Carbon nanotubes and carbon nanotube resin composition

Abstract

Carbon nanotubes subjected to a graphitization treatment and then subjected to a pulverization treatment, wherein the particle diameter D50 of said carbon nanotubes is 100 µm or less.

Description

Carbon nanotubes and carbon nanotube resin compositions

[0001] The present invention relates to carbon nanotubes (hereinafter sometimes referred to as CNTs) and carbon nanotube resin compositions.

[0002] Currently, lithium-ion secondary batteries (hereinafter sometimes referred to as Libs) are used in mobile phones, notebook computers, digital cameras, or automobiles (e.g., EVs, HEVs, P-HEVs), etc., and have made great contributions to improving the productivity of various industries or enhancing the quality of life. As the electrodes of Libs, a positive electrode in which an electrode composite material composed of a positive electrode active material containing lithium ions, a conductive assistant, and an organic binder is fixed to a current collector of a metal foil, and a negative electrode formed by fixing an electrode composite material composed of a negative electrode material capable of inserting and extracting lithium ions, a conductive assistant, and an organic binder, etc., to the surface of an aggregate of metal foils.

[0003] Usually, lithium transition metal composite oxides such as lithium cobaltate are used as the positive electrode material, but these have low electron conductivity, and sufficient battery performance cannot be obtained when used alone. Therefore, by adding carbon black (hereinafter sometimes referred to as CB), especially acetylene black or ketjen black, etc., as a conductive assistant, the conductivity is improved and the internal resistance of the electrode is reduced. On the other hand, graphite is usually used as the negative electrode active material, and improvements are being made every day, such as improving the charge and discharge characteristics by adding CB as a conductive assistant here. However, the Lib industry or its user side is demanding even higher-performance products such as further increased capacity, higher safety, and further reduction of cycle degradation due to repeated charge and discharge.

[0004] One measure to address this demand is the use of CNTs as a conductive additive, particularly in the positive electrode. This involves blending CNTs with conventional carbon binders (CBs) or using CNTs alone. For example, Non-Patent Literature 1 reports that in a system where CNTs were blended with acetylene black in a 100:1 ratio, "the CNTs acted like a conductive binder, lowering the initial resistivity, improving the bonding strength, mitigating the breakage of the acetylene black due to expansion and contraction during charging and discharging, and significantly improving the cycle characteristics." Based on these numerous achievements, the use of CNTs in lithium-ion batteries (LIBs) has spread rapidly and they are now widely used in LIBs for automobiles. On the other hand, it is also true that LIB manufacturers, especially in Japan, are demanding "10-year, 100,000 km" guarantees from material manufacturers. Among the materials, the metal catalyst contained in the CNTs is considered a particular problem.

[0005] There are two main methods for manufacturing multilayer carbon nanotubes (CNTs): chemical vapor deposition (QV) and physical vapor deposition (PHV). Chemical vapor deposition is suitable for mass production because the reactor can be scaled up relatively easily. Chemical vapor deposition can be broadly divided into two methods. One method involves dissolving a metal compound or co-catalyst such as sulfur in hydrocarbons such as benzene, supplying hydrogen as a carrier gas to a reactor at over 1000°C, and generating the catalyst and growing the CNTs in situ (floating catalyst method). The other method involves adding a pre-prepared supported catalyst to a reactor heated to 500°C to 700°C, and supplying hydrocarbons such as ethylene with a mixed gas such as hydrogen or nitrogen to cause a reaction (supported catalyst method). Most multilayer CNTs currently on the market are produced using the supported catalyst method. CNTs obtained using the supported catalyst method do not have deposition of pyrolysis carbon due to the self-decomposition reaction of hydrocarbons and are relatively crystalline, so there is no need for the heat treatment required for graphitization that is essential in the floating catalyst method. On the other hand, because no heat treatment is performed for graphitization, typically 3-6% of the catalyst metal remains in the CNTs obtained by the supported catalyst method.

[0006] When CNTs containing metal impurities are used as a conductive additive in the positive electrode, transition metals such as iron or cobalt may be oxidized and leached out because the positive electrode is in an oxidizing atmosphere. Similarly, when CNTs containing metal impurities are used in the negative electrode, metal deposition may occur. In the worst case, 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, the precipitates can grow significantly during battery use, potentially rupturing the separator. To create a highly safe lithium-ion battery that is free from internal short circuits and localized overheating or degradation, it has become crucial to reduce the metal impurities in the CNTs used as conductive additives as much as possible, and to use CNTs with excellent dispersibility.

