WO2024122469A1 - Method for producing graphite intercalation compound - Google Patents

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 compound

The present invention relates to a method for producing graphite intercalation compounds.

Graphite intercalation compounds (GICs) are known in which atoms or molecules are inserted between the graphite layers of a layered structure. Such GICs exhibit a variety of properties depending on the type of atoms or molecules inserted between the graphite layers and the position (staging) between the graphite layers at which they are inserted. They are expected to have a wide range of applications, including as battery electrode materials, superconducting materials, gas storage materials, thermoelectric conversion materials, magnetic materials, and conductive materials.

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

For example, Patent Document 1 discloses a method for producing GICs by contacting an intercalation compound with graphite in the presence of a supercritical fluid that easily penetrates between graphite layers (layer planes). When the compounds that make up the supercritical fluid are in a gaseous state at room temperature and pressure, the intercalation 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 normal. This makes it possible to extract the graphite intercalation compound from the compounds that made up the supercritical fluid. This method is said to be able to uniformly insert the intercalation compound or ions derived from it between the graphite layers, making it easy to produce GICs with uniform quality.

JP 2011-213583 A

As mentioned above, there is a need for a manufacturing method that can efficiently synthesize large amounts of homogeneous GIC.

The present invention was made in consideration of the above circumstances, and its purpose is to provide a method for manufacturing a GIC that allows any intercalation material to be easily inserted between the graphite layers.

The method for producing graphite intercalation compounds according to the present invention is characterized by reacting an intercalation material with graphite in the presence of Na. This feature makes it possible to easily synthesize large quantities of homogeneous GIC.

In the above-mentioned invention, the graphite may be in the form of a powder. This feature makes it possible to synthesize a large amount of homogeneous GIC more efficiently.

In the above-mentioned invention, the insertion material may be an alkali metal, which is mixed with the graphite and reacted. Alternatively, the insertion material may be either an alkaline earth metal or a lanthanoid, which is mixed with the graphite and heated to react. With these features, it is possible to efficiently synthesize high-quality GICs with a specified insertion material inserted.

The above-mentioned 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 large amounts of homogeneous GIC without the need for high temperatures that make it difficult to obtain a pure sample.

The above-mentioned 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.

3 is a photograph showing a step of a method for producing a graphite intercalation compound as an example of the present invention. 1 is a photograph showing the appearance of a GIC synthesized using Li as an intercalation material. 1 shows XRD patterns illustrating the reaction processes of Li-GIC. 1 is an X-ray diffraction (XRD) pattern showing the relationship between the composition of the raw materials of Li-GIC and the stage. FIG. 1 is a diagram illustrating the stages of GIC. 1 is an XRD pattern showing the relationship between the composition of raw materials of K-GIC and the stage. 1 shows XRD patterns of (a) a product containing Na-GIC (NaC _y ) previously obtained, (b) a product obtained by adding Li to this, and (c) a product obtained by adding K. FIG. 1 is a partial enlarged overlay of XRD patterns of products containing Na-GIC obtained by changing the formulation. XRD patterns of products with Ca, Sr, and Ba as intercalation materials, where the black circles are diffraction peaks due to graphite intercalation compounds such as _CaC6 . 1 is a graph showing the temperature dependence of magnetic susceptibility of _CaC6 . FIG. 1 is a side cross-sectional view of a die used to remove Na. 1 is a photograph showing the appearance of a quartz tube used for removing Na. 1 shows XRD patterns of products with Sm, Eu, and Yb as intercalation materials, where the black circles are diffraction peaks due to graphite intercalation compounds such as _SmC6 .

Below, a method for producing GIC (graphite intercalation compound), which is one embodiment of the present invention, will be explained with reference to FIG. 1.

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

For example, as shown in FIG. 1, graphite 11, sodium 12 (Na), and intercalation material 13 are placed in a mortar 10, and the graphite and intercalation material are kneaded into the sodium, which is a soft metal, to synthesize a GIC.

