Carbon nanotube dispersion

Abstract

To provide a carbon nanotube dispersion excellent in dispersibility.SOLUTION: A carbon nanotube dispersion includes at least carbon nanotubes, oxidized cellulose nanofibers and water. The oxidized cellulose nanofibers have a peak height ratio (C=O / C-O) of a peak (1062 cm-1 vicinity) height derived from C-O in an infrared spectroscopy spectrum to a peak (1610 cm-1 vicinity) height derived from C=O of 0.70 or less, or the peak height ratio (C=O / O-H) of a peak (3340 cm-1 vicinity) height derived from O-H to a peak (1610 cm-1 vicinity) height derived from C=O of 1.35 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a carbon nanotube dispersion that serves as a raw material for manufacturing electrodes and other components of lithium-ion batteries. [Background technology]

[0002] With the spread of electric vehicles and the miniaturization, weight reduction, and increased performance of portable devices such as mobile phones and notebook computers, there is a growing demand for rechargeable batteries with high energy density and higher capacity. Against this backdrop, lithium-ion rechargeable batteries, which use non-aqueous electrolytes due to their high energy density and high voltage characteristics, are increasingly being used in many devices. Studies have been conducted to explore how using carbon nanotube dispersions as negative and positive electrode materials in lithium-ion secondary batteries can improve conductivity, reduce electrode resistance, and efficiently form conductive networks with small amounts of material. Recently, carbon nanotube dispersions using cellulose nanofibers as dispersants have also become known.

[0003] For example, Patent Document 1 discloses a carbon nanotube dispersion containing carbon nanotubes, cellulose nanofibers, and a dispersion medium, in order to provide a carbon nanotube dispersion that suppresses aggregation of carbon nanotubes and exhibits high dispersion stability, wherein the cellulose nanofibers are fine cellulose fibers having a maximum fiber diameter of 1000 nm or less and a number average fiber diameter of 2 nm or more and 150 nm or less, wherein some of the hydroxyl groups of the fine cellulose fibers are replaced with at least one functional group selected from the group consisting of carboxyl groups and aldehyde groups, and have a cellulose type I crystal structure.

[0004] Patent Document 2 discloses a nanomaterial composition that can improve the surface hardness when formed into a molded article, characterized by comprising a dispersion medium and cellulose nanofibers and carbon nanotubes dispersed in the dispersion medium.

[0005] Furthermore, Patent Document 3 discloses a dispersion stabilizer for electrode coating liquids for energy storage devices, which provides excellent dispersion stability for electrode active materials and conductive materials, and enables the production of uniform electrodes even when using a dispersion device with weak shear force. The dispersion stabilizer contains cellulose fibers that satisfy the following conditions: (a) the number average width of the shorter side is 2 to 200 nm; (b) the aspect ratio is 7.5 to 250; and (c) the cellulose has type I crystals and the degree of crystallinity is 70% to 95%. In addition, a dispersion liquid for electrode coating is disclosed, which further has (d) an anionic functional group, and (e) the anionic functional group is a carboxyl group and its content is 1.2 to 2.5 mmol / g.

[0006] Furthermore, Patent Document 4 discloses an electrode binder composition that provides electrodes that exhibit high durability even when using active materials with large volume changes, electrodes for energy storage devices made using the same, and energy storage devices equipped with the electrodes for energy storage devices, characterized in that the electrode binder composition contains (A) at least one or more polymer components selected from the group consisting of fluorine-based polymers, butadiene-based polymers, and thermoplastic elastomers, (B) a fibrous nanocarbon material with an average fiber diameter of 0.5 nm to 20 nm and a fiber length of 0.5 μm to 1 mm, (C) a cellulose material, (D) nanocellulose fibers, and (E) water, wherein the mass ratio of (A) to (B) is (A) / (B) = 60 / 40 to 98 / 2.

[0007] Patent document 5 discloses a dispersion containing a dispersion medium, metal-containing oxidized cellulose nanofibers containing metals other than sodium in the form of salts, and single-walled nanotubes. While carboxylated cellulose nanofibers obtained by TEMPO oxidation are described as the oxidized cellulose nanofibers, their properties are not described. TEMPO oxidation of cellulose nanofibers is usually carried out as completely as possible.

[0008] Furthermore, Non-Patent Document 1, "FY2017 Performance Evaluation Project for Cellulose Nanofiber-Utilizing Products (FY2017 Ministry of the Environment Commissioned Project) (Identification of Issues for the Practical Application of Lithium-ion Batteries for Idling Stop Vehicles Using Cellulose Nanofibers) Results Report, March 16, 2018: Daiichi Kogyo Seiyaku Co., Ltd.", describes how applying cellulose nanofibers to the manufacture of lithium-ion battery electrodes makes it possible to make the cathode coating liquid aqueous, and how it has been found to significantly improve battery degradation associated with charging and discharging, as well as how the discharge capacity retention rate during cycles is improved.

