Carbon nanotube dispersion

Abstract

Provided is a carbon nanotube dispersion liquid having excellent dispersibility. A carbon nanotube dispersion liquid, which contains at least carbon nanotubes, oxidized cellulose nanofibers, and water, and which is characterized in that the oxidized cellulose nanofibers having a fiber length of 50-250 nm account for 85% or more of the oxidized cellulose nanofibers.

Description

Technical Field

[0001] The present invention relates to a carbon nanotube dispersion liquid as a raw material for manufacturing electrodes of a lithium ion battery or the like. Background Art

[0002] With the popularization of electric vehicles, the miniaturization, light weight, and high performance of portable devices such as mobile phones and notebook personal computers, there is a demand for secondary batteries with high energy density, and further, for high-capacity secondary batteries. Against this background, due to the characteristics of high energy density and high voltage, lithium ion secondary batteries using non-aqueous electrolytes are used in many devices.

[0003] The following studies have been conducted: By using a carbon nanotube dispersion liquid or the like in the negative electrode material and the positive electrode material used in these lithium ion secondary batteries, good conductive performance can be achieved, the electrode resistance can be reduced, and a conductive network can be formed in a small amount and efficiently. Recently, a carbon nanotube dispersion liquid using cellulose nanofibers as a dispersant has also been known.

[0004] For example, in Patent Document 1, in order to provide a carbon nanotube dispersion liquid that suppresses the aggregation of carbon nanotubes and exhibits high dispersion stability, a carbon nanotube dispersion liquid is provided, which contains carbon nanotubes, cellulose nanofibers, and a dispersion medium. The cellulose nanofibers are fine cellulose fibers having a maximum fiber diameter of 1000 nm or less, a number average fiber diameter of 2 nm or more and 150 nm or less, and a part of the hydroxyl groups of the fine cellulose fibers is substituted with at least one functional group selected from the group consisting of a carboxyl group and an aldehyde group, and has a cellulose I-type crystal structure or the like.

[0005] In Patent Document 2, in order to provide a nanomaterial composition capable of improving the surface hardness when forming a molded body, a nanomaterial composition is disclosed, which is characterized by containing: a dispersion medium; and cellulose nanofibers and carbon nanotubes dispersed in the dispersion medium.

[0006] In addition, in Patent Document 3, in order to provide a composition for a dispersion stabilizer for an electrode coating liquid of a power storage device, which has excellent dispersion stability of an electrode active material and a conductive material and can produce a uniform electrode even with a dispersion device using a weak shearing force, an electrode coating dispersion liquid is disclosed, which contains: a cellulose fiber that satisfies (a) the number average width on the shorter width side is 2 to 200 nm, (b) the aspect ratio is 7.5 or more and 250 or less, (c) has a cellulose I-type crystal and its crystallinity satisfies 70% or more and 95% or less, as a dispersant for an electrode coating liquid of a power storage device, and further (d) has an anionic functional group, (e) the anionic functional group is a carboxyl group, and its content is 1.2 to 2.5 mmol / g.

[0007] Furthermore, in Patent Document 4, in order to provide a binder composition for an electrode, an electrode for a power storage device produced using the same, and a power storage device including the electrode for a power storage device that can obtain an electrode exhibiting high durability even when using an active material with a large volume change, the following binder composition for an electrode, etc. are disclosed. The binder composition for an electrode is characterized by containing: (A) at least one or two or more polymer components selected from the group consisting of fluorine-based polymers, butadiene-based polymers, and thermoplastic elastomers, (B) a fibrous carbon nanomaterial having an average fiber diameter of 0.5 nm or more and 20 nm or less and a fiber length of 0.5 μm or more and 1 mm or less, (C) a cellulose material, (D) nanofibrillated cellulose, and (E) water, wherein the mass ratio of (A) to (B) is (A) / (B) = 60 / 40 to 98 / 2.

[0008] Patent Document 5 discloses a dispersion containing: a dispersion medium, metal-containing oxidized cellulose nanofibers containing a metal other than sodium in the form of a salt, and single-walled nanotubes. As the oxidized cellulose nanofibers, carboxylated cellulose nanofibers obtained by TEMPO oxidation are described, but their properties are not described. For the TEMPO oxidation of cellulose nanofibers, the oxidation reaction is usually carried out as completely as possible.

[0009] Furthermore, as described in the "Report on the Performance Evaluation Project of Cellulose Nanofiber-Utilizing Products in 2017 (Consignment Business of the Ministry of the Environment in 2017) (Extraction of Issues for the Practical Application of Idling Stop Vehicle Lithium-Ion Batteries Using Cellulose Nanofibers)", published by Daiichi Pharmaceutical Co., Ltd. on March 16, 2018, by applying cellulose nanofibers in the manufacture of electrodes for lithium-ion batteries, it is possible to make the positive electrode coating liquid aqueous, and it has been found that there is a significant improvement in battery deterioration during charge and discharge. In addition, an increase in the discharge capacity retention rate during cycling, etc. is described.

[0010] However, for these carbon nanotube dispersions, etc. in Patent Documents 1 to 5, Non-Patent Document 1, etc., there are still problems such as the dispersion decreasing over time, or it being difficult to highly balance stability and electrical conductivity. In particular, currently, there are problems such as spending time in the dispersion process to prevent insufficient dispersion when mixing cellulose nanofibers, or having a high resistance value of the battery due to including a large amount of binder to improve the durability of the battery, and further improvement is desired.

[0011] Prior Art Documents

[0012] Patent Documents

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-206412 (claims, examples, etc.)

[0014] Patent Document 2: Japanese Patent Application Laid-Open No. 2020-019924 (claims, examples, etc.)

[0015] Patent Document 3: Japanese Patent Application Laid-Open No. 2010-254546 (claims, examples, etc.)

[0016] Patent Document 4: Japanese Patent Application Laid-Open No. 2007-169120 (claims, examples, etc.)

