Carbon nanotube dispersion liquid, resin composition, mixture slurry, electrode film, and lithium ion secondary battery

Abstract

This carbon nanotube dispersion liquid contains carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), and is characterized in that X / Y satisfies 0.8 ≤ X / Y ≤ 5.5 when X represents the storage elastic modulus of the carbon nanotube dispersion liquid at a strain of 0.1%, and Y represents the storage elastic modulus at a strain of 1%. Due to this configuration, a carbon nanotube dispersion liquid having improved dispersibility and viscosity stability is provided.

Description

Carbon nanotube dispersion, resin composition, composite slurry, electrode film, and lithium ion secondary battery

[0001] The present invention relates to a carbon nanotube dispersion, a resin composition, a composite slurry, an electrode film, and a lithium ion secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have been attracting attention with the widespread use of mobile phones and notebook personal computers. A lithium-ion secondary battery typically comprises a negative electrode made of a carbonaceous material, a positive electrode containing an active material that reversibly transfers lithium ions, and a non-aqueous electrolyte in which these are immersed. The positive electrode is produced by applying a composite slurry made of the active material, a conductive material, and a binder to a current collector plate.

[0003] Conductive materials include carbon black, ketjen black, fullerene, graphene, and fine carbon materials. In particular, carbon nanotubes (hereinafter also referred to as CNTs), a type of fine carbon fiber, are tubular carbons with a diameter of 1 μm or less. Due to their unique structure and high conductivity, they are being considered for use as conductive materials in lithium-ion secondary batteries. Among these, multi-walled CNTs with an outer diameter of several nanometers to several tens of nanometers are relatively inexpensive and are expected to be put to practical use.

[0004] The use of CNTs with a small average outer diameter allows for efficient formation of a conductive network with a small amount, thereby reducing the amount of conductive material contained in the positive and negative electrodes of lithium-ion secondary batteries. However, CNTs with a small average outer diameter have a strong cohesion force and are difficult to disperse, making it difficult to obtain a CNT dispersion with sufficient dispersibility.

[0005] To address the above-mentioned problems, methods for stabilizing the dispersion of carbon nanotubes using various dispersants have been proposed, such as a method using polyvinylpyrrolidone as a dispersant (Patent Document 1) and a method using polyvinyl alcohol (hereinafter also referred to as "PVA") as a dispersant (Patent Documents 2 to 4).

[0006] JP 2005-162877 A JP 2014-193996 A JP 2020-011934 A JP 2021-050106 A

[0007] An object of the present invention is to provide a carbon nanotube dispersion liquid having improved dispersibility and viscosity stability. Another object of the present invention is to provide a resin composition, a composite slurry, and a lithium ion secondary battery containing the carbon nanotube dispersion liquid.

[0008] As a result of extensive research, the present inventors have found that the above object can be achieved by using PVA as a dispersant for carbon nanotubes and setting the ratio X / Y of the storage modulus X at a strain of 0.1% and the storage modulus Y at a strain of 1% of the carbon nanotube dispersion within a predetermined range.

[0009] That is, the above object is to provide the carbon nanotube dispersion liquid comprising: [1] carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), wherein, when the storage modulus of the carbon nanotube dispersion liquid at a strain of 0.1% is X and the storage modulus of the carbon nanotube dispersion liquid at a strain of 1% is Y, X / Y satisfies 0.8≦X / Y≦5.5; [2] the carbon nanotube dispersion liquid of [1], wherein the polyvinyl alcohol (B) has at least one anionic group selected from the group consisting of a carboxy group and a sulfonic acid group, and the modification amount of the anionic group is 0.5 mol % or more and 10 mol % or less; [3] the carbon nanotube dispersion liquid of [1] or [2], wherein the polyvinyl alcohol (B) has a quaternary ammonium salt group, and the modification amount is 0.1 mol % or more and 10 mol % or less; [4] the carbon nanotube dispersion liquid of any of [1] to [3], wherein the saponification degree of the polyvinyl alcohol (B) is 82 mol % or more and 95 mol % or less; [5] The carbon nanotube dispersion of any one of [1] to [4], wherein the viscosity-average degree of polymerization of the polyvinyl alcohol (B) is 250 or more and less than 450; [6] The carbon nanotube dispersion of any one of [1] to [5], wherein the solvent (C) is a water-soluble organic solvent; [7] The carbon nanotube dispersion of any one of [1] to [6], wherein the solvent (C) is N-methyl-2-pyrrolidone; [8] The carbon nanotube dispersion of any one of [1] to [7], wherein the water content is 50 parts by mass or less per 100 parts by mass of the carbon nanotube dispersion; [9] A resin composition comprising the carbon nanotube dispersion of any one of [1] to [8], and a binder resin (D);

[10] A composite slurry comprising the resin composition of [9], and an active material (E);

[11] An electrode film obtained by forming the composite slurry of

[10] into a film;

[12] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein at least one of the positive electrode and the negative electrode comprises the electrode film of

[11] .

[0010] According to the present invention, it is possible to provide a carbon nanotube dispersion liquid with improved dispersibility and viscosity stability, as well as a resin composition, composite slurry, and lithium ion secondary battery containing the carbon nanotube dispersion liquid.

[0011] Hereinafter, embodiments of the present invention will be described in detail. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0012] <Carbon Nanotube Dispersion> The carbon nanotube dispersion of the present invention is a carbon nanotube dispersion containing carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), in which X / Y satisfies 0.8≦X / Y≦5.5, where X is the storage modulus at 0.1% strain and Y is the storage modulus at 1% strain. Hereinafter, the carbon nanotube dispersion may be referred to as a "CNT dispersion," carbon nanotubes (A) as "CNT(A)," and polyvinyl alcohol (B) as "PVA(B)."

