Carbon nanotube dispersion liquid, electrolyte membrane, and separator for battery

Abstract

To provide a carbon nanotube dispersion liquid capable of suppressing tendency of generating dendrite caused by repeating electric charge and discharge.SOLUTION: The carbon nanotube dispersion liquid is characterized by including two or more kinds of carbon nanotubes selected from the group A consisting of the following A), B), and C), a cellulose derivative and water. (Group A): A) a carbon nanotube of a fiber length of from 4 to 100 μm and a fiber width of from 1 to 5 nm, B) a carbon nanotube of a fiber length of from 30 to 20,000 μm and a fiber width of from 3 to 60 nm, and C) a carbon nanotube of a fiber length of 10 μm or less and a fiber width of from 10 to 30 nm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon nanotube dispersion, an electrode membrane, and a battery separator. [Background technology]

[0002] In recent years, interest in energy storage technology has been growing. As the range of applications expands to include the energy sources for notebook personal computers and electric vehicles, there is a growing demand for higher energy density in batteries used as power sources for such electronic devices. To achieve higher energy density, lithium secondary batteries using metallic lithium as the negative electrode have been investigated (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-146980 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when metallic lithium is used in the negative electrode, repeated charge and discharge causes dendrites (resinous deposits of metallic lithium) to form on the surface of the negative electrode, which has a significant impact on battery performance.

[0005] An object of the present invention is to provide a carbon nanotube dispersion liquid that can suppress dendrites that tend to form due to repeated charge and discharge, and to provide an electrode film and a battery separator that use this carbon nanotube dispersion liquid. [Means for solving the problem]

[0006] As a result of extensive research, the present inventors have found that the above problems can be solved by using specific carbon nanotubes, and have thus completed the present invention.

[0007] That is, according to the present invention, (1) A carbon nanotube dispersion liquid, characterized by containing two or more kinds of carbon nanotubes selected from Group A consisting of the following A), B), and C), a cellulose derivative, and water: (Group A) A) Carbon nanotubes with fiber lengths of 4 μm to 100 μm and fiber widths of 1 nm to 5 nm B) Carbon nanotubes with fiber lengths of 30 μm to 20,000 μm and fiber widths of 3 nm to 60 nm C) Carbon nanotubes with a fiber length of 10 μm or less and a fiber width of 10 nm to 30 nm (2) A carbon nanotube dispersion comprising two or more types of carbon nanotubes selected from Group B consisting of the following D), E), and F), a cellulose derivative, and water: (Group B) D) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum greater than 16 E) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum of 1 to 3 F) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum of 0.2 to 0.9 (However, the intensity ratio G / D is determined by the Raman spectrum at 1570 cm -1 ~1620cm -1 The maximum intensity of the G-band scattered light peak in the range of 1320 cm is defined as G. -1 ~1370cm -1 The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.) (3) The carbon nanotube dispersion liquid according to (1), wherein the carbon nanotubes are two or more types selected from Group C consisting of the following A'), B'), and C'): (Group C) A') Carbon nanotubes according to A), in which the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is greater than 16. B') Carbon nanotubes according to B), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 1 to 3. C') Carbon nanotubes according to C), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 0.2 to 0.9. (However, the intensity ratio G / D is determined by the Raman spectrum at 1570 cm -1 ~1620cm -1 The maximum intensity of the G-band scattered light peak in the range of 1320 cm is defined as G. -1 ~1370cm -1 The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.) (4) The carbon nanotube dispersion liquid according to any one of (1) to (3), wherein the content of the carbon nanotubes is 0.5% by mass to 6.0% by mass based on the total amount of the carbon nanotube dispersion liquid. (5) The carbon nanotube dispersion liquid according to any one of (1) to (3), further comprising a cellulose derivative, the content of the cellulose derivative being 0.01% by mass to 2.0% by mass relative to the total amount of the carbon nanotube dispersion liquid. (6) The carbon nanotube dispersion according to any one of (1) to (3), which is a dispersion for a secondary battery separator. (7) The carbon nanotube dispersion according to (4), which is a dispersion for a secondary battery separator. (8) The carbon nanotube dispersion according to (5), which is a dispersion for a secondary battery separator. (9) The carbon nanotube dispersion liquid according to any one of (1) to (3), which is a dispersion liquid for a secondary battery electrode. (10) The carbon nanotube dispersion according to (4), which is a dispersion for a secondary battery electrode. (11) The carbon nanotube dispersion liquid according to (5), which is a dispersion liquid for a secondary battery electrode. (12) An electrode film formed using the carbon nanotube dispersion liquid for an electrode according to any one of (1) to (3). (13) An electrode film formed using the carbon nanotube dispersion liquid according to (4). (14) An electrode film formed using the carbon nanotube dispersion liquid according to (5). (15) A secondary battery separator coated with the dispersion for a secondary battery separator according to (6). is provided. [Effects of the Invention]

[0008] The carbon nanotube dispersion, electrode film, and battery separator of the present invention can suppress dendrites that tend to form due to repeated charge and discharge. DETAILED DESCRIPTION OF THE INVENTION