In this case, it is preferable that the graphite 11 is in powder form, since this allows for efficient synthesis of GIC. Note that there is no need 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.

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.

The intercalation material 13 is a material that is inserted between graphene layers (hereinafter simply referred to as between layers) in the graphite 11 that is composed of stacked graphene layers, and can be, for example, any one of an alkali metal, an alkaline earth metal, and a lanthanide, or a combination of these.

As the alkali metal, Li, Na, K, Rb, and Cs can be suitably used. In addition, since the alkali metal is soft, it can be mixed by putting it into the mortar 10 as a lump metal piece and kneading it with a pestle. The purity of the raw material can be about 99%. When these are used as the insertion material, GIC can be synthesized by mixing at room temperature.

As alkaline earth metals, Ca, Sr, and Ba can be preferably used. As lanthanides, Sm, Eu, and Yb can be preferably used. Alkaline earth metals and lanthanides are relatively hard, and it is difficult to grind them from a lump with a pestle. Therefore, it is recommended to crush them into powder with a diameter of about 100 μm using a file or the like and then put them into the mortar 10. The purity of the raw material can be about 99%. When these are used as the insertion material, GIC can be synthesized by mixing the raw materials and holding them in an environment where they are heated to a predetermined temperature that can cause the reaction to proceed. This predetermined temperature may be about 300 to 400°C, but synthesis is possible in a sufficiently short time even at a low temperature of 250°C or less. In general, it is preferable that the temperature is 100°C or higher, taking into account the synthesis time. For example, if mixing and heating are repeated at 250°C, the reaction between the raw materials is generally completed in a total of about several hours.

In any of the above cases, the synthesis time required to obtain GIC can be anywhere from a few minutes to a few hours at most, which is significantly shorter than the conventional synthesis time of GIC, which takes 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. In addition, there is no need for precise temperature and composition control, as was necessary in conventional methods, and no complicated processes or special equipment are required. As a result, synthesis can be performed simply and efficiently in a short time, and it is possible to synthesize homogeneous and large quantities of GIC, making it convenient for industrial GIC production.

The above manufacturing method uses Na, so Na remains mixed in the synthesized GIC. If Na is not necessary, a step of removing Na can be added. For example, there is a method of melting the Na by heating and then centrifuging, a method of forming a powder containing GIC into a pellet while heating it and then pushing out the liquid Na (details will be described later), and a method of heating the resulting pellet to evaporate the Na (details will be described later).

The following describes the results of actually synthesizing GIC.

[Example 1]
As shown in Figure 2, Li was used as an intercalation 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 were put into a mortar. Then, mixing was performed by kneading with a pestle. Then, the raw materials, which were initially mainly black, the color of graphite, reacted and gradually took on a metallic luster. This metallic luster was initially silver, and as mixing proceeded, it gradually took on a yellowish hue, which is a characteristic of _LiC6 .

As shown in FIG. 3, the products were analyzed by X-ray diffraction (XRD) in each process until the synthesis of _LiC6 . It was found that the reaction proceeds as follows. That is, in the initial stage of mixing, _LiC6 , _LiC12 , and _NaCy are produced, and unreacted C, Li, and Na are also present. As the reaction proceeds, unreacted C and Li decrease, _NaCy also decreases, and _LiC6 and _LiC12 increase. As the reaction proceeds, _LiC12 starts to decrease, _LiC6 increases, and finally the mixture becomes _LiC6 and Na, and the reaction is almost completed. When about 0.1 g of raw material was used, the time required for the reaction to be completed was about 20 minutes. In the figure, 2θ is the diffraction angle. Also, an X-ray source using Cuα1 is shown.