[0013] In the CNT dispersion of the present invention, when the storage modulus at 0.1% strain is X and the storage modulus at 1% strain is Y, it is important that the ratio of X to Y, X / Y, satisfies 0.8≦X / Y≦5.5, preferably 0.9≦X / Y≦5.0, and more preferably 1.0≦X / Y≦4.0. If X / Y is less than 0.8, the CNTs form a pseudo-aggregated structure, which may result in an uneven coating surface when a resin composition obtained by adding a binder resin to a CNT dispersion is applied to a metal electrode. On the other hand, if X / Y exceeds 5.5, the CNTs are insufficiently dispersible, resulting in poor viscosity stability of the CNT dispersion. The storage modulus X at 0.1% strain and the storage modulus Y at 1% strain are values ​​measured using a rheometer, as described in the Examples below. X / Y can be adjusted by the modified species and amount of PVA (B), the viscosity average degree of polymerization, the degree of saponification, the content of PVA (B) in the CNT dispersion, the type of solvent, and the like.

[0014] Although there are no particular restrictions on the content of CNT(A) in the CNT dispersion, it is preferable that the CNT(A) be contained in an amount of 0.2 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, and even more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the CNT dispersion. When the content of CNT(A) is within the above range, the CNTs are more likely to form a conductive network, and the viscosity of the CNT dispersion is superior.

[0015] The content of PVA (B) in the CNT dispersion is not particularly limited, but is preferably 1 to 200 parts by mass, more preferably 5 to 100 parts by mass, and even more preferably 10 to 60 parts by mass, of PVA (B) relative to 100 parts by mass of CNT (A). Having the PVA (B) content within the above range can further suppress CNT aggregation and improve the viscosity of the CNT dispersion. Furthermore, from the viewpoint of the viscosity stability of the CNT dispersion over time, the content of PVA (B) in the CNT dispersion is preferably 0.01 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, even more preferably 0.08 to 2 parts by mass, and even more preferably 0.1 to 1 part by mass relative to 100 parts by mass of the CNT dispersion.

[0016] To obtain a CNT dispersion, it is preferable to carry out a process of dispersing CNT (A) in a solvent (C). The dispersing device used for this process is not particularly limited. As the dispersing device, a dispersing machine typically used for pigment dispersion or the like can be used. Examples of the dispersing device include, but are not limited to, mixers such as a disperser, a homomixer, and a planetary mixer; homogenizers; colloid mills; media-type dispersing machines such as a cone mill, a ball mill, and a sand mill; medialess dispersing machines such as a wet jet mill; and roll mills.

[0017] (Carbon Nanotube (A)) CNT (A) has a cylindrical shape with planar graphite wound around it, and may be either a single-walled CNT or a multi-walled CNT, or a combination of these. A single-walled CNT has a structure with one layer of graphite wound around it. A multi-walled CNT has a structure with two or more layers of graphite wound around it. The sidewall of CNT (A) may have a graphite structure, or it may be a CNT with sidewalls having an amorphous structure, for example. The CNT (A) may also be surface-treated. The CNT (A) may also be a CNT derivative to which a functional group such as a carboxyl group has been added. The CNT (A) may also be a CNT encapsulating a substance such as an organic compound, a metal atom, or a fullerene.

[0018] The shape of the CNT (A) is not particularly limited, and may be needle-like, cylindrical tubular, fishbone-like (fishbone or cup stacked type), playing card-like (platelet), coil-like, etc., among which needle-like or cylindrical tubular is preferred. The CNT (A) may have a single shape or may contain two or more shapes.

[0019] Examples of the form of CNT (A) include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, carbon nanofibers, etc. CNT (A) may have any one of these forms alone or a combination of two or more of these forms.

[0020] The fiber length of the CNT (A) is preferably 50 nm or more and 5000 nm or less, and may be 80 nm or more and 2000 nm or less, or may be 100 nm or more and 1000 nm or less.

[0021] The average outer diameter of the CNTs (A) is preferably 2 to 25 nm, more preferably 5 to 20 nm, and even more preferably 5 to 15 nm. The average outer diameter of the CNTs (A) is determined as follows: First, the CNTs (A) are observed and photographed using a transmission electron microscope. Next, 300 CNTs (A) are randomly selected from the photograph, and the outer diameter of each is measured. Next, the average outer diameter (nm) of the CNTs (A) is calculated as the number average of the outer diameters.

[0022] When the CNT (A) is cylindrical, its average diameter is preferably 3 nm to 25 nm, more preferably 5 nm to 20 nm, and even more preferably 10 nm to 15 nm. When the CNT (A) is cylindrical, its average length is preferably 20 nm to 100 nm, more preferably 30 nm to 60 nm, and even more preferably 35 nm to 50 nm.

[0023] In addition to CNT (A), the CNT dispersion may optionally contain other conductive materials. Examples of other conductive materials include metal powders such as gold, silver, copper, silver-plated copper powder, silver-copper composite powder, silver-copper alloy, amorphous copper, nickel, chromium, palladium, rhodium, ruthenium, indium, silicon, aluminum, tungsten, molybdenum, and platinum; inorganic powders coated with these metals; powders of metal oxides such as silver oxide, indium oxide, tin oxide, zinc oxide, and ruthenium oxide; inorganic powders coated with these metal oxides; and carbon materials such as carbon black and graphite. These other conductive materials may be used alone or in combination. When using other conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant.

