Binder for non-aqueous electrolyte secondary battery, binder composition for non-aqueous electrolyte secondary battery, anode slurry for non-aqueous electrolyte secondary battery, anode for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery and method of manufacturing binder for non-aqueous electrolyte secondary battery

Abstract

To provide a binder for a non-aqueous electrolyte secondary battery, which is improved in adhesiveness of an electrode mixture layer and a collector (a peeling strength of an electrode) and capable of reducing internal resistance of a battery further than the prior arts.SOLUTION: There is provided a particulate binder having core-shell structure comprising a hydrophobic core and a hydrophilic shell. The hydrophobic core includes one or more selected from among an aromatic vinyl monomer derived unit, an unsaturated carboxylic acid alkyl ester monomer derived unit, a (meth) acrylic acid-based monomer derived unit and an unsaturated carboxylic amide monomer derived unit. The hydrophilic shell includes an acrylic acid-based monomer derived unit, a styrene sulfonic sodium monomer derived unit and an acrylonitril-based monomer derived unit. In the binder for the non-aqueous electrolyte secondary battery, when a mass of the binder is defined as 100 mass%, a content of the hydrophobic core is 90 mass% or more to 97 mass% or less, a content of the hydrophilic shell is 3 mass% or more to 10 mass% or less and an average particle diameter of the binder which is measured by a wet laser diffraction scattering method is 250 nm or more to 1,000 nm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a binder for non-aqueous electrolyte secondary batteries, a binder composition for non-aqueous electrolyte secondary batteries, a negative electrode slurry for non-aqueous electrolyte secondary batteries containing the binder or binder composition for non-aqueous electrolyte secondary batteries, a negative electrode for non-aqueous electrolyte secondary batteries formed using the negative electrode slurry for non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery including the negative electrode for non-aqueous electrolyte secondary batteries. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are widely used in mobile batteries, personal computers, automobiles, etc. As their use has expanded in recent years, there has been a growing demand in the market for rapid charging, and there is a strong demand for lithium-ion batteries with even higher output and reduced internal resistance.

[0003] Therefore, as in Patent Document 1, it has been proposed to reduce the internal resistance by using an aqueous dispersion binder composed mainly of 2-ethylhexyl acrylate and styrene synthesized by emulsion polymerization using a low-molecular surfactant. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6007263 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the aqueous dispersion binder described in Patent Document 1 is used, the peel strength between the electrode mixture layer and the current collector is insufficient, which can lead to problems such as part of the mixture layer peeling off from the current collector during the process of cutting the electrode to a specified size, or the mixture layer falling off during the process of transporting the electrode at high speed using rolls. The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a binder for a nonaqueous electrolyte secondary battery that has excellent adhesion between an electrode mixture layer and a current collector (electrode peel strength) and can reduce the internal resistance of the battery more than conventional binders. [Means for solving the problem]

[0006] That is, the binder, binder composition, negative electrode slurry, negative electrode, secondary battery, and method for producing the binder according to the present invention are as follows. [1] A particulate binder having a core-shell structure with a hydrophobic core and a hydrophilic shell, the hydrophobic core comprises at least one selected from the group consisting of units derived from aromatic vinyl monomers, units derived from unsaturated carboxylic acid alkyl ester monomers, units derived from (meth)acrylic acid monomers, and units derived from unsaturated carboxylic acid amide monomers; the hydrophilic shell comprises a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrene sulfonate monomer, and a unit derived from an acrylonitrile-based monomer; When the mass of the binder is taken as 100 mass%, the content of the hydrophobic core is 90 mass% or more and 97 mass% or less, and the content of the hydrophilic shell is 3 mass% or more and 10 mass% or less, The binder for a non-aqueous electrolyte secondary battery has an average particle size of 250 nm or more and 1000 nm or less as measured by a wet laser diffraction scattering method. [2] The hydrophilic shell is formed from a water-soluble polymer, the water-soluble polymer contains a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrene sulfonate monomer, and a unit derived from an acrylonitrile-based monomer, The binder for a non-aqueous electrolyte secondary battery according to [1], wherein the contents of the units derived from an acrylic acid-based monomer, the units derived from a sodium styrene sulfonate monomer, and the units derived from an acrylonitrile-based monomer in the water-soluble polymer are such that, when the mass of the water-soluble polymer is taken as 100 mass%, the content of the units derived from an acrylic acid-based monomer is 10% by mass or more and 50% by mass or less, the content of the units derived from a sodium styrene sulfonate monomer is 10% by mass or more and 50% by mass or less, and the content of the acrylonitrile-based monomer units is 10% by mass or more and 50% by mass or less. [3] The binder for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the molecular weight of the water-soluble polymer is 300,000 or more and 2,000,000 or less. [4] The binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [3], wherein the hydrophobic core is made of a hydrophobic polymer containing a unit derived from an aromatic vinyl monomer, a unit derived from an unsaturated carboxylic acid alkyl ester monomer, a unit derived from a (meth)acrylic acid monomer, and a unit derived from an unsaturated carboxylic acid amide monomer. [5] The binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein the hydrophobic core and the hydrophilic shell are chemically bonded to each other. [6] The binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein no non-reactive surfactant is added. [7] A binder composition for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [6] and an aqueous solvent. [8] A negative electrode slurry for a non-aqueous electrolyte secondary battery, comprising the binder composition for a non-aqueous electrolyte secondary battery according to [7] and a negative electrode active material. [9] A negative electrode for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [6] and a negative electrode active material.

[10] The negative electrode for a non-aqueous electrolyte secondary battery according to [9], further containing a water-soluble polymer compound.

[11] The negative electrode for a non-aqueous electrolyte secondary battery according to

[10] , wherein the water-soluble polymer compound is an alkali metal salt of carboxymethyl cellulose.

[12] A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of [9] to

[11] .

[13] A method for producing a core-shell structured particulate binder having a hydrophobic core and a hydrophilic shell, comprising: synthesizing a hydrophobic polymer constituting the hydrophobic core in the presence of a water-soluble polymer constituting the hydrophilic shell; the water-soluble polymer contains a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrenesulfonate monomer, and a unit derived from an acrylonitrile-based monomer.

[14] The method for producing a binder for a non-aqueous electrolyte secondary battery according to

[13] , wherein the hydrophobic polymer is formed by polymerizing one or more monomers selected from the group consisting of an aromatic vinyl monomer, an unsaturated carboxylic acid alkyl ester monomer, a (meth)acrylic acid monomer, and an unsaturated carboxylic acid amide monomer.

[15] The method for producing a binder for a non-aqueous electrolyte secondary battery according to

[13] or

[14] , wherein the content of the hydrophobic core is 90% by mass or more and 97% by mass or less, and the content of the hydrophilic shell is 3% by mass or more and 10% by mass or less, when the mass of the binder is 100% by mass.

[16] The method for producing a binder for a non-aqueous electrolyte secondary battery according to any one of

[13] to

[15] , wherein the binder has an average particle size of 250 nm or more and 1000 nm or less as measured by a wet laser diffraction scattering method. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery that has excellent adhesion between an electrode mixture layer and a current collector and can reduce the internal resistance of the battery compared to conventional batteries. DETAILED DESCRIPTION OF THE INVENTION

[0008] A specific configuration of a secondary battery according to one embodiment of the present invention will be described below. <1. Nonaqueous electrolyte secondary battery> The lithium ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The ultimate charge voltage (oxidation-reduction potential) of this lithium ion secondary battery is, for example, 4.0 V (vs. Li / Li+) to 5.0 V, in particular 4.2 V to 5.0 V. The shape of the lithium ion secondary battery is not particularly limited, and may be, for example, cylindrical, prismatic, laminate, or button-shaped.

