Carbon nanotubes and their uses

Abstract

To provide carbon nanotubes, a carbon nanotube dispersion, a carbon nanotube resin composition, and a mixture slurry for reducing the risk of a short circuit or the like caused by a catalyst or the like containing a metal component and for obtaining an electrode film and a battery with high conductivity, and to provide a secondary battery with excellent rate and cycle characteristics, and a vehicle equipped with the secondary battery.SOLUTION: Carbon nanotubes satisfy (1) to (3) as follows: (1) the content of cobalt atoms is 1,000 ppm or more and 30,000 ppm or less; (2) particles containing metal atoms are contained and the mass magnetization of the particles is 100 emu / g or less; and (3) when a maximum peak intensity at 1,560 cm-1 to 1,600 cm-1 of a Raman spectrum is G and a maximum peak intensity at 1,310 cm-1 to 1,350 cm-1 is D, the G / D ratio is 0.7 or more and 1.5 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to carbon nanotubes, and more particularly to carbon nanotubes, a carbon nanotube dispersion, a resin composition containing the carbon nanotube dispersion and a binder resin, a composite slurry containing the carbon nanotube dispersion, a binder resin, and an active material, an electrode film containing carbon nanotubes, a secondary battery using an electrode film containing carbon nanotubes, and a vehicle including the secondary battery. [Background technology]

[0002] Carbon nanotubes have a cylindrical structure of graphite layers and are chemically stable, conductive, and mechanically strong, and therefore are used in a variety of applications, such as electronic materials, structural materials, and paints. Specific applications of carbon nanotubes include electrode materials, semiconductor materials, filler materials, short needle probes for microscopes, adsorption materials, and filter materials. In particular, electrode materials using carbon nanotubes have excellent conductivity, and are therefore expected to be applied to secondary batteries, fuel cells, electric double layer capacitors, and the like.

[0003] With the spread of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices, there is a demand for secondary batteries with high input / output characteristics or high energy density, as well as for higher capacity. Against this background, lithium-ion secondary batteries, in particular, are increasingly being used in many devices.

[0004] Carbon black, ketjen black, graphene, fine carbon materials, and the like are used as conductive additives in secondary batteries. Carbon nanotubes, a type of fine carbon fiber, are particularly widely used. For example, adding carbon nanotubes to electrode active materials can reduce electrode resistance, improve the load resistance of the battery, increase the material strength of the electrode, and increase the expansion and contraction properties of the electrode, thereby improving the rate characteristics and cycle life of the secondary battery. Among these, multi-walled carbon nanotubes with outer diameters of 5 nm to several tens of nm are relatively inexpensive and are becoming widely used.

[0005] Carbon nanotubes can generally be produced by methods such as arc discharge, laser evaporation, and chemical vapor deposition. Of these, chemical vapor deposition is the most suitable for mass production from the standpoint of productivity and economy, and is therefore widely used. In chemical vapor deposition, carbon nanotubes are produced by reacting a carbon source gas with catalyst particles containing metal components such as iron, cobalt, and nickel. Therefore, carbon nanotubes obtained by chemical vapor deposition contain catalyst particles containing metal components such as iron, cobalt, and nickel, or particles such as carbides or oxides derived from the catalyst particles. When carbon nanotubes containing catalyst particles containing metal components are used in secondary batteries, the metal components can dissolve and precipitate, causing problems such as short-circuiting the battery. A short-circuiting battery can lead to serious accidents such as fire or explosion. Therefore, several methods have been proposed for purifying carbon nanotubes to remove catalyst particles containing metal components.

[0006] For example, Patent Document 1 describes a method for purifying carbon nanotubes by adding acid to unpurified carbon nanotubes containing metal catalysts and metal carbides as impurities to dissolve the impurities. However, it is known that methods for removing metal components using acid, such as those described in Patent Document 1, also oxidize part of the carbon nanotubes, resulting in a decrease in properties such as conductivity. Patent Documents 2 to 4 also describe methods for evaporating and removing metals by baking carbon nanotubes at high temperatures in the presence of halogen gas. When purified carbon nanotubes are used in secondary batteries, problems caused by catalysts containing metal components can be prevented. Carbon nanotubes purified by this method have improved crystallinity and a higher G / D ratio calculated from Raman spectra. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-198611 [Patent Document 2] Japanese Patent Application Publication No. 2019-210173 [Patent Document 3] Re-tabled publication 2008 / 126532 [Patent Document 4] International Publication No. 2017 / 048053 Summary of the Invention [Problem to be solved by the invention]

[0008] In the purification methods described in Patent Documents 2 to 4, carbon nanotubes are calcined at high temperatures in the presence of halogen gas, which improves the crystallinity of the carbon nanotubes and thus the electrical conductivity of the carbon nanotubes themselves. However, because the carbon nanotubes become hard and brittle, when they are used as a conductive additive in electrode films of secondary batteries and the like, it becomes difficult to form a well-developed conductive network in the electrode film, which can lead to deterioration in electrode resistance. Furthermore, the use of halogen gas at high temperatures raises safety and environmental concerns, as well as problems such as increased manufacturing costs.

[0009] The problem to be solved by the present invention is to provide carbon nanotubes, a carbon nanotube dispersion, a carbon nanotube resin composition, and a composite slurry for obtaining a highly conductive electrode film and battery that reduce the risk of short circuits caused by catalysts containing metal components, etc. More specifically, the problem to be solved by the present invention is to provide a secondary battery and a vehicle equipped with a secondary battery that have excellent rate characteristics and cycle characteristics. [Means for solving the problem]

[0010] The inventors of the present invention conducted extensive research to solve the above problems and found that by using carbon nanotubes with a cobalt atom content of 1,000 ppm to 30,000 ppm and with mass magnetization and G / D ratios within specific ranges, it is possible to reduce the risk of short circuits and obtain an electrode film with excellent conductivity, and a secondary battery with excellent rate and cycle characteristics. Based on this discovery, the inventors have made the present invention.

[0011] That is, the present invention includes the following embodiments: The embodiments of the present invention are not limited to the following.

[0012] The present invention relates to carbon nanotubes that satisfy the following (1) to (3): (1) The cobalt atom content is 1,000 ppm or more and 30,000 ppm or less. (2) The particle contains metal atoms, and the mass magnetization of the particle is 100 emu / g or less. (3) 1,560 cm of the Raman spectrum -1 ~1,600cm -1 The maximum peak intensity at G is 1,310 cm -1 ~1,350cm -1 When the maximum peak intensity at is D, the G / D ratio is 0.7 or more and 1.5 or less.

[0013] The present invention relates to the above carbon nanotube, wherein the content of iron atoms is equal to or greater than 0 ppm and equal to or less than 10,000 ppm.

[0014] The present invention uses a material with a volume resistivity of 1.0×10 -2 Over 2.0 x 10 -2 The following relates to the above carbon nanotubes:

[0015] The present invention is directed to a method for producing a cellulose ester having a BET specific surface area of ​​150 m 2 / g or more 800m 2 / g or less.

[0016] The present invention relates to the above-mentioned carbon nanotube, which has an average outer diameter of 5 nm or more and 15 nm or less.

[0017] The present invention relates to the above carbon nanotubes, wherein the cobalt atoms include components derived from catalyst particles for synthesizing the carbon nanotubes.

[0018] The present invention relates to a carbon nanotube dispersion liquid containing the above-mentioned carbon nanotubes, a solvent, and a dispersant.

[0019] The present invention relates to the carbon nanotube dispersion liquid, wherein the solvent is an amide organic solvent or water.

[0020] The present invention provides a carbon nanotube resin composition, which further contains a binder in addition to the carbon nanotube dispersion liquid.

[0021] The present invention relates to a composite slurry containing the carbon nanotube resin composition and an active material.

[0022] The present invention relates to an electrode film containing the carbon nanotubes.

[0023] The present invention relates to a secondary battery including the electrode film.

[0024] The present invention relates to a vehicle including the secondary battery.