[0017] Patent Document 5: Japanese Patent Application Laid-Open No. 2021-57271 (claims, examples, etc.)

[0018] Non-Patent Document 1: Report of the Results of the "Extraction of Issues for the Practical Application of an Idle Stop Vehicle Lithium-Ion Battery Using Cellulose Nanofibers" in the Performance Evaluation Business Commissioned for Cellulose Nanofiber Utilization Products in Fiscal Year 2017 (Business Commissioned by the Ministry of the Environment in Fiscal Year 2017), March 16, 2018, Daiichi Kogyo Seiyaku Co., Ltd. Summary of the Invention

[0019] Problems to be Solved by the Invention

[0020] The present invention has been made to solve the above-mentioned conventional problems, etc., and the problem thereof is to provide a carbon nanotube dispersion liquid having excellent dispersibility. In particular, the object is to provide a means for solving the problems that when carbon nanotubes are dispersed in a dispersion medium such as water, the carbon nanotubes aggregate and the dispersion is insufficient, or a long time is required in the dispersion process to achieve complete dispersion.

[0021] Solutions to the Problems

[0022] The inventors of the present invention have conducted in-depth research on the above problems, and as a result, have found that a 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, thereby completing the present invention.

[0023] That is, the carbon nanotube dispersion liquid of the present invention is a carbon nanotube dispersion liquid characterized by containing at least carbon nanotubes, oxidized cellulose nanofibers, and water,

[0024] wherein, among the oxidized cellulose nanofibers, those having a fiber length of 50 nm to 250 nm account for 85% or more.

[0025] Preferably, the carbon nanotube dispersion liquid is such that the number-average fiber length L of the aforementioned oxidized cellulose nanofibers is 100 nm to 150 nm, and the ratio (Ll / L) of the length-weighted average fiber length Ll to the number-average fiber length L is 1.0 to 1.4.

[0026] In addition, it is a carbon nanotube dispersion liquid in which the crystallinity of the aforementioned oxidized cellulose nanofibers as measured by X-ray diffraction is less than 70%.

[0027] In addition, it is the following carbon nanotube dispersion liquid: the peak height ratio (C=O / C-O) of the peak derived from C=O (near 1610 cm -1 -1) to the peak derived from C-O (near 1062 cm -1 -1) in the infrared absorption spectrum of the aforementioned oxidized cellulose nanofibers is 0.70 or less, or the peak height ratio (C=O / O-H) of the peak derived from C=O (near 1610 cm -1 -1) to the peak derived from O-H (near 3340 cm -1 -1) is 1.35 or less.

[0028] The carbon nanotube dispersion liquid of the present invention is suitable for a coating liquid for forming an electrode layer.

[0029] It is known that carboxylated oxidized cellulose nanofibers are obtained by TEMPO-oxidizing cellulose nanofibers, but usually this is the result of the TEMPO oxidation reaction proceeding as completely as possible. It is not controlled in such a way as to adjust the TEMPO oxidation reaction so that the oxidized cellulose nanofibers have specific physical properties. On the other hand, in the present invention, it is important that the TEMPO oxidation reaction of cellulose nanofibers does not proceed completely so that the oxidized cellulose nanofibers have specific physical properties.

[0030] Effects of the Invention

[0031] According to the present invention, the following effects can be exhibited: in the dispersion of carbon nanotubes in a solvent, the dispersion of carbon nanotubes is easy, and the dispersion process can be completed in a short time. The dispersibility of the carbon nanotube dispersion liquid of the present invention is excellent. It is useful as a raw material for fuel cells, various electrodes, electromagnetic wave shielding materials, conductive resins, components for field emission displays, etc. 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.

[0032] The objects and effects of the present invention are particularly identified and obtained by using the constituent elements and combinations indicated in the claims. Both the above general description and the following detailed description are illustrative and explanatory, and do not limit the present invention described in the claims. Detailed Description of the Invention

[0033] Hereinafter, embodiments of the present invention will be described in detail. However, it should be noted that the protection scope of the present invention is not limited to the embodiments described in detail below, but covers the invention described in the claims and equivalents thereof. In addition, the present invention can be implemented based on the content disclosed in this specification and common technical knowledge in this field (including design matters and common general knowledge).

[0034] 〈Carbon nanotube dispersion liquid〉

[0035] The carbon nanotube dispersion liquid of the present invention is characterized by containing at least carbon nanotubes, specific oxidized cellulose nanofibers, and water.

[0036] 〈Carbon nanotubes (CNT)〉

[0037] As the carbon nanotubes (CNT) used in the present invention, as long as they substantially have a shape in which one plane of graphite is rolled into a cylindrical shape, there is no particular limitation, and single-walled CNTs in which one plane of graphite is rolled into one layer, and multi-walled CNTs in which it is rolled into two or more layers can both be used.

[0038] In addition, examples of the form of carbon nanotubes include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultra-fine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, and these can be used individually or in combination of two or more (hereinafter, also simply referred to as "at least one kind").

[0039] Furthermore, from the viewpoints of the viscosity, conductivity, and stability of the dispersion liquid, 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 still more preferably 3 nm or more and 15 nm or less.

[0040] In the present invention, the average outer diameter of the carbon nanotubes refers to the arithmetic mean of the outer shapes of a sufficient number of n measured by an image at a magnification of 100,000 times or more using a transmission electron microscope.

[0041] In addition, the purity of the carbon nanotubes used in the present invention is preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass. It should be noted that for the purity of the carbon nanotubes, the ash content measured according to JIS K1469 and JIS K6218 is used as an impurity and calculated based on this impurity amount.

[0042] As the carbon nanotubes (CNTs) that can be specifically used, for example, 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) manufactured by Cnano, 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), MEIJOeDIPS EC2.0 (average outer diameter 2.0 nm) manufactured by Meijo Nano Carbon Co., Ltd., KORBON-A7 (average outer diameter 1.2 nm) manufactured by KORBON, NFT-7 (average outer diameter 30 nm) manufactured by KOATSU GAS KOGYO CO., LTD., NFT-15 (average outer diameter 30 nm) manufactured by KOATSU GAS KOGYO CO., LTD., etc. can be used, and at least one of them is used.