[0009] The carbon nanotube dispersion of the present invention is described below. The carbon nanotube dispersion of the present invention is characterized by containing two or more types of carbon nanotubes selected from Group A consisting of the following A), B), and C), a cellulose derivative, and water. (Group A) A) Carbon nanotubes with fiber lengths of 4 μm to 100 μm and fiber widths of 1 nm to 5 nm B) Carbon nanotubes with fiber lengths of 30 μm to 20,000 μm and fiber widths of 3 nm to 60 nm C) Carbon nanotubes with a fiber length of 10 μm or less and a fiber width of 10 nm to 30 nm

[0010] The carbon nanotube dispersion of the present invention is characterized by containing two or more types of carbon nanotubes selected from Group B consisting of the following D), E), and F), a cellulose derivative, and water. (Group B) D) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum greater than 16 E) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum of 1 to 3 F) Carbon nanotubes with a G / D ratio of the G band to the D band in the Raman spectrum of 0.2 to 0.9 (However, the intensity ratio G / D is determined by the Raman spectrum at 1570 cm -1 ~1620cm -1 The maximum intensity of the G-band scattered light peak in the range of 1320 cm is defined as G. -1 ~1370cm -1 The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.)

[0011] (carbon nanotubes) The carbon nanotubes (hereinafter sometimes referred to as "CNTs") used in the carbon nanotube dispersion (hereinafter sometimes referred to as "dispersion") of the present invention are two or more types selected from Group A consisting of A), B), and C) below, or two or more types selected from Group B consisting of D), E), and F) below. Here, the carbon nanotubes can be used as a conductive additive.

[0012] (Group A) Carbon nanotubes A), B) and C) included in group A are A) Carbon nanotubes having a fiber length of 4 μm to 100 μm, preferably 4 μm to 80 μm, more preferably 5 to 60 μm, and a fiber width of 1 nm to 5 nm, preferably 1.5 nm to 5 nm. B) Carbon nanotubes having a fiber length of 30 μm to 20,000 μm, preferably 30 μm to 10,000 μm, more preferably 50 μm to 3,000 μm, and a fiber width of 3 nm to 60 nm, preferably 3 nm to 30 nm, more preferably 3 nm to 15 nm. C) Carbon nanotubes having a fiber length of 10 μm or less and a fiber width of 10 nm to 30 nm, preferably 12 nm to 25 nm In the present invention, when carbon nanotubes of group A are used, two or more types are selected from A) to C).

[0013] The fiber length and fiber width of the carbon nanotubes were measured using images from an electron microscope, and the arithmetic mean values ​​of the fiber length and fiber width of a sufficient number of samples were calculated. In the present invention, a three-dimensional crosslinking effect is obtained by selecting two or more types of carbon nanotubes from Group A. This effect creates appropriate voids that do not hinder the movement of lithium ions, and also creates appropriate surface irregularities that improve wettability with the electrolyte.

[0014] The carbon nanotube content in the carbon nanotube dispersion of the present invention is preferably 0.5 to 1.5 mass %, more preferably 0.5 to 1.2 mass %, based on the total mass of the dispersion when carbon nanotubes A) and B) are used, preferably 2.0 to 6.0 mass %, more preferably 3.0 to 5.0 mass %, based on the total mass of the dispersion when carbon nanotubes B) and C) are used, and preferably 2.0 to 6.0 mass %, more preferably 3.0 to 5.0 mass %, based on the total mass of the dispersion when carbon nanotubes A) and C) are used. Furthermore, when carbon nanotubes A), B), and C) are used, the content is preferably 2.0 to 6.0 mass %, more preferably 4.0 to 5.0 mass %.

[0015] Furthermore, when carbon nanotubes A) and B) are used without carbon nanotube C), the mass ratio of carbon nanotubes A) to B) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes B) and C) are used without carbon nanotube A), the mass ratio of carbon nanotubes B) to C) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes A) and C) are used without carbon nanotube B), the mass ratio of carbon nanotubes A) to C) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. Furthermore, when carbon nanotubes A), B) and C) are used, the carbon nanotubes B) can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, and the carbon nanotubes C) can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, per 100 parts by mass of carbon nanotubes A).

[0016] (Group B) Carbon nanotubes D), E) and F) included in group B are D) Carbon nanotubes having a peak intensity ratio G / D of G band to D band in Raman spectrum of greater than 16, preferably greater than 30, more preferably greater than 50. E) Carbon nanotubes having a peak intensity ratio G / D of G band to D band in a Raman spectrum of 1 to 3, preferably 1.0 to 2.6 F) Carbon nanotubes having a peak intensity ratio G / D of G band to D band in a Raman spectrum of 0.2 to 0.9, preferably 0.5 to 0.85 In the present invention, when carbon nanotubes of group B are used, two or more types are selected from D) to F).

[0017] Here, the intensity ratio G / D is the intensity at 1570 cm -1 ~1620cm -1 The maximum intensity of the G-band scattered light peak in the range of 1320 cm is defined as G. -1 ~1370cm-1 The maximum intensity of the D-band scattered light peak in the range is defined as D, and the ratio is expressed as the ratio. Raman spectra can be measured by detecting Raman scattered light using a Raman spectrometer. In the present invention, by selecting two or more types of carbon nanotubes from Group B, the effects of fewer defects, high resistance to high voltage driving, and high conductivity over a long period of time can be obtained.

