Method for producing graphite by low-temperature sintering of non-graphitizable carbon

Abstract

Provided is a method for producing graphite in a low-temperature process using non-graphitizable carbon. Non-graphitizable carbon to which magnesium (Mg) has been added is sintered at 1000-1700°C, and then the magnesium is removed from the sintered body to produce graphite. The amount of Mg added to the non-graphitizable carbon is 5-12 mass%. The sintering time for the non-graphitizable carbon to which Mg has been added is within 10 minutes. Spark plasma sintering (SPS) is used to perform the sintering in a short time. As the non-graphitizable carbon, activated carbon or a material carbonized by firing waste plastic such as a PET bottle is used. Magnesium and magnesium oxide are removed from the sintered body by dissolving the sintered body in an acid such as hydrochloric acid, or by controlling the sintering temperature and sintering pressure.

Description

Method for producing graphite by low-temperature sintering of non-graphitizable carbon

[0001] The present invention relates to a method for producing graphite by low-temperature sintering of non-graphitizable carbon (hard carbon).

[0002] Graphite has excellent electrical conductivity, thermal conductivity, heat resistance, lubricity, and acid and alkali resistance, and these properties are used to make steel, castings, refractories, carbon brushes, automotive parts, batteries, and lubricants. Graphite, in particular, has excellent crystallinity and electrical conductivity, and is used as an anode material for lithium-ion batteries.

[0003] Graphite has a relatively high specific capacity (theoretical value) of approximately 370 mAh / g (mAh: a unit representing the ability to continuously pass current for one hour), making it ideal for improving battery capacity. While graphite is ideal when battery capacity is a priority, other graphite-based materials include non-graphitizable carbon (hard carbon) and easily graphitizable carbon (soft carbon), which has properties intermediate between graphite and hard carbon, when battery charge / discharge and durability are important. Graphite has a crystalline structure in which carbon atoms are stacked, and is composed of layers of graphene, in which carbon atoms are regularly arranged at the vertices of each hexagon. In contrast, hard carbon is carbon with an irregular structure obtained by heat treatment of wood or thermosetting resin in an inert gas atmosphere. Graphene is randomly stacked and aggregated, with fine voids present in the gaps. It has a low density and a very high porosity. Due to its irregular structure, hard carbon generally does not graphitize even when subjected to heat treatment. Soft carbon has a structure somewhere between graphite and hard carbon, and is obtained by heat treating pitch (a graphite manufacturing material made from pitch coke and coal tar pitch, a petroleum by-product) or thermoplastic resin at high temperatures, and has an extremely small porosity.

[0004] Graphite is broadly divided into natural graphite and synthetic graphite. Synthetic graphite is obtained by heating soft carbon such as coke or coal tar pitch at temperatures above 3000°C for several weeks. The typical manufacturing method for graphite involves the following steps 1) to 5). 1) Coke is crushed and mixed to a specified particle size ratio, and pitch is added and mixed. The resulting paste is molded to a specified size (molding process). 2) It is carbonized by heat treatment at approximately 1000°C (calcination process). To prevent cracks and pores, the material is repeatedly fired and cooled for approximately one month. 3) During the firing and re-calcination processes, the volatile components of the pitch are removed, creating pores, which are then filled with pitch to increase density (filling process). 4) It is carbonized by another heat treatment (below 800°C) for approximately one week (re-calcination process). 5) After re-firing, the graphite is graphitized by heat treatment at about 3000°C for about one week to one month using Joule heat (graphitization process). As described above, conventional methods for producing synthetic graphite require synthesis through long-term sintering at high temperatures over several months, and because it is obtained by heating soft carbon such as coke or petroleum at temperatures above 3000°C for several weeks, a large amount of electricity, for example, about 13 to 14 MW / h, is required to produce one ton of graphite.

[0005] For example, Patent Document 1 discloses a method for producing graphite particles, which includes an activation treatment step of producing activated carbon by activating a raw material carbon composition containing soft carbon, and a heat treatment step of heat treating the activated carbon in an inert gas atmosphere at a temperature of 2000 to 3000°C to grow graphite crystals. The method for producing graphite particles in Patent Document 1 also requires heat treatment at a temperature of 2000 to 3000°C.

[0006] Furthermore, Patent Document 2 discloses a graphitization process for producing porous graphite, in which a carbon member having micropores is obtained using components other than carbon, such as aluminum, boron, silicon, potassium, calcium, iron, magnesium, and manganese, and then graphitized. However, the graphitization process in Patent Document 2 involves preparing a precursor made of a compound, alloy, or non-equilibrium alloy containing carbon and a major component other than carbon, immersing the prepared carbon-containing precursor material in a metal bath having a freezing point lower than the melting point of the carbon-containing material, and selectively eluting the major component other than carbon from the carbon-containing material into the metal bath to obtain a porous, bulky carbon member having micropores. The resulting carbon member is then heated for graphitization, and the heating temperature is set to 2000°C or higher, particularly 2400°C or higher, in order to increase crystallinity, reduce electrical resistivity, and increase electrical conductivity (see paragraphs 0026 to 0033 of Patent Document 2).