[0024] The method for producing the CNTs (A) is not particularly limited, and the CNTs may be produced by any method.

[0025] (Polyvinyl alcohol (B)) PVA (B) is a polymer having vinyl alcohol units as structural units. PVA (B) is obtained by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer, which is a raw material monomer for PVA (B), and the saponified PVA (B) may contain vinyl ester units in addition to vinyl alcohol units.

[0026] Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, and vinyl versatate, with vinyl acetate being preferred.

[0027] In one embodiment of the present invention, the PVA (B) preferably has at least one anionic group selected from the group consisting of a carboxy group and a sulfonic acid group.

[0028] The method for introducing the anionic group into PVA (B) is not particularly limited. However, in the production process of PVA (B), it is preferable to copolymerize a vinyl ester monomer, which is a raw material for PVA (B), with an unsaturated monomer having an anionic group, followed by saponification to obtain PVA (B), which is a saponified copolymer having an anionic group. Among unsaturated monomers having an anionic group, examples of unsaturated monomers having a carboxy group include fumaric acid, maleic acid, itaconic acid, maleic anhydride, fumaric anhydride, trimellitic anhydride, and itaconic anhydride. Among these monomers, maleic anhydride, half esters derived from maleic anhydride, and itaconic acid are preferred in terms of availability and copolymerizability. Examples of unsaturated monomers having a sulfonic acid group include ethylenesulfonic acid, allylsulfonic acid, methallylsulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid. Among these monomers, 2-acrylamido-2-methylpropanesulfonic acid, allylsulfonic acid, and methallylsulfonic acid are preferred from the viewpoints of availability and copolymerizability. In particular, from the viewpoint of adjusting X / Y in the CNT dispersion, the anionic group-containing PVA (B) is preferably at least one selected from the group consisting of a saponified copolymer of vinyl acetate and itaconic acid, and a saponified copolymer of vinyl acetate and maleic anhydride.

[0029] The amount of modification of the anionic groups in the anionic group-containing PVA (B) is preferably 0.5 mol% to 10 mol%, more preferably 1 mol% to 8 mol%, and even more preferably 3 mol% to 7 mol%. Having the amount of modification of the anionic groups in the PVA (B) within the above range results in better dispersibility of the carbon nanofibers, lower resistance of the lithium ion secondary battery, better solubility in solvents such as N-methyl-2-pyrrolidone (NMP), and easier preparation of the carbon nanofiber slurry. The amount of modification of the anionic groups in the PVA (B) refers to the ratio of monomer units having anionic groups to all monomer units in the anionic group-containing PVA (B).

[0030] In another preferred embodiment of the present invention, the PVA (B) preferably has a quaternary ammonium salt group.

[0031] The method for introducing the quaternary ammonium salt group into PVA (B) is not particularly limited. However, in the production process of PVA (B), it is preferable to copolymerize a vinyl ester monomer, which is a raw material for PVA (B), with an unsaturated monomer having a quaternary ammonium salt group, followed by saponification to obtain PVA (B), which is a saponified copolymer having a quaternary ammonium salt group. Examples of unsaturated monomers having a quaternary ammonium salt group include trimethyl-3-(1-(meth)acrylamide-1,1-dimethylpropyl)ammonium chloride, trimethyl-3-(1-(meth)acrylamide-1,1-dimethylethyl)ammonium chloride, trimethyl-3-[(methacryloylamino)propyl]ammonium chloride, N-vinylimidazole, and quaternized products of N-vinyl-N-methylimidazole. Among these monomers, trimethyl-3-[(methacryloylamino)propyl]ammonium chloride is preferred from the viewpoints of availability and copolymerizability. In particular, from the viewpoint of adjusting the X / Y ratio of the CNT dispersion, the PVA (B) having a quaternary ammonium salt group is preferably a saponified copolymer of vinyl acetate and trimethyl-3-[(methacryloylamino)propyl]ammonium chloride.

[0032] The amount of modification of the quaternary ammonium base in the PVA (B) is preferably 0.1 mol% to 10 mol%, more preferably 1 mol% to 8 mol%, and even more preferably 3 mol% to 7 mol%. When the amount of modification of the quaternary ammonium base in the PVA (B) is within the above range, the dispersibility of the CNTs is improved, the resistance of the lithium ion secondary battery is reduced, the solubility in NMP used as a solvent is improved, and the preparation of the CNT dispersion is made easier. The amount of modification of the quaternary ammonium base in the PVA (B) refers to the ratio of monomer units having a quaternary ammonium base to all monomer units in the PVA (B) having a quaternary ammonium base.

[0033] As described above, the PVA (B) having an anionic group and / or a quaternary ammonium salt group can be obtained, for example, by polymerizing a vinyl ester monomer and an unsaturated monomer having an anionic group and / or a quaternary ammonium salt group together, followed by saponification.

[0034] When (co)polymerizing vinyl ester monomers, known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization can be used. Among these, bulk polymerization, which involves polymerization without a solvent, or solution polymerization, which involves polymerization in a solvent such as alcohol, is typically used, while emulsion polymerization is used to obtain polymers with a high degree of polymerization. Examples of alcohols used as solvents during solution polymerization include lower alcohols such as methanol, ethanol, and propanol, with methanol being preferred. Examples of initiators used in polymerization include known azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and peroxide initiators such as benzoyl peroxide and n-propyl peroxydicarbonate. The polymerization temperature is not particularly limited, but a range of -30 to 150°C is appropriate.