[0018] The carbon nanotube content in the carbon nanotube dispersion of the present invention is preferably 0.5 to 1.5 mass% and more preferably 0.5 to 1.2 mass% relative to the total dispersion when carbon nanotubes D) and E) are used, preferably 2.0 to 6.0 mass% and more preferably 3.0 to 5.0 mass% relative to the total dispersion when carbon nanotubes E) and F) are used, and preferably 2.0 to 6.0 mass% and more preferably 3.0 to 5.0 mass% relative to the total dispersion when carbon nanotubes D) and F) are used. Furthermore, when carbon nanotubes D), E), and F) are used, the content is preferably 2.0 to 6.0 mass% and more preferably 4.0 to 5.0 mass% relative to the total dispersion.

[0019] Furthermore, when carbon nanotubes D) and E) are used without blending carbon nanotube F), the mass ratio of carbon nanotubes D) to E) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes E) and F) are used without blending carbon nanotube D), the mass ratio of carbon nanotubes E) to F) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes D) and F) are used without blending carbon nanotube E), the mass ratio of carbon nanotubes D) to F) is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. Furthermore, when carbon nanotubes D), E) and F) are used, carbon nanotubes E) can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, and carbon nanotubes F) can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, per 100 parts by mass of carbon nanotubes D).

[0020] Furthermore, when carbon nanotubes A), B), and C) constituting the above-mentioned Group A are used as the carbon nanotubes used in the present invention, carbon nanotubes A), B), and C) may be carbon nanotubes A'), B'), and C') shown in Group C below, respectively. (Group C) A') Carbon nanotubes according to A), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is greater than 16, preferably greater than 30, more preferably greater than 50. B') Carbon nanotubes according to B), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 1 to 3, preferably 1.0 to 2.6. C') Carbon nanotubes according to C), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 0.2 to 0.9, preferably 0.5 to 0.85.

[0021] When carbon nanotubes of group C are used in the present invention, two or more types are selected from A') to C'). The intensity ratio G / D is the same as that explained for group B above. In the present invention, by selecting two or more types of carbon nanotubes from Group C, effects such as improved wettability to the electrolyte due to a three-dimensional crosslinking effect and high long-term conductivity can be obtained.

[0022] The carbon nanotube content in the carbon nanotube dispersion of the present invention is preferably 0.5 to 1.5 mass%, more preferably 0.5 to 1.2 mass%, based on the total dispersion when carbon nanotubes A') and B') are used, 2.0 to 6.0 mass%, more preferably 3.0 to 5.0 mass%, based on the total dispersion when carbon nanotubes B') and C') are used, and 2.0 to 6.0 mass%, more preferably 3.0 to 5.0 mass%, based on the total dispersion when carbon nanotubes A') and C') are used. Furthermore, the carbon nanotubes content in the carbon nanotube dispersion of the present invention is preferably 2.0 to 6.0 mass%, more preferably 4.0 to 5.0 mass%, based on the total dispersion when carbon nanotubes A'), B'), and C') are used. Furthermore, when carbon nanotubes A') and B') are used without carbon nanotube C'), the mass ratio of carbon nanotubes A') to B') is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes B') and C') are used without carbon nanotube A'), the mass ratio of carbon nanotubes B') to C') is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. When carbon nanotubes A') and C') are used without carbon nanotube B'), the mass ratio of carbon nanotubes A') to C') is preferably 1:99 to 99:1, and more preferably 10:90 to 90:10. Furthermore, when carbon nanotubes A'), B') and C') are used, carbon nanotubes B') can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, and carbon nanotubes C') can be blended in an amount of preferably 1 to 99 parts by mass, more preferably 10 to 90 parts by mass, per 100 parts by mass of carbon nanotubes A').

[0023] The carbon nanotubes (CNTs) used in the present invention are not particularly limited as long as they have a cylindrical shape formed by essentially rolling up one surface of graphite. Either single-walled CNTs, in which one surface of graphite is rolled up in one layer, or multi-walled CNTs, in which one surface of graphite is rolled up in two or more layers, can be used.

[0024] Furthermore, examples of the form of carbon nanotubes include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, and these may be used alone or in combination of two or more types (hereinafter simply referred to as "at least one type").

[0025] Furthermore, from the viewpoint of the viscosity, conductivity, and stability of the dispersion, the average outer diameter of the carbon nanotubes is preferably 1 nm or more and 90 nm or less, more preferably 3 nm or more and 30 nm or less, and even more preferably 3 nm or more and 15 nm or less.

[0026] In the present invention, the average outer diameter of carbon nanotubes refers to the arithmetic mean value of a sufficient number n of outer diameters measured using an image of a transmission electron microscope at a magnification of 100,000 or more.

[0027] The purity of the carbon nanotubes used in the present invention is preferably 90 to 100% by mass, particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with ash content measured in accordance with JIS K 1469 or JIS K 6218 being considered as impurities.

