Evaluation method for carbon nanotube powder and production method and production system for carbon nanotube dispersion liquid using the evaluation method

Abstract

To provide a method for evaluating carbon nanotube powder which can select carbon nanotube powder suitable for dispersion treatment.SOLUTION: An evaluation method includes the steps of: acquiring a dry particle size distribution (D10) of carbon nanotube powder; acquiring information on whether or not at least one of the following conditions (1) and (2) is satisfied; and evaluating dispersibility of the carbon nanotube powder using the dry particle size distribution (D10) and the information: the condition (1): the carbon nanotube powder has a first peak in a diffraction angle 2θ=8° to 12° and a second peak in a diffraction angle 2θ=24° to 27° in powder X-ray diffraction; and the condition (2): the carbon nanotube powder has cohesive force as determined by a powder rheometer of 1 kPa or more and 10 kPa or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] TECHNICAL FIELD Embodiments of the present disclosure relate to a method for evaluating carbon nanotube powder, and a method and system for producing a carbon nanotube dispersion liquid using this evaluation method. [Background technology]

[0002] Carbon nanotubes have a cylindrical structure of graphite layers and are chemically stable, electrically conductive, and mechanically strong, and therefore are used in a variety of applications, such as electronic materials, structural materials, and paints. Specific applications of carbon nanotubes include electronic materials, semiconductor materials, filler materials, short needle probes for microscopes, adsorption materials, and filter materials. In particular, electrode materials using carbon nanotubes have excellent electrical conductivity, and are therefore expected to be applied to secondary batteries, fuel cells, electric double layer capacitors, and the like.

[0003] With the widespread use of electric vehicles and the trend toward smaller, lighter, and more powerful mobile phones, secondary batteries with high energy density and even higher capacity are required. Against this background, non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, which use non-aqueous electrolytes, are increasingly being used in many devices due to their high energy density and high voltage characteristics.

[0004] Electrodes for secondary batteries are fabricated by coating a current collector with a composite slurry containing a positive electrode active material or a negative electrode active material, a conductive material, a binder resin, and the like. While carbon black, graphene, fine carbon materials, and the like have been used as conductive agents, the use of carbon nanotubes, a type of fine carbon fiber, has been investigated to further improve conductivity and thereby increase battery capacity. For example, adding carbon nanotubes to a positive electrode can improve the conductivity of the electrode film and reduce electrode resistance. Multiwalled carbon nanotubes with outer diameters of several nanometers to several tens of nanometers are relatively inexpensive and are increasingly being put to practical use. Using carbon nanotubes with a small average outer diameter and long fiber length allows for efficient formation of a conductive network even in small amounts, thereby enabling the development of high-capacity secondary batteries. However, carbon nanotubes with these characteristics exhibit strong cohesion, making it difficult to further enhance the dispersibility of carbon nanotube dispersions.

[0005] Patent Document 1 proposes providing a high-concentration, low-viscosity carbon nanotube dispersion containing 2% by mass or more of carbon nanotubes by specifying a wettability index, which is the mass of absorbed solvent relative to the mass of carbon nanotubes. In Patent Document 1, the wettability index is controlled by dry-pulverizing carbon nanotubes using a milling device containing metal or ceramic balls. In a specific example, batch-type dry pulverization is performed using an attrition mill for 10 to 140 minutes. Patent Document 2 also proposes providing a carbon nanotube paint that imparts jet black and gloss to resin molded products, automobile bodies, and the like, using carbon nanotubes with specified average outer diameters and surface oxygen concentrations. In Patent Document 2, the surface oxygen concentration of carbon nanotubes is controlled by pulverizing untreated carbon nanotubes using a grinding medium without the use of a liquid medium. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent No. 10-2125933 [Patent Document 2] Japanese Patent Application Publication No. 2020-029372 Summary of the Invention [Problem to be solved by the invention]

[0007] When carbon nanotubes are dispersed in a dispersion medium, the high aspect ratio of carbon nanotubes causes strong physical bonds between them due to van der Waals forces, making it difficult for the carbon nanotubes to entangle or disentangle, resulting in difficulty in spreading in the dispersion medium. Furthermore, dispersing carbon nanotubes in a dispersion medium increases the viscosity of the dispersion, making it difficult to obtain a dispersion containing uniformly dispersed, high-concentration carbon nanotubes. Obtaining a high-concentration dispersion that combines fluidity and storage stability is particularly challenging. To overcome the inherent dispersion difficulties of carbon nanotubes due to their high aspect ratio, techniques such as those described in Patent Documents 1 and 2 have been proposed that dry-process carbon nanotubes using grinding media to improve the wettability of carbon nanotubes to solvents, thereby obtaining a low-viscosity carbon nanotube dispersion containing 2% or more carbon nanotubes at high concentrations. However, there remains room for further research into the formation of a fully developed conductive network in electrode films using carbon nanotubes, which is due to the inherent high aspect ratio of carbon nanotubes.

[0008] Carbon nanotube powder is optionally subjected to dry processing and then dispersed in a dispersion medium to produce a carbon nanotube dispersion. In the dispersion process, improving the initial dispersibility is crucial, as high viscosity can easily lead to a decline in production efficiency. Therefore, there is a need to establish a method for evaluating carbon nanotube powders to be subjected to dispersion processing and to select carbon nanotube powders suitable for dispersion processing using this evaluation method.

[0009] One of the objects of the present disclosure is to provide a method for evaluating carbon nanotube powder that can select carbon nanotube powder suitable for dispersion processing, a method for producing a carbon nanotube dispersion liquid using this evaluation method, a method for producing a carbon nanotube resin composition, a method for producing a composite slurry, a method for producing an electrode film, and a method for producing a secondary battery, as well as a system for producing a carbon nanotube dispersion liquid using this evaluation method. [Means for solving the problem]

[0010] To improve the initial dispersibility of a carbon nanotube dispersion and achieve dispersion stability, it is important to eliminate the high aspect ratio of the carbon nanotubes, improve the wettability of the carbon nanotubes, and reduce the viscosity of the dispersion. On the other hand, to fully form a well-developed conductive network in an electrode film using carbon nanotubes, it is important to disperse the carbon nanotubes while maintaining a certain level of their inherent high aspect ratio and to stabilize the dispersion state. There is a trade-off between the fluidity and storage stability of the dispersion and its conductivity. The inventors conducted extensive research aimed at solving the above-mentioned problems. They focused on the influence of the properties of the carbon nanotube powder used in the dispersion process on the initial dispersibility. They found that by satisfying certain conditions for the carbon nanotube powder properties, the initial dispersibility of the carbon nanotube powder can be improved while maintaining a certain level of high aspect ratio, thereby increasing the efficiency of the subsequent dispersion process and shortening the dispersion time. By establishing these conditions, a method for evaluating raw materials for carbon nanotube powder can be provided.

[0011] By using the evaluation method of the present disclosure, it is possible to improve the initial dispersibility in the manufacturing method of a carbon nanotube dispersion and further shorten the dispersion time of the dispersion treatment. By evaluating the carbon nanotube powder evaluated by the evaluation method of the present disclosure, it is possible to manufacture a carbon nanotube dispersion, a resin composition, and a composite slurry with good dispersion stability. Furthermore, by using these compositions with good carbon nanotube dispersibility, it is possible to manufacture an electrode film with a developed conductive network because a high aspect ratio can be maintained at a certain level or more, and it is also possible to manufacture a secondary battery with high output, high capacity, and long life.

[0012] That is, the present disclosure relates to the following embodiments, however, the embodiments of the present disclosure are not limited to the following.

[0013] [1] A step of obtaining a dry particle size distribution (D10) of the carbon nanotube powder; A step of acquiring information on whether or not at least one of the following conditions (1) and (2) is satisfied: An evaluation method comprising a step of evaluating the dispersibility of carbon nanotube powder using the dry particle size distribution (D10) and the information. Condition (1): In the powder X-ray diffraction of the carbon nanotube powder, a first peak is present at a diffraction angle 2θ=8° to 12° and a second peak is present at a diffraction angle 2θ=24° to 27°. Condition (2) The cohesive strength of the carbon nanotube powder measured by a powder rheometer is 1 kPa or more and 10 kPa or less.

[0014] [2] The evaluation method according to [1], wherein the evaluation step evaluates the dispersibility of the carbon nanotube powder as being good when the dry particle size distribution (D10) is 110 μm or less and at least one of the conditions (1) and (2) is satisfied.

[0015] [3] The evaluation method according to [1] or [2], wherein the evaluation step evaluates the dispersibility of the carbon nanotube powder as good if the dry particle size distribution (D10) is 40 μm or more and 110 μm or less and satisfies at least one of the conditions (1) and (2).

[0016] [4] The evaluation method according to any one of [1] to [3], wherein, in the condition (2), the cohesive strength of the carbon nanotube powder measured by a powder rheometer is 5 kPa or more and 10 kPa or less.

[0017] [5] A method for producing a carbon nanotube dispersion, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of [1] to [4] above, obtaining an evaluated carbon nanotube powder; and obtaining a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium.

[0018] [6] A method for producing a carbon nanotube resin composition, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of [1] to [4] above, providing an evaluated carbon nanotube powder, and obtaining a resin composition containing the evaluated carbon nanotube powder, a dispersion medium, and a binder resin.

[0019] [7] A method for producing a composite slurry, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of [1] to [4] above, providing an evaluated carbon nanotube powder, and obtaining a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material.

[0020] [8] A method for producing an electrode film, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of [1] to [4] above, providing the evaluated carbon nanotube powder; and obtaining an electrode film using a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material.

[0021] [9] A method for producing a secondary battery, comprising obtaining an electrode film for at least one of a positive electrode and a negative electrode according to the method for producing an electrode film according to [8].

[0022]

[10] A system for producing a carbon nanotube dispersion, An evaluation device for evaluating a carbon nanotube powder according to the evaluation method according to any one of [1] to [4], and obtaining an evaluated carbon nanotube powder; and A carbon nanotube dispersion production system including a production apparatus for producing a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium. [Effects of the Invention]

[0023] According to embodiments of the present disclosure, it is possible to provide a method for evaluating carbon nanotube powder that can select carbon nanotube powder suitable for dispersion processing, a method for producing a carbon nanotube dispersion liquid using this evaluation method, a method for producing a carbon nanotube resin composition, a method for producing a composite slurry, a method for producing an electrode film, and a method for producing a secondary battery, as well as a system for producing a carbon nanotube dispersion liquid using this evaluation method. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of a production system for a carbon nanotube dispersion liquid according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, several embodiments of the present disclosure will be described in detail. However, the present invention is not limited to the following embodiments, and the present invention also includes embodiments that are implemented within the scope of the present disclosure.

