Carbon nanotube dispersion liquid

Abstract

Provided are: a carbon nanotube dispersion liquid in which carbon nanotubes are dispersed while suppressing shortening of the lengths of the carbon nanotubes; negative electrode slurry using the carbon nanotube dispersion liquid; a negative electrode; and a secondary battery.　Provided are: a carbon nanotube dispersion liquid comprising carbon nanotubes, a dispersant, and a dispersion medium, wherein 50% or more of bundles of the carbon nanotubes have a length of 10 μm or more; negative electrode slurry containing the carbon nanotube dispersion liquid; a negative electrode; and a secondary battery.

Description

Carbon nanotube dispersion

[0001] The present invention relates to a carbon nanotube dispersion, a negative electrode slurry, a negative electrode, a secondary battery, and a method for observing components dispersed in the dispersion.

[0002] Carbon nanotubes (CNTs) are a carbon allotrope with a seamless, hollow tube structure formed by rolling graphene sheets into a cylindrical shape. Taking advantage of their mechanical strength, electrical conductivity, and thermal conductivity, carbon nanotubes are used as a conductive additive in the electrodes of lithium-ion secondary batteries.

[0003] The positive and negative electrodes of secondary batteries are generally composed of a current collector, an active material, a conductive additive (also referred to as a conductive material), and a binder. For example, Patent Document 1 proposes a negative electrode for secondary batteries in which a negative electrode active material slurry containing a negative electrode active material, single-walled carbon nanotubes (carbon nanotubes) as a conductive material, and a binder is applied to a copper current collector, and a lithium-ion secondary battery using the negative electrode for secondary batteries. In the negative electrode for secondary batteries, carbon nanotubes are expected to be a material that connects particles of the negative electrode active material and forms a conductive network.

[0004] Special Publication No. 2024-512901

[0005] When using carbon nanotubes, it is necessary to prepare a dispersion by mixing carbon nanotubes and a dispersant in a dispersion medium (see, for example, Patent Document 1). However, during the process of preparing the dispersion, the carbon nanotubes are cut and shortened, resulting in a problem that the carbon nanotubes are short in the negative electrode for a secondary battery or in the secondary battery, making it impossible to form a sufficient conductive network between the negative electrode active materials. Therefore, the present invention relates to a carbon nanotube dispersion in which carbon nanotubes are dispersed while preventing the carbon nanotubes from shortening, and to a negative electrode slurry, a negative electrode, and a secondary battery that use the carbon nanotube dispersion.

[0006] As a result of extensive research, the inventors discovered a carbon nanotube dispersion liquid in which the length of the carbon nanotubes is 10 μm or more, and also found that using this carbon nanotube dispersion liquid in a negative electrode for a secondary battery improves the capacity and cycle characteristics of the secondary battery, thereby completing the present invention.

[0007] That is, the present invention relates to the following [1] to

[18] . [1] A carbon nanotube dispersion comprising carbon nanotubes, a dispersant, and a dispersion medium, wherein 50% or more of the carbon nanotube bundles have a length of 10 μm or more. [2] The carbon nanotube dispersion according to [1], wherein the average length of the bundles is 15 to 1000 μm. [3] The carbon nanotube dispersion according to [1] or [2], wherein the average length of the bundles is 6% or more of the average length of the carbon nanotubes at the time of production. [4] The carbon nanotube dispersion according to any of [1] to [3], wherein the carbon nanotubes are multi-walled carbon nanotubes. [5] The carbon nanotube dispersion according to any of [1] to [4], wherein the dispersant is at least one selected from vinyl polymers, cellulose polymers, fluorine-based polymers, acrylic polymers, and nitrile rubbers. [6] The carbon nanotube dispersion liquid according to any one of [1] to [5], wherein the dispersion medium is at least one selected from water, lower alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, ether-based organic solvents, ester-based organic solvents, amide-based organic solvents, and sulfoxide-based organic solvents.

[0008] [7] A method for producing a carbon nanotube dispersion containing carbon nanotubes, a dispersant, and a dispersion medium, wherein 50% or more of the bundles of the carbon nanotubes have a length of 10 μm or more, the method comprising the step of immersing a carbon nanotube forest formed on a substrate in a solution containing the dispersant and the dispersion medium. [8] A method for producing a carbon nanotube dispersion according to [7], further comprising the step of separating the carbon nanotubes from the substrate after the immersion step, and then adding the dispersant and the dispersion medium to the carbon nanotubes and performing a kneading treatment.

[0009] [9] A negative electrode slurry comprising a negative electrode active material, a binder, and the carbon nanotube dispersion liquid according to any one of [1] to [6].

[10] A negative electrode slurry comprising a negative electrode active material, a binder, carbon nanotubes, a dispersant, and a dispersion medium, wherein the median bundle length of the carbon nanotubes is longer than the average particle size of the negative electrode active material.

[0010]

[11] A method for producing a negative electrode slurry, comprising a step of mixing the carbon nanotube dispersion liquid according to any one of [1] to [6] above or the carbon nanotube dispersion liquid obtained by the production method according to [7] or [8] above, a negative electrode active material, and a binder.

[0011]

[12] A negative electrode comprising a negative electrode current collector and a coating film of the negative electrode slurry according to [9] or

[10] formed on the negative electrode current collector.

[13] A secondary battery comprising the negative electrode according to

[12] .

[0012]

[14] A method for observing a component dispersed in a dispersion, the method comprising the steps of dropping the dispersion onto a substrate that has been subjected to functionalization treatment, drying the dispersion on the substrate, and observing the substrate after drying.

[15] The observation method according to

[14] , in which at least one selected from the shape, length, and diameter of the component dispersed in the dispersion is observed.

[16] The observation method according to

[14] or

[15] , in which the substrate is a silicon substrate.

[17] The observation method according to any one of

[14] to

[16] , in which the functionalization treatment is an excimer light irradiation treatment.

[18] The observation method according to any one of

[14] to

[17] , in which the concentration of the component dispersed in the dispersion is adjusted to 0.00001 to 1 mass %.

[0013] According to the present invention, it is possible to provide a carbon nanotube dispersion liquid containing long carbon nanotubes. When the carbon nanotube dispersion liquid of the present invention is used in the negative electrode of a secondary battery, the carbon nanotubes can form a conductive network with excellent conductivity between the negative electrode active materials, thereby providing a secondary battery that exhibits excellent cycle characteristics.

