Method for manufacturing silicon clathrate

Abstract

To provide a method for producing silicon clathrate that allows for the safe recovery of highly reactive metallic sodium (Na). [Solution] A method for producing silicon clathrate, comprising the step of heating the Na-Si alloy and the NH4Cl in a reaction system in which a Na-Si alloy containing Na and Si and NH4Cl coexist in non-contact manner, thereby reacting the Na vaporized from the Na-Si alloy with the vaporized NH4Cl.

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing silicon clathrate. [Background technology]

[0002] Silicon clathrates are known to enclose other metals within the internal space of polyhedra formed by silicon. Among these, research has been reported primarily on silicon clathrate I and silicon clathrate II.

[0003] Silicon clathrate I is a structure in which a dodecahedron, in which one sodium atom is encased by 20 silicon atoms, and a tetrahedron, in which one sodium atom is encased by 24 silicon atoms, share faces. x Si 46 It is represented by the following empirical formula. Na is present in all polyhedral cages that make up silicon clathrate I.

[0004] Silicon clathrate II is formed when a dodecahedron of Si and a hexahedron of Si share faces, and Na x Si 136 It is represented by the following empirical formula, where x satisfies 0 ≤ x ≤ 24. That is, Na may or may not be present in the polyhedral cage that constitutes silicon clathrate II.

[0005] Such silicon clathrates are expected to be used as anode materials in lithium-ion secondary batteries.

[0006] Patent Document 1 discloses "a method for producing silicon clathrate II, characterized by heating the Na-Si alloy in a reaction system in which a Na-Si alloy containing Na and Si and a Na getter agent coexist in a non-contact manner, thereby reacting the Na vaporized from the Na-Si alloy with the Na getter agent." [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-031349 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, when manufacturing silicon clathrate, there was a problem in that Na was detached from the Zintl phase Na-Si alloy, resulting in the formation of highly reactive metallic Na (sodium).

[0009] One embodiment of this disclosure aims to solve the problem of providing a method for producing silicon clathrate that can safely recover highly reactive metallic sodium (Na). [Means for solving the problem]

[0010] The means for solving the above problems include the following embodiments. <1> A method for producing silicon clathrate, comprising the step of heating the Na-Si alloy and the NH4Cl in a reaction system in which the Na-Si alloy containing Na and Si and NH4Cl coexist in a non-contact manner, thereby reacting the Na vaporized from the Na-Si alloy with the vaporized NH4Cl. <2> The heating temperature is between 350°C and 400°C. <1> A method for producing silicon clathrate as described above. <3> Heating is carried out under atmospheric pressure. <1> or <2> A method for producing silicon clathrate as described above. <4> The process further includes washing the reaction products. <1> ~ <3> A method for manufacturing silicon clathrate as described in any one of the following. <5> Washing is done with water. <4> A method for producing silicon clathrate as described above. [Effects of the Invention]

[0011] According to an embodiment of the present disclosure, there is provided a method for producing a silicon clathrate that can safely recover highly reactive metallic sodium (Na). [Brief Description of the Drawings]

[0012] [Figure 1] FIG. 1 is an XRD pattern of the sample after washing. [Mode for Carrying Out the Invention]

[0013] Hereinafter, the method for producing a silicon clathrate of the present disclosure will be described.

[0014] <Method for Producing Silicon Clathrate> The method for producing a silicon clathrate of the present disclosure is a method comprising the step of, in a reaction system where a Na-Si alloy containing Na and Si and NH4Cl coexist in a non-contact state, heating the Na-Si alloy and the NH4Cl to cause Na vaporized from the Na-Si alloy to react with the vaporized NH4Cl, which is a method for producing a silicon clathrate.

[0015] By including the above step, Na vaporized from the Na-Si alloy by heating reacts with NH4Cl, which is a Na getter agent vaporized by heating, to form NaCl. Therefore, highly reactive metallic sodium (Na) can be safely recovered.

[0016] The Na-Si alloy contains Na and Si, and the composition of Na and Si is Na y Si 136 represented by (y>24). As the Na-Si alloy, it is preferable to use one in which Na is present in a larger amount than Si, that is, one in which the composition of Na and Si is Na z Si represented by (z>1). The Na-Si alloy can be produced by melting and alloying, for example, pieces of metallic sodium (Na) and Si powder under an inert gas atmosphere.

