Carbon nanotube dispersion, conductive paste using the same, electrode paste for secondary battery, electrode for secondary battery, and secondary battery

Abstract

To provide a carbon nanotube dispersion liquid that is suitable for manufacturing a secondary battery such as a lithium ion battery, a conductive paste, an electrode paste for a secondary battery, an electrode for a secondary battery, and a secondary battery.SOLUTION: A carbon nanotube dispersion liquid according to the present invention includes at least a carbon nanotube, a water-soluble polymer material, and a dispersion medium, and when X is the median diameter of the carbon nanotube measured by the dynamic light scattering method, and a is the median diameter of the dispersion limit, 1.0<X / a≤2.0, and X<1.0 μm are satisfied.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon nanotube dispersion liquid having excellent stability and conductive performance, a conductive paste using the dispersion liquid, a secondary battery electrode paste suitable for producing secondary batteries such as lithium ion batteries, a secondary battery electrode, and a secondary battery. [Background technology]

[0002] With the spread of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices such as mobile phones and laptop computers, secondary batteries with high energy density and even higher capacity are required. Against this background, lithium-ion secondary batteries using nonaqueous electrolytes are increasingly being used in many devices due to their high energy density and high voltage. Research is being conducted into the use of carbon nanotube dispersions in the negative and positive electrode materials used in these lithium-ion secondary batteries, particularly in the positive electrode material, which can provide good conductive performance, reduce electrode resistance, and efficiently form a conductive network with a small amount of material.

[0003] For example, Patent Document 1 discloses a carbon nanotube dispersion containing carbon nanotubes, a dispersant, and a dispersion medium, in which the geometric standard deviation (σD) of the distribution of outer diameters of the carbon nanotubes is 1.25 to 1.70, and the average particle diameter (D50) of the carbon nanotubes in the dispersion as measured by a dynamic light scattering method is 400 nm or less. Patent Document 2 describes a carbon nanotube (A) containing a carbon nanotube, a water-soluble resin, and water, the carbon nanotube (A) being single-walled and having an average outer diameter of 0.5 to 5 nm and a specific surface area of ​​400 to 800 m 2 / g, containing 400 parts by mass or more and 2000 parts by mass or less of a water-soluble resin (B) relative to 100 parts by mass of the carbon component of the carbon nanotubes (A), and the 50% particle diameter (D50 diameter) of the carbon nanotube dispersion calculated by laser diffraction particle size distribution measurement is 1.5 to 40 μm, Patent Document 3 discloses an aqueous dispersion of carbon nanotubes, which is a dispersion containing carbon nanotubes having an average outer diameter of 3 nm or less and a dispersant, and which is characterized in that the average particle diameter measured by a dynamic light scattering method is 200 nm or more and 1500 nm or less, and the dispersant is an ionic dispersant. Patent Document 4 describes a method for dispersing carbon nanotubes in a dispersion medium, in which the carbon nanotubes have an average diameter of 0.4 to 5 mm and a specific gravity of 2 to 3 g / cm3 in the presence of a surfactant. 3 The present invention also discloses a method for dispersing carbon nanotubes, characterized by using beads of the formula: Patent Document 5 discloses that carbon nanotubes having a number average fiber diameter of 100 nm or more and carbon nanotubes having a number average fiber diameter of 30 nm or less are mixed in a 1-methyl-2-pyrrolidone solution of polyaniline using a wet jet mill, thereby obtaining a carbon nanotube dispersion in which both carbon nanotubes are dispersed in the 1-methyl-2-pyrrolidone solution of polyaniline.

[0004] However, the carbon nanotube dispersions of Patent Documents 1 to 5 tend to lose their dispersibility over time, making it difficult to achieve both high stability and conductive performance. Currently, there is a strong demand for carbon nanotube dispersions that can achieve both high stability and conductive performance, conductive pastes using such dispersions, electrode pastes for secondary batteries and electrodes for secondary batteries, and secondary batteries such as lithium-ion batteries that use such electrodes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-206412 A (claims, examples, etc.) [Patent Document 2] JP 2020-019924 A (claims, examples, etc.) [Patent Document 3] JP 2010-254546 A (claims, examples, etc.) [Patent Document 4] JP 2007-169120 A (claims, examples, etc.) [Patent Document 5] JP 2015-117150 A (claims, examples, etc.) Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention seeks to solve the above-mentioned conventional problems, and aims to provide a carbon nanotube dispersion liquid that can achieve both high stability and conductive performance, a conductive paste using the same, an electrode paste for secondary batteries and an electrode for secondary batteries, and a secondary battery such as a lithium-ion battery using the electrode. [Means for solving the problem]

[0007] As a result of intensive research into the above-mentioned conventional problems, the inventors have discovered that the above-mentioned desired carbon nanotube dispersion, a conductive paste using the same, a secondary battery electrode paste and a secondary battery electrode, and a secondary battery such as a lithium ion battery using the same can be obtained by using a carbon nanotube dispersion containing at least carbon nanotubes, a water-soluble polymer material, and a dispersion medium, the dispersion being prepared so that the median diameter (D50) of the carbon nanotubes in the dispersion is within a predetermined range as measured by dynamic light scattering, and have thus completed the present invention.

