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

Abstract

An aqueous composition for the positive electrode of a lithium ion secondary battery, the aqueous composition containing a water-soluble polymer (A) comprising constituent units corresponding to a monomer having a sulfonic acid group, wherein the total amount of the constituent units corresponding to a monomer having a sulfonic acid group in the water-soluble polymer (A) is 25-100 mass% with respect to the whole water-soluble polymer (A).

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, especially 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, examples using celluloses, polycarboxylic acid compounds, and the like as the water-soluble polymer have been disclosed (see Patent Documents 1 and 2 below).

[0006] JP 2005-63825 A JP 2006-134777 A

[0007] Regarding the thickener contained in the aqueous paste for the positive electrode, Patent Documents 1 and 2 exemplify water-soluble polymers such as celluloses such as carboxymethyl cellulose (CMC), polyacrylic acid compounds, and compounds having a vinylpyrrolidone structure, and in practice, cellulose compounds are used. However, these compounds are not necessarily sufficient in terms of the stability over time of the aqueous formulation, and there is room for improvement.

[0008] In order to solve the above-mentioned problems, an object of the present invention is to provide an aqueous composition for a positive electrode of a lithium ion secondary battery, which is an aqueous composition and has excellent stability over time, as well as a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

[0009] <1> An aqueous composition for a positive electrode of a lithium ion secondary battery, comprising: an active material; and a water-soluble polymer (A) containing structural units corresponding to a monomer having a sulfonic acid group, wherein the total amount of structural units in the water-soluble polymer (A) corresponding to the monomer having a sulfonic acid group is 25% by mass or more and 100% by mass or less, based on the total amount of the water-soluble polymer (A). <2> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <1>, further comprising a water-soluble polymer (B) (excluding those corresponding to the water-soluble polymer (A)) containing structural units having a carboxyl group. <3> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <1> or <2>, wherein the water-soluble polymer (A) has a weight-average molecular weight of 50,000 or more and 5,000,000 or less. <4> The aqueous composition for a positive electrode of a lithium ion secondary battery according to any one of <1> to <3>, further comprising a conductive aid. <5> The aqueous composition for a lithium ion secondary battery positive electrode according to <4>, wherein a content of the conductive additive in the composition is 3.5 mass % or more and 10.0 mass % or less with respect to the total amount of non-volatile components in the composition. <6> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <5>, further comprising a particulate polymer. <7> The aqueous composition for a lithium ion secondary battery positive electrode according to <6>, wherein the particulate polymer contains a (meth)acrylic acid ester monomer. <8> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <5>, further comprising: a conductive additive; and a particulate 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 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 0.10 mass% to 15.00 mass% in total relative to the total amount of the particulate polymer.<9> The aqueous composition for a lithium ion secondary battery positive electrode according to <6> or <7>, wherein the particulate polymer contains structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the total content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10% by mass or more and 15.00% by mass or less, relative to the total amount of the particulate polymer. <10> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <6> to <9>, wherein the particulate polymer contains a monobasic acid (salt) and / or a dibasic acid (salt). <11> The aqueous composition for a lithium ion secondary battery positive electrode according to <8>, wherein the product of the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive and the content ratio (mass%) of the total amount of structural units derived from the ethylenically unsaturated carboxylic acid (salt) in the particulate polymer, (O×COOH), is 0.030 to 10.00. <12> The aqueous composition for a lithium ion secondary battery positive electrode according to <8> or <9>, wherein 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 polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate polymer is 0.0050 to 0.1000. <13> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <6> to <12>, wherein the particulate polymer contains a crosslinkable monomer. <14> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <13>, further containing a preservative. <15> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <14>, wherein the active material is a lithium phosphate-based compound. <16> A positive electrode for a lithium ion secondary battery, comprising the aqueous composition for a lithium ion secondary battery positive electrode according to any one of <1> to <15>. <17> A lithium ion secondary battery, comprising the positive electrode for a lithium ion secondary battery according to <16>.

[0010] According to the present invention, it is possible to provide an aqueous composition for a lithium ion secondary battery positive electrode, which is an aqueous composition and has excellent stability, as well as a lithium ion secondary battery positive electrode and a lithium ion secondary battery using the same.

[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 thereof. In this specification, "(meth)acrylic" means "acrylic" and its corresponding "methacrylic". Furthermore, in this specification, "to" means that the numerical values ​​at both ends thereof are included as the upper and lower limits, unless otherwise specified.

[0012] First Embodiment Aqueous Composition for Positive Electrode of Lithium-Ion Secondary Battery An aqueous composition for a positive electrode of a lithium-ion secondary battery according to a first embodiment includes an active material and a water-soluble polymer (A) (hereinafter, sometimes simply referred to as "water-soluble polymer (A)") containing structural units corresponding to monomers having sulfonic acid groups, wherein the total amount of structural units corresponding to the monomers having sulfonic acid groups in the water-soluble polymer (A) is 25% by mass or more and 100% by mass or less relative to the total amount of the water-soluble polymer (A). Hereinafter, the aqueous composition for a positive electrode of a lithium-ion secondary battery according to the first embodiment will sometimes be referred to as the "aqueous composition for positive electrode."

[0013] The term "water-soluble polymer" in the first embodiment includes a water-soluble copolymer and is synonymous with the term "water-soluble (co)polymer" in the second to fourth embodiments described below. The term "sulfonic acid group" in the first embodiment includes a sulfonate group and is synonymous with the term "sulfonic acid (salt) group" in the second to fourth embodiments described below. The term "structural unit corresponding to a monomer" in the first embodiment is sometimes referred to as "structural unit derived from a monomer." The term "water-soluble polymer (A) comprising a structural unit corresponding to a monomer having a sulfonic acid group" in the first embodiment is synonymous with the term "water-soluble (co)polymer (A) comprising a structural unit derived from a monomer having a sulfonic acid (salt) group" in the second embodiment described below and the term "water-soluble (co)polymer comprising a structural unit derived from a monomer having a sulfonic acid (salt) group" in the third and fourth embodiments described below. The "water-soluble polymer (B) containing a structural unit having a carboxyl group (excluding those corresponding to the water-soluble polymer (A))" in the first embodiment is synonymous with the "water-soluble (co)polymer (B) containing a structural unit having a carboxyl group (excluding those corresponding to the water-soluble (co)polymer (A))" in the second embodiment described below. The "particulate polymer" in the first embodiment includes particulate copolymers, and is synonymous with the "particulate (co)polymer" in the second to fourth embodiments described below.

[0014] The aqueous composition for positive electrodes of the first embodiment contains a water-soluble resin containing a specific amount of sulfonic acid groups, thereby improving the stability of viscosity and hysteresis over time. As a result, even when using an aqueous composition for positive electrodes that has been used for a certain period of time, it is possible to efficiently produce lithium-ion secondary battery positive electrodes that have a uniform thickness during line coating and that have excellent uniformity between products.

[0015] The aqueous composition for a positive electrode of the first 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 an active material and a water-soluble polymer (A). The aqueous composition for a positive electrode of the first embodiment may further contain a binder, a conductive aid, a preservative, etc.

[0016] (Active Material) The active material used in the aqueous composition for a positive electrode of the first 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 the first 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. 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.) 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.

[0017] 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.

[0018] - 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 the first 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.

[0019] 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, relative to 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 the first 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.

[0020] (Water-soluble polymer (A)) The aqueous composition for a positive electrode of the first embodiment includes a water-soluble polymer (A) including a structural unit corresponding to a monomer having a sulfonic acid group. The water-soluble polymer (A) of the first embodiment can serve as a thickener for the composition. In the water-soluble resin of the first embodiment, "water-soluble" means that when 0.5 g of the polymer is dissolved in 100 g of water at 25°C, the insoluble content is less than 3.0 mass%. Note that when the solubility of a water-soluble polymer in water changes depending on the pH of the water, if there is a pH range in which the water-soluble polymer is water-soluble, the polymer can be considered to be water-soluble.

[0021] In the water-soluble resin of the first embodiment, the "structural unit corresponding to a monomer having a sulfonic acid group" refers to a structural unit in a polymer having a structure corresponding to a monomer having a sulfonic acid group. Hereinafter, the structural unit corresponding to a monomer having a sulfonic acid group may be referred to as a "sulfonic acid group-containing unit."

[0022] In the first embodiment, the "sulfonic acid group" is "-SO ３ -R" where R is a hydrogen atom, an alkali metal (Na, K, Li, etc.), an ammonium (NH ３ Examples of the sulfonic acid group in the first embodiment are not particularly limited, but include, for example, —SO ３ H, -SO ３ Na, -SO ３ K, -SO ３ Li, -S.O. ３ NH ３The monomer having a sulfonic acid group is not particularly limited, but 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 suitable polyoxyethylene sulfonates 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, 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.

[0023] The water-soluble polymer (A) in the first embodiment may contain, in addition to the sulfonic acid group-containing unit, a structural unit corresponding to another polymerizable monomer (hereinafter, sometimes referred to as "another monomer"). The other monomer is not particularly limited, but examples thereof include a polymerizable compound that does not contain a sulfonic acid group but contains a hydrophilic group other than a sulfonic acid group. Examples of other monomers include monomers having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, 2-acryloyloxyethyl succinic acid, and vinyl benzoic 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 and 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 n-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 sulfonic acid group-containing monomer in the first 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 and 2-hydroxyethyl (meth)acrylate. One type of other monomer from the above examples may be used alone, or two or more types may be used in combination.

[0024] In the first embodiment, the total amount of sulfonic acid group-containing units in the water-soluble polymer (A) is 25% by mass or more and 100% by mass or less, based on the total amount of the water-soluble polymer (A), from the viewpoint of stability over time of viscosity and hysteresis. The lower limit of the total amount of sulfonic acid group-containing units in the water-soluble polymer (A) in the first embodiment is more preferably 27% by mass or more, and even more preferably 29% by mass or more.

[0025] When the water-soluble polymer (A) in the first embodiment contains a structural unit corresponding to another monomer, the content of the structural unit corresponding to the other monomer in the water-soluble polymer (A) is preferably 75% by mass or less, more preferably 73% by mass or less, and particularly preferably 71% by mass or less, based on the total amount of the water-soluble polymer (A).

