Method for producing graphite intercalation compound

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

Method for producing graphite intercalation compounds

[0001] The present invention relates to a method for producing a graphite intercalation compound.

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

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

[0004] For example, Patent Document 1 discloses a method for producing GICs by contacting graphite with an insertion compound in the presence of a supercritical fluid that easily penetrates between graphite layers (between layer planes). When the compound constituting the supercritical fluid is gaseous at room temperature and atmospheric pressure, the insertion compound is inserted between the graphite layers at a predetermined temperature and pressure that indicates the supercritical state of the supercritical fluid, and then the temperature and pressure are returned to room temperature and atmospheric pressure. This method allows the graphite intercalation compound to be extracted from the compounds that constituted the supercritical fluid. It is claimed that this method allows the insertion compound or ions derived therefrom to be uniformly inserted between the graphite layers, making it possible to easily produce GICs with uniform quality.

[0005] JP 2011-213583 A

[0006] As described above, there is a need for a production method that can efficiently synthesize a large amount of homogeneous GIC.

[0007] The present invention has been made in view of the above circumstances, and its object is to provide a method for producing a GIC that allows any intercalation material to be easily intercalated between graphite layers.

[0008] The method for producing a graphite intercalation compound according to the present invention is characterized by reacting an intercalation material with graphite in the presence of Na. This feature allows for the easy synthesis of a large amount of homogeneous GIC.

[0009] In the above-described invention, the graphite may be in the form of powder. This feature allows for the efficient synthesis of a large amount of homogeneous GIC.

[0010] In the above-described invention, the intercalation material may be an alkali metal, which is mixed with the graphite and reacted. Alternatively, the intercalation material may be either an alkaline earth metal or a lanthanoid, which is mixed with the graphite and heated to react. According to this feature, high-quality GICs intercalated with a predetermined intercalation material can be efficiently synthesized.

[0011] The above-described invention may be characterized in that the reaction is carried out by heating to a predetermined temperature of 250° C. or less. This characteristic makes it possible to efficiently synthesize a large amount of homogeneous GIC without requiring high temperatures that make it difficult to obtain a pure sample.

[0012] The above-described invention may be characterized by including a step of removing Na after mixing. With this feature, it is possible to efficiently synthesize a large amount of homogeneous GIC.

[0013] 1 is a photograph showing one step of a manufacturing method of a graphite intercalation compound as an example according to the present invention; 2 is a photograph of the appearance of GIC synthesized using Li as an intercalation material; 3 is an XRD pattern showing each reaction process of Li-GIC; 4 is an X-ray diffraction (XRD) pattern showing the relationship between the composition of raw materials and the stage of Li-GIC; 5 is a diagram explaining the stages of GIC; 6 is an XRD pattern showing the relationship between the composition of raw materials and the stage of K-GIC; (a) Na-GIC (NaC ｙ (b) the product obtained by adding Li to the product (c) the product obtained by adding K to the product (b). The XRD patterns are superimposed enlarged partial XRD patterns of products containing Na-GIC obtained by changing the formulation. The XRD patterns are of products with Ca, Sr, and Ba as intercalation materials. The black circles indicate CaC６ These are diffraction peaks due to graphite intercalation compounds such as CaC ６ 1 is a graph showing the temperature dependence of magnetic susceptibility of SmC. 2 is a side cross-sectional view of a die used to remove Na. 3 is a photograph of the appearance of a quartz tube used to remove Na. 4 is an XRD pattern of a product using Sm, Eu, and Yb as insertion materials. 5 is a graph showing the temperature dependence of magnetic susceptibility of SmC. ６ These are diffraction peaks due to graphite intercalation compounds such as

[0014] A method for producing a GIC (graphite intercalation compound) according to one embodiment of the present invention will be described below with reference to FIG.

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

[0016] For example, as shown in FIG. 1, graphite 11, sodium 12 (Na), and an intercalation material 13 are placed in a mortar 10, and the graphite and intercalation material are kneaded into the soft metal sodium so as to react with each other, thereby synthesizing a GIC.

[0017] In this case, it is preferable that the graphite 11 is powder, since this allows for efficient synthesis of GIC. Note that it is not necessary to use special, expensive graphite 11 that is particularly high in purity or highly oriented; for example, a commercially available product with a purity of about 99.99% is sufficient.

