Binder composition for positive electrode of non-aqueous secondary battery, conductive material dispersion for positive electrode of non-aqueous secondary battery, slurry composition for positive electrode of non-aqueous secondary battery, positive electrode for non-aqueous secondary battery, and non-aqueous secondary battery

Abstract

Provided is a binder composition for the positive electrode of a nonaqueous secondary battery, the binder composition containing a polymer. The polymer contains nitrile-group-containing monomer units, has a weight-average molecular weight of 300,000 or less, and has a sulfur content of 500 ppm or more.

Description

[Technical Field]

[0001] The present invention relates to a binder composition for the positive electrode of a non-aqueous secondary battery, a conductive material dispersion for the positive electrode of a non-aqueous secondary battery, a slurry composition for the positive electrode of a non-aqueous secondary battery, a positive electrode for a non-aqueous secondary battery, and a non-aqueous secondary battery. [Background technology]

[0002] Non-aqueous secondary batteries (non-aqueous electrolyte secondary batteries), such as lithium-ion secondary batteries, are small, lightweight, have high energy density, and can be repeatedly charged and discharged, making them suitable for a wide range of applications. Therefore, in recent years, improvements to battery components such as electrodes have been considered to further enhance the performance of non-aqueous secondary batteries.

[0003] Here, the positive electrode for a non-aqueous secondary battery typically comprises a current collector and an electrode composite layer (positive electrode composite layer) formed on the current collector. This positive electrode composite layer is formed using a slurry composition obtained by dispersing, for example, a positive electrode active material and a binder composition containing a binder in a dispersion medium.

[0004] In recent years, in order to achieve further improvements in the performance of non-aqueous secondary batteries, attempts have been made to improve the binder composition used to form the electrode composite layer, which is a component of the electrodes of non-aqueous secondary batteries.

[0005] Specifically, various binder compositions for non-aqueous secondary battery electrodes containing nitrile group-containing monomer units and hydrogenated butadiene units have been studied and provided (see, for example, Patent Documents 1 to 3). Furthermore, polymers containing nitrile group-containing monomer units and hydrogenated butadiene units have various industrially advantageous attributes and have therefore been improved in various ways. For example, Patent Document 4 proposes that when copolymerizing a conjugated diene monomer and an α,β-unsaturated nitrile monomer in the presence of a molecular weight modifier containing a mercapto group to obtain a polymer, the residual amount of the molecular weight modifier should be 40 ppm or less. [Prior art documents]

Patent Document

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Here, from the viewpoint of improving the productivity and performance of secondary batteries, it is necessary to enhance the dispersibility of a conductive material dispersion obtained by mixing such a binder composition with carbon nanotubes as a conductive material. In addition, for the resulting secondary battery, even after being stored at a high temperature, it is necessary that there is little deterioration when comparing the discharge capacity before and after storage, that is, it has excellent high - temperature storage characteristics.

[0008] However, with conventional binder compositions and the like that have been conventionally studied and used, it has not been possible to achieve both enhancing the dispersibility of the obtained conductive material dispersion and enhancing the high - temperature storage characteristics of the obtained secondary battery at a high level.

[0009] Therefore, an object of the present invention is to provide a non - aqueous secondary battery positive electrode binder composition capable of forming a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high - temperature storage characteristics. Another object of the present invention is to provide a conductive material dispersion for a non - aqueous secondary battery positive electrode with excellent dispersibility, which can form a secondary battery with excellent high - temperature storage characteristics. Furthermore, an object of the present invention is to provide a slurry composition for a non - aqueous secondary battery positive electrode capable of forming a secondary battery with excellent high - temperature storage characteristics. Another object of the present invention is to provide a non-aqueous secondary battery positive electrode capable of forming a secondary battery having excellent high-temperature storage characteristics and a secondary battery having excellent high-temperature storage characteristics. **Means for Solving the Problems**

[0010] The inventors of the present invention conducted intensive studies to achieve the above object. As a result, the inventors have found that a binder composition for a non-aqueous secondary battery positive electrode containing a polymer containing a nitrile group-containing monomer unit and having a weight average molecular weight and a sulfur content within a predetermined range can form a conductive material dispersion excellent in dispersibility and a secondary battery excellent in high-temperature storage characteristics, and thus completed the present invention.

[0011] That is, the present invention aims to advantageously solve the above problems. The binder composition for a non-aqueous secondary battery positive electrode of the present invention is a binder composition for a non-aqueous secondary battery positive electrode containing a polymer, wherein the polymer contains a nitrile group-containing monomer unit, the weight average molecular weight of the polymer is 300,000 or less, and the sulfur content of the polymer is 500 ppm or more. Thus, according to the binder composition for a non-aqueous secondary battery positive electrode containing a polymer containing a nitrile group-containing monomer unit and having a weight average molecular weight and a sulfur content within a predetermined range, a conductive material dispersion excellent in dispersibility and a secondary battery excellent in high-temperature storage characteristics can be formed. In the present invention, the "monomer unit" of the polymer means "a repeating unit derived from the monomer contained in the polymer obtained using the monomer". In addition, the weight average molecular weight and the sulfur content of the polymer can be measured according to the methods described in the examples of this specification.

[0012] In the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention, it is preferable that the polymer contains linear alkylene structural units having 4 or more carbon atoms in a proportion of 30% to 80% by mass. If the polymer contains linear alkylene structural units having 4 or more carbon atoms in a proportion of 30% to 80% by mass, the dispersibility of the conductive material can be improved when the binder composition is prepared. Furthermore, the "percentage of linear alkylene structural units with 4 or more carbon atoms" in polymers is, 1 It can be measured using nuclear magnetic resonance (NMR) methods such as 1H-NMR.

[0013] Furthermore, in the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention, it is preferable that the polymer contains the nitrile group-containing monomer units in a proportion of 10% by mass or more and 55% by mass or less. If the polymer contains the nitrile group-containing monomer units in a proportion of 10% by mass or more and 55% by mass or less, the dispersibility of the conductive material can be improved when the binder composition is prepared. Furthermore, the "percentage of nitrile group-containing monomer units" in polymers is, 1 It can be measured using nuclear magnetic resonance (NMR) methods such as 1H-NMR.

[0014] Furthermore, in the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention, it is preferable that the iodine value of the polymer is 5 mg / 100 mg or more and 100 mg / 100 mg or less. If the iodine value of the polymer is 5 mg / 100 mg or more and 100 mg / 100 mg or less, the dispersibility of the conductive material can be improved when the binder composition is prepared. The iodine value of the polymer can be measured according to the method described in the examples.

[0015] Furthermore, this invention aims to advantageously solve the above problems, and the conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention is characterized by containing a conductive material containing carbon nanotubes, a dispersion medium, and any of the above-described binder compositions for the positive electrode of a non-aqueous secondary battery. In this way, by using any of the above-described binder compositions for the positive electrode of a non-aqueous secondary battery in combination with carbon nanotubes as a conductive material, a conductive material dispersion with excellent dispersibility can be obtained, which can form a secondary battery with excellent high-temperature storage characteristics.

[0016] In this case, it is preferable that the conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention has a volume-average particle diameter D50 value in the particle size distribution of the dispersed carbon nanotubes that is 0.1 μm or more and 5.0 μm or less. If the volume-average particle diameter D50 value of the dispersed carbon nanotubes in the conductive material dispersion is 0.1 μm or more and 5.0 μm or less, the dispersibility of carbon nanotubes in the conductive material dispersion can be further improved. The volume-average particle size D50 in the particle size distribution of dispersed carbon nanotubes can be measured according to the method described in the examples of this specification.