[0035] Any of the conventionally known methods for saponifying vinyl ester polymers can be suitably used. Typically, however, a method is employed in which an alcohol solution of a vinyl ester polymer is saponified using an alkaline or acidic catalyst. Methanol is a suitable alcohol for the saponification solvent. Furthermore, the saponification solvent may be an anhydride or one containing a small amount of water, depending on the purpose. It may also contain other organic solvents such as methyl acetate or ethyl acetate. The saponification temperature is typically selected from the range of 10 to 70°C. Preferred saponification catalysts are alkaline catalysts such as sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium methoxide. The amount of saponification catalyst used is determined appropriately depending on the desired degree of saponification and the water content, but it is desirable for the molar ratio of the saponification catalyst to the vinyl ester units in the polymer to be 0.001 or more, preferably 0.002 or more.

[0036] The saponification degree of PVA (B) is preferably 82 mol% or more and 95 mol% or less, more preferably 83 mol% or more and 93 mol% or less, even more preferably 84 mol% or more and less than 92 mol%, even more preferably 84 mol% or more and less than 90 mol%, and particularly preferably 84 mol% or more and 89 mol% or less. When the saponification degree of PVA (B) is within the above range, the adsorption performance of PVA (B) to CNT is improved, the CNT dispersion stability is superior, and the solubility of PVA (B) in solvents is increased, making it easier to prepare a CNT dispersion. The saponification degree of PVA is a value measured in accordance with JIS K6726:1994.

[0037] The viscosity-average degree of polymerization (hereinafter sometimes simply referred to as degree of polymerization) of PVA (B) is preferably 250 to 450, more preferably 260 to 370, and even more preferably 270 to 350. When the degree of polymerization is within the above range, the adsorption layer of PVA (B) adsorbed to the CNT becomes thicker, the CNT dispersibility becomes better, and the viscosity of the CNT dispersion becomes more appropriate, making it easier to form a uniform film. The viscosity-average degree of polymerization of PVA is a value measured in accordance with JIS K 6726:1994.

[0038] The PVA (B) may contain other monomer units in addition to the vinyl alcohol unit, the vinyl ester unit, the unsaturated monomer having an anionic group, and the unsaturated monomer having a quaternary ammonium salt group. Examples of such other monomers include α-olefins such as ethylene, propylene, n-butene, and isobutylene; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, and diacetone (meth)acrylate. Examples of suitable compounds include (meth)acrylamide compounds such as acrylamide, N-methylol (meth)acrylamide, and derivatives thereof; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; and isopropenyl acetate. These compounds may be used alone or in combination of two or more. The content of other monomers is preferably 10 mol% or less based on the total monomer units of the PVA (B). In this disclosure, "(meth)acrylic" is a general term for methacrylic and acrylic.

[0039] The PVA (B) may have a total content of vinyl alcohol units and vinyl ester units of 80 mol% or more, 90 mol% or more, 95 mol% or more, 98 mol% or more, or 99 mol% or more, based on all monomer units. The PVA (B) may be a PVA having no carboxy group, a PVA having no sulfonic acid group, or a PVA having no quaternary ammonium salt group.

[0040] (Solvent (C)) The solvent (C) is not particularly limited as long as it is capable of dispersing CNT (A), and may be at least one selected from the group consisting of water and organic solvents. The solvent (C) may be a mixed solvent of two or more organic solvents, or a mixed solvent of water and one or more organic solvents. The solvent (C) is preferably an organic solvent, and more preferably a water-soluble organic solvent.

[0041] Examples of water-soluble organic solvents include monohydric alcohol-based organic solvents (methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, benzyl alcohol, etc.), polyhydric alcohol-based organic solvents (ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, etc.), polyhydric alcohol ether-based organic solvents (ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, Ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, ethylene glycol monophenyl ether, propylene glycol monophenyl ether, etc.), amine organic solvents (ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, tetramethylpropylenediamine, etc.), amide organic solvents (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocyclic organic solvents (cyclohexylpyrrolidone, 2-oxazolidone, 1,Examples of suitable organic solvents include 3-dimethyl-2-imidazolidinone, γ-butyrolactone, etc.), sulfoxide-based organic solvents (dimethyl sulfoxide, etc.), sulfone-based organic solvents (hexamethylphosphorotriamide, sulfolane, etc.), lower ketone-based organic solvents (acetone, methyl ethyl ketone, etc.), tetrahydrofuran, urea, and acetonitrile. Among these, amide-based organic solvents are more preferred, and among amide-based organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferred. From the viewpoint of battery performance and ease of handling, the content of the amide-based organic solvent is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, even more preferably 80 parts by mass or more, particularly preferably 90 parts by mass or more, and even more particularly preferably 95 parts by mass or more, per 100 parts by mass of the CNT dispersion. The content of the amide-based organic solvent may be 99 parts by mass or less, per 100 parts by mass of the CNT dispersion. In one embodiment, the amount of N-methyl-2-pyrrolidone is preferably within the above range. In another embodiment, from the viewpoint of battery performance and ease of handling, the content of water contained in the CNT dispersion is preferably 90 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 30 parts by mass or less, and even more preferably 10 parts by mass or less, relative to 100 parts by mass of the CNT dispersion. It is particularly preferable that the content is 0 parts by mass, i.e., the CNT dispersion is substantially free of water.

[0042] <Resin Composition> A resin composition according to one embodiment of the present invention contains the CNT dispersion and the binder resin (D), i.e., contains CNT (A), PVA (B), a solvent (C), and a binder resin (D).