[0014] 1 shows the results of measuring the water contact angle of a silicon wafer surface before and after functionalization treatment. SEM observation images of carbon nanotube dispersions. (a) SEM observation images of carbon nanotube dispersions of Example 1 and Example 2. (c) SEM observation image of negative electrode slurry of Comparative Example 1. (a) SEM observation images of Example 4 and Example 4 and Example 3. (b) SEM observation images of negative electrodes of Example 7 and Comparative Examples 10 and 11. Results of a cycle test of Example 6 are shown. Results of a cycle test of Comparative Example 8 are shown. Results of a cycle test of Example 6 and Comparative Examples 7 and 9 are shown. Results of a cycle test of Example 7 and Comparative Examples 10 and 11 are shown. Results of a cycle test of Example 6 are shown. Results of a cycle test of Comparative Example 8 are shown. SEM images of negative electrodes of Example 6 and Comparative Example 8 before the cycle test. SEM images of negative electrodes of Example 6 and Comparative Example 8 after the cycle test.

[0015] The upper and / or lower limits of the numerical ranges described herein can be arbitrarily combined to define a preferred range. For example, the upper and lower limits of the numerical ranges can be arbitrarily combined to define a preferred range, the upper limits of the numerical ranges can be arbitrarily combined to define a preferred range, and the lower limits of the numerical ranges can be arbitrarily combined to define a preferred range.

[0016] Throughout this specification, singular terms should be understood to include the plural concept unless otherwise stated. Therefore, singular articles (e.g., "a," "an," "the," etc. in English) should be understood to include the plural concept unless otherwise stated.

[0017] In this specification, various components may be used independently, either singly or in combination of two or more, unless otherwise specified.

[0018] In this specification, the term "carbon nanotube" refers to a cylindrical structure made of graphene sheets. Carbon nanotubes are sometimes referred to as "CNT."

[0019] In this specification, the length of the bundle of carbon nanotubes can be measured by the method described later in "Evaluation of the length of the bundle of carbon nanotubes in a dispersion liquid."

[0020] In this specification, unless otherwise specified, the "carbon nanotube dispersion liquid" does not contain an active material. The active material is a material that is the basis of the battery reaction. When a negative electrode active material is contained, it is referred to as "negative electrode slurry."

[0021] In this specification, the term "secondary battery" generally refers to an electricity storage device that can be repeatedly charged and discharged. Secondary batteries include storage batteries (i.e., chemical batteries) such as lithium-ion secondary batteries, nickel-metal hydride batteries, and nickel-cadmium batteries, and capacitors (i.e., physical batteries) such as electric double-layer capacitors.

[0022] <Carbon Nanotube Dispersion> The carbon nanotube dispersion of the present invention contains carbon nanotubes, a dispersant, and a dispersion medium. At least 50% of the carbon nanotube bundles have a length of 10 μm or more. Here, "%" refers to the ratio of the number of carbon nanotube bundles having a length of 10 μm or more to the total number of carbon nanotube bundles. The percentage of carbon nanotube bundles having a length of 10 μm or more is 50% or more, but is preferably 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 93% or more, 96% or more, 98% or more, or 99% or more. The upper limit is 100%. In the examples described below, a carbon nanotube dispersion was obtained in which 100% of the bundles had a length of 10 μm or more. Preferably, 50-100%, 55-100%, 60-100%, 65-100%, 70-100%, 75-100%, 80-100%, 85-100%, 90-100%, 93-100%, 96-100%, 98-100%, or 99-100% of the carbon nanotubes are 10 μm or longer in length.

[0023] In the carbon nanotube dispersion of the present invention, 50% or more of the bundles have a length of 10 μm or more, but 50% or more of the bundles may have a length of 13 μm or more, 15 μm or more, 17 μm or more, 20 μm or more, 30 μm or more, 40 μm or more, 50 μm or more, 55 μm or more, 60 μm or more, 65 μm or more, 70 μm or more, 75 μm or more, 80 μm or more, 85 μm or more, or 90 μm or more. Preferred embodiments of the percentage of carbon nanotube bundles having a length of at least the above length include ranges similar to those of the preferred embodiments of the percentage of carbon nanotube bundles having a length of at least 10 μm, such as 50 to 100%, 55 to 100%, 60 to 100%, 65 to 100%, 70 to 100%, 75 to 100%, 80 to 100%, 85 to 100%, 90 to 100%, 93 to 100%, 96 to 100%, 98 to 100%, or 99 to 100%. Regarding the percentage of carbon nanotube bundles, the upper limit of the bundle length is not particularly limited, but is preferably 500 μm or less, 450 μm or less, 400 μm or less, 350 μm or less, or 300 μm or less. An example of a case where the upper limit of the length of the bundle is specified is that 50% or more of the bundles of carbon nanotubes have a length of 10 μm or more and 500 μm or less.

[0024] In the carbon nanotube dispersion, the average length of the carbon nanotube bundles is preferably 15 to 1000 μm, and more preferably 15 to 500 μm. The average length of the carbon nanotube bundles is preferably 17 μm or more, 20 μm or more, 30 μm or more, 40 μm or more, 50 μm or more, 60 μm or more, 70 μm or more, 80 μm or more, or 90 μm or more. The upper limit is preferably 500 μm or less, 400 μm or less, 300 μm or less, 250 μm or less, or 235 μm or less.

[0025] When the average length of the carbon nanotube bundles is 15 μm or more, the upper limit is preferably less than 50 μm, 40 μm or less, or 35 μm or less. Specifically, the average length of the carbon nanotube bundles is preferably 15 to less than 50 μm, 17 to 40 μm, or 20 to 35 μm.

[0026] When the average length of the carbon nanotube bundles is 50 μm or more, the upper limit is preferably 500 μm or less, 400 μm or less, 300 μm or less, 250 μm or less, or 235 μm or less. Specifically, the average length of the carbon nanotube bundles is preferably 50 to 500 μm, 60 to 400 μm, 70 to 300 μm, 80 to 250 μm, or 90 to 235 μm.

