Aqueous composition for positive electrode for lithium-ion secondary battery, positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

This aqueous composition for a positive electrode for a lithium-ion secondary battery contains a conductive auxiliary agent and a particulate (co)polymer, in which; when elements present on the surface of the conductive auxiliary agent are measured by an X-ray photoelectron spectroscopic analysis method and the abundance ratio (mass %) of oxygen atoms and the abundance ratio (mass %) of carbon atoms are calculated on the basis of peak areas, the abundance ratio of the oxygen atoms on the surface of the conductive auxiliary agent is within a predetermined numerical range, and the ratio of the abundance ratio of the oxygen atoms present on the surface to the abundance ratio of the carbon atoms present on the surface is within a predetermined numerical range; and the particulate (co)polymer includes a predetermined amount of a constitutional unit derived from ethylenically unsaturated carboxylic acid (carboxylate).

Description

Aqueous composition for lithium ion secondary battery positive electrode, lithium ion secondary battery positive electrode, and lithium ion secondary battery

[0001] The present invention relates to an aqueous composition for a positive electrode of a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged. In recent years, with growing concern about environmental issues, secondary batteries are increasingly being used not only in electronic devices such as mobile phones and laptops, but also in fields such as automobiles and aircraft. In response to this growing demand for secondary batteries, active research is being conducted. Among secondary batteries, lithium-ion batteries, which are lightweight, compact, and have high energy density, have attracted attention from various industries and are being actively developed.

[0003] Lithium-ion batteries are primarily composed of a positive electrode, an electrolyte, a negative electrode, and a separator. Among these, electrodes (positive and negative electrodes) are formed by applying an electrode composition onto a current collector. Among the electrode compositions, the positive electrode composition used to form the positive electrode primarily comprises a positive electrode active material (hereinafter sometimes simply referred to as "active material"), a conductive additive, a binder, and a solvent. Polyvinylidene fluoride (PVDF) is typically used as the binder for the positive electrode composition, and N-methyl-2-pyrrolidone (NMP) is typically used as the solvent. The reasons for their use in the positive electrode composition include: (i) PVDF is chemically and electrically stable, and NMP is a solvent that dissolves PVDF and is stable over time; and (ii) nickel-cobalt-manganese (NCM)-based active materials, which are commonly used as positive electrode active materials, are known to be prone to degradation in water, particularly those with a high nickel content, necessitating the use of an organic solvent.

[0004] In recent years, interest in the handleability of solvents has been growing from various viewpoints. From such viewpoints, since NMP is difficult to handle and the use of solvents with excellent handleability is desired, and since PVDF may be subject to PFAS (fluorinated organic compounds) regulations, there has been a demand for aqueous compositions that are not subject to PFAS regulations and do not use organic solvents, even for electrode compositions.

[0005] Under these circumstances, various research and development efforts have been made on aqueous positive electrode compositions and thickeners and binders that can be used in such compositions. Progress has been made in the development of positive electrodes formed from aqueous pastes for positive electrodes containing a positive electrode active material, a water-dispersible elastomer, and a water-soluble polymer that acts as a thickener, as a composition for forming the positive electrode of a secondary battery. Among these, a conductive adhesive composition for electrochemical element electrodes has been disclosed, which contains conductive carbon, a particulate copolymer (A) containing dibasic acid monomer units, a particulate copolymer (B) containing ethylenically unsaturated carboxylic acid amide derivative units, and a dispersion medium (see Patent Document 1 below).

[0006] International Publication No. 2013 / 018887

[0007] The conductive adhesive composition for electrochemical element electrodes described in Patent Document 1 is said to contribute to improving the adhesion between the current collector and the electrode composition layer. However, the inventors have discovered through their research that combining a specific conductive additive with a specific binder causes the conductive additive in the aqueous composition to aggregate, thereby changing the viscosity of the aqueous composition over time. Furthermore, when these aqueous compositions are used to prepare positive electrodes for lithium ion secondary batteries, the coating film formed by applying the aqueous composition to a current collector and drying the composition may suffer from defects, which is known as electrode cracking. It has been found that there is still room for improvement in these respects.

[0008] The present invention has been made in view of the problems associated with the above-mentioned conventional techniques, and has an object to provide an aqueous composition for a positive electrode of a lithium ion secondary battery that has excellent viscosity stability over time and can suppress the occurrence of cracks in the electrode, as well as a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery that use the aqueous composition.

[0009] <1> An aqueous composition for a lithium ion secondary battery positive electrode, comprising: a conductive additive; and a particulate (co)polymer; wherein, when elements present on a surface of the conductive additive are measured by X-ray photoelectron spectroscopy and the abundance ratio (mass%) of oxygen atoms and the abundance ratio (mass%) of carbon atoms are calculated based on peak areas, the abundance ratio of oxygen atoms on the surface of the conductive additive is 0.30 mass% to 2.00 mass% and the ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms is 0.0020 to 0.0250; and the particulate (co)polymer contains structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the content ratio of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.1 mass% to 15.00 mass% in total relative to the total amount of the particulate (co)polymer. <2> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <1>, wherein the structural units derived from the ethylenically unsaturated carboxylic acid (salt) have a total content of 0.10% by mass or more and 5.00% by mass or less, relative to the total amount of the particulate (co)polymer. <3> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <1> or <2>, wherein the structural units derived from the ethylenically unsaturated carboxylic acid (salt) include a monobasic acid (salt) and / or a dibasic acid (salt). <4> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <3>, wherein the structural units derived from the monobasic acid (salt) have a total content of 0.01% by mass or more and 2.00% by mass or less, relative to the total amount of the particulate (co)polymer, and / or the structural units derived from the dibasic acid (salt) have a total content of 0.10% by mass or more and 5.00% by mass or less, relative to the total amount of the particulate (co)polymer. <5> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <4>, wherein the product of the abundance ratio (% by mass) of oxygen atoms on the surface of the conductive additive and the content ratio (% by mass) of the total amount of structural units derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer (O×COOH) is 0.030 to 10.00.<6> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <5>, wherein the ratio (O / bound COOH) of the proportion (mass %) of oxygen atoms on the surface of the conductive additive to the proportion (bound COOH) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is 0.0050 to 0.1000. <7> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <6>, wherein the particulate (co)polymer further contains a structural unit derived from a crosslinkable monomer. <8> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <7>, wherein the proportion (mass %) of oxygen atoms on the surface of the conductive additive is 0.5 mass % or more and 2.0 mass % or less. <9> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <8>, wherein the content of the conductive aid is 3.5% by mass or more and 10.0% by mass or less, based on the total amount of non-volatile components in the aqueous composition. <10> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <9>, further comprising a water-soluble (co)polymer. <11> The aqueous composition for a lithium ion secondary battery positive electrode according to <10>, wherein the water-soluble (co)polymer contains a structural unit derived from a monomer having a sulfonic acid (salt) group. <12> The aqueous composition for a lithium ion secondary battery positive electrode according to <11>, wherein the content of the structural unit derived from the monomer having a sulfonic acid (salt) group is 25% by mass or more and 100% by mass or less, based on the total solids content of the water-soluble (co)polymer. <13> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <12>, further comprising a preservative. <14> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <13>, further comprising an active material, wherein the active material comprises a lithium phosphate compound. <15> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <14>, further comprising a plasticizer. <16> The plasticizer has a solubility in water at 25°C of 5 g / 100 gH. ２<17> The aqueous composition for a lithium ion secondary battery positive electrode according to <15>, wherein the plasticizer comprises one or more selected from the group consisting of triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, glycerin, γ-butyrolactone, 1,3-propanediol, and 1,3-butanediol. <18> A lithium ion secondary battery positive electrode comprising the aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <17>. <19> A lithium ion secondary battery comprising the lithium ion secondary battery positive electrode according to <18>.

[0010] The present invention can provide an aqueous composition for a positive electrode of a lithium ion secondary battery that has excellent viscosity stability over time and can suppress cracking of the electrode, as well as a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery that use the aqueous composition.

[0011] Hereinafter, an embodiment of the present invention (hereinafter also referred to as "the present embodiment") will be described in detail. Note that the present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist of the present invention.

[0012] In this specification, "(meth)acrylic" means "acrylic" and its corresponding "methacrylic." In addition, in this specification, unless otherwise specified, the numerical values ​​on both ends of the range "to" are included as the upper and lower limits.

[0013] In this specification, the term "(co)polymer" means at least one of a homopolymer of a monomer and a copolymer of a plurality of types of monomers.

[0014] <Aqueous Composition for Lithium-Ion Secondary Battery Positive Electrode> The aqueous composition for a lithium-ion secondary battery positive electrode of the present embodiment includes a conductive additive and a particulate (co)polymer. When elements present on a surface of the conductive additive are measured by X-ray photoelectron spectroscopy and the abundance ratio (mass%) of oxygen atoms and the abundance ratio (mass%) of carbon atoms are calculated based on peak areas, the abundance ratio of oxygen atoms on the surface of the conductive additive is 0.30 mass% or more and 2.00 mass% or less, and the ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms is 0.0020 or more and 0.0250 or less, the particulate (co)polymer includes structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the content ratio of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.1 mass% or more and 15.00 mass% or less in total, relative to the total amount of the particulate (co)polymer. Hereinafter, the aqueous composition for a positive electrode of a lithium ion secondary battery according to this embodiment may be referred to as the "aqueous composition for a positive electrode."

[0015] The aqueous composition for positive electrodes of this embodiment includes a conductive additive and a particulate (co)polymer. The conductive additive has a surface oxygen atom content within a predetermined range, the surface oxygen atom content ratio to the carbon atom content ratio within a predetermined range, and the particulate (co)polymer contains a predetermined amount of structural units derived from an ethylenically unsaturated carboxylic acid (salt). This prevents aggregation of the conductive additive in the aqueous composition for positive electrodes, resulting in excellent stability over time in viscosity and hysteresis. Furthermore, the aqueous composition for positive electrodes, when applied and dried, exhibits improved flexibility. This allows for efficient production of lithium-ion secondary battery positive electrodes with uniform thickness and excellent product-to-product uniformity during line coating, even when using an aqueous composition for positive electrodes that has been used for a certain period of time. Furthermore, this prevents cracking of the lithium-ion secondary battery positive electrodes, contributing to improved product yield.