[0026] —Content— From the viewpoint of the viscosity and hysteresis stability over time of the aqueous composition for a positive electrode, the content of the water-soluble polymer (A) in the aqueous composition for a positive electrode is preferably 0.5% by mass or more and 5.0% by mass or less, more preferably 0.7% by mass or more and 4.5% by mass or less, and particularly preferably 1.0% by mass or more and 4.0% by mass or less, relative to the total amount of non-volatile components in the composition.

[0027] - pH - In the first embodiment, the pH of the water-soluble polymer (A) is preferably 4.0 or more and 11.0 or less, more preferably 4.5 or more and 10.7 or less, and particularly preferably 5.0 or more and 10.5 or less. Here, the "pH of the water-soluble polymer" refers to the pH after adjusting the water-soluble 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.

[0028] - Molecular Weight - In the first embodiment, the weight-average molecular weight of the water-soluble polymer (A) 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, from the viewpoints of viscosity and ease of handling of the aqueous composition for a positive electrode. The weight-average molecular weight of the water-soluble polymer (A) can be measured by the method described in the examples below, and can be appropriately adjusted by known means.

[0029] —Viscosity of aqueous composition for positive electrode— From the viewpoint of ease of handling, the viscosity of the aqueous composition for positive electrode in the first embodiment is preferably 300 mPa·s or more and 20,000 mPa·s or less, more preferably 500 mPa·s or more and 10,000 mPa·s or less, even more preferably 1,000 mPa·s or more and 8,000 mPa·s or less, and particularly preferably 1,000 mPa·s or more and 6,000 mPa·s or less.

[0030] The water-soluble polymer (A) in the first embodiment may contain a preservative. As the preservative, for example, those described below can be used alone or in combination.

[0031] -Method for Producing Water-Soluble Polymer (A)- The water-soluble polymer (A) in the first 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 usually be the same as the ratio of the structural units in the water-soluble polymer. Examples of aqueous solvents include those described below. The polymerization method for the water-soluble polymer (A) in the first 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 polymer in the first embodiment can be any method such as ionic polymerization, radical polymerization, or living radical polymerization. In the synthesis method for the water-soluble polymer (A) in the first 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 and the type of polymerization initiator. Typically, the polymerization temperature is about 30°C or higher, and the polymerization time is about 0.5 to 30 hours.

[0032] (Water-soluble polymer (B) containing a structural unit having a carboxyl group) The aqueous composition for a positive electrode of the first embodiment may further contain, in addition to the water-soluble polymer (A), a water-soluble polymer (B) containing a structural unit having a carboxyl group (excluding those corresponding to the water-soluble polymer (A)) (hereinafter, this may be simply referred to as "water-soluble polymer (B)"). Note that the "water-soluble (co)polymer (B)" described in the second embodiment below can be used as the water-soluble polymer (B).

[0033] (Dispersion Medium) The aqueous composition for a positive electrode of the first 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 thereof 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.

[0034] 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.

[0035] (Binder) The aqueous composition for a positive electrode of the first embodiment may contain a binder. As the binder, a known binder that can be dispersed in an aqueous solvent and used in the aqueous composition for a positive electrode can be appropriately selected and used. The binder is not particularly limited, and examples thereof include water-insoluble particulate polymers that are dispersed in water. One type of particulate polymer may be used alone, or two or more types may be used in combination. Addition of the particulate polymer is expected to impart flexibility to the positive electrode and improve yield.

[0036] Examples of the particulate polymer include polymers having a predetermined structural unit. The structural unit of the particulate polymer is not particularly limited, and examples thereof include structural units derived from the following monomers. For example, (meth)acrylic acid ester monomers such as 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; aromatic vinyl monomers such as styrene and vinyl naphthalene; and nitrile group-containing monomers such as acrylonitrile. aromatic divinyl monomers such as divinylbenzene; polyalkylene glycol mono(meth)acrylate monomers such as polyethylene glycol mono(meth)acrylate and polypropylene glycol mono(meth)acrylate; alkylene glycol di(meth)acrylates such as 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. Cole di(meth)acrylate monomers; polyol poly(meth)acrylate monomers such as trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol di(meth)acrylate, and pentaerythritol tetra(meth)acrylate; allyl compound monomers such as diallyl fumarate, diaryl itaconate, and triallyl isocyanurate; vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltriethoxysilane; Silane-based monomers such as silane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, and 3-acryloxypropyltriethoxysilane; aliphatic conjugated diene-based monomers such as 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene;Monomers having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and 2-acryloyloxyethyl succinic acid; 3-allyloxy-2-hydroxypropanephosphoric acid, 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, diisopropyl-2-methacryloyloxyethyl phosphate, mono-n-butyl-2-methacryloyloxyethyl phosphate, di-n-butyl-2-methacryloyloxyethyl phosphate, monobutoxyethyl-2-methacryloyloxyethyl phosphate, dibutoxyethyl-2-methacryloyloxyethyl phosphate unsaturated carboxylic acid amide monomers such as (meth)acrylamide and N,N-dimethyl(meth)acrylamide; unsaturated monomers containing a hydroxyalkyl group such as β-hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate and hydroxybutyl(meth)acrylate; unsaturated monomers having an epoxy group such as glycidyl(meth)acrylate; and monomers having a sulfonic acid group such as vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfoethyl methacrylate, sulfopropyl methacrylate, sulfobutyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid and 3-allyloxy-2-hydroxypropanesulfonic acid. The above-mentioned monomers having a carboxyl group, monomers having a phosphate group, and monomers having a sulfonic acid group may have the carboxyl group, phosphate group, or sulfonic acid group in the form of a salt, such as a lithium salt, sodium salt, potassium salt, or ammonium salt of the carboxyl group, phosphate group, or sulfonic acid group. As for the other monomers, one of the above-mentioned examples may be used alone, or two or more of them may be used in combination.

[0037] The particulate polymer may further contain a crosslinkable monomer. When the particulate polymer contains a crosslinkable monomer, the mixture layer does not dissolve even in the electrolyte solution, and good battery characteristics can be maintained. The crosslinkable monomer is not particularly limited, and may be a structural unit having a crosslinkable group among the structural units of the particulate polymer exemplified above, such as a structural unit derived from the following monomers. For example, alkylene glycol di(meth)acrylate monomers such as 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; trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol di(meth)acrylate, and pentaerythritol tetra(meth)acrylate; Examples of the other monomer include polyol poly(meth)acrylate monomers such as acrylate; allyl compound monomers such as diallyl fumarate, diaryl itaconate, and triallyl isocyanurate; and silane-based monomers such as vinyltrimethoxysilane, vinyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, and 3-acryloxypropyltriethoxysilane. One type of the other monomers listed above may be used alone, or two or more types may be used in combination.

[0038] -Average particle size- The average particle size of the binder (particulate 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.

[0039] From the viewpoint of the strength and flexibility of a positive electrode obtained using the aqueous composition for a positive electrode, the content of the binder (particulate polymer) in the aqueous composition for a positive electrode is preferably 0.5% by mass or more and 10.0% by mass or less, more preferably 1.0% by mass or more and 9.0% by mass or less, and particularly preferably 1.5% by mass or more and 7.0% by mass or less, relative to the total amount of non-volatile components in the composition.

[0040] -pH- In the first embodiment, the pH of the binder (particulate polymer) dispersion 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, from the viewpoint of the storage stability of the binder dispersion. The glass transition temperature of the particulate polymer in the first embodiment is preferably -75°C or more, more preferably -70°C or more, particularly preferably -65°C or more, from the viewpoint of the flexibility and windability of the electrode, and the binding strength between the electrode active material layer and the current collector, and is preferably 50°C or less, more preferably 40°C or less, even more preferably 35°C or less, and particularly preferably 30°C or less. The glass transition temperature of the particulate polymer can be adjusted by combining various monomers. The glass transition temperature can be obtained by measuring with a differential scanning calorimeter (DSC) in accordance with ASTM D3418-15. A DDSC curve can also be obtained by differentiating the DSC curve. When the DDSC curve has a plurality of peaks, the glass transition temperature obtained from the peak corresponding to the temperature range where the slope of the DSC curve is maximum is the glass transition temperature in this specification.

[0041] -Method for Producing Binder Dispersion- The binder (particulate polymer) dispersion in the first 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 the seed particles can also be obtained by conventional emulsion polymerization. Furthermore, known methods can be used for polymerization, and the binder (particulate polymer) 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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. Examples of chain transfer agents 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; nucleus-substituted α-methylstyrene dimers 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. Examples of chelating agents that can be used include sodium ethylenediaminetetraacetate and pentasodium diethylenetriaminepentaacetate. Examples of pH adjusters that can be used include sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydroxide, sodium hydrogencarbonate, disodium hydrogenphosphate, 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.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. 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.

[0048] (Conductive Aid) The aqueous composition for a positive electrode of the first embodiment may contain 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 the first embodiment, the discharge rate characteristics and the like of a lithium ion secondary battery can be improved.

[0049] The conductive additive in the first embodiment is not particularly limited, and may be appropriately selected from known conductive additives such as carbon black, furnace black, acetylene black, ketjen black, graphite, vapor-grown carbon fiber, carbon nanotubes, and other conductive carbons. One type of conductive additive may be used alone, or two or more types may be used in combination at any ratio.

[0050] From the viewpoints 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 of the conductive additive in the aqueous composition for a positive electrode 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.

[0051] When the positive electrode aqueous composition of the first embodiment includes a conductive additive and a particulate polymer, for example, 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 may be 0.30% by mass or more and 2.00% by mass or less, and the ratio (O / C) of the abundance ratio (mass%) of atoms to the abundance ratio (mass%) of carbon atoms may be 0.0020 to 0.0250 or less, and the particulate polymer may include structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the total content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) may be 0.10% by mass or more and 15.00% by mass or less relative to the total amount of the particulate polymer. In addition to the above-mentioned conductive additive and particulate polymer, the conductive additive and the particulate polymer may be the "conductive additive" and "particulate (co)polymer" described in the fourth embodiment described below.