[0043] The content of these carbon nanotubes (CNTs) can be set appropriately according to the use, and there is no particular limitation.

[0044] For example, in the case of being used for conductive paste, electrode paste for secondary battery, electrode for secondary battery, etc., from the aspects of balancing high stability and conductive performance and the viscosity during the manufacture of the dispersion liquid, its content is preferably set to 0.1 to 15.0% by mass, more preferably 0.1 to 10.0% by mass, still more preferably 0.5 to 8.0% by mass, 1.0 to 6.0% by mass, and particularly 2.0 to 5.0% by mass with respect to the total amount of the dispersion liquid.

[0045] In this way, by setting the content of the carbon nanotubes (CNTs) to 0.1% by mass or more, sufficient conductivity can be ensured. On the other hand, by setting it to 15.0% by mass or less, the stability of the dispersion liquid and good conductivity can be ensured.

[0046] 〈Oxidized cellulose nanofibers (CeNF)〉

[0047] The oxidized cellulose nanofibers (CeNF) of the present invention are used as a dispersant for the carbon nanotube dispersion liquid, and are oxidized cellulose nanofibers that satisfy the following requirements.

[0048] The oxidized cellulose nanofibers used in the present invention preferably have a crystallinity of less than 70% when measured by X-ray diffraction method. The crystallinity is more preferably 25% or more and less than 70%.

[0049] When the crystallinity is less than 70%, moderate oxidation treatment and defibrillation are carried out, and the carbon nanotubes have good dispersibility. If the crystallinity exceeds 70%, the carbon nanotubes are prone to insufficient defibrillation and the dispersibility is also insufficient.

[0050] The oxidized cellulose nanofibers used preferably have cellulose I-type crystals.

[0051] The crystallinity of the oxidized cellulose of the present invention is the cellulose I-type crystallinity calculated by the Segal method from the diffraction intensity value based on the X-ray diffraction method, and can be obtained by the following formula (1).

[0052] Crystallinity (%) = 〔(I 22.6 -I 18.5 ) / I 22.6 〕×100…(1)

[0053] In the above formula (1), I 22.6 represents the diffraction intensity of the crystal plane (002 plane) (diffraction angle 2θ = 22.6°) in X-ray diffraction, and I 18.5 represents the diffraction intensity of the amorphous part (diffraction angle 2θ = 18.5°). It should be noted that cellulose I-type refers to the crystalline form of natural cellulose, and the cellulose I-type crystallinity refers to the proportion of the cellulose I-type crystalline region in the whole cellulose.

[0054] Regarding the mechanism by which the dispersibility of carbon nanotubes is improved when the crystallinity of the oxidized cellulose nanofibers is low, it is not yet clear, but due to the carboxyl group or its salt generated by the oxidation reaction through the hydroxymethyl group of the cellulose nanofibers, the higher-order structure of the cellulose nanofibers changes, resulting in a decrease in crystallinity. At the same time, it is speculated that the cellulose nanofibers are easily permeated into the carbon nanotubes, whereby the carbon nanotubes can be easily dispersed in the solvent.

[0055] Among the oxidized cellulose nanofibers incorporated in the carbon nanotube dispersion, those with a fiber length of 50 nm to 250 nm preferably account for more than 85% of the whole, and more preferably those with a fiber length of 100 nm to 150 nm account for more than 30% of the whole.

[0056] In addition, the oxidized cellulose nanofibers incorporated in the carbon nanotube dispersion preferably have a number-average fiber length L of 100 nm to 150 nm, and the ratio (Ll / L) of the length-weighted average fiber length Ll to the number-average fiber length L is 1.0 to 1.4.

[0057] The fiber length of oxidized cellulose nanofibers can be measured by a transmission electron microscope (TEM). Based on the TEM images, a histogram of the fiber length of the oxidized cellulose nanofibers can be created, for example, at a step of 50 nm, and the proportion of those within the range of 50 nm to 250 nm or within the range of 100 nm to 150 nm in the whole can be calculated, thereby evaluating the distribution of the fiber length. Additionally, based on the histogram of the aforementioned fiber length, the number-average fiber length L, the length-weighted average fiber length Ll, and the ratio of the length-weighted average fiber length Ll to the number-average fiber length L (Ll / L) can be obtained.

[0058] When the fiber length of the oxidized cellulose nanofibers is within the aforementioned range, the dispersibility of the carbon nanotubes is good. The mechanism is not clear and is presumed to be related to the permeability of the oxidized cellulose nanofibers to the carbon nanotubes.

[0059] Furthermore, a certain amount of alcoholic hydroxyl groups remain in the used oxidized cellulose nanofibers, whereby the molecules are hydrogen-bonded to each other more strongly, resulting in structural restorability. Additionally, when coating the composition, the viscosity decreases during coating, making it easy to coat evenly, and the viscosity recovers after coating, and a uniform state can be maintained thereafter. Also, by making the viscosity recover (increase) after coating, the material is less likely to aggregate during drying and warping is less likely to occur. The alcoholic hydroxyl groups are detected by infrared spectroscopy.

[0060] Preferably, for the oxidized cellulose nanofibers, the peak height ratio (C=O / C-O) of the peak derived from C=O (around 1610 cm -1 -1) to the peak height of the peak derived from C-O (around 1062 cm -1 -1) in the infrared spectroscopy is 0.70 or less, or the peak height ratio (C=O / O-H) of the peak derived from C=O (around 1610 cm-1) to the peak height of the peak derived from O-H (around 3340 cm -1 -1) is 1.35 or less.

[0061] When the above peak height ratio (C=O / C-O) is 0.70 or less, or the above peak height ratio (C=O / O-H) is 1.35 or less, it can be used as TEMPO-oxidized cellulose with good dispersibility of carbon nanotubes.

