Method for evaluating carbon material dispersion liquid, method for manufacturing carbon material dispersion liquid, and manufacturing system

Abstract

To provide an evaluation method for accurately evaluating the degree of mixing of a foreign substance in a carbon material dispersion liquid.SOLUTION: A method for evaluating a carbon material dispersion liquid includes the steps of: filtering the carbon material dispersion liquid with a filter; irradiating the filter after the filtering with X-rays to obtain the X-ray transmission distribution; detecting metal particles on the basis of X-ray transmissivity of a substance obtained from the X-ray transmission distribution; and evaluating the degree of mixing of a foreign substance in the carbon material dispersion liquid on the basis of information on the detected metal particles.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for evaluating a carbon material dispersion liquid, a method for manufacturing a carbon material dispersion liquid, and a manufacturing system.

Background Art

[0002] Carbon material dispersion liquids are in demand in a wide range of technical fields and are mixed with other materials and provided as products such as molded articles, coating films, and compositions. As a raw material for various products, it is desirable that no foreign substances such as metals are mixed in addition to the carbon material. Since the carbon material exhibits black color and the carbon material itself may contain metals, it is difficult to extract and remove foreign substances such as metal particles from the carbon material dispersion liquid.

[0003] Some methods for removing metal foreign substances in the manufacturing process of carbon material dispersion liquids are known. On the other hand, when the carbon material adheres to metal foreign substances, there is a problem that it is difficult to remove the metal foreign substances separately from the carbon material by magnetic separation, washing, etc.

[0004] Patent Document 1 (Japanese Patent Application Laid-Open No. 2022-046307) discloses a method using a primary filter and a secondary filter as a technique for efficiently removing foreign substances, particularly non-magnetic foreign substances, mixed in a carbonaceous material dispersion. Patent Document 2 (Japanese Patent Application Laid-Open No. 2022-042691) discloses a conductive material dispersion containing carbon fibers and capable of achieving both good dispersibility and stability. Among them, as a method for removing metal foreign substances from the conductive material dispersion, a method of removing by magnetic force and a method using a filtration filter are disclosed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Since it is difficult to completely avoid the mixing of raw materials and foreign substances from the manufacturing process into the carbon material dispersion liquid, there is a technique for evaluating the degree of foreign substance mixing in the carbon material dispersion liquid and using it for determining good products, grading, etc. As an evaluation method, magnetic separation is used to separate metal foreign substances from the carbon material dispersion liquid, followed by extraction, washing, filtration, and drying. This process is repeated, and then the metal foreign substances are identified visually or using an optical microscope to confirm the foreign substances in the carbon material dispersion liquid. However, this evaluation method has a complicated procedure and requires time, so a new procedure is expected.

[0007] One object of the present disclosure is to provide an evaluation method for accurately evaluating the degree of foreign substance mixing in a carbon material dispersion liquid. Another object is to provide a manufacturing method and a manufacturing system for a carbon material dispersion liquid using this evaluation method.

Means for Solving the Problems

[0008] Some embodiments of the present disclosure are as follows. However, the present invention is not limited to the following and may include various embodiments. [1] A method for evaluating a carbon material dispersion liquid, comprising a step of filtering the carbon material dispersion liquid with a filter, a step of irradiating the filter after filtration with X-rays to obtain an X-ray transmission distribution, a step of detecting metal particles based on the X-ray transmissibility of the substance obtained from the X-ray transmission distribution, and a step of evaluating the degree of foreign substance mixing in the carbon material dispersion liquid based on the information of the detected metal particles.

[0009] [2] The method according to [1], further comprising a step of measuring metal particle distribution information including at least one of the number and size of the detected metal particles, and the step of evaluating the degree of foreign substance mixing in the carbon material dispersion liquid is performed based on the measured metal particle distribution information. [3] The method according to [2], wherein the metal particle distribution information includes the number and size of the detected metal particles. [4] The step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid is the method described in [1], which evaluates the degree of foreign matter contamination in the carbon material dispersion liquid using the criterion that the number of metal particles with a size of 20 μm or more is 100 particles / kg or less. [5] The filter is the filter described in [4] that can filter out particles with a size of 20 μm or more. [6] The step of obtaining the X-ray transmission distribution is performed using an X-ray transmission device or an X-ray CT device, and is the method described in any one of [1] to [5].

[0010] [7] A method for manufacturing a carbon material dispersion liquid, including a step of providing a carbon material dispersion liquid containing a carbon material and a dispersion medium, and a step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid using the evaluation method described in any one of [1] to [6]. [8] The method described in [7] further includes a step of setting foreign matter removal conditions for the carbon material dispersion liquid based on the degree of foreign matter contamination in the carbon material dispersion liquid.

[0011] [9] A system for manufacturing a carbon material dispersion liquid, including a manufacturing device for manufacturing a carbon material dispersion liquid containing a carbon material and a dispersion medium, and an evaluation device for evaluating the degree of foreign matter contamination in the carbon material dispersion liquid using the evaluation method described in any one of [1] to [6] for an evaluation sample taken from the carbon material dispersion liquid housed in, supplied to, or discharged from the manufacturing device.

[10] The system described in [9] further includes a control device for setting foreign matter removal conditions for the carbon material dispersion liquid based on the degree of foreign matter contamination in the carbon material dispersion liquid. [Effects of the Invention]

[0012] According to one of the embodiments of the present disclosure, an evaluation method capable of accurately evaluating the degree of foreign matter contamination in a carbon material dispersion liquid can be provided. In addition, a manufacturing method and a manufacturing system for a carbon material dispersion liquid using this evaluation method can be provided. [Brief Description of the Drawings]

[0013]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0014] Hereinafter, using some embodiments of the present disclosure, a method for evaluating a carbon material dispersion, a method for manufacturing a carbon material dispersion using this evaluation method, and a manufacturing system will be described in detail. However, the present disclosure is not limited to these embodiments, and various modifications are possible.

[0015] "Method for Evaluating Carbon Material Dispersion" One of some embodiments is a method for evaluating a carbon material dispersion, which includes a step (1) of filtering the carbon material dispersion with a filter, a step (2) of irradiating the filter after filtration with X-rays to obtain an X-ray transmission distribution, a step (3) of detecting metal particles based on the X-ray transmissibility of the substance obtained from the X-ray transmission distribution, and a step (4) of evaluating the degree of foreign matter contamination in the carbon material dispersion based on the information of the detected metal particles.

