Manufacturing method of negative electrode active material containing silicon clathrate ii

Abstract

To provide a manufacturing method of a negative electrode active material containing a silicon clathrate II having excellent production efficiency.SOLUTION: A manufacturing method of a silicon clathrate II includes: a housing step of housing a raw material containing a Na material and a Si material and a Na getter agent in the same container in a state of being separated from each other by a predetermined sample distance; an alloying step of obtaining a Na-Si alloy by heating the raw material housed in the container in the housing step to a first temperature; and a silicon clathrate generation step of generating the silicon clathrate II by reacting Na vaporized from the Na-Si alloy with a Na gettering agent by decompressing the inside of the container and heating the inside of the container to a second temperature while accommodating the Na-Si alloy obtained in the alloying step in the container.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a negative electrode active material including silicon clathrate II.

Background Art

[0002] A clathrate compound is a compound in which guest atoms are encapsulated in a three-dimensional cage structure formed by host atoms. As a silicon clathrate in which the host atom is silicon, for example, a type II silicon clathrate (so-called silicon clathrate II) in which sodium as a guest atom is encapsulated in a cubic crystal structure formed by sharing faces of a dodecahedron of Si atoms and an octahedron of Si atoms is known.

[0003] Since silicon clathrate II can release the encapsulated sodium while maintaining its cage structure by heat treatment, its utilization as a negative electrode active material for secondary batteries has been studied.

[0004] Patent Document 1 describes a method for manufacturing silicon clathrate II from a Na—Si alloy containing Na and Si. Specifically, Na and Si are melted to produce a Na—Si alloy, and then the Na—Si alloy and a predetermined Na getter agent are placed in a reaction vessel and heated while reducing the pressure in the reaction vessel to produce silicon clathrate II.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, in the conventional manufacturing method, Na and Si contained in a container are heated and melted to produce a Na-Si alloy. After the Na-Si alloy in the container has cooled, the produced Na-Si alloy is taken out of the container in an inert atmosphere, and the taken-out Na-Si alloy and a Na getter agent are accommodated in another container and heat-treated to synthesize silicon class rate II. Therefore, there is room for improvement in the conventional manufacturing method in terms of manufacturing efficiency.

[0007] The present disclosure provides a method for manufacturing a negative electrode active material including silicon class rate II with excellent manufacturing efficiency.

Means for Solving the Problems

[0008] One aspect of the present disclosure provides the following method for manufacturing silicon class rate II, method for manufacturing a negative electrode active material, method for manufacturing a negative electrode, and method for manufacturing a secondary battery. [1] A housing step of housing a raw material containing a Na material and a Si material and a Na getter agent in the same container in a state where they are separated by a predetermined sample interval distance, An alloying step of obtaining a Na-Si alloy by heating the raw material housed in the container in the housing step to a first temperature, While the Na-Si alloy obtained in the alloying step is housed in the container, the pressure inside the container is reduced and the inside of the container is heated to a second temperature, so that Na vaporized from the Na-Si alloy reacts with the Na getter agent to produce silicon class rate II. A method for manufacturing a negative electrode active material including silicon class rate II, comprising a silicon class rate generation step. [2] In the alloying step, the pressure inside the container is controlled to a first pressure so that the diffusion distance of Na is less than the sample interval distance, In the silicon class rate generation step, the pressure inside the container is controlled to a second pressure lower than the first pressure so that the diffusion distance of Na is equal to or greater than the sample interval distance. The method for manufacturing a negative electrode active material including silicon class rate II according to [1]. [3] The method for manufacturing a negative electrode active material including silicon class rate II according to [1] or [2], wherein the first temperature is 280°C or higher and 400°C or lower. [4] The sample distance is 7 mm or less, The method for producing a negative electrode active material containing silicon class rate II according to any one of [1] to [3], wherein the first temperature is 280°C or higher and 360°C or lower. [5] In the accommodating step, in the first accommodating portion that holds the raw material in the container, the raw material is arranged in layers, The method for producing a negative electrode active material containing silicon class rate II according to any one of [1] to [4], wherein the layer thickness of the raw material arranged in layers is 10 mm or less. [6] In the accommodating step, in the container, a plurality of first accommodating portions that hold the raw material and a plurality of second accommodating portions that hold the Na getter agent are alternately arranged in the first direction. The method for producing a negative electrode active material containing silicon class rate II according to any one of [1] to [5].

