CN111656575A - Positive electrode for lithium ion secondary battery - Google Patents

Positive electrode for lithium ion secondary battery Download PDF

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CN111656575A
CN111656575A CN201980009851.3A CN201980009851A CN111656575A CN 111656575 A CN111656575 A CN 111656575A CN 201980009851 A CN201980009851 A CN 201980009851A CN 111656575 A CN111656575 A CN 111656575A
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positive electrode
secondary battery
ion secondary
polymer particles
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隐岐一雄
平石笃司
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Kao Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • H01M4/622—Binders being polymers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F2/00—Processes of polymerisation
    • C08F2/12—Polymerisation in non-solvents
    • C08F2/16—Aqueous medium
    • C08F2/22—Emulsion polymerisation
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F220/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride ester, amide, imide or nitrile thereof
    • C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F220/04—Acids; Metal salts or ammonium salts thereof
    • C08F220/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F220/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride ester, amide, imide or nitrile thereof
    • C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F220/52—Amides or imides
    • C08F220/54—Amides, e.g. N,N-dimethylacrylamide or N-isopropylacrylamide
    • C08F220/56—Acrylamide; Methacrylamide
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0402—Methods of deposition of the material
    • H01M4/0404—Methods of deposition of the material by coating on electrode collectors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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Abstract

In one embodiment, a positive electrode for a lithium ion secondary battery is provided, which has charge/discharge characteristics equivalent to those of a positive electrode for a lithium ion secondary battery produced by a conventional nonaqueous process and is obtained by an aqueous process using an aqueous binder. One aspect of the present application relates to a current collector comprisingAnd a composite material layer formed on the current collector, wherein the composite material layer contains, as a binder, polymer particles containing a structural unit (A) derived from a compound represented by formula (I) below and a structural unit (B) derived from at least 1 compound selected from a compound represented by formula (II) below, a compound represented by formula (III) below and an unsaturated dibasic acid, the content of the structural unit (A) in all the structural units of the polymer particles is 50 to 99.9 mass%, and the content of the structural unit (B) in all the structural units of the polymer particles is 0.1 to 20 mass%.

Description

Positive electrode for lithium ion secondary battery
Technical Field
The present application relates to a positive electrode for a lithium ion secondary battery.
Background
Lithium ion batteries have higher weight and energy density per unit volume than lead storage batteries, nickel metal hydride batteries, and the like, and therefore contribute to reduction in size and weight of mounted electronic devices. In recent years, hybrid vehicles and electric vehicles have been increasingly used as measures for zero emission of vehicles, and improvement of performance of lithium ion batteries has become a key point for improvement of fuel efficiency and extension of driving distance.
A lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, a nonaqueous electrolytic solution, a separator, and the like. The positive electrode is produced by a nonaqueous process in which a positive electrode active material, a conductive material, and a binder are dispersed in an organic solvent such as N-methylpyrrolidone, a positive electrode composite material is prepared, the positive electrode composite material is applied to the surface of a current collector, and the solvent is volatilized. On the other hand, the negative electrode is produced by an aqueous process in which an aqueous emulsion of styrene-butadiene copolymer rubber produced by an emulsion polymerization method is used as a binder, and a carboxymethyl cellulose aqueous solution having a thickening effect and a thickener are mixed together with a negative electrode active material (for example, patent document 1).
Documents of the prior art
Patent document
Patent document 1: japanese patent No. 3101775
Disclosure of Invention
Problems to be solved by the invention
In recent years, it has been desired to adopt an aqueous process such as a negative electrode for a positive electrode, from the viewpoint of reducing the production cost of an electrode by reducing the use of an organic solvent, and improving the environmental load and the working environment.
However, when styrene-butadiene copolymer rubber (SBR) which is an aqueous binder mainly used in the negative electrode is directly applied to the positive electrode, electrochemical resistance of the polymer tends to be poor and charge-discharge characteristics tend to be degraded.
The present invention provides, in one or more embodiments, a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery obtained by an aqueous process using an aqueous binder, which have charge and discharge characteristics equivalent to those of a positive electrode for a lithium-ion secondary battery produced by a conventional nonaqueous process.
Means for solving the problems
The present invention relates, in one aspect, to a positive electrode for a lithium ion secondary battery (hereinafter also referred to as "the positive electrode of the present invention") including a current collector and a composite material layer formed on the current collector, the composite material layer including, as a binder, polymer particles containing a structural unit (a) derived from a compound represented by formula (I) below and a structural unit (B) derived from at least 1 compound selected from a compound represented by formula (II) below, a compound represented by formula (III) below and an unsaturated dibasic acid, the content of the structural unit (a) in all the structural units of the polymer particles being 50 mass% or more and 99.9 mass% or less, and the content of the structural unit (B) in all the structural units of the polymer particles being 0.1 mass% or more and 20 mass% or less.
[ chemical formula 1]
Figure BDA0002598440090000021
[ in the formula (I), R1Represents a hydrogen atom or a methyl group. R2Represents a linear or branched alkyl group having 1 to 6 carbon atoms or-CH2OR3At least 1 kind of (1). R3Represents a straight-chain alkyl group or a branched-chain alkyl group having 4 to 6 carbon atoms. X represents-O-or-NH-.]
[ chemical formula 2]
Figure BDA0002598440090000022
[ in the formula (II), R1Represents a hydrogen atom or a methyl group, and M represents a hydrogen atom or a cation.]
[ chemical formula 3]
Figure BDA0002598440090000031
[ in the formula (III), R1Represents a hydrogen atom or a methyl group; x represents-O-or-NH-. R4Is selected from- (CH)2)nOH、-R5SO3M、-R6N(R7)(R8) and-R6N+(R7)(R8)(R9)·Y-At least 1 kind of (1). n is 1 to 4 inclusive. R5Represents a linear alkylene group or a branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. R6Represents a linear alkylene group or a branched alkylene group having 1 to 4 carbon atoms. R7And R8The same or different, each represents a straight-chain or branched-chain alkyl group having 1 to 3 carbon atoms. R9Represents a straight-chain alkyl group or a branched-chain alkyl group having 1 to 3 carbon atoms. Y is-Represents an anion.]
In one aspect, the present invention relates to a method for manufacturing a positive electrode for a lithium ion secondary battery, the method including: a step of applying an aqueous slurry containing a positive electrode active material and polymer particles used in the positive electrode of the present application on a current collector and drying the slurry.
The present application relates, in one aspect, to a lithium ion secondary battery comprising the positive electrode of the present application.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present application, the following effects can be exhibited in one aspect: the positive electrode for a lithium ion secondary battery obtained by an aqueous process using an aqueous binder can be provided, which has charge/discharge characteristics equivalent to those of a positive electrode for a lithium ion secondary battery produced by a conventional nonaqueous process.
