Method for producing carbon material granule

Abstract

To provide carbon material granules capable of reducing metallic impurities and excellent in dispersibility.SOLUTION: A carbon material granule contains carbon black with a particle size of 500 μm or less according to the method described in JIS K-6219-4, and carbon nanotubes that satisfy either one of condition 1 or condition 2 below, as well as a solvent-soluble polymer attached to the carbon black and the carbon nanotubes. (Condition 1) Carbon nanotubes that have undergone graphitization treatment and subsequently grinding treatment, where the particle size D50 of the carbon nanotubes is 100 μm or less. (Condition 2) Carbon nanotubes to which graphitization treatment has been applied, either in the form of carbon nanotube powder or carbon nanotube granules that have undergone grinding treatment.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon material granule and a method for producing the carbon material granule. [Background technology]

[0002] Lithium-ion secondary batteries (hereinafter sometimes referred to as Lib) are used in mobile phones, laptops, digital cameras, and automobiles (e.g., EVs, HEVs, and P-HEVs), and have made a significant contribution to improving productivity in various industries and the quality of life. Lib electrodes consist of a positive electrode, in which an electrode mixture consisting of a positive electrode active material containing lithium ions, a conductive additive, and an organic binder is fixed to a metal foil current collector, and a negative electrode, in which an electrode mixture consisting of a negative electrode material that can insert and remove lithium ions, a conductive additive, and an organic binder is fixed to the surface of a metal foil aggregate.

[0003] Typically, lithium transition metal composite oxides such as lithium cobalt oxide are used as positive electrode materials, but these have low electronic conductivity and do not provide sufficient battery performance when used alone. Therefore, carbon black (hereinafter sometimes referred to as CB), especially acetylene black or ketjen black, is added as a conductive additive to improve conductivity and reduce the internal resistance of the electrode. However, the Lib industry and its users are demanding even higher performance products, such as higher capacity, higher safety, and reduced cycle degradation due to repeated charging and discharging.

[0004] One of the measures to meet this demand is to blend carbon nanotubes (hereinafter referred to as CNTs) with conventional CB as a conductive additive, especially for the positive electrode, or to use CNTs alone.In addition, improvements are also being attempted, such as using a granulated product made by uniformly blending CB and CNTs and then adding a polymer.

[0005] There is a method for producing carbon material granules (see Patent Document 1) that includes the steps of dry-pulverizing and mixing CB granules and CNT granules so as to satisfy predetermined conditions to obtain a mixture, dissolving a specific solvent-soluble polymer in a solvent to prepare a binder solution, and adding a predetermined amount of the binder solution to the mixture while mixing and granulating to obtain carbon material granules.

[0006] When CNTs containing metal impurities are used as a conductive additive for the positive electrode, the positive electrode is in an oxidizing atmosphere, which can lead to the oxidation and leaching of transition metals such as iron or cobalt. Furthermore, when CNTs containing metal impurities are used for the negative electrode, metal deposition can occur. In the worst case scenario, the leached or deposited metals can rupture the separator membrane, resulting in an internal short circuit. Furthermore, even if these metal impurities are initially minute, they can grow into large deposits over time and rupture the separator. To create a safe LIB that is free of internal short circuits, localized heat generation, or degradation, it is important to minimize the metal impurities in the CNTs used as conductive additives and to use CNTs with excellent dispersibility. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 7126666 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a carbon material granule that can reduce metal impurities and has excellent dispersibility, and a method for producing the carbon material granule. [Means for solving the problem]

[0009] According to the present invention, there are provided the following carbon material granules and a method for producing the carbon material granules. [1] A composition comprising: carbon black having a particle size of 500 μm or less as measured by the method described in JIS K-6219-4; carbon nanotubes satisfying either the following condition 1 or the following condition 2; and a solvent-soluble polymer attached to the carbon black and the carbon nanotubes. Carbon material granules. (Condition 1) Carbon nanotubes that have been subjected to graphitization and then crushing, and the particle size of the carbon nanotubes is 50 is 100 μm or less. (Condition 2) The carbon nanotubes are either powdered carbon nanotubes or granular carbon nanotubes that have been pulverized and then graphitized. [2] In the carbon material granules according to [1], the solvent-soluble polymer is a water-soluble polymer, The solvent is water. Carbon material granules. [3] A method for producing the carbon material granules according to [1] or [2], a step of mixing carbon black having a particle size of 500 μm or less according to the method described in JIS K-6219-4 with carbon nanotubes satisfying either the condition 1 or the condition 2 to obtain a mixture; dissolving the solvent-soluble polymer in a solvent to prepare a binder solution; and a step of adding the binder solution little by little to the mixture, mixing the mixture, and granulating the mixture to obtain carbon material granules. A method for producing carbon material granules. [4] A method for producing the carbon material granules according to [1] or [2], a step of mixing carbon black having a particle size of 500 μm or less according to the method described in JIS K-6219-4 with carbon nanotubes satisfying either the condition 1 or the condition 2 to obtain a mixture; dissolving the solvent-soluble polymer in a solvent to prepare a binder solution; mixing the binder solution with the mixture to obtain a wet mixture; and granulating the wet mixture to obtain carbon material granules. A method for producing carbon material granules. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to provide a carbon material granule that can reduce metal impurities and has excellent dispersibility, and a method for producing the carbon material granule. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a conceptual diagram showing an example of an extrusion granulator used in the present embodiment. [Figure 2] 1 is a photograph showing the dispersibility of the resin compositions obtained in Example 1, Comparative Examples 1 and 2, and Reference Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. Hereinafter, the carbon material granules may also be simply referred to as "granules".