[0007] "Toyama Prefectural Industrial Technology Center Research Report No. 29 (2015)," published by Toyama Prefectural Industrial Technology Center, pp. 97-98.

[0008] The present invention aims to provide carbon nanotubes and carbon nanotube resin compositions that can reduce metal impurities and have excellent dispersibility.

[0009] The present invention provides the following carbon nanotubes and carbon nanotube resin compositions: [1] Carbon nanotubes that have been subjected to graphitization treatment and then pulverized, wherein the particle size D of the carbon nanotubes ５０ [2] Carbon nanotubes having a particle size D of 100 μm or less. [3] Carbon nanotubes having been subjected to graphitization treatment on carbon nanotube powder or carbon nanotube granules that have been subjected to pulverization treatment. [4] In the carbon nanotubes described in [2], the particle size D of the carbon nanotubes ５０ A carbon nanotube having a diameter of 100 μm or less. [4] A carbon nanotube resin composition comprising a carbon nanotube according to any one of [1] to [3] and a binder resin.

[0010] According to one aspect of the present invention, it is possible to provide carbon nanotubes and carbon nanotube resin compositions that can reduce metal impurities and have excellent dispersibility.

[0011] These are photographs showing the dispersibility of the resin compositions obtained in Examples 1-5, Comparative Examples 1-3, and Reference Examples 1 and 2.

[0012] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0013] [Carbon Nanotubes] First, the carbon nanotubes according to this embodiment will be described. The carbon nanotubes according to this embodiment are carbon nanotubes that have been subjected to graphitization treatment and then pulverization treatment, and the particle size D of these carbon nanotubes ５０ The particle size is 100 μm or less. Another embodiment of the carbon nanotube according to this embodiment is a carbon nanotube that has been subjected to a graphitization treatment after being subjected to a pulverization treatment of carbon nanotube powder or carbon nanotube granules. The reason why this embodiment can reduce metal impurities and obtain carbon nanotubes with excellent dispersibility is not entirely clear, but the inventors speculate as follows.

[0014] First, the reason why metal impurities can be reduced is that the CNTs are subjected to graphitization treatment. Graphitization treatment is a heat treatment at a high temperature of, for example, 2000°C or higher, and this graphitization treatment can remove catalyst-derived metal impurities (catalyst metals) mixed in with the CNTs. In contrast, other methods for removing catalyst-derived metal impurities mixed in with CNTs include (i) immersing the CNTs in an acidic solution containing at least one of the following: sulfuric acid, hydrochloric acid, and hydrofluoric acid to dissolve and remove the metals, or (ii) irradiating the CNTs with microwaves to convert the metal impurities into metal oxides, which are inactive at the operating voltage of the secondary battery and do not dissolve in the electrolyte. Of these, the acid treatment method in (i) above has drawbacks. Even if heat treatment (below 600°C) is performed after pickling, acid ions remain on the CNT surface, which may corrode the positive electrode material and may also damage the CNTs, causing lattice defects. Furthermore, since the catalyst metal is covered with a graphite layer, treatment using only an acid solution is insufficient because the graphite layer prevents the acid solution from reaching the catalyst metal, making it impossible to completely dissolve and remove the catalyst metal. In addition, the microwave irradiation method described in (ii) above has the problem of oxidizing the CNTs and reducing their conductivity. Thus, the method of graphitizing the CNTs is optimal as a method for removing catalyst-derived metal impurities (catalyst metals) mixed in with the CNTs.

[0015] On the other hand, when CNTs are subjected to graphitization treatment, there is a problem that crystallization of the CNTs progresses, reducing their dispersibility in the resin. However, in this embodiment, the CNTs are subjected to pulverization treatment before or after graphitization treatment to become fine powder, or the graphitization treatment is performed on CNTs that are already in the form of fine powder. By making them into fine powder in this way, the dispersibility of the CNTs in the resin is improved. Therefore, the inventors surmise that the carbon nanotubes according to this embodiment can reduce metal impurities and have excellent dispersibility.