[0062] On the other hand, when the above peak height ratio (C=O / C-O) exceeds 0.70, or the above peak height ratio (C=O / O-H) exceeds 1.35, it becomes a state of over-oxidation, which has an adverse effect on the dispersibility of the carbon nanotubes.

[0063] The above peak height ratio (C=O / C-O) is preferably 0.25 or more and 0.70 or less, or the above peak height ratio (C=O / O-H) is more preferably 0.45 or more and 1.35 or less.

[0064] In the present invention, it is preferable that the above-mentioned peak height ratio (C=O / C-O) is 0.70 or less, or the above-mentioned peak height ratio (C=O / O-H) is 1.35 or less. Although the peaks may sometimes be affected by noise due to the mixing of impurities derived from the material during analysis, it is sufficient to satisfy any one of the above-mentioned peak height ratios.

[0065] From the aspect of the dispersibility of carbon nanotubes, it is further preferable that the above-mentioned peak height ratio (C=O / C-O) is 0.70 or less, and the above-mentioned peak height ratio (C=O / O-H) is 1.35 or less.

[0066] In order to make the above-mentioned peak height ratio (C=O / C-O) 0.70 or less, or the above-mentioned peak height ratio (C=O / O-H) 1.35 or less, it can be obtained by controlling the amount of carboxyl groups by adjusting the addition amount and reaction time of the co-oxidant used in the oxidation process of cellulose fibers during the following manufacturing process or the like.

[0067] The infrared absorption spectrum of oxidized cellulose nanofibers can be measured using an infrared spectrometer. In the present invention, infrared rays are irradiated on the oxidized cellulose nanofibers, the infrared absorption intensity of each functional group at the characteristic wavelength is measured, and the ratio of the absorption intensity is compared as the peak height ratio.

[0068] As the oxidized cellulose nanofibers, those in which the hydroxyl group at the 6-position of the glucose unit is selectively oxidized to have a carboxyl group (COOH) are preferred. The carboxyl group can be neutralized to a carboxylate group (COOX, where X represents a cation forming a salt with the carboxylic acid). Regarding the fact that the oxidized cellulose is formed by selectively oxidizing the hydroxyl group at the 6-position on the glucose unit, for example, it can be confirmed by 13 the C-NMR chart. It should be noted that the oxidized cellulose nanofibers may have an aldehyde group or a ketone group while having a carboxyl group and / or a carboxylate group, but it is preferred that they substantially do not have an aldehyde group and a ketone group.

[0069] The content of the carboxyl group of the above-mentioned oxidized cellulose nanofibers (hereinafter referred to as the amount of carboxyl groups) is preferably adjusted to 0.5 mmol / g to 3.0 mmol / g relative to the absolute dry mass of the cellulose nanofibers. When the amount of carboxyl groups is within the above range, the dispersibility is good when using a carbon nanotube dispersion to prepare a coating liquid for an electrode material.

[0070] For the determination of the carboxyl group content of the above-mentioned oxidized cellulose nanofibers, for example, 60 mL of a 0.5 to 1% by mass slurry is prepared from a cellulose sample whose dry mass has been accurately weighed. After adjusting the pH to approximately 2.5 with a 0.1 M hydrochloric acid aqueous solution, a 0.05 M sodium hydroxide aqueous solution is added dropwise, and the conductivity is measured. The measurement is continued until the pH reaches approximately 11. The carboxyl group content can be determined by the following formula (2) based on the amount of sodium hydroxide (V) consumed during the neutralization stage of the weak acid where the change in conductivity is mitigated.

[0071] Carboxyl group content (mmol / g) = V (mL) × [0.05 / cellulose mass]... (2)

[0072] It should be noted that the adjustment of the carboxyl group content can be carried out by controlling factors such as the addition amount of the co-oxidant and the reaction time used in the oxidation process of cellulose fibers, as described later.

[0073] The above-mentioned oxidized cellulose fibers can be obtained by a manufacturing method including the following steps: an oxidation reaction step (1) in which natural cellulose fibers are used as raw materials, an N-oxy compound is used as an oxidation catalyst in water, and a co-oxidant is allowed to act to oxidize the natural cellulose fibers to obtain a reaction product; a purification step (2) in which impurities are removed from the reaction product and water is impregnated; and a dispersion step (3) in which the reaction product impregnated with water is dispersed in a solvent.

[0074] (1) Oxidation reaction step

[0075] After dispersing the natural cellulose fibers and the N-oxy compound in water (dispersion medium), a co-oxidant is added to start the reaction. During the reaction, a 0.5 M sodium hydroxide aqueous solution is added dropwise to maintain the pH at 10 to 11, and a co-oxidant is added. Here, the co-oxidant is not a substance that directly oxidizes cellulose hydroxyl groups, but a substance that oxidizes the N-oxy compound used as an oxidation catalyst.

[0076] The above-mentioned natural cellulose fibers refer to purified cellulose fibers separated from the cellulose biosynthesis systems of plants, animals, and bacterial biogels. More specifically, examples include softwood pulp, hardwood pulp, cotton linter, cotton lint and other cotton-based pulps, wheat straw pulp, bagasse pulp and other non-wood pulps, bacterial cellulose fibers (BC), cellulose fibers separated from ascidians, cellulose fibers separated from seaweeds, etc. These can be used alone or in combination of two or more. Among these, softwood pulp, hardwood pulp, cotton linter, cotton lint and other cotton-based pulps, wheat straw pulp, bagasse pulp and other non-wood pulps are preferred.

[0077] If the above-mentioned natural cellulose fibers are subjected to treatments such as beating to increase the surface area, the reaction efficiency can be improved and the productivity can be increased, so it is preferred. In addition, when the above-mentioned natural cellulose fibers are used and stored without drying (never dry) after separation and purification, since they are in a state where the aggregate of microfibers is easily swollen, the reaction efficiency can be improved and the number-average fiber diameter after the fibrillation treatment can be reduced, so it is preferred.