[0008] That is, the carbon nanotube dispersion of the present invention contains at least carbon nanotubes, a water-soluble polymer material, and a dispersion medium, and when the median diameter of the carbon nanotubes measured by the dynamic light scattering method is X and the dispersion limit median diameter is a, 1.0 < X / a ≤ 2.0, and X < 1.0 μm. The water-soluble polymer material is preferably at least one polymer material selected from nonionic water-soluble polymers and anionic water-soluble polymers. In the present invention, the conductive paste is characterized by containing at least the carbon nanotube dispersion having the above configuration, and the electrode paste for secondary batteries is characterized by containing at least the carbon nanotube dispersion having the above configuration and an active material for secondary batteries. The electrode for secondary batteries of the present invention is characterized by using the electrode paste for secondary batteries having the above configuration, and the secondary battery is characterized by using the electrode for secondary batteries having the above configuration.

Effects of the Invention

[0009] According to the present invention, it is possible to provide a carbon nanotube dispersion capable of achieving both high stability and conductive performance, a conductive paste using the same, an electrode paste for secondary batteries, an electrode for secondary batteries, and a secondary battery such as a lithium-ion battery suitable for using this electrode. The objects and effects of the present invention are recognized and obtained by using the components and combinations particularly pointed out in the claims. Both the above general description and the following detailed description are exemplary and explanatory, and do not limit the present invention described in the claims.

Brief Description of the Drawings

[0010] [Figure 1] It is an explanatory diagram for explaining the dispersion limit median diameter a in the present invention.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail. However, it should be noted that the technical scope of the present invention is not limited to the embodiments described in detail below, but extends to the invention described in the claims and its equivalents. Further, the present invention can be implemented based on the content disclosed in this specification and common technical knowledge in the art (including design matters and self-evident matters).

[0012] 〈Carbon nanotube dispersion liquid〉 The carbon nanotube dispersion liquid of the present invention contains at least carbon nanotubes, a water-soluble polymer material, and a dispersion medium. When the median diameter of the carbon nanotubes measured by the dynamic light scattering method is X and the dispersion limit median diameter is a, 1.0 < X / a ≤ 2.0, and X < 1.0 μm.

[0013] 〈Carbon nanotube (CNT)〉 The carbon nanotube (CNT) used in the present invention is not particularly limited as long as it has a shape formed by winding one plane of substantially graphite into a cylindrical shape. Either a single-layer CNT formed by winding one plane of graphite into one layer or a multi-layer CNT formed by winding two or more layers can be used. In addition, examples of the form of the carbon nanotube include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultra-fine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. These can be used individually or in combination of two or more kinds (hereinafter simply referred to as "at least one kind").

[0014] Furthermore, from the viewpoints of the viscosity, conductivity, and stability of the dispersion liquid, the average outer diameter of the carbon nanotubes is preferably 1 nm or more and 90 nm or less, more preferably 3 nm or more and 30 nm or less, and even more preferably 3 nm or more and 15 nm or less. In the present invention, the average outer diameter of the carbon nanotubes refers to the arithmetic average value of the outer shapes of a sufficient number of n measured using an image with a magnification of 100,000 times or more of a transmission electron microscope. The purity of the carbon nanotubes used in the present invention is preferably 90 to 100% by mass, particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with ash content measured in accordance with JIS K1469 or JIS K6218 being considered as an impurity.

[0015] Specific examples of carbon nanotubes (CNTs) that can be used include at least one of FloTube9000 (average outer diameter 11 nm) manufactured by Cnano Corporation and MEIJOeDIPS EC2.0 (average outer diameter 2.0 nm) manufactured by Meijo Nano Carbon Co., Ltd.

[0016] The content of these carbon nanotubes (CNTs) can be set to a suitable content depending on the application, and is not particularly limited. For example, when used in a conductive paste, a secondary battery electrode paste, a secondary battery electrode, or the like, the content is preferably 0.1 to 15.0 mass % relative to the total amount of the dispersion, from the viewpoint of achieving both high stability and conductive performance, and from the viewpoint of viscosity during dispersion production, and is more preferably 0.1 to 10.0 mass %, even more preferably 0.5 to 8.0 mass %, 1.0 to 6.0 mass %, and particularly preferably 2.0 to 5.0 mass %. By making the carbon nanotube (CNT) content 0.1% by mass or more, sufficient conductivity can be ensured, while by making it 15.0% by mass or less, the stability of the dispersion and good conductivity can be ensured.

