Method for producing graphite intercalation compounds

Abstract

Provided is a method for producing a GIC whereby a given insertion material can be easily inserted between layers of graphite. The insertion material is reacted with graphite in the presence of Na. The insertion material may be an alkali metal, an alkaline earth metal, and lanthanide.

Description

[Technical Field]

[0001] This invention relates to a method for producing graphite intercalation compounds. [Background technology]

[0002] Graphite intercalation compounds (GICs) are known in which atoms or molecules are inserted between layers of graphite. Such GICs exhibit various properties depending on the type of atoms or molecules inserted between the graphite layers and their position (staging) within those layers, and are expected to have a wide range of applications, including battery electrode materials, superconducting materials, gas storage materials, thermoelectric conversion materials, magnetic materials, and conductive materials.

[0003] On the other hand, various methods have been devised to produce GIC, including long-duration gas-phase reaction methods using special (expensive) graphite, molten salt methods, and solid-phase reactions at high temperatures, such as around 500°C, where it is difficult to obtain pure samples. However, it has been difficult to efficiently synthesize homogeneous and large quantities of GIC, as GIC is often only produced from a portion of the raw materials or only small amounts of GIC are obtained.

[0004] For example, Patent Document 1 discloses a method for producing GIC by contacting graphite with an insertion compound in the presence of a supercritical fluid that easily penetrates between layers (between layers) of graphite. If the compound constituting the supercritical fluid is a gas at room temperature and atmospheric pressure, the insertion compound can be inserted between the layers of graphite under a predetermined temperature and pressure that indicates the supercritical state of the supercritical fluid, and then returned to room temperature and atmospheric pressure. This allows the graphite interlayer compound to be extracted from the compound that constituted the supercritical fluid. According to this method, it is possible to easily produce GIC with uniform quality by uniformly inserting the insertion compound or ions derived therefrom between the layers of graphite. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-213583 [Overview of the project] [Problems that the invention aims to solve]

[0006] As mentioned above, there is a need for a manufacturing method that can efficiently synthesize homogeneous and large quantities of GIC.

[0007] The present invention has been made in view of the above circumstances, and its objective is to provide a method for manufacturing GIC that allows any inserting material to be easily inserted between layers of graphite. [Means for solving the problem]

[0008] The method for producing graphite intercalation compounds according to the present invention is characterized by reacting graphite with an insertion material in the presence of Na. This characteristic allows for the easy synthesis of homogeneous and large quantities of GIC.

[0009] In the invention described above, the graphite may be characterized as being in powder form. With this characteristic, homogeneous and large quantities of GIC can be synthesized more efficiently.

[0010] In the invention described above, the insert material may be an alkali metal, and may be characterized by being mixed with the graphite and reacted. Alternatively, the insert material may consist of either an alkaline earth metal or a lanthanide, and may be characterized by being mixed with the graphite and reacted by heating. According to such features, high-quality GIC with a predetermined insert material can be efficiently synthesized.

[0011] The invention described above may be characterized by heating to a predetermined temperature of 250°C or lower to carry out the reaction. With this characteristic, high temperatures that make it difficult to obtain a pure sample are not required, and a homogeneous and large quantity of GIC can be efficiently synthesized.