[0007] Furthermore, Patent Document 3 discloses a method for producing a structure in which aggregate carbide and vapor-grown graphite are integrated, by sealing aggregate calcined so as to contain residual hydrogen in a graphite container and performing hot isostatic pressing (HIP) treatment using isotropic gas pressure, and depositing vapor-grown graphite around and inside the filler using gases such as hydrocarbons and hydrogen generated from the filler as raw materials.

[0008] Also, Non-Patent Document 1 describes ion-catalyzed graphitization for the synthesis of nanostructured graphitic carbon, in which it is disclosed that transition metals such as nickel, cobalt, and iron function as catalysts for the growth of nanostructured graphitic carbon. In particular, in the case of iron catalysts, the graphitization process is carried out at relatively low temperatures of 800°C or 900°C due to the relatively small size of the nanoparticles. However, there are problems such as a large amount of metal remaining in the final product and a relatively low crystallinity due to the nanostructure of the product.

[0009] The closest technology to the present invention is disclosed in Non-Patent Document 2, which discloses a method for producing graphite in the presence of a magnesium catalyst through a low-temperature process at 800 to 1000°C. Specifically, powder mixtures of non-graphitizable carbon (C) and metallic magnesium (Mg) in a molar ratio of 1:1 (C:Mg = 1:1) are sealed in a stainless steel container in a glove box filled with argon gas, and the container is heated to 800 to 1000°C to promote graphitization. In Non-Patent Document 2, the produced graphite is highly crystalline, but has small seed particles on the graphite surface, which are composed of special Voronoi tessellation domains. Each seed particle is composed of a spherical graphite shell, with hexagonal graphite plates attached vertically to its interior, producing graphite with a high surface area through a low-temperature process far below the temperatures conventionally used for synthetic graphite. However, while the manufacturing method described in Non-Patent Document 2 can produce graphite through a low-temperature process at 800-1000°C in the presence of a magnesium catalyst, it involves heating a 1:1 mixture of C:Mg in a sealed stainless steel container, which essentially leaves a very large amount of metal remaining in the final product. Furthermore, because the stainless steel container must be sealed in a glove box, the manufacturing procedure is complicated and unsuitable for mass production, despite the high crystallinity. In other words, the manufacturing method described in Non-Patent Document 2 produces graphite through a low-temperature process by heating the mixture in a closed system where no substances leak out of the system. Furthermore, the process of dissolving the metallic magnesium remaining in the sample with acid creates numerous pores and voids, resulting in porous graphite.

[0010] Japanese Patent Application Laid-Open No. 2008-247643 Republished No. 2016 / 039268 Republished No. 2010 / 137592

[0011] RD Hunter et al., "Ion-catalyzed graphitization for the synthesis of nanostructured graphitic carbons", J. Mater. Chem. A 2022, 10, 4489.L. Zhao et al., "Voronoi-tessellated graphite produced by low-temperature catalytic graphitization from renewable resources", ChemSusChem 2017, 10 3409.

[0012] As described above, the manufacturing method of Non-Patent Document 2 involves producing graphite through a low-temperature process by heating a powder mixture of non-graphitizable carbon (C) and Mg in a molar ratio of 1:1 (C:Mg = 1:1) in a closed system. Since the molar ratio is 1:1, a mixture with a mass ratio of C:Mg = 12 (g):24.3 (g) is heated. That is, the manufacturing method of Non-Patent Document 2 involves graphitizing non-graphitizable carbon in liquid magnesium in a closed system. While this method allows for the production of graphite through a low-temperature process and has high crystallinity, it suffers from problems such as the complicated manufacturing procedure making it unsuitable for mass production. In light of this situation, the present invention aims to provide a method for producing graphite through a low-temperature process using non-graphitizable carbon (hard carbon).

[0013] In order to solve the above problems, the method for producing graphite of the present invention produces graphite by sintering magnesium-added non-graphitizable carbon at 1000 to 1700°C and removing magnesium and magnesium oxide from the sintered body. This production method makes it possible to graphitize non-graphitizable carbon at a low temperature of 1000 to 1700°C, rather than a high-temperature process near 3000°C. Note that the term "graphite" used in this specification is intended to include graphite-like materials. It is preferable to sinter magnesium-added non-graphitizable carbon at 1050 to 1650°C, as this allows the graphite structure to be formed more stably.