Effect of the Invention

[0009] According to the present disclosure, a method for producing a negative electrode active material containing silicon class rate II with excellent production efficiency is provided.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings as appropriate. However, the present disclosure is not limited to the following embodiments.

[0012] The numerical range "x to y" described in the present disclosure includes the lower limit x and the upper limit y within that range. And, by arbitrarily combining these upper and lower limit values, as well as the numerical values listed in the examples, a numerical range can be constituted. Furthermore, a numerical value arbitrarily selected from within the numerical range can be used as the upper and lower limit numerical values.

[0013] [Method for Producing a Negative Electrode Active Material Containing Silicon Class Rate II] The method for producing a negative electrode active material containing silicon class rate II according to this embodiment (hereinafter, also simply referred to as "the production method according to this embodiment") includes the following accommodation step, alloying step, and silicon class rate generation step. In the production method according to this embodiment, the Na-Si alloy obtained in the alloying step and the Na getter agent accommodated in the accommodation step are continuously subjected to the silicon class rate generation step in succession to the alloying step without taking them out of the container after the completion of the alloying step.

[0014] Accommodation step: A step of accommodating a raw material containing a Na material and a Si material and a Na getter agent in the same container while separating them by a predetermined sample interval. Alloying step: A step of obtaining a Na-Si alloy by heating the raw material accommodated in the container in the accommodation step to a first temperature. Silicon class rate generation step: While the Na-Si alloy obtained in the alloying step is accommodated in the container, the pressure inside the container is reduced and the inside of the container is heated to a second temperature, so that Na vaporized from the Na-Si alloy reacts with the Na getter agent to generate silicon class rate II.

[0015] The manufacturing method according to this embodiment can obtain silicon class rate II with a high yield while being excellent in manufacturing efficiency compared with the conventional method. The reasons for such an effect are as follows. When obtaining a Na-Si alloy from raw materials containing a Na material and a Si material, if a Na getter agent is accommodated in the same container in a state of being in contact with the raw materials, there is a risk that Na atoms will be adsorbed by the Na getter agent during the alloying reaction. As a result, the yield of silicon class rate II obtained through class rating decreases. In the manufacturing method according to this embodiment, the raw materials of the Na-Si alloy used in the alloying step (that is, the raw materials containing the Na material and the Si material) and the Na getter agent used in the silicon class rate generation step following the alloying step are accommodated in the same container in a state where they are separated by a predetermined sample-to-sample distance. The sample-to-sample distance is, for example, 0.14 μm or more. Also, the sample-to-sample distance may be, for example, 20 mm or less. This sample-to-sample distance is set based on the diffusion distance of Na atoms obtained from the relational expression of the mean free path of the following formula (1). That is, the diffusion distance of Na atoms in the alloying step is calculated based on the heating temperature of the raw materials required for the alloying reaction to obtain the Na-Si alloy and the pressure inside the container, and the sample-to-sample distance between the raw materials and the Na getter agent is made larger than the calculated diffusion distance of Na atoms, and the sample-to-sample distance in the accommodation step is set. Thereby, it is possible to suppress the adsorption of Na atoms by the Na getter agent during the alloying reaction. Also, in the manufacturing method according to this embodiment, the Na-Si alloy obtained in the alloying step and the Na getter agent accommodated in the accommodation step are continuously subjected to the silicon class rate generation step without being taken out of the container after the completion of the alloying step. In the silicon class rate generation step, the temperature and pressure inside the container are controlled so that the diffusion distance of Na atoms is larger than the sample-to-sample distance between the raw materials and the Na getter agent set in the accommodation step. Here, the distance between the Na-Si alloy and the Na getter agent accommodated in the container in the silicon class rate generation step is equal to the sample-to-sample distance between the raw materials and the Na getter agent set in the accommodation step.Since the heating temperature required for the silicon class rate generation process is determined to be a predetermined temperature, the diffusion distance of Na atoms in the silicon class rate generation process is adjusted by controlling the pressure in the container. Specifically, the pressure in the container in the silicon class rate generation process is made lower than the pressure in the container in the alloying process so that the diffusion distance of Na atoms in the silicon class rate generation process is equal to or greater than the sample interval distance set in the accommodation process. As a result, in the silicon class rate generation process, the adsorption of Na atoms from the Na-Si alloy to the Na getter agent disposed at a predetermined sample interval distance is promoted. As a result, the manufacturing method according to the present embodiment can continuously perform the synthesis and class rate formation of the Na-Si alloy in a state where a raw material containing a Na material and a Si material and a Na getter material are accommodated in the same container, and further, silicon class rate II can be obtained with a high yield.