[0012] In the invention described above, the invention may be characterized by comprising a step of removing Na after mixing. According to this feature, a large amount of homogeneous GIC can be synthesized efficiently. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] [Figure 1] It is a photograph showing one step of a method for producing a graphite intercalation compound as an embodiment according to the present invention. [Figure 2] It is an appearance photograph of a GIC synthesized using Li as an intercalation material. [Figure 3] It is an XRD pattern showing each reaction process of Li-GIC. [Figure 4] It is an X-ray diffraction (XRD) pattern showing the relationship between the blending ratio of raw materials for Li-GIC and the stage. [Figure 5] It is a diagram explaining the stage of GIC. [Figure 6] It is an XRD pattern showing the relationship between the blending ratio of raw materials for K-GIC and the stage. [Figure 7] These are XRD patterns of (a) a product containing Na-GIC (NaCy) obtained in advance, a product obtained by adding Li thereto (b), and a product obtained by adding K thereto (c). [Figure 8] It is a superimposed view of partially enlarged XRD patterns of products containing Na-GIC obtained with different blending ratios. [Figure 9] These are XRD patterns of products using Ca, Sr, and Ba as intercalation materials. Here, the black circles indicate diffraction peaks attributed to graphite intercalation compounds such as CaC6. [Figure 10] It is a graph showing the temperature dependence of the magnetic susceptibility of CaC6. [Figure 11] It is a side cross-sectional view of a die used for removing Na. [Figure 12] It is an appearance photograph of a quartz tube used for removing Na. [Figure 13] These are XRD patterns of products using Sm, Eu, and Yb as intercalation materials. Here, the black circles indicate diffraction peaks attributed to graphite intercalation compounds such as SmC6. [Modes for carrying out the invention]

[0014] Below, a method for producing GIC (graphite intercalation compound), which is one embodiment of the present invention, will be described with reference to Figure 1.

[0015] The method for producing GIC according to this embodiment involves reacting graphite and an intercalate material in the presence of Na.

[0016] For example, as shown in Figure 1, GIC can be synthesized by placing graphite 11, sodium 12 (Na), and an insert material 13 in a mortar 10 and mixing them so that they react, kneading the graphite and insert material into the sodium, which is a soft metal.

[0017] In this case, it is preferable that the graphite 11 is in powder form so that GIC can be synthesized efficiently. Furthermore, it is not necessary to use special and expensive graphite 11 that is particularly high in purity or highly oriented; for example, commercially available graphite with a purity of around 99.99% is sufficient.

[0018] Sodium 12 can be ground in the mortar 10 and may be added as a lump of metal. To maintain the "presence of Na," mixing should be carried out in an inert gas atmosphere such as nitrogen or argon, or in a vacuum, to prevent oxidation of Na. In this embodiment, the mortar 10 was placed inside a glove box and used for manufacturing. In industrial manufacturing, a sealed ball mill or the like can be used.

[0019] The insert material 13 is a material inserted between the graphene layers (hereinafter simply referred to as "interlayers") in graphite 11, which is composed of layers of graphene, and can be, for example, an alkali metal, an alkaline earth metal, or a lanthanide, or a combination thereof.

[0020] Suitable alkali metals include Li, Na, K, Rb, and Cs. Since alkali metals are soft, they can be mixed by placing them in a mortar 10 as chunks and kneading them with a pestle. A purity of approximately 99% is sufficient for the raw materials. When these are used as inserts, GIC can be synthesized by mixing at room temperature.

[0021] Suitable alkaline earth metals include Ca, Sr, and Ba. Suitable lanthanides include Sm, Eu, and Yb. Alkaline earth metals and lanthanides are relatively hard, making it difficult to grind them from lumps with a mortar and pestle. Therefore, it is preferable to crush them into powders of about 100 μm in diameter using a file or the like before adding them to the mortar 10. The purity of the raw materials should be around 99%. When these are used as insert materials, GIC can be synthesized by mixing the raw materials and holding them in an environment heated to a predetermined temperature that allows the reaction to proceed. This predetermined temperature may be around 300-400°C, but synthesis is possible in a sufficiently short time even at low temperatures of 250°C or below. Generally, it is preferable to have a temperature of 100°C or higher, considering the synthesis time. For example, if mixing and heating are repeated at 250°C, the reaction between the raw materials will be largely completed in a total of several hours.

[0022] In all of the above cases, the synthesis time required to obtain GIC can be reduced to a few minutes to at most a few hours, which is significantly shorter than the conventional GIC synthesis time, which takes several days to several weeks. Furthermore, it does not require high temperatures that make it difficult to obtain pure samples, and as mentioned above, it enables the synthesis of GIC by heating at room temperature to around 250°C. In addition, it does not require the precise temperature and composition control that was necessary in conventional methods, nor does it require complex processes or special equipment. As such, synthesis can be performed simply, quickly, and efficiently, and it is possible to synthesize homogeneous and large quantities of GIC, making it suitable for industrial GIC production.

