JP7683196B2 - Carbon nanotube dispersion, and resin composition, conductive film, composite material slurry, electrode, and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

The present invention relates to a boron-containing carbon nanotube dispersion. More specifically, the present invention relates to a resin composition containing the carbon nanotube dispersion and a resin, a composite slurry containing the carbon nanotube dispersion, a resin, and an active material, a conductive film and an electrode film formed by forming them into a film shape, and a nonaqueous electrolyte secondary battery comprising an electrode film and an electrolyte.

In recent years, electronics has developed remarkably, and conductive materials used in various electronic devices and batteries are now required to be smaller, lighter, less expensive, and have a longer life under various usage environments. To date, various conductive carbon materials with low volume resistivity, such as graphite and carbon nanotubes, have been considered as conductive carbon materials. In addition, research has been reported on carbon nanotubes, in which carbon nanotubes and boron compounds are subjected to high-temperature heat treatment at up to 3000° C. to dope the carbon nanotubes with boron and improve their conductivity (Patent Document 1). By doping carbon nanotubes with 3% or more of boron, conductivity at a volume resistivity level of 10 ⁻² Ω·cm has been obtained. However, further improvement of the conductivity of conductive materials is a challenge.
On the other hand, when conductive materials are used in various electronic devices and batteries, a dispersion liquid in which the conductive material is dispersed in a solvent or the like is often used, but the biggest problem is that the dispersion can cause a decrease in properties such as a decrease in conductivity, etc. Therefore, it is very important to prepare a dispersion liquid that can obtain the desired properties.

For example, in the case of lithium-ion secondary batteries, carbon materials such as graphite, which have a large charge/discharge capacity per unit mass at a base potential close to that of lithium, are used as negative electrode materials in lithium-ion secondary batteries. However, these electrode materials are used to the point where the charge/discharge capacity per mass is close to the theoretical value, and the energy density per mass of the battery is approaching its limit. Therefore, in order to increase the utilization rate of the electrode, attempts are being made to reduce the conductive additives and binders that do not contribute to the discharge capacity.

Carbon black, Ketjen black, graphene, fine carbon materials, etc. are used as conductive additives, and carbon nanotubes, a type of fine carbon fiber, are particularly widely used. For example, adding carbon nanotubes to negative electrode active materials such as graphite and silicon reduces the resistance of the electrode, improves the internal resistance of the battery, increases the strength of the electrode, and increases the expansion and contraction properties of the electrode, thereby improving the cycle life of lithium-ion secondary batteries (Patent Documents 2 to 4). Also, studies are being conducted on reducing electrode resistance by adding carbon nanotubes to the positive electrode (Patent Documents 5 and 6).

When carbon nanotubes with a small average outer diameter are used, a conductive network can be efficiently formed with a small amount, and the amount of conductive auxiliary material contained in the positive and negative electrodes of lithium-ion secondary batteries can be reduced. It is also known that the same effect can be obtained when carbon nanotubes with a long fiber length are used (Patent Document 7). However, carbon nanotubes with these characteristics have strong cohesive forces and are difficult to disperse, so it has not been possible to obtain a carbon nanotube dispersion liquid with sufficient dispersibility.

Therefore, various proposals have been made. For example, a method of dispersing and stabilizing carbon nanotubes using a dispersant has been disclosed, and a dispersion liquid using a polymer-based dispersant such as a water-soluble polymer polyvinylpyrrolidone in water and NMP (N-methyl-2-pyrrolidone) has been disclosed (Patent Documents 5, 6, and 8). However, in Patent Document 5, an electrode is produced using carbon nanotubes with an outer diameter of 10 to 150 nm, but the resistance of the electrode is high, which is a problem. In addition, Patent Document 6 discloses a dispersion liquid using carbon nanotubes with a small DBP oil absorption, but although the dispersibility is improved, the improvement in conductivity is insufficient. Patent Document 8 discloses a dispersion liquid using single-walled carbon nanotubes, but it is difficult to disperse carbon nanotubes at a high concentration in a solvent. Patent Document 9 discloses a dispersion liquid using double-walled carbon nanotubes, but since the carbon nanotubes are oxidized and dispersed using an ultrasonic homogenizer, not only does the conductivity decrease, but the carbon nanotubes are also insufficiently dispersed at a high concentration in a solvent. Patent Document 10 discloses a carbon nanotube dispersion in which carbon nanotubes with an outer diameter of 150 nm are dispersed to a fiber length of approximately 2 to 7 μm using a ball mill-type disperser in order to produce carbon nanotubes suitable as a conductive assistant in lithium-ion secondary batteries; however, it was necessary to use a large amount of carbon nanotubes to obtain sufficient conductivity.
Therefore, obtaining a carbon nanotube dispersion liquid with good electrical conductivity and in which the carbon nanotubes are uniformly dispersed at a high concentration has been an important challenge for expanding the range of applications.

JP 2009-256118 A Japanese Unexamined Patent Publication No. 4-155776 Japanese Patent Application Publication No. 4-237971 JP 2004-178922 A JP 2011-70908 A JP 2014-19619 A JP 2012-221672 A JP 2005-162877 A JP 2010-254546 A JP 2014-182892 A

The object of the present invention is to provide a carbon nanotube dispersion, a carbon nanotube resin composition, and a composite slurry having good dispersibility in order to obtain a conductive film or an electrode film having high conductivity and adhesion. Furthermore, the object of the present invention is to provide a nonaqueous electrolyte secondary battery having excellent rate characteristics and cycle characteristics.

The present invention relates to a carbon nanotube dispersion liquid containing boron-containing carbon nanotubes and a solvent, characterized in that the complex modulus of the carbon nanotube dispersion liquid measured by dynamic viscoelasticity measurement is less than 200 Pa, and the phase angle at a frequency of 1 Hz is 10° or more.

The present invention also relates to the carbon nanotube dispersion liquid, characterized in that the boron content of the boron-containing carbon nanotubes is 0.01 to 3 mol%.

The present invention also relates to the carbon nanotube dispersion liquid, which is characterized in that the content of boron element doped to substitute for carbon element on the surface of the boron-containing carbon nanotubes is 0.01 to 1 mol %.

The present invention also relates to the carbon nanotube dispersion liquid, which further contains a dispersant.

The present invention also relates to a carbon nanotube resin composition comprising the carbon nanotube dispersion and a binder.

The present invention also relates to a conductive film that is a coating film of the carbon nanotube dispersion liquid or the carbon nanotube resin composition.

The present invention also relates to a composite slurry that contains the carbon nanotube resin composition and an active material.

The present invention also relates to an electrode film that is a coating film of the composite slurry.

The present invention also relates to a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein at least one of the positive electrode and the negative electrode contains the electrode film.

By using the carbon nanotube dispersion of the present invention, a conductive film with excellent conductivity, a resin composition with excellent conductivity and adhesion, a composite slurry, and an electrode can be obtained. In addition, a non-aqueous electrolyte secondary battery with excellent rate characteristics and cycle characteristics can be obtained. Therefore, the carbon nanotube dispersion of the present invention can be used in various fields of application where high conductivity, adhesion, and durability are required.

FIG. 1 is a diagram showing an example of peaks in a B1s spectrum in XPS and peak separation.

The carbon nanotube dispersion, resin composition, conductive film, composite slurry, electrode, and nonaqueous electrolyte secondary battery of the present invention will be described in detail below. In this specification, boron-containing carbon nanotubes may be referred to as carbon nanotubes or CNTs.

<Boron-containing carbon nanotubes>
First, the shape of the carbon nanotube of the present invention will be described. The carbon nanotube has a shape in which planar graphite is rolled into a cylindrical shape. A single-walled carbon nanotube has a structure in which one layer of graphite is rolled. A multi-walled carbon nanotube has a structure in which two or more layers of graphite are rolled. The sidewall of the carbon nanotube does not have to have a graphite structure. For example, a carbon nanotube having a sidewall with an amorphous structure can also be used as the carbon nanotube. Furthermore, these carbon nanotubes may be a mixture of carbon nanotubes having a plurality of total numbers.

The shape of the carbon nanotubes of the present invention is not limited. Examples of such shapes include various shapes including needle-like, cylindrical tube-like, fishbone-like (fishbone or cup stacked type), and coil-like. In addition, the carbon nanotubes may be secondary aggregates in the form of plates or platelets obtained by dry processing of cylindrical tube-like carbon nanotubes. The shape of the carbon nanotubes in the present invention is preferably needle-like or cylindrical tube-like. The carbon nanotubes may be of a single shape or a combination of two or more shapes.

The carbon nanotubes of the present invention may have, for example, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. The carbon nanotubes may have any one of these forms or a combination of two or more of these forms.

Next, the boron-containing form of the boron-containing carbon nanotube of the present invention will be described. The boron content in the boron-containing carbon nanotube (the boron content in the entire material) is preferably 0.005 to 10 mol%, more preferably 0.01 to 3 mol%, and even more preferably 0.1 to 1 mol%. When the boron content is 0.005 mol% or more, the boron doping effect is easily obtained and the conductivity is improved. When the boron content is less than 10 mol%, the conductivity is less likely to decrease due to the inhibition of electron transfer. Furthermore, the inclusion of boron increases the affinity with organic solvents, dispersants, resins, etc., and improves the dispersibility of the dispersion liquid. On the other hand, a high boron content tends to increase the hardness of the carbon nanotubes, improving durability against external forces such as impacts and coating strength, and enabling the conductive assistant to maintain contact during the expansion and contraction of the electrode active material associated with charging and discharging the secondary battery. However, if the content exceeds 3 mol%, dispersion tends to become difficult, and from the standpoint of the strength and dispersibility of the carbon nanotubes, the boron content is preferably 3 mol% or less, and more preferably 1 mol% or less.
The boron content in the boron-containing carbon nanotubes can be determined by methods such as ICP emission spectrometry and ICP mass spectrometry. One example is a measurement method in accordance with JIS-R7223. The boron content in the boron-containing carbon nanotubes refers to the amount of boron alone contained on the surface or inside of the carbon material, boron doped to replace carbon elements in the carbon material, or the total amount of boron in the boron compound.

Next, we will explain the boron that is doped to replace the carbon elements on the surface of the boron-containing carbon nanotube of the present invention. The content of boron that is doped to replace the carbon elements on the surface of the boron-containing carbon nanotube can be determined by a method such as X-ray photoelectron spectroscopy (XPS).

The B1s spectrum of boron obtained by XPS measurement appears in the binding energy range of the B1s electron of boron element (around 185 to 197 eV), and is known to be roughly divided into four components. The binding energy values (peak tops) of each component are 186 to 187 eV for boron clusters, 187 to 188 eV for boron carbide, 188 to 189.3 eV for boron doped to replace carbon elements with a hexagonal mesh surface as a basic skeleton (BC ₃ ), 189.5 to 190.5 eV for various boron oxides BC ₂ O, 191.5 to 192 eV for BCO ₂ , and 192.5 to 193 eV for B ₂ O _3. When these peaks overlap, the ratio can be determined by fitting each component as a Gaussian function, optimizing the peak intensity, peak position, and full width at half maximum as parameters, and separating the peaks. Therefore, the state of boron on the surface of the boron-containing carbon nanotube can be analyzed by performing peak separation of B1s.
Therefore, the bond energy value of boron doped to substitute for carbon elements on the surface of the boron-containing carbon nanotube of the present invention appears in the range of 188 to 189.3 eV.

The boron-containing carbon nanotubes of the present invention are carbon materials whose basic skeleton is a hexagonal carbon mesh surface, and this can be confirmed by Raman spectroscopy and X-ray diffraction measurement.

In Raman spectroscopy, for example, by confirming the G band (1560-1620 cm ^-1 ) using an excitation laser wavelength of 532 nm, it can be confirmed that the carbon material has a hexagonal carbon mesh plane as its basic skeleton. Furthermore, when the intensity ratio (G/D ratio) to the D band (1310-1370 cm ^-1 ), which indicates defects in the carbon material, is high, the boron-doped carbon material has a high degree of crystallinity and high electrical conductivity. The G/D ratio is preferably 0.6 or more, and more preferably 0.8 or more.

In X-ray diffraction (XRD) measurements, the XRD pattern of boron-containing carbon nanotubes obtained using CuKα radiation as an X-ray source shows a (002) plane diffraction peak at a diffraction angle (2θ) of approximately 24.0 to 27.0°, confirming that the material is a carbon material with a hexagonal carbon mesh plane as its basic skeleton.

The average outer diameter of the boron-containing carbon nanotubes of the present invention is preferably 1 to 250 nm, more preferably 2 to 50 nm, more preferably 5 to 25 nm, and even more preferably 5 to 15 nm. When the average outer diameter of the carbon nanotubes is within the above range, the surface of the electrode active material is easily covered with the carbon nanotubes, improving the electrical conductivity and adhesion of the electrode film.

The outer diameter and average outer diameter of the boron-containing carbon nanotubes of the present invention are determined as follows. First, the 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.

The boron-containing carbon nanotube of the present invention preferably has 3 to 30 walls, more preferably 3 to 20 walls, and even more preferably 3 to 10 walls.