[0009] (1-1. Positive electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector may be any conductive material, for example, a plate or foil, and is preferably made of aluminum, stainless steel, nickel-plated steel, or the like. The positive electrode mixture layer contains at least a positive electrode active material, and may further contain a conductive agent and a positive electrode binder.

[0010] The positive electrode active material is, for example, a lithium-containing transition metal oxide or solid solution oxide, and is not particularly limited as long as it is a material that can electrochemically absorb and release lithium ions. Examples of lithium-containing transition metal oxides include Li 1.0 Ni 0.88 Co 0.1 Al 0.01 Mg 0.01 O2, but also LiCoO2 and other Li-Co based composite oxides, LiNi x Co y Mn z Examples of solid solution oxides include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. a Mn x Co y Ni zO2 (1.150≦a≦1.430, 0.45≦x≦0.6, 0.10≦y≦0.15, 0.20≦z≦0.28), LiMn 1.5 Ni 0.5 Examples include O4, etc. The content (content ratio) of the positive electrode active material is not particularly limited, and may be any content that is applicable to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery. These compounds may be used alone or in combination.

[0011] The conductive agent is not particularly limited as long as it is capable of increasing the conductivity of the positive electrode. Specific examples of the conductive agent include one or more selected from the group consisting of carbon black, natural graphite, artificial graphite, and fibrous carbon. Examples of the carbon black include furnace black, channel black, thermal black, ketjen black, and acetylene black. Examples of the fibrous carbon include carbon nanotubes, graphene, and carbon nanofibers. The content of the conductive agent is not particularly limited as long as it is a content that can be applied to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery.

[0012] Examples of the positive electrode binder include fluorine-containing resins such as polyvinylidene fluoride, ethylene-containing resins such as styrene-butadiene rubber, ethylene-propylene-diene terpolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, polyvinyl alcohol, carboxymethyl cellulose or carboxymethyl cellulose derivatives (such as carboxymethyl cellulose salts), and nitrocellulose. The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material and the conductive agent to the positive electrode current collector. For example, the negative electrode binder described below may be used as the positive electrode binder.

[0013] (1-2. Negative electrode) The negative electrode includes, for example, a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode current collector may be any conductive material, and is preferably, for example, in the form of a plate or foil, and is made of copper, stainless steel, nickel-plated steel, or the like. The negative electrode mixture layer includes, for example, a negative electrode active material and a negative electrode binder. The configuration of this negative electrode mixture layer is a characteristic feature of this embodiment, and will be described in detail later.

[0014] (1-3. Separator) The separator is not particularly limited, and any separator suitable for use in lithium ion secondary batteries may be used. As the separator, a porous membrane or a nonwoven fabric, which exhibits excellent high-rate discharge performance, is preferably used alone or in combination. Resins constituting the separator include, for example, polyolefin resins typified by polyethylene, polypropylene, etc., polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene di ... copolymer), vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymerExamples of the separator include vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, etc. The porosity of the separator is not particularly limited, and the porosity of the separators of conventional lithium ion secondary batteries can be applied as desired.

[0015] The separator surface may have a heat-resistant layer containing inorganic particles to improve heat resistance, or a layer containing an adhesive to bond with the electrodes and fix the battery element. Examples of inorganic particles include Al2O3, AlOOH, Mg(OH)2, and SiO2. Examples of adhesives include vinylidene fluoride-hexafluoropropylene copolymers, acid-modified vinylidene fluoride polymers, and styrene-(meth)acrylic acid ester copolymers.

[0016] (1-4.Non-aqueous electrolyte) The nonaqueous electrolyte may be the same as any nonaqueous electrolyte conventionally used in lithium ion secondary batteries, and has a composition in which an electrolyte salt is contained in a nonaqueous solvent that is a solvent for the electrolyte. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; methylformate, methylacetate, methylbutyrate, ethylpropionate, and propylpropionate. propionate, tetrahydrofuran or its derivatives, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane (1,4-dibutoxyethane, or methyldiglyme, ethers such as ethylene glycol monopropyl ether and propylene glycol monopropyl ether, nitriles such as acetonitrile and benzonitrile, dioxolane or its derivatives, ethylene sulfide, sulfolane, sultone or its derivatives, etc., can be used alone or in combination of two or more. When two or more of the nonaqueous solvents are used in combination, the mixing ratio of the nonaqueous solvents can be the same as that used in conventional lithium ion secondary batteries.

[0017] Examples of the electrolyte salt include LiClO4, LiBF4, LiAsF6, LiPF6, and LIPF6-x (C n F 2n+1 )x[However, 1 <x<6、n=1or2]、LiSCN、LiBr、LiI、Li2SO4、Li2B 10 Cl 10, inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K) such as NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H5)4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phtalate, lithium stearyl sulfonic acid Examples of suitable ionic salts include organic ionic salts such as lithium, octyl sulfonic acid lithium, and dodecyl benzenesulfonic acid lithium. These ionic compounds can be used alone or in combination of two or more. The concentration of the electrolyte salt may be the same as that of nonaqueous electrolytes used in conventional lithium-ion secondary batteries and is not particularly limited. In this embodiment, it is preferable to use a nonaqueous electrolyte containing the aforementioned lithium compound (electrolyte salt) at a concentration of approximately 0.8 mol / L to 1.5 mol / L.

[0018] Various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative electrode additives, positive electrode additives, ester-based additives, carbonate-based additives, sulfate-based additives, phosphate-based additives, borate-based additives, acid anhydride-based additives, and electrolyte-based additives. Any one of these additives may be added to the non-aqueous electrolyte, or multiple additives may be added to the non-aqueous electrolyte.

[0019] <2. Method for manufacturing a nonaqueous electrolyte secondary battery according to this embodiment> Next, a method for manufacturing a lithium-ion secondary battery will be described. The positive electrode is fabricated as follows. First, a positive electrode active material, a conductive agent, and a positive electrode binder are mixed in a desired ratio and dispersed in a positive electrode slurry solvent (e.g., N-methyl-2-pyrrolidone) to form a positive electrode slurry. Next, this positive electrode slurry is applied to a positive electrode current collector and dried to form a positive electrode mixture layer. The application method is not particularly limited. Possible application methods include a knife coater method, a gravure coater method, a reverse roll coater, and a slit die coater. The following application steps are also performed in the same manner. Next, the positive electrode mixture layer is pressed using a press to a desired density. This completes the fabrication of the positive electrode.

[0020] The negative electrode is fabricated in the same manner as the positive electrode. First, a mixture of materials constituting the negative electrode mixture layer is dispersed in a solvent for the negative electrode slurry (for example, an aqueous solvent such as water) to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to a negative electrode current collector and dried to form a negative electrode mixture layer. Next, the negative electrode mixture layer is pressed to a desired density using a press. This completes the negative electrode.

[0021] Next, the separator is sandwiched between the positive electrode and the negative electrode to produce an electrode structure. The electrode structure is then processed into a desired shape (e.g., cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, a nonaqueous electrolyte is injected into the container, impregnating the pores in the separator and the gaps in the positive and negative electrodes with the electrolyte. This completes the production of a lithium-ion secondary battery.

[0022] 3. Characteristic Configuration of the Non-Aqueous Electrolyte Secondary Battery According to the Present Embodiment The negative electrode mixture layer, which is a feature of the nonaqueous electrolyte secondary battery according to this embodiment, and the negative electrode slurry, binder composition, and binder used to form the negative electrode mixture layer will be described in detail below.

