Production method for porous clathrate silicon

Abstract

To produce porous clathrate silicon having a large pore volume.SOLUTION: A production method for porous clathrate Si includes: a reduction step; an alloying step: a clathrate production step; and a porosity formation step. The reduction step includes obtaining a Si-MgO complex including Si and MgO. The alloying step includes reacting the Si-MgO complex and a Na source to thereby produce NaSi, resulting in a NaSi-MgO complex. The clathrate production step includes heating the NaSi-MgO complex to clathrate NaSi with desorption of Na, resulting in a clathrate Si-MgO complex including II-type clathrate Si and MgO. The porosity formation step includes removing MgO from the clathrate Si-MgO complex for porosity formation.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for producing porous clathrate silicon. [Background technology]

[0002] Patent Document 1 discloses a technique for applying porous clathrate silicon, which is a porous body of type II clathrate silicon, as a negative electrode active material for lithium ion secondary batteries. Clathrate silicon is a compound that includes other metals in the space of a polyhedron formed by Si atoms. Type II clathrate silicon has the formula Na x Si 136 It is a type of clathrate silicon represented by the formula (0≦x≦24) and has a structure in which a dodecahedron of Si atoms and a decahedron of Si atoms share a face. Na may or may not be present in the space between the polyhedra that constitute type II clathrate silicon.

[0003] Silicon has a higher theoretical capacity than graphite and other materials, and its use as a negative electrode active material is expected to increase the capacity of lithium-ion secondary batteries. Meanwhile, silicon has the property of significantly expanding when it absorbs charge carriers such as lithium ions. In lithium-ion secondary batteries, excessive volumetric changes in the negative electrode active material during charging and discharging can reduce the durability of the device. Patent Document 1 proposes that the negative electrode active material, which is made of type II clathrate silicon, is made into a porous material with fine pores, thereby suppressing excessive volumetric changes during charging and discharging. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-043948 Summary of the Invention [Problem to be solved by the invention]

[0005] Type II clathrate silicon can be formed by reacting a Na source and a Si source to produce NaSi, and then heating the NaSi to form a clathrate. In Patent Document 1, porous silicon is used as the Si source to impart pores to the produced type II clathrate silicon. That is, porous NaSi formed from a Na source and porous silicon is heated to form a clathrate, thereby producing porous clathrate silicon. In this case, when the porous NaSi is formed, the porous silicon expands when Na is absorbed, causing the pores in the porous silicon to shrink or collapse. In addition, when the porous NaSi is heated to form a clathrate, the NaSi softens, causing the pores in the porous NaSi to shrink or collapse. As a result, the amount of pores in the produced porous clathrate silicon is reduced. [Means for solving the problem]

[0006] A method for producing porous clathrate silicon that solves the above-mentioned problems includes a reduction step in which a Si-MgO composite containing Si and MgO is obtained by contacting Mg vapor with a Si raw material containing silicon oxide under reduced pressure and under conditions where the Mg vapor pressure is equal to or lower than the equilibrium pressure of the following reaction formula (1); an alloying step in which the Si-MgO composite is obtained by reacting the Si-MgO composite with a Na source to produce NaSi, thereby obtaining a NaSi-MgO composite; a clathrate generation step in which the NaSi-MgO composite is heated to clathrate the NaSi and desorb Na, thereby obtaining a clathrate Si-MgO composite containing type II clathrate silicon and MgO; and a porosification step in which MgO is removed from the clathrate Si-MgO composite to make it porous.

[0007] [ka] In one embodiment, the clathrate generation step includes heating the NaSi-MgO composite in the presence of a Na getter agent that captures Na derived from NaSi.

[0008] The above-described manufacturing method according to one embodiment further comprises a Na removal step of reducing the amount of Na remaining in the porous clathrate silicon after the porosity-forming step. In one aspect of the above manufacturing method, the clathrate generation step includes heating the NaSi-MgO composite in contact with a Na getter agent that captures Na derived from NaSi, and the Na removal step includes heating the porous clathrate silicon after the porosification step in contact with the Na getter agent, and the Na getter agent used in the Na removal step has a higher reactivity with Na than the Na getter agent used in the clathrate generation step. [Effects of the Invention]

[0009] According to the present invention, porous clathrate silicon having a large amount of pores can be produced. [Brief explanation of the drawings]

[0010] [Figure 1] Figure 1 shows the temperature-Mg vapor pressure curve for reaction formula (1). [Figure 2] FIG. 2 is an explanatory diagram of the reduction step. [Figure 3] FIG. 3 is a schematic diagram of the reduction step, alloying step, clathrate generation step, and porosity formation step. [Figure 4] FIG. 4 shows the X-ray diffraction pattern of the example. [Figure 5] FIG. 5 shows the pore size distribution of the examples and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will now be described with reference to the drawings. Hereinafter, porous clathrate silicon and type II clathrate silicon may be referred to as porous clathrate Si and type II clathrate Si, respectively.