[0027] In the carbon nanotube dispersion, the average length of the carbon nanotube bundles is preferably 6% or more of the average length of the carbon nanotubes at the time of production. Here, the average length of the carbon nanotubes at the time of production refers to the average length of the carbon nanotubes constituting the carbon nanotube forest immediately after production when carbon nanotubes are produced by the CVD method described below. The average length of the carbon nanotube bundles is preferably 7% or more, 8% or more, 9% or more, 10% or more, 20% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 90% or more of the average length of the carbon nanotubes at the time of production. Furthermore, the average length of the carbon nanotube bundles is preferably 100% or less, less than 100%, 98% or less, 96% or less, 94% or less, or 92% or less of the average length of the carbon nanotubes at the time of production.

[0028] When the average length of the carbon nanotube bundles is less than 50 μm, the average length of the carbon nanotube bundles is preferably less than 25%, 23% or less, 20% or less, 17% or less, or 15% or less of the average length of the carbon nanotubes as produced, more specifically, preferably 7 to less than 25%, 7 to 23%, 8 to 20%, 9 to 17%, or 10 to 15%.

[0029] When the average length of the carbon nanotube bundles is 50 μm or more, the average length of the carbon nanotube bundles is preferably 100% or less, less than 100%, 98% or less, 96% or less, 94% or less, or 92% or less of the average length of the carbon nanotubes at the time of manufacture. Specifically, the average length is preferably 30 to 100%, 30 to less than 100%, 35 to 98%, 40 to 98%, 45 to 98%, 50 to 96%, 55 to 96%, 60 to 96%, 65 to 94%, 70 to 94%, 75 to 94%, 80 to 92%, or 90 to 92%. The thickness of the carbon nanotube bundles is not particularly limited, but is typically 20 nm to 1 μm.

[0030] The carbon nanotubes may be either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), or a mixture of these. Single-walled carbon nanotubes have one tube layer, while multi-walled carbon nanotubes have a multi-layer structure with two or more tube layers, preferably 2 to 8 layers, and more preferably 3 to 7 layers. From the viewpoint of improving and uniforming the productivity and quality (length, diameter, purity, etc.) of carbon nanotubes, multi-walled carbon nanotubes are preferred.

[0031] The average diameter of the carbon nanotubes is preferably 6 to 10 nm, and can be measured by observing the morphology of the carbon nanotubes using a transmission electron microscope (TEM).

[0032] The carbon purity of the carbon nanotubes is preferably 95 to 99.999%, more preferably 96 to 99.99%, even more preferably 97 to 99.9%, particularly preferably 98 to 99.9%, and most preferably 99 to 99.9%.

[0033] The crystallinity (D / G ratio) of the carbon nanotubes is preferably 0.5 to 1.0, more preferably 0.6 to 0.8.

[0034] The carbon nanotube content in the carbon nanotube dispersion is 0.00001 to 3 mass %, 0.0001 to 2 mass %, 0.001 to 1.5 mass %, 0.01 to 1.3 mass %, or 0.1 to 1.1 mass %.

[0035] Carbon nanotubes can be obtained by, for example, an arc discharge method, a laser ablation method, a chemical vapor deposition (CVD) method, etc. From the viewpoint of improving productivity and uniformity of quality (length, diameter, purity, etc.) of carbon nanotubes, the CVD method is preferred.

[0036] CVD utilizes a chemical reaction of a carbon-containing source gas to grow countless carbon nanotubes (carbon nanotube forests) perpendicular to the substrate surface. The substrate material is not particularly limited, and examples include materials that can withstand high temperatures, such as stainless steel. Examples of carbon-containing source gases include hydrocarbons, alcohols, and carbon monoxide. Catalysts include metals such as iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), gold (Au), and alloys thereof, as well as precursors thereof (e.g., oxides or compounds). The catalyst is preferably formed as a catalyst layer on the substrate surface by coating or sputtering. The CVD temperature is preferably 600 to 850°C, more preferably 650 to 800°C. The pressure is not particularly limited, and the process can be performed under atmospheric pressure, reduced pressure, or increased pressure.

[0037] In the carbon nanotube dispersion liquid, the dispersant disperses the carbon nanotubes in the dispersion medium. The dispersant is not particularly limited, and for example, at least one selected from vinyl polymers, cellulose polymers, fluorine-based polymers, acrylic polymers, and nitrile rubbers can be used. Examples of vinyl polymers include polyvinyl alcohol, polyvinylpyrrolidone, and polyvinyl methyl ether. Examples of cellulose polymers include alkyl celluloses such as methyl cellulose and ethyl cellulose, hydroxyalkyl celluloses such as hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropylmethyl cellulose, and carboxymethyl cellulose or salts thereof. Examples of salts include alkali metal salts such as sodium salts and potassium salts, and ammonium salts. Examples of fluorine-based polymers include polyvinylidene fluoride and fluorinated ethylene-propylene copolymers. Examples of acrylic polymers include sodium polyacrylate, polyethyl acrylate, and polyacrylamide. Examples of nitrile rubbers include acrylonitrile-butadiene rubber and hydrogenated acrylonitrile-butadiene rubber. The dispersant is preferably at least one selected from vinyl polymers, cellulose polymers, and nitrile rubbers, and more preferably at least one selected from polyvinylpyrrolidone, carboxymethyl cellulose or a salt thereof, and hydrogenated acrylonitrile-butadiene rubber.

[0038] The weight average molecular weight of the dispersant is not particularly limited, and may be within a range known for dispersants of conductive assistants, for example.

[0039] The content of the dispersant in the carbon nanotube dispersion liquid is, in terms of the mass ratio of carbon nanotubes to dispersant, carbon nanotubes:dispersant, for example, 1:0.1 to 1:10, 1:0.2 to 1:5, 1:0.5 to 1:2, or 1:0.8 to 1:1.25.

[0040] Examples of dispersion media include polar solvents and nonpolar solvents. Examples include water, lower alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ketone-based, ether-based, ester-based, amide-based, and sulfoxide-based organic solvents, as well as mixtures thereof. Examples of lower alcohols include methanol, ethanol, and isopropanol. Examples of aliphatic hydrocarbons include hexane and heptane. Examples of aromatic hydrocarbons include toluene and xylene. Examples of halogenated hydrocarbons include methylene chloride and chloroform. Examples of ketone-based organic solvents include acetone and methyl ethyl ketone. Examples of ether-based organic solvents include tetrahydrofuran, diethylene glycol dimethyl ether, and diethylene glycol diethyl ether. Examples of ester-based organic solvents include ethyl acetate, butyl acetate, and γ-butyl lactone. Examples of amide-based organic solvents include N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), and N-ethylpyrrolidone. Examples of sulfoxide-based organic solvents include dimethyl sulfoxide. From the viewpoints of the stability of the carbon nanotube dispersion and the ease of availability of the dispersion medium, the dispersion medium is preferably at least one selected from water, lower alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, ether-based organic solvents, ester-based organic solvents, amide-based organic solvents, and sulfoxide-based organic solvents. In another embodiment, the dispersion medium is at least one selected from water, methanol, ethanol, heptane, toluene, diethylene glycol diethyl ether, ethyl acetate, butyl acetate, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide, and more preferably at least one selected from water and N-methylpyrrolidone. Examples of water include ion-exchanged water, RO water, distilled water, pure water, and ultrapure water.