Thus, in the reaction in which the intercalation material Li is intercalated into graphite, Na is thought to function like a catalyst that accelerates the reaction. In this embodiment, the time required to manufacture GIC is very short compared to conventional manufacturing methods. The reason for this is thought to be that NaC _y produced by the reaction of Na and C acts as a reaction intermediate in the catalytic reaction, lowering the activation energy for the intercalation of the intercalation material between the layers of graphite. In addition, for example, even if K is used instead of Na and Li is mixed, only KC ₆ is produced, and GIC (Li-GIC) in which Li is the intercalation material could not be synthesized. From this, it can be said that Na works uniquely. Since Na easily produces GIC by reacting with graphite, Na-GIC (NaC _y ) is first produced by mixing, but since Na is unstable compared to other GICs, it is thought that when Li or other intercalation materials are present in the vicinity, it replaces those elements to produce more stable GIC such as LiC ₆ .

In addition, in light of these functions of Na, it is surmised that even if a small amount of Na is used as a raw material, it is possible to synthesize GIC over a period of time. On the other hand, increasing the amount of Na can increase the efficiency of GIC synthesis to a certain extent, but this effect is thought to saturate. Taking into account the need to remove Na, it is also important to select a well-balanced amount of Na.

In addition, as shown in FIG. 4, a component analysis was performed by X-ray diffraction (XRD) on a sample containing Li-GIC synthesized by changing the composition of the raw materials. Here, about 0.1 g of raw materials were mixed for about 20 minutes. As a result, when the raw materials were mixed in a molar ratio of Li:C:Na=1:6:1 (lower part of the figure), LiC ₆ was generated as GIC, and when this ratio was 1:12:2 (upper part of the figure), LiC ₁₂ was generated as GIC. LiC ₆ has a stage 1 structure, and LiC ₁₂ has a stage 2 structure. In this way, it is possible to manipulate the stage by mixing the amount of the intercalation material and graphite so that it becomes the element ratio of the structure to be obtained. Here, it was shown that in Li-GIC, the stage 1 and stage 2 structures can be controlled by the composition of the raw materials.

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

[Example 2]
As shown in FIG. 6, it was confirmed whether the stages of K-GIC could be manipulated. Here, about 0.1 g of raw materials blended to give K:C:Na=1:x:x/6 were mixed at room temperature for about 20 minutes in the same manner as above to synthesize K-GIC. It was confirmed that K-GIC of KC ₈ (stage 1) could be obtained when x=8, KC ₂₄ (stage 2) when x=24, KC ₃₆ (stage 3) when x=36, and KC ₄₈ (stage 4) when x=48 could be obtained. In other words, it was shown that the structure of stages 1 to 4 can be controlled in K-GIC by blending the raw materials.

[Example 3]
Next, the case where the order of feeding ingredients was changed was examined.

As shown in FIG. 7(a), when the raw materials previously weighed so that C:Na=3:1 were mixed in a mortar, a mixture of Na-GIC (NaC _y ) and Na was obtained. Then, as shown in FIG. 7(b), Li was further added to the mixture of Na-GIC and Na to make Li:C:Na=1:6:2, and LiC ₆ was obtained. Also, as shown in FIG. 7(c), when K was further added to the mixture of Na-GIC and Na to make K:C:Na=1:8:2.67, KC ₈ was obtained. In this way, even if Na-GIC was generated in advance and the insertion material was added later, GIC could be synthesized from each insertion material. This shows the reaction process in which Na-GIC becomes an intermediate reactant in the catalytic reaction, and more stable Li-GIC and K-GIC are generated, as described above.

In addition, in order to estimate the value of y for the previously generated Na-GIC, i.e., _NaCy , the product was investigated by changing the value of z in the mixture ratio of the raw materials C and Na, C:Na=z:1.