[0016] According to this evaluation method, the degree of foreign matter contamination in the carbon material dispersion can be accurately evaluated.

[0017] Since the carbon material exhibits black color, it is difficult to distinguish other solid components from the carbon material. Therefore, conventionally, magnetic substances have been separated from the carbon material dispersion by magnetism, and whether the separated evaluation sample is a metal particle has been determined visually or by an optical microscope. In one embodiment, by filtering the carbon material dispersion with a filter, large particle diameter metal particles to be detected are left on the filter, and by obtaining the X-ray transmission distribution of the filter after filtration and detecting the metal particles, it is possible to evaluate whether large particle diameter metal particles are present in the carbon material dispersion. The X-ray transmission distribution can detect metal particles from the carbon material based on the X-ray transmissibility of the substance.

[0018] After small particle-sized metal particles are removed by a filter, large particle-sized metal particles that may be contained in the carbon material dispersion can be detected from the X-ray transmission distribution. Metals with a large atomic number are likely to absorb X-rays due to their high density, while carbon with an atomic number smaller than that of metals is less likely to absorb X-rays due to its low density. Therefore, in the X-ray transmission distribution, metal particles can be detected based on X-ray transmissibility. At this time, metal particles and carbon materials are deposited on the filter, but metal particles can be detected from the carbon materials based on X-ray transmissibility.

[0019] Furthermore, the degree of foreign matter contamination in the carbon material dispersion can be evaluated in more detail based on the number, size, etc. of the metal particles remaining on the filter. For example, from the particle size distribution of the metal particles remaining on the filter, if the proportion of the number of large particle-sized metal particles in the carbon material dispersion is large, it can be used for an evaluation of being defective.

[0020] In the present disclosure, large particle-sized metal particles are appropriately set according to the purpose of evaluation. For example, the pore diameter of the filter is appropriately set according to the purpose of evaluation so that large particle-sized metal particles can be filtered out from small particle-sized metal particles.

[0021] Hereinafter, step (1) of filtering the carbon material dispersion with a filter will be described. The carbon material dispersion is an object to be evaluated, contains a carbon material and a dispersion medium, and further has the potential to contain foreign matters, specifically, the potential to contain metal particles as foreign matters. The carbon material dispersion may further contain a dispersant from the viewpoint of the dispersibility of the carbon material.

[0022] The carbon material dispersion may contain, as the carbon material, fibrous carbon materials such as carbon nanotubes, carbon nanofibers, and carbon fibers; graphite, carbon black, etc. alone or in combination of two or more.

[0023] The dispersion medium of the carbon material dispersion is not limited as long as it can disperse the carbon material, and may be water or an organic solvent. The carbon material dispersion may be an aqueous dispersion or a non-aqueous dispersion. The carbon material dispersion may have a water content of 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, and may be a non-aqueous dispersion substantially free of water.

[0024] The carbon material dispersion may contain, as the organic solvent, an amide-based organic solvent such as N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam; an alcohol such as ethanol, propanol, butanol, methyl cellosolve, butyl cellosolve, propylene glycol monomethyl ether, etc., alone or in combination of two or more.

[0025] The dispersant is not particularly limited as long as it can stabilize the dispersion of the carbon material in the carbon material dispersion, and a surfactant, a resin-type dispersant, etc. can be used.

[0026] Surfactants are mainly classified into anionic, cationic, nonionic and amphoteric. A suitable type of dispersant can be used in a suitable blending amount according to the properties required for the dispersion of the carbon material.

[0027] When selecting an anionic surfactant, its type is not particularly limited. Specifically, fatty acid salts, polysulfonate salts, polycarboxylate salts, alkyl sulfate esters, alkylaryl sulfonate salts, alkylnaphthalene sulfonate salts, dialkyl sulfonate salts, dialkyl sulfosuccinate salts, alkyl phosphate salts, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkylaryl ether sulfates, naphthalene sulfonic acid formalin condensates, polyoxyethylene alkyl phosphate sulfonate salts, glycerol borate fatty acid esters, and polyoxyethylene glycerol fatty acid esters can be mentioned, but it is not limited thereto. Further specifically, sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium polyoxyethylene lauryl ether sulfate, polyoxyethylene nonylphenyl ether sulfate ester salt, and sodium salt of β-naphthalene sulfonic acid formalin condensate can be mentioned, but it is not limited thereto.

[0028] In addition, as cationic surfactants, there are alkylamine salts and quaternary ammonium salts. Specifically, stearylamine acetate, trimethyl coconut ammonium chloride, trimethyl tallow ammonium chloride, dimethyldioleyl ammonium chloride, methyloleyl diethanol chloride, tetramethylammonium chloride, lauryl pyridinium chloride, lauryl pyridinium bromide, lauryl pyridinium disulfate, cetyl pyridinium bromide, 4-alkyl mercaptopyridine, poly(vinylpyridine)-dodecyl bromide, and dodecylbenzyltriethylammonium chloride can be mentioned, but it is not limited thereto. Also, as amphoteric surfactants, aminocarboxylate salts can be mentioned, but it is not limited thereto.

[0029] In addition, examples of nonionic surfactants include, but are not limited to, polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, polyoxyethylene phenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and alkyl allyl ethers. Specifically, polyoxyethylene lauryl ether, sorbitan fatty acid esters, and polyoxyethylene octyl phenyl ether are included, but are not limited thereto.

[0030] The selected surfactant is not limited to a single surfactant. Therefore, it is also possible to use a combination of two or more surfactants. For example, a combination of an anionic surfactant and a nonionic surfactant, or a combination of a cationic surfactant and a nonionic surfactant can be used. The blending amount at that time is preferably a suitable blending amount for each surfactant component. As the combination, a combination of an anionic surfactant and a nonionic surfactant is preferred. The anionic surfactant is preferably a polycarboxylate. The nonionic surfactant is preferably polyoxyethylene phenyl ether.