[0052] In the positive electrode aqueous composition of the first embodiment, the particulate polymer may contain a constituent unit derived from an ethylenically unsaturated carboxylic acid (salt), and the content of the constituent unit derived from the ethylenically unsaturated carboxylic acid (salt) may be 0.10 mass% or more and 15.00 mass% or less in total with respect to the total amount of the particulate polymer. In addition to the particulate polymer described above, the particulate polymer may be a "particulate (co)polymer" described in the second or fourth embodiment described later.

[0053] In the positive electrode aqueous composition of the first embodiment, the particulate polymer may contain a monobasic acid (salt) and / or a dibasic acid (salt). As the monobasic acid (salt) and dibasic acid (salt), the "monobasic acid (salt)" and "dibasic acid (salt)" described in the fourth embodiment described later can be used.

[0054] In the positive electrode aqueous composition of the first embodiment, 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 an ethylenically unsaturated carboxylic acid (salt) contained in the particulate polymer may be 0.030 to 10.00. Note that, in addition to the conductive additives described above, the conductive additive and the particulate polymer may also be the "conductive additive" described in the fourth embodiment described below.

[0055] In the positive electrode aqueous composition of the first embodiment, the ratio (O / bound COOH) of the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive and the content ratio (bound COOH) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate polymer may be 0.0050 to 0.1000. Note that, in addition to the conductive additives described above, the "conductive additive" described in the fourth embodiment described below may be used as the conductive additive and the particulate polymer.

[0056] (Preservative) The aqueous composition for a positive electrode of the first embodiment may contain a preservative. The preservative may be contained in the water-soluble polymer (A) as described above. By containing a preservative, the aqueous composition for a positive electrode of the first 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, and thiocyanates, 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, and 2,2-dibromo-2-nitroethanol.

[0057] 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.

[0058] <Positive electrode for lithium ion secondary battery> The positive electrode for lithium ion secondary battery of the first embodiment contains the aqueous composition for positive electrode of the first embodiment. In other words, the positive electrode for lithium ion secondary battery of the first embodiment can be produced using the aqueous composition for positive electrode of the first embodiment. The method for producing the positive electrode for lithium ion secondary battery of the first embodiment is not particularly limited, but for example, the positive electrode for lithium ion secondary battery can be produced by applying the aqueous composition for positive electrode of the first embodiment to a positive electrode current collector, heating, and drying. The positive electrode current collector is not particularly limited, but for example, aluminum foil can be used.

[0059] The method for applying the aqueous composition for a positive electrode of the first embodiment to the positive electrode current collector is not particularly limited, and any coater head such as a reverse roll coater, a comma bar coater, a gravure coater, or an air-knife coater can be used. The method for drying the coated film of the aqueous composition for a positive electrode of the first embodiment is not particularly limited, and examples of the drying method include leaving the composition 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 the first embodiment is not particularly limited, and can be, for example, 60°C to 150°C.

[0060] <Lithium-ion secondary battery> The lithium-ion secondary battery of the first embodiment includes the positive electrode for the lithium-ion secondary battery of the first embodiment. Typical components of the lithium-ion secondary battery of the first 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 the first embodiment may be any battery as long as at least the positive electrode, which is a main component thereof, is obtained using an aqueous composition for a positive electrode.

[0061] The method for manufacturing the lithium ion secondary battery of the first embodiment is not particularly limited, but may include, for example, a method in which a negative electrode and a positive electrode of the first 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.

[0062] Second Embodiment As described above, the aqueous composition for a positive electrode of a lithium ion secondary battery described in the first embodiment may further contain a water-soluble polymer (B) containing a structural unit having a carboxyl group (excluding those corresponding to the water-soluble polymer (A)). The aqueous composition for a positive electrode of a lithium ion secondary battery according to the second embodiment includes an active material, a water-soluble (co)polymer (A) containing a structural unit derived from a monomer having a sulfonic acid (salt) group, and a water-soluble (co)polymer (B) containing a structural unit having a carboxyl group (excluding those corresponding to the water-soluble (co)polymer (A)), 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 (A). Hereinafter, the description regarding the water-soluble polymer (B) can also be applied to the aqueous composition for a positive electrode of the first embodiment. The aqueous composition for a lithium ion secondary battery positive electrode of the second embodiment is similar to those described in the other embodiments in terms of the configuration other than the water-soluble (co)polymer (B) described below.

[0063] For example, the slurry composition described in JP 2014-165108 A is said to be able to achieve both high levels of cycle characteristics and adhesion. However, the inventors have found through their investigations that, depending on the combination of specific components in the slurry composition, viscosity changes over time occur due to aggregation of the components in the slurry composition, and that when the slurry is used to manufacture a positive electrode for a lithium ion secondary battery, there are issues such as insufficient electrode adhesion. These findings indicate that conventional compositions for lithium ion secondary battery positive electrodes still have room for improvement in terms of viscosity stability over time, electrode adhesion, etc.

[0064] From the above viewpoints, the aqueous compositions for positive electrodes according to the first and second embodiments, which contain the water-soluble (co)polymer (B), have excellent viscosity stability over time and electrode adhesion, and a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the composition can be provided.

[0065] (Water-soluble (co)polymer (B)) The aqueous composition for a positive electrode of the second embodiment may contain a water-soluble (co)polymer (B) (excluding those corresponding to the water-soluble (co)polymer (A)) containing a structural unit having a carboxyl group. The water-soluble (co)polymer (B) of the second embodiment can serve as a thickener for the aqueous composition for a positive electrode.

[0066] In this specification, the term "structural unit having a carboxyl group" is a term that encompasses a structural unit derived from a monomer having a carboxyl group, a structural unit derived from a (co)polymer having a carboxyl group, or an alkali metal salt thereof (e.g., Na, K). Therefore, the water-soluble (co)polymer (B) in the second embodiment is understood to be a water-soluble resin that has at least one or both of a structural unit derived from a monomer having a carboxyl group and a structural unit derived from a (co)polymer having a carboxyl group.

[0067] The structural unit having a carboxyl group is not particularly limited as long as it is a structural unit that contains a carboxyl group in the structural unit that constitutes the polymer, and examples thereof include glucose units that contain a carboxyl group in part of the glucose units that constitute cellulose, and (meth)acrylic acid units. From the viewpoint of improving the viscosity and hysteresis stability over time of the aqueous composition for positive electrodes, glucose units that contain a carboxyl group in part of the glucose units that constitute cellulose are preferred. The water-soluble (co)polymer (B) of the second embodiment may contain one type of the structural unit listed above alone, or two or more types in any combination and ratio.

[0068] Specific examples of the water-soluble (co)polymer (B) include, but are not limited to, cellulose derivatives having a carboxyl group, poly(meth)acrylic acid, and sodium poly(meth)acrylate. Among these, carboxymethyl cellulose and alkali metal salts of carboxymethyl cellulose (e.g., sodium salt), which are cellulose derivatives having a carboxyl group, are preferred. The water-soluble (co)polymer (B) can be used alone or in any combination and ratio of two or more.

[0069] Known commercially available water-soluble (co)polymers (B) include, for example, the Sunrose MAC (registered trademark) series, Sunrose (registered trademark) F, A, and SLD series manufactured by Nippon Paper Industries Co., Ltd., and the Cellogen (registered trademark) series manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.

[0070] The content of the structural unit having a carboxyl group in the water-soluble (co)polymer (B) in the second embodiment is not particularly limited, but from the viewpoint of viscosity and hysteresis stability over time, it is preferably 25% by mass or more and 100% by mass or less, and the lower limit is more preferably 27% by mass or more, and even more preferably 29% by mass or more, based on the total solid content of the water-soluble (co)polymer (B). The content of the structural unit having a carboxyl group in the water-soluble (co)polymer (B) in the second embodiment can be measured by absorbance measurement.

[0071] When the water-soluble (co)polymer (B) in the second 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 (B). The lower limit is not particularly limited as long as it is more than 0% by mass.

[0072] - Content - The content of the water-soluble (co)polymer (B) in the water-soluble (co)polymer in the second embodiment is not particularly limited, but from the viewpoints of the viscosity and hysteresis stability over time of the aqueous composition for the positive electrode and electrode adhesion, the content is preferably 5% by mass or more and 70% by mass or less, more preferably 10% by mass or more and 60% by mass or less, and particularly preferably 20% by mass or more and 50% by mass or less, relative to the total amount of non-volatile components of the water-soluble (co)polymer.

[0073] The content of the water-soluble (co)polymer (B) in the aqueous composition for a positive electrode is not particularly limited, but from the viewpoints of the viscosity and hysteresis stability over time of the aqueous composition for a positive electrode and electrode adhesion, the content is preferably from 0.3 mass % to 5.0 mass %, more preferably from 0.4 mass % to 1.5 mass %, and particularly preferably from 0.45 mass % to 1.0 mass %, relative to the total amount of non-volatile components in the aqueous composition.

[0074] -pH- In the second embodiment, the pH of the water-soluble (co)polymer (B) is not particularly limited, but is preferably 4.0 to 11.0, more preferably 4.5 to 10.7, and particularly preferably 5.0 to 10.5. Here, the "pH of the water-soluble (co)polymer (B)" refers to the pH after adjusting the water-soluble (co)polymer aqueous solution with water to a solids content of 1% 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.

[0075] - Molecular Weight - The weight average molecular weight (Mw) of the water-soluble (co)polymer (B) in the second embodiment is not particularly limited, but from the viewpoint of the viscosity stability of the aqueous composition for the positive electrode, it is preferably from 10,000 to 1,000,000, more preferably from 30,000 to 700,000, and particularly preferably from 50,000 to 500,000. The weight average molecular weight (Mw) of the water-soluble (co)polymer (B) can be measured using gel permeation chromatography (GPC).