[0017] <Water-soluble polymer materials> The water-soluble polymer material used in the present invention is not particularly limited as long as it is soluble in water or in the dispersion medium (solvent other than water) used. Examples of water-soluble polymeric materials include natural polymeric materials such as proteins and starches, as well as synthetic polymeric materials such as polyacrylic acid, polyacrylamide, polyoxyethylene oxide, polyvinylpyrrolidone, polyvinyl alcohol, polyvinylamide, and polyamines. These water-soluble polymeric materials function as dispersants and binders for carbon nanotubes (CNTs). Furthermore, the water-soluble polymer material that dissolves in a solvent other than water may not only be completely dissolved, but also partially dissolved. By using such a water-soluble polymer, it becomes possible to stably disperse carbon nanotubes (CNTs) without impairing their electrical conductivity. As the water-soluble polymer material, it is particularly preferable to use at least one selected from nonionic water-soluble polymers and anionic water-soluble polymers, which are particularly suitable for stably dispersing carbon nanotubes (CNTs) without impairing their electrical conductivity.

[0018] Examples of nonionic water-soluble polymers that can be used include at least one of polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, polyoxyethylene phenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters and alkyl allyl ethers, glycerol borate fatty acid esters and polyoxyethylene glycerol fatty acid esters, xanthan gum, welan gum, succinoglycan, polyvinyl alcohol (PVAL or PVOH), polyvinylpyrrolidone, and polyvinyl acetal.

[0019] Examples of anionic water-soluble polymers that can be used include at least one of fatty acids and salts thereof, polysulfonic acids and salts thereof, polycarboxylic acids and salts thereof, alkyl sulfates and salts thereof, alkylaryl sulfonic acids and salts thereof, alkylnaphthalenesulfonic acids and salts thereof, dialkylsulfonic acids and salts thereof, dialkylsulfosuccinic acids and salts thereof, alkylphosphoric acids and salts thereof, polyoxyethylene alkyl ether sulfates and salts thereof, polyoxyethylene alkylaryl ether sulfates and salts thereof, naphthalenesulfonic acid-formalin condensates and salts thereof, polyoxyethylene alkylphosphoric acid sulfonic acids and salts thereof, acrylic polymers such as styrene-acrylic resins, and cellulose-based polymers such as carboxymethylcellulose or its sodium (Na) salt.

[0020] Preferably, polyvinylpyrrolidone is used as the nonionic water-soluble polymer, and acrylic polymers and cellulose polymers are used as the anionic water-soluble polymers, in order not to inhibit conductivity.

[0021] The (total) content of these water-soluble polymer materials can be set to a suitable content depending on the application, and is not particularly limited. For example, when used in a conductive paste, a secondary battery electrode paste, a secondary battery electrode, or the like, the content is preferably 0.005 to 20 mass % relative to the total amount of the dispersion, and particularly preferably 0.025 to 16 mass %, in order to achieve both high stability and conductive performance. By making the content of these polymer materials 0.005% by mass or more, the dispersion stability of the carbon nanotubes (CNTs) becomes good, while by making the content 20% by mass or less, the stability of the dispersion liquid and good conductivity can be ensured.

[0022] <Dispersion medium> The dispersion medium used in the present invention is not particularly limited, and may be water (purified water, distilled water, pure water, ultrapure water, etc.), organic solvents, or the like, as long as it can dissolve at least a portion of water-soluble polymer materials such as nonionic water-soluble polymers and anionic water-soluble polymers. The dispersion medium may be used alone or in combination of two or more kinds. A combination of water and an organic solvent may be used by adjusting appropriately within a range where they can be mixed.

[0023] Examples of the organic solvent that can be used include aromatics, alcohols, polyhydric alcohols, glycol ethers, esters, etc. These solvents may be used alone or in combination.

[0024] Examples of aromatic compounds that can be used include benzyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, propylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monophenyl 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, alkylsulfonic acid phenyl ester, butyl phthalate, ethylhexyl phthalate, tridecyl phthalate, ethylhexyl trimellitate, diethylene glycol dibenzoate, and dipropylene glycol dibenzoate.

[0025] Examples of alcohols that can be used include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, n-hexanol, methylamyl alcohol, 2-ethylbutanol, n-heptanol, 2-heptanol, 3-heptanol, n-octanol, 2-octanol, 2-ethylhexanol, 3,5,5-trimethylhexanol, nonanol, n-decanol, undecanol, n-decanol, trimethylnonyl alcohol, tetradecanol, heptadecanol, cyclohexanol, and 2-methylcyclohexanol.

