Coating liquid for electrode layer formation

Abstract

To provide a slurry for electrodes suitable for manufacturing electrodes such as lithium-ion batteries.SOLUTION: A coating liquid for electrode layer formation is a carbon nanotube dispersion coating liquid containing at least carbon nanotube, oxidized cellulose nanofiber, active material, binder, and water. The oxidized cellulose nanofiber is characterized in that, in the infrared spectrum, the peak height ratio (C=O / C-O) between the height of the peak derived from C-O (near 1062 cm-1) and the height of the peak derived from C=O (near 1610 cm-1) is 0.70 or less, or the peak height ratio (C=O / O-H) between the height of the peak derived from O-H (near 3340 cm-1) and the height of the peak derived from C=O (near 1610 cm-1) is 1.35 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a coating liquid for forming electrode layers, which is a raw material for manufacturing electrodes of lithium-ion batteries and the like. [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 leads to increased 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] Report on the "Identification of Challenges for the Practical Application of Lithium-ion Batteries for Idling Stop Vehicles Using Cellulose Nanofibers" from the FY2017 Performance Evaluation Project for Cellulose Nanofiber-Utilizing Products (FY2017 Ministry of the Environment Commissioned Project), March 16, 2018, Daiichi Kogyo Seiyaku Co., Ltd. [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The present invention aims to solve the above-mentioned conventional problems, and in particular, to provide a means to solve the problem that when carbon nanotubes are dispersed in a dispersion medium such as water, carbon nanotubes aggregate and the dispersion is insufficient, or that the dispersion process takes a long time to complete the dispersion. [Means for solving the problem]

[0012] As a result of diligent research into the above-mentioned problems, the inventors of this invention have found that a carbon nanotube dispersion meeting the above objective can be obtained by a carbon nanotube dispersion containing at least carbon nanotubes, oxidized cellulose nanofibers with specific properties, and water, and have thus completed the present invention.

[0013] In other words, the coating solution of the present invention is a carbon nanotube dispersion coating solution comprising at least carbon nanotubes, oxidized cellulose nanofibers, an active material, a binder, and water, Oxidized cellulose nanofibers produce a CO-derived peak (1062 cm⁻¹) in the 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 (Near) Height and peak originating from C=O (1610cm) -1 This is a coating liquid for forming an electrode layer, characterized in that the peak height ratio (C=O / OH) with respect to the surrounding height is 1.35 or less.

[0014] It is known that carboxylated oxidized cellulose nanofibers can be obtained by TEMPO-oxidizing cellulose nanofibers. Usually, this is the case where the TEMPO oxidation reaction is carried out completely as much as possible. The TEMPO oxidation reaction is not adjusted to control the oxidized cellulose nanofibers to have specific physical properties. On the other hand, in the present invention, it is important not to completely carry out the TEMPO oxidation reaction of cellulose nanofibers so that the oxidized cellulose nanofibers have specific physical properties.

Advantages of the Invention

[0015] According to the present invention, there is provided a coating liquid for forming an electrode layer, which can obtain a homogeneous film having a predetermined conductivity with good coating properties.

[0016] The object and advantages of the present invention are recognized and obtained by using the components and combinations particularly pointed out in the claims. Both the above general description and the following detailed description are exemplary and explanatory, and do not limit the present invention described in the claims.

Modes for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be described in detail. However, it should be noted that the technical scope of the present invention is not limited to the embodiments described in detail below, and extends to the invention described in the claims and its equivalents. Further, the present invention can be implemented based on the content disclosed in this specification and common technical knowledge in the art (including design matters and self-evident matters).

[0018] 〈Carbon Nanotube Dispersion〉 The carbon nanotube dispersion of the present invention is characterized by containing at least carbon nanotubes, specific oxidized cellulose nanofibers, and water. <于 <于

[0019] <于 〈Carbon Nanotube (CNT)〉 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 surface of graphite into a tube. Single-walled CNTs, in which a single surface of graphite is wound in one layer, and multi-walled CNTs, in which two or three or more layers of graphite are wound, 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 10 nm) from Nanocyl, Baytubes C150P (average outer diameter 11 nm) from Bayer, FloTube 9000 (average outer diameter 19 nm), FloTube 7320 (average outer diameter 9 nm), FloTube 7010 (average outer diameter 9 nm), FloTube 6810 (average outer diameter 8 nm), FloTube 6120 (average outer diameter 8 nm), FloTube 6100 (average outer diameter 8 nm), FloTube 2020 (average outer diameter 4 nm) 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] 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.

[0025] 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. Furthermore, 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 aggregation and warping during drying. The alcoholic hydroxyl groups can be detected by infrared spectroscopy.

[0026] The CO-derived peak in the infrared spectral spectrum (1062 cm⁻¹) -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 The peak height ratio (C=O / OH) between the height of the peak (near 1610 cm⁻¹) originating from C=O 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.