[0016] (Graphitization Treatment) In this embodiment, the graphitization treatment is performed by heat treatment at a temperature of 2000°C to 3000°C in a non-oxidizing atmosphere. More specifically, the graphitization treatment is carried out by heating and holding the CNTs at 2000°C or higher in an oxygen-free atmosphere (for example, in a nitrogen stream, in a vacuum, or in carbon powder (also known as nail powder, usually coke powder or CB is used)). In this case, since the yield of CNTs obtained will decrease if oxygen is present in the atmosphere, it is desirable to keep the oxygen concentration in the atmosphere to 1% by volume or less. When performing this graphitization treatment on a large scale, it is advantageous to use an Acheson electric furnace. On the other hand, when performing it on a small scale, a predetermined amount of CNTs is filled into a graphite pipe, which is then buried in carbon powder, and an alternating current is passed through both ends of the pipe. Thus, in the graphitization treatment, it is preferable to heat the CNTs to 2000°C to 3000°C, more preferably to 2500°C to 2900°C, and particularly preferably to 2700°C to 2900°C. When heated within this temperature range, the crystallites of the CNTs are rearranged and their shape changes, the wire diameter becomes slightly smaller, but the crystallinity grows. Furthermore, the higher the treatment temperature, the more the functional groups or metallic impurities on the surface can be reduced. However, it is thought that the higher the treatment temperature, the stronger the entanglement of primary or secondary aggregates present in the CNTs before graphitization becomes due to the heat treatment, resulting in poor dispersibility when compounded with resins and the like.

[0017] (Grinding Process) In this embodiment, the grinding process can be dry grinding or wet grinding, and can be used depending on the purpose. The reason why the dispersibility of graphitized CNTs decreases is thought to be because the complex and strong entanglement of CNT fibers could not be untangled by ordinary dispersers or dispersing methods. To untangle this strong entanglement, it is preferable to use a powerful grinding method. In this embodiment, it is preferable to use dry grinding. In the case of dry grinding, the grinder used will differ depending on the desired particle size and particle size distribution. For example, if the purpose is fine grinding of tens of micrometers or less, a jet mill, pin mill, vibrating ball mill, or planetary mill can be used. Among the manufacturers of grinders, jet mill type grinders include Seishin Corporation, Aisin Technologies Corporation, and Earth Technica Corporation. Pin mill manufacturers include Makino Sangyo Co., Ltd., Nishimura Machinery Works Co., Ltd., and Hosokawa Micron Corporation. Furthermore, manufacturers of impeller mills include Seishin Corporation and Earth Technica Corporation.

[0018] (Particle size of CNTs) Particle size D of carbon nanotubes according to this embodiment ５０ The particle size D must be 100 μm or less. Furthermore, if the carbon nanotube according to this embodiment is a carbon nanotube powder that has been subjected to graphitization treatment, then this particle size D ５０ The particle size is preferably 100 μm or less. The particle size distribution of CNTs is determined by the laser diffraction / scattering method specified in ISO 13320. A laser micronizer LMS-3000 (manufactured by Seishin Corporation) was used as the measuring instrument. The measurable range of this instrument is 0.01 to 3500 μm. The aqueous dispersion medium was prepared by adding 0.05 g of polyoxyethylene alkyl ether (product name Emulgen 705, manufactured by Kao Corporation) as a surfactant to 50 mL of pure water. For the measurement, 10 mg of CNTs was weighed into a 20 mL vial, 10 mL of the aqueous dispersion medium was added, and then dispersed in an ultrasonic disperser for about 10 minutes. The optical model of the measuring instrument was set to the refractive index of CNTs to 1.520 and water to 1.333 for the measurements. Preferred particle size D of CNTs５０ The particle size is between 10 μm and 100 μm, preferably between 10 μm and 50 μm. If the particle size is larger than 50 μm, many aggregates will be present, resulting in poor dispersibility. Furthermore, processing to make the particle size smaller than 10 μm is also a process that cuts the CNT fibers, which is undesirable because it worsens conductivity.