[0076] - Viscosity - The viscosity of the water-soluble (co)polymer (B) is not particularly limited and can be set appropriately depending on the desired performance. From the viewpoint of viscosity stability, the viscosity of the water-soluble (co)polymer (B) is preferably 10 mPa·s or more and 10,000 mPa·s or less, more preferably 50 mPa·s or more and 7,000 mPa·s or less, and even more preferably 100 mPa·s or more and 5,000 mPa·s or less. The viscosity can be measured by measuring a 1% aqueous solution of the water-soluble (co)polymer (B) at 25°C using a rheometer "MCR-102" manufactured by Anton Paar.

[0077] In the second embodiment, the content ratio of the water-soluble (co)polymer (A) to the water-soluble (co)polymer (B) is not particularly limited, but from the viewpoints of the viscosity and hysteresis stability over time of the aqueous composition for a positive electrode and electrode adhesion, the content ratio is preferably 1.0:9.0 to 9.0:1.0, more preferably 1.0:4.0 to 8.0:1.0, and even more preferably 1.0:1:0 to 7.0:1.0, calculated as solids.

[0078] (Particulate (co)polymer) The content of the structural units derived from an ethylenically unsaturated carboxylic acid (salt) is not particularly limited, but is generally 0.10% by mass or more and 15.00% by mass or less, preferably 0.20% by mass or more and 10.00% by mass or less, more preferably 0.50% by mass or more and 7.50% by mass or less, and even more preferably 1.00% by mass or more and 4.00% by mass or less, relative to the total amount of the particulate (co)polymer. When the content of the structural units derived from an ethylenically unsaturated carboxylic acid (salt) is within the above range, aggregation in the aqueous composition is suppressed, and the aqueous composition exhibits excellent stability over time in terms of viscosity and hysteresis. Therefore, when the aqueous composition is applied to a current collector, it can be applied while maintaining appropriate fluidity, and handling tends to be excellent. Furthermore, adhesion to electrodes also tends to be improved. The content of the structural units derived from an ethylenically unsaturated carboxylic acid (salt) can be measured by potentiometric titration of the particulate (co)polymer.

[0079] Third Embodiment The aqueous compositions for a lithium-ion secondary battery positive electrode described in the first and second embodiments may further contain a particulate polymer. The aqueous composition for a lithium-ion secondary battery positive electrode of the third embodiment includes a particulate (co)polymer and a water-soluble (co)polymer containing a structural unit derived from a monomer having a sulfonic acid (salt) group, 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 amount of the water-soluble (co)polymer. The following descriptions regarding the particulate (co)polymer can also be applied to the aqueous compositions for a positive electrode of the first and second embodiments. The aqueous composition for a lithium-ion secondary battery positive electrode of the third embodiment is similar to the aqueous compositions described in the other embodiments, except for the particulate (co)polymer described below.

[0080] Regarding the thickener contained in the aqueous composition for the positive electrode, Japanese Patent Application Laid-Open No. 2014-165108 cites examples of water-soluble polymers such as celluloses (e.g., carboxymethyl cellulose (CMC)), polyacrylic acid compounds, and compounds having a vinylpyrrolidone structure, and in practice, cellulose compounds are used. However, these compounds are not necessarily sufficient in terms of the stability over time of the aqueous composition, and there is room for improvement. Furthermore, when these aqueous compositions are used to prepare a lithium-ion secondary battery positive electrode, the coating film formed by applying the aqueous composition to a current collector and drying it may suffer from defects, which is known as electrode cracking. In this regard, there is also room for improvement.

[0081] From the above viewpoints, the aqueous compositions for positive electrodes of the first and third embodiments described above, which contain a particulate (co)polymer, have excellent viscosity stability over time and can suppress the occurrence of cracks in the electrode, and it is possible to provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the composition.

[0082] (Particulate (co)polymer) In the third embodiment, the content of the structural units derived from the monobasic acid (salt) monomer is not particularly limited, but from the viewpoint of particle dispersion stability, it is preferably 0.1 mass % or more and 20.0 mass % or less, more preferably 0.1 mass % or more and 10.0 mass % or less, even more preferably 0.2 mass % or more and 5.0 mass % or less, and particularly preferably 0.2 mass % or more and 2.0 mass % or less, relative to the total amount of the particulate (co)polymer. When the content of the structural units derived from the monobasic acid (salt) monomer is within the above range, the aqueous composition for a positive electrode of the third embodiment suppresses aggregation in the aqueous composition and can be applied to a current collector while maintaining appropriate fluidity, and therefore tends to be easy to handle and also tends to be excellent in storage stability.

[0083] In the third embodiment, the content of the constitutional units derived from the dibasic acid (salt) monomer is not particularly limited, but from the viewpoint of the mechanical strength of the particles, it is preferably 0.1 mass % or more and 10.0 mass % or less, more preferably 0.5 mass % or more and 5.0 mass % or less, even more preferably 1.0 mass % or more and 4.5 mass % or less, and particularly preferably 1.5 mass % or more and 3.5 mass % or less, relative to the total amount of the particulate (co)polymer. When the content of the constitutional units derived from the dibasic acid (salt) monomer is within the above range, aggregation in the aqueous composition is suppressed, and when applied to a current collector, the composition can be applied while maintaining appropriate fluidity, which tends to improve handleability and storage stability.

[0084] Fourth Embodiment The aqueous composition for a lithium ion secondary battery positive electrode described in the first to third embodiments may contain a conductive additive and a particulate (co)polymer. Further, according to a fourth embodiment, there is provided an aqueous composition for a lithium ion secondary battery positive electrode, the aqueous composition 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% 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 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% or more and 15.00 mass% or less, relative to the total amount of the particulate (co)polymer. The following descriptions regarding the conductive additive and the particulate (co)polymer can be applied as they are to the aqueous compositions for positive electrodes of the first to third embodiments. The aqueous composition for positive electrodes of lithium ion secondary batteries of the third embodiment is similar to those described in the other embodiments in terms of the configuration other than the conductive additive and the particulate (co)polymer described below.

[0085] For example, the conductive adhesive composition for electrochemical element electrodes described in International Publication No. 2013 / 018887 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 a positive electrode for a lithium ion secondary battery, 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.

[0086] From the above viewpoints, the aqueous compositions for positive electrodes according to the first to fourth embodiments, which contain a conductive additive and a particulate (co)polymer, have excellent stability of viscosity over time and can suppress the occurrence of cracks in the electrode, and it is possible to provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the composition.

[0087] The aqueous composition for positive electrodes of the fourth embodiment may include a conductive additive and a particulate polymer, wherein the proportion of oxygen atoms present on the surface of the conductive additive is within a predetermined range, the ratio of the proportion of oxygen atoms present on the surface to the proportion of carbon atoms is 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, and improved flexibility of a coating film formed by applying and drying the aqueous composition for positive electrodes. This allows for efficient production of lithium-ion secondary battery positive electrodes that are uniform in thickness during line coating and have excellent product-to-product uniformity, 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.

[0088] (Conductive Aid) The conductive aid in the fourth embodiment is not particularly limited, and can be appropriately selected and used from known conductive aids such as furnace black, carbon black, acetylene black, ketjen black, graphite, vapor-grown carbon fiber, carbon nanotubes, and other conductive carbons. The conductive aid may be used alone or in combination of two or more types in any ratio. Among these, from the viewpoint of cost benefits, it is preferable that the conductive aid contains furnace black, and it is particularly preferable that the conductive aid contains furnace black as an essential component.

[0089] In the fourth embodiment, the proportion of oxygen atoms present on the surface of the conductive additive may be 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%) may be 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 an aqueous solvent 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.

[0090] 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.

[0091] (Particulate (co)polymer) The aqueous composition for a positive electrode of the fourth embodiment may contain, as the particulate polymer, a constituent unit derived from an ethylenically unsaturated carboxylic acid (salt). By containing the conductive additive and the particulate polymer, the aqueous composition for a positive electrode can suppress aggregation of the conductive additive in the aqueous composition, and the aqueous composition has excellent stability over time of viscosity and hysteresis.

[0092] In the fourth 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.

[0093] In the fourth 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.

[0094] In the fourth 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.

[0095] 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.

[0096] 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

[0097] 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.

[0098] The particulate (co)polymer in the fourth 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.

[0099] 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.

[0100] 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.

[0101] 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 the fourth 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 aqueous 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).

[0102] 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.

[0103] 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 embodiments may contain known additives in addition to the components described above.

[0104] The embodiments will be described in more detail below with reference to examples, but the embodiments are not limited to these examples.

[0105] Examples 1 to 14, Comparative Examples 1 to 3 Preparation Examples of Water-Soluble Polymer (A) Preparation Example 1 667 parts by mass of ion-exchanged water and 100 parts by mass of sodium p-styrenesulfonate were added to a reactor, and the sodium p-styrenesulfonate was dissolved in the ion-exchanged water. After nitrogen substitution, the mixture was heated to 50°C and maintained at that temperature. 1.05 parts by mass of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50; polymerization initiator) was dissolved in 10.5 parts by mass of ion-exchanged water and charged to the reactor. After charging an aqueous solution of 2,2'-azobis(2-methylpropionamidine) dihydrochloride, the mixture was heated to 80°C and maintained at 80°C for 3 hours to complete the polymerization. The mixture was then cooled to room temperature, and water was added to adjust the solids content to 10% by mass. Furthermore, 0.02% by mass of 2-methyl-4-isothiazolin-3-one (preservative) was added to the obtained aqueous water-soluble polymer solution to obtain a water-soluble polymer 1. The physical properties of the obtained water-soluble polymer 1 are shown in Table 1.

[0106] [Preparation Examples 2 to 6] In each preparation example, polymerization was completed in the same manner as in Preparation Example 1, except that the amount of sodium p-styrenesulfonate was changed as shown in Table 1. Thereafter, after cooling to room temperature, water and sodium hydroxide were added so as to achieve the solid content and pH shown in Table 1 (sodium hydroxide was added in an amount such that the pH would be as shown in Table 1 when the obtained aqueous water-soluble polymer solution was diluted to 5% by mass). Furthermore, 0.02% by mass of 2-methyl-4-isothiazolin-3-one was added to the obtained aqueous water-soluble polymer solution, thereby obtaining water-soluble polymers 2 to 6. The physical properties of water-soluble polymers 2 to 6 are shown in Table 1.