[0026] Examples of polyhydric alcohols that can be used include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, 3-methyl-1,3-butanediol, triethylene glycol, dipropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, hexylene glycol, and octylene glycol.

[0027] Examples of glycol ethers that can be used include methyl isopropyl ether, ethyl ether, ethyl propyl ether, ethyl butyl ether, isopropyl ether, butyl ether, hexyl ether, 2-ethylhexyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylbutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, 3-methyl-3-methoxy-1-butanol, 3-methoxy-1-butanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, propylene glycol tertiary butyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monobutyl ether, and tetrapropylene glycol monobutyl ether.

[0028] Examples of esters include propylene glycol methyl ether acetate, propylene glycol diacetate, 3-methyl-3-methoxybutyl acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether acetate, butyl formate, isobutyl formate, isoamyl formate, propyl acetate, butyl acetate, isopropyl acetate, isobutyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, propyl propionate, isobutyl propionate, isoamyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, propyl isobutyrate, Examples of usable surfactants include methyl valerate, ethyl valerate, propyl valerate, methyl isovalerate, ethyl isovalerate, propyl isovalerate, methyl trimethylacetate, ethyl trimethylacetate, propyl trimethylacetate, methyl caproate, ethyl caproate, propyl caproate, methyl caprylate, ethyl caprylate, propyl caprylate, methyl laurate, ethyl laurate, methyl oleate, ethyl oleate, caprylic acid triglyceride, tributyl acetate citrate, octyl oxystearate, propylene glycol monoricinoleate, methyl 2-hydroxyisobutyrate, and 3-methoxybutyl acetate.

[0029] Furthermore, at least one of amine-based compounds such as ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, tetramethylpropylenediamine, amide-based compounds such as N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, heterocyclic compounds such as cyclohexylpyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, γ-butyrolactone, sulfoxide-based compounds such as dimethylsulfoxide, sulfone-based compounds such as hexamethylphosphorotriamide, sulfolane, urea, acetonitrile, etc. can be used.

[0030] Additives according to the intended use may be added to the carbon nanotube dispersion of the present invention. For example, sodium carboxymethyl cellulose as a thickener, anti-settling agent, wetting agent, emulsifier, anti-dripping agent, defoaming agent, leveling agent, plasticizer, anti-mold and anti-algae agent, antibacterial agent, etc. can be mentioned.

[0031] The carbon nanotube dispersion of the present invention is characterized in that when the median diameter of carbon nanotubes measured by the dynamic light scattering method is X and the dispersion limit median diameter is a, 1.0 < X / a ≤ 2.0 and X < 1.0 μm.

[0032] 〈Median diameter X〉 In the present invention (including the examples and comparative examples described later), the median diameter X of carbon nanotubes measured by the dynamic light scattering method refers to the scattering intensity reference median diameter (median diameter by the cumulant analysis method, particle diameter at which the cumulative frequency is 50%, D50) measured in an environment at 25°C, and can be measured using, for example, FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.). In the measurement, it can be appropriately diluted to an optimal concentration according to the dispersion medium used in the carbon nanotube dispersion liquid.

[0033] 〈Dispersion limit median diameter a〉 In the present invention, the dispersion limit median diameter a can be defined as the median diameter of carbon nanotubes when, in the relationship between the dispersion time and the median diameter of carbon nanotubes, the rate of change of the median diameter ((rate of change) = (amount of change in median diameter) / (amount of change in dispersion time)) changes from negative to zero and reaches the minimum median diameter. (See Figure 1) Also, the dispersion limit median diameter a is specified from the composition of the carbon nanotube dispersion liquid in the present invention and the dispersion conditions for obtaining the carbon nanotube dispersion liquid.

[0034] In the present invention, when the median diameter X of carbon nanotubes measured by the above dynamic light scattering method and the dispersion limit minimum median diameter a satisfy 1.0 < X / a ≤ 2.0 and X < 1.0 μm, a carbon nanotube dispersion liquid with high compatibility between stability and conductivity can be obtained.

[0035] The state where carbon nanotubes are independent and separated one by one without aggregation is favorable in terms of conductivity, but it is difficult to achieve both the stability of the carbon nanotube dispersion liquid. The inventors have found that, for the first time, high compatibility between stability and conductivity can be achieved when the median diameter X of carbon nanotubes measured by the above dynamic light scattering method and the dispersion limit median diameter a satisfy 1.0 < X / a ≤ 2.0 and X < 1.0 μm.