[0012] <Method of manufacturing silicon-based active material> The method for producing porous clathrate Si of this embodiment includes a reduction step, an alloying step, a clathrate production step, and a porosity-imparting step, which will be described below.

[0013] [Reduction process] (Si raw material) The reduction step is a step of obtaining a Si-MgO composite containing Si and MgO by a reduction reaction in which a raw material containing silicon oxide (hereinafter referred to as a Si raw material) is brought into contact with Mg vapor to reduce the silicon oxide. Examples of the Si raw material include SiO and SiO2. It is preferable to use SiO as the Si raw material.

[0014] The Si raw material may contain components other than silicon oxide. The proportion of silicon oxide in the Si raw material is, for example, 50 mass % or more, preferably 90 mass % or more, and more preferably 99 mass % or more.

[0015] The Si raw material is preferably in powder form. In this case, the average particle size of the Si raw material is, for example, 3 μm or less, preferably 1 μm or less. The average particle size of the Si raw material is, for example, 0.5 μm or more.

[0016] (Pressure conditions) The first pressure condition for the reduction reaction is a reduced pressure. The reduced pressure is a pressure of less than 1 atmosphere (101,325 Pa). The reduced pressure is, for example, preferably 100 Pa or less, and more preferably 20 Pa or less. As the pressure in the reduction step decreases, Mg vapor is more likely to diffuse within the reaction system. This increases the chance of contact between the Si raw material and Mg vapor, and as a result, the reduction reaction in the reduction step is more likely to proceed. Furthermore, since the reduction reaction is more likely to proceed, the temperature required to proceed with the reduction step can be lowered.

[0017] The second pressure condition for the reduction reaction is a Mg vapor pressure equal to or lower than the equilibrium pressure of the following reaction formula (1).

[0018] [ka] Reaction formula (1) shows the reversible decomposition reaction that decomposes Mg2Si into Mg vapor and Si. Figure 1 shows the temperature-Mg vapor pressure curve for reaction formula (1). Mg vapor pressures below the equilibrium pressure of reaction formula (1) are lower than curve 1. At Mg vapor pressures below the equilibrium pressure of reaction formula (1), the forward reaction that decomposes Mg2Si into Mg vapor and Si proceeds, while the reverse reaction that produces Mg2Si from Mg vapor and Si does not proceed, or proceeds very little. Therefore, the reaction of Si produced from the Si raw material SiO or SiO2 by the reduction reaction with Mg vapor to produce Mg2Si can be suppressed.

[0019] (How to generate magnesium vapor) The method for generating Mg vapor is not particularly limited as long as it is possible to bring Mg vapor into contact with the Si raw material so as to satisfy the second pressure condition. For example, Mg vapor can be generated by heating a reducing agent serving as a Mg source, such as an Mg alloy or metallic Mg.

[0020] Among these, a method using an Mg alloy (hereinafter referred to as a low equilibrium pressure Mg alloy) in which the equilibrium pressure of the reaction to generate Mg vapor is equal to or lower than the equilibrium pressure of reaction formula (1) is particularly preferred. In the reaction system in which Mg vapor is generated from the low equilibrium pressure Mg alloy, the Mg vapor pressure does not exceed the equilibrium pressure of reaction formula (1). Therefore, the reaction to generate Mg vapor from the low equilibrium pressure Mg alloy and the reaction in which Mg vapor is brought into contact with the Si raw material can be carried out in the same reaction system. In this case, the equipment used in the reduction step can be simplified.

[0021] Examples of low equilibrium pressure Mg alloys include Mg2Si, MgCa alloy, MgCu2, MgNi2, and MgSn alloy. Table 1 below shows the equilibrium pressures at temperatures of 600°C, 700°C, and 800°C for the reaction that generates Mg vapor from low equilibrium pressure Mg alloys. The Mg vapor pressure values ​​in Table 1 are expressed in common logarithms. The values ​​for Mg2Si in Table 1 are the equilibrium pressures of reaction formula (1).

[0022] Among low equilibrium pressure Mg alloys, those in which the equilibrium pressure of the reaction that generates Mg vapor is close to the equilibrium pressure of reaction formula (1) are preferred.