[0025] The present invention relates to a method for producing carbon nanotubes, the carbon nanotubes containing particles containing metal atoms, and the carbon nanotubes are heated at 200°C or higher and 500°C or lower in an oxygen-containing atmosphere to oxidize the particles containing metal atoms. [Effects of the Invention]

[0026] According to an embodiment of the present invention, it is possible to provide carbon nanotubes, a carbon nanotube dispersion, a carbon nanotube resin composition, and a composite slurry for obtaining a highly conductive electrode film and a battery that reduces the risk of short circuits due to catalyst particles containing metal components, etc. Furthermore, according to an embodiment of the present invention, it is possible to provide a highly conductive electrode film, a secondary battery with excellent rate characteristics and cycle characteristics, and a vehicle equipped with the secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0027] The carbon nanotube according to the embodiment of the present invention will be described in detail below. The present invention is not limited to the following embodiment, and the present invention also includes embodiments that are implemented within the scope of the present invention.

[0028] (1) Carbon nanotubes Carbon nanotubes have a cylindrical shape with planar graphite rolled up. Carbon nanotubes may also contain single-walled carbon nanotubes. Single-walled carbon nanotubes have a structure in which one layer of graphite is rolled up. Multi-walled carbon nanotubes have a structure in which two or more layers of graphite are rolled up. Furthermore, the sidewalls of carbon nanotubes do not have to have a graphite structure. For example, carbon nanotubes with sidewalls having an amorphous structure can also be used as carbon nanotubes.

[0029] The shape of the carbon nanotubes is not limited. Examples of such shapes include needle-like, cylindrical tube-like, fishbone-like (fishbone or cup stacked), trump-like (platelet) and coil-like. In this embodiment, the shape of the carbon nanotubes is preferably needle-like or cylindrical tube-like. The carbon nanotubes may have a single shape or a combination of two or more shapes.

[0030] Examples of the form of carbon nanotubes include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. Carbon nanotubes may have any of these forms alone or in combination.

[0031] The purity of carbon nanotubes is expressed as a value (% by mass) obtained by subtracting the ash content (% by mass) from the mass of the carbon nanotubes. The ash content (% by mass) of carbon nanotubes can be measured, for example, in accordance with JIS K 6218-2. The ash content of carbon nanotubes is a non-flammable component containing metal components and the like. From the viewpoint of electrical conductivity, the purity of carbon nanotubes is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more, based on the mass of the carbon nanotubes. Furthermore, the content of non-flammable components contained in the carbon nanotubes is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0032] The carbon nanotubes of the present invention are preferably synthesized by chemical vapor deposition using catalyst particles containing a metal component, and preferably contain particles derived from the catalyst particles. The catalyst particles preferably contain cobalt atoms and aluminum atoms, and may also contain iron atoms. The total amount of cobalt atoms and aluminum atoms contained in the catalyst particles is preferably 50 mass% or more, more preferably 80 mass% or more, based on the mass of the metal components of the catalyst particles. The catalyst particles may also contain other metal atoms such as nickel, silicon, manganese, etc. When the catalyst particles contain silicon, silica is preferred. Iron atoms or other metal atoms may be mixed into the carbon nanotubes due to wear from metals such as stainless steel used in synthesis equipment, filling equipment, or piping. The carbon nanotubes may also contain iron atoms or other metal atoms not derived from the catalyst particles.

[0033] The carbon nanotubes contain 1,000 ppm to 30,000 ppm of cobalt atoms, based on the mass of the carbon nanotubes. They may contain 10,000 ppm or less of iron atoms, or may contain no iron atoms. The cobalt atom content of the carbon nanotubes is preferably 3,000 ppm or more, more preferably 5,000 ppm or more, and even more preferably 8,000 ppm or more. The cobalt atom content is preferably 20,000 ppm or less, more preferably 15,000 ppm or less, and even more preferably 10,000 ppm or less. The cobalt atoms in the carbon nanotubes can exist as a magnetic substance, such as a metal or alloy, or as a non-magnetic substance, such as an oxide or carbide. When catalyst particles containing cobalt atoms are used in the carbon nanotube production process, if the cobalt atom content of the carbon nanotubes is below the above-mentioned lower limit, the productivity of the carbon nanotubes may be significantly reduced. Also, if the cobalt atom content of the carbon nanotubes is above the above-mentioned upper limit, there is a concern that the risk of short-circuiting in the battery may increase if the cobalt atom content is high as a magnetic material, and there is a concern that the resistance of the battery may increase if the cobalt atom content is high as a non-magnetic material. Methods for adjusting the amount of cobalt atoms contained in carbon nanotubes to the above range include increasing the yield of carbon nanotubes per mass of catalyst particles during the carbon nanotube production process, and reducing the proportion of cobalt atoms in the catalyst particles.

[0034] The iron atom content of carbon nanotubes is preferably 10,000 ppm or less, more preferably 500 ppm or less, even more preferably 1,000 ppm or less, and particularly preferably 500 ppm or less. Alternatively, carbon nanotubes may not contain iron atoms. Iron atoms in carbon nanotubes can exist as magnetic substances such as metals or alloys, or as non-magnetic substances such as oxides or carbides. If the iron atom content of the carbon nanotubes exceeds the upper limit, iron-containing components may dissolve and precipitate in the battery, causing a short circuit in the battery. Methods for adjusting the amount of iron atoms contained in the carbon nanotubes to the above range include increasing the carbon nanotube yield per mass of catalyst particles in the carbon nanotube production process, using catalyst particles that do not contain iron atoms, reducing the proportion of iron atoms in the catalyst particles, and reducing the amount of iron atoms mixed in during the carbon nanotube production process. The content of metal components in carbon nanotubes can be analyzed using inductively coupled plasma (ICP).

[0035] Metal components contained in carbon nanotubes may be removed by known methods to the extent that the effects of the present invention are not impaired. When removing by acid treatment, the acid used may be any acid capable of dissolving the metal components contained in the carbon nanotubes. For example, inorganic acids or carboxylic acids are preferred, and among inorganic acids, hydrochloric acid, sulfuric acid, and nitric acid are particularly preferred. The acid treatment of carbon nanotubes is preferably carried out in a liquid phase, and it is more preferred to disperse and / or mix the carbon nanotubes in the liquid phase. After the acid treatment, the carbon nanotubes are preferably washed with water and dried.

[0036] The carbon nanotubes contain particles containing metal atoms with a mass magnetization of 100 emu / g or less. The mass magnetization of the particles containing metal atoms is preferably 70 emu / g or less, more preferably 50 emu / g or less. The particles containing metal atoms are preferably particles derived from catalyst particles. The mass magnetization of particles containing metal atoms is the magnetic strength (magnetization) based on the mass of the particles containing metal atoms, and increases with the magnetic strength of the magnetic material and the amount of magnetic material components contained in the carbon nanotubes. If the mass magnetization of the particles exceeds the above range, the battery is more likely to short-circuit. The mass magnetization of the particles can be measured while they are still contained in the carbon nanotubes and converted to a value per mass of the particles, for example, by the method described in the Examples. The mass magnetization of the carbon nanotubes containing the particles is preferably 1.5 emu / g or less, more preferably 1.0 emu / g or less, and even more preferably 0.8 emu / g or less. Methods for reducing the mass magnetization of particles containing metal atoms include demagnetizing some or all of the metal atoms contained in the particles by oxidizing or carbonizing them, with oxidation being particularly preferred. Examples of methods for oxidizing particles include heating in air at 200°C to 500°C. If the heating temperature is too low, the metal atoms contained in the particles may not be sufficiently oxidized. If the heating temperature is too high, the carbon nanotubes may burn and disappear. If heated at a high temperature that does not cause combustion, the carbon nanotubes may become highly crystalline, hard, and brittle, making it difficult to form good conductive paths in the electrode film and reducing the conductivity of the electrode film. From the above perspective, the temperature at which carbon nanotubes are heated in air is preferably 200°C to 500°C, more preferably 200°C to 400°C, and even more preferably 200°C to 350°C. Furthermore, in order to ensure that oxidation reaches the center of the particles, it is preferable to hold the mixture for a certain period of time, and the holding time is preferably 0.5 hours or more, more preferably 1 hour or more, even more preferably 2 hours or more, and particularly preferably 6 hours or more. If the holding time is insufficient, oxidation will be difficult to achieve to the center of the particles.