[0009] However, carbon nanotube dispersions such as those described in Patent Documents 1-5 and Non-Patent Document 1 still have challenges, such as a decrease in dispersibility over time and difficulty in achieving a high level of both stability and conductivity. In particular, when mixing with cellulose nanofibers, the dispersion process takes time to prevent insufficient dispersion, and the inclusion of a large amount of binder to improve battery durability results in high battery resistance. Further improvements are urgently needed. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2017-206412 (Claims, Examples, etc.) [Patent Document 2] Japanese Patent Publication No. 2020-019924 (Claims, Examples, etc.) [Patent Document 3] Japanese Patent Publication No. 2010-254546 (Claims, Examples, etc.) [Patent Document 4] Japanese Patent Publication No. 2007-169120 (Claims, Examples, etc.) [Patent Document 5] Japanese Patent Publication No. 2021-57271 (Claims, Examples, etc.) [Non-Patent Document 1] Heisei 29 (FY2017) Commissioned Performance Evaluation Project for Cellulose Nanofiber Utilized Products (Commissioned Business of the Ministry of the Environment Japan in Heisei 29) "Extraction of Issues toward Practical Application of Lithium-ion Batteries for Idling Stop Vehicles Using Cellulose Nanofibers" Result Report, March 16, Heisei 30 (2018), Dai-ichi Kogyo Seiyaku Co., Ltd. Summary of the Invention Problem to be Solved by the Invention

[0011] The present invention is intended to solve the above-mentioned conventional problems, and particularly aims to provide a means for solving the problems that when dispersing carbon nanotubes in a dispersion medium such as water, the carbon nanotubes aggregate to result in insufficient dispersion, or the dispersion step takes a long time to achieve complete dispersion. Means for Solving the Problem

[0012] As a result of intensive studies on the above problems, the present inventors have found that the carbon nanotube dispersion liquid for the above object can be obtained by a composition containing at least carbon nanotubes, oxidized cellulose nanofibers having specific physical properties, and water, and have completed the present invention.

[0013] That is, the carbon nanotube dispersion liquid of the present invention is a carbon nanotube dispersion liquid containing at least carbon nanotubes, oxidized cellulose nanofibers, and water, wherein for the oxidized cellulose nanofibers, the peak height ratio (C=O / C-O) of the height of a C-O-derived peak (at 1062 cm -1 -1 vicinity) to the height of a C=O-derived peak (at 1610 cm -1 -1 vicinity) is 0.70 or less, or the peak height ratio (C=O / O-H) of the height of an O-H-derived peak (at 3340 cm -1 -1 vicinity) to the height of a C=O-derived peak (at 1610 cm -1 -1 vicinity) is 1.35 or less A carbon nanotube dispersion liquid characterized by .

[0014] It is known that carboxylated oxidized cellulose nanofibers can be obtained by TEMPO oxidation of cellulose nanofibers, but typically this TEMPO oxidation reaction is allowed to proceed completely as far as possible. No attempt has been made to adjust the TEMPO oxidation reaction to control oxidized cellulose nanofibers to have specific physical properties. On the other hand, in the present invention, it is important not to allow the TEMPO oxidation reaction of cellulose nanofibers to proceed completely, such that the oxidized cellulose nanofibers have specific physical properties. [Effects of the Invention]

[0015] According to the present invention, when dispersing carbon nanotubes in a solvent, the advantageous effect is obtained that dispersion of carbon nanotubes is easy, and the dispersion step can be completed in a short time. The carbon nanotube dispersion liquid of the present invention has excellent dispersibility, and is useful as a raw material for fuel cells, various electrodes, electromagnetic wave shielding materials, conductive resins, members for field emission displays, and the like. In particular, by using the carbon nanotube dispersion liquid of the present invention, a coating liquid for forming an electrode layer suitable for manufacturing an electrode (positive electrode or negative electrode) of a lithium-ion secondary battery can be provided.

[0016] The objects and effects of the present invention will be appreciated and attained, in particular, by employing the constituent elements and combinations thereof pointed out in the claims. Both the foregoing general description and the following detailed description are exemplary and explanatory, and do not limit the present invention as recited in the claims. [Mode for Carrying Out the Invention]

[0017] Hereinafter, embodiments of the present invention will be described in detail. It should be noted, however, that the technical scope of the present invention is not limited to the embodiments described in detail below, and extends to the invention recited in the claims and equivalents thereof. The present invention can also be implemented based on the content disclosed herein and common technical knowledge in this field (including design matters and matters obvious to a person skilled in the art).

[0018] <Carbon nanotube dispersion> The carbon nanotube dispersion of the present invention is characterized by comprising at least carbon nanotubes, specific oxidized cellulose nanofibers, and water.

[0019] <Carbon nanotubes (CNTs)> The carbon nanotubes (CNTs) used in the present invention are not particularly limited as long as they have a shape that is substantially formed by winding a single sheet of graphite into a tube. Single-walled CNTs, which are made by winding a single sheet of graphite into one layer, and multi-walled CNTs, which are made by winding two or three or more layers of graphite, can all be used. Furthermore, examples of carbon nanotube forms include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, but are not limited to these. These may be used individually or in combination of two or more types (hereinafter simply referred to as "at least one type").

[0020] Furthermore, the average outer diameter of the carbon nanotubes is preferably 1 nm to 90 nm, more preferably 3 nm to 30 nm, and even more preferably 3 nm to 15 nm, from the viewpoint of the viscosity, conductivity, and stability of the dispersion. In this invention, the average outer diameter of carbon nanotubes refers to the arithmetic mean of a sufficient number of outer diameters (n) measured using images at a magnification of 100,000 times or more with a transmission electron microscope. Furthermore, the purity of the carbon nanotubes used in this invention is preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with the ash content measured in accordance with JIS K 1469 or JIS K 6218 being treated as an impurity.