[0023] [Table 1] For reference, Table 1 also shows the equilibrium pressures of the reaction generating Mg vapor from metallic Mg at each of the temperatures mentioned above. As shown in Table 1, the equilibrium pressure of the reaction generating Mg vapor from metallic Mg is higher than the equilibrium pressure of reaction formula (1). Therefore, the Mg vapor pressure in the reaction system generating Mg vapor from metallic Mg is higher than the equilibrium pressure of reaction formula (1). When using a method for generating Mg vapor from metallic Mg, a process is performed to reduce the Mg vapor pressure in the reaction system. For example, a metal that bonds more easily with Mg than Si, such as Ca, Cu, Sn, or Ni, is separately placed in the reaction system. The same applies when using a method for generating Mg vapor from an Mg alloy, where the equilibrium pressure of the reaction generating Mg vapor exceeds the equilibrium pressure of reaction formula (1).

[0024] The amount of reducing agent used as a Mg source, such as an Mg alloy or metallic Mg, is an amount that generates more than 1 molar equivalent of Mg vapor relative to the number of moles of SiO or SiO2 contained in the Si raw material. In other words, the amount of reducing agent is at least an amount that can reduce all of the SiO or SiO2 contained in the Si raw material. For example, the amount of reducing agent that generates 1 molar equivalent of Mg vapor relative to the number of moles of SiO or SiO2 contained in the Si raw material is taken as the reference amount. In this case, the amount of reducing agent is at least 1 time the reference amount, preferably at least 1.1 times, more preferably at least 1.2 times, and even more preferably at least 1.3 times. Furthermore, the amount of reducing agent is, for example, no more than 2 times the reference amount.

[0025] (Temperature conditions) The temperature of the reduction reaction is the temperature at which Mg vapor exists below the equilibrium pressure of reaction formula (1). By adjusting the temperature of the reduction reaction, the pore size distribution of the silicon-based active material can be controlled. The temperature of the reduction reaction is, for example, 900°C or lower, preferably 800°C or lower, and more preferably 750°C or lower. The temperature of the reduction reaction is, for example, 500°C or higher, preferably 600°C or higher, and more preferably 650°C or higher.

[0026] (Reaction time) The reaction time of the reduction reaction can be appropriately selected depending on the pressure conditions and the temperature conditions, and is, for example, 3 hours or more and 24 hours or less.

[0027] (Specific example of reduction process) A specific example of the reduction step will be described with reference to FIG. The stainless steel reaction vessel 10 is configured to allow gas to pass between the inside and outside of the vessel. A raw material tray 11 and a reduction tray 12 are housed inside the reaction vessel 10. The raw material tray 11 is placed on a leg 11a that stands upright from the bottom of the reaction vessel 10. The raw material tray 11 is a mesh-like tray that is breathable. A powdered Si raw material A is placed in the raw material tray 11.

[0028] The reduction tray 12 is disposed below the raw material tray 11 at the bottom of the reaction vessel 10. The powdered low equilibrium pressure Mg alloy B is disposed in the reduction tray 12. Therefore, in the reaction vessel 10, the Si raw material A and the low equilibrium pressure Mg alloy B are disposed in a non-contact state.

[0029] The reaction vessel 10 containing the Si raw material A and the low equilibrium pressure Mg alloy B is placed in a vacuum furnace 13. The vacuum furnace 13 is then depressurized and heated to a temperature at which Mg vapor is generated from the low equilibrium pressure Mg alloy B. The Mg vapor generated from the low equilibrium pressure Mg alloy B diffuses into the reaction vessel 10. When the Mg vapor comes into contact with the Si raw material A in the raw material tray 11, the SiO or SiO2 contained in the Si raw material A is reduced to Si and MgO. After a predetermined reaction time has elapsed, the reaction vessel 10 is removed from the vacuum furnace 13, and the Si-MgO composite generated in the raw material tray 11 of the reaction vessel 10 is collected.

[0030] [Alloying process] The alloying step is a step of producing NaSi by reacting Si contained in the Si-MgO composite with a Na source to obtain a NaSi-MgO composite.

[0031] Examples of the Na source include metallic Na, NaH, and metallic Na dispersions in which metallic Na particles are dispersed in oil. The Na source is not particularly limited, and any source that introduces minimal impurities into the resulting NaSi may be used.

[0032] The reaction conditions for the alloying reaction are not particularly limited, and reaction conditions used in a known alloying reaction for producing NaSi from Si and a Na source can be applied. The alloying reaction is preferably carried out in an inert gas atmosphere such as Ar gas.