[0014] As described above, non-graphitizable carbon is a hard carbon obtained by heat-treating wood or thermosetting resin, and has a turbostratic structure in which graphene is randomly stacked and aggregated. However, it is amorphous carbon that does not undergo conversion from the turbostratic structure to a graphite structure even when heated at around 3000°C under normal pressure or reduced pressure, and does not become graphite even when heated in an inert gas, and has an irregular structure that suppresses the development of a graphite structure. Non-graphitizable carbon is obtained by heat-treating wood or thermosetting resin (e.g., phenolic resin, furfuryl alcohol resin, etc.) in a vacuum or inert gas atmosphere, and examples of such carbon include activated carbon and soot, and can be obtained by burning and carbonizing waste plastic such as PET bottle containers, or activated carbon derived from palm kernels.

[0015] Furthermore, non-graphitizable carbon to which magnesium (Mg) has been added is a mixture of powdered metallic magnesium and similarly powdered non-graphitizable carbon (such as soot or activated carbon), and the amount of Mg added to the non-graphitizable carbon may be 5 to 15% by mass, and preferably 5 to 12% by mass. Furthermore, since it is better to have a high degree of crystallinity of graphite and to add as little Mg as possible as an impurity, 7 to 10% by mass is more preferable. If the amount of Mg added is less than 5% by mass, the crystalline structure of graphite does not appear, and if the amount of Mg added is 11% by mass or more, the degree of crystallinity does not improve any more.

[0016] Furthermore, the sintering time for Mg-added non-graphitizable carbon is preferably within 10 minutes, more preferably within 300 seconds (5 minutes), and particularly preferably within 60 seconds. Here, a shorter sintering time for Mg-added non-graphitizable carbon is preferable. Non-graphitizable carbon can be sintered using any of the following methods: hot press sintering, hot isostatic pressing (HIP), spark plasma sintering (SPS), electric current sintering, hot rolling, hot extrusion, and hot forging. However, to prevent oxidation of the added Mg, sintering is preferably performed in a vacuum or in an inert gas such as nitrogen or argon. Even when heated in a vacuum or in an inert gas, materials tend to dissipate or evaporate to some degree during heating, resulting in sintering in a type of open or closed system. SPS processing is particularly suitable for short-term sintering. SPS is a process of sintering using mechanical pressure and pulse current heating. In addition to thermal and mechanical energy, sintering is achieved using electromagnetic energy from pulse current and discharge plasma energy between the particles of the material to be sintered, allowing the material to reach a predetermined temperature in a few seconds. In SPS, the material can be heated to 1000-1700°C in a few seconds and sintered. The pressure used in SPS is 20 MPa or higher.

[0017] In the SPS treatment, sintering of non-graphitizable carbon is preferably carried out at 1000 to 1700°C and under a high pressure of 100 MPa or more. After sintering non-graphitizable carbon to which magnesium has been added, removing the magnesium and magnesium oxide from the sintered body usually requires dissolving the sintered body in hydrochloric acid or the like to remove the magnesium (Mg) and magnesium oxide (MgO). However, by sintering the non-graphitizable carbon at 1000°C or more and then holding it under a low pressure of 10 MPa or less, the Mg and MgO can be sublimated and removed without being dissolved in hydrochloric acid or the like. That is, by sintering the non-graphitizable carbon at 1000 to 1700°C and under a high pressure of 100 MPa or more, and then holding it under a low pressure of 10 MPa or less after sintering, the Mg and MgO can be sublimated and removed from the sintered body.

[0018] Furthermore, in the SPS treatment, the non-graphitizable carbon is sintered at 1600 to 1700°C under a low pressure of 20 to 40 MPa, and then held at a high pressure of 1000 to 1700°C and 100 MPa or more. This allows the Mg and MgO to be sublimated and removed by the SPS treatment at 1600°C or more under low pressure, and then held at a high pressure of 1000 to 1700°C and 100 MPa or more, thereby improving the degree of graphitization.

[0019] The non-graphitizable carbon used as a material may also be produced by burning and carbonizing waste plastic. Waste plastic refers to plastic products such as PET bottles discarded after use, as well as plastic generated during their manufacturing process. As long as the waste plastic can be carbonized, the atmosphere does not matter. It can be burned in air or in an inert gas atmosphere such as nitrogen or argon.

[0020] In the present invention, the step of removing magnesium and magnesium oxide from the sintered body can be performed by dissolving the sintered body in acid. The acid that can be used may be either strong or weak acid, but hydrochloric acid or sulfuric acid can be used to remove the magnesium and magnesium oxide contained in the sintered body. Furthermore, the sintered body from which magnesium and magnesium oxide have been removed can be used as an anode material for lithium-ion batteries. Because graphite has a layered structure, it can efficiently store ions during charging, making it suitable for use as an anode material for lithium-ion batteries.