[0028] Specific examples of usable carbon nanotubes (CNTs) include NC7000 (average outer diameter 10 nm) manufactured by Nanocyl, Baytubes C150P (average outer diameter 11 nm) manufactured by Bayer, FloTube 9000 (average outer diameter 19 nm), FloTube 7320 (average outer diameter 9 nm), FloTube 7010 (average outer diameter 9 nm), FloTube 6810 (average outer diameter 8 nm), FloTube 6120 (average outer diameter 8 nm), FloTube 6100 (average outer diameter 8 nm), FloTube 2020 (average outer diameter 4 nm) manufactured by Cnano, and MEIJO eDIPS manufactured by Meijo Nanocarbon. At least one of EC2.0 (average outer diameter 2.0 nm), KORBON-A7 (average outer diameter 1.2 nm) manufactured by KORBON Co., Ltd., NTF-7 (average outer diameter 30 nm) manufactured by Koatsu Gas Kogyo Co., Ltd., and NTF-15 (average outer diameter 30 nm) manufactured by Koatsu Gas Kogyo Co., Ltd. can be used.

[0029] (cellulose derivatives) The carbon nanotube dispersion of the present invention contains a cellulose derivative. The cellulose derivative is used as a dispersant. The dispersant is a solvent-soluble polymer that disperses carbon nanotubes well in a solvent to form a stable slurry.

[0030] Examples of cellulose derivatives include carboxymethyl cellulose (CMC), cellulose nanofiber, hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, hydroxyethyl ethyl methyl cellulose, and hydroxypropyl methyl cellulose, with carboxymethyl cellulose (CMC) and cellulose nanofiber being preferred.

[0031] Examples of carboxymethyl cellulose (CMC) include those with a mass average molecular weight of 300,000 or less, which are used alone, or those with a mass average molecular weight of 1,000,000 to 3,000,000, and the like. Examples of cellulose nanofibers (CeNF) include those with a crystallinity of 70% or less as measured by X-ray diffraction.

[0032] The crystallinity of the cellulose is the cellulose type I crystallinity calculated by the Segal method from the diffraction intensity value obtained by X-ray diffraction, and can be determined by the following formula (1). Cellulose type I crystallinity (%) = [(I22.6 - I18.5) / I22.6] × 100 ... (1)

[0033] In the above formula (1), I22.6 represents the diffraction intensity of the lattice plane (002 plane) (diffraction angle 2θ=22.6°) in X-ray diffraction, and I18.5 represents the diffraction intensity of the amorphous portion (diffraction angle 2θ=18.5°).

[0034] Here, type I cellulose refers to the crystalline form of natural cellulose, and type I cellulose crystallinity refers to the proportion of crystalline regions in the total cellulose.

[0035] The content of the dispersant contained in the carbon nanotube dispersion of the present invention is preferably 0.01 mass % to 2.0 mass %, more preferably 0.05 mass % to 1.5 mass %, and even more preferably 0.06 mass % to 1.25 mass %, relative to the total amount of the carbon nanotube dispersion. When the content of the dispersant is within this range, the bundles of the carbon nanotubes can be loosened, the carbon nanotubes can be stably dispersed, and the dispersant does not inhibit the conductivity in the electrode.

[0036] (solvent) The carbon nanotube dispersion of the present invention contains water as a solvent. That is, the solvent used in the carbon nanotube dispersion of the present invention is an aqueous solvent containing water. As the aqueous solvent, water (e.g., ion-exchanged water, distilled water, tap water, etc.) or a mixed solvent of water and a water-soluble solvent can be used.

[0037] Examples of water-soluble solvents that can be used in the aqueous mixed solvent include ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,5-hexanediol, 3-methyl-1,3-butanediol, 2-methylpentane-2,4-diol, and 3-methylpentane-1,3,5-hexanediol. alkylene glycols such as hexanetriol and 1,2,3-hexanetriol; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; glycerols such as glycerol, diglycerol and triglycerol; lower alkyl ethers of glycols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether and diethylene glycol mono-n-butyl ether; N-methyl-2-pyrrolidone and 1,3-dimethyl-2-imidalidinone.

[0038] When a mixed solvent is used, the content of the water-soluble solvent in the mixed solvent is preferably 0.1 to 30% by mass, more preferably 1 to 20% by mass, based on the total amount of the mixed solvent. By adjusting the content within this range, the fluidity of the dispersion can be made as desired. <pH adjuster> The carbon nanotube dispersion of the present invention preferably has a pH of 4 to 9, and preferably contains a pH adjuster, from the viewpoints of current collector corrosion and composition stability.

[0039] Examples of pH adjusters that can be used include at least one of ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, alkali metal salts of carbonate or phosphate such as sodium carbonate, and alkali metal hydroxides such as sodium hydroxide.

[0040] Furthermore, additives may be added to the carbon nanotube dispersion of the present invention depending on the intended use, such as anti-settling agents, wetting agents, emulsifiers, anti-sagging agents, anti-foaming agents, leveling agents, and plasticizers.