[0019] (Raw Material CNTs) As for the raw material CNTs, the fiber diameter is 0.3 nm, which is feasible to manufacture with modern technology, but it may be thinner than 0.3 nm. Furthermore, as the fiber diameter increases beyond 50 nm, the electrical and mechanical properties tend to decrease, and when it exceeds 100 nm, it tends to lose its advantages over CB or carbon nanofibers. In addition, in this embodiment, from the viewpoint of efficiently forming a three-dimensional network with CNTs, the fiber diameter of the CNTs is more preferably 3 nm to 50 nm, even more preferably 5 nm to 40 nm, and particularly preferably 10 nm to 30 nm. The fiber length of the CNTs is related to conductivity, mechanical properties, or dispersibility. The fiber length of the CNTs is preferably 0.1 μm to 2000 μm, and more preferably 1 μm to 1000 μm. As the fiber length decreases, conductivity or mechanical properties tend to become less pronounced. As the fiber length increases, the entanglement of fibers becomes stronger, leading to an increase in poorly dispersed clumps and increased fiber breakage during kneading and dispersion, which is undesirable. The aspect ratio of CNTs is approximately 10 to 10000. Furthermore, a structure in which hexagonal mesh graphite sheets form a cylindrical shape is preferably used as the CNT. The CNT can be single-layer or multi-layer, and can be selected according to the final purpose. There are also no restrictions on the manufacturing method of the CNT. Examples of CNT manufacturing methods include thermal decomposition by contacting a carbon-containing gas with a catalyst, arc discharge by generating an arc discharge between carbon rods, laser evaporation by irradiating a carbon target with a laser, CVD by reacting a carbon source gas at high temperature in the presence of metal nanoparticles, and HiPco by decomposing carbon monoxide under high pressure. Additionally, metal atoms may be doped into the CNTs.

[0020] [Carbon Nanotube Resin Composition] Next, the carbon nanotube resin composition according to this embodiment will be described. The carbon nanotube resin composition according to this embodiment contains the carbon nanotubes described above and a resin. According to the carbon nanotubes according to this embodiment, metal impurities can be reduced and dispersibility is excellent, so a carbon nanotube resin composition useful as a constituent material for lithium-ion secondary batteries can be made using the carbon nanotubes according to this embodiment.

[0021] Examples of resins include polyolefin resins, polyhalogenated olefin resins, polyester resins, polyamide resins, polyimide resins, polyether resins, polyvinyl resins, polystyrene resins, polyvinyl alcohol resins, polymethacrylate resins, polyurethane resins, polyepoxy resins, polyphenol resins, polyurea resins, and polyethersulfone resins.

[0022] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" refer to mass unless otherwise specified.

[0023] [Preparation Example 1] A graphitization apparatus was prepared in which electrodes were attached to both ends of a hollow graphite pipe with an inner diameter of 4 cm and a length of 100 cm, and alternating current was applied to directly heat the pipe. Approximately 15 g of pelletized CNT granules "K-Nanos-100T" manufactured by Kumhopetrochemical was packed into a cylindrical graphite container with an inner diameter of 3 cm, an outer diameter of 3.5 cm, and a length of 10 cm, and then inserted into the center of the graphite pipe. Nitrogen gas was then introduced into the inside of the graphite pipe. After that, alternating current was applied to the graphite pipe, and a heat treatment (graphitization treatment) was performed at a temperature of 2000°C for 30 minutes to produce graphitized CNTs.

[0024] [Preparation Example 2] CNTs were prepared in the same manner as in Preparation Example 1, except that the processing temperature for the graphitization treatment was changed to 2500°C.

[0025] [Preparation Example 3] CNTs were prepared in the same manner as in Preparation Example 1, except that the processing temperature for the graphitization treatment was changed to 2900°C.

[0026] [Preparation Example 4] "K-Nanos-100T," a pelletized CNT granule manufactured by Kumhope Petrochemical, was prepared and crushed using a jet mill "PJM-80" manufactured by Nippon Pneumatic Co., Ltd. to produce crushed CNTs. Subsequently, using these crushed CNTs instead of the CNT granules, graphitized CNTs were prepared in the same manner as in Preparation Example 1, except that the processing temperature for graphitization was changed to 2900°C.

[0027] [Preparation Example 5] Kumhopetrochemical's CNT powder "K-Nanos-100P" was prepared. Then, using this CNT powder instead of granular CNTs, graphitized CNTs were prepared in the same manner as in Preparation Example 1, except that the processing temperature for the graphitization treatment was changed to 2900°C.

[0028] [Example 1] The graphitized CNTs obtained in Preparation Example 3 were crushed using a jet mill "PJM-80" manufactured by Nippon Pneumatic Co., Ltd. to produce graphitized and crushed CNTs. The manufacturing conditions for the CNTs are as shown in Table 1 below. A resin composition was prepared by mixing 2% of the obtained CNTs with polypropylene resin ("J229E" manufactured by Prime Polymer Co., Ltd.) and mixing at 210°C for 4 minutes using a Plastmill.

[0029] [Examples 2-5, Comparative Examples 1-3, and Reference Examples 1 and 2] CNTs and resin compositions were prepared in the same manner as in Example 1, except that each CNT was subjected to the respective treatments according to the manufacturing conditions shown in Table 1.

