Slurry composition for non-aqueous secondary battery electrodes, electrodes and secondary batteries

Abstract

To provide a slurry composition for non-aqueous secondary battery electrodes that exhibits excellent storage stability, coating properties, and dispersibility of active materials, and in which the layer formed from the slurry composition has excellent adhesion to the current collector, low internal resistance when used as an electrode, and excellent cycle characteristics when used as a battery. [Solution] A slurry composition for a non-aqueous secondary battery electrode comprising an active material (a), polymer particles (b), and a liquid medium (c), wherein the polymer particles (b) include constituent units derived from an ethylenically unsaturated carboxylic acid ester monomer (ma) represented by the following general formula (1) and constituent units derived from an ethylenically unsaturated monomer (mb) having a logarithm of the octanol / water partition coefficient (Log Kow) of 2.0 or more, and the slurry composition for a non-aqueous secondary battery electrode is characterized in that when the viscosity of an aqueous dispersion at 25°C with a concentration of the polymer particles (b) of 2% by mass is V1 at pH 3 and V2 at pH 8, the relationship V2 / V1 > 50 is satisfied.

Description

[Technical Field]

[0001] This invention relates to a slurry composition for forming electrodes for a non-aqueous secondary battery. Furthermore, this invention relates to electrodes and a secondary battery formed from this slurry composition. [Background technology]

[0002] Non-aqueous 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. Among these, lithium-ion secondary batteries (hereinafter abbreviated as "LIB") are widely used in mobile applications such as laptops and smartphones due to their high power output. Furthermore, LIBs are now also used in automotive applications, and there is a demand for development products that exhibit good cycle characteristics.

[0003] A lithium-ion battery (LIB) is mainly composed of a positive electrode, electrolyte, negative electrode, and separator. The electrode is made by coating a slurry composition onto a current collector. Conventionally, both sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) are often used in aqueous electrode slurry compositions. The roles of the electrode slurry composition include (1) storage stability, (2) coating properties when applied to the current collector, (3) dispersibility of the active material, and (4) adhesion to the current collector when forming the electrode composite layer. However, since CMC and SBR are insulators, it is desirable to reduce their usage as they increase the internal resistance of the battery, and there has been a need to develop a material that combines the roles of both.

[0004] To address the above-mentioned problems, for example, Patent Document 1 discloses the use of an aqueous dispersion containing an alkali-soluble polymer. Furthermore, Patent Document 2 discloses the use of a binder composition containing a copolymer of a hydroxyl group-containing monomer and an acid group-containing monomer. However, in Patent Document 1, the volume resistivity of the electrode composite layer was not necessarily sufficient, and in Patent Document 2, the dispersibility of the active material was unsatisfactory. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2013-062080 [Patent Document 2] Japanese Patent Publication No. 2023-116666 [Overview of the project] [Problems that the invention aims to solve]

[0006] In addition to the characteristics (1) to (4) listed in the background technology, there is currently no electrode slurry composition that has a low volume resistivity when used as an electrode composite layer. Therefore, the problem that the present invention aims to solve is to provide a non-aqueous secondary battery electrode slurry composition that has excellent storage stability, coating properties and dispersibility of active materials, the layer formed from the slurry composition has excellent adhesion to the current collector, the internal resistance when used as an electrode is low, and the cycle characteristics when used as a battery is excellent. [Means for solving the problem]

[0007] The inventors of this invention have diligently conducted research to solve the above-mentioned problems, and as a result, have arrived at this invention. In other words, the present invention relates to a slurry composition for a non-aqueous secondary battery electrode comprising an active material (a), polymer particles (b), and a liquid medium (c), wherein the polymer particles (b) include constituent units derived from an ethylenically unsaturated carboxylic acid ester monomer (ma) represented by the following general formula (1) and constituent units derived from an ethylenically unsaturated monomer (mb) having a logarithm of the octanol / water partition coefficient (Log Kow) of 2.0 or higher, and the slurry composition for a non-aqueous secondary battery electrode is characterized in that when the viscosity of an aqueous dispersion at 25°C with a concentration of the polymer particles (b) of 2% by mass is V1 at pH 3 and V2 at pH 8, the relationship V2 / V1 > 50 is satisfied. General formula (1) CH2=CR-C(=O)-O-Xn-R' (In the formula, R represents a hydrogen atom or a methyl group, R' represents a hydrogen atom or an alkyl group having 1 to 18 carbon atoms, X represents an oxyethylene group or an oxypropylene group, and n represents an integer from 1 to 30.)

[0008] Furthermore, the present invention further comprises constituent units derived from an ethylenically unsaturated carboxylic acid monomer (mc), wherein the polymer particles (b) contain, in 100% by mass, constituent units derived from the ethylenically unsaturated carboxylic acid ester monomer (ma), 3 to 60% by mass derived from the ethylenically unsaturated monomer (mb), and 20 to 60% by mass derived from the ethylenically unsaturated carboxylic acid monomer (mc), as described above, in the slurry composition for non-aqueous secondary battery electrodes.

[0009] Furthermore, the present invention relates to an electrode for a non-aqueous secondary battery having a layer formed from the above-mentioned slurry composition for electrodes of a non-aqueous secondary battery and a current collector.