[0021] Examples of carbon nanotubes (CNTs) that can be specifically used include NC7000 (average outer diameter 10nm) from Nanocyl, BaytubesC150P (average outer diameter 11nm) from Bayer, FloTube9000 (average outer diameter 19nm), FloTube7320 (average outer diameter 9nm), FloTube7010 (average outer diameter 9nm), FloTube6810 (average outer diameter 8nm), FloTube6120 (average outer diameter 8nm), FloTube6100 (average outer diameter 8nm), FloTube2020 (average outer diameter 4nm) from Canno, and MEIJOeDIPS from Meijo Nanocarbon. At least one of the following can be used: EC2.0 (average outer diameter 2.0 nm), KORBON-A7 from KORBON Corporation (average outer diameter 1.2 nm), NFT-7 from High Pressure Gas Industry Co., Ltd. (average outer diameter 30 nm), or NFT-15 from High Pressure Gas Industry Co., Ltd. (average outer diameter 30 nm).

[0022] The carbon nanotube (CNT) content can be set to an appropriate level depending on the application, and is not particularly limited. For example, when used in conductive pastes, electrode pastes for secondary batteries, electrodes for secondary batteries, etc., in order to achieve both high stability and conductive performance, and in terms of viscosity during dispersion production, the content is preferably 0.1 to 15.0% by mass of the total amount of the dispersion, more preferably 0.1 to 10.0% by mass, more preferably 0.5 to 8.0% by mass, 1.0 to 6.0% by mass, and particularly preferably 2.0 to 5.0% by mass. By setting the carbon nanotube (CNT) content to 0.1% by mass or more, sufficient conductivity can be ensured, while setting it to 15.0% by mass or less ensures the stability of the dispersion and good conductivity.

[0023] <Oxidized Cellulose Nanofiber (CeNF)> The oxidized cellulose nanofiber (CeNF) of the present invention is used as a dispersant for carbon nanotube dispersions and is an oxidized cellulose nanofiber that satisfies the following requirements.

[0024] The oxidized cellulose nanofibers used in the present invention preferably have a crystallinity of less than 70% when measured by X-ray diffractometry. The crystallinity is more preferably 25% or more and less than 70%. When the crystallinity is less than 70%, appropriate oxidation treatment and defibrillation are performed, resulting in good dispersibility of carbon nanotubes. When the crystallinity exceeds 70%, insufficient defibrillation of carbon nanotubes tends to occur, and the dispersibility also becomes insufficient. The oxidized cellulose nanofibers used preferably have cellulose type I crystals.

[0025] The crystallinity of oxidized cellulose in the present invention is the cellulose type I crystallinity calculated by the Segal method from diffraction intensity values obtained by X-ray diffractometry, and can be obtained by the following formula (1). [Formula 1] Crystallinity (%) = [(I 22.6 - I 18.5 ) / I 22.6 × 100 …(1) In the above formula (1), I 22.6 is the diffraction intensity of the lattice plane (002 plane) (diffraction angle 2θ = 22.6°) in X-ray diffraction, and I 18.5 is the diffraction intensity of the amorphous portion (diffraction angle 2θ = 18.5°). Note that cellulose type I refers to the crystal form of natural cellulose, and cellulose type I crystallinity means the proportion of cellulose type I crystal regions in the entire cellulose.

[0026] When the crystallinity of oxidized cellulose nanofibers is low, the mechanism by which the dispersibility of carbon nanotubes improves is not clear. The higher-order structure of cellulose nanofibers is changed by carboxylic acid groups or salts thereof generated by the oxidation reaction of methylol groups of cellulose nanofibers, resulting in a decrease in crystallinity. It is presumed that at the same time, cellulose nanofibers easily penetrate into carbon nanotubes, thereby allowing carbon nanotubes to be easily dispersed in a solvent.

[0027] In the carbon nanotube dispersion, it is preferable that 85% or more of the oxidized cellulose nanofibers have a fiber length of 50 nm to 250 nm, and it is more preferable that 30% or more have a fiber length of 100 nm to 150 nm. Furthermore, the oxidized cellulose nanofibers blended into the carbon nanotube dispersion preferably have a number-average fiber length L of 100 nm to 150 nm, and a ratio of the number-average fiber length L to the length-weighted average fiber length Ll (Ll / L) of 1.0 to 1.4.

[0028] The fiber length of oxidized cellulose nanofibers can be measured using a transmission electron microscope (TEM). By creating a histogram of the fiber lengths of the oxidized cellulose nanofibers from the TEM image, for example in 50 nm increments, and calculating the proportion of fibers with lengths in the range of 50 nm to 250 nm or 100 nm to 150 nm, the distribution of fiber lengths can be evaluated. Furthermore, from the fiber length histogram, the number-average fiber length L, the length-weighted average fiber length Ll, and the ratio of the number-average fiber length L to the length-weighted average fiber length Ll (Ll / L) can be determined.

[0029] When the fiber length of oxidized cellulose nanofibers is within the aforementioned range, the dispersibility of carbon nanotubes is good. Although the mechanism is not clear, it is presumed to be related to the permeability of oxidized cellulose nanofibers into carbon nanotubes.

[0030] Furthermore, the oxidized cellulose nanofibers used retain a certain amount of alcoholic hydroxyl groups, which allows for stronger hydrogen bonding between molecules and provides structural resilience. Additionally, during coating, the viscosity decreases, making uniform application easier. After coating, the viscosity recovers, maintaining a uniform state thereafter. The recovery (increase) of viscosity after coating also reduces material agglomeration and warping during drying. The alcoholic hydroxyl groups can be detected by infrared spectroscopy.