[0016] The aqueous composition for a positive electrode of this embodiment is a slurry composition for a positive electrode of a lithium ion secondary battery, and contains an aqueous solvent as a dispersion medium in addition to a conductive additive and a particulate (co)polymer. The aqueous composition for a positive electrode of this embodiment may further contain an active material, a water-soluble (co)polymer, a preservative, a plasticizer, etc.

[0017] (Conductive Aid) The aqueous composition for a positive electrode of this embodiment contains a conductive aid. The conductive aid serves to improve electrical contact between active materials. By including the conductive aid in the aqueous composition for a positive electrode of this embodiment, the discharge rate characteristics and the like of a lithium ion secondary battery can be improved.

[0018] The conductive additive in this embodiment is not particularly limited, and can be appropriately selected from known conductive additives such as furnace black, carbon black, acetylene black, ketjen black, graphite, vapor-grown carbon fiber, carbon nanotubes, and other conductive carbons. The conductive additives may be used alone or in combination of two or more in any ratio. Among these, from the viewpoint of cost benefits, it is preferable for the conductive additive to contain furnace black, and it is particularly preferable for the conductive additive to contain furnace black as an essential component.

[0019] In this embodiment, the proportion of oxygen atoms present on the surface of the conductive additive is 0.30% by mass or more and 2.00% by mass or less, and the ratio (O / C) of the proportion of oxygen atoms (mass%) to the proportion of carbon atoms (mass%) is 0.0020 or more and 0.0250 or less. The proportion of oxygen atoms present on the surface of the conductive additive is preferably 0.40% by mass or more and 2.00% by mass or less, and more preferably 0.50% by mass or more and 2.00% by mass or less. If the proportion of oxygen atoms present on the surface of the conductive additive is less than 0.30% by mass, dispersibility in aqueous solvents is deteriorated, and if it exceeds 2.00% by mass, performance as a conductive additive is deteriorated, resulting in a decrease in battery performance when a lithium ion secondary battery positive electrode is produced. The ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms is preferably 0.0030 or more and 0.0230 or less, more preferably 0.0040 or more and 0.0220 or less, and even more preferably 0.0050 or more and 0.0210 or less. When the abundance ratio of oxygen atoms present on the surface of the conductive additive and the ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms are within the above ranges, aggregation of the conductive additive in the aqueous composition tends to be suppressed, and the aqueous composition tends to have excellent viscosity and hysteresis stability over time.

[0020] In this specification, the abundance ratios of oxygen atoms and carbon atoms present on the surface of the conductive additive can be measured by X-ray photoelectron spectroscopy (XPS). For example, the abundance ratio of oxygen atoms (mass%) can be determined based on the peak area of ​​the O1s spectrum (525-545 eV) in XPS analysis, and the abundance ratio of carbon atoms (mass%) can be determined based on the peak area of ​​the C1s spectrum (280-300 eV) in XPS analysis. The O / C ratio can then be calculated from the abundance ratios of oxygen atoms and carbon atoms determined, respectively.

[0021] The content of the conductive additive in the aqueous composition for a positive electrode is not particularly limited, but from the viewpoint of improving the viscosity and hysteresis of the aqueous composition for a positive electrode and the electrical contact between the positive electrode active materials, the content is preferably 3.5% by mass or more and 10.0% by mass or less, more preferably 3.6% by mass or more and 9.0% by mass or less, even more preferably 3.8% by mass or more and 8.0% by mass or less, and particularly preferably 4.1% by mass or more and 7.0% by mass or less, relative to the total amount of non-volatile components in the composition.

[0022] The content of furnace black in the conductive additive is not particularly limited, but from the viewpoint of improving the viscosity and hysteresis of the aqueous composition for a positive electrode and the electrical contact between positive electrode active materials, and from the viewpoint of cost benefits, the content is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, further preferably 95% by mass or more and 100% by mass or less, and particularly preferably 100% by mass, relative to the total amount of the conductive additive.

[0023] The content of furnace black in the aqueous composition for a positive electrode is not particularly limited, but from the viewpoint of improving the viscosity and hysteresis of the aqueous composition for a positive electrode and the electrical contact between positive electrode active materials, the content is preferably 3.5% by mass or more and 10.0% by mass or less, more preferably 3.6% by mass or more and 9.0% by mass or less, even more preferably 3.8% by mass or more and 8.0% by mass or less, and particularly preferably 4.1% by mass or more and 7.0% by mass or less, relative to the total amount of non-volatile components in the composition.

[0024] (Particulate (co)polymer) The aqueous composition for a positive electrode of this embodiment contains a particulate (co)polymer. The particulate (co)polymer can serve as a binder for the aqueous composition for a positive electrode. The particulate (co)polymer is not particularly limited as long as it contains a structural unit derived from an ethylenically unsaturated carboxylic acid (salt), and any known particulate (co)polymer that can be dispersed in an aqueous solvent and used in the aqueous composition for a positive electrode can be appropriately selected and used. One type of particulate (co)polymer may be used alone, or two or more types may be used in combination. When the aqueous composition for a positive electrode contains the conductive additive and the particulate (co)polymer, aggregation of the conductive additive in the aqueous composition can be suppressed, and the aqueous composition has excellent stability over time in viscosity and hysteresis.

[0025] In this embodiment, the content of the structural units derived from an ethylenically unsaturated carboxylic acid (salt) is, relative to the total amount of the particulate (co)polymer, from 0.10% by mass to 15.00% by mass, preferably from 0.20% by mass to 10.00% by mass, more preferably from 0.50% by mass to 7.50% by mass, and even more preferably from 1.00% by mass to 4.00% by mass. When the content of the structural units derived from an ethylenically unsaturated carboxylic acid (salt) is within the above range, the aqueous composition has excellent stability over time in viscosity and hysteresis, and can be applied to a current collector while maintaining appropriate fluidity, which tends to result in excellent handleability.

[0026] In this embodiment, the value (O × COOH) obtained by multiplying the proportion (mass%) of oxygen atoms on the surface of the conductive additive in the aqueous composition for a positive electrode by the proportion (mass%) of the total amount of structural units derived from an ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is not particularly limited, but is preferably 0.030 to 10.00, more preferably 0.100 to 7.00, even more preferably 0.300 to 6.00, and particularly preferably 0.600 to 5.00. If the value (O × COOH) obtained by multiplying the proportion (mass%) of oxygen atoms on the surface of the conductive additive by the proportion (mass%) of the total amount of structural units derived from an ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is less than 0.030, adhesion to the electrode is insufficient, causing cracking of the electrode. If the value exceeds 10.00, aggregation occurs in the aqueous composition, and the viscosity and hysteresis of the aqueous composition change over time, resulting in instability.

[0027] In this embodiment, the ratio (O / bound COOH) of the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive to the content ratio (bound COOH) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is not particularly limited, but is preferably 0.0050 to 0.1000, more preferably 0.0065 to 0.0900, and even more preferably 0.0080 to 0.0800. When the ratio (O / bound COOH) of the proportion of oxygen atoms present on the surface of the conductive additive (mass %) to the proportion of monomers derived from ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer relative to the total amount of monomers derived from ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is within the above range, adhesion to the electrode tends to be excellent and the occurrence of cracks in the electrode tends to be suppressed, and aggregation in the aqueous composition tends to be suppressed, resulting in excellent viscosity and hysteresis stability over time of the aqueous composition.

[0028] In this specification, the content ratio (bound COOH) of the monomer derived from an ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer relative to the total amount of the monomer derived from an ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is determined as follows.

[0029] The acid value of the surface of the particulate (co)polymer is determined by conductometric titration, and the acid value is converted into the content of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer, and calculated using the following formula: Bound COOH = Content (parts by mass) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) converted from the acid value / Content (parts by mass) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) charged before polymerization × 100

[0030] The sample preparation method and titration conditions for conductometric titration are as follows. A particulate (co)polymer with a solids mass of 2 g was diluted to 10% with distilled water and centrifuged using an ultracentrifuge (Optima L-90K, manufactured by Beckman Coulter) to separate it into a particle phase and an aqueous phase. Distilled water was added to the sedimented sample after the aqueous phase was removed, and the sample was redispersed using a shaker. The process of removing the aqueous phase by centrifugation and redispersing was repeated a total of three times. 5 g of ion exchange resin (DIAION SK1B, manufactured by Mitsubishi Chemical) was added to the particulate (co)polymer from which the aqueous phase had been removed, and the mixture was stirred for 5 minutes, and the ion exchange resin was removed by filtration. This procedure was repeated until the pH no longer changed. The solids content of the particulate (co)polymer from which the aqueous phase had been removed after ion exchange was measured. 15 g of the particulate (co)polymer from which the aqueous phase had been removed was weighed into a 100 ml beaker and diluted to 50 g with distilled water. The solution was set in an automatic titrator (Hiranuma Sangyo Co., Ltd.: COM-1750, electrode used: TPT-351) and conductometric titration was carried out with 0.1N potassium hydroxide while stirring, the end point was detected by the V1 intersection detection method, and the acid value was determined.

[0031] The particulate (co)polymer in this embodiment is not particularly limited as long as it contains a structural unit derived from an ethylenically unsaturated carboxylic acid (salt), and the ethylenically unsaturated carboxylic acid (salt) may contain, for example, a monobasic acid (salt) and / or a dibasic acid (salt). One of these may be used alone, or two or more may be used in any ratio.

[0032] The monobasic acid (salt) is not particularly limited, but examples thereof include monomers having a carboxyl group such as (meth)acrylic acid, crotonic acid, vinylbenzoic acid, and cinnamic acid, monomers having a sulfonic acid group such as styrenesulfonic acid, methallylsulfonic acid, allylsulfonic acid, and vinylsulfonic acid, and salts thereof (e.g., sodium salts, potassium salts, and ammonium salts). Among these, (meth)acrylic acid is preferred. Of these, one type may be used alone, or two or more types may be used in any ratio.