[0021] In the graphite production method of the present invention, it is preferable to add boron to non-graphitizable carbon in addition to magnesium and then sinter the carbon. By further adding boron and sintering the carbon, the sintered body from which magnesium and magnesium oxide have been removed can have an increased interplanar spacing of graphite, making it suitable for use as a negative electrode material for lithium-ion batteries. The increased interplanar spacing of graphite improves the properties of the negative electrode material for lithium-ion batteries.

[0022] The method for producing graphite of the present invention can produce graphite in a low-temperature process using non-graphitizable carbon (hard carbon), and can produce graphite in a low-temperature process in a short time, which has the effect of significantly reducing the energy required to produce graphite.

[0023] Flow diagram of the graphite production method of the present invention Schematic diagram of the graphite production method Schematic diagram of a spark plasma sintering apparatus Flow diagram of producing graphite using activated carbon Figure showing the results of X-ray diffraction measurement for the sample of Example 1 Figure showing the results of X-ray diffraction measurement after immersing the sintered body in dilute hydrochloric acid Figure showing the results of X-ray diffraction measurement (logarithmic display) Figure showing the results of X-ray diffraction measurement (comparison with graphite powder) for a sample sintered at 160°C Flow diagram of producing graphite using PET bottles (waste plastic) Figure showing the results of X-ray diffraction measurement when PET bottles are used as the raw material in Example 2 Figure showing the results of X-ray diffraction measurement (logarithmic display) Transmission electron microscope (TEM) image (magnification 150,000 times) High-resolution transmission electron microscope (HR-TEM) image (magnification 1 100,000 times magnification) Figure showing X-ray diffraction measurement results according to different amounts of Mg added when using PET. Production flow diagram of graphite in Example 3 (removal of MgO by controlling sintering temperature and pressure) Figure showing X-ray diffraction measurement results showing MgO removal by controlling sintering temperature and pressure. Illustrative diagram of an example of control of sintering temperature and pressure where MgO is not removed. Production flow diagram of graphite in Example 4 (improvement of crystallinity by holding at high temperature and high pressure) Figure showing X-ray diffraction measurement results showing improvement of crystallinity by holding at high temperature and high pressure. Production flow diagram of graphite in Example 5 (improvement of crystallinity by holding at low temperature and high pressure) Figure showing X-ray diffraction measurement results showing improvement of crystallinity by holding at low temperature and high pressure. Figure showing change in interplanar spacing of graphite with addition of boron.

[0024] The flow and schematic diagram of the graphite production method of the present invention are described with reference to FIGS. 1 and 2. First, non-graphitizable carbon (hard carbon) is powdered 1, and 5 to 12% by mass of Mg powder 5 is added and mixed using a mortar 4 (Step S01). The mixed powder is then filled into a die 6 using a spark plasma sintering apparatus 10 and heated in a vacuum at 1000 to 1700°C for several tens of seconds to 10 minutes to produce a sintered body 2 (Step S02). The sintering atmosphere can be either vacuum or inert gas. Spark plasma sintering is used because it is preferable for the heat treatment to reach the desired temperature in a short time. The sintered body 2 is then placed in a glass container 7, dissolved in an acid solution 8 such as dilute hydrochloric acid, and the magnesium salt is removed (Step S03). The graphite 3 (residue) that remains undissolved in the acid solution is then obtained (Step S04).

[0025] Figure 3 is a schematic diagram of a spark plasma sintering apparatus. In the spark plasma sintering apparatus 10, a material 14 to be sintered is placed inside a die 6 in a chamber 11. Here, a mixture of non-graphitizable carbon and Mg powder is filled inside the die 6. While applying pressure with a punch 16, a pulse current is passed through the die 6, and spark plasma sintering is performed at a temperature of 1000 to 1700°C in a vacuum or in an inert gas atmosphere such as argon, nitrogen, or helium. Figure 3 also shows pressure shafts, electrodes 12, and spacers 13 that support the upper and lower punches 16 of the die 6.

[0026] An example of an embodiment of the present invention will be described below with reference to the drawings. Note that the scope of the present invention is not limited to the following examples and illustrated examples, and many modifications and variations are possible.

[0027] In this example, graphite was prepared by the method of the present invention using commercially available activated carbon (Nacalai Tesque, Inc., product code 07909-65, average particle size: approximately 40 μm) as the non-graphitizable carbon, and its physical properties were evaluated. As shown in Figure 4 , graphite was prepared from activated carbon by adding 11% by mass of Mg to the activated carbon (Step S11). Sintered bodies were then prepared by spark plasma sintering (SPS) at three different temperatures (1070°C, 1160°C, and 1320°C) for several tens of seconds (Step S12). Since the added Mg was mixed into the sintered body, the sintered body was dissolved in dilute hydrochloric acid to produce magnesium chloride, a metal salt. The magnesium chloride was then dissolved in the dilute hydrochloric acid to remove the Mg from the sintered body (Step S13). Because graphite is insoluble in dilute hydrochloric acid, the remaining sample (graphite) was obtained (Step S14). The sintering conditions, density (g / cm ) of the prepared samples (Nos. 1 to 7) were as follows: ３ ), electrical resistivity (μΩm), and electrical conductivity (S / m) are shown in Table 1 below. Here, the SPS temperature means the set temperature in the spark plasma sintering method.