Drawings
Fig. 1 is a graph showing the results of charge and discharge tests of example 1, comparative example 1, and reference example 1.
FIG. 2 is a graph showing the results of cyclic voltammetry in example 1.
FIG. 3 is a graph showing the results of cyclic voltammetry in comparative example 1.
FIG. 4 is a graph showing the results of cyclic voltammetry in reference example 1.
Detailed Description
The present application is based on the following insight: by incorporating the specific polymer particles in the positive electrode composite material layer, a positive electrode having charge and discharge characteristics equivalent to those of a positive electrode for a lithium ion secondary battery produced by a conventional nonaqueous process can be obtained by an aqueous process.
That is, the present application relates, in one aspect, to a positive electrode for a lithium ion secondary battery, comprising a current collector and a composite material layer formed on the current collector, wherein the composite material layer comprises, as a binder, polymer particles containing a structural unit (a) derived from a compound represented by formula (I) and a structural unit (B) derived from at least 1 compound selected from a compound represented by formula (II), a compound represented by formula (III), and an unsaturated dibasic acid, the content of the structural unit (a) in the entire structural units of the polymer particles is 50 mass% or more and 99.9 mass% or less, and the content of the structural unit (B) in the entire structural units of the polymer particles is 0.1 mass% or more and 20 mass% or less.
The detailed mechanism of the effect of the present application is not clear, but is presumed as follows.
It can be considered that: in the present application, by including the specific polymer particles in the positive electrode composite material layer, the electrode of the present application can withstand a high-voltage atmosphere at a charge/discharge site when used in a lithium ion secondary battery. This makes it possible to charge and discharge while suppressing a decrease in battery characteristics.
However, it is assumed that the present application is not limited to these mechanisms.
Hereinafter, the positive electrode for a lithium ion secondary battery according to the present invention will be specifically described.
[ composite Material layer ]
The composite material layer used for the positive electrode of the present application contains a binder. The binder functions to fix the active material to the surface of the current collector. The binder may be a water-based binder in one or more embodiments. The aqueous binder means a binder dispersed in an aqueous medium. Examples of the aqueous medium include water such as distilled water, ion-exchanged water, and ultrapure water.
(Polymer particles)
In the present application, the polymer particles included in the composite material layer as the binder include a structural unit (a) and a structural unit (B). The structural units (A) and (B) are each a structural unit derived from a monofunctional monomer of a compound described later. The monofunctional monomer means a monomer having 1 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)"). The monomer (A) may be used alone in 1 kind, or may be used in combination of 2 or more kinds.
[ chemical formula 4]
Figure BDA0002598440090000051
In the formula (I), R is R from the viewpoint of ease of synthesis1Represents a hydrogen atom or a methyl group. From the viewpoints of charge-discharge characteristics, affinity for an electrolyte, and binder physical properties, R2Represents a linear or branched alkyl group having 1 to 6 carbon atoms or-CH2OR3At least 1 kind of (b), 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 further preferable. R3Represents a straight-chain alkyl group or a branched-chain alkyl group having 4 to 6 carbon atoms. X represents-O-or-NH-. In this applicationR in the above formula (I), the following formula (II) and formula (III)1Each independently.
Examples of the monomer (a) include alkyl (meth) acrylates selected from methyl (meth) acrylate, ethyl acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, sec-butyl (meth) acrylate, tert-butyl (meth) acrylate, n-pentyl (meth) acrylate, n-hexyl (meth) acrylate and the like; cycloalkyl-containing esters of (meth) acrylic acid such as cyclohexyl (meth) acrylate; 1 or a combination of 2 or more of monofunctional (meth) acrylamides such as methyl (meth) acrylamide, ethyl (meth) acrylamide, n-propyl (meth) acrylamide, isopropyl (meth) acrylamide, n-butyl (meth) acrylamide, isobutyl (meth) acrylamide, sec-butyl (meth) acrylamide, tert-butyl (meth) acrylamide, n-pentyl (meth) acrylamide, n-hexyl (meth) acrylamide, and cyclohexyl (meth) acrylamide. Among these, from the viewpoint of charge and discharge characteristics, affinity for an electrolyte solution, and physical properties of a binder, a combination of 1 or 2 or more selected from Methyl Methacrylate (MMA), Ethyl Methacrylate (EMA), n-Butyl Methacrylate (BMA), Ethyl Acrylate (EA), and n-Butyl Acrylate (BA) is preferable. In the present application, (meth) acrylate means methacrylate or acrylate, and (meth) acrylamide means methacrylamide or acrylamide.
The content of the structural unit (a) in all the structural units of the polymer particles in the present application is 50 mass% or more, preferably 60 mass% or more, more preferably 70 mass% or more, further preferably 80 mass% or more, and further preferably 90 mass% or more from the viewpoint of ease of synthesis, and is 99.9 mass% or less, preferably 99.5 mass% or less, more preferably 99 mass% or less, further preferably 98 mass% or less, and further preferably 97 mass% or less from the same viewpoint. More specifically, the content of the structural unit (a) 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, more preferably 70% by mass or more and 99% by mass or less, still more preferably 80% by mass or more and 98% by mass or less, and still more preferably 90% by mass or more and 97% by mass or less. The content of the structural unit (a) can be determined by a known analytical method or analytical apparatus. When the structural unit (a) contains 2 or more structural units derived from the monomer (a), the content of the structural unit (a) means the total content thereof.
< structural Unit (B) >
The structural unit (B) is a structural unit derived from at least 1 compound (hereinafter also referred to as a monomer (B)) selected from the group consisting of a compound represented by the following formula (II) (hereinafter also referred to as a "monomer (B1)"), a compound represented by the following formula (III) (hereinafter also referred to as a "monomer (B2)") and an unsaturated dibasic acid (hereinafter also referred to as a "monomer (B3)"), and is preferably a structural unit derived from a monomer (B1) from the viewpoints of charge and discharge characteristics, affinity for an electrolyte solution and binder physical properties. The monomer (B) may be used alone in 1 kind, or may be used in combination of 2 or more kinds.
[ chemical formula 5]
Figure BDA0002598440090000061
In the above formula (II), R is R from the viewpoint of ease of synthesis1Is a hydrogen atom or a methyl group. From the viewpoint of charge-discharge characteristics, dispersion stability, and binder physical properties, M is a hydrogen atom or a cation. The cation is preferably at least one of an alkali metal ion and an ammonium ion, more preferably at least 1 selected from the group consisting of an ammonium ion, a lithium ion, a sodium ion, and a potassium ion, and even more preferably at least one of a lithium ion and a sodium ion, from the viewpoints of charge and discharge characteristics, dispersion stability, and binder physical properties.