[0107] The abbreviations in Table 1 represent the following compounds: NaSS: sodium p-styrenesulfonate MAA: methacrylic acid HEMA: 2-hydroxyethyl methacrylate

[0108] (Molecular Weight) The molecular weight of each water-soluble polymer was evaluated using GPC. The water-soluble polymer was diluted with ion-exchanged water to approximately 0.1% by mass and passed through a 45 μm filter to prepare a measurement sample. Measurements were performed under the following conditions: column temperature: 40°C, eluent: pH 9.0 buffer solution (manufactured by Kishida Chemical Co., Ltd., containing sodium tetraborate and boric acid), eluent flow rate: 0.6 ml / min, and columns: Asahipak GF-7M HQ and GF-210 HQ. Sodium polyacrylate was used for the calibration curve, and the obtained weight average molecular weight value was used as the molecular weight.

[0109] <Preparation Examples of Particulate Polymers> [Preparation Example 7] A reactor was charged with 130 parts by mass of ion-exchanged water and 0.05 parts by mass of an aqueous solution of sodium alkyldiphenyl ether sulfonate (DOW CHEMICAL, Dowfax 2A1) in terms of solids content, and the mixture was then purged with nitrogen and heated to 70°C while stirring. Next, 10 parts by mass of a 5% aqueous solution of sodium persulfate was added to the reactor. A mixture of 84 parts by mass of n-butyl acrylate, 15 parts by mass of 2-hydroxyethyl methacrylate, 1 part by mass of trimethylolpropane triacrylate, 0.15 parts by mass of the aqueous solution of sodium alkyldiphenyl ether sulfonate in terms of solids content, and 125 parts by mass of ion-exchanged water was emulsified using a homogenizer, and the resulting emulsion was added dropwise over 2.5 hours while maintaining the temperature at 70°C. After completion of the dropwise addition, polymerization was continued at 70°C for 2 hours, after which the temperature was raised to 80°C and maintained for 1 hour to complete the polymerization. Thereafter, the mixture was cooled to room temperature, and the pH was adjusted to 8.0 and the solid content to 20% with a 5% aqueous sodium hydroxide solution and water, and 0.05% by mass of 1,2-benzisothiazolin-3-one was added to the aqueous dispersion of the particulate polymer, followed by filtration through a mesh with an opening of 74 μm to obtain Particulate Polymer 1. The physical properties of Particulate Polymer 1 were as shown in Table 2.

[0110] Preparation Example 8 Particulate polymer 2 was obtained in the same manner as in Preparation Example 7, except that the monomer composition was changed as shown in Table 1. The physical properties of particulate polymer 2 were as shown in Table 1.

[0111] Preparation Example 9: A reactor was charged with 120 parts by mass of ion-exchanged water, 0.02 parts by mass of an aqueous solution of sodium alkyl diphenyl ether sulfonate (DOW CHEMICAL, Dowfax 2A1) in terms of solids, and 1.7 parts by mass of itaconic acid. The mixture was then purged with nitrogen and heated to 80°C while stirring. Next, 4 parts by mass of a 5% aqueous solution of sodium persulfate was added to the reactor. A mixture of 68.6 parts by mass of 2-ethylhexyl acrylate, 20.5 parts by mass of styrene, 8 parts by mass of methyl methacrylate, 1 part by mass of methacrylic acid, 0.2 parts by mass of 1,9-nonanediol dimethacrylate, 0.4 parts by mass of the aqueous solution of sodium alkyl diphenyl ether sulfonate in terms of solids, and 38 parts by mass of ion-exchanged water was emulsified using a homogenizer, and the resulting emulsion was added dropwise over 2.5 hours while maintaining the temperature at 80°C. After the dropwise addition was completed, polymerization was continued at 80°C for 1 hour, and then the temperature was raised to 85°C and maintained at that temperature for 2 hours to complete the polymerization. Thereafter, the mixture was cooled to room temperature, and the pH was adjusted to 8.0 and the solid content to 30% with a 10% 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 polymer, and the mixture was filtered through a mesh with an opening of 74 µm to obtain Particulate Polymer 3. The physical properties of Particulate Polymer 3 were as shown in Table 2.

[0112] (Particle diameter) The particle diameters of the particulate polymers 1 and 2 were measured using a concentrated particle size analyzer "FPAR-1000" manufactured by Otsuka Electronics Co., Ltd., which uses dynamic light scattering as its measurement principle. The measurement was performed using a concentrated probe under conditions of 25°C and a light intensity of 20,000 to 50,000, and the value at 50% of the cumulative particle diameter of the obtained scattering intensity distribution data was taken as the particle diameter. The results are shown in Table 2.

[0113] The abbreviations in Table 2 represent the following compounds: 2EHA: 2-ethylhexyl acrylate BA: n-butyl acrylate ST: styrene MMA: methyl methacrylate IA: itaconic acid MAA: methacrylic acid HEMA: 2-hydroxyethyl methacrylate A-TMPT: trimethylolpropane triacrylate 1,9-ND: 1,9-nonanediol dimethacrylate

[0114] <Preparation of Aqueous Composition for Positive Electrode> [Example 1] 1.5 parts by mass (solids content) of water-soluble polymer 1 and 5 parts by mass of acetylene black (Denka Black Li-400, manufactured by Denka Corporation; "Li-400" in the table below) as a conductive additive were added to a planetary mixer equipped with a disperser, and the mixture was stirred and mixed for 10 minutes. Next, 100 parts by mass of lithium iron phosphate (P600A, manufactured by Pulead Technology Industry Co., Ltd., particle size 1 μm; "Pulead P600A" in the table below) as a positive electrode active material was added, and the mixture was stirred and mixed for 30 minutes. Ion-exchanged water was then added to achieve the solid content shown in Table 3, and the mixture was stirred and mixed for 10 minutes to obtain an aqueous composition for positive electrode (Aqueous Composition 1).

[0115] [Examples 2 to 6, 9 to 12, Comparative Examples 1 to 3] In each example, aqueous compositions 2 to 6 and 9 to 12 were obtained in the same manner as in Example 1, except that the substances such as the water-soluble polymer were changed as shown in Table 3.

[0116] [Example 7] Each substance such as the water-soluble polymer was changed as shown in Table 3, and 100 parts by mass of lithium iron phosphate was added and stirred and mixed for 30 minutes in the same procedure as in Example 1. Next, ion-exchanged water was added so that the particulate polymer 1 became 3.5 parts by mass in terms of solid content and the solid content shown in Table 3 was obtained, and the mixture was stirred and mixed for 10 minutes to obtain aqueous composition 7.

[0117] Example 8 Aqueous composition 8 was obtained in the same manner as in Example 7, except that the substances such as the water-soluble polymer and the particulate polymer were changed as shown in Table 3.

[0118] Examples 13 and 14 Aqueous compositions 13 and 14 were obtained in the same manner as in Example 7, except that the materials such as the water-soluble polymer and the particulate polymer were changed as shown in Table 3.

[0119] (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 was increasing. The hysteresis was calculated using a shear rate of 4.2±0.2 (1 / s) and was calculated as (rising viscosity - falling viscosity) / rising viscosity × 100 (%).

[0120] (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 as (rising viscosity - falling viscosity) / rising viscosity x 100 (%) using a shear rate of 4.2 ± 0.2 (1 / s).

[0121] (Preparation of Secondary Battery Positive Electrode) The aqueous composition was applied to one side of a 20 μm-thick aluminum foil serving 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 nonvolatile content of the aqueous composition for the positive electrode was 12 mg / cm ２ The density of the mixture layer after roll pressing was 2.2 g / cm ３ The composition and the positive electrode for a secondary battery obtained as described above were used to evaluate various physical properties, which will be described later.

[0122] (Cracks during electrode punching) The prepared secondary battery positive electrodes were punched out into 30 mm x 30 mm squares using a Thomson blade (mirror-finished blade), and the degree of peeling of the electrode layer after punching was evaluated according to the following criteria: 5: No peeling 4: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was within 1 mm 3: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was more than 1 mm and within 5 mm 2: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was more than 5 mm and within 10 mm 1: The shortest distance from the closest point in the plane of the exposed current collecting foil to the periphery was more than 10 mm

[0123]

[0124] Details of each component in Table 3 are shown below. Super P Li: Carbon black (manufactured by Imerys). Water-soluble polymer 7: PS-100 (polystyrene sodium sulfonate, manufactured by Tosoh Finechem Co., Ltd.) was used as water-soluble polymer 7. The pH was 10.0, and the molecular weight measured by the above-mentioned method was 540,000. Water-soluble polymer 8: Sodium hydroxide was added to a poly(2-acrylamido-2-methyl-1-propanesulfonic acid) solution (manufactured by Aldrich), and 0.02% by mass of 2-methyl-4-isothiazolin-3-one was added to the water-soluble polymer aqueous solution to adjust the pH to 7.0, which was used as water-soluble polymer 8. The molecular weight measured by the above-mentioned method was 760,000. Water-soluble polymer 9: Sunrose MAC-350HC (carboxymethyl cellulose, manufactured by Nippon Paper Industries Co., Ltd.) was used as water-soluble polymer 9. Water-soluble polymer 10: Water and sodium hydroxide were added to polyacrylic acid (manufactured by Aldrich, molecular weight 1,250,000) to adjust the pH to 8.6, and this was used as water-soluble polymer 10. Water-soluble polymer 11: PS-1 (polystyrenesodium sulfonate, manufactured by Tosoh Finechem Co., Ltd.) was used as water-soluble polymer 11. The pH was 10.0, and the molecular weight measured by the above-mentioned method was 26,000. Water-soluble polymer 12: PS-5 (polystyrenesodium sulfonate, manufactured by Tosoh Finechem Co., Ltd.) was used as water-soluble polymer 12. The pH was 10.0, and the molecular weight measured by the above-mentioned method was 93,500. Water-soluble polymer 13: PS-50 (polystyrenesodium sulfonate, manufactured by Tosoh Finechem Co., Ltd.) was used as water-soluble polymer 13. The pH was 10.0, and the molecular weight measured by the above-mentioned method was 370,000.