[0033] Examples of dibasic acids (salts) include itaconic acid, fumaric acid, maleic acid, citraconic acid, muconic acid, and salts thereof (e.g., sodium salt, potassium salt, and ammonium salt). In the case of a dibasic acid salt, only one of the carboxyl groups in the monomer may be in the form of a salt, or both may be in the form of a salt. Among these, it is preferable to include at least one selected from the group consisting of itaconic acid and itaconate salts (e.g., sodium salt, potassium salt, and ammonium salt), and more preferably itaconic acid or sodium itaconate. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0034] The content of the structural units derived from a monobasic acid (salt) is not particularly limited. From the viewpoint of particle dispersion stability, the content is preferably 0.01% by mass or more and 2.00% by mass or less, more preferably 0.10% by mass or more and 1.75% by mass or less, even more preferably 0.20% by mass or more and 1.50% by mass or less, and particularly preferably 0.20% by mass or more and 1.25% by mass or less, relative to the total amount of the particulate (co)polymer. When the content of the structural units derived from a monobasic acid (salt) is within the above range, the aqueous composition for a positive electrode of this embodiment tends to suppress aggregation in the aqueous composition, and to exhibit excellent stability over time in the viscosity and hysteresis of the aqueous composition, as well as excellent storage stability. Furthermore, when applied to a current collector, the composition can be applied while maintaining appropriate fluidity, and therefore tends to have excellent handleability. The content of the structural units derived from a monobasic acid (salt) can be measured by pyrolysis gas chromatography (pyrolysis GC-MS).

[0035] The content of the dibasic acid (salt)-derived structural units is not particularly limited. From the viewpoint of the mechanical strength of the particles, the content is preferably 0.10% by mass or more and 5.00% by mass or less, more preferably 0.50% by mass or more and 4.50% by mass or less, even more preferably 1.00% by mass or more and 4.00% by mass or less, and particularly preferably 1.50% by mass or more and 3.50% by mass or less, relative to the total amount of the particulate (co)polymer. When the content of the dibasic acid (salt)-derived structural units is within the above range, aggregation in the aqueous composition is suppressed, and the aqueous composition tends to have excellent stability over time in terms of viscosity and hysteresis, and excellent storage stability. Furthermore, when applied to a current collector, the composition can be applied while maintaining appropriate fluidity, and therefore tends to have excellent handleability. The content of the dibasic acid (salt)-derived structural units can be measured by liquid chromatography.

[0036] In addition to the structural units derived from the ethylenically unsaturated carboxylic acid (salt), the particulate (co)polymer may also contain structural units derived from the following monomers: (meth)acrylic acid ester monomers, aromatic vinyl monomers, polyalkylene glycol mono(meth)acrylate monomers, polyol poly(meth)acrylate monomers, allyl compound monomers, silane-based monomers, aliphatic conjugated diene-based monomers, monomers having a phosphate group, monomers having a hydroxyl group, monomers having a nitrile group, unsaturated monomers containing a hydroxyalkyl group, crosslinkable monomers, etc. These monomers may be used alone or in combination of two or more from the above-listed examples.

[0037] The (meth)acrylic acid ester monomer is not particularly limited, but examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, octadecyl (meth)acrylate, and tetradecyl (meth)acrylate. Among these, 2-ethylhexyl (meth)acrylate and / or butyl (meth)acrylate are preferred, and 2-ethylhexyl acrylate and / or n-butyl acrylate are more preferred. One of these may be used alone, or two or more may be used in any ratio.

[0038] The content ratio of the structural unit derived from the (meth)acrylic acid ester monomer is not particularly limited, but because copolymerization is easy, it is preferably 1.0 mass% or more and 95.0 mass% or less, more preferably 20.0 mass% or more and 93.0 mass% or less, even more preferably 40.0 mass% or more and 91.0 mass% or less, and particularly preferably 60.0 mass% or more and 89.0 mass% or less, relative to the total amount of the particulate (co)polymer.

[0039] Examples of the aromatic vinyl monomer include styrene and vinyl naphthalene, and among these, styrene is preferred. One of these may be used alone, or two or more may be used in any ratio.

[0040] The content ratio of the structural unit derived from the aromatic vinyl monomer is not particularly limited, but because copolymerization is easy, it is preferably 1.0 mass% or more and 40.0 mass% or less, more preferably 2.0 mass% or more and 35.0 mass% or less, even more preferably 5.0 mass% or more and 30.0 mass% or less, and particularly preferably 10.0 mass% or more and 25.0 mass% or less, relative to the total amount of the particulate (co)polymer.

[0041] The polyalkylene glycol mono(meth)acrylate monomer is not particularly limited, but examples thereof include polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0042] The allyl compound monomer is not particularly limited, but examples thereof include diallyl fumarate, diaryl itaconate, triallyl isocyanurate, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0043] The aliphatic conjugated diene monomer is not particularly limited, but examples thereof include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0044] The monomer having a phosphate group is not particularly limited, but examples thereof include 3-allyloxy-2-hydroxypropane phosphate, dioctyl-2-methacryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethyl phosphate, monomethyl-2-methacryloyloxyethyl phosphate, dimethyl-2-methacryloyloxyethyl phosphate, monoethyl-2-methacryloyloxyethyl phosphate, diethyl-2-methacryloyloxyethyl phosphate, monoisopropyl-2-methacryloyloxyethyl phosphate, Examples of such esters include ethyl phosphate, diisopropyl-2-methacryloyloxyethyl phosphate, mono-n-butyl-2-methacryloyloxyethyl phosphate, di-n-butyl-2-methacryloyloxyethyl phosphate, monobutoxyethyl-2-methacryloyloxyethyl phosphate, dibutoxyethyl-2-methacryloyloxyethyl phosphate, mono(2-ethylhexyl)-2-methacryloyloxyethyl phosphate, di(2-ethylhexyl)-2-methacryloyloxyethyl phosphate, etc. Among these, one type may be used alone, or two or more types may be used in any desired ratio.

[0045] The monomer having a hydroxyl group is not particularly limited, but examples thereof include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0046] The monomer having a nitrile group is not particularly limited, but examples thereof include acrylonitrile, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0047] The crosslinkable monomer is not particularly limited, and examples thereof include aromatic divinyl monomers, alkylene glycol di(meth)acrylate monomers, polyol poly(meth)acrylate monomers, silane-based monomers, unsaturated carboxylic acid amide monomers, and unsaturated monomers having an epoxy group. Among these, it is preferable to include at least one selected from the group consisting of alkylene glycol di(meth)acrylate monomers and polyol poly(meth)acrylate monomers. One of these may be used alone, or two or more may be used in any ratio. By using such a crosslinkable monomer, for example, the aqueous composition for a positive electrode of this embodiment can ensure resistance to an electrolyte solution, resulting in excellent battery cycle life and capacity retention.

[0048] The aromatic divinyl monomer is not particularly limited, but examples thereof include divinylbenzene.

[0049] The alkylene glycol di(meth)acrylate monomer is not particularly limited, but examples thereof include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and 1,9-nonanediol di(meth)acrylate. Of these, 1,9-nonanediol di(meth)acrylate is preferred, and 1,9-nonanediol dimethacrylate is more preferred. Of these, one type may be used alone, or two or more types may be used in any ratio.

[0050] The polyol poly(meth)acrylate monomer is not particularly limited, but examples thereof include trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol di(meth)acrylate, and pentaerythritol tetra(meth)acrylate. Among these, trimethylolpropane tri(meth)acrylate is preferred, and trimethylolpropane triacrylate is more preferred. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0051] The silane monomer is not particularly limited, but examples thereof include vinyltrimethoxysilane, vinyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltriethoxysilane, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0052] The unsaturated carboxylic acid amide monomer is not particularly limited, but examples thereof include (meth)acrylamide, N,N-dimethyl(meth)acrylamide, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0053] The unsaturated monomer having an epoxy group is not particularly limited, but examples thereof include glycidyl (meth)acrylate, etc. Among these, one type may be used alone, or two or more types may be used in any ratio.

[0054] The content of the structural unit derived from the crosslinkable monomer is not particularly limited, but is preferably 0.01% by mass or more and 5.0% by mass or less, more preferably 0.02% by mass or more and 3.0% by mass or less, even more preferably 0.03% by mass or more and 1.0% by mass or less, and particularly preferably 0.04% by mass or more and 0.60% by mass or less, relative to the total amount of the particulate (co)polymer. When the content of the structural unit derived from the crosslinkable monomer is within the above range, the aqueous composition for positive electrodes of this embodiment can ensure resistance to an electrolyte while maintaining flexibility as a binder, thereby resulting in excellent battery cycle life and capacity retention. Furthermore, when applied to a current collector, the composition can be applied while maintaining appropriate fluidity, and therefore tends to be easy to handle.

[0055] -Average particle size- The average particle size of the particulate (co)polymer is not particularly limited, but from the viewpoint of the strength and flexibility of a positive electrode obtained using the aqueous composition for a positive electrode, it is preferably 50 nm to 1,000 nm, more preferably 80 nm to 700 nm, even more preferably 100 nm to 500 nm, and particularly preferably 130 nm to 400 nm. The average particle size of the binder can be measured by the method described in the examples below, and can be appropriately adjusted by known means.

[0056] - Content ratio - The content ratio of the particulate (co)polymer is not particularly limited, but from the viewpoint of the strength and flexibility of the positive electrode, it is preferably from 0.5% by mass to 10.0% by mass, more preferably from 1.0% by mass to 9.0% by mass, even more preferably from 1.5% by mass to 8.0% by mass, and particularly preferably from 2.0% by mass to 7.0% by mass, relative to the total amount of non-volatile components in the aqueous composition for the positive electrode.

[0057] -pH- In view of the storage stability of the binder dispersion, the pH of the particulate (co)polymer dispersion in the present embodiment is preferably 4.0 or more and 11.0 or less, more preferably 5.0 or more and 10.0 or less, and particularly preferably 5.5 or more and 9.5 or less.

[0058] -Glass Transition Temperature- The glass transition temperature of the particulate polymer in this embodiment is preferably -75°C or higher, more preferably -70°C or higher, particularly preferably -65°C or higher, and is preferably 50°C or lower, more preferably 40°C or lower, even more preferably 35°C or lower, and particularly preferably 30°C or lower, from the viewpoints of the flexibility and windability of the electrode, and the binding strength between the electrode active material layer and the current collector. The glass transition temperature of the particulate polymer can be adjusted by combining various monomers. The glass transition temperature is obtained by measuring with a differential scanning calorimeter (DSC) in accordance with ASTM D3418-15. The DSC curve can also be obtained by differentiating the DSC curve. When the DSC curve has multiple peaks, the glass transition temperature obtained from the peak corresponding to the temperature range in which the slope of the DSC curve is maximum is the glass transition temperature in this specification.