[0031] Specific examples of the resin type dispersant include fluororesins, cellulose derivatives (such as cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethyl hydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, polyacrylonitrile polymers, and the like. Particularly, fluororesins, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers are preferred.

[0032] The carbon material dispersion may contain other optional components. Examples of other optional components include a dispersant, a binder resin, a wetting agent, a pH adjuster, a wetting and penetrating agent, a leveling agent, and the like.

[0033] The carbon material dispersion preferably has a solid content of 0.5 to 30% by mass, 1 to 25% by mass, or 2 to 20% by mass. When filtering with a filter, the carbon material dispersion may be diluted. For dilution, the solvent contained in the carbon material dispersion may be used.

[0034] The carbon material dispersion may have an average particle diameter of 0.3 to 3 μm, 0.5 to 2.5 μm, or 0.6 to 2 μm. Here, the average particle diameter is the 50% integrated value (D50) based on volume by the laser diffraction / scattering method. In the present disclosure, the average particle diameter of the carbon material dispersion may be measured using the carbon material dispersion as a sample in that state, or alternatively, a sample obtained by diluting the carbon material dispersion with a diluting solvent according to the measuring apparatus may be used to measure the particle diameter. As the diluting solvent, the dispersion medium contained in the carbon material dispersion may be used.

[0035] The filter varies depending on the application, but for example, it may be a filter capable of filtering particles having a size of 100 μm or more, 50 μm or more, or 20 μm or more, further 15 μm or more, or 10 μm or more. The filter preferably has a pore diameter of 100 μm or more, 50 μm or more, or 20 μm or more, further 15 μm or more, or 10 μm or more. For example, the pore diameter of the filter may be 100 μm to 1,000 μm, 50 μm to 500 μm, or 20 μm to 200 μm. As the filter, a surface filter (a filter that mainly captures particulate matter in a fluid on the filter surface) can be used, and a membrane filter or the like can be used. Note that the material of the filter is preferably X-ray permeable for detecting metal particles, and may be, for example, a resin material, a fiber material, or the like.

[0036] The metal particles that can be contained in the carbon material dispersion include, for example, those derived from the raw materials of the carbon material, those derived from other raw materials other than the carbon material, those derived from the manufacturing process of the carbon material dispersion, and the like. Among them, in the manufacturing process of the carbon material dispersion, a storage device, a dispersion device, piping, etc. are used. Therefore, there is a possibility that metal particles are mixed into the carbon material dispersion as foreign substances such as wear powder from these metal members. Against such a background, it is preferable that metal particles made of a metal having a specific gravity greater than that of Ti can be detected from the X-ray transmission distribution. For example, the metals constituting the metal particles include iron, copper, cobalt, nickel, chromium, zinc, molybdenum, etc., alloys containing these, and further metal compounds containing these. Examples of the metal compounds include metal oxides, metal nitrides, metal sulfides, etc. containing these metals. For example, low-magnetic stainless steel (SUS) is difficult to be removed by magnetic separation treatment and may be mixed into the product. However, in this evaluation method, metal particles can be detected according to the size of the metal particles regardless of the strength of magnetism, and it is also useful for detecting stainless steel (SUS).

[0037] The carbon material dispersion may contain metal particles of various sizes from small particle diameters to large particle diameters. Among them, large particle diameter metal particles may be exposed from the film surface and cause a decrease in film-forming properties when forming a thin film using the carbon material dispersion, and further may come into contact with the adjacent metal layer and cause a short circuit. For example, in a secondary battery, although a separator is interposed between the electrodes, when manufacturing an electrode body using the carbon material dispersion, metal particles may protrude from the electrode body and break through the separator, causing a short circuit (internal short circuit) with the other electrode body.

[0038] As such a secondary battery, it is preferable that the electrode body does not contain metal particles with a large particle diameter. For example, in the carbon material dispersion for the electrode body, the maximum particle diameter of the metal particles that can be contained is preferably less than 100 μm, more preferably less than 50 μm, and even more preferably less than 20 μm. From another index, in the carbon material dispersion for the electrode body, the metal particles that can be contained are preferably in a range where they can be filtered through a filter capable of filtering particles of 100 μm or more, 50 μm or more, or 20 μm or more. Therefore, the evaluation method according to one embodiment is useful for the carbon material dispersion for secondary batteries.

[0039] Hereinafter, the process (2) of irradiating the filter after filtration with X-rays to obtain an X-ray transmission distribution will be described. On the filter after filtration, the filtered large-particle-diameter particles are retained. Examples of the large-particle-diameter particles include aggregates of carbon materials, carbon material particles with a large particle diameter, metal particles with a large particle diameter, resin particles with a large particle diameter, and the like. These may be mixed in from the raw materials and the manufacturing process. By irradiating the filter after filtration with X-rays, an X-ray transmission distribution can be obtained.

[0040] The X-ray irradiation can be performed using an X-ray transmission device or an X-ray CT device. The X-ray irradiation is preferably performed after drying the filter after filtration in order to exclude the influence of moisture. The dried filter may be directly subjected to X-ray irradiation, or the filter may be cut, folded, etc. into a shape suitable for X-ray irradiation.

[0041] The X-ray transmission device can obtain an X-ray transmission image that two-dimensionally observes the filter after filtration from one direction. Since the measurement time for one field of view is short and the measurement field of view is wide, the pretreatment process before measurement is simpler, and observation can be performed without concern about foreign matter contamination from the pretreatment process. Also, since the data volume is small, the load for data storage is small.

[0042] The X-ray CT device can observe three-dimensionally (stereoscopically) by capturing X-ray transmission images of the filter after filtration from multiple directions, so the three-dimensional shape of the metal particles can be captured.

[0043] As measurement conditions for the X-ray transmission distribution, for example, it can be carried out under conditions such as a tube voltage of 60 kV and a tube current of 40 μA using an X-ray transmission device "Cheetah EVO" (COMET TECHNOLOGY JAPAN CO., LTD.). It can also be carried out under conditions such as a tube voltage of 80 kV and a tube current of 50 μA using an X-ray CT device "CT / CT Lab HX100" (RIGAKU CORPORATION). Note that the X-ray transmission distribution can be appropriately set to such an extent that metal particles of a desired size can be properly detected in the measurement object.