[0041] In addition to the above components, the carbon nanotube dispersion of the present invention may optionally contain other conductive aids such as carbon black, surfactants, antifoaming agents, pH adjusters, viscosity adjusters, tackifiers, crosslinking agents for polymers, preservatives, antioxidants, etc.

[0042] In this specification, the length of the carbon nanotube bundles in a carbon nanotube dispersion can be calculated by observing the morphology of the carbon nanotube bundles in an observation sample obtained by dropping the carbon nanotube dispersion onto a functionalized substrate and drying the substrate. Normally, when a carbon nanotube dispersion is dropped onto a substrate and then dried, droplets such as water droplets (structures that rise perpendicular to the substrate surface) dry, causing the carbon nanotubes to fold or aggregate after drying. In contrast, by functionalizing the substrate, the dispersion does not bulge when dropped onto the substrate, but spreads over the substrate. That is, the dispersion on the substrate becomes very thin. As a result, the carbon nanotubes are less likely to fold or aggregate after drying, making it possible to measure the length of each carbon nanotube bundle.

[0043] The morphological observation of an observation sample using a functionalized substrate can also be applied to dispersions other than carbon nanotube dispersions. Therefore, according to the present invention, there is provided a method for observing a component dispersed in a dispersion (hereinafter also referred to as a dispersed component), which includes the steps of dropping the dispersion onto a functionalized substrate, drying the dispersion on the substrate, and observing the substrate after drying.

[0044] <Method for observing components dispersed in a dispersion> In the method for observing components dispersed in a dispersion of the present invention, the dispersion is a solution in which the dispersed components are dispersed as fine particles or the like in a liquid dispersion medium. There are no particular limitations on the type of liquid or the type of dispersed components. The dispersion is preferably a carbon nanotube dispersion, and in one aspect, it is a carbon nanotube dispersion containing the carbon nanotubes described above, a dispersant, and a dispersion medium, and in which 50% or more of the carbon nanotube bundles have a length of 10 μm or more.

[0045] The substrate to be subjected to the functionalization treatment is preferably a silicon substrate. Examples of the functional group include polar groups such as a hydroxyl group, a carbonyl group, an epoxy group, and a carboxyl group. The functionalization treatment can be performed, for example, by irradiation with ultraviolet light. The amount of ultraviolet light to be irradiated and the irradiation time are not particularly limited. Examples of ultraviolet light sources include low-pressure mercury lamps, high-pressure mercury lamps, excimer lasers, excimer lamps, and metal halide lamps. Irradiation with excimer light (e.g., wavelength 172 nm) from an excimer lamp is preferred.

[0046] Such functionalization treatment improves, for example, the hydrophilicity of the substrate surface. The improved hydrophilicity can be confirmed by measuring the contact angle of the substrate surface with water. Compared to the contact angle before the functionalization treatment, the contact angle after the functionalization treatment is reduced. The contact angle with water after the functionalization treatment is preferably 15° or less, 12° or less, 10° or less, or 8° or less.

[0047] The concentration of the dispersed component (carbon nanotubes in the case of a carbon nanotube dispersion) in the dispersion liquid to be dropped onto the substrate is preferably 0.00001 to 1 mass %, or 0.0001 to 0.1 mass %. The observation method of the present invention may further include a step of adjusting the concentration of the dispersed component in the dispersion liquid to 0.00001 to 1 mass %. The amount of the dispersion liquid to be dropped is not particularly limited.

[0048] The method for drying the dispersion dropped onto the functionalized substrate is not particularly limited, and examples thereof include natural drying, ventilation drying, heat drying, and vacuum drying.

[0049] When observing the substrate after drying, the shape and diameter of the dispersed components can be observed in addition to the length. For the observation, for example, a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning probe microscope (SPM), a laser confocal microscope, etc. can be used. A scanning electron microscope (SEM) is preferred.

[0050] <Method for Producing Carbon Nanotube Dispersion> The carbon nanotube dispersion of the present invention can be obtained, for example, by kneading carbon nanotubes, a dispersant, and a dispersion medium using a kneading device. Known devices may be used as the kneading device. Examples include a kneader, planetary mixer, extruder, roll mill, ball mill, bead mill, sand mill, ultrasonic homogenizer, high-pressure homogenizer, and attritor. From the viewpoint of increasing the length of the carbon nanotube bundle, a roll mill is preferred. Examples of roll mills that can be used include a two-roll mill and a three-roll mill. The order of treatment of each component is not particularly limited, and the components can be treated in any order. The treatment may also be carried out in multiple batches.

[0051] From the viewpoint of economy, the temperature during kneading is preferably room temperature (for example, 15 to 40° C.).

[0052] From the viewpoint of increasing the length of the carbon nanotube bundles, the carbon nanotube dispersion of the present invention is preferably obtained by a production method including a step of immersing a carbon nanotube forest formed on a substrate in a solution containing a dispersant and a dispersion medium. The carbon nanotube forest formed on a substrate can be obtained by the CVD method described above. The dispersant and dispersion medium are as described above. Hereinafter, in this specification, the step of immersing a carbon nanotube forest formed on a substrate in a solution containing a dispersant and a dispersion medium is also referred to as the immersion step.

[0053] In the immersion step, the amount of the solution containing the dispersant and the dispersion medium in which the carbon nanotube forest is immersed is not particularly limited as long as it is an amount that allows the carbon nanotube forest formed on the substrate to be sufficiently saturated with the solution, for example, 1.1 to 1000 times by mass, or 1.5 to 500 times by mass, of the dispersant and the dispersion medium that constitute the carbon nanotube dispersion.