[0016] [Number] [In formula (1), l represents the diffusion distance of Na atoms, R represents the gas constant, P represents the pressure, T represents the temperature (350 ° C), and N A represents Avogadro's number, and d represents the diameter of Na atoms.]

[0017] (Accommodation process) Hereinafter, the accommodation process will be described in detail.

[0018] FIG. 1 is a schematic cross-sectional view showing the manufacturing method according to the present embodiment. In the manufacturing method according to the present embodiment, a reaction apparatus having a container 10 and a heating furnace 20 is used. Although not shown in the figure, the container 10 has a function of discharging the gas inside the container to the outside of the container and a function of introducing the gas into the container. A raw material basket 12 (first accommodating portion) is arranged in the container 10. In this step, as shown in FIG. 1(a), in the raw material basket 12, a raw material 14 containing a Na material and a Si material is accommodated in a layered state (a state having a certain thickness) as a raw material for generating a Na—Si alloy in the alloying step. Further, a Na getter agent 18 is accommodated at the bottom of the container 10 in a layered state (a state having a certain thickness). That is, the raw material 14 and the Na getter agent 18 are accommodated in the same container 10. The raw material 14 and the Na getter agent 18 are arranged inside the same container 10 while being separated by a preset predetermined sample interval distance L. The sample interval distance L is, for example, the shortest distance between the raw material 14 and the Na getter agent.

[0019] The sample interval distance L between the raw material 14 and the Na getter agent 18 inside the container 10 is preferably 1 μm or more, more preferably 0.1 mm or more, and still more preferably 0.5 mm or more. By setting the sample interval distance L to such a value, the yield of silicon class rate II can be further improved. The sample interval distance L between the raw material 14 and the Na getter agent 18 inside the container 10 is preferably 17 mm or less, more preferably 7 mm or less, and still more preferably 3 mm or less. By setting the sample interval distance L to such a value, the amount of the raw material 14 that can be accommodated in the container 10 can be increased. As a result, the yield of the silicon class rate II can be improved.

[0020] The main surfaces of the layered raw material 14 and the layered Na getter agent 18 face each other. In plan view, the area of the main surface of the raw material 14 that overlaps with the main surface of the Na getter agent 18 may be 70 area % or more, 80 area % or more, 90 area % or more, 95 area % or more, or 100 area %.

[0021] Examples of the Na material contained in the raw material 14, which is a raw material for producing the Na-Si alloy, include metallic sodium and sodium hydride.

[0022] Examples of the Si material contained in the raw material 14, which is a raw material for producing the Na-Si alloy, include silicon, amorphous silicon, polysilicon, ferrosilicon, and porous silicon.

[0023] The blending amount of the Si material in the raw material 14 may be 80 to 120 parts by mass with respect to 100 parts by mass of the Na material.

[0024] The content of the Na material and the Si material in the raw material 14 may be 90% by mass or more, 95% by mass or more, 99% by mass or more, or 100% by mass based on the total amount of the raw material 14.

[0025] The layer thickness t of the raw material 14 accommodated in layers in the raw material basket 12 is preferably 10 mm or less, more preferably 5 mm or less, and still more preferably 4 mm or less. Thereby, the reaction between the Na vapor and the Na getter agent 18 is promoted in the silicon class rate generation step, and the yield of the silicon class rate II is further improved.

[0026] The Na getter agent means a material that can react with zero-valent Na. The Na getter agent means a material that can react with zero-valent Na and has a lower vapor pressure than the vapor pressure of metallic Na.

[0027] Examples of the Na getter agent 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.

[0028] Examples of the Na getter agent include WO2, WO3, MoO3, ZnO, FeO, VO, V2O3, TiO2, SiO, SiO2, Al2O3, WS2, MoS2, ZnS, FeS, TiS2, SiS2, Al2S3, and Fe3O4. Among them, those selected from WO3, MoO3, ZnO, FeO, V2O3, TiO2, SiO, Al2O3, and Fe3O4 are preferred, and those selected from WO3, MoO3, FeO, TiO2, and Fe3O4 are more preferred.