[0059] -Method for Producing Particulate (Co)polymer Dispersion- The binder (particulate polymer) dispersion in this embodiment is not particularly limited, and can be prepared, for example, by emulsion polymerization or suspension polymerization using raw material monomers, or by dispersing a solution-polymerized copolymer in water and removing the solvent. Suitable seed particles can be used during polymerization of the monomers, and seed particles can also be obtained by conventional emulsion polymerization. Furthermore, known methods can be used for polymerization, and the dispersion can be produced by appropriately using a polymerization initiator, a reducing agent, a chain transfer agent, a chelating agent, a pH adjuster, an emulsifier, a preservative, an antifoaming agent, and the like in an aqueous medium. The aqueous medium can be the water-based dispersion medium described above.

[0060] The emulsifier is not particularly limited, but for example, anionic surfactants, nonionic surfactants, amphoteric surfactants, reactive surfactants, etc. can be used alone or in combination of two or more kinds.

[0061] The anionic surfactant is not particularly limited, but examples thereof include non-reactive alkyl sulfates, polyoxyethylene alkyl ether sulfates, alkylbenzene sulfonates, alkylnaphthalene sulfonates, alkyl sulfosuccinates, alkyldiphenyl ether disulfonates, naphthalenesulfonate-formalin condensates, polyoxyethylene polycyclic phenyl ether sulfates, polyoxyethylene distyrenated phenyl ether sulfates, fatty acid salts, alkyl phosphates, and polyoxyethylene alkylphenyl ether sulfates.

[0062] The nonionic surfactant is not particularly limited, but examples thereof include polyoxyethylene alkyl ethers, polyoxyalkylene alkyl ethers, polyoxyethylene polycyclic phenyl ethers, polyoxyethylene distyrenated phenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerin fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene alkylamines, alkylalkanolamides, and polyoxyethylene alkylphenyl ethers.

[0063] The amphoteric surfactant is not particularly limited, but examples thereof include betaines such as lauryl betaine and stearyl betaine, and amino acid surfactants such as lauryl-β-alanine, stearyl-β-alanine, and lauryldi(aminoethyl)glycine.

[0064] The reactive surfactant is not particularly limited, but is preferably, for example, an ethylenically unsaturated monomer having a sulfonic acid group, a sulfonate group, or a sulfate ester group, or a salt thereof, and is preferably a compound having a sulfonic acid group or a group that is an ammonium salt or alkali metal salt thereof (an ammonium sulfonate group or an alkali metal sulfonate group). Specific examples include alkylaryl sulfosuccinates (for example, "ELEMINOL (trademark) JS-2" and "ELEMINOL (trademark) JS-5" manufactured by Sanyo Chemical Industries, Ltd.; "LATEMUL (trademark) S-120", "LATEMUL (trademark) S-180A", and "LATEMUL (trademark) S-180" manufactured by Kao Corporation); polyoxyethylene alkylpropenylphenyl ether sulfates (for example, "AQUALON (trademark) HS-10" and "AQUALON (trademark) HS-1025" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.); α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-polyoxyethylene sulfates (for example, "ADEKA REASOAP (trademark) SE-1025N" manufactured by Asahi Denka Kogyo Co., Ltd.); ammonium = α -sulfonato-ω-1-(allyloxymethyl)alkyloxypolyoxyethylene (for example, "Aqualon (trademark) KH-10" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-hydroxypolyoxyethylene (for example, "ADEKA REASOAP (trademark) NE-20," "ADEKA REASOAP (trademark) NE-30," and "ADEKA REASOAP (trademark) NE-40" manufactured by Asahi Denka Kogyo Co., Ltd.), polyoxyethylene alkylpropenyl phenyl ether (for example, "Aqualon (trademark) RN-10," "Aqualon (trademark) RN-20," "Aqualon (trademark) RN-30," and "Aqualon (trademark) RN-50" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and the like. The amount of the emulsifier used is preferably 0.01% by mass or more and 5.0% by mass or less, more preferably 0.05% by mass or more and 3.0% by mass or less, still more preferably 0.1% by mass or more and 2.0% by mass or less, and most preferably 0.15% by mass or more and 1.0% by mass or less, relative to 100% by mass of the total monomers. The emulsifier may be used alone or in combination of two or more kinds.

[0065] The polymerization initiator is not particularly limited, and examples thereof include water-soluble polymerization initiators such as sodium persulfate, potassium persulfate, ammonium persulfate, and 2,2′-azobis(2-methylpropionamidine) dihydrochloride; oil-soluble polymerization initiators such as benzoyl peroxide, lauryl peroxide, and t-butyl peroxybenzoate; and redox polymerization initiators in combination with reducing agents such as sodium sulfite, ascorbic acid and its salts, erythorbic acid and its salts, and Rongalit, which can be used alone or in combination.

[0066] Examples of the chain transfer agent that can be used alone or in combination include mercaptan chain transfer agents such as n-butyl mercaptan, t-butyl mercaptan, t-dodecyl mercaptan, n-octyl mercaptan, and n-lauryl mercaptan; dimers of nucleus-substituted α-methylstyrene such as α-methylstyrene dimer; disulfides such as tetramethylthiuradium disulfide and tetraethylthiuradium disulfide; halogenated derivatives of hydrocarbons such as carbon tetrachloride and carbon tetrabromide; and 2-ethylhexyl thioglycolate.

[0067] As the chelating agent, sodium ethylenediaminetetraacetate, pentasodium diethylenetriaminepentaacetate, etc. can be used.

[0068] Examples of pH adjusters include sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydroxide, sodium bicarbonate, disodium hydrogen phosphate, and amines such as monoethanolamine, dimethylethanolamine, triethanolamine, and triethylamine. Sodium hydroxide, potassium hydroxide, and lithium hydroxide are preferred, and sodium hydroxide and lithium hydroxide are more preferred. These can be used alone or in combination.

[0069] Examples of preservatives include isothiazoline compounds, phenols and alkali metal salts thereof, chlorinated quinones, nitro group-containing compounds, amines, amides, iodine-containing compounds, thiazoles, thiocyanates, etc., such as 5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one, 1,2-benzisothiazolin-3-one, 2,2-dibromo-3-nitrilopropionamide, 2-n-octyl 4-isothiazolin-3-one, 2-bromo-2-nitropropane-1,3-diol, 2,2-dibromo-2-nitroethanol, etc. These can be used alone or in combination.

[0070] Examples of the defoaming agent include mineral oil-based defoaming agents, polyether-based defoaming agents, silicone-based defoaming agents, and emulsion-based defoaming agents. For example, NOPTAM 777F (manufactured by San Nopco), ADEKA CORPORATION B-925, ADEKA CORPORATION B-940, and ADEKA CORPORATION B-556 can be used.

[0071] (Water-Soluble (Co)Polymer) The aqueous composition for a positive electrode of this embodiment may contain a water-soluble (co)polymer. The water-soluble (co)polymer is not particularly limited, and any known water-soluble (co)polymer that can be used in aqueous compositions can be appropriately selected and used. The water-soluble (co)polymer of this embodiment can serve as a thickener for the aqueous composition for a positive electrode.

[0072] As used herein, the term "water-soluble" in relation to a water-soluble (co)polymer means that the insoluble content is less than 3.0 mass% when 0.5 g of the (co)polymer is dissolved in 100 g of water at 25° C. In addition, when the solubility of a water-soluble (co)polymer in water varies depending on the pH of the water, if there is a pH range in which the water-soluble (co)polymer is water-soluble, the (co)polymer can be considered to be water-soluble.

[0073] The water-soluble (co)polymer may contain, for example, a structural unit derived from a monomer having a sulfonic acid (salt) group, or may contain a structural unit derived from another polymerizable monomer (hereinafter, sometimes referred to as "another monomer"). From the viewpoint of improving the viscosity and hysteresis stability over time of the aqueous composition for a positive electrode and improving electrode adhesion, it is preferable that the water-soluble (co)polymer contain a structural unit derived from a monomer having a sulfonic acid (salt) group.

[0074] In this specification, the term "structural unit derived from a monomer having a sulfonic acid (salt) group" refers to a structural unit in a polymer having a chemical structure derived from a monomer having a sulfonic acid (salt) group.

[0075] In this specification, the term "sulfonic acid (salt) group" includes a sulfonic acid group and a sulfonate salt group. ３ H, and the sulfonate group is —SO ３ The sulfonate group is a monovalent substituent represented by —R. The sulfonate group is an alkali sulfonate group (—SO ) where R is an alkali metal (Na, K, Li, etc.). ３ -Na ＋ , -SO ３ -K ＋ , -SO ３ -Li ＋ etc.), even if R is ammonium (NH ４ ) which is an ammonium sulfonate (—SO ３ -NH ４ ＋ ) may also be used.

[0076] The monomer having a sulfonic acid (salt) group is not particularly limited, and examples thereof include monomers in which one of the conjugated double bonds of a diene compound such as isoprene or butadiene is sulfonated; vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfoethyl methacrylate, sulfopropyl methacrylate, sulfobutyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, and the like, and examples of the salts thereof include lithium salts, sodium salts, potassium salts, and ammonium salts. Furthermore, reactive emulsifiers having a so-called sulfonic acid group, such as alkylaryl sulfosuccinates (for example, "ELEMINOL (trademark) JS-2" and "ELEMINOL (trademark) JS-5" manufactured by Sanyo Chemical Industries, Ltd.; "LATEMUL (trademark) S-120", "LATEMUL (trademark) S-180A", and "LATEMUL (trademark) S-180" manufactured by Kao Corporation); polyoxyethylene alkylpropenylphenyl ether sulfates (for example, "AQUALON (trademark) HS-10" and "LATEMUL (trademark) S-180" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.); Examples of the suitable hydroxyl groups include α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-polyoxyethylene sulfate (for example, ADEKA REASOAP™ SE-1025N manufactured by Asahi Denka Kogyo Co., Ltd.); and ammonium α-sulfonato-ω-1-(allyloxymethyl)alkyloxypolyoxyethylene (for example, AQUALON™ KH-10 manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.). From the viewpoint of improving the viscosity and hysteresis stability over time of the aqueous composition for the positive electrode and electrode adhesion, preferred are styrene sulfonic acid, vinyl sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and their lithium salts, sodium salts, potassium salts, and ammonium salts, and more preferred are styrene sulfonic acid, and their lithium salts, sodium salts, potassium salts, and ammonium salts. Styrene sulfonic acid and its sodium salt, sodium styrene sulfonate, are even more preferred. The water-soluble (co)polymer of this embodiment may contain one type of structural unit derived from the above-listed monomers alone, or two or more types in any combination and ratio.