[0033] The temperature of the alloying reaction is a temperature below the melting point of NaSi, for example, 300°C or higher and 500°C or lower. The temperature of the alloying reaction is preferably 350°C or higher. The temperature of the alloying reaction is preferably 450°C or lower. The reaction time of the alloying reaction can be appropriately selected depending on the reaction temperature and the like. The reaction time of the reduction reaction is, for example, 3 hours or higher and 24 hours or lower.

[0034] [Clatrate generation process] The clathrate formation step is a step of heating the NaSi-MgO composite to form a clathrate of NaSi contained in the NaSi-MgO composite and desorbing Na to form type II clathrate Si. x Si 136 This means that the crystal structure is transformed into a crystal structure represented by (0≦x≦24).

[0035] The clathrate formation step produces a clathrate Si-MgO composite containing type II clathrate Si and MgO. The reaction conditions for the formation reaction are not particularly limited, and reaction conditions used in known formation reactions for forming type II clathrate Si from NaSi can be applied. The formation reaction is preferably carried out in an inert gas atmosphere such as Ar gas.

[0036] In the above-mentioned production reaction, it is preferable to promote the desorption of Na from NaSi by using a Na getter agent that captures Na derived from NaSi. Methods for capturing Na using a Na getter agent include a gas phase method that captures Na vapor and a solid phase method that captures Na while maintaining a solid state.

[0037] In the gas-phase method, a Na getter agent is placed in the reaction system without contacting the NaSi-MgO composite. Then, the NaSi-MgO composite is heated to capture Na vapor generated from NaSi by the Na getter agent. This reduces the partial pressure of Na in the reaction system, accelerating the desorption of Na from NaSi. The heating temperature t in the gas-phase method depends on the pressure conditions in the reaction system, but examples include 100°C≦t≦500°C, 200°C≦t≦400°C, and 250°C≦t≦350°C. In order to vaporize the desorbed Na, it is preferable to create a reduced pressure environment in the reaction system. The pressure P in the reaction system is, for example, P<10 5 Pa, P ≤ 10 4 Pa, P ≤ 10 3 Pa, P ≤ 10 2Examples include P≦10 Pa and P≦10 Pa. When the heating temperature t is low, the pressure P in the reaction system needs to be low.

[0038] The Na getter agent used in the vapor phase method is a material that can react with zero-valent Na and has a vapor pressure lower than that of metallic Na. Examples of the Na getter agent include WO2, WO3, MoO3, ZnO, FeO, VO, VO3, TiO2, SiO, SiO2, Al2O3, WS2, MoS2, ZnS, FeS, TiS2, SiS2, and Al2S3. Only one Na getter agent may be used, or multiple types may be used in combination.

[0039] In the solid-phase method, a Na getter agent is placed in contact with the NaSi-MgO composite in a reaction system. For example, a powdered NaSi-MgO composite and a powdered Na getter agent are mixed and placed in the reaction system. Then, by heating the NaSi-MgO composite, the Na in the contacting NaSi reacts with the Na getter agent, resulting in desorption of Na from the NaSi. The heating temperature t in the solid-phase method is not particularly limited as long as it is a temperature at which the reaction between the Na derived from NaSi and the Na getter agent proceeds, but it is preferably 400°C or less. Specifically, examples of the heating temperature t include 200°C≦t≦400°C, 270°C≦t≦360°C, 270°C≦t<310°C, and 270°C≦t≦300°C. In the solid-phase method, since there is no need to vaporize the desorbed Na, the reaction can be carried out under normal pressure.

[0040] The Na getter agent used in the solid-phase method is a material that can react with NaSi to receive Na from NaSi. Examples of the Na getter agent include CaCl2, AlF3, CaBr2, CaI2, Fe3O4, FeO, MgCl2, ZnO, ZnCl2, and MnCl2. Only one type of Na getter agent may be used, or multiple types may be used in combination.

[0041] [Porous process] The porosity-producing step is a step in which the clathrate Si-MgO composite is treated with acid to remove MgO. By removing MgO from the clathrate Si-MgO composite, the desired porous clathrate Si is obtained. Furthermore, when a solid-phase method is used in the clathrate production step, the acid treatment also removes the reaction product of Na and the Na getter agent, as well as the remaining Na getter agent.

[0042] The acid used in the porosity-forming step is not particularly limited, and any acid capable of removing MgO may be used. Examples of acids used in the washing step include hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the acid used in the washing step can be appropriately selected depending on the type of acid.

[0043] The porosity-imparting step may, if necessary, further include a step of washing the porous clathrate Si after the acid treatment with distilled water, alcohol, etc. The porosity-imparting step may also, if necessary, include a step of drying the porous clathrate Si after washing with distilled water, alcohol, etc.