[0029] As the Na getter agent, one type may be used alone or two or more types may be combined.

[0030] The amount of the Na getter agent contained in the container may be 235 to 620 parts by mass with respect to 100 parts by mass of the Na material. The Na getter agent may be placed in a mesh basket in the container 10 and accommodated in layers in the mesh basket.

[0031] The material of the container 10 may be, for example, stainless steel.

[0032] The raw material basket 12 may be a mesh basket. In the manufacturing method according to this embodiment, the raw materials are accommodated in a basket, but the one for accommodating the raw material 14 is not limited to a basket. For example, the raw material 14 may be placed on a mesh tray. The material of the raw material basket 12 may be, for example, stainless steel.

[0033] (Alloying step) Hereinafter, the alloying step will be described in detail.

[0034] In this step, as shown in FIG. 1(b), the raw material 14 in the container 10 is heat-treated at the first temperature to cause an alloying reaction between the Na raw material and the Si raw material of the raw material 14, and the Na-Si alloy 15 is obtained.

[0035] The first temperature may be 250 °C or higher, preferably 280 °C or higher, and more preferably 300 °C or higher. Thereby, the alloying reaction between the Na raw material and the Si raw material of the raw material 14 can be promoted. The first temperature may be 800 °C or lower, preferably 400 °C or lower, and more preferably 360 °C or lower. Thereby, the yield of silicon class rate II obtained in the silicon class rate generation step following the alloying step can be further improved.

[0036] In this step, by increasing the pressure in the container 10 (hereinafter also referred to as "the first pressure"), the diffusion distance of Na is made smaller than the sample interval distance L set in the accommodation step. That is, the first pressure is preferably a value at which the diffusion distance of Na is less than the sample interval distance L. Thereby, the yield of silicon class rate II can be improved. The first pressure may be, for example, 10 5 Pa or higher.

[0037] The heating time of the raw material 14 may be, for example, 1 to 40 hours.

[0038] This step may be carried out in an inert gas atmosphere by introducing an inert gas into the container 10.

[0039] In this step, the sample interval distance L between the raw material and the Na getter agent is the same as the sample interval distance L in the accommodation step.

[0040] The Na—Si alloy obtained in this step has a composition of Na and Si of Na y Si 136 represented by (24 < y). As the Na—Si alloy, it is preferable that Na is present in excess of Si, that is, the composition of Na and Si is Na z Si (1 < z).

[0041] In the Na-Si alloy, other elements than Na and Si may be present without departing from the gist of the present invention. Examples of other elements include Li, K, Rb, Cs, and Ba that can be substituted for Na, and Ga and Ge that can be substituted for Si in silicon class rate II.

[0042] (Silicon class rate generation step) Hereinafter, the silicon class rate generation step will be described in detail.

[0043] This step is a silicon class rate generation step that is continuously executed following the alloying step without taking out the Na-Si alloy obtained in the alloying step and the Na getter agent accommodated in the accommodation step from the container after the completion of the alloying step. In the silicon class rate generation step, heat treatment of the Na-Si alloy in the container at a second temperature is performed. When the Na-Si alloy in the container is heat-treated at the second temperature, gaseous Na(g) is generated as shown in the following reaction formula, and the gaseous Na(g) is captured by the Na getter agent present in the container. As a result, the following reaction formula can be made to proceed favorably to the right side. Also, in the manufacturing method according to the present embodiment, the reaction proceeds under relatively low Na partial pressure conditions. Therefore, there is an advantage that the generation of silicon class rate I is suppressed and silicon class rate II is preferentially manufactured. In this step, as shown in FIG. 1(c), a silicon material 16 containing silicon class rate II is obtained. Na-Si alloy ←→ silicon class rate II + Na(g)

[0044] At the start of this step, the Na-Si alloy obtained in the alloying step and the Na getter agent are accommodated in the container 10. The sample-to-sample distance between the Na-Si alloy and the Na getter agent in the container 10 is the same as the sample-to-sample distance L between the raw material and the Na getter agent in the accommodation step.

[0045] The pressure inside the container 10 in this process (hereinafter, also referred to as "the second pressure") is adjusted to a value lower than the first pressure so that the diffusion distance of Na becomes equal to or greater than the sample distance between the raw material and the Na getter agent set in the accommodation process. Thereby, the yield of silicon class rate II can be improved. The second pressure is, for example, less than 10 5 Pa, 10 4 Pa or less, 10 3 Pa or less, 10 2 Pa or less, or may be 10 Pa or less. The second pressure is, for example, 10 -2 Pa or more, or 10 -1 Pa or more.

