Single-walled carbon nanotube dispersion for producing electrode

Abstract

Provided is a slurry for producing an electrode, the slurry being suitable for producing, inter alia, an electrode in a lithium ion secondary battery or an easy-to-handle carbon nanotube dispersion. There is provided a carbon nanotube dispersion for manufacturing an electrode, the carbon nanotube dispersion containing at least single-walled carbon nanotubes, carboxymethyl cellulose and / or a salt thereof, and an aqueous solvent, wherein the content amount for the single-walled carbon nanotubes is 0.1-1.5 mass% with respect to the entire dispersion, the carboxymethyl cellulose and / or the salt thereof has a weight-average molecular weight of 300,000 or more and 2,000,000 or less, the loss tangent of the carboxymethyl cellulose and / or the salt thereof at a shear stress of 1.0 Pa and an angular frequency of 100 rad / s in a 3 mass% concentration aqueous solution is 0.45 or greater, and the content amount for the carboxymethyl cellulose and / or the salt thereof is 0.7-1.2 mass times that of the single-walled carbon nanotubes.

Description

Single-walled carbon nanotube dispersion for electrode fabrication

[0001] The present invention relates to an electrode-producing slurry suitable for producing electrodes for secondary batteries such as lithium-ion secondary batteries, and to a dispersion of carbon nanotubes (sometimes abbreviated as CNT) used in the production of the electrode-producing slurry.

[0002] With the widespread use of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices such as mobile phones and laptop computers, secondary batteries with high energy density, as well as higher output and longer life, are being demanded. Against this background, non-aqueous secondary batteries, including lithium-ion batteries using non-aqueous electrolytes, have come to be used in many devices due to their high energy density and high voltage, and their development is being actively pursued.

[0003] Carbon electrodes are commonly used for electrodes in non-aqueous secondary batteries, including lithium-ion batteries. Carbon electrodes are fabricated by applying a dispersion of a carbon material to an electrode substrate and drying it. Carbon materials include carbon black and carbon nanotubes. Carbon nanotubes are classified into single-walled carbon nanotubes and multi-walled carbon nanotubes based on their higher-order structure, and both are commercially available.

[0004] Patent Document 1 discloses a carbon nanotube dispersion containing single-walled carbon nanotubes, carboxymethyl cellulose and / or a salt thereof, and water, wherein the content of the single-walled carbon nanotubes is 0.47 to 1.00 mass %, the carboxymethyl cellulose and / or salt thereof includes at least one type having an etherification degree of 0.65 to 0.85 and a weight-average molecular weight of 120,000 to 250,000, and the content of the carboxymethyl cellulose and / or salt thereof is 120 to 220 parts by mass per 100 parts by mass of the single-walled carbon nanotubes.

[0005] Patent Document 2 discloses a carbon nanotube dispersion containing carbon nanotubes, carboxymethyl cellulose or a salt thereof, and water, wherein the carboxymethyl cellulose or the salt thereof has a weight-average molecular weight of 10,000 to 100,000 and a degree of etherification of 0.5 to 0.9, and the product (X×Y) of the complex elastic modulus X (Pa) and the phase angle Y (°) of the carbon nanotube dispersion is 100 or more and 1,500 or less.

[0006] Patent Document 3 discloses a carbon nanotube dispersion for a lithium ion battery electrode, which contains a dispersion resin (A), carbon nanotubes (B), and water, wherein the dispersion resin (A) contains a polar functional group-containing resin (a), and a carbon nanotube dispersion for a lithium ion battery electrode, which contains a polar functional group-containing resin (a) that is a carboxymethylcellulose. However, none of Patent Documents 1 to 3 discloses the viscoelastic properties of carboxymethylcellulose.