[0054] The content of the dispersant in the solution containing the dispersant and the dispersion medium is not particularly limited as long as a carbon nanotube dispersion liquid can be obtained.

[0055] The carbon nanotube forest formed on the substrate is preferably immersed from the carbon nanotube forest side in a solution containing a dispersant and a dispersion medium, from the viewpoint of sufficiently impregnating the carbon nanotube forest formed on the substrate with the dispersant. The immersion time is, for example, 10 seconds to 24 hours, 30 seconds to 12 hours, 50 seconds to 1 hour, or 1 minute to 10 minutes. The immersion temperature is, for example, room temperature (15 to 40°C). The immersion step may be performed once or multiple times (for example, two or three times).

[0056] The immersion process allows the carbon nanotubes to be impregnated with the dispersant. After the immersion process, it is preferable to separate the carbon nanotubes from the substrate, then add a dispersant and a dispersion medium to the carbon nanotubes, and perform a kneading process using the kneading device. The dispersant and dispersion medium used here may be the same type as the dispersant and dispersion medium used in the immersion process, or may be different types. Furthermore, the amounts of dispersant and dispersion medium added are not particularly limited as long as a carbon nanotube dispersion liquid can be obtained. After the kneading process, the carbon nanotube dispersion liquid may be diluted with a dispersion medium, if necessary.

[0057] The carbon nanotube dispersion of the present invention thus obtained can be used, for example, for positive and negative electrodes of secondary batteries such as lithium ion secondary batteries, antistatic materials, electronic components, a substitute for transparent electrodes (ITO films), electromagnetic wave shields, etc. In particular, it is suitable for use in negative electrodes of secondary batteries.

[0058] <Negative Electrode Slurry> The negative electrode slurry of the present invention contains a negative electrode active material, a binder, and a carbon nanotube dispersion. The carbon nanotube dispersion is as described above. The content of carbon nanotubes in the negative electrode slurry is 0.01 to 5 mass%, preferably 0.05 to 1 mass%, and more preferably 0.08 to 0.6 mass%, per 100 mass% of the total of the negative electrode active material, the binder, and the carbon nanotubes. The content of the dispersant in the negative electrode slurry, in terms of the mass ratio of carbon nanotubes to dispersant, is, for example, 1:0.1 to 1:10, 1:0.2 to 1:5, 1:0.5 to 1:2, or 1:0.8 to 1:1.25.

[0059] Examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, expanded graphite, activated carbon, carbon fiber, coke, soft carbon, and hard carbon; silicon alloys; Si; SiC; SiO x Si compounds such as (0.5≦x≦1.5); Li x Fe2O3 (0≦x≦1), Li x WO2 (0≦x≦1), Sn x Me 1-x Me' y O z Examples of materials that can be used include metal composite oxides such as (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of Groups 1, 2, and 3 of the periodic table, and halogens; 0<x≦1; 1≦y≦3; 1≦z≦8); lithium metal; lithium alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; and Li-Co-Ni-based materials.

[0060] The negative electrode active material preferably contains a silicon-based negative electrode, such as a silicon alloy, Si, or a Si compound, because it exhibits high capacity characteristics. Silicon-based negative electrodes undergo large volume changes during charge and discharge, which tends to reduce cycle performance. However, by using the carbon nanotube dispersion of the present invention in the negative electrode of a secondary battery, a sufficient conductive network can be formed between the negative electrode active materials, thereby improving cycle performance. The Si and Si compounds may be coated with carbon. Examples of such carbon include carbon black, such as furnace black and acetylene black; graphene; and graphite, such as natural graphite, artificial graphite, expanded graphite, soft carbon, and hard carbon. The content of at least one selected from Si, Si compounds, and carbon-coated materials thereof in the negative electrode active material is 1% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 50% by mass or more, and 100% by mass or less, 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, or 50% by mass or less.

[0061] The average particle size of the negative electrode active material is not particularly limited and may be within the conventional range in the field of negative electrode active materials, and is generally 25 μm or less, preferably 22 μm or less. The average particle size of the negative electrode active material is determined by measuring the particle size distribution of the negative electrode active material on a volume basis by a laser diffraction / scattering method, and determining the particle size (D50) at which the cumulative frequency in the particle size distribution is 50% by volume.

[0062] The content of the negative electrode active material in the negative electrode slurry is 90 to 99.9 mass%, preferably 92 to 99.5 mass%, and more preferably 94 to 99 mass%, per 100 mass% of the total of the negative electrode active material, the binder, and the carbon nanotubes.

[0063] The binder binds the negative electrode active material, the conductive additive, etc., and binds the negative electrode active material and the negative electrode current collector. There are no particular limitations on the binder, and examples of the binder that can be used include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose or a salt thereof, starch, polyethylene, polypropylene, vinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, polyacrylonitrile, and mixtures thereof.

[0064] As shown in FIG. 4 in the Examples below, the median length of the carbon nanotube bundles in the anode slurry of the present invention is longer than the average particle size of the anode active material, and the carbon nanotubes contact multiple particles of the anode active material. Therefore, the carbon nanotubes can form a sufficient conductive network between the anode active material. The anode slurry of the present invention is preferably an anode slurry for secondary batteries. In the anode slurry of the present invention, 50% or more of the carbon nanotube bundles have a length of 10 μm or more. Here, "%" refers to the ratio of the number of carbon nanotube bundles having a length of 10 μm or more to the total number of carbon nanotube bundles. The percentage of carbon nanotube bundles having a length of 10 μm or more is 50% or more, but is preferably 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 93% or more, 96% or more, 98% or more, or 99% or more. The upper limit is 100%. In the examples described below, negative electrode slurries were obtained in which 100% of the bundles had lengths of 10 μm or more. Preferably, 50 to 100%, 55 to 100%, 60 to 100%, 65 to 100%, 70 to 100%, 75 to 100%, 80 to 100%, 85 to 100%, 90 to 100%, 93 to 100%, 96 to 100%, 98 to 100%, or 99 to 100% of the carbon nanotubes had lengths of 10 μm or more.