[0026] The carbon nanotube dispersion and carbon nanotube resin composition refer to the state before the active material is added. In this respect, the carbon nanotube dispersion and carbon nanotube resin composition are distinguished from composite slurries containing active material. That is, the carbon nanotube dispersion and carbon nanotube resin composition are substantially free of active material. This concept excludes states in which an active material is intentionally added to the carbon nanotube dispersion and carbon nanotube resin composition. The active material may be present in an amount of 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, or even 0% by mass, relative to the total mass of the carbon nanotube dispersion and carbon nanotube resin composition.

[0027] In this specification, carbon nanotubes may be referred to as "CNTs" and N-methyl-2-pyrrolidone may be referred to as "NMP." In addition, in this specification, a carbon nanotube dispersion may be referred to as a "CNT dispersion" or simply as a "dispersion."

[0028] <Evaluation method for carbon nanotube powder> According to one of some embodiments, an evaluation method can be provided, comprising the steps of acquiring the dry particle size distribution (D10) of the carbon nanotube powder, acquiring information on whether or not at least one of the following conditions (1) and (2) is satisfied, and evaluating the dispersibility of the carbon nanotube powder using the acquired dry particle size distribution (D10) and information. Condition (1): In the powder X-ray diffraction of the carbon nanotube powder, a first peak is present at a diffraction angle 2θ=8° to 12° and a second peak is present at a diffraction angle 2θ=24° to 27°. Condition (2) The cohesive strength of the carbon nanotube powder measured by a powder rheometer is 1 kPa or more and 10 kPa or less.

[0029] This evaluation method allows for the selection of CNT powder raw materials that are suitable for dispersibility. In particular, it allows for the selection of CNT powder that can shorten the dispersion time during dispersion processing. By using CNT powders evaluated and selected according to this evaluation method, a CNT dispersion with good dispersibility can be obtained.

[0030] The process for obtaining the dry particle size distribution (D10) of the CNT powder is described below. This process is based on the fact that the initial dispersibility of the CNT powder can be estimated from the dry particle size distribution (D10) of the CNT powder. The dry particle size distribution (D10) of the CNT powder is used as an evaluation index, based on the fact that the initial dispersibility of the CNT powder can be improved by controlling the inclusion of small particle diameter CNT particles. If the content of small particle diameter CNTs is too low, a large amount of dispersion medium is incorporated into the CNTs when mixing them with the dispersion medium, which is undesirable because it promotes high viscosity and low fluidity. If a large amount of small particle diameter CNTs is included, the viscosity of the CNT dispersion can be reduced at the initial stage of dispersion, improving the initial dispersibility. Improving the initial dispersibility increases the efficiency of the subsequent dispersion process and shortens the dispersion process time.

[0031] For example, a standard for evaluating good initial dispersibility of CNT powder is that the dry particle size distribution (D10) is 110 μm or less. To evaluate better initial dispersibility, this standard may be that the dry particle size distribution (D10) is 100 μm or less, 95 μm or less, or 90 μm or less. While the lower limit is not particularly limited, the dry particle size distribution (D10) may be, for example, 1 to 110 μm or 10 to 100 μm.

[0032] As another example, a standard for evaluating whether the initial dispersibility of a CNT powder is good can be a dry particle size distribution (D10) of 40 μm or more and 110 μm or less. By controlling the amount of fine powder so that it contains small CNT particles but does not contain excessive amounts of fine powder, the initial dispersibility can be evaluated more precisely. The dry particle size distribution (D10) of the CNT powder is preferably 40 μm or more, from the viewpoint of suppressing a decrease in conductivity. To evaluate a better initial dispersibility, this standard may be a dry particle size distribution (D10) of 50 to 100 μm, 55 to 95 μm, or 60 to 90 μm.

[0033] The dry particle size distribution (D10) of CNT powder was measured as follows. A dry dispersion unit for dry powder dispersion (Malvern Panalytical's Aero S standard type venturi) was connected to a dry particle size distribution measuring device (Malvern Panalytical's Mastersizer 3000). 0.1 to 1 g of a sample for dry particle size distribution analysis of the carbon material was placed in the dry dispersion unit, and measurements were taken from 0.1 μm to 5000 μm. A general-purpose analysis model was used in dry fine powder mode, and volume distribution analysis was performed. The dry particle size distribution (D10) of CNT powder can be measured by following the detailed measurements in the Examples.

[0034] The process of acquiring information on whether or not at least one of the conditions (1) and (2) is satisfied will be described below.

[0035] <Condition (1)> Condition (1) is a condition in which the powder X-ray diffraction of the CNT powder shows a first peak at a diffraction angle 2θ = 8° to 12° and a second peak at a diffraction angle 2θ = 24° to 27°. Under condition (1), the second peak is derived from the CNT. The first peak is derived from graphene oxide. By incorporating (introducing) graphene oxide into the CNT powder along with CNTs, high dispersibility can be achieved while maintaining a high aspect ratio. Although the details are unclear, it is thought that the CNT dispersibility is improved for the following reasons: CNTs can be dispersed in a dispersion medium by shear force or impact force, but graphene oxide is tougher than CNTs, and since graphene oxide, a planar or point-like carbon material, acts as a type of grinding medium, the inclusion of graphene oxide can promote CNT dispersion.

[0036] The measured powder X-ray diffraction chart is observed to determine whether or not there is a first peak at 2θ=8° to 12°. At least one peak should be present at 2θ=8° to 12°, but two or more peaks may be present. The measured powder X-ray diffraction chart is also observed to determine whether or not there is a second peak at 2θ=24° to 27°. At least one peak should be present at 2θ=24° to 27°, but two or more peaks may be present.

[0037] Regarding condition (1), the method for measuring powder X-ray diffraction of CNT powder is as follows: CNT powder is placed on a glass sample plate and flattened using a slide glass to prepare a sample for powder X-ray diffraction analysis. The sample for powder X-ray diffraction analysis is then placed in a fully automated multipurpose X-ray diffractometer (SmartLab, manufactured by Rigaku), and the angle is adjusted from 10° to 35° to confirm the presence or absence of peaks. The presence or absence of peaks can be confirmed by measuring in accordance with the detailed examples.

[0038] <Condition (2)> Condition (2) is that the cohesive strength of the CNT powder measured by a powder rheometer is 1 kPa or more and 10 kPa or less. Condition (2) utilizes the fact that the weak cohesion of CNT powder, i.e., the ease with which aggregates break down, directly affects initial dispersibility, allowing for control of the initial dispersibility of the CNT powder. Maintaining a certain level of cohesion measured by a powder rheometer can further improve the wettability of the CNT powder and dispersion medium at the initial stage of dispersion. When the cohesion of the CNT powder is within the above range, particle size reduction during the dispersion process is promoted, shortening the process time required to release the entrapped dispersion medium, promoting low viscosity, and improving initial dispersibility. Improving initial dispersibility increases the efficiency of the subsequent dispersion process and shortens the dispersion process time. This improves the production efficiency of CNT dispersions.

[0039] Regarding condition (2), the cohesive strength of the CNT powder measured using a powder rheometer may be 1 kPa to 6 kPa, 2 kPa to 5 kPa, or 3 kPa to 5 kPa in order to obtain better initial wettability and aggregate disintegration. These ranges are suitable for a method in which the CNT powder is mixed with a dispersion medium and dispersed without being dry-pulverized.

[0040] In another example, regarding condition (2), the cohesive strength of the CNT powder measured by a powder rheometer may be 5 kPa to 9.5 kPa, 6 kPa to 9.5 kPa, or 6.5 kPa to 9 kPa to obtain better initial wettability and aggregate disintegration. These ranges are suitable for a method in which the CNT powder is dry-pulverized and mixed with a dispersion medium for dispersion treatment.

[0041] Regarding condition (2), the cohesion force of the CNT powder was measured using a powder rheometer as follows. The measurement was performed using an MCR302e (Anton Paar). 2.5 g of CNT powder was placed in a dedicated aluminum container (C-CC27 / D / Al) and compressed at a pressure of 12 kPa using a compression cylinder jig. The jig was then replaced with a wing-shaped jig for measuring cohesion force, and the jig was moved at 125 μm / s and 0.1 revolutions / min to penetrate the measurement object, measuring the torque when shearing was applied, and the maximum peak was taken as the cohesion force. The temperature control device used was a C-PTD200, set to 25°C.

[0042] The process of evaluating the dispersibility of CNT powder using the acquired dry particle size distribution (D10) and information is described below.

[0043] Based on information on whether at least one of conditions (1) and (2) is satisfied, when these conditions are satisfied, the dispersibility of the CNT powder can be evaluated as good in combination with the dry particle size distribution (D10). For example, the combination of the dry particle size distribution (D10) and condition (1) defines the amount of fine powder and graphene oxide in the CNT powder, allowing the dispersibility of the CNT powder to be evaluated. The combination of the dry particle size distribution (D10) and condition (2) defines the amount of fine powder and cohesion in the CNT powder, allowing the dispersibility of the CNT powder to be evaluated.

[0044] The evaluation method may acquire information on whether or not either one of the above conditions (1) and (2) is satisfied, along with the dry particle size distribution (D10) of the CNT powder, or may acquire information on whether or not both conditions (1) and (2) are satisfied.

[0045] Conditions (1) and (2) are mutually different indicators, but due to the above-described effects, they can be combined with the dry particle size distribution (D10) to evaluate the tendency of dispersibility of CNT powder. Therefore, it is advisable to select one or both of conditions (1) and (2) in consideration of the dispersing device, dispersion conditions, dispersion medium, etc., and evaluate the CNT powder.

[0046] The method for evaluating the dispersibility of CNT powder can be used to evaluate raw CNT powder in a manufacturing method for a CNT dispersion liquid containing CNT powder and a dispersion medium. This evaluation method can be used in both a method in which the raw CNT powder is directly mixed with a dispersion medium and dispersed, and a method in which the raw CNT powder is dry-pulverized and then mixed with a dispersion medium and dispersed. In the following explanation, the dispersion process using CNT powder and a dispersion medium is also referred to as wet dispersion.

[0047] CNTs are difficult to disperse due to interactions caused by van der Waals forces, so wettability can be controlled by physically crushing them using dry milling. However, dry milling reduces the aspect ratio of CNTs, hindering the formation of a conductive network and reducing conductive performance, which is considered to be an issue. The evaluation method disclosed herein can be used to evaluate the initial dispersibility of raw CNT powder when it is dispersed through dry milling.