The BET specific surface area of the boron-containing carbon nanotubes of the present invention is preferably 10 to 1500 ^{m 2} /g, more preferably 150 to 800 ^{m 2} /g, and even more preferably 200 to 700 m ² /g.

The volume resistivity of the boron-containing carbon nanotube of the present invention is preferably 1.0×10 ⁻² Ω·cm or less, more preferably 5.0×10 ⁻³ Ω·cm or less, and even more preferably 3.0×10 ⁻³ Ω·cm or less. The volume resistivity of the carbon nanotube can be measured using a powder resistivity measuring device (Loresta GP-powder resistivity measuring system MCP-PD-51, manufactured by Nitto Seiko Analytech Co., Ltd.).

The carbon purity of the boron-containing carbon nanotubes of the present invention is expressed as the content (%) of carbon atoms in the carbon nanotubes. The carbon purity is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more, relative to 100% by mass of the carbon nanotubes.

The amount of metal contained in the boron-containing carbon nanotubes of the present invention is preferably less than 10% by mass, more preferably less than 5% by mass, and even more preferably less than 2% by mass, based on 100% by mass of the carbon nanotubes. Examples of metals contained in the carbon nanotubes include metals and metal oxides used as catalysts when synthesizing carbon nanotubes. Specific examples include metals such as cobalt, nickel, aluminum, magnesium, silica, manganese, and molybdenum, metal oxides, and composite oxides of these.

The boron-containing carbon nanotubes of the present invention usually exist as secondary particles. The shape of these secondary particles may be, for example, a state in which carbon nanotubes, which are general primary particles, are intricately entangled. They may also be an aggregate of linear carbon nanotubes. Secondary particles that are an aggregate of linear carbon nanotubes are easier to disentangle than entangled ones. Furthermore, linear ones have better dispersibility than entangled ones, so they can be used preferably as carbon nanotubes.

The boron-containing carbon nanotubes of the present invention may be surface-treated carbon nanotubes. The carbon nanotubes may also be carbon nanotube derivatives to which functional groups such as carboxyl groups have been added. Carbon nanotubes that encapsulate organic compounds, metal atoms, or substances such as fullerenes may also be used.

The boron-containing carbon nanotubes of the present invention may be carbon nanotubes produced by any method. Carbon nanotubes can generally be produced by, but are not limited to, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion methods. For example, carbon nanotubes can be produced by contacting a carbon source with a catalyst at 500 to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. The carbon source may be at least one of a hydrocarbon and an alcohol.

The raw material gas that serves as the carbon source for the boron-containing carbon nanotubes of the present invention can be any conventionally known gas. For example, the raw material gas that contains carbon can be, but is not limited to, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohol. From the viewpoint of ease of use, it is preferable to use at least one of a hydrocarbon and an alcohol as the raw material gas.

<Method for producing boron-containing carbon nanotubes>
The method for producing the boron-containing carbon nanotube in the present invention is not particularly limited, but it can be synthesized by heating a carbon source and a boron source. Specifically, it can be synthesized by a method of mixing a carbon nanotube as a carbon source and a boron source such as a boron compound (boron carbide, etc.) and heating the mixture to produce a boron-containing carbon nanotube, a method of heating a carbon source such as an aromatic hydrocarbon gas and a boron source such as boron gas on a metal catalyst supported on a substrate to perform chemical vapor deposition (CVD), a method of heating a carbon source such as a hydrocarbon gas and a boron source such as boron gas on a metal catalyst suspended by spraying or the like to perform catalytic chemical vapor deposition (CCVD), a method of ion-implanting boron into a carbon source, etc.

Next, the boron source used in the production of boron-containing carbon nanotubes will be described. The boron source is not particularly limited, but examples thereof include boron carbide, boron oxide, boron nitride, metal borides, boron oxoacids, boranes, and boron-containing organic compounds.
Specifically, boron carbide includes B ₄ C (B ₁₂ C ₃ ), B ₁₂ C ₂ (B ₆ C), etc.
Boron _oxides include _BC2O , _BCO2 , _{B2O2 , B2O3} , _{B4O3 , B4O5} , _etc.
For boron nitride, BN, etc.
Metal borides include _AlB2 , CoB, FeB, _MgB2 , NiB, _TiB2 , etc.
Boron oxo acids include orthoboric acid, metaboric acid, tetraboric acid, etc.
Boranes include monoborane, diborane, decaborane, etc.
Examples of boron-containing organic compounds include borate esters such as trimethyl borate and triethyl borate, substituted boranes such as triethylborane and triphenylborane, and boronic acids such as phenylboronic acid and phenylboronic acid esters.

The raw material composition ratio of the carbon source and the boron source for producing boron-containing carbon nanotubes is not particularly limited, but the ratio of the boron source to 100 parts by mass of the carbon source is 0.01 to 300 parts by mass, preferably 0.05 to 100 parts by mass, and more preferably 0.1 to 50 parts by mass.

The method for mixing the carbon source and the boron source is not particularly limited, but is preferably dry mixing or wet mixing. As a mixing device, the following dry mixing device or wet mixing device can be used.

Examples of dry mixing devices include roll mills such as two-roll and three-roll devices, high-speed agitators such as Henschel mixers and super mixers, fluid energy pulverizers such as Micronizers and jet mills, attritors, particle composite devices manufactured by Hosokawa Micron Corporation such as "Nanocure,""Nobilta," and "Mechanofusion," and powder surface modification devices manufactured by Nara Machinery Manufacturing Co., Ltd. such as "Hybridization System,""MechanoMicros," and "Miraro."
In addition, when using a dry mixer, the other raw materials may be added directly in powder form to the base raw material powder, but in order to prepare a more uniform mixture, the other raw materials may be dissolved or dispersed in a small amount of solvent in advance and then added while breaking up the agglomerated particles of the base raw material powder. In order to further increase the efficiency of the process, the raw materials may be heated.

Examples of wet mixing devices include mixers such as Disper, Homomixer, or Planetary Mixer, homogenizers such as M-Technique's "Clearmix" or PRIMIX's "Filmix", Red Devil's paint conditioner, ball mill, sand mills such as Shinmaru Enterprises'"Dynomill", attritor, or Coball Mill, media-type dispersers, wet jet mills such as Genus'"GenusPY", Sugino Machine's "Starburst", Nanomizer's "Nanomizer", media-less dispersers such as M-Technique's "Clear SS-5" or Nara Machinery's "Micros", or other roll mills, kneaders, etc., but are not limited thereto. In addition, it may be preferable to use a wet mixing device that has been treated to prevent metal from entering the device.
For example, when using a media type disperser, it is preferable to use a method using a disperser whose agitator and vessel are made of ceramic or resin, or to use a disperser whose metallic agitator and vessel surface are treated with tungsten carbide spraying, resin coating, or the like. As the media, it is preferable to use ceramic beads such as glass beads, zirconia beads, or alumina beads. Also, when using a roll mill, it is preferable to use a ceramic roll. Only one type of dispersing device may be used, or multiple types of devices may be used in combination.

If the raw materials do not dissolve or disperse uniformly, a dispersant may be added as necessary to improve the wettability and dispersibility of each raw material in the solvent, and then dispersed and mixed.

The conditions for heat-treating the mixture of the carbon source and the boron source vary depending on the types and amounts of the carbon source and the boron source used as raw materials, and are not particularly limited, but the heating temperature is 1000 to 3000° C., preferably 1100 to 2500° C., and more preferably 1200 to 2000° C. In addition, the heating time is not particularly limited, but is 10 minutes to 72 hours, and preferably 30 minutes to 10 hours.
The atmosphere in the heat treatment step is preferably an inert gas atmosphere such as nitrogen or argon, or a vacuum atmosphere, in order to prevent side reactions such as oxidation of the raw materials.
The heat treatment step may be not only a treatment step performed in one stage at a constant atmosphere, temperature and time, but also a treatment step performed in multiple stages with different atmospheres, temperatures and times.

<Solvent>
Next, the solvent used in the carbon nanotube dispersion of the present invention will be described. The solvent is not particularly limited as long as it is a solvent in which carbon nanotubes can be dispersed, but is preferably water or a mixed solvent consisting of one or more organic solvents, and more preferably water or a solvent consisting of one or more water-soluble organic solvents, or a mixed solvent consisting of two or more water-soluble organic solvents. In addition, when a dispersant described later is used, it is preferable that the solvent is one in which the dispersant can be partially or completely dissolved, and although the solvent is not particularly limited, it is preferable that the solvent contains water or a water-soluble organic solvent.

Examples of the organic solvent that can be used include alcohols (e.g., methanol), polyhydric alcohols (e.g., ethylene glycol), polyhydric alcohol ethers (e.g., ethylene glycol monomethyl ether), amines (e.g., ethanolamine), amides (e.g., N-methyl-2-pyrrolidone (NMP)), heterocyclics (e.g., γ-butyrolactone), sulfoxides (e.g., dimethyl sulfoxide), sulfones (e.g., sulfolane), aromatics (e.g., toluene, xylene), hydrocarbons (e.g., hexane), lower ketones (e.g., acetone), and esters (e.g., ethyl acetate).
Preferred water-soluble organic solvents include 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, 1,3-dimethyl-2-imidazolidinone, γ-butyrolactone, etc.), sulfoxides (dimethyl sulfoxide, etc.), sulfones (hexamethylphosphorotriamide, sulfolane, etc.), alcohols (methanol, ethanol, etc.), polyhydric alcohols (ethylene glycol, diethylene glycol, etc.), lower ketones (acetone, methyl ethyl ketone, etc.), and others, such as tetrahydrofuran, urea, and acetonitrile.

<Dispersant>
The dispersant in the present invention is not particularly limited as long as it can disperse and stabilize the carbon nanotubes, and a surfactant or a resin-type dispersant can be used. Surfactants are mainly classified into anionic, cationic, nonionic, and amphoteric. Depending on the characteristics required for dispersing the carbon nanotubes, a suitable type of dispersant can be used in a suitable amount.

When selecting an anionic surfactant, the type is not particularly limited. Specific examples include, but are not limited to, fatty acid salts, polysulfonates, polycarboxylates, alkyl sulfates, alkylaryl sulfonates, alkylnaphthalenesulfonates, dialkylsulfonates, dialkylsulfosuccinates, alkyl phosphates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkylaryl ether sulfates, naphthalenesulfonate formalin condensates, polyoxyethylene alkyl phosphate sulfonates, glycerol borate fatty acid esters, and polyoxyethylene glycerol fatty acid esters. Specific examples include, but are not limited to, sodium dodecylbenzenesulfonate, sodium laurate sulfate, sodium polyoxyethylene lauryl ether sulfate, polyoxyethylene nonylphenyl ether sulfate, and sodium salt of β-naphthalenesulfonate formalin condensates.

Cationic surfactants include alkylamine salts and quaternary ammonium salts. Specific examples include, but are not limited to, stearylamine acetate, trimethyl coconut ammonium chloride, trimethyl tallow ammonium chloride, dimethyldioleyl ammonium chloride, methyl oleyl diethanol chloride, tetramethyl ammonium chloride, lauryl pyridinium chloride, lauryl pyridinium bromide, lauryl pyridinium disulfate, cetyl pyridinium bromide, 4-alkyl mercaptopyridine, poly(vinylpyridine)-dodecyl bromide, and dodecylbenzyl triethyl ammonium chloride.

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. Specific examples include, but are not limited to, polyoxyethylene lauryl ethers, sorbitan fatty acid esters, and polyoxyethylene octylphenyl ethers. Amphoteric surfactants include, but are not limited to, amino carboxylates.

The surfactant may be used alone or in combination of two or more kinds. For example, a combination of an anionic surfactant and a nonionic surfactant, or a combination of a cationic surfactant and a nonionic surfactant may be used. In this case, it is preferable to use an appropriate amount for each surfactant component. A combination of an anionic surfactant and a nonionic surfactant is preferable. The anionic surfactant is preferably a polycarboxylate. The nonionic surfactant is preferably polyoxyethylene phenyl ether.

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, polyvinyl butyral, polyvinyl pyrrolidone, polyacrylonitrile polymers, various rubbers, etc. In particular, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyacrylonitrile polymers, and various rubbers (e.g. partially hydrogenated nitrile rubber) are preferred.

The resin-type dispersant is more preferably made of a polymer containing one or more structural units selected from the group consisting of a nitrile group-containing structural unit, a carboxyl group-containing structural unit, a hydroxyl group-containing structural unit, and a heterocyclic ring-containing structural unit, which enhances the adsorption to boron-containing carbon nanotubes and the affinity to the medium. The polymer preferably contains an alkylene structure in the main chain, and by having any of the structures of a nitrile group, a carboxyl group, a hydroxyl group, and a heterocyclic ring, which have strong polarization, the adsorption to carbon nanotubes and the affinity to the medium can be enhanced, and the carbon nanotubes can be stably present in the medium. In addition, the polymer is more preferably made of two or more structural units selected from the group consisting of a nitrile group-containing structural unit, a carboxyl group-containing structural unit, a hydroxyl group-containing structural unit, and a heterocyclic ring-containing structural unit, which further enhances the adsorption to carbon nanotubes and the affinity to the medium.