[0044] Hereinafter, the process (3) of detecting metal particles based on the X-ray transmissibility of the substance obtained from the X-ray transmission distribution will be described.

[0045] From the obtained X-ray transmission distribution, substances with high X-ray transmissibility and substances with low X-ray transmissibility can be discriminated based on the difference in X-ray transmissibility. In the X-ray transmission distribution, particulate regions with low X-ray transmissibility can be detected as metal particles. Also, as described above, particulate regions with high X-ray transmissibility can be mainly judged as carbonaceous materials. Also, those with high X-ray transmissibility such as resin particles can be distinguished and judged from metal particles. Further, those in which regions with low X-ray transmissibility are observed to be dispersed are those in which small particle-sized metal particles are attached to and dispersed in carbonaceous materials or the like, and can be distinguished and judged from large particle-sized metal particles.

[0046] As a method of detecting metal particles based on X-ray transmissibility from the X-ray transmission distribution, a method of discriminating whether it is a metal particle from the shading of the image of the X-ray transmission distribution can be mentioned. Specifically, it can be carried out using Winroof (MITANI SHOKAI CO., LTD.) or the like as image analysis software.

[0047] Hereinafter, the process (4) of evaluating the degree of foreign matter contamination in the carbonaceous material dispersion based on the information of the detected metal particles will be described. Examples of the information of the detected metal particles include the presence or absence of metal particles, the number of metal particles, the size of metal particles, the shape of metal particles, etc. The information of the metal particles may be used alone or in combination of two or more.

[0048] Since large particle-sized metal particles can be retained on the filter after filtration, the presence or absence of foreign matter contamination in the carbon material dispersion can be evaluated from the presence or absence of the detected metal particles. Further, by measuring the number or size of the metal particles retained on the filter, the degree of foreign matter contamination in the carbon material dispersion can be evaluated in more detail from this result. Further, by measuring the number and size of the metal particles retained on the filter, the degree of foreign matter contamination in the carbon material dispersion can be evaluated in more detail using the particle size distribution.

[0049] The evaluation method may further include a step of measuring metal particle distribution information including at least one of the number and size of the detected metal particles, and the step of evaluating the degree of foreign matter contamination in the carbon material dispersion may be performed based on the measured metal particle distribution information. The metal particle distribution information may include the number and size of the detected metal particles. The particle size distribution of the metal particles can be obtained from the information on the number and size.

[0050] As a method for measuring the number of metal particles from the X-ray transmission distribution, for example, a method may be used in which a threshold value is set in the image shading, and a particle-shaped region having an image density equal to or higher than the threshold value is counted as a metal particle. Here, the threshold value is set to the image density corresponding to the metal to be detected. This method may be performed by image analysis software.

[0051] As a method for measuring the size of metal particles from the X-ray transmission distribution, for example, a method may be used in which a threshold value is set in the image shading, and the size of a particle-shaped region having an image density equal to or higher than the threshold value is measured. Here, the threshold value is set to the image density corresponding to the metal to be detected. The size measurement may be the major axis, minor axis, equivalent circle diameter, area, etc. of the particle shape. This method may be performed by image analysis software.

[0052] In addition, as a method for obtaining the average value of the sizes of a plurality of metal particles in a visual field, it may be the arithmetic average value of the major axis, minor axis, equivalent circle diameter, area, etc. Further, as a method for obtaining the particle size distribution of a plurality of metal particles in a visual field, it may be based on the number of the major axis, minor axis, equivalent circle diameter, area, etc., or may be based on the area or volume. It may be appropriately set according to the intended evaluation.

[0053] As an example of a method using this evaluation method, a predetermined amount of a carbon material dispersion is filtered through a filter, and by obtaining the X-ray transmission distribution and metal particle distribution information of the entire image of the filter after filtration, it can be used for quantitative analysis of metal particles that may be contained in the predetermined amount of the carbon material dispersion. For example, information on the number of metal particles (pieces / Kg) having a predetermined size or more per unit mass of the carbon material dispersion can be obtained. This information is useful for evaluating product defects. For example, the degree of foreign matter contamination of the carbon material dispersion can be evaluated using a criterion that the number of metal particles having a size of 20 μm or more is 100 pieces / Kg or less. This criterion is more preferably 80 pieces / Kg or less, 60 pieces / Kg or less, 40 pieces / Kg or less, 20 pieces / Kg or less, or 10 pieces / Kg or less.

[0054] This evaluation method can be used for evaluating samples of carbon material dispersions and evaluating products. This evaluation method can be used for total inspection of products. For example, the total amount of the carbon material dispersion may be supplied to a filter and evaluated using the filter after filtration. Further, this evaluation method can be used for sampling inspection of products. For example, during the manufacturing process, during product inspection, etc., an evaluation sample may be taken from the carbon material dispersion for evaluation. During the manufacturing process, this evaluation method may be used at one location or two or more locations before, after, or during the supply of the carbon material dispersion to a disperser. Thereby, the presence or absence or information of metal particles that may be mixed in from the disperser can be obtained. Also, when using the carbon material dispersion as a raw material, this evaluation method can be used for total inspection or sampling inspection during inspection of the raw material, during use, etc. Since this evaluation method uses filtration of a filter, non-destructive inspection of the carbon material dispersion is possible, it is applicable to various stages, and repeated evaluation is possible at a plurality of stages.

[0055] "Method for Producing Carbon Material Dispersion Liquid" One of several embodiments is a method for producing a carbon material dispersion liquid, including a step of providing a carbon material dispersion liquid containing a carbon material and a dispersion medium, and a step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid using the evaluation method according to the above-described embodiments. A method for producing a carbon material dispersion liquid can be provided.

[0056] According to this method for producing a carbon material dispersion liquid, the degree of foreign matter contamination in the carbon material dispersion liquid can be accurately evaluated, and a carbon material dispersion liquid can be provided.

[0057] The carbon material dispersion liquid contains a carbon material and a dispersion medium, and may optionally further contain other components such as a dispersant. Details are as described above. As a method for providing a carbon material dispersion liquid, a method of mixing the components together or separately and performing a dispersion treatment may be used.