[0007] Japanese Patent Publication No. 7194860 Japanese Patent Application Laid-Open No. 2023-24526 International Publication No. 2023 / 286793

[0008] When preparing a carbon nanotube dispersion, powdered carbon nanotubes are generally mixed into a solvent and dispersed using a disperser. This process often results in high viscosity, making it difficult to prepare a uniform carbon nanotube dispersion. Furthermore, when the carbon nanotube dispersion is stored for a long period of time, the carbon nanotubes separate and the viscosity of the dispersion increases, making it difficult to handle.

[0009] Furthermore, the carbon nanotube dispersion is generally applied uniformly to a current collector such as aluminum foil and then dried to produce an electrode for a lithium ion secondary battery, etc. In this case, depending on the state of the carbon nanotube dispersion, it may be difficult to apply it to the current collector, or the surface resistivity of the produced electrode may become high, making it difficult to produce a suitable electrode.

[0010] An object of the present invention is to provide a carbon nanotube dispersion liquid that is easy to handle, and to provide a slurry for producing electrodes that is suitable for producing carbon electrodes for lithium ion secondary batteries and the like.

[0011] The inventors investigated the above-mentioned problems by focusing on the viscoelastic properties of carboxymethyl cellulose, and discovered that a suitable carbon nanotube dispersion can be obtained by combining single-walled carbon nanotubes with a specific carboxymethyl cellulose dispersant.Based on this finding, they completed the present invention.

[0012] The present invention relates to a carbon nanotube dispersion for electrode fabrication, which contains at least single-walled carbon nanotubes, carboxymethyl cellulose and / or a salt thereof, and an aqueous solvent, wherein the content of the single-walled carbon nanotubes is 0.1 mass % or more and 1.5 mass % or less relative to the total dispersion, and the carboxymethyl cellulose and / or salt thereof has a weight average molecular weight of 300,000 or more and 2,000,000 or less, and a loss tangent of 0.45 or more at a shear stress of 1.0 Pa and an angular frequency of 100 rad / s when the carboxymethyl cellulose and / or salt thereof is a 3 mass % aqueous solution, and the content of the carboxymethyl cellulose and / or salt thereof is 0.7 times or more and 1.2 times by mass relative to the single-walled carbon nanotubes.

[0013] The present invention also relates to a slurry for producing an electrode, which contains the carbon nanotube dispersion for producing an electrode and at least a positive electrode active material or a negative electrode active material.

[0014] The carbon nanotube dispersion liquid for producing an electrode or the slurry for producing an electrode of the present invention may contain auxiliary agents such as conductive particles and binders as components other than those mentioned above.

[0015] The carbon nanotube dispersion for electrode production of the present invention is a slurry that is highly stable during storage and exhibits almost no increase in viscosity even after long-term storage, making the carbon nanotube dispersion of the present invention highly convenient for industrial use.

[0016] Furthermore, the slurry prepared using the carbon nanotube dispersion for electrode fabrication of the present invention was easy to knead. Furthermore, the carbon electrode fabricated from the slurry prepared using the carbon nanotube dispersion of the present invention had extremely low surface resistivity. Therefore, this slurry is useful as a raw material for fabricating electrodes for secondary batteries such as lithium-ion batteries.

[0017] <Carbon Nanotubes> The carbon nanotubes used in the present invention are single-walled carbon nanotubes consisting of cylindrical carbon nanotubes with nanometer-sized diameters. Single-walled carbon nanotubes have different dispersion characteristics from multi-walled carbon nanotubes. Single-walled carbon nanotubes are more flexible than multi-walled carbon nanotubes, and when used as a conductive agent for electrodes, they have excellent battery performance (cyclability).

[0018] The single-walled carbon nanotubes used in the present invention are preferably 300 mm or less. ２ / g to 2000m ２ / g, more preferably 500m ２ / g~1500m ２ / g, more preferably 800m ２ / g~1300m ２ The BET specific surface area of ​​the single-walled carbon nanotube can be measured using a specific surface area measuring device.