The nitrile group-containing structural unit is a structural unit containing a nitrile group, and preferably contains a structural unit containing an alkylene structure substituted with a substituent containing a nitrile group. The alkylene structure is preferably a linear or branched alkylene structure. The number of nitrile groups contained in the nitrile group-containing structural unit is preferably one or two, and more preferably one. There are no particular limitations on the method for introducing the nitrile group-containing structural unit into the polymer, but for example, a method of preparing a polymer by a polymerization reaction of a monomer containing a nitrile group can be preferably used.

Examples of monomers containing nitrile groups include acrylonitrile, methacrylonitrile, fumaronitrile, etc., and one type can be used alone or two or more types can be used in combination. In particular, from the viewpoint of increasing the intermolecular force between polymers and/or between the polymer and the dispersed substance (adsorbed substance), ease of availability of raw materials, and reactivity, it is preferable that the monomer containing a nitrile group is acrylonitrile. The content of the nitrile group-containing structural unit is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass). In addition, it is preferably 100% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. It is believed that by setting the content of the nitrile group-containing structural unit within the above range, it is possible to control the adsorption to the dispersed substance and the affinity to the dispersion medium, and the dispersed substance can be stably present in the dispersion medium. It is also possible to control the affinity of the polymer with the electrolyte, which is believed to prevent problems such as the polymer dissolving in the electrolyte inside the battery and increasing the resistance of the electrolyte.

The carboxyl group-containing structural unit is a structural unit containing a carboxyl group, and preferably contains a structural unit containing an alkylene structure substituted with a substituent containing a carboxyl group. The alkylene structure is preferably a linear or branched alkylene structure. The number of carboxyl groups contained in the carboxyl group-containing structural unit is preferably one or two, and more preferably one. The method of introducing the carboxyl group-containing structural unit into the polymer is not particularly limited, but examples include a method of preparing a polymer by polymerization reaction of a monomer containing a carboxyl group, or a method of preparing a polymer by polymerization reaction of a monomer containing a functional group other than a carboxyl group, and modifying it to a carboxyl group.

Examples of monomers containing a carboxyl group include unsaturated fatty acids such as (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, 2-(meth)acryloyloxyethyl phthalate, and 2-(meth)acryloyloxypropyl phthalate. A carboxyl group-containing monomer may also be obtained by hydrolyzing the carbamoyl group of a polymer obtained by polymerization of a monomer containing a carbamoyl group such as (meth)acrylamide. The carboxyl group-containing monomer is preferably an unsaturated fatty acid, more preferably (meth)acrylic acid, and even more preferably acrylic acid.

The content of the carboxyl group-containing structural unit is preferably 60% by mass or more, more preferably 80% by mass or more, and preferably 98% by mass or less, and may be 100% by mass, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass), from the viewpoint of having a moderate affinity with the solvent. In addition, if one or more types selected from the group of nitrile group-containing structural units, hydroxyl group-containing structural units, and heterocyclic ring-containing structural units are further contained, the affinity between the carbon nanotube and the medium is increased, which is more preferable. When one or more types selected from the group of nitrile group-containing structural units, hydroxyl group-containing structural units, and heterocyclic ring-containing structural units are further contained, the content of the carboxyl group-containing structural unit is preferably 3% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass). In addition, from the viewpoint of electrolyte resistance, it is preferably 50% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less.

The hydroxyl group-containing structural unit is a structural unit containing a hydroxyl group, and preferably contains a structural unit containing an alkylene structure substituted with a substituent containing a hydroxyl group. The alkylene structure is preferably a linear or branched alkylene structure. The number of hydroxyl groups contained in the hydroxyl group-containing structural unit is preferably one or two, and more preferably one. The method of introducing the hydroxyl group-containing structural unit into the polymer is not particularly limited, but examples include a method of preparing a polymer by polymerization reaction of a monomer containing a hydroxyl group, or a method of preparing a polymer by polymerization reaction of a monomer containing a functional group other than a hydroxyl group, and modifying it to a hydroxyl group.

Examples of monomers containing a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerol mono(meth)acrylate, 4-hydroxyvinylbenzene, 2-hydroxy-3-phenoxypropyl acrylate, and caprolactone adducts of these monomers (number of moles added is 1 to 5). The monomer containing a hydroxyl group is preferably a hydroxyalkyl (meth)acrylate, more preferably 2-hydroxyethyl (meth)acrylate, and even more preferably 2-hydroxyethyl acrylate.

As a method for preparing a polymer by polymerization reaction of a monomer containing a functional group other than a hydroxyl group and modifying it to a hydroxyl group, for example, there is a method in which the acetyl group of polyvinyl acetate obtained by polymerizing vinyl acetate is saponified with an alkali such as sodium hydroxide to convert it to a hydroxyl group (saponification reaction). The saponification reaction rate (degree of saponification) can be controlled as desired by changing the concentration of sodium hydroxide and the treatment time.

In addition, for the purpose of increasing the affinity between the carbon nanotube and the medium, the hydroxyl group in the polymer may be reacted with an aldehyde compound and modified into an acetal group (acetalization). The aldehyde compound used in the acetalization reaction may be, for example, a linear, branched, cyclic saturated, unsaturated, or aromatic aldehyde compound having 1 to 15 carbon atoms, but is not limited thereto. Specific examples include formaldehyde, acetaldehyde, propionyl aldehyde, n-butyl aldehyde, isobutyl aldehyde, tert-butyl aldehyde, and benzaldehyde. These aldehyde compounds may be used alone or in combination of two or more. From the viewpoint of versatility, linear, branched, cyclic saturated, unsaturated, or aromatic aldehyde compounds having 1 to 10 carbon atoms are preferred, and linear aldehyde compounds having 1 to 4 carbon atoms are more preferred. By changing the aldehyde compound and treatment time, the acetalization reaction rate (degree of acetalization) can be controlled as desired.

The content of the hydroxyl group-containing structural unit is preferably 80% by mass or more, more preferably 85% by mass or more, and more preferably 99.8% by mass or less, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass). However, when one or more selected from the group consisting of a nitrile group-containing structural unit, a hydroxyl group-containing structural unit, and a heterocyclic ring-containing structural unit are further contained, the content is preferably 5% by mass or more, preferably 95% by mass or less, and more preferably 85% by mass or less. By setting the content in the above range, it is possible to strengthen the polarization and increase the affinity to the carbon nanotube and the medium. It is also preferable from the viewpoint of electrolyte resistance. For the same reasons as those for the content of the hydroxyl group-containing structural unit, the content of the acetal group is preferably set within the preferred range of the content of the hydroxyl group-containing structural unit.

The heterocycle-containing structural unit is a structural unit containing a heterocycle, and more preferably a structural unit containing an alkylene structure substituted with a substituent containing a heterocycle. The alkylene structure is preferably a linear or branched alkylene structure. The heterocycle contained in the heterocycle-containing structural unit may be a monocyclic structure or a condensed ring structure, but is preferably a monocyclic structure. The number of heterocycles contained in the heterocycle-containing structural unit is preferably one or two, more preferably one. The heterocycle contains an atom other than carbon in the ring-constituting atoms, and for example, contains one or more nitrogen, oxygen, sulfur atoms, etc. As the atom other than carbon constituting the ring, nitrogen or oxygen is preferable, and nitrogen is more preferable. When the ring-constituting atoms contain an atom other than carbon, polarization occurs in the heterocycle, and it becomes possible to strongly act on the carbon nanotube. The method of introducing a heterocycle into a polymer is not particularly limited, but for example, a method of preparing a polymer by a polymerization reaction of a monomer containing a heterocycle can be used.

As a monomer containing a heterocycle, an N-vinyl cyclic amide structural unit is preferred, and examples thereof include N-vinyl-2-pyrrolidone, N-vinyl-ε-caprolactam, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-1,3-oxazin-2-one, and N-vinyl-3,5-morpholinedione. In particular, N-vinyl-2-pyrrolidone is preferred from the viewpoint of improving battery characteristics. These can be used alone or in combination of two or more kinds.

The content of the heterocycle-containing structural unit is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass), from the viewpoint of enhancing the effect on the carbon nanotubes, and may be 100% by mass.

The polymer may further contain, as other structural units, one or more structural units selected from the group consisting of active hydrogen group-containing structural units (excluding carboxyl groups and hydroxyl groups), basic group-containing structural units, and ester group-containing structural units. By selecting and incorporating the above structural units in accordance with the properties such as hydrophilicity, hydrophobicity, acidity, and basicity of the substrate to which the conductive material dispersion of the present invention is applied or the material to be mixed, the conductive material dispersion of the present invention can be used for various purposes.

The active hydrogen group-containing structural unit is a structural unit that has, for example, a primary amino group, a secondary amino group, a mercapto group, or the like as an active hydrogen group. Here, "primary amino group" means -NH2 (amino group), and "secondary amino group" means a group in which one hydrogen atom on a primary amino group is replaced with an organic residue such as an alkyl group. However, in this specification, the primary amino group and secondary amino group in an acid amide are not included in the active hydrogen group.

The basic group-containing structural unit is a structural unit that has a basic group. Examples of basic groups include tertiary amino groups and amide groups. Note that structural units that have a primary amino group and structural units that have a secondary amino group can also be included in the basic group-containing structural units, but in the present invention, they are treated as the active hydrogen group-containing structural units and are not included in the basic group-containing structural units.

The ester group-containing structural unit is a structural unit having a structure represented by (R1) ₂ C=CH-CO-O-R2 (wherein R1 is a hydrogen atom or a methyl group, at least one of which is a hydrogen atom, and R2 is an alkyl group which may have a substituent).
In addition, those containing the active hydrogen group or basic group as the substituent of the alkyl group are treated as the active hydrogen group-containing structural unit or the basic group-containing structural unit, and are not included in the ester group-containing structural unit.

The method for producing the polymer is not particularly limited, and examples thereof include solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, and precipitation polymerization, with solution polymerization or precipitation polymerization being preferred. The polymerization reaction system may include addition polymerization such as ionic polymerization, free radical polymerization, and living radical polymerization, with free radical polymerization or living radical polymerization being preferred. Examples of the radical polymerization initiator include peroxides and azo-based initiators. When polymerizing the polymer, a molecular weight regulator such as a chain transfer agent can be used.

When radical polymerization is performed, the polymerization initiator may be, but is not limited to, di-t-butyl peroxide, lauroyl peroxide, stearyl peroxide, benzoyl peroxide, t-butyl peroxy neodecane, t-butyl peroxy pivalate, dilauroyl peroxide, dicumyl peroxide, t-butylperoxy-2-ethylhexanoate, 1,1-bis(t-butylperoxy)-3,3,5-trimethyl Examples of the initiator include organic peroxides such as cyclohexane and 1,1-bis(t-butylperoxy)cyclohexane, and general azo-based radical polymerization initiators such as azobisisobutyronitrile, azobisisovaleronitrile, 1,1-azobis(1-cyclohexanecarbonitrile), 2,2'-azobis-4-methoxy-2,4-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, and 2,2'-azobis-2-methylbutyronitrile. These initiators may be used alone or in combination of two or more. These polymerization initiators are generally used in an amount of 1% by mass or less based on the total mass of all monomers used (i.e., when the total mass of all monomers is taken as 100% by mass), and can be appropriately selected taking into consideration the temperature at which the polymerization is performed and the half-life of the initiator.

In the polymer production process, the molecular weight of the polymer produced can be controlled using a chain transfer agent or the like within the scope of the present invention. Examples of chain transfer agents include alkyl mercaptans such as octyl mercaptan, nonyl mercaptan, decyl mercaptan, dodecyl mercaptan, and 3-mercapto-1,2-propanediol, thioglycolic acid esters such as octyl thioglycolate, nonyl thioglycolate, and 2-ethylhexyl thioglycolate, 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, -pinene, and β-pinene. In terms of ease of handling and stability, 3-mercapto-1,2-propanediol, thioglycolic acid esters, 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, -pinene, and β-pinene are particularly preferred. These can be added appropriately depending on the required molecular weight. Specifically, the amount of chain transfer agent used is preferably 0.01 to 4 mass%, and more preferably 0.1 to 2 mass%, based on the mass of the polymer (i.e., when the mass of the polymer is 100 mass%).

Other molecular weight control methods include changing the polymerization method, adjusting the amount of polymerization initiator, and changing the polymerization temperature. These molecular weight control methods may be used alone or in combination of two or more methods.

When a polymer is used as a dispersant in the carbon nanotube dispersion, the content of the polymer is preferably determined according to the specific surface area and wettability of the carbon nanotubes, and is preferably 2% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the mass of the conductive material (i.e., when the mass of the carbon nanotubes is taken as 100% by mass), and is preferably 250% by mass or less, more preferably 150% by mass or less, and even more preferably 100% by mass or less.

The dispersant preferably contains at least a polymer. The dispersant may further contain any polymer, any copolymer, etc. The polymer content in the dispersant is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The polymer content in the dispersant may be 100% by mass, in which case the dispersant consists of only the polymer.