[0043] To obtain the resin composition of the present invention, it is preferable to mix and homogenize the carbon nanotube dispersion and the binder resin (D). As a mixing method, various conventionally known methods can be used. The resin composition can be prepared using the dispersing device described above for the carbon nanotube dispersion.

[0044] (Binder Resin (D)) Examples of the binder resin (D) include (co)polymers obtained by (co)polymerizing at least one monomer selected from the group consisting of ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, and vinyl pyrrolidone; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, and fluororesins; cellulose resins such as carboxymethyl cellulose; rubbers such as styrene-butadiene rubber and fluororubber; and conductive resins such as polyaniline and polyacetylene. Furthermore, from the viewpoint of durability, polymeric compounds having fluorine atoms in the molecule, such as polyvinylidene fluoride (PVDF), polyvinyl fluoride, and tetrafluoroethylene, are preferred. Modified products, mixtures, and copolymers of these resins may also be used.

[0045] The weight average molecular weight of the binder resin (D) is preferably 10,000 to 2,000,000, more preferably 100,000 to 1,000,000, and even more preferably 200,000 to 1,000,000. When the molecular weight is in the above range, the adhesiveness is more excellent.

[0046] <Composite Slurry> The composite slurry, which is one embodiment of the present invention, contains the above-described resin composition and the active material (E), that is, contains CNTs (A), PVA (B), a solvent (C), a binder resin (D), and an active material (E).

[0047] To obtain the composite slurry, it is preferable to add the active material (E) to the above-mentioned resin composition and then perform a dispersion treatment. The dispersion device used for such treatment is not particularly limited. The composite slurry can be obtained using the dispersion device described above for the carbon nanotube dispersion liquid.

[0048] The amount of the active material (E) in the composite slurry is preferably 20 to 85% by mass, and more preferably 40 to 85% by mass, relative to 100% by mass of the composite slurry.

[0049] The amount of CNT (A) in the composite slurry is preferably 0.05 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, and even more preferably 0.1 to 3 parts by mass, relative to 100 parts by mass of the active material (E).

[0050] The amount of binder resin (D) in the composite slurry is preferably 0.5 to 20 parts by mass, more preferably 1 to 10 parts by mass, and particularly preferably 1 to 5 parts by mass, per 100 parts by mass of the active material (E).

[0051] (Active Material (E)) The active material (E) is a material that is the basis of the battery reaction. The active material (E) is divided into a positive electrode active material and a negative electrode active material based on the electromotive force.

[0052] The positive electrode active material is not particularly limited, but may be a metal compound such as a metal oxide or metal sulfide capable of doping or intercalating lithium ions, or a conductive polymer. Examples include oxides of metals such as Al, Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides. Specific examples include MnO, V ２ O ５ , V ６ O １３ , TiO ２ transition metal oxide powders such as lithium nickel oxide, lithium cobalt oxide, lithium manganate, and lithium manganate with a spinel structure; lithium iron phosphate-based materials, which are phosphate compounds with an olivine structure; TiS ２, and transition metal sulfide powders such as FeS. Conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can also be used. The above-mentioned inorganic compounds and organic compounds may be mixed and used. A preferred embodiment of the positive electrode active material is a composite oxide of lithium containing a metal such as Al, Fe, Co, Ni, or Mn, more preferably a composite oxide of lithium containing any of Al, Co, Ni, and Mn, and even more preferably a composite oxide of lithium containing Co, Ni, and / or Mn, and among these, lithium cobalt oxide (LiCoO ２ When these active materials are used, better effects can be obtained.

[0053] The negative electrode active material is not particularly limited as long as it can dope or intercalate lithium ions. For example, metal Li, its alloys such as tin alloys, silicon alloys, and lead alloys, Li ｘ Fe ２ O ３ , Li ｘ Fe ３ O ４ Examples of the negative electrode active material include metal oxides such as lithium titanate, lithium vanadate, and lithium silicate, conductive polymers such as polyacetylene and poly-p-phenylene, amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, carbonaceous powders such as natural graphite, carbon black, mesophase carbon black, resin-baked carbon materials, vapor-grown carbon fibers, and carbonaceous fibers. These negative electrode active materials can be used alone or in combination.

[0054] The BET specific surface area of ​​the active material (E) is 0.1 to 10 m ２ / g, and 0.2 to 5m ２ / g, and more preferably 0.3 to 3m ２ / g is more preferable.

[0055] The average particle size of the active material (E) is preferably in the range of 0.05 to 100 μm, more preferably in the range of 0.1 to 50 μm, where the average particle size of the active material (E) is the average value of particle sizes measured by an electron microscope.

[0056] <Electrode Film> The electrode film according to one embodiment of the present invention is obtained by forming the composite slurry into a film, and may be, for example, a coating film formed by applying the composite slurry onto a current collector and drying the same to form an electrode composite layer. In another embodiment, the electrode film may be an electrode film containing the above-mentioned CNTs (A), PVA (B), binder resin (D), and active material (E).

[0057] The material and shape of the current collector used in the electrode film are not particularly limited, and can be appropriately selected from those suitable for various secondary batteries. For example, the material of the current collector can be metals or alloys such as aluminum, copper, nickel, titanium, and stainless steel. Furthermore, while flat foils are generally used, current collectors with roughened surfaces, perforated foils, and mesh-shaped current collectors can also be used.

[0058] The method for applying the composite slurry onto the current collector is not particularly limited, and any known method can be used, such as die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, electrostatic painting, etc., and the drying method can be, but is not particularly limited to, standing drying, a blower dryer, a hot air dryer, an infrared heater, a far-infrared heater, etc.