[0065] In the carbon nanotube dispersion of the present invention, 50% or more of the bundles have a length of 10 μm or more, but 50% or more of the bundles may have a length of 11 μm or more, 12 μm or more, or 13 μm or more. Preferred embodiments of the percentage of carbon nanotube bundles having a length of 10 μm or more include ranges similar to the preferred embodiments of the percentage of carbon nanotube bundles having a length of 10 μm or more, such as 50 to 100%, 55 to 100%, 60 to 100%, 65 to 100%, 70 to 100%, 75 to 100%, 80 to 100%, 85 to 100%, 90 to 100%, 93 to 100%, 96 to 100%, 98 to 100%, or 99 to 100%. In the percentage of the carbon nanotube bundles, the upper limit of the length of the bundles is not particularly limited, but is preferably 100 μm or less, 80 μm or less, 60 μm or less, 40 μm or less, or 30 μm or less. An example of a case where the upper limit of the bundle length is specified is that 50% or more of the carbon nanotube bundles have a length of 10 μm or more and 100 μm or less.

[0066] The negative electrode slurry of the present invention may contain, in addition to the above components, a protective colloid, an antibacterial agent, an antifungal agent, an antioxidant, a colorant, an antifoaming agent, a crosslinking agent, an emulsion stabilizer, a chelating agent, and the like, if desired.

[0067] <Method for Producing Negative Electrode Slurry> The negative electrode slurry of the present invention can be obtained, for example, by mixing and stirring a negative electrode active material, a binder, and a carbon nanotube dispersion. The order in which the components are mixed is not particularly limited, and they can be mixed in any order. For example, a method can be used in which the carbon nanotube dispersion and the negative electrode active material are mixed and stirred, and then the binder is mixed. Another example can be a method in which the negative electrode active material and the binder are mixed, and then the mixture is mixed with the carbon nanotube dispersion. The stirring device is not particularly limited as long as the effects of the present invention are not impaired, and known devices can be used.

[0068] <Negative electrode> The negative electrode of the present invention includes a negative electrode current collector and a coating film of the above-described negative electrode slurry formed on the negative electrode current collector. The coating film of the negative electrode slurry can be formed by applying the negative electrode slurry to the current collector and drying it. The coating film constitutes a negative electrode mixture layer. The negative electrode of the present invention is preferably a negative electrode for a secondary battery.

[0069] The current collector is not particularly limited, and conductive materials such as titanium, titanium alloys, aluminum, aluminum alloys, copper, nickel, stainless steel, nickel-plated steel, and carbon are used. The current collector may be in the form of a foil, plate, mesh, three-dimensional mesh, foam, or nonwoven fabric, and may be either porous or non-porous. The conductive material may be surface-treated to improve adhesion and electrical properties. The thickness of the current collector, for example, in the case of a foil, is generally 5 μm to 30 μm.

[0070] The negative electrode slurry can be applied using, for example, a bar coater, die coater, slit coater, comma coater, gravure coater, blade coater, or roll coater. The negative electrode slurry is applied to at least one surface of the current collector. It may also be applied to both surfaces of the current collector. The drying method after application is not particularly limited, and examples include natural drying, ventilation drying, heat drying, vacuum drying, infrared heating, and far-infrared heating. The drying temperature is preferably 105°C or less or 100°C or less, and the drying time is preferably 1 second to 5 hours. After drying, a rolling treatment such as a plate press or a roll press may be performed.

[0071] <Secondary Battery> The secondary battery of the present invention includes the above-described negative electrode. The secondary battery may also include a positive electrode, an electrolyte (electrolyte solution or solid electrolyte), and a separator. Known materials may be used for the positive electrode, electrolyte, and separator. For example, the positive electrode includes a positive electrode current collector, a positive electrode active material, a binder, and a conductive additive. The positive electrode active material is not particularly limited, and for example, a lithium-containing composite oxide or the like may be used. The positive electrode current collector, binder, and conductive additive are also not particularly limited, and conventionally known materials may be used. The electrolyte may be an electrolytic solution or a solid electrolyte. The electrolytic solution is not particularly limited, and for example, a non-aqueous solvent containing a lithium salt is used. The separator is not particularly limited, and for example, a nonwoven fabric such as a polyethylene nonwoven fabric, a polymer film, or the like may be used.

[0072] The secondary battery can be formed into various shapes depending on the intended use, such as a paper type, a cylindrical type, a button type, a laminated type, etc. The secondary battery of the present invention has excellent cycle characteristics and is therefore useful as a battery for electric vehicles, portable devices, etc.

[0073] The present invention will be explained in more detail below based on examples, but the present invention is not limited to these examples.

[0074] [Production Example 1] Production of a carbon nanotube forest A wafer coated with a catalyst for carbon nanotube growth was prepared, and vertically aligned carbon nanotubes were grown from the catalyst by chemical vapor deposition to produce a vertically aligned carbon nanotube forest oriented perpendicular to the wafer. The carbon nanotubes constituting the carbon nanotube forest had an average length of 250 μm per tube, a carbon nanotube diameter of 6 to 10 nm, a purity of ≥ 99.8%, and a crystallinity (D / G ratio) of 0.6 to 0.8.

[0075] Example 1 Preparation of Carbon Nanotube Dispersion Approximately 0.6 g of the carbon nanotube forest formed on the wafer obtained in Production Example 1 was scraped off from the wafer using a scraper to obtain carbon nanotubes.

[0076] (Mixing with a Three-Roll Roll) A carboxymethylcellulose sodium aqueous solution (CMC Na (manufactured by Hayashi Pure Chemical Industries, Ltd.): 0.6 g, pure water: 59.4 g) and pure water were added to the carbon nanotubes and mixed with a three-roll roll to obtain a carbon nanotube dispersion. A dispersion containing 0.8 wt % or more of carbon nanotubes is referred to as a high-concentration carbon nanotube dispersion.

[0077] (Concentration Adjustment) Pure water was added to the high-concentration carbon nanotube dispersion to obtain a carbon nanotube dispersion with a carbon nanotube concentration of 0.4 wt %.

[0078] Example 2 Preparation of Carbon Nanotube Dispersion (Impregnation Step) Approximately 0.6 g of the carbon nanotube forest formed on the wafer obtained in Production Example 1 was immersed and coated from the carbon nanotube forest side into an aqueous solution of sodium carboxymethylcellulose (1.0 g of CMC Na (manufactured by Hayashi Pure Chemical Industries, Ltd.), 114 g of pure water). The wafer was immersed thoroughly until all air bubbles were removed from between the carbon nanotube forest, and the aqueous solution of sodium carboxymethylcellulose was impregnated. The impregnated carbon nanotube forest was scraped off from the wafer using a scraper to obtain impregnated carbon nanotubes.