[0077] Specific examples of the water-soluble (co)polymer include, but are not limited to, poly(sodium styrene sulfonate), poly(sodium vinyl sulfonate), and poly(2-acrylamido-2-methylpropanesulfonic acid). Among these, poly(sodium styrene sulfonate) is preferred. The water-soluble (co)polymer may be used alone or in any combination and ratio of two or more kinds.

[0078] The water-soluble (co)polymer in this embodiment may contain, in addition to the constituent units derived from the monomer having a sulfonic acid (salt) group, constituent units derived from other polymerizable monomers (hereinafter, sometimes referred to as "other monomers"). The other monomers are not particularly limited, but examples thereof include polymerizable compounds that do not contain a sulfonic acid (salt) group but contain a hydrophilic group other than a sulfonic acid (salt) group. Examples of other monomers include monomers having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and 2-acryloyloxyethyl succinic acid, monomers having a hydroxyl group such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate; 3-allyloxy-2-hydroxypropane phosphate, dioctyl-2-methacryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethyl phosphate, monomethyl-2-methacryloyloxyethyl phosphate, dimethyl-2-methacryloyloxyethyl phosphate, monoethyl-2-methacryloyloxyethyl phosphate, and methyl-2-methacryloyloxyethyl phosphate. Examples of the monomer include monomers having a phosphate group, such as mono-n-butyl-2-methacryloyloxyethyl phosphate, diethyl-2-methacryloyloxyethyl phosphate, monoisopropyl-2-methacryloyloxyethyl phosphate, diisopropyl-2-methacryloyloxyethyl phosphate, mono-n-butyl-2-methacryloyloxyethyl phosphate, di-n-butyl-2-methacryloyloxyethyl phosphate, monobutoxyethyl-2-methacryloyloxyethyl phosphate, dibutoxyethyl-2-methacryloyloxyethyl phosphate, mono(2-ethylhexyl)-2-methacryloyloxyethyl phosphate, and di(2-ethylhexyl)-2-methacryloyloxyethyl phosphate.Also, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate (meth)acrylic acid alkyl esters such as tetradecyl (meth)acrylate and stearyl (meth)acrylate; polyalkylene glycol mono(meth)acrylate monomers such as polyethylene glycol mono(meth)acrylate and polypropylene glycol mono(meth)acrylate; and monomers such as styrene, 1,3-butadiene, (meth)acrylonitrile, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, and glycidyl (meth)acrylate can also be used within the range that does not impair water solubility. Although not particularly limited, examples of other monomers that can be combined with the monomer containing a sulfonic acid (salt) group in this embodiment are, from the viewpoint of improving the viscosity and hysteresis stability over time of the aqueous composition for the positive electrode without impairing water solubility, preferably acrylic acid, itaconic acid, fumaric acid, methacrylic acid, 2-hydroxyethyl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide, and styrene, more preferably acrylic acid, methacrylic acid, 2-hydroxyethyl (meth)acrylate, and (meth)acrylamide, and even more preferably methacrylic acid. As the other monomer, one type from the above-mentioned examples may be used alone, or two or more types may be used in combination.

[0079] The water-soluble (co)polymer in this embodiment may include a water-soluble (co)polymer containing a structural unit having a carboxyl group other than the structural unit derived from the other monomer. The water-soluble (co)polymer containing a structural unit having a carboxyl group is not particularly limited, but examples thereof include cellulose derivatives having glucose units having a carboxyl group in some of the glucose units constituting cellulose. Among these, carboxymethyl cellulose and carboxymethyl cellulose alkali metal salts (e.g., sodium salts) are preferred. The water-soluble (co)polymer may be used alone or in any combination and ratio of two or more.

[0080] Known commercially available water-soluble (co)polymers include, for example, p-NASS (sodium polystyrene sulfonate) and PSS (polystyrene sulfonic acid) manufactured by Tosoh Finechem Co., Ltd. In addition to commercially available products, water-soluble (co)polymers prepared as described below may also be used.

[0081] The content of the structural unit derived from a monomer having a sulfonic acid (salt) group in the water-soluble (co)polymer of this embodiment is not particularly limited, but from the viewpoints of viscosity, stability of hysteresis over time, and electrode adhesion, it is preferably 25% by mass or more and 100% by mass or less, and the lower limit of the content is more preferably 27% by mass or more, and even more preferably 29% by mass or more, relative to the total solid content of the water-soluble (co)polymer. The content of the structural unit derived from a monomer having a sulfonic acid (salt) group in the water-soluble (co)polymer of this embodiment can be measured by liquid chromatography (LC-MS).

[0082] When the water-soluble (co)polymer in this embodiment contains a structural unit derived from another monomer, the content of the structural unit derived from the other monomer is not particularly limited, but is preferably 75% by mass or less, more preferably 73% by mass or less, and particularly preferably 71% by mass or less, relative to the total solid content of the water-soluble (co)polymer. The lower limit is not particularly limited, as long as it is more than 0% by mass.

[0083] The content of the water-soluble (co)polymer in the aqueous composition for a positive electrode is not particularly limited, but from the viewpoint of the viscosity and hysteresis stability over time of the aqueous composition for a positive electrode, the content is preferably from 0.1 mass % to 5.0 mass %, more preferably from 0.3 mass % to 4.5 mass %, and particularly preferably from 0.5 mass % to 4.0 mass %, relative to the total amount of non-volatile components in the aqueous composition.

[0084] The pH of the water-soluble (co)polymer in this embodiment is not particularly limited, but is preferably 4.0 to 11.0, more preferably 4.5 to 10.7, even more preferably 5.0 to 10.5, and particularly preferably 6.5 to 10.5. Here, the "pH of the water-soluble (co)polymer" refers to the pH after adjusting the water-soluble (co)polymer aqueous solution with water to a solids content of 5% by mass. The pH can be adjusted with, for example, sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydroxide, sodium bicarbonate, disodium hydrogen phosphate, amines such as monoethanolamine, dimethylethanolamine, triethanolamine, and triethylamine, and is preferably sodium hydroxide, potassium hydroxide, or lithium hydroxide, and more preferably sodium hydroxide or lithium hydroxide. These can be used alone or in combination.

[0085] The weight-average molecular weight (Mw) of the water-soluble (co)polymer in this embodiment can be appropriately set depending on the desired performance and is not particularly limited, but from the viewpoints of the viscosity, hysteresis, viscosity stability, and electrode adhesion of the aqueous composition for the positive electrode, it is preferably from 50,000 to 5,000,000, more preferably from 100,000 to 3,000,000, even more preferably from 200,000 to 1,000,000, and particularly preferably from 500,000 to 1,000,000. The weight-average molecular weight (Mw) of the water-soluble (co)polymer can be measured using gel permeation chromatography (GPC) or the like.

[0086] The water-soluble (co)polymer in this embodiment may contain a preservative. As the preservative, for example, those described below can be used alone or in combination.

[0087] -Method for Producing Water-Soluble (Co)Polymer- The water-soluble (co)polymer of this embodiment is not particularly limited, and can be produced, for example, by polymerizing a monomer composition containing the above-mentioned monomers in an aqueous solvent. In this case, the ratio (by mass) of each monomer in the monomer composition can typically be the same as the ratio of the constituent units in the water-soluble (co)polymer. Examples of aqueous solvents include those described below. The polymerization method for the water-soluble (co)polymer of this embodiment is not particularly limited, and any method, such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization, can be used. Furthermore, the polymerization method for the water-soluble (co)polymer of this embodiment can be any method, such as ionic polymerization, radical polymerization, or living radical polymerization. In the synthesis method for the water-soluble (co)polymer of this embodiment, the polymer can be produced by appropriately adding a polymerization initiator, a molecular weight modifier, a chelating agent, a pH adjuster, or the like. Examples of polymerization initiators include organic peroxides such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, and 3,3,5-trimethylhexanoyl peroxide; azo compounds such as α,α'-azobisisobutyronitrile and 2,2'-azobis(2-methylpropionamidine) dihydrochloride; and persulfate compounds such as ammonium persulfate, potassium persulfate, and sodium persulfate. One type of polymerization initiator may be used alone, or two or more types may be used in any ratio. The polymerization temperature and polymerization time can be selected as desired depending on the polymerization method, the type of polymerization initiator, and other factors. Typically, the polymerization temperature is about 30°C or higher, and the polymerization time is about 0.5 to 30 hours.

[0088] (Dispersion Medium) The aqueous composition for a positive electrode of this embodiment may contain an aqueous solvent as the dispersion medium. The aqueous solvent is not particularly limited as long as it is a solvent that can be used in an aqueous composition for a positive electrode. For example, an aqueous solvent having a boiling point of 80°C or higher at normal pressure is preferred. The aqueous solvent is not particularly limited, but examples include water; alcohols such as ethyl alcohol, isopropyl alcohol, and normal propyl alcohol; glycol ethers such as propylene glycol monomethyl ether, methyl cellosolve, ethyl cellosolve, ethylene glycol tertiary butyl ether, butyl cellosolve, 3-methoxy-3-methyl-1-butanol, ethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and dipropylene glycol monomethyl ether; ketones such as diacetone alcohol and γ-butyrolactone; and ethers such as 1,3-dioxolane, 1,4-dioxolane, and tetrahydrofuran. As the aqueous solvent, water is preferred from the viewpoints of ease of handling and ease of application to aqueous compositions for a positive electrode. The aqueous solvent may be used alone or in any combination of two or more kinds in any ratio.