As shown in FIG. 8, when the XRD patterns of the obtained product were enlarged and overlapped in the range of 20≦2θ≦40, no NaC _y peak was observed at z=32, and finally began to appear at z=14-16, and at z=12, _{the NaC y peak} was predominant. At z=10, only the Na peak became large, so it is considered that Na reacts with graphite in a form that is not observed as a diffraction peak up to the ratio of y=z=12. Considering that high-stage GIC (n=6-8 and y=48-64) of NaC _y is considered to be stable, it is considered that Na is not only inserted between the graphite layers, but also adsorbed on the graphite surface. The NaC _y peak is wider than the C peak, so it is considered that NaC _y of different stages is mixed.

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

Figure 9 shows the XRD patterns of the products obtained as described above. As can be seen from these, the compounds _CaC6 , _SrC6 , and _BaC6 were produced by heating at 250°C. In the sample that produced _CaC6 , unreacted Ca and _NaCy were present, but by repeating the pellet crushing, mixing, and heat treatment several times, it was possible to react almost all of the Ca and _NaCy .

As shown in FIG. 10, the temperature dependence of the magnetic susceptibility of _CaC6 obtained as described above was measured. In general, _CaC6 is known as a superconductor with the highest superconducting critical temperature of 11.5K among GICs. As is clear from the figure, _CaC6 obtained by the above method shows a superconducting transition at about 11.5K. In other words, it was confirmed that this _CaC6 is a material equivalent to the _CaC6 previously reported.

[Removal of Na]
As described above, when GIC is synthesized by the method according to this embodiment, the product contains Na. Therefore, two examples of the process for removing Na will be described below.

Referring to FIG. 11, a method of causing Na to flow out of a pellet when forming a product 25 containing GIC and Na obtained by synthesizing GIC into a pellet will be described. In detail, the product 25 is held in a die 31 and heated together with a compression rod 32 to a temperature equal to or higher than the melting point of Na (98°C) using a hot plate or the like. The heating temperature can be, for example, 200°C. Next, these are placed on a press table 33 on which a spacer ring 34 is placed, and the product 25 is compressed by the compression rod 32. Then, 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 the upper end side of the die 31, and Na can be removed from the product 25. By adding such a process, a sintered pellet of GIC can be obtained from which about 90% of the Na remaining in the product 25 has been removed.

As shown in FIG. 12, Na can also be evaporated and removed by heating. In detail, the product 25 is placed inside the heating section 41 at one end of the 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, and the evaporated Na condenses in the condensation section 42 at the other end. This makes it possible to remove Na from the product 25 placed on the heating section 41. With this method, it is possible to remove most of the Na remaining in the product 25.

[Example 5]
In the case where lanthanoids (Ln), Sm, Eu, and Yb were selected as the intercalation material, the raw materials were weighed so that Ln:C:Na=1:6:2, mixed, molded into pellets, and sealed in a stainless steel tube. In addition, the tube containing the pellets was heated in the same manner as in Example 4, but since it is less reactive than alkaline earth metals, it was heated at 275°C for 6 hours, and further, in order to increase the purity, the sample was mixed once more and heated at 275°C for another 6 hours.

13 shows the XRD patterns after removing Na from each of the products obtained by the above-mentioned method. Here, too, the compounds _SmC6 , _EuC6 , and _YbC6 were produced, respectively.

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

The above describes representative embodiments of the present invention, but the present invention is not necessarily limited to these, and a person 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.

10 Mortar 11 Graphite 12 Sodium 13 Insertion material

Claims (6)

1. A method for producing graphite intercalation compounds, characterized by reacting an intercalation material with graphite in the presence of Na.
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 intercalation material is an alkali metal, which is mixed with and reacted with the graphite.
4. The method for producing a graphite intercalation compound according to claim 2, characterized in that the intercalation material is made of either an alkaline earth metal or a lanthanide, and is mixed with the graphite and heated to cause a reaction.
5. The method for producing a graphite intercalation compound according to claim 4, characterized in that the reaction is carried out by heating to a predetermined temperature of 250°C or less.
6. 2. The method for producing a graphite intercalation compound according to claim 1, further comprising the step of removing Na after the mixing.
   

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