[0079] (Mixing with a Three-Roll Roll) A carboxymethylcellulose sodium aqueous solution (CMC Na (manufactured by Hayashi Pure Chemical Industries, Ltd.): 0.25 g, pure water: 24.75 g) and pure water were added to the impregnated carbon nanotubes, and the mixture was mixed with a three-roll roll to obtain a high-concentration carbon nanotube dispersion.

[0080] (Concentration Adjustment) Pure water was added to the high-concentration carbon nanotube dispersion to obtain a carbon nanotube dispersion with a carbon nanotube concentration of 0.4 wt %.

[0081] Example 3 Preparation of Carbon Nanotube Dispersion (Impregnation Step) Approximately 0.6 g of the carbon nanotube forest formed on the wafer obtained in Production Example 1 was immersed and coated from the carbon nanotube forest side into a hydrogenated acrylonitrile-butadiene rubber solution (1.0 g of HNBR Therban 3406 (manufactured by Arlanxeo), 49 g of NMP). The wafer was thoroughly immersed until all air bubbles were removed from between the carbon nanotube forests, and the hydrogenated acrylonitrile-butadiene rubber solution was impregnated. The impregnated carbon nanotube forest was scraped off from the wafer using a scraper to obtain impregnated carbon nanotubes.

[0082] (Mixing with a Three-Roll Roll) A hydrogenated acrylonitrile-butadiene rubber solution (HNBR Therban 3406 (manufactured by Arlanxeo): 2.0 g, NMP: 23 g) and NMP were added to the impregnated carbon nanotubes, and the mixture was mixed with a three-roll roll to obtain a high-concentration carbon nanotube dispersion.

[0083] (Concentration Adjustment) NMP was added to the high-concentration carbon nanotube dispersion to obtain a carbon nanotube dispersion with a carbon nanotube concentration of 0.4 wt %.

[0084] Comparative Example 1 A water dispersion of single-walled carbon nanotubes (TUBALL manufactured by OCSiAl) (dispersant: aqueous solution of sodium carboxymethyl cellulose (CMC Na) (carbon nanotube concentration 0.4 wt %)) (commercially available) was used.

[0085] [Evaluation of the length of carbon nanotube bundles in the dispersion] A silicon wafer was irradiated with excimer light (172 nm) using an excimer lamp to functionalize the surface of the silicon wafer. Water was dropped onto the silicon wafer before and after the functionalization treatment, and the contact angle was measured. The results are shown in Figure 1. The contact angle before the functionalization treatment was 25.694°, and the contact angle after the treatment was 6.385°.

[0086] 0.002 g of the carbon nanotube dispersion prepared in each of the Examples and Comparative Examples was diluted with 5 to 10 g of water or MNP. The same water or MNP was used as the dispersion medium used to prepare the dispersion. One drop of the diluted dispersion was placed on a functionalized silicon wafer. The silicon wafer was placed on a hot plate and the dispersion was dried at 100°C. A metal coating for SEM observation was applied, and observations were performed using an SEM (JSM-7001F, manufactured by JEOL Ltd.). Observed images are shown in Figure 2 ((a) Example 1, (b) Example 2) and Figure 3 ((c) Comparative Example 1). The lengths of the carbon nanotube bundles were measured from the images using ImageJ (image processing software), and the results are shown in Tables 1 to 3.

[0087]

[0088]

[0089]

[0090] The median length of the carbon nanotube bundles in Example 1 was 23.384 μm and the average length was 23.898 μm, the median length of the carbon nanotube bundles in Example 2 was 269.681 μm and the average length was 222.780 μm, and the median length of the carbon nanotube bundles in Comparative Example 1 was 3.229 μm and the average length was 3.969 μm.

[0091] [Examples 4 and 5, Comparative Examples 2 to 6] Preparation of Negative Electrode Slurry The following components were used as raw materials: Negative electrode active material: carbon-coated SiO (SDCF-Si, average particle size 5 μm, manufactured by Tech One Co., Ltd.), SiO powder (SiO-P-101, average particle size 1 μm, manufactured by Japan NER Co., Ltd.), graphite powder (average particle size 8 μm (used in Example 5 and Comparative Examples 5 and 6) or 20 μm (used in Comparative Example 4), manufactured by Ito Graphite Industries Co., Ltd.).

[0092] CNT dispersion: carbon nanotube dispersion obtained in Example 1 (CNT concentration: 0.4 wt%), single-walled carbon nanotube aqueous dispersion of Comparative Example 1 (TUBALL manufactured by OCSiAl, CNT concentration: 0.4 wt%, dispersant: sodium carboxymethyl cellulose) Binder: PP (polypropylene) binder (solid concentration: 15 wt%) Pure water

[0093] (Slurry Preparation) A negative electrode slurry was prepared by the following method so as to have the composition shown in Table 4. The measured negative electrode active material, CNT dispersion liquid, or pure water were kneaded and stirred until the appearance became uniform. Next, a PP binder was added to the kneaded and stirred negative electrode active material and CNT dispersion liquid, and the mixture was kneaded and stirred until the appearance became uniform to prepare a negative electrode slurry.

[0094]

[0095] [Evaluation of the Length of Carbon Nanotube Bundles in Negative Electrode Slurry] The silicon wafer was irradiated with excimer light (172 nm) using an excimer lamp to subject the surface of the silicon wafer to functionalization treatment.

[0096] 0.002 g of the negative electrode slurry prepared in Example 4 and Comparative Example 3 was diluted with 5 to 10 g of water. One drop of the diluted negative electrode slurry was placed on a functionalized silicon wafer. The silicon wafer was placed on a hot plate and the dispersion was dried at 100°C. A metal coating for SEM observation was applied, and observations were performed using an SEM (JSM-7001F, manufactured by JEOL Ltd.). Observed images are shown in Figure 4 ((a) Example 4, (b) Comparative Example 3). The lengths of the carbon nanotube bundles were measured from the images using ImageJ (image processing software), and the results are shown in Tables 5 and 6.

[0097]

[0098]

[0099] The median length of the carbon nanotube bundles in Example 4 was 19.804 μm and the average length was 19.130 μm. The median length of the carbon nanotube bundles in Comparative Example 3 was 6.933 μm and the average length was 14.840 μm.

