Carbon nanotube dispersion

Abstract

Provided are a carbon nanotube dispersion, a negative electrode slurry, and a negative electrode usable for a lithium ion secondary battery or the like, and a secondary battery exhibiting high cycle characteristics.　The carbon nanotube dispersion contains carbon nanotubes, a dispersant, and a dispersion medium. The carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes.

Description

Carbon nanotube dispersion

[0001] The present invention relates to a carbon nanotube dispersion, a negative electrode slurry, a negative electrode, and a secondary battery.

[0002] Carbon nanotubes (CNTs) are a type of carbon allotrope that have a seamless, closed hollow tube structure formed by rolling graphene sheets into a cylindrical shape. Carbon nanotubes are broadly classified into single-walled carbon nanotubes (SWCNTs), which have a single tube layer, and multi-walled carbon nanotubes (MWCNTs), which have two or more tube layers.

[0003] Carbon nanotubes are used as a conductive additive in the electrodes of lithium-ion secondary batteries, taking advantage of their properties such as mechanical strength, electrical conductivity, and thermal conductivity (see, for example, Patent Document 1). In particular, lithium-ion secondary batteries using single-walled carbon nanotubes, which have a small average outer diameter and a large specific surface area, are known to have excellent cycle characteristics due to the efficient formation of a conductive network.

[0004] JP 2023-153456 A

[0005] However, due to the expanded use of lithium ion secondary batteries, there is a demand for improvements in battery characteristics, such as improved cycle characteristics, more than ever before. Therefore, the present invention relates to providing a carbon nanotube dispersion, a negative electrode slurry, and a negative electrode that can be used in lithium ion secondary batteries, etc. The present invention also relates to providing a secondary battery that exhibits high cycle characteristics.

[0006] As a result of extensive research, the inventors have discovered that the capacity and cycle characteristics of a secondary battery can be improved by using a carbon nanotube dispersion containing single-walled carbon nanotubes and multi-walled carbon nanotubes in the negative electrode of a secondary battery, and have completed the present invention.

[0007] That is, the present invention relates to the following [1] to [9]. [1] A carbon nanotube dispersion comprising carbon nanotubes, a dispersant, and a dispersion medium, wherein the carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes. [2] The carbon nanotube dispersion according to [1], wherein the average length of the carbon nanotube bundles is 8 μm or more. [3] The carbon nanotube dispersion according to [1] or [2], wherein the average length of the multi-walled carbon nanotube bundles is longer than the average length of the single-walled carbon nanotube bundles. [4] The carbon nanotube dispersion according to any of [1] to [3], wherein the content of single-walled carbon nanotubes in the carbon nanotubes is 0.01 to 30 mass %. [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 according to any one of [1] to [5], wherein the dispersion medium is at least one selected from water, a lower alcohol, an aliphatic hydrocarbon, an aromatic hydrocarbon, an ether-based organic solvent, an ester-based organic solvent, an amide-based organic solvent, and a sulfoxide-based organic solvent. [7] A negative electrode slurry comprising a negative electrode active material, a binder, carbon nanotubes, a dispersant, and a dispersion medium, wherein the carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes. [8] A negative electrode comprising a negative electrode current collector and a coating film of the negative electrode slurry according to [7] formed on the negative electrode current collector. [9] A secondary battery comprising the negative electrode according to [8].

[0008] According to the present invention, a carbon nanotube dispersion containing single-walled carbon nanotubes and multi-walled carbon nanotubes is used in a negative electrode for a secondary battery, whereby a secondary battery exhibiting high cycle characteristics can be provided.

[0009] 1 shows an SEM image of a carbon nanotube dispersion liquid. 2 shows an SEM image of a negative electrode. 3 shows the results of a cycle test of Example 3. 4 shows the results of a cycle test of Reference Example 2. 5 shows the results of a cycle test of Comparative Example 2.

[0010] 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.

[0011] 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.

[0012] In this specification, various components may be used independently, either singly or in combination of two or more, unless otherwise specified.