[0125] The results shown in Table 3 indicate that the aqueous compositions of the Examples, in which the water-soluble resin of this embodiment was used with lithium iron phosphate as the active material, exhibited superior viscosity change and hysteresis change after one day compared to the aqueous composition of Comparative Example 1, in which carboxymethyl cellulose was used as the thickener. Furthermore, the aqueous composition of Comparative Example 2, in which polyacrylic acid was used as the thickener, was confirmed to have undergone phase separation after one day. Furthermore, the aqueous composition of Comparative Example 3, in which a water-soluble resin in which the total amount of sulfonic acid group-containing units in the water-soluble polymer (A) was less than 25% by mass relative to the active material lithium iron phosphate, exhibited a particularly large viscosity change after one day, indicating poor stability.

[0126] Examples a1 to a18, Comparative Examples a1 to a2 Preparation Examples of Particulate (Co)polymers Preparation Example a1 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 with stirring and maintained at that temperature. To this mixture, 0.2 parts by mass of sodium persulfate was added. 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, along with 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 obtained monomer emulsion was added dropwise, and the dropwise addition was completed over 2.5 hours while maintaining the internal temperature at 80°C, after which polymerization was continued for 1 hour. Thereafter, the internal temperature was raised from 80°C to 85°C and maintained 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 opening of 74 μm to obtain particulate (co)polymer a1. The physical properties of the particulate (co)polymer a1 were as shown in Table 4. Furthermore, the acid amount of the obtained particulate (co)polymer a1 was 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 the particulate (co)polymer a1.

[0127] [Preparation Examples a2 to a10 and a13] Particulate (co)polymers a2 to a10 and a13 were obtained in the same manner as in Preparation Example a1, except that the monomer composition was changed as shown in Table 1. The physical properties of the particulate (co)polymers a2 to a10 and a13 were as shown in Table 4.

[0128] (Particle diameter) The particle diameters of the particulate (co)polymers a1 to a10 and a13 were measured by the method described above. The results are shown in Table 4.

[0129] (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.

[0130] The abbreviations in Table 4 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.

[0131] <Preparation of aqueous composition for positive electrode> [Example a1] To a planetary mixer equipped with a disperser, 4 parts by mass, converted to solids content, of the particulate polymer a1, 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 a solids content shown in Tables 5 and 6, followed by stirring, mixing, and degassing for 30 minutes to obtain an aqueous composition for positive electrode a1.

[0132] [Examples a2 to a18, Comparative Examples a1 to a2] In each example, aqueous compositions a2 to a18 and a22 to a23 were obtained in the same manner as in Example a1, except that the types and compositions of the conductive additive, particulate (co)polymer, and water-soluble (co)polymer were changed as shown in Tables 5 and 6.

[0133] <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 5 and 6.

[0134] (Viscosity and hysteresis of aqueous composition for positive electrode) The viscosity and hysteresis of the obtained aqueous composition were measured by the above-mentioned method. 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 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

[0135] (Changes in viscosity and hysteresis of aqueous composition for positive electrode over time) The changes in viscosity and hysteresis of the aqueous composition for positive electrode over time were measured using the method described above. 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

[0136] (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

[0137] (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

[0138] (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%

[0139]

[0140] Details of each component in Tables 5 and 6 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")

[0141] From the results of Examples a1 to a18 shown in Tables 5 and 6, when a conductive additive and a particulate (co)polymer are included, and 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% or more and 2.00 mass% or less, and the ratio of the abundance ratio (mass%) of oxygen atoms to the abundance ratio (mass%) of carbon atoms (O / C) is 0.0020 or more and 0.0250 or less, the particulate (co)polymer contains structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10 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 has improved stability of the stirred slurry and suppressed cracking of the electrode, compared to Comparative Examples a1 and a2.

[0142] Examples b1 to b17, Comparative Examples b1 to b3 Preparation Examples of Water-Soluble (Co)Polymer (A) Preparation Example b1: 667 parts by mass of ion-exchanged water, 75 parts by mass of sodium p-styrenesulfonate, and 25 parts of methacrylic acid were added to a reactor. The sodium p-styrenesulfonate was dissolved in the ion-exchanged water, and the atmosphere was purged with nitrogen. The temperature was raised to 50°C and maintained at this temperature. 1.05 parts by mass of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50; polymerization initiator) was dissolved in 10.5 parts by mass of ion-exchanged water and the mixture was added to the reactor. After adding an aqueous solution of 2,2'-azobis(2-methylpropionamidine) dihydrochloride, the mixture was heated to 80°C and maintained at 80°C for 3 hours to complete the polymerization. The mixture was then cooled to room temperature, and water was added to adjust the solids content to 10% by mass. Furthermore, 0.02% by mass of 2-methyl-4-isothiazolin-3-one (preservative) was added to the obtained aqueous water-soluble polymer solution to obtain a water-soluble (co)polymer (A-4). The physical properties of the obtained water-soluble polymer (A-4) are shown in Table 7.

[0143] [Preparation Examples b2 and b3] In each preparation example, polymerization was completed in the same manner as in Preparation Example b1, except that the amounts of sodium p-styrenesulfonate and methacrylic acid were changed as shown in Table 7. Thereafter, after cooling to room temperature, water and sodium hydroxide were added so as to achieve the solid content and pH shown in Table 7 (sodium hydroxide was added in an amount such that the pH would be as shown in Table 7 when the obtained water-soluble polymer aqueous solution was diluted to 5% by mass). Furthermore, 0.02% by mass of 2-methyl-4-isothiazolin-3-one was added to the obtained water-soluble polymer aqueous solution, thereby obtaining water-soluble (co)polymers (A-5) and (A-6). The physical properties of the water-soluble (co)polymers (A-5) and (A-6) are shown in Table 7. The abbreviations in Table 7 represent the following compounds: NaSS: sodium p-styrenesulfonate MAA: methacrylic acid

[0144] (Molecular Weight of Water-Soluble (Co)polymer (A)) The molecular weight of each water-soluble (co)polymer (A) was evaluated by the method using GPC described above.

[0145] (Molecular Weight of Water-Soluble (Co)Polymer (B)) The molecular weight of each water-soluble (co)polymer (B) was evaluated using gel filtration chromatography (GFC). The water-soluble polymer was diluted with ion-exchanged water to approximately 0.1% by mass and passed through a 45 μm filter to prepare a measurement sample. Measurement was performed under the following conditions: column temperature: 40°C, eluent: 100 mmol sodium nitrate aqueous solution, eluent flow rate: 1.0 ml / min, column: Shodex OHpak SB-803 HQ + SB-805 HQ, pullulan was used for the calibration curve, and the obtained weight average molecular weight value was taken as the molecular weight.

[0146] <Preparation Examples of Binder (Particulate (Co)polymer)> [Preparation Example b1] 1.75 parts by mass of itaconic acid, 0.015 parts by mass of dodecyl diphenyl ether sulfonic acid sodium salt, and 120 parts by mass of ion-exchanged water were added to a reactor, and the mixture was heated to and maintained at 80°C while stirring. To this mixture, 0.05 parts by mass of sodium persulfate was added. Next, a monomer solution prepared by mixing 73 parts by mass of 2-ethylhexyl acrylate, 8 parts by mass of methyl methacrylate, 17 parts by mass of styrene, and 0.25 parts by mass of 1,9-nonanediol dimethacrylate, 0.45 parts by mass of dodecyl diphenyl ether sulfonic acid sodium salt, and 55 parts by mass of ion-exchanged water were added. The resulting monomer mixture was emulsified using a homogenizer to obtain a monomer emulsion. The resulting monomer emulsion was added dropwise to a subsequent reactor over 2.5 hours while maintaining the internal temperature at 80°C, and polymerization was then continued for another hour. Thereafter, 0.03 parts by mass of Perbutyl Z (NOF Corporation), 0.03 parts by mass of anhydrous zinc bicarbonate sodium sulfate, and 19 g of ion-exchanged water were added, and the mixture was maintained for 30 minutes to complete the polymerization, yielding polymer particles. 0.0005 parts by mass of methylisothiazoline was added as an additive to 100 parts by mass of the solid content of the obtained polymer particles, and then sodium hydroxide was added dropwise to obtain a particulate (co)polymer b1 adjusted to pH 8.0 and a solid content of 20% by mass. The physical properties of the particulate (co)polymer b1 were as shown in Table 8.

[0147] [Preparation Example b2] A particulate (co)polymer b2 was obtained in the same manner as in Preparation Example b1, except that the monomer composition was changed as shown in Table 8. The physical properties of the particulate (co)polymer b2 are as shown in Table 8.

[0148] (Particle Diameter) The particle diameters of the particulate (co)polymers b1 and b2 were measured by the method described above.

[0149] The abbreviations in Table 8 represent the following compounds: St: styrene, 2-EHA: 2-ethylhexyl acrylate, MMA: methyl methacrylate, MAA: methacrylic acid, IA: itaconic acid, 1,9-ND: 1,9-nonanediol dimethacrylate.

[0150] <Preparation of aqueous composition for positive electrode> [Example b1] A rotation-revolution mixer (manufactured by Thinky Corporation) was charged with 3.5 parts by mass, in terms of solid content, of particulate (co)polymer b1, 1.5 parts by mass, in terms of solid content, of sodium polystyrene sulfonate (manufactured by Tosoh Finechem Corporation, product name "PS-100") as the water-soluble (co)polymer (A), 0.5 parts by mass, in terms of solid content, of carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., product name "Sunrose MAC (registered trademark) 350HC") as the water-soluble (co)polymer (B), 100 parts by mass of lithium iron phosphate (manufactured by Pulead Technology Industry, P600A, particle size 1 μm) as the positive electrode active material, and furnace black (manufactured by Imerys, Super 5 parts by mass of ammonium phosphate (P—Li) was added, and ion-exchanged water was further added to the mixture so as to obtain the solid content shown in Table 9. The mixture was stirred and mixed for 30 minutes and degassed to obtain an aqueous composition b1 for positive electrode.