[0078] As the cellulose raw material, regenerated cellulose obtained by dissolving cellulose in certain solvents such as cuprammonium solution and morpholine derivatives and then spinning, and microfibrillated cellulose obtained by depolymerizing cellulose by subjecting the above-mentioned cellulose raw material to hydrolysis, alkali hydrolysis, enzymatic hydrolysis, explosion treatment, mechanical treatment such as a vibratory ball mill, etc. are particularly preferred.

[0079] The dispersion medium of the natural cellulose fibers in the above reaction is water, and the concentration of the natural cellulose fibers in the reaction aqueous solution can be arbitrary as long as it can achieve sufficient diffusion of the reagent (natural cellulose fibers). Usually, it is about 5% or less relative to the mass of the reaction aqueous solution, and the reaction concentration can be increased by using a device with strong mechanical stirring force.

[0080] In addition, as the above-mentioned N-oxy compound, for example, compounds having a nitroxy radical that are usually used as oxidation catalysts can be cited. The above-mentioned N-oxy compound is preferably a water-soluble compound, particularly preferably piperidine nitroxyl radical, and particularly preferably 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) or 4-acetamido-TEMPO. The addition amount of the above-mentioned N-oxy compound is not particularly limited as long as it is a catalytic amount capable of oxidizing cellulose as a raw material. For example, relative to 1 g of dry cellulose, it is preferably 0.01 to 10 mmol, more preferably 0.02 to 1 mmol, and further preferably 0.05 to 0.5 mmol. In addition, it is preferably about 0.1 to 4 mmol / L relative to the reaction system.

[0081] As the above-mentioned co-oxidant, for example, hypohalous acid or its salts, halous acid or its salts, perhalic acid or its salts, hydrogen peroxide, organic peracids, etc. can be cited. These can be used alone or in combination of two or more. Among them, alkali metal hypohalites such as sodium hypochlorite and sodium hypobromite are preferred. Moreover, when using the above-mentioned sodium hypochlorite, from the aspect of reaction rate, it is preferred to carry out the reaction in the presence of an alkali metal bromide such as sodium bromide. The addition amount of the above-mentioned alkali metal bromide is about 1 to 40 times the molar amount relative to the above-mentioned N-oxy compound, preferably about 10 to 20 times the molar amount.

[0082] The pH of the above reaction aqueous solution is preferably maintained in the range of about 8 to 11. The temperature of the aqueous solution is selected from the range of about 4 to 40 °C. In order to obtain a desired amount of carboxyl group, etc., the degree of oxidation is controlled by the addition amount of the co-oxidant and the reaction time. The reaction time in the oxidation reaction can be appropriately set according to the degree of oxidation, usually 0.5 to 6 hours, for example, about 1 to 4 hours. In addition, the oxidation reaction can also be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the completion of the reaction in the first stage is oxidized again under the same or different reaction conditions, whereby oxidation can be efficiently carried out without being hindered by the sodium chloride by-produced in the reaction in the first stage.

[0083] In addition to these, the degree of oxidation and hydrolysis of cellulose molecules can be controlled by controlling factors such as the addition amount of the co-oxidant and the pH of the reaction aqueous solution, and each property of the oxidized cellulose nanofibers can be arbitrarily set. Generally, for the TEMPO oxidation of cellulose nanofibers, the oxidation reaction proceeds as completely as possible. However, in the present invention, it is important not to over-perform the TEMPO oxidation so that the oxidized cellulose nanofibers have the aforementioned specific physical properties.

[0084] (2) Purification process

[0085] Next, purification is carried out for the purpose of removing unreacted co-oxidants (such as hypochlorous acid), various by-products, etc. The reactant fibers are usually not completely dispersed into nanofiber units at this stage, so a high-purity (99 mass% or more) dispersion of the reactant fibers and water can be prepared by a usual purification method, that is, by repeatedly performing water washing and filtration.

[0086] For the purification method in the above purification process, any device can be used as long as it can achieve the above purpose, such as a method using centrifugal dehydration (for example, a continuous decanter). The water dispersion of the reactant fibers obtained in this way is in the range of approximately 10 mass% to 50 mass% in terms of the solid component (cellulose fiber) concentration in the extruded state. Considering the subsequent dispersion process, if the solid component concentration becomes higher than 50 mass%, extremely high energy is required for dispersion, so it is not preferred.

[0087] (3) Dispersion process (fine particle size treatment process)

[0088] Disperse the impregnated and water-containing reactant (aqueous dispersion) obtained in the above purification process in a dispersion medium and perform a dispersion treatment. The viscosity increases with the treatment, and a dispersion of micro-fine cellulose fibers can be obtained. It should be noted that as the cellulose fibers are micro-fined, cutting also occurs in the longitudinal direction of the cellulose fibers. Therefore, the aspect ratio of the cellulose fibers can be arbitrarily set by controlling the degree of the micro-fine treatment (for example, the treatment shear force, treatment pressure, treatment times, treatment time, etc. of the disperser). Thereafter, if necessary, the above cellulose fibers can be dried. As the drying method for the dispersion of the cellulose fibers, for example, in the case where the dispersion medium is water, spray drying, freeze drying method, vacuum drying method, etc. can be used. In the case where the dispersion medium is a mixed solution of water and an organic solvent, a drying method using a drum dryer, a spray drying method using a spray dryer, etc. can be used. It should be noted that the dispersion of the cellulose fibers can be used in the state of a dispersion without drying.

[0089] As the disperser used in the above dispersion process, by using a homogenizing mixer, high-pressure homogenizer, ultra-high-pressure homogenizer, ultrasonic dispersing processor, stirrer, disk refiner, conical refiner, double-disk refiner, sand mill, etc. that have a beating ability under strong force during high-speed rotation, more efficient and highly refined downsizing can be achieved, and an aqueous lubricant composition can be economically advantageously obtained, which is preferable in this regard. It should be noted that as the above disperser, for example, a screw-type mixer, paddle mixer, dispersion-type mixer, turbine-type mixer, disperser, propeller mixer, kneader, blender, homogenizer, ultrasonic homogenizer, colloid mill, pebble mill, beads mill crusher, etc. can also be used. In addition, two or more dispersers can be used in combination.