[0041] (Method of manufacturing carbon nanotube dispersion liquid) The carbon nanotube dispersion of the present invention can be produced, for example, by adding a carbon material containing carbon nanotubes, a dispersant, a liquid medium (water), and other components used as needed, stirring and mixing, and then carrying out a dispersion step.

[0042] The dispersion treatment of the dispersion liquid can be carried out using, for example, an ultrasonic disperser, mixers such as a Disper, a homomixer, a rotation-revolution mixer, a Henschel mixer, or a planetary mixer; media-type dispersers such as a paint conditioner, colloid mills, a bead mill, a ball mill, a sand mill, an attritor, a pearl mill, or a Co-ball mill; media-less dispersers such as a (high-pressure) homogenizer, a wet jet mill, a wet cavitation mill, a thin film rotary high-speed mixer, or a cone mill; or other dispersing devices such as a roll mill. In view of the stability of the dispersing action and the dispersing efficiency, preferred dispersing devices are a (high-pressure) homogenizer, a wet cavitation mill, and a bead mill.

[0043] Shear rate of the carbon nanotube dispersion of the present invention: 38.3 s-1 The viscosity at 25°C is preferably 5 to 500,000 mPa·s, more preferably 10 to 10,000 mPa·s, even more preferably 20 to 1,000 mPa·s, and even more preferably 30 to 700 mPa·s. The viscosity of the carbon nanotube dispersion can be measured using an E-type rotational viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.). When the viscosity of the carbon nanotube dispersion for an electrode is within this range, the fluidity is high, and it becomes possible to uniformly coat the current collector with carbon nanotubes and the like at a high concentration.

[0044] (Applications of carbon nanotube dispersion liquid) The carbon nanotube dispersion of the present invention may be applied to a separator to form a film on the separator, or may be mixed with an electrode active material and components such as a binder used as needed to form an electrode slurry (a positive electrode slurry or a negative electrode slurry) to form an electrode film (a positive electrode or a negative electrode). When coated on a separator, a conductive layer can be formed that does not impair the function of the separator's pores or its wettability with the electrolyte.

[0045] (lithium-ion secondary battery) The carbon nanotube dispersion of the present invention can be used in batteries, preferably lithium ion secondary batteries. A lithium ion secondary battery usually comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and it is preferable that at least one of the positive electrode, the negative electrode, and the separator has a layer formed using the carbon nanotube dispersion of the present invention.

[0046] The carbon nanotube dispersion liquid of the present invention can be used to prepare both negative electrodes and positive electrodes, but is particularly suitable for preparing negative electrodes. That is, one aspect of the present invention is a negative electrode slurry containing the carbon nanotubes, a dispersant, an aqueous solvent, and a negative electrode active material.

[0047] (Negative electrode) The negative electrode may be prepared by using a metal such as lithium in the form of a metal foil or the like, or by applying a negative electrode slurry containing a negative electrode active material onto a current collector and drying the applied slurry. For example, when a lithium foil is used, the thickness of the lithium foil is preferably 100 μm to 300 μm, and more preferably 180 μm to 200 μm.

[0048] The negative electrode slurry contains a negative electrode active material, a binder used as needed, and a conductive material. The negative electrode slurry may also contain the carbon nanotube dispersion of the present invention.

[0049] (Negative electrode active material) When a negative electrode slurry is used, the negative electrode active material contained in the negative electrode slurry can be any negative electrode active material without any particular limitation, and examples thereof include carbon-based materials such as graphite; metal oxide-based active material particles such as titanium oxide, silicon oxide, and tin oxide; and silicon-based active material particles, and metal oxide-based negative electrode active material particles can be preferably used.

[0050] The titanium oxide is not particularly limited as long as it is capable of absorbing and releasing lithium, but examples that can be used include spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxides, titanium dioxide (TiO2(B)) having a monoclinic crystal structure, and anatase-type titanium dioxide.

[0051] The content of the negative electrode active material in the negative electrode slurry is preferably 30 to 60 mass % and more preferably 35 to 55 mass % based on the total mass of the negative electrode slurry. When the content of the negative electrode active material in the negative electrode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the produced electrode.

[0052] (binding material) Examples of binders that can be used in the negative electrode slurry include various emulsion-type polymers. Specifically, fluorine-based emulsion-type polymers such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), elastomer-based emulsion-type polymers such as ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), and styrene-butadiene rubber (SBR), and acrylic emulsion-type polymers can be used. Furthermore, solutions of acrylic acid-based resins and cellulose-based resins can also be used as soluble polymers. Two or more binders may be used in combination.

[0053] The amount of binder used is preferably 0.05 to 5 parts by mass, more preferably 0.1 to 4.5 parts by mass, and even more preferably 0.5 to 4.5 parts by mass, per 100 parts by mass of the active material in the negative electrode slurry. When the amount of binder added is within this range, an electrode with high adhesion to the current collector is obtained without adversely affecting the battery capacity or charge / discharge characteristics.

[0054] (Conductive materials) Usable conductive materials include conductive carbon particles and conductive carbon fibers made of graphite-type carbonaceous materials. Usable conductive carbon particles include carbon black particles such as acetylene black and ketjen black. Conductive carbon fibers include carbon nanofibers other than carbon nanotubes.