[0013] [Carbon material granules] First, the carbon material granules according to this embodiment will be described. The granulated material according to this embodiment contains CB having a particle size equal to or smaller than a specific particle size, CNTs that satisfy specific conditions, and a solvent-soluble polymer attached to the CB and the CNTs. The reason why the present embodiment can reduce metal impurities and obtain carbon material granules with excellent dispersibility is not entirely clear, but the present inventors speculate as follows.

[0014] First, the reason for the reduction of metal impurities is that the CNTs are graphitized. Graphitization is a high-temperature heat treatment, typically at 2000°C or higher, which removes catalyst-derived metal impurities (catalytic metals) from the CNTs. Alternative methods for removing catalyst-derived metal impurities from the CNTs include (i) immersing the CNTs in an acidic solution containing at least one of sulfuric acid, hydrochloric acid, and hydrofluoric acid to dissolve and remove the metal, or (ii) irradiating the CNTs with microwaves to convert the metal impurities into metal oxides that are inactive at the operating voltage of the secondary battery and do not dissolve in the electrolyte. Among these methods, acid treatment (i) leaves acid ions on the CNT surface even after heat treatment (below 600°C) after pickling, which can corrode the positive electrode material and potentially damage the CNTs and cause lattice defects. Furthermore, because the surface of the catalytic metal is covered with a graphite layer, treatment using an acid solution alone is not sufficient to completely dissolve and remove the catalytic metal because the acid solution is blocked by the graphite layer and does not reach the catalytic metal.Furthermore, the microwave irradiation method (ii) above has the problem of oxidizing the CNTs and reducing their electrical conductivity. Thus, graphitization of CNTs is an optimal method for removing catalyst-derived metal impurities (catalytic metals) that are present in the CNTs.

[0015] On the other hand, when CNTs are graphitized, there is a problem that the crystallization of the CNTs progresses, resulting in a decrease in dispersibility in resin. However, in this embodiment, the CNTs are pulverized before or after the graphitization process to produce a fine powder, or the CNTs that have already been made into a fine powder are graphitized. By converting the CNTs into a fine powder in this way, the dispersibility of the CNTs in resin is improved. Therefore, the inventors believe that the granulated material according to this embodiment can reduce metal impurities and has excellent dispersibility.

[0016] (carbon black) The CB used in this embodiment has a particle size of 500 μm or less according to the method described in JIS K-6219-4. CNT particle size distribution is measured according to JIS K-6219-4, "Method for determining the size distribution of granulated particles." The measuring device uses a classification method that uses stacked mesh sieves. Depending on the method of vibration, there are sonic vibration, low-tap, and electromagnetic types. Manufacturers of sonic vibration types include Hatsuratsu Co., Ltd. and Seishin Enterprise Co., Ltd. Manufacturers of low-tap sieves include the aforementioned two companies, as well as CMT Co., Ltd. and AS ONE Corporation. Manufacturers of electromagnetic shaking sieves include Tsutsui Rikagakuhin Co., Ltd. Taking the low-tap type manufactured by CMT Co., Ltd. as an example, the specific measurement method involves stacking four to six 200mm diameter mesh sieves on the low-tap. The most common combination of mesh types is 10 mesh (1000 μm opening), 30 mesh (500 μm), 60 mesh (250 μm), and 100 mesh (150 μm), but measurements may also be performed using 86 mesh (2000 μm) and 149 mesh (100 μm). To measure, a tray is attached to the bottom, 100 g of granulated product is placed on the top mesh sieve, the lid is then attached, and the sieve is shaken for 1 minute at 290 rpm, 28 mm amplitude, and 156 tpm. The granulated product clogging the top and mesh openings of each sieve is then scraped off, weighed, and the particle size distribution calculated. The value for the mesh with the smallest opening among all the meshes is the particle size of the pulverized product. The preferred particle size after pulverization, as measured by a rotary tap classification method, is preferably 10 μm to 500 μm, and more preferably 25 μm to 250 μm. If the particle size exceeds 500 μm, the number of agglomerates increases, resulting in poor dispersibility and poor uniform mixing of the CB and CNT. Furthermore, processing to reduce the particle size to less than 10 μm is not easy due to industrial-scale production, and even if it were possible, it would require a long processing time and be impractical. Furthermore, this process cuts the CNT fibers, which is undesirable because it reduces conductivity. Furthermore, when pulverizing CB and CNT after blending, it is preferable to make the particle size approximately the same as that of the CNT.