[0028]

[0029] Figure 5 shows the results of X-ray diffraction measurements for each sample. Figure 5 (a) shows the results for the activated carbon used as the raw material (magnified 10 times due to low intensity in the graph), while (b) to (f) show the results for samples of activated carbon containing 11% Mg sintered using spark plasma sintering (SPS) at the temperatures shown in the figure ((b): 870°C, (c): 1070°C, (d): 1160°C, (e): 1320°C, (f): 1480°C). The numbers 002 and 004 in Figure 5 represent the diffraction index of graphite. As shown in Figure 5, with increasing sintering temperature, the peak positions of 002 and 004, which correspond to the diffraction between two-dimensional carbon network planes, shift toward higher angles and the peak width becomes sharper, indicating the progression of graphitization. Furthermore, the diffraction angle of the 002 peak revealed that the interplanar distance between the two-dimensional layers of the sintered sample was 0.336 nm. This value is almost identical to the interplanar spacing of graphite (0.335 nm), confirming the progress of graphitization. Furthermore, since the sintered body of activated carbon to which Mg was added contains unreacted Mg and MgO produced during the sintering process, their diffraction peaks were also confirmed.

[0030] Figure 6 shows the results of X-ray diffraction measurements of the SPS sintered bodies of Samples Nos. 3 to 7 in Table 1 above, after they were immersed in dilute hydrochloric acid to dissolve Mg and MgO. By immersing the sintered bodies in dilute hydrochloric acid, the Mg and MgO contained in the sintered bodies become magnesium chloride and dissolve in the dilute hydrochloric acid. As a result, peaks related to Mg and MgO are no longer observed, and it was found that the samples were composed almost exclusively of carbon.

[0031] Figure 7 shows the X-ray diffraction measurement results of Figure 6 in a logarithmic scale to emphasize weak peaks. Figure 7 shows the diffraction indices of 100, 101, 110, 112, and 006 in addition to the diffraction indices of 002 and 004 of graphite.

[0032] Figure 8, like Figure 7, shows the X-ray diffraction measurement results (logarithmic scale). It compares the X-ray diffraction measurement results of Sample No. 6, sintered by SPS at 1160°C (see (a) in Figure 8), with the X-ray diffraction measurement results of commercially available graphite powder (see (b) in Figure 8). Sample No. 6, sintered by SPS at 1160°C, did not exhibit the peaks of diffraction indices 102, 103, 104, and 105 observed in graphite powder, confirming that the crystal structures of the two samples are not identical. The absence of these peaks suggests that the two-dimensional layers are partially rotated and that there is no regularity in the direction perpendicular to the two-dimensional layers. Therefore, structurally, the sintered body obtained by the present invention is not completely identical to the structure of graphite. However, as described in the explanation of FIG. 5, the interlayer angle of the two-dimensional layers is almost the same as that of ordinary graphite, and the electrical conductivity is also as high as that of ordinary graphite (see Table 1 above). Therefore, it is safe to consider that the sintered body obtained by the present invention has almost the same structure and properties as ordinary graphite.

[0033] In this example, graphite was produced by the method of the present invention using non-graphitizable carbon obtained by heating and carbonizing PET (Polyethylene terephthalate) bottles. The physical properties were evaluated. Specifically, PET bottles, which are waste plastics, were cut into approximately 2 mm square pieces. The temperature was raised to 400°C at a rate of 30°C per minute under atmospheric pressure, held for 1 hour, and then further raised to 1000°C at a rate of 30°C per minute, and held for 15 minutes. Graphite was then produced using the resulting carbonized material. The recovery rate of the carbon source from the heating process of the PET bottles was approximately 41% in terms of carbon in the PET, as calculated below.