[0010] Furthermore, the present invention relates to a non-aqueous secondary battery comprising the above-mentioned electrodes for non-aqueous secondary batteries. [Effects of the Invention]

[0011] The present invention provides a slurry composition for non-aqueous secondary battery electrodes that exhibits excellent storage stability, coating properties, and dispersibility of active materials, and in which the layer formed from the slurry composition has excellent adhesion to the current collector, low internal resistance when used as an electrode, and excellent cycle characteristics when used as a battery. [Modes for carrying out the invention]

[0012] The slurry composition for non-aqueous secondary battery electrodes and the electrodes for non-aqueous secondary batteries of the present invention will be described below. However, the present invention is not limited thereto. Each component can be replaced with any other component that can perform a similar function, or any component can be added.

[0013] In this specification, numerical ranges specified using "~" include the numbers before and after "~" as the lower and upper limits. Furthermore, in this specification, "film" and "sheet" are not distinguished by thickness. Furthermore, in this specification, when "(meth)acrylamide" and "(meth)acrylate" are used, unless otherwise specified, they refer to "acrylamide or methacrylamide" and "acrylate or methacrylate," respectively, and ethylenically unsaturated monomer means a monomer having an ethylenically unsaturated double bond. Furthermore, in this specification, when "acrylic acid and / or methacrylic acid" is used, unless otherwise specified, it shall refer to both "acrylic acid and methacrylic acid" and "acrylic acid or methacrylic acid."

[0014] The logarithm of the octanol / water partition coefficient is sometimes abbreviated as "Log Kow". The ethylenically unsaturated carboxylic acid monomer (ma) represented by general formula (1), the ethylenically unsaturated monomer (mb) with a Log Kow of 2.0 or higher, the ethylenically unsaturated carboxylic acid monomer (mc), and the other monomer (md) may be abbreviated as monomer (ma), monomer (mb), monomer (mc), and monomer (md), respectively. Furthermore, the slurry composition for non-aqueous secondary battery electrodes, the electrode for non-aqueous secondary battery, and the non-aqueous secondary battery of the present invention may be abbreviated as slurry composition, electrode, and battery, respectively. Furthermore, unless otherwise noted, each of the components mentioned herein may be used independently, individually, or in combination of two or more.

[0015] Furthermore, a non-aqueous secondary battery refers to a secondary battery that does not use water as an electrolyte, and examples include lithium-ion secondary batteries (LIBs), sodium-ion secondary batteries, and magnesium secondary batteries. In this specification, LIBs are described as an example of a non-aqueous secondary battery, but it goes without saying that the binder dispersion for non-aqueous secondary battery separators, slurry composition for non-aqueous secondary battery separators, and non-aqueous secondary battery separators of the present invention can be applied to non-aqueous secondary batteries other than LIBs.

[0016] ≪Slurry Composition for Non-aqueous Secondary Battery Electrodes≫ The slurry composition for non-aqueous secondary battery electrodes contains an active material (a), polymer particles (b), and a liquid medium (c). The polymer particles (b) contain a structural unit derived from an ethylenically unsaturated carboxylic acid ester monomer (ma) represented by the general formula (1) and a structural unit derived from an ethylenically unsaturated monomer (mb) having a logarithm of the octanol / water partition coefficient of 2.0 or more. When the viscosity of an aqueous dispersion with the concentration of the polymer particles (b) being 2% by mass in a state of pH 3 at 25°C is V1 and the viscosity in a state of pH 8 is V2, the relational expression V2 / V1 > 50 is satisfied. Thereby, a slurry composition for non-aqueous secondary battery electrodes excellent in the dispersibility of the active material can be obtained, and an electrode for non-aqueous secondary battery using this has a small internal resistance.

[0017] <Active material (a)> Examples of the active material (a) include a positive electrode active material and a negative electrode active material. As the positive electrode active material, metal compounds such as metal oxides and metal sulfides, conductive polymers, etc. can be used. Examples of the metal oxide or metal compound include oxides of transition metals such as Fe, Co, Ni, Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides. Specific examples of the metal oxide or metal compound include transition metal oxide powders such as MnO, V2O5, V6O 13 , TiO2, etc., composite oxide powders of lithium and transition metals such as layered lithium nickelate, lithium cobaltate, lithium manganate, spinel-structured lithium manganate, etc., lithium iron phosphate-based materials which are lithium acid compounds with an olivine structure, and transition metal sulfide powders such as TiS2, FeS, etc.

[0018] On the other hand, examples of negative electrode active materials include alloys such as metallic Li, its alloys tin alloy, silicon alloy, and lead alloy; metal oxides such as lithium titanate, lithium vanadate, and lithium siliconate; conductive polymers such as polyacetylene and poly-p-phenylene; amorphous carbonaceous materials such as soft carbon and hard carbon; carbonaceous powders such as artificial graphite and natural graphite, such as high-graphitization carbon materials; carbon black, mesophase carbon black, resin-fired carbon materials, vapor-grown carbon fibers, and carbon fibers; and silicon-based materials such as silicon (Si), SiO, and SiOx.

[0019] <Polymer particles (b)> The polymer particles (b) contain constituent units derived from an ethylenically unsaturated carboxylic acid ester monomer (ma) represented by general formula (1), and constituent units derived from an ethylenically unsaturated monomer (mb) with a Log Kow of 2.0 or higher, and may further contain constituent units derived from an ethylenically unsaturated carboxylic acid monomer (mc). In addition, the polymer particles (b) may also contain constituent units derived from other monomers (md) copolymerizable with monomers (ma), (mb), and (mc).

[0020] [Ethylene-unsaturated carboxylic acid monomer (ma)] The monomer (ma) is an ethylenically unsaturated carboxylic acid ester monomer represented by the following general formula (1). General formula (1) CH2=CR-C(=O)-O-Xn-R' (In the formula, R represents a hydrogen atom or a methyl group, R' represents a hydrogen atom or an alkyl group having 1 to 18 carbon atoms, X represents an oxyethylene group or an oxypropylene group, and n represents an integer from 1 to 30.)