[0019] In the carbon nanotube dispersion of the present invention, the content of single-walled carbon nanotubes to be blended is 0.1% by mass or more and 1.5% by mass or less, more preferably 0.2% by mass or more and 1.0% by mass or less, based on the total amount of the carbon nanotube dispersion. When the carbon nanotube content is within this range, the flowability of the carbon nanotube dispersion is good, the dispersion can be uniformly applied to a current collector, and the performance of a secondary battery electrode made from the carbon nanotube dispersion can be favorably obtained.

[0020] The single-walled carbon nanotubes used in the present invention have a peak intensity ratio G / D in Raman spectroscopy of preferably 10 or more and 200 or less, more preferably 30 or more and 150 or less. Here, the peak intensity ratio G / D is the ratio between 1570 cm and 1570 cm in Raman spectroscopy. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range of G, 1320 cm －１ ~1370cm －１ The maximum value of the D-band scattered light peak intensity within the range is defined as D, and the G / D ratio is expressed as D. The Raman scattered light peak intensity can be measured using a Raman spectrometer.

[0021] The single-walled carbon nanotubes used in the present invention preferably have an average fiber width of 0.5 nm to 10 nm, more preferably 1.0 nm to 5.0 nm. The average fiber width of carbon nanotubes refers to the average value of the outer diameter of a sufficient number (n) of fibers measured using an electron microscope. Carbon nanotubes can generally be produced by laser ablation, arc discharge, thermal CVD, plasma CVD, or combustion methods.

[0022] The low-oxygen atmosphere is preferably an atmosphere of an inert gas such as a rare gas typified by argon gas or nitrogen gas, and the oxygen concentration in the atmosphere is preferably 1% by volume or less. In the contact reaction, a reducing gas may be added to the atmosphere to activate the catalyst. The reducing gas may be, for example, hydrogen or ammonia, preferably hydrogen.

[0023] A carbon-containing gas is used as a carbon source raw material. Examples include hydrocarbons, carbon monoxide, and alcohols, which can be used alone or in combination of two or more. Among these, one or more selected from saturated or unsaturated hydrocarbons and alcohols are preferred. Examples of hydrocarbons include methane, propane, butane, ethylene, and acetylene, with ethylene being preferred. When ethylene is used as a raw material gas serving as a carbon source, it is preferred to bring the ethylene into contact with a catalyst and react it at 600 to 800°C, particularly 650 to 750°C, in an atmosphere with an oxygen concentration of 1% by volume or less.

[0024] The catalyst for the catalytic reaction is preferably a mixture of an active component such as cobalt, nickel, or iron and a catalyst support such as magnesium, aluminum, or silicon, which is then molded and pulverized. Particularly preferred is a powdery catalyst obtained by mixing and molding a metal oxide containing cobalt as the active component and magnesium as the catalyst support, and then pulverizing the mixture.

[0025] When a hydrocarbon is used as the raw material gas, the reaction operating conditions may vary depending on the size of the reaction vessel and the amount of catalyst in the reaction vessel. However, when the amount of carbon nanotubes produced per gram of catalyst is Y (g) and the contact reaction time between the catalyst and the hydrocarbon is Z (minutes), it is preferable to adjust the amount of catalyst and / or the flow rate of the hydrocarbon to be supplied so that Y / Z (g / minute) satisfies 1.5≦Y / Z≦2.7.

[0026] The carbon nanotubes used in the present invention are preferably those that have been subjected to one or a combination of the following treatments after production: pulverization, classification, and demetallization. The treatments allow the properties of the carbon nanotubes to be adjusted.

[0027] 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 mill, bead mill, cone mill, wet jet mill, or thin film rotary high-speed mixer.

[0028] The classification operation is an operation for making the size of carbon nanotubes uniform. Applicable classification operations include devices that utilize gravity, inertial force, or centrifugal force, or devices that utilize filters, and can be used in either dry or wet methods. The demetallization operation is an operation for removing metal components contained in carbon nanotubes. Since carbon nanotubes contain metal components contained in the catalyst used in their production, it is preferable to remove the metal components from the carbon nanotubes.