When M in the formula (II) is a cation, the monomer (B1) may be a monomer obtained by neutralizing a monomer in which M is a hydrogen atom with a base (ammonia, sodium hydroxide, lithium hydroxide, potassium hydroxide, etc.), or a monomer obtained by forming a structural unit of a polymer obtained by polymerizing a monomer in which M is a hydrogen atom and then neutralizing the monomer. From the viewpoint of controlling the polymerization reaction, a monomer obtained by forming a structural unit of a polymer after polymerization and then neutralizing the structural unit is preferable.
Examples of the monomer (B1) include 1 or a combination of 2 or more selected from acrylic acid, methacrylic acid and salts thereof. Examples of the salt include at least 1 selected from ammonium salts, sodium salts, lithium salts, and potassium salts.
[ chemical formula 6]
Figure BDA0002598440090000071
In the above formula (III), R1Represents a hydrogen atom or a methyl group; x represents-O-or-NH-. R4Is selected from- (CH)2)nOH、-R5SO3M、-R6N(R7)(R8) and-R6N+(R7)(R8)(R9)·Y-At least 1 kind of (1). n represents an average molar number of addition and is 1 to 4. R5Represents a linear alkylene group or a branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. Examples of the cation include the same cations as those of M in the above formula (II). In the present application, M in formula (II) and formula (III) are each independently. R6Represents a linear alkylene group or a branched alkylene group having 1 to 4 carbon atoms. R7And R8The same or different, each represents a straight-chain or branched-chain alkyl group having 1 to 3 carbon atoms. R9Represents a straight-chain alkyl group or a branched-chain alkyl group having 1 to 3 carbon atoms. Y is-Represents an anion. Examples of the anion include halide ions such as chloride ion, bromide ion, and fluoride ion; sulfate ions; phosphate ions, and the like.
When M in the formula (III) is a cation, the monomer (B2) may be a monomer obtained by neutralizing a monomer in which M is a hydrogen atom with a base, or a monomer obtained by forming a structural unit of a polymer obtained by polymerizing a monomer in which M is a hydrogen atom and then neutralizing the monomer. From the viewpoint of controlling the polymerization reaction, a monomer obtained by forming a structural unit of a polymer after polymerization and then neutralizing the structural unit is preferable.
When X in the above formula (III) is — O-, the monomer (B2) may be a hydroxyl group-containing ester of (meth) acrylic acid selected from hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate and the like, from the viewpoint of ease of synthesis; and at least 1 of nitrogen atom-containing esters of (meth) acrylic acid such as dimethylaminoethyl (meth) acrylate, dimethylaminopropyl (meth) acrylate, and trimethylaminoethyl (meth) acrylate, preferably at least 1 selected from hydroxyethyl (meth) acrylate and hydroxypropyl (meth) acrylate, and more preferably at least one of hydroxyethyl methacrylate and hydroxyethyl acrylate.
When X in the above formula (III) is-NH-, the monomer (B2) may, for example, be 2-acrylamido-2-methylpropanesulfonic Acid (AMPS).
The monomer (B3) is an unsaturated dibasic acid, and from the viewpoint of ease of synthesis, for example, at least 1 selected from unsaturated dibasic acids having 4 to 12 carbon atoms and salts thereof is included, the number of carbon atoms is preferably 4 to 8, and the number of carbon atoms is more preferably 4 to 6.
From the viewpoint of ease of synthesis, the monomer (B3) may be maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, 2-glutaconic acid, 3-hexenedioic acid, or a salt thereof, preferably at least 1 selected from the group consisting of maleic acid, fumaric acid, itaconic acid, and a salt thereof, and more preferably at least one of maleic acid and a salt thereof.
When the monomer (B3) is a salt of an unsaturated dibasic acid, the salt is preferably at least 1 selected from the group consisting of an ammonium salt, a lithium salt, a sodium salt and a potassium salt, and more preferably at least one selected from the group consisting of a lithium salt and a sodium salt, from the viewpoints of dispersion stability and binder physical properties.
The salt of the unsaturated dibasic acid of the monomer (B3) may be obtained by neutralizing the unsaturated dibasic acid with a base, or may be obtained by polymerizing the unsaturated dibasic acid and then neutralizing the unsaturated dibasic acid. From the viewpoint of controlling the polymerization reaction, a salt obtained by forming a structural unit of the polymer after polymerization and then neutralizing the structural unit is preferred.
The content of the structural unit (B) in the entire structural units of the polymer particles in the present application is 0.1 mass% or more, preferably 0.5 mass% or more, more preferably 1 mass% or more, further preferably 2 mass% or more, and further preferably 3 mass% or more from the viewpoints of charge and discharge characteristics, dispersion stability, and binder properties, and is 20 mass% or less, preferably 15 mass% or less, more preferably 10 mass% or less, further preferably 8 mass% or less, and further preferably 6 mass% or less from the same viewpoints. More specifically, the content of the structural unit (B) is 0.1% by mass or more and 20% by mass or less, 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, further preferably 2% by mass or more and 8% by mass or less, and further preferably 3% by mass or more and 6% by mass or less. The content of the structural unit (B) can be determined by a known analytical method or analytical apparatus. When the structural unit (B) contains 2 or more structural units derived from the monomer (B), the content of the structural unit (B) means the total content thereof.
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 application is preferably 500 or less, more preferably 100 or less, and further preferably 50 or less from the viewpoints of charge and discharge characteristics, dispersion stability, and binder physical properties, and is preferably 5 or more, more preferably 10 or more, and further preferably 20 or more from the viewpoints of dispersion stability and binder physical properties. 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 further preferably 20 or more and 50 or less.
The polymer particles in the present application may contain other structural units than the structural unit (a) and the structural unit (B) described above within a range in which the effects of the present application are not impaired. The other structural unit may be a structural unit of a monomer copolymerizable with the monomers (a) and (B) (hereinafter also referred to as "monomer (C)") (hereinafter also referred to as "structural unit (C)"). The monomer (C) may be used alone in 1 kind, or may be used in combination of 2 or more kinds. Examples of the monomer (C) include crosslinkable monomers.
From the viewpoint of charge/discharge characteristics, affinity for an electrolyte solution, and binder physical properties, the total content of the structural units (a) and (B) in all the structural units of the polymer particles in the present application is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 100% by mass.
In one or more embodiments, the polymer particles herein do not contain structural units derived from a crosslinkable monomer.
In one or more other embodiments, the polymer particles herein comprise structural units derived from a crosslinkable monomer. Examples of the crosslinkable monomer include crosslinkable monomers having 2 or more vinyl groups, and specifically, polyfunctional (meth) acrylates. When the polymer particles in the present application contain a structural unit derived from a polyfunctional (meth) acrylate, the content of the structural unit derived from a polyfunctional (meth) acrylate in the entire structural units of the polymer particles in the present application 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 one or more embodiments, from the viewpoint of battery characteristics. In addition, from the viewpoint of battery characteristics, the content of the structural unit derived from the polyfunctional (meth) acrylate in all the structural units of the polymer particles in the present application is preferably 2 mol% or less, more preferably 1 mol% or less, and even more preferably 0.5 mol% or less, with respect to the total mole number of the structural unit (a) and the structural unit (B) in the other one or more embodiments.