[0090] (4) Reduction process

[0091] In the present invention, the oxidized cellulose nanofibers preferably further undergo a reduction reaction after the above oxidation reaction process. 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, common reducing agents can be used, and preferably LiBH 4 , NaBH 3 CN, NaBH 4 , etc. Among them, from the aspects of cost and availability, NaBH 4 is preferred.

[0092] Regarding the amount of the reducing agent, based on the dry weight of the oxidized cellulose nanofibers, it is preferably in the range of 0.1 to 4% by mass, and particularly preferably in the range of 1 to 3% by mass. The reaction is carried out at room temperature or a temperature slightly higher than room temperature, usually for 10 minutes to 10 hours, preferably for 30 minutes to 2 hours.

[0093] Through the above reduction process, the aldehyde groups and ketone groups contained in the oxidized cellulose nanofibers can be converted into hydroxyl groups.

[0094] The oxidized cellulose nanofibers satisfying the respective characteristics of the present invention can be manufactured by adopting the following means / methods, etc. during the above manufacturing process. In addition, when there are commercially available products having the above respective characteristics, they can be used.

[0095] In order to set the crystallinity of the oxidized cellulose nanofibers, the distribution of fiber lengths, the peak height ratio (C=O / C-O) or (C=O / O-H) in the infrared absorption spectrum within a specified range, it can be adjusted by controlling the addition amount of the co-oxidant, the reaction time, the fibrillation time, etc. during the oxidation reaction of the cellulose nanofibers in the above manufacturing method.

[0096] In the present invention, the content of the oxidized cellulose nanofibers contained in the carbon nanotube dispersion can be set to an appropriate content according to the use.

[0097] For example, in the case of being used for conductive pastes, electrode pastes for secondary batteries, electrodes for secondary batteries, etc., from the aspects of achieving both high stability and conductive performance and the viscosity during the manufacture of the dispersion, the content of the oxidized cellulose nanofibers contained in the carbon nanotube dispersion is preferably set to 0.1 to 2% by mass relative to the amount of the active material, and ideally, it is further preferably set to 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.

[0098] By making the content of the oxidized cellulose nanofibers 0.1% by mass or more relative to the active material, sufficient dispersibility in the carbon nanotube dispersion and cycle characteristics as a battery can be ensured. On the other hand, by setting it to 2% by mass or less, the stability of the dispersion and good conductivity can be ensured.

[0099] The dispersion medium (the remaining part of the dispersion) in the carbon nanotube dispersion of the present invention is water (for example, purified water, distilled water, pure water, ultrapure water, tap water, ion-exchanged water, etc.). In addition to water, water-soluble solvents can also be used.

[0100] Examples of water-soluble solvents that can be used include, for example, alkylene glycols such as ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2 - propylene glycol, 1,3 - propylene glycol, 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, 3 - methylpentane - 1,3,5 - triol, 1,2,3 - hexanetriol; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; glycerols such as glycerol, diglycerol, and triglycerol; lower alkyl ethers of diols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, and diethylene glycol mono - n - butyl ether; at least one of N - methyl - 2 - pyrrolidone, 1,3 - dimethyl - 2 - imidazolidinone, etc.

[0101] In addition, for example, water - soluble solvents such as amides such as dimethylformamide and dimethylacetamide, and ketones such as acetone can also be mixed.

[0102] The content of these water - soluble solvents varies with the adjustment of the solid components of the dispersion. Relative to the total amount of the carbon nanotube dispersion, it is preferably 0.1 to 7% by mass, and from the aspects of improving the miscibility of the slurry and storage stability, it is more preferably less than 10% by mass, and further preferably 0.1 to 5% by mass.

[0103] From the aspects of storage stability and preventing the growth of miscellaneous bacteria, it is preferred that the carbon nanotube dispersion of the present invention contains a preservative. In addition, additives suitable for the use can also be added. For example, as thickeners, sodium carboxymethyl cellulose, anti - settling agents, wetting agents, emulsifiers, anti - sagging agents, defoaming agents, leveling agents, plasticizers, etc. can be cited.

[0104] For the production of the carbon nanotube dispersion of the present invention, for example, at least carbon nanotubes, oxidized cellulose nanofibers with the above characteristics, water, and plate - shaped graphite, water - soluble solvents, preservatives, etc. can be put in, stirred / mixed, and then obtained through a dispersion process.

[0105] For the dispersion treatment of the above - mentioned dispersion, for example, mixers such as ultrasonic dispersers, dispersers, homogenizing mixers, rotation - revolution mixers, Henschel mixers, planetary mixers, (high - pressure) homogenizers, paint conditioners, colloid mills, bead mills, conical mills, ball mills, sand mills, grinders, pearl mills, CoBall Mills and other media - type dispersers, wet jet mills, film - rotating high - speed mixers and other medium - less dispersers, and other roll mills and other dispersion devices can be used for dispersion treatment, but it is not limited to these.

[0106] As a preferred dispersion device, from the aspects of stability and dispersion efficiency, a thin-film rotary high-speed mixer, a bead mill, etc. are preferred.

[0107] In addition, for the carbon nanotube dispersion of the present invention, from the aspect of obtaining the fluidity of the electrode paste prepared by adding it, etc., the viscosity (mPa·s) at 25°C with an E-type rotational viscometer [manufactured by Toki Sangyo Co., Ltd., TV-25] and a rotor (1°34’×R24mm) at a rotational speed of 10 rpm is preferably 5 to 700, and more preferably 5 to 200.