[0055] (Other ingredients) In addition to the above components, the negative electrode slurry of the present invention may further contain a leveling agent, a solid electrolyte, an antiseptic, and the like, as appropriate.

[0056] (Production of negative electrode slurry) There are no limitations on the conditions for producing the negative electrode slurry, and the negative electrode slurry can be produced by mixing a negative electrode active material, if necessary a binder, a conductive material, the carbon nanotube dispersion of the present invention, and a solvent, and stirring or the like if necessary to homogenize the mixture.

[0057] (Manufacturing of negative electrodes) When a metal such as lithium is used in the form of a metal foil, the negative electrode can be used as is. The negative electrode slurry using the carbon nanotube dispersion liquid of the present invention can be used to prepare an electrode as follows. First, the electrode slurry is applied onto a current collector. The current collector is a conductive member that serves as an electrode substrate for a secondary battery such as a lithium-ion secondary battery. The material and shape of the current collector used as the electrode substrate are not particularly limited, and can be appropriately selected to suit the secondary battery to be used. Examples of materials for the current collector include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. Furthermore, as the shape of the current collector, a flat metal foil is generally used, but a foil with a roughened surface, a perforated foil, or a mesh-like foil can also be used.

[0058] Examples of methods for applying the electrode slurry to a current collector include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. After coating, the surface may be smoothed using a lithographic press, a calendar roll, or the like.

[0059] Next, the current collector coated with the electrode slurry is dried, thereby producing an electrode in which an electrode film is formed on the current collector. The electrode slurry after application can be dried using natural drying, a blower dryer, a hot air dryer, an infrared heater, a far infrared heater, or the like. The thickness of the electrode to be fabricated, including the thickness of the current collector, is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less. An electrode produced from an electrode slurry using the carbon nanotube dispersion of the present invention has a low electrical resistance and is suitable for use as an electrode for a lithium ion secondary battery or the like.

[0060] (separator) As the separator, a conventionally known separator can be used, for example, a microporous membrane made of polyethylene, polypropylene, polyvinylidene chloride, or a multilayer composite membrane of these materials.

[0061] In addition, in the present invention, it is preferable to provide a layer formed by coating and drying the carbon nanotube dispersion liquid of the present invention on at least one of the negative electrode side and the positive electrode side of the separator, and it is more preferable to provide the above layer at least on the negative electrode side of the separator.

[0062] The layer formed on the separator by coating and drying the carbon nanotube dispersion of the present invention functions as a conductive layer that does not interfere with the function of the separator's pores or wettability with the electrolyte, and is particularly effective in suppressing dendrites when lithium metal is used as the negative electrode. The carbon nanotube dispersion coating and drying methods for forming this conductive layer can be the same as those described above for producing the negative electrode. The thickness of the film (conductive layer) formed on the separator after drying is preferably 0.1 μm or more and 5 μm or less, and more preferably 0.5 μm or more and 2 μm or less.

[0063] (manufacturing lithium-ion secondary batteries) A lithium ion secondary battery is composed of a positive electrode, a negative electrode, a separator, and an electrolyte solution, and can be produced by stacking the positive electrode and the negative electrode with the separator between them, rolling or folding this into a battery shape, placing it in a battery container, injecting the electrolyte solution into the battery container, and sealing it. The shape may be cylindrical, prismatic, gum-shaped, coin-shaped, button-shaped, pin-shaped, paper-shaped, or any other shape suitable for the intended use.

[0064] The positive electrode may be a positive electrode made from the carbon nanotube dispersion of the present invention, and the negative electrode may be a metal such as lithium foil or a negative electrode made from the above-mentioned negative electrode slurry. Furthermore, it is preferable to use a separator provided with a film made from the carbon nanotube dispersion of the present invention.

[0065] The electrolyte solution may be a non-aqueous solvent containing an electrolyte dissolved therein. Examples of the non-aqueous solvent include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and aprotic polar solvents such as acetonitrile and other nitriles. These solvents may be used singly or in combination of two or more.

[0066] As the electrolyte, a Li salt in which ions are movable can be used, such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl4, LiHF2, LiSCN, and LiBPh4 (wherein Ph represents a phenyl group). [Example]

[0067] The present invention will be described below using examples, but the present invention is not limited to the following examples.

[0068] The carbon nanotubes used in the present invention are produced, for example, by a fluidization method in which a raw material gas is introduced into a high-temperature chamber containing a powdered catalyst and grown on the catalyst surface in a fluidized state. Alternatively, they are produced by a substrate method in which a raw material gas is introduced into a high-temperature chamber containing a stationary substrate with a catalyst attached and grown chemically on the substrate. Carbon nanotubes produced by the above method or any other method can be used as long as they have the properties specified in the present invention.

[0069] The carbon nanotubes used in the present invention are preferably those that have been subjected to one or a combination of the following treatments: pulverization, classification, and demetallization. These treatments allow various properties of the carbon nanotubes to be adjusted.