[0034] <Calculation formula for carbon conversion in PET> Chemical formula: C １０ H ８ O ４ , Molecular weight (1 repeating unit): 192.17 Molecular weight of carbon only: 120.1 Theoretical yield: 120.1 / 192.17 x 100 = 62.5% Experimental yield: Raw material: 8.0174 g, Yield after heating: 2.0631 g Yield: 2.0631 / 8.0174 x 100 = 25.7% Recovery rate: 25.7 / 62.5 x 100 = approx. 41%

[0035] Figure 9 shows the flow chart for producing graphite from PET bottles. As shown in Figure 9, a PET bottle was cut and crushed into approximately 2 mm squares (Step S21). The temperature was then increased to 400 °C at a rate of 30 °C per minute under atmospheric pressure, and the temperature was maintained for 1 hour for carbonization (Step S22). The temperature was then increased to 1000 °C at a rate of 30 °C per minute under atmospheric pressure, and the resulting carbide was then heated in air for 15 minutes for complete carbonization (Step S23). 10% by mass of Mg was then added to the resulting carbide (Step S24). The Mg-added carbide was then sintered using spark plasma sintering (SPS) at 1020 °C for several tens of seconds (Step S25). The sintered body was then dissolved in dilute hydrochloric acid to produce magnesium chloride, a metal salt. The magnesium chloride was then dissolved in the dilute hydrochloric acid to remove the Mg from the sintered body (Step S26). The remaining sample (graphite) that did not dissolve in the dilute hydrochloric acid was collected (Step S27). The sintering conditions and density (g / cm ３ ), electrical resistivity (μΩm), and electrical conductivity (S / m) are shown in Table 2 below.

[0036]

[0037] Figure 10 compares the results of X-ray diffraction measurements using PET bottles and activated carbon as raw materials. When using PET bottles, carbonized PET bottles were added with 10% Mg, and the resulting carbonized material was prepared by spark plasma sintering at 1020°C for several tens of seconds, followed by washing with dilute hydrochloric acid. When using activated carbon, activated carbon was added with 11% Mg, and a sintered body was prepared by spark plasma sintering at 1160°C for several tens of seconds, followed by washing with dilute hydrochloric acid. Figure 10 shows no difference in the X-ray diffraction results, and it was found that even when PET was used as a carbon source, graphitization proceeded with the addition of 10% Mg and SPS treatment at approximately 1000°C.

[0038] Figure 11 shows the X-ray diffraction measurement results of Figure 10 in a logarithmic scale to emphasize weak peaks. From Figure 11, the diffraction indices of 002, 004, 100, 101, 110, 112, and 006 of graphite were confirmed.

[0039] Figure 12 shows a transmission electron microscope (TEM) image (magnification: 150,000 times) of graphite obtained using PET as the carbon source. It was found that even when PET was used as the carbon source, graphitization proceeded through the addition of 10% Mg and SPS treatment at approximately 1,000°C. Figure 12 shows that the flake-like sample was overlapping.

[0040] Figure 13 shows a high-resolution transmission electron microscope (HR-TEM) image (magnification: 1,000,000 times) of graphite obtained using PET as the carbon source. Crystal lattice fringes can be seen in Figures 13(1) and 13(2), indicating high crystallinity. Since each lattice fringe has a length of about 5 to 10 nm, the minimum size of graphitized crystals is estimated to be about 5 to 10 nm.

[0041] Figure 14 shows the results of X-ray diffraction measurements using PET with different amounts of Mg added. The raw material was a carbide obtained by adding 10% Mg to carbonized PET bottles. The spark plasma sintering conditions were a sintering temperature of 1100°C, a sintering pressure of 115 MPa, and a sintering time of 10 seconds. Figure 14 shows the dependence of the graphitization process on the amount of Mg added. Below 5%, there is no graphite peak, and the crystalline structure of graphite does not appear. It can also be seen that even with increasing Mg addition, the degree of crystallinity almost reaches a plateau at around 11%. All samples were washed with hydrochloric acid to remove Mg and MgO.

[0042] For Samples Nos. 3 to 7 in Table 1 of Example 1 and the sample in Example 2, the results of X-ray diffraction measurements of the sintered body after immersion in dilute hydrochloric acid to dissolve Mg and MgO are shown. By immersing the sintered body in dilute hydrochloric acid, the Mg and MgO contained in the sintered body became magnesium chloride and dissolved in the dilute hydrochloric acid, eliminating the peaks related to Mg and MgO. In other words, the hydrochloric acid washing procedure enabled the production of graphite free of residual Mg and MgO. This Example 3 demonstrates that a graphite-like material free of residual Mg and MgO can be obtained by controlling the sintering temperature and pressure without the need for hydrochloric acid washing. Figure 15 shows the production flow of graphite in Example 3. Figure 16 also shows the results of X-ray diffraction measurements demonstrating the removal of MgO by controlling the sintering temperature and sintering pressure. Figure 16(1) shows a linear vertical axis, while Figure 16(2) shows a logarithmic vertical axis to emphasize weak peaks.