[0017] Examples of CB include those obtained by thermal decomposition methods such as the thermal method or the acetylene decomposition method, incomplete combustion methods such as the oil furnace method, and those obtained by heavy oil gasification processes such as the Texas process, the Fazer process, and the Shell process. These may be used alone or in combination of two or more. Specific examples include the #4000 and #5000 series manufactured by Tokai Carbon Co., Ltd., the #3000 series manufactured by Mitsubishi Chemical Corporation, FX, HS, Denka Black, etc. manufactured by Denka Co., Ltd., the Conductex series manufactured by Birla Carbon, the Vulcan series and LITX series manufactured by Cabot Corporation, the ENSACO series and SuperP-Li series manufactured by Imerys GC, and Printex L manufactured by Orion Engineer Carbons.

[0018] (carbon nanotubes) The CNTs used in this embodiment satisfy either the following condition 1 or the following condition 2. (Condition 1) Carbon nanotubes that have been subjected to graphitization and then crushing, and the particle size of the carbon nanotubes is 50 is 100 μm or less. (Condition 2) The carbon nanotubes are either powdered carbon nanotubes or granular carbon nanotubes that have been pulverized and then graphitized.

[0019] (Graphitization treatment) The graphitization treatment in this embodiment is a heat treatment at a temperature of 2000° C. or higher and 3000° C. or lower in a non-oxidizing atmosphere. More specifically, graphitization is performed by heating and holding CNTs at 2000°C or higher in an oxygen-free atmosphere (for example, in a nitrogen stream, a vacuum, or carbon powder (also known as packing powder, and usually made from coke or CB)). Since the presence of oxygen in the atmosphere reduces the yield of CNTs, it is desirable to keep the oxygen concentration in the atmosphere below 1% by volume. For large-scale graphitization, it is advantageous to use an Acheson-type electric furnace. On the other hand, for small-scale processing, a predetermined amount of CNTs can be filled into a graphite pipe, immersed in carbon powder, and then an alternating current can be passed through both ends of the pipe. Thus, in the graphitization process, CNTs are preferably heated to 2000°C to 3000°C, more preferably 2500°C to 2900°C, and particularly preferably 2700°C to 2900°C. Heating within this temperature range causes the CNT crystallites to rearrange and change shape, resulting in a slight decrease in diameter but increased crystallinity. Furthermore, the higher the treatment temperature, the more effectively the functional groups and metal impurities on the surface can be reduced. However, it is believed that the higher the treatment temperature, the stronger the entanglements of the primary and secondary aggregates present in the CNTs before graphitization become due to the heat treatment, resulting in poor dispersibility when blended with resins, etc.

[0020] (Crushing process) The pulverization process in this embodiment includes dry pulverization and wet pulverization, which can be used depending on the purpose. The reason why the dispersibility of graphitized CNTs decreases is believed to be that the complex and strong entanglements of the CNT fibers cannot be disentangled using a conventional dispersing machine or dispersion method. To disentangle these strong entanglements, it is preferable to use a powerful pulverization method. In this embodiment, it is preferable to use dry milling. In the case of dry milling, the mill used varies depending on the target particle size and particle size distribution. For example, when the goal is to achieve fine milling to a size of several tens of μm or less, a jet mill, pin mill, vibration ball mill, or planetary mill is used. Among mill manufacturers, examples of jet mill-type mill manufacturers include Seishin Enterprise Co., Ltd., Aisin Nano Technologies Co., Ltd., and Earth Technica Co., Ltd. Examples of pin mill manufacturers include Makino Sangyo Co., Ltd., Nishimura Machinery Works, Ltd., and Hosokawa Micron Corporation. Examples of impeller mill manufacturers include Seishin Enterprise Co., Ltd. and Earth Technica Co., Ltd.

[0021] (CNT particle size) The particle size D of CNTs that satisfies the above condition 1 50 The particle size D of the CNTs that satisfies the above condition 2 must be 100 μm or less. 50 is preferably 100 μm or less. The particle size distribution of CNTs was measured using the laser diffraction / scattering method specified in ISO 13320. The measurement was performed using a Laser Micronsizer LMS-3000 (Seishin Enterprise Co., Ltd.). The measurable range of this device is 0.01 to 3500 μm. The aqueous dispersion medium was prepared by adding 0.05 g of polyoxyethylene alkyl ether (Emulgen 705, product name, Kao Corporation) as a surfactant to 50 mL of pure water. For the measurement, 10 mg of CNTs were weighed into a 20 mL vial, 10 mL of aqueous dispersion medium was added, and the CNTs were dispersed for approximately 10 minutes using an ultrasonic disperser. The optical model of the measurement was set to a refractive index of 1.520 for CNTs and 1.333 for water. CNT optimum particle size D 50 is 10 μm or more and 100 μm or less, and preferably 10 μm or more and 50 μm or less. If it is larger than 50 μm, many agglomerates will be present, and dispersibility will be poor. Furthermore, processing to make it finer than 10 μm is also a processing to cut the CNT fibers, which is not preferable because it will deteriorate the conductivity.