[0023] (3-1. Negative electrode mixture layer) As described above, the negative electrode mixture layer is formed by applying the negative electrode slurry to one or both sides of the negative electrode current collector, drying, and pressing. In the drying step, the solvent for the negative electrode slurry volatilizes, and solid components such as the negative electrode active material and the negative electrode binder dispersed in the negative electrode slurry remain on the negative electrode current collector to form the negative electrode mixture layer. Therefore, the content ratio of each component in the negative electrode mixture layer directly reflects the content ratio of the solid components in the negative electrode slurry.

[0024] The negative electrode mixture layer has an areal capacity of 3.5 mAh / cm after pressing. 2 More than 10mAh / cm 2 To form such a negative electrode mixture layer, the negative electrode slurry is applied to the negative electrode at an areal density of 5 mg / cm on one side, for example. 2 More than 25mg / cm 2 It is preferable to coat with a coating weight of 10 mg / cm or less. 2 More than 20mg / cm 2 It is more preferable to apply the coating in the following amount: The thickness of the negative electrode mixture layer after drying and pressing is preferably, for example, 50 μm or more and 150 μm or less as the thickness on one side. By setting the coating amount and thickness of the negative electrode mixture layer within these ranges, the charge / discharge capacity of the lithium ion secondary battery can be further increased, which is preferable.

[0025] The negative electrode mixture layer may further contain a conductive agent, such as those exemplified as those used in the positive electrode.

[0026] (3-2. Negative electrode slurry) The negative electrode slurry may contain, for example, a negative electrode active material and a negative electrode binder, and may further contain a negative electrode slurry solvent that disperses or dissolves the negative electrode active material and the negative electrode binder.

[0027] The negative electrode active material contains, for example, a graphite-based active material containing graphite.

[0028] Specific examples of the graphite-based active material include those containing one or more active materials selected from the group consisting of artificial graphite obtained by graphitizing graphite precursors such as coal-based or petroleum-based raw coke, calcine coke, and needle coke, mesophase carbons such as mesophase microspheres and bulk mesophase at 1500°C or higher, preferably 2800°C to 3200°C, flake, block, or granulated natural graphite, mixtures of artificial graphite and natural graphite, and natural graphite coated with artificial graphite. These may be chemically or physically treated, and examples of the treatment methods include pulverization, classification, granulation, lamination, compression, compounding, mixing, coating, oxidation, vapor deposition, mechanochemical treatment, de-edging, spheroidization, curvature, and heat treatment. In the case of artificial graphite, these treatments can be carried out in combination either before or after the graphitization treatment. Specific examples of the artificial graphite include, but are not limited to, MCMB, MCF, and MAG.

[0029] In addition to the above-mentioned negative electrode active materials, for example, Si-based active materials containing silicon, Sn-based active materials (for example, a mixture of fine particles of tin (Sn) or tin oxide with a graphite-based active material, fine particles of tin, an alloy based on tin), metallic lithium and Li4Ti5O 12 The negative electrode may further contain one or more negative electrode active materials selected from the group consisting of titanium oxide-based compounds such as titanium oxide, lithium nitride, and the like.

[0030] When the solids concentration of the negative electrode slurry is set to 40% by mass or more and 60% by mass or less, the viscosity of the negative electrode slurry as measured by a Brookfield viscometer at 25°C is preferably 1000 mPa·s or more and 8000 mPa·s or less, and more preferably 1500 mPa·s or more and 7000 mPa·s or less. The viscosity of the negative electrode slurry may be adjusted, for example, by the amount of the water-soluble polymer compound added. It is preferable that the viscosity change little over time after the preparation of the negative electrode slurry and be maintained within the aforementioned range.

[0031] (3-3. Binders for non-aqueous electrolyte secondary batteries) The negative electrode mixture layer and negative electrode slurry of the secondary battery according to this embodiment contain, as the negative electrode binder that binds the negative electrode active materials together and the negative electrode mixture layer onto the negative electrode current collector, a particulate binder having a core-shell structure including a hydrophobic core and a hydrophilic shell that surrounds the hydrophobic core (the binder for non-aqueous electrolyte secondary batteries according to the present invention), and a water-soluble polymer compound binder that is a binder made of a water-soluble polymer compound.

[0032] The water-soluble polymer compound is not particularly limited as long as it can impart good viscosity to the negative electrode slurry and can suppress sedimentation of the negative electrode active material even after a long time has passed since the negative electrode slurry was prepared. For example, the water-soluble polymer compound is an alkali metal salt of carboxymethyl cellulose.

[0033] The content of the water-soluble polymer compound binder in the negative electrode slurry is preferably 0.5% by mass or more and 2.0% by mass or less, where the total mass of the negative electrode active material, the binder for a non-aqueous electrolyte secondary battery, and the water-soluble polymer compound binder is taken as 100% by mass. A content of the water-soluble polymer compound binder of 0.5% by mass or more is preferable because it can impart good viscosity to the negative electrode slurry for coating the negative electrode mixture layer and suppress sedimentation of the negative electrode active material during storage of the negative electrode slurry. Furthermore, a content of the water-soluble polymer compound binder of 2.0% by mass or less is preferable because it does not impart excessive viscosity to the negative electrode slurry for coating the negative electrode mixture layer, allowing the slurry to be coated in a high solids state and efficiently dried to obtain a negative electrode.

[0034] The content of the binder for a non-aqueous electrolyte secondary battery in the negative electrode slurry is preferably 1.0% by mass or more and 3.0% by mass, where the total mass of the negative electrode active material, the binder for a non-aqueous electrolyte secondary battery, and the water-soluble polymer compound binder is taken as 100% by mass. A content of the binder for a non-aqueous electrolyte secondary battery of 1.0% by mass or more is preferred because cracking and peeling of the negative electrode mixture layer can be further suppressed when the negative electrode mixture layer is coated, dried, and rolled (pressed). Furthermore, a content of the binder for a non-aqueous electrolyte secondary battery of 3.0% by mass or less is preferred because excessive increases in electrode resistance can be suppressed, resulting in a negative electrode with good cycle performance.

[0035] The binder for a non-aqueous electrolyte secondary battery has a surface formed by the hydrophilic shell, and is therefore a water-dispersible fine particle that can be uniformly dispersed in an aqueous solvent such as water, which is the solvent for the negative electrode slurry. The aqueous solvent refers to, in addition to the water described above, a mixed solution of water and a water-miscible organic solvent, with a water content of 50% by mass or more. Examples of the water-miscible organic solvent include water-soluble alcohols.

[0036] The binder for a non-aqueous electrolyte secondary battery has an average particle size measured by a wet laser diffraction scattering method of 250 nm or more and 1000 nm or less, and preferably 300 nm or more and 750 nm or less. It is preferable that the average particle size of the binder for a non-aqueous electrolyte secondary battery is 250 nm or more, since this can improve the adhesion between the electrode mixture layer and the current collector (the peel strength of the electrode) and reduce the internal resistance of the battery. Also, it is preferable that the average particle size of the binder for a negative electrode is 1000 nm or less, since this can further suppress the occurrence of cracks in the electrode during the coating and drying process of the negative electrode mixture slurry.

[0037] The hydrophobic core is formed of a hydrophobic polymer and may be spherical. The hydrophobic polymer is, for example, a polymer containing one or more selected from the group consisting of units derived from an aromatic vinyl monomer, units derived from an unsaturated carboxylic acid alkyl ester monomer, units derived from a (meth)acrylic acid monomer, and an unsaturated carboxylic acid amide monomer. The hydrophobic polymer preferably contains units derived from an aromatic vinyl monomer, units derived from an unsaturated carboxylic acid alkyl ester monomer, units derived from a (meth)acrylic acid monomer, and an unsaturated carboxylic acid amide monomer.

