JP2019133908A - Lithium ion secondary battery positive electrode - Google Patents

Abstract

To provide a lithium ion secondary battery positive electrode obtained by means of an aqueous process that uses an aqueous binder.SOLUTION: In a lithium ion secondary battery positive electrode, a mixture layer includes, as a binder, polymer particles that contain: a structural unit (A) derived from a compound that is represented by formula (I); and a structural unit (B) derived from at least one type of compound selected from compounds represented by formula (II), compounds represented by formula (III), and unsaturated dibasic acids. The content of a structural unit (A) of the total structural units of the polymer particles is 50 mass% or more and 99.9 mass% or less, and the content of a structural unit (B) is 0.1 mass% or more and 20 mass% or less.SELECTED DRAWING: None

Description

  The present disclosure relates to a positive electrode for a lithium ion secondary battery.

  Lithium ion batteries have a higher energy density per weight and volume than lead-acid batteries and nickel metal hydride batteries, which contributes to the reduction in size and weight of on-board electronic devices. In recent years, hybrid vehicles and electric vehicles have become widespread as efforts toward zero emission of automobiles, and improving the performance of lithium-ion batteries is an important key for improving fuel efficiency and extending mileage.

  A lithium ion secondary battery is generally composed of members such as a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator. Of these, the positive electrode is produced by a non-aqueous process in which the positive electrode active material and conductive material are dispersed in an organic solvent such as N-methylpyrrolidone together with a binder to prepare a positive electrode mixture, which is applied to the current collector surface, and the solvent is volatilized. Has been. On the other hand, the negative electrode is manufactured by an aqueous process in which an aqueous emulsion of styrene-butadiene copolymer rubber manufactured by an emulsion polymerization method is used as a binder, and an aqueous carboxymethyl cellulose solution having a thickening action is mixed with a negative electrode active material together with a thickener. (For example, Patent Document 1).

Japanese Patent No. 3101775

In recent years, since the electrode manufacturing cost can be reduced by reducing the use of organic solvents, and the environmental load and working environment can be improved, it has been desired to employ an aqueous process such as a negative electrode for the positive electrode.
However, when styrene-butadiene copolymer rubber (SBR), which is a water-based binder mainly employed in the negative electrode, is applied to the positive electrode as it is, the polymer has poor electrochemical resistance and the charge / discharge characteristics tend to deteriorate.

  In one or a plurality of embodiments of the present disclosure, the lithium ion obtained by an aqueous process using an aqueous binder having charge / discharge characteristics equivalent to a positive electrode for a lithium ion secondary battery produced by a conventional non-aqueous process is provided. A positive electrode for an ion secondary battery and a lithium ion secondary battery are provided.

In one aspect, the present disclosure is a positive electrode for a lithium ion secondary battery including a current collector and a composite material layer formed on the current collector, wherein the composite material layer has the following formula (I ) Derived from the compound represented by formula (II) and at least one selected from the compound represented by formula (II) below, the compound represented by formula (III) below, and an unsaturated dibasic acid The polymer particle containing the structural unit (B) derived from the compound is used as a binder, and the content of the structural unit (A) in all the structural units of the polymer particle is 50% by mass or more and 99.9% by mass or less. The positive electrode for a lithium ion secondary battery (hereinafter referred to as “the positive electrode of the present disclosure”), wherein the content of the structural unit (B) in all the structural units of the polymer particles is 0.1% by mass or more and 20% by mass or less. Say).
[In the formula (I), R ¹ represents a hydrogen atom or a methyl group. R ² represents at least one selected from a linear or branched alkyl group having 1 to 6 carbon atoms and —CH ₂ OR ³ . R ³ represents a linear or branched alkyl group having 4 to 6 carbon atoms. X represents —O— or —NH—. ]
[In the formula (II), R ¹ represents a hydrogen atom or a methyl group, and M represents a hydrogen atom or a cation. ]
[In Formula (III), R ¹ represents a hydrogen atom or a methyl group, and X represents —O— or —NH—. R ⁴ is — (CH ₂ ) _n OH, —R ⁵ SO ₃ M, —R ⁶ N (R ⁷ ) (R ⁸ ), and —R ⁶ N ⁺ (R ⁷ ) (R ⁸ ) (R ⁹ ). Y ^- represents at least one member selected from. n is 1 or more and 4 or less. R ⁵ represents a linear or branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. R ⁶ represents a linear or branched alkylene group having 1 to 4 carbon atoms. R ⁷ and R ⁸ are the same or different and represent a linear or branched alkyl group having 1 to 3 carbon atoms. R ⁹ represents a linear or branched alkyl group having 1 to 3 carbon atoms. Y ⁻ represents an anion. ]

  In one aspect, the present disclosure provides a method for producing a positive electrode according to the present disclosure, which includes a step of applying and drying a positive electrode active material and an aqueous slurry containing polymer particles used in the positive electrode according to the present disclosure on a current collector. It is related with the manufacturing method of the positive electrode for lithium ion secondary batteries containing.

  In one aspect, the present disclosure relates to a lithium ion secondary battery including the positive electrode of the present disclosure.

  According to the present disclosure, in one aspect, the lithium ion secondary battery obtained by an aqueous process using an aqueous binder having charge / discharge characteristics equivalent to that of a positive electrode for a lithium ion secondary battery produced by a conventional non-aqueous process. The effect that the positive electrode for secondary batteries can be provided can be produced.

FIG. 1 is a graph showing the results of charge / discharge tests of Example 1, Comparative Example 1 and Reference Example 1. FIG. 2 is a graph showing the results of the cyclic voltammetry test of Example 1. FIG. 3 is a graph showing the results of the cyclic voltammetry test of Comparative Example 1. FIG. 4 is a graph showing the results of the cyclic voltammetry test in Reference Example 1.

  In the present disclosure, a positive electrode having charge / discharge characteristics equivalent to a positive electrode for a lithium ion secondary battery produced by a conventional non-aqueous process can be obtained by an aqueous process by including predetermined polymer particles in the positive electrode mixture layer. Based on this knowledge.

  That is, in one aspect, the present disclosure is a positive electrode for a lithium ion secondary battery including a current collector and a composite material layer formed on the current collector, wherein the composite material layer has the above formula. At least selected from the structural unit (A) derived from the compound represented by (I), the compound represented by the above formula (II), the compound represented by the above formula (III), and the unsaturated dibasic acid The polymer particle containing the structural unit (B) derived from one compound is included as a binder, and the content of the structural unit (A) in all the structural units of the polymer particle is 50% by mass or more and 99.9% by mass or less. And the content of the structural unit (B) in all the structural units of the polymer particles is from 0.1% by mass to 20% by mass with respect to the positive electrode for a lithium ion secondary battery.

The details of the mechanism of the effect of the present disclosure are not clear, but the following is presumed.
In the present disclosure, by including predetermined polymer particles in the positive electrode mixture layer, it is possible to withstand a high voltage atmosphere in a charge / discharge field when the electrode of the present disclosure is used in a lithium ion secondary battery. Thereby, it is thought that charging / discharging becomes possible, suppressing the fall of a battery characteristic.
However, these are estimations, and the present disclosure is not limited to these mechanisms.

  Hereinafter, the positive electrode for a lithium ion secondary battery of the present disclosure will be specifically described.