[0043] In Example 3, similar to Example 2, a charcoal obtained by heating and carbonizing a PET bottle was used as the non-graphitizable carbon. In the production flow shown in Figure 15 , the addition of 10% by mass of Mg to the non-graphitizable carbon (Step S31) is the same as Steps S21 to S24 shown in Figure 9 for Example 2. The Mg-added charcoal was sintered using SPS at 1600 °C and 115 MPa for 10 seconds (Step S32). The sintered body was then held at 1600 °C and 2 MPa for 10 minutes (Step S33) to sublimate the Mg and MgO, yielding graphite (Step S34). As shown in Figure 16 , immediately after SPS treatment at high temperature (1600 °C) and high pressure (SPS pressure 115 MPa), Mg and MgO peaks were still observable, indicating their presence (see the lower plots in Figures 16 (1) and (2)). The Mg and MgO peaks are indicated by ▼ and ●, respectively). On the other hand, by subjecting the sample to low pressure (SPS pressure 2 MPa) and high temperature (1600°C) for 10 minutes immediately after SPS treatment, the Mg and MgO peaks disappeared (see the upper plots in Figures 16(1) and 16(2); all peaks other than those of Mg and MgO are due to graphite). Therefore, even without the hydrochloric acid washing step, by controlling the sintering temperature and pressure, graphite without residual Mg and MgO could be obtained.

[0044] On the other hand, in samples subjected to SPS at sintering temperatures below 1600°C, even when the sample was subsequently subjected to low pressure (SPS pressure 2 MPa) and high temperature (1600°C) for 10 minutes, MgO remained and was not removed (see Figure 17). Figure 17 shows the results of X-ray diffraction measurements under controlled sintering temperature and sintering pressure. Figure 17(1) shows a linear vertical axis, while Figure 17(2) shows a logarithmic vertical axis to highlight weak peaks. From Figure 17, Mg is relatively reduced, but a peak for MgO can be observed, indicating that MgO remains.

[0045] This Example 4 describes the improvement in crystallinity achieved by maintaining the material at high temperature (1600°C) and high pressure (SPS pressure: 115 MPa). Figure 18 shows the graphite production flow for this Example 4. Figure 19 shows the results of X-ray diffraction measurements demonstrating the improvement in crystallinity achieved by maintaining the material at high temperature and high pressure. Figure 19(1) shows the vertical axis as a linear plot, while Figure 19(2) shows the vertical axis as a logarithmic plot to highlight weak peaks. In this Example 4, similar to Example 2, a charcoal obtained by heating and carbonizing a PET bottle was used as the non-graphitizable carbon. In the production flow shown in Figure 18, the addition of 10% by mass of Mg to the non-graphitizable carbon (step S41) is the same as steps S21 to S24 shown in Figure 9 for Example 2. The Mg-added charcoal was sintered using SPS at 1600°C and low pressure (28 MPa) for 10 seconds (step S42). In the sintered body produced by SPS treatment at low pressure (28 MPa), Mg and MgO can be sublimated (step S43). Then, in order to increase the crystallinity of the graphite, the sintered body was held at 1600°C and 115 MPa for 10 minutes (step S44), thereby obtaining graphite with improved crystallinity.

[0046] As shown in Figure 19, immediately after SPS treatment at high temperature (1600°C) and low pressure (SPS pressure 28 MPa), no peaks for Mg or MgO were observed, indicating that no Mg or MgO remained (see the lower plots in Figures 19(1) and 19(2)). By subjecting the sample to high pressure (SPS pressure 115 MPa) and high temperature (1600°C) for 10 minutes immediately after SPS treatment, the crystallinity of the graphite was further improved (see the upper plots in Figures 19(1) and 19(2)). Therefore, even without the hydrochloric acid washing step, graphite free of residual Mg and MgO could be obtained by controlling the sintering temperature and pressure.

[0047] This Example 5 describes the improvement in crystallinity achieved by SPS treatment at high temperature (1600°C) and low pressure (SPS pressure: 28 MPa), followed immediately by holding at low temperature (1200°C) and high pressure (SPS pressure: 115 MPa) for 10 minutes. Figure 20 shows the graphite production flow for this Example 5. Figure 21 shows the results of X-ray diffraction measurements demonstrating the improvement in crystallinity achieved by holding at low temperature and high pressure. Figure 21(1) shows a linear representation of the vertical axis, while Figure 21(2) shows a logarithmic representation of the vertical axis to highlight weak peaks. In this Example 5, graphite was produced using commercially available activated carbon (Kuraray Co., Ltd., Kuraray Coal (registered trademark) PDX-1, average particle size: 140 μm) as the non-graphitizable carbon. As shown in Figure 20, 11% by mass of Mg was added to non-graphitizable carbon (Step S51), and the Mg-added carbide was subjected to SPS at 1600°C under low pressure (28 MPa) for 10 seconds to produce a sintered body (Step S52). The sintered body produced by SPS at low pressure (28 MPa) was able to sublimate Mg and MgO (Step S53). To increase the crystallinity of the graphite, the sintered body was then held at a low temperature (1200°C) under 115 MPa for 10 minutes (Step S54), yielding graphite with improved crystallinity.