[0048] Initial dispersion of CNTs in a dispersion medium is difficult because, immediately after adding and mixing the CNTs and dispersion medium, the dispersion medium is absorbed into the CNTs, resulting in a shortage of dispersion medium in the system. This leads to a high viscosity and low fluidity of the dispersion system, which is thought to hinder initial dispersion due to factors such as reduced mixing efficiency, system inhomogeneity, and reduced dispersion efficiency of the disperser. Therefore, suppressing the increase in viscosity of the dispersion liquid and maintaining high fluidity leads to improved production efficiency in the initial dispersion stage. According to the evaluation method disclosed herein, the rate at which the absorbed dispersion medium is released or the amount of dispersion medium initially absorbed into the CNTs can be estimated by combining the degree of CNT particle size reduction with at least one of conditions (1) and (2), thereby evaluating initial dispersibility. For example, when the dry particle size distribution (D10) of the CNT powder is 110 μm or less, the rate at which the absorbed dispersion medium is released due to the reduced CNT particle size increases, or the amount of dispersion medium initially absorbed decreases, maintaining low viscosity and improving initial dispersibility. By improving the initial dispersibility, the efficiency of the subsequent dispersion treatment is increased, and the dispersion treatment time can be shortened.

[0049] We believe that adjusting the aspect ratio by dry milling has a similar effect in that it reduces the amount of dispersion medium that is incorporated into the CNTs when the CNT powder is mixed with the dispersion medium.

[0050] <Method of manufacturing carbon nanotube dispersion> According to one of several embodiments, there can be provided a method for producing a carbon nanotube dispersion, the method comprising: evaluating a carbon nanotube powder according to the evaluation method described above to obtain an evaluated carbon nanotube powder; and obtaining a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium. Here, the details of the evaluation method are as described above.

[0051] According to this manufacturing method, the dispersibility is judged to be good or bad based on the evaluation results, and the conditions for the dispersion treatment are appropriately set, thereby making it possible to obtain a CNT dispersion liquid with good dispersion stability. For example, by performing the dispersion treatment using a CNT powder with good dispersibility based on the evaluation results, it is possible to obtain a CNT dispersion liquid with good dispersion stability while shortening the dispersion treatment time.

[0052] One example is a method for producing a carbon nanotube dispersion liquid, which includes evaluating a carbon nanotube powder according to the evaluation method described above, obtaining an evaluated carbon nanotube powder, and dispersing the evaluated carbon nanotube powder and a dispersion medium. Here, the details of the evaluation method are as described above.

[0053] According to one example of the method for producing this CNT dispersion, the CNT powder is selected using the evaluation method described above, and the CNT powder to be subjected to the dispersion treatment is suitable for initial dispersibility, thereby making it possible to provide a CNT dispersion with good dispersibility.

[0054] Another example can provide a method for producing a carbon nanotube dispersion, which includes evaluating a carbon nanotube powder according to the evaluation method described above, obtaining an evaluated carbon nanotube powder, dry-pulverizing the evaluated carbon nanotube powder, and dispersing the dry-pulverized evaluated carbon nanotube powder and a dispersion medium. Here, details of the evaluation method are as described above.

[0055] According to another example of the method for producing this CNT dispersion, CNT powder is selected using the evaluation method described above, and the CNT powder that is subjected to dry grinding and then wet dispersion is suitable for initial dispersibility, making it possible to provide a CNT dispersion with good dispersibility.

[0056] The CNT dispersion contains CNTs and a dispersion medium, and may optionally further contain other components such as a dispersant.

[0057] The dispersion medium may include a non-aqueous dispersion medium. The non-aqueous dispersion medium is not particularly limited, but is preferably a high-dielectric constant solvent, and preferably includes a solvent consisting of any one of high-dielectric constant solvents or a mixed solvent consisting of two or more of high-dielectric constant solvents. In addition, the high-dielectric constant solvent may be mixed with one or more other solvents.

[0058] Examples of high-dielectric-constant solvents that can be used include amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocyclics (cyclohexylpyrrolidone, γ-butyrolactone, etc.), sulfoxides (dimethyl sulfoxide, etc.), sulfones (hexamethylphosphorotriamide, sulfolane, etc.), lower ketones (acetone, methyl ethyl ketone, etc.), carbonates (diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate), and others, such as tetrahydrofuran, urea, and acetonitrile. The relative dielectric constant of the high-dielectric-constant solvent can be a value listed in a solvent handbook or the like, and is preferably 2.5 or more at 20°C.

[0059] From the viewpoint of the solubility of the dispersant and binder resin, or the wettability of the CNTs to the dispersion medium, the non-aqueous dispersion medium preferably contains an amide-based organic solvent, and more preferably contains at least one selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone.

[0060] The content of the amide organic solvent is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the total mass of the non-aqueous dispersion medium. From the viewpoint of ease of handling, it is more preferable to use N-methyl-2-pyrrolidone alone.

[0061] The CNT dispersion may contain a dispersant. There are no particular restrictions on the dispersant, but one that can stabilize the dispersion of CNTs in the CNT dispersion is preferred. While either a resin-type dispersant or a surfactant can be used as the dispersant, resin-type dispersants are preferred because they have a strong adsorption force to CNTs and provide good dispersion stability. Depending on the properties required for CNT dispersion, an appropriate type of dispersant can be used in an appropriate amount.

[0062] As the resin-type dispersant, a (meth)acrylic polymer, a polymer derived from an ethylenically unsaturated hydrocarbon, a cellulose derivative, a copolymer thereof, or the like can be used.

[0063] Examples of polymers derived from ethylenically unsaturated hydrocarbons include polyvinyl alcohol resins, polyvinylpyrrolidone resins, polyacrylonitrile resins, nitrile rubbers, etc. Examples of polyvinyl alcohol resins include polyvinyl alcohol, modified polyvinyl alcohols having functional groups other than hydroxyl groups (e.g., acetyl groups, sulfo groups, carboxy groups, carbonyl groups, amino groups), polyvinyl alcohols modified with various salts, other anion- or cation-modified polyvinyl alcohols, and polyvinyl acetals (polyvinyl acetoacetal, polyvinyl butyral, etc.) modified with aldehydes (acetoacetal-modified, butyral-modified, etc.). Polyacrylonitrile resins may be polyacrylonitrile homopolymers, polyacrylonitrile copolymers, or modified versions thereof. Preferred examples include polyacrylonitrile resins containing at least one active hydrogen group (e.g., hydroxyl, carboxy, primary amino, secondary amino, or mercapto groups), basic groups, and alkyl groups derived from (meth)acrylic acid alkyl esters or α-olefins. For example, the acrylonitrile copolymer described in JP 2020-163362 A can be used. Nitrile rubbers include acrylonitrile butadiene rubber and hydrogenated acrylonitrile butadiene rubber. Cellulose derivatives include cellulose acetate, cellulose butyrate, cyanoethyl cellulose, ethylhydroxyethyl cellulose, nitrocellulose, methylcellulose, ethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethylcellulose, carboxymethylcellulose, and copolymers thereof. In addition, dispersants described in International Publication No. 2008 / 108360, JP 2018-192379, JP 2019-087304, JP 2020-011934, and JP 2009-026744 may be used, but are not limited to these.Particularly preferred are methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyacrylonitrile homopolymers, polyacrylonitrile copolymers, and hydrogenated acrylonitrile butadiene rubber. Polymers in which other substituents have been introduced into a portion of these polymers, modified polymers, etc. may also be used.

[0064] The weight-average molecular weight of the resin-type dispersant is preferably 500,000 or less, more preferably 300,000 or less, from the viewpoint of the affinity balance between the CNTs to be dispersed and the non-aqueous dispersion medium, and from the viewpoint of resistance to the electrolyte. It is also preferably 3,000 or more, more preferably 5,000 or more. The resin-type dispersant may be used alone or in combination of two or more. The weight-average molecular weight is the weight-average molecular weight in terms of polystyrene determined by gel permeation chromatography (GPC).

[0065] Commercially available polyvinyl alcohol resins include Kuraray Poval (polyvinyl alcohol resin manufactured by Kuraray), Gohsenol, Gohsenex (polyvinyl alcohol resin manufactured by Nippon Synthetic Chemical Industry), Denka Poval (polyvinyl alcohol resin manufactured by Denka), and J-Poval (polyvinyl alcohol resin manufactured by Nippon Acetate Poval Corporation), and various grades are available. Modified polyvinyl alcohols having various functional groups are also available. Polyvinyl alcohols synthesized by known synthesis methods may also be used. Specific examples of commercially available polyvinylpyrrolidone resins include Luvitec K17 (K value: 15.0 to 19.0, low molecular weight), K30 (K value: 27.0 to 33.0), K90 (K value: 88.0 to 92.0), K90HM (K value: 92.0 to 96.0, high molecular weight) (manufactured by BASF Japan), K15, K30, K90, K120 (manufactured by ISP), polyvinylpyrrolidone K30 (K value: 27.0 to 33.0), K90 (K value: 88.0 to 96.0) (manufactured by Nippon Shokubai), and PVP. Examples include K12 (K value 10-14), K15 (K value 13-19), K30 (K26-K35), K60 (K value 50-62), and K90 (K value 88-100) (manufactured by DSP Gokei Food & Chemical). To prevent an increase in viscosity, the polyvinylpyrrolidone preferably has a K value of 150 or less, more preferably a K value of 100 or less, and even more preferably a K value of 85 or less. Commercially available nitrile rubbers include Therban (hydrogenated nitrile rubber manufactured by Arlanxeo), Baymod (nitrile rubber manufactured by Arlanxeo), Zetpole (hydrogenated nitrile rubber manufactured by Nippon Zeon), and Nipole NBR (nitrile rubber manufactured by Nippon Zeon), and various grades with different nitrile ratios, hydrogenation rates, molecular weights, etc. are available. Furthermore, those synthesized by known synthesis methods may also be used.

[0066] Surfactants may be used in place of or in addition to the resin-type dispersants described above. Surfactants are classified into anionic, cationic, amphoteric ionic surfactants and nonionic surfactants.

[0067] The CNT dispersion may further contain a base. The inclusion of a base in the CNT dispersion is preferable because it increases the wettability of the CNTs in the dispersion medium, thereby improving dispersibility or dispersion stability. The base to be added may be, for example, a basic compound derived from a metal or a basic compound not derived from a metal, and at least one selected from the group consisting of inorganic bases and inorganic metal salts, metal alkoxides, organic bases, and salts thereof can be used. The total amount of these is preferably 0.001 to 0.1 mass%, more preferably 0.005 to 0.05 mass%, of the total amount of the CNT dispersion. Adding too much base may cause corrosion of the dispersion device and / or the inside of the battery.

[0068] Here, the water content of the added base is preferably less than 5% by mass, more preferably less than 3% by mass, and even more preferably less than 2% by mass. If the added base contains a large amount of water, the adsorption of the dispersant to the CNTs may decrease, making it difficult to stably maintain the CNTs in the non-aqueous dispersion medium. By keeping the water content within the above range, the problem of gelation of the CNT dispersion during storage can be prevented.