[Composite layer]
The composite material layer used for the positive electrode of the present disclosure includes a binder. The binder plays a role of keeping the active material on the surface of the current collector. In one or a plurality of embodiments, the binder may be an aqueous binder. An aqueous binder refers to a binder that is dispersed in an aqueous medium. Examples of the aqueous medium include water such as distilled water, ion exchange water, and ultrapure water.

(Polymer particles)
In the present disclosure, the polymer particles contained in the composite material layer as a binder include the structural unit (A) and the structural unit (B). Each of the structural units (A) and (B) is a structural unit derived from a monofunctional monomer of a compound described later. A monofunctional monomer refers to a monomer having one unsaturated bond.

<Structural unit (A)>
The structural unit (A) is a structural unit derived from a compound represented by the following formula (I) (hereinafter also referred to as “monomer (A)”). A monomer (A) may be used individually by 1 type, and may be used together 2 or more types.

In formula (I), R ¹ represents a hydrogen atom or a methyl group from the viewpoint of ease of synthesis. R ² is at least one selected from linear or branched alkyl groups having 1 to 6 carbon atoms and —CH ₂ OR ³ from the viewpoints of charge / discharge characteristics, affinity for an electrolytic solution, and binder properties. A linear or branched alkyl group having 1 to 6 carbon atoms is more preferable, and a linear or branched alkyl group having 1 to 4 carbon atoms is more preferable. R ³ represents a linear or branched alkyl group having 4 to 6 carbon atoms. X represents —O— or —NH—. In the present disclosure, R ¹ in Formula (I), Formula (II), and Formula (III), which will be described later, is independent.

  Examples of the monomer (A) include methyl (meth) acrylate, ethyl acrylate, normal propyl (meth) acrylate, isopropyl (meth) acrylate, normal butyl (meth) acrylate, isobutyl (meth) acrylate, and secondary butyl (meth) acrylate. Alkyl ester (meth) acrylates such as tertiary butyl (meth) acrylate, normal pentyl (meth) acrylate, and normal hexyl (meth) acrylate; cycloalkyl group-containing esters (meth) acrylates such as cyclohexyl (meth) acrylate; methyl ( (Meth) acrylamide, ethyl (meth) acrylamide, normal propyl (meth) acrylamide, isopropyl (meth) acrylamide, normal butyl (meth) acrylic Monofunctional (meth) such as ruamide, isobutyl (meth) acrylamide, secondary butyl (meth) acrylamide, tertiary butyl (meth) acrylamide, normal pentyl (meth) acrylamide, normal hexyl (meth) acrylamide, and cyclohexyl (meth) acrylamide Examples thereof include one or a combination of two or more selected from acrylamide. Among these, methyl methacrylate (MMA), ethyl methacrylate (EMA), normal butyl methacrylate (BMA), ethyl acrylate (EA), and normal butyl acrylate (from the viewpoints of charge / discharge characteristics, affinity for the electrolyte, and binder properties One or a combination of two or more selected from BA) is preferred. In the present disclosure, (meth) acrylate means methacrylate or acrylate, and (meth) acrylamide means methacrylamide or acrylamide.

  In the present disclosure, the content of the structural unit (A) in all the structural units of the polymer particles is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more from the viewpoint of ease of synthesis. Preferably, 80% by mass or more is more preferable, 90% by mass or more is more preferable, and from the same viewpoint, it is 99.9% by mass or less, preferably 99.5% by mass or less, and preferably 99% by mass or less. More preferably, 98 mass% or less is still more preferable, and 97 mass% or less is still more preferable. More specifically, the content of the structural unit (A) is 50% by mass to 99.9% by mass, preferably 60% by mass to 99.5% by mass, and more preferably 70% by mass to 99% by mass. The following is more preferable, 80 mass% or more and 98 mass% or less is still more preferable, and 90 mass% or more and 97 mass% or less is still more preferable. The content of the structural unit (A) can be determined by a known analysis method or analyzer. When a structural unit (A) consists of a structural unit derived from 2 or more types of monomers (A), content of a structural unit (A) says those total content.

<Structural unit (B)>
The structural unit (B) is a compound represented by the following formula (II) (hereinafter also referred to as “monomer (B1)”), and a compound represented by the following (III) (hereinafter also referred to as “monomer (B2)”). ) And unsaturated dibasic acid (hereinafter also referred to as “monomer (B3)”), a structural unit derived from at least one compound (hereinafter also referred to as monomer (B)), charge / discharge characteristics, electrolyte solution The structural unit (B) is preferably a structural unit derived from the monomer (B1) from the viewpoints of affinity to the binder and physical properties of the binder. A monomer (B) may be used individually by 1 type, and may be used together 2 or more types.

In the formula (II), R ¹ is a hydrogen atom or a methyl group from the viewpoint of ease of synthesis. M is a hydrogen atom or a cation from the viewpoint of charge / discharge characteristics, dispersion stability, and binder physical properties. The cation is preferably at least one of alkali metal ions and ammonium ions, more preferably at least one selected from ammonium ions, lithium ions, sodium ions and potassium ions from the viewpoints of charge / discharge characteristics, dispersion stability and binder physical properties. More preferably, at least one of lithium ions and sodium ions is used.

  When M in the formula (II) is a cation, the monomer (B1) is, for example, a monomer in which M is a hydrogen atom in an alkali (ammonia, sodium hydroxide, lithium hydroxide, potassium hydroxide, etc.) It may be a sum, or it may be neutralized after becoming a structural unit of a polymer obtained by polymerizing a monomer in which M is a hydrogen atom. From the viewpoint of controlling the polymerization reaction, it is preferably neutralized after becoming a polymer structural unit after polymerization.

  Examples of the monomer (B1) include one or a combination of two or more selected from acrylic acid, methacrylic acid, and salts thereof. Examples of the salt include at least one selected from an ammonium salt, a sodium salt, a lithium salt, and a potassium salt.

In the formula (III), R ¹ represents a hydrogen atom or a methyl group, and X represents —O— or —NH—. R ⁴ is — (CH ₂ ) _n OH, —R ⁵ SO ₃ M, —R ⁶ N (R ⁷ ) (R ⁸ ), and —R ⁶ N ⁺ (R ⁷ ) (R ⁸ ) (R ⁹ ). Y ^- represents at least one member selected from. n represents an average added mole number and is 1 or more and 4 or less. R ⁵ represents a linear or branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. Examples of the cation include the same as the cation of M in the above formula (II). In the present disclosure, M in formula (II) and formula (III) is independent of each other. R ⁶ represents a linear or branched alkylene group having 1 to 4 carbon atoms. R ⁷ and R ⁸ are the same or different and represent a linear or branched alkyl group having 1 to 3 carbon atoms. R ⁹ represents a linear or branched alkyl group having 1 to 3 carbon atoms. Y ⁻ represents an anion. Examples of the anion include halide ions such as chloride ions, bromide ions, and fluoride ions; sulfate ions; phosphate ions;

  When M in the formula (III) is a cation, the monomer (B2) may be, for example, a monomer in which M is a hydrogen atom neutralized with an alkali, or M is a hydrogen atom. It may be neutralized after becoming a structural unit of a polymer obtained by polymerizing a certain monomer. From the viewpoint of controlling the polymerization reaction, it is preferably neutralized after becoming a polymer structural unit after polymerization.