[0017] Furthermore, this invention aims to advantageously solve the above-mentioned problems, and the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention is characterized by containing an electrode active material and any of the conductive material dispersions described above. If the slurry composition for the positive electrode contains the conductive material dispersion described above, a secondary battery with excellent high-temperature storage characteristics can be formed.

[0018] Furthermore, this invention aims to advantageously solve the above-mentioned problems, and the positive electrode for a non-aqueous secondary battery of the present invention is characterized by comprising a positive electrode composite layer formed using a slurry composition for a non-aqueous secondary battery positive electrode. In this way, by using a positive electrode having a positive electrode composite layer formed using the above-mentioned slurry composition for a positive electrode, a non-aqueous secondary battery with excellent high-temperature storage characteristics can be stably obtained.

[0019] Furthermore, this invention aims to advantageously solve the above-mentioned problems, and the non-aqueous secondary battery of the present invention is characterized by comprising the above-described positive electrode for non-aqueous secondary batteries, a negative electrode, an electrolyte, and a separator. In this way, by using the above-described positive electrode for non-aqueous secondary batteries, a non-aqueous secondary battery with excellent high-temperature storage characteristics can be obtained. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a binder composition for a positive electrode of a non-aqueous secondary battery that can form a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous conductive material dispersion for the positive electrode of a secondary battery that has excellent dispersibility and can form a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous slurry composition for a positive electrode of a secondary battery that can form a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a positive electrode for a non-aqueous secondary battery that can form a secondary battery with excellent high-temperature storage characteristics, and a secondary battery with excellent high-temperature storage characteristics. [Modes for carrying out the invention]

[0021] Embodiments of the present invention will be described in detail below. Herein, the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention can be used in the preparation of the conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention. Furthermore, the conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention can be used in the preparation of the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention. Moreover, the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention can be used when manufacturing the positive electrode of a non-aqueous secondary battery such as a lithium-ion secondary battery. Furthermore, the non-aqueous secondary battery of the present invention is characterized by using a positive electrode for a non-aqueous secondary battery of the present invention formed using the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention.

[0022] (Binder composition for positive electrodes of non-aqueous secondary batteries) The binder composition for the positive electrode of a non-aqueous secondary battery of the present invention is a binder composition for the positive electrode of a non-aqueous secondary battery that contains a predetermined polymer. Here, the predetermined polymer is characterized by containing nitrile group-containing monomer units, having a weight-average molecular weight of 300,000 or less, and having a sulfur content of 500 ppm or more. Since the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention contains a polymer that satisfies the above predetermined composition and properties, it can form a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high-temperature storage characteristics. In addition to the above predetermined polymer, the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention may optionally contain a solvent and other components.

[0023] <polymer> The polymer is a component that can effectively disperse carbon nanotubes as a conductive material in a conductive material dispersion prepared using a binder composition. Furthermore, in a positive electrode manufactured by forming a positive electrode composite layer on a current collector using a non-aqueous slurry composition for secondary battery positive electrodes containing the conductive material dispersion, the polymer is a component that can suppress the occurrence of side reactions by protecting the surface of substances that contribute to electrochemical reactions, such as the conductive material and positive electrode active material. This can improve the high-temperature storage characteristics of the resulting secondary battery. In addition, the polymer can also function to retain components contained in the positive electrode composite layer so that they do not detach from the positive electrode composite layer.

[0024] <Composition of polymer> The polymer must contain nitrile group-containing monomer units, and preferably also contains linear alkylene structural units having 4 or more carbon atoms. Furthermore, optionally, the polymer may also contain repeating units other than nitrile group-containing monomer units and linear alkylene structural units having 4 or more carbon atoms.

[0025] [Nitrile group-containing monomer unit] Examples of nitrile group-containing monomers that can form nitrile group-containing monomer units include α,β-ethylenically unsaturated nitrile monomers. The α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, but examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. Among these, acrylonitrile and methacrylonitrile are preferred as nitrile group-containing monomers, and acrylonitrile is more preferred. These can be used individually or in combination of two or more types.

[0026] Furthermore, the amount of nitrile group-containing monomer units in the polymer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, preferably 55% by mass or less, and more preferably 40% by mass or less, when the total repeating units (sum of structural units and monomer units) in the polymer are taken as 100% by mass. If the amount of nitrile group-containing monomer units in the polymer is within the above range, the dispersibility of the conductive material can be improved when a binder composition is prepared.

[0027] [Linear alkylene structural units with 4 or more carbon atoms] The linear alkylene structural unit having 4 or more carbon atoms has the general formula: -C n H 2n -[where n is an integer of 4 or more] is a repeating unit composed only of linear alkylene structures. When the polymer contains linear alkylene structural units with 4 or more carbon atoms, the dispersibility of the conductive material can be improved when it is mixed with carbon nanotubes as a conductive material to prepare a conductive material dispersion.

[0028] The method for introducing linear alkylene structural units having 4 or more carbon atoms into the polymer is not particularly limited, but for example, the following methods (1) or (2): (1) A method for preparing a polymer from a monomer composition containing a conjugated diene monomer, and converting the conjugated diene monomer units into linear alkylene structural units having 4 or more carbon atoms by hydrogenation of the polymer. (2) Method for preparing a polymer from a monomer composition containing a 1-olefin monomer having 4 or more carbon atoms. These include (1), which is preferred because it facilitates the production of polymers.

[0029] In other words, the linear alkylene structural unit having 4 or more carbon atoms is preferably a structural unit obtained by hydrogenating a conjugated diene monomer unit (conjugated diene hydride unit), and more preferably a structural unit obtained by hydrogenating a 1,3-butadiene unit (1,3-butadiene hydride unit). Examples of 1-olefin monomers having four or more carbon atoms include 1-butene and 1-hexene. These conjugated diene monomers and 1-olefin monomers can be used individually or in combination of two or more.

[0030] Examples of conjugated diene monomers that can be used in the method described in (1) above include conjugated diene compounds such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Among these, 1,3-butadiene is preferred. That is, the linear alkylene structural unit having 4 or more carbon atoms is preferably a structural unit obtained by hydrogenating a conjugated diene monomer unit (conjugated diene hydride unit), and more preferably a structural unit obtained by hydrogenating a 1,3-butadiene unit (1,3-butadiene hydride unit). The hydrogenation can be carried out using known methods as described later.

[0031] Furthermore, if the conjugated diene monomer units are not completely hydrogenated when linear alkylene structural units having 4 or more carbon atoms are introduced into the polymer via the method described in (1) above, conjugated diene monomer units may remain in the polymer. In other words, the polymer may contain conjugated diene monomer units as arbitrary repeating units.

[0032] The content of linear alkylene structural units having 4 or more carbon atoms in the polymer is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, when the total repeating units (sum of structural units and monomer units) in the polymer are taken as 100% by mass. If the content of linear alkylene structural units having 4 or more carbon atoms in the polymer is within the above range, the dispersibility of the conductive material can be further improved when a binder composition is prepared.

[0033] Furthermore, if the polymer contains conjugated diene monomer units, it is preferable that the total content ratio of conjugated diene monomer units and linear alkylene structural units having 4 or more carbon atoms in the polymer satisfies the above preferred range.

[0034] -Other repeating units- Furthermore, monomers that can form monomer units other than nitrile group-containing monomer units, linear alkylene structural units having 4 or more carbon atoms, and conjugated diene units (hereinafter sometimes referred to as "other monomers") are not particularly limited, but include (meth)acrylic acid ester monomers; styrene monomers; polymerizable monomers having hydrophilic groups; and the like. These monomers can be used individually or in combination of two or more. In this invention, "(meth)acrylic" means acrylic and / or methacrylic.