[0013] In this specification, the term "carbon nanotube" refers to a cylindrical structure made of graphene sheets. Carbon nanotubes are sometimes referred to as "CNT."

[0014] As used herein, "single-walled carbon nanotubes" refer to carbon nanotubes with a single tube layer, while "multi-walled carbon nanotubes" refer to carbon nanotubes with a multi-layer structure having two or more tube layers, for example, 2 to 20 layers, preferably 2 to 8 layers, and more preferably 3 to 7 layers.

[0015] 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."

[0016] 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."

[0017] 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.

[0018] <Carbon Nanotube Dispersion> The carbon nanotube dispersion of the present invention contains carbon nanotubes, a dispersant, and a dispersion medium. The carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0019] In the carbon nanotube dispersion, the average length of the carbon nanotube bundles is preferably 8 μm or more from the viewpoint of improving the cycle characteristics of the secondary battery. The average length of the carbon nanotube bundles is preferably 12 μm or more, more preferably 15 μm or more, and particularly preferably 18 μm or more. There is no particular upper limit, but the upper limit is preferably 300 μm or less, 200 μm or less, 100 μm or less, 60 μm or less, 50 μm or less, or 40 μm or less. Specifically, the average length of the carbon nanotube bundles is 8 to 60 μm, preferably 12 to 50 μm.

[0020] In the carbon nanotube dispersion, the average length of the bundles of single-walled carbon nanotubes and the average length of the bundles of multi-walled carbon nanotubes may be the same or different. In one preferred embodiment, the average length of the bundles of single-walled carbon nanotubes is shorter than the average length of the bundles of multi-walled carbon nanotubes, in other words, the average length of the bundles of multi-walled carbon nanotubes is longer than the average length of the bundles of single-walled carbon nanotubes.

[0021] In the carbon nanotube dispersion of the present invention, from the viewpoint of improving the cycle characteristics of a secondary battery, the average length of the bundles of multi-walled carbon nanotubes is preferably 9 μm or more, 13 μm or more, 16 μm or more, or 19 μm or more, and is preferably 300 μm or less, 200 μm or less, 100 μm or less, 60 μm or less, 50 μm or less, or 40 μm or less. The average length of the bundles of multi-walled carbon nanotubes is preferably 9 to 300 μm, 13 to 200 μm, 13 to 100 μm, 16 to 60 μm, 19 to 50 μm, or 19 to 40 μm.

[0022] In the carbon nanotube dispersion of the present invention, from the viewpoint of improving the cycle characteristics of the secondary battery and from the viewpoint of carbon nanotube productivity, the average length of the bundles of single-walled carbon nanotubes is preferably 0.5 μm or more, 1.0 μm or more, 1.5 μm or more, or 2.0 μm or more, and preferably 15 μm or less, 12 μm or less, 10 μm or less, 8 μm or less, 7 μm or less, or 6 μm or less. The average length of the bundles of single-walled carbon nanotubes is preferably 0.5 to 15 μm, 1.0 to 12 μm, 1.5 to 10 μm, 1.5 to 8 μm, 2.0 to 7 μm, or 2.0 to 6 μm. The thickness of the bundles of carbon nanotubes is not particularly limited, but is usually 20 nm to 1 μm.

[0023] In this specification, the length of the carbon nanotube bundles in a carbon nanotube dispersion can be measured by dropping the carbon nanotube dispersion onto a functionalized substrate (e.g., a silicon substrate irradiated with excimer light (e.g., wavelength 172 nm) from an excimer lamp) and drying the substrate to be observed, and then observing the morphology of the carbon nanotube bundles in the observed sample. Specifically, a method described in the Examples below can be mentioned. When a carbon nanotube dispersion is prepared by mixing a single-walled carbon nanotube dispersion and a multi-walled carbon nanotube dispersion, the average length of the single-walled carbon nanotube bundles and the average length of the multi-walled carbon nanotube bundles in the carbon nanotube dispersion are approximately the same as the average length of the carbon nanotube bundles in each dispersion used for mixing. The concentration of carbon nanotubes dropped onto the substrate is preferably 0.00001 to 1 mass %, or 0.0001 to 0.1 mass %. The amount of dispersion to be dropped is not particularly limited, but may be, for example, one drop.