When X in the formula (III) is —O—, the monomer (B2) is a hydroxyl group-containing ester (hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate) or the like from the viewpoint of ease of synthesis. Meth) acrylate; and nitrogen atom-containing ester (meth) acrylate such as dimethylaminoethyl (meth) acrylate, dimethylaminopropyl (meth) acrylate, and trimethylammonioethyl (meth) acrylate; At least one selected from hydroxyethyl (meth) acrylate and hydroxypropyl (meth) acrylate is preferable, and at least one of hydroxyethyl methacrylate and hydroxyethyl acrylate is more preferable.
When X in the formula (III) is -NH-, examples of the monomer (B2) include 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

  Monomer (B3) is an unsaturated dibasic acid, and from the viewpoint of ease of synthesis, for example, at least one selected from unsaturated dibasic acids having 4 to 12 carbon atoms and salts thereof may be mentioned, The number of carbon atoms is preferably 4 or more and 8 or less, and more preferably 4 or more and 6 or less.

  Examples of the monomer (B3) include maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, 2-pentenedioic acid, 3-hexenedioic acid, and salts thereof from the viewpoint of ease of synthesis. At least one selected from maleic acid, fumaric acid, itaconic acid, and salts thereof is preferable, and at least one of maleic acid and salts thereof is more preferable.

  When the monomer (B3) is a salt of an unsaturated dibasic acid, the salt is preferably at least one selected from ammonium salt, lithium salt, sodium salt and potassium salt from the viewpoint of dispersion stability and binder physical properties, At least one of a lithium salt and a sodium salt is more preferable.

  The unsaturated dibasic acid salt of the monomer (B3) may be one obtained by neutralizing an unsaturated dibasic acid with an alkali, or neutralized after polymerization using the unsaturated dibasic acid. May be used. From the viewpoint of controlling the polymerization reaction, it is preferably neutralized after becoming a polymer structural unit after polymerization.

  The content of the structural unit (B) in all the structural units of the polymer particles in the present disclosure is 0.1% by mass or more and 0.5% by mass from the viewpoints of charge / discharge characteristics, dispersion stability, and binder physical properties. Or more, preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 3% by mass or more, and from the same viewpoint, it is 20% by mass or less, preferably 15% by mass or less. 10 mass% or less is more preferable, 8 mass% or less is still more preferable, and 6 mass% or less is still more preferable. More specifically, the content of the structural unit (B) is 0.1 to 20% by mass, preferably 0.5 to 15% by mass, and preferably 1 to 10% by mass. The following is more preferable, 2% by mass or more and 8% by mass or less is further preferable, and 3% by mass or more and 6% by mass or less is more preferable. The content of the structural unit (B) can be determined by a known analysis method or analyzer. When a structural unit (B) consists of a structural unit derived from 2 or more types of monomers (B), content of a structural unit (B) says those total content.

  The ratio (A / B) of the content of the structural unit (A) to the content of the structural unit (B) in the polymer particles in the present disclosure is 500 or less from the viewpoint of charge / discharge characteristics, dispersion stability, and binder physical properties. Preferably, 100 or less is more preferable, 50 or less is more preferable, and from the viewpoint of dispersion stability and binder physical properties, 5 or more is preferable, 10 or more is more preferable, and 20 or more is more preferable. More specifically, the content ratio (A / B) is preferably 5 or more and 500 or less, more preferably 10 or more and 100 or less, and still more preferably 20 or more and 50 or less.

  The polymer particles in the present disclosure may contain other structural units other than the structural unit (A) and the structural unit (B) as long as the effects of the present disclosure are not impaired. The other structural unit is a structural unit (hereinafter also referred to as “structural unit (C)”) of a monomer copolymerizable with the monomers (A) and (B) (hereinafter also referred to as “monomer (C)”). I just need it. A monomer (C) may be used individually by 1 type, and may be used together 2 or more types. Examples of the monomer (C) include a crosslinkable monomer.

  The total content of the structural units (A) and (B) in all the structural units of the polymer particles in the present disclosure is preferably 80% by mass or more from the viewpoints of charge / discharge characteristics, affinity for an electrolytic solution, and binder physical properties, 90 mass% or more is more preferable, and 100 mass% is still more preferable.

The polymer particle in this indication does not contain the structural unit derived from a crosslinkable monomer in one or some embodiment.
The polymer particle in this indication contains the structural unit derived from a crosslinking | crosslinked monomer in other one or some embodiment. As a crosslinkable monomer, the crosslinkable monomer which has 2 or more of vinyl groups is mentioned, Specifically, polyfunctional (meth) acrylate is mentioned. When the polymer particle in the present disclosure includes a polyfunctional (meth) acrylate-derived structural unit, the content of the polyfunctional (meth) acrylate-derived structural unit in all the structural units of the polymer particle in the present disclosure is one or more. In the embodiment, from the viewpoint of battery characteristics, it is preferably 2% by mass or less, more preferably 1% by mass or less, and still more preferably less than 0.5% by mass. In addition, the content of the structural unit derived from the polyfunctional (meth) acrylate in all the structural units of the polymer particles in the present disclosure is the structural unit (A) and the structural unit (A) in the one or more embodiments from the viewpoint of battery characteristics. 2 mol% or less is preferable with respect to the total number of moles of the structural unit (B), more preferably 1 mol% or less, and even more preferably 0.5 mol% or less.

[Method for producing polymer particles]
The polymer particle in this indication can be manufactured by copolymerizing a monomer (A), a monomer (B), and another monomer (C) as needed, for example. That is, this indication is related with a manufacturing method of polymer particles including a polymerization process which polymerizes a monomer mixture containing monomer (A) and monomer (B), and monomer (C) as needed. Examples of the polymerization method include known polymerization methods such as an emulsion polymerization method, a solution polymerization method, a suspension polymerization method, and a bulk polymerization method, and the emulsion polymerization method is preferred from the viewpoint of ease of production of the polymer.

  In the present disclosure, the content of the structural unit (A) in all the structural units of the polymer particles can be regarded as a ratio of the usage amount of the monomer (A) to the total amount of the monomer used for polymerization. The content of the structural unit (B) in all the structural units of the polymer particles can be regarded as a ratio of the amount of the monomer (B) used relative to the total amount of monomers used for polymerization. The ratio (A / B) of the content of the structural unit (A) to the structural unit (B) is regarded as the ratio of the usage amount of the monomer (A) to the usage amount of the monomer (B) in the total amount of monomers used for polymerization. Can do. The total content of the structural unit (A) and the structural unit (B) in all the structural units of the polymer particles is regarded as a ratio of the total usage amount of the monomer (A) and the monomer (B) to the total amount of monomers used for polymerization. Can do. The content of the structural unit (C) in the polymer particles can be regarded as a ratio of the amount of the monomer (C) used to the total number of moles of the monomers (A) and (B) used for the polymerization.

  Examples of the emulsion polymerization method include a known method using an emulsifier and a method substantially not using an emulsifier, a so-called soap-free emulsion polymerization method. From the viewpoint of charge / discharge characteristics and reduction of ionic resistance of the positive electrode, the soap-free method is used. An emulsion polymerization method is preferred. Examples of the polymer particles in the present disclosure include a polymer obtained by emulsion polymerization, preferably soap-free emulsion polymerization, of a monomer mixture containing the monomer (A), the monomer (B) and, if necessary, another monomer (C). Particles.