[0059] Furthermore, after the mixture slurry is applied onto the current collector, it may be rolled using a lithographic press, a calendar roll, etc. The thickness of the electrode mixture layer is preferably 1 to 500 μm, and more preferably 10 to 300 μm.

[0060] <Lithium-ion secondary battery> A lithium-ion secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte in which lithium ions are movable, and at least one of the positive electrode and the negative electrode includes the above-described electrode film. The lithium-ion secondary battery may use an electrolytic solution in which the electrolyte is dissolved in a non-aqueous solvent.

[0061] As the positive electrode, an electrode film prepared by coating and drying a composite slurry containing a positive electrode active material can be used.

[0062] As the negative electrode, an electrode film prepared by coating and drying a composite slurry containing a negative electrode active material can be used.

[0063] As the electrolyte, various known electrolytes can be used. For example, LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiCF ３ SO ３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ３ C, LiI, LiBr, LiCl, LiAlCl, LiHF ２ , LiSCN, LiBPh ４ (wherein Ph is a phenyl group), but is not limited to these.

[0064] The non-aqueous solvent is not particularly limited, but examples thereof include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes 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. These solvents may be used alone or in combination of two or more.

[0065] The lithium ion secondary battery preferably includes a separator. The separator is preferably, for example, a nonwoven fabric or film containing at least one material selected from the group consisting of polyethylene, polypropylene, and polyamide, or may be one that has been subjected to a hydrophilic treatment. For example, the separator is preferably a porous polypropylene film.

[0066] The structure of the lithium ion secondary battery is not particularly limited, but it is usually composed of a positive electrode, a negative electrode, and a separator that is provided as needed, and can be in various shapes depending on the purpose of use, such as a paper type, a cylindrical type, a button type, or a laminated type.

[0067] A preferred embodiment of the present invention is a carbon nanotube dispersion liquid containing carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), wherein, when the storage modulus of the carbon nanotube dispersion liquid at a strain of 0.1% is X and the storage modulus of the carbon nanotube dispersion liquid at a strain of 1% is Y, X / Y satisfies 0.8≦X / Y≦5.5, and the polyvinyl alcohol (B) is at least one selected from the group consisting of polyvinyl alcohol having a quaternary ammonium base (preferably a saponified copolymer of vinyl acetate and trimethyl-3-[(methacryloylamino)propyl]ammonium chloride), a saponified copolymer of vinyl acetate and itaconic acid, and a saponified copolymer of vinyl acetate and maleic anhydride. A preferred embodiment of the present invention is a carbon nanotube dispersion liquid containing carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), wherein, when the storage modulus of the carbon nanotube dispersion liquid at a strain of 0.1% is X and the storage modulus of the carbon nanotube dispersion liquid at a strain of 1% is Y, X / Y satisfies 0.8≦X / Y≦5.5, and the polyvinyl alcohol (B) is a polyvinyl alcohol having a quaternary ammonium base (preferably a copolymer of vinyl acetate and trimethyl-3-[(methacryloylamino)propyl]ammonium chloride). a saponified copolymer of vinyl acetate and itaconic acid, and a saponified copolymer of maleic anhydride; the solvent (C) contains an amide organic solvent (preferably N-methyl-2-pyrrolidone), and the amount of the amide organic solvent (preferably N-methyl-2-pyrrolidone) is 50 parts by mass or more (preferably 60 parts by mass or more, 70 parts by mass or more, 80 parts by mass or more, 90 parts by mass or more, or 95 parts by mass or more) per 100 parts by mass of the carbon nanotube dispersion.A preferred embodiment of the present invention is a carbon nanotube dispersion liquid containing carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), wherein X / Y satisfies 0.8≦X / Y≦5.5, where X is the storage modulus of the carbon nanotube dispersion liquid at a strain of 0.1% and Y is the storage modulus of the carbon nanotube dispersion liquid at a strain of 1%, and the polyvinyl alcohol (B) is a polyvinyl alcohol having a quaternary ammonium base (preferably a saponified copolymer of vinyl acetate and trimethyl-3-[(methacryloylamino)propyl]ammonium chloride), a saponified copolymer of vinyl acetate and itaconic acid, or a saponified copolymer of vinyl acetate and maleic anhydride. the solvent (C) comprises an amide organic solvent (preferably N-methyl-2-pyrrolidone), the amount of the amide organic solvent (preferably N-methyl-2-pyrrolidone) is 50 parts by mass or more (preferably 60 parts by mass or more, 70 parts by mass or more, 80 parts by mass or more, 90 parts by mass or more, or 95 parts by mass or more) per 100 parts by mass of the carbon nanotube dispersion, and the content of water contained in the carbon nanotube dispersion is 90 parts by mass or less (preferably 50 parts by mass or less, 30 parts by mass or less, 10 parts by mass or less, or 0 parts by mass) per 100 parts by mass of the carbon nanotube dispersion.

[0068] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples as long as it does not depart from the gist of the invention. In the examples, an unsaturated monomer having an anionic group or a cationic group for introducing an anionic group or a cationic group into PVA (B) may be referred to as a "modified species." Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

[0069] [Viscosity Measurement of CNT Dispersion] The CNT dispersion was left to stand in a thermostatic chamber at 20°C for at least 1 hour, and then thoroughly stirred. The viscosity was measured at a stirring speed of 60 rpm using a viscometer (Viscometer, Model BL, manufactured by Toki Sangyo Co., Ltd.). Note that a lower viscosity indicates better dispersibility.