[0108] The carbon nanotube dispersion of the present invention can be used as a coating liquid (hereinafter, also referred to as an electrode paste) for forming an electrode layer suitable for manufacturing an electrode (positive electrode or negative electrode) of a lithium ion secondary battery. When coating for forming an electrode, carbon nanotubes as a conductive material and an active material are uniformly dispersed, and the dispersion stability is excellent. Moreover, it does not have an adverse effect on the resistance value of the electrode itself, etc., and is an electrode composition suitable for manufacturing battery electrodes such as high-efficiency lithium ion batteries. The electrode layer obtained from this coating liquid for forming an electrode layer highly maintains the cycle characteristics and self-discharge characteristics, and can take into account high stability and conductive performance, and does not have an adverse effect on the entry and exit of Li + plasma and the resistance value of the electrode.

[0109] 〈Positive electrode paste〉

[0110] The positive electrode paste can be prepared by mixing a positive electrode active material in the carbon nanotube dispersion of the present invention.

[0111] As the positive electrode active material that can be used, as long as it is a general positive electrode active material (an active material that allows lithium ions to reversibly enter and exit) that can be used in the positive electrode of a lithium ion battery, it can be used without particular limitation.

[0112] For example, lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt composite oxides, lithium-nickel-aluminum composite oxides, lithium-nickel-cobalt-aluminum composite oxides, lithium-nickel-manganese-cobalt composite oxides, lithium-nickel-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, etc., composite oxides of lithium and transition metals, TiS 2 , FeS, MoS 2 and other transition metal sulfides, MnO, V 2 O 5 , V 6 O 13 , TiO 2Transition metal oxides such as, and olivine-type lithium phosphorus oxides. The olivine-type lithium phosphorus oxides may contain, for example: at least one element selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe; lithium; phosphorus, and oxygen. In order to improve their properties, a part of these elements may be partially replaced with other elements.

[0113] As a preferred active material for the positive electrode, it is a lithium-nickel composite oxide. More preferably, the lithium-nickel composite oxide preferably has the formula: LiNi X M1 Y M2 Z O 2 (M1 and M2 are at least one or more metal elements selected from the group consisting of Al, B, alkali metals, alkaline earth metals, and transition metals, 0.8 ≤ X ≤ 1.0, 0 ≤ Y ≤ 0.2, 0 ≤ Z ≤ 0.2) The lithium-nickel composite oxide shown.

[0114] These active materials for the positive electrode can be used alone or in combination of two or more.

[0115] From the aspects of ensuring the battery capacity and ensuring the fluidity of the slurry, in the positive electrode slurry of the present invention, the content of the above-mentioned active material for the positive electrode is preferably 50 to 70% by mass, more preferably 50 to 63% by mass, based on the total amount of the positive electrode slurry.

[0116] In addition, the content of carbon nanotubes is preferably 0.5 to 10% by mass, more preferably 0.5 to 7% by mass, based on the total amount of the positive electrode slurry in terms of the solid content.

[0117] The positive electrode slurry of the present invention contains carbon nanotubes and an active material for the positive electrode, and may further contain a solid electrolyte such as a sulfide solid electrolyte, an oxide solid electrolyte, a dry polymer electrolyte, a gel polymer electrolyte, a quasi-solid-state electrolyte, etc. as needed.

[0118] 〈Negative electrode slurry〉

[0119] The negative electrode slurry can be prepared by mixing an active material for the negative electrode in the carbon nanotube dispersion liquid of the present invention.

[0120] As the active material for the negative electrode that can be used, in addition to graphite, there is no particular limitation as long as it is a non-conductive material. For example, metal oxide-based active material particles, silicon-based active material particles, especially metal oxide-based negative electrode active material particles can be used.

[0121] As the negative electrode active material particles of the metal oxide system, for example, titanium oxide can be used. As the titanium oxide, as long as it can occlude / discharge lithium, there is no particular limitation. For example, spinel-type lithium titanate, orthorhombic lithium titanate, titanium-containing metal composite oxide, titanium dioxide (TiO 2 (B)) having a monoclinic crystal structure, and anatase-type titanium dioxide can be used.

[0122] As the spinel-type lithium titanate, Li 4 +xTi 5 O 12 (x varies in the range of -1 ≤ x ≤ 3) due to the charge-discharge reaction can be cited. As the orthorhombic lithium titanate, Li 2 +yTi 3 O 7 (y varies in the range of -1 ≤ y ≤ 3) due to the charge-discharge reaction can be cited. As TiO 2 (B) and anatase-type titanium dioxide, Li 1 +zTiO 2 (z varies in the range of -1 ≤ z ≤ 0) due to the charge-discharge reaction can be cited.

[0123] As the titanium-containing metal composite oxide, a metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe can be cited. As the metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe, for example, 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) can be cited.

[0124] Such a metal composite oxide preferably has a microstructure with low crystallinity, in which a crystalline phase and an amorphous phase coexist, or an amorphous phase exists alone. By having a microstructure, the cycle performance can be further improved.

[0125] From the aspects of ensuring battery capacity and ensuring the fluidity of the slurry, in the slurry for the negative electrode of the present invention, the content of the above-mentioned negative electrode active material relative to the total amount of the slurry for the negative electrode is preferably 30 to 60% by mass, and more preferably 35 to 55% by mass, which is ideal.

[0126] In addition, the content of the carbon nanotube dispersion liquid relative to the total amount of the slurry for the negative electrode in terms of the solid component amount is preferably 0.5 to 10% by mass, and more preferably 0.5 to 7% by mass, which is ideal.

[0127] The slurry for the negative electrode of the present invention can contain carbon nanotubes and the negative electrode active material, and according to needs, can also contain solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, dry polymer electrolytes, gel polymer electrolytes, and quasi-solid electrolytes.

[0128] By using the slurry for the positive electrode and the slurry for the negative electrode obtained above, an electrode for a secondary battery can be obtained.

[0129] Examples

[0130] Examples 1 to 3, Comparative Example 1

[0131] Hereinafter, the present invention will be described by way of examples, but the present invention is not limited to these examples.