[0023] In the manufacturing method described above, sodium (Na) is used, so the synthesized GIC will contain residual Na. If Na is not required, a step to remove Na can be added. For example, this could involve melting the Na by heating and then centrifuging it, or heating the powder containing the GIC and forming it into pellets to extrude the liquid Na (details will be described later), or heating the resulting pellets to evaporate the Na (details will be described later).

[0024] Next, we will explain the results of actually synthesizing the GIC.

[0025] [Example 1] As shown in Figure 2, Li was used as an insert material and mixed with graphite and Na. The raw materials were weighed to a molar ratio of Li:C:Na = 1:6:2 and placed in a mortar. The mixture was then kneaded with a pestle. The raw materials, which initially exhibited mainly black color due to the graphite, gradually developed a metallic luster as the reaction progressed. This metallic luster was initially silvery, and as the mixing continued, it gradually took on the yellowish tint characteristic of LiC6.

[0026] As shown in Figure 3, X-ray diffraction (XRD) analysis was performed on the products at each stage of the synthesis of LiC6. It was found that the reaction proceeded as follows: In other words, in the initial stage of mixing, LiC6 and LiC 12 NaC y A compound is formed, along with unreacted C, Li, and Na. As the reaction proceeds, the amount of unreacted C and Li decreases, and NaC is also present. y Also decreased, LiC6 and LiC 12 It increases. As the reaction proceeds, LiC 12 The amount of sodium decreased, while the amount of LiC6 increased, eventually resulting in a mixture of LiC6 and Na, indicating that the reaction was largely complete. When approximately 0.1 g of raw materials was used, the reaction took approximately 20 minutes to complete. In the figure, 2θ represents the diffraction angle. An X-ray source using Cuα1 is also shown.

[0027] As described above, in the reaction of inserting Li, which is the intercalation material, into graphite, Na is considered to function like a catalyst that accelerates the reaction. In the present example, the time required for producing GIC is extremely shortened compared to conventional production methods. The reason for this is that NaC generated by the reaction between Na and C y acts as a reaction intermediate in the catalytic reaction, and is considered to reduce the activation energy for intercalation of the intercalation material into the graphite interlayers. Furthermore, for example, even when Li is mixed using K instead of Na, only KC₆ is generated, and GIC using Li as the intercalation material (Li-GIC) could not be synthesized. From this, it can be said that Na functions specifically. Since Na readily forms GIC through reaction with graphite, mixing first produces Na-GIC (NaC y ), but since Na-GIC is unstable compared to other GICs, when Li or other intercalation materials are present in the surrounding environment, it is considered that Na is replaced by these elements to form more stable GIC such as LiC₆.

[0028] Furthermore, in view of such a function of Na, it is presumed that even if the amount of Na used as a raw material is small, GIC can be similarly synthesized by taking sufficient time. On the other hand, when the amount of Na is increased, the synthesis efficiency of GIC can be increased to a certain extent, but this effect is considered to become saturated. Considering also the removal of Na, it can be said that selecting a well-balanced amount of Na is also important.

[0029] Furthermore, as shown in Fig. 4, component analysis by X-ray diffractometry (XRD) was performed on samples containing Li-GIC synthesized by changing the blending ratio of raw materials. Here, approximately 0.1 g of raw materials are used, and mixing is performed for approximately 20 minutes. As a result, when the molar ratio of raw materials is Li:C:Na = 1:6:1 (lower panel of the figure), LiC₆ is generated as GIC, and when this ratio is 1:12:2 (upper panel of the figure), LiC 12 is generated as GIC. LiC₆ has a stage 1 structure, and LiC 12This has the structure of Stage 2. In this way, it is possible to manipulate the stages by adjusting the amount of insert material and graphite so that the elemental ratio of the desired structure is achieved. Here, we have shown that in Li-GIC, the structures of Stage 1 and Stage 2 can be controlled by adjusting the raw material composition.