[0038] Examples of the aromatic vinyl monomer include styrene, p-methylstyrene, m-methylstyrene, pt-butylstyrene, mt-butylstyrene, etc. Among these, styrene is preferred. Examples of the unsaturated carboxylic acid alkyl ester monomer include 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, octyl (meth)acrylate, butyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, etc. Among these, 2-ethylhexyl (meth)acrylate is preferred. Examples of the (meth)acrylic acid monomer include (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid. Examples of the unsaturated carboxylic acid amide monomer include (meth)acrylamide, (meth)N-methylacrylamide, (meth)Nt-butylacrylamide, (meth)N-hydroxymethylacrylamide, and (meth)N-butoxymethylacrylamide. Other copolymerizable monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, glycidyl (meth)acrylate, methoxyethyl (meth)acrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, and divinylbenzene.

[0039] The hydrophilic shell is formed so as to cover the surface of the hydrophobic shell, and enables the hydrophobic core to be dispersed in an aqueous solvent, and is made of a water-soluble polymer.

[0040] The water-soluble polymer contains an acrylic acid-sodium styrene sulfonate-acrylonitrile terpolymer (also referred to as an AA-NaSS-AN copolymer). The AA-NaSS-AN copolymer contains units derived from an acrylic acid monomer, units derived from a sodium styrene sulfonate monomer, and units derived from an acrylonitrile monomer.

[0041] The AA-NaSS-AN copolymer preferably has, for example, a content of units derived from acrylic acid-based monomers of 10% by mass or more and 50% by mass or less, a content of units derived from sodium styrenesulfonate monomers of 10% by mass or more and 50% by mass or less, and a content of units derived from acrylonitrile-based monomers of 10% by mass or more and 50% by mass or less. The AA-NaSS-AN copolymer may contain units derived from other monomers copolymerizable with the acrylic acid-based monomer units, sodium styrenesulfonate monomer units, and / or acrylonitrile-based monomer units, and the content of the units derived from the other monomers is preferably in the range of more than 0% by mass to 20% by mass or less.

[0042] It is preferable that the content of the structural units derived from the acrylic acid monomer in the AA-NaSS-AN copolymer is 10% by mass or more, since this makes the acrylic acid-acrylonitrile copolymer more soluble in water, thereby further improving the dispersibility of the negative electrode active material and the storage stability of the negative electrode slurry.It is also preferable that the content of the structural units derived from the acrylic acid monomer in the AA-NaSS-AN copolymer is 50% by mass or less, since this further reduces the occurrence of cracks during the coating and drying processes of the negative electrode slurry.

[0043] It is preferable that the content of units derived from sodium styrenesulfonate monomer in the AA-NaSS-AN copolymer is 10% by mass or more, since this can further suppress swelling of the negative electrode binder in the electrolyte solution and further improve high-temperature cycle performance.Also, it is preferable that the content of units derived from sodium styrenesulfonate monomer in the AA-NaSS-AN copolymer is 50% by mass or less, since this can further suppress cracking during the coating and drying processes of the negative electrode slurry.

[0044] It is preferable that the content of units derived from acrylonitrile-based monomers in the AA-NaSS-AN copolymer be 10% by mass or more, since this further improves the adhesion of the negative electrode mixture layer to the negative electrode current collector.Also, it is preferable that the content of units derived from acrylonitrile-based monomers in the AA-NaSS-AN copolymer be 50% by mass or less, since this makes the AA-NaSS-AN copolymer more soluble in water, and this further improves the dispersibility of the negative electrode active material and the storage stability of the negative electrode slurry.

[0045] It is preferable that the content of constituent units derived from other monomers copolymerizable with the acrylic acid-based monomer, sodium styrenesulfonate monomer, and acrylonitrile-based monomer, which are raw materials for the AA-NaSS-AN-based copolymer, is 20 mass% or less, because this can prevent cracks from occurring during the application and drying steps of the negative electrode slurry and prevent the negative electrode mixture layer from peeling off from the negative electrode current collector during charging.

[0046] The acrylic acid monomer preferably contains at least one selected from the group consisting of (meth)acrylic acid, metal salts of (meth)acrylic acid, ammonium salts of (meth)acrylic acid, and amine salts of (meth)acrylic acid.

[0047] As the metal salt of (meth)acrylic acid, it is preferable to use, for example, an alkali metal salt of (meth)acrylic acid. Specific examples of the metal salt of (meth)acrylic acid include sodium acrylate, lithium acrylate, potassium acrylate, calcium acrylate, magnesium acrylate, sodium methacrylate, lithium methacrylate, potassium methacrylate, calcium methacrylate, etc. Among these, it is more preferable to use sodium acrylate.

[0048] Examples of the ammonium salt of (meth)acrylic acid include ammonia-neutralized (meth)acrylic acid, monoethanolamine-neutralized (meth)acrylic acid, diethanolamine-neutralized (meth)acrylic acid, and hydroxylamine-neutralized (meth)acrylic acid. Among these, it is more preferable to use ammonia-neutralized (meth)acrylic acid.

[0049] The other monomer copolymerizable with the acrylic acid monomer, the sodium styrenesulfonate monomer, and the acrylonitrile monomer in the AA-NaSS-AN copolymer is preferably, for example, a water-soluble monomer. Specific examples of the water-soluble monomer include (meth)acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, N-isopropylacrylamide, N-(2-hydroxyethyl)acrylamide, N-(2-hydroxyethyl)methacrylamide, 2-hydroxyethyl (meth)acrylate, vinylpyrrolidone, vinylacetamide, vinylformamide, and vinyl alcohol.

[0050] The weight-average molecular weight of the AA-NaSS-AN copolymer is preferably 300,000 or more and 2,000,000 or less, more preferably 1,600,000 or less, and even more preferably 1,500,000 or less. This weight-average molecular weight can be measured by gel permeation chromatography (GPC), which will be described in detail in the Examples.

[0051] In order to achieve a weight-average molecular weight within the above-mentioned range, it is preferable that the AA-NaSS-AN copolymer does not contain crosslinkable monomer units as the units derived from the other monomers. By not adding crosslinkable monomer units, it is possible to prevent a decrease in the water solubility of the copolymer and thus a decrease in cycle performance due to a loss of dispersibility of the active material. The crosslinkable monomer unit broadly refers to a compound having at least two or more ethylenically unsaturated bonds, and specific examples include methylenebisacrylamide, ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and divinylbenzene.

[0052] The viscosity of an 8% by mass aqueous solution of the AA-NaSS-AN copolymer at 25° C. measured with a Brookfield viscometer is preferably in the range of 500 mPa·s to 20,000 mPa·s. A viscosity of 500 mPa·s or higher for the AA-NaSS-AN copolymer aqueous solution is preferred because it improves the adhesion of the negative electrode mixture layer to the negative electrode current collector and results in better cycle performance. A viscosity of 20,000 mPa·s or lower for the AA-NaSS-AN copolymer aqueous solution is preferred because it prevents excessive viscosity from being imparted to the negative electrode slurry used to form the negative electrode mixture layer, allowing the slurry to be applied in a high solids state and efficiently dried to obtain a negative electrode. The solids refer to the residue remaining after removing the solvent from a slurry, aqueous solution, aqueous dispersion, or the like. The solids in the negative electrode slurry are considered to be the same mass as the negative electrode mixture layer in its dried state.