  When using an emulsifier in the polymerization step, the emulsifier is preferably a water-soluble emulsifier from the viewpoint of polymerization stability. Examples of the water-soluble emulsifier include at least one surfactant selected from anionic surfactants, nonionic surfactants and reactive surfactants from the viewpoint of polymerization stability. From the viewpoint of suppression, a reactive surfactant is preferable. A reactive surfactant refers to a surfactant that is incorporated into a polymer during polymerization.

  Examples of the anionic surfactant include alkyl aryl sulfonates such as sodium dodecylbenzene sulfonate; alkyl sulfonate esters such as sodium lauryl sulfonate; alkyl sulfate esters such as sodium lauryl sulfate; polyoxyethylene lauryl ether And anionic surfactants having a polyoxyethylene group such as sodium sulfate and sodium polyoxyethylene nonylphenyl ether sulfate.

  Examples of the nonionic surfactant include polyoxyethylene alkylphenyl ethers such as polyoxyethylene nonylphenyl ether; polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether; and the like.

  Examples of the reactive surfactant include reactive emulsifiers having a vinyl polymerizable double bond in the molecule, and specific examples include polyoxyalkylene alkenyl ethers.

  The amount of the emulsifier used in the emulsion polymerization is preferably 0.05% by mass or less, more preferably 0.02% by mass or less, and more preferably 0.01% by mass or less, based on the total amount of monomers, from the viewpoint of suppressing a decrease in binding properties. Is more preferable, and substantially 0% by mass is even more preferable. In the present disclosure, the amount of the emulsifier used in the emulsion polymerization can be the amount of the surfactant used in the polymerization step.

  In the polymerization step, a polymerization initiator can be used. As the polymerization initiator, a water-soluble polymerization initiator is preferable from the viewpoint of polymerization stability. Examples of the water-soluble polymerization initiator include, from the viewpoint of polymerization stability, persulfates such as potassium persulfate and ammonium persulfate; peroxides such as hydrogen peroxide and t-butyl hydroperoxide; and the like. Persulfate is preferable, and ammonium persulfate is more preferable.

  The amount of the polymerization initiator used in the polymerization step is preferably 0.01 mol% or more, more preferably 0.05 mol% or more, still more preferably 0.1 mol% or more, and more preferably 5 mol%, based on the total amount of monomers. % Or less is preferable, 3 mol% or less is more preferable, and 1 mol% or less is still more preferable.

  In the polymerization step, water such as ion exchange water can be used as a solvent. The usage-amount of the water in the said superposition | polymerization process can be 40 mass parts or more and 1500 mass parts or less (6.25-71.4 mass% in polymerization solid content) with respect to 100 mass parts of monomer whole quantity, for example.

  In the polymerization step, a reducing agent that can be used in combination with a polymerization initiator can be used. Examples of the reducing agent include sulfites and pyrosulfites.

  In the polymerization step, a chain transfer agent can be used. As the chain transfer agent, a known chain transfer agent can be used. For example, isopropyl alcohol, n-dodecyl mercaptan, octyl mercaptan, tert-butyl mercaptan, thioglycolic acid, thiomalic acid, thiosalicylic acid, mercaptoethanol, etc. A mercapto compound is mentioned.

  What is necessary is just to set suitably as polymerization conditions according to the kind of polymerization initiator, monomer, solvent, etc. to be used. For example, the polymerization reaction can be performed in a temperature range of 60 to 100 ° C. in a nitrogen atmosphere, and the polymerization time can be set to 0.5 to 20 hours, for example.

  The arrangement of each structural unit constituting the polymer particle in the present disclosure may be random, block, or graft. The composition analysis of the polymer can be performed by, for example, NMR spectrum, UV-vis spectrum, IR spectrum, affinity chromatography and the like.

  The average particle size of the polymer particles in the present disclosure is preferably 0.2 μm or more, more preferably more than 0.3 μm, and more preferably 1 μm or less from the viewpoints of charge / discharge characteristics, affinity for an electrolytic solution, and binder properties. 0.9 μm or less is more preferable, 0.8 μm or less is more preferable, and less than 0.7 μm is still more preferable. More specifically, the average particle size of the polymer particles is preferably 0.2 μm or more and 1 μm or less, more preferably more than 0.2 μm and 0.9 μm or less, still more preferably more than 0.2 μm and 0.8 μm or less, and more preferably 0.2 μm. It is more preferably less than 0.7 μm and even more preferably more than 0.3 μm and less than 0.7 μm.

  The content of the polymer particles in the composite layer in the present disclosure is preferably 0.5% by mass or more, more preferably 1% by mass or more, and further preferably 1.5% by mass or more from the viewpoints of binding properties and battery capacity. From the same viewpoint, it is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 5% by mass or less. More specifically, the content of the polymer particles in the composite material layer is preferably 0.5% by mass to 15% by mass, more preferably 1% by mass to 10% by mass, and more preferably 1.5% by mass to 5%. A mass% or less is more preferable.

  The surface tension of the polymer particle dispersion in which the polymer particles in the present disclosure are dispersed in an aqueous medium is preferably 55 mN / m or more, more preferably 60 mN / m or more, and 72 mN / m from the viewpoint of improving binder physical properties and battery characteristics. The following is preferred. The surface tension can be measured by the method described in the examples. More specifically, the surface tension of the polymer particle dispersion is preferably 55 mN / m or more and 72 mN / m or less, and more preferably 60 mN / m or more and 72 mN / m or less.

  The glass transition point (Tg) of the polymer particles in the present disclosure is preferably −30 ° C. or higher, more preferably −20 ° C. or higher, from the viewpoints of binding properties and battery characteristics, and similar viewpoints. Therefore, 30 ° C. or lower is preferable, 25 ° C. or lower is more preferable, and 20 ° C. or lower is still more preferable. More specifically, the Tg of the polymer particles is preferably −30 ° C. or higher and 30 ° C. or lower, more preferably −20 ° C. or higher and 25 ° C. or lower, and further preferably −20 ° C. or higher and 20 ° C. or lower.

  As one embodiment of the composite material layer in the present disclosure, for example, one containing the above-described polymer particles, a positive electrode active material, and optional components (for example, a conductive material and a thickener) added as necessary. Can be mentioned. The mass ratio of each component contained in the composite material layer can be arbitrarily adjusted according to the suitability of the battery.

(Positive electrode active material)
The positive electrode active material may be any active material capable of occluding and releasing lithium and capable of charge / discharge reaction. For example, lithium such as lithium iron phosphate, LiCoO ₂ , LiNiO ₂ , Li ₂ MnO ₄ Metal composite oxide is mentioned. These compounds may be partially element-substituted. The average particle diameter of the positive electrode active material can be, for example, 2 μm or more and 40 μm or less.

  In the present disclosure, the content of the positive electrode active material in the composite material layer is preferably 80% by mass or more, more preferably 90% by mass or more, and more preferably 90% by mass or more, from the viewpoint of increasing the battery capacity. From the viewpoint of improving binding properties, 99% by mass or less is preferable, and 98% by mass or less is more preferable. More specifically, the content of the positive electrode active material is preferably 80% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less.