[0024] The average diameter of the single-walled carbon nanotubes is preferably 1 to 4 nm. In this specification, the average diameter of the carbon nanotubes can be measured by observing the morphology of the carbon nanotubes using a transmission electron microscope (TEM).

[0025] The carbon purity of the single-walled carbon nanotubes is preferably 90 to 99.9%, and the crystallinity (D / G ratio) of the single-walled carbon nanotubes is preferably 0.009 to 1.0, and more preferably 0.01 to 0.5.

[0026] The average diameter of the multi-walled carbon nanotubes is preferably 6 to 10 nm.

[0027] The carbon purity of the multi-walled 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%.

[0028] The crystallinity (D / G ratio) of the multi-walled carbon nanotubes is preferably 0.5 to 1.0, more preferably 0.6 to 0.8.

[0029] In the carbon nanotube dispersion, the content of single-walled carbon nanotubes in the carbon nanotubes is preferably 0.01 to 30 mass%, more preferably 5 to 28 mass%, and even more preferably 10 to 24 mass%. In the carbon nanotube dispersion, the content of multi-walled carbon nanotubes in the carbon nanotubes is preferably 70 to 99.99 mass%, more preferably 72 to 95 mass%, and even more preferably 76 to 90 mass%.

[0030] Carbon nanotubes can be obtained by, for example, arc discharge, laser ablation, chemical vapor deposition (CVD), etc. From the viewpoint of improving productivity and uniformity of quality (length, diameter, purity, etc.) of carbon nanotubes, CVD is preferred. Single-walled carbon nanotubes may be produced directly, or may be produced by producing multi-walled carbon nanotubes and then removing the layers of the tube structure while leaving one layer.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] The carbon nanotube dispersion of the present invention can be obtained, for example, by kneading carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), a dispersant, and a dispersion medium using a kneading device. The order of processing the components is not particularly limited, and processing can be performed in any order. Processing can also be performed multiple times. For example, a single-walled carbon nanotube dispersion containing single-walled carbon nanotubes, a dispersant, and a dispersion medium, and a multi-walled carbon nanotube dispersion containing multi-walled carbon nanotubes, a dispersant, and a dispersion medium are separately prepared, and then the single-walled carbon nanotube dispersion and the multi-walled carbon nanotube dispersion are mixed. Another example is a method in which single-walled carbon nanotubes and multi-walled carbon nanotubes are mixed as needed, and then kneaded with a dispersant and a dispersion medium. Yet another example is a method in which a dispersion containing either single-walled carbon nanotubes or multi-walled carbon nanotubes, a dispersant, and a dispersion medium is prepared, and the other single-walled carbon nanotubes and multi-walled carbon nanotubes are dispersed in the dispersion.

[0038] Known kneading devices may be used. Examples include kneaders, planetary mixers, extruders, roll mills, ball mills, bead mills, sand mills, ultrasonic homogenizers, high-pressure homogenizers, and attritors. From the viewpoint of increasing the length of the carbon nanotube bundles, roll mills are preferred. As the roll mill, a two-roll mill, a three-roll mill, or the like can be used. From the viewpoint of economy, the temperature during kneading is preferably room temperature (e.g., 15 to 40°C).

[0039] The carbon nanotube dispersion of the present invention 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 can be suitably used for negative electrodes of secondary batteries.

[0040] <Negative electrode slurry> The negative electrode slurry of the present invention contains a negative electrode active material, a binder, carbon nanotubes, a dispersant, and a dispersion medium. The carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes. The negative electrode slurry of the present invention is preferably a negative electrode slurry for a secondary battery.