[0046] The second temperature may be, for example, 100 °C or higher, 200 °C or higher, or 250 °C or higher. The second temperature may be, for example, 500 °C or lower, preferably 400 °C or lower, and more preferably 350 °C or lower. In particular, by setting the second temperature to 400 °C or lower, the generation of diamond-structured Si crystals can be suppressed. As a result, the obtained silicon class rate II can be suitably used as a negative electrode active material for secondary batteries such as lithium ion secondary batteries, and power storage devices such as electric double layer capacitors and lithium ion capacitors.

[0047] This process may be carried out in an inert gas atmosphere by introducing an inert gas into the container 10.

[0048] In the manufacturing method according to this embodiment, the silicon material 16 containing silicon class rate II obtained in the above silicon class rate generation step and a new Na getter agent are coexisted in the container 10, and the silicon material 16 is heated to further reduce the amount of Na remaining in the silicon class rate II. A treatment may be performed.

[0049] Silicon class rate II can be suitably used as a negative electrode active material for secondary batteries such as lithium ion secondary batteries, and power storage devices such as electric double layer capacitors and lithium ion capacitors. The lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolytic solution and a separator, or a positive electrode, a negative electrode and a solid electrolyte.

[0050] When silicon class rate II is used as the negative electrode active material, the composition formula of silicon class rate II is Na x Si 136 For x, it may be 0 or more. Preferably x is 10 or less, more preferably 7 or less, still more preferably 5 or less, even more preferably 3 or less, particularly preferably 2 or less, and most preferably 1 or less. Thereby, charge carriers such as lithium can move within the polyhedral cage of silicon class rate II from which Na has detached. As a result, the degree of expansion of the negative electrode active material is suppressed.

[0051] Silicon class rate II can be used, for example, in applications such as thermoelectric elements, light-emitting elements, and light-absorbing elements.

[0052] (Washing step) In the silicon material 16 containing silicon class rate II obtained in the silicon class rate generation step, by-products other than silicon class rate II such as Na and NaOH may be attached. The manufacturing method according to the present embodiment may further include a washing step of washing the silicon material 16 in order to remove the by-products attached to the silicon material 16 obtained in the silicon class rate generation step.

[0053] In the washing step, the silicon material 16 is washed using a solvent capable of dissolving the by-products. As the solvent, for example, an acidic aqueous solution is used. The concentration of the acid in the acidic aqueous solution is preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass, and still more preferably 1 to 4% by mass.

[0054] After the washing step, it is preferable to remove water from the silicon material 16 by filtration and drying.

[0055] The silicon material 16 is preferably made into a powder with a certain particle size distribution through pulverization and classification. The preferred average particle size of the silicon material 16 is preferably in the range of 1 to 30 μm, more preferably in the range of 2 to 20 μm, and even more preferably in the range of 3 to 15 μm. The average particle size means D50 when the sample is measured with a general laser diffraction particle size distribution measuring device.

[0056] The manufacturing method of silicon class rate II according to one embodiment has been described above. However, the manufacturing method of silicon class rate II of the present disclosure is not limited to the above embodiment. For example, in the above embodiment, the layered raw material and the layered Na getter agent are respectively arranged in the container in a horizontal position so that the main surfaces are along the horizontal direction. However, the layered raw material and the layered Na getter agent may be respectively arranged in the container in a vertical position so that the main surfaces are along the vertical direction.

[0057] Also, in the accommodation step of the above embodiment, one layered raw material and one layered Na getter agent are respectively arranged in the container of the reaction device. However, it is not limited thereto. In the accommodation step, a plurality of first accommodation parts each holding a layered raw material and a plurality of second accommodation parts each holding a layered Na getter agent are prepared, and in the same container, the plurality of first accommodation parts and the plurality of second accommodation parts are alternately arranged side by side in the first direction. FIG. 2 is a schematic cross-sectional view showing the state after the silicon class rate generation step when two layered raw materials and three layered Na getter agents are accommodated in the accommodation step. In the two raw material baskets 12, the silicon material 16 containing silicon class rate II can be obtained. Since a plurality of first accommodation parts holding raw materials are accommodated in the container so as to efficiently utilize the volume in the container 10, the amount of silicon class rate II that can be obtained in one production increases. As a result, the production efficiency can be further improved.