[0132] As the oxidized cellulose nanofibers (CeNF), the following CeNF-1 and CeNF-2 were used.

[0133] (Method for measuring crystallinity)

[0134] Using a bench-top X-ray diffractometer (manufactured by Rigaku Corporation, MiniFlex600), at a tube voltage of 40 Kv, a tube current of 15 mA, a measurement range of 2θ = 5 to 35°, and a scanning speed of 10° / min, the crystallinity of the oxidized cellulose nanofibers was calculated by the Segal method using X-ray diffraction.

[0135] The measurement results of the crystallinity of the oxidized cellulose nanofibers used are as follows.

[0136] *CeNF-1: Crystallinity 54%

[0137] *CeNF-2: Crystallinity 78%

[0138] (Method for measuring fiber length)

[0139] Based on the images measured using a transmission electron microscope (manufactured by Hitachi High-Tech Corporation, H-7650; TEM), the fiber length of the oxidized cellulose nanofibers was measured, and the proportion of the number of those with a fiber length of 50 nm to 250 nm or 100 nm to 150 nm in the total number was calculated. The number n was set to 100. In addition, based on the distribution, the number-average fiber length L and the length-weighted average fiber length Ll were obtained, and the ratio (Ll / L) of the length-weighted average fiber length Ll to the number-average fiber length L was calculated.

[0140] The measurement results of the fiber length of the oxidized cellulose nanofibers used are as follows.

[0141] *CeNF-1: 90% with a fiber length of 50 nm to 250 nm, 35% with a fiber length of 100 nm to 150 nm, and the ratio Ll / L of the length-weighted average fiber length Ll to the number-average fiber length L is 1.3

[0142] *CeNF-2: 60% with a fiber length of 50 nm to 250 nm, 13% with a fiber length of 100 nm to 150 nm, and the ratio Ll / L of the length-weighted average fiber length Ll to the number-average fiber length L is 1.6

[0143] (The method for measuring the peak height ratio (C=O / C-O) of the peak derived from C=O (near 1610 cm -1 in the infrared absorption spectrum) and the peak derived from C-O (near 1062 cm -1 ), as well as the peak height ratio (C=O / O-H) of the peak derived from C=O (near 1610 cm-1) and the peak derived from O-H (near 3340 cm -1 ))

[0144] The infrared spectrum of the naturally dried cellulose nanofibers was measured using an infrared spectrophotometer (manufactured by Thermo SCIENTIFIC, NICOLET iZ10). The maximum absorption of each functional group in the obtained infrared spectrum at the characteristic wavelength was taken as the peak, and the ratio of the respective absorption ratios was taken as the peak height ratio.

[0145] *CeNF-1: The peak height ratio (C=O / C-O) is 0.61, and the peak height ratio (C=O / O-H) is 1.16

[0146] *CeNF-2: The peak height ratio (C=O / C-O) is 0.72, and the peak height ratio (C=O / O-H) is 1.39

[0147] [Preparation of Carbon Nanotube Dispersion]

[0148] To a prescribed amount of carbon nanotubes, plate-shaped graphite (manufactured by Nippon Graphite Industries, Ltd., J-SP-α), and the aforementioned oxidized cellulose nanofibers CNT-A or CNT-B were added separately to distilled water, and a stirring operation was performed using a dispersion device (a bead mill using zirconia beads with a circumferential speed set to 10 m / s) to obtain a carbon nanotube dispersion. The compounding composition of the carbon nanotube dispersion is shown in Table 1 below.

[0149] (Evaluation of dispersibility)

[0150] The obtained carbon nanotube dispersion was observed by SEM. The number of solids with a maximum diameter of 500 nm or more seen within a 5-μm square frame was counted. Observation was performed in 10 grids, and the average value of the counted number of solids was obtained, and the dispersibility was evaluated according to the following evaluation criteria. This solid is an undispersed carbon nanotube aggregate.

[0151] Evaluation criteria:

[0152] ○: The number of counted solids is less than 5

[0153] △: The number of counted solids is 6 or more and less than 10

[0154] ×: The number of counted solids is 10 or more

[0155] These evaluation results are shown in Table 1 below.

[0156] [Table 1]

[0157]

[0158] From the evaluation results in Table 1, it is clear that the carbon nanotube dispersion of the present invention has good dispersibility.

[0159] Industrial applicability

[0160] The carbon nanotube dispersion of the present invention has excellent dispersibility of carbon nanotubes and can be used as a raw material for manufacturing electrodes for fuel cells, various electrodes, electromagnetic wave shielding materials, conductive resins, components for field emission displays, etc., particularly electrodes for lithium ion secondary batteries.

Claims

1. A carbon nanotube dispersion liquid, characterized in that, it at least contains carbon nanotubes, oxidized cellulose nanofibers and water, more than 85% of the oxidized cellulose nanofibers have a fiber length of 50 to 250 nm.

2. The carbon nanotube dispersion liquid according to claim 1, wherein, the number average fiber length L of the oxidized cellulose nanofibers is 100 nm to 150 nm, and the ratio (Ll / L) of the length weighted average fiber length Ll to the number average fiber length L is 1.0 to 1.

4.

3. The carbon nanotube dispersion liquid according to claim 1, wherein, the crystallinity of the oxidized cellulose nanofibers measured by X-ray diffraction method is less than 70%.

4. The carbon nanotube dispersion liquid according to claim 1, wherein, The peak height ratio (C=O / C-O) of the peak derived from C=O (around 1610 cm -1 -1) to the peak derived from C-O (around 1062 cm -1 -1) in the infrared absorption spectrum of the oxidized cellulose nanofibers is 0.70 or less, or the peak height ratio (C=O / O-H) of the peak derived from C=O (around 1610 cm -1 -1) to the peak derived from O-H (around 3340 cm -1 -1) is 1.35 or less.

5. A coating liquid for forming an electrode layer, which is made by using the carbon nanotube dispersion liquid according to any one of claims 1 to 4.

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