[0151] [Examples b2 to b17, Comparative Examples b1 to b3] In each example, aqueous compositions b2 to b20 were obtained in the same manner as in Example b1, except that the types and compositions of the water-soluble (co)polymers (A) and (B) and the particulate (co)polymer were changed as shown in Tables 9 and 10.

[0152] <Preparation of Positive Electrode for Lithium-Ion Secondary Battery>

[0153] The resulting aqueous composition for a positive electrode was compression-molded using the method described above, and the electrode adhesion was evaluated using the resulting electrode. The electrode thus obtained was used as a secondary battery positive electrode. The aqueous composition for a positive electrode and the secondary battery positive electrode obtained as described above were used to evaluate various physical properties, as described below. The results are shown in Tables 9 and 10.

[0154] (Viscosity and hysteresis of aqueous composition for positive electrode) The viscosity and hysteresis of the obtained aqueous composition were measured by the above-mentioned method. 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 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

[0155] (Changes in viscosity and hysteresis of aqueous composition for positive electrode over time) The changes in viscosity and hysteresis of the aqueous composition for positive electrode over time were measured using the method described above. 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

[0156] (Stability of Agitated Slurry) The stability of the agitated slurry was measured by the method described above and judged according to the criteria described above.

[0157] (Initial electrode adhesion) The surface of the obtained electrode was visually observed, and the degree of peeling of the electrode layer was judged according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: No peeling B: No defects within the coating surface, but defects at the edges of the coating surface (less than 5 locations) C: No defects within the coating surface, but defects at the edges of the coating surface (5 locations or more) D: Defects within the coating surface

[0158] (Electrode Adhesion of Slurry After Stirring) After stirring using the method for stirring slurry stability, the slurry was used to prepare a positive electrode for a lithium ion secondary battery. The surface of the obtained electrode was visually observed, and the degree of peeling of the electrode layer was judged according to the following criteria. A rating of C or higher was considered pass, and D was considered fail. A: No peeling B: No defects within the coating surface, but defects at the edges of the coating surface (less than 5 locations) C: No defects within the coating surface, but defects at the edges of the coating surface (5 locations or more) D: Defects within the coating surface

[0159]

[0160] Details of each component in Tables 9 and 10 are shown below. Active material: lithium iron phosphate (manufactured by Pulead Technology Industry, product name "P600A", particle size 1 μm) Conductive additive: furnace black (manufactured by Imerys, Super P-Li) Water-soluble (co)polymer (A-1): sodium polystyrene sulfonate aqueous solution (manufactured by Tosoh Finechem, product name "PS-100", weight average molecular weight (Mw): 540,000, pH: 10.0) Water-soluble (co)polymer (A-2): sodium polystyrene sulfonate aqueous solution (manufactured by Tosoh Finechem, product name "PS-50", weight average molecular weight (Mw): 500,000, pH: 10.0) Water-soluble (co)polymer (A-3): sodium polystyrene sulfonate aqueous solution (manufactured by Tosoh Finechem, product name "PS-5", weight average molecular weight (Mw): 75,000, pH: 10.0) Water-soluble (co)polymer (A-7): Polystyrene sulfonic acid aqueous solution (Sigma-Aldrich, poly(4-styrene sulfonic acid) solution weight average molecular weight (Mw): 75,000, pH: 3.0) Water-soluble (co)polymer (B-1): Carboxymethyl cellulose (Nippon Paper Industries, product name "Sunrose MAC (registered trademark) 350HC", weight average molecular weight (Mw): 340,000, pH: 7.0) Water-soluble (co)polymer (B-2): Carboxymethyl cellulose (Dai-ichi Kogyo Seiyaku, product name "Cellogen WS-C", weight average molecular weight (Mw): 90,000, pH: 7.0)

[0161] The results of Examples b1 to b17 shown in Tables 9 and 10 indicate that aqueous compositions for lithium ion secondary battery positive electrodes, which comprise an active material, a water-soluble (co)polymer (A) containing structural units derived from a monomer having a sulfonic acid (salt) group, and a water-soluble (co)polymer (B) containing structural units having a carboxyl group (excluding those corresponding to the water-soluble (co)polymer (A)), in which the content of the structural units derived from the monomer having a sulfonic acid (salt) group is 25% by mass or more and 100% by mass or less relative to the total solids content of the water-soluble (co)polymer (A), exhibit improved stirred slurry stability and electrode adhesion of the slurry both initially and after stirring, compared to Comparative Examples b1 to b3. In particular, Comparative Example b1, which does not contain the water-soluble (co)polymer (B), exhibits inferior stirred slurry stability and electrode adhesion of the slurry after stirring. Furthermore, Comparative Example b2, which does not contain the water-soluble (co)polymer (A), exhibits inferior viscosity change and hysteresis after one day. Furthermore, it can be seen that Comparative Example b3, in which the content of the structural unit derived from a monomer having a sulfonic acid (salt) group is 25 mass% or less relative to the total solid content of the water-soluble (co)polymer (A), is inferior in viscosity change and hysteresis after one day, stability of the stirred slurry, and electrode adhesion of the slurry at the initial stage and after stirring.

[0162] Examples c1 to c11, Comparative Examples c1 to c3 Preparation Examples of Water-Soluble (Co)Polymers Preparation Example c1 As the water-soluble (co)polymer c1, an aqueous solution of sodium polystyrene sulfonate (manufactured by Tosoh Finechem Corporation, product name "PS-100", weight average molecular weight: 540,000, pH: 10.0) was used.

[0163] [Preparation Example c2] 667 parts by mass of ion-exchanged water and 75 parts by mass of sodium p-styrenesulfonate were added to a reactor, and the sodium p-styrenesulfonate was dissolved in ion-exchanged water. After nitrogen substitution, 25 parts by mass of methacrylic acid was added, and the internal temperature was raised to and maintained at 50°C. 1.05 parts by mass of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50; polymerization initiator) was dissolved in 10.5 parts by mass of ion-exchanged water and added to the reactor. After adding an aqueous solution of 2,2'-azobis(2-methylpropionamidine) dihydrochloride, the internal temperature was raised to 80°C and maintained at 80°C for 3 hours to complete the polymerization. Thereafter, the mixture was cooled to room temperature, and water was added so that the solids content was 10% by mass. Furthermore, 0.02% by mass of 2-methyl-4-isothiazolin-3-one (preservative) was added to the obtained aqueous solution of the water-soluble (co)polymer to obtain a water-soluble (co)polymer c2. The physical properties of the obtained water-soluble (co)polymer c2 are shown in Table 11.

[0164] [Preparation Examples c3 to c4] In each preparation example, water-soluble (co)polymers c3 and c4 were obtained in the same manner as in Preparation Example c2, except that the amounts of sodium p-styrenesulfonate and methacrylic acid were changed as shown in Table 11. The physical properties of the water-soluble (co)polymers c3 and c4 were as shown in Table 11.

[0165] Preparation Example c5 Carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., product name "Sunrose MAC (registered trademark) 350HC") was used as the water-soluble (co)polymer c5.

[0166] The abbreviations in Table 11 represent the following compounds: PNaSS: Polystyrenesulfonate sodium (manufactured by Tosoh Finechem Corporation, product name "PS-100", weight average molecular weight: 540,000, pH: 10.0) NaSS: Sodium p-styrenesulfonate MAA: Methacrylic acid CMC: Carboxymethylcellulose (manufactured by Nippon Paper Industries Co., Ltd., product name "Sunrose MAC (registered trademark) 350HC")

[0167] (Molecular Weight) The molecular weight of each of the water-soluble (co)polymers c1 to c4 was evaluated by the method using GPC described above. The molecular weight of the water-soluble (co)polymer c5 was evaluated by the method using GFC described above.

[0168] (pH) The pH of the water-soluble (co)polymers c1 to c4 is the pH of an aqueous solution of the water-soluble (co)polymer when the solid content is 5% by mass, and the pH of the water-soluble (co)polymer c5 is the pH of an aqueous solution of the water-soluble (co)polymer when the solid content is 1% by mass.

[0169] <Preparation Examples of Particulate (Co)polymers> [Preparation Example c6] 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 parts by mass of methyl methacrylate, and 0.05 parts by mass of 1,9-nonanediol dimethacrylate was then added, along with 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 dropwise 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 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 an opening of 74 μm to obtain particulate (co)polymer c1. The physical properties of the particulate (co)polymer c1 are shown in Table 12.

[0170] [Preparation Examples c7 to c11] Particulate (co)polymers c2 to c6 were obtained in the same manner as in Preparation Example c6, except that the monomer composition was changed as shown in Table 12. The physical properties of the particulate (co)polymers c2 to c6 were as shown in Table 12.

[0171] (Particle diameter) The particle diameters of the particulate (co)polymers c1 to c6 were measured by the method described above. The results are shown in Table 12.

[0172] (pH) The pH of the particulate (co)polymer is the pH of an aqueous dispersion of the particulate (co)polymer when the solid content is 20% by mass.

[0173] The abbreviations in Table 12 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, HEMA: hydroxyethyl methacrylate, 1,9-ND: 1,9-nonanediol dimethacrylate.

[0174] <Preparation of aqueous composition for positive electrode> [Example c1] 2.35 parts by mass, in terms of solid content, of particulate polymer c1, 0.75 parts by mass, in terms of solid content, of water-soluble (co)polymer c1, 0.75 parts by mass, in terms of solid content, of water-soluble (co)polymer c5, 100 parts by mass of lithium iron phosphate (P600A, manufactured by Pulead Technology Industry, particle size 1 μm) as a positive electrode active material, and 5 parts by mass of furnace black (Super P-Li, manufactured by Imerys) as a conductive additive were added to a planetary mixer equipped with a disperser, and ion-exchanged water was further added so as to achieve a solid content shown in Table 13. The mixture was stirred, mixed, and degassed for 30 minutes to obtain aqueous composition c1 for positive electrode.