[0053] When the mass of the binder for a non-aqueous electrolyte secondary battery is taken as 100 mass %, the content of the hydrophobic core is 90 mass % or more and 97 mass % or less, and the content of the hydrophilic shell is 3 mass % or more and 10 mass % or less. It is preferable that the content of the hydrophobic core in the binder for non-aqueous electrolyte secondary batteries is 97% by mass or less and the content of the hydrophilic shell is 3% by mass or more, since peeling of the negative electrode mixture layer from the negative electrode current collector during charge and discharge can be suppressed.Furthermore, it is preferable that the content of the hydrophobic core in the binder for non-aqueous electrolyte secondary batteries is 90% by mass or more and the content of the hydrophilic shell is 10% by mass or less, since excellent adhesion between the electrode mixture layer and the current collector (electrode peel strength) can be achieved and the internal resistance of the battery can be reduced.

[0054] <4. Method for producing binder for non-aqueous electrolyte secondary battery according to this embodiment> The negative electrode binder (binder for non-aqueous electrolyte secondary batteries) according to this embodiment can be produced, for example, by the following steps.

[0055] (4-1. Synthesis of Water-Soluble Copolymer) Acrylic acid monomers, sodium styrenesulfonate monomers, acrylonitrile monomers, and optionally other polymerizable monomers are mixed in a flask and stirred thoroughly in an aqueous solution containing sodium hydroxide and / or ammonia, etc., as needed. The atmosphere in the flask is then replaced with nitrogen and heated to 60°C. Once the solution temperature reaches 60°C, a reaction initiator is added to initiate polymerization. By allowing the reaction to proceed with stirring, an aqueous solution of a water-soluble copolymer can be obtained.

[0056] (4-2. Synthesis of binder for non-aqueous electrolyte secondary battery) The aqueous solution of the water-soluble polymer obtained as described above and the monomers used to prepare the hydrophobic polymer are placed in a flask, and the flask is then purged with nitrogen and heated to 60°C. When the solution temperature reaches 60°C, a reaction initiator is added to initiate polymerization. The polymerization reaction is carried out with stirring in the presence of the water-soluble copolymer, synthesizing the hydrophobic polymer in a stably dispersed state in water, thereby obtaining an aqueous dispersion of a binder for non-aqueous electrolyte secondary batteries. During the polymerization reaction to form the hydrophobic polymer, a portion of the water-soluble polymer may form chemical bonds with the hydrophobic polymer, such as ionic bonds, hydrogen bonds, or covalent bonds. Alternatively, the hydrophobic polymer and the water-soluble polymer may be fixed to each other by other methods, such as an anchor effect, which occurs when the hydrophobic polymer and the water-soluble polymer become entangled.

[0057] Examples of reaction initiators for the water-soluble polymer and the hydrophobic polymer that can be used include azo compounds such as α,α'-azobisisobutyronitrile, 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide), and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and persulfates such as ammonium persulfate and potassium persulfate.

[0058] The synthesized binder for non-aqueous electrolyte secondary batteries can be obtained by removing the solvent, such as water, and drying it, but it may also be used as a binder composition for non-aqueous electrolyte secondary batteries as an aqueous dispersion. Using the aqueous dispersion as a binder composition for non-aqueous electrolyte secondary batteries not only saves the trouble of removing the solvent or re-swelling the binder for non-aqueous electrolyte secondary batteries with a solvent when used, but also allows the synthesized binder for non-aqueous electrolyte secondary batteries to be used as is in a negative electrode slurry, etc., which is preferable. Note that the solvent for the binder composition for non-aqueous electrolyte secondary batteries can be the same as that used for the negative electrode slurry described above.

[0059] 5. Effects of this embodiment The binder for a non-aqueous electrolyte secondary battery, the binder composition for a non-aqueous electrolyte secondary battery, the negative electrode slurry, and the negative electrode mixture layer and negative electrode formed from the negative electrode slurry as described above can provide the following effects. the binder for a non-aqueous electrolyte secondary battery is in the form of particles having a core-shell structure with a hydrophobic core and a hydrophilic shell, the hydrophobic core comprises at least one selected from the group consisting of units derived from aromatic vinyl monomers, units derived from unsaturated carboxylic acid alkyl ester monomers, units derived from (meth)acrylic acid monomers, and units derived from unsaturated carboxylic acid amide monomers; the hydrophilic shell comprises a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrene sulfonate monomer, and a unit derived from an acrylonitrile-based monomer; When the mass of the binder for a non-aqueous electrolyte secondary battery is taken as 100 mass%, the content of the hydrophobic core is 90 mass% or more and 97 mass% or less, and the content of the hydrophilic shell is 3 mass% or more and 10 mass% or less, The binder for a non-aqueous electrolyte secondary battery has an average particle size of 250 nm or more and 1000 nm or less as measured by a wet laser diffraction scattering method, and therefore has excellent adhesion between the electrode mixture layer and the current collector, and can reduce the internal resistance of the battery compared to conventional methods. [Example]

[0060] The present invention will be described in more detail below based on specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples. In this example, first, binders A to F for non-aqueous electrolyte secondary batteries were synthesized as shown in Table 1. Next, negative electrode slurries containing these binders A to F for non-aqueous electrolyte secondary batteries were prepared, and secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 5 were fabricated, and these secondary batteries were evaluated. Each example and comparative example will be described below.

[0061] (Synthesis of Water-Soluble Polymers) 39.0 g of acrylic acid, 39.0 g of sodium p-styrenesulfonate, 42.0 g of acrylonitrile, 202.9 g of ion-exchanged water, and 1975 mg of 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) were charged into a flask and polymerized at 85°C for at least 4 hours. The reaction mixture was then concentrated by heated vacuum distillation to remove unreacted monomers. Ammonia water and ion-exchanged water were then added to adjust the solids concentration and pH of the aqueous polymer solution, resulting in a water-soluble polymer solution with a pH of 7.5 and a solids content of 9.5% by mass. The resulting water-soluble polymer had a weight-average molecular weight of 900,000 and a molecular weight distribution of 3.1. The viscosity of the resulting 9.5% by mass aqueous solution of the water-soluble polymer was 9200 mPa·s at 25°C and 30 rpm using a Brookfield viscometer. Because almost no unreacted monomer was removed, the content of the raw materials used was used directly as the percentage of units derived from each monomer in the water-soluble polymer. In this example, the molecular weight of the water-soluble polymer used is 900,000, but it goes without saying that other molecular weights may also be used. A water-soluble polymer with a molecular weight of 300,000 or more is preferable because it makes it possible to make a negative electrode composite layer containing a binder for a non-aqueous electrolyte secondary battery produced using this water-soluble polymer less likely to peel from the negative electrode current collector. Furthermore, a water-soluble polymer with a molecular weight of 2,000,000 or less is preferable because it allows the water-soluble polymer to be produced by a known method without any special ingenuity, and it also prevents the concentration of the water-soluble polymer from becoming extremely low when it is made into an aqueous solution.

[0062] (Synthesis of binder A for non-aqueous electrolyte secondary batteries) 57.89 g of the water-soluble polymer aqueous solution described above with a solids concentration of 9.5% by mass and a pH of 7.5, 52.25 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 51.89 g of styrene, and 0.36 g of 1,4-butanediol diacrylate), and 396.5 g of ion-exchanged water were charged into a flask. The temperature inside the system was raised to 65°C, and then an initiator solution of 942 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder A for nonaqueous electrolyte secondary batteries. This solution contained a hydrophobic core formed by the copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic copolymer, and a hydrophilic shell formed by the water-soluble polymer, resulting in a stable dispersion of binder A for non-aqueous electrolyte secondary batteries in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 19.3% by mass. The reaction solution was then concentrated by heated, reduced-pressure distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder A for non-aqueous electrolyte secondary batteries (binder composition A for non-aqueous electrolyte secondary batteries) containing 20% ​​solids by mass and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition A for non-aqueous electrolyte secondary batteries with a solids content of 20% by mass was 300 mPa·s at 25°C and 30 rpm using a B-type viscometer. In this step, almost no unreacted monomer was removed, so the content of the raw materials used was used as it was as the content ratio of the units derived from each monomer in the hydrophobic polymer or the binder for non-aqueous electrolyte secondary batteries. The relationship between the content of the raw materials and the content of each component in the hydrophobic polymer or the binder for non-aqueous electrolyte secondary batteries is also the same for other binders for non-aqueous electrolyte secondary batteries described below. Furthermore, even when the binder for a non-aqueous electrolyte secondary battery prepared as described above was centrifuged at 10,000 rpm for about 5 minutes using a centrifuge, the hydrophobic copolymer forming the hydrophobic core and the hydrophilic copolymer forming the hydrophilic shell did not separate, which revealed that the hydrophobic core and the hydrophilic shell were at least partially bonded to each other via some kind of chemical bond.