The pH of the carbon nanotube dispersion of the present invention is preferably 7.0 or more, more preferably 8.0 or more, and even more preferably 9.0 or more. Also, it is preferably 12.0 or less, and more preferably 11.0 or less. It is believed that by adjusting the pH within the above range, the wettability of the carbon nanotubes can be improved, and further, the action as a dispersant for the polymer can be enhanced. If the pH is outside the above range, the dispersibility of the carbon nanotubes may decrease, or the binder may gel. In addition, when used for battery applications, problems such as corrosion of various raw materials and exterior materials in the battery may easily occur.
The pH of the carbon nanotube dispersion liquid can be measured using a general pH meter when it contains water as a solvent. On the other hand, when it does not substantially contain water as a solvent, for example, when only NMP is selected as the solvent, the carbon nanotube dispersion liquid is prepared by adding water to the carbon nanotube dispersion liquid so that the solid concentration after adding water is 50% when the solid concentration before adding water is 100%, and the pH value is measured using a general pH meter, and can be measured, for example, by the following method.
Water is added to a carbon nanotube dispersion having a solid content of 5% while stirring the carbon nanotube dispersion with a disperser or the like so that the solid content of the carbon nanotube dispersion becomes 2.5%. After stirring uniformly, the pH of the carbon nanotube dispersion can be measured at 25° C. using a benchtop pH meter (Seven Compact S220 Expert Pro, manufactured by Mettler Toledo).
In the present invention, "substantially free of water" means that water is not intentionally added. The water content is preferably less than 5% by mass, more preferably less than 1% by mass, and even more preferably less than 0.5% by mass, based on the mass of the solvent.

The pH of the carbon nanotube dispersion is not particularly limited, but can be adjusted by (1) the amount of metal hydroxide contained in the carbon nanotubes, (2) the type and amount of functional groups on the carbon nanotube surface, and (3) the type and amount of base added. By adjusting the pH by combining factors such as (1) to (3) above, it is possible to obtain a carbon nanotube dispersion that not only improves the wettability of the carbon nanotubes, but also has excellent dispersibility and stability.

As mentioned above, metals, metal oxides, and metal hydroxides used as catalysts in the manufacturing process remain in the carbon nanotubes. The pH can be adjusted by appropriately adjusting the amount of metals remaining in the carbon nanotubes, particularly the content of metal hydroxides (1) above. The amount of remaining metals and carbon purity of carbon nanotubes, as well as the pH of the carbon nanotube dispersion liquid containing them, can be adjusted by conventionally known purification methods.

The functional group on the carbon nanotube surface in (2) above is not particularly limited, but examples thereof include carboxyl groups, sulfo groups, and hydroxyl groups. There is no particular limit to the method of introducing functional groups into the carbon nanotube. For example, to introduce carboxyl groups into the carbon nanotube, it is sufficient to heat the carbon nanotube together with an acid having an oxidizing effect. This operation is relatively easy and is preferable because it can add highly reactive carboxyl groups. Examples of acids having an oxidizing effect include concentrated nitric acid, hydrogen peroxide, a mixture of sulfuric acid and nitric acid, aqua regia, etc. In particular, when concentrated nitric acid is used, the concentration is preferably 5% by mass or more, and more preferably 60% by mass or more. Heating may be performed by a conventional method, but the temperature is preferably below the boiling point of the acid used. For example, the range of 50 to 130°C is preferable for concentrated nitric acid. In addition, the heating time is preferably in the range of 30 minutes to 20 hours, and more preferably in the range of 1 hour to 8 hours. It is preferable that the carbon nanotube does not have an acidic functional group such as a carboxyl group or a sulfo group. If the carbon nanotube dispersion contains a large number of acidic functional groups, there is a risk that it will gel during storage.

The base added to adjust the pH in (3) above is not particularly limited, and specifically, at least one base selected from the group consisting of inorganic bases, inorganic metal salts, organic hydroxides, and other organic bases can be used.

Examples of inorganic bases and inorganic metal salts include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, or borates of alkali metals or alkaline earth metals; and ammonium hydroxide. Among these, chlorides, hydroxides, carbonates, and alkoxides of alkali metals and alkaline earth metals are preferred from the viewpoint of easily supplying cations. Examples of hydroxides of alkali metals include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of hydroxides of alkaline earth metals include calcium hydroxide and magnesium hydroxide. Examples of carbonates of alkali metals include lithium carbonate, lithium hydrogen carbonate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, and potassium hydrogen carbonate. Examples of the alkoxide of an alkali metal include lithium methoxide, lithium ethoxide, lithium n-butoxide, lithium t-butoxide, potassium methoxide, potassium ethoxide, potassium n-butoxide, potassium t-butoxide, sodium methoxide, sodium ethoxide, sodium n-butoxide, and sodium t-butoxide. The number of carbon atoms of the alkoxide may be 5 or more. Examples of the alkoxide of an alkaline earth metal include magnesium methoxide, magnesium ethoxide, magnesium n-butoxide, and magnesium t-butoxide. The number of carbon atoms of the alkoxide may be 5 or more.
Among these, lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, sodium carbonate, lithium t-butoxide, potassium t-butoxide, and sodium t-butoxide are more preferable. The metal contained in the inorganic base and of the present invention may be a transition metal.

An organic hydroxide is a salt containing an organic cation and a hydroxide ion. Examples of organic hydroxides include trimethyl-2-hydroxyethylammonium hydroxide, tetramethylammonium hydroxide, cetyltrimethylammonium hydroxide, hexadecyltrimethylammonium hydroxide, trimethylphenylammonium hydroxide, 3-trifluoromethyl-phenyltrimethylammonium hydroxide, and benzyltrimethylammonium hydroxide. Among these, trimethyl-2-hydroxyethylammonium hydroxide and tetramethylammonium hydroxide are particularly preferred.

Other organic bases include methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, dioctylamine, trioctylamine, aminoethanol, aminopropanol, aminobutanol, and 2-methoxyethylamine. These organic bases have high solubility in the electrolyte, so using too much of them may reduce battery performance. In addition, these compounds are prone to decomposition, so the decomposition products may remain in the coating film, and if they are present in the battery, the initial capacity may decrease.

The amount of base used is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, based on the mass of the polymer. The amount of base used is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, based on the mass of the polymer. If the amount used is too large, the stability of the resulting carbon nanotube dispersion may be poor. Furthermore, it may cause corrosion of the dispersion device and/or the inside of the battery.

The reason why the dispersibility is improved by adjusting the pH to a predetermined value is unclear, but the following factors are considered. The reasons why the dispersibility is improved are not limited to the factors (1) to (3) listed below.
(1) Improve the dispersibility of the polymer.
By adjusting pH to a predetermined value, the polymer containing one or more selected from the group consisting of nitrile group-containing structural unit, carboxyl group-containing structural unit, hydroxyl group-containing structural unit, and heterocyclic ring-containing structural unit can increase the adsorption power to the dispersed material.In addition, since the functional group of the structural unit can form hydrogen bond, the crosslinked structure by hydrogen bond is introduced into the molecule of the polymer, and it can be three-dimensionally adsorbed to the dispersed material, and it is considered that the dispersion liquid having excellent not only dispersibility but also stability can be obtained.
(2) It reduces the solution viscosity of the polymer.
When a polymer is dissolved in a solvent and used, if the viscosity of the polymer solution is low, the dispersant can easily penetrate into the carbon nanotubes, which have strong cohesive forces, and it is believed that a uniform dispersion can be obtained.
(3) Improve the wettability of carbon nanotubes.
When dispersing carbon nanotubes, the carbon nanotubes are wetted with a solvent to reduce the cohesive force between the carbon nanotubes, and then crushed and stabilized to form a dispersion. Carbon nanotubes have significantly lower wettability than other conductive materials, so they need to be pretreated, such as chemically or mechanically crushed, to improve their wettability, but these treatments may reduce their conductivity. It is believed that by adjusting the pH to a specified value, the wettability of the carbon nanotubes can be dramatically improved without impairing their conductivity.

The carbon nanotube dispersion of the present invention may contain a defoaming agent in addition to the dispersant. Any defoaming agent having a defoaming effect, such as a commercially available defoaming agent, a wetting agent, a hydrophilic organic solvent, or a water-soluble organic solvent, may be used as the defoaming agent. One type of defoaming agent may be used alone, or a combination of two or more types may be used.
For example, alcohols: ethanol, propanol, isopropanol, butanol, octyl alcohol, hexadecyl alcohol, acetylene alcohol, ethylene glycol monobutyl ether, methyl cellosolve, butyl cellosolve, propylene glycol monomethyl ether, acetylene glycol, polyoxyalkylene glycol, propylene glycol, and other glycols;
Fatty acid esters: diethylene glycol laurate, glycerin monoricinoleate, alkenylsuccinic acid derivatives, sorbitol monolaurate, sorbitol trioleate, polyoxyethylene monolaurate, polyoxyethylene sorbitol monolaurate, natural waxes, etc.
Amide-based: polyoxyalkylene amide, acrylate polyamine, etc.
Phosphate esters: tributyl phosphate, sodium octyl phosphate, etc.
Metal soaps: aluminum stearate, calcium oleate, etc.
Oils and fats: animal and vegetable oils, sesame oil, castor oil, etc.
Mineral oils: kerosene, paraffin, etc.
Silicone-based: dimethylsilicone oil, silicone paste, silicone emulsion, organic modified polysiloxane, fluorosilicone oil, etc.

<Other conductive materials>
The carbon nanotube dispersion of the present invention may contain other conductive materials in addition to the carbon nanotubes. Examples of the other conductive materials include metal powders such as gold, silver, copper, silver-plated copper powder, silver-copper composite powder, silver-copper alloy, amorphous copper, nickel, chromium, palladium, rhodium, ruthenium, indium, silicon, aluminum, tungsten, molbutene, and platinum, inorganic powders coated with these metals, powders of metal oxides such as silver oxide, indium oxide, tin oxide, zinc oxide, and ruthenium oxide, inorganic powders coated with these metal oxides, and carbon materials such as carbon black and graphite. The other conductive materials may be used alone or in combination of two or more. When using the other conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant. The other conductive materials are substances (materials) different from the electrode active material described later.

Examples of carbon black include acetylene black, furnace black, hollow carbon black, channel black, thermal black, and ketjen black. Carbon black may be neutral, acidic, or basic, and oxidation-treated carbon black or graphitized carbon black may be used.

<Carbon nanotube dispersion liquid>
The carbon nanotube dispersion of the present invention contains boron-containing carbon nanotubes and a solvent, and preferably further contains a dispersant.

To obtain the carbon nanotube dispersion of the present invention, it is preferable to carry out a process for dispersing boron-containing carbon nanotubes in a solvent. There are no particular limitations on the dispersing device used for carrying out such a process.

As the dispersing device, a dispersing machine that is normally used for dispersing pigments, etc., can be used. For example, mixers such as a disperser, homomixer, and planetary mixer, homogenizers (Advanced Digital Sonifer (registered trademark), MODEL 1000 manufactured by BRANSON) 450DA, M Technique's "Clearmix", PRIMIX's "Filmix", Silverson's "Abramix", etc.), paint conditioners (Red Devil), colloid mills (PUC's "PUC Colloid Mill", IKA's "Colloid Mill MK"), cone mills (IKA's "Cone Mill MKO", etc.), ball mills, sand mills (Shinmaru Enterprises' "Dyno Mill", etc.), attritors, pearl mills (Eirich's "DCP Mill", etc.), media-type dispersers such as Coball mills, wet jet mills (Genus' "Genus PY", Sugino Machine's "Starburst", Nanomizer's "Nanomizer", etc.), M Technique's "Clear SS-5", Nara Machine's "MICROS", and other media-less dispersers, as well as other roll mills, but are not limited to these.

The amount of boron-containing carbon nanotubes in the carbon nanotube dispersion of the present invention is preferably 0.2 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, and more preferably 0.5 to 3.0 parts by mass, per 100 parts by mass of the carbon nanotube dispersion.

The amount of dispersant used in the carbon nanotube dispersion of the present invention is preferably 0.1 to 100 parts by mass, more preferably 5 to 80 parts by mass, and even more preferably 20 to 50 parts by mass, per 100 parts by mass of boron-containing carbon nanotubes.

The dispersibility of carbon nanotubes in a carbon nanotube dispersion can be evaluated by the complex modulus and phase angle measured by dynamic viscoelasticity measurement. The complex modulus indicates the hardness of the carbon nanotube dispersion, and tends to be smaller as the dispersibility of the carbon nanotubes improves and the viscosity decreases. However, when the fiber length of the carbon nanotubes is large or the structure length of the carbon black is large, the complex modulus may be a high value even if the carbon nanotubes are uniformly and stably dissolved in the medium, due to the structural viscosity of the carbon nanotubes themselves. The complex modulus of the carbon nanotube dispersion of the present invention measured by dynamic viscoelasticity measurement is less than 200 Pa, preferably less than 50 Pa, and more preferably less than 20 Pa. It is also preferably 1 Pa or more, and more preferably 5 Pa or more.