[0030] As shown in Figure 5, the structure of stage n refers to a structure in which n layers of graphene 21 are arranged between the interlayers of graphite 11 into which the insert material 13 is inserted (see Figure (a)). For example, the structure of stage 1 is such that, as a result of inserting the insert material 13 between all the interlayers of graphene 21, there is one layer of graphene 21 between the interlayers into which the graphene 21 is inserted (see Figure (b)). The structure of stage 2 is such that there are two layers of graphene 21 between the interlayers into which the insert material 13 is inserted (see Figure (c)). In stage 3, there are three layers of graphene 21 between the interlayers into which the insert material is inserted (see Figure (d)).

[0031] [Example 2] As shown in Figure 6, we confirmed whether the stages could be manipulated for K-GIC. Here, approximately 0.1 g of raw materials, formulated so that K:C:Na = 1:x:x / 6, were mixed at room temperature for approximately 20 minutes, as described above, to synthesize K-GIC. When x=8, KC8 (stage 1) was produced, and when x=24, KC 24 (Stage 2), KC when x=36 36 (Stage 3), KC when x=48 48 It was confirmed that K-GIC of stage 4 could be obtained. In other words, it was shown that even in K-GIC, the structure of stages 1 to 4 can be controlled by the raw material formulation.

[0032] [Example 3] Next, we examined the results when the order in which the raw materials were added was changed.

[0033] As shown in Figure 7(a), when the raw materials, which had been weighed in advance to have a C:Na ratio of 3:1, were mixed in a mortar, Na-GIC(NaC y A mixture of ) and Na was obtained. Then, as shown in Figure (b), Li was added to the mixture of Na-GIC and Na so that Li:C:Na = 1:6:2 and mixed, yielding LiC6. Also, as shown in Figure (c), K was added to the mixture of Na-GIC and Na so that K:C:Na = 1:8:2.67 and mixed, yielding KC8. In this way, even if Na-GIC was produced in advance and the insert material was added later, it was possible to synthesize GICs with each insert material. This indicates a reaction process in which, as described above, Na-GIC acts as an intermediate reactant in the catalytic reaction, producing more stable Li-GIC and K-GIC.

[0034] Furthermore, the Na-GIC, i.e., NaC, that was pre-generated as described above, y However, in order to estimate the value of y, the product was investigated by changing the value of z in the mixing ratio of the raw materials C and Na, C:Na=z:1.

[0035] As shown in Figure 8, when the XRD patterns of the obtained products were enlarged and superimposed in the range of 20≦2θ≦40, NaC was found at z=32. y No peak was observed, and NaC was finally detected at z=14~16. y The peak begins to appear, and at z=12 it is almost NaC y The peak for NaC became dominant. At z=10, only the Na peak became large, suggesting that up to a ratio of approximately y=z=12, Na reacts with graphite in a form that is not observed as a diffraction peak. y Considering that high-stage GICs (n=6~8 and y=48~64) are considered stable, it is thought that in addition to being inserted between graphite layers, Na also exists adsorbed on the graphite surface. y The peak of is wider than the peak of C, indicating that it is a different stage of NaC y It is thought that these are mixed together.

[0036] [Example 4] When alkaline earth metals (AE), specifically Ca, Sr, and Ba, were selected as the insert material, the raw materials were weighed and mixed in a ratio of AE:C:Na = 1:6:2, formed into pellets, and vacuum-sealed inside a quartz tube. Furthermore, the quartz tube containing the pellets was heated at 250°C for 2 hours.

[0037] Figure 9 shows the XRD patterns of each product obtained as described above. As can be seen, heating at 250°C produced the compounds CaC6, SrC6, and BaC6, respectively. In the sample that produced CaC6, unreacted Ca and NaC y Although it exists, by repeating the crushing, mixing, and heat treatment of the pellets several times, Ca and NaC y It was also possible to react almost the entire amount.

[0038] As shown in Figure 10, the temperature dependence of the magnetic susceptibility of the CaC6 obtained as described above was measured. Generally, CaC6 is known as a superconductor with the highest superconducting critical temperature among GICs, at 11.5 K. As is clear from the figure, the CaC6 obtained by the above method shows a superconducting transition around 11.5 K. In other words, this CaC6 was confirmed to be equivalent to the CaC6 reported to date.