[0029] <Carboxymethylcellulose> In the present invention, carboxymethylcellulose and / or its salts are used as dispersants. Carboxymethylcellulose and / or its salts (hereinafter sometimes referred to as CMC) are polymers having a cellulose structure in which carboxymethyl groups are bonded to some of the hydroxy groups of the glucose units that constitute the CMC. CMC may contain a residue in the form of a carboxyl group or its salt in its structure.

[0030] Examples of carboxymethylcellulose salts containing the above residues include alkali metal salts such as sodium salt, lithium salt, and potassium salt, alkaline earth metal salts such as calcium salt and magnesium salt, and organic salts such as ammonium salt, alkylamine salt, and alkanolamine salt. Only one of these salts may be contained, or two or more salts may be contained. Among these, alkali metal salts are preferred, and sodium salt is more preferred.

[0031] The carboxymethyl cellulose and / or its salt (CMC) used in the present invention has a weight average molecular weight (Mw) of 300,000 to 2,000,000, preferably 500,000 to 1,000,000. The weight average molecular weight of CMC can be measured using GPC (Gel Permeation Chromatography).

[0032] The CMC used in the present invention has the above-mentioned weight-average molecular weight, and also has the property that, when it is a 3% by mass aqueous solution, it has a loss tangent (tanδ) of 0.45 or more, preferably 0.50 or more, at a shear stress of 1.0 Pa and an angular frequency of 100 rad / s. The loss tangent is measured by using a dynamic viscoelasticity measuring device to apply a sinusoidal force and a shear force of a predetermined frequency as stress to a 3% by mass aqueous CMC solution sample, detecting the resulting deformation (strain) of the sample, and calculating the loss tangent from the strain. By using a CMC with such properties, a dispersing force is efficiently applied to the carbon nanotubes during the dispersion process, resulting in a good slurry.

[0033] The carbon nanotube dispersion of the present invention has excellent stability when stored for a long period of time by using a CMC that has a weight average molecular weight within the above range and gives a loss tangent (tan δ) equal to or greater than the above lower limit.

[0034] The degree of etherification of the CMC is preferably 0.5 to 0.9, more preferably 0.6 to 0.8. By using a CMC with such a degree of etherification, the storage stability of the carbon nanotube dispersion can be further improved, and the battery performance can be improved when the CMC is used as an electrode coating material.

[0035] The amount of CMC blended is 0.7 to 1.2 times by mass, more preferably 0.8 to 1.1 times by mass, relative to the mass of the single-walled carbon nanotubes. When the amount of CMC blended is within this range, the dispersibility of the single-walled carbon nanotubes in the carbon nanotube dispersion can be exhibited, and the performance of the electrode produced from the carbon nanotube dispersion can be ensured. <Solvent>

[0036] An aqueous solvent is used for the carbon nanotube dispersion of the present invention. For example, distilled water, ion-exchanged water, tap water, industrial water, etc. can be used as the solvent. Among these, distilled water and ion-exchanged water are preferred. Also, an aqueous solution obtained by mixing water with a highly hydrophilic organic solvent such as alcohol, ether, ester, or amide can be used.

[0037] <Preparation of Carbon Nanotube Dispersion> The carbon nanotube dispersion of the present invention can be produced through a dispersion step in which at least single-walled carbon nanotubes, the specific CMC, and a solvent are mixed and homogenized. When dispersing carbon nanotubes in a solvent, it is preferable to perform premixing (hereinafter referred to as "premixing") in which powdered carbon nanotubes are mixed into the solvent, and then disperse the premixed mixture using a disperser.

[0038] Dispersion can be carried out using an ultrasonic disperser, mixers such as a Disper, a Homomixer, a rotation-revolution mixer, a Henschel mixer, or a planetary mixer, a (high-pressure) homogenizer, a paint conditioner, colloid mills, media-type dispersers such as a bead mill, a cone mill, a ball mill, a sand mill, an attritor, a pearl mill, or a Coball mill, media-less dispersers such as a wet jet mill or a thin film rotary high-speed mixer, or other dispersing devices such as a roll mill. From the viewpoints of the stability of the dispersing action and dispersion efficiency, preferred dispersing devices are a bead mill-type disperser and an ultra-high-pressure wet atomizer.