[0044] [Other processes] The method for producing porous clathrate Si may include steps other than the reduction step, alloying step, clathrate production step, and porosity-inducing step described above. For example, after the porosity-inducing step, a Na removal step may be performed to further reduce the amount of Na remaining in the porous clathrate Si.

[0045] In the Na removal step, first, the same process as in the clathrate generation step is performed. Then, a cleaning process is performed as necessary. For example, in a reaction system, porous clathrate Si and a Na getter agent are placed in contact with each other. Then, the porous clathrate Si is heated, and the Na contained in the porous clathrate Si is desorbed by reacting with the Na getter agent. The Na getter agent used here may be the same as or different from the Na getter agent used in the clathrate generation step.

[0046] When a solid-phase method is employed in the clathrate production step, it is preferable to perform a Na removal step, i.e., to remove Na in multiple steps. In the solid-phase method, the reaction between Na and the Na getter agent is an exothermic reaction that releases heat. If the generated type II clathrate Si is excessively heated in the clathrate production step, its crystal structure may change. By removing Na in multiple steps and reducing the amount of Na reacted each time, it is possible to prevent the generated type II clathrate Si from being excessively heated due to the exothermic reaction between Na and the Na getter agent.

[0047] Furthermore, when a solid phase method is used in both the clathrate production step and the Na removal step, it is preferable that the Na getter agent used in the Na removal step has a higher reactivity with Na than the Na getter agent used in the clathrate production step, which more significantly prevents the produced type II clathrate Si from being excessively heated.

[0048] The Na removal step may, as needed, include a process of washing the reaction product of Na and the Na getter agent and the remaining Na getter agent with an acid or the like. The Na removal step may, as needed, also include a process of further washing the solid content generated after the acid treatment with distilled water, alcohol, or the like. The Na removal step may, as needed, also include a process of drying the washed solid content.

[0049] The Na removal step may be repeated multiple times. The Na removal step may be a step for further reducing the amount of Na remaining in the clathrate Si-MgO composite between the clathrate production step and the porosity-forming step. In this case, the Na removal step may be performed both between the clathrate production step and the porosity-forming step and after the porosity-forming step.

[0050] <Porous clathrate Si> The porous clathrate Si produced by the production method of this embodiment is a particle of porous silicon containing silicon as a main component.

[0051] The silicon content of the porous clathrate Si is, for example, 70 mass % or more, preferably 90 mass % or more. The silicon content is, for example, 100 mass % or less. The silicon content refers to the mass ratio of silicon atoms contained in the porous clathrate Si.

[0052] The porous clathrate Si contains type II clathrate Si. The proportion of type II clathrate Si in the silicon contained in the porous clathrate Si is, for example, 90% by mass or more, preferably 95% by mass or more. The proportion is, for example, 100% by mass or less. The porous clathrate Si may contain crystalline silicon.

[0053] The average particle diameter of the porous clathrate Si is, for example, 1 μm or more and 5 μm or less. The average particle diameter of the porous clathrate Si is preferably 1 μm or more, more preferably 1.5 μm or more. The average particle diameter of the porous clathrate Si is preferably 5 μm or less, more preferably 4 μm or less. In this specification, the "average particle diameter" refers to the median diameter (d 50 ) means

[0054] Porous clathrate Si has pores that are connected three-dimensionally. The pore size (diameter) of the pores in porous clathrate Si is, for example, 1 nm or more and 1000 nm or less. Porous clathrate Si is, for example, a microporous material with pores having a diameter of less than 2 nm, a mesoporous material with pores having a diameter of 2 to 50 nm, or a macroporous material with pores having a diameter of more than 50 nm.

[0055] An example of porous clathrate Si has pores with a diameter of 100 nm or less. The amount of pores with a diameter of 100 nm or less is, for example, 0.2 cm 3 / g or more 0.9cm 3 The amount of pores with a diameter of 100 nm or less is preferably 0.3 cm 3 / g or more, and more preferably 0.4 cm 3The amount of pores with a diameter of 100 nm or less is preferably 0.8 cm 3 / g or less, and more preferably 0.7 cm 3 / g or less.

[0056] An example of porous clathrate Si has pores with a pore diameter of 30 nm or less. The amount of pores with a pore diameter of 30 nm or less is, for example, 0.1 cm 3 / g or more 0.5cm 3 The amount of pores with a diameter of 30 nm or less is preferably 0.2 cm 3 / g or more, and more preferably 0.3 cm 3 The amount of pores with a diameter of 30 nm or less is preferably 0.45 cm 3 / g or less, and more preferably 0.4 cm 3 / g or less.