[0022] (Raw material CNT) The fiber diameter of CNTs used as raw materials is 0.3 nm, which is the maximum diameter that can be produced using current technology, but it can also be thinner than 0.3 nm. As the fiber diameter increases beyond 50 nm, the electrical and mechanical properties tend to decrease, and as the diameter increases beyond 100 nm, the superiority over CB or carbon nanofibers tends to be lost. Furthermore, in this embodiment, from the viewpoint of allowing the CNTs to efficiently form a three-dimensional network, the fiber diameter of the CNTs is more preferably 3 nm or more and 50 nm or less, even more preferably 5 nm or more and 40 nm or less, and particularly preferably 10 nm or more and 30 nm or less. The fiber length of CNTs is related to their electrical conductivity, mechanical properties, and dispersibility. The fiber length of CNTs is preferably 0.1 μm or more and 2000 μm or less, and more preferably 1 μm or more and 1000 μm or less. As the fiber length decreases, it tends to be more difficult for electrical conductivity or mechanical properties to be exhibited. As the fiber length increases, the fibers become more entangled, resulting in more poorly dispersed agglomerates and more fiber breakage during kneading and dispersion, which is undesirable. The aspect ratio of CNTs is approximately 10 to 10,000. Furthermore, a structure in which hexagonal mesh graphite sheets are formed into a cylindrical shape is preferably used as the CNT. The CNTs may be either single-walled or multi-walled, and the structure can be selected depending on the final purpose. Furthermore, there are no limitations on the method for producing the CNTs. Examples of methods for producing CNTs include the pyrolysis method in which a carbon-containing gas is brought into contact with a catalyst, the arc discharge method in which an arc discharge is generated between carbon rods, the laser evaporation method in which a carbon target is irradiated with a laser, the CVD method in which a carbon source gas is reacted at high temperature in the presence of metal particles, and the HiPco method in which carbon monoxide is decomposed under high pressure. Furthermore, CNTs may be doped with metal atoms.

[0023] (CNT content) In the granulated product according to this embodiment, the amount of CNT is preferably 5% by mass or more and 40% by mass or less, and more preferably 10% by mass or more and 35% by mass or less, relative to 100% by mass of the total amount of CB and CNT. If the amount of CNTs is equal to or less than the upper limit, the dispersibility of CNTs can be further improved, and if the amount of CNTs is equal to or more than the lower limit, the conductivity can be further improved.

[0024] (solvent-soluble polymer) The solvent-soluble polymer used in this embodiment is attached to the CB and CNT. Carbon nanotubes are said to have safety issues due to poor workability caused by their low bulk density and environmental pollution caused by their tendency to scatter. To solve these issues, in this embodiment, a solvent-soluble polymer is attached to the CNTs and CB.

[0025] The solvent-soluble polymer can be any polymer that dissolves in water, organic solvents, and mixtures thereof. The solvent can be water, organic solvents, or mixtures thereof, with water being the most preferred. When water is used as the solvent, the solvent-soluble polymer is a water-soluble polymer. Examples of the solvent-soluble polymer include polymer surfactants and high molecular weight polymers. Examples of surfactants include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants, which may be used alone or in combination of two or more. Examples of high molecular weight polymers include ether-based polymers (such as polyethylene glycol (polyethylene oxide) and polypropylene glycol), vinyl-based polymers (such as polyvinyl alcohol, polyvinyl acetate, and polyvinylpyrrolidone), acrylamide-based polymers (such as polyacrylamide), amine-based polymers (such as polyethyleneimine and polybutyleneimine), cellulose-based polymers (such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methylcellulose), and starch-based polymers (such as oxidized starch and gelatin). These may be used alone or in combination of two or more. Among these, from the viewpoint of reducing scattering or improving dispersibility, it is more preferable to use glycol-based polymers, and it is particularly preferable to use polyethylene oxide.

[0026] In the granulated product according to this embodiment, the amount of the solvent-soluble polymer is preferably 1 part by mass or more and 30 parts by mass or less, and more preferably 3 parts by mass or more and 15 parts by mass or less, per 100 parts by mass of the total amount of carbon black and carbon nanotubes.