[0058] Examples of the dispersion treatment include methods of treating with a dispersion device such as a dry and / or wet method. As the dispersion device, a disperser commonly used for general pigment dispersion or the like can be used. For example, mixers such as a disper, a homomixer, a planetary mixer, homogenizers (Advanced Digital Sonifer (registered trademark) manufactured by BRANSON, MODEL 450DA, "Creamix" manufactured by M. TECHNIC, "Filmix" manufactured by PRIMIX, "Abramix" manufactured by SILVERSON, etc.), paint conditioners (manufactured by Red Devil), colloid mills ("PUC colloid mill" manufactured by PUC, "Colloid mill MK" manufactured by IKA), cone mills ("Cone mill MKO" manufactured by IKA, etc.), ball mills, sand mills ("Dynomill" manufactured by Shinmaru Enterprises, etc.), attritors, pearl mills ("DCP mill" manufactured by Ehrlich, etc.), media type dispersers such as coball mills, high-pressure homogenizers ("Genius PY" manufactured by Genius, "Starburst" manufactured by Sugino Machine, "Nanomizer" manufactured by Nanomizer, etc.), media-less dispersers such as "Cream SS-5" manufactured by M. TECHNIC, "MICROS" manufactured by Nara Machinery, and other roll mills.

[0059] For the obtained carbon material dispersion liquid, the degree of foreign matter contamination is evaluated using the evaluation method according to the above-described embodiment. The evaluation sample may be a part taken out from the carbon material dispersion liquid. For example, the carbon material dispersion liquid may be taken out from at least one of a dispersion device, a supply pipe for supplying raw materials to the dispersion device, and a discharge pipe for discharging the carbon material dispersion liquid from the dispersion device and used for evaluation. For the taken-out evaluation sample, the ratio of metal particles in the carbon material dispersion liquid can be estimated by converting the ratio of metal particles to the evaluation sample.

[0060] Based on the evaluation result obtained by this evaluation method, in the manufacturing method of the carbon material dispersion liquid, it is possible to judge good products and defective products. This evaluation result can also be reflected in the manufacturing conditions of the carbon material dispersion liquid. For example, when judged as a defective product, the carbon material dispersion liquid may be recirculated to the foreign matter removal process. Thus, it may further include a step of setting the foreign matter removal conditions of the carbon material dispersion liquid based on the degree of foreign matter contamination of the carbon material dispersion liquid.

[0061] Also, based on the evaluation result obtained by this evaluation method, it is possible to provide a product with an evaluation label attached to the carbon material dispersion liquid.

[0062] "Manufacturing System of Carbon Material Dispersion Liquid" One of several embodiments is a system for manufacturing a carbon material dispersion liquid, including a manufacturing device for manufacturing a carbon material dispersion liquid containing a carbon material and a dispersion medium, and an evaluation device for evaluating the degree of foreign matter contamination of the carbon material dispersion liquid using the evaluation method according to the above-described evaluation method for an evaluation sample taken out from the carbon material dispersion liquid housed in, supplied to, or discharged from the manufacturing device. A manufacturing system of the carbon material dispersion liquid can be provided.

[0063] According to this manufacturing system of the carbon material dispersion liquid, the degree of foreign matter contamination of the carbon material dispersion liquid can be accurately evaluated and the carbon material dispersion liquid can be provided.

[0064] The following will be described using a schematic configuration diagram showing an example of the manufacturing system shown in FIG. 1. Note that the present invention is not limited to the examples shown in the drawings. In FIG. 1, the carbon material manufacturing system 100 includes a carbon material manufacturing apparatus 10 and an evaluation apparatus 20. The carbon material manufacturing apparatus 10 includes a storage apparatus 1 and a dispersion apparatus 2. The storage apparatus 1 stores a raw material mixture or a pre-dispersion liquid of the carbon material dispersion liquid. The dispersion apparatus 2 performs dispersion processing on these. The carbon material manufacturing apparatus 10 includes a supply pipe 3 that supplies raw materials of the carbon material dispersion liquid from the storage apparatus 1 to the dispersion apparatus 2, and a discharge pipe 4 that discharges the carbon material dispersion liquid from the dispersion apparatus 2 and supplies it to the next process.

[0065] The evaluation apparatus 20 is connected to the supply pipe 3, the dispersion apparatus 2, and the discharge pipe 4 respectively, and takes out an evaluation sample at each position. Using the obtained evaluation sample, evaluation is performed according to the above-described procedure, and the degree of foreign matter contamination in the carbon material dispersion liquid is evaluated.

[0066] In another embodiment, the evaluation apparatus 20 may be connected to any one or two of the supply pipe 3, the dispersion apparatus 2, and the discharge pipe 4. In still another embodiment, when the evaluation apparatus 20 is connected to two or more of the supply pipe 3, the dispersion apparatus 2, and the discharge pipe 4, the main body of the evaluation apparatus 20 may be one, and the connection positions may be two or more. In still another embodiment, the evaluation apparatus 20 may be connected at two or more different positions within one apparatus such as the dispersion apparatus 2, and evaluation samples may be taken from two or more locations.

[0067] The manufacturing system 100 may further include a control apparatus 50. The control apparatus 50 can set foreign matter removal conditions for the carbon material dispersion liquid based on the degree of foreign matter contamination in the carbon material dispersion liquid. For example, when the control apparatus 50 determines that the ratio of metal particles that may be contained in the carbon material dispersion liquid is defective based on the evaluation result information from the evaluation apparatus 20, the control apparatus 50 can perform control such as recirculating the carbon material dispersion liquid to a foreign matter removal apparatus (not shown).

[0068] Although not shown, as another embodiment, a total inspection of the carbon material dispersion is performed. For example, a filter may be attached in at least one of the flow paths of the supply pipe and the discharge pipe connected to the dispersion device, the dispersion process may be performed for a certain period, and then the filter may be taken out and used for evaluation. As yet another embodiment, a product inspection is performed while the product is discharged from the dispersion device and packaged. Also, an acceptance inspection of the carbon material dispersion can be performed at the receiving destination of the product. The present evaluation method can be used in these inspections.

[0069] "Carbon material dispersion for secondary battery" The evaluation method of the carbon material dispersion according to one embodiment can be used for the evaluation method of the carbon material dispersion for secondary batteries. Also, this evaluation method can be used for the manufacturing method of the carbon material dispersion for secondary batteries and the manufacturing system of the carbon material dispersion for secondary batteries. The carbon material dispersion for secondary batteries will be described below.