(Conductive material)
The conductive material is used to efficiently perform a charge / discharge reaction and enhance conductivity. Examples of the conductive material include carbon materials such as acetylene black, ketjen black, and graphite, and these can be used alone or in combination of two or more.

  The content of the conductive material in the composite layer in the present disclosure is preferably 0.5% by mass or more, more preferably 1% by mass or more from the viewpoint of improving conductivity, and 10% by mass from the viewpoint of improving battery capacity. % Or less is preferable and 5 mass% or less is more preferable. More specifically, the content of the conductive material is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

(Thickener)
Examples of the thickening agent include thickening polysaccharides, alginic acid, carboxymethylcellulose, starch, polyacrylic acid, polyvinyl alcohol, and polyvinylpyrrolidone. Among these, carboxymethyl cellulose is preferable from the viewpoint of assisting the binder action of the polymer particles.

  The content of the thickener in the composite layer in the present disclosure is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and 0.8% by mass from the viewpoints of binding properties and battery characteristics. The above is more preferable, 10 mass% or less is preferable, 8 mass% or less is more preferable, and 5 mass% or less is still more preferable. More specifically, the content of the thickener is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 8% by mass or less, and 0.8% by mass or more and 5% by mass or less. The following is more preferable.

(binder)
The composite material layer in the present disclosure can further contain a conventionally known binder as a binder component in addition to the polymer particles described above.

[Current collector]
The current collector used for the positive electrode of the present disclosure can be selected from materials having conductivity, and examples thereof include metal foils such as copper foil, aluminum foil, and stainless steel foil.

[Method for producing positive electrode for lithium ion secondary battery]
For the positive electrode of the present disclosure, for example, an aqueous slurry (positive electrode mixture paste) containing the above-described positive electrode active material, the above-described polymer particles, and an aqueous medium described later is prepared, and this aqueous slurry is applied to a current collector. It is obtained by removing the aqueous medium in the slurry by drying. That is, this indication WHEREIN: The manufacturing method (1) of the positive electrode for lithium ion secondary batteries including the process which apply | coats the aqueous slurry containing the positive electrode active material mentioned above and the polymer particle mentioned above on an electrical power collector, and dries. Hereinafter, it is also referred to as “a manufacturing method of the present disclosure”.

  The production method of the present disclosure may further include a step of blending the polymer particles, the positive electrode active material, and an aqueous medium to prepare the aqueous slurry (a blending step). The said mixing | blending can be performed using well-known mixing apparatuses, such as a stirrer, a disper, a homomixer, for example. The compounding amount of each component in the blending step can be the same as the content of each component of the above-mentioned composite material layer.

  The production method of the present disclosure may include a polymerization step of polymerizing a monomer mixture containing the monomer (A) and the monomer (B) and, if necessary, the monomer (C) to obtain polymer particles. About the polymerization method, the kind of each component which can be used for superposition | polymerization in the superposition | polymerization process of this indication, and the usage-amount, it can be made to be the same as that of the superposition | polymerization process of the manufacturing method of the polymer particle mentioned above.

(Aqueous medium)
Examples of the aqueous medium include water such as ion exchange water, distilled water, and ultrapure water. Examples of the form of the polymer particles present in the aqueous slurry include a polymer particle dispersion in which the polymer particles are dispersed in an aqueous medium, or a polymer particle emulsion in which the polymer particles are emulsified and dispersed in water. . As the polymer particle dispersion, for example, a mixed liquid containing polymer particles obtained by the emulsion polymerization method described above can be used as it is.

(Optional component)
The aqueous slurry may contain an optional component in addition to the polymer particles and the aqueous medium as long as the effects of the present disclosure are not impaired. Examples of optional components include surfactants, the above-described thickeners, antifoaming agents, and neutralizing agents.

  Examples of the surfactant include known surfactants. For example, surfactants that can be used as the above-described emulsifier may be used. The content of the surfactant in the aqueous slurry is preferably 0.05% by mass or less, more preferably 0.02% by mass or less, based on the total solid content of the aqueous slurry, from the viewpoint of improving binder physical properties and battery characteristics. Preferably, 0.01 mass% or less is further more preferable, and substantially 0 mass% is still more preferable. In the present disclosure, the surfactant content in the aqueous slurry also includes an emulsifier-derived surfactant used in emulsion polymerization.

[Lithium ion secondary battery]
In one embodiment, the present disclosure relates to a lithium ion secondary battery (hereinafter, also referred to as “lithium ion secondary battery of the present disclosure”) including the positive electrode of the present disclosure. As one embodiment of the lithium ion secondary battery of the present disclosure, one having the positive electrode, the negative electrode, the electrolytic solution, and the separator of the present disclosure may be mentioned. A negative electrode, electrolyte solution, and a separator may not be specifically limited, A well-known thing can be used.

  The lithium ion secondary battery of the present disclosure is charged at a current of 0.5 C to a voltage of 3.0 V to 4.2 V and discharged at a current of 0.5 C to a voltage of 4.2 V to 3 V in an environment of 30 ° C. From the viewpoint of extending the life of the battery, the capacity retention rate after 50 cycles is preferably 60% or more, more preferably 80% or more, with respect to the capacity (100%) of the first cycle, 93 % Or more is more preferable.

The present disclosure further relates to one or more embodiments described below.
<1> A positive electrode for a lithium ion secondary battery comprising a current collector and a composite material layer formed on the current collector,
The composite layer includes a structural unit (A) derived from a compound represented by the following formula (I), a compound represented by the following formula (II), a compound represented by the following formula (III), and Including as a binder polymer particles containing a structural unit (B) derived from at least one compound selected from saturated dibasic acids,
The content of the structural unit (A) in all the structural units of the polymer particles is 50% by mass or more and 99.9% by mass or less,
The positive electrode for lithium ion secondary batteries whose content of the structural unit (B) in all the structural units of the said polymer particle is 0.1 to 20 mass%.
[In the formula (I), R ¹ represents a hydrogen atom or a methyl group. R ² represents at least one selected from a linear or branched alkyl group having 1 to 6 carbon atoms and —CH ₂ OR ³ . R ³ represents a linear or branched alkyl group having 4 to 6 carbon atoms. X represents —O— or —NH—. ]
[In the formula (II), R ¹ represents a hydrogen atom or a methyl group, and M represents a hydrogen atom or a cation. ]
[In Formula (III), R ¹ represents a hydrogen atom or a methyl group, and X represents —O— or —NH—. R ⁴ is — (CH ₂ ) _n OH, —R ⁵ SO ₃ M, —R ⁶ N (R ⁷ ) (R ⁸ ), and —R ⁶ N ⁺ (R ⁷ ) (R ⁸ ) (R ⁹ ). Y ^- represents at least one member selected from. n is 1 or more and 4 or less. R ⁵ represents a linear or branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. R ⁶ represents a linear or branched alkylene group having 1 to 4 carbon atoms. R ⁷ and R ⁸ are the same or different and represent a linear or branched alkyl group having 1 to 3 carbon atoms. R ⁹ represents a linear or branched alkyl group having 1 to 3 carbon atoms. Y ⁻ represents an anion. ]