[0039] The amount of single-walled carbon nanotubes to be blended in the carbon nanotube dispersion of the present invention is preferably 0.1 mass % or more and 1.5 mass % or less, more preferably 0.2 mass % or more and 1.3 mass % or less, based on the total amount of the dispersion. When the blending amount of single-walled carbon nanotubes in the dispersion is within this range, a carbon nanotube dispersion that can be used for electrode slurries can be obtained.

[0040] Carbon nanotube dispersions often have the property that, when stored for a long period of time, their viscosity increases significantly, making them difficult to handle. In contrast, the carbon nanotube dispersion of the present invention does not exhibit a significant increase in viscosity even when stored for a long period of time, and is highly stable during storage. For this reason, the carbon nanotube dispersion of the present invention is suitable for industrial use as a slurry for producing electrodes.

[0041] <Slurry for Electrode Fabrication> A preferred use form of the carbon nanotube dispersion for electrode fabrication of the present invention is a slurry for electrode fabrication obtained by adding at least a positive electrode active material or a negative electrode active material to the carbon nanotube dispersion for electrode fabrication and mixing it. That is, the present invention relates to a slurry for electrode fabrication containing the carbon nanotube dispersion for electrode fabrication and at least a positive electrode active material or a negative electrode active material.

[0042] The electrode-fabricating slurry of the present invention may further contain conductive particles and a binder. The electrode-fabricating slurry of the present invention is used as a raw material for producing a positive electrode or a negative electrode of a secondary battery such as a lithium-ion battery. Hereinafter, an embodiment of producing a negative electrode using the electrode-fabricating slurry of the present invention will be described.

[0043] The slurry for producing a negative electrode of the present invention contains the carbon nanotube dispersion liquid having the above-described configuration and at least a negative electrode active material. Examples of the negative electrode active material include metal oxide-based active material particles, silicon-based active material particles, and spherical graphite. In particular, metal oxide-based negative electrode active material particles are preferably used.

[0044] As the metal oxide-based negative electrode active material particles, for example, titanium oxide can be used. The titanium oxide is not particularly limited as long as it can absorb and release lithium, but preferred are spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxide, and titanium dioxide (TiO) having a monoclinic crystal structure. ２ (B)) and anatase type titanium dioxide can be used.

[0045] The spinel-type lithium titanate includes Li ４＋ｘ Ti ５ O １２ (x varies in the range of -1≦x≦3 depending on the charge / discharge reaction). Ramsdellite-type lithium titanate includes Li ２＋ｙ Ti ３ O ７ (y varies in the range of -1≦y≦3 depending on the charge / discharge reaction). ２ (B) and anatase type titanium dioxide include Li １＋ｚ TiO２ (z varies in the range of −1≦z≦0 depending on the charge / discharge reaction).

[0046] Examples of titanium-containing metal composite oxides include metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe. Examples of metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe include TiO ２ -P ２ O ５ , TiO ２ -V ２ O ５ , TiO ２ -P ２ O ５ -SnO ２ , TiO ２ -P ２ O ５ -MeO (Me is at least one element selected from the group consisting of Cu, Ni and Fe).

[0047] Such a metal composite oxide preferably has a microstructure with low crystallinity, in which a crystalline phase and an amorphous phase coexist, or in which an amorphous phase exists alone, which can further improve cycle performance.

[0048] In the negative electrode slurry of the present invention, the content of the negative electrode active material is preferably 30 to 60 mass %, more preferably 35 to 55 mass %, based on the total amount 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 dispersion can be maintained while ensuring the performance of the produced electrode.