[0070] The carbon material dispersion for secondary batteries contains a carbon material and a solvent, and may further contain optional components such as a dispersant. As the carbon material for the secondary battery, fibrous carbon materials such as carbon nanotubes, carbon nanofibers, and carbon fibers; graphite, carbon black, etc. can be included alone or in combination of two or more. Among them, carbon nanotubes are materials that are used as conductive aids in the electrode body and are expected to improve battery performance. Since carbon nanotubes are in the form of fine particles, their dispersion stability is controlled, while the control of foreign matter contamination from raw materials and manufacturing processes is also important from the perspective of the performance of secondary batteries. The carbon nanotube dispersion will be described below.

[0071] Examples of carbon nanotubes include single-walled carbon nanotubes having a structure in which a planar graphene layer is wound into a cylindrical shape, multi-walled carbon nanotubes having a structure in which two or more layers of graphene are wound, etc., and these may be mixed. The side wall of the carbon nanotube may be a graphite structure or may not be a graphite structure. For example, a carbon nanotube having a side wall with an amorphous structure can also be used as the carbon material. Further, carbon materials other than carbon nanotubes, such as graphite and carbon black, may be further included in combination with the carbon nanotubes.

[0072] The solvent is not particularly limited, and for example, it may be one kind or a combination of two or more kinds selected from those described in the above carbon material dispersion. The dispersant is not particularly limited, and for example, it may be one kind or a combination of two or more kinds selected from those described in the above carbon material dispersion. From the viewpoint of dispersibility, resin-type dispersants are preferable as dispersants suitable for carbon nanotubes. Examples thereof include fluororesins, methylcellulose, ethylcellulose, carboxymethylcellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, polyacrylonitrile-based polymers, and the like.

[0073] Examples of the metal that can be contained in the carbon nanotube dispersion include metals contained in the carbon nanotube itself, metal particles that are substances different from the carbon nanotube, and the like. Examples of the type of metal include those described in the above carbon material dispersion. According to the evaluation method according to one embodiment, as described above, metal particles can be detected from the carbon nanotube in the carbon nanotube dispersion.

[0074] The solid content of the carbon nanotube dispersion is preferably 0.5 to 15% by mass, 1 to 12% by mass, or 2 to 10% by mass.

[0075] The average particle size of the carbon nanotube dispersion may be 0.3 to 3 μm, 0.5 to 2.5 μm, or 0.6 to 2.0 μm. Here, the average particle size is the 50% integrated value (D50) based on volume by the laser diffraction / scattering method. In the present disclosure, the average particle size of the carbon nanotube dispersion may be measured with the carbon nanotube dispersion used as a sample in that state, or alternatively, the particle size may be measured using a sample in which the carbon nanotube dispersion is diluted with a diluting solvent according to the measuring apparatus. As the diluting solvent, it is preferable to use the dispersion medium contained in the carbon nanotube dispersion.

[0076] Since it is difficult to detect metal particles from these carbon nanotubes within the ranges of these solid contents, average particle sizes, or combinations thereof, the evaluation method according to one embodiment is useful.

[0077] In the evaluation method of the carbon material dispersion for a secondary battery, it is preferable to evaluate the degree of foreign matter contamination in the carbon material dispersion using the criterion that the number of metal particles having a size of 100 μm or more, 50 μm or more, or 20 μm or more is 100 or less per Kg.

[0078] When using this criterion, when producing a film-shaped electrode body using a composite slurry containing the carbon material dispersion for a secondary battery, it is possible to prevent the incorporation of metal particles with a large particle size into the electrode body, maintain the smoothness of the surface of the electrode body, and further prevent the protrusion of metal particles from the surface of the electrode body. For example, since the protrusion of metal particles can cause a short circuit between electrodes, it is better to eliminate this possibility. From this perspective, in the evaluation method of the carbon material dispersion, it is more preferable to use the criterion that the number of metal particles having a size of 20 μm or more is 100 or less per Kg. Further, from the viewpoints of the safety and electrical performance of the secondary battery, in the evaluation method of the carbon material dispersion, it is even more preferable to use the criterion that the number of metal particles having a size of 20 μm or more is 80 or less per Kg, 60 or less per Kg, 40 or less per Kg, 20 or less per Kg, or 10 or less per Kg.

[0079] To evaluate this standard, the filter used in the evaluation method may be a filter capable of filtering particles with a size of 100 μm or more, 50 μm or more, or 20 μm or more. For details of the filter, those described for the carbon material dispersion liquid above can be cited.

[0080] Using the carbon material dispersion liquid, a resin composition, a composite material slurry, an electrode body, and a secondary battery can be manufactured. Preferably, carbon nanotubes may be used as the carbon material.

[0081] The resin composition includes a carbon material dispersion liquid and a binder. The binder is a resin for binding substances such as carbon materials. Examples of the binder may include carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, and derivatives thereof.

[0082] The composite material slurry includes a carbon material dispersion liquid, a binder, and an electrode active material. The electrode active material is a material that serves as the basis for the battery reaction. The electrode active material may be either a positive electrode active material or a negative electrode active material.

[0083] The electrode body may be a coating film of the above composite material slurry. The electrode body may be a dried coating film formed by coating the current collector with the composite material slurry. The thickness of the electrode body is generally 1 μm or more and 500 μm or less, preferably 5 μm or more and 300 μm or less, 10 μm or more and 100 μm or less, or 20 μm or more and 50 μm or less.

[0084] The secondary battery includes a negative electrode, a positive electrode, an electrolyte, and a separator, and at least one of the negative electrode and the positive electrode may be the above electrode body. The secondary battery may be a non-aqueous electrolyte secondary battery, and specifically may be a lithium ion secondary battery.

Examples

[0085] Examples of the present invention are given below, but the present invention is not limited to the following examples. Unless otherwise specified, "parts" represents "parts by mass" and "%" represents "% by mass".

[0086] <Preparation of Evaluation Sample A> The following model metal powder was added to and mixed with the carbon nanotube dispersion composition prepared by the following procedure to obtain Evaluation Sample A. The model metal powder was mixed so as to be 0.0002 parts by mass with respect to 100 parts by mass of the carbon nanotube dispersion composition.