The phase angle means the phase shift of the stress wave when the strain applied to the CNT dispersion is a sine wave, and indicates the ease of flow. If it is a pure elastic body, the phase angle will be 0° because it will be a sine wave in phase with the applied strain. On the other hand, if it is a pure viscous body, the stress wave will be 90° advanced. In a typical viscoelasticity measurement sample, the phase angle will be a sine wave greater than 0° and less than 90°, and if the dispersibility of the CNT in the CNT dispersion is good, the phase angle will approach 90° for a pure viscous body. However, as with the complex elastic modulus, if the conductive material itself has structural viscosity, the phase angle may be a low value even if the conductive material is uniformly and stably dissolved in the medium. The phase angle at a frequency of 1 Hz in the dynamic viscoelasticity measurement of the CNT dispersion of the present invention is 10° or more, and preferably 20° or more. It is also preferably less than 75°, and more preferably less than 65°.

By uniformly and well dispersing conductive materials with long fiber lengths (carbon nanotubes, etc.) or long structure lengths (carbon black) while maintaining the length at a certain level, a well-developed conductive network is formed. Therefore, it is not enough for the conductive material dispersion to simply have low viscosity and good (apparent) dispersibility; it is particularly effective to judge the dispersion state by combining the complex modulus and phase angle with conventional indices such as viscosity. By setting the complex modulus and phase angle within the above ranges, a conductive material dispersion with good conductivity and electrode strength can be obtained. The complex modulus and phase angle of the conductive material dispersion can be measured by the method described in the examples.

The fiber length of the carbon nanotubes in the carbon nanotube dispersion of the present invention is preferably 0.1 to 5 μm, more preferably 0.2 to 3.5 μm, and even more preferably 0.3 to 2.5 μm.

The cumulative particle size D50 of the carbon nanotube dispersion of the present invention is preferably 100 to 5000 nm, and more preferably 300 to 3000 nm. The cumulative particle size D50 of the carbon nanotube dispersion can be measured using a particle size distribution meter (Microtrac Bell Co., Ltd., Nanotrac UPA, model UPA-EX).

The viscosity of the carbon nanotube dispersion of this embodiment, which contains 0.5 parts by mass or more and 3.0 parts by mass or less of carbon nanotubes per 100 parts by mass of the carbon nanotube dispersion, is preferably 10 mPa·s or more and less than 10,000 mPa·s, more preferably 10 mPa·s or more and less than 5,000 mPa·s, and even more preferably 10 mPa·s or more and less than 2,000 mPa·s, as measured at 60 rpm using a B-type viscometer.

The carbon nanotube dispersion of the present invention not only achieves both excellent dispersibility and conductivity, but also improves durability against external forces such as impacts and coating strength, making it suitable for use as a dispersion for forming conductive wiring or transparent conductive films, or as a dispersion for forming electrochemical devices such as secondary batteries, capacitors, and sensors.

<Carbon nanotube resin composition>
The carbon nanotube resin composition of the present invention contains carbon nanotubes, a solvent, and a binder.

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

<Binder>
The binder of the present invention is a resin for binding substances such as carbon nanotubes together.

The binder of the present invention is not particularly limited and can be appropriately selected according to the desired physical properties. For example, 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, vinyl pyrrolidone, etc. as constituent units; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicon resins, fluororesins; cellulose resins such as carboxymethylcellulose; rubbers such as styrene butadiene rubber and fluororubber; conductive resins such as polyaniline and polyacetylene. Modified bodies, mixtures, and copolymers of these resins may also be used. Among these, when used as a binder resin for the positive electrode film, polymers or copolymers having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene, are preferred from the perspective of resistance. Furthermore, when used as a binder resin for the negative electrode film, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, etc., which have good adhesion, are preferred.

The weight average molecular weight of the binder resin is preferably 10,000 to 2,000,000, more preferably 100,000 to 1,000,000, and even more preferably 200,000 to 1,000,000.

Carboxymethylcellulose as a binder resin preferably has a high viscosity. For example, the viscosity of a 1% aqueous solution is preferably 500 to 6000 mPa·s, and more preferably 1000 to 3000 mPa·s. The viscosity of a 1% aqueous solution of carboxymethylcellulose can be measured at 25°C with a rotor speed of 60 rpm using a B-type viscometer.

Carboxymethyl cellulose as a binder resin preferably has a high degree of etherification. For example, the degree of etherification is preferably 0.6 to 1.5, and more preferably 0.8 to 1.2.

The type and amount of the binder are appropriately selected according to the properties of the coexisting materials, such as carbon nanotubes and active materials. For example, the amount of carboxymethyl cellulose used is preferably 0.5 to 3.0% by mass, and more preferably 1.0 to 2.0% by mass, assuming that the mass of the active material is 100% by mass.

Styrene-butadiene rubber that is generally used as an electrode binder can be used as long as it is an oil-in-water emulsion. The amount of styrene-butadiene rubber used is preferably 0.5 to 3.0% by mass, and more preferably 1.0 to 2.0% by mass, assuming that the mass of the active material is 100% by mass.

Regarding the amount of polyacrylic acid used, when the mass of the active material is taken as 100 mass%, the proportion of polyacrylic acid is preferably 1 to 25 mass%, and more preferably 5 to 20 mass%.

<Compound Slurry>
The composite slurry of the present invention contains carbon nanotubes, a solvent, a binder, and an active material.

<Active material>
The active material of the present invention is a material that is the basis of a battery reaction. Based on the electromotive force, active materials are divided into positive electrode active materials and negative electrode active materials.

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. For example, an oxide of a transition metal such as Fe, Co, Ni, or Mn, a composite oxide with lithium, or an inorganic compound such as a transition metal sulfide may be used. Specifically, a transition metal oxide powder such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , or TiO ₂ , a composite oxide powder of lithium and a transition metal such as lithium nickel oxide having a layered structure, lithium cobalt oxide, lithium manganate, or lithium manganate having a spinel structure, a lithium iron phosphate material which is a phosphoric acid compound having an olivine structure, or a transition metal sulfide powder such as TiS ₂ or FeS may be used. In addition, a conductive polymer such as polyaniline, polyacetylene, polypyrrole, or polythiophene may be used. In addition, the above-mentioned inorganic compounds and organic compounds may be mixed and used.

The negative electrode active material is not particularly limited as long as it can dope or intercalate lithium ions. For example, metal Li, its alloys such as tin alloys, silicon alloys, lead alloys, and the like, metal oxides such as Li _x Fe ₂ O ₃ , Li _x Fe ₃ O ₄ , Li _x WO ₂ (x is a number of 0<x<1), lithium titanate, lithium vanadate, and lithium silicate, conductive polymers such as polyacetylene and poly-p-phenylene, amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, carbonaceous powders such as natural graphite, carbon black, mesophase carbon black, resin-sintered carbon materials, vapor-grown carbon fibers, and carbon fibers, and other carbonaceous materials, can be used alone or in combination. These negative electrode active materials can also be used alone or in combination.

As the negative electrode active material of the present invention, a silicon-based negative electrode active material, which is a negative electrode active material containing silicon such as a silicon alloy or lithium silicate, is preferred.

Examples of silicon-based negative electrode active materials include so-called metallurgical grade silicon produced by reducing silicon dioxide with carbon, industrial grade silicon in which impurities have been reduced from metallurgical grade silicon by acid treatment or unidirectional solidification, high purity silicon with different crystal states such as high purity single crystal, polycrystalline, and amorphous produced from silane obtained by reacting silicon, and silicon in which industrial grade silicon has been made highly pure by sputtering or EB deposition (electron beam deposition) while simultaneously adjusting the crystal state and precipitation state.

Other examples include silicon oxide, which is a compound of silicon and oxygen, silicon and various alloys, and silicon compounds in which the crystalline state of these is adjusted by a quenching method or the like. Among these, silicon-based negative electrode active materials having a structure in which silicon nanoparticles are dispersed in silicon oxide and the outside is coated with a carbon film are preferred.

In addition to the silicon-based negative electrode active material, the negative electrode active material of the present invention preferably uses amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, or carbonaceous powder such as natural graphite. Among these, it is preferable to use carbonaceous powder such as artificial graphite and natural graphite.

The amount of silicon-based negative electrode active material is preferably 3 to 50 mass %, and more preferably 5 to 25 mass %, assuming that the carbonaceous powder, such as artificial graphite or natural graphite, is 100 mass %.

The BET specific surface area of the active material of the present invention is preferably from 0.1 to 10 m ² /g, more preferably from 0.2 to 5 m ² /g, and even more preferably from 0.3 to 3 m ² /g.

The average particle size of the active material of the present invention is preferably within the range of 0.5 to 50 μm.
It is more preferably 2 to 20 μm. The average particle size of the active material as used herein means the average value of particle sizes measured by an electron microscope.

<Method for producing composite slurry>
The composite slurry of the present invention can be prepared by various conventionally known methods, such as a method of adding an active material to a carbon nanotube resin composition, or a method of adding an active material to a carbon nanotube dispersion and then adding a binder.

To obtain the composite slurry of the present invention, it is preferable to add an active material to the carbon nanotube resin composition and then carry out a dispersion process. The dispersion device used for such a process is not particularly limited. The composite slurry can be obtained using the dispersion device described above for the carbon nanotube dispersion liquid.

The amount of active material in the composite slurry of the present invention is preferably 20 to 85 parts by mass, more preferably 30 to 75 parts by mass, and even more preferably 40 to 70 parts by mass, per 100 parts by mass of the composite slurry.

The amount of carbon nanotubes in the composite slurry of the present invention is preferably 0.01 to 10 parts by mass, more preferably 0.02 to 5 parts by mass, and even more preferably 0.03 to 1 part by mass, per 100 parts by mass of active material.

The amount of binder in the composite slurry of the present invention is preferably 0.5 to 30% by mass, more preferably 1 to 25% by mass, and particularly preferably 2 to 20% by mass, relative to 100% by mass of the active material.

The amount of solids in the composite slurry of the present invention is preferably 30 to 90% by mass, more preferably 30 to 80% by mass, and more preferably 40 to 75% by mass, relative to 100% by mass of the composite slurry.

<Conductive Film and Electrode Film>
The conductive film of the present invention is formed by forming a carbon nanotube dispersion liquid or a carbon nanotube resin composition into a film shape. For example, it is a coating film formed by coating a carbon nanotube dispersion liquid or a carbon nanotube resin composition on a sheet-like substrate and drying it. The sheet-like substrate is not particularly limited, and a conductive or non-conductive substrate can be used, and the conductive substrate can be a current collector described below.
The electrode film of the present invention is formed by forming a composite material slurry into a film shape, for example, a coating film formed by coating a composite material slurry on a current collector and drying the coating film to form an electrode composite material layer.

The material and shape of the current collector used in the electrode film of the present invention are not particularly limited, and can be appropriately selected from those suitable for various secondary batteries. For example, the material of the current collector can be metals or alloys such as aluminum, copper, nickel, titanium, or stainless steel. In addition, the shape of the current collector is generally a flat foil, but current collectors with a roughened surface, perforated foils, and mesh-shaped current collectors can also be used.

There are no particular limitations on the method for applying the composite slurry onto the current collector, 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, leaving to dry, using a blower dryer, a hot air dryer, an infrared heater, or a far-infrared heater.

After coating, a rolling process using a lithographic press or a calendar roll may be performed. 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.

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrolyte.

The positive electrode can be made by applying a composite slurry containing a positive electrode active material onto a current collector and drying it to create an electrode film.

The negative electrode can be made by applying a composite slurry containing a negative electrode active material onto a current collector and drying it to create an electrode film.

As the electrolyte, various conventionally known electrolytes capable of moving ions can be used. For example, those containing lithium salts such as LiBF ₄ , LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , Li(CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , Li(CF ₃ SO ₂ ) ₃ C, LiI, LiBr, LiCl, LiAlCl, LiHF ₂ , LiSCN, or LiBPh ₄ (wherein Ph is a phenyl group) can be used, but are not limited thereto, and those containing sodium salts or calcium salts can also be used. It is preferable that the electrolyte is dissolved in a non-aqueous solvent and used as an electrolytic solution.

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 of two or more.

The non-aqueous electrolyte secondary battery of the present invention preferably includes a separator. Examples of separators include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those that have been subjected to a hydrophilic treatment, but are not limited to these.

The structure of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but is usually composed of a positive electrode, a negative electrode, and a separator that is provided as necessary, 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.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, so long as it does not depart from the gist of the invention. In the examples, "carbon nanotubes" may be abbreviated as "CNT." Unless otherwise specified, "parts" refers to "parts by mass."

<CNT>
The CNTs used in the examples and comparative examples described below and their physical properties will be described below.

(Boron-containing CNTs)
An example of CNT production will be described below.