[0100] [Examples 6 and 7, Comparative Examples 7 to 11] Preparation of Negative Electrodes Approximately 0.5 g of each of the negative electrode slurries prepared in Examples 4 and 5 and Comparative Examples 2 to 6 was dropped onto 30 mm x 50 mm x 18 μm thick copper foil that had been degreased with alcohol and applied using a 50 μm bar coater. The copper foil coated with the slurry using the bar coater was dried in a thermostatic chamber (75°C) for approximately 1 hour to harden the slurry and obtain a negative electrode. The negative electrode slurries prepared in the examples exhibited excellent coatability to the copper foil. The negative electrodes prepared using the negative electrode slurries prepared in Examples 4 and 5 are referred to as Examples 6 and 7, respectively. The negative electrodes prepared using the negative electrode slurries prepared in Comparative Examples 2 to 6 are referred to as Comparative Examples 7 to 11, respectively. The surface resistance of each negative electrode was measured using a four-terminal method, and the results are shown in Table 7.

[0101]

[0102] SEM images of the negative electrodes produced in Example 7 and Comparative Examples 10 and 11 are shown in FIG.

[0103] [Cycle Test: Three-Electrode Measurement] The negative electrodes produced in Examples 6 and 7 and Comparative Examples 7 to 11 were used as the working electrodes, metallic lithium was used as the counter electrode (cathode), and metallic lithium was used as the reference electrode. The working electrode was cut to 5 x 10 mm, the counter electrode to 10 mm square, and the reference electrode to 5 x 10 mm, and these were fixed to plate electrodes and placed in a three-electrode cell. Electrolyte (1 mol / L, LiPF6) was poured into the cell so that the three electrodes were immersed. Terminals were fixed to the plate electrodes, and the cells were connected to a potentiostat, after which a cyclic voltammetry / charge-discharge cycle test was performed.

[0104] For the negative electrodes produced in Example 6 and Comparative Example 8, a test was performed by performing 100 charge-discharge cycles at a scan speed of 100 mV / sec to fully activate the negative electrode active material, followed by 170 charge-discharge cycles at a scan speed of 10 mV / sec. The test results for the negative electrode of Example 6 are shown in FIG. 6 , and the test results for the negative electrode of Comparative Example 8 are shown in FIG. 7 . For the negative electrodes produced in Example 6 and Comparative Examples 7 and 9, a test was performed by performing 100 charge-discharge cycles at a scan speed of 100 mV / sec to fully activate the negative electrode active material, followed by three charge-discharge cycles at a scan speed of 10 mV / sec. The test results are shown in FIG. 8 . The negative electrodes prepared in Example 7 and Comparative Examples 10 and 11 were subjected to 100 charge-discharge cycles at a scan speed of 100 mV / sec to fully activate the negative electrode active material, and then to three charge-discharge cycles at a scan speed of 10 mV / sec. The test results are shown in FIG.

[0105] Furthermore, the negative electrodes produced in Example 6 and Comparative Example 8 were tested by performing 1,000 charge-discharge cycles at a scan speed of 100 mV / sec. The test results for the negative electrode of Example 6 are shown in Figure 10, and the test results for the negative electrode of Comparative Example 8 are shown in Figure 11. Table 8 shows the results of each cycle test, expressed as a relative value with the initial (first cycle) area set to 100.

[0106]

[0107] FIG. 12 shows SEM images of the negative electrodes produced in Example 6 and Comparative Example 8 before the cycle test, and FIG. 13 shows SEM images of the negative electrodes after the cycle test.

Claims

1. A carbon nanotube dispersion comprising carbon nanotubes, a dispersant, and a dispersion medium, wherein 50% or more of the bundles of the carbon nanotubes have a length of 10 μm or more.

2. The carbon nanotube dispersion according to claim 1, wherein the average length of the bundles is 15 to 1000 μm.

3. The carbon nanotube dispersion according to claim 1, wherein the average length of the bundles is 6% or more of the average length of the carbon nanotubes at the time of production.

4. The carbon nanotube dispersion according to claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes.

5. The carbon nanotube dispersion liquid according to claim 1, wherein the dispersant is at least one selected from the group consisting of vinyl polymers, cellulose polymers, fluorine polymers, acrylic polymers, and nitrile rubbers.

6. The carbon nanotube dispersion liquid according to claim 1, wherein the dispersion medium is at least one selected from the group consisting of water, lower alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, ether-based organic solvents, ester-based organic solvents, amide-based organic solvents, and sulfoxide-based organic solvents.

7. A method for producing a carbon nanotube dispersion liquid containing carbon nanotubes, a dispersant, and a dispersion medium, wherein 50% or more of the bundles of carbon nanotubes are 10 μm or longer in length, the method comprising the step of immersing a carbon nanotube forest formed on a substrate in a solution containing the dispersant and the dispersion medium.

8. The method for producing a carbon nanotube dispersion liquid according to claim 7, further comprising the steps of: separating the carbon nanotubes from the substrate after the immersion step; and then adding a dispersant and a dispersion medium to the carbon nanotubes and kneading them.

9. A negative electrode slurry comprising a negative electrode active material, a binder, and the carbon nanotube dispersion liquid according to claim 1.

10. A negative electrode slurry comprising a negative electrode active material, a binder, carbon nanotubes, a dispersant, and a dispersion medium, wherein the median bundle length of the carbon nanotubes is longer than the average particle size of the negative electrode active material.

11. A method for producing a negative electrode slurry, comprising the step of mixing the carbon nanotube dispersion liquid according to claim 1, a negative electrode active material, and a binder.

12. A negative electrode comprising a negative electrode current collector and a coating film of the negative electrode slurry according to claim 9 formed on the negative electrode current collector.

13. A secondary battery comprising the negative electrode according to claim 12.

14. A method for observing a component dispersed in a dispersion liquid, comprising the steps of dropping the dispersion liquid onto a substrate that has been subjected to a functionalization treatment, drying the dispersion liquid on the substrate, and observing the substrate after drying.

15. The observation method according to claim 14, wherein at least one selected from the shape, length and diameter of the component dispersed in the dispersion liquid is observed.

16. The observation method according to claim 14, wherein the substrate is a silicon substrate.

17. The observation method according to claim 14, wherein the functionalization treatment is an excimer light irradiation treatment.

18. The observation method according to claim 14, further comprising a step of adjusting the concentration of the dispersed component in the dispersion to 0.00001 to 1% by mass.

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