[0041] In the negative electrode slurry of the present invention, the average length of the carbon nanotube bundles is preferably 7 μm or more, more preferably 10 μm or more, and particularly preferably 13 μm or more. The upper limit is not particularly limited, but is preferably 300 μm or less, 200 μm or less, 100 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less. Specifically, the average length of the carbon nanotube bundles is 7 to 50 μm, preferably 10 to 40 μm. The length of the carbon nanotube bundles in the negative electrode slurry can be measured by the same method as that for the length of the carbon nanotube bundles in the carbon nanotube dispersion liquid. Specifically, the method described in the Examples below can be used.

[0042] The carbon nanotubes, dispersant, and dispersion medium are as described above. The content of the 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 the carbon nanotubes to the 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 of mixing the components 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 carbon nanotube dispersion is mixed. As the carbon nanotube dispersion, a carbon nanotube dispersion containing single-walled carbon nanotubes and multi-walled carbon nanotubes can be prepared in advance, or a carbon nanotube dispersion containing single-walled carbon nanotubes and a carbon nanotube dispersion containing multi-walled carbon nanotubes can be prepared separately and mixed during the production of the negative electrode slurry. 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.

[0050] <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.

[0051] 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.

[0052] 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.

[0053] <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.

[0054] 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.

[0055] The present invention will be explained in more detail below based on examples, but the present invention is not limited to these examples.

[0056] [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 were multi-walled carbon nanotubes, each with an average length of 250 μm, a carbon nanotube diameter of 6 to 10 nm, a carbon purity of ≥ 99.8%, and a crystallinity (D / G ratio) of 0.6 to 0.8.

[0057] Production Example 2 Preparation of Multi-Walled 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.

[0058] (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.

[0059] (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 %. The obtained carbon nanotube dispersion is also referred to as a multi-walled carbon nanotube dispersion.

[0060] [Single-walled carbon nanotube dispersion] A dispersion of single-walled carbon nanotubes (TUBALL manufactured by OCSiAl) (dispersant and dispersion medium: carboxymethyl cellulose sodium (CMC Na) and water (carbon nanotube concentration 0.4 wt%)) (commercially available) was used. The carbon nanotubes contained in the dispersion were a mixture of single-walled carbon nanotubes and multi-walled carbon nanotubes, and it was confirmed by optical analysis that the proportion of single-walled carbon nanotubes was 50 mass% or more.

[0061] Example 1 Preparation of Carbon Nanotube Dispersion The multi-walled carbon nanotube dispersion obtained in Production Example 2 and a single-walled carbon nanotube dispersion were mixed in a mass ratio of 8:2 to prepare a carbon nanotube dispersion (blend). In the carbon nanotube dispersion (blend), the amount of single-walled carbon nanotubes was 10 to 20 wt % and the amount of multi-walled carbon nanotubes was 80 to 90 wt % based on 100 wt % of carbon nanotubes.

[0062] [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 contact angle before the functionalization treatment was 25.694°, and the contact angle after the treatment was 6.385°.

[0063] 0.002 g of the multi-walled carbon nanotube dispersion prepared in Production Example 2, the single-walled carbon nanotube dispersion, or the carbon nanotube dispersion (blend) prepared in Example 1 was diluted with 5 to 10 g of water. 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 observation was performed using an SEM (JSM-7001F, manufactured by JEOL Ltd.). 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.

[0064]

[0065]

[0066]

[0067] The median bundle length of the carbon nanotubes in the multi-walled carbon nanotube dispersion was 23.384 μm, and the average length was 23.898 μm. The median bundle length of the carbon nanotubes in the single-walled carbon nanotube dispersion was 3.229 μm, and the average length was 3.969 μm. The median bundle length of the carbon nanotubes in the carbon nanotube dispersion (blend) of Example 1 was 28.607 μm, and the average length was 27.901 μm.

[0068] Each carbon nanotube dispersion was dropped onto a silicon wafer and dried to obtain a carbon nanotube film. The surface resistance was measured by a four-terminal method, and the resistance was 1 Ω / sq. for the carbon nanotube dispersion (blend) prepared in Example 1, 23 Ω / sq. for the multi-walled carbon nanotube dispersion prepared in Production Example 2, and 3 Ω / sq. for the single-walled carbon nanotube dispersion. The results of SEM observation of the samples whose resistance was measured are shown in FIG. 1.