[0069] Examples of metal alkoxides include lithium methoxide, lithium ethoxide, lithium-n-butoxide, lithium-t-butoxide, potassium methoxide, potassium ethoxide, potassium-n-butoxide, potassium-t-butoxide, sodium methoxide, sodium ethoxide, sodium-n-butoxide, and sodium-t-butoxide. The alkoxide may have 5 or more carbon atoms. Sodium-t-butoxide is particularly preferred. Examples of alkaline earth metal alkoxides include magnesium methoxide, magnesium ethoxide, magnesium-n-butoxide, and magnesium-t-butoxide. The alkoxide may have 5 or more carbon atoms. The metal contained may be a transition metal.

[0070] Examples of inorganic bases and inorganic metal salts include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, borates, and ammonium hydroxide of alkali metals or alkaline earth metals. Among these, alkali metal or alkaline earth metal hydroxides are preferred from the viewpoint of easy supply of cations. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include calcium hydroxide and magnesium hydroxide. Among these, it is more preferable to use at least one selected from the group consisting of lithium hydroxide, sodium hydroxide, and potassium hydroxide. The metal contained in the inorganic base may be a transition metal.

[0071] Examples of organic bases include primary, secondary, and tertiary amine compounds (e.g., alkylamines and aminoalcohols) having 1 to 40 carbon atoms, which may have a substituent, and organic hydroxides. Examples of primary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as propylamine, butylamine, isobutylamine, octylamine, 2-ethylhexylamine, and laurylamine; aminoalcohols such as 2-aminoethanol and 3-aminopropanol; and 3-ethoxypropylamine. Examples of secondary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as dibutylamine, diisobutylamine, N-methylhexylamine, and dioctylamine, and aminoalcohols such as 2-methylaminoethanol. Examples of tertiary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as triethylamine, tributylamine, N,N-dimethylbutylamine, N,N-diisopropylethylamine, dimethyloctylamine, trioctylamine, and dimethyldecylamine, triethanolamine, and 2-(dimethylamino)ethanol. The organic hydroxide is a salt containing an organic cation and a hydroxide ion. Examples of the organic hydroxide include trimethyl-2-hydroxyethylammonium hydroxide, tetramethylammonium hydroxide, trimethylphenylammonium hydroxide, 3-trifluoromethyl-phenyltrimethylammonium hydroxide, and benzyltrimethylammonium hydroxide. Among these, from the viewpoint of the action on CNTs, it is more preferable to use at least one selected from the group consisting of 2-aminoethanol, 3-aminopropanol, triethanolamine, and trimethyl-2-hydroxyethylammonium hydroxide.

[0072] The CNT dispersion may further contain an acid. It is preferable to include an acid in the CNT dispersion, since this can suppress the polymerization of the resin and the associated increase in viscosity and gelation of the lithium-ion battery positive electrode composite slurry. The acid to be added may be either an inorganic acid or an organic acid. The total amount of these is preferably 0.001 to 1.0 mass %, more preferably 0.005 to 0.5 mass %, based on the total amount of the CNT dispersion. Adding too much acid may reduce the adsorption and dispersion ability of the dispersant to the CNTs.

[0073] Examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. Examples of organic acids include carboxylic acid compounds and sulfonic acid compounds. Examples of carboxylic acid compounds include formic acid, acetic acid, propionic acid, butyric acid, tartaric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, (meth)acrylic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, and fluoroacetic acid. Examples of sulfonic acid compounds include methanesulfonic acid, paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. Furthermore, anhydrides, hydrates, or partial salts of the above acids can also be used. These can be used alone or in combination of two or more.

[0074] The CNT dispersion may contain optional components such as wetting agents, wetting penetrants, leveling agents, and other additives as needed, as long as the objective of the present invention is not impaired. The optional components can be added at any timing, such as before preparing the CNT dispersion, during mixing, after mixing, or a combination of these.

[0075] Since the inclusion of optional components does not impair the object of the present invention and the CNT dispersion is substantially free of active materials, the content of CNTs and dispersant in the CNT dispersion is 60 mass % or more, preferably 70 mass % or more, more preferably 80 mass % or more, even more preferably 90 mass % or more, and even more preferably 95 mass % or more, based on the non-volatile components of the CNT dispersion, i.e., the solid content of the CNT dispersion, and may be 100 mass % or less.

[0076] Carbon black can be included as a conductive material other than CNT. It is preferable that carbon black be included in the CNT dispersion within a range that does not impair the effects of the present invention, and the CNT content is preferably 1 to 80 mass %, more preferably 1 to 50 mass %, of the total mass of the carbon black and CNT. If the above range is satisfied, even if carbon black is included, the dispersion state of the CNT can be controlled, and high dispersibility and flowability can be maintained. Furthermore, the carbon black content is preferably 20 mass % or less, more preferably 15 mass % or less, of the total mass of the CNT dispersion.

[0077] The CNT powder to be evaluated may be a production lot of CNT powder. Although the initial dispersibility may be reduced depending on the properties of the CNT powder production lot, by evaluating and selecting the CNT powder using the evaluation method of this embodiment, it is possible to distinguish between good and bad CNT powders due to the production lot.

[0078] Alternatively, the CNT powder to be evaluated may be a batch unit of a dispersion treatment of CNT powder. By evaluating and selecting the CNT powder for each dispersion treatment batch unit using the evaluation method of the present disclosure, it is possible to distinguish between good and bad batches.

[0079] The CNT powder to be evaluated should be one immediately before being subjected to a dispersion treatment. Satisfying at least one of the conditions (1) and (2) of the evaluation method can improve dispersibility. These conditions (1) and (2) may also be combined for evaluation.

[0080] In another example, if dry pulverization is performed before the dispersion treatment, the CNT powder immediately before being subjected to dry pulverization should be evaluated. By satisfying at least one of the conditions (1) and (2) of the evaluation method, dispersibility can be improved. Evaluation may also be performed to determine whether both conditions (1) and (2) are satisfied. By dry pulverizing an evaluated CNT powder that satisfies at least one of conditions (1) and (2), the CNT powder immediately before being subjected to wet dispersion will satisfy more preferred ranges for conditions (1) and (2), thereby further improving dispersibility.

[0081] Dry pulverization is explained below. The step of dry pulverizing CNT powder can be carried out by utilizing impact and / or shear force. Dry pulverization can be carried out, for example, in the absence of water and / or organic solvent. Dry pulverization is preferable in that it does not require post-processing steps such as drying the solvent after pulverization and defibrating the dried and aggregated fibers. The components to be fed into the pulverizer are not limited to CNTs alone, and optional components such as lubricants, dispersants, and surface treatment agents may also be added, but it is preferable to use only CNTs.

[0082] The equipment used for dry pulverization is preferably an equipment that performs dry processing by impact and / or shear force. From the viewpoint of wettability and incorporation of CNTs into a dispersion medium when wet-dispersing CNTs after dry pulverization, impact processing is preferred, and processing using pulverization media is particularly preferred.

[0083] When using a milling device, the milling process can be performed using any of the batch, pass, and circulation methods. However, the pass or circulation method is preferred due to the ease of control of the physical properties of the CNTs, with the circulation method being more preferred. The batch method is a method in which processing is performed using only the dispersion device itself, without using piping or other devices. The pass method is a method in which the milling device itself is equipped with a tank that supplies CNTs via piping and a tank that receives CNTs, and the CNTs are passed through the milling device itself. The circulation method is a method in which CNTs that have passed through the dispersion device itself are returned to the tank that supplies CNTs and processed while circulating. In both cases, the longer the processing time, the more the milling process progresses; therefore, the pass or circulation method can be repeated until the desired state is achieved, and the processing volume can be increased by changing the tank size or processing time. Specific milling devices include, for example, a bead mill, a ball mill, and an attrition mill. However, from the perspectives of CNT conductivity, dispersibility, and uniformity of milling degree, dry milling using a pass or circulation bead mill is preferred.

[0084] In a pass-type or circulation-type milling process, the desired CNTs can be obtained by controlling the mass of CNTs fed into the milling device per unit time, the type, diameter, and amount of milling media, the milling temperature, and other factors. One example is a circulation-type bead mill, where CNT raw materials are fed and dry-milled. Methods for controlling the milling temperature include cooling with a cooling jacket attached to the milling chamber, cooling outside the bead mill, or a combination of both. From the perspective of cooling efficiency, cooling with a cooling jacket attached to the milling chamber is preferred. The milling temperature can be measured using a thermometer attached to any piping. Specifically, it can be confirmed by measuring the powder outlet temperature using a thermometer attached to the bead mill outlet. For example, the powder outlet temperature should be controlled to 45±10°C.

[0085] When dry milling is performed using milling media, ceramic materials are preferred, with zirconia being more preferred, to prevent the incorporation of metal impurities such as iron into the CNTs and to enhance milling process strength. The milling media diameter is preferably 1 mm or more, more preferably 3 mm or more, and even more preferably 5 mm or more. A milling media diameter of at least the above range increases the collision energy with the CNTs, thereby improving dry milling efficiency. Furthermore, the milling media diameter is preferably 15 mm or less, more preferably 10 mm or less, and even more preferably 8 mm or less. A milling media diameter of at most the above range reduces wear due to collisions between the milling media. While two or more different milling media diameters can be used together as long as the milling media diameter is within the above range, it is preferable to use only one type of milling media diameter because wear due to collisions between beads can cause contamination. Furthermore, milling media used in dry milling of CNTs can wear and crack with repeated use, which can lead to poor dispersion and contamination. Therefore, it is recommended to classify and clean the milling media before use.

[0086] Wet dispersion is described below. Wet dispersion of CNTs involves mixing and dispersing CNTs with a dispersion medium. The above-mentioned dispersant may be further included in order to stabilize the dispersion of CNTs in the CNT dispersion liquid. For example, it is preferable to finely disperse the CNTs, dispersion medium, and dispersant by dispersing them using a dispersing device. The dispersing device used for this process is not particularly limited. The dispersion process may be a multi-stage process of two or more steps, with the timing of adding the materials used being adjusted as desired.

[0087] Examples of dispersing devices used to disperse CNTs include kneaders, roll mills, planetary mixers, ball mills, sand mills, bead mills, attritors, high-shear mixers, high-pressure homogenizers, and ultrasonic homogenizers. Among these, a high-shear mixer, high-pressure homogenizer, bead mill, ultrasonic homogenizer, or a combination of these is preferred to finely disperse CNTs in the CNT dispersion and achieve favorable dispersibility. In particular, a high-shear mixer is used in the initial dispersion step to promote wetting of the CNTs and disintegrate coarse particles, followed by a high-pressure homogenizer to maintain the CNT aspect ratio. Using a high-pressure homogenizer in multiple stages via circulation dispersion can further enhance CNT dispersibility. The pressure when using a high-pressure homogenizer is preferably 60 to 150 MPa, more preferably 60 to 120 MPa.