[0037] The G / D ratio of carbon nanotubes is determined by Raman spectroscopy. Various laser wavelengths are used in Raman spectroscopy, but in this embodiment, 532 nm and 632 nm are used. -1 The Raman shift observed around 1,350 cm is called the G band derived from graphite. -1 The Raman shift observed around this band is called the D band, which is derived from defects in amorphous carbon and graphite. The higher the G / D ratio of a carbon nanotube, the higher its crystallinity. Furthermore, sintering carbon nanotubes at high temperatures tends to increase the G / D ratio. Carbon nanotubes have a Raman spectrum of 1,560 to 1,600 cm -1 The maximum peak intensity in the range of 1,310 to 1,350 cm is G. -1When the maximum peak intensity within this range is D, the G / D ratio is 0.7 or more and 1.5 or less. The G / D ratio of the carbon nanotubes is preferably 0.8 or more, and more preferably 0.9 or more. Furthermore, the G / D ratio of the carbon nanotubes is preferably 1.4 or less, and more preferably 1.3 or less. When the G / D ratio of the carbon nanotubes is within the above range, the conductivity and dispersibility of the carbon nanotubes are excellent, and therefore, the rate characteristics and cycle characteristics of a secondary battery using an electrode film made from the carbon nanotube dispersion are improved.

[0038] The volume resistivity of carbon nanotubes is 1.0×10 -2 It is preferable that the resistance is Ω·cm or more, and 2.0×10 -2 It is preferable that the resistance is Ω·cm or less, and 1.8×10 -2 It is more preferable that the resistance is Ω·cm or less, and 1.5×10 -2 When the volume resistivity of the carbon nanotubes is in the above range, the conductivity of the carbon nanotubes is excellent, and therefore the rate characteristics and cycle characteristics of a secondary battery using an electrode film made from the carbon nanotube dispersion are improved. The volume resistivity of the carbon nanotubes can be measured using a powder resistivity measuring device (Loresta GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0039] The BET specific surface area of ​​carbon nanotubes is 150m 2 / g or more is preferable, and 180m 2 / g or more. The BET specific surface area of ​​the carbon nanotubes is preferably 800 m 2 / g or less, and 2 / g or less is more preferable, and 400m 2 / g or less is even more preferable. The BET specific surface area of ​​carbon nanotubes is calculated by the BET method using nitrogen adsorption measurements. There is often a correlation between the specific surface area of ​​carbon nanotubes and the average outer diameter of carbon nanotubes; the smaller the specific surface area, the larger the outer diameter of the carbon nanotubes, and the fewer the number of carbon nanotubes per mass. On the other hand, the larger the specific surface area of ​​carbon nanotubes, the smaller the outer diameter of the carbon nanotubes, and the more the number of carbon nanotubes per mass. If the specific surface area of ​​carbon nanotubes is below the above range, the number of carbon nanotubes per mass is small, making it difficult to efficiently form a conductive network, and the rate characteristics and cycle characteristics of the battery are likely to deteriorate. On the other hand, if the specific surface area of ​​carbon nanotubes exceeds the above range, the carbon nanotubes have a strong cohesive force, making dispersion difficult, and the carbon nanotubes may aggregate in the electrode film, making it difficult to form a good conductive network.

[0040] The average outer diameter of the carbon nanotubes is preferably 5 nm or more, more preferably 8 nm or more, and even more preferably 10 nm or more. The average outer diameter of the carbon nanotubes is preferably 15 nm or less. If the average outer diameter of the carbon nanotubes exceeds the above range, the number of carbon nanotubes per mass decreases, which may make it difficult to efficiently form a conductive network. On the other hand, if the average outer diameter of the carbon nanotubes is below the above range, the carbon nanotubes have a strong cohesive force, making dispersion difficult. The carbon nanotubes may aggregate in the electrode film, making it difficult to form a good conductive network.

[0041] The standard deviation of the average outer diameter of the carbon nanotubes is preferably 2 nm to 8 nm, more preferably 3 nm to 6 nm. If the standard deviation of the average outer diameter of the carbon nanotubes is large, it may be difficult to efficiently form a conductive network, or the carbon nanotubes may become entangled and aggregate in the carbon nanotube dispersion, electrode slurry, and / or electrode film, making it impossible to form a good conductive network.

[0042] The outer diameter and average outer diameter of carbon nanotubes can be calculated as follows: First, carbon nanotubes are observed and photographed using a transmission electron microscope. Next, 300 carbon nanotubes are randomly selected from the photograph and the outer diameter of each is measured. Next, the average outer diameter (nm) of the carbon nanotubes is calculated as the number average of the outer diameters.

[0043] (2) Solvent The carbon nanotube dispersion of the present invention contains carbon nanotubes and a solvent. The solvent is not particularly limited as long as it is capable of dispersing carbon nanotubes, but is preferably a mixed solvent consisting of one or more of water and a water-soluble organic solvent.

[0044] Examples of water-soluble organic solvents include alcohols (methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, benzyl alcohol, etc.), polyhydric alcohols (ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, etc.), polyhydric alcohol ethers (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, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, propylene ... 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.), amines (ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, tetramethylpropylenediamine, etc.), amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocyclics (cyclohexylpyrrolidone, 2-oxazolidone, 1,Examples of solvents that can be used include solvents based on 3-dimethyl-2-imidazolidinone, γ-butyrolactone, etc., sulfoxides (dimethyl sulfoxide, etc.), sulfones (hexamethylphosphorotriamide, sulfolane, etc.), lower ketones (acetone, methyl ethyl ketone, etc.), and tetrahydrofuran, urea, acetonitrile, etc. Among these, water or an amide-based organic solvent is more preferred, and among the amide-based organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferred.

[0045] When only an amide-based organic solvent is used as the solvent, the water content in the solvent is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0046] (3) Dispersant The carbon nanotube dispersion may contain a dispersant. The dispersant is not particularly limited as long as it can stabilize the dispersion of carbon nanotubes. Examples of dispersants that can be used include surfactants and resin-type dispersants, with resin-type dispersants being preferred. The surfactant is preferably one that reduces the surface tension of the solvent, and examples include anionic, cationic, nonionic, and amphoteric surfactants. Depending on the properties required for the intended use of the carbon nanotube dispersion, a suitable type of dispersant can be used in a suitable amount. Furthermore, multiple dispersants may be used in combination, and an acid or base may be used as a dispersion stabilizer.

[0047] Examples of anionic surfactants include, but are not limited to, fatty acid salts, polysulfonates, polycarboxylates, alkyl sulfates, alkylaryl sulfonates, alkylnaphthalenesulfonates, dialkylsulfonates, dialkylsulfosuccinates, alkylphosphates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkylaryl ether sulfates, naphthalenesulfonate-formalin condensates, polyoxyethylene alkylphosphate sulfonates, glycerol borate fatty acid esters, and polyoxyethylene glycerol fatty acid esters. Examples of anionic surfactants include, but are not limited to, sodium dodecylbenzenesulfonate, sodium laurate sulfate, sodium polyoxyethylene lauryl ether sulfate, polyoxyethylene nonylphenyl ether sulfate, and the sodium salt of β-naphthalenesulfonate-formalin condensates. Cationic surfactants include alkylamine salts and quaternary ammonium salts, including, but not limited to, stearylamine acetate, trimethylcoconut ammonium chloride, trimethyltallow ammonium chloride, dimethyldioleylammonium chloride, methyloleyldiethanol chloride, tetramethylammonium chloride, laurylpyridinium chloride, laurylpyridinium bromide, laurylpyridinium disulfate, cetylpyridinium bromide, 4-alkylmercaptopyridine, poly(vinylpyridine)-dodecyl bromide, and dodecylbenzyltriethylammonium chloride. Amphoteric surfactants include, but are not limited to, aminocarboxylates. Nonionic surfactants include, but are not limited to, polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, polyoxyethylene phenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and alkyl allyl ethers, particularly, but not limited to, polyoxyethylene lauryl ethers, sorbitan fatty acid esters, and polyoxyethylene octylphenyl ethers.

[0048] Specific examples of resin-type dispersants include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethylhydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol polymers, polyvinyl butyral polymers, polyvinyl pyrrolidone polymers, hydrogenated nitrile butadiene rubbers, polyacrylonitrile polymers, etc. Particularly preferred are methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol polymers, polyvinyl butyral polymers, polyvinyl pyrrolidone polymers, hydrogenated nitrile butadiene rubbers, polyacrylonitrile polymers, etc.