[Production example (A)]
A catalyst for CNT synthesis was prepared by the method described in paragraph [0117] of JP 2018-150218 A. Then, a quartz glass heat-resistant plate on which 1.0 g of the catalyst for CNT synthesis was sprayed was placed in the center of a horizontal reaction tube with an internal volume of 10 L, which can be pressurized and heated by an external heater. The reaction tube was evacuated while nitrogen gas was injected, and the air in the reaction tube was replaced with nitrogen gas, and the atmosphere in the horizontal reaction tube had an oxygen concentration of 1% by volume or less. Next, the horizontal reaction tube was heated with an external heater until the central temperature in the horizontal reaction tube reached 680°C. After reaching 680°C, propane gas was introduced into the reaction tube as a carbon source at a flow rate of 2L per minute, and contact reaction was carried out for 2 hours. After the reaction was completed, the gas in the reaction tube was replaced with nitrogen gas, and the gas in the reaction tube was replaced with nitrogen gas, and the reaction tube was cooled to 100°C or less and taken out, thereby obtaining CNT (A).

[Production example (B)]
A catalyst for CNT synthesis was prepared by the method described in paragraphs [0147] and [0148] of JP 2019-108256 A. Then, a quartz glass heat-resistant dish on which 1 g of the catalyst for CNT synthesis was sprayed was placed in the center of a horizontal reaction tube with an internal volume of 10 L, which can be pressurized and heated by an external heater. The reaction tube was evacuated while injecting nitrogen gas, and the air in the reaction tube was replaced with nitrogen gas, and the horizontal reaction tube was heated until the atmospheric temperature reached 700 ° C. After reaching 700 ° C., ethylene gas was introduced into the reaction tube as a hydrocarbon at a flow rate of 2 L per minute and reacted for 15 minutes. After the reaction was completed, the gas in the reaction tube was replaced with nitrogen gas, and the reaction tube was cooled to 100 ° C. or less and taken out to obtain CNT (B).

[Production Example (1)]
95 parts of CNT (A) as a carbon source, 5 parts of boric acid as a boron source, and 9,900 parts of toluene/ethanol (=1/1, weight ratio) as a solvent were mixed and wet-mixed by ultrasonic treatment, and then the mixture was heated and dried at 65° C. to remove the solvent, thereby producing precursor (1).
Next, the precursor (1) was filled into a graphite crucible and heated in a firing furnace under an argon atmosphere at a heating rate of 10°C/min until the temperature inside the furnace reached 1700°C. Then, heat treatment was performed at 1700°C for 2 hours to obtain boron-containing CNTs (1).

[Production Example (2)]
92.5 parts of 100P (Kumho Petrochemical carbon nanotube) as a carbon source and 7.5 parts of boric acid as a boron source were mixed and subjected to a dry treatment using Mechanofusion (Hosokawa Micron Corporation) to prepare a precursor (2).
Next, the precursor (2) was filled into a graphite crucible and heated in a firing furnace under an argon atmosphere at a heating rate of 10°C/min until the temperature inside the furnace reached 1650°C. Then, heat treatment was performed at 1650°C for 2 hours to obtain boron-containing CNTs (2).

[Production Examples (3), (5), (8), and (9)]
Boron-containing CNTs (3), (5), (8), and (9) were obtained in the same manner as in Production Example 2, except that the carbon source, boron source, and firing temperature were changed as shown in Table 1.

[Production Examples (4), (6), and (7)]
Boron-containing CNTs (4), (6), and (7) were obtained in the same manner as in Production Example 1, except that the carbon source, boron source, solvent, and firing temperature were changed as shown in Table 1.

(Content of boron element in CNT)
The boron content in the CNTs was measured using ICP emission spectroscopy (SPECTROARCOS FHS12, manufactured by Spectro Corporation). The obtained value indicates the amount (mol%) of boron contained in the entire CNT material.

(Substituted boron element amount of CNT)
The amount of substituted boron (mol %) on the surface of the CNTs was measured using XPS (K-Alpha, manufactured by Thermo Fisher Scientific).
The spectrum of boron 1s electrons appears at binding energies of 185 to 194 eV, so the state of boron on the surface can be evaluated by determining the peak area.
In detail, boron clusters appear at 186 to 187 eV, boron carbide at 187 to 188 eV, boron element doped to substitute for carbon element with a hexagonal mesh surface as a basic skeleton (substitutional boron on the surface) at 188 to 189.3 eV, various boron oxides BC2O at 189.5 to 190.5 eV, BCO2 at 191.5 to 192 eV, and B2O3 at 192.5 to 193 eV. Therefore, by performing peak separation of boron 1s, the doping state of boron on the surface of CNT can be analyzed, and the amount of substitutional boron in CNT can be measured.

(Average outer diameter of CNTs)
Using an electronic balance, 0.2 g of CNT was weighed into a sample bottle, 200 mL of toluene was added, and then an ultrasonic homogenizer (Advanced Digital Sonifer (registered trademark), MODEL 450DA, manufactured by BRANSON) was used to perform dispersion treatment under ice cooling at an amplitude of 50% for 5 minutes to prepare a CNT dispersion. Thereafter, the CNT dispersion was appropriately diluted, several μL was dropped onto a collodion film, and the mixture was dried at room temperature, and then directly observed using a transmission electron microscope (H-7650, manufactured by Hitachi, Ltd.). The observation was performed at a magnification of 50,000 times, and multiple photographs containing 10 or more CNTs were taken within the field of view, and the outer diameters of 300 CNTs randomly extracted were measured, and the average value was taken as the average outer diameter (nm) of the CNTs.

(BET specific surface area of CNT)
0.03 g of CNT was weighed out using an electronic balance, and then dried while degassing for 15 minutes at 110° C. Thereafter, the BET specific surface area of the CNT was measured using a fully automatic specific surface area measuring device (MOUNTECH, HM-model 1208).

(G/D ratio of CNTs)
CNT was placed in a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measured using a laser wavelength of 532 nm. The measurement conditions were: capture time 60 seconds, number of integrations 2, neutral density filter 10%, objective lens magnification 20 times, confocus hole 500, slit width 100 μm, measurement wavelength 100 to 3000 cm ⁻¹ . The CNT for measurement was separated onto a slide glass and flattened using a spatula. Of the peaks obtained, the maximum peak intensity in the range of 1560 to 1600 cm ⁻¹ in the spectrum was taken as G, the maximum peak intensity in the range of 1310 to 1350 cm ⁻¹ was taken as D, and the ratio of G/D was taken as the G/D ratio of the CNT.

(Volume resistivity of CNT)
Using a powder resistivity measurement system (manufactured by Nitto Seiko Analytech, MCP-PD51 type),
The volume resistivity of the CNT was measured. After the CNT was placed in a measurement cell, the volume resistivity was measured while applying a load, and the volume resistivity value of the CNT at a density of 1 g/ ^cm3 was evaluated.

<Dispersant>
The dispersants used in the examples and comparative examples described later are shown below.

Dispersant (1): Acrylonitrile-conjugated diene rubber (Synthesis Example 1 below)
Dispersant (2): Polyvinylpyrrolidone (K-30, manufactured by Nippon Shokubai Co., Ltd.)
Dispersant (3): Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval PVA-403)
Dispersant (4): Therban (registered trademark) 3406 (manufactured by ARLANXEO)
Dispersant (5): Zetpole (registered trademark) 2000L (manufactured by Zeon Corporation)
Dispersant (6): Polyvinyl butyral (manufactured by Sekisui Chemical Co., Ltd., S-LEC BL-10)
Dispersant (7): Acrylonitrile-acrylic acid copolymer (Synthesis Example 2 below)
Dispersant (8): Carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., APP-084)
Dispersant (9): Carboxymethylcellulose ammonium salt (Metolose (registered trademark) SM-4, manufactured by Shin-Etsu Kogyo Co., Ltd.)
Dispersant (10): Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval SD1000)
Dispersant (11): Carboxymethylcellulose sodium salt (Sunrose (registered trademark) F01MC, manufactured by Nippon Paper Industries Co., Ltd.)

[Synthesis Example (1)]
Into a stainless steel polymerization reactor, 35 parts of acrylonitrile, 65 parts of 1,3-butadiene, 3 parts of potassium soap oleate, 0.3 parts of azobisisobutyronitrile, 0.55 parts of t-dodecyl mercaptan, and 200 parts of ion-exchanged water were added. Under a nitrogen atmosphere, polymerization was carried out at 45°C for 20 hours while stirring, and the polymerization was terminated at a conversion rate of 90%. Unreacted monomers were removed by vacuum stripping to obtain an acrylonitrile-conjugated diene rubber latex with a solid content concentration of about 30%. Subsequently, ion-exchanged water was added to the latex to adjust the total solid content concentration to 12%, and the latex was placed in a 1L autoclave equipped with a stirrer, and nitrogen gas was passed through it for 10 minutes to remove dissolved oxygen in the contents. A catalyst solution prepared by dissolving 75 mg of palladium acetate as a hydrogenation catalyst in 180 mL of ion-exchanged water to which nitric acid was added in an amount four times the moles of palladium was added was added to the autoclave. After replacing the inside of the autoclave with hydrogen gas twice, the contents of the autoclave were heated to 50°C while being pressurized with hydrogen gas up to 3MPa, and hydrogenation reaction was carried out for 6 hours. After that, the contents were returned to room temperature, the inside of the autoclave was made into a nitrogen atmosphere, and the solid content was dried to recover dispersant (1). The hydrogenation rate of dispersant (1) was 99.6%, and the weight average molecular weight (Mw) was 150,000.

[Synthesis Example (2)]
A reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer was charged with 100 parts of acetonitrile and substituted with nitrogen gas. The reaction vessel was heated to 75°C, and a mixture of 90.0 parts of acrylonitrile, 10.0 parts of acrylic acid, and 5.0 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by NOF Corp.; V-65) was added dropwise over 3 hours to carry out a polymerization reaction. After completion of the dropwise addition, the reaction was continued for another hour at 75°C, and then 0.5 parts of Perbutyl O was added, and the reaction was continued for another hour at 75°C. Thereafter, the conversion rate was confirmed to exceed 98% by non-volatile content measurement, and the dispersion medium was completely removed by vacuum concentration to obtain a dispersant (7). The weight average molecular weight (Mw) of the dispersant (7) was 45,000.

<CNT dispersion>
Next, the preparation of the CNT dispersion liquid and the method for evaluating the dispersion liquid will be described.

[Example 1-N1]
0.9 parts of dispersant (1), 94.064 parts of NMP (N-methylpyrrolidone) as a solvent, and 0.036 parts of NaOH as an additive were added to a stainless steel container, and the mixture was stirred with a disperser until it became uniform. Then, 5 parts of boron-containing CNT (1) were added while stirring with a disperser, and a square-hole high shear screen was attached to a high shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 8,500 rpm until the entire mixture became uniform and the dispersion particle size was 250 μm or less as measured by a grind gauge. Next, the liquid to be dispersed was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) through a pipe, and a 20-pass 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, to obtain a CNT dispersion (N1).

[Examples 1-N2 to N15]
According to the compositions shown in Table 3, CNT dispersions (N2) to (N15) were obtained in the same manner as in Example 1-N1.

[Comparative Example 1-N1]
According to the composition shown in Table 3, CNT, NMP, a dispersant, and 140 parts of zirconia beads (bead diameter 0.3 mmφ) were charged into a glass bottle, and a dispersion treatment was carried out for 8 hours using a paint conditioner manufactured by Red Devil Co., Ltd., and then the zirconia beads were separated to obtain a CNT dispersion liquid (N16).

[Comparative Example 1-N2]
According to the composition shown in Table 3, a dispersant and NMP were added to a stainless steel container, and the concentration was adjusted by adding NMP, and the mixture was stirred with a disperser until it was uniform. Then, boron-containing CNT (6) was added while stirring with a disperser, and a square-hole high shear screen was attached to a high shear mixer, and the mixture was dispersed in a batch manner at a speed of 8,500 rpm until the entire mixture became uniform and the dispersion particle size was 250 μm or less as measured by a grind gauge. Next, the liquid to be dispersed was supplied from the stainless steel container to a high-pressure homogenizer through a pipe, and a five-pass 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, to obtain a CNT dispersion (N17).

[Comparative example 1-N3]
According to the composition shown in Table 3, boron-containing CNT, NMP, a dispersant, and 140 parts of zirconia beads (bead diameter 0.3 mmφ) were charged into a glass bottle, and a dispersion treatment was carried out for 2 hours using a paint conditioner manufactured by Red Devil Co., Ltd., and then the zirconia beads were separated to obtain a CNT dispersion liquid (N18).

[Example 1-W1]
97.7 parts of ion-exchanged water, 0.45 parts of dispersant (2), and 0.05 parts of defoamer (A) (SN DEFORMER 1312, manufactured by San Nopco Ltd.) were added to a stainless steel container, and the mixture was stirred with a disperser until it was uniform. Then, while stirring with a disperser, 1.8 parts of boron-containing CNT (1) were added and further stirred with a disperser until it was uniform. After that, a dispersion treatment was performed using a high-pressure homogenizer (Starburst 10, manufactured by Sugino Machine Co., Ltd.). The dispersion treatment was performed using a single nozzle chamber, with a nozzle diameter of 0.17 mm and a pressure of 100 MPa, and 5 passes were performed to obtain a CNT dispersion liquid (W1).