[0036] When the median diameter X of the carbon nanotube dispersion is such that X / a = 1.0, it corresponds to a state where the carbon nanotubes are independent and separated one by one without aggregation. However, as described above, it is difficult to achieve a high level of both stability and conductivity. Also, when 2.0 < X / a, the dispersion of the carbon nanotubes is insufficient or in a state of excessive dispersion, and both conductivity and stability decrease, which is not preferable. Furthermore, when X satisfies X < 1.0 μm (1000 nm), it is possible to most effectively achieve both stability and conductivity. When X ≥ 1.0 μm (1000 nm), the stability decreases, which is not preferable.

[0037] In the present invention, the carbon nanotube dispersion having the above characteristics can be prepared by dispersing each component having the above blending characteristics under suitable conditions using a dispersion device as shown below. For example, as the dispersion device, a disperser commonly used for pigment dispersion and the like can be used. For example, mixers such as a disper, a homomixer, a planetary mixer, a Henschel mixer, a planetary mixer, etc., (high-pressure) homogenizers, paint conditioners, colloid mills, bead mills, cone mills, ball mills, sand mills, attritors, pearl mills, coball mills, etc. media type dispersers, media-less dispersers such as wet jet mills, thin-film swirling high-speed mixers, and other roll mills, etc. can be mentioned, but it is not limited thereto. As a preferable dispersion device, from the viewpoints of stability and dispersion efficiency, a thin-film swirling high-speed mixer, a bead mill, etc. are preferable.

[0038] In the present invention, the dispersion limit median diameter a of the carbon nanotube dispersion varies slightly depending on the above-described blending composition and the type of disperser used. Preferably, a disperser is selected and the dispersion limit median diameter a when dispersing without changing the dispersion conditions performed by that disperser is measured. For example, when using a bead mill, it is desirable to fix conditions such as the bead diameter and filling rate of the media, the dispersion peripheral speed, the flow rate of the dispersion liquid introduced into the disperser, the flow rate and temperature of the cooling water, etc. and disperse and specify. In addition, in the present invention, when the dispersion elapsed time when the median diameter of the carbon nanotube dispersion limit is a is defined as Ta, it is more preferable to use a carbon nanotube dispersion liquid that is dispersed in a dispersion time less than Ta and has a median diameter range of the present invention, where 1.0 < X / a ≦ 2.0 and X < 1.0 μm.

[0039] In addition, from the viewpoints of stability and handling, the viscosity of the carbon nanotube dispersion liquid of the present invention is preferably such that the viscosity value measured with a cone and plate viscometer (rotor rotation speed 10 rpm (shear rate 38.3 s -1 ), temperature 25°C) is 1 to 10,000 mPa·s, and more preferably 1 to 5,000 mPa·s.

[0040] The carbon nanotube dispersion liquid of the present invention configured as described above can achieve both high stability and conductive performance. Since this carbon nanotube dispersion liquid has excellent performance that is not found in the prior art, it can be used in conductive pastes, electrode pastes for secondary batteries, electrodes for secondary batteries, and secondary batteries such as lithium ion batteries using such electrodes.

[0041] 〈Conductive Paste〉 The conductive paste of the present invention is characterized by containing at least the carbon nanotube dispersion liquid having the above-described configuration, and can be used in conductive resin products, conductive adhesives, printed wiring applications, and the like. As this conductive paste, at least a resin component can be added to the carbon nanotube dispersion liquid that can achieve both the above-described high stability and conductive performance. As the resin component, for example, thermosetting resins, ultraviolet curable resins, and the like can be used.

[0042] 〈Electrode Paste for Secondary Battery, Electrode for Secondary Battery〉 The electrode paste for a secondary battery of the present invention is characterized by containing at least the carbon nanotube dispersion liquid having the above-described configuration and an active material for a secondary battery, and the electrode for a secondary battery of the present invention is characterized by using the electrode paste for a secondary battery having the above-described configuration.

[0043] The carbon nanotube dispersion liquid having the above-mentioned constitution can be used as it is, or after dilution or concentration. The carbon nanotubes contained in the secondary battery electrode paste and secondary battery electrode are appropriately adjusted to an optimal content depending on the electrode characteristics, the battery capacity after cell formation, the charge / discharge characteristics, etc., but it is desirable for them to be contained in an amount of 1 to 15 parts by mass.