[0031] Oxidized cellulose nanofibers exhibit a CO-derived peak (1062 cm⁻¹) in their infrared spectroscopy spectrum. -1 (Near) Height and peak originating from C=O (1610cm) -1 The peak height ratio (C=O / CO) to the surrounding height is 0.70 or less, or the peak originating from OH (3340cm) -1 It is preferable that the peak height ratio (C=O / OH) between the height of the peak (near 1610 cm-1) originating from C=O and the height of the peak (near 1610 cm-1) is 1.35 or less. If the above peak height ratio (C=O / CO) is 0.70 or less, or the above peak height ratio (C=O / OH) is 1.35 or less, it can be used as TEMPO-oxidized cellulose with good carbon nanotube dispersibility. On the other hand, if the peak height ratio (C=O / CO) exceeds 0.70, or if the peak height ratio (C=O / OH) exceeds 1.35, it indicates an excessively oxidized state, which negatively affects the dispersibility of carbon nanotubes. The above peak height ratio (C=O / CO) is preferably 0.25 or more and 0.70 or less, or more preferably the above peak height ratio (C=O / OH) is 0.45 or more and 1.35 or less.

[0032] In this invention, it is preferable that the peak height ratio (C=O / CO) is 0.70 or less, or that the peak height ratio (C=O / OH) is 1.35 or less. During analysis, the peak may be affected by noise due to the inclusion of impurities originating from the material, but it is sufficient to satisfy either of the above peak height ratios. From the viewpoint of carbon nanotube dispersibility, it is even more preferable that the above peak height ratio (C=O / CO) is 0.70 or less and the above peak height ratio (C=O / OH) is 1.35 or less. To achieve a peak height ratio (C=O / CO) of 0.70 or less, or a peak height ratio (C=O / OH) of 1.35 or less, the amount of carboxyl groups can be controlled by adjusting the amount of co-oxidant added and the reaction time used in the oxidation process of cellulose fibers during the manufacturing process described later.

[0033] The infrared spectral spectrum of oxidized cellulose nanofibers can be measured using an infrared spectrometer. In this invention, infrared light is irradiated onto oxidized cellulose nanofibers, and the infrared absorption intensity at wavelengths characteristic of each functional group is measured. The ratio of these absorption intensities is then compared as the peak height ratio.

[0034] Preferably, the oxidized cellulose nanofiber has a carboxylic acid group (COOH) formed by selective oxidation of the hydroxyl group at position 6 of the glucose unit. Alternatively, the carboxylic acid group may be neutralized with a carboxylic acid base (COOX, where X represents a cation that forms a salt with the carboxylic acid). The fact that the oxidized cellulose has a hydroxyl group at position 6 on the glucose unit selectively oxidized is, for example, 13 This can be confirmed by a 1C-NMR chart. Note that oxidized cellulose nanofibers may have aldehyde groups or ketone groups along with carboxylic acid groups and / or carboxylic acid bases, but it is preferable that they substantially lack aldehyde groups and ketone groups.

[0035] The carboxylate group content (hereinafter referred to as the carboxyl group content) of the above-mentioned oxidized cellulose nanofibers is preferably adjusted to be between 0.5 mmol / g and 3.0 mmol / g relative to the oven-dry mass of the cellulose nanofibers. When the carboxyl group content is within the above range, the dispersibility is good when a coating solution for electrode materials is prepared using the carbon nanotube dispersion.

[0036] To measure the amount of carboxyl groups in the above-mentioned oxidized cellulose nanofibers, for example, prepare 60 mL of a 0.5-1% by mass slurry from a cellulose sample whose dry mass has been accurately weighed. Adjust the pH to approximately 2.5 with a 0.1 M hydrochloric acid aqueous solution, then add a 0.05 M sodium hydroxide aqueous solution dropwise and measure the electrical conductivity. Continue the measurement until the pH reaches approximately 11. From the amount of sodium hydroxide consumed (V) during the neutralization stage of the weak acid, where the change in electrical conductivity is gradual, the amount of carboxyl groups can be determined according to the following formula (2). [Formula 2] Carboxyl group content (mmol / g) = V (mL) × [0.05 / cellulose mass] ... (2) Furthermore, the amount of carboxyl groups can be adjusted by controlling factors such as the amount of co-oxidant added and the reaction time used in the oxidation process of cellulose fibers, as will be described later.

[0037] The above-mentioned oxidized cellulose fibers can be obtained by a manufacturing method comprising: an oxidation reaction step (1) in which natural cellulose fibers are used as raw materials and oxidized in water using an N-oxyl compound as an oxidation catalyst and a co-oxidant to obtain a reaction product; a purification step (2) in which impurities are removed and a water-impregnated reaction product is obtained; and a dispersion step (3) in which the water-impregnated reaction product is dispersed in a solvent.

[0038] (1) Oxidation reaction process After dispersing natural cellulose fibers and an N-oxyl compound in water (dispersion medium), a co-oxidant is added to initiate the reaction. During the reaction, a 0.5 M sodium hydroxide aqueous solution is added dropwise to maintain the pH at 10-11 while the co-oxidant is added. Here, the co-oxidant is not a substance that directly oxidizes the hydroxyl groups of cellulose, but rather a substance that oxidizes the N-oxyl compound used as an oxidation catalyst.