[0027] [Method of manufacturing carbon material granules] Next, a method for producing a granulated product according to this embodiment will be described. Examples of the method for producing the granulated product according to this embodiment include the first production method and the second production method shown below.

[0028] (First method for producing carbon material granules) The first method for producing a granulated material according to this embodiment includes a step of grinding and mixing CNT and CB granules as necessary (grinding and mixing step), a step of dissolving a solvent-soluble polymer in a solvent to prepare a binder solution (solution preparation step), and a step of adding the binder solution little by little to the mixture of CB and CNT while mixing, and granulating to obtain a carbon material granulated material (granule preparation step). The granulated material according to the present embodiment can be produced by the first method for producing a granulated material according to the present embodiment. However, the method for producing a granulated material according to the present embodiment is not limited to the method for producing a granulated material according to the present embodiment. For example, in the grinding and mixing step, CNT granules and CB are used and ground, but the grinding process may be omitted by using equivalent CNT or CB after the grinding process.

[0029] (Crushing and mixing process) In the grinding and mixing step, the CB granules are ground to a specific particle size or less, and the CNTs are ground as necessary to satisfy condition 1 or condition 2, and then mixed to obtain a mixture. Here, the order of dry grinding and mixing is not particularly limited. (1) The CB granules and CNTs may be dry ground separately and then mixed, or (2) the CB granules and CNTs may be mixed and then dry ground. There are two types of pulverization methods, dry pulverization and wet pulverization, which are used depending on the purpose. In the present invention, dry pulverization is used, but in the case of dry pulverization, the pulverizer used varies depending on the target particle size and particle size distribution. For example, (1) when the target is medium pulverization (10 mm or less), an impeller mill, pin mill, or roller mill is used, and (2) when the target is fine pulverization (several tens of μm or less), a jet mill, ball mill, vibration ball mill, or planetary mill is used. Among crusher manufacturers, those that manufacture jet mill-type crushers include Seishin Enterprise Co., Ltd., Aisin Nano Technologies Co., Ltd., and Earth Technica Co., Ltd. Pin mill manufacturers include Makino Sangyo Co., Ltd., Nishimura Machinery Works, Ltd., and Hosokawa Micron Co., Ltd. Impeller mill manufacturers include Seishin Enterprise Co., Ltd. and Earth Technica Co., Ltd. Furthermore, manufacturers of sanitary rotary crushers include Aishin Sangyo Co., Ltd. and Tokuju Kosakusho Co., Ltd. Among these, the sanitary type crusher manufactured by Aishin Sangyo Co., Ltd. and the Randle Mill (RM-1N type) manufactured by Tokuju Kosakusho Co., Ltd. are capable of directly inserting materials from the raw material hopper into a mixer or twin-screw extruder while crushing them in a sealed state.

[0030] (Solution preparation process) In the solution preparation step, a solvent-soluble polymer is dissolved in a solvent to prepare a binder solution. The solvent-soluble polymer and the solvent are as described above. The concentration of the solvent-soluble polymer in the binder solvent is preferably 1% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 5% by mass or less. If the concentration of the solvent-soluble polymer is equal to or higher than the lower limit, the solvent-soluble polymer can be impregnated onto the carbon material more efficiently. On the other hand, if the concentration of the solvent-soluble polymer exceeds the upper limit, the polymer does not sufficiently penetrate into the carbon material, and the effect of expelling air present on the surface or in pores, which is detrimental to the conductive performance, decreases, which tends to result in a decrease in the conductivity. Adding the solvent-soluble polymer at the lowest possible concentration allows it to penetrate more easily into the voids in the carbon material, enabling a uniform coating over the entire carbon material. Adding a surfactant to the binder solution also facilitates penetration of the binder solution into the carbon material.

[0031] (Granule preparation process) In the granule preparation step, the binder solution is added little by little to the mixture of CB and CNT while being mixed to obtain carbon material granules. The mixing devices used here can be broadly divided into batch and continuous types. Representative batch types include a Henschel type agitation mixer and a batch type Loedige mixer. An example of a continuous type is a twin-screw pin mixer that mixes by rotating twin screws.

[0032] Examples of Henschel mixers include the "High Speed Mixer" series manufactured by EarthTechnica, the "SPG" series manufactured by Technopaudal, the "FM Mixer" manufactured by Nippon Coke & Engineering Co., Ltd., the "SMB" or "SM" series manufactured by Kawata, and the "VG" series manufactured by Powrex. There are various models of Loedige mixers sold by Matsubo Corporation, from the M20 to the M8000D. An example of the biaxial pin type is the "Dow Pelletizer" manufactured by Shin-Nichinan Co., Ltd.