[ method for producing Polymer particles ]
The polymer particles in the present application can be produced by, for example, copolymerizing the monomer (a) and the monomer (B), and if necessary, another monomer (C). That is, the present application relates, in one aspect, to a method for producing polymer particles, which includes: a polymerization step of polymerizing a monomer mixture containing the monomer (A), the monomer (B) and, if necessary, the monomer (C). Examples of the polymerization method include known polymerization methods such as emulsion polymerization, solution polymerization, suspension polymerization, and bulk polymerization, and emulsion polymerization is preferred from the viewpoint of ease of production of the polymer.
In the present application, the content of the structural unit (A) in the whole structural units of the polymer particles can be regarded as the ratio of the amount of the monomer (A) to the total amount of the monomers used for polymerization. The content of the structural unit (B) in the whole structural units of the polymer particles can be regarded as the ratio of the amount of the monomer (B) used to the total amount of the monomers used for polymerization. The content ratio (A/B) of the structural unit (A) to the structural unit (B) can be regarded as the ratio of the amount of the monomer (A) to the amount of the monomer (B) in the total amount of the monomers used for polymerization. The total content of the structural unit (a) and the structural unit (B) in all the structural units of the polymer particles can be regarded as the ratio of the total amount of the monomer (a) and the monomer (B) used to the total amount of the monomers used for polymerization. The content of the structural unit (C) in the polymer particles can be regarded as the ratio of the amount of the monomer (C) used for polymerization to the total mole number of the monomers (a) and (B).
The emulsion polymerization method includes a known method using an emulsifier and a method using substantially no emulsifier, so-called soap-free emulsion polymerization method, and from the viewpoint of charge and discharge characteristics and reduction in ionic resistance of the positive electrode, soap-free emulsion polymerization is preferable. The polymer particles in the present application include, for example, polymer particles obtained by emulsion polymerization, preferably soap-free emulsion polymerization, of a monomer mixture containing the monomer (a) and the monomer (B), and if necessary, another monomer (C).
When an emulsifier is used in the polymerization step, the emulsifier is preferably a water-soluble emulsifier from the viewpoint of polymerization stability. The water-soluble emulsifier includes at least 1 surfactant selected from anionic surfactants, nonionic surfactants, and reactive surfactants from the viewpoint of polymerization stability, and is preferably a reactive surfactant from the viewpoint of suppressing a decrease in adhesion. Reactive surfactant means a surfactant that enters the polymer during polymerization.
Examples of the anionic surfactant include alkyl aryl sulfonates such as sodium dodecylbenzenesulfonate; alkylsulfonate salts such as sodium lauryl sulfonate, alkylsulfate salts such as sodium lauryl sulfate, etc.; and anionic surfactants having a polyoxyethylene group such as sodium polyoxyethylene lauryl ether 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.
Examples of the reactive surfactant include a reactive emulsifier having a vinyl-polymerizable double bond in the molecule, and specifically, a polyoxyalkylene alkenyl ether and the like.
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, even more preferably 0.01% by mass or less, and even more preferably substantially 0% by mass, based on the total amount of the monomers, from the viewpoint of suppressing the decrease in the adhesion. In the present application, the amount of the emulsifier used in the emulsion polymerization may be the amount of the surfactant used in the polymerization step.
In the polymerization step, a polymerization initiator may be used. The polymerization initiator is preferably a water-soluble polymerization initiator 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 tert-butyl hydroperoxide, etc., preferably persulfates, more preferably ammonium persulfate.
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, and still more preferably 0.1 mol% or more, and preferably 5 mol% or less, more preferably 3 mol% or less, and still more preferably 1 mol% or less, based on the total amount of the monomers.
In the polymerization step, water such as ion-exchanged water may be used as a solvent. The amount of water used in the polymerization step may be, for example, 40 to 1500 parts by mass (6.25 to 71.4% by mass based on the amount of the polymerized solid content) per 100 parts by mass of the total amount of the monomers.
In the polymerization step, a reducing agent that can be used in combination with a polymerization initiator may be used. Examples of the reducing agent include sulfite and pyrosulfite.
In the polymerization step, a chain transfer agent may be used. As the chain transfer agent, known chain transfer agents can be used, and examples thereof include mercapto compounds such as isopropyl alcohol, n-dodecyl mercaptan, octyl mercaptan, tert-butyl mercaptan, thioglycolic acid, thiomalic acid, thiosalicylic acid, and mercaptoethanol.
The polymerization conditions may be appropriately set depending on the kind of the polymerization initiator, the monomer, the solvent, and the like used. For example, the polymerization reaction may be carried out at a temperature of 60 to 100 ℃ in a nitrogen atmosphere, and the polymerization time may be set to, for example, 0.5 to 20 hours.
The arrangement of each structural unit constituting the polymer particle in the present application may be any of random, block, or graft. The composition analysis of the polymer can be performed by, for example, NMR spectroscopy, UV-vis spectroscopy, IR spectroscopy, affinity chromatography, or the like.
From the viewpoint of charge and discharge characteristics, affinity for an electrolyte solution, and binder physical properties, the average particle diameter of the polymer particles in the present application is preferably 0.2 μm or more, more preferably more than 0.3 μm, and is preferably 1 μm or less, more preferably 0.9 μm or less, further preferably 0.8 μm or less, and still further preferably less than 0.7 μm. More specifically, the average particle diameter 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, still more preferably more than 0.2 μm and less than 0.7 μm, and still more preferably more than 0.3 μm and less than 0.7 μm.
The content of the polymer particles in the composite material layer in the present application is preferably 0.5% by mass or more, more preferably 1% by mass or more, and still more preferably 1.5% by mass or more from the viewpoints of adhesiveness and battery capacity, and is preferably 15% by mass or less, more preferably 10% by mass or less, and still more preferably 5% by mass or less from the same viewpoint. More specifically, the content of the polymer particles in the composite material 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 further preferably 1.5% by mass or more and 5% by mass or less.
The surface tension of the polymer particle dispersion obtained by dispersing the polymer particles in the present application in an aqueous medium is preferably 55mN/m or more, more preferably 60mN/m or more, and preferably 72mN/m or less, from the viewpoint of improving the binder physical properties and the battery characteristics. The surface tension can be measured by the method described in examples. More specifically, the surface tension of the polymer particle dispersion is preferably 55mN/m or more and 72mN/m or less, and more preferably 60mN/m or more and 72mN/m or less.