[0021] In general formula (1), R' is a hydrogen atom or an alkyl group having 1 to 18 carbon atoms, preferably a hydrogen atom. The alkyl group having 1 to 18 carbon atoms is preferably a linear alkyl group having 1 to 18 carbon atoms, and more preferably has 1 carbon atom. Furthermore, n is preferably 3 to 20, and more preferably 5 to 15. This results in excellent dispersibility of the active material when it is used in a slurry composition. Furthermore, the constituent units derived from monomers (ma) are preferably 3 to 30% by mass of the total constituent units of polymer particles (b) by 100% by mass. With such a structure, the storage stability and dispersibility of the active material are excellent when preparing the slurry composition.

[0022] [Ethylene-unsaturated monomers (mb)] Monomer (mb) is an ethylenically unsaturated monomer with a Log Kow of 2.0 or higher, and is an ethylenically unsaturated monomer other than monomer (ma) and monomer (mc). Preferably, the Log Kow of monomer (mb) is between 2.23 and 4.01. This allows for the acquisition of an electrode with low internal resistance that does not completely dissolve even under alkaline conditions. Log Kow is expressed by (Equation 2) below and is used as an indicator of whether a certain compound X is more likely to be distributed into the aqueous phase or the oil phase (octanol). Log Kow can be calculated from experiments such as the flask shaking method and HPLC method, and it can also be calculated from simulations using chemical structures, such as the YMB method (physical property estimation function) of the Hansen solubility parameter software HSPiP. (Formula 2) Log Kow = Log(Concentration of compound X in the octanol phase / Concentration of compound X in the aqueous phase)

[0023] Examples of monomers (mb) include butyl acrylate (Log Kow = 2.23), 2-ethylhexyl acrylate (Log Kow = 4.01), n-butyl methacrylate (Log Kow = 2.84), t-butyl methacrylate (Log Kow = 2.67), 2-ethylhexyl methacrylate (Log Kow = 5.59), lauryl methacrylate (Log Kow = 6.60), styrene (Log Kow = 3.06), α-methylstyrene (Log Kow = 3.06), diallyl phthalate (Log Kow = 3.00), etc., but are not limited to these. In particular, materials other than ethylenically unsaturated carboxylic acid monomers are preferred, and styrene is more preferred, because they provide good dispersibility of the active material when used in a slurry composition. Furthermore, the constituent units derived from monomers (mb) are preferably 3 to 60% by mass of the total constituent units of the polymer particles (b) by 100% by mass. This results in excellent dispersibility and coating properties of the active material when used in a slurry composition.

[0024] [Ethylene-unsaturated carboxylic acid monomers (mc)] Examples of monomers (mc) include acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and β-(meth)acryloxyethyl monoester succinate. Among these, acrylic acid and / or methacrylic acid are preferred due to their good polymerization stability. The constituent units derived from monomers (mc) are preferably 20 to 60% by mass of the total constituent units of polymer particles (b) by 100% by mass. This results in excellent storage stability and coating properties when used as a slurry composition.

[0025] [Other monomers (md)] Other monomers (md) include, for example, methyl (meth)acrylate, ethyl (meth)acrylate, methoxyethyl (meth)acrylate, hydroxyethyl (meth)acrylate, 4-hydroxybutyl acrylate, (meth)acrylonitrile, (meth)acrylamide, isopropylacrylamide, diacetoneacrylamide, 2-acetoacetoxyethyl methacrylate, glycidyl methacrylate, trifluoroethyl acrylate, vinyl chloride, vinyl acetate, and the like.

[0026] [Method for producing polymer particles (b)] The polymerization method for producing the polymer particles (b) may be any of the following methods: solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, etc. However, emulsion polymerization is preferred because it yields uniform particles. When producing polymer particles (b) by emulsion polymerization, it is preferable to synthesize them in the presence of at least one of a surfactant and a protective colloid. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants, but anionic surfactants and / or nonionic surfactants are preferred.

[0027] As a non-reactive surfactant, anionic surfactants are preferably those whose main skeleton is a sulfosuccinate ester, alkyl ether, alkylphenyl ether, alkylphenyl ester, or phosphate ester. Specific examples of anionic surfactants include higher fatty acid salts such as sodium oleate, alkylaryl sulfonates such as dodecylbenzenesulfonic acid, alkyl sulfate esters such as sodium lauryl sulfate, polyoxyethylene alkyl ether sulfates such as sodium polyoxyethylene lauryl ether sulfate, alkyl sulfosuccinate esters such as sodium monooctyl sulfosuccinate and their derivatives, and polyoxyethylene distyrenated phenyl ether sulfates.

[0028] As a non-reactive surfactant, nonionic surfactants are preferably those whose main skeleton is an alkyl ether, alkylphenyl ether, or alkyl ester. Specific examples of nonionic surfactants include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether and polyoxyethylene stearyl ether, polyoxyethylene alkylphenyl ethers such as polyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether, sorbitan higher fatty acid esters such as sorbitan monolaurate, sorbitan monostearate, and sorbitan trioleate, polyoxyethylene sorbitan higher fatty acid esters such as polyoxyethylene sorbitan monolaurate and polyoxyethylene sorbitan monostearate, polyoxyethylene higher fatty acid esters such as polyoxyethylene monolaurate and polyoxyethylene monostearate, glycerin higher fatty acid esters such as oleic acid monoglyceride and stearate monoglyceride, polyoxyethylene polyoxypropylene block copolymer, and polyoxyethylene distyrenated phenyl ether.