[0030] [Evaluation of CNTs and Resin Compositions] The properties (ash content, dispersibility, particle size) of CNTs and resin compositions were evaluated using the following methods. The results are shown in Table 1. (1) Ash Content The ash content of CNTs was measured in accordance with JIS K-6218-2 (ASTM D-1506). Specifically, the ash content (unit: %) was determined from the amount of residue after the sample was completely ashed in a porcelain crucible at 750°C or 825°C. The degree of progress of the graphitization treatment can be determined from the ash content; the lower the ash content, the more advanced the graphitization treatment. Also, the lower the ash content, the fewer metal impurities there are in the CNTs. (2) Dispersibility The dispersibility of the resin composition was evaluated by melt-pressing the resin composition to create thin sections and observing these sections using transmitted light under a microscope (magnification 50x and 200x). The degree of dispersion was evaluated on a scale of 0 to 10. Furthermore, the larger this value, the better the dispersibility. The observed photographs are shown in Figure 1. (3) Particle size D ５０ The particle size distribution of CNTs was determined using the laser diffraction / scattering method specified in ISO 13320. A laser micronizer LMS-3000 (manufactured by Seishin Corporation) was used as the measuring instrument. From the obtained particle size distribution, the particle size D was determined. ５０ The value obtained was obtained.

[0031]

[0032] From the results shown in Table 1, the following was found: The amount of metal impurities, as seen from the ash content, was found to be reliably reduced by graphitization treatment at 2000°C, 2500°C, or 2900°C. Furthermore, although 2000°C is a higher temperature than the melting point of iron impurities (1535°C) that constitute the metal impurities, the amount of impurities was not reduced to a significant extent. On the other hand, it was found that the dispersibility when compounded with polypropylene resin deteriorated drastically with high-temperature heating (see Comparative Examples 1-3). The inventors speculate that the reason for the deterioration in dispersibility is as follows: Since the CNT granules were granulated by compression granulation methods (roller compactor method or briquette machine method), it is thought that the entanglement of secondary aggregates on the micron order is considerably strong. In addition, it is thought that the graphitization treatment at temperatures above 2000°C (especially above 2500°C) completely eliminated the various functional groups that were present in small amounts on the CNT surface, that the development of crystallites eliminated the reactive active sites that were present in disordered areas of the crystallites, and furthermore, that the bonds of the aggregates, which had been strengthened by mechanical compression, were further strengthened by the heat treatment, making them difficult to disperse.

[0033] The CNTs obtained in Example 1 were subjected to graphitization treatment followed by grinding treatment. The grinding treatment significantly improved dispersibility, but it did not reach the level of the raw material CNTs before graphitization treatment (see Reference Example 2, Comparative Example 3, and Example 1). The CNTs obtained in Example 2 were subjected to grinding treatment followed by graphitization treatment. It was found that the dispersibility was better than that of Example 1 (see Examples 1 and 2). The CNTs obtained in Example 3 were obtained by further grinding the CNTs obtained in Example 2. The dispersibility was the best among graphitized products using CNT granules as raw material, and was close to that of the raw material CNTs (see Reference Example 2 and Example 3). However, although not measured this time, it is suspected that the CNT fibers may have been cut because jet mill grinding was performed twice. The CNTs obtained in Example 4 were obtained by graphitizing CNT powder. The dispersibility was significantly better than that of the 2900°C graphitized CNT granules, and was found to be almost equivalent to that of the 2900°C graphitized CNT granules after pulverization (see Comparative Example 3, and Examples 2 and 4). The CNTs obtained in Example 5 were obtained by further pulverizing the CNTs obtained in Example 4. The dispersibility was at almost the same level as that of the raw material CNTs (see Reference Examples 1 and 2, and Example 5).

[0034] The carbon nanotubes of the present invention are useful as constituent materials for paints, inks, and resin molded products, and are particularly useful as conductive additives for electrode mixtures for lithium-ion secondary batteries.

Claims

1. Carbon nanotubes that have been subjected to graphitization treatment and then pulverized, wherein the particle size D of the carbon nanotubes ５０ Carbon nanotubes with a diameter of 100 μm or less.

2. Carbon nanotubes that have been subjected to graphitization treatment, either as carbon nanotube powder or as carbon nanotube granules that have been pulverized.

3. The carbon nanotube according to claim 2, wherein the particle size D of the carbon nanotube ５０ Carbon nanotubes with a diameter of 100 μm or less.

4. A carbon nanotube resin composition comprising a carbon nanotube according to any one of claims 1 to 3 and a binder resin.

Images