[0088] Dispersion methods using a dispersing device include batch dispersion, pass dispersion, and circulation dispersion. Any of these methods may be used, or two or more methods may be combined. Batch dispersion is a method in which dispersion is performed using only the dispersing device itself, without using piping or the like. Because it is easy to handle, it is preferred for small-scale production. Pass dispersion is a dispersion method in which the dispersing device itself is equipped with a tank that supplies the dispersion liquid via piping and a tank that receives the dispersion liquid, and the dispersion passes through the dispersing device itself. Furthermore, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersing device itself is returned to the tank that supplies the dispersion liquid and dispersed while circulating. In both methods, the longer the processing time, the more the dispersion progresses; therefore, the pass or circulation can be repeated until the desired dispersion state is achieved, and the processing volume can be increased by changing the tank size or processing time. Pass dispersion is preferred over circulation dispersion because it is easier to achieve a uniform dispersion state. Circulation dispersion is preferred over pass dispersion because the operation and production equipment are simpler.

[0089] In the dispersion process, the disintegration of agglomerated particles and the loosening, wetting, and stabilization of CNTs proceed sequentially or simultaneously, and the final dispersion state varies depending on how the processes proceed. However, dispersibility can be improved by using CNTs that satisfy at least one of conditions (1) and (2) as the dispersed material.

[0090] The dispersing device used in wet dispersion may be equipped with a heat exchanger or a coolant supply mechanism for cooling the CNT dispersion. By pre-cooling the CNT dispersion or crude dispersion that has reached a high temperature after dispersion, it is possible to suppress the generation of bubbles in the CNT dispersion and also to suppress the remaining solid content on the walls of a dispersion tank, etc. The liquid temperature of the CNT dispersion is not particularly limited, but is preferably controlled to 25 to 75°C, and more preferably 40 to 65°C.

[0091] In the production of the CNT dispersion, the CNT powder to be evaluated is not particularly limited, and will be described below.

[0092] The CNT powder to be evaluated may be a mixture of single-walled CNTs and multi-walled CNTs. Single-walled CNTs have a structure in which one layer of graphite is wound into a cylindrical shape. Multi-walled CNTs have a structure in which two or three or more layers of graphite are wound into a cylindrical shape. Furthermore, the sidewalls of the CNTs do not have to have a graphite structure. For example, CNTs with sidewalls having an amorphous structure can also be used as CNTs. The CNT powder to be evaluated is preferably a powder of multi-walled CNTs, and the number of CNT layers is preferably 3 to 30, more preferably 3 to 20, and even more preferably 3 to 10.

[0093] The CNTs may be produced by any method. Generally, CNTs can be produced by laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion methods, but are not limited to these. Surface-treated CNTs may also be used. CNTs may be CNT derivatives to which functional groups, such as carboxyl groups, have been added. CNTs containing organic compounds, metal atoms, or substances, such as fullerenes, may also be used.

[0094] The average outer diameter of the CNT powder to be evaluated is preferably 3 nm or more, more preferably 5 nm or more. It is also preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 15 nm or less. When the average outer diameter of the CNTs is 15 nm or less, the number of CNTs per mass can be ensured, allowing for efficient formation of a conductive network. The average outer diameter of the CNTs can be calculated by first observing and photographing the CNTs using a transmission electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the outer diameter of each.

[0095] The average fiber length of the CNT powder to be evaluated is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. It is also preferably 5.0 μm or less, more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. The average fiber length of the CNTs can be calculated by first observing and photographing the CNTs using a scanning electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the fiber length of each.

[0096] The aspect ratio is the fiber length of a CNT divided by its outer diameter. A typical aspect ratio can be calculated using the average fiber length and average outer diameter. A conductive material with a higher aspect ratio can achieve higher conductivity when formed into an electrode. The aspect ratio of the CNT powder to be evaluated is preferably 30 or greater, more preferably 50 or greater, and even more preferably 80 or greater. It is also preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less. The aspect ratio of a CNT can be calculated by first observing and photographing the CNTs using a scanning electron microscope, then randomly selecting 300 CNTs from the photograph and measuring the aspect ratio of each.

[0097] The carbon purity of the CNT powder to be evaluated is expressed as the content (mass%) of carbon atoms in the CNT powder. The carbon purity is preferably 80 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 98 mass% or more, relative to 100 mass% of the CNT powder. By keeping the carbon purity within the above range, problems such as short circuits caused by the formation of dendrites due to impurities such as metal catalysts can be prevented. CNTs that have been subjected to a high-purification treatment to remove or reduce impurities such as metal catalysts and increase the carbon purity may be used. The method of high-purification treatment is not particularly limited, and known methods can be used.

[0098] In the CNT powder to be evaluated, the Co content is preferably 15,000 ppm or less, more preferably 13,500 ppm or less, even more preferably 12,500 ppm or less, and even more preferably 11,500 ppm or less. In the CNT powder to be evaluated, the Al content is preferably 8000 ppm or less, more preferably 7000 ppm or less, even more preferably 6000 ppm or less, and even more preferably 5000 ppm or less. The content of metals contained in CNTs can be calculated, for example, by acid-decomposing the CNTs, extracting the metals contained in the CNTs, and analyzing the extract using high-frequency inductively coupled plasma (ICP).

[0099] It is more preferable that the average outer diameter, average fiber length, and aspect ratio described above each be maintained within the preferred ranges described above while satisfying at least one of conditions (1) and (2).It is more preferable that the carbon purity, Co content, and Al content described above each be maintained within the preferred ranges described above while satisfying at least one of conditions (1) and (2).

[0100] The specific surface area of ​​the CNT powder to be evaluated is 100 m 2 / g or more, and 150m 2 / g or more is more preferable, and 200m 2 / g or more is more preferable. 2 / g or less, and 2 / g or less is more preferable, and 600m 2 / g or less is even more preferable. The specific surface area of ​​the CNTs is calculated by the BET method using nitrogen adsorption measurements. When the average outer diameter, average fiber length, aspect ratio, and specific surface area are within the above ranges, it becomes easier to form a well-developed conductive path in the electrode.

[0101] The bulk density of the CNT powder to be evaluated was 0.020 g / cm 3 It is preferable that the concentration is 0.030 g / cm or more. 3 It is more preferable that the density is 0.145 g / cm or more. 3 Preferably, it is 0.130 g / cm or less. 3 More preferably, it is 0.125 g / cm or less. 3 It is even more preferable that:

[0102] In this disclosure, the bulk density of CNT powder refers to the loose bulk density when loosely packed. The bulk density can be determined by allowing CNTs to freely fall into an open-top container of a specified capacity until they overflow, leaving the container to stand, leveling off the raised CNT powder on the upper surface with a leveling plate, measuring the mass of the CNTs, and dividing the mass by the volume of the container.

[0103] A specific method for measuring bulk density is to allow CNT powder to fall freely into a 30 ml stainless steel cylindrical container, remove any excess mass from the top of the container, and then calculate the mass of the CNT powder and divide it by the volume of the container. To break down aggregates formed during storage of the CNT powder, and allow the resulting sample to flow freely into the measurement container until it overflows, a sufficient amount of sample for the test is passed through a 0.5 mm sieve.

[0104] It is more preferable that the specific surface area and bulk density are maintained within the above-mentioned preferred ranges while satisfying at least one of the conditions (1) and (2).

[0105] The dry particle size distribution (D50) of the CNT powder to be evaluated is preferably 420 μm or less, more preferably 400 μm or less, even more preferably 360 μm or less, and particularly preferably 330 μm or less. The dry particle size distribution (D50) is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 25 μm or more. The dry particle size distribution (D50) is measured in the same manner as the dry particle size distribution (D10) described above, and is the particle size at which the volume-based integrated value reaches 50%.

[0106] The 50% particle size (D50) of a CNT dispersion, which is the particle size at which the cumulative volumetric frequency reaches 50% in a laser diffraction / scattering particle size distribution, is preferably 0.1 to 20 μm, more preferably 0.2 to 10 μm, and even more preferably 0.4 to 5 μm. The 50% particle size (D50) determined by a laser diffraction / scattering particle size distribution allows for estimation of the particle size of CNT aggregated particles based on the scattered light intensity distribution of the particles, and serves as an index for evaluating the dispersibility of a CNT dispersion. By ensuring that the 50% particle size (D50) is within the above range, a CNT dispersion with good dispersibility can be provided. If the 50% particle size (D50) is below the above range, aggregated carbon nanotubes will be present, while if it is above the above range, numerous finely cut carbon nanotubes will be present, potentially making it difficult to form an efficient conductive network. A specific method for measuring the 50% particle size (D50) of a CNT dispersion is described in the Examples.

[0107] In the production of a CNT dispersion, if the CNT powder does not satisfy the dry particle size distribution (D10) or does not satisfy either of the conditions (1) or (2), the CNT powder may be reprocessed to satisfy the conditions. Examples of reprocessing methods include size selection by sieving, dry crushing, high-temperature treatment, and halogenation treatment.

[0108] <Carbon nanotube dispersion> According to one of some embodiments, it is possible to provide a carbon nanotube dispersion liquid produced by the above-described method for producing a carbon nanotube dispersion liquid.

[0109] Preferably, the CNT dispersion is obtained using a CNT powder that has a dry particle size distribution (D10) of 110 μm or less and satisfies at least one of conditions (1) and (2) in the evaluation method of the present disclosure. This CNT dispersion eliminates factors that reduce the initial dispersibility of the CNT powder used, thereby suppressing the formation of CNT aggregates in the dispersion. Furthermore, the good initial dispersibility also suppresses the thickening of the dispersion. Therefore, by using a CNT powder that satisfies at least one of conditions (1) and (2), a CNT dispersion with good dispersion stability and storage stability can be provided.

[0110] Furthermore, in the CNT resin composition and composite slurry obtained using this CNT dispersion, the dispersibility of CNTs can be maintained well. In the electrode film obtained using these, aggregation can be suppressed while maintaining the aspect ratio of the CNTs, resulting in a state of good distribution uniformity. In the electrode obtained using this electrode film and the secondary battery including the electrode, the conductive network of CNTs can be maintained well, thereby improving battery performance.

[0111] <Carbon nanotube resin composition> According to one of several embodiments, a method for producing a carbon nanotube resin composition can be provided, including evaluating a carbon nanotube powder according to the evaluation method described above to provide the evaluated carbon nanotube powder, and obtaining a resin composition containing the evaluated carbon nanotube powder, a dispersion medium, and a binder resin. Details of the evaluation method are as described above. According to another embodiment, a carbon nanotube resin composition produced by the method for producing a carbon nanotube resin composition described above can be provided. Preferably, the CNT resin composition is obtained using a CNT powder in the evaluation method of the present disclosure that has a dry particle size distribution (D10) of 110 μm or less and satisfies at least one of conditions (1) and (2).