[0029] Reactive surfactants can also be used. Here, a reactive surfactant is a surfactant that has one or more radically polymerizable unsaturated double bonds in its molecule. As these reactive surfactants, compounds in which a radically polymerizable unsaturated double bond is bonded to a surfactant (preferably an anionic surfactant or a nonionic surfactant) can be used.

[0030] Examples of protective colloids include water-soluble polymer compounds such as polyvinyl alcohol, carboxymethylcellulose, xanthan gum, starch, self-emulsifying and dispersible polyester resins, and water-soluble polyester resins.

[0031] It is preferable to use 0.1 to 5 parts by mass of surfactant or protective colloid per 100 parts by mass of monomer mixture. This allows for the synthesis of particles with excellent polymerization stability.

[0032] Known oil-soluble polymerization initiators and water-soluble polymerization initiators can be used as radical polymerization initiators in polymerization reactions. The radical polymerization initiator is preferably used in an amount of 0.1 to 1.0 parts by mass, and more preferably 0.1 to 0.6 parts by mass, per 100 parts by mass of the total amount of the monomer mixture. This allows for the synthesis of particles with excellent polymerization stability.

[0033] Examples of oil-soluble polymerization initiators include organic peroxides such as benzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy(2-ethylhexanoate), tert-butyl peroxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide; and azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitride.

[0034] In polymerization, it is preferable to use a water-soluble polymerization initiator. Suitable water-soluble polymerization initiators include conventionally known ones such as ammonium persulfate (APS), potassium persulfate (KPS), sodium persulfate (NPS), hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride.

[0035] During polymerization, a reducing agent may be used in combination with the polymerization initiator. Examples of reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts such as formaldehyde sulfoxylate; reducing inorganic compounds such as sodium thiosulfate, sodium sulfite, sodium bisulfite, and sodium metabisulfite; and ferrous chloride, longalite, and thiourea dioxide.

[0036] Furthermore, when polymerizing monomers, buffers, chain transfer agents, etc., can be used as needed. Examples of buffers include sodium acetate, sodium citrate, and sodium bicarbonate. Examples of chain transfer agents include octyl mercaptan, tertial decyl mercaptan, lauryl mercaptan, stearyl mercaptan, 2-ethylhexyl mercaptoacetate, octyl mercaptoacetate, 2-ethylhexyl mercaptopropionate, and octyl mercaptopropionate.

[0037] While water is preferred as the dispersion medium during polymerization, an aqueous medium in which a water-soluble solvent is dissolved in water can also be used. Examples of water-soluble solvents include alcohols, glycols, cellosolves, amino alcohols, amines, ketones, carboxylic acid amides, phosphate amides, sulfoxides, carboxylic acid esters, phosphate esters, ethers, and nitriles.

[0038] The aqueous dispersion of polymer particles (b) exhibits low viscosity under acidic conditions, as it is thought to form a state where many acidic functional groups are present inside the particles. However, under alkaline conditions, the acidic functional groups are neutralized, causing the particles to swell and resulting in high viscosity. If the hydrophilicity of polymer particles (b) is too high, the particles will swell even under acidic conditions, increasing the viscosity of the slurry composition and potentially failing to satisfy the relationship V2 / V1 > 50. On the other hand, if the hydrophilicity of polymer particles (b) is too low, swelling of the particles will be suppressed even under alkaline conditions, resulting in a low viscosity of the slurry composition and potentially failing to satisfy the relationship V2 / V1 > 50. Therefore, it is preferable that polymer particles (b) have an appropriate balance between hydrophobicity and hydrophilicity. The polymer particles (b) satisfy the relationship V2 / V1 > 50 when the viscosity of the aqueous dispersion at a concentration of 2% by mass is V1 at pH 3 and the viscosity at pH 8 is V2 at 25°C. A V2 / V1 value of 100 or more is more preferable, as this results in good viscosity when used as a slurry composition, leading to excellent coating properties.

[0039] It is preferable that the total light transmittance of the aqueous dispersion of polymer particles (b) at pH 8 is less than 90%. By doing so, the slurry composition containing polymer particles (b) is less likely to clog the pores of the active material (a), and an electrode with excellent internal resistance can be obtained. In this specification, total light transmittance can be determined by spectrally separating the light source to the measurement wavelength using a diffraction grating, and calculating the ratio of light (l) transmitted through the sample to the light incident on the sample (l0), and is expressed by the following (Equation 3). (Formula 3) Total light transmittance (%)=(l / l0)×100

[0040] The average particle diameter of polymer particles (b) is preferably 10 to 500 nm, and more preferably 50 to 200 nm. In this specification, the average particle diameter refers to the particle diameter (median diameter, D) at which the cumulative value from the smallest particle diameter side reaches 50% in the volume-based particle diameter distribution obtained by laser diffraction and scattering. 50 This value shall be referred to as the average particle diameter.

[0041] <Liquid media (c)> The liquid medium (c) can be any medium that disperses the active material (a) and polymer particles (b), and an aqueous medium is preferred, and water is particularly preferred, but a solvent can also be used if necessary. Examples of solvents include alcohols, glycols, cellosolves, amino alcohols, amines, ketones, carboxylic acid amides, phosphate amides, sulfoxides, carboxylic acid esters, phosphate esters, ethers, nitriles, and the like.