[0035] Here, the (meth)acrylic acid ester monomers include alkyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; Examples include alkyl methacrylates such as ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate.

[0036] Furthermore, polymerizable monomers having hydrophilic groups include monomers having carboxylic acid groups, monomers having sulfonic acid groups, monomers having phosphate groups, and monomers having hydroxyl groups.

[0037] Examples of monomers having a carboxylic acid group include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides, and their derivatives. Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid. Examples of monocarboxylic acid derivatives include 2-ethyl acrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, and β-diaminoacrylic acid. Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methyl maleic acid, dimethyl maleic acid, phenyl maleic acid, chloro maleic acid, dichloro maleic acid, fluoromaleic acid, and maleic acid esters such as methyl allyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of dicarboxylic acid acid anhydrides include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Furthermore, as monomers having a carboxylic acid group, acid anhydrides that generate a carboxyl group by hydrolysis can also be used. Other examples include monoesters and diesters of α,β-ethylenically unsaturated polycarboxylic acids such as monoethyl maleate, diethyl maleate, monobutyl maleate, dibutyl maleate, monoethyl fumarate, diethyl fumarate, monobutyl fumarate, dibutyl fumarate, monocyclohexyl fumarate, dicyclohexyl fumarate, monoethyl itaconate, diethyl itaconate, monobutyl itaconate, and dibutyl itaconate.

[0038] Examples of monomers having a sulfonic acid group include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, ethyl (meth)acrylate-2-sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-alyloxy-2-hydroxypropanesulfonic acid. In this invention, "(meth)allyl" means allyl and / or metallyl.

[0039] Examples of monomers containing a phosphate group include 2-(meth)acryloyloxyethyl phosphate, methyl-2-(meth)acryloyloxyethyl phosphate, and ethyl-(meth)acryloyloxyethyl phosphate. In this invention, "(meth)acryloyl" means acryloyl and / or methacryloyl.

[0040] Examples of the monomer having a hydroxyl group include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-buten-1-ol, and 5-hexen-1-ol; alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; general formula: CH2=CR 1 -COO-(C n H 2n O) m -H (where m is an integer from 2 to 9, n is an integer from 2 to 4, and R 1Polyalkylene glycols represented by (where represents hydrogen or a methyl group) and (meth)acrylic acid esters; mono(meth)acrylic acid esters of dihydroxy esters of dicarboxylic acids such as 2-hydroxyethyl-2'-(meth)acryloyl oxyphthalate and 2-hydroxyethyl-2'-(meth)acryloyl oxysuccinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; alkyl groups such as (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether Examples include mono(meth)allyl ethers of alkylene glycols; polyoxyalkylene glycol mono(meth)allyl ethers such as diethylene glycol mono(meth)allyl ether and dipropylene glycol mono(meth)allyl ether; mono(meth)allyl ethers of halogen and hydroxy-substituted (poly)alkylene glycols such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether, and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol and their halogen-substituted derivatives; and (meth)allyl thioethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl thioether and (meth)allyl-2-hydroxypropyl thioether.

[0041] Furthermore, if the polymer contains other monomer units, it is preferable that the proportion of other monomer units in the polymer be 10% by mass or less. This is because limiting the proportion of other monomer units to 10% by mass or less improves the dispersibility of the conductive material when a binder composition is prepared. Needless to say, the polymer may not contain other monomer units.

[0042] <Method for preparing polymers> The method for producing the polymer described above is not particularly limited, and any of the following methods can be used, for example, solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization. Furthermore, addition polymerization methods such as ionic polymerization, radical polymerization, and living radical polymerization can be used as polymerization methods. In addition, known polymerization initiators, such as redox polymerization initiators containing iron-based compounds, can be used as polymerization initiators.

[0043] Furthermore, it is preferable to use a molecular weight modifier having a sulfur-containing group such as a mercapto group during polymerization. Examples of compounds having a mercapto group that can be used as molecular weight modifiers include compounds having 8 to 12 carbon atoms such as octyl mercaptan, 2,2,4,6,6-pentamethyl-4-heptanethiol, 2,4,4,6,6-pentamethyl-2-heptanethiol, 2,3,4,6,6-pentamethyl-2-heptanethiol, 2,3,4,6,6-pentamethyl-3-heptanethiol, t-dodecyl mercaptan, and n-dodecyl mercaptan; and compounds having a mercapto group such as 2,2,4,6,6-pentamethyl-4-octanthiol, 2,2,4,6,6,8,8-heptamethyl-4-nonanthiol, bis(2-mercaptoethyl) sulfide, methyl 3-mercaptopropionate, and 1-butanethiol. Among these, compounds having a mercapto group with 8 to 12 carbon atoms are preferred, and t-dodecyl mercaptan is more preferred. Furthermore, the amount of the compound having a mercapto group used as a molecular weight adjuster can be determined so that the sulfur content in the resulting polymer reaches a desired value.

[0044] Furthermore, when producing the polymer described above by the method described in (1) above, it is preferable to use radical polymerization using a redox polymerization initiator containing an iron-based compound as the polymerization method for the polymer to be hydrogenated. The redox polymerization initiator is not particularly limited, and for example, a combination of cumene hydroperoxide, monosodium iron ethylenediaminetetraacetate, sodium hydroxymethanesulfinate, and tetrasodium ethylenediaminetetraacetate (EDTA·4Na) can be used. Also, when producing the polymer described above by the method described in (1) above, it is preferable to perform emulsion polymerization, then coagulate with a coagulant and recover the product, and then hydrogenate the recovered product (optionally after carrying out a "double decomposition reaction" described later). Furthermore, hydrogenation can be carried out using known hydrogenation methods such as oil-layer hydrogenation or aqueous-layer hydrogenation. In addition, any known selective hydrogenation catalyst can be used as the catalyst for hydrogenation, including palladium-based and rhodium-based catalysts. Two or more of these may be used in combination.

[0045] Furthermore, the hydrogenation of the polymer may be carried out using, for example, the method described in Japanese Patent No. 4509792. Specifically, the hydrogenation of the polymer may be carried out after a double decomposition reaction of the polymer in the presence of a catalyst and a co(co-)olefin. Here, known ruthenium-based catalysts can be used as catalysts for the double decomposition reaction. In particular, it is preferable to use Grubbs catalysts such as bis(tricyclohexylphosphine)benzylideneruthenium dichloride or 1,3-bis(2,4,6-trimethylphenyl)-2-(imidazolidinylidene)(dichlorophenylmethylene)(tricyclohexylphosphine)ruthenium as catalysts for the double decomposition reaction. As coolefins, olefins having 2 to 16 carbon atoms such as ethylene, isobutane, and 1-hexane can be used. Furthermore, as hydrogenation catalysts when hydrogenation is carried out after the double decomposition reaction, known homogeneous hydrogenation catalysts such as Wilkinson catalyst ((PPh3)3RhCl) can be used.

[0046] <Properties of the polymer> [Weight-average molecular weight of polymers] The weight-average molecular weight of the polymer must be 300,000 or less, as described above. In addition, it is preferable that the weight-average molecular weight of the polymer be 2,000 or more, more preferably 7,000 or more, even more preferably 8,500 or more, preferably 150,000 or less, more preferably 100,000 or less, and even more preferably 10,000 or less. If the weight-average molecular weight of the polymer is within the above range, the dispersibility of carbon nanotubes in a conductive material dispersion containing carbon nanotubes as a conductive material can be improved. The average molecular weight of the polymer can be controlled, for example, by adjusting the amount of molecular weight adjusting agent added during polymerization.