[0049] (4) Carbon nanotube dispersion The carbon nanotube dispersion liquid can be produced, for example, by dispersing carbon nanotubes in a solvent. The dispersant may be added once or in multiple batches at any timing during the dispersion process. The dispersion method for carrying out such a process is not particularly limited.

[0050] Dispersion methods include, for example, methods using various dispersers such as a disperser (disperser), homogenizer, high-shear mixer, kneader, two-roll mill, three-roll mill, ball mill, horizontal sand mill, vertical sand mill, annular bead mill, attritor, planetary mixer, or high-pressure homogenizer. While the disperser is not particularly limited, a high-pressure homogenizer is preferred in order to adjust the fiber length of the carbon nanotubes in the carbon nanotube dispersion to a desired range. It is also more preferred to select and combine multiple dispersers. For example, a high-shear mixer is used in the initial dispersion step to promote wetting of the carbon nanotubes and break down coarse particles and agglomerates, followed by a high-pressure homogenizer to maintain the fiber length of the carbon nanotubes. The pressure when using the high-pressure homogenizer is not particularly limited, but is preferably 60 to 150 MPa, more preferably 60 to 120 MPa.

[0051] The solid content of the carbon nanotube dispersion liquid is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 2% by mass or more, relative to 100% by mass of the carbon nanotube dispersion liquid. The solid content of the carbon nanotube dispersion liquid is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less, relative to 100% by mass of the carbon nanotube dispersion liquid.

[0052] The amount of dispersant in the carbon nanotube dispersion is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, relative to 100% by mass of carbon nanotubes, from the viewpoint of the chargeability, dispersibility, and dispersion stability of the carbon nanotubes. Also, the amount of dispersant in the carbon nanotube dispersion is preferably 300% by mass or less, more preferably 100% by mass or less, and even more preferably 50% by mass or less, relative to 100% by mass of carbon nanotubes, from the viewpoint of electrical conductivity.

[0053] The average fiber length of the carbon nanotubes in the carbon nanotube dispersion is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. The average fiber length of the carbon nanotubes in the carbon nanotube dispersion is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 2 μm or less. If the average fiber length of the carbon nanotubes is below the above range, it may be difficult to effectively form a conductive network between the active materials. If the average fiber length of the carbon nanotubes exceeds the above range, the carbon nanotubes may become entangled and aggregate in the carbon nanotube dispersion, electrode slurry, and / or electrode film, making it difficult to form a good conductive network.

[0054] The average fiber length of carbon nanotubes in a carbon nanotube dispersion can be determined as follows: First, carbon nanotubes are observed and photographed using a scanning electron microscope. Next, the fiber length of the carbon nanotubes can be measured in the observation photograph to confirm the average fiber length. The average fiber length is determined by randomly selecting 300 carbon nanotubes in the observation photograph, measuring the fiber length of each, and averaging the measured values. Note that the average fiber length of carbon nanotubes in an embodiment of the present invention is the average fiber length in the carbon nanotube dispersion, i.e., the average fiber length after the carbon nanotube dispersion has been prepared.

[0055] Carbon nanotube dispersions may contain particulate metal foreign matter and dissolved metal ions as metal components. The metal foreign matter is a metal component present in particulate form in the carbon nanotube dispersion, specifically, metals such as copper, iron, nickel, chromium, aluminum, magnesium, silica, manganese, and molybdenum, metal oxides, and composite oxides thereof. Carbon nanotubes, dispersants, and other materials may contain metal foreign matter resulting from their respective manufacturing processes, and metal foreign matter may also be mixed in during the manufacturing process of the carbon nanotube dispersion. If metal foreign matter is present inside a battery, the battery is more likely to short-circuit, so removing the metal foreign matter is extremely important from a safety perspective. The process for producing a carbon nanotube dispersion preferably includes a step of removing contaminants such as metallic foreign matter (metallic foreign matter removal step) at any timing. From the viewpoint of efficiency, the metallic foreign matter removal step is preferably carried out during and / or at the end of the dispersion step of the carbon nanotube dispersion. The metallic foreign matter removal step may be carried out multiple times.

[0056] In the metal foreign matter removal step, the method for removing metal foreign matter from the carbon nanotube dispersion liquid is not particularly limited, and examples thereof include a method of removal by filtration using a filter, a method of removal by a vibrating sieve, a method of removal by centrifugation, a method of removal by magnetic force, etc. Among these, since metal foreign matter such as iron and chromium is magnetic, a method of removal by magnetic force is preferred, and a method of combining a step of removal by magnetic force and a step of removal by filtration using a filter is more preferred.

[0057] The method of removal using magnetic force is not particularly limited as long as it can remove metal foreign matter, but considering productivity and removal efficiency, a method of removing the metal foreign matter by passing the carbon nanotube dispersion through a magnetic filter placed in the carbon nanotube dispersion production line is preferred. The step of removing metallic foreign matter from a carbon nanotube dispersion using a magnetic filter is preferably carried out by passing the carbon nanotube dispersion through a magnetic filter that generates a magnetic field with a magnetic flux density of 1,000 gauss or more. Because a low magnetic flux density reduces the efficiency of removing metal components, the magnetic flux density is preferably 5,000 gauss or more, more preferably 10,000 gauss or more in consideration of removing stainless steel, which has low magnetic properties, and most preferably 12,000 gauss or more. When a magnetic filter is installed in a production line, it is preferable to include a process upstream of the magnetic filter that uses a filter such as a cartridge filter to remove coarse foreign matter or metal particles. This is because coarse metal particles may pass through the magnetic filter depending on the filtration flow rate. Furthermore, although a magnetic filter is effective even when used for a single filtration, a circulating type is more preferable. This is because a circulating type improves the efficiency of metal particle removal. When a magnetic filter is placed in a production line for a carbon nanotube dispersion, the location of the magnetic filter is not particularly limited, but it is preferably placed immediately before filling a container with the carbon nanotube dispersion, or before the filter if a filtration step using a filtration filter is performed before filling the container, in order to prevent metal components from being mixed into the product if they are detached from the magnetic filter.

[0058] The content of metal components in a carbon nanotube dispersion can be analyzed using inductively coupled plasma (ICP) after drying the carbon nanotube dispersion to remove the solvent. The content of metal components detected by ICP analysis includes metal foreign matter present in particulate form and dissolved metal ions. That is, the amount of metal foreign matter in a carbon nanotube dispersion that has undergone a metal foreign matter removal process includes metal foreign matter that has not been completely removed and dissolved metal ions.

[0059] The content of the iron and chromium metal components contained in the carbon nanotube dispersion liquid is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less, relative to 100 mass % of the carbon nanotube dispersion liquid. By keeping the content of the metal components within the above range, side reactions in the electrode are less likely to occur, making it easier to obtain a battery with better conductivity.

[0060] The carbon nanotube dispersion may further contain one or more carbon materials such as carbon black and graphite as a conductive material. Among these conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant. Examples of carbon black include acetylene black, furnace black, hollow carbon black, channel black, thermal black, and ketjen black. Furthermore, the carbon black may be neutral, acidic, or basic, and oxidation-treated carbon black and graphitization-treated carbon black may also be used.

[0061] (4) Binder The binder is a resin that binds various substances together in the electrode film. Examples of binders include polymers or copolymers containing 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 as structural units; 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. Modified resins, mixtures, and copolymers of these resins are also acceptable. In particular, polymeric compounds containing fluorine atoms in the molecule are preferred from the standpoint of durability, and the use of polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene, and the like is preferred.

[0062] The weight-average molecular weight of the binder is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. The weight-average molecular weight of the binder is preferably 2,000,000 or less, more preferably 1,000,000 or less, and particularly preferably 1,000,000 or less. If the weight-average molecular weight is below the above range, the resistance and adhesion of the binder may decrease. If the weight-average molecular weight is above the above range, the resistance and adhesion of the binder may improve, but the viscosity of the binder itself may increase, reducing workability, and the binder may act as a flocculant, causing significant aggregation of dispersed particles.

[0063] (5) Carbon nanotube resin composition The carbon nanotube resin composition contains carbon nanotubes, a solvent, a dispersant, and a binder. The carbon nanotube resin composition is preferably produced by mixing and homogenizing a carbon nanotube dispersion with a binder, or the binder may be dissolved in advance. The binder may be added at any timing during the process of producing the carbon nanotube dispersion. Various conventionally known methods may be used as the mixing method. The carbon nanotube resin composition can be produced using the dispersion device described above for the carbon nanotube dispersion. The carbon nanotube resin composition may contain one type of binder, or two or more types may be used in combination. Furthermore, the process of producing the carbon nanotube resin composition may include a metal foreign matter removal step.