[0049] The content of the single-walled carbon nanotubes in the negative electrode slurry is preferably 0.05 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 1 part by mass, relative to 100 parts by mass of the negative electrode active material. When the content of the negative electrode active material in the negative electrode slurry is within this range, the fluidity of the dispersion can be maintained while ensuring the performance of the carbon electrode to be produced.

[0050] The slurry for producing a negative electrode of the present invention contains the carbon nanotube dispersion liquid and the negative electrode active material having the above-described configuration, and may also contain a solid electrolyte such as a sulfide solid electrolyte, an oxide solid electrolyte, a dry polymer electrolyte, a gel polymer electrolyte, or a quasi-solid electrolyte, as necessary.

[0051] <Conductive Particles> The slurry for producing a carbon electrode of the present invention can further contain conductive particles. By adding conductive particles, the conductivity of the secondary battery electrode produced from the dispersion can be increased. The conductive particles have a specific gravity difference of ±0.2 g / cm with respect to the single-walled carbon nanotubes contained in the carbon nanotube dispersion. ３ A slurry having a specific gravity within this range is particularly preferred because it is less likely to separate due to differences in specific gravity when stored as a slurry for producing carbon electrodes.

[0052] The conductive particles that can be used are preferably conductive carbon particles made of graphite-type carbonaceous material, and preferably include carbon black such as acetylene black, ketjen black, etc. The amount of the conductive particles to be mixed is preferably 0.5 to 10 mass %, more preferably 0.5 to 7 mass %, and even more preferably 0.5 to 5 mass %, relative to 100 parts by mass of the negative electrode active material.

[0053] <Binder> The electrode preparation slurry preferably further contains a binder. Examples of binders that can be used include fluororesins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; polyolefin resins such as polyethylene and polypropylene; polyimide resins; polyvinylpyrrolidone; polyvinyl alcohol resins; acrylic resins; styrene-butadiene rubber (SBR); and carboxymethyl cellulose. Two or more binders may be used in combination.

[0054] 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 slurry for producing each electrode for a secondary battery. When the amount of binder added is within this range, an electrode with high adhesion to the current collector can be obtained without adversely affecting the battery capacity or charge / discharge characteristics. In addition to the above components, the slurry for producing an electrode can also appropriately contain a leveling agent, a solid electrolyte, a preservative, and the like.

[0055] The electrode-forming slurry can be prepared by mixing a carbon nanotube dispersion, an active material for a positive electrode or a negative electrode of a secondary battery, and optionally, conductive particles, a binder, a solvent, and / or other components. For example, a twin-screw kneader can be used for the mixing operation.

[0056] <Preparation of Electrodes> The electrode preparation slurry of the present invention is applied to a current collector, which is a conductive member of a secondary battery such as a lithium-ion secondary battery, and then dried to prepare a positive or negative electrode. The electrode preparation slurry of the present invention is a dispersion liquid with low viscosity even at high concentrations, so it can be uniformly applied to the current collector. The resulting positive or negative electrode achieves high output and battery performance that can withstand repeated charge and discharge over a long period of time as an electrode for a secondary battery.

[0057] More specifically, a carbon electrode can be prepared from the electrode preparation slurry for a positive electrode or a negative electrode of the present invention as follows. First, the carbon electrode preparation slurry is applied to a current collector. The current collector is a component 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 can be appropriately selected from those suitable for 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-preparing 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-preparing slurry is dried. Drying methods that can be used include natural drying and forced drying using a blower dryer, a hot air dryer, an infrared heater, or a far-infrared heater. This produces an electrode in which an electrode film is formed on the current collector.

[0060] The thickness of the electrode formed on the current collector is preferably 1 μm or more and 500 μm or less, more preferably 10 μm or more and 300 μm or less. The electrode formed preferably has a surface resistivity of less than 5.0 kΩ / □, more preferably less than 1.0 kΩ / □. The surface resistivity can be measured using a resistivity meter.

[0061] An electrode produced from a slurry for producing an electrode obtained by using the carbon nanotube dispersion liquid for producing an electrode of the present invention has a low electrical resistance value and is suitable for use as a positive electrode or a negative electrode of a secondary battery such as a lithium ion battery.