[0047] [Sulfur content of polymer] As mentioned above, the sulfur content of the polymer must be 500 ppm or more. In addition, it is preferable that the sulfur content of the polymer be 1,000 ppm or more, more preferably 3,000 ppm or more, even more preferably 5,000 ppm or more, preferably 20,000 ppm or less, and more preferably 10,000 ppm or less. The inclusion of sulfur atoms in the polymer molecule can increase its oxidation resistance. Furthermore, such a polymer protects the surface of the positive electrode active material and the conductive material carbon nanotubes in the positive electrode of a secondary battery, thereby suppressing side reactions inside the secondary battery and consequently improving the high-temperature storage characteristics of the secondary battery. It is preferable that the sulfur atoms are not simply attached to the polymer, but are bonded to the polymer chains of the polymer. By keeping the sulfur content of the polymer within the above range, the high-temperature storage characteristics of the secondary battery can be further improved. The sulfur content of the polymer can be controlled, for example, based on the amount of a molecular weight adjusting agent containing sulfur-containing groups such as mercapto groups added during polymerization.

[0048] [Iodine value of polymers] The polymer preferably has an iodine value of 5 mg / 100 mg or more, more preferably 10 mg / 100 mg or more, more preferably 20 mg / 100 mg or more, more preferably 100 mg / 100 mg or less, more preferably 80 mg / 100 mg or less, and even more preferably 70 mg / 100 mg or less. If the iodine value of the polymer is within the above range, the dispersibility of carbon nanotubes in a conductive material dispersion containing carbon nanotubes as a conductive material can be improved. The iodine value of the polymer can be controlled, for example, based on the amount of hydrogenation catalyst used when hydrogenating the polymer.

[0049] <Solvent> Organic solvents can be used as the solvent to be included in the binder composition for the positive electrode of non-aqueous secondary batteries. Examples of organic solvents include N-methylpyrrolidone (NMP), N,N-dimethylformamide, and acetone. Among these, N-methylpyrrolidone (NMP) is preferred from the viewpoint of the stability of the binder composition for the positive electrode of non-aqueous secondary batteries.

[0050] <Other ingredients> The binder composition for the positive electrode of a non-aqueous secondary battery of the present invention may contain, in addition to the above components, components such as reinforcing agents, leveling agents, viscosity modifiers, and electrolyte additives. These are not particularly limited as long as they do not affect the battery reaction, and known components, such as those described in International Publication No. 2012 / 115096, can be used. Furthermore, these components may be used individually or in combination of two or more components in any ratio.

[0051] <Preparation of binder composition for positive electrode of non-aqueous secondary battery> If the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention contains a solvent, it can be prepared by dissolving or dispersing the polymer described above in the solvent. Specifically, the binder composition for the positive electrode of a non-aqueous secondary battery can be prepared by mixing the above-mentioned components with the solvent using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, lye crusher, ultrasonic disperser, homogenizer, planetary mixer, or film mixer. Furthermore, if the polymer used is prepared as an aqueous dispersion, the polymer may be solidified with a coagulant, the water removed, and then mixed with an organic solvent to form a binder composition, or the aqueous dispersion of the polymer and the organic solvent may be mixed, and then the water removed to form a binder composition for the positive electrode of a non-aqueous secondary battery.

[0052] (Conductive material dispersion for non-aqueous secondary battery positive electrode) The conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention contains a conductive material containing carbon nanotubes, a dispersion medium, and the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention described above. Because the conductive material dispersion of the present invention contains the binder composition for the positive electrode of a non-aqueous secondary battery of the present invention described above, it is possible to form a secondary battery that has excellent dispersibility and excellent high-temperature storage characteristics.

[0053] <Conductive material> The conductive material dispersion for the positive electrode of a non-aqueous secondary battery of the present invention requires the inclusion of carbon nanotubes as a conductive material. Here, the conductive material is a component that can be blended to promote electrical contact between electrode active materials in the electrode composite layer. Other conductive materials besides carbon nanotubes include conductive carbon materials such as carbon black (e.g., acetylene black, Ketjenblack®, furnace black, etc.), graphite, carbon fibers other than carbon nanotubes, and carbon flakes; and various metal fibers and foils. These can be used individually or in combination of two or more types.

[0054] Furthermore, the BET specific surface area of ​​the conductive material is preferably 100 m². 2 / g or more, comfortable 150m 2 It is 1 / g or more, and usually 2500m 2 It is less than or equal to / g. If the BET specific surface area of ​​the conductive material is greater than or equal to the above lower limit, good interaction with the polymer contained in the binder composition of the present invention can occur, further improving the high-temperature storage characteristics of the resulting secondary battery. Good conductive paths can be formed in the electrode composite layer, further improving the output characteristics of the secondary battery. Also, if the BET specific surface area of ​​the conductive material is less than or equal to the above upper limit, aggregation of the conductive material can be suppressed, and the dispersibility of the conductive material can be ensured.

[0055] Normally, carbon nanotubes tend to aggregate and are difficult to disperse. However, the conductive material dispersion of the present invention uses a binder composition containing a polymer that satisfies the predetermined composition and properties described above, so carbon nanotubes can be dispersed well and stably.

[0056] <Dispersion medium> As the dispersion medium, various organic solvents listed in the <solvent> section above can be used. The amount of dispersion medium can be determined so that the solid content concentration of the conductive material dispersion is, for example, 1% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more, for example 30% by mass or less, preferably 25% by mass or less, and more preferably 20% by mass or less.

[0057] <Other ingredients> Other components that can be incorporated into the conductive material dispersion are not particularly limited and include those similar to the other components that can be incorporated into the binder composition of the present invention. Furthermore, these other components may be used individually or in combination of two or more components in any ratio.

[0058] <D50 value of carbon nanotubes in conductive material dispersion> The volume-average particle size D50 of the dispersed carbon nanotubes in a conductive material dispersion is preferably 0.1 μm or more and 5.0 μm or less, more preferably 4.0 μm or less, and even more preferably 2.0 μm or less. If the D50 value of the carbon nanotubes in the conductive material dispersion is within the above range, the dispersibility of the conductive material dispersion can be further improved.

[0059] <Method for manufacturing conductive material dispersion> A conductive material dispersion can be prepared by mixing the binder composition of the present invention described above with a conductive material, a dispersion medium, and any other components. If the binder composition contains a solvent, the solvent can be used as is as the dispersion medium. The mixing method is not particularly limited, but includes the method described above in the section on "Preparation of Binder Composition for Non-Aqueous Secondary Battery Positive Electrode". Here, the content ratio of the conductive material to the polymer of the specific composition described above in the conductive material dispersion is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, preferably 100 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 30 parts by mass or less, with the conductive material content being 100 parts by mass and the polymer content being 15 parts by mass or more.

[0060] (Slurry composition for positive electrodes of non-aqueous secondary batteries) The slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention comprises a positive electrode active material and the conductive material dispersion of the present invention described above. That is, the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention comprises a positive electrode active material, a polymer, carbon nanotubes as a conductive material, and a solvent, and further contains any other components. By using the slurry composition for the positive electrode of a non-aqueous secondary battery of the present invention, a secondary battery with excellent high-temperature storage characteristics can be formed.

[0061] <Conductive material dispersion> As the conductive material dispersion, the conductive material dispersion of the present invention described above is used.