[0044] As the active material used in the electrode paste for the secondary battery, either a positive electrode active material or a negative electrode active material can be used. The positive electrode active material for secondary batteries is not particularly limited, and any ordinary positive electrode active material (active material that allows lithium ions to reversibly enter and exit) that can be used for the positive electrode of a lithium ion battery can be used. For example, composite oxides of lithium and transition metals such as lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-cobalt-manganese-aluminum composite oxide; transition metal sulfides such as TiS2, FeS, and MoS2; MnO, V---2O5, and V6O 13Examples of suitable lithium phosphate compounds include transition metal oxides such as TiO2 and olivine-type lithium phosphate compounds. The olivine-type lithium phosphate compounds contain at least one element selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, as well as lithium, phosphorus, and oxygen. These compounds may have some of the elements partially substituted with other elements to improve their properties. A preferred positive electrode active material for a secondary battery is a lithium-nickel composite oxide, and more preferably, the lithium-nickel composite oxide is represented by the formula: LiNi X M1 Y M2 Z A lithium-nickel composite oxide represented by O2 (M1 and M2 are at least one metal element selected from Al, B, alkali metals, alkaline earth metals, and transition metals, 0.8≦X≦1.0, 0≦Y≦0.2, 0≦Z≦0.2) is desirable. These active materials for the positive electrode of a secondary battery may be used alone or in combination of two or more.

[0045] The negative electrode active material for secondary batteries is not particularly limited, and any ordinary negative electrode active material that can be used for the negative electrode of a lithium ion battery can be used. Examples of usable negative electrode active materials for secondary batteries include inorganic compounds such as lithium metal, lithium alloys, and tin compounds; carbonaceous materials capable of absorbing and releasing lithium ions; composite oxides containing multiple elements; and conductive polymers. Examples of carbonaceous materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, enable charging and discharging at high operating voltages, suppress self-discharge when using a lithium salt as a supporting electrolyte, and reduce irreversible capacity during charging. Examples of composite oxides include lithium-titanium composite oxide and lithium-vanadium composite oxide. Of these, metal oxide materials and carbonaceous materials are preferred as negative electrode active materials for secondary batteries from the standpoint of safety.

[0046] Moreover, the electrode paste for a secondary battery preferably contains the carbon nanotube dispersion liquid having the above-mentioned configuration, an active material for a positive electrode or a negative electrode for a secondary battery, and further a binding material (binder). Examples of binders that can be used include fluororesins such as polyimide resins, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers, polyolefin resins such as polyethylene and polypropylene, polyvinylpyrrolidone, polyvinyl alcohol, styrene-butadiene rubber (SBR), and acrylic resins. Two or more of these binders may be mixed and used. The amount of these binders to be added is preferably 0.2 to 3.0 parts by mass, more preferably 0.5 to 2.5 parts by mass, based on the total amount of the electrode paste for secondary batteries, from the viewpoints of adhesion to the current collector foil, battery capacity after cell formation, and charge / discharge characteristics.

[0047] Furthermore, various solvents may be added to the electrode paste for secondary batteries. Examples of the solvent include water (purified water, ion-exchanged water, distilled water, ultrapure water, etc.), aromatic solvents, alcohols, polyhydric alcohols, ether solvents, glycol ether solvents, ester solvents, amine solvents, amide solvents, heterocyclic solvents, sulfoxide solvents, and sulfone solvents. These solvents may be used alone or in combination of two or more. The amount of these solvents is preferably 0.5 to 80 parts by mass, more preferably 1 to 70 parts by mass, based on the total amount of the electrode paste for secondary batteries, in view of the need to achieve an appropriate viscosity when applying the electrode paste. Furthermore, in addition to the carbon nanotube dispersion, active material, and binder, a leveling agent, solid electrolyte material, and the like may be appropriately blended within a range that does not impair the effects of the present invention.

[0048] The electrode paste for a secondary battery configured in this manner can be prepared by mixing the carbon nanotube dispersion liquid, a positive or negative electrode active material for a secondary battery, a binder, a solvent, etc., using, for example, a biaxial kneader. The obtained secondary battery electrode paste is applied to a current collector, which is a conductive member of a lithium ion secondary battery, and then dried to obtain a desired positive electrode and negative electrode for a lithium ion secondary battery. In this invention, a secondary battery electrode paste and secondary battery electrodes are obtained that realize battery performance that can withstand repeated charge and discharge over a long period of time.