[0062] Example 1 <Preparation of Carbon Nanotube Dispersion> Sodium carboxymethyl cellulose (manufactured by Daicel Millize Co., Ltd., tan δ: 3.72; CMC-1) was added as a dispersant to ion-exchanged water as a solvent, and single-walled carbon nanotubes (manufactured by Meijo Nanocarbon Co., Ltd., EC2.0P; Carbon Nanotube A) were then added. Bead dispersion was carried out using zirconia beads with a diameter of 0.5 mm in a disperser (manufactured by Shinmaru Enterprises Co., Ltd., DYNO-MILL), to prepare a carbon nanotube dispersion for electrode fabrication. The blending ratios of the components contained in the carbon nanotube dispersion are as shown in Table 1. Evaluation of the loss tangent of the CMC by dynamic viscoelasticity measurement and evaluation of the storage stability of the prepared CNT dispersion were performed as follows.

[0063] [Loss Tangent of CMC] The loss tangent of the sodium carboxymethylcellulose used was evaluated by the following method. An aqueous solution of 3% by mass of sodium carboxymethylcellulose was prepared and allowed to stand without applying a load. Next, the loss tangent (tan δ) was measured using a dynamic viscoelasticity measuring device (manufactured by Anton Paar, product name: MCR) under the conditions of a measurement temperature of 25°C, a shear stress of 1.0 Pa, and an angular frequency of 100 rad / s. The loss tangent of each CMC is shown in Table 1.

[0064] [Storage Stability of CNT Dispersion] The storage stability of the carbon nanotube dispersion for electrode fabrication was evaluated by the following method. The obtained carbon nanotube dispersion for electrode fabrication was filled into a container, and immediately measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd., TV-22 model) at a sample temperature of 40°C, a cone angle of 1°34', and a shear rate of 38.3 s －１ The viscosity was measured under the conditions above and recorded as the initial viscosity (mPa s). Next, the container was left to stand at 40°C for one week without applying any load. The viscosity was then measured by the method described above and recorded as the storage viscosity (mPa s).

[0065] The value (percentage) obtained by dividing the storage viscosity by the initial viscosity was taken as the viscosity increase rate over time. If the viscosity increase rate over time was less than 140%, i.e., the increase in viscosity value was less than 40% of the viscosity before storage, the viscosity increase of the dispersion was considered to be small and rated as "good." If the viscosity increase rate over time was 140% or more, i.e., the increase in viscosity value was 40% or more of the viscosity before storage, the viscosity increase of the dispersion was considered to be large and rated as "bad." The evaluation results of the storage stability of the CNT dispersion are shown in Table 1.

[0066] <Preparation of Slurry for Electrode Production> A negative electrode active material (LTO-2S, manufactured by Beiterui New Energy Materials Co., Ltd.), the carbon nanotube dispersion obtained above, and styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC1380, manufactured by Daicel Corporation) as binders were blended in amounts such that, relative to 100 parts by mass of the negative electrode active material, the carboxymethyl cellulose was 1.5 parts by mass, the carbon nanotubes were 0.3 parts by mass, and the styrene butadiene rubber was 1.25 parts by mass, and the mixture was kneaded until completely uniform using a mixer (Thinky Corporation, Awatori Rentaro ARE-310) to prepare a slurry for electrode (negative electrode). The kneadability of the electrode production slurry was evaluated as follows.

[0067] <Electrode Preparation> The resulting electrode preparation slurry was applied to a soda glass test piece using a 50 μm thick applicator. The coated glass piece was then dried under reduced pressure using a hot plate at 90° C. for 10 minutes to prepare an electrode for a lithium battery positive electrode. The surface resistivity of the electrode was evaluated as follows.