[0062] <Cathode active material> The positive electrode active material is a material that transfers electrons in the positive electrode of a secondary battery. And, for example, as the positive electrode active material for a lithium-ion secondary battery, usually, a material that can occlude and release lithium is used. Incidentally, hereinafter, as an example, the positive electrode active material in the case where a non-aqueous secondary battery is a lithium-ion secondary battery will be described, but the present invention is not limited to the following example.

[0063] Specifically, the positive electrode active material for a lithium-ion secondary battery is not particularly limited, and includes lithium-containing cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxides of Co-Ni-Mn, lithium-containing composite oxides of Ni-Mn-Al, lithium-containing composite oxides of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), lithium-excess spinel compounds represented by Li 1+x Mn 2-x O4 (0 < X < 2), Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2, LiNi 0.5 Mn 1.5 O4 and other known positive electrode active materials. Incidentally, as the lithium-containing composite oxides of Co-Ni-Mn, Li(Ni 0.6 Co 0.2 Mn 0.2 )O2, Li(Ni 0.5 Co 0.2 Mn 0.3 )O2, Li(Ni 1 / 3 Co 1 / 3 Mn 1 / 3 )O2 and the like can be mentioned. Among the above, from the viewpoint of improving the battery capacity and the like of the secondary battery, as the positive electrode active material, lithium-containing cobalt oxide (LiCoO2), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxides of Co-Ni-Mn, Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2 or LiNi0.5 Mn 1.5 It is preferable to use O4, and more preferable to use a lithium-containing composite oxide of Co-Ni-Mn. Furthermore, the amount and particle size of the positive electrode active material are not particularly limited and can be the same as those of conventionally used positive electrode active materials.

[0064] <Content ratio> In the slurry composition, the content ratio of the conductive material is preferably 0.01 parts by mass or more and 20 parts by mass or less, based on the content of the positive electrode active material at 100 parts by mass. If the ratio of the conductive material is above the lower limit, electrical contact between the positive electrode active materials can be promoted. Furthermore, if the amount of conductive material is below the upper limit, the coating properties of the slurry composition can be improved. Furthermore, the preferred content ratio of the polymer in the slurry composition may be within a preferred range that can be derived from the preferred range of polymer relative to the conductive material, as described above in the section on <Method for producing conductive material dispersion>, and the content ratio between the positive electrode active material and the conductive material, as explained at the beginning of this paragraph.

[0065] <Other ingredients> Other components that can be incorporated into the slurry composition are not particularly limited and include those similar to those that can be incorporated into the binder composition of the present invention. Furthermore, these other components may be used individually or in combination of two or more components in any ratio. In particular, when the slurry composition for the positive electrode of a secondary battery is a slurry composition for the positive electrode of a lithium-ion secondary battery, it is preferable to use a fluorine-containing polymer such as polyvinylidene fluoride (PVdF) as a binder in addition to the polymer described above. If the slurry composition contains one or more binders in addition to the polymer described above, the proportion of the polymer described above may be 5 to 50 parts by mass, with the total content of such binders being 100 parts by mass. If the content of the polymer described above in the slurry composition is above the lower limit, the high-temperature storage characteristics of the resulting secondary battery can be further improved. Also, if the content of the polymer described above is below the upper limit, good adhesive strength can be achieved in the electrode composite layer.

[0066] <Method for producing slurry composition> The slurry composition described above can be prepared by dissolving or dispersing each of the above components in a solvent such as an organic solvent. For example, it is preferable to prepare the slurry composition of the present invention by adding a positive electrode active material, a solvent, and optional components to the conductive dispersion described above and mixing them using the known method described above.

[0067] (Positive electrode for non-aqueous secondary batteries) The positive electrode for a non-aqueous secondary battery of the present invention comprises a positive electrode composite layer formed using the slurry composition for a non-aqueous secondary battery positive electrode of the present invention. More specifically, the positive electrode for a secondary battery of the present invention comprises a positive electrode composite layer formed on a current collector using the slurry composition for a positive electrode of the present invention. That is, the electrode composite layer contains at least an electrode active material, a polymer, and carbon nanotubes as a conductive material. The components contained in the electrode composite layer are those contained in the slurry composition for secondary batteries, and the preferred ratio of these components is the same as the preferred ratio of each component in the slurry composition. Furthermore, by using the positive electrode for non-aqueous secondary batteries of the present invention, it is possible to form a secondary battery with excellent high-temperature storage characteristics.

[0068] <Manufacturing method for positive electrodes for non-aqueous secondary batteries> The positive electrode for a non-aqueous secondary battery of the present invention is manufactured, for example, by a step of applying the above-described slurry composition for a non-aqueous secondary battery positive electrode onto a current collector (coating step) and a step of drying the slurry composition applied onto the current collector to form a positive electrode composite layer on the current collector (drying step). Furthermore, the positive electrode for non-aqueous secondary batteries of the present invention can also be manufactured by preparing composite particles by drying and granulating the above-described slurry composition for the positive electrode of a non-aqueous secondary battery, and then forming a positive electrode composite layer on a current collector using these composite particles.

[0069] [Coating process] The method for applying the above-mentioned non-aqueous secondary battery positive electrode slurry composition onto the current collector is not particularly limited and any known method can be used. Specifically, the application method can be the doctor blade method, dip method, reverse roll method, direct roll method, gravure method, extrusion method, brush application method, etc. In this case, the non-aqueous secondary battery positive electrode slurry composition may be applied to only one side of the current collector or to both sides. The thickness of the slurry film on the current collector before drying after application can be appropriately set according to the thickness of the positive electrode composite layer obtained after drying.

[0070] Here, the current collector to which the slurry composition for the positive electrode of a non-aqueous secondary battery is applied is a material that is electrically conductive and electrochemically durable. Specifically, the current collector can be made of, for example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. Among these, aluminum foil is particularly preferred as the current collector used for the positive electrode. Note that one type of material may be used alone, or two or more types may be used in any ratio.

[0071] [Drying process] The method for drying the slurry composition for the positive electrode of a non-aqueous secondary battery on a current collector is not particularly limited and known methods can be used, such as drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams. By drying the slurry composition for the positive electrode of a non-aqueous secondary battery on a current collector in this way, a positive electrode composite layer is formed on the current collector, and a positive electrode for a non-aqueous secondary battery comprising a current collector and a positive electrode composite layer can be obtained.

[0072] Furthermore, in the method for manufacturing a positive electrode for a non-aqueous secondary battery of the present invention, after the drying process, the positive electrode composite layer may be subjected to pressure treatment using a mold press or a roll press. Pressure treatment can improve the adhesion between the positive electrode composite layer and the current collector. Furthermore, if the positive electrode composite layer contains a curable polymer, it is preferable to cure the polymer after the positive electrode composite layer has been formed.

[0073] (Non-aqueous secondary battery) The non-aqueous secondary battery of the present invention comprises a positive electrode, a negative electrode, an electrolyte, and a separator, and uses the positive electrode for non-aqueous secondary batteries of the present invention as the positive electrode. Furthermore, because the non-aqueous secondary battery of the present invention is equipped with the positive electrode for non-aqueous secondary batteries of the present invention, it exhibits excellent high-temperature storage characteristics. In the following explanation, we will describe the case where the non-aqueous secondary battery is a lithium-ion secondary battery as an example, but the present invention is not limited to the example described below.

[0074] <Negative electrode> As the negative electrode, a known negative electrode can be used. Specifically, as the negative electrode, for example, a negative electrode made of a thin plate of metallic lithium or a negative electrode made by forming a negative electrode composite layer on a current collector can be used. Furthermore, the current collector can be made of metal materials such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, or platinum. The negative electrode composite layer can be a layer containing a negative electrode active material and a binder. Moreover, the binder is not particularly limited and any known material can be used.