[0175] [Examples c2 to c11, Comparative Examples c1 to c3] In each example, aqueous compositions c2 to c14 were obtained in the same manner as in Example c1, except that the types and compositions of the water-soluble (co)polymer and particulate (co)polymer were changed as shown in Table 13.

[0176] <Preparation of a Lithium-Ion Secondary Battery Positive Electrode> The aqueous composition for the positive electrode obtained was compression-molded using the method described above, and the resulting electrode was evaluated for cracking. The electrode thus obtained was used as a positive electrode for a secondary battery. The aqueous composition for the 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 Table 13.

[0177] (Viscosity and hysteresis of aqueous composition for positive electrode) The viscosity and hysteresis of the obtained aqueous composition were measured by the above-mentioned method. 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 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

[0178] (Changes in viscosity and hysteresis of aqueous composition for positive electrode over time) The changes in viscosity and hysteresis of the aqueous composition for positive electrode over time were measured using the method described above. 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

[0179] (Stability of Agitated Slurry) The stability of the agitated slurry was measured by the method described above and judged according to the criteria described above.

[0180] (Electrode Cracking) Electrode cracking was measured by the method described above and judged according to the criteria described above.

[0181] (Electrolyte-Insoluble Content) The electrolyte-insoluble content was measured by the method described above and judged according to the criteria described above.

[0182]

[0183] Details of each component in Table 13 are shown below. Active material: lithium iron phosphate (manufactured by Pulead Technology Industry, product name "P600A", particle size 1 μm) Conductive additive: furnace black (manufactured by Imerys, Super P-Li)

[0184] The results of Examples c1 to c11 shown in Table 13 indicate that aqueous compositions for lithium ion secondary battery positive electrodes, which include a particulate (co)polymer and a water-soluble (co)polymer containing structural units derived from a monomer having a sulfonic acid (salt) group, and in which the content of the structural units derived from the monomer having the sulfonic acid (salt) group is 25% by mass or more and 100% by mass or less relative to the total amount of the water-soluble (co)polymer, suppress electrode cracking compared to Comparative Examples c1 to c3. Furthermore, Comparative Example c2, which does not include a water-soluble (co)polymer containing structural units derived from a monomer having a sulfonic acid (salt) group, and Comparative Example c3, which includes a water-soluble (co)polymer containing less than 25% by mass of structural units derived from a monomer having a sulfonic acid (salt) group, are found to have poor stability during slurry stirring.

[0185] The disclosures of Japanese Patent Application No. 2023-115758 filed on July 14, 2023, Japanese Patent Application No. 2024-053460 filed on March 28, 2024, Japanese Patent Application No. 2024-053275 filed on March 28, 2024, and Japanese Patent Application No. 2024-053283 filed on March 28, 2024 are incorporated herein by reference in their entirety. In addition, all documents, patent applications, and technical standards described in the specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually indicated to be incorporated by reference.

[0186] Examples of means for solving the problems of the aqueous compositions for lithium ion secondary battery positive electrodes, the lithium ion secondary battery positive electrodes, and the lithium ion secondary batteries according to the second to fourth embodiments will be described below.

[0187] <a1> 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% 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 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% or more and 15.00 mass% or less, relative to the total amount of the particulate (co)polymer. <a2> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <a1>, 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. <a3> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <a1> or <a2>, wherein the structural units derived from the ethylenically unsaturated carboxylic acid (salt) include a monobasic acid (salt) and / or a dibasic acid (salt). <a4> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <a3>, 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. <a5> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a4>, 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.<a6> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a5>, wherein the ratio (O / bound COOH) of the abundance rate (mass %) of oxygen atoms on the surface of the conductive additive to the content rate (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. <a7> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a6>, wherein the particulate (co)polymer further contains a structural unit derived from a crosslinkable monomer. <a8> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a7>, wherein the abundance rate of oxygen atoms on the surface of the conductive additive is 0.5 mass % or more and 2.0 mass % or less. <a9> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a8>, wherein the content of the conductive aid is 3.5% by mass or more and 10.0% by mass or less, relative to the total amount of non-volatile components in the aqueous composition. <a10> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a9>, further comprising a water-soluble (co)polymer. <a11> The aqueous composition for a lithium ion secondary battery positive electrode according to <a10>, wherein the water-soluble (co)polymer contains a structural unit derived from a monomer having a sulfonic acid (salt) group. <a12> The aqueous composition for a lithium ion secondary battery positive electrode according to <a11>, 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, relative to the total solids content of the water-soluble (co)polymer. <a13> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a12>, further comprising a preservative. <a14> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a13>, further comprising an active material, wherein the active material comprises a lithium phosphate compound. <a15> A lithium ion secondary battery positive electrode, comprising the aqueous composition for a lithium ion secondary battery positive electrode according to any one of <a1> to <a14>.<a16> A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to <a15>.

[0188] <b1> An aqueous composition for a positive electrode of a lithium ion secondary battery, comprising: an active material; a water-soluble (co)polymer (A) containing structural units derived from a monomer having a sulfonic acid (salt) group; and a water-soluble (co)polymer (B) (excluding those falling under the category of the water-soluble (co)polymer (A)) containing structural units having a carboxyl group, wherein the content of the structural units 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 (A). <b2> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <b1>, wherein the water-soluble (co)polymer (A) has a pH of 4.0 or more and 11.0 or less. <b3> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <b2> or <b3>, wherein the water-soluble (co)polymer (B) has a pH of 4.0 or more and 11.0 or less. <b4> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b3>, wherein the content ratio of the water-soluble (co)polymer (A) to the water-soluble (co)polymer (B) is 1:9 to 9:1, calculated as solids. <b5> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b4>, further comprising a conductive aid. <b6> The aqueous composition for a lithium ion secondary battery positive electrode according to <b5>, 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. <b7> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b6>, further comprising a particulate (co)polymer. <b8> The aqueous composition for a lithium ion secondary battery positive electrode according to <b7>, wherein the particulate (co)polymer contains structural units derived from an ethylenically unsaturated carboxylic acid (salt), and the content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10 mass % or more and 15.00 mass % or less in total, relative to the total amount of the particulate (co)polymer. <b9> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b8>, further containing a preservative. <b10> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b9>, wherein the active material contains a lithium phosphate-based compound.<b11> A positive electrode for a lithium ion secondary battery, comprising the aqueous composition for a lithium ion secondary battery positive electrode according to any one of <b1> to <b10>. <b12> A lithium ion secondary battery, comprising the positive electrode for a lithium ion secondary battery according to <b11>.

[0189] <c1> An aqueous composition for a positive electrode of a lithium ion secondary battery, comprising: a particulate (co)polymer; and a water-soluble (co)polymer containing structural units derived from a monomer having a sulfonic acid (salt) group, wherein the content of the structural units 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 the water-soluble (co)polymer. <c2> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <c1>, wherein the particulate (co)polymer contains structural units derived from a dibasic acid (salt) monomer. <c3> The aqueous composition for a positive electrode of a lithium ion secondary battery according to <c1> or <c2>, wherein the particulate (co)polymer contains structural units derived from a crosslinkable monomer. <c4> The aqueous composition for a positive electrode of a lithium ion secondary battery according to any one of <c1> to <c3>, wherein the water-soluble (co)polymer has a pH of 4.0 or more and 11.0 or less. <c5> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <c1> to <c4>, further comprising a conductive aid. <c6> The aqueous composition for a lithium ion secondary battery positive electrode according to <c5>, wherein the content of the conductive aid is 3.5 mass % or more and 10.0 mass % or less, based on the total amount of non-volatile components in the aqueous composition. <c7> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <c1> to <c6>, further comprising a preservative. <c8> The aqueous composition for a lithium ion secondary battery positive electrode according to any one of <c1> to <c7>, further comprising an active material, wherein the active material comprises a lithium phosphate compound. <c9> A lithium ion secondary battery positive electrode comprising the aqueous composition for a lithium ion secondary battery positive electrode according to any one of <c1> to <c8>. <c10> A lithium ion secondary battery comprising the lithium ion secondary battery positive electrode according to <c9>.

Claims

1. An active material; a water-soluble polymer (A) containing a structural unit corresponding to a monomer having a sulfonic acid group; Including, the total amount of structural units corresponding to the monomers having a sulfonic acid group in the water-soluble polymer (A) is 25% by mass or more and 100% by mass or less with respect to the total amount of the water-soluble polymer (A), The weight average molecular weight of the water-soluble polymer (A) is 200,000 or more and 1,000,000 or less. An aqueous composition for use in a positive electrode of a lithium ion secondary battery.

2. A water-soluble polymer (B) containing a structural unit having a carboxyl group (however, excluding those corresponding to the water-soluble polymer (A)). ) The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

3. Further comprising a conductive aid, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

4. The content of the conductive additive in the composition 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 composition; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 3 .

5. Further comprising a particulate polymer, The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

6. The particulate polymer contains a (meth)acrylic acid ester monomer. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 5 .

7. Further comprising a conductive additive and a particulate polymer, 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 polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid (salt), the content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10% by mass or more and 15.00% by mass or less in total relative to the total amount of the particulate polymer; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

8. the particulate polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid (salt), the content of the structural units derived from the ethylenically unsaturated carboxylic acid (salt) is 0.10% by mass or more and 15.00% by mass or less in total relative to the total amount of the particulate polymer; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 5 .

9. The particulate polymer contains a monobasic acid (salt) and / or a dibasic acid (salt). The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 8 .

10. 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 polymer is 0.030 to 10.00; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 7 .

11. the ratio (O / bound COOH) of the abundance ratio (mass%) of oxygen atoms on the surface of the conductive additive and the content ratio (bound COOH) of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) bound to the particulate polymer relative to the total amount of the monomer derived from the ethylenically unsaturated carboxylic acid (salt) contained in the particulate polymer is 0.0050 to 0.1000; The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 7 .

12. The particulate polymer contains a crosslinkable monomer. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 5 .

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

14. The active material is a lithium phosphate compound. The aqueous composition for a positive electrode of a lithium ion secondary battery according to claim 1 .

15. 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 14.

16. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 15.

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