<2> The content of the structural unit (A) in all the structural units of the polymer particles is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and further 80% by mass or more. Preferably, 90 mass% or more is still more preferable, The positive electrode for lithium ion secondary batteries as described in <1>.
<3> The content of the structural unit (A) in all the structural units of the polymer particles is 99.9% by mass or less, preferably 99.5% by mass or less, more preferably 99% by mass or less, and 98% by mass. % Or less, more preferably 97% by mass or less, and the positive electrode for a lithium ion secondary battery according to <1> or <2>.
<4> The content of the structural unit (A) in all the structural units of the polymer particles is 50% by mass or more and 99.9% by mass or less, preferably 60% by mass or more and 99.5% by mass or less, and 70% by mass. % To 99% by mass is more preferable, 80% by mass to 98% by mass is more preferable, and 90% by mass to 97% by mass is even more preferable. The lithium ion according to any one of <1> to <3> Secondary battery positive electrode.
<5> The content of the structural unit (B) in all the structural units of the polymer particles is 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass or more, and 2% by mass. % Or more, more preferably 3% by mass or more, and the positive electrode for a lithium ion secondary battery according to any one of <1> to <4>.
<6> The content of the structural unit (B) in all the structural units of the polymer particles is 20% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, and further more preferably 8% by mass or less. The positive electrode for a lithium ion secondary battery according to any one of <1> to <5>, preferably 6% by mass or less.
<7> The content of the structural unit (B) in all the structural units of the polymer particles is 0.1% by mass to 20% by mass, preferably 0.5% by mass to 15% by mass, and preferably 1% by mass. % To 10% by mass, more preferably 2% to 8% by mass, still more preferably 3% to 6% by mass, and the lithium ion according to any one of <1> to <6> Secondary battery positive electrode.
<8> The ratio (A / B) of the content of the structural unit (A) to the content of the structural unit (B) in the polymer particles is preferably 500 or less, more preferably 100 or less, and even more preferably 50 or less. <1> to the positive electrode for a lithium ion secondary battery according to any one of <7>.
<9> The ratio (A / B) of the content of the structural unit (A) to the content of the structural unit (B) in the polymer particles is preferably 5 or more, more preferably 10 or more, and even more preferably 20 or more. <1> to the positive electrode for lithium ion secondary batteries in any one of <8>.
<10> The ratio (A / B) of the content of the structural unit (A) to the content of the structural unit (B) in the polymer particles is preferably 5 or more and 500 or less, more preferably 10 or more and 100 or less, and 20 or more. The positive electrode for a lithium ion secondary battery according to any one of <1> to <9>, further preferably 50 or less.
<11> The lithium according to any one of <1> to <10>, wherein the surface tension of the polymer particle dispersion in which the polymer particles are dispersed in an aqueous medium is preferably 55 mN / m or more, more preferably 60 mN / m or more. Positive electrode for ion secondary battery.
<12> The positive electrode for a lithium ion secondary battery according to any one of <1> to <11>, wherein the surface tension of the polymer particle dispersion is preferably 72 mN / m or less.
<13> The surface tension of the polymer particle dispersion is preferably 55 mN / m or more and 72 mN / m or less, more preferably 60 mN / m or more and 72 mN / m or less, and the lithium ion according to any one of <1> to <12> Secondary battery positive electrode.
<14> The lithium according to any one of <1> to <13>, wherein the total content of the structural unit (A) and the structural unit (B) in all the structural units of the polymer particles is 80% by mass or more. Positive electrode for ion secondary battery.
<15> In the formula (I), R ¹ is a hydrogen atom or a methyl group, R ² is a linear or branched alkyl group having 1 to 6 carbon atoms, and X is —O—.
The structural unit B is a structural unit derived from the compound represented by the formula (II). In the formula (II), R ¹ is a hydrogen atom or a methyl group, and M is a hydrogen atom or a cation. The positive electrode for a lithium ion secondary battery according to any one of <1> to <14>.
<16> The polymer particles are selected from the compound represented by the formula (I), the compound represented by the formula (II), the compound represented by the formula (III), and an unsaturated dibasic acid. The lithium ion secondary battery according to any one of <1> to <15>, which is a polymer particle obtained by emulsion polymerization of a monomer mixture containing at least one compound selected from the above and a polyfunctional monomer selectively. Positive electrode.
<17> The amount of the emulsifier used in the emulsion polymerization is preferably 0.05% by mass or less, more preferably 0.02% by mass or less, still more preferably 0.01% by mass or less, substantially based on the total amount of monomers. The positive electrode for a lithium ion secondary battery according to any one of <16>, wherein 0% by mass is even more preferable.
<18> The positive electrode for a lithium ion secondary battery according to any one of <16> or <17>, wherein the emulsion polymerization is soap-free emulsion polymerization.
<19> The positive electrode for a lithium ion secondary battery according to any one of <1> to <18>, wherein the polymer particles do not include a structural unit derived from a crosslinkable monomer.
<20> The positive electrode for a lithium ion secondary battery according to any one of <1> to <19>, wherein the average particle diameter of the polymer particles is preferably 0.2 μm or more, more preferably more than 0.3 μm.
<21> The average particle diameter of the polymer particles is preferably 1 μm or less, more preferably 0.9 μm or less, still more preferably 0.8 μm or less, and even more preferably less than 0.7 μm, any one of <1> to <20> A positive electrode for a lithium ion secondary battery according to claim 1.
<22> The average particle size of the polymer particles is preferably 0.2 μm or more and 1 μm or less, more preferably more than 0.2 μm and 0.9 μm or less, still more preferably more than 0.2 μm and 0.8 μm or less, and more than 0.2 μm and 0. The positive electrode for a lithium ion secondary battery according to any one of <1> to <21>, more preferably less than 7 μm, and still more preferably more than 0.3 μm and less than 0.7 μm.
<23> The content of the polymer particles in the composite layer is preferably 0.5% by mass or more, more preferably 1% by mass or more, and further preferably 1.5% by mass or more, from <1> to <22> The positive electrode for lithium ion secondary batteries in any one.
<24> The content of the polymer particles in the composite material layer is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 5% by mass or less, according to any one of <1> to <23>. Positive electrode for lithium ion secondary battery.
<25> The content of the polymer particles in the composite layer is preferably 0.5% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less, and 1.5% by mass or more and 5% by mass or less. Is more preferable, The positive electrode for lithium ion secondary batteries in any one of <1> to <24>.
<26> A method for producing a positive electrode for a lithium ion secondary battery according to any one of <1> to <25>,
Lithium comprising a step of applying an aqueous slurry containing a positive electrode active material and polymer particles used in the positive electrode for a lithium ion secondary battery according to any one of <1> to <25> on a current collector and drying the lithium A method for producing a positive electrode for an ion secondary battery.
<27> A lithium ion secondary battery including the positive electrode for a lithium ion secondary battery according to any one of <1> to <25>.

  Hereinafter, although an example explains this indication, this indication is not limited to this.

1. Measurement of each parameter [Measurement of average particle size of polymer particles]
The average particle diameter of the polymer particles is measured by diluting with a dispersion medium (water) at room temperature until reaching a predetermined light intensity range of the apparatus using a laser diffraction particle size measuring instrument (LA-920 manufactured by Horiba Seisakusho). did. The results are shown in Table 1.

[Measurement of surface tension]
A polymer particle dispersion adjusted to 20 ° C. (liquid diluted with pure water to a solid content of 0.03% by mass) is placed in a petri dish and subjected to the Wilhelmi method (a method in which a platinum plate is immersed and pulled at a constant speed). The surface tension was measured using a surface tension meter (“CBVP-Z” manufactured by Kyowa Interface Chemical Co., Ltd.). The results are shown in Table 1.