[0063] (Synthesis of binder B for non-aqueous electrolyte secondary batteries) 57.89 g of the aforementioned water-soluble polymer aqueous solution with a solids concentration of 9.5% by mass and a pH of 7.5, 52.25 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 51.89 g of styrene, and 0.36 g of 1,4-butanediol diacrylate), and 396.5 g of ion-exchanged water were charged into a flask. The temperature inside the system was raised to 65°C, and then an initiator solution of 850 mg of 4,4'-azobis(4-cyanovaleric acid) dissolved in 20.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder B for non-aqueous electrolyte secondary batteries. This solution contained a hydrophobic core formed by copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer, and was covered by a hydrophilic shell formed by a water-soluble polymer, resulting in a stable dispersion of binder B for non-aqueous electrolyte secondary batteries in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 19.2% by mass. The reaction solution was then concentrated by heated, vacuum distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder B for non-aqueous electrolyte secondary batteries (binder composition B for non-aqueous electrolyte secondary batteries) containing 20% ​​solids by mass and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition B for non-aqueous electrolyte secondary batteries with a solids content of 20% by mass was 370 mPa·s at 25°C and 30 rpm using a Brookfield viscometer.

[0064] (Synthesis of binder C for non-aqueous electrolyte secondary batteries) A flask was charged with 89.19 g of the aforementioned water-soluble polymer aqueous solution with a solids concentration of 9.5% by mass and a pH of 7.5, 52.25 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 51.89 g of styrene, and 0.36 g of 1,4-butanediol diacrylate), and 385.3 g of ion-exchanged water. The temperature inside the system was raised to 65°C, and then an initiator solution containing 942 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder C for non-aqueous electrolyte secondary batteries. This solution contained a hydrophobic core formed by the copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic copolymer, and a hydrophilic shell formed by the water-soluble polymer, resulting in a stable dispersion of binder C for non-aqueous electrolyte secondary batteries in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 19.4% by mass. The reaction solution was then concentrated by heated, vacuum distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder C for non-aqueous electrolyte secondary batteries (binder composition C for non-aqueous electrolyte secondary batteries) containing 20% ​​solids by mass and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition C for non-aqueous electrolyte secondary batteries with a solids content of 20% by mass was 480 mPa·s at 25°C and 30 rpm using a B-type viscometer.

[0065] (Synthesis of binder D for non-aqueous electrolyte secondary batteries) A flask was charged with 99.42 g of the aforementioned water-soluble polymer aqueous solution with a solids concentration of 9.5% by mass and a pH of 7.5, 42.5 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 42.2 g of styrene, and 0.30 g of 1,4-butanediol diacrylate), and 302.6 g of ion-exchanged water. The temperature inside the system was raised to 65°C, and then an initiator solution containing 876 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder D for non-aqueous electrolyte secondary batteries. This solution contained a hydrophobic core formed by copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer, and was covered by a hydrophilic shell formed by the water-soluble polymer, resulting in a stable dispersion of binder D for non-aqueous electrolyte secondary batteries in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 19.0% by mass. The reaction solution was then concentrated by heated, reduced-pressure distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder D for non-aqueous electrolyte secondary batteries (binder composition D for non-aqueous electrolyte secondary batteries) containing 20% ​​solids by mass and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition D for non-aqueous electrolyte secondary batteries with a solids content of 20% by mass was 570 mPa·s at 25°C and 30 rpm using a B-type viscometer.

[0066] (Synthesis of binder composition E for non-aqueous electrolyte secondary batteries) A flask was charged with 157.89 g of the water-soluble copolymer solution (9.5% solids by mass, pH 7.5), 30.0 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 29.79 g of styrene, and 0.21 g of 1,4-butanediol diacrylate), and 272.1 g of ion-exchanged water. The temperature inside the system was raised to 65°C, and then an initiator solution (831 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water) was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder E for non-aqueous electrolyte secondary batteries. This solution was a stable dispersion of binder E for non-aqueous electrolyte secondary batteries in water, with the hydrophobic polymer produced by copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer. The hydrophobic polymer was coated with a water-soluble polymer, resulting in binder E for non-aqueous electrolyte secondary batteries being dispersed stably in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 15.0% by mass. The reaction solution was then concentrated by heated, vacuum distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder E for non-aqueous electrolyte secondary batteries (binder composition E for non-aqueous electrolyte secondary batteries) containing 15% solids by mass and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition E for non-aqueous electrolyte secondary batteries with a solids content of 15% by mass was 350 mPa·s at 25°C and 30 rpm using a B-type viscometer.

[0067] (Synthesis of binder composition F for non-aqueous electrolyte secondary batteries) A flask was charged with 270.68 g of the aforementioned water-soluble polymer aqueous solution with a solids concentration of 9.5% by mass and a pH of 7.5, 30.0 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 29.79 g of styrene, and 0.21 g of 1,4-butanediol diacrylate), and 230.6 g of ion-exchanged water. The temperature inside the system was raised to 65°C, and then an initiator solution containing 773 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder F for non-aqueous electrolyte secondary batteries. This solution was a stable dispersion of binder F for non-aqueous electrolyte secondary batteries in water, with the hydrophobic polymer produced by copolymerization of 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer. The hydrophobic polymer was coated with a water-soluble polymer, resulting in binder F for non-aqueous electrolyte secondary batteries being dispersed stably in water. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 15.0% by mass. The reaction solution was then concentrated by heated, vacuum distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder F for non-aqueous electrolyte secondary batteries containing 15% solids by mass and having a pH of 7.5 (binder composition F for non-aqueous electrolyte secondary batteries). The viscosity of the resulting aqueous dispersion of binder composition F for non-aqueous electrolyte secondary batteries with a solids content of 15% by mass was 670 mPa·s at 25°C and 30 rpm using a B-type viscometer.

[0068] (Synthesis of binder composition G for non-aqueous electrolyte secondary batteries) 57.89 g of the water-soluble polymer aqueous solution (9.5% solids by mass, pH 7.5) described above, 52.25 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 51.89 g of styrene, and 0.36 g of 1,4-butanediol diacrylate), 2.1 g of sodium dodecylbenzenesulfonate, and 396.5 g of ion-exchanged water were charged into a flask. The temperature inside the system was raised to 65°C, and then an initiator solution of 942 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder G for non-aqueous electrolyte secondary batteries. This solution was prepared by copolymerizing 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer, to produce a hydrophobic polymer. The hydrophobic polymer was then coated with a water-soluble polymer and a non-reactive surfactant, sodium dodecylbenzenesulfonate, resulting in a stable dispersion of binder G for non-aqueous electrolyte secondary batteries in water. The non-reactive surfactant is a surfactant that does not undergo a polymerization reaction with the hydrophobic polymer and water-soluble polymer that form the core and shell of the binder. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 19.6% by mass. The reaction solution was then concentrated by heated, vacuum distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder G for non-aqueous electrolyte secondary batteries (binder composition G for non-aqueous electrolyte secondary batteries) containing 20% ​​solids by mass and having a pH of 7.5. The viscosity of the obtained aqueous dispersion of Binder Composition G for Non-aqueous Electrolyte Secondary Batteries with a solid content of 20 mass % was 330 mPa·s at 25° C. and 30 rpm using a Brookfield viscometer.