[0064] (6) Active material Active materials are the materials that form the basis of battery reactions. Active materials are divided into positive electrode active materials and negative electrode active materials based on their electromotive force. 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 transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides. Specific examples include MnO, VO, and VO. 13 Examples of suitable materials include transition metal oxide powders such as TiO2, composite oxide powders of lithium and transition metals such as layered lithium nickel oxide, lithium cobalt oxide, lithium manganate, and spinel-structured lithium manganate, lithium iron phosphate-based materials that are phosphate compounds with an olivine structure, and transition metal sulfide powders such as TiS2 and FeS. Conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can also be used. Mixtures of the above inorganic and organic compounds may also be used. The positive electrode active material is preferably a composite oxide of lithium containing a transition metal such as Al, Fe, Co, Ni, or Mn, more preferably a composite oxide of lithium containing any of Al, Co, Ni, or Mn, and particularly preferably a composite oxide of lithium containing Ni and / or Mn. When these active materials are used, particularly good effects can be obtained. As the negative electrode active material, those capable of doping or intercalating lithium ions can be used. For example, metallic Li, alloy systems such as its alloys like tin alloy, silicon alloy, lead alloy, etc., Li X Fe2O3, Li X Fe3O4, Li X WO2 (x is a number where 0 < x < 1.), metal oxide systems such as lithium titanate, lithium vanadate, lithium silicate, etc., conductive polymer systems such as polyacetylene, poly-p-phenylene, etc., amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, carbon black, mesophase carbon black, resin-fired carbon materials, gas-phase grown carbon fibers, carbon fibers and other carbon-based materials can be mentioned. These negative electrode active materials can also be used alone or in combination of two or more.

[0065] The BET specific surface area of the active material is preferably 0.1 to 10 m 2 / g, more preferably 0.2 to 5 m 2 / g, and even more preferably 0.3 to 3 m 2 / g. The average particle diameter of the active material is preferably within the range of 0.05 to 100 μm, and more preferably within the range of 0.1 to 50 μm. The average particle diameter of the active material as referred to in this specification is the average value of the particle diameters measured by an electron microscope for the active material.

[0066] [[ID=二十三]](7) Composite material slurry The composite material slurry contains carbon nanotubes, a solvent, a dispersant, a binder, and an active material. The composite material slurry is preferably produced by adding and dispersing an active material to a carbon nanotube resin composition. Also, the active material may be added at any timing in the process of producing a carbon nanotube dispersion liquid or the process of producing a carbon nanotube resin composition. The dispersion device used for performing such a process is not particularly limited, and the dispersion devices exemplified in the production of a carbon nanotube dispersion liquid can be used.

[0067] The amount of active material contained in the composite slurry is preferably 20% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry. The amount of active material contained in the composite slurry is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite slurry. A content within the above range is preferable from the viewpoints of coatability, productivity, and uniformity of the electrode film. The amount of carbon nanotubes contained in the composite slurry is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, relative to 100% by mass of the active material, and is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of the active material. The amount of binder contained in the composite slurry is preferably 0.5% by mass or more, more preferably 1% by mass or more, relative to 100% by mass of the active material, and is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the active material. The solid content of the composite slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry, and is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite slurry. The water content of the composite slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0068] (7) Electrode film According to one embodiment of the present invention, the electrode film is a coating film formed by coating a carbon nanotube dispersion, a carbon nanotube resin composition, or a composite slurry containing carbon nanotubes on a current collector and then drying the coating film. 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 current collector material can be metals or alloys such as aluminum, copper, nickel, titanium, or 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.

[0069] The method for applying the carbon nanotube dispersion, carbon nanotube resin composition, or composite slurry containing carbon nanotubes 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 leaving the material to dry, using a blower dryer, a hot air dryer, an infrared heater, and a far-infrared heater.

[0070] After coating and drying, the electrode mixture layer may be rolled using a lithographic press, a calendar roll, etc. The thickness of the electrode mixture layer is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0071] (8) Secondary battery The electrode film can be used as an electrode of a secondary battery, and is particularly preferably used as an electrode of a nonaqueous electrolyte secondary battery that uses an organic electrolyte. A nonaqueous electrolyte secondary battery is a battery that includes a positive electrode, a negative electrode, and an electrolyte containing an organic electrolyte. The electrode film can be used for either the positive electrode, the negative electrode, or both. According to one embodiment of the present invention, an electrode film obtained by coating a current collector with a composite slurry containing a positive electrode active material and drying the coated current collector can be used as a positive electrode. Furthermore, according to one embodiment of the present invention, an electrode film obtained by applying a composite slurry containing a negative electrode active material to a current collector and drying the applied composite slurry can be used as a negative electrode. Furthermore, according to one embodiment of the present invention, an electrode film obtained by coating a current collector with a carbon nanotube dispersion or a carbon nanotube resin composition and drying it can be used as a current collector with an underlayer.

[0072] As the electrolyte, various conventionally known materials capable of ion mobility can be used. Examples include, but are not limited to, those containing lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, or LiBPh4 (where Ph is a phenyl group). Those containing sodium salts can also be used. The electrolyte is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution.

[0073] Non-aqueous solvents include, but are not limited to, 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.

[0074] The 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.

[0075] The structure of the secondary battery is not particularly limited. Usually, it is composed of a positive electrode, a negative electrode, and a separator provided as needed, and can have various shapes according to the purpose of use, such as a paper type, a cylindrical type, a button type, a laminated type, etc.

Examples

[0076] The present invention will be described more specifically with reference to the following examples. The present invention is not limited to the following examples as long as it does not exceed the gist. Unless otherwise specified, "parts" represents "parts by mass" and "%" represents "% by mass". In addition, carbon nanotubes may be referred to as "CNT", carbon nanotube dispersions may be referred to as "CNT dispersions", and carbon nanotube resin compositions may be referred to as "CNT resin compositions". In addition, carbon nanotubes heat-treated in air may be referred to as "calcined CNT".

[0077] The physical properties of CNTs used in the following examples and comparative examples were measured by the following methods.

[0078] <Measurement of cobalt and iron atom contents in CNT> CNTs were acid-decomposed using a microwave sample pretreatment device (ETHOS1 manufactured by Milestone General Co., Ltd.) to extract the metals contained in the CNTs. Then, analysis was performed using a multi-type ICP emission spectroscopic analyzer (720-ES manufactured by Agilent Co., Ltd.) to calculate the cobalt and iron atom contents of the CNTs.

[0079] <Measurement of G / D ratio of CNT> CNTs were placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measured using a laser wavelength of 532 nm. The measurement conditions were an acquisition time of 60 seconds, an integration number of 2 times, a dimming filter of 10%, an objective lens magnification of 20 times, a confocal hole of 500, a slit width of 100 μm, and a measurement wavelength of 100 to 3000 cm -1 was used. The CNTs for measurement were separated on a slide glass and flattened using a spatula. Among the obtained peaks, in the spectrum, 1560 to 1600 cm -1Within the range, the maximum peak intensity was G, 1310~1350 cm -1 Within the range, the maximum peak intensity was D, and the ratio of G / D was defined as the G / D ratio of CNT.

[0080] <Measurement of BET specific surface area of CNT> Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 0.03 g of CNT was weighed, and then it was dried while degassing at 110 °C for 15 minutes. Subsequently, the BET specific surface area of CNT was measured using a fully automatic specific surface area measuring device (HM-model1208, manufactured by MOUNTECH).

[0081] <Measurement of water content in CNT dispersion> The water content in the CNT dispersion was measured using a trace water content measuring device (CA200, manufactured by Nitto Seiko Analytic Co., Ltd.). The measurement was carried out by setting the vaporization device (VA200, manufactured by Nitto Seiko Analytic Co., Ltd.) at 230 °C, and placing 0.5 g to 1.0 g of CNT at the sample installation location in the vaporization device.