[0027] 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 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.

[0028] 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.

[0029] Preferably, the fiber length of the oxidized cellulose nanofibers blended into the coating solution is 50 nm to 250 nm, accounting for 85% or more of the total, and more preferably 100 nm to 150 nm, accounting for 30% or more of the total. 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.

[0030] 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.

[0031] 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.

[0032] 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 coating solution.

[0033] To measure the amount of carboxyl groups in the above-mentioned oxidized cellulose nanofibers, for example, prepare 60 mL of a 0.5-1% 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 (1). [Formula 1] Carboxyl group content (mmol / g) = V (mL) × [0.05 / cellulose mass] ... (1) 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.

[0034] 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.

[0035] (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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] (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.

[0045] (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.

[0046] 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.

[0047] (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.

[0048] 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.

[0049] Furthermore, oxidized cellulose nanofibers in which the peak height ratio (C=O / CO) or (C=O / OH) in the infrared spectroscopic spectrum is within a specific range, or the fiber length is within a specific range, can be obtained by selecting raw materials, controlling the amount of co-oxidant added and the reaction time during the oxidation of cellulose nanofibers in the manufacturing method, and controlling the defibrillation time.

[0050] Oxidized cellulose nanofibers that satisfy the properties of the present invention can be manufactured by employing the above-mentioned means and methods during production, and commercially available products having each of these properties can also be used.

[0051] In the present invention, the content of oxidized cellulose nanofibers in the coating solution 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 coating solution 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.

[0052] 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 coating solution. On the other hand, by setting it to 2% by mass or less, stability of the dispersion and good conductivity can be ensured.

[0053] The dispersion medium (the remainder of the dispersion) in the coating solution of the present invention 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.

[0054] 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 coating solution, 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.

[0055] The coating liquid 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.

[0056] The coating liquid of the present invention can be produced, for example, by adding at least carbon nanotubes, oxidized cellulose nanofibers having the above properties, water, as well as graphite on a flat plate, a water-soluble solvent, a preservative, etc., stirring and mixing, and then dispersing. 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.

[0057] In addition, from the viewpoint of obtaining the fluidity of the electrode slurry prepared by adding this coating liquid, the viscosity (mPa·s) at 25°C when using an E-type rotational viscometer [manufactured by Toki Sangyo Co., Ltd., TV-25] with a rotor (1°34’×R24mm) rotational speed of 10 rpm is preferably 5 to 700, and more preferably 5 to 200.

[0058] The coating liquid of the present invention can be used for a coating liquid for forming an electrode layer (hereinafter, also referred to as an electrode slurry) suitable for manufacturing an electrode (positive electrode or negative electrode) of a lithium-ion secondary battery. When coating for electrode formation, carbon nanotubes serving as a conductive material and an active material are uniformly dispersed, and it has excellent dispersion stability. Moreover, it does not adversely affect the resistance value of the electrode itself, etc., and becomes 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 can achieve both high stability and conductive performance while highly maintaining cycle characteristics and self-discharge characteristics, and does not adversely affect the insertion and extraction of ions such as Li + and the reduction of the resistance value of the electrode.

[0059] 〈Positive electrode slurry〉 The positive electrode slurry is the coating liquid of the present invention containing a positive electrode active material. The positive electrode active material that can be used is not particularly limited as long as it is a normal positive electrode active material (an active material that reversibly allows lithium ions to enter and exit) that can be used for the positive electrode of a lithium-ion battery, and can be used without particular limitation.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] <Slurry for negative electrode> The negative electrode slurry is a coating liquid of the present invention containing a negative electrode active material. 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.

[0065] As for spinel-type lithium titanate, Li4+xTi5O 12 Examples 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).

[0066] 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.

[0067] 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 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 the negative electrode slurry. The negative electrode slurry of the present invention contains a negative electrode active material in a carbon nanotube dispersion having the above configuration, 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.

[0068] By using the above-mentioned positive electrode slurry and negative electrode slurry, electrodes for secondary batteries can be obtained. Preferably, the above-mentioned positive electrode slurry and negative electrode slurry further contain a binder. Examples of binders that can be used include polyimide resins, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, tetrafluoroethylene-perfluorovinyl ether copolymers, and other fluororesins; polyethylene, polypropylene and other polyolefin resins; polyvinylpyrrolidone, polyvinyl alcohol, styrene-butadiene rubber (SBR), and acrylic resins. Two or more of these binders may be used in combination. From the viewpoint of adhesion to the current collector foil, and the battery capacity and charge / discharge characteristics after cell formation, it is preferable that the amount of these binders be 0.2 to 3.0% by mass, more preferably 0.5 to 2.5% by mass, relative to the total amount of slurry for each electrode of the secondary battery.