[0075] <Electrolyte> Typically, an organic electrolyte is used, which is obtained by dissolving a supporting electrolyte in an organic solvent. For example, lithium salts are used as supporting electrolytes. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Among these, LiPF6, LiClO4, and CF3SO3Li are preferred, with LiPF6 being particularly preferred, because they are easily soluble in the solvent and exhibit a high degree of dissociation. Note that one type of electrolyte may be used alone, or two or more types may be used in any ratio. Generally, the lithium ion conductivity tends to increase as the supporting electrolyte with a higher degree of dissociation is used, so the lithium ion conductivity can be adjusted by the type of supporting electrolyte.

[0076] The organic solvent used in the electrolyte is not particularly limited as long as it can dissolve the supporting electrolyte, but suitable examples include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. A mixture of these solvents may also be used. The concentration of the electrolyte in the electrolyte can be adjusted as appropriate.

[0077] <Separator> The separator is not particularly limited, and for example, those described in Japanese Patent Publication No. 2012-204303 can be used. Among these, a microporous membrane made of polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it allows for a thinner overall film thickness of the separator, thereby increasing the ratio of electrode active material in the secondary battery and thus increasing the capacity per unit volume.

[0078] <Manufacturing method for secondary batteries> The secondary battery of the present invention can be manufactured, for example, by stacking a positive electrode and a negative electrode with a separator in between, winding or folding them as needed according to the battery shape, placing them in a battery container, injecting an electrolyte into the battery container, and sealing it. To prevent pressure rise inside the secondary battery, overcharging and discharging, etc., an overcurrent prevention element such as a fuse or PTC element, expanded metal, lead plates, etc. may be provided as needed. The shape of the secondary battery may be any of the following: coin type, button type, sheet type, cylindrical type, rectangular type, flat type, etc. [Examples]

[0079] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" used to express quantities refer to mass unless otherwise specified. In the examples and comparative examples, various attributes and evaluations were measured or performed as follows.

[0080] <Weight-average molecular weight of polymers> The weight-average molecular weight (Mw) of the polymer was measured by gel permeation chromatography (GPC) using a 10 mM LiBr-DMF solution under the following measurement conditions. • Separation column: Shodex KD-806M (manufactured by Showa Denko Corporation) • Detector: Differential refractometer detector RID-10A (manufactured by Shimadzu Corporation) • Eluent flow rate: 0.3 mL / min Column temperature: 40°C • Standard polymer: TSK standard polystyrene (manufactured by Tosoh Corporation) <Sulfur content of polymer> The NMP solutions of the polymers obtained in the examples and comparative examples were distilled under reduced pressure to remove the solvent and obtain samples. Approximately 0.02 g of the sample was weighed onto a magnetic board and combusted in an automatic combustion apparatus (Yanaco), after which the amount of sulfur contained was quantified by ion chromatography (Metrohm 930 Compact IC Flex). The amount of sulfur contained was quantified as the amount of sulfur contained per gram of polymer mass (μg), i.e., the amount based on the mass of the polymer (ppm). <Iodine value of polymers> 100 g of aqueous dispersions of the polymers prepared in the examples and comparative examples were coagulated with 1 L of methanol, and then vacuum-dried at 60°C for 12 hours. The iodine value of the resulting dried polymer was then measured according to JIS K6235 (2006). <D50 of carbon nanotubes in conductive material dispersion> In the examples and comparative examples, the volume-based D50 diameter of carbon nanotubes in a conductive material dispersion was measured using a dry integrated particle size distribution analyzer (Nikki Co., Ltd., "Microtrac MT3200II") with a dispersion air pressure of 0.02 MPa.

[0081] <Dispersibility of conductive materials in conductive material dispersions> The viscosity of the conductive material dispersions prepared in the examples and comparative examples was measured using a rheometer (MCR302, manufactured by Anton Paar) at a temperature of 25°C and a shear rate of 0.1 s. -1 Viscosity was measured under the following conditions and evaluated according to the following criteria. At the same solid content concentration, the lower the viscosity, the better the dispersibility of the conductive material in the conductive material dispersion. A: Viscosity less than 250 Pa·s B: Viscosity between 250 Pa·s and less than 500 Pa·s C: Viscosity between 500 Pa·s and less than 750 Pa·s D: Viscosity of 750 Pa·s or higher <Viscosity stability of conductive material dispersions> The viscosity η0 of the conductive material dispersions obtained in the examples and comparative examples was measured using a Type B viscometer (Toki Sangyo Co., Ltd., product name "TVB-10", rotation speed: 60 rpm). Next, the conductive material dispersions whose viscosity was measured were stirred for 24 hours using a planetary mixer (rotation speed: 60 rpm), and the viscosity η1 of the conductive material dispersion after stirring was measured using the same Type B viscometer (rotation speed: 60 rpm). The viscosity retention rate Δη of the conductive material dispersion before and after stirring was calculated as η1 / η0 × 100 (%), and the viscosity stability of the conductive material dispersion was evaluated according to the following criteria. The temperature during viscosity measurement was 25°C. A viscosity retention rate Δη value closer to 100% indicates better viscosity stability of the conductive material dispersion. A: Viscosity maintenance rate Δη is 90% or more and 110% or less B: Viscosity retention rate Δη is 80% or more but less than 90% C: Viscosity retention rate Δη is 70% or more but less than 80%

[0082] <High temperature storage characteristics> The lithium-ion secondary batteries prepared in the examples and comparative examples were left to stand for 5 hours at 25°C after electrolyte injection. Next, they were charged to a cell voltage of 3.65V using a constant current method at 25°C and 0.2C, and then aged for 12 hours at 60°C. Then, they were discharged to a cell voltage of 3.00V using a constant current method at 25°C and 0.2C. Subsequently, CC-CV charging (upper limit cell voltage 4.20V) was performed with a constant current of 0.2C, and CC discharge was performed to a cell voltage of 3.00V with a constant current of 0.2C. This charging and discharging at 0.2C was repeated three times. The discharge capacity of the third discharge at 0.2C was defined as the initial capacity CX. Subsequently, CC-CV charging (upper limit cell voltage 4.20V) was performed with a constant current of 0.2C. Next, the lithium-ion secondary batteries were stored for 4 weeks in an inert oven with a nitrogen atmosphere at 60°C. Subsequently, the battery was discharged to a cell voltage of 3.00V using a constant current method at 0.2C, and the discharge capacity at this time was defined as CY. The high-temperature capacity retention rate, expressed as (CY / CX) × 100 (%), was calculated and evaluated according to the following criteria. A higher high-temperature capacity retention rate indicates less degradation of the lithium-ion secondary battery during high-temperature storage (i.e., superior high-temperature storage characteristics). A: High-temperature capacity retention rate is 90% or higher B: High-temperature capacity retention rate is 85% or more but less than 90% C: High-temperature capacity retention rate is 80% or more but less than 85% D: High-temperature capacity retention rate is less than 80%