[0042] <Optional ingredients> The slurry composition of the present invention may also contain various additives such as conductive additives, film-forming additives, defoaming agents, leveling agents, preservatives, crosslinking agents, dispersants, and pH adjusters, as long as they do not interfere with the effects of the invention. Furthermore, the slurry composition of the present invention may contain binders such as SBR as needed, but since the polymer particles (b) also serve as a binder, it is possible to reduce the amount of non-conductive components added, thereby forming electrodes with excellent internal resistance.

[0043] <Method for producing slurry composition for non-aqueous secondary battery electrodes> The slurry composition can be obtained, for example, by dispersing the active material (a) and the polymer particles (b) in a liquid medium (c). Optional components may be added during dispersion. The preferred mixing ratio is active material (a) / polymer particles (b) / liquid medium (c) / optional component = 30-60 / 0.1-5 / 30-60 / 0-5 (by mass ratio).

[0044] The slurry composition for non-aqueous secondary battery electrodes can be manufactured using known dispersers and mixers. Specific mixing devices include, for example, mixers such as dispersers, homomixers, and planetary mixers; homogenizers; media-type dispersers such as paint conditioners, ball mills, sand mills, attritors, pearl mills, and co-ball mills; media-less dispersers such as jet mills; and other types of roll mills. Furthermore, it is preferable to use a disperser that has been treated to prevent metal contamination from the disperser. As the media, it is preferable to use ceramic beads such as glass beads, zirconia beads, and alumina beads. Only one type of disperser may be used, or a combination of multiple types of dispersers may be used.

[0045] ≪Electrodes for non-aqueous secondary batteries≫ The electrode for a non-aqueous secondary battery of the present invention is obtained by applying a slurry composition to a current collector. The layer formed by applying the slurry composition to the current collector is sometimes referred to as the "composite layer" or "electrode composite layer." As the current collector, a current collector applicable to various secondary batteries can be appropriately selected. Examples of materials for the current collector include metals such as aluminum, copper, nickel, titanium, and stainless steel, and alloys of these metals. In the case of a lithium-ion battery (LIB), it is preferable to use a current collector made of aluminum for the positive electrode and a current collector made of copper for the negative electrode. In terms of shape, a flat foil is generally used, but current collectors with roughened surfaces, perforated foils, and mesh-shaped current collectors can also be used. The thickness of the current collector is preferably 5 to 50 μm.

[0046] ≪Nonaqueous secondary battery≫ The non-aqueous secondary battery of the present invention will be described using a lithium-ion battery (LIB) as an example. The LIB comprises a battery body having at least a positive electrode, a negative electrode, and a separator provided between the positive and negative electrodes, and an electrolyte impregnated into the battery body.

[0047] The electrolyte is a liquid obtained by dissolving a lithium-containing electrolyte in a non-aqueous solvent. Specific examples of electrolytes include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, and LiBPh4 (where Ph represents a phenyl group).

[0048] Examples of non-aqueous solvents include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyl lactone, γ-valerolactone, and γ-octanoic lactone; glimes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile.

[0049] Furthermore, the electrolyte can also be used as a polymer electrolyte in the form of a gel by being held in a polymer matrix. Examples of polymer matrices include acrylic resins having polyalkylene oxide segments, polyphosphazene resins having polyalkylene oxide segments, and polysiloxane resins having polyalkylene oxide segments.

[0050] Non-aqueous secondary batteries using the above-mentioned components offer excellent safety and battery characteristics. The non-aqueous secondary battery of this invention can be used for industrial, automotive, and mobile applications. [Examples]

[0051] The present invention will be described in more detail below with reference to examples, but these examples do not limit the scope of the present invention in any way. Unless otherwise specified, in the following examples, "parts" refers to "parts by mass," "%" refers to "mass %," the numbers in the table represent the mass of solids, and blank spaces indicate that the material is not used.

[0052] [Synthesis Example 1] In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 140 parts of water and 1 part of RA-9607 (manufactured by Nippon Emulsifier Co., Ltd.) as a surfactant were charged. Separately, a pre-emulsion was prepared by mixing 20 parts of Bremmer® AE-400 (manufactured by NOF Corporation) as an ethylenically unsaturated carboxylic acid monomer (ma), 3 parts of styrene as an ethylenically unsaturated monomer (mb), 2 parts of acrylic acid and 35 parts of methacrylic acid as ethylenically unsaturated carboxylic acid monomers (mc), 40 parts of methyl methacrylate as other monomers (md), 70 parts of water, and 1 part of RA-9607 (manufactured by Nippon Emulsifier Co., Ltd.) as a surfactant. After raising the internal temperature of the reaction vessel to 76°C under a nitrogen stream, 3.75 parts of an 8% aqueous solution of ammonium persulfate were added and the mixture was stirred for 1 minute. Subsequently, while maintaining the reaction vessel at 76°C, the pre-prepared pre-emulsion was added dropwise over 2 hours, and stirring was continued for another 2 hours. After confirming that the conversion rate exceeded 99% by measuring the solid content, the internal temperature was cooled to 40°C. The solid content was adjusted to 28% with water to obtain an aqueous dispersion of polymer particles (b-1) with a pH of 2.6 and a particle size of 130 nm. The viscosity (V1) of the aqueous dispersion of the obtained polymer particles (b-1), adjusted to pH 3 by adding 2% solids and ammonia water, was 3 mPa·s. Similarly, the viscosity (V2) of the aqueous dispersion adjusted to pH 8 by adding ammonia water was 1840 mPa·s, the total light transmittance at that time was 85%, and the V1 / V2 value was 613. The methods for measuring average particle size, solid content, pH, viscosity, and total light transmittance are as follows.