[0057] In this specification, the pore volume means the pore volume per unit mass calculated based on the BJH (Barret-Joyner-Halenda) method. Porous clathrate Si can be used for various purposes. Examples of uses of porous clathrate Si include a negative electrode material for a power storage device such as a lithium-ion secondary battery. The power storage device is, for example, a secondary battery such as a nickel-metal hydride secondary battery or a lithium-ion secondary battery. The power storage device may also be an all-solid-state battery or an electric double layer capacitor.

[0058] <effect> Next, the operation of this embodiment will be described. As shown in Figure 3, in the manufacturing method of this embodiment, a Si-MgO composite is formed in the reduction step, and then in the subsequent alloying step, the Si contained in the Si-MgO composite is alloyed by reacting with a Na source to form a NaSi-MgO composite. Next, in the clathrate generation step, the NaSi contained in the NaSi-MgO composite is clathrated by heating to form a clathrate Si-MgO composite. Then, in the porosity generation step, MgO is removed from the clathrate Si-MgO composite to form pores, thereby producing porous clathrate Si. According to the manufacturing method of this embodiment, the pores do not shrink or collapse in the alloying step and clathrate generation step, so porous clathrate Si with a large pore volume can be manufactured.

[0059] More specifically, in the conventional technology, a porous Si body is used as the Si to be alloyed with Na in the alloying step, and then in the subsequent clathrate production step, the porous NaSi body obtained in the alloying step is clathrated to produce porous clathrate Si.

[0060] In contrast, in the manufacturing method of this embodiment, in the alloying step, a Si-MgO composite is used as the Si to be alloyed with Na, rather than a porous Si body. Then, in the subsequent clathrate generation step, a NaSi-MgO composite is clathrated, rather than a porous NaSi body. The Si-MgO composite is a solid body in which MgO is distributed in a dispersed state in a Si matrix. In other words, the Si-MgO composite has a porous body formed from Si, and the pores of the porous body are filled with MgO. Similarly, the NaSi-MgO composite is a solid body in which MgO is distributed in a dispersed state in a NaSi matrix. In other words, the NaSi-MgO composite has a porous body formed from NaSi, and the pores of the porous body are filled with MgO.

[0061] Then, in the clathrate generation step, the porous portion of the NaSi-MgO composite formed by NaSi is converted to clathrate Si, and then in the subsequent porosity formation step, the MgO filling the pores is removed. This results in porous clathrate Si, a porous body of clathrate Si. In this way, in the manufacturing method of this embodiment, the solid Si-MgO composite is alloyed, the solid NaSi-MgO composite is clathrated, and then the MgO is removed to form pores.

[0062] When an alloying process is performed using a porous Si body as in the prior art, the pores in the porous Si body shrink or collapse due to the expansion that occurs when Na is absorbed into Si. In addition, the porous NaSi obtained by the alloying process is softened by heating during clathration, which causes the pores in the porous NaSi body to shrink or collapse. As a result, the amount of pores in the finally obtained porous clathrate silicon is reduced.

[0063] In contrast, when the alloying process is performed using a Si-MgO composite as in this embodiment, even if the Si in the Si-MgO composite expands when Na is absorbed, the portion formed by MgO remains intact without shrinking or collapsing. Similarly, even if NaSi in the NaSi-MgO composite softens due to heating in the clathrate formation process, the portion formed by MgO remains intact without shrinking or collapsing. In other words, the dispersion state of MgO within the composite is maintained in both the alloying process and the clathrate formation process. Therefore, removing MgO from the clathrate Si-MgO composite forms pores equivalent in number to the portion formed by MgO. Note that MgO is a stable substance with respect to Na and therefore does not participate in the Na elimination reaction in the clathrate formation process.

[0064] As described above, in the manufacturing method of this embodiment, the alloying step and the clathrate formation step are performed using a solid composite containing MgO. Therefore, neither the expansion caused by absorption of Na into Si nor the softening of NaSi due to heating during clathration has a significant effect on the removal of MgO and the formation of pores. Therefore, according to the manufacturing method of this embodiment, the reduction in pores during the alloying step and the clathrate formation step can be suppressed, and porous clathrate Si with a large amount of pores can be manufactured.

[0065] <Effects> According to this embodiment, the following effects can be obtained. (1) A method for producing porous clathrate Si includes a reduction step, an alloying step, a clathrate formation step, and a porosity-improving step. The reduction step is a step for obtaining a Si-MgO composite containing Si and MgO. The alloying step is a step for producing NaSi by reacting the Si-MgO composite with a Na source to obtain a NaSi-MgO composite. The clathrate formation step is a step for heating the NaSi-MgO composite to clathrate NaSi and desorb Na, thereby obtaining a clathrate Si-MgO composite containing type II clathrate Si and MgO. The porosity-improving step is a step for removing MgO from the clathrate Si-MgO composite to make it porous. According to the above configuration, porous clathrate Si with a large amount of pores can be produced.