[Example 1-W3, W4, W6, W7, W9, W11, W12, W15]
CNT dispersions (W3), (W4), (W6), (W7), (W9), (W11), (W12), and (W15) were obtained in the same manner as in Example 1-W1, except that the CNT type, CNT addition amount, ion-exchanged water addition amount, dispersant type, dispersant addition amount, defoamer addition amount, and number of passes were changed to those shown in Table 5.

[Example 1-W2]
A glass bottle was charged with 2 parts of the boron-containing CNT (1), 0.5 parts of the dispersant (7), 0.05 parts of the defoamer (A) (SN DEFORMER 1312 manufactured by SAN NOPCO LTD.), 97.45 parts of ion-exchanged water, and 140 parts of zirconia beads (bead diameter 0.3 mmφ), and the mixture was dispersed for 4 hours using a paint conditioner manufactured by Red Devil Co., Ltd., and the zirconia beads were separated to obtain a CNT dispersion (W2).

[Example 1-W5, W8, W10, W13, W14]
CNT dispersions (W5), (W8), (W10), (W13), and (W14) were obtained in the same manner as in Example 1-W2, except that the CNT type, CNT addition amount, ion-exchanged water addition amount, dispersant type, dispersant addition amount, antifoaming agent addition amount, and dispersion time were changed to those shown in Table 5.

[Comparative Example 1-W1]
According to the composition shown in Table 5, CNT, ion-exchanged water, a dispersant, an antifoaming agent, and 140 parts of zirconia beads (bead diameter 0.3 mmφ) were charged into a glass bottle, and a dispersion treatment was carried out for 4 hours using a paint conditioner manufactured by Red Devil Co., Ltd., and then the zirconia beads were separated to obtain a CNT dispersion (W16).

[Comparative Example 1-W2]
97.85 parts of ion-exchanged water, 0.1 parts of dispersant (8), and 0.05 parts of defoamer (A) (SN DEFORMER 1312, manufactured by San Nopco Ltd.) were added to a stainless steel container, and the mixture was stirred with a disperser until it was uniform. Then, while stirring with a disperser, 2 parts of boron-containing CNT (9) were added and further stirred with a disperser until it was uniform. Then, a dispersion process was performed with a high-pressure homogenizer. The dispersion process was performed using a single nozzle chamber, with a nozzle diameter of 0.17 mm and a pressure of 100 MPa, and a CNT dispersion liquid (W17) was obtained by one pass process.

[Comparative example 1-W3]
According to the composition shown in Table 5, boron-containing CNT, ion-exchanged water, a dispersant, an antifoaming agent, and 140 parts of zirconia beads (bead diameter 0.3 mmφ) were charged into a glass bottle, and a dispersion treatment was carried out for 0.5 hours using a paint conditioner manufactured by Red Devil Co., Ltd., and then the zirconia beads were separated to obtain a CNT dispersion (W18).

(Measurement of Complex Modulus and Phase Angle of CNT Dispersion)
The complex modulus and phase angle of the CNT dispersion were evaluated by performing dynamic viscoelasticity measurements at 25°C, a frequency of 1 Hz, and a strain rate in the range of 0.01% to 5% using a rheometer (RheoStress1 rotational rheometer manufactured by Thermo Fisher Scientific Co., Ltd.) with a cone of 60 mm diameter and 2°. The smaller the complex modulus obtained, the better the dispersibility, and the larger the complex modulus, the worse the dispersibility. In addition, the larger the phase angle obtained, the better the dispersibility, and the smaller the phase angle, the worse the dispersibility.
Criteria (complex elastic modulus)
◎: Less than 20 Pa (very good)
○: 20 Pa or more and less than 50 Pa (excellent)
○△: 50 Pa or more and less than 100 Pa (good)
△: 100 Pa or more, less than 200 Pa (acceptable)
×: 200 Pa or more (defective)
Judgment criteria (phase angle)
◎: 30° or more (excellent)
○: 20° or more and less than 30° (good)
△: 10° or more and less than 20° (acceptable)
×: Less than 10° (bad)

(Viscosity of Dispersion)
After leaving the CNT dispersion in a thermostatic chamber at 25°C for at least 1 hour, the CNT dispersion was thoroughly stirred and immediately subjected to measurement with a B-type viscometer rotor at a rotation speed of 60 rpm. The rotors used in the measurement were 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.
Judgment criteria: ◎: Less than 2000 mPa·s (excellent)
○: 2000 mPa·s or more and less than 5,000 mPa·s (good)
△: 5,000 mPa·s or more and less than 10,000 mPa·s (acceptable)
×: 10,000 mPa·s or more, settling or separation (bad)

(Evaluation of Dispersion Stability)
The storage stability was evaluated based on the change in the liquid properties after storing the conductive material dispersion at 50° C. for 7 days. The change in the liquid properties was judged based on the ease of stirring when stirred with a spatula.
Judgment criteria: ○: No problem (good)
△: Viscosity has increased but gelation has not occurred (passable)
×: Gelled (very poor)

(CNT fiber length in dispersion)
The CNT dispersion was diluted with the solvent used in preparing the CNT dispersion so that the CNT concentration was 0.01% by mass, and several μL was dropped onto a mica substrate, followed by drying in an electric oven at 120° C. to prepare a substrate for observing the CNT fiber length. The surface of the substrate prepared for observing the CNT fiber length was then sputtered with platinum. Further, the substrate was observed using a SEM. The observation was performed at a magnification of 5,000 times or 20,000 times according to the fiber length of the CNT, and multiple photographs containing 10 or more CNTs were taken within the field of view, and the fiber lengths of 100 CNTs randomly extracted were measured, and the average value was taken as the CNT fiber length (μm) in the CNT dispersion.

(Measurement of particle size distribution of dispersion liquid)
The CNT dispersion was allowed to stand in a thermostatic chamber at 25° C. for 1 hour or more, and then thoroughly stirred and diluted. The cumulative particle size D50 of the CNT dispersion was then measured using a particle size distribution meter (Microtrac-Bell Corporation, Nanotrac UPA, model UPA-EX).

(Volume resistivity of CNT coating film)
The CNT dispersion was applied onto a PET substrate using an applicator, and then dried in an oven at 120°C for 30 minutes to obtain a CNT coating film (film thickness 3 µm). The surface resistivity (Ω/□) of the coating film after drying was then measured using Loresta GP, MCP-T610 (manufactured by Nitto Seiko Analytech Co., Ltd.). After measurement, the surface resistivity was multiplied by the thickness of the CNT coating film formed on the PET substrate to obtain the volume resistivity (Ω·cm) of the coating film. The thickness of the coating film was calculated by subtracting the film thickness of the PET substrate from the average value measured at five points in the film using a film thickness meter, and the volume resistivity (Ω·cm) of the coating film was calculated. The criteria for evaluation are shown below.
Judgment criteria: ⊚: Volume resistivity (Ω cm) of the CNT coating film is less than 7 × 10 ^-3 (excellent)
◯: The volume resistivity (Ω·cm) of the CNT coating film is 7×10 ⁻³ or more and less than 1×10 ⁻² (good)
△: The volume resistivity (Ω·cm) of the CNT coating film is 1×10 ⁻² or more and less than 2×10 ⁻² (acceptable)
×: The volume resistivity (Ω·cm) of the CNT coating film is 2×10 ⁻² or more (poor)

From the results of the above examples, it became clear that the carbon nanotube dispersion liquid having a complex modulus and phase angle within the range of the present invention has superior conductivity compared to the comparative examples. It is believed that the carbon nanotubes in the coating film of the carbon nanotube dispersion liquid of the present invention were able to form a conductive network more efficiently than those of the comparative examples 1-N2, 1-N3, 1-W2, and 1-W3, but the conductivity of the carbon nanotubes in the dispersion liquid is low even though the dispersibility of the carbon nanotubes in the dispersion liquid is good in the comparative examples 1-N1 and 1-W1. Here, when comparing Example 1-W2 with Comparative Example 1-W1, it became clear that the carbon nanotube fiber length of the dispersion liquid in Example 1-W2 was longer and the particle size distribution was smaller, although the carbon nanotube dispersion liquid was prepared under similar conditions except for the carbon nanotubes. This is considered to be because the boron-containing carbon nanotubes in Example 1-W2 are harder than the boron-free carbon nanotubes in the comparative example and have high durability against energy such as impacts applied during dispersion of the carbon nanotubes, making the carbon nanotubes less likely to break and allowing them to be unraveled while maintaining their fiber length. As a result, it is believed that the carbon nanotube dispersion of the present invention was able to form a good conductive network in the coating film of the carbon nanotubes. On the other hand, when comparing Examples 1-N1, 1-N5, and 1-N10, it was also found that differences in the boron content of the carbon nanotubes also had a significant effect on the dispersion state of the carbon nanotube dispersion, and also affected the conductivity of the coating film. Although the details of these have not been clarified, it is believed that with carbon nanotubes with a high boron content, if the hardness is too high, it becomes difficult to separate the carbon nanotubes, and sufficient dispersibility cannot be obtained, which also affected the conductivity of the coating film.

<Preparation and evaluation of composite slurry and electrodes>
Next, the preparation and evaluation methods of the composite slurry, electrodes, and batteries will be described.

[Example 2-N1]
CNT dispersion (1) and NMP in which 8% by mass of PVDF (Solef#5130, Solvey Co., Ltd.) was dissolved were added so that the composition (mass ratio) of CNT, PVDF, and active material was 0.25/1.5/98.25, and then the mixture was stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky Corporation, Awatori Rentaro, ARE-310). Then, electrode active material NCM523 (Nippon Chemical Industry Co., Ltd., composition: LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) was added, and the mixture was stirred at 2000 rpm for 20 minutes using a planetary centrifugal mixer. Further, NMP was added so that the solid content of the positive electrode mixture slurry was 75 mass %, and the mixture was stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer to obtain a positive electrode mixture slurry (1).

[Examples 2-N2 to N15, Comparative Examples 2-N1 to N3]
Positive electrode composite slurries (N2) to (N18) were obtained in the same manner as in Example 2-1, except that the materials shown in Table 7 were used.

[Example 2-W1]
7.5 parts by mass of CNT dispersion (W1), 12.5 parts by mass of an aqueous solution in which 2% by mass of CMC (manufactured by Daicel FineChem Co., Ltd., #1190) was dissolved, and 4.9 parts by mass of ion-exchanged water were weighed. Then, using a rotation and revolution mixer, the mixture was stirred at 2000 rpm for 30 seconds to obtain a CNT resin composition (W1). Then, 2.4 parts by mass of silicon monoxide (manufactured by Osaka Titanium Technology Co., Ltd., SILICON MONOOXIDE, SiO 1.3C 5 μm) was added, and the mixture was stirred at 2000 rpm for 30 seconds using a rotation and revolution mixer. Furthermore, 21.9 parts by mass of artificial graphite (manufactured by Nippon Graphite Industries Co., Ltd., CGB-20) was added, and the mixture was stirred at 2000 rpm for 30 seconds using a rotation and revolution mixer. Thereafter, 0.78 parts by mass of styrene-butadiene emulsion (TRD2001, manufactured by JSR Corporation) was further added, and the mixture was stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer to obtain a negative electrode mixture slurry (W1).

[Examples 2-W2 to W15, Comparative Examples 2-W1 to W3]
Negative electrode composite slurries (W2) to (W18) were obtained in the same manner as in Example 2-W1, except that the materials were changed to those shown in Table 7 and the amounts of the CNT resin composition and the CNT dispersion liquid added were adjusted so that the CNT was 0.27 parts by mass in 100 parts by mass of the composite slurry.