[0017] The Na-Si alloy may contain elements other than Na and Si without departing from the scope of the present disclosure. Examples of other elements include Li (lithium), K (potassium), Rb (rubidium), Cs (cesium) and Ba (barium) that can be substituted for Na, and Ga (gallium) and Ge (germanium) that can be substituted for Si in silicon clathrates.

[0018] As NH4Cl, in addition to solid NH4Cl, those mixed with water to form an aqueous solution or a slurry can be vaporized (sublimated) in a reactor before use. There is no particular limitation on the ammonium chloride used, and naturally occurring products, products obtained by the ammonia-soda process, or products obtained by double decomposition of ammonium sulfate and sodium chloride, etc., can be used.

[0019] The amount of NH4Cl used can be appropriately determined according to the amount of Na contained in the Na-Si alloy. From the viewpoint of recovering the generated Na, it is preferable to use a large excess relative to the Na-Si alloy. Ammonium chloride may be used in combination with other Na getter agents (Na trapping agents) as long as it does not depart from the scope of the present disclosure.

[0020] Examples of other Na getter agents other than NH4Cl include metal oxides other than alkali metals and alkaline earth metals, metal sulfides other than alkali metals and alkaline earth metals, and metal halides other than alkali metals and alkaline earth metals.

[0021] Specific examples of other Na getter agents include WO2, WO3, MoO3, ZnO, FeO, VO, V2O3, TiO2, SiO, SiO2, Al2O3, WS2, MoS2, ZnS, FeS, TiS2, SiS2, Al2S3. Among them, those selected from WO3, MoO3, ZnO, FeO, V2O3, TiO2, SiO and Al2O3 are preferred, and those selected from WO3, MoO3, FeO and TiO2 are more preferred. However, it is preferable to use ammonium chloride alone from the viewpoints that the reaction product is safe, easy to remove, inexpensive and excellent in handleability.

[0022] In the production method of the present disclosure, the heating temperature is preferably not lower than 350°C and lower than 400°C, more preferably not lower than 350°C and lower than 390°C, and even more preferably not lower than 350°C and lower than 380°C. When the heating temperature is within the above range, Na is vaporized from the Na-Si alloy, ammonium chloride is vaporized, and the vaporized Na is reacted with the vaporized NH4Cl, while suppressing the formation of silicon clathrate II and Si crystals with a diamond-type structure, thereby making it easy to obtain only silicon clathrate I. Compared with ordinary Si having a diamond-type structure and silicon clathrate II, silicon clathrate I has a larger volume, and when used as a negative electrode material for lithium ion secondary batteries, it can suppress volume expansion during metal ion insertion. It has also been reported that silicon clathrate I exhibits superconductivity and hydrogen storage properties that ordinary Si does not have.

[0023] In the production method of the present disclosure, heating is preferably performed under atmospheric pressure. When the heating temperature is set to a temperature lower than the range of not lower than 350°C and lower than 400°C, the heating can also be performed under reduced pressure. As for the pressure under reduced pressure, 10 5 Pa or lower, 10 4 Pa or lower, 10 3 Pa or lower can be exemplified.

[0024] The step of reacting Na vaporized from the Na-Si alloy with vaporized NH4Cl may be performed as a single step to produce silicon clathrate; alternatively, silicon clathrate with reduced Na content obtained in a preceding step and NH4Cl may coexist in a non-contact state, and the silicon clathrate with reduced Na content and NH4Cl are heated to react Na vaporized from said silicon clathrate with vaporized NH4Cl, thereby producing silicon clathrate.

[0025] In addition to NaCl generated by the reaction, unreacted Na, NaOH, NH3 and the like may adhere to the silicon clathrate produced through the production method of the present disclosure, so it is preferable to further include a step of washing the reaction product to remove these impurities.

[0026] From the viewpoint of simplicity, washing is preferably carried out with water. However, it is not limited to water, and washing may also be carried out with an acidic aqueous solution. In this case, the concentration of the acid in the acidic aqueous solution is preferably 0.1% to 10% by mass, more preferably 0.5% to 5.0% by mass, and even more preferably 1% to 4% by mass, based on the total mass of the aqueous solution.

[0027] After washing, it is preferable to filter and dry the material to remove water.

[0028] According to the method described above, silicon clathrate can be produced while safely recovering highly reactive metallic sodium (Na).