[0068] [Coatability of Slurry for Electrode Creation] The obtained slurry for electrode creation was applied to a test piece made of soda glass using a 50 μm thick applicator. The obtained coating film was visually observed, and the coatability was evaluated according to the following criteria. ◯: No pinholes or uneven coating occurred on the film surface, and the coating was good. ×: Pinholes or uneven coating occurred on the film surface. The evaluation results of the coatability of the slurry for electrode creation are shown in Table 1.

[0069] [Surface Resistivity of Electrode] The surface resistivity (kΩ / □) of the prepared electrode was measured using a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd., Loresta GP, MCP-T610, four-point probe, ASP pin spacing 5 mm). The measurement results of the surface resistivity of the electrode prepared from the electrode preparation slurry are shown in Table 1.

[0070] Examples 2 to 4 and Comparative Examples 1 to 5 Carbon nanotube dispersions were prepared in the same manner as in Example 1, except that the type and amount of CMC dispersant were changed to those shown in Table 1. In particular, as the CMC, the same carboxymethylcellulose sodium as in Example 1 (manufactured by Daicel Millize Co., Ltd., tanδ: 3.72; CMC-1) as well as carboxymethylcellulose sodium having a loss tangent tanδ within the range of the present invention (manufactured by Daicel Millize Co., Ltd., tanδ: 0.53; CMC-2) or carboxymethylcellulose sodium having a loss tangent tanδ outside the range of the present invention (manufactured by Daicel Millize Co., Ltd., tanδ: 0.43; CMC-3) were used.

[0071] Furthermore, using the obtained carbon nanotube dispersion liquid, an electrode fabrication slurry was prepared by the same method as in Example 1, and an electrode (negative electrode) was fabricated. The surface resistivity (kΩ / □) of the fabricated electrode was measured by the same method as in Example 1. The compositions (mass %) and evaluation results of the carbon nanotube dispersion liquids of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 1.

[0072]

[0073] The carbon nanotube dispersions of Examples 1 to 4, which are embodiments of the present invention, had excellent storage stability, as shown in Table 1. Furthermore, the electrode fabrication slurries prepared using the carbon nanotube dispersions of Examples 1 to 4 had excellent kneadability, and the electrodes fabricated from the electrode fabrication slurries exhibited low surface resistivity.

[0074] On the other hand, the carbon nanotube dispersions of Comparative Examples 1 to 4, which are not embodiments of the present invention, were poor in storage stability or sometimes had unevenness on the coated surface of the electrode slurry, as shown in Table 1. Electrodes fabricated from the electrode fabrication slurries prepared using the carbon nanotube dispersions of Comparative Examples 1 to 5 sometimes exhibited low surface resistivity.

[0075] The electrode-fabricating slurry of the present invention can be suitably used for producing electrodes such as electrodes for lithium secondary batteries. The carbon nanotube dispersion of the present invention can be suitably used for preparing a slurry for producing an electrode.

Claims

1. A carbon nanotube dispersion for electrode fabrication containing at least single-walled carbon nanotubes, carboxymethyl cellulose and / or a salt thereof, and an aqueous solvent, wherein the content of the single-walled carbon nanotubes is 0.1% by mass or more and 1.5% by mass or less relative to the total dispersion, and the carboxymethyl cellulose and / or salt thereof has a weight-average molecular weight of 300,000 to 2,000,000, and a loss tangent of 0.45 or more at a shear stress of 1.0 Pa and an angular frequency of 100 rad / s when in a 3% by mass aqueous solution, and the content of the carboxymethyl cellulose and / or salt thereof is 0.7 times by mass or more and 1.2 times by mass more than the single-walled carbon nanotubes.

2. The carbon nanotube dispersion liquid for electrode fabrication according to claim 1, wherein the single-walled carbon nanotube slurry has a peak intensity ratio G / D in Raman spectroscopy of 30 to 150. (However, the intensity ratio G / D is determined by the Raman spectroscopy.) －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. A slurry for producing an electrode, comprising the carbon nanotube dispersion liquid for producing an electrode according to claim 1 or 2, and at least a positive electrode active material or a negative electrode active material.

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