[0112] The binder resin is not particularly limited as long as it is typically used as a binder resin for paints, and can be appropriately selected depending on the purpose. The binder resin may be a resin that bonds between substances such as active materials, CNTs, and other conductive materials. Examples of binder resins include homopolymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, and vinyl pyrrolidone as structural units; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, and fluororesins; cellulose resins such as carboxymethyl cellulose or salts thereof; elastomers such as hydrogenated or non-hydrogenated styrene-butadiene rubber and fluororubber; and conductive resins such as polyaniline and polyacetylene. Modified versions of these resins and copolymers of these resins may also be used. The binder resin may be used alone or in combination of two or more. Among these, when used as a binder resin for the positive electrode of a secondary battery, homopolymers or copolymers having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, polytetrafluoroethylene, and modified products and copolymers thereof, are preferred in terms of durability. Furthermore, when used as a binder resin for the negative electrode of a secondary battery, CMC (carboxymethyl cellulose or its salt), hydrogenated or non-hydrogenated styrene-butadiene rubber, polyacrylic acid, and the like, which have good adhesion, are preferred.

[0113] In the CNT resin composition, the binder resin content is preferably 0.5 to 30 mass %, more preferably 0.5 to 25 mass %, relative to the total amount of nonvolatile content of the CNT resin composition.

[0114] The CNT resin composition is preferably obtained by adding a binder resin to a CNT dispersion and mixing. The binder resin may be used in the form of a solution (varnish) in which the binder resin is dispersed or dissolved in a dispersion medium, but is preferably used in a dispersion medium-free state, more preferably in a solid state, and particularly preferably in powder form. By adding a powdered binder resin to a CNT dispersion, the amount of non-aqueous dispersion medium does not increase when the binder resin is added, making it possible to provide a CNT resin composition with a high binder resin concentration and non-volatile content concentration. The form of the binder resin is not particularly limited and may be, for example, pellets, powder, granules, flakes, chunks, or chopped fibers.

[0115] There are no particular limitations on the method for adding the binder resin to the CNT dispersion, but it is preferable to add the binder resin while stirring the CNT dispersion using a stirring device. The stirring device is not particularly limited, but a disperser (stirring blade) or the like is generally used. There are various types of stirrers, such as propeller-type and turbine-type, and there are no particular limitations on their shape as long as they can stir the CNT resin composition homogeneously. The stirring speed can be adjusted appropriately by adjusting the size and rotation speed of the stirring blade. There are no particular limitations on the stirring speed as long as it is within the range allowed by the actual operating process.

[0116] <Compound slurry> According to one of several embodiments, a method for producing a composite slurry can be provided, including evaluating a carbon nanotube powder according to the evaluation method described above to provide the evaluated carbon nanotube powder, and obtaining a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material. Details of the evaluation method are as described above. According to another embodiment, a composite slurry produced by the above-described method for producing a composite slurry can be provided. Preferably, the composite slurry is obtained using a CNT powder in the evaluation method of the present disclosure that has a dry particle size distribution (D10) of 110 μm or less and satisfies at least one of conditions (1) and (2).

[0117] The active material may be either a positive electrode active material or a negative electrode active material. The composite slurry can be used as a composite slurry for a secondary battery electrode, and may be either a positive electrode composite slurry for a secondary battery or a negative electrode composite slurry for a secondary battery.

[0118] The composite slurry may contain other optional components as needed, as long as they do not impair the object of the present invention. The optional components can be added at any time, such as before preparing the composite slurry, during mixing, after mixing, or a combination thereof. The optional components may be those described above for the CNT dispersion.

[0119] The active material may be a positive electrode active material or a negative electrode active material. In this disclosure, the positive electrode active material and the negative electrode active material may be simply referred to as "active material." The active material is a material that is the basis of the battery reaction, and is broadly classified into a positive electrode active material and a negative electrode active material based on the electromotive force. The composite slurry is preferably in a slurry state to improve uniformity and processability.

[0120] The positive electrode active material is not particularly limited, but for example, metal compounds such as metal oxides and metal sulfides that can reversibly dope or intercalate lithium ions can be used. For example, lithium manganese composite oxides (e.g., Li xMn2O4 or Li x (MnO2), lithium nickel composite oxide (e.g., Li x NiO2), lithium cobalt composite oxide (Li x CoO2), lithium nickel cobalt composite oxide (e.g., Li x Ni 1-y Co y O2), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co 1-y O2), lithium nickel manganese cobalt composite oxide (e.g., Li x Ni y Co z Mn 1-y-z O2), spinel type lithium manganese nickel composite oxide (e.g., Li x Mn 2-y Ni y O4), etc., composite oxide powders of lithium and transition metals, lithium phosphate powders having an olivine structure (e.g., Li x FePO4, Li x Fe 1-y Mn y PO4, Li x CoPO4, etc.), manganese oxide, iron oxide, copper oxide, nickel oxide, vanadium oxides (e.g., V2O5, V6O 13 ), etc., transition metal oxide powders such as titanium oxide, iron sulfate (Fe2(SO4)3), transition metal sulfide powders such as TiS2 and FeS, etc. However, x, y, z are numbers, 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < y + z < 1. These cathode active materials can also be used alone or in combination of two or more. Among these active materials, in particular, active materials containing Ni and / or Mn (especially when the total amount of Ni and / or Mn in the transition metal is 50 mol% or more) tend to have high basicity due to components derived from raw materials or elution of metal ions, and due to this influence, gelation of the binder resin and deterioration of the dispersion state are likely to occur. Therefore, in the case of a battery containing an active material containing Ni and / or Mn, the present embodiment is particularly effective.

[0121] The negative electrode active material is not particularly limited. For example, it can be metallic Li capable of reversibly doping or intercalating lithium ions, or its alloy, tin alloy, silicon alloy negative electrode, Li X Metal oxide systems such as TiO2, LiXFe2O3, LiXFe3O4, LiXWO2, conductive polymers such as polyacetylene and poly-p-phenylene, artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, and resin-fired carbon materials can be used. However, x is a number and 0 < x < 1. These negative electrode active materials can be used alone or in combination of two or more. Particularly when using a silicon alloy negative electrode, although the theoretical capacity is large, the volume expansion is extremely large. Therefore, it is preferably used in combination with artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, resin-fired carbon materials, etc.

[0122] The content of CNT in the composite material slurry is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more based on the mass of the active material (taking the mass of the active material as 100% by mass). Also, it is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. If it exceeds the above range, the filling amount of the active material in the electrode decreases, leading to a decrease in the battery capacity. Also, if it is below the above range, the conductivity of the electrode and the battery may become insufficient.

[0123] The content of the dispersant in the composite material slurry is preferably 0.01% by mass or more, more preferably 0.02% by mass or more based on the mass of the active material (taking the mass of the active material as 100% by mass). Also, it is preferably 10% by mass or less, more preferably 5% by mass or less.

[0124] The content of the binder resin in the composite slurry is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the mass of the active material (the mass of the active material being 100% by mass), and is preferably 20% by mass or less, more preferably 10% by mass or less.

[0125] The nonvolatile content in the composite slurry is preferably 40% by mass or more, more preferably 50% by mass or more, based on the mass of the composite slurry (the mass of the composite slurry being 100% by mass), and is preferably 90% by mass or less, more preferably 85% by mass or less.

[0126] A preferred method for preparing the composite slurry is to add an active material to the CNT resin composition and stir the mixture. The stirring device used for stirring is not particularly limited. Examples of suitable stirring devices include a disperser and a homogenizer.

[0127] The composite slurry can further contain carbon black, which functions as a conductive material. The carbon black is preferably included in the composite slurry within a range that does not impair the effects of the present invention. The CNT content is preferably 1 to 80 mass%, more preferably 1 to 50 mass%, of the total amount of the conductive material containing carbon black and CNT. Within these ranges, the dispersion state of the CNTs can be controlled, and high dispersibility and flowability can be maintained even when the binder resin is included. Furthermore, the carbon black content is preferably 10 mass% or less, more preferably 5 mass% or less, of the total mass of the composite slurry. The method for producing the composite slurry includes a step of adding an active material to the CNT resin composition, but can also include adding carbon black before, after, simultaneously with, or a combination of these steps. Alternatively, the active material may be added to a CNT resin composition that already contains carbon black, or the active material and carbon black may be further added to a CNT resin composition that already contains carbon black.

[0128] <Electrode film> According to one of several embodiments, a method for producing an electrode film can be provided, including evaluating a carbon nanotube powder according to the evaluation method described above to provide the evaluated carbon nanotube powder, and obtaining an electrode film using a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material. Details of the evaluation method are as described above. According to another embodiment, an electrode film produced by the electrode film production method described above can be provided. Preferably, the electrode film is obtained using a CNT powder in the evaluation method of the present disclosure that has a dry particle size distribution (D10) of 110 μm or less and satisfies at least one of conditions (1) and (2).

[0129] The electrode film may be a coating film of a composite slurry. The electrode film contains CNTs, a binder resin, and an active material, and may optionally further contain additives such as a dispersant. The active material may be either a positive electrode active material or a negative electrode active material. The electrode film can be used as an electrode film for a secondary battery electrode, and may be either an electrode film for a secondary battery positive electrode or a secondary battery negative electrode.

[0130] An electrode can be provided using an electrode film. The electrode may include a current collector and an electrode film. The electrode can be obtained by applying a composite slurry to a current collector and drying the applied film. A positive electrode can be provided using an electrode film containing a positive electrode active material, and a negative electrode can be provided using an electrode film containing a negative electrode active material. These positive electrodes and negative electrodes can be used as the positive electrode and negative electrode of a secondary battery, respectively.

[0131] The material and shape of the current collector are not particularly limited and can be appropriately selected to suit various secondary batteries. Examples of current collector materials include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. While flat foils are generally used, current collectors with roughened surfaces, perforated foils, and mesh-like current collectors can also be used. The thickness of the current collector is preferably about 0.5 to 30 μm.

[0132] The method for applying the composite slurry to the current collector is not particularly limited, and known methods can be used. Specific examples include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. Drying methods after application include, but are not limited to, standing drying, air blowing, hot air drying, infrared heating, and far-infrared heating. Furthermore, after application of the composite slurry, rolling may be performed using a lithographic press, a calendar roll, or the like. The thickness of the electrode film is, for example, 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0133] <Secondary battery> According to one of some embodiments, a secondary battery can be provided, which includes a positive electrode and a negative electrode, at least one of which includes an electrode film obtained using a carbon nanotube dispersion, a carbon nanotube resin composition, or a composite slurry. Details of the carbon nanotube dispersion, the resin composition, and the composite slurry are as described above. The positive electrode and the negative electrode may each include a current collector and an electrode film. Details are as described above. The secondary battery is preferably a non-aqueous electrolyte secondary battery, and more preferably a lithium ion secondary battery.