The glass transition temperature (Tg) of the polymer particles in the present application is preferably-30 ℃ or higher, more preferably-20 ℃ or higher from the viewpoint of the adhesiveness and the battery characteristics, and is preferably 30 ℃ or lower, more preferably 25 ℃ or lower, and further preferably 20 ℃ or lower from the same viewpoint. More specifically, the Tg of the polymer particles is preferably from-30 ℃ to 30 ℃, more preferably from-20 ℃ to 25 ℃, and still more preferably from-20 ℃ to 20 ℃.
As one embodiment of the composite material layer in the present application, for example, a composite material layer containing the above-described polymer particles, a positive electrode active material, and optional components (for example, a conductive material and a thickener) added as needed can be cited. The mass ratio of each component contained in the composite material layer can be arbitrarily adjusted according to the use suitability of the battery.
(Positive electrode active Material)
The positive electrode active material may be any active material that can store and release lithium and can cause a charge-discharge reaction, and examples thereof include lithium iron phosphate and LiCoO2、LiNiO2、Li2MnO4And the like. These compounds may be partially substituted with elements. The average particle diameter of the positive electrode active material may be, for example, 2 μm or more and 40 μm or less.
The content of the positive electrode active material in the composite material layer in the present application is preferably 80 mass% or more, more preferably 90 mass% or more from the viewpoint of increasing the battery capacity, and is preferably 99 mass% or less, more preferably 98 mass% or less from the viewpoint of improving the adhesion of the composite material layer to the current collector. More specifically, the content of the positive electrode active material is preferably 80 mass% or more and 99 mass% or less, and more preferably 90 mass% or more and 98 mass% or less.
(conductive Material)
The conductive material is used for effectively carrying out charge-discharge reaction and improving conductivity. Examples of the conductive material include carbon materials such as acetylene black, ketjen black, and graphite, and 2 or more of them may be used alone or in combination.
The content of the conductive material in the composite material layer in the present application is preferably 0.5% by mass or more, more preferably 1% by mass or more from the viewpoint of improving the conductivity, and is preferably 10% by mass or less, more preferably 5% by mass or less from the viewpoint of improving the battery capacity. 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.
(thickening agent)
Examples of the thickener include thickening polysaccharides, alginic acid, carboxymethyl cellulose, starch, polyacrylic acid, polyvinyl alcohol, and polyvinyl pyrrolidone. Among them, carboxymethyl cellulose is preferable from the viewpoint of assisting the binder action of the polymer particles.
From the viewpoint of the adhesiveness and the battery characteristics, the content of the thickener in the composite material layer in the present application is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 0.8% by mass or more, and is preferably 10% by mass or less, more preferably 8% by mass or less, and further preferably 5% by mass or less. 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 further preferably 0.8% by mass or more and 5% by mass or less.
(Binder)
The composite material layer in the present application may further contain a conventionally known binder as a binder component in addition to the polymer particles.
[ Current collector ]
The current collector used in the positive electrode of the present invention may 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 ]
The positive electrode of the present application can be obtained, for example, by preparing an aqueous slurry (positive electrode composite material paste) containing the positive electrode active material, the polymer particles, and an aqueous medium described later, applying the aqueous slurry to a current collector, and drying and removing the aqueous medium in the aqueous slurry. That is, the present application relates, in one aspect, to a method for producing a positive electrode for a lithium-ion secondary battery (hereinafter, also referred to as "the production method of the present application"), including: and a step of applying an aqueous slurry containing the positive electrode active material and the polymer particles onto a current collector and drying the applied aqueous slurry.
The manufacturing method of the present application may further include: and a step (mixing step) of preparing the aqueous slurry by mixing the polymer particles, the positive electrode active material, and an aqueous medium. The blending may be carried out using a known mixing device such as a stirrer, a distributor, or a homomixer. The amount of each component to be blended in the blending step may be the same as the content of each component in the composite material layer.
The manufacturing method of the present application may include: a polymerization step of polymerizing a monomer mixture containing the monomer (a), the monomer (B), and if necessary, the monomer (C) to obtain polymer particles. The polymerization method in the polymerization step of the production method of the present application, the kinds and amounts of each component usable for polymerization may be the same as those in the polymerization step of the production method of the polymer particles.
(aqueous Medium)
Examples of the aqueous medium include water such as ion-exchanged 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 polymer particles are dispersed in an aqueous medium, and a polymer particle emulsion in which the polymer particles are emulsified and dispersed in water. As the polymer particle dispersion, for example, a mixed solution containing polymer particles obtained by the above emulsion polymerization method can be used as it is.
(optional ingredients)
The aqueous slurry may contain optional components in addition to the polymer particles and the aqueous medium, within a range not impairing the effects of the present invention. Examples of the optional components include surfactants, the above thickeners, defoaming agents, and neutralizing agents.
The surfactant includes known surfactants, and examples of the surfactant include surfactants usable as the emulsifier. From the viewpoint of improving the binder physical properties and the battery characteristics, 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, even more preferably 0.01% by mass or less, and even more preferably substantially 0% by mass, based on the total solid content of the aqueous slurry. In the present application, the content of the surfactant in the above aqueous slurry further includes a surfactant from an emulsifier used in the emulsion polymerization.
[ lithium ion Secondary Battery ]
The present application relates, in one aspect, to a lithium ion secondary battery (hereinafter also referred to as "lithium ion secondary battery of the present application") comprising the positive electrode of the present application. As one embodiment of the lithium ion secondary battery of the present application, a battery having the positive electrode, the negative electrode, the electrolytic solution, and the separator of the present application is exemplified. The negative electrode, the electrolyte, and the separator are not particularly limited, and known materials can be used.
In the lithium ion secondary battery of the present invention, from the viewpoint of extending the life of the battery, the capacity retention rate when the operation of charging the battery at a current of 0.5C to a voltage of 3.0V to 4.2V and discharging the battery at a current of 0.5C to a voltage of 4.2V to 3V is repeated for 50 cycles is preferably 60% or more, more preferably 80% or more, and still more preferably 93% or more, with respect to the capacity (100%) of the 1 st cycle.
The present application is also directed to one or more of the following embodiments.
<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 material layer contains, as a binder, polymer particles containing a structural unit (A) derived from a compound represented by the following formula (I) and a structural unit (B) derived from at least 1 compound selected from a compound represented by the following formula (II), a compound represented by the following formula (III) and an unsaturated dibasic acid,
the content of the structural unit (A) in all the structural units of the polymer particles is 50 to 99.9 mass%,
the content of the structural unit (B) in all the structural units of the polymer particles is 0.1 mass% or more and 20 mass% or less.
[ chemical formula 7]
Figure BDA0002598440090000161
[ in the formula (I), R1Represents a hydrogen atom or a methyl group. R2Represents a linear or branched alkyl group having 1 to 6 carbon atoms or-CH2OR3At least 1 kind of (1). R3Represents a straight-chain alkyl group or a branched-chain alkyl group having 4 to 6 carbon atoms. X represents-O-or-NH-.]