[0049] The material and shape of the current collector used in the electrode are not particularly limited and can be appropriately selected from those suitable for various secondary batteries. Examples of materials for the current collector include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. While flat foils are generally used, current collectors with roughened surfaces, perforated foils, and mesh-shaped current collectors can also be used. The method for applying the electrode paste onto the current collector is not particularly limited, and any known method can be used. Specific examples include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic painting. Drying methods that can be used include, but are not limited to, standing to dry, a blower dryer, a hot air dryer, an infrared heater, and a far-infrared heater. After coating, the coating may be rolled using a lithographic press, a calendar roll, etc. The thickness of the electrode material layer is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0050] <Secondary batteries, lithium-ion secondary batteries> The secondary battery of the present invention is characterized by using the above-mentioned secondary battery electrode, and preferably is a lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, in which the above-mentioned secondary battery electrode is used as the positive electrode and the negative electrode. The use of the electrode in a lithium ion secondary battery will be described below. The positive electrode may be prepared by applying the above-mentioned electrode paste containing a positive electrode active material onto the current collector and drying the applied paste. The negative electrode may be prepared by applying an electrode paste containing a negative electrode active material onto a current collector and drying the applied paste.

[0051] Various conventionally known electrolytes capable of ion mobility can be used. Examples include, but are not limited to, lithium salts such as LiBF, LiClO, LiPF, LiAsF, LiSbF, LiCFSO, Li(CFSO)N, LiCFSO, Li(CFSO)C, LiI, LiBr, LiCl, LiAlCl, LiHF, LiSCN, or LiBPh (where Ph is a phenyl group). The electrolyte is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution.

[0052] 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.

[0053] In the present invention, the lithium ion secondary battery preferably includes a separator, such as, but not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those which have been subjected to a hydrophilic treatment. 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.

[0054] The lithium ion secondary battery or the like that is the secondary battery of the present invention configured in this manner can provide a secondary battery that achieves battery performance that can withstand repeated charge and discharge over a long period of time. [Example]

[0055] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0056] [Examples 1 to 11 and Comparative Examples 1 to 5: Preparation of Carbon Nanotube Dispersions] (Determining the dispersion limit median diameter a) The blending composition shown in Table 1 below [amounts of carbon nanotubes, dispersant (polymer material), and dispersion medium] was dispersed using a horizontal bead mill (dispersion device A) or a thin film rotary high-speed mixer (dispersion device B), and samples were taken at regular intervals to measure the median diameter of the carbon nanotubes using dynamic light scattering, and the dispersion limit median diameter a was determined for the examples and comparative examples. The dispersion conditions in the dispersion devices A and B are as follows: Dispersion device A: Zirconia beads with a diameter of φ0.5 mm, dispersion time 0 to 360 minutes Dispersion device B: Mixer rotation speed 40 rpm, dispersion time 0 to 360 minutes

[0057] Example 1 Dispersion was carried out for a dispersion time Tx (minutes) using a dispersion device and the composition shown in Table 1 below (amounts of carbon nanotubes, dispersant (polymer material), and dispersion medium), to obtain a carbon nanotube dispersion. The median diameter X (nm) of the obtained carbon nanotube dispersion measured by dynamic light scattering and X / a, which shows the relationship between a and X, are shown in Table 1 below. The stability and conductivity were evaluated by the following methods and according to the following evaluation criteria. The evaluation results are shown in Table 1 below.

[0058] (Examples 2 to 9, 11, Comparative Examples 1 to 5) The results obtained in the same manner as in Example 1 are shown in Table 1 below.

[0059] Example 10 The results obtained in the same manner as in Example 1, except that dispersion was performed using Dispersion Apparatus B, are shown in Table 1 below.

[0060] (Method for measuring median diameter X) The median diameter was measured by dynamic light scattering (25°C) using cumulant analysis.

[0061] (Method for measuring dispersion viscosity) A cone-plate viscometer (manufactured by Toki Sangyo Co., Ltd., 1°34′R24 cone) was used at 25°C and 10 rpm (shear rate 38.3 s -1 The viscosity was measured under the following conditions:

[0062] (Method for evaluating stability) (1) The rate of change in viscosity over time, (2) the appearance of the liquid over time, and (3) the state of the liquid when it was made into a film after aging were evaluated, and the stability was comprehensively evaluated based on the results [(1) to (3)] using the following evaluation criteria.

[0063] (1) Evaluation method for the rate of change in viscosity over time The completed dispersion was left in a 25°C environment for one week, and the change in viscosity value determined by the viscosity measurement method described above was used to determine the rate of change (after one week / initial value), and the rate of viscosity change over time was evaluated according to the following evaluation criteria. Evaluation criteria: A: Change rate is less than 200% B: Change rate is 200% or more but less than 500% C: Change rate is 500% or more

[0064] (2) Appearance of the liquid over time The appearance of the dispersion liquid after being left in the 25°C environment described in (1) above for one week was evaluated by a sensory test according to the following criteria. Evaluation criteria: A: Uniform, with no separation or concentration difference B: Some separation and concentration differences are observed, but homogenization is easily achieved by re-mixing. C: The carbon nanotubes settle and cannot be homogenized by re-mixing.