[0082] <Measurement of average outer diameter of CNT> Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 0.2 g of CNT was weighed into a 450 mL SM sample bottle (manufactured by Sansho Co., Ltd.), 200 mL of toluene was added, and an ultrasonic homogenizer (Advanced Digital Sonifer (registered trademark), MODEL 450DA, manufactured by BRANSON) was used to perform a dispersion treatment under ice cooling at an amplitude of 50% for 5 minutes to prepare a CNT dispersion. Subsequently, the CNT dispersion was appropriately diluted, a few μL was dropped in the form of a collodion film, dried at room temperature, and then observed directly using a transmission electron microscope (H-7650, manufactured by Hitachi, Ltd.). The observation was carried out at a magnification of 50,000 times. A plurality of photos containing 10 or more CNTs in the visual field were taken, and the outer diameters of 300 arbitrarily extracted CNTs were measured, and the average value was defined as the average outer diameter (nm) of CNT.

[0083] <Measurement of volume resistivity of CNT> Using a powder resistivity measuring device (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: Loresta GP Powder Resistivity Measuring System MCP-PD-51), with a sample mass of 1.2 g, and using a powder probe unit (four-probe ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm), with the applied voltage limiter set at 90 V, the volume resistivity [Ω·cm] of the conductive powder under various pressures was measured. At a density of 1 g / cm 3 The values of the volume resistivity of CNTs were evaluated.

[0084] <Measurement of the mass magnetization of particles containing metal atoms in CNTs> Using a high-sensitivity vibrating sample magnetometer (TM-VSM311483-HGC type), 0.1 g of mass was placed in a powder container, and the magnetization (emu / g) per gram of CNT was measured under the measurement conditions of a measurement magnetic field of 30 kOe, a measurement time of 30 minutes, a sample vibration amplitude of 1.0 mp-p, a lock-in amplifier sensitivity of 0.2 mV, a lock-in amplifier time constant of 100 msec, and a full-loop measurement. Also, the ash content (%) of CNTs measured in accordance with JIS K 6218-2 was taken as the amount of particles containing metal atoms in CNTs, and the mass magnetization of the particles containing metal atoms was calculated from the following formula 1. (Formula 1) (Mass magnetization of particles) = (Mass magnetization of CNTs) ÷ (Ash content) × 100 (emu / g) Criteria for the mass magnetization of particles ◎: 50 emu / g or less ○: 51 emu / g or more and 100 emu / g or less ×: 101 emu / g or more

[0085] <cnt> In the examples described below, the following CNTs were used. CNT(A): Multi-walled carbon nanotube (JEIO, JENOTUBE10B) CNT(B): Multi-walled carbon nanotubes (LG Chem, BT1001) CNT(C): Multi-walled carbon nanotubes (JEIO, JENOTUBE6A) CNT(D): Multi-walled carbon nanotubes (Kumho Petrochemical, 100T)

[0086] (Example 1-1) 10 g of CNTs (A) were weighed using an electronic balance (Sartorius, MSA225S100DI) and placed in an alumina crucible SSA-HB4 (Nikkato), which was then placed in a muffle furnace (Yamato Scientific, FO510). The temperature was increased to 330°C at a rate of 60°C / min in an air atmosphere, and calcined at 330°C for 18 hours to obtain calcined CNTs (A-1).

[0087] (Examples 1-2 to 9, 11, and 12, and Comparative Examples 1-1 and 1-2) Sintered CNTs (A-2 to 9, B-1, C-1, A-11, 12) were obtained in the same manner as in Example 1-1, except that the type of CNT, the sintering temperature, and the sintering time shown in Table 1 were changed.

[0088] (Examples 1-10) 10 g of CNT (A) was weighed into a 1 L glass container, and 500 g of 20% hydrochloric acid (Fujifilm Wako Pure Chemical Industries, Ltd.) was added, followed by thorough stirring using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and subjected to vacuum filtration using a membrane filter. After repeating the dilution and filtration process, the CNT was transferred to a PTFE tray. After drying at 140°C using an oven, fired CNT (A-10) was obtained using the same method as in Example 1-1.

[0089] (Comparative Examples 1-3, 8, 9, and 11) CNTs (A), (B), (C), and (D) were designated A-0, B-0, C-0, and D-0, respectively.

[0090] (Comparative Examples 1-4) 10 g of CNT (A) was weighed into a 1 L glass container, and 500 g of 20% hydrochloric acid (Fujifilm Wako Pure Chemical Industries, Ltd.) was added, followed by thorough stirring using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and subjected to vacuum filtration using a membrane filter. After repeated dilution and filtration, the CNT was transferred to a PTFE tray and dried in an oven at 140 °C to obtain CNT (A-13).

[0091] (Comparative Examples 1-5) CNTs (A-14) were obtained in the same manner as in (Comparative Example 1-4), except that 20% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was changed to 60% nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0092] (Comparative Examples 1-6) 1000 g of CNT (A) was weighed into a 7 L carbon heat-resistant container, and the heat-resistant container containing the CNT was placed in a furnace. Nitrogen gas was then introduced into the furnace, and the air inside the furnace was evacuated while maintaining positive pressure. After the oxygen concentration inside the furnace reached 0.1% or less, the furnace was heated to 3000°C over 30 hours and then held at 3000°C for 1 hour. Heating inside the furnace was then stopped, and the sample was cooled to obtain CNT (A-15).

[0093] (Comparative Examples 1-7, 10, and 12) CNT(A), CNT(C), and CNT(D) are the same as those in JP 2019-210173 A

[0094] CNTs (A-16), (C-2), and (D-1) were obtained by the method described in .

[0094] Table 1 shows the CNTs used in Examples 1-1 to 1-12 and Comparative Examples 1-1 to 1-10, and the evaluation results thereof.

[0095] [Table 1]

[0096] <Dispersant> In the examples described below, the following dispersants were used. PVP: Polyvinylpyrrolidone (Nippon Shokubai Co., Ltd., K-30, weight average molecular weight 40,000) PVB: Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd., BL-10, calculated molecular weight 15,000) H-NBR: Hydrogenated nitrile butadiene rubber (dispersant 6 described in paragraph

[0194] of Japanese Patent No. 6933285)

[0097] Example 2-1 200 g of CNT (A-1) was placed in a heat-resistant Teflon container and dried in an electric oven at 140°C ±5°C for 24 hours. The mixture was then cooled in an open dry chamber (manufactured by Daikin Industries, Ltd., HRG-50A) to produce dried CNT (A-1) with a moisture content of 300 ppm or less. Following the composition shown in Table 3, 96.4 parts by mass of NMP (manufactured by Kishida Chemical Co., Ltd., for LBG, moisture content 100 ppm or less) and 0.6 parts by mass of dispersant (PVP) were added to a stainless steel container placed in the open dry chamber and stirred until uniform. Next, 1.5 parts by mass of dried CNT (A-1) was added while stirring with a disperser. A high-shear mixer (L5M-A, manufactured by Silverson) equipped with a square-hole high-shear screen was used. The mixture was batch-dispersed at 8,600 rpm until the mixture was uniform and the dispersion particle size measured by a grind gauge was 250 μm or less. Next, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer via piping, and a circulation-type dispersion process was performed. The dispersion process was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After the viscosity of the dispersion liquid measured at 60 rpm using a B-type viscometer (TOKI SANGYO, VISCOMETER, MODEL: BL) reached 3,000 mPa·s or less, 0.5 parts by mass of dried CNT (A-1) was added to the stainless steel container while stirring with a disperser, and the circulation-type dispersion process was performed again using the high-pressure homogenizer. After circulation-type dispersion using the high-pressure homogenizer until the viscosity reached 3,000 mPa·s or less, 0.5 parts by mass of dried CNT (A-1) was added to the stainless steel container while stirring with a disperser. This process was repeated three times (a total of 3.0 parts by mass of dried CNT (A-1) was added). Subsequently, the mixture was subjected to 15 passes of dispersion treatment using a high-pressure homogenizer, and then passed through a mag filter with a surface magnetic flux density of 17,000 gauss and a nylon mesh with 20 μm openings to obtain a CNT dispersion (A-1-1) containing 3.0 parts by mass of CNT (A-1).

[0098] (Examples 2-2 to 2-17), (Comparative Examples 2-1 to 2-12) Except for changing the type and addition amount of CNTs, the type and addition amount of dispersants, and the addition amount of NMP described in Table 2-1, CNT dispersions (A-0) to (D-1) were obtained in the same manner as in Example 1-1. CNTs (A-0) to (D-1) were prepared by drying CNTs (A-0) to (D-1) in the same manner as in Example 2-1.