[0083] (Example 1) <Preparation of polymers> In the reactor, 200 parts of deionized water, 25 parts of a 10% aqueous solution of sodium dodecylbenzenesulfonate, 36 parts of acrylonitrile as a nitrile group-containing monomer unit, and 7.90 parts of t-dodecyl mercaptan as a molecular weight modifier were charged in order. Next, the internal gas was replaced three times with nitrogen, and then 64 parts of 1,3-butadiene as a conjugated diene monomer were charged. The reactor was then kept at 5°C, and 0.03 parts of cumene hydroperoxide as a polymerization initiator, appropriate amounts of a reducing agent and a chelating agent were charged. The polymerization reaction was continued with stirring, and when the polymerization conversion rate reached 80%, 0.1 parts of a 10% aqueous solution of hydroquinone as a polymerization termination agent was added to stop the polymerization reaction. Next, residual monomers were removed at a water temperature of 80°C to obtain an aqueous dispersion of the polymer precursor. To obtain the target polymer aqueous dispersion, the aqueous dispersion of the precursor was subjected to a hydrogenation reaction at a hydrogen pressure of 3 MPa and a temperature of 50°C for 6 hours. This reaction was carried out by adding the aqueous dispersion and a palladium catalyst (a solution of 1% palladium acetate acetone solution mixed with an equal weight of deionized water) to an autoclave so that the palladium content relative to the weight of solids in the obtained aqueous dispersion of the precursor was 3,000 ppm. Subsequently, the contents were allowed to return to room temperature, the system was subjected to a nitrogen atmosphere, and then concentrated using an evaporator until the solid content concentration reached 40% to obtain a concentrated aqueous dispersion. Next, 200 parts of N-methylpyrrolidone were added to 100 parts of the concentrated aqueous dispersion. After evaporating all the water and residual monomers under reduced pressure, the N-methylpyrrolidone was evaporated to obtain an 8% by mass NMP solution (binder composition) of the polymer. The weight-average molecular weight of the obtained polymer was measured. The results are shown in Table 1.

[0084] <Preparation of conductive material dispersion> Carbon nanotubes as conductive materials (specific surface area: 250 m²) 26.0 parts of ( / g), 1.2 parts (solid content equivalent) of the above binder composition, and 92.8 parts of NMP were stirred using a disperser (3000 rpm, 10 minutes). Then, a bead mill using 1 mm diameter zirconia beads was used to mix the mixture at a peripheral speed of 8 m / s for 1 hour to produce a conductive material dispersion with a solid content of 7.2 mass%. The dispersibility and viscosity stability of the conductive material in the obtained conductive material dispersion were then evaluated according to the above procedure. The results are shown in Table 1.

[0085] <Preparation of slurry composition for secondary battery positive electrode and manufacture of positive electrode> In the conductive material dispersion described above, a ternary active material having a layered structure (LiNi 0.6 Co 0.2 Mn 0.2 A slurry for the positive electrode was prepared by adding 98.0 parts of O2 (average particle size: 10 μm), 1.0 part of polyvinylidene fluoride as a binder, 1.0 part of the above conductive material dispersion (in terms of solid content), and NMP, and mixing them in a planetary mixer (60 rpm, 30 minutes). The amount of NMP added was adjusted so that the viscosity of the resulting positive electrode slurry composition (measured using a single cylindrical rotational viscometer in accordance with JIS Z8803:1991; temperature: 25°C, rotation speed: 60 rpm) was within the range of 4000 to 5000 mPa·s. <Formation of the positive electrode> As a current collector, a 20 μm thick aluminum foil was prepared. The positive electrode slurry obtained as described above was coated onto one side of the aluminum foil using a comma coater, resulting in a basis weight of 20 mg / cm² after drying. 2 The material was applied to the surface, dried at 90°C for 20 minutes, then at 120°C for 20 minutes, and finally heat-treated at 60°C for 10 hours to obtain a cathode base. This cathode base was rolled using a roll press to obtain a density of 3.2 g / cm³. 3 A sheet-like positive electrode was fabricated consisting of a positive electrode composite layer and aluminum foil. The sheet-like positive electrode was then cut to a width of 48.0 mm and a length of 47 cm to be used as a positive electrode for a lithium-ion secondary battery. <Fabrication of negative electrodes for lithium-ion secondary batteries> In a 5 MPa pressure vessel equipped with a stirrer, 33 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 3.5 parts of itaconic acid as a carboxylic acid group-containing monomer, 63.5 parts of styrene as an aromatic vinyl monomer, 0.4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of deionized water, and 0.5 parts of potassium persulfate as a polymerization initiator were added and thoroughly stirred. Polymerization was then started by heating to 50°C. When the polymerization conversion rate reached 96%, the mixture was cooled to stop the polymerization reaction, and a mixture containing particulate binder (styrene-butadiene copolymer) was obtained. A 5% aqueous sodium hydroxide solution was added to the above mixture to adjust the pH to 8, and unreacted monomers were removed by heated vacuum distillation. The mixture was then cooled to below 30°C to obtain an aqueous dispersion containing a binder for the negative electrode. In a planetary mixer, 48.75 parts of artificial graphite and 48.75 parts of natural graphite were added as negative electrode active materials, along with 1 part of carboxymethylcellulose (equivalent to solid content) as a thickening agent. The mixture was then diluted with deionized water to a solid content concentration of 60%, and kneaded at a rotation speed of 45 rpm for 60 minutes. Subsequently, 1.5 parts (equivalent to solid content) of the aqueous dispersion containing the negative electrode binder obtained as described above was added, and the mixture was kneaded at a rotation speed of 40 rpm for 40 minutes. Finally, deionized water was added to achieve a viscosity of 3000 ± 500 mPa·s (measured with a B-type viscometer at 25°C and 60 rpm) to prepare the negative electrode slurry composition. The above-mentioned negative electrode slurry composition was applied to the surface of a 15 μm thick copper foil, which served as the current collector, using a comma coater, with a coating amount of 10 ± 0.5 mg / cm². Subsequently, the copper foil coated with the negative electrode slurry composition was transported at a speed of 400 mm / min in an oven at 80°C for 2 minutes, and then in an oven at 110°C for another 2 minutes, thereby drying the slurry composition on the copper foil and obtaining a negative electrode base with a negative electrode composite layer formed on the current collector. This negative electrode raw material is rolled in a roll press until it has a density of 1.6 g / cm³. 3 A sheet-like negative electrode was fabricated consisting of a negative electrode composite layer and aluminum foil. The sheet-like negative electrode was then cut to a width of 50.0 mm and a length of 52 cm to be used as a negative electrode for a lithium-ion secondary battery. <Manufacturing of lithium-ion secondary batteries> The fabricated positive electrode and negative electrode for lithium-ion secondary batteries were placed facing each other with their electrode mixture layers facing each other, and a 15 μm thick separator (microporous polyethylene membrane) was interposed between them. The mixture was then wound around a 20 mm diameter core to obtain a wound body. The resulting wound body was then compressed from one direction at a speed of 10 mm / second until its thickness reached 4.5 mm. The compressed wound body was elliptical in plan view, and its ratio of major axis to minor axis (major axis / minor axis) was 7.7. In addition, a 1.0 M LiPF6 solution was prepared as the electrolyte (solvent: a mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC) = 3 / 7 (volume ratio), additive: containing 2 vol% vinylene carbonate (solvent ratio)). Subsequently, the compressed coil was placed in an aluminum laminate case along with 3.2 g of electrolyte. Nickel lead wires were then connected to designated locations on the negative electrode and aluminum lead wires to designated locations on the positive electrode. The opening of the case was then sealed with heat to obtain the lithium-ion secondary battery, which serves as the electrochemical element of the present invention. This lithium-ion secondary battery was a pouch type with a width of 35 mm, a height of 60 mm, and a thickness of 5 mm, and its nominal capacity was 700 mAh. The high-temperature storage characteristics of the obtained lithium-ion secondary batteries were evaluated. The results are shown in Table 1.