[Measurement of glass transition point (Tg)]
The polymer particle dispersion was dried at 40 ° C. for 3 days to obtain a film having a thickness of 1 mm. This film was vacuum-dried for 12 hours in an 80 ° C. vacuum dryer. Using the differential scanning calorimeter ("DSC7000X" manufactured by Hitachi High-Tech Science Co., Ltd.) at a measurement temperature of -80 ° C to 180 ° C and a temperature increase rate of 5 ° C / min, according to JIS K7121, Tg was measured and the results are shown in Table 1.

2. Preparation of polymer particle dispersions a, b, d to h and non-aqueous binder c The following raw materials were used for the preparation of polymer particle dispersions a, d to h shown in Table 1.
<Monomer (A)> [The following R ¹ and R ² represent the symbols in formula (I)]
MMA: Methyl methacrylate (manufactured by Wako Pure Chemical Industries) (R ¹ : CH ₃ , R ² : CH ₃ )
EA: Ethyl acrylate (manufactured by Wako Pure Chemical Industries) (R ¹ : H, R ² : C ₂ H ₅ )
BA: Butyl acrylate (manufactured by Wako Pure Chemical Industries, Ltd.) (R ¹ : H, R ² : C ₄ H ₉ )
<Monomer (B)>
AA: Acrylic acid (manufactured by Wako Pure Chemical Industries, Ltd.) (R ¹ : H) (R ¹ means a symbol in formula (II))
AMPS: 2-acrylamido-2-methylpropanesulfonic acid (manufactured by Tokyo Chemical Industry Co., Ltd.) (R ¹ : H, X = —NH—, R ⁴ = —C (CH ₃ ) ₂ CH ₂ —SO ₃ H) [R ¹ , X, and R ⁴ represent symbols in formula (III)]
<Monomer (C)>
EGDMA: Ethylene glycol dimethacrylate (manufactured by Wako Pure Chemical Industries)
<Polymerization initiator>
APS: ammonium persulfate <neutralized salt>
Na: Sodium <Emulsifier>
Sodium dodecylbenzenesulfonate

(Polymer particle dispersion a)
First, MMA 74g and EA 120g as a monomer (A), AA 6g as a monomer (B), and ion-exchanged water 340g are put into a glass 4-neck separable flask having an internal volume of 1 L, and are kept under a nitrogen atmosphere for a certain period of time ( 0.5 hour). And after heating up the reaction solution in a flask to about 70 degreeC, the polymerization initiator solution which melt | dissolved 1g of APS in 10g of ion-exchange water is added in a flask, The reaction solution in a flask is 70-75 degreeC vicinity. By maintaining for 6 hours, polymerization and aging were performed to obtain a polymer particle dispersion. Thereafter, the polymer particle dispersion in the flask is cooled to room temperature, neutralized by adding 29.14 g of 1N NaOH aqueous solution, and then aggregates are removed using a 200 mesh filter cloth so that the concentration is about 40% by mass. The polymer particle dispersion a was obtained. Table 1 shows the amount and type of each component used in the preparation of the polymer particle dispersion a.

(Polymer particle dispersion d, f, g, h)
The same as the polymer particle dispersion a, except that the monomers (A) and (B) as structural units shown in Table 1 were changed, and the type of neutralized salt was changed as shown in Table 1. Polymer particle dispersions d, f, g and h were obtained by the method. Table 1 shows the amounts and types of the components used for the preparation of the obtained polymer particle dispersions.

(Polymer particle dispersion e)
194 g of EA as monomer (A), 6 g of AA as monomer (B), 1.0 g of EGDMA as monomer (C) [0.5 mol% based on the total number of moles of monomers (A) and (B)], and ions 340 g of exchanged water was placed in a glass 4-neck separable flask having an internal volume of 1 L, and stirred for a certain time (0.5 hours) under a nitrogen atmosphere. And after heating up the reaction solution in a flask to about 70 degreeC, the polymerization initiator solution which melt | dissolved 1g of APS in 10g of ion-exchange water is added in a flask, The reaction solution in a flask is 70-75 degreeC vicinity. By maintaining for 6 hours, polymerization and aging were performed to obtain a polymer particle dispersion. Thereafter, the polymer particle dispersion in the flask is cooled to room temperature, neutralized by adding 29.14 g of 1N LiOH aqueous solution, and then aggregates are removed using a 200 mesh filter cloth, and the concentration is 30 to 35% by mass. The polymer particle dispersion e was obtained by concentrating to a degree. Table 1 shows the amount and type of each component used for the preparation of the polymer particle dispersion e.

(Polymer particle dispersion b)
The following SBR was used for the polymer particle dispersion b.
SBR: Styrene butadiene rubber (manufactured by Nippon Zeon, “BM-400B”, solid content: 40% by mass)

(Non-aqueous binder c)
The following PVDF was used for the non-aqueous binder c.
PVDF: N-methylpyrrolidone solution of polyvinylidene fluoride (“KF polymer L # 1120”, solid content: 12% by mass)

3. Preparation of positive electrode for lithium ion secondary battery (Examples 1 to 6, Comparative Example 1 and Reference Example 1)
(Positive electrode of Example 1)
Conductive material (acetylene black, manufactured by Denka, “Li-100”) 0.33 g and 1.5% thickener (carboxymethylcellulose sodium, manufactured by Wako Pure Chemical Industries, Ltd.), 4.4 g, and positive electrode active material (“NCM523”) ”, Manufactured by Nippon Chemical Industry Co., Ltd., composition: LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) 10.23 g was mixed to prepare slurry [1], and then 1.5% sodium carboxymethylcellulose (CMC) was added to the slurry [1]. ) 2.93 g and 1.52 g of water were added and mixed to prepare slurry [2]. Next, 0.83 g of the prepared polymer particle dispersion a was mixed to prepare a positive electrode mixture paste. Table 2 shows the content of each component in the positive electrode mixture paste (in terms of solid content). “Awatori Nertaro (ARV-310)” was used for mixing each component.
Next, a positive electrode mixture paste is applied onto a 10 μm thick stainless steel foil (manufactured by ASONE) so that the positive electrode capacity density is 1.0 mAh / cm ^2, and is used at 100 ° C. for 12 hours using a vacuum dryer. It dried and produced the positive electrode of Example 1 in which the compound-material layer was formed on the electrical power collector.

(Positive electrode of Examples 2-6)
Except having used the polymer particle dispersion shown in Table 2 as a binder, it carried out similarly to Example 1, and produced the positive electrode of Examples 2-6.

(Positive electrode of Comparative Example 1)
A positive electrode of Comparative Example 1 was produced in the same manner as in Example 1 except that the polymer particle dispersion b was used in place of the polymer particle dispersion a. Table 2 shows the content of each component in the positive electrode mixture paste of Comparative Example 1 (in terms of solid content).

