, wherein the amount of surfactant blended is 2.5 parts by mass or less per 100 parts by mass of the copolymer.

6. An electrode composition comprising an electrode binder according to any one of claims 1 to 5 and an active material.

7. Current collector and, An electrode mixture layer is provided on the current collector and is formed from the electrode composition described in claim 6, An electrode equipped with

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