[0087] "Preparation of Carbon Nanotube Dispersion Composition" Into a stainless steel container, 92 parts of N-methyl-2-pyrrolidone (NMP) and 8 parts of dispersant (A) were added, and using a disper, it was stirred under the condition of 80 °C. After the dispersant (A) was completely dissolved, it was passed through a nylon mesh with an opening size of 48 μm, and then an 8% solution of dispersant (A) was prepared through a high magnetic force mag filter (manufactured by Aisin, surface magnetic flux density 17000 gauss). Then, into the stainless steel container, 89.5 parts of N-methyl-2-pyrrolidone (NMP) and 7.5 parts of the 8% solution of dispersant (A) were added, and it was stirred with a disper until it became uniform. Then, 3 parts of carbon nanotubes (C) were taken and added while stirring with a disper. A fine emulsifier screen was attached to a high shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was carried out at a speed of 9000 rpm until the whole became uniform and the dispersion particle size became 200 μm or less as measured by a grind gauge. Then, a carbon nanotube preliminary dispersion (C) was prepared through a high magnetic force mag filter (manufactured by Aisin, surface magnetic flux density 17000 gauss). Further thereafter, the carbon nanotube preliminary dispersion was fed, and a circulating dispersion treatment with a residence time of 15 minutes (bead filling rate 80%, peripheral speed 12 m / s) was carried out using a bead mill (manufactured by Asazawa Fine Tech Co., Ltd., Mugunflow (registered trademark)) filled with zirconia beads having a diameter of 1.0 mmφ. Subsequently, the liquid to be dispersed was supplied to a high-pressure homogenizer (manufactured by Sugino Machine, Starburst Turbo), and a 15-pass dispersion treatment was carried out. The dispersion treatment was carried out using a single nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 Mpa. Then, the liquid to be dispersed was supplied to an electromagnet (manufactured by Daiho Magnetic Co., Ltd., EMF-100S, magnetic flux density 16000 gauss, spatial volume 1.7 L, diameter: 10 cm, thickness: 1.3 cm, equipped with 31 grid screens), and after a 3-pass treatment was carried out, it was passed through two depth filters (manufactured by 3M, PP non-woven fabric depth cartridge NT-T series, filtration accuracy 20 μm) installed in series to prepare a carbon nanotube dispersion composition.

[0088] The raw materials are as follows. Dispersant (A): Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN35052, Mooney viscosity 20, weight average molecular weight 110,000, alkylene structural unit 66% by mass, content of nitrile group-containing structural unit 34% by mass).

[0089] Carbon nanotube (C): Prepared by the following procedure. 1 kg of carbon nanotubes (manufactured by JEIO, JENOTUBE10B) was weighed into a 100 L glass container, 50 kg of 20% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Corporation) was added, and then it was sufficiently stirred using a stirrer under the condition of 25°C. Thereafter, it was sufficiently diluted with ion-exchanged water and vacuum filtration was performed using a membrane filter. After repeating the dilution and filtration operations, the carbon nanotubes were transferred to a PTFE bat and then dried at 80°C using an oven to obtain carbon nanotubes (C1). The average outer diameter of the carbon nanotubes (C) by transmission electron microscope observation was 10 nm, the BET specific surface area was 210 m 2 / g, and the carbon purity was 99.5% by mass.

[0090] "Model metal powder (iron powder)" KUAMET6B2 (trade name) Manufactured by Epson Atmix Corporation D50 = 20 - 25 μm (volume distribution)

[0091] <Preparation of target sample B> The above model metal powder (KUAMET6B2 (trade name)) was added and mixed into N-methyl-2-pyrrolidone (NMP) to obtain target sample B. "Composition" NMP 99.9998 parts by mass Model metal powder 0.0002 parts by mass Total amount 100 parts by mass

[0092] <Evaluation method> "Apparatus A" The X-ray transmission apparatus and measurement conditions are as follows. X-ray transmission apparatus: X-ray transmission apparatus "Cheetah EVO", Comet Technology Japan Co., Ltd. X-ray measurement conditions: tube voltage 60 kV, tube current 40 μA, field of view 6 mm × 6 mm.

[0093] "Device B" The X-ray CT device and measurement conditions are as follows. X-ray CT device: Industrial 3D micro X-ray "CT / CT Lab HX100", Rigaku Corporation. X-ray measurement conditions: tube voltage 80 kV, tube current 50 μA, field of view Φ5.5 mm × 3.7 mm.

[0094] "Device C": Optical microscope device: "DSX510" manufactured by OLYMPUS.

[0095] "Preparation of Samples for X-ray Measurement" 50 g of evaluation sample A was passed through a mesh filter (mesh size 5 μm), and the residue filtered by the filter was collected together with the filter. The collected filter was dried at 60 °C until there was no change in mass. Target sample B was prepared in the same manner as target sample A.

[0096] In X-ray transmission device A, the dried planar filter was used as the sample for X-ray measurement. In X-ray CT device B, the dried planar filter was folded, and a folded filter of Φ5 mm × 10 mm was used as the sample for X-ray measurement. In optical microscope device C, the dried planar filter was used as the sample for optical microscope measurement.

[0097] (1) Consideration of X-ray Inspection 1 Using evaluation sample A and target sample B respectively, the evaluation method was carried out, and the distribution of the evaluated sizes and numbers was obtained. The results are shown graphically in Figure 1. "Evaluation Method 1A" In evaluation method 1A, using X-ray transmission device A, the X-ray transmission distributions of evaluation sample A and target sample B were obtained. From the X-ray transmission distributions, metal particles were detected from carbon nanotube particles under the following conditions. Then, the sizes and numbers of the metal particles were measured.

[0098] The detection of metal particles from carbon nanotube particles is determined from the image density in the X-ray transmission distribution based on X-ray permeability. Specifically, since metal particles have low X-ray permeability, they are distinguished from other regions in the X-ray transmission distribution and observed as the shape of metal particles. The X-ray transmission distribution is measured continuously from the edge of the filter in a field of view with a resolution capable of measuring the target particles to obtain the X-ray transmission distribution of the entire filter. From this, the number of metal particles is calculated, and the frequency (%) of the number of metal particles with respect to all particles is determined. The particle diameter of the metal particles is determined as the major axis of the particles observed in the X-ray transmission distribution. From these results, a particle size distribution of the metal particles is created. Specifically, a particle size distribution is created using image analysis software: Winroof (Miyaya Trading Co., Ltd.). The results are shown in Fig. 1.