[0033] When continuous mixing is performed in the granulation preparation process, using a pin-type mixer as an example, the process is as follows: A fixed amount of CNT and CB mixture is charged into the device with a rotating body through an inlet, a binder solution is added through an inlet after the inlet and mixed, and the granules are removed through an outlet and dried in the drying process described below. Mixing performance is adjusted by the residence time within the device. The longer the residence time, the more spherical the mixture obtained. If the desired granules cannot be obtained, granulation may be performed by increasing the number of pin-type mixers in series to two. The rotation speed of the rotor is preferably 500 rpm to 3000 rpm, and more preferably 1000 rpm to 2000 rpm.

[0034] On the other hand, when batch mixing is performed in the granulation preparation process, taking a Henschel mixer as an example, the process is as follows: That is, as in the continuous process, a predetermined amount of pulverized carbon nanotube and carbon black powder is charged into the mixer, and the mixture is stirred with a rotor blade. A binder solution is added little by little, and while checking the mixed state, solvent is added. When the desired granule particle size is reached, the granules are removed and dried in the drying process described below. The rotation speed of the rotor blade is preferably 300 rpm or more and 2500 rpm or less, and more preferably 500 rpm or more and 2000 rpm or less.

[0035] After the granule preparation step, a step of drying the carbon material granules (drying step) is carried out. Vacuum drying, hot air drying, etc. can be used for drying. As hot air dryers, vibration / fluidized dryers, fluidized dryers, box dryers, and dryer-type dryers can be used. On the other hand, as vacuum (reduced pressure) dryers, vacuum tray dryers, reduced pressure outer mixer dryers, and box dryers can be used.

[0036] The drying temperature is preferably a temperature at which the solvent-soluble polymer does not deteriorate, and therefore there is an optimum temperature or a maximum temperature depending on the type of solvent-soluble polymer, but generally, the temperature is preferably from 40 to 200° C., more preferably from 50 to 150° C., and particularly preferably from 60 to 100° C. The drying time, although depending on the drying temperature, is usually from 1 to 20 hours, and preferably from 2 to 10 hours.

[0037] (Second method for producing carbon material granules) The second method for producing a granulated material according to this embodiment includes the steps of grinding and mixing CNT and CB granules as needed (grinding and mixing step), dissolving a solvent-soluble polymer in a solvent to prepare a binder solution (solution preparation step), adding the binder solution while mixing the CB and CNT to obtain a wet mixture (granulation precursor) (wet mixture preparation step), and granulating the wet mixture to obtain a carbon material granule (granule preparation step). The second manufacturing method differs from the first manufacturing method in the preparation step for preparing the granules. In the following description, differences from the first manufacturing method will be mainly described, and overlapping descriptions will be omitted or simplified.

[0038] (Crushing and mixing process) The grinding and mixing step is generally as described above. However, the CB and CNTs can be mixed by feeding the pulverized material into a mixer / granulator such as a Henschel mixer or a Loedige mixer, and then mixing while adding water in which the polymer has been dissolved. Alternatively, the material can be directly fed into a mixer / granulator such as a Henschel mixer without being pulverized, and pulverized in the granulator by high-speed stirring without adding water in which the polymer has been dissolved, and then water in which the polymer has been dissolved is added to produce a wet mixture that serves as a granulation precursor.

[0039] (Solution preparation process) The solution preparation step is as described above.

[0040] (Wet mixture preparation process) In the mixture preparation step, the mixture obtained in the pulverization and mixing step is mixed with the binder solution obtained in the solution preparation step to obtain a wet mixture. The amount of the binder solution is preferably adjusted depending on the amount of the solvent-soluble polymer, that is, the amount of the solvent-soluble polymer is preferably 0.01 to 15 parts by mass, more preferably 0.1 to 12 parts by mass, and particularly preferably 2 to 10 parts by mass, relative to 100 parts by mass of the total amount of carbon black and carbon nanotubes.

[0041] In this embodiment, a shearing treatment for shearing the CB and CNT may be performed during or after the wet mixture preparation step. However, in this embodiment, the CB and CNT are sufficiently pulverized in the aforementioned pulverization and mixing step, so this shearing treatment is not necessarily required.

[0042] (Granule preparation process) In the granule preparation step, the wet mixture obtained in the wet mixture preparation step is granulated to obtain carbon material granules. Granulating the wet mixture enables continuous granulation, thereby improving the production efficiency of the granules. Examples of granulators used here include extrusion granulators and shear crushing granulators. Among these, extrusion granulators (see Figure 1, which is a model of the interior of an extrusion granulator) are particularly preferred from the perspective of production efficiency. As shown in Figure 1, an extrusion granulator is equipped with a screw case 1, a screw 2, an extractor blade 3, a screen 4, and a screen holder 5. Extrusion granulators are primarily screw-type extrusion granulators, available in single-screw and twin-screw configurations. They are broadly classified into two types, front extruders and side extruders, depending on the location of the extrusion screen die. Single-screw extruders generally have a screen die attached to the front end of the granulation chamber. Twin-screw extruders, on the other hand, often have screen dies attached to both sides of the granulation chamber. Comparing the features of single-screw and twin-screw extrusions, the extrusion pressure is stronger in single-screw extrusions, allowing for the production of relatively large particle sizes and high hardness granules. Twin-screw extrusion is good for small particle size products, and although the particle strength is weaker, it is excellent in terms of production efficiency. Examples of screw-type extrusion granulator equipment and their manufacturers include the Pelletter Double EXD type or Fineluzer EXR type manufactured by Dalton Co., Ltd., the Granumaster manufactured by Okawara Manufacturing Co., Ltd., and the Extrude Mix EM manufactured by Hosokawa Micron Corporation. Examples of shear crushing granulators include the Speed Mill HM series manufactured by Fuji Yakuhin Kikai Co., Ltd., and the Chopper Mill manufactured by Nippon Pneumatic Mfg. Co., Ltd.