[0066] (2) The clathrate formation process involves heating the NaSi-MgO composite in the presence of a Na getter that captures Na from NaSi. In this case, the desorption of Na from NaSi is promoted, thereby efficiently promoting the clathration of NaSi.

[0067] (3) A Na removal step is provided to reduce the amount of Na remaining in the porous clathrate Si after the porosity-imparting step, which makes it easy to adjust the amount of Na remaining in the porous clathrate Si.

[0068] (4) The clathrate generation process involves heating the NaSi-MgO composite in contact with a Na getter agent that captures Na derived from NaSi. The Na removal process involves heating the porous clathrate silicon after the porosification process in contact with a Na getter agent. The Na getter agent used in the Na removal process has a higher reactivity with Na than the Na getter agent used in the clathrate generation process.

[0069] According to the above configuration, by removing Na in multiple steps and reducing the amount of Na reacted each time, it is possible to prevent the generated type II clathrate Si from being excessively heated due to the exothermic reaction between Na and the Na getter agent in the solid-phase method. [Example]

[0070] <Example> (Synthesis of Mg2Si) Si powder (30 g) of 300 μm or less and metallic Mg powder (53.5 g) of 180 μm or less were mixed. The resulting mixture was placed in a stainless steel container with a lid and heated at 600 °C for 6 hours in an Ar atmosphere to obtain powdered Mg2Si.

[0071] (Reduction process) As shown in FIG. 2, a Si raw material (10 g of quartz (SiO2) powder with an average particle size of 0.8 μm) was placed in a raw material tray 11 in a stainless steel reaction vessel 10, and Mg2Si (16.6 g) was placed in a reduction tray 12 in the reaction vessel 10. The reaction vessel 10 was placed in a vacuum furnace 13, and the vacuum furnace 13 was heated to 650°C for 12 hours while evacuating with a rotary pump. The pressure inside the vacuum furnace 13 at this time was 1 Pa. After the heat treatment, the powdered Si-MgO composite was collected from the raw material tray 11.

[0072] (Alloying process) The Si-MgO composite (10 g) and NaH (2.6 g) were mixed using a cutter mill, and the resulting mixture was heated at 420°C for 20 hours in an Ar atmosphere to obtain a powder containing the NaSi-MgO composite.

[0073] (Clatrate generation process (solid phase method)) The total amount of powder containing the NaSi-MgO composite and AlF (2.8 g) were mixed using a cutter mill, and the resulting mixture was heated at 320 °C for 40 hours in an Ar atmosphere to obtain a powder containing the clathrate Si-MgO composite.

[0074] (Porous process) The powder (total amount) containing the clathrate Si-MgO composite was added to 200 ml of a 15% by mass HCl aqueous solution cooled to 0°C, and after stirring for 15 hours while maintaining the temperature at 0°C, the solid matter was recovered by filtration under reduced pressure. The recovered solid matter was washed with distilled water and ethanol, and vacuum dried at 120°C for 12 hours to obtain powdered porous clathrate Si.

[0075] (Na removal process (solid phase method)) The obtained porous clathrate Si (4 g) and ZnCl2 (2.2 g) were mixed using a cutter mill. The obtained mixture was heated at 320°C for 15 hours under an Ar atmosphere. The heated reaction product was added to 100 ml of aqueous nitric acid solution and stirred for 30 minutes, after which the solid matter was collected by vacuum filtration. The aqueous nitric acid solution was a mixture of pure water (90 ml) and nitric acid 1.38 (10 ml). The collected solid matter was washed with distilled water and ethanol and vacuum dried at 120°C for 12 hours to obtain powdered porous clathrate Si. This was the porous clathrate Si of this example.

[0076] <Comparative Example> The comparative example differs from the example in the order of the alloying step, clathrate generation step, and porosity generation step. In the comparative example, the porosity generation step is performed after the reduction step and before the alloying step and clathrate generation step. In other words, in the comparative example, the Si-MgO composite obtained by the reduction step is first made porous, and then alloyed and clathrated.

[0077] (Reduction process) In the same manner as in Example 1, a powdered Si—MgO composite was obtained. (Porous process) The Si-MgO composite (total amount) was added to 200 ml of a 15% by mass HCl aqueous solution cooled to 0°C, and after stirring for 15 hours while maintaining the temperature at 0°C, the solid content was recovered by filtration under reduced pressure. The recovered solid content was washed with distilled water and ethanol, and vacuum dried at 120°C for 12 hours to obtain powdered porous silicon.