(Positive electrode of Reference Example 1)
Conductive material (acetylene black, manufactured by Denka, “HS-100”) 0.33 g, non-aqueous binder c 3.5 g, and positive electrode active material (“NCM523”, manufactured by Nippon Chemical Industry Co., Ltd., composition: LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) 10.23 g and 2 g of a solvent (N methylpyrrolidone, manufactured by Wako Pure Chemical Industries, Ltd.) were mixed to prepare slurry [1]. Next, 2 g of a non-aqueous binder c was mixed to prepare a non-aqueous positive electrode mixture paste. Table 2 shows the content of each component in the non-aqueous positive electrode mixture paste (in terms of solid content).
Next, a non-aqueous positive electrode mixture paste is applied onto a 10 μm thick stainless steel foil (manufactured by ASONE) so that the positive electrode capacity density is 1.0 mAh / cm ^2, and is applied at 100 ° C. using a vacuum dryer. The positive electrode of Reference Example 1 in which the positive electrode mixture layer was formed on the current collector was prepared by drying for 12 hours.

4). Production of Coin Cell The positive electrodes of Examples 1 to 6, Comparative Example 1 and Reference Example 1 were each punched and pressed to a diameter of 13 mm. Then, on each pressed positive electrode, a separator (made by Hosen) having a diameter of 19 mm and a coin-shaped metal lithium foil having a diameter of 15 mm and a thickness of 0.5 were arranged as a negative electrode, thereby producing a 2032 type coin cell. As the electrolytic solution, 1M LiPF6 EC / DEC (volume ratio) = 3/7 was used.

5). Evaluation The charge / discharge characteristics of Example 1, Comparative Example 1 and Reference Example 1 were evaluated by the following charge / discharge test and electrochemical resistance test by cyclic voltammetry.

[Charge / discharge test]
Using the produced coin cell, a charge / discharge test was performed in an environment of 30 ° C. Charging was performed at a constant current of 0.1 C up to 4.2 V (0.5 C after the fourth cycle) and then at a constant voltage for 10 minutes. The discharge was performed at a constant current of 0.1 C up to 3.0 V (0.5 C after the fourth cycle). This charging / discharging was repeated 50 cycles. The results are shown in FIG. As shown in FIG. 1, Example 1 using predetermined polymer particles as an aqueous binder was found to have charge / discharge cycle characteristics comparable to those of Reference Example 1 using a non-aqueous binder.

[Electrochemical resistance test by cyclic voltammetry]
A coin cell using the positive electrode of Example 1, Comparative Example 1 and Reference Example 1 was manufactured by the same manufacturing method as the coin cell used in the charge / discharge test, and cyclic voltammetry was measured using a potentiogalvanostat under the following measurement conditions. did. The respective cyclic voltammograms are shown in FIGS.

<Measurement conditions>
Sweep voltage 2.5V ~ 4.5V
Sweep speed 0.1mV / s
4 sweeps

  As shown in FIGS. 2 to 4, the cyclic voltammograms of Example 1 and Reference Example 1 had substantially the same profile. On the other hand, the cyclic voltammogram of Comparative Example 1 shows that a reduction peak is observed in the vicinity of 3.5 V, and that the peak intensity of the oxidation peak observed in the vicinity of 3.8 V decreases with each cycle. Compared to Example 1, the electrochemical durability was not sufficient.

[Capacity maintenance rate]
When the discharge capacity at the fourth cycle was SmAh / g and the discharge capacity at the 50th cycle was FmAh / g in the charge / discharge test, the capacity retention rate was calculated by the following formula.
Capacity maintenance rate (%) = F ÷ S × 100

  From the above, the positive electrode of the present disclosure can obtain charge / discharge characteristics equivalent to those of a positive electrode for a lithium ion secondary battery produced by a conventional non-aqueous process by using predetermined polymer particles as an aqueous binder. all right.

  The positive electrode of the present disclosure is useful as a positive electrode of a lithium ion secondary battery.

Claims (10)

A positive electrode for a lithium ion secondary battery, comprising: a current collector; and a composite material layer formed on the current collector,
The composite layer includes a structural unit (A) derived from a compound represented by the following formula (I), a compound represented by the following formula (II), a compound represented by the following formula (III), and Including as a binder polymer particles containing a structural unit (B) derived from at least one compound selected from saturated dibasic acids,
The content of the structural unit (A) in all the structural units of the polymer particles is 50% by mass or more and 99.9% by mass or less,
The positive electrode for lithium ion secondary batteries whose content of the structural unit (B) in all the structural units of the said polymer particle is 0.1 to 20 mass%.
[In the formula (I), R ¹ represents a hydrogen atom or a methyl group. R ² represents at least one selected from a linear or branched alkyl group having 1 to 6 carbon atoms and —CH ₂ OR ³ . R ³ represents a linear or branched alkyl group having 4 to 6 carbon atoms. X represents —O— or —NH—. ]
[In the formula (II), R ¹ represents a hydrogen atom or a methyl group, and M represents a hydrogen atom or a cation. ]
[In Formula (III), R ¹ represents a hydrogen atom or a methyl group, and X represents —O— or —NH—. R ⁴ is — (CH ₂ ) _n OH, —R ⁵ SO ₃ M, —R ⁶ N (R ⁷ ) (R ⁸ ), and —R ⁶ N ⁺ (R ⁷ ) (R ⁸ ) (R ⁹ ). Y ^- represents at least one member selected from. n is 1 or more and 4 or less. R ⁵ represents a linear or branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. R ⁶ represents a linear or branched alkylene group having 1 to 4 carbon atoms. R ⁷ and R ⁸ are the same or different and represent a linear or branched alkyl group having 1 to 3 carbon atoms. R ⁹ represents a linear or branched alkyl group having 1 to 3 carbon atoms. Y ⁻ represents an anion. ]   The positive electrode for a lithium ion secondary battery according to claim 1, wherein the polymer particle dispersion in which the polymer particles are dispersed in an aqueous medium has a surface tension of 55 mN / m or more.   The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein a total content of the structural unit (A) and the structural unit (B) in all the structural units of the polymer particles is 80% by mass or more. In the formula (I), R ¹ is a hydrogen atom or a methyl group, R ² is a linear or branched alkyl group having 1 to 6 carbon atoms, and X is —O—.
The structural unit B is a structural unit derived from the compound represented by the formula (II). In the formula (II), R ¹ is a hydrogen atom or a methyl group, and M is a hydrogen atom or a cation. The positive electrode for lithium ion secondary batteries in any one of Claim 1 to 3.   The polymer particle is at least one selected from the compound represented by the formula (I), the compound represented by the formula (II), the compound represented by the formula (III), and an unsaturated dibasic acid. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode is a polymer particle obtained by emulsion polymerization of a monomer mixture containing a seed compound and a polyfunctional monomer selectively.   The positive electrode for a lithium ion secondary battery according to claim 5, wherein the amount of the emulsifier used in the emulsion polymerization is 0.05% by mass or less based on the total amount of the monomers.   The positive electrode for a lithium ion secondary battery according to claim 5, wherein the emulsion polymerization is soap-free emulsion polymerization.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein an average particle size of the polymer particles is 0.2 µm or more and less than 0.7 µm. A method for producing a positive electrode for a lithium ion secondary battery according to claim 1,
A step of applying an aqueous slurry containing a positive electrode active material and polymer particles used in the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 8 onto a current collector and drying the lithium ion secondary battery. A method for producing a positive electrode for a secondary battery.   The lithium ion secondary battery containing the positive electrode for lithium ion secondary batteries in any one of Claim 1 to 8.