[0065] (3) State of the liquid when made into a film after aging The completed dispersion was applied to one side of a PET film (Lumirror #100-T60, Toray Industries, Inc.) using an applicator with a gap of 50 μm, and then dried at a temperature of 80°C. The state of the obtained film was subjected to a sensory evaluation according to the following evaluation criteria. A: A uniform and smooth coating film can be obtained. B: Several agglomerates are observed, or there is a slight difference in concentration within the surface. C: The surface is rough or the paint does not spread evenly

[0066] Stability criteria: ◎: All three items in the evaluation results (1) to (3) above were rated A. 〇: Two of the evaluation results (1) to (3) above received an A rating, and no C ratings were received. △: 0 to 1 A rating among the evaluation results of (1) to (3) above, and no C rating ×: One or more items in the evaluation results (1) to (3) above received a C rating

[0067] (Conductivity evaluation method) The completed dispersion was applied to one side of a PET film (Lumirror #100-T60, Toray Industries, Inc.) using an applicator with a gap of 50 μm, then dried at a temperature of 80°C, and the resistance value of the resulting film was measured. The resistance value was measured as sheet resistance using an apparatus consisting of a four-point probe with a probe spacing of 10 mm and a Milliohm HiTester 3227 (Hioki E.E.), and the conductivity was evaluated according to the following evaluation criteria. Evaluation criteria: ◎: Sheet resistance is less than 250Ω / □ 〇: Sheet resistance is 250Ω / □ or more and less than 400Ω / □ △: Sheet resistance is 400Ω / □ or more and less than 800Ω / □ ×: Sheet resistance is 800Ω / □ or more

[0068] [Table 1]

[0069] As is clear from the evaluation results in Table 1, the carbon nanotube dispersions of Examples 1 to 11, which are within the scope of the present invention, were excellent in both stability and conductivity. In contrast, the carbon nanotube dispersions of Comparative Examples 1 to 5 were unable to satisfy the above characteristics. [Industrial Applicability]

[0070] The carbon nanotube dispersion liquid has excellent stability and conductive properties, and is useful as a material for fuel cells, various electrodes, electromagnetic wave shielding materials, conductive resins, field emission display components, and the like. In particular, it can be used to produce an electrode paste suitable for producing electrodes for lithium ion secondary batteries, etc., and can realize excellent battery performance that can withstand repeated charging and discharging over a long period of time.

Claims

1. A carbon nanotube dispersion liquid containing at least 0.1 to 15.0 mass % of carbon nanotubes, 0.05 to 20.0 mass % of a water-soluble polymer material, and a dispersion medium, wherein the carbon nanotube dispersion liquid is obtained such that, when the median diameter of carbon nanotubes measured by dynamic light scattering is X and the dispersion limit median diameter under the following conditions is a, 1.0 < X / a ≦ 2.0 and X < 1.0 μm. conditions: A blended composition containing at least carbon nanotubes, a water-soluble polymer material, and a dispersion medium is dispersed using a dispersion device selected from Group A below, without changing the dispersion conditions of the dispersion device, including the conditions of Group B below. The median diameter of the carbon nanotubes sampled and measured at any time intervals by dynamic light scattering is measured at a rate of change (change in median diameter / change in dispersion time) that changes from negative to zero and is the smallest among the median diameters at each time point. This median diameter is defined as the dispersion limit median diameter a. Dispersion device of group A: Disperser, homo mixer, planetary mixer, (high pressure) homogenizer, paint conditioner, colloid mill, bead mill, cone mill, ball mill, sand mill, attritor, pearl mill, co-ball mill, roll mill, wet jet mill, thin film swirling high-speed mixer Conditions for Group B: For mixer dispersers (Disper, Homo Mixer, Rotating / Revolving Mixer, Henschel Mixer, Planetary Mixer): Mixer rotation speed For media-type dispersers (bead mills, cone mills, ball mills, sand mills, attritors, pearl mills, and co-ball mills): media bead diameter, media filling rate, dispersion peripheral speed For media-less dispersers (wet jet mills, thin film swirling high-speed mixers): Mixer rotation speed

2. 2. The carbon nanotube dispersion liquid according to claim 1, wherein the water-soluble polymer material is one or more polymer materials selected from the group consisting of nonionic water-soluble polymers and anionic water-soluble polymers.

3. A conductive paste comprising at least the carbon nanotube dispersion liquid according to claim 1 or 2.

4. 3. An electrode paste for a secondary battery, comprising at least the carbon nanotube dispersion liquid according to claim 1 or 2 and an active material for a secondary battery.

5. 5. An electrode for a secondary battery, comprising the electrode paste for a secondary battery according to claim 4.

6. A secondary battery using the electrode for secondary batteries according to claim 5.

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