[0099] "Evaluation Method 1B" In Evaluation Method 1B, the X-ray transmission distributions of the evaluation sample A and the target sample B were obtained using the X-ray CT apparatus B. Metal particles were detected from the carbon nanotube particles in the X-ray transmission distribution. Then, the size and number of the metal particles were measured. The conditions are the same as those in the above Evaluation Method 1A.

[0100] From the obtained results, since the particle size distributions of the evaluation sample A and the target sample B are the same, it can be seen that metal particles can be detected from the carbon nanotube particles.

[0101] (2) Consideration of X-ray inspection 2 Two types of evaluation methods, X-ray inspection and inspection by optical microscope, were carried out to obtain the evaluated size and number distributions. The evaluation results of the X-ray inspection used the results of the evaluation sample A used in Evaluation Method 1A and Evaluation Method 1B. The results are shown graphically in Fig. 2. "Evaluation Method 2C" In Evaluation Method 2C, model metal powder was set on a glass substrate, and inspection by optical microscope was carried out. Targeting model metal powder of 20 μm or more, 616 particles were photographed, and the obtained photographed images were measured in length using image analysis software: Winroof (Miyaya Trading Co., Ltd.) to obtain the size and number distributions.

[0102] Regarding the obtained results, from the results of Evaluation Method 1A and Evaluation Method 2C, since the X-ray transmission device A and the optical microscope device C showed similar particle size distributions, it can be seen that the X-ray transmission device can perform length measurement similar to that of an optical microscope. From the results of Evaluation Method 1B and Evaluation Method 2C, it was observed that in the X-ray CT device B, the frequency of smaller particle diameters increased and the frequency of larger particle diameters decreased compared to the optical microscope device C. This is considered to be because the X-ray transmission device performs two-dimensional measurement, while the X-ray CT device performs three-dimensional measurement, thus distinguishing the overlap of metal powders. It is advisable to adopt a measurement device suitable for the evaluation purpose.

[0103] (4) Consideration of X-ray Inspection 3 Using Evaluation Sample A, the evaluation method was carried out, and the number of particles of 20 μm or more was calculated. The results are shown in Table 1. "Evaluation Method 3A" In Evaluation Method 3A, using the X-ray transmission device A, the X-ray transmission distribution of Evaluation Sample A was obtained. From the X-ray transmission distribution, metal particles were detected from carbon nanotube particles. Then, the number of metal particles of 20 μm or more was measured. Other conditions are the same as those in the above Evaluation Method 1A.

[0104] "Evaluation Method 3B" In Evaluation Method 3B, using the X-ray CT device B, the X-ray transmission distribution of the test Evaluation Sample A was obtained. From the X-ray transmission distribution, metal particles were detected from carbon nanotube particles. Then, the number of metal particles of 20 μm or more was measured. The conditions are the same as those in the above Evaluation Method 1A.

[0105] Evaluation Methods 3A and 3B were repeated three times (N = 3), and the average value and standard deviation were obtained. The error rate was obtained from the following formula. Error rate (%) = ((maximum value - minimum value) / median value) × 100

[0106]

Table 1

[0107] From the obtained results, both evaluation methods 3A and 3B had a small error rate and were suitable for practical use.

[0108] From the above experimental results, it can be seen that according to the evaluation method according to one embodiment, it is possible to evaluate the degree of contamination of metal particles that may be contained in the carbon material dispersion. For example, the degree of foreign matter contamination of the carbon material dispersion can be evaluated using a criterion that the number of metal particles having a size of 20 μm or more is 100 pieces / kg or less.

Description of Reference Numerals

[0109] 1 housing device, 2 dispersion device, 3 supply pipe, 4 discharge pipe, 10 carbon material production device, 20 evaluation device, 50 control device, 100 system

Claims

1. A method for evaluating a carbon material dispersion liquid, comprising: a step of filtering the carbon material dispersion liquid with a filter; a step of irradiating the filter after filtration with X-rays to obtain an X-ray transmission distribution; a step of detecting metal particles based on the X-ray transmissibility of the substance obtained from the X-ray transmission distribution; and a step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid based on the information of the detected metal particles.

2. The method according to claim 1, further comprising a step of measuring metal particle distribution information including at least one of the number and size of the detected metal particles, wherein the step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid is performed based on the measured metal particle distribution information.

3. The method according to claim 2, wherein the metal particle distribution information includes the number and size of the detected metal particles.

4. The method according to claim 1, wherein the step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid is performed using a criterion that the number of metal particles having a size of 20 μm or more is 100 particles / kg or less.

5. The method according to claim 4, wherein the filter is a filter capable of filtering out particles having a size of 20 μm or more.

6. The method according to claim 1, wherein the step of obtaining the X-ray transmission distribution is performed using an X-ray transmission device or an X-ray CT device.

7. A method for producing a carbon material dispersion liquid, comprising: a step of providing a carbon material dispersion liquid containing a carbon material and a dispersion medium; and a step of evaluating the degree of foreign matter contamination in the carbon material dispersion liquid using the evaluation method according to any one of claims 1 to 6.

8. The method according to claim 7, further comprising a step of setting foreign matter removal conditions for the carbon material dispersion liquid based on the degree of foreign matter contamination in the carbon material dispersion liquid.

9. A system for producing a carbon material dispersion liquid, comprising: a production device for producing a carbon material dispersion liquid containing a carbon material and a dispersion medium; and an evaluation device for evaluating the degree of foreign matter contamination in the carbon material dispersion liquid using the evaluation method according to any one of claims 1 to 6 for an evaluation sample taken from the carbon material dispersion liquid accommodated in, supplied to, or discharged from the production device.

10. The system according to claim 9, further comprising a control device for setting foreign matter removal conditions for the carbon material dispersion liquid based on the degree of foreign matter contamination in the carbon material dispersion liquid.

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