[0078] (Alloying process) Porous silicon (1 g) and NaH (2.6 g) were mixed using a cutter mill, and the resulting mixture was heated at 420°C for 20 hours in an Ar atmosphere to obtain a powder containing porous NaSi.

[0079] (Clatrate generation process (solid phase method)) The powder containing porous NaSi (total amount) and AlF3 (2.8 g) were mixed using a cutter mill. The resulting mixture was heated at 320°C for 40 hours under an Ar atmosphere to obtain a powder containing porous silicon clathrate. The resulting powder was added to a cleaning solution (200 ml) and stirred for 30 minutes, after which the solid matter was collected by vacuum filtration. The cleaning solution used was a mixture of pure water (180 ml) and nitric acid (20 ml) mixed with aluminum chloride hexahydrate (20 g). The collected solid matter was washed with distilled water and ethanol and vacuum dried at 120°C for 12 hours to obtain powdered porous clathrate Si.

[0080] (Na removal process (solid phase method)) The obtained porous clathrate Si (2.9 g) and ZnCl2 (2.2 g) were mixed using a cutter mill. The obtained mixture was heated at 320°C for 15 hours under an Ar atmosphere. The heated reaction product was added to 100 ml of aqueous nitric acid solution and stirred for 30 minutes, after which the solid matter was recovered by vacuum filtration. The aqueous nitric acid solution was a mixture of pure water (90 ml) and nitric acid 1.38 (10 ml). The recovered solid matter was washed with distilled water and ethanol and vacuum dried at 120°C for 12 hours to obtain powdered porous clathrate Si. This was the porous clathrate Si of the comparative example.

[0081] <Analysis by X-ray diffraction method> The porous clathrate Si of the example was analyzed by the X-ray diffraction method. The X-ray diffraction pattern of the example is shown in FIG. 4. From the X-ray diffraction pattern, it can be confirmed that the porous clathrate Si of the example is composed of type II clathrate Si and contains almost no crystalline silicon. Although the illustration is omitted, the analysis result by the X-ray diffraction method of the comparative example was the same as that of the example.

[0082] <Analysis of pores> Using a specific surface area and pore size distribution analyzer, the nitrogen adsorption / desorption isotherms of the particles of the porous clathrate Si of the example and the comparative example were measured by the gas adsorption method. Based on the obtained nitrogen adsorption / desorption isotherms, using the BJH method and the BET method, a pore size distribution plot was created to determine the pore volume for each size. The pore size distributions of the example and the comparative example are shown in FIG. 5. As shown in FIG. 5, the pore volume of the example is significantly larger than that of the comparative example. As an example, referring to the cumulative pore volume of 100 nm or less in the pore size distribution plot of the example, the value of the comparative example is 0.246 cm 3 / g, while the value of the example is 0.554 cm 3 / g, which is more than twice the value of the comparative example.

Claims

1. a reduction step of bringing a silicon raw material containing silicon oxide into contact with Mg vapor under reduced pressure and at a Mg vapor pressure equal to or lower than the equilibrium pressure of the following reaction formula (1) to obtain a Si-MgO composite containing Si and MgO; an alloying step of reacting the Si—MgO composite with a Na source to produce NaSi and obtain a NaSi—MgO composite; a clathrate generation step of heating the NaSi-MgO composite to form a clathrate of NaSi and desorb Na, thereby obtaining a clathrate Si-MgO composite containing type II clathrate silicon and MgO; and removing MgO from the clathrate Si-MgO composite to make it porous. 【Chemistry 1】

2. 2. The method for producing porous clathrate silicon according to claim 1, wherein the clathrate generation step includes heating the NaSi-MgO composite in the presence of a Na getter agent that captures Na derived from NaSi.

3. 2. The method for producing porous clathrate silicon according to claim 1, further comprising a Na removal step of reducing the amount of Na remaining in the porous clathrate silicon after the porosity-imparting step.

4. the clathrate generation step includes heating the NaSi-MgO composite in a state in which the NaSi-MgO composite is in contact with a Na getter agent that captures Na derived from NaSi; the Na removal step includes heating the porous clathrate silicon after the porosification step in a state where the Na getter agent is in contact with the porous clathrate silicon, 4. The method for producing porous clathrate silicon according to claim 3, wherein the Na gettering agent used in the Na removal step has a higher reactivity with Na than the Na gettering agent used in the clathrate production step.

Images