[0069] In electrode slurries, the dissolution of binders, particularly binder components, can be hindered, leading to precipitation. However, the electrode slurry of the present invention does not hinder the dissolution of binders.

[0070] Furthermore, various solvents may be added to each of the electrode slurries described above. Examples of solvents include water (purified water, deionized water, distilled water, ultrapure water, etc.), aromatic solvents, alcohols, polyhydric alcohols, ether solvents, glycol ether solvents, ester solvents, amine solvents, amide solvents, heterocyclic solvents, sulfoxide solvents, and sulfone solvents. These solvents may be used individually or in mixtures of two or more. The amount of these solvents is preferably 0.5 to 80% by mass, more preferably 1 to 70% by mass, relative to the total amount of the slurry for each electrode of the secondary battery, in order to achieve the appropriate viscosity when coating each electrode slurry. Furthermore, in addition to the graphite dispersion for battery electrodes, the active materials, and the binder, the slurry for each electrode can be appropriately blended with a leveling agent, a solid electrolyte, and the like, to the extent that it does not impair the effects of the present invention.

[0071] The electrodes for each secondary battery configured in this way can be prepared by mixing the carbon nanotube dispersion, the active materials for the positive or negative electrode of the secondary battery, a binder, a solvent, etc., using, for example, a twin-screw kneader. By applying the obtained slurries for each electrode of the secondary battery onto a current collector, which is a conductive component of the lithium-ion secondary battery, and drying them, predetermined positive and negative electrodes for the lithium-ion secondary battery can be obtained. Thus, in this invention, slurries for each electrode of a secondary battery and electrodes for a secondary battery that can withstand repeated charging and discharging over a long period of time 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 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

[0074] (CO-derived peak in infrared spectroscopy 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

[0075] Examples, Comparative Examples [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 CeNF-1 or CeNF-2 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.

[0076] [Preparation of coating solution for electrode layer formation] The carbon nanotube dispersion obtained above was mixed with an active material, a conductive additive, a dispersant, and a preservative, and dispersed using a dispersion apparatus (a bead mill using φ1.0 mm zirconia beads; peripheral speed set to 10 m / s). The resulting dispersion was further transferred to a planetary mixer, a rubber component was added as a binder, and the composition was kneaded for 120 minutes at a rotational speed of 10 rpm to obtain a coating liquid for electrode layer formation. Lithium titanium oxide was used as the active material. Graphite was used as the conductive additive. Carboxymethylcellulose was used as the dispersion aid. An organic nitrogen sulfur compound was used as the preservative. Styrene butadiene rubber (SBR) was used as the binder. The compound composition is shown in Table 1.

[0077] (Evaluation of coating liquid for electrode layer formation) The obtained electrode layer forming coating solution was applied to one side of a PET film (Lumirror #100-T60, Toray Industries) using an applicator with a 50 μm gap, and then dried at 80°C to obtain an electrode layer film. The basis weight stability (thickness unevenness) and warpage of the obtained film were evaluated visually. For thickness unevenness or warpage of the film, ○ was used if no unevenness was observed, △ if slight unevenness was observed, and × if significant unevenness was observed. The resistance value was measured as sheet resistance using a device consisting of a 4-probe probe with a probe spacing of 10 mm and a measuring instrument (HIOKI E.E. CORPORATION, Milliohm HiTester 3227). If the sheet resistance is 1.0 kΩ / □ or less, it can be confirmed that the conductivity is excellent. In addition, the obtained electrode layer forming coating solution was filtered through a mesh with a mesh opening of 50 μm. ○ was used if no insoluble matter was observed in the mesh, and × was used if insoluble matter was observed. The main component of the insoluble matter was SBR rubber, which was blended as a binder. These evaluation results are shown in Table 1.

[0078] [Table 1]

[0079] As is clear from the evaluation results in Table 1 above, the film obtained using the electrode layer forming coating liquid within the scope of the present invention was excellent, achieving a high degree of compatibility between coating properties and conductivity without impairing the predetermined properties. In contrast, the film obtained using the coating liquid of Comparative Example 1 lacked uniformity. [Industrial applicability]

[0080] The coating solution of the present invention can be used in the manufacture of electrodes (positive or negative electrodes) for lithium-ion secondary batteries.

Claims

1. A carbon nanotube dispersion coating solution comprising at least carbon nanotubes, oxidized cellulose nanofibers, an active material, a binder, 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 the 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 A coating liquid for forming an electrode layer, characterized in that the peak height ratio (C = O / O - H) to the surrounding height is 1.35 or less.

2. The coating liquid according to claim 1, wherein 85% or more of the oxidized cellulose nanofibers have a fiber length of 50 to 250 nm.

3. The coating liquid according to claim 1 or 2, wherein the binder is synthetic rubber.

4. An electrode composition comprising the coating liquid described in claim 1.

Images