[0070] The pulverization operation is an operation for pulverizing carbon nanotubes to an appropriate size. Applicable pulverization operations include dry pulverization using a pin mill, pulverizer, hammer mill, jet mill, ball mill, Henschel mixer, or attritor, and wet pulverization using an ultrasonic disperser, disperser, homomixer, planetary mixer, high-pressure homogenizer, paint conditioner, colloid mills, bead mill, cone mill, wet jet mill, or thin-film rotary high-speed mixer.

[0071] The classification operation is an operation for adjusting the size of carbon nanotubes. Applicable classification operations can be carried out using a device that utilizes gravity, inertial force, or centrifugal force, or a device that utilizes a filter, in a dry method or a wet method.

[0072] Carbon nanotubes may contain heavy metals of Group VIII, Group VIIA, and Group VIA elements, such as Fe, Co, and Ni, due to their manufacturing process. The demetallization process removes the amount of heavy metals contained in carbon nanotubes to adjust their content. Applicable demetallization processes include acid treatment, base treatment, and calcination in an inert atmosphere, and can be carried out singly or in combination. The amount of heavy metals contained in carbon nanotubes can be measured by calcining the carbon nanotubes, extracting them with an acidic aqueous solution, and then measuring the amount using a CP optical emission spectrometer.

[0073] The total content of heavy metals contained in the carbon nanotubes after the demetallization operation is preferably less than 5000 ppm, more preferably less than 3000 ppm, and even more preferably less than 1000 ppm.

[0074] If the total content of heavy metals contained in the carbon nanotubes is 5000 ppm or more, this is undesirable because the heavy metals will dissolve from the electrodes into the electrolyte in the produced secondary battery, reducing the charge / discharge cycle characteristics and making it impossible to achieve a long battery life.

[0075] The fiber lengths and fiber diameters of the carbon nanotubes A to C used in Examples 1-1 to 1-6 and Comparative Examples 1-1 to 1-4 were measured as follows. Carbon nanotubes were observed using a scanning electron microscope (Hitachi High-Tech Corporation, S-3400N; SEM), and the arithmetic mean fiber length of 10 carbon nanotubes was calculated using images at 1000x magnification. Carbon nanotubes were also observed using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the arithmetic mean fiber width of 10 carbon nanotubes was calculated using images at 50,000x magnification.

[0076] (Example 1-1) <Preparation of carbon nanotube dispersion> 0.08 parts of carbon nanotubes A (fiber length 4 μm to 100 μm, fiber width 1 nm to 5 nm), 0 parts of carbon nanotubes B (fiber length 30 μm to 20,000 μm, fiber width 3 nm to 60 nm), 3.92 parts of carbon nanotubes C (fiber length 10 μm or less, fiber width 10 nm to 30 nm), 0.6 parts of carboxymethylcellulose sodium salt (Cellogen 7A, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (sometimes abbreviated as "CMC-Na") as a dispersant, and the remainder was mixed and stirred (premixed) for 1 hour using a disperser at a stirring speed (300 rpm) that did not introduce bubbles, to obtain a premixed liquid.

[0077] The premixed liquid was put into a disperser (DYNO-MILL, manufactured by Shinmaru Enterprises Co., Ltd.), and then 970 parts by mass of zirconia beads (bead diameter Φ0.5 mm) were added, followed by dispersion treatment at a peripheral speed of 14 m / s. The beads were then separated to obtain a carbon nanotube dispersion in which the carbon nanotubes were uniformly dispersed. The obtained carbon nanotube dispersion was measured at a shear rate of 38.3 s using an E-type rotational viscometer (TV-22, manufactured by Toki Sangyo Co., Ltd.). -1 The viscosity values ​​were measured at 25°C.

[0078] <Surface resistivity of separator coating film> The carbon nanotube dispersion was manually coated onto a polypropylene separator using a bar coater (manufactured by Yasuda Seiki Seisakusho) and dried in an oven at 60°C for 3-4 minutes to form a 0.5-3 μm film. The surface resistivity (Ω / □) of the resulting film was measured using a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., Loresta GP, MCP-T610, four-point probe, ASP pin spacing 5 mm). The measurement results for the surface resistivity of the separator coating film are shown in Table 1.

[0079] <Air permeability resistance> The carbon nanotube dispersion was manually coated onto a polypropylene separator using a bar coater (manufactured by Yasuda Seiki Seisakusho) and then dried in an oven at 60°C for 3-4 minutes to form a film with a thickness of 0.5-3 μm. A Gurley densometer was used to measure the time (seconds) it took for 100 ml of air to permeate, and the air permeation resistance was evaluated according to the following criteria. The results are shown in Table 1. The measuring device used was a No. 323 AUTO Gurley densometer (manufactured by Yasuda Seiki Seisakusho). A: The pores on the separator surface are not obstructed and sufficient air resistance is exhibited. B: The pores on the separator surface are partially hidden. The air resistance shows a certain value. C: Covers the pores on the separator surface. It takes a long time for air to permeate.

[0080] <Coatability evaluation> The obtained carbon nanotube dispersion was applied to a separator as described above. The obtained coating film was visually observed and the coating properties were evaluated according to the following criteria. The results are shown in Table 1. A+: No pinholes or uneven coating on the film surface, good. A: There are few pinholes or uneven coating on the film surface. B: There are some pinholes or uneven coating on the film surface.