[0070] [Temporal Viscosity Stability of CNT Dispersion Liquid] The ratio β / α between the viscosity α immediately after the preparation of the CNT dispersion liquid and the viscosity β after the CNT dispersion liquid was left standing at 20°C for 7 days was calculated, and the obtained value of β / α was used to evaluate the stability of the CNT dispersion liquid over time using the following four-level scale. ＋ A: 1 or more and less than 1.2 A: 1.2 or more and less than 1.5 B: 1.5 or more and less than 2.0 C: 2.0 or more

[0071] [Measurement of storage modulus of CNT dispersion] The CNT dispersion was left to stand in a thermostatic bath at 20°C for 1 hour, and then thoroughly stirred. The storage modulus X at a strain of 0.1% and the storage modulus Y at a strain of 1% were measured using a rheometer (MCR102: manufactured by Anton Paar). A PP50 was used as the measurement plate. X / Y was calculated from the obtained values ​​of the storage modulus X and Y.

[0072] [Measurement of AC Resistance of Lithium-Ion Secondary Battery] A coin-type lithium-ion secondary battery was charged at a 0.1 C rate using a Solartron 1480 potentiostat, then held at 100% SOC for 1 hour, and the AC resistance was evaluated using an AC impedance analyzer. The AC resistance was fitted with an appropriate equivalent circuit, and the negative electrode resistance was separately evaluated. The battery was then discharged at a 0.1 C rate. The upper potential limit was 4.2 V and the lower potential limit was 2.7 V, and the charge capacity and discharge capacity were measured. The voltage at 50% SOC was obtained as the average voltage during discharge.

[0073] [Synthesis of PVA-1] 448 parts of vinyl acetate, 1,152 parts of methanol, and 2.1 parts of itaconic acid were charged into a 5-liter reactor equipped with a reflux condenser, a stirrer, a thermometer, a nitrogen inlet, a feed port for post-addition liquid, and a pump. The polymerization solution was purged with nitrogen while stirring. The polymerization solution was heated, and when the temperature reached a constant temperature of 60°C, 1.3 parts of 2,2'-azobisisobutyronitrile (hereinafter sometimes abbreviated as AIBN) was added to initiate polymerization. From the start of polymerization, 71 parts of a methanol solution containing 3% itaconic acid was added to the system, and the reaction was continued while analyzing the solids concentration in the system. After 5 hours, the reactor was cooled to terminate the polymerization. The conversion at the time of termination of polymerization was 65%. The resulting polymerization paste was added dropwise to n-hexane to precipitate the polymer. The precipitated polymer was recovered, dissolved in acetone, and precipitated in n-hexane. This reprecipitation-purification procedure was repeated three times. The polymer was then dissolved in acetone and added dropwise to distilled water, purified by boiling, and dried at 60° C. to obtain purified polyvinyl acetate (hereinafter sometimes abbreviated as PVAc).

[0074] Next, a 45% methanol solution of purified PVAc was prepared. While stirring this methanol solution of purified PVAc at 40°C, a 10% methanol solution of sodium hydroxide was added so that the molar ratio of sodium hydroxide to vinyl acetate units in the PVAc was 0.03, and a saponification reaction was carried out for 60 minutes. The resulting gel-like substance was crushed, subjected to Soxhlet washing with methanol for 3 days, and then dried under reduced pressure at 80°C for 3 days to obtain purified PVA. The saponification degree of the PVA was measured according to the standard method of JIS K6726, and was found to be 88 mol%. Furthermore, proton NMR spectroscopy revealed that the amount of itaconic acid modification in the purified PVA was 0.5 mol% relative to the total monomer units. Hereinafter, the PVA (B) obtained above will be referred to as PVA-1.

[0075] [Synthesis of PVA-2 to PVA-14] Various PVA-2 to PVA-14 were prepared in the same manner as PVA-1, except that the polymerization reaction conditions (amounts of vinyl acetate and methanol charged, type and amount of modified species, and polymerization rate) and saponification conditions were changed as shown in Table 1. The properties of each PVA obtained are shown in Table 1. In Table 1, itaconic acid is represented as "IA," maleic anhydride as "Man," and trimethyl-3-[(methacryloylamino)propyl]ammonium chloride as "MAPTAC."

[0076] Example 1 (Preparation of CNT Dispersion and Measurement of Physical Properties) A ​​glass bottle was charged with 1.0 part of multi-walled carbon nanotubes ("LUCAN BT1003M" manufactured by LG Chemical Co., Ltd.; average diameter 12 nm, average length 40 μm) as CNT (A), 0.4 parts of PVA-1, and 98.6 parts of NMP (N-methyl-2-pyrrolidone) as solvent (C), and the mixture was dispersed with zirconia beads (bead diameter 0.5 mmφ) using a paint conditioner for 8 hours. The zirconia beads were then separated to obtain CNT Dispersion-1. The storage modulus of CNT Dispersion-1 was such that the value X at 0.1% strain was 0.16 Pa, the value Y at 1% strain was 0.03 Pa, and the ratio X / Y was 5.3. The viscosity of the obtained CNT Dispersion-1 was 36 mPa·s. Furthermore, the viscosity stability over time was evaluated as B. The results are shown in Table 2.

[0077] Examples 2 to 11, Comparative Examples 1 to 5 CNT Dispersion-2 to CNT Dispersion-16 were obtained in the same manner as in Example 1, except that the types and contents of CNT (A), PVA (B), and solvent (C) were changed as shown in Table 2. The viscosity, viscosity stability over time, and storage modulus of the obtained CNT dispersions were measured. The results are shown in Table 2.