[0086] (Examples 2-4) The amount of t-dodecyl mercaptan added as a molecular weight modifier during polymer preparation was adjusted so that the sulfur content and weight-average molecular weight of the resulting polymer were as shown in Table 1. Specifically, by reducing the amount of t-dodecyl mercaptan, the sulfur content of the resulting polymer was reduced and the weight-average molecular weight was increased, while by increasing the amount of t-dodecyl mercaptan, the sulfur content of the resulting polymer was increased and the weight-average molecular weight was decreased. Except for these points, various operations and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0087] (Example 5) Except for using polymers prepared as described below, various operations and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1. <Preparation of polymers> [Preparation of polymerization precursors] In a metal bottle, 0.2 parts sodium carbonate was dissolved in 200 parts deionized water. 2.5 parts potassium caprate (a soap made from a 10-carbon fatty acid) was added to the metal bottle as an emulsifier. Furthermore, an aqueous solution containing 1.0 part naphthalene sulfonic acid formaldehyde polycondensate was added as a dispersant. Then, 35 parts acrylonitrile as a nitrile group-containing monomer and 0.5 parts t-dodecyl mercaptan as a molecular weight modifier were added sequentially to the metal bottle. After replacing the gas inside the metal bottle with nitrogen three times, 64 parts 1,3-butadiene as a conjugated diene monomer were added. The metal bottle was kept at 5°C, and a redox polymerization initiator consisting of 0.1 parts cumene hydroperoxide, 0.01 parts ethylenediaminetetraacetate monosodium iron hydrate, 0.03 parts hydroxymethanesulfinate sodium dihydrate, and 0.02 parts EDTA·4Na·4H2O was added. The polymerization reaction was carried out for 16 hours while maintaining the temperature at 5°C. After reaching a degree of polymerization of 90%, 0.1 parts of hydroxylamine sulfate and 0.03 parts of diethylhydroxylamine were added as polymerization termination agents to stop the polymerization reaction. The residual monomers were removed using a rotary evaporator at a water temperature of 60°C to obtain a latex polymer (nitrile rubber) containing conjugated diene monomer units and nitrile group-containing monomer units. The composition of the nitrile rubber was 36% by mass of acrylonitrile monomer units and 64% by mass of 1,3-butadiene monomer units, and the latex concentration was 25% by mass. A portion of the obtained latex was then added to an aqueous solution of magnesium sulfate as a coagulant in an amount equivalent to 1.0% by mass relative to the nitrile rubber content, and the latex was coagulated by stirring. After that, it was filtered while washing with water, and the obtained coagulated material was vacuum dried at a temperature of 60°C for 12 hours to obtain nitrile rubber as a polymer precursor. [Double decomposition of polymer precursors] Next, 9 parts of the polymer precursor obtained above were dissolved in 141 parts of monochlorobenzene and added to the reactor. After heating the reactor to 80°C, 2 L of a monochlorobenzene solution containing bis(tricyclohexylphosphine)benzylideneruthenium dichloride as a Grubbs catalyst was added so that the amount of Grubbs catalyst was 0.25 parts per 100 parts of polymer precursor. The reactor was then pressurized to 3.5 MPa with ethylene as the coolefin, and the polymer was subjected to a double decomposition reaction at a stirring speed of 600 rpm. During the reaction, the temperature was kept constant using a cooling coil connected to a temperature control device and a thermal sensor. [Hydrogenation reaction] Subsequently, the reactor was degassed three times with H2 at 0.7 MPa while continuing to stir. Then, the reactor temperature was raised to 130°C, and 1 L of monochlorobenzene solution containing Wilkinson catalyst and triphenylphosphine was added to the reactor. The amount of Wilkinson catalyst was 0.075 parts and the amount of triphenylphosphine was 1 part per 100 parts of polymer. The temperature was then raised to 138°C, and the hydrogenation reaction of the polymer was carried out under a hydrogen pressure (gauge pressure) of 8.4 MPa, and the reaction was terminated when the iodine value reached 5.0 mg / 100 mg. After the reaction was complete, 0.2 parts of activated carbon with an average diameter of 15 μm was added to the reactor and stirred for 30 minutes. Then, the solution was filtered through a filter with a pore size of 5 μm. Steam was then introduced into the filtrate, and monochlorobenzene was recovered and removed by steam distillation. The precipitated polymer (hydrogenated polymer) was separated, dried, and recovered. Next, N-methylpyrrolidone was added to the dried polymer to obtain an 8% by mass NMP solution of the polymer (binder composition). The weight-average molecular weight of the obtained polymer was measured. The results are shown in Table 1.

[0088] (Comparative Examples 1-2) By reducing the amount of t-dodecyl mercaptan used as a molecular weight modifier during polymer preparation, the sulfur content of the resulting polymer was reduced while increasing its weight-average molecular weight. Except for these points, various operations and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0089] [Table 1]

[0090] Table 1 shows that the binder compositions for positive electrodes of non-aqueous secondary batteries, prepared in Examples 1 to 5, which contained polymers with a weight-average molecular weight of 300,000 or less and a sulfur content of 500 ppm or more, and which included nitrile group-containing monomer units, were able to form a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high-temperature storage characteristics. Conversely, the binder compositions for positive electrodes of non-aqueous secondary batteries, prepared in Comparative Examples 1 to 2, which contained polymers whose weight-average molecular weight and sulfur content were both outside the above ranges, were unable to form a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high-temperature storage characteristics. [Industrial applicability]

[0091] According to the present invention, it is possible to provide a binder composition for a positive electrode of a non-aqueous secondary battery that can form a conductive material dispersion with excellent dispersibility and a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous conductive material dispersion for the positive electrode of a secondary battery that has excellent dispersibility and can form a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous slurry composition for a positive electrode of a secondary battery that can form a secondary battery with excellent high-temperature storage characteristics. Furthermore, according to the present invention, it is possible to provide a positive electrode for a non-aqueous secondary battery that can form a secondary battery with excellent high-temperature storage characteristics, and a secondary battery with excellent high-temperature storage characteristics.

Claims

1. A binder composition for the positive electrode of a non-aqueous secondary battery, comprising a polymer, The polymer contains nitrile group-containing monomer units, and The weight-average molecular weight of the polymer is 2,000 or more and 300,000 or less, and the sulfur content of the polymer is 500 ppm or more and 20,000 ppm or less. Binder composition for the positive electrode of non-aqueous secondary batteries.

2. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the polymer contains linear alkylene structural units having 4 or more carbon atoms in a proportion of 30% by mass or more and 80% by mass or less.

3. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1 or 2, wherein the polymer contains the nitrile group-containing monomer units in a proportion of 10% by mass or more and 55% by mass or less.

4. A binder composition for a positive electrode of a non-aqueous secondary battery according to any one of claims 1 to 3, wherein the iodine value of the polymer is 5 mg / 100 mg or more and 100 mg / 100 mg or less.

5. A dispersion of conductive material for a non-aqueous secondary battery positive electrode, comprising a conductive material containing carbon nanotubes, a dispersion medium, and a binder composition for a non-aqueous secondary battery positive electrode according to any one of claims 1 to 4.

6. The conductive material dispersion for a non-aqueous secondary battery cathode according to claim 5, wherein the value of the volume-average particle diameter D50 in the particle size distribution of the dispersed carbon nanotubes is 0.1 μm or more and 5.0 μm or less.

7. A slurry composition for a positive electrode of a non-aqueous secondary battery, comprising an electrode active material and a conductive material dispersion for a positive electrode of a non-aqueous secondary battery according to claim 5 or 6.

8. A positive electrode for a non-aqueous secondary battery, comprising a positive electrode composite layer formed using the slurry composition for a non-aqueous secondary battery positive electrode described in claim 7.

9. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in claim 8.

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