[0069] [Example 2, Comparative Example 1, Reference Example 1] 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.)

[0070] CNT dispersion: multi-walled carbon nanotube dispersion prepared in Production Example 2, single-walled carbon nanotube dispersion, or carbon nanotube dispersion (blend) prepared in Example 1 Binder: PP (polypropylene) binder (solid concentration 15 wt %) Pure water

[0071] (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 was kneaded and stirred using a triple roll 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.

[0072]

[0073] [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.

[0074] 0.002 g of the negative electrode slurry prepared in Example 2 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 negative electrode slurry was dried at 100°C. A metal coating for SEM observation was applied, and observation was performed using an SEM (JSM-7001F, manufactured by JEOL Ltd.). The length of the carbon nanotube bundles was measured from the image using ImageJ (image processing software), and the results are shown in Table 5.

[0075]

[0076] The median length of the carbon nanotube bundles in Example 2 was 11.203 μm, and the average length was 15.240 μm.

[0077] [Example 3, Reference Example 2, Comparative Example 2] Production of Negative Electrode Approximately 0.5 g of each of the negative electrode slurries prepared in Example 2, Reference Example 1, and Comparative Example 1 was dropped onto a 30 mm × 50 mm × 18 μm thick copper foil that had been degreased with alcohol, and the resulting mixture was 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, thereby obtaining a negative electrode. The negative electrode slurries prepared in the examples had excellent coatability to copper foil. The negative electrode using the negative electrode slurry prepared in Example 2 is referred to as Example 3. The negative electrodes using the negative electrode slurries prepared in Reference Example 1 and Comparative Example 1 are referred to as Reference Example 2 and Comparative Example 2, respectively. The surface resistance of each negative electrode was measured using a four-terminal method, and the results are shown in Table 6.

[0078]

[0079] SEM images of the negative electrodes produced in Example 3, Reference Example 2, and Comparative Example 2 are shown in FIG.

[0080] [Cycle Test: Three-Electrode Measurement] The negative electrodes produced in Example 3, Reference Example 2, and Comparative Example 2 were used as the working electrode, metallic lithium as the counter electrode (cathode), and metallic lithium as the reference electrode. The working electrode, counter electrode, and reference electrode were cut into 5 × 10 mm pieces, 10 mm square pieces, and 5 × 10 mm pieces, respectively, and fixed to plate electrodes. These were then 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, followed by a cyclic voltammetry / charge-discharge cycle test.

[0081] The test was carried out by performing 1000 charge / discharge cycles at a scan speed of 100 mV / sec. The test results for the negative electrode of Example 3 are shown in Figure 3, the test results for the negative electrode of Reference Example 2 are shown in Figure 4, and the test results for the negative electrode of Comparative Example 2 are shown in Figure 5. Table 7 shows the results of each cycle test, expressed as a relative value with the initial (first cycle) area set to 100.

[0082]

Claims

1. A carbon nanotube dispersion comprising carbon nanotubes, a dispersant, and a dispersion medium, wherein the carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes.

2. The carbon nanotube dispersion according to claim 1, wherein the average length of the bundles of carbon nanotubes is 8 μm or more.

3. The carbon nanotube dispersion liquid according to claim 1, wherein the average length of the bundles of multi-walled carbon nanotubes is longer than the average length of the bundles of single-walled carbon nanotubes.

4. The carbon nanotube dispersion according to claim 1, wherein the content of single-walled carbon nanotubes in the carbon nanotubes is 0.01 to 30% by mass.

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 negative electrode slurry comprising a negative electrode active material, a binder, carbon nanotubes, a dispersant, and a dispersion medium, wherein the carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes.

8. A negative electrode comprising a negative electrode current collector and a coating film of the negative electrode slurry according to claim 7 formed on the negative electrode current collector.

9. A secondary battery comprising the negative electrode according to claim 8.

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