[0029] Silicon materials containing silicon clathrate can be used as negative electrode active materials in secondary batteries such as lithium-ion secondary batteries, and in energy storage devices such as electric double-layer capacitors and lithium-ion capacitors.

[0030] Silicon materials, including silicon clathrate, can also be used in applications such as thermoelectric elements, light-emitting elements, and light-absorbing elements. [Examples]

[0031] The embodiments of this disclosure will be specifically described below with reference to examples. The embodiments of this disclosure are not limited to the following examples.

[0032] <Example 1> -Manufacturing of Na-Si alloy- Under an argon atmosphere, metallic sodium (Na) and silicon (Si) powders were weighed in a 1:1 molar ratio, placed in a boron nitride (BN) crucible, sealed in a stainless steel (SUS) tube, and heated at 450°C for 48 hours to synthesize the alloy. In the resulting Na-Si alloy, the composition ratio of Na to Si was 1:1.

[0033] -Manufacturing of silicon clathrate- The reaction was carried out using a simple glove box with an argon atmosphere connected to a tubular furnace. Na-Si alloy was placed downstream of the tubular furnace and NH4Cl was placed upstream of the tubular furnace in a 1:1 molar ratio. The downstream and upstream sections of the tubular furnace were heated to 350°C, and the NH4Cl vaporized (sublimated) at 350°C was brought into contact with the Na-Si alloy from the upstream side and held for 1 hour. After that, the sample was collected and XRD measurements were performed using a powder X-ray diffractometer.

[0034] -Cleaning process- After washing the collected samples with water, XRD measurements were performed again. The results are shown in Figure 1.

[0035] <Example 2> In the production of silicon clathrate, the silicon clathrate was produced and measured in the same manner as in Example 1, except that the downstream section of the tubular furnace was heated to 400°C.

[0036] <Example 3> In the production of silicon clathrate, the silicon clathrate was produced and measured in the same manner as in Example 1, except that the downstream section of the tubular furnace was heated to 450°C.

[0037] <Example 4> In the production of silicon clathrate, the silicon clathrate was manufactured and measured in the same manner as in Example 1, except that the downstream section of the tubular furnace was heated to 500°C.

[0038] <Comparative Example 1> In the production of silicon clathrate, the silicon clathrate was manufactured and measured in the same manner as in Example 1, except that the downstream section of the tubular furnace was heated to 550°C.

[0039] <Rating> XRD measurements before washing confirmed that all samples contained NaCl after the reaction. This demonstrated that highly reactive metallic sodium (Na) can be safely recovered as a stable salt.

[0040] Figure 1 shows the XRD pattern of the sample after washing with water. As shown in Figure 1, it was confirmed that silicon clathrate I (Type I) grows in a single phase when the heating temperature of the downstream section is 350°C. If the reaction temperature is further increased, and the heating temperature of the downstream part of the tubular furnace is 400°C, then NaSi 14 However, at 450°C, silicon clathrate II (Type II) was produced as a by-product. When the heating temperature of the downstream part of the tubular furnace was 500°C, the growth of Si resulting from the decomposition of silicon clathrate was observed. Although not shown in the diagram, at 550°C, a single phase of Si was formed.

[0041] Since it was found that the Na-Si alloy is converted to silicon clathrate I at 350°C, the same temperature as the vaporization (sublimation) of NH4Cl, the Na-Si alloy and NH4Cl were mixed and heated at 350°C in a single electric furnace, rather than using a double electric furnace with upstream and downstream sections of a tubular furnace. In this case as well, it was confirmed that silicon clathrate I could be synthesized while safely recovering metallic Na (sodium).

Claims

1. Na-Si alloy containing Na and Si and NH ４ In a reaction system in which Cl and the Na-Si alloy and the NH coexist in non-contact, ４ By heating the Cl, the Na vaporized from the Na-Si alloy is vaporized into the NH ４ Includes a step of reacting with Cl, A method for manufacturing silicon clathrate.

2. A method for producing silicon clathrate according to claim 1, wherein the heating temperature is 350°C or higher and less than 400°C.

3. A method for producing silicon clathrate according to claim 1, wherein heating is carried out under atmospheric pressure.

4. A method for producing silicon clathrate according to claim 1, further comprising the step of washing the reaction product.

5. A method for producing silicon clathrate according to claim 4, wherein the washing is performed with water.

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