[0039] The above-mentioned natural cellulose fibers refer to refined cellulose fibers isolated from cellulose biosynthesis systems such as plant, animal, and bacterial-produced gels. More specifically, examples include coniferous pulp, hardwood pulp, cotton pulp such as cotton linters and cotton lint, non-wood pulp such as straw pulp and bagasse pulp, bacterial cellulose fibers (BC), cellulose fibers isolated from sea squirts, and cellulose fibers isolated from seaweed. These can be used individually or in combination of two or more. Among these, coniferous pulp, hardwood pulp, cotton pulp such as cotton linters and cotton lint, and non-wood pulp such as straw pulp and bagasse pulp are preferred.

[0040] The above-mentioned natural cellulose fibers are preferable because subjecting them to treatments that increase their surface area, such as beating, can improve reaction efficiency and increase productivity. Furthermore, using the above-mentioned natural cellulose fibers that have been isolated, purified, and then stored without drying (never-dry) is preferable because the microfibril clusters are in a state where they swell easily, thereby improving reaction efficiency and reducing the number-average fiber diameter after micronization treatment.

[0041] Particularly preferred cellulose raw materials include regenerated cellulose obtained by dissolving cellulose in some solvent such as copper ammonia solution or morpholine derivatives and then spinning it, and fine cellulose obtained by depolymerizing the above cellulose raw material by hydrolysis, alkaline hydrolysis, enzymatic decomposition, explosion treatment, or mechanical treatment such as a vibrating ball mill.

[0042] In the above reaction, the dispersion medium for natural cellulose fibers is water, and the concentration of natural cellulose fibers in the reaction aqueous solution can be any concentration, as long as sufficient diffusion of the reagent (natural cellulose fibers) is possible. Typically, it is about 5% or less of the mass of the reaction aqueous solution, but the reaction concentration can be increased by using equipment with strong mechanical stirring power.

[0043] Furthermore, examples of the above N-oxyl compounds include compounds having a nitroxyl radical, which is generally used as an oxidation catalyst. The above N-oxyl compound is preferably a water-soluble compound, and among these, piperidine nitroxyl radicals are preferred, and 2,2,6,6-tetramethylpiperidinooxyl radical (TEMPO) or 4-acetamide-TEMPO are particularly preferred. The addition of the above N-oxyl compound is not particularly limited as long as it is a catalytic amount that can oxidize the raw material cellulose. For example, 0.01 to 10 mmol is preferred, 0.02 to 1 mmol is more preferred, and 0.05 to 0.5 mmol is even more preferred, per 1 g of oven-dried cellulose. Also, about 0.1 to 4 mmol / L relative to the reaction system is good.

[0044] Examples of the above co-oxidants include hypohalous acid or its salts, haloous acid or its salts, perhalous acid or its salts, hydrogen peroxide, and perorganic acids. These can be used alone or in combination of two or more. Among these, alkali metal hypohalite salts such as sodium hypochlorite and sodium hypobromite are preferred. When sodium hypochlorite is used, it is preferable to carry out the reaction in the presence of alkali metal bromide such as sodium bromide, from the viewpoint of reaction rate. The amount of alkali metal bromide added is about 1 to 40 times the molar amount relative to the N-oxyl compound, preferably about 10 to 20 times the molar amount.

[0045] The pH of the above reaction aqueous solution is preferably maintained in the range of approximately 8 to 11. The temperature of the aqueous solution is selected from the range of approximately 4 to 40°C. To obtain the desired amount of carboxyl groups, the degree of oxidation is controlled by the amount of co-oxidant added and the reaction time. The reaction time in the oxidation reaction can be appropriately set depending on the degree of oxidation, and is usually 0.5 to 6 hours, for example, about 1 to 4 hours. The oxidation reaction may also be carried out in two stages. For example, by oxidizing the cellulose obtained by filtering after the completion of the first stage reaction under the same or different reaction conditions, it is possible to efficiently oxidize it without being inhibited by the salt produced as a by-product in the first stage reaction.

[0046] In addition to these factors, the degree of oxidation and hydrolysis of cellulose molecules can be controlled by controlling factors such as the amount of co-oxidant added and the pH of the reaction aqueous solution, thereby arbitrarily setting the various properties of the oxidized cellulose nanofibers. Normally, TEMPO oxidation of cellulose nanofibers is carried out to the extent that the oxidation reaction can be completed. However, in the present invention, it is important not to proceed too far with TEMPO oxidation so that the oxidized cellulose nanofibers have the specific physical properties described above.

[0047] (2) Purification process Next, the mixture is purified to remove unreacted co-oxidants (such as hypochlorous acid) and various by-products. Since the reactant fibers are not usually dispersed into nanofiber units at this stage, a high-purity (99% by mass or more) dispersion of reactant fibers and water can be obtained by repeating the usual purification method, i.e., washing with water and filtering. The purification method used in the above purification process can be any apparatus that can achieve the above-mentioned objectives, such as a method utilizing centrifugal dehydration (for example, a continuous decanter). The aqueous dispersion of reactant fibers obtained in this way has a solid content (cellulose fiber) concentration in the range of approximately 10% to 50% by mass when squeezed. Considering the subsequent dispersion process, a solid content concentration higher than 50% by mass is undesirable because it would require extremely high energy for dispersion.