[0018] Sodium 12 can be added as chunks of metal pieces since it can be ground in the mortar 10. To maintain the state of "in the presence of Na," mixing is performed in an inert gas atmosphere such as nitrogen or argon, or in a vacuum to prevent oxidation of Na. In this example, the mortar 10 was placed inside a glove box and used for production. For industrial production, a sealable ball mill or the like can be used.

[0019] The intercalation material 13 is a material to be intercalated between graphene layers (hereinafter simply referred to as interlayer) in the graphite 11, which is composed of stacked graphene layers, and may be, for example, any one of an alkali metal, an alkaline earth metal, and a lanthanoid, or a combination thereof.

[0020] Suitable alkali metals include Li, Na, K, Rb, and Cs. Because alkali metals are soft, they can be mixed by adding them as chunks of metal pieces to the mortar 10 and kneading them with a pestle. The purity of the raw material can be about 99%. When these metals are used as intercalation materials, GIC can be synthesized by mixing them 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 and difficult to grind from a lump using a pestle. Therefore, they are preferably crushed into powder with a diameter of approximately 100 μm using a file or similar tool before being placed in the mortar 10. The purity of the raw material can be approximately 99%. When these are used as intercalation materials, GIC can be synthesized by mixing the raw materials and then maintaining them in an environment heated to a predetermined temperature that allows the reaction to proceed. This predetermined temperature may be approximately 300-400°C, but synthesis can be achieved in a sufficiently short time even at temperatures as low as 250°C or lower. Generally, a temperature of 100°C or higher is preferable, taking into account the synthesis time. For example, by repeatedly mixing and heating at 250°C, the reaction between the raw materials is generally completed within a total of several hours.

[0022] In either of the above cases, the synthesis time required to obtain GIC can be as short as a few minutes to a few hours at most, which is significantly shorter than the conventional synthesis time of GIC, which requires several days to a few weeks. Furthermore, high temperatures that make it difficult to obtain a pure sample are not required, and as mentioned above, GIC can be synthesized by heating at room temperature to about 250°C. Furthermore, the precise temperature and composition control required in conventional methods is not required, and complicated processes and special equipment are not required. Because of these features, efficient synthesis can be performed simply and quickly, and it is possible to synthesize homogeneous GIC in large quantities, making this method convenient for industrial GIC production.

[0023] In the above-described manufacturing method, Na is used, so Na remains mixed in the synthesized GIC. If Na is unnecessary, a step of removing Na can be added. For example, there are a method of melting Na by heating and then centrifuging, a method of forming a powder containing GIC into pellets while heating them and then extruding the liquid Na (details will be described later), and a method of heating the resulting pellets to evaporate the Na (details will be described later).

[0024] The results of actually synthesizing GIC will be described below.

[0025] [Example 1] As shown in Figure 2, Li was used as an intercalating material and mixed with graphite and Na. The raw materials were weighed out so that the molar ratio of Li:C:Na was 1:6:2 and placed in a mortar. Then, the materials were mixed by kneading with a pestle. Then, the raw materials, which initially showed mainly black, the color of graphite, reacted and gradually took on a metallic luster. This metallic luster was initially silver, and as the mixing proceeded, it gradually became LiC. ６ It has taken on the characteristic yellow color.

[0026] As shown in FIG. ６ In each step of the synthesis, the products were analyzed by X-ray diffraction (XRD). It was found that the reaction proceeded as follows. ６ , LiC １２ , NaC ｙ is produced, and unreacted C, Li, and Na are also present. As the reaction progresses, the unreacted C and Li decrease and NaC ｙ Also decreased, LiC ６ and LiC １２ As the reaction progresses, LiC １２ has started to decrease, and LiC ６ increases, and finally LiC ６ The reaction was almost complete, resulting in a mixture of Na and Cu. When approximately 0.1 g of raw material was used, the reaction took approximately 20 minutes to complete. In the figure, 2θ is the diffraction angle. The figure also shows an X-ray source using Cuα1.

[0027] In this way, it is thought that Na functions like a catalyst to accelerate the reaction in the reaction of inserting Li, which is the insertion material, into graphite. In this example, the time required to manufacture GIC is much shorter than in the conventional manufacturing method. The reason for this is that NaC, which is generated by the reaction of Na and C, ｙ It is thought that K acts as a reaction intermediate in the catalytic reaction, lowering the activation energy for the insertion of the intercalation material between the layers of graphite. ６ It was not possible to synthesize GIC with Li as an intercalation material (Li-GIC). This suggests that Na acts uniquely. Since Na easily generates GIC by reacting with graphite, it is necessary to first mix Na-GIC (NaC ｙ ) is produced, but it is unstable compared to other GICs, so when Li or other intercalation materials are present in the vicinity, it replaces those elements to produce the more stable LiC ６ It is thought that GICs such as the above are generated.