[0043] After the granule preparation step, a step of drying the carbon material granules (drying step) may be carried out as needed. The drying step is as described above.

[0044] [How to use carbon material granules] Next, a method for using the granulated product according to this embodiment will be described. The granulated material according to the present embodiment can reduce metal impurities and has excellent dispersibility, and therefore, the granulated material according to the present embodiment can be used to prepare a carbon material-containing resin composition that is useful as a constituent material for lithium ion secondary batteries.

[0045] Examples of the resin include polyolefin resin, polyhalogenated olefin resin, polyester resin, polyamide resin, polyimide resin, polyether resin, polyvinyl resin, polystyrene resin, polyvinyl alcohol resin, polymethacrylate resin, polyurethane resin, polyepoxy resin, polyphenol resin, polyurea resin, and polyether sulfone resin. [Example]

[0046] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" are by mass unless otherwise specified.

[0047] [Preparation Example 1] A graphitization apparatus was prepared, in which electrodes were attached to both ends of a hollow graphite pipe measuring 4 cm in inner diameter and 100 cm in length, and the pipe could be directly heated by passing an alternating current through it. Approximately 15 g of Kumho Petrochemical's pelletized CNT granules (raw CNTs) "K-Nanos-100T" were placed in a cylindrical graphite container measuring 3 cm in inner diameter, 3.5 cm in outer diameter, and 10 cm in length. This was then inserted approximately into the center of the graphite pipe, and nitrogen gas was introduced into the graphite pipe. An alternating current was then passed through the graphite pipe, and the pipe was subjected to a heat treatment (graphitization treatment) at 2900 °C for 30 minutes to produce graphitized CNTs.

[0048] [Preparation Example 2] "K-Nanos-100T," a pelletized CNT granule (raw CNT) manufactured by Kumho Petrochemical Co., Ltd., was pulverized using a jet mill "PJM-80" manufactured by Nippon Pneumatic Co., Ltd. to produce a pulverized product of raw CNT.

[0049] [Preparation Example 3] The graphitized CNT obtained in Preparation Example 1 was pulverized using a jet mill "PJM-80" manufactured by Nippon Pneumatic Co., Ltd. to produce a pulverized product of graphitized CNT. The particle size distribution of the crushed graphitized CNT was measured using the laser diffraction and scattering method specified in ISO 13320. The measuring instrument used was a Laser Micronsizer LMS-3000 (manufactured by Seishin Enterprise Co., Ltd.). From the obtained particle size distribution, the particle size D 50 The particle size of graphitized CNTs, D 50 was 35 μm.

[0050] [Reference examples 1~4] A resin composition was prepared by blending 2% of each of the following CNTs into polypropylene resin (Prime Polymer's "J229E") and mixing it at 210°C and 100 rpm for 4 minutes using a Plastomill. Reference example 1: Raw material CNT Reference Example 2: Graphitized CNT obtained in Preparation Example 1 Reference Example 3: Crushed raw CNT obtained in Preparation Example 2 Reference Example 4: Crushed graphitized CNT obtained in Preparation Example 3

[0051] [Example 1 and Comparative Examples 1 and 2] (Preparation of carbon material granules) First, the carbon material and binder solution materials shown below were prepared. Specifically, the binder solution was produced by adding 120 g of a solvent-soluble polymer (polyethylene oxide (PEO): molecular weight 100,000 to 200,000, trade name "ALKOX R-150", manufactured by Meisei Chemical Industry Co., Ltd.) to 3,480 g of water, and mixing at 3,000 rpm for 5 minutes using a high-speed homomixer (manufactured by Chuo Rika Co., Ltd., LZB14-HM-1) to obtain the binder solution. Next, carbon material granules were produced. Specifically, 360 g of CB (Li435, manufactured by Denka Co., Ltd.) and various CNTs pulverized (particle size after pulverization: 40 μm) using a pin mill (DD-2-3.7) manufactured by Seishin Enterprise Co., Ltd. were added to a Lödige mixer (manufactured by Lödige, model M20, volume 20 L). While stirring at 250 rpm, 3600 g of an aqueous polymer solution was sprayed from the upper inlet and mixed for 15 minutes. After that, the spraying of the aqueous solution was stopped and the mixture was stirred for 15 minutes, followed by particle size regulation, to obtain wet carbon material granules. The mixture was then dried in a hot air dryer to obtain carbon material granules. Example 1: 30% crushed graphitized CNT / 70% crushed CB Comparative Example 1: Graphitized CNT 30% / CB crushed product 70% Comparative Example 2: 30% crushed raw CNT / 70% crushed CB