[0069] (Synthesis of binder composition H for non-aqueous electrolyte secondary batteries) 2.5 g of sodium dodecylbenzenesulfonate, 50.0 g of hydrophobic polymer raw materials (2-ethylhexyl acrylate, 49.6 g of styrene, and 0.4 g of 1,4-butanediol diacrylate), and 390.0 g of ion-exchanged water were charged into a flask. The temperature inside the system was raised to 65°C, and then an initiator solution consisting of 896 mg of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate dissolved in 10.0 g of ion-exchanged water was added. Polymerization was carried out at 65°C for 12 hours or more, yielding a milky white dispersion. This milky white dispersion was a solution containing binder H for non-aqueous electrolyte secondary batteries. This solution was a stable dispersion of binder H for non-aqueous electrolyte secondary batteries in water, formed by copolymerizing 2-ethylhexyl acrylate, styrene, and 1,4-butanediol diacrylate, which were added as raw materials for the hydrophobic polymer. The hydrophobic polymer was coated with the non-reactive surfactant dodecylbenzenesulfonic acid, resulting in binder H for non-aqueous electrolyte secondary batteries. The non-volatile content of the aqueous dispersion after the reaction was measured and found to be 20.3% by mass. The reaction solution was then concentrated by heated, reduced-pressure distillation to remove unreacted monomers, and ammonia water and ion-exchanged water were added to adjust the solids concentration and pH to obtain an aqueous dispersion of binder H for non-aqueous electrolyte secondary batteries (binder composition H for non-aqueous electrolyte secondary batteries) containing 20% ​​by mass of solids and having a pH of 7.5. The viscosity of the resulting aqueous dispersion of binder composition F for non-aqueous electrolyte secondary batteries with a solids content of 20% by mass was 80 mPa·s at 25°C and 30 rpm using a B-type viscometer.

[0070] <Measurement of average particle size> The volume average particle diameter of the binder composition for a non-aqueous electrolyte secondary battery, whose concentration was adjusted to give a scattering intensity of 2%, was measured in ion-exchanged water using a wet laser diffraction / scattering particle size analyzer, Mastersizer 3000E, manufactured by Malvern Instruments. The results are shown in Table 1 below.

[0071] [Table 1]

[0072] Example 1 A lithium ion secondary battery was fabricated by the following procedure using the binder composition A for non-aqueous electrolyte secondary batteries obtained as described above. (Negative electrode production) Graphite, binder composition A for non-aqueous electrolyte secondary batteries, and sodium carboxymethyl cellulose were dispersed and mixed in ion-exchanged water, which was a solvent for the negative electrode slurry, in a mass ratio of 97.2:2.0:0.8 as solid contents to prepare a negative electrode mixture slurry. After drying, the coating amount (areal density) of the negative electrode mixture layer was 12 mg / cm per side. 2 This negative electrode mixture slurry was applied to both sides of a 10 μm-thick copper foil current collector using a reverse roll coater so that the density of the negative electrode mixture layer became 1.60 g / cc, and then dried at 120° C. Thereafter, the negative electrode mixture was rolled using a roll press machine so that the density of the negative electrode mixture layer became 1.60 g / cc, thereby producing a double-sided negative electrode.

[0073] (Positive electrode production) Li 1.0 Ni 0.88 Co 0.1 Al 0.01 Mg 0.01 The positive electrode slurry was prepared by dispersing and mixing O2, acetylene black, and polyvinylidene fluoride in N-methyl-2-pyrrolidone, a solvent for the positive electrode slurry, in a powder mass ratio of 97.7:1.0:1.3. The coating amount (areal density) of the positive electrode mixture layer after drying was 19.8 mg / cm per side. 2 The slurry was applied to both sides of an aluminum current collector foil serving as a positive electrode current collector using a reverse roll coater so that the density of the mixture layer became 3.70 g / cc, and the mixture was dried. Thereafter, the mixture was pressed using a roll press machine so that the density of the mixture layer became 3.70 g / cc, thereby producing a positive electrode.

[0074] (Secondary battery cell manufacturing) The negative and positive electrodes were cut into 29mm x 29mm and 27mm x 27mm squares, respectively, and nickel and aluminum lead wires were welded to them. Afterwards, nine positive electrodes were sandwiched between ten negative electrodes via a polyethylene porous separator to produce an electrode stack. The electrode stack was then housed in an aluminum laminate film with the lead wires extended to the outside, and an electrolyte was poured into the cell, which was then vacuum-sealed to produce a pre-charged secondary battery cell. The electrolyte was a 20 / 40 / 40 (volume ratio) mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, dissolved in 1M LiPF6 and 1% by mass of vinylene carbonate. The amount of electrolyte poured was 1.25g. The designed capacity of the fabricated battery was 500mAh.

[0075] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that binder composition B for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0076] Example 3 A secondary battery was fabricated in the same manner as in Example 1, except that binder composition C for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0077] Example 4 A secondary battery was fabricated in the same manner as in Example 1, except that binder composition D for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0078] <Comparative Example 1> A secondary battery was fabricated in the same manner as in Example 1, except that binder composition E for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0079] <Comparative Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that binder composition F for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0080] <Comparative Example 3> A secondary battery was fabricated in the same manner as in Example 1, except that binder composition G for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0081] <Comparative Example 4> A secondary battery was fabricated in the same manner as in Example 1, except that binder composition H for non-aqueous electrolyte secondary batteries was used instead of binder composition A for non-aqueous electrolyte secondary batteries.

[0082] <Comparative Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that the binder composition A for non-aqueous electrolyte secondary batteries was replaced with a commercially available modified styrene butadiene rubber (hereinafter referred to as SBR).

[0083] <Evaluation of negative electrodes and secondary batteries> (negative electrode peel strength) Each of the negative electrodes produced in Examples 1 to 4 and Comparative Examples 1 to 5 before and after rolling was cut into a strip measuring 25 mm wide and 100 mm long. The active material side was then attached to a stainless steel plate using double-sided tape, with the adherend facing up, to prepare a sample for evaluating negative electrode peel strength. The sample was mounted on a peel tester (Shimadzu EZ-S, manufactured by Shimadzu Corporation) and the 180-degree peel strength was measured. The results are shown in Table 2.

[0084] (DC resistance) The secondary battery cells fabricated in Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to constant current charging at a design capacity of 0.1 CA up to 4.25 V in a thermostatic chamber at 25°C, followed by constant voltage charging at 4.25 V down to 0.05 CA. They were then subjected to constant current discharging at 0.1 CA down to 2.8 V. Furthermore, two cycles of constant current charging at 0.2 CA, constant voltage charging at 0.05 CA, and constant current discharging at 0.2 CA were performed in a thermostatic chamber at 25°C, with a charge cut-off voltage of 4.25 V and a discharge cut-off voltage of 2.8 V. The secondary battery was then charged at a constant current of 0.2 CA at 25°C to 50% charge, and then discharged at constant currents of 0.2 CA, 0.5 CA, 1 CA, and 2 CA for 10 seconds each. The current value after 10 seconds was plotted on the horizontal axis and the voltage on the vertical axis. A straight line was drawn from the plot using linear approximation, and the slope was taken as the resistance value. The results are shown in Table 2.