[0053] <Average particle size> For an aqueous dispersion containing 2% polymer particles (b), the volume-based particle size distribution was determined using a dynamic light scattering analyzer ("NANOTRAC WAVE II," manufactured by Microtrac Bell Co., Ltd.), and the particle size (median diameter, D) at which the cumulative value from the smallest particle size side reached 50% was determined. 50 The average particle size was defined as follows.

[0054] <Solid content> The solid content of polymer particles (b) in the aqueous dispersion was determined by weighing 1 g of the sample into a metal container and measuring the residue after baking it in a 150°C oven for 20 minutes.

[0055] <ph> For an aqueous dispersion containing 2% polymer particles (b), the hydrogen ion concentration was measured at 25°C using a glass electrode type hydrogen ion meter (Horiba, Ltd. "D-51").

[0056] <Viscosity> A viscometer (TVB-10, manufactured by Toki Sangyo Co., Ltd.) was used to measure the viscosity of an aqueous dispersion containing 2% polymer particles (b) (temperature: 25°C, spindle rotation time: 60 seconds, spindle rotation speed: 60 rpm).

[0057] <Total light transmittance> For an aqueous dispersion containing 2% polymer particles (b), the haze meter (SH7000, manufactured by Nippon Denshoku Industries Co., Ltd.) was used to measure the total light transmittance (%) by placing the sample in a 1 cm square quartz cell. The total light transmittance of water was set to 100%, and measurements were taken at wavelengths of 380 to 780 nm.

[0058] [Synthesis Examples 2-12] Aqueous dispersions of polymer particles (b-2 to b-12) were obtained using the same method as in Synthesis Example 1, except for changes to the materials and formulations shown in Table 1. The abbreviations in the table are as follows:

[0059] <Ethylene-unsaturated carboxylic acid monomer (ma)> AE-400: Bremmer® AE-400 (manufactured by NOF Corporation) (log Kow 0.47, monomer in general formula (1) where R is a hydrogen atom, R' is a hydrogen atom, X is an oxyethylene group, and n is 10) P: Bremmer® P (manufactured by NOF Corporation) (log Kow 1.10, a monomer in general formula (1) where R is a methyl group, R' is a hydrogen atom, X is an oxypropylene group, and n is 1) AME-400: Bremmer® AME-400 (manufactured by NOF Corporation) (log Kow 0.39, monomer in general formula (1) where R is a hydrogen atom, R' is a methyl group, X is an oxyethylene group, and n is 9) PME-1000: Brenmer (registered trademark) PME-1000 (manufactured by Nippon Oil Corporation) (monomer with log Kow 5.07, where in general formula (1), R is a methyl group, R’ is a methyl group, X is an oxyethylene group, and n is 23) PSE-1300: Brenmer (registered trademark) PSE-1300 (manufactured by Nippon Oil Corporation) (monomer with log Kow 16.95, where in general formula (1), R is a methyl group, R’ is an octadecyl group, X is an oxyethylene group, and n is 30)

[0060] <Ethylenically unsaturated monomer (mb)> St: Styrene (log Kow 3.06) BA: Butyl acrylate (log Kow 2.23) 2EHA: 2-Ethylhexyl acrylate (log Kow 4.01) 2EHMA: 2-Ethylhexyl methacrylate (log Kow 5.59) DAP: Diallyl phthalate (log Kow 3.00) <Ethylenically unsaturated carboxylic acid monomer (mc)> AA: Acrylic acid (log Kow 0.20) MAA: Methacrylic acid (log Kow 0.82) <Other monomers (md)> MMA: Methyl methacrylate (log Kow 1.13) EA: Ethyl acrylate (log Kow 1.16) HEMA: 2-Hydroxyethyl methacrylate (log Kow 0.70)

[0061] <Method for calculating logKow> The log Kow value of the monomer was calculated by converting the structural formula of the above compound into Smiles notation and inputting it using the YMB method (physical property estimation function) of the Hansen solubility parameter software HSPiP (ver. 5.2.05).

[0062] [Synthesis Example 13] 760 parts of water were charged into a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus. Separately, an aqueous monomer solution was prepared by mixing 10 parts of Brembo® AE-400 (manufactured by NOF Corporation) and 10 parts of Brembo® P (manufactured by NOF Corporation) as ethylenically unsaturated carboxylic acid monomers (ma), 2 parts of diallyl phthalate as ethylenically unsaturated monomer (mb), 50 parts of acrylic acid as ethylenically unsaturated carboxylic acid monomer (mc), 28 parts of hydroxyethyl methacrylate as other monomer (md), and 70 parts of water. After raising the internal temperature of the reaction vessel to 76°C under a nitrogen stream, 3.75 parts of an 8% aqueous solution of ammonium persulfate were added and stirred for 1 minute. Then, while maintaining the reaction vessel at 76°C, the pre-prepared aqueous monomer solution was added dropwise over 2 hours, and stirring was continued for another 2 hours. After confirming that the conversion rate exceeded 99% by measuring the solid content, the internal temperature was cooled to 40°C. The solid content was adjusted to 10% with water to obtain an aqueous solution of the water-soluble polymer (b-13). The viscosity (V1) of an aqueous solution of the obtained water-soluble polymer (b-13) with a solid content of 2% and adjusted to pH 3 by adding ammonia water was 2200 mPa·s, and the viscosity (V2) of an aqueous solution of the same substance adjusted to pH 8 by adding ammonia water was 2420 mPa·s, the total light transmittance at which point was 100%, and the V1 / V2 value was 1.1. Furthermore, the solid content, pH, viscosity, and total light transmittance were measured in the same manner as in Synthesis Example 1.