[0134] According to another embodiment, a method for manufacturing a secondary battery can be provided, which includes obtaining an electrode film for at least one of a positive electrode and a negative electrode according to the above-described method for manufacturing an electrode film. Preferably, the secondary battery is obtained using a CNT powder that has a dry particle size distribution (D10) of 110 μm or less and satisfies at least one of conditions (1) and (2) in the evaluation method of the present disclosure.

[0135] The positive electrode can be prepared by coating a current collector with a composite slurry containing a positive electrode active material and drying the coated film. The negative electrode can be prepared by coating a current collector with a composite slurry containing a negative electrode active material and drying the coated film. The positive electrode active material and the negative electrode active material can be prepared by the above-mentioned methods. The composite slurry can be prepared by the above-mentioned methods.

[0136] The electrolyte of the secondary battery may be any of a liquid electrolyte, a gel electrolyte, and a solid electrolyte. For example, the liquid electrolyte may contain an electrolyte salt such as a lithium salt and a non-aqueous dispersion medium.

[0137] As the electrolyte salt, various conventionally known salts capable of transferring ions can be used. Examples include, but are not limited to, lithium salts such as LiBF, LiClO, LiPF, LiAsF, LiSbF, LiCFSO, Li(CFSO)N, LiCFSO, Li(CFSO)C, LiI, LiBr, LiCl, LiAlCl, LiHF, LiSCN, and LiBPh (wherein Ph is a phenyl group). The electrolyte salt is preferably dissolved in a non-aqueous dispersion medium and used as an electrolytic solution.

[0138] The non-aqueous dispersion medium is not particularly limited, but examples thereof include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile. These solvents may be used alone or in combination.

[0139] The secondary battery preferably includes a separator, such as, but not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and nonwoven fabrics obtained by subjecting these to hydrophilic treatment.

[0140] The structure of the secondary battery is not particularly limited. In one embodiment, the nonaqueous electrolyte secondary battery may generally include a positive electrode, a negative electrode, and a separator, if necessary. The nonaqueous electrolyte secondary battery may be in various shapes depending on the purpose of use, such as a paper type, a cylindrical type, a button type, or a laminated type.

[0141] The electrode films obtained using the CNT powders according to some embodiments have excellent conductivity and can therefore be used not only in secondary batteries but also in fuel cells, electric double layer capacitors, etc. These secondary batteries, fuel cells, electric double layer capacitors, etc. can be applied to small terminal devices such as electric vehicles and mobile phones.

[0142] <Carbon nanotube dispersion liquid manufacturing system> In one embodiment, a carbon nanotube dispersion production system can be provided, which includes an evaluation device that evaluates a carbon nanotube powder using the evaluation method described above to obtain an evaluated carbon nanotube powder, and a production device that produces a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium. Here, details of the evaluation method are as described above.

[0143] According to this CNT dispersion production system, it is possible to improve the initial dispersibility of the CNT dispersion and provide a CNT dispersion with good dispersion stability and storage stability.

[0144] An example of a manufacturing system will be described below using a schematic configuration diagram shown in FIG. 1. Note that the present disclosure is not limited to the example shown in the drawing. In FIG. 1, a CNT dispersion system 100 includes a manufacturing apparatus 10 that manufactures a CNT dispersion, and an evaluation apparatus 20. The manufacturing apparatus 10 includes a dry-milling apparatus 1 and a dispersion treatment apparatus 2. The system 100 may further include a storage apparatus 30. The storage apparatus 30 stores CNT powder, which is a raw material. The manufacturing apparatus 10 includes a supply pipe 11a that supplies CNT powder from the dry-milling apparatus 11 to the dispersion treatment apparatus 12, and a discharge pipe 12a that discharges the CNT dispersion from the dispersion treatment apparatus 12 and supplies it to the next process.

[0145] The evaluation device 20 extracts an evaluation sample from the raw CNT powder while it is being supplied from the storage device 30 to the manufacturing device 10. The obtained evaluation sample is evaluated according to the evaluation method of the present disclosure. If the evaluation sample satisfies at least one of conditions (1) and (2) and whether the dry particle size distribution (D50) of the evaluation sample is within a preferred range, the raw CNT powder is supplied to the manufacturing device 10 and subjected to dispersion treatment. Depending on the evaluation results of the dry particle size distribution (D50) of the evaluation sample and conditions (1) and (2), the raw CNT powder may be directly dispersed in the dispersion treatment device 12 in the manufacturing device 10 without passing through the dry grinding device 11.

[0146] In another embodiment, the evaluation device 20 may be arranged by being connected to the supply pipe 11a immediately before the dispersion processing device 2. Alternatively, the evaluation device 20 may be arranged at two or more locations, including a position between the storage device 30 and the dry-grinding device 11 and a position on the supply pipe 11a immediately before the dispersion processing device 2.

[0147] The system 100 may further include a control device (not shown). The control device determines whether the evaluation sample satisfies at least one of conditions (1) and (2) in the evaluation device 20. For example, if the control device determines that the evaluation sample satisfies at least one of conditions (1) and (2), it supplies the CNT powder to be evaluated to the manufacturing device 10. On the other hand, if the control device determines that the evaluation sample does not satisfy both conditions (1) and (2), it determines that the CNT powder to be evaluated is defective. CNT powder determined to be defective may be re-evaluated after reprocessing, or may be provided as a lower-grade product. [Example]

[0148] The present disclosure will be explained in more detail below with reference to examples. The present disclosure is not limited to the following examples as long as it does not deviate from the gist of the disclosure. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." The blending amounts in the tables are in parts by mass, and all amounts other than the solvent are calculated as non-volatile content. Blank spaces in the tables indicate that no blending has been performed.

[0149] <Preparing raw CNT> Several raw CNTs with the following properties and different production lots were prepared. Each raw CNT was evaluated for its dry particle size distribution, powder X-ray diffraction, and CNT cohesion strength. Bulk density: 0.030~0.070g / cm 3 Powder resistivity: 1.5×10 ―2 ~2.1×10 ―2 Ω cm Co content: 7000~15000ppm Al content: 3000~5000ppm Dry particle size distribution (D50): 60~425μm BET specific surface area: 165~300m 2 / g

[0150] [Dry particle size distribution measurement] A dry dispersion unit for dry powder dispersion (Malvern Panalytical Aero S standard type venturi) was connected to a dry particle size distribution measuring device (Malvern Panalytical Mastersizer 3000), and 0.1 to 1 g of a sample for dry particle size distribution analysis of the carbon material was placed into the dry dispersion unit, and measurements were taken under the following conditions from 0.1 μm to 5000 μm. The analysis model was a general-purpose, dry fine powder mode, and volume distribution analysis was performed. Other detailed measurement conditions are shown below. Particle type: Aspherical, Refractive index: 1.76, Absorption coefficient: 1, Density: 1g / cm 3 ,Background measurement time: 10 seconds, Vision measurement time: 300 seconds, Number of measurements: 6, Lower limit of scattering intensity: 2%, Upper limit of scattering intensity: 5%, Scattering intensity filtering enabled with timeout 10 seconds

[0151] In the examples, the dry particle size distribution (D10) of the carbon nanotubes was evaluated according to the following criteria. A: 40μm or more and 90μm or less B: More than 90μm and less than 100μm C: More than 100μm and less than 110μm D: More than 110μm

[0152] [Powder X-ray diffraction measurement] The CNTs were placed in the recess of a glass sample plate (outer diameter 5.0 cm x 3.5 cm, thickness 3 mm, sample area 2.0 cm x 2.0 cm, thickness 2 mm) and flattened using a glass slide. The CNT powder X-ray diffraction analysis sample was then placed in a fully automated multipurpose X-ray diffractometer (SmartLab, Rigaku Co., Ltd.) and scanned from 10° to 35° to check for the presence or absence of peaks. Sampling was performed every 0.01°, with a scan speed of 1° / min. The voltage was 40 kV, the current was 40 mA, and the X-ray source was CuKα radiation.

[0153] [Measurement of CNT cohesion] The measuring device was measured using an MCR302e (Anton Paar). 2.5 g of CNT powder was placed in a dedicated aluminum container (C-CC27 / D / Al), and compressed at a pressure of 12 kPa using a cylinder jig for compression. Then, it was replaced with a wing-shaped jig for measuring cohesive force, and while moving the jig at 125 μm / s and 0.1 rotation / min, it was penetrated into the measurement target, and the torque when shear was applied was measured, and the maximum peak was taken as the cohesive force. The temperature control device used was a C-PTD200, and the temperature was set to 25°C.

[0154] In the examples and comparative examples, the following materials were used · H-NBR: Therban(R) AT3404, manufactured by ARLANXEO, hydrogenated acrylonitrile-butadiene rubber · PVdF: solef5130, manufactured by Solvay, polyvinylidene fluoride · NCM: Celsid NMC (LiNi 0.6 Co 0.2 Mn 0.2 O2, manufactured by Nippon Chemical Industry · LFP: HED(trademark) LFP-400 (lithium iron phosphate, manufactured by BASF)

[0155] <Production of CNT Dispersion Liquid> The CNT dispersion liquid was produced using the above-mentioned evaluated raw material CNT powder by the following procedure.

[0156] (Example 1: CNT Dispersion Liquid 1) 93.97 parts by mass of N-methyl-2-pyrrolidone (NMP) was placed in a stainless steel jacketed tank and heated to 50°C. While stirring with a disper, 1 part by mass of a dispersant (H-NBR) and 0.03 part by mass of an additive (NaOH) were added, and then stirred for 1 hour to dissolve the dispersant (H-NBR) to prepare a dispersant solution.

[0157] The CNT dispersion was prepared as follows. While stirring the dispersant solution in the previously prepared stainless steel jacketed tank with a disper, 5 parts by mass of CNT (CNT1) was added. A square hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 8,000 rpm for 5 minutes. By repeating the above CNT addition and dispersion steps 5 times, the total amount of CNT (5 parts by mass) was added, and the whole became uniform. Batch dispersion was performed with a high-shear mixer until the dispersion particle size became 250 μm or less using a grind gauge with a maximum groove depth of 300 μm. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (Starburst Turbo HJP-17007, manufactured by Sugino Machine) through a pipe, and circulation-type dispersion treatment was performed. The dispersion treatment was performed using a single nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 MPa. Pass-type dispersion treatment was performed until the 50% particle size (D50) in the wet dispersion particle size of the dispersion liquid reached 1.0 μm or less, and CNT dispersion liquid 1 (CNT concentration 5% by mass) was obtained. When the wet particle size distribution of CNT dispersion liquid 1 was measured, D50 was 0.98 μm. The measurement of D50 was as follows. Also, the number of dispersion passes of the high-pressure homogenizer required until D50 of CNT dispersion liquid 1 reached the end point was 10 times.

[0158] Except for changing CNT1 to the CNT types shown in Table 2, CNT dispersions 2 to 7 were obtained in the same manner as CNT dispersion liquid 1.