[0028] In addition, in view of the function of Na, it is presumed that GIC can be similarly synthesized by using a small amount of Na as a raw material over a period of time. On the other hand, increasing the amount of Na can increase the synthesis efficiency of GIC to a certain extent, but this effect is thought to saturate. Taking into account the removal of Na, it can be said that it is also important to select a well-balanced amount of Na.

[0029] Furthermore, as shown in Figure 4, component analysis was performed by X-ray diffraction (XRD) on samples containing Li-GIC synthesized by changing the blend of raw materials. Here, approximately 0.1 g of raw materials were mixed for approximately 20 minutes. As a result, when the blend of raw materials was Li:C:Na = 1:6:1 in molar ratio (lower part of the figure), LiC was found as GIC. ６ When this ratio is set to 1:12:2 (upper part of the figure), LiC is produced as GIC. １２ LiC ６ has a stage 1 structure, and LiC １２has a stage 2 structure. In this way, it is possible to manipulate the stage by blending the amounts of intercalation material and graphite to achieve the element ratio of the desired structure. Here, we demonstrated that in Li-GIC, the stage 1 and stage 2 structures can be controlled by blending the raw materials.

[0030] 5, the stage n structure refers to a structure in which n layers of graphene 21 are arranged between the interlayers of graphene 21 of graphite 11 where an intercalation material 13 is inserted (see FIG. 5(a)). For example, the stage 1 structure is a structure in which an intercalation material 13 is inserted between all the interlayers of graphene 21, resulting in one layer of graphene 21 being arranged between the interlayers where the graphene 21 is inserted (see FIG. 5(b)). The stage 2 structure is a structure in which two layers of graphene 21 are arranged between the interlayers where the intercalation material 13 is inserted (see FIG. 5(c)). The stage 3 structure is a structure in which three layers of graphene 21 are arranged between the intercalation layers (see FIG. 5(d)).

[0031] [Example 2] As shown in Figure 6, it was confirmed whether the stage could be operated for K-GIC. Here, K-GIC was synthesized by mixing about 0.1 g of raw materials blended so that K:C:Na = 1:x:x / 6 at room temperature for about 20 minutes in the same manner as above. When x = 8, KC ８ (Stage 1), KC when x = 24 ２４ (Stage 2), KC when x = 36 ３６ (Stage 3), KC when x = 48 ４８ It was confirmed that K-GIC of stage 1 (stage 4) could be obtained. In other words, it was shown that the structure of K-GIC can also be controlled to any of stages 1 to 4 by adjusting the composition of the raw materials.

[0032] Example 3 Next, a case where the order of adding raw materials was changed was examined.

[0033] As shown in FIG. 7(a), when raw materials were weighed in advance so that C:Na=3:1, and mixed in a mortar, Na-GIC (NaC ｙAs shown in Figure 1(b), when Li was further added to the mixture of Na-GIC and Na so that the ratio of Li:C:Na was 1:6:2, LiC ６ As shown in Fig. 1(c), when K was further added to the mixture of Na-GIC and Na so that the ratio of K:C:Na was 1:8:2.67, KC ８ was obtained. In this way, even if Na-GIC was produced in advance and the insertion material was added later, GIC could be synthesized using each insertion material. This indicates the reaction process in which Na-GIC acts as an intermediate reactant in the catalytic reaction, producing the more stable Li-GIC and K-GIC, as described above.

[0034] In addition, the Na-GIC previously produced as described above, i.e., NaC ｙ However, in order to estimate the value of y, the value of z in the mixture ratio of raw materials C and Na, C:Na=z:1, was changed and the product was investigated.

[0035] As shown in FIG. 8, the XRD patterns of the obtained product were enlarged and overlapped in the range of 20≦2θ≦40. ｙ The peak of NaC was not observed at z = 14 to 16. ｙ The peak of NaC begins to appear, and at z = 12 ｙ At z = 10, only the Na peak becomes large, so it is thought that up to the ratio of y = z = 12, Na reacts with graphite in a form that is not observed as a diffraction peak. ｙ Considering that high-stage GIC (n = 6 to 8 and y = 48 to 64) is considered to be stable, it is thought that Na is not only inserted between the graphite layers, but also adsorbed on the graphite surface. ｙ The peak of is wider than the peak of C, which indicates that it is NaC of a different stage. ｙ It is thought that there is a mixture of

[0036] [Example 4] When alkaline earth metals (AE), Ca, Sr, and Ba were selected as the insertion materials, the raw materials were weighed so that the AE:C:Na ratio was 1:6:2, mixed, molded into pellets, and vacuum-sealed in a quartz tube. The quartz tube containing the pellets was then heated at 250°C for 2 hours.