[0063] [Table 1]

[0064] [Example 1] <Preparation of slurry composition for negative electrode> A slurry composition (1) for the anode was obtained by mixing 98 parts of CGB-20 (manufactured by Nippon Graphite Industries Co., Ltd.), a carbonaceous powder of natural graphite, as the active material (a), 2 parts of polymer particles (b-1) obtained in Synthesis Example 1 as the solid content, 3 parts of 25% aqueous ammonia, and 97 parts of water as the liquid medium (c) in a planetary mixer.

[0065] <Fabrication of the negative electrode> The obtained negative electrode slurry composition (1) was applied to an 18 μm thick copper foil, which would serve as the current collector, using a doctor blade, and then heated and dried at 80°C. The application amount was 6.5 mg / cm² per unit area of ​​the electrode. 2 The mixture was adjusted to achieve the following: After heating and drying, it was rolled using a roll press, resulting in a density of 1.6 g / cm³ in the asphalt layer. 3 A negative electrode was fabricated. The storage stability, active material dispersibility, and coating properties of the slurry composition, as well as the adhesion and volume resistivity of the electrode composite layer, were evaluated using the following methods. The results are shown in Table 2.

[0066] <Storage stability> The slurry composition was stored at 25°C, and the presence or absence of aggregation, precipitation, or separation was visually confirmed. [Evaluation Criteria] ◎: No abnormalities (aggregation, precipitation, or separation) were observed for more than two weeks from the start of storage. Excellent condition. ○: No abnormalities were observed between one and two weeks from the start of storage. Good. △: Any abnormality was observed between 4 days and less than 1 week from the start of storage. Still usable. ×: Any abnormality was observed within 3 days of the start of storage. This poses a practical problem.

[0067] <Dispersibility of active materials in slurry compositions> The dispersibility of the active material in the slurry composition was evaluated using a grind gauge with a maximum groove depth of 100 μm, in accordance with the method compliant with JIS K5600-2-5. [Evaluation Criteria] ◎: The point where spots begin to appear is less than 80 μm. Excellent. ○: The spots where the spots begin to appear are 80 μm or larger and less than 90 μm. Good. △: The point where spots begin to appear is 90 μm or larger but less than 100 μm. Usable. ×: The spots where the speckles begin to appear are 100 μm or larger. This poses a practical problem.

[0068] <Coating properties of slurry compositions> The coating properties of the slurry composition were visually observed using the obtained negative electrode. [Evaluation Criteria] ◎: The film thickness across the coated area is uniform, and no repelling is observed on the copper foil current collector. Excellent. ○: Less than 5% of the coated area shows uneven coating or repellency. Good. △: Coating unevenness or repellency is observed in 5% to less than 10% of the coated area. Usable. ×: Uneven coating or repellency is observed in more than 10% of the coated area. This presents a practical problem.

[0069] <Adhesion of electrode composite layer> The obtained negative electrode was cut to a size of 25 mm wide x 100 mm long, and the current collector side and the stainless steel plate were bonded together with double-sided adhesive tape. A 20 mm wide cellophane tape was attached to the asphalt layer side and roll-pressed with a load of 1 kg. After standing for 24 hours under conditions of 25°C and 50% humidity, one end of the cellophane tape was pulled in a 180° direction, and the peel strength was measured using a tensile testing machine (Shimadzu Corporation "AGS-X") (peeling speed: 50 mm / min). [Evaluation Criteria] ◎: Peel strength of 10 N / m or higher. Excellent. ○: Peel strength of 8 N / m or more, and less than 10 N / m. Good. △: Peel strength of 6 N / m or more, and less than 8 N / m. Usable. ×: Peel strength is less than 6 N / m. This poses a practical problem.

[0070] <Volume resistivity of electrode composite layer> The obtained negative electrode slurry composition was then measured using an applicator to obtain a basis weight of 6.5 mg / cm³ per unit of electrode. 2 After coating on a polyethylene terephthalate (PET) film so as to achieve the following, the coating film was dried at 80°C. Then, using a resistivity meter ("LORESTA-GP MCP-T610" manufactured by Mitsubishi Chemical Analytech Co., Ltd.), the surface resistivity (Ω / □) of the dried coating film was measured. After the measurement, the thickness of the composite layer formed on the PET film was multiplied to obtain the volume resistivity (Ω·cm). The thickness of the electrode composite layer was obtained by subtracting the film thickness of the PET film from the average value measured at any three locations in the electrode film using a film thickness meter ("DIGIMICRO MH-15M" manufactured by NIKON Corporation) as the thickness of the composite layer. [Evaluation Criteria] ◎: Volume resistivity is less than 0.15 Ω·cm. Good. 〇: Volume resistivity is 0.15 Ω·cm or more and less than 0.18. Good. △: Volume resistivity is 0.18 Ω·cm or more and less than 0.20. Usable. ×: Volume resistivity is 0.20 Ω·cm or more. There are practical problems.

[0071] [Fabrication of the Positive Electrode] As the active material, 93 parts of LiNi 0.5 Mn 0.3 Co 0.2 O₂, 4 parts of acetylene black as the conductive agent, 3 parts of polyvinylidene fluoride as the binder, and 45 parts of N-methylpyrrolidone were put in and mixed to prepare a positive electrode slurry composition. The obtained positive electrode slurry composition was applied onto an aluminum foil with a thickness of 20 μm serving as the current collector using a doctor blade, and then heated and dried at 80°C so that the coating amount per unit area of the electrode was adjusted to be 18 mg / cm 2 . Further, rolling treatment was performed by roll pressing to fabricate a positive electrode with a density of the composite layer of 3.1 g / cm 3 .