(Volume resistivity of positive electrode mixture layer)
The positive electrode composite slurry was applied to a PET substrate using an applicator so that the electrode weight was 20 mg/ ^cm2 , and then dried in an electric oven at 120°C for 30 minutes to obtain a positive electrode. Thereafter, the surface resistivity (Ω/□) of the coating film after drying was measured using (Nitto Seiko Analytech Co., Ltd.: Loresta GP, MCP-T610). After the measurement, the surface resistivity (Ω/□) of the electrode film for the positive electrode was determined by multiplying the thickness of the electrode composite layer formed on the PET substrate. The thickness of the electrode composite layer was calculated by subtracting the film thickness of the PET substrate from the average value measured at five points in the electrode film using a film thickness meter, and the volume resistivity (Ω/cm) of the composite layer was calculated. The criteria are shown below, and the results are shown in Table 7.
Judgment criteria: The volume resistivity (Ω cm) of the positive electrode mixture layer is less than 8 (excellent)
◯: The volume resistivity (Ω cm) of the positive electrode mixture layer is 8 or more and less than 12 (good)
△: The volume resistivity (Ω cm) of the positive electrode mixture layer is 12 or more and less than 16 (acceptable)
×: The volume resistivity (Ω cm) of the positive electrode mixture layer is 16 or more (poor)

(Peel strength of positive electrode mixture layer)
The positive electrode composite slurry was applied to an aluminum foil using an applicator so that the electrode weight was 20 mg/ ^cm2 , and the coating was dried in an oven at 120°C for 30 minutes. Then, two 90 mm x 20 mm rectangles were cut with the coating direction as the long axis. The peel strength was measured using a tensile tester and evaluated by a 180-degree peel test method. Specifically, a double-sided tape of 100 mm x 30 mm size was attached to a stainless steel plate, and the prepared battery electrode composite layer was adhered to the other side of the double-sided tape and peeled off while pulling from the bottom to the top at a constant speed (50 mm/min). The average value of the stress at this time was taken as the peel strength. The criteria are shown below, and the results are shown in Table 87.
◎: Peel strength (N/cm) of the positive electrode mixture layer is 0.3 or more (excellent)
◯: The peel strength (N/cm) of the positive electrode mixture layer is 0.25 or more and less than 0.3 (good)
△: The peel strength (N/cm) of the positive electrode mixture layer is 0.2 or more and less than 0.25 (acceptable)
×: The peel strength (N/cm) of the positive electrode mixture layer is less than 0.2 (poor)

(Volume resistivity of negative electrode mixture layer)
The volume resistivity (Ω cm) of the composite layer was measured in the same manner as for the positive electrode composite layer, except that the negative electrode composite slurry was used and the electrode weight was 8 mg/cm ^2. The criteria for evaluation are shown below, and the results are shown in Table 7.
Criteria : Volume resistivity (Ω cm) of the negative electrode mixture layer is less than 0.12 (excellent)
◯: The volume resistivity (Ω cm) of the negative electrode mixture layer is 0.12 or more and less than 0.15 (good)
△: The volume resistivity (Ω cm) of the negative electrode mixture layer is 0.15 or more and less than 0.25 (acceptable)
×: The volume resistivity (Ω cm) of the negative electrode mixture layer is 0.25 or more (poor)

(Peel strength of negative electrode mixture layer)
The peel strength of the negative electrode composite layer was measured in the same manner as for the positive electrode composite layer, except that the negative electrode composite slurry was applied to the copper foil using an applicator so that the electrode weight was 8 mg/cm ^2. The evaluation criteria are shown below, and the results are shown in Table 7.
Criteria : Peel strength (N/cm) of the negative electrode mixture layer is 0.5 or more (excellent)
◯: The peel strength (N/cm) of the negative electrode mixture layer is 0.4 or more and less than 0.5 (good)
△: The peel strength (N/cm) of the negative electrode mixture layer is 0.3 or more and less than 0.4 (acceptable)
×: Peel strength (N/cm) of the negative electrode mixture layer is less than 0.3 (bad)

<Preparation of non-aqueous electrolyte secondary battery and evaluation of its characteristics>
[Examples 3-N1 to N15, 3-W1 to W15, Comparative Examples 3-N1 to N3, 3-W1 to W3]
The positive and negative electrodes for evaluation were prepared by the following method. The positive electrode composite slurry was applied to an aluminum foil having a thickness of 20 μm as a current collector using an applicator, and then dried in an oven at 120 ° C. for 30 minutes to prepare an electrode with a weight of 20 mg / cm ² , and then rolled by a roll press to prepare a positive electrode with a positive electrode composite layer having a density of 3.1 g / cm ^3. The negative electrode composite slurry was applied to a copper foil having a thickness of 20 μm as a current collector using an applicator, and then dried in an oven at 80 ° C. for 30 minutes to prepare an electrode with a weight of 10 mg / cm ² , and then rolled by a roll press to prepare a negative electrode with a negative electrode composite layer having a density of 1.6 g / cm ³ .
The positive electrode shown in Table 7 and the standard negative electrode shown below, or the negative electrode shown in Table 7 and the standard positive electrode shown below, were punched out to 50 mm x 45 mm and 45 mm x 40 mm, respectively, and the separator (porous polypropylene film) inserted therebetween was inserted into an aluminum laminate bag, and dried in an electric oven at 70 ° C. for 1 hour. Then, in a glove box filled with argon gas, 2 mL of electrolyte was injected, and the aluminum laminate bag was sealed to prepare nonaqueous electrolyte secondary batteries (N1) to (N18), (W1) to (W18). The electrolyte was a nonaqueous electrolyte prepared by mixing ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a ratio of 1:1:1 (volume ratio) to prepare a mixed solvent, and further adding 1 part of VC (vinylene carbonate) to 100 parts of the electrolyte as an additive, and then dissolving LiPF ₆ at a concentration of 1M.

[Preparation of standard negative electrode]
Acetylene black (Denka Co., Ltd., Denka Black (registered trademark) HS-100), CMC (Daicel FineChem Co., Ltd., carboxymethylcellulose #1190), and water were added to a plastic container with a capacity of 150 mL, and then the mixture was stirred for 30 seconds at 2,000 rpm using a rotation and revolution mixer. Furthermore, artificial graphite (Nippon Graphite Industries Co., Ltd., CGB-20) was added as a negative electrode active material, and the mixture was stirred for 150 seconds at 2,000 rpm using a rotation and revolution mixer. SBR (JSR Corporation, TRD2001, solid content 48% dispersion) was then added, and the mixture was stirred for 30 seconds at 2,000 rpm using a rotation and revolution mixer to obtain a standard negative electrode composite slurry. The solid content of the standard negative electrode composite slurry was 48% by mass. The solid content ratio of the negative electrode active material:conductive material:CMC:SBR in the standard negative electrode mixture slurry was 97:0.5:1:1.5.
Next, the standard negative electrode composite slurry was applied to a copper foil having a thickness of 20 μm as a current collector using an applicator, and then dried in an electric oven at 80° C. for 30 minutes to adjust the weight per unit area of the electrode to 10 mg/cm ^2. Further, rolling treatment was performed using a roll press to produce a standard negative electrode (SA) with a negative electrode composite layer density of 1.6 g/cm ³ .

[Preparation of standard positive electrode]
A positive electrode ^active material (BASF Toda Battery Materials LLC, HED (registered trademark) NCM-111 1100) 93 parts by mass, acetylene black (Denka Co., Ltd., Denka Black (registered trademark) HS100) 4 parts by mass, and PVDF (Kureha Battery Materials Japan Co., Ltd., Kureha KF Polymer W # 1300) 3 parts by mass were added to a plastic container with a capacity of 150 cm 3, and then mixed with a spatula until the powder was uniform. Then, 20.5 parts by mass of NMP was added, and the mixture was stirred at 2000 rpm for 30 seconds using a rotation and revolution mixer. Then, the mixture in the plastic container was mixed with a spatula until it was uniform, and the mixture was stirred at 2000 rpm for 30 seconds using the rotation and revolution mixer. Further, 14.6 parts by mass of NMP was added, and the mixture was stirred at 2000 rpm for 30 seconds using the rotation and revolution mixer. Finally, the mixture was stirred at 3000 rpm for 10 minutes using a high-speed stirrer to obtain a standard positive electrode composite slurry. The standard positive electrode composite slurry was then applied to an aluminum foil having a thickness of 20 μm as a current collector using an applicator, and then dried in an electric oven at 120° C. for 30 minutes to adjust the weight per unit area of the electrode to 20 mg/cm ^2. A rolling process was then performed using a roll press to produce a standard positive electrode (SC) with a composite layer density of 3.1 g/cm ³ .

(Evaluation of rate characteristics of non-aqueous electrolyte secondary batteries)
The non-aqueous electrolyte secondary battery was placed in a thermostatic chamber at 25°C, and charge/discharge measurements were performed using a charge/discharge device (SM-8, manufactured by Hokuto Denko Corporation). After constant current/constant voltage charging (cut-off current 1mA (0.02C)) at a charge end voltage of 4.3V at a charge current of 10mA (0.2C), constant current discharging was performed at a discharge end voltage of 3V at a discharge current of 10mA (0.2C). After repeating this operation three times, constant current/constant voltage charging (cut-off current (1mA (0.02C))) was performed at a charge end voltage of 4.3V at a charge current of 10mA (0.2C), and constant current discharging was performed at discharge currents of 0.2C and 3C until the discharge end voltage reached 3.0V, and the discharge capacity was determined. The rate characteristic can be expressed by the ratio of the 0.2C discharge capacity to the 3C discharge capacity, as shown in the following formula 1. The criteria for judgment are shown below, and the results are shown in Table 7.
(Formula 1)
Rate characteristic = 3C discharge capacity / 3rd 0.2C discharge capacity × 100 (%)
Judgment criteria: ◎: Rate characteristics are 80% or more (very good)
○: Rate characteristics are 70% or more and less than 80% (excellent)
◯△: Rate characteristics are 60% or more and less than 70% (good)
△: Rate characteristics are 50% or more and less than 60% (acceptable)
×: Rate characteristic is less than 50% (bad)

(Method for Evaluating Cycle Characteristics of Non-Aqueous Electrolyte Secondary Battery)
The non-aqueous electrolyte secondary battery was placed in a thermostatic chamber at 40° C., and charge/discharge measurements were performed using a charge/discharge device (SM-8, manufactured by Hokuto Denko Corporation). After constant-current constant-voltage charging (cutoff current 2.5 mA (0.05 C)) at a charge current of 25 mA (0.5 C) and a charge end voltage of 4.3 V, constant-current discharging was performed at a discharge current of 25 mA (0.5 C) and a discharge end voltage of 3 V. This operation was repeated 200 times. The cycle characteristics can be expressed by the ratio of the third 0.5 C discharge capacity at 25° C. to the 200th 0.5 C discharge capacity, as shown in the following formula 2. The criteria for judgment are shown below, and the results are shown in Table 7.
(Formula 2)
Cycle characteristic = 3rd 0.5C discharge capacity / 200th 0.5C discharge capacity × 100 (%)
Evaluation criteria: Cycle characteristics are 80% or more (excellent)
◯: Cycle characteristics are 70% or more and less than 80% (excellent)
◯△: Cycle characteristics are 60% or more and less than 70% (good)
△: Cycle characteristics are 60% or more and less than 70% (acceptable)
×: Cycle characteristics are less than 60% (bad)

From the results of the above examples, it was found that by using a carbon nanotube dispersion liquid whose complex modulus and phase angle are within the range of the present invention, a lithium ion secondary battery having superior rate characteristics and cycle characteristics compared to the comparative examples can be obtained.
The carbon nanotube dispersion of the present invention has low viscosity and good dispersibility, and further has good dispersion stability, so that the carbon nanotubes can efficiently form a conductive network in the electrode, which is thought to have resulted in good cycle characteristics. However, in Comparative Example 3-N1 and Comparative Example 3-W1, although the carbon nanotube dispersion has good dispersibility, it is thought that sufficient rate characteristics could not be obtained due to insufficient conductivity. In addition, in Comparative Example 3-N1 and Comparative Example 3-W1, even though a conductive network in the electrode was formed, the cycle characteristics were poor compared to the Examples. These results are considered to be due to the fact that the boron-containing carbon nanotubes of the present invention are harder than the carbon nanotubes not containing boron, so that the conductive network of the carbon nanotubes can be maintained without collapsing against the repeated expansion and contraction of the active material in the electrode during charging and discharging of the battery, thereby improving the cycle characteristics. On the other hand, when comparing Examples 2-N1, 2-N5, and 2-N10, or Examples 3-N1, 3-N5, and 3-N10, it was also found that the difference in boron content of the carbon nanotubes affects the conductivity and peel strength of the electrode (composite layer) and the battery characteristics (rate characteristics, cycle characteristics).
From the above, it has become clear that the present invention can provide a lithium ion secondary battery having high capacity, high output and high durability, which are difficult to achieve with conventional carbon nanotube dispersion liquids.

1: Peak of boron cluster (186-187 eV) 2: Peak of boron carbide (187-188 eV) 3: Peak of boron doped to substitute for carbon elements with a basic skeleton of a hexagonal mesh face (188.2-189.3 eV) 4: Peak of boron oxide BC ₂ O (189.5-190.5 eV) 5: Peak of boron oxide BCO ₂ (191.5-192 eV) 6: Peak of boron oxide B ₂ O ₃ (192.5-193 eV)

Claims (7)

A carbon nanotube dispersion comprising boron-containing carbon nanotubes (excluding branched vapor-grown carbon fibers having a cylindrical hollow structure, an outer fiber diameter of 0.5 μm or less, an aspect ratio of 10 or more, and a compaction resistivity of 0.02 Ω cm or less, and single-walled carbon nanotubes) and a solvent, characterized in that the carbon nanotube dispersion has a complex modulus of less than 200 Pa as measured by dynamic viscoelasticity measurement, a phase angle of 10° or more at a frequency of 1 Hz, a boron content of the boron -containing carbon nanotubes of 0.01 to 3 mol%, and a content of boron element doped to substitute for carbon element on the surface of the boron-containing carbon nanotubes of 0.01 to 1 mol% . 2. The carbon nanotube dispersion according to claim 1, further comprising a dispersant. A carbon nanotube resin composition comprising the carbon nanotube dispersion liquid according to claim 1 or 2 and a binder. A conductive film, which is a coating film of the carbon nanotube dispersion liquid according to claim 1 or 2 , or the carbon nanotube resin composition according to claim 3 . A composite slurry comprising the carbon nanotube resin composition according to claim 3 and an active material. An electrode film, which is a coating film of the composite slurry according to claim 5 . 7. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein at least one of the positive electrode and the negative electrode contains the electrode film according to claim 6 .