[0089] (Active Material) The aqueous composition for a positive electrode of this embodiment may contain an active material. The active material used in the aqueous composition for a positive electrode of this embodiment is a material that can be used as a positive electrode active material. The positive electrode active material is an electrode active material used in a positive electrode, and is a material that transfers electrons at the positive electrode of a lithium ion secondary battery. The active material of this embodiment is not particularly limited, and examples thereof include lithium-containing composite metal oxides having a layered structure, lithium-containing composite metal oxides having a spinel structure, and lithium-containing composite metal oxides having an olivine structure.

[0090] Examples of lithium-containing composite metal oxides having a layered structure include lithium-containing cobalt oxide (LiCoO ２ ), lithium-containing nickel oxide (LiNiO ２), Co—Ni—Mn lithium composite oxide, Ni—Mn—Al lithium composite oxide, Ni—Co—Al lithium composite oxide, etc. Examples of lithium-containing composite metal oxides having a spinel structure include lithium manganate (LiMn ２ O ４ ), or Li[Mn ３／２ M １／２ ]O ４ (wherein M is Cr, Fe, Co, Ni, Cu, etc.)

[0091] Examples of lithium-containing composite metal oxides having an olivine structure include Li ｘ MPO ４ (wherein M represents at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo, and X represents a number satisfying 0≦X≦2.) Other examples include transition metal oxides, transition metal sulfides, and lithium-containing composite metal oxides of lithium and a transition metal.

[0092] Among these examples of active materials, although not particularly limited, from the viewpoints of long life, safety, and easy availability of the positive electrode for lithium ion secondary batteries, olivine-type lithium phosphate compounds are preferred, specifically LiFePO ４ (lithium iron phosphate), LiMnPO ４ (Lithium manganese phosphate), LiMn （１－ｙ） Fe ｙ P.O. ４ (lithium manganese iron phosphate) (y represents a number satisfying 0≦y≦1) is preferred, and LiFePO ４ These active materials may be coated with carbon to enhance the electron conductivity.

[0093] - Particle size - The particle size of the active material is expressed as a volume-based D50 particle size and is not particularly limited, but in order to facilitate obtaining an appropriate viscosity of the aqueous composition for the positive electrode and to improve Li ion conductivity, it is preferably 0.6 μm or more and 8.0 μm or less, more preferably 0.65 μm or more and 7.0 μm or less, even more preferably 0.7 μm or more and 6.0 μm or less, and particularly preferably 0.75 μm or more and less than 5.0 μm. In this embodiment, a laser light diffraction / scattering particle size analyzer (Microtrac MT3300EXII, manufactured by Microtrac-Bell Co., Ltd.) was used to measure the 50% cumulative diameter d50 obtained from the measured particle size distribution of the positive electrode active material, and this was used as the average particle size.

[0094] The content of the active material in the aqueous composition for a positive electrode is preferably 70% by mass or more and 99.5% by mass or less, more preferably 80% by mass or more and 97% by mass or less, and particularly preferably 85% by mass or more and 95% by mass or less, based on the total amount of nonvolatile components in the composition, in order to obtain high capacity and high load characteristics in a lithium-ion secondary battery using a positive electrode obtained using the aqueous composition for a positive electrode of this embodiment. Here, the term "nonvolatile components in the composition" refers to nonvolatile components excluding volatile components such as water in the aqueous composition for a positive electrode, and refers to the components remaining after drying when the aqueous composition for a positive electrode is dried at 130°C under normal pressure for 1 hour.

[0095] From the viewpoints of maintaining the viscosity and hysteresis stability of the aqueous composition for a positive electrode over time and achieving a uniform thickness during line coating, the content of the dispersion medium in the aqueous composition for a positive electrode is preferably 35% by mass or more and 65% by mass or less, more preferably 37% by mass or more and 64% by mass or less, and particularly preferably 39% by mass or more and 63% by mass or less, relative to the total amount of the composition.

[0096] (Preservative) The aqueous composition for a positive electrode of this embodiment may contain a preservative. The preservative may be contained in the water-soluble polymer as described above. By containing a preservative, the aqueous composition for a positive electrode of this embodiment can further improve the stability over time of viscosity and hysteresis. The preservative is not particularly limited, but examples thereof include isothiazolinone compounds, phenols and alkali metal salts thereof, chlorinated quinones, nitro group-containing compounds, amines, amides, iodine-containing compounds, thiazoles, thiocyanates, and the like, and examples thereof include 5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one, 1,2-benzisothiazolin-3-one, 2,2-dibromo-3-nitrilopropionamide, 2-n-octyl 4-isothiazolin-3-one, 2-bromo-2-nitropropane-1,3-diol, 2,2-dibromo-2-nitroethanol, and the like.

[0097] From the viewpoints of the stability of viscosity and hysteresis over time and the binding strength of the electrode, the content of the preservative in the aqueous composition for a positive electrode is preferably from 0.00001% by mass to 1% by mass, more preferably from 0.00002% by mass to 0.5% by mass, and particularly preferably from 0.00004% by mass to 0.1% by mass, relative to the total amount of non-volatile components in the composition.

[0098] (Plasticizer) The aqueous composition for a positive electrode of this embodiment may contain a plasticizer for the particulate (co)polymer and / or the water-soluble (co)polymer. By containing a plasticizer in the aqueous composition for a positive electrode of this embodiment, drying shrinkage stress that occurs when the electrode is dried can be alleviated, and cracking of the electrode tends to be further suppressed.

[0099] The plasticizer is not particularly limited, but is preferably a substance that penetrates between the polymer chains of the particulate (co)polymer and / or the water-soluble (co)polymer to loosen the bonding force between the polymer chains, thereby lowering the glass transition point and making the polymer flexible. Examples of the plasticizer include monohydric alcohols, dihydric or higher polyhydric alcohols, esters, carbonate esters, ethers, etc., and one of these can be used alone or two or more can be used in combination.

[0100] The monohydric alcohols are not particularly limited, but examples thereof include 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, tert-butanol, 1-pentanol, 2-pentanol, 3-pentanol, isopentanol, neopentanol, 1-hexanol, 2-hexanol, 3-hexanol, cyclohexanol, 1-heptanol, 2-heptanol, 3-heptanol, 1-octanol, 2-octanol, 3-octanol, 1- Examples of the alcohol include nonanol, 2-nonanol, 1-decanol, 2-decanol, 1-undecanol, 1-dodecanol, 2-methyl-1-propanol, 3-methyl-1-propanol, 2-ethyl-1-butanol, 2-ethyl-1-pentanol, 2,2-dimethyl-1-propanol, 3-methyl-2-butanol, 2-propyl-1-hexanol, cyclopentanol, 1-dodecyl alcohol, tridecyl alcohol, myristyl alcohol, and palmityl alcohol.

[0101] The dihydric or higher polyhydric alcohols are not particularly limited, but examples thereof include ethylene glycol, propylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, cyclohexane-1,2-diol, cyclohexane-1,3-diol, cyclohexane-1,4-diol, glycerin, triethylene glycol, tetraethylene glycol, dipropylene glycol, and triptyrenic acid. Examples of the alkyl acrylate copolymer include pyrene glycol, neopentyl glycol, pinacol, 1,2,3-propanetriol, 1,2,4-butanetriol, 1,2,5-pentanetriol, pentaerythritol, sorbitol, 1,2,3,4-butanetetraol, erythritol, xylitol, mannitol, 2-methyl-2,4-pentanediol, 1,2-cyclopentanediol, 1,2-cycloheptanediol, 1,2-decanediol, 1,10-decanediol, dimethylpropanediol, cyclohexanedimethanol, 1,3-cyclopentanediol, and 2-methyl-1,3-propanediol.

[0102] The esters are not particularly limited, but examples thereof include ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, octyl acetate, nonyl acetate, decyl acetate, dodecyl acetate, methyl adipate, ethyl adipate, propyl adipate, butyl adipate, pentyl adipate, hexyl adipate, octyl adipate, nonyl adipate, decyl adipate, dodecyl adipate, methyl succinate, ethyl succinate, propyl succinate, butyl succinate, pentyl succinate, hexyl succinate, octyl succinate, and methyl glycol. acid esters, ethyl glycolate esters, propyl glycolate esters, butyl glycolate esters, pentyl glycolate esters, γ-butyrolactone, γ-hexanolactone, γ-octanolactone, δ-decanolactone, δ-dodecanolactone, ε-dodecanolactone, ε-laurinlactone, γ-phenyllactone, γ-methylbutyrolactone, δ-nonanolactone, ε-caprolactone, γ-cyclohexanolactone, δ-cyclopentenyllactone, γ-cyclooctanolactone, ε-cyclododecanolactone, γ-caprolactone, ε-hexalactone, γ-valeriolactone, δ-cyclodecanolactone, and γ-creolactone.

[0103] The carbonate esters are not particularly limited, but examples thereof include ethylene carbonate, propylene carbonate, methyl ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, ethyl propyl carbonate, ethyl butyl carbonate, dibutyl carbonate, ethyl pentyl carbonate, propyl pentyl carbonate, ethyl hexyl carbonate, propyl hexyl carbonate, methyl hexyl carbonate, ethyl octyl carbonate, propyl octyl carbonate, diethylene glycol dimethyl ether carbonate, diethylene glycol diethyl ether carbonate, triethylene glycol dimethyl ether carbonate, triethylene glycol diethyl ether carbonate, tetraethylene glycol dimethyl ether carbonate, tetraethylene glycol diethyl ... Examples of the carbonates include ethylene glycol dimethyl ether carbonate, tetraethylene glycol diethyl ether carbonate, dipropylene glycol dimethyl ether carbonate, dipropylene glycol diethyl ether carbonate, tripropylene glycol dimethyl ether carbonate, tripropylene glycol diethyl ether carbonate, methoxyethyl carbonate, methoxypropyl carbonate, ethoxyethyl carbonate, ethoxypropyl carbonate, methoxybutyl carbonate, ethoxybutyl carbonate, propoxyethyl carbonate, propoxypropyl carbonate, propoxybutyl carbonate, fluorocarbonates, ethyl methacrylate carbonate, methyl methacrylate carbonate, and ethyl acrylate carbonate.