[0078] [Example 12] (Preparation of composite slurry) Volume 150 cm ３Into a plastic container, 10.6 parts by mass of a PVDF solution prepared by dissolving PVDF (Kureha KF Polymer (registered trademark) W1100 manufactured by Kureha Corporation) in NMP as a binder resin (D) and adjusting the concentration to 8% by mass was weighed. Thereafter, 0.5 parts by mass of CNT dispersion liquid-1 was added, and the mixture was stirred at 2000 rpm for 30 seconds using a centrifugal mixer (Thinky Corporation's Awatori Rentaro (registered trademark) ARE-310). Further, 6.5 parts by mass of CNT dispersion liquid-1 was added, and the mixture was stirred at 2000 rpm for 30 seconds using the mixer to obtain a CNT resin composition. Further, lithium cobalt oxide (LiCoO ２ ) (HLC-22 manufactured by Honjo Chemical Co., Ltd.; average particle size 6.6 μm, specific surface area 0.62 m ２ 55.1 parts by mass of PEG-100 / g was added, and the mixture was stirred at 2000 rpm for 2.5 minutes using the mixer. Finally, 2.3 parts by mass of NMP was added, and the mixture was stirred at 2000 rpm for 2.5 minutes using the mixer to obtain a composite slurry.

[0079] (Preparation of Electrode Film) The above composite slurry was applied to the electrode using an applicator so that the weight per unit area of ​​the electrode was 20 mg / cm ２ After coating on an aluminum foil so as to obtain an electrode film, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes.

[0080] (Fabrication of Lithium-Ion Secondary Battery) The above electrode film was subjected to rolling treatment using a roll press (3-ton hydraulic roll press manufactured by Thank Metal Co., Ltd.) to obtain a composite layer having a density of 3.1 g / cm ３ The positive electrode and the negative electrode (lithium foil) were punched out to the desired size, and a separator (porous polypropylene film) was inserted between them. The resulting mixture was dried in an electric oven at 60°C for 1 hour. After that, in a glove box filled with argon gas, an electrolytic composition (a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1) was added to LiPF ６The battery was placed in a coin-type battery cell holder and sealed using a crimping machine to prepare a lithium-ion secondary battery. The AC resistance of the resulting lithium-ion secondary battery was 8.8 Ω.

[0081] [Examples 13 to 22, Comparative Examples 6 to 10] Lithium ion secondary batteries were obtained in the same manner as in Example 12, except that the type of CNT dispersion was changed to that shown in Table 3. The AC resistance values ​​of the obtained lithium ion secondary batteries were measured. The results are shown in Table 3.

[0082] The results in Table 2 show that CNT dispersions in which the ratio X / Y, where X is the storage modulus at 0.1% strain and Y is the storage modulus at 1% strain, is 0.8≦X / Y≦5.5, have low viscosity and excellent dispersibility, as well as excellent viscosity stability over time (Examples 1 to 11). On the other hand, CNT dispersions in which X / Y exceeds 5.5 have poor viscosity stability over time (Comparative Examples 1 to 5).

[0083] The results in Table 3 show that the lithium ion secondary batteries produced using CNT dispersions satisfying 0.8≦X / Y≦5.5 have low AC resistance values ​​(Examples 12 to 22). On the other hand, the lithium ion secondary batteries produced using CNT dispersions that do not satisfy 0.8≦X / Y≦5.5 have high AC resistance values ​​(Comparative Examples 6 to 10).

[0084]

[0085]

[0086]

Claims

1. A carbon nanotube dispersion comprising carbon nanotubes (A), polyvinyl alcohol (B), and a solvent (C), wherein X / Y satisfies 0.8≦X / Y≦5.5, where X is a storage modulus of the carbon nanotube dispersion at a strain of 0.1% and Y is a storage modulus of the carbon nanotube dispersion at a strain of 1%.

2. The carbon nanotube dispersion liquid according to claim 1, wherein the polyvinyl alcohol (B) has at least one anionic group selected from the group consisting of a carboxy group and a sulfonic acid group, and the modification amount of the anionic group is 0.5 mol % or more and 10 mol % or less.

3. The carbon nanotube dispersion liquid according to claim 1, wherein the polyvinyl alcohol (B) has a quaternary ammonium base and the degree of modification is 0.1 mol % or more and 10 mol % or less.

4. The carbon nanotube dispersion liquid according to claim 1, wherein the degree of saponification of the polyvinyl alcohol (B) is 82 mol % or more and 95 mol % or less.

5. The carbon nanotube dispersion according to claim 1, wherein the polyvinyl alcohol (B) has a viscosity average degree of polymerization of 250 or more and less than 450.

6. The carbon nanotube dispersion liquid according to claim 1, wherein the solvent (C) is a water-soluble organic solvent.

7. The carbon nanotube dispersion according to claim 1, wherein the solvent (C) is N-methyl-2-pyrrolidone.

8. The carbon nanotube dispersion according to claim 1, wherein the water content is 50 parts by mass or less per 100 parts by mass of the carbon nanotube dispersion.

9. A resin composition comprising the carbon nanotube dispersion liquid according to any one of claims 1 to 8 and a binder resin (D).

10. A composite slurry comprising the resin composition according to claim 9 and an active material (E).

11. An electrode film obtained by forming the composite slurry according to claim 10 into a film.

12. A lithium ion secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein at least one of the positive electrode and the negative electrode comprises the electrode film according to claim 11.

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