[ chemical formula 8]
Figure BDA0002598440090000162
[ in the formula (II), R1Represents a hydrogen atom or a methyl group; m represents a hydrogen atom or a cation.]
[ chemical formula 9]
Figure BDA0002598440090000163
[ in the formula (III), R1Represents a hydrogen atom or a methyl group; x represents-O-or-NH-. R4Is selected from- (CH)2)nOH、-R5SO3M、-R6N(R7)(R8) and-R6N+(R7)(R8)(R9)·Y-At least 1 kind of (1). n is 1 to 4 inclusive. R5Represents a linear alkylene group or a branched alkylene group having 1 to 5 carbon atoms. M represents a hydrogen atom or a cation. R6Represents a linear alkylene group or a branched alkylene group having 1 to 4 carbon atoms. R7And R8The same or different, each represents a straight-chain or branched-chain alkyl group having 1 to 3 carbon atoms. R9Represents a straight-chain alkyl group or a branched-chain alkyl group having 1 to 3 carbon atoms. Y is-Represents an anion.]
<2> the positive electrode for a lithium-ion secondary battery according to <1>, wherein 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, still more preferably 80% by mass or more, and still more preferably 90% by mass or more.
<3> the positive electrode for a lithium-ion secondary battery as stated in <1> or <2>, wherein 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, further preferably 98% by mass or less, and still further preferably 97% by mass or less.
<4> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <3>, wherein 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, more preferably 70% by mass or more and 99% by mass or less, further preferably 80% by mass or more and 98% by mass or less, and further preferably 90% by mass or more and 97% by mass or less.
<5> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <4>, wherein 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, further preferably 2% by mass or more, and still further preferably 3% by mass or more.
<6> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <5>, wherein 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, still more preferably 8% by mass or less, and still more preferably 6% by mass or less.
<7> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <6>, 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, 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, further preferably 2% by mass or more and 8% by mass or less, and further preferably 3% by mass or more and 6% by mass or less.
<8> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <7>, wherein a ratio (a/B) of a content of the structural unit (a) to a content of the structural unit (B) in the polymer particles is preferably 500 or less, more preferably 100 or less, and still more preferably 50 or less.
<9> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <8>, wherein a ratio (a/B) of a content of the structural unit (a) to a content of the structural unit (B) in the polymer particles is preferably 5 or more, more preferably 10 or more, and further preferably 20 or more.
<10> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <9>, wherein a ratio (a/B) of a content of the structural unit (a) to a 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 further preferably 20 or more and 50 or less.
<11> the positive electrode for a lithium-ion secondary battery 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 55mN/m or more, more preferably 60mN/m or more.
<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 72mN/m or less.
<13> the positive electrode for a lithium ion secondary battery according to any one of <1> to <12>, wherein the surface tension of the polymer particle dispersion is preferably 55mN/m or more and 72mN/m or less, more preferably 60mN/m or more and 72mN/m or less.
<14> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <13>, 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.
<15>According to<1>～<14>The positive electrode for a lithium ion secondary battery as described in any one of the above formulas (I), wherein R is1Is a hydrogen atom or a methyl group, R2Is a straight-chain alkyl group or a branched-chain alkyl group having 1 to 6 carbon atoms, X is-O-,
the structural unit B is a structural unit derived from a compound represented by the formula (II) in which R is1Is a hydrogen atom or a methyl group, and M is a hydrogen atom or a cation.
<16> the positive electrode for a lithium ion secondary battery according to any one of <1> to <15>, wherein the polymer particles are polymer particles obtained by emulsion polymerization of a monomer mixture containing a compound represented by the formula (I), at least 1 compound selected from a compound represented by the formula (II), a compound represented by the formula (III), and an unsaturated dibasic acid, and optionally a polyfunctional monomer.
<17> the positive electrode for a lithium ion secondary battery according to <16>, wherein 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, further preferably 0.01% by mass or less, and further preferably substantially 0% by mass, based on the total amount of the monomers.
<18> the positive electrode for a lithium ion secondary battery as stated in <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 contain 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 positive electrode for a lithium-ion secondary battery according to any one of <1> to <20>, wherein 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 still more preferably less than 0.7 μm.
<22> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <21>, wherein the average particle diameter 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, further preferably more than 0.2 μm and 0.8 μm or less, further preferably more than 0.2 μm and less than 0.7 μm, further preferably more than 0.3 μm and less than 0.7 μm.
<23> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <22>, wherein the content of the polymer particles in the composite material layer is preferably 0.5% by mass or more, more preferably 1% by mass or more, and still more preferably 1.5% by mass or more.
<24> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <23>, wherein 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 still more preferably 5% by mass or less.
<25> the positive electrode for a lithium-ion secondary battery according to any one of <1> to <24>, wherein the content of the polymer particles in the composite material 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 still more preferably 1.5% by mass or more and 5% by mass or less.
<26> A method for producing a positive electrode for a lithium ion secondary battery, which is the method for producing a positive electrode for a lithium ion secondary battery according to any one of <1> to <25>,
it includes: and a step of applying an aqueous slurry containing a positive electrode active material and the polymer particles used in the positive electrode for a lithium-ion secondary battery according to any one of <1> to <25> to a current collector and drying the slurry.
<27> a lithium-ion secondary battery comprising the positive electrode for a lithium-ion secondary battery as defined in any one of <1> to <25 >.
Examples
The present application will be described below with reference to examples, but the present application is not limited thereto.
1. Measurement of the respective parameters
[ measurement of average particle diameter of Polymer particles ]
The average particle diameter of the polymer particles was measured by using a particle diameter measuring instrument (LA-920, manufactured by horiba, Ltd.) by laser diffraction method, and diluting the polymer particles with a dispersion medium (water) at room temperature until the amount of light reached a specific light amount range of the apparatus. The results are shown in Table 1.
[ measurement of surface tension ]
The polymer particle dispersion adjusted to 20 ℃ (a liquid obtained by diluting with pure water so as to reach 0.03 mass% in terms of solid content) was put into a petri dish, and the surface tension was measured by the Wilhemly method (a method of dipping a platinum plate and pulling it at a constant speed) using a surface tensiometer (CBVP-Z, manufactured by coyote interfacial chemical corporation). The results are shown in Table 1.
[ measurement of glass transition temperature (Tg) ]
The polymer particle dispersion was dried at 40 ℃ for 3 days to obtain a film having a thickness of 1 mm. The film was dried under vacuum in a vacuum desiccator at 80 ℃ for 12 hours. The film-shaped resin thus obtained was measured according to JIS K7121 at a measurement temperature of-80 to 180 ℃ and a temperature rise rate of 5 ℃/min using a differential scanning calorimeter ("DSC 7000X" manufactured by Hitachi high-tech Co., Ltd., measurement Tg., and the results are shown in Table 1.