[0099] The compositions and evaluation results of the CNT dispersions prepared in Examples 2-1 to 2-17 and Comparative Examples 2-1 to 2-12 are shown in Table 2-2.

[0100]

Table 2-1

[0101]

Table 2-2

[0102] The evaluation of the CNT dispersions was carried out by the following method.

[0103] <Measurement of the content of cobalt and iron atoms in the CNT dispersion> The CNT dispersion was placed in a heat-resistant container made of Teflon (registered trademark) and dried in a hot air oven at 140 ± 5 °C to prepare a solid sample. The solid content (mass%) of the CNT dispersion was calculated from the yield of the solid sample. The obtained solid sample was acid-digested using a microwave sample pretreatment device (manufactured by Milestone General, ETHOS1) to extract the metals contained in the CNT dispersion composition. Then, analysis was performed using a multi-type ICP emission spectroscopic analyzer (manufactured by Agilent, 720-ES) to calculate the content of cobalt and iron atoms in the solid sample. Furthermore, the content of cobalt and iron atoms in the CNT dispersion was calculated from the solid content of the CNT dispersion.

[0104] <Measurement of the initial viscosity of the CNT dispersion> After leaving the CNT dispersion in a thermostat at 25°C for 1 hour, the CNT dispersion was thoroughly stirred and then immediately set in a B-type viscometer. The viscosity was measured at a rotor rotation speed of 60 rpm 1 minute after the start of rotation. The rotor used for the measurement was No. 1 when the viscosity value was less than 100 mPa·s, No. 2 when it was 100 or more and less than 500 mPa·s, No. 3 when it was 500 or more and less than 2000 mPa·s, and No. 4 when it was 2000 or more and less than 10000 mPa·s, respectively. Initial Viscosity Judgment Criteria ◎: 100 mPa·s or more and 500 mPa·s or less (excellent) 〇: Exceeding 500 mPa·s and less than 2000 mPa·s (good) ×: Less than 100 mPa·s or 2000 mPa·s or more (poor)

[0105] <Evaluation of Thixotropy (TI value) of CNT Dispersion> After leaving the CNT dispersion in a thermostat at 25°C for 24 hours, the CNT dispersion was thoroughly stirred and then immediately set in a B-type viscometer. The viscosity was measured at a rotor rotation speed of 6 rpm 1 minute after the start of rotation. Further, the rotor rotation speed was changed to 60 rpm and the viscosity 1 minute after the start of rotation was measured. The viscosity at a rotor rotation speed of 6 rpm was designated as V6 (mPa·s), and the viscosity at a rotor rotation speed of 60 rpm was designated as V 60 (mPa·s), and the TI value was calculated as V6 / V 60 as such. TI Value Judgment Criteria ◎: 2 or more and less than 4 (excellent) 〇: 4 or more and less than 7 (good) ×: Less than 2 or exceeding 7 (not acceptable)

[0106] <Measurement of Complex Elastic Modulus and Phase Angle of CNT Dispersion> The complex elastic modulus and phase angle of the CNT dispersion were evaluated by performing dynamic viscoelasticity measurements at 25 °C and a frequency of 1 Hz in the range of strain rates from 0.01% to 5% using a rheometer (RheoStress 1 rotational rheometer manufactured by Thermo Fisher Scientific Co., Ltd.) with a cone having a diameter of 60 mm and an angle of 2°.

[0107] <Measurement of Particle Size Distribution of CNT Dispersion> After leaving the CNT dispersion in a thermostatic bath at 25 °C for 1 hour or more, the CNT dispersion was sufficiently stirred and diluted, and then the cumulative particle diameters D50 and D90 of the CNT dispersion were measured using a particle size distribution analyzer (Nanotrac UPA, model UPA - EX manufactured by Microtrac Bell Corporation). The refractive index of the CNT particles was 1.8 and the shape was non - spherical. The refractive index of the solvent was 1.47. During the measurement, the concentration of the CNT dispersion was diluted so that the value of the loading index was in the range of 0.8 to 1.2. Criteria for Particle Size ◎: D50 value is 100 μm or more and 300 μm or less (excellent) 〇: D50 value exceeds 300 μm and is 600 μm or less (good) ×: D50 value is less than 100 μm or exceeds 600 μm (poor)

[0108] [[ID=1​​​​​​​(Examples 3-2 to 3-17), (Comparative Examples 3-1 to 3-12) CNT resin compositions (A-2) to (D-1) and composite slurries (A-2) to (D-1) were obtained in the same manner as in Example 3-1, except that CNT dispersions (A-2) to (D-1) shown in Table 3 were used instead of the CNT dispersion (A-1-1).

[0110] [Table 3]

[0111] (Example 4-1) The composite slurry (A-1-1) 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 aluminum foil so that the coating was as follows: the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes to obtain an electrode film (A-1-1). The electrode film (A-1-1) was then rolled using a roll press (Thank Metal, 3t hydraulic roll press) to obtain a positive electrode (A). The weight per unit area of ​​the composite layer was 20 mg / cm. 2 The density of the composite layer after the rolling treatment was 3.1 g / cc.

[0112] (Examples 4-2 to 4-17), (Comparative Examples 4-1 to 4-12) Electrode membranes (A-2) to (D-2) and positive electrodes (A-2) to (D-1) were produced in the same manner as in Example 4-1, except that composite slurries (A-2) to (D-2) shown in Table 4 were used instead of composite slurry (A-1-1).

[0113] The electrode films obtained in the above examples were evaluated by the following methods.

[0114] <Volume resistivity of electrode film> The composite slurry was applied to the electrode using an applicator so that the weight per unit area was 20 mg / cm 2 After coating the aluminum foil so that the coating was uniform, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. The surface resistivity (Ω / □) of the dried coating was then measured using a Mitsubishi Chemical Analytech Loresta GP MCP-T610. After measurement, this value was multiplied by the thickness of the electrode composite layer formed on the aluminum foil to obtain the volume resistivity (Ω·cm) of the electrode film. The thickness of the electrode composite layer was measured at three points on the electrode film using a film thickness meter (NIKON DIGIMICRO MH-15M) and the average value was calculated by subtracting the film thickness of the aluminum foil from the average value to obtain the volume resistivity (Ω·cm) of the electrode film. Volume resistivity criteria ◎: 6Ω cm or less (excellent) ○: Over 6Ω·cm and 10Ω·cm or less (Good) ×: Over 10 Ω·cm (unacceptable)

[0115] <Peel strength of electrode film> The composite slurry was applied to the electrode using an applicator so that the weight per unit area was 20 mg / cm 2 After coating the aluminum foil so that the coating was uniform, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. Two 90mm x 20mm rectangles were then cut, with the coating direction as the long axis. Peel strength was measured using a tabletop tensile tester (Strograph E3, manufactured by Toyo Seiki Seisaku-sho, Ltd.) using a 180° peel test method. Specifically, a 100mm x 30mm double-sided tape (No. 5000NS, manufactured by Nitoms Inc.) was attached to a stainless steel plate, and the prepared battery electrode composite layer was attached to the other side of the double-sided tape. The aluminum foil was pulled from bottom to top at a constant speed (50mm / min) while being peeled off. The average stress value during this process was recorded as the peel strength. Peel strength criteria ◎: 0.7N / cm or more (excellent) ○: 0.5N / cm or more and less than 0.7N / cm (good) ×: Less than 0.5N / cm (unacceptable)

[0116] Table 4 shows the evaluation results of the electrode films obtained in Examples 4-1 to 4-17 and Comparative Examples 4-1 to 4-12.