[0037] Figure 9 shows the XRD patterns of the products obtained as described above. As can be seen from the XRD patterns, heating at 250°C resulted in the formation of CaC ６ , SrC ６ , BaC ６ The compounds CaC ６ In the sample that produced the unreacted Ca and NaC ｙ However, by repeatedly crushing and mixing the pellets and heat treating them several times, Ca and NaC ｙ It was also possible to react almost the entire amount.

[0038] As shown in FIG. 10, the CaC ６ The temperature dependence of the magnetic susceptibility of CaC ６ is known as a superconductor with the highest superconducting critical temperature of 11.5K among GICs. ６ As is clear from the figure, this CaC ６ is the CaC reported so far. ６ It was confirmed to be the same substance as

[0039] [Removal of Na] As described above, when GIC is synthesized by the method of this example, the product contains Na. Therefore, two examples of the step of removing Na will be described below.

[0040] Referring to FIG. 11 , a method for extracting sodium from a pellet containing GIC and sodium obtained by synthesizing GIC is described. Specifically, the product 25 is held in a die 31 and heated together with a compression rod 32 to a temperature above the melting point of sodium (98°C) using a hot plate or similar device. The heating temperature can be, for example, 200°C. The product 25 is then placed on a press table 33 with a spacer ring 34, and the product 25 is compressed with the compression rod 32. As a result, molten sodium 26 flows out through the gap between the die 31 and the compression rod 32, into the spacer ring 34, and toward the upper end of the die 31, thereby removing sodium from the product 25. By adding this process, a sintered GIC pellet can be obtained in which approximately 90% of the sodium remaining in the product 25 has been removed.

[0041] As shown in Fig. 12, Na can also be evaporated and removed by heating. Specifically, the product 25 is placed inside a heating section 41 at one end of a quartz tube 40, which is 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 a condenser section 42 at the other end. This makes it possible to remove Na from the product 25 placed on the heating section 41. By using this method, it is possible to remove most of the Na remaining in the product 25.

[0042] [Example 5] When lanthanides (Ln), Sm, Eu, and Yb were selected as the intercalation material, the raw materials were mixed so that the ratio of Ln:C:Na was 1:6:2, and then molded into pellets. The pellets were then sealed in a stainless steel tube. As in Example 4, the tube containing the pellets was heated. However, since lanthanides are less reactive than alkaline earth metals, they were heated at 275°C for 6 hours. Furthermore, to improve purity, the sample was mixed again and heated at 275°C for another 6 hours.

[0043] FIG. 13 shows the XRD patterns of the products obtained after removing Na by the above-mentioned method. ６ , EuC ６ , YbC ６ Each of the compounds was produced.

[0044] As described above, according to the production method in which graphite and an intercalation material are reacted in the presence of Na, any intercalation material can be easily intercalated between the layers of graphite to synthesize GIC.

[0045] Although typical 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 appended claims.

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

Claims

1. A method for producing a graphite intercalation compound in which an intercalation material consisting of any one of an alkali metal, an alkaline earth metal, and a lanthanoid, or a combination thereof, is intercalated between graphene layers, the method comprising: A method for producing a graphite intercalation compound, comprising the steps of reacting the intercalation material with graphite in the presence of Na, and then removing the Na.

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

3. 3. The method for producing a graphite intercalation compound according to claim 1, wherein the intercalation material is an alkali metal, and is mixed with and reacted with the graphite.

4. 3. The method for producing a graphite intercalation compound according to claim 2, wherein the intercalation material is one of an alkaline earth metal and a lanthanoid, and is mixed with the graphite and heated to react with it.

5. 5. The method for producing a graphite intercalation compound according to claim 4, wherein the reaction is carried out by heating to a predetermined temperature of 250° C. or less.

6. A method for producing a graphite intercalation compound as described in claim 1, characterized in that a periodic structure is formed in which a predetermined number of graphene layers are arranged between the inserted layers of the insertion material.