[0048] (3) Dispersion process (micronization process) The water-impregnated reaction product (aqueous dispersion) obtained in the purification process described above is dispersed in a dispersion medium for dispersion treatment. The viscosity increases during the treatment, and a dispersion of finely processed cellulose fibers can be obtained. Since the fine processing of cellulose fibers also causes cutting in the longitudinal direction of the cellulose fibers, the aspect ratio of the cellulose fibers can be arbitrarily set by controlling the degree of fine processing (for example, the processing shear force of the disperser, processing pressure, number of processing cycles, processing time, etc.). After that, the cellulose fibers may be dried as needed. As a drying method for the cellulose fiber dispersion, for example, if the dispersion medium is water, spray drying, freeze-drying, or vacuum drying can be used, and if the dispersion medium is a mixed solution of water and an organic solvent, drying by drum dryer or spray drying by spray dryer can be used. The cellulose fiber dispersion may also be used in its dispersed state without drying.

[0049] The dispersants used in the above dispersion process are preferably powerful devices with beating capabilities, such as homomixers, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersion machines, beaters, disc refiners, conical refiners, double-disc refiners, and grinders, which allow for more efficient and advanced downsizing, and thus enable the production of water-containing lubricant compositions at an economically advantageous rate. Other dispersants may include, for example, screw mixers, paddle mixers, disperser mixers, turbine mixers, dispersers, propeller mixers, kneaders, blenders, homogenizers, ultrasonic homogenizers, colloid mills, pebble mills, and bead mill grinders. Furthermore, two or more types of dispersants may be used in combination.

[0050] (4) Reduction process In the present invention, it is preferable to further reduce the oxidized cellulose nanofibers after the oxidation reaction step described above. Specifically, the fine oxidized cellulose fibers after the oxidation reaction are dispersed in purified water, the pH of the aqueous dispersion is adjusted to about 10, and a reduction reaction is carried out using various reducing agents. As the reducing agent used in the present invention, general-purpose agents can be used, but LiBH4, NaBH3CN, NaBH4, etc. are preferred. Among these, NaBH4 is preferred from the viewpoint of cost and availability.

[0051] The amount of reducing agent is preferably in the range of 0.1 to 4% by mass, and particularly preferably in the range of 1 to 3% by mass, based on the dry weight of oxidized cellulose nanofibers. The reaction is carried out at room temperature or slightly above room temperature, usually for 10 minutes to 10 hours, preferably 30 minutes to 2 hours. The above reduction process can convert the aldehyde groups and ketone groups contained in the oxidized cellulose nanofibers into hydroxyl groups.

[0052] Oxidized cellulose nanofibers that satisfy the properties of the present invention can be manufactured by employing the following means and methods during the manufacturing process described above. Furthermore, if commercially available products possessing the above properties exist, they can be used.

[0053] To set the crystallinity, fiber length distribution, and peak height ratio (C=O / CO) or (C=O / OH) in the infrared spectroscopic spectrum of oxidized cellulose nanofibers within a predetermined range, these can be adjusted in the above-mentioned manufacturing method by controlling the amount of co-oxidant added during the oxidation reaction of cellulose nanofibers, the reaction time, and the defibrillation time.

[0054] In the present invention, the content of oxidized cellulose nanofibers in the carbon nanotube dispersion can be set to a suitable content depending on the application. For example, when used in conductive pastes, electrode pastes for secondary batteries, or electrodes for secondary batteries, the content of oxidized cellulose nanofibers in the carbon nanotube dispersion is preferably 0.1 to 2% by mass relative to the amount of active material, more preferably 0.1 to 1.5% by mass, more preferably 0.1 to 1% by mass, and particularly preferably 0.1 to 0.8% by mass, in order to achieve both high stability and conductive performance, and to maintain viscosity during dispersion production.

[0055] By setting the content of oxidized cellulose nanofibers to 0.1% by mass or more relative to the active material, sufficient dispersibility and cycle characteristics as a battery can be ensured in the carbon nanotube dispersion. On the other hand, by setting it to 2% by mass or less, stability and good conductivity of the dispersion can be ensured.

[0056] In the carbon nanotube dispersion of the present invention, the dispersion medium (the remainder of the dispersion) is water (for example, purified water, distilled water, pure water, ultrapure water, tap water, ion-exchanged water, etc.), and in addition to water, water-soluble solvents can be used. Examples of water-soluble solvents that can be used 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-triol. Examples include alkylene glycols such as 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, and at least one of N-methyl-2-pyrrolidone and 1,3-dimethyl-2-imidalidinone.

[0057] In addition, water-soluble solvents such as amides (dimethylformamide, dimethylacetamide, etc.) and ketones (acetone, etc.) can also be mixed in. The content of these water-soluble solvents varies depending on the adjustment of the solid content of the dispersion, but is preferably 0.1 to 7% by mass relative to the total amount of carbon nanotube dispersion, more preferably less than 10% by mass, and even more preferably 0.1 to 5% by mass, from the viewpoint of improving the mixability of the slurry and storage stability.

[0058] The carbon nanotube dispersion of the present invention preferably contains a preservative for the sake of storage stability and prevention of bacterial growth. In addition, other additives may be added depending on the application. Examples include sodium carboxymethylcellulose as a thickener, anti-settling agents, wetting agents, emulsifiers, anti-sagging agents, defoaming agents, leveling agents, and plasticizers.