[0081] (Examples 1-2 to 1-4, Comparative Examples 1-1 to 1-4) A carbon nanotube dispersion was prepared in the same manner as in Example 1-1, except that the types and amounts of the carbon nanotubes, dispersant, and solvent were changed to those shown in Table 1. The viscosity of the carbon nanotube dispersion, the surface resistivity and air resistance of the separator coating film, and the coatability of the carbon nanotube dispersion were measured and evaluated in the same manner as in Example 1-1. The results are shown in Table 1.

[0082] [Table 1]

[0083] Furthermore, the peak intensity ratio G / D in the Raman spectrum of carbon nanotubes A to C was measured as follows. After dispersing carbon nanotubes in water, the dispersion was applied to the test piece with an applicator and dried at 80 °C. After that, a Raman spectrum was measured by mapping in a 100 μm square area using a Raman spectrometer (Thermo Fisher Scientific, DXR2xi). -1 The absorbance G at the peak top position within the range of 1310 to 1350 cm -1 The absorbance D at the peak top position within this range was measured, and the G / D ratio was calculated.

[0084] As a result, the peak intensity ratio G / D in the Raman spectrum of carbon nanotube A was greater than 16, the peak intensity ratio G / D in the Raman spectrum of carbon nanotube B was 1 to 3, and the peak intensity ratio G / D in the Raman spectrum of carbon nanotube C was 0.2 to 0.9. [Industrial Applicability]

[0085] The carbon nanotube dispersion of the present invention can be suitably used for preparing a separator slurry, particularly for a separator. Furthermore, an electrode slurry obtained by mixing the carbon nanotube dispersion of the present invention with an electrode active material can be suitably used for producing an electrode such as an electrode for a lithium secondary battery.

Claims

1. A carbon nanotube dispersion liquid comprising two or more types of carbon nanotubes selected from Group A consisting of the following A), B), and C), a cellulose derivative, and water: (Group A) A) Carbon nanotubes with a fiber length of 4 μm to 100 μm and a fiber width of 1 nm to 5 nm B) Carbon nanotubes having a fiber length of 30 μm to 20,000 μm and a fiber width of 3 nm to 60 nm C) Carbon nanotubes with a fiber length of 10 μm or less and a fiber width of 10 nm to 30 nm

2. A carbon nanotube dispersion liquid comprising two or more types of carbon nanotubes selected from Group B consisting of the following D), E) and F), a cellulose derivative, and water. (Group B) D) Carbon nanotubes having a peak intensity ratio G / D of the G band to the D band in the Raman spectrum of greater than 16 E) Carbon nanotubes having a peak intensity ratio G / D of the G band to the D band in the Raman spectrum of 1 to 3. F) Carbon nanotubes having a peak intensity ratio G / D of the G band to the D band in the Raman spectrum of 0.2 to 0.

9. (However, the intensity ratio G / D is 1570 cm in the Raman spectrum. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range of 1320 cm －１ ~1370cm －１ The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.)

3. 2. The carbon nanotube dispersion liquid according to claim 1, wherein the carbon nanotubes are two or more types selected from Group C consisting of the following A'), B') and C'): (Group C) A') Carbon nanotubes according to A), in which the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is greater than 16 B') Carbon nanotubes according to B), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 1 to 3. C') Carbon nanotubes according to C), wherein the peak intensity ratio G / D of the G band to the D band in the Raman spectrum is 0.2 to 0.

9. (However, the intensity ratio G / D is 1570 cm in the Raman spectrum. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range of 1320 cm －１ ~1370cm －１ The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.)

4. 4. The carbon nanotube dispersion according to claim 1, wherein the carbon nanotube content is 0.5% by mass to 6.0% by mass based on the total amount of the carbon nanotube dispersion.

5. The carbon nanotube dispersion liquid according to any one of claims 1 to 3, further comprising a cellulose derivative, wherein the content of the cellulose derivative is 0.01 mass % to 2.0 mass % relative to the total amount of the carbon nanotube dispersion liquid for an electrode.

6. 4. The carbon nanotube dispersion according to claim 1, which is a dispersion for a secondary battery separator.

7. The carbon nanotube dispersion according to claim 4, which is a dispersion for a secondary battery separator.

8. The carbon nanotube dispersion according to claim 5, which is a dispersion for a secondary battery separator.

9. 4. The carbon nanotube dispersion liquid according to claim 1, which is a dispersion liquid for a secondary battery electrode.

10. The carbon nanotube dispersion liquid according to claim 4, which is a dispersion liquid for a secondary battery electrode.

11. The carbon nanotube dispersion liquid according to claim 5, which is a dispersion liquid for a secondary battery electrode.

12. An electrode film formed using the carbon nanotube dispersion liquid according to any one of claims 1 to 3.

13. An electrode film formed using the carbon nanotube dispersion liquid according to claim 4 .

14. An electrode film formed using the carbon nanotube dispersion liquid according to claim 5 .

15. A secondary battery separator coated with the dispersion for a secondary battery separator according to claim 6.

Images