[0159] <50% Particle Size (D50) of CNT Dispersion> The wet particle size of the CNT dispersion was evaluated by the 50% particle size (D50). The 50% particle size (D50) was measured using a particle size distribution measuring device (Partical LA-960V2, manufactured by HORIBA). The operating conditions for circulation / ultrasound were circulation speed: 3, ultrasound intensity: 7, ultrasound time: 1 minute, stirring speed: 1, and stirring mode: continuous. Also, during air bleeding, ultrasound operation was performed at an ultrasound intensity of 7 and an ultrasound time of 5 seconds. The refractive index of NMP was 1.468, the refractive index of water was 1.333, and the refractive index of the carbon material was 1.92. The measurement was performed after diluting the measurement sample so that the transmittance of the red laser diode was 60 to 80%, and the particle size standard was the volume standard.

[0160] [Evaluation of dispersibility] In the wet dispersion, the number of dispersion passes was counted from the start of dispersion with a high-pressure homogenizer until the 50% particle size (D50) of the wet dispersion particle size reached the end point. The number of dispersion passes was calculated assuming that the number of dispersion passes for CNT Dispersion 1 was 100%, and the dispersibility of the carbon nanotubes was evaluated according to the following criteria. A: 100% or less B: More than 100% and less than 115% C: More than 115% and less than 130% D: Over 130%

[0161] [Table 1]

[0162] [Table 2]

[0163] From the above, it can be seen that by using the evaluation method of the present disclosure to evaluate CNT powder based on its dry particle size distribution (D10) and whether it satisfies at least one of conditions (1) and (2), it is possible to prepare CNT powder suitable for the dispersibility of a CNT dispersion.

[0164] <Production of Resin Composition> 20 parts by mass of CNT dispersion 1 (1 part by mass of CNT) was placed in a stainless steel jacketed tank, and the temperature of the jacketed tank was heated to 55°C so that the liquid temperature measured with a thermocouple thermometer reached 55°C. 3 parts by mass of PVdF as a binder resin was added little by little while stirring with a disperser, taking care not to cause the PVdF to form aggregates. The temperature was adjusted to maintain a liquid temperature of 55°C as measured with a thermocouple thermometer, and the mixture was stirred with a disperser for 4 hours until the binder resin was completely dissolved, yielding resin composition 1. Resin compositions 2 to 7 were obtained in the same manner as resin composition 1, except that the CNT dispersion was changed to the one shown in Table 2.

[0165] <Production of composite slurry> Resin composition 1 was placed in a plastic container, and NCM was added as the positive electrode active material. The mixture was stirred at 2,000 rpm for 5 minutes using a centrifugal mixer (Thinky, Awatori Rentaro, ARE-310) to obtain positive electrode composite slurry 1. The nonvolatile content of the positive electrode composite slurry was 78 mass %, and the amounts of CNT, PVdF, and NCM were adjusted so that the total solid content of the positive electrode composite slurry was 100 mass parts, respectively. Cathode composite slurries 2 to 7 were obtained in the same manner as positive electrode composite slurry 1, except that the resin composition was changed to the one shown in Table 2.

[0166] <Secondary battery manufacturing> <Production of positive electrodes> The positive electrode composite slurry 1 was applied to a 20 μm thick aluminum foil using an applicator, and then heated and dried in an electric oven at 120±5°C for 25 minutes to prepare an electrode film. The electrode film was then rolled using a roll press (Thank Metal, 3 ton hydraulic roll press) to obtain a positive electrode 1. The weight per unit area of ​​the composite layer was 20 mg / cm. 2 The density of the composite layer after rolling is 3.2 g / cm 3 Positive electrodes 2 to 7 were obtained in the same manner as positive electrode 1, except that the positive electrode mixture slurry was changed to the positive electrode mixture slurry shown in Table 2.

[0167] <Production of standard anodes> A 150 ml plastic container was charged with 0.5 parts of acetylene black (Denka Black® HS-100, manufactured by Denka), 1 part of MAC500LC (carboxymethylcellulose sodium salt, Sunrose Special Type MAC500L, manufactured by Nippon Paper Industries Co., Ltd., 100% nonvolatile content), and 98.4 parts of water, and the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). 97 parts by mass of artificial graphite (CGB-20, manufactured by Nippon Graphite Industries Co., Ltd.) was then added as an active material, and the mixture was stirred at 2,000 rpm for 150 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Next, 3.1 parts of SBR (styrene butadiene rubber, TRD2001, non-volatile content 48%, manufactured by JSR) was added, and the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain a standard negative electrode composite slurry. The non-volatile content of the standard negative electrode composite slurry was set to 50% by mass.

[0168] The above standard negative electrode composite slurry was applied to a 20 μm thick copper foil current collector using an applicator, and then dried in an electric oven at 80°C ± 5°C for 25 minutes to obtain a coating weight per unit area of ​​the electrode of 10 mg / cm. 2 Further, a rolling treatment was carried out using a roll press (Thank Metal, 3-ton hydraulic roll press) to adjust the density of the composite layer to 1.6 g / cm. 3 A standard negative electrode was fabricated.

[0169] The fabricated positive electrodes (positive electrodes 1 to 7) and standard negative electrodes were punched out to 50 mm x 45 mm and 45 mm x 40 mm, respectively. The punched positive electrodes and standard negative electrodes, along with a separator (porous polypropylene film) inserted between them, were placed in an aluminum laminate bag and dried in an electric oven at 70 °C for 1 hour. Then, in a glove box filled with argon gas, 2 mL of electrolyte (a nonaqueous electrolyte prepared by mixing ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1, with 1 part by mass of vinylene carbonate added per 100 parts by mass as an additive, followed by dissolving LiPF6 at a concentration of 1 M) was poured into the bag. The aluminum laminate was then sealed to fabricate secondary batteries.

[0170] <Battery performance evaluation> <Rate characteristics of secondary batteries> The resulting secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge / discharge measurements using a charge / discharge device (Hokuto Denko, SM-8). The battery was charged at a constant current / constant voltage of 10 mA (0.2 C) with a charge cutoff voltage of 4.3 V (cutoff current: 1 mA (0.02 C)), followed by a constant current discharge at a discharge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.3 V (cutoff current: 1 mA 0.02 C). This procedure was repeated three times, followed by a constant current / constant voltage charge at a charge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.3 V (cutoff current: 1 mA 0.02 C). The battery was then discharged at constant currents of 0.2 C and 3 C until the discharge cutoff voltage reached 3.0 V, and the discharge capacity was calculated. The rate characteristics can be expressed as the ratio of the 0.2 C discharge capacity to the 3 C discharge capacity, as shown in Equation 1. (Formula 1) Rate characteristic = 3C discharge capacity / 3rd 0.2C discharge capacity × 100 (%) The higher the rate characteristic, the more desirable it is. [Evaluation criteria] A: 80% or more B: 60% or more but less than 80% C: 40% or more but less than 60% D: Less than 40%

[0171] (Cycle characteristics of secondary batteries) The resulting secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge / discharge measurements using a charge / discharge device (Hokuto Denko, SM-8). Constant-current, constant-voltage charging (cutoff current 2.5 mA (0.05 C)) was performed at a charge current of 25 mA (0.5 C) with a charge cutoff voltage of 4.3 V, followed by constant-current discharging at a discharge current of 25 mA (0.5 C) with a discharge cutoff voltage of 3 V. This procedure was repeated 200 times. The cycle characteristics can be expressed as the ratio of the 3rd 0.5 C discharge capacity to the 200th 0.5 C discharge capacity at 25°C, using the following formula: (Formula 2) Cycle characteristic = 200th 0.5C discharge capacity / 3rd 0.5C discharge capacity × 100 (%) Higher cycle characteristics are more desirable. [Evaluation criteria] A: 85% or more B: 80% or more but less than 85% C: 50% or more but less than 80% D: Less than 50%

[0172] From the above, it can be seen that by using the evaluation method of the present disclosure to evaluate CNT powder based on its dry particle size distribution (D10) and whether it satisfies at least one of conditions (1) and (2), and by using CNT powder that is evaluated as having good dispersibility, it is possible to provide a secondary battery with excellent battery performance.

Claims

1. Obtaining the dry particle size distribution (D10) of the carbon nanotube powder; A step of acquiring information on whether or not at least one of the following conditions (1) and (2) is satisfied: An evaluation method comprising a step of evaluating the dispersibility of carbon nanotube powder using the dry particle size distribution (D10) and the information. Condition (1): In powder X-ray diffraction of the carbon nanotube powder, the carbon nanotube powder has a first peak at a diffraction angle 2θ=8° to 12° and a second peak at a diffraction angle 2θ=24° to 27°. Condition (2) The cohesive strength of the carbon nanotube powder measured by a powder rheometer is 1 kPa or more and 10 kPa or less.

2. 2. The evaluation method according to claim 1, wherein the evaluation step evaluates the dispersibility of the carbon nanotube powder as being good when the dry particle size distribution (D10) is 110 μm or less and at least one of the conditions (1) and (2) is satisfied.

3. 2. The evaluation method according to claim 1, wherein the evaluation step evaluates the dispersibility of the carbon nanotube powder as being good when the dry particle size distribution (D10) is 40 μm or more and 110 μm or less and at least one of the conditions (1) and (2) is satisfied.

4. 2. The evaluation method according to claim 1, wherein the condition (2) is that the cohesive strength of the carbon nanotube powder measured with a powder rheometer is 5 kPa or more and 10 kPa or less.

5. A method for producing a carbon nanotube dispersion, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of claims 1 to 4, obtaining an evaluated carbon nanotube powder, and obtaining a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium.

6. A method for producing a carbon nanotube resin composition, comprising: evaluating a carbon nanotube powder according to the evaluation method of any one of claims 1 to 4, providing an evaluated carbon nanotube powder, and obtaining a resin composition containing the evaluated carbon nanotube powder, a dispersion medium, and a binder resin.

7. A method for producing a composite slurry, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of claims 1 to 4, providing an evaluated carbon nanotube powder, and obtaining a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material.

8. A method for manufacturing an electrode film, comprising: evaluating a carbon nanotube powder according to the evaluation method described in any one of claims 1 to 4, providing an evaluated carbon nanotube powder, and obtaining an electrode film using a composite slurry containing the evaluated carbon nanotube powder, a dispersion medium, a binder resin, and an electrode active material.

9. A method for producing a secondary battery, comprising obtaining an electrode film for at least one of a positive electrode and a negative electrode according to the method for producing an electrode film according to claim 8.

10. A system for producing a carbon nanotube dispersion, comprising: An evaluation device that evaluates a carbon nanotube powder according to the evaluation method of any one of claims 1 to 4 and obtains an evaluated carbon nanotube powder; and A carbon nanotube dispersion production system including a production apparatus for producing a carbon nanotube dispersion containing the evaluated carbon nanotube powder and a dispersion medium.

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