[0085] (Evaluation of mixture layer peeling after first charge) The secondary battery cells fabricated in Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to constant current charging at a design capacity of 0.1 CA up to 4.25 V in a thermostatic chamber at 25°C, followed by constant voltage charging at 4.25 V down to 0.05 CA. The secondary batteries were then disassembled in a dry room, and the condition of the negative electrode was visually observed and evaluated according to the following criteria. The results are shown in Table 2. ○: No peeling of the negative electrode mixture layer from the base copper foil ×: Peeling of the negative electrode mixture layer from the copper foil substrate.

[0086] [Table 2]

[0087] (Discussion of the results of the Examples and Comparative Examples) A comparison of Examples 1 to 4 with Comparative Examples 1 to 4 in Table 2 reveals that, according to Examples 1 to 4, by setting the content of the water-soluble polymer in the binder composition for a non-aqueous electrolyte secondary battery in the range of 3 to 10% and setting the average particle size of the binder for a non-aqueous electrolyte secondary battery to 250 nm or more, the peel strength of the negative electrode mixture layer from the negative electrode current collector and the resistance value of the battery can be significantly improved compared to the Comparative Examples.

[0088] In particular, in comparison with Comparative Examples 3 and 4, it was found that the average particle size of the binder for non-aqueous electrolyte secondary batteries was likely to be small because a non-reactive surfactant, which is a low-molecular-weight emulsifier, was used in addition to or instead of the water-soluble polymer when synthesizing the binder for non-aqueous electrolyte secondary batteries. Compared with Examples 1 to 4, the results showed inferior peel strength and resistance. From these results, it is considered preferable that the binder for non-aqueous electrolyte secondary batteries is produced under conditions in which no non-reactive surfactant is added, as in Examples 1 to 4, and that the binder for non-aqueous electrolyte secondary batteries is preferably substantially free of a non-reactive surfactant.

[0089] In this example, the composition of the binder for non-aqueous electrolyte secondary batteries and the content of the water-soluble polymer in the binder composition for non-aqueous electrolyte secondary batteries are set to a range of 3 to 10%, and the particle size of the binder for non-aqueous electrolyte secondary batteries is increased. It is believed that the anchor effect facilitates adhesion between the negative electrode composite layer and the negative electrode current collector when they are pressed together. Furthermore, a larger particle size of the binder for non-aqueous electrolyte secondary batteries can reduce the number of binder particles for non-aqueous electrolyte secondary batteries present in the negative electrode composite layer, thereby minimizing the inhibition of ion conduction in the negative electrode composite layer and reducing the resistance value of the negative electrode. While this suggests that a larger particle size of the binder for non-aqueous electrolyte secondary batteries is preferable, it has been confirmed that a particle size of 1000 nm or less is preferable from the viewpoint of suppressing aggregation of the binder particles for non-aqueous electrolyte secondary batteries.

[0090] Furthermore, when compared with SBR (Comparative Example 5), which has conventionally been used as a binder for nonaqueous electrolyte secondary batteries, the nonaqueous electrolyte secondary batteries fabricated in Examples 1 to 4 had performance in terms of the peel strength of the negative electrode mixture layer from the negative electrode current collector that was equal to or greater than that of batteries using SBR, and were able to reduce the resistance value compared to batteries using SBR.

Claims

1. a particulate binder having a core-shell structure with a hydrophobic core and a hydrophilic shell, the hydrophobic core comprises at least one unit selected from the group consisting of a unit derived from an aromatic vinyl monomer, a unit derived from an unsaturated carboxylic acid alkyl ester monomer, a unit derived from a (meth)acrylic acid monomer, and a unit derived from an unsaturated carboxylic acid amide monomer, the hydrophilic shell comprises a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrene sulfonate monomer, and a unit derived from an acrylonitrile-based monomer; When the mass of the binder is taken as 100 mass%, the content of the hydrophobic core is 90 mass% or more and 97 mass% or less, and the content of the hydrophilic shell is 3 mass% or more and 10 mass% or less, The binder for a non-aqueous electrolyte secondary battery has an average particle size of 250 nm or more and 1000 nm or less as measured by a wet laser diffraction scattering method.

2. the hydrophilic shell comprises a water-soluble polymer; the water-soluble polymer contains the units derived from the acrylic acid-based monomer, the units derived from the sodium styrene sulfonate monomer, and the units derived from the acrylonitrile-based monomer, 2. The binder for a non-aqueous electrolyte secondary battery according to claim 1, wherein the contents of the units derived from the acrylic acid-based monomer, the units derived from the sodium styrene sulfonate monomer, and the units derived from the acrylonitrile-based monomer in the water-soluble polymer are such that, when the mass of the water-soluble polymer is taken as 100 mass%, the content of the units derived from the acrylic acid-based monomer is 10% by mass or more and 50% by mass or less, the content of the units derived from the sodium styrene sulfonate monomer is 10% by mass or more and 50% by mass or less, and the content of the acrylonitrile-based monomer units is 10% by mass or more and 50% by mass or less.

3. 3. The binder for a non-aqueous electrolyte secondary battery according to claim 1, wherein the water-soluble polymer has a molecular weight of 300,000 or more and 2,000,000 or less.

4. 2. The binder for a non-aqueous electrolyte secondary battery according to claim 1, wherein the hydrophobic core comprises a hydrophobic polymer including units derived from the aromatic vinyl-based monomer, units derived from the unsaturated carboxylic acid alkyl ester monomer, units derived from the (meth)acrylic acid-based monomer, and units derived from the unsaturated carboxylic acid amide monomer.

5. The binder for a non-aqueous electrolyte secondary battery according to claim 1 , wherein the hydrophobic core and the hydrophilic shell are chemically bonded to each other.

6. 2. The binder for a non-aqueous electrolyte secondary battery according to claim 1, which is additive-free and does not contain a non-reactive surfactant.

7. A binder composition for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery according to claim 1 and an aqueous solvent.

8. A negative electrode slurry for a non-aqueous electrolyte secondary battery, comprising the binder composition for a non-aqueous electrolyte secondary battery according to claim 7 and a negative electrode active material.

9. A negative electrode for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery according to claim 1 and a negative electrode active material.

10. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 9 , further comprising a water-soluble polymer compound.

11. 11. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 10, wherein the water-soluble polymer compound is an alkali metal salt of carboxymethyl cellulose.

12. A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 9 to 11.

13. 1. A method for producing a core-shell structured particulate binder having a hydrophobic core and a hydrophilic shell, comprising: synthesizing a hydrophobic polymer constituting the hydrophobic core in the presence of a water-soluble polymer constituting the hydrophilic shell; the water-soluble polymer contains a unit derived from an acrylic acid-based monomer, a unit derived from a sodium styrenesulfonate monomer, and a unit derived from an acrylonitrile-based monomer.

14. 14. The method for producing a binder for a non-aqueous electrolyte secondary battery according to claim 13, wherein in the step of synthesizing the hydrophobic polymer, the hydrophobic polymer is synthesized by polymerizing one or more monomers selected from the group consisting of an aromatic vinyl-based monomer, an unsaturated carboxylic acid alkyl ester monomer, a (meth)acrylic acid-based monomer, and an unsaturated carboxylic acid amide monomer.

15. 14. The method for producing a binder for a nonaqueous electrolyte secondary battery according to claim 13, wherein, when the mass of the binder is taken as 100 mass%, a content of the hydrophobic core is 90 mass% or more and 97 mass% or less, and a content of the hydrophilic shell is 3 mass% or more and 10 mass% or less.

16. 14. The method for producing a binder for a non-aqueous electrolyte secondary battery according to claim 13, wherein the binder has an average particle size of 250 nm or more and 1000 nm or less as measured by a wet laser diffraction scattering method.

Images