[0052] (Preparation of Resin Composition) A resin composition was prepared by blending 2% of each of the following CNTs into polypropylene resin (Prime Polymer's "J229E") and mixing it at 210°C and 100 rpm for 4 minutes using a Plastomill.

[0053] [Evaluation of CNTs, carbon material granules, and resin compositions] The properties (ash content, dispersibility, volume resistivity) of the CNTs, carbon material granules, and resin composition were evaluated by the following methods. The results are shown in Table 1. (1) Ash content The ash content of the CNT or carbon material granules was measured in accordance with JIS K-6218-2 (ASTM D-1506). Specifically, the sample was placed in a porcelain crucible and completely incinerated at 750°C or 825°C, and the ash content (unit: %) was calculated from the amount of residue. The ash content indicates the progress of the graphitization process, with the lower the ash content, the more progressed the graphitization process. Furthermore, the lower the ash content, the fewer metal impurities there are in the CNT or carbon material granules. (2) Dispersibility The dispersibility of the resin composition was evaluated by melt-pressing the resin composition to prepare thin flakes, and observing the flakes under a microscope (magnifications of 50x and 200x) using transmitted light. The degree of dispersion was evaluated on a scale of 0 to 10. The higher the value, the better the dispersibility. A photograph of the observation (magnification of 50x) is shown in Figure 2. (3) Volume resistivity Using a Labo Plastomill, three resin compositions were prepared by mixing polycarbonate resin (Teijin's "Panlite L-1225WP") with the carbon material granules in amounts of 1.5%, 1.7%, and 2%. The mixtures were kneaded at 280°C and 100 rpm for 4 minutes to prepare resin compositions. The volume resistivity of the resin compositions was then measured.

[0054] [Table 1]

[0055] The results shown in Table 1 reveal the following: That is, the blended granules of pulverized graphitized CNT and CB (Example 1) had significantly improved dispersibility and volume resistivity compared to the granules of unpulverized graphitized CNT and CB (Comparative Example 1). It was found that, despite the significant reduction in catalytic metals due to the graphitization treatment, the blended granules of CNT and CB that had not been subjected to the graphitization treatment also exhibited performance similar to that of the blended granules of CNT and CB that had not been subjected to the graphitization treatment (Comparative Example 2) in terms of dispersibility and volume resistivity. From the above, it was confirmed that the carbon material granules according to the present invention can reduce metal impurities and have excellent dispersibility. [Explanation of symbols]

[0056] 1...Screw case, 2...Screw, 3...Extract blade, 4...Screen, 5...Screen holder.

Claims

1. A carbon nanotube composition comprising: carbon black having a particle size of 500 μm or less as measured by the method described in JIS K-6219-4; carbon nanotubes satisfying either one of the following conditions 1 and 2; and a solvent-soluble polymer attached to the carbon black and the carbon nanotubes: Carbon material granules. (Condition 1) Carbon nanotubes that have been subjected to graphitization and then crushing, and the particle size D of the carbon nanotubes ５０ is 100 μm or less. (Condition 2) The carbon nanotubes are either powdered carbon nanotubes or granular carbon nanotubes that have been pulverized and then graphitized.

2. The carbon material granules according to claim 1, the solvent-soluble polymer is a water-soluble polymer, The solvent is water. Carbon material granules.

3. A method for producing the carbon material granules according to claim 1 or 2, comprising: a step of mixing carbon black having a particle size of 500 μm or less according to the method described in JIS K-6219-4 with carbon nanotubes that satisfy either the condition 1 or the condition 2 to obtain a mixture; dissolving the solvent-soluble polymer in a solvent to prepare a binder solution; and a step of adding the binder solution little by little to the mixture, mixing the mixture, and granulating the mixture to obtain carbon material granules. A method for producing carbon material granules.

4. A method for producing the carbon material granules according to claim 1 or 2, comprising: a step of mixing carbon black having a particle size of 500 μm or less according to the method described in JIS K-6219-4 with carbon nanotubes that satisfy either the condition 1 or the condition 2 to obtain a mixture; dissolving the solvent-soluble polymer in a solvent to prepare a binder solution; mixing the binder solution with the mixture to obtain a wet mixture; and granulating the wet mixture to obtain carbon material granules. A method for producing carbon material granules.

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