[0059] The carbon nanotube dispersion of the present invention can be produced, for example, by adding at least carbon nanotubes, oxidized cellulose nanofibers having the above-mentioned properties, water, as well as a flat graphite plate, a water-soluble solvent, a preservative, etc., stirring and mixing, and then dispersing the mixture. The dispersion of the above-mentioned dispersions can be carried out using, but is not limited to, ultrasonic dispersers, mixers such as dispersers, homomixers, rotational mixers, Henschel mixers, and planetary mixers, media-type dispersers such as (high-pressure) homogenizers, paint conditioners, colloid mills, bead mills, cone mills, ball mills, sand mills, attritors, pearl mills, and coball mills, media-less dispersers such as wet jet mills and thin-film swirling high-speed mixers, and other dispersion devices such as roll mills. Preferred dispersion devices, from the standpoint of stability and dispersion efficiency, include thin-film swirling high-speed mixers and bead mills.

[0060] Furthermore, in order to obtain the fluidity of the electrode slurry prepared by adding the carbon nanotube dispersion of the present invention, it is preferable that the viscosity (mPa·s) at 25°C at a rotor rotation speed of 10 rpm using an E-type rotational viscometer [Toki Sangyo Co., Ltd., TV-25] is 5 to 700, and more preferably 5 to 200.

[0061] The carbon nanotube dispersion of the present invention can be used as a coating liquid for forming electrode layers (hereinafter also referred to as electrode slurry) suitable for the manufacture of electrodes (positive or negative electrodes) of lithium-ion secondary batteries. When coating for electrode formation, it uniformly disperses the conductive carbon nanotubes and active material, and exhibits excellent dispersion stability. Moreover, it does not adversely affect the resistance value of the electrode itself, and is a suitable electrode composition for the manufacture of battery electrodes such as high-efficiency lithium-ion batteries. The electrode layer obtained from this electrode layer forming coating liquid can achieve both high stability and conductivity while maintaining high cycle characteristics and self-discharge characteristics. + It does not adversely affect the exchange of ions or the resistance of the electrodes.

[0062] <Slurry for positive electrode> The cathode slurry can be prepared by mixing the cathode active material with the carbon nanotube dispersion of the present invention. The positive electrode active material that can be used is not particularly limited to any ordinary positive electrode active material (an active material that allows lithium ions to enter and exit reversibly) that can be used as the positive electrode of a lithium-ion battery.

[0063] For example, lithium-transition metal composite oxides such as lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-cobalt-manganese-aluminum composite oxide, etc., transition metal sulfides such as TiS2, FeS, MoS2, MnO, V---2O5, V6O 13 Examples include transition metal oxides such as TiO2 and olivine-type lithium phosphate oxides. Olivine-type lithium phosphate oxides, for example, contain at least one element from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, along with lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements to improve their properties.

[0064] A preferred positive electrode active material is a lithium-nickel composite oxide, and more preferably, the lithium-nickel composite oxide is of the formula: LiNi X M1 Y M2 Z A lithium-nickel composite oxide represented as O2 (where M1 and M2 are at least one metallic element from among Al, B, alkali metals, alkaline earth metals, and transition metals, with 0.8 ≤ X ≤ 1.0, 0 ≤ Y ≤ 0.2, and 0 ≤ Z ≤ 0.2) is preferred. These positive electrode active materials may be used individually or in combination of two or more types.

[0065] In the cathode slurry of the present invention, the content of the above-mentioned positive electrode active material is preferably 50 to 70% by mass, and more preferably 50 to 63% by mass, relative to the total amount of the positive electrode slurry, in order to ensure battery capacity and slurry fluidity. Furthermore, the carbon nanotube content is preferably 0.5 to 10% by mass, and more preferably 0.5 to 7% by mass, in terms of solid content relative to the total amount of cathode slurry.

[0066] The cathode slurry of the present invention contains carbon nanotubes and a cathode active material, and may optionally contain a solid electrolyte such as a sulfide solid electrolyte, oxide solid electrolyte, dry polymer electrolyte, gel polymer electrolyte, or pseudo-solid electrolyte.

[0067] <Slurry for negative electrode> The negative electrode slurry can be prepared by mixing the negative electrode active material with the carbon nanotube dispersion of the present invention. In addition to graphite, there are no particular restrictions on the negative electrode active material that can be used. Any non-conductive material can be used, such as metal oxide-based active material particles, silicon-based active material particles, and especially metal oxide-based negative electrode active material particles. As 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 is capable of intercalating and deintercalating lithium, but examples of usable titanium oxides 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.

[0068] As for spinel-type lithium titanate, Li4+xTi5O 12Examples include (where x changes in the range of -1 ≤ x ≤ 3 due to charge-discharge reactions). Ramsdelite-type lithium titanate includes Li2 + yTi3O7 (where y changes in the range of -1 ≤ y ≤ 3 due to charge-discharge reactions). TiO2(B) and anatase-type titanium dioxide include Li1 + zTiO2 (where z changes in the range of -1 ≤ z ≤ 0 due to charge-discharge reactions).

[0069] 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 TiO2-P2O5, TiO2-V2O5, TiO2-P2O5-SnO2, and TiO2-P2O5-MeO (where Me is at least one element selected from the group consisting of Cu, Ni, and Fe). Such metal composite oxides preferably have low crystallinity and a microstructure in which a crystalline phase and an amorphous phase coexist, or in which the amorphous phase exists alone. This microstructure can further improve the cycling performance.