[0048] As shown in Figure 21, immediately after SPS treatment at high temperature (1600°C) and low pressure (SPS pressure 28 MPa), no Mg or MgO peaks were observed, indicating that no Mg or MgO remained, but a broad component remained, indicating that graphitization had not progressed completely (see the lower plots in Figures 21(1) and 21(2)). By subjecting the sample to high pressure (SPS pressure 115 MPa) and low temperature (1200°C) for 10 minutes immediately after SPS treatment, the crystallinity of the graphite was further improved (see the upper plots in Figures 21(1) and 21(2)). Therefore, even without the hydrochloric acid washing step, graphite free of residual Mg or MgO could be obtained by controlling the sintering temperature and pressure.

[0049] In Example 6, we describe the change in graphite interplanar spacing associated with the addition of boron to non-graphitizable carbon in addition to the addition of magnesium. In this Example 6, commercially available activated carbon (Kuraray Co., Ltd., Kuraray Coal (registered trademark) PDX-1, average particle size: 140 μm) was used as the non-graphitizable carbon. This activated carbon was doped with 11% Mg by mass and sintered at 1600°C, 115 MPa, and 10 seconds using SPS. Figure 22 shows the results of X-ray diffraction measurements, illustrating the change in graphite interplanar spacing associated with boron addition. As shown in Figure 22, the interplanar spacing (d value) increased from 0 wt%, to 3.44 nm, and then to 3.46 nm, indicating that the interplanar spacing of the resulting graphite increased as the boron addition amount increased from 0 wt%, to 2 wt%, and then to 5 wt%. This increase in interplanar spacing is expected to improve the negative electrode characteristics of lithium-ion batteries. As described above, from the diffraction angle of the 002 peak, 2θ=26.6°, and the interplanar spacing of graphite is 0.335 nm.

[0050] The present invention is useful as a method for producing synthetic graphite.

[0051] REFERENCE SIGNS LIST 1 Hard carbon powder 2 Sintered body 3 Graphite 4 Mortar 5 Magnesium powder 6 Die 7 Glass container 8 Acidic solution 10 Spark plasma sintering apparatus 11 Chamber 12 Pressurizing shaft and electrode 13 Spacer 14 Material to be sintered 16 Punch

Claims

1. A method for producing graphite, comprising sintering non-graphitizable carbon to which magnesium has been added at 1000 to 1700°C, and removing magnesium and magnesium oxide from the sintered body.

2. The method for producing graphite according to claim 1, characterized in that the amount of magnesium added to the non-graphitizable carbon is 5 to 12 mass %.

3. The method for producing graphite according to claim 2, characterized in that the sintering time of the non-graphitizable carbon is within 10 minutes.

4. A method for producing graphite according to claim 2, characterized in that the sintering time of the non-graphitizable carbon is within 60 seconds.

5. A method for producing graphite according to claim 4, characterized in that the non-graphitizable carbon is sintered using a spark plasma sintering method.

6. A method for producing graphite according to claim 5, characterized in that the sintering of non-graphitizable carbon is carried out under high pressure of 100 MPa or more.

7. A method for producing graphite according to claim 6, characterized in that the step of removing magnesium and magnesium oxide from the sintered body comprises holding the sintered body at 1600 to 1700°C and under a low pressure of 10 MPa or less after sintering.

8. The method for producing graphite according to claim 5, characterized in that the non-graphitizable carbon is sintered at 1600-1700°C under a low pressure of 20-40 MPa, and then maintained at 1000-1700°C under a high pressure of 100 MPa or more.

9. A method for producing graphite according to any one of claims 1 to 8, characterized in that the sintering of non-graphitizable carbon is carried out in a vacuum atmosphere of 10 Pa or less.

10. A method for producing graphite according to any one of claims 1 to 8, characterized in that the sintering of non-graphitizable carbon is carried out in a non-closed system.

11. A method for producing graphite according to any one of claims 1 to 8, characterized in that the non-graphitizable carbon is obtained by burning and carbonizing waste plastic.

12. A method for producing graphite as set forth in any one of claims 1 to 6, characterized in that the step of removing magnesium and magnesium oxide from the sintered body comprises dissolving the sintered body in acid.

13. A method for producing graphite according to any one of claims 1 to 8, characterized in that the sintered body from which magnesium and magnesium oxide have been removed is used as a negative electrode material for lithium ion batteries.

14. A method for producing graphite according to any one of claims 1 to 8, characterized in that boron is added to the non-graphitizable carbon in addition to magnesium, and then the carbon is sintered.

15. The method for producing graphite according to claim 14, characterized in that the sintered body from which magnesium and magnesium oxide have been removed has an increased interplanar spacing of graphite and is used as a negative electrode material for lithium ion batteries.

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