[0117] [Table 4]

[0118] <Preparation of standard negative electrode> A 150 ml plastic container was charged with 0.5 parts by weight of acetylene black (Denka Black® HS-100, manufactured by Denka), 1 part by weight of MAC500LC (carboxymethylcellulose sodium salt, Sunrose Special Type MAC500L, manufactured by Nippon Paper Industries Co., Ltd., 100% nonvolatile content), and 98.4 parts by weight of water, and then stirred at 2000 rpm for 30 seconds using a centrifugal mixer (Thinky Awatori Rentaro, ARE-310). Further added were 92 parts by weight of artificial graphite (Nippon Graphite Industries, CGB-20) and 5 parts by weight of silicon (Osaka Titanium Technology, SILICON MONOOXIDE SiO 1.3C 5 μm, 100% nonvolatile content) as active materials, and the mixture was stirred at 3000 rpm for 10 minutes using a high-speed mixer. Next, 3.1 parts by mass of SBR (TRD2001, manufactured by JSR Corporation) was added, and the mixture was stirred for 30 seconds at 2000 rpm using the planetary centrifugal mixer to obtain a standard negative electrode composite slurry. The standard negative electrode composite slurry was then applied to an applicator to form a standard negative electrode composite slurry having a weight per unit area of ​​8 mg / cm. 2 After coating on copper foil so that the density of the composite layer became 1.6 g / cm, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. Then, the coating was rolled using a roll press (Thank Metal Co., Ltd., 3 ton hydraulic roll press) until the density of the composite layer became 1.6 g / cm. 3 A standard negative electrode was fabricated.

[0119] (Example 5-1) The positive electrode (A-1-1) and standard negative electrode were punched out to 45 mm x 40 mm and 50 mm x 45 mm, respectively. The separator (porous polypropylene film) between them was inserted into an aluminum laminate bag and dried in an electric oven at 60 °C for 1 hour. Then, in an argon-filled glove box, 2 mL of electrolyte (a nonaqueous electrolyte prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio) mixture, adding 2 parts by weight of VC (vinylene carbonate) per 100 parts by weight of the mixed solvent, and then dissolving LiPF6 at a concentration of 1 M) was poured into the bag. The aluminum laminate was then sealed to prepare a laminate-type lithium-ion secondary battery (A-1-1).

[0120] (Examples 5-2 to 5-17), (Comparative Examples 5-1 to 5-12) Laminated lithium ion secondary batteries (A-0) to (D-1) were produced in the same manner as in Example 5-1, except that the positive electrodes shown in Table 5 were used instead of the positive electrode (A-1-1).

[0121] The laminated lithium ion secondary batteries produced in the Examples and Comparative Examples were evaluated by the following methods.

[0122] <Evaluation of rate characteristics of lithium-ion secondary batteries> The laminated lithium-ion secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge / discharge measurements using a charge / discharge device (Hokuto Denko Corporation, SM-8). The battery was charged at a constant current / constant voltage of 10 mA (0.2 C) with a charge cutoff voltage of 4.2 V (cutoff current of 1.0 mA (0.02 C)), followed by a constant current discharge at a discharge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.2 V (cutoff current of 1.0 mA (0.02 C)). This procedure was repeated three times, followed by a constant current / constant voltage charge at a charge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.2 V (cutoff current of 1.0 mA (0.02 C)). The battery was then discharged at both 0.2 C and 3 C until the discharge cutoff voltage of 2.5 V was reached, and the discharge capacity was calculated. The rate characteristics were calculated using the following equation (2), where the ratio of the discharge capacity at 0.2 C to the discharge capacity at 3 C was defined as the capacity retention ratio. (Formula 2) Capacity retention rate = 3C discharge capacity / 3rd 0.2C discharge capacity × 100 (%) Rate characteristic criteria ◎: Capacity retention rate 80% or more (excellent) ○: Capacity retention rate 70% or more but less than 80% (good) △: Capacity retention rate is 60% or more but less than 70% (acceptable) ×: Capacity retention rate less than 60% (unacceptable)

[0123] <Evaluation of cycle characteristics of lithium-ion secondary batteries> The laminated lithium-ion secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge / discharge measurements using a charge / discharge device (Hokuto Denko Corporation, SM-8). A constant-current / constant-voltage charge (cutoff current: 1.25 mA (0.025 C)) was performed at a charge current of 50 mA (1 C) with a charge cutoff voltage of 4.2 V, followed by a constant-current discharge at a discharge current of 50 mA (1 C) with a discharge cutoff voltage of 2.5 V. This procedure was repeated 200 times. 1 C was defined as the current value required to discharge the theoretical capacity of the positive electrode in 1 hour. The cycle characteristics were calculated using the following equation (3), where the ratio of the discharge capacity at 1 C for the third charge at 25°C to the discharge capacity at 1 C for the 200th charge was used as the capacity retention ratio. (Equation 3) Capacity retention rate = 3rd 1C discharge capacity / 200th 1C discharge capacity × 100 (%) Cycle characteristics criteria ◎: Capacity retention rate of 90% or more (excellent) ○: Capacity retention rate 85% or more but less than 90% (good) △: Capacity retention rate 80% or more but less than 85% (acceptable)

[0124] <Short circuit evaluation of lithium-ion secondary batteries> The laminated lithium-ion secondary battery was placed in a 60°C thermostatic chamber and charged at a constant current of 25 mA (0.5 C) to a cut-off voltage of 4.4 V using a charge / discharge device (Hokuto Denko Corporation, SM-8). It was then charged at a constant voltage of 4.4 V for 48 hours. It was then transferred to a 25°C thermostatic chamber and discharged at a constant current of 25 mA (0.5 C) to a cut-off voltage of 3.0 V. It was then charged at a constant current / constant voltage of 25 mA (0.5 C) to a cut-off voltage of 3.85 V (cut-off current: 1.25 mA (0.025 C)). The battery was then left in the 25°C thermostatic chamber for 80 hours, and the voltage drop (V) from 3.85 V was measured. After the test, the lithium-ion secondary battery was disassembled and the separator and negative electrode surfaces were examined. Short-circuit evaluation criteria ◎: Voltage drop less than 0.012V (excellent) 〇: Voltage drop 0.012V or more but less than 0.014V (good) ×: Voltage drop of 0.014V or more (not permitted)

[0125] The evaluation results of the laminated lithium ion secondary batteries produced in Examples 5-1 to 5-17 and Comparative Examples 5-1 to 5-12 are shown in Table 5. Furthermore, when the secondary batteries of Comparative Examples 5-1, 5-3, 5-8, 5-9, and 5-11 were disassembled after the short circuit evaluation, metal deposition was observed on the negative electrode and separator.

[0126] [Table 5] < / cnt>

Claims

1. Carbon nanotubes that satisfy the following (1) to (4): (1) The cobalt atom content is 1,000 ppm or more and 12,500 ppm or less, and the iron atom content is 3,500 ppm or less. (2) The composition contains particles containing cobalt atoms and / or iron atoms, and the mass magnetization of the particles is 100 emu / g or less. (3) 1,560 cm in the Raman spectrum -1 ~1,600cm -1 The maximum peak intensity at G is 1,310 cm -1 ~1,350cm -1 When the maximum peak intensity at (4) The ash content measured according to JIS K 6218-2 is 1.9% or less.

2. Volume resistivity is 1.0 x 10 -2 Above 2.0 x 10 -2 2. The carbon nanotube according to claim 1, wherein the viscosity is Ω·cm or less.

3. BET specific surface area is 150m 2 / g or more 800m 2 The carbon nanotube according to claim 1 or 2, wherein the molecular weight is 1 / g or less.

4. 4. The carbon nanotubes according to claim 1, wherein the average outer diameter is 5 nm or more and 15 nm or less.

5. 5. The carbon nanotube according to claim 1, wherein the cobalt atoms include components derived from catalyst particles for synthesizing the carbon nanotube.

6. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of claims 1 to 5, a solvent, and a dispersant.

7. 7. The carbon nanotube dispersion according to claim 6, wherein the solvent is an amide organic solvent or water.

8. A carbon nanotube resin composition comprising the carbon nanotube dispersion liquid according to claim 6 or 7, and further comprising a binder.

9. A composite slurry comprising the carbon nanotube resin composition according to claim 8 and an active material.

10. An electrode film comprising the carbon nanotube according to any one of claims 1 to 5.

11. A secondary battery comprising the electrode film according to claim 10.

12. A vehicle comprising the secondary battery according to claim 11.

13. A method for producing carbon nanotubes as described in any one of claims 1 to 5, wherein the carbon nanotubes contain particles containing cobalt atoms and / or iron atoms, and the carbon nanotubes are heated in an oxygen-containing atmosphere at a temperature of 200°C or higher and 500°C or lower to oxidize the particles containing cobalt atoms and / or iron atoms.

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