[0104] The ethers are not particularly limited, but examples thereof include diethyl ether, dimethyl ether, dipropyl ether, diisopropyl ether, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, tripropylene glycol dimethyl ether, tripropylene glycol diethyl ether, methoxypropyl ether, methoxyethyl ether, ethoxyethyl ether, ethoxypropyl ether, methoxy butyl ether, ethoxybutyl ether, propoxyethyl ether, propoxypropyl ether, propoxybutyl ether, 1,4-dioxane, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and dimethoxypropane.

[0105] From the viewpoint of being able to further suppress cracking of the electrode, the plasticizer of the present embodiment preferably contains one or more selected from the group consisting of dihydric or higher polyhydric alcohols, esters, and ethers, among the above plasticizers, and more preferably contains one or more selected from the group consisting of triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, glycerin, γ-butyrolactone, 1,3-propanediol, and 1,3-butanediol.

[0106] The plasticizer may have solubility in water. The solubility is not particularly limited, but for example, the solubility in water at 25°C may be 5 g / 100 gH２ It is preferable that the viscosity is 0 or more, and more preferably 10 g / 100 gH ２ The water solubility of the plasticizer is 0 or more. When the plasticizer has water solubility, the plasticizer is uniformly dispersed in the aqueous composition for the positive electrode, which tends to more effectively suppress cracking of the electrode. The water solubility of the plasticizer can be measured, for example, by the method described in the Examples.

[0107] From the viewpoint of more suitably suppressing cracking of a positive electrode obtained using the aqueous composition for a positive electrode of this embodiment, the content of the plasticizer in the aqueous composition for a positive electrode is preferably 1% by mass or more and 10% by mass or less, more preferably 1.5% by mass or more and 8% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less, relative to 100% by mass of the total amount of the composition including the solvent.

[0108] <Positive electrode for lithium ion secondary battery> The positive electrode for lithium ion secondary battery of this embodiment contains the aqueous composition for positive electrode of this embodiment. In other words, the positive electrode for lithium ion secondary battery of this embodiment can be produced using the aqueous composition for positive electrode of this embodiment. The method for producing the positive electrode for lithium ion secondary battery of this embodiment is not particularly limited. For example, the positive electrode for lithium ion secondary battery can be produced by applying the aqueous composition for positive electrode of this embodiment to a positive electrode current collector, heating it, and drying it. The positive electrode current collector is not particularly limited, but aluminum foil, for example, can be used.

[0109] The method for applying the aqueous composition for a positive electrode of this embodiment to a positive electrode current collector is not particularly limited, and any coater head such as a reverse roll coater, comma bar coater, gravure coater, or air knife coater can be used. The method for drying the coated film of the aqueous composition for a positive electrode of this embodiment is not particularly limited, and examples of the method include standing to dry, blowing air drying, hot air drying, an infrared heater, and a far-infrared heater. The drying temperature for the coated film of the aqueous composition for a positive electrode of this embodiment is not particularly limited, and can be, for example, 60°C to 150°C. <Lithium Ion Secondary Battery> The lithium ion secondary battery of this embodiment includes the positive electrode for the lithium ion secondary battery of this embodiment. Typical components of the lithium ion secondary battery of this embodiment include a negative electrode, a negative electrode current collector, a positive electrode, a positive electrode current collector, a separator, and an electrolyte. The lithium ion secondary battery of this embodiment may be obtained by using at least the positive electrode, which is its main component, using the aqueous composition for a positive electrode.

[0110] The method for manufacturing the lithium ion secondary battery of this embodiment is not particularly limited, but may include, for example, a method in which a negative electrode and a positive electrode of this embodiment are placed opposite each other via a separator, an electrolyte is injected, and the resulting mixture is sealed. The negative electrode and the electrolyte are not particularly limited, and any electrolyte that is applicable to lithium ion secondary batteries may be appropriately selected and used. For example, the electrolyte may be LiClO ４ , LiBF ４ , LiPF ６ The organic solvent is not particularly limited, but examples thereof include ethers, ketones, lactones, nitriles, amines, amides, carbonates, and chlorinated hydrocarbons, and representative examples thereof include tetrahydrofuran, acetonitrile, butyronitrile, propylene carbonate, ethylene carbonate, and diethyl carbonate. These organic solvents are used alone or in combination of two or more.

[0111] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. For example, the aqueous composition for a positive electrode of the present embodiments may contain known additives and the like in addition to the components described above.

[0112] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples in any way.

[0113] <Preparation Examples of Particulate (Co)polymers> [Preparation Example 1] 0.015 parts by mass of dodecyl diphenyl ether sulfonate sodium salt, 3 parts by mass of itaconic acid, and 122 parts by mass of ion-exchanged water were added to a reactor, and the mixture was heated to 80°C while stirring and maintained at that temperature. To this was added 0.2 parts by mass of sodium persulfate. A monomer solution containing 67.45 parts by mass of 2-ethylhexyl acrylate, 1 part by mass of acrylic acid, 20.5 parts by mass of styrene, 8.0 parts by mass of methyl methacrylate, and 0.05 parts by mass of 1,9-nonanediol dimethacrylate was added, followed by 0.4 parts by mass of dodecyl diphenyl ether sulfonate sodium salt, 0.8 parts by mass of sodium persulfate, and 55 parts by mass of ion-exchanged water. The resulting monomer mixture was emulsified using a homogenizer to obtain a monomer emulsion. The resulting monomer emulsion was added dropwise, and the internal temperature was maintained at 80°C. After the addition was completed over 2.5 hours, polymerization was continued for 1 hour. Thereafter, the internal temperature was raised from 80°C to 85°C and maintained at this temperature for 3 hours to complete the polymerization. Thereafter, the mixture was cooled to room temperature, and the pH was adjusted to 8.0 and the solids content to 20% with a 5% aqueous sodium hydroxide solution and water. 0.05% by mass of 1,2-benzisothiazolin-3-one was added to the aqueous dispersion of the particulate (co)polymer, and the mixture was filtered through a mesh with a mesh size of 74 μm to obtain particulate (co)polymer 1. The physical properties of particulate (co)polymer 1 are shown in Table 1. Furthermore, the acid amounts of the obtained particulate (co)polymer 1 were quantified by acid titration, and the results were that itaconic acid was 3% by mass and acrylic acid was 1.1% by mass relative to the total amount of particulate (co)polymer 1.

[0114] [Preparation Examples 2 to 15] Particulate (co)polymers 2 to 15 were obtained in the same manner as in Preparation Example 1, except that the monomer composition was changed as shown in Table 1. The physical properties of the particulate (co)polymers 2 to 15 were as shown in Table 1. Note that, in Preparation Example 15, the monomer composition was the same as in Preparation Example 14, and the average particle diameter was adjusted to be as shown in Table 1 by adjusting the amount of dodecyl diphenyl ether sulfonate sodium salt.

[0115] (Particle size) The particle sizes of particulate (co)polymers 1 to 15 were measured using a concentrated particle size analyzer "FPAR-1000" manufactured by Otsuka Electronics, which uses dynamic light scattering as its measurement principle. Measurements were performed using a concentrated probe at 25°C and a light intensity of 20,000 to 50,000, and the value at 50% of the cumulative particle size in the obtained scattering intensity distribution data was taken as the particle size. The results are shown in Table 1.

[0116] (pH) The pH of the particulate (co)polymer is the pH of an aqueous solution of the particulate (co)polymer when the solid content is 5% by mass.

[0117] The abbreviations in the table represent the following compounds: St: styrene, 2-EHA: 2-ethylhexyl acrylate, BA: n-butyl acrylate, MMA: methyl methacrylate, AA: acrylic acid, MAA: methacrylic acid, IA: itaconic acid, AN: acrylonitrile, HEMA: 2-hydroxyethyl methacrylate, 1,9-ND: 1,9-nonanediol dimethacrylate, TMPT-A: trimethylolpropane triacrylate

[0118] <Preparation of aqueous composition for positive electrode> [Example 1] To a planetary mixer equipped with a disperser, 4 parts by mass, converted to solids content, of particulate polymer 1, 1.5 parts by mass, converted to solids content, of sodium polystyrene sulfonate (manufactured by Tosoh Finechem Corporation, product name "PS-100") as a water-soluble (co)polymer, 100 parts by mass of lithium iron phosphate (manufactured by Pulead Technology Industry, P600A, particle size 1 μm) as a positive electrode active material, and 5 parts by mass of furnace black (manufactured by Imerys, Super P-Li) as a conductive additive were added, and ion-exchanged water was further added so as to achieve the solids content shown in Table 2. The mixture was stirred, mixed, and degassed for 30 minutes to obtain aqueous composition for positive electrode 1.

[0119] [Examples 2 to 34, Comparative Examples 1 and 2] In Examples 2 to 23 and Comparative Examples 1 and 2, aqueous compositions 2 to 25 were obtained in the same manner as in Example 1, except that the types and compositions of the conductive assistant, particulate (co)polymer, and water-soluble (co)polymer were changed as shown in Tables 2 and 3. Furthermore, in Examples 24 to 34, the types and compositions of the conductive assistant, particulate (co)polymer, and water-soluble (co)polymer were changed as shown in Table 4, and aqueous compositions 26 to 36 were obtained in the same manner as in Example 1, except that the plasticizers shown in Table 4 were added.

[0120] <Preparation of Positive Electrode for Lithium Ion Secondary Battery> The obtained aqueous composition for the positive electrode was applied to one side of a 20 μm thick aluminum foil as a positive electrode current collector using a die coater, dried at 100° C. for 10 minutes, and then compression molded using a roll press. At this time, the amount of the active material applied to the positive electrode was 12 mg / cm. ２ , the density of the electrode layer after roll pressing was 2.2 g / cm ３ The electrode was then evaluated for cracking using the resulting electrode. The electrode thus obtained was used as a positive electrode for a secondary battery. The aqueous composition for a positive electrode and the positive electrode for a secondary battery obtained as described above were used to evaluate various physical properties, as described below. The results are shown in Tables 2 to 4.