2. Preparation of Polymer particle dispersions a, b, d to h and nonaqueous Binder c
The following raw materials were used for the preparation of the polymer particle dispersions a and d to h shown in Table 1.
<Monomer (A)>[ the following R1、R2X is a symbol in the formula (I)]
MMA: methyl methacrylate (Wako pure chemical industries, Ltd.) (R)1：CH3、R2：CH3、X：O)
EA: ethyl acrylate (manufactured by Wako pure chemical industries, Ltd.) (R)1：H、R2：C2H5、X：O)
BA: butyl acrylate (manufactured by Wako pure chemical industries, Ltd.) (R)1：H、R2：C4H9、X：O)
< monomer (B) >
AA: acrylic acid (manufactured by Wako pure chemical industries, Ltd.) (R)1：H、M：H)[R1M is a symbol in the formula (II)]
AMPS: 2-acrylamido-2-methylpropanesulfonic acid (manufactured by Tokyo chemical industries, Ltd.) (R)1：H、X：-NH-、R4：-C(CH3)2CH2-SO3H)[R1、X、R4Is a symbol in the formula (III)]
< monomer (C) >
EGDMA: ethylene glycol dimethacrylate (manufactured by Wako pure chemical industries, Ltd.)
< polymerization initiator >
APS: ammonium persulfate
< neutralization salt >
Na: sodium salt
< emulsifiers >
Sodium dodecyl benzene sulfonate
(Polymer particle Dispersion a)
First, 74g of MMA and 120g of EA as a monomer (A), 6g of AA as a monomer (B) and 340g of ion-exchanged water were put into a glass four-neck separable flask having an internal volume of 1L, and stirred for a certain period of time (0.5 hour) under a nitrogen atmosphere. Then, the reaction solution in the flask was heated to about 70 ℃ and then a polymerization initiator solution prepared by dissolving 1g of APS in 10g of ion-exchanged water was added to the flask, and the reaction solution in the flask was kept at about 70 to 75 ℃ for 6 hours to polymerize/mature, thereby obtaining a polymer particle dispersion. Thereafter, the polymer particle dispersion in the flask was cooled to room temperature, 29.14g of a 1N NaOH aqueous solution was added to neutralize the dispersion, and then a 200mesh filter cloth was used to remove aggregates and concentrate the mixture until the concentration reached about 40 mass%, thereby obtaining a polymer particle dispersion a. The amounts and kinds of the respective components used for the preparation of the polymer particle dispersion a are shown in table 1.
(Polymer particle Dispersion d, f, g, h)
Polymer particle dispersions d, f, g, and h were obtained by the same method as polymer particle dispersion a except that monomers (a) and (B) forming the structural units shown in table 1 were changed and the kind of the neutralizing salt was changed as shown in table 1. The amounts and kinds of the respective components used for the preparation of the respective polymer particle dispersions obtained are shown in table 1.
(Polymer particle Dispersion e)
EA 194g as a monomer (A), AA 6g as a monomer (B), EGDMA 1.0g as a monomer (C) [ 0.5 mol% based on the total mole number of the monomers (A) and (B) ], and 340g of ion-exchanged water were put into a glass four-neck separable flask having an internal volume of 1L, and stirred under a nitrogen atmosphere for a certain period of time (0.5 hour). Then, the reaction solution in the flask was heated to about 70 ℃ and then a polymerization initiator solution prepared by dissolving 1g of APS in 10g of ion-exchanged water was added to the flask, and the reaction solution in the flask was kept at about 70 to 75 ℃ for 6 hours to polymerize/mature, thereby obtaining a polymer particle dispersion. Thereafter, the polymer particle dispersion in the flask was cooled to room temperature, 29.14g of a 1N LiOH aqueous solution was added to neutralize the polymer particle dispersion, and then a 200mesh filter cloth was used to remove aggregates, and the polymer particle dispersion was concentrated to a concentration of about 30 to 35 mass%, thereby obtaining a polymer particle dispersion e. The amounts and kinds of the respective components used for the preparation of the polymer particle dispersion e are shown in table 1.
(Polymer particle Dispersion b)
The following SBR was used for the polymer particle dispersion b.
SBR: styrene butadiene rubber (manufactured by ZEON, Japan, "BM-400B", solid content: 40 mass%)
(nonaqueous Binder c)
The following PVDF was used for the nonaqueous binder c.
PVDF: n-methylpyrrolidone solution of polyvinylidene fluoride (manufactured by KUREHA, "KF Polymer L # 1120", solid content 12% by mass)
[ Table 1]
Figure BDA0002598440090000231
3. Production of Positive electrode for lithium ion Secondary Battery (examples 1 to 6, comparative example 1 and reference example 1)
(Positive electrode of example 1)
A conductive material (acetylene black, manufactured by DENKA, "Li-100") 0.33g, a 1.5% thickener (sodium carboxymethylcellulose, manufactured by Wako pure chemical industries, Ltd.) aqueous solution 4.4g, and a positive electrode active material ("NCM 523", manufactured by Nippon chemical industries, Ltd.) having a composition of LiNi0.5Co0.2Mn0.3O2)10.23g were mixed to prepare slurry [ 1]]Then, to the slurry [ 1]]2.93g of 1.5% sodium carboxymethylcellulose (CMC) and 1.52g of water were added to the mixture, and the mixture was mixed to prepare slurry [ 2]]. Subsequently, 0.83g of the prepared polymer particle dispersion a was mixed to prepare a positive electrode composite paste. The contents (in terms of solid content) of the respective components in the positive electrode composite paste are shown in table 2. Mixing the above components, and soaking and taking Tailang (ARV-310))”。
Then, on a stainless steel foil (manufactured by AS ONE) having a thickness of 10 μm, a positive electrode capacity density of 1.0mAh/cm was obtained2The positive electrode composite material paste was applied and dried at 100 ℃ for 12 hours using a vacuum dryer, thereby producing a positive electrode of example 1 in which a composite material layer was formed on a current collector.
(Positive electrodes of examples 2 to 6)
Positive electrodes of examples 2 to 6 were produced in the same manner as in example 1, except that the polymer particle dispersions shown in table 2 were used as the binder.
(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 instead of the polymer particle dispersion a. Table 2 shows the content (in terms of solid content) of each component in the positive electrode composite paste of comparative example 1.
(Positive electrode of reference example 1)
A conductive material (acetylene black, manufactured by DENKA, "HS-100") 0.33g, a nonaqueous binder c 3.5g, and a positive electrode active material ("NCM 523", manufactured by Nippon chemical industries, Ltd., composition: LiNi)0.5Co0.2Mn0.3O2)10.23g of this solution and 2g of a solvent (N-methylpyrrolidone, Wako pure chemical industries, Ltd.) were mixed to prepare a slurry [ 1]]. Subsequently, 2g of the nonaqueous binder c was mixed to prepare a nonaqueous positive electrode composite paste. The content (in terms of solid content) of each component in the nonaqueous positive electrode composite paste is shown in table 2.