[0072] [Fabrication of the Non-aqueous Secondary Battery] The positive and negative electrodes were punched out to 45mm x 40mm and 50mm x 45mm, respectively. The positive and negative electrodes were placed facing each other with a protective separator in between and inserted into an aluminum laminate bag. After vacuum drying, an electrolyte (a non-aqueous electrolyte prepared by dissolving LiPF6 at a concentration of 1M in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 2:3) was injected, and the aluminum laminate was sealed to fabricate a laminate-type non-aqueous secondary battery. The cycle characteristics of the non-aqueous secondary battery were measured by the method described below. The results are shown in Table 2.

[0073] <Cycle Characteristics> Constant current and constant voltage charging (cutoff current 0.6mA) was performed in a 50°C constant temperature bath with a charging current of 60mA and a charging termination voltage of 4.2V. Then, constant current discharge was performed with a discharge current of 60mA until the charging termination voltage reached 3.0V, and the initial discharge capacity was determined. This charge-discharge cycle was performed 200 times, and the discharge capacity retention rate (percentage of the discharge capacity after the 10th discharge relative to the initial discharge capacity) was calculated. A higher discharge capacity retention rate indicates better cycle characteristics. [Evaluation Criteria] ◎: Discharge capacity retention rate is 90% or higher. Excellent. ○: Discharge capacity retention rate is 85% or more but less than 90%. Good. △: Discharge capacity retention rate is 80% or more but less than 85%. Usable for practical purposes. ×: Discharge capacity retention rate is less than 80%. This poses a practical problem.

[0074] [Examples 2-11, Comparative Examples 1-6] Except for the changes in composition and blending amounts (parts by mass) shown in Table 2, a slurry composition for non-aqueous secondary battery electrodes, electrodes for non-aqueous secondary batteries, and non-aqueous secondary batteries were prepared in the same manner as in Example 1. In Comparative Example 3, however, the coating properties of the slurry composition were poor, and it was not possible to obtain an electrode composite layer.

[0075] The following was found from the results of the examples and comparative examples. The aqueous dispersion of the present invention can be used to prepare an electrode composition by kneading it with an electrode active material, thereby forming an electrode that exhibits excellent volume resistivity and cycle characteristics while possessing dispersibility and viscosity adjustment functions for the active material. On the other hand, in Comparative Example 1, polymer particles (b-10) that did not contain ethylenically unsaturated carboxylic acid monomers (ma) as a component were used, resulting in problems with the dispersibility of the active material in the slurry composition, leading to deterioration of the volume resistivity and cycle characteristics of the electrode composite layer. In Comparative Examples 2 and 4, polymer particles (b-11) and water-soluble polymer (b-13) that did not contain ethylenically unsaturated monomers (mb) with a logarithmic octanol / water partition coefficient of 2.0 or higher as a component were used, respectively, resulting in the water-soluble polymer covering the surface of the active material, leading to a decrease in volume resistivity and cycle characteristics. In Comparative Example 3, polymer particles (b-12) with poor viscosity under alkaline conditions were used, causing the slurry composition to separate and preventing the formation of an electrode composite layer. In Comparative Example 5, CMC was used, but it was shown to have poor adhesion of the electrode composite layer. In Comparative Example 6, SBR was used in addition to CMC, but the volume resistivity and cycle characteristics deteriorated due to the increased amount of non-conductive component in the electrode composite layer.

[0076] [Table 2] < / ph>

Claims

1. A slurry composition for a non-aqueous secondary battery electrode comprising an active material (a), polymer particles (b), and a liquid medium (c), wherein the polymer particles (b) include constituent units derived from an ethylenically unsaturated carboxylic acid ester monomer (ma) represented by the following general formula (1) and constituent units derived from an ethylenically unsaturated monomer (mb) having a logarithm of the octanol / water partition coefficient (Log Kow) of 2.0 or more, and the slurry composition for a non-aqueous secondary battery electrode is characterized in that when the viscosity of an aqueous dispersion at 25°C with a concentration of the polymer particles (b) of 2% by mass is V1 at pH 3 and V2 at pH 8, the relationship V2 / V1 > 50 is satisfied. General formula (1) CH 2 =CR-C(=O)-O-Xn-R’ (In the formula, R represents a hydrogen atom or a methyl group, R' represents a hydrogen atom or an alkyl group having 1 to 18 carbon atoms, X represents an oxyethylene group or an oxypropylene group, and n represents an integer from 1 to 30.)

2. The slurry composition for non-aqueous secondary battery electrodes according to claim 1, wherein the polymer particles (b) further contain constituent units derived from ethylenically unsaturated carboxylic acid monomers (mc), and of 100% by mass of the total constituent units, 3 to 30% by mass are constituent units derived from the ethylenically unsaturated carboxylic acid ester monomer (ma), 3 to 60% by mass are constituent units derived from the ethylenically unsaturated monomer (mb), and 20 to 60% by mass are constituent units derived from the ethylenically unsaturated carboxylic acid monomers (mc).

3. An electrode for a non-aqueous secondary battery having a layer formed from the slurry composition for electrodes of a non-aqueous secondary battery according to claim 1 or 2, and a current collector.

4. A non-aqueous secondary battery comprising the electrode for a non-aqueous secondary battery described in claim 3.

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