[0121] (Viscosity and hysteresis of aqueous composition for positive electrode) The viscosity and hysteresis of the obtained aqueous composition were measured using an Anton Paar MCR-102 rheometer. Measurements were performed at 25°C using a CP-50-1 cone plate at a shear rate range of 1 to 100 (1 / s). The initial viscosity was taken as the value of 1 (1 / s) when the shear rate increased. The hysteresis was calculated using a shear rate of 4.2±0.2 (1 / s) and calculated as (viscosity when increasing - viscosity when decreasing) / viscosity when increasing × 100 (%). The absolute value of the hysteresis was evaluated according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: 0% or more but less than 20% B: 20% or more but less than 40% C: 40% or more but less than 60% D: 60% or more or immobilization due to aggregation

[0122] (Changes in Viscosity and Hysteresis of Aqueous Composition for Positive Electrode Over Time) The aqueous composition was placed in a container and left to stand at 25°C for one day while rotating with a mix rotor. The viscosity and hysteresis of the aqueous composition were measured under the same conditions as described above. The viscosity change after one day was calculated as (initial viscosity - viscosity after one day) / initial viscosity x 100 (%). The hysteresis change after one day was calculated using a shear rate of 4.2 ± 0.2 (1 / s) and calculated as (viscosity during rise - viscosity during fall) / viscosity during rise x 100 (%). The absolute values ​​of the rate of change in viscosity and hysteresis over time were evaluated according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: 0% or higher but less than 20% B: 20% or higher but less than 40% C: 40% or higher but less than 60% D: 60% or higher or immobilization due to aggregation

[0123] (Stability of stirred slurry) 30 g of the prepared slurry was placed in a 50 mL plastic bottle and stirred with a mix rotor at 40 rpm for 24 hours. After stirring, the viscosity of the slurry was measured with a rheometer, and the viscosity change rate was calculated using the following formula and evaluated according to the following criteria. Viscosity change rate of slurry after stirring = (initial viscosity - viscosity after 1 day of stirring) / initial viscosity x 100 (%) The calculated absolute value of the viscosity change rate of the slurry after stirring was evaluated according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: less than 20% B: 20% or more but less than 40% C: 40% or more but less than 60% D: 60% or more or immobilization due to aggregation

[0124] (Electrode cracking) The obtained electrode was punched out into a 30 mm x 30 mm square using a Thomson blade (mirror-finished blade), and the surface of the electrode layer after punching was visually observed to determine the degree of peeling according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: No peeling B: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was within 1 mm C: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was more than 1 mm but not more than 5 mm D: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was more than 5 mm

[0125] (Electrolyte Insoluble Content) The obtained particulate (co)polymer was left to stand in an oven at 130°C for 1 hour to dry. A film of the particulate (co)polymer obtained by drying was cut out and used as a sample for measurement. The mass of the film (Wa: unit g) at that time was measured. Thereafter, the sample was placed in a 50 mL vial together with 20 g of a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1:2), and the mixed solvent was allowed to penetrate at 60°C for 7 days. The sample was then removed and washed with the mixed solvent. Thereafter, the sample was left to stand in an oven at 150°C for 1 hour, and the mass of the sample was measured (Wc: unit g). The insoluble content of the particulate (co)polymer in the electrolyte was calculated using the following formula and evaluated according to the following criteria: Electrolyte Insoluble Content (%) = (Wc / Wa) x 100 A: 97% or more B: 90% or more but less than 97% C: 85% or more but less than 90% D: Less than 85%

[0126]

[0127]

[0128] Details of each component in the table are shown below. Active material: lithium iron phosphate (manufactured by Pulead Technology Industry, product name "P600A", particle size 1 μm) Conductive additives: Conductive additive A (surface oxygen atom abundance ratio: 0.60 mass%, O / C: 0.0061) Conductive additive B (surface oxygen atom abundance ratio: 0.88 mass%, O / C: 0.0090) Conductive additive C (surface oxygen atom abundance ratio: 2.00 mass%, O / C: 0.0206) Conductive additive D (surface oxygen atom abundance ratio: 0.54 mass%, O / C: 0.0054) Conductive additive E (surface oxygen atom abundance ratio: 0.19 mass%, O / C: 0.0019) Water-soluble (co)polymer: p-NaSS: sodium polystyrene sulfonate (manufactured by Tosoh Finechem Co., Ltd., product name "PS-100", weight average molecular weight: 540,000, pH: 10.0) CMC: carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., product name "Sunrose MAC (registered trademark) 350HC") Plasticizer: TEGDM: triethylene glycol dimethyl ether (manufactured by Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH２ 0 or more) TeEGDM: tetraethylene glycol dimethyl ether (manufactured by Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH ２ 0 or more) Glycerin (Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH ２ 0 or more) GBL: γ-butyrolactone (manufactured by Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH ２ 0 or more) 1,3-PG: 1,3-propanediol (manufactured by Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH ２ 0 or more) 1,3-BG: 1,3-butanediol (manufactured by Tokyo Chemical Industry Co., Ltd., solubility in water at 25°C: 10 g / 100 gH ２ 0 or more)

[0129] <Method for measuring the solubility of a plasticizer in water> 500 g of water was added to a 1 L glass beaker, and the mixture was stirred at 100 rpm using a rotor (made of fluororesin (PTFE), φ8 mm x 50 mm) while being kept warm in a thermostatic bath at 25°C. 0.1 g of each plasticizer was added, and after 10 minutes, it was visually confirmed whether the mixture was uniformly transparent. Additional 0.1 g of each plasticizer was added until the mixture was no longer uniformly transparent, and the solubility in water was measured. If the mixture was still uniformly transparent even after adding 50 g of plasticizer, the measurement was stopped at that point, and the solubility in water at 25°C was determined to be 10 g / 100 gH. ２ If the solution is not uniformly transparent when 0.1 g of plasticizer is added, the solubility in water at 25°C is 0.02 g / 100 g H ２ Water: Pure water with a conductivity of 1 μS / cm or less at 25°C, produced using a pure water device "G-10DSTSET" manufactured by Organo Corporation

[0130] The results of Examples 1 to 34 shown in Tables 2 to 4 show that an aqueous composition for a lithium ion secondary battery positive electrode, which comprises a conductive additive and a particulate (co)polymer, and which includes: a conductive additive; and a particulate (co)polymer; and when elements present on the surface of the conductive additive are measured by X-ray photoelectron spectroscopy and the abundance ratio (mass%) of oxygen atoms and the abundance ratio (mass%) of carbon atoms are calculated based on the peak areas, the abundance ratio of oxygen atoms on the surface of the conductive additive is 0.30 mass% to 2.00 mass%, and the ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms is 0.0020 to 0.0250; and the particulate (co)polymer includes structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the content ratio of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10 mass% to 5.00 mass% in total, relative to the total amount of the particulate (co)polymer, has improved agitated slurry stability and suppressed electrode cracking, compared to Comparative Examples 1 and 2.

[0131] The disclosure of Japanese Patent Application No. 2024-053283, filed on March 28, 2024, is incorporated herein by reference in its entirety. In addition, all documents, patent applications, and technical standards mentioned in the specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A conductive additive and a particulate (co)polymer are included, The conductive additive includes furnace black, When elements present on the surface of the conductive additive are measured by X-ray photoelectron spectroscopy and the abundance ratio (mass%) of oxygen atoms and the abundance ratio (mass%) of carbon atoms are calculated based on peak areas, the abundance ratio of oxygen atoms on the surface of the conductive additive is 0.30 mass% or more and 2.00 mass% or less, and the ratio (O / C) of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms is 0.0020 or more and 0.0250 or less, the particulate (co)polymer comprises a structural unit derived from an ethylenically unsaturated carboxylic acid (salt) and a structural unit derived from an aromatic vinyl monomer, and the structural unit derived from the ethylenically unsaturated carboxylic acid (salt) comprises a structural unit derived from a dibasic acid (salt); the content ratio of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 2.00% by mass or more and 15.00% by mass or less in total relative to the total amount of the particulate (co)polymer, the content ratio of the structural unit derived from the aromatic vinyl monomer is 1.0 mass% or more and 40.0 mass% or less with respect to the total amount of the particulate (co)polymer; An aqueous composition for use in a positive electrode of a lithium ion secondary battery.

2. the content ratio of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 2.00 mass% or more and 5.00 mass% or less in total with respect to the total amount of the particulate (co)polymer; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

3. the structural unit derived from the ethylenically unsaturated carboxylic acid (salt) further contains a monobasic acid (salt); The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

4. the content ratio of the structural units derived from the monobasic acid (salt) is 0.01% by mass or more and 2.00% by mass or less in total relative to the total amount of the particulate (co)polymer, and / or the content ratio of the structural units derived from the dibasic acid (salt) is 0.10% by mass or more and 5.00% by mass or less in total relative to the total amount of the particulate (co)polymer; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 3 .

5. The value (O × COOH) obtained by multiplying the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive by the content ratio (mass%) of the total amount of structural units derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is 0.030 to 10.00; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

6. the ratio (O / bound COOH) of the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive to the content ratio (bound COOH) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate (co)polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate (co)polymer is 0.0050 to 0.1000; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

7. The particulate (co)polymer further contains a constituent unit derived from a crosslinkable monomer. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

8. The abundance ratio of oxygen atoms on the surface of the conductive additive is 0.50 mass% or more and 2.00 mass% or less. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

9. The content of the conductive auxiliary agent is 3.5% by mass or more and 10.0% by mass or less with respect to the total amount of non-volatile components in the aqueous composition. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

10. Further comprising a water-soluble (co)polymer, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

11. the water-soluble (co)polymer contains a constituent unit derived from a monomer having a sulfonic acid (salt) group; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 10.

12. the content of the structural unit derived from the monomer having a sulfonic acid (salt) group is 25% by mass or more and 100% by mass or less based on the total amount of solids in the water-soluble (co)polymer; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 11.

13. further comprising a preservative, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

14. further comprising an active material, The active material includes a lithium phosphate compound. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

15. further comprising a plasticizer, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

16. The solubility of the plasticizer in water at 25°C is 5 g / 100 gH 2 O or higher, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 15.

17. the plasticizer comprises one or more selected from the group consisting of triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, glycerin, γ-butyrolactone, 1,3-propanediol, and 1,3-butanediol; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 15.

18. A positive electrode for a lithium ion secondary battery, comprising the aqueous composition for a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 17.

19. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 18.

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