Then, on a stainless steel foil (manufactured by AS ONE) having a thickness of 10 μm, a positive electrode capacity density of 1.0mAh/cm was obtained2The nonaqueous positive electrode composite material paste was applied and dried at 100 ℃ for 12 hours using a vacuum dryer, thereby producing a positive electrode of reference example 1 in which a positive electrode composite material layer was formed on a current collector.
[ Table 2]
Figure BDA0002598440090000251
4. Fabrication of coin-type 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, a 19 mm-diameter separator (manufactured by baoquan) and a 15 mm-diameter coin-shaped lithium metal foil having a thickness of 0.5 were disposed as a negative electrode on each of the positive electrodes obtained by pressing, and a 2032 coin-shaped battery cell was produced. The electrolyte used was 1M LiPF6 EC/DEC (volume ratio) 3/7.
5. Evaluation of
The charge and discharge characteristics of example 1, comparative example 1, and reference example 1 were evaluated by the following electrochemical resistance test based on a charge and discharge test and cyclic voltammetry.
[ Charge/discharge test ]
The coin-type cell thus produced was used to perform a charge/discharge test in an environment of 30 ℃. Charging is set as follows: constant current charging was performed at 0.1C (0.5C after the 4 th cycle) until 4.2V, and then constant voltage charging was performed for 10 minutes. The discharge was performed at 0.1C (0.5C after the 4 th cycle) and constant current discharge was performed until 3.0V. This charge and discharge was repeated for 50 cycles. The results are shown in FIG. 1. As shown in fig. 1, it can be seen that: in example 1 in which specific polymer particles were used as an aqueous binder, the charge-discharge cycle characteristics were comparable to those of reference example 1 in which a nonaqueous binder was used.
[ electrochemical resistance test based on cyclic voltammetry ]
Coin-type cells using the positive electrodes of example 1, comparative example 1, and reference example 1 were prepared by the same preparation method as the coin-type cells used in the charge and discharge test, and cyclic voltammetry was measured using a potentiostat under the following measurement conditions. Each cyclic voltammogram is shown in FIGS. 2 to 4.
< measurement conditions >
Scanning voltage: 2.5V-4.5V
Scanning speed: 0.1mV/s
The scanning times are as follows: 4 cycles
As shown in FIGS. 2 to 4, the cyclic voltammograms of example 1 and reference example 1 are substantially the same curves. On the other hand, in the cyclic voltammogram of comparative example 1, a reduction peak was observed in the vicinity of 3.5V, or the peak intensity of an oxidation peak observed in the vicinity of 3.8V was decreased every time the cycle was repeated, and the electrochemical durability was insufficient as compared with example 1 and reference example 1.
[ Capacity Retention ratio ]
In the above charge and discharge test, when the discharge capacity at the 4 th cycle was sma h/g and the discharge capacity at the 50 th cycle was FmAh/g, the capacity retention rate was calculated by the following formula.
Capacity retention rate (%) ═ F ÷ sx 100
In conclusion: the positive electrode of the present application can obtain charge and discharge characteristics equivalent to those of a positive electrode for a lithium ion secondary battery produced by a conventional nonaqueous process, by using specific polymer particles as an aqueous binder.
Industrial applicability
The positive electrode of the present application is useful as a positive electrode of a lithium ion secondary battery.

Claims (10)

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 material layer contains, as a binder, polymer particles containing a structural unit (A) derived from a compound represented by the following formula (I) and a structural unit (B) derived from at least 1 compound selected from a compound represented by the following formula (II), a compound represented by the following formula (III) and an unsaturated dibasic acid,
the content of the structural unit (A) in all the structural units of the polymer particles is 50 to 99.9 mass%,
the content of the structural unit (B) in all the structural units of the polymer particles is 0.1 to 20 mass%,
Figure FDA0002598440080000011
in the formula (I), R1Represents a hydrogen atom or a methyl group; r2Represents a number of carbon atoms selected fromIs a straight-chain or branched alkyl group of 1 to 6 inclusive, and-CH2OR3At least 1 of; r3Represents a straight-chain alkyl group or a branched-chain alkyl group having 4 to 6 carbon atoms; x represents-O-or-NH-,
Figure FDA0002598440080000012
in the formula (II), R1Represents a hydrogen atom or a methyl group; m represents a hydrogen atom or a cation,
Figure FDA0002598440080000013
in the formula (III), R1Represents a hydrogen atom or a methyl group; x represents-O-or-NH-; r4Is selected from- (CH)2)nOH、-R5SO3M、-R6N(R7)(R8) and-R6N+(R7)(R8)(R9)·Y-At least 1 of; n is 1 or more and 4 or less; r5Represents a linear alkylene group or a branched alkylene group having 1 to 5 carbon atoms; m represents a hydrogen atom or a cation; r6Represents a linear alkylene group or a branched alkylene group having 1 to 4 carbon atoms; r7And R8The same or different, and represents a straight-chain or branched-chain alkyl group having 1 to 3 carbon atoms; r9Represents a straight-chain alkyl group or a branched-chain alkyl group having 1 to 3 carbon atoms; y is-Represents an anion.
2. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein a surface tension of a polymer particle dispersion in which the polymer particles are dispersed in an aqueous medium is 55mN/m or more.
3. The positive electrode for a lithium-ion secondary battery according to claim 1 or 2, 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 mass% or more.
4. The positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 3, wherein R in the formula (I)1Is a hydrogen atom or a methyl group; r2A linear or branched alkyl group having 1 to 6 carbon atoms; x is-O-,
the structural unit B is a structural unit derived from the compound represented by the formula (II) in which R is1Is a hydrogen atom or a methyl group; m is a hydrogen atom or a cation.
5. The positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein the polymer particles are polymer particles obtained by emulsion polymerization of a monomer mixture containing the compound represented by formula (I), at least 1 compound selected from the compound represented by formula (II), the compound represented by formula (III), and an unsaturated dibasic acid, and optionally a polyfunctional monomer.
6. 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.
7. The positive electrode for a lithium-ion secondary battery according to any one of claims 5 or 6, wherein the emulsion polymerization is soap-free emulsion polymerization.
8. The positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 7, wherein the polymer particles have an average particle diameter of 0.2 μm or more and less than 0.7 μm.
9. A method for producing a positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 8,
it includes: 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 to a current collector and drying the slurry.
10. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 8.
CN201980009851.3A 2018-01-26 2019-01-24 Positive electrode for lithium ion secondary battery Pending CN111656575A (en)

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