[0070] In the negative electrode slurry of the present invention, the content of the negative electrode active material is preferably 30 to 60% by mass, and more preferably 35 to 55% by mass, relative to the total amount of the negative electrode slurry, in order to ensure battery capacity and slurry fluidity. Furthermore, the carbon nanotube dispersion content is preferably 0.5 to 10% by mass, and more preferably 0.5 to 7% by mass, relative to the total amount of the negative electrode slurry, in terms of solid content. The negative electrode slurry of the present invention contains carbon nanotubes and a negative electrode active material, and may optionally contain a solid electrolyte such as a sulfide solid electrolyte, oxide solid electrolyte, dry polymer electrolyte, gel polymer electrolyte, or pseudo-solid electrolyte.

[0071] By using the positive electrode slurry and negative electrode slurry obtained above, electrodes for secondary batteries can be obtained. [Examples]

[0072] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. CeNF-1 and CeNF-2 were used as oxidized cellulose nanofibers (CeNF).

[0073] (Method for measuring crystallinity) Using a desktop X-ray diffractometer (MiniFlex600, manufactured by Rigaku Corporation), the crystallinity of oxidized cellulose nanofibers was calculated by the Segal method using X-ray diffraction with a tube voltage of 40 kV, a tube current of 15 mA, a measurement range of 2θ = 5 to 35°, and a scan speed of 10° / min. The results of the crystallinity measurement of the oxidized cellulose nanofibers used are as follows: *CeNF-1: Crystallinity 54% *CeNF-2: Crystallinity 78%

[0074] (Method for measuring fiber length) The fiber length of oxidized cellulose nanofibers was measured from images obtained using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the proportion of nanofibers with lengths of 50 nm to 250 nm or 100 nm to 150 nm relative to the total number was calculated. The sample size (n) was set to 100. Furthermore, the number-average fiber length L and the length-weighted average fiber length Ll were determined from this distribution, and the ratio of the number-average fiber length L to the length-weighted average fiber length Ll (Ll / L) was calculated. The measurement results for the fiber lengths of the oxidized cellulose nanofibers used are as follows. *CeNF-1: 90% of fibers have a length of 50nm to 250nm, 35% have a length of 100nm to 150nm, the ratio of number-average fiber length L to length-weighted average fiber length Ll is Ll / L 1.3 *CeNF-2: 60% of fibers have a length of 50nm to 250nm, 13% have a length of 100nm to 150nm, the ratio of number-average fiber length L to length-weighted average fiber length Ll is Ll / L 1.6

[0075] (CO-derived peak in infrared spectral spectrum (1062 cm) -1 (Near) Height and peak originating from C=O (1610cm) -1 The ratio of peak height to (near) height (C=O / CO) and the peak originating from OH (3340cm) -1 (Method for measuring the peak height ratio (C=O / OH) between the height of the peak originating from C=O (around 1610 cm⁻¹) and the height of the peak originating from C=O.) The infrared spectra of naturally dried cellulose nanofibers were measured using an infrared spectrophotometer (Thermo SCIENTIFIC, NICOLET iZ10). The maximum absorption at wavelengths characteristic of each functional group in the obtained infrared spectra was defined as the peak, and the ratio of their respective absorption ratios was defined as the peak height ratio. *CeNF-1: Peak height ratio (C=O / CO) 0.61, Peak height ratio (C=O / OH) 1.16 *CeNF-2: Peak height ratio (C=O / CO) 0.72, Peak height ratio (C=O / OH) 1.39

[0076] [Preparation of carbon nanotube dispersion] Determined amounts of carbon nanotubes, flat graphite (manufactured by Nippon Graphite Co., Ltd., J-SP-α), and the aforementioned oxidized cellulose nanofibers CNT-A or CNT-B were added to distilled water, and the mixture was stirred using a dispersion device (a bead mill using φ1.0 mm zirconia beads; the peripheral speed was set to 10 m / s) to obtain a carbon nanotube dispersion. The composition of the carbon nanotube dispersion is shown in Table 1 below.

[0077] (Evaluation of variance) The obtained carbon nanotube dispersion was observed using a scanning electron microscope (SEM). The number of solids with a maximum diameter of 500 nm or more found within a 5 μm square frame was counted. Observations were performed on 10 grids, and the average number of counted solids was calculated. The dispersibility was evaluated according to the following criteria. The solids found were undispersed carbon nanotube aggregates. Evaluation criteria: ○: The number of counted individuals is less than 5. △: The number of counted individuals is 6 or more but less than 10. ×: The number of counted individuals is 10 or more. These evaluation results are shown in Table 1 below.

[0078] [Table 1]

[0079] As is clear from the evaluation results in Table 1, the carbon nanotube dispersion of the present invention exhibited good dispersibility. [Industrial applicability]

[0080] The carbon nanotube dispersion of the present invention exhibits excellent carbon nanotube dispersibility and can be used in fuel cells, various electrodes, electromagnetic shielding materials, conductive resins, and components for field emission displays, in particular as a raw material for manufacturing electrodes for lithium-ion secondary batteries.

Claims

1. A carbon nanotube dispersion comprising at least carbon nanotubes, oxidized cellulose nanofibers, and water, Oxidized cellulose nanofibers produce a C-O origin peak (1062 cm⁻¹) in the infrared spectroscopy spectrum. －１ (Nearby) Height and peak originating from C=O (1610 cm) －１ The ratio of peak height to surrounding height (C = O / C - O) is 0.70 or less, or the peak originating from O-H (3340 cm) －１ (Nearby) Height and peak originating from C=O (1610 cm) －１ The ratio of peak height to surrounding height (C = O / O - H) is 1.35 or less. A carbon nanotube dispersion characterized by the following features.

2. An electrode composition characterized by using the carbon nanotube dispersion described in claim 1.

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