[0039] [Regarding the removal of sodium] As described above, when GIC is synthesized using the method of this embodiment, the product contains Na. Therefore, two examples of steps for removing Na will be described below.

[0040] Referring to Figure 11, a method for removing Na from pellets when forming the product 25 containing GIC and Na obtained by synthesizing GIC into pellets will be described. Specifically, the product 25 is held in a die 31 and heated together with a compression rod 32 on a hot plate or the like to a temperature above the melting point of Na (98°C). The heating temperature can be, for example, 200°C. Next, these are placed on a press table 33 with a spacer ring 34 on it, and the product 25 is compressed with the compression rod 32. As a result, the molten Na 26 flows out from the gap between the die 31 and the compression rod 32 to the inside of the spacer ring 34 and to the upper end of the die 31, thereby removing Na from the product 25. By adding this step, sintered pellets of GIC from which approximately 90% of the Na remaining in the product 25 has been removed can be obtained.

[0041] As shown in Figure 12, Na can also be evaporated and removed by heating. Specifically, the product 25 is placed inside the heating section 41 at one end of a quartz tube 40 and vacuum-sealed, and heated to a temperature at which Na evaporates, for example, 250°C. At this time, the other end is kept at room temperature, so that the evaporated Na condenses in the condensation section 42 at the other end. This removes Na from the product 25 placed in the heating section 41. By this method, almost all of the Na remaining in the product 25 can be removed.

[0042] [Example 5] In the case where lanthanides (Ln), specifically Sm, Eu, and Yb, were selected as the insert material, the raw materials were weighed and mixed so that Ln:C:Na = 1:6:2, formed into pellets, and sealed inside a stainless steel tube. Furthermore, similar to Example 4, the tube containing the pellets was heated, but because it is less reactive than alkaline earth metals, it was heated at 275°C for 6 hours. In addition, to further improve purity, the sample was mixed again and heated at 275°C for another 6 hours.

[0043] Figure 13 shows the XRD patterns after removing Na from each of the obtained products using the method described above. Here again, the compounds SmC6, EuC6, and YbC6 were produced, respectively.

[0044] As described above, by the manufacturing method in which graphite and an insertion material are reacted in the presence of Na, any insertion material can be easily inserted between layers of graphite to synthesize GIC.

[0045] Although representative embodiments of the present invention have been described above, the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the attached claims. [Explanation of Symbols]

[0046] 10 Mortar and pestle 11 Graphite 12 Sodium 13 Insertion material

Claims

1. A method for producing a graphite intercalation compound by inserting an insert material consisting of an alkali metal, an alkaline earth metal, or a lanthanide, or a combination thereof, between graphene layers, The process includes a reaction step in which the insertion material is reacted with graphite in the presence of Na, followed by a step in which Na is removed. The reaction step involves weighing Na, graphite, and the insertion material in a predetermined ratio, mixing the graphite and the insertion material with the soft metal Na and reacting them, while simultaneously generating a reaction intermediate of Na-GIC produced by the reaction of Na and C, and reacting the graphite with the insertion material. A method for producing a graphite intercalation compound, characterized in that the predetermined ratio is at least the mixing ratio of C and Na that produce the reaction intermediate.

2. The method for producing a graphite intercalation compound according to claim 1, characterized in that the graphite is in powder form.

3. The method for producing a graphite intercalation compound according to claim 1 or 2, characterized in that the insert material is an alkali metal and is reacted with the graphite.

4. The method for producing a graphite intercalation compound according to claim 2, characterized in that the insert material consists of either an alkaline earth metal or a lanthanide, and is mixed with the graphite and heated to cause a reaction.

5. A method for producing a graphite intercalation compound according to claim 4, characterized by heating to a predetermined temperature of 250°C or lower to carry out the reaction.

6. A method for producing a graphite intercalation compound according to claim 1, characterized in that a periodic structure is formed by arranging a predetermined number of graphene layers between the intercalations in which the insert material is inserted.

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