[0058] [Manufacturing method of negative electrode] The method for manufacturing a negative electrode according to this embodiment includes a step of manufacturing a negative electrode active material by the method for manufacturing a negative electrode active material including silicon class rate II according to the above embodiment, and a step of manufacturing a negative electrode for a battery using the manufactured negative electrode active material.

[0059] [Method for manufacturing a secondary battery] The method for manufacturing a secondary battery according to this embodiment includes a step of manufacturing a negative electrode for a battery by the method for manufacturing a negative electrode according to the above embodiment, and a step of manufacturing a secondary battery using the manufactured negative electrode for a battery.

Examples

[0060] Hereinafter, the present disclosure will be specifically described with reference to examples, but the present disclosure is not limited to these examples.

[0061] (Example 1) A Na ingot cut to about 5 mm square was prepared as the Na material. Silicon powder was prepared as the Si material. The Na ingot and the silicon powder were mixed with a wonder crusher. The blending amount of the silicon powder was set to 102 parts by mass with respect to 100 parts by mass of the Na ingot. The Na ingot was blended in three equal amounts, and the mixture was performed three times for 1 minute at a wonder crusher strength of 5. Thus, a raw material containing the Na material and the Si material was obtained.

[0062] As shown in Fig. 1(a), Fe3O4 was arranged in layers at the bottom of a stainless-steel container as a Na getter agent (accommodation step). The amount of the Na getter agent was arranged to be 284 parts by mass with respect to 100 parts by mass of the Na raw material. A stainless-steel mesh basket was placed on the Na getter agent. The raw materials were accommodated in layers in the basket. The sample interval distance between the Na getter agent and the raw materials was set to 1 mm. Then, the container was covered and set in a furnace. After evacuating the inside of the container, Ar was flowed and the raw materials were fired. The firing conditions were under atmospheric pressure (first pressure), heating temperature of 300 °C (first temperature), and heating time of 3 hours. Thus, a Na-Si alloy was obtained (alloying step). Then, without taking out the Na-Si alloy and the Na getter agent from the container, the inside of the container was depressurized, and the Na-Si alloy was fired at a pressure of 20 Pa (second pressure), a heating temperature of 350 °C (second temperature), and a heating time of 10 hours. Thus, a silicon material containing silicon class rate II was obtained (silicon class rate generation step). The container was cooled to room temperature, the pressure was increased to atmospheric pressure, and the silicon material was taken out of the container.

[0063] (Example 2) A silicon material was obtained in the same manner as in Example 1 except that the first temperature was 330 °C and the second temperature was 350 °C. Regarding the series of operations of charging the raw materials, the alloying step, the silicon class rate generation step, and taking out the silicon material in this example, a graph with the elapsed time on the horizontal axis and the temperature inside the container on the vertical axis is shown in Fig. 3(a). Also, regarding the series of operations, a graph with the elapsed time on the horizontal axis and the pressure inside the container on the vertical axis is shown in Fig. 3(b).

[0064] (Example 3) A silicon material was obtained in the same manner as in Example 1 except that the first temperature was 350 °C and the second temperature was 350 °C.

[0065] (Example 4) A silicon material was obtained in the same manner as in Example 1 except that the first temperature was 380 °C and the second temperature was 350 °C.

[0066] (Example 5) A silicon material was obtained in the same manner as in Example 1, except that the sample distance between the Na getter agent and the raw material was set to 5 mm.

[0067] (Example 6) A silicon material was obtained in the same manner as in Example 5, except that the first temperature was set to 330°C and the second temperature was set to 350°C.

[0068] (Example 7) A silicon material was obtained in the same manner as in Example 5, except that the first temperature was set to 350°C and the second temperature was set to 350°C.

[0069] (Example 8) A silicon material was obtained in the same manner as in Example 5, except that the first temperature was set to 380°C and the second temperature was set to 350°C.

[0070] (Example 9) A silicon material was obtained in the same manner as in Example 5, except that the first temperature was set to 430°C and the second temperature was set to 350°C.

[0071] (Example 10) A silicon material was obtained in the same manner as in Example 9, except that the sample distance between the Na getter agent and the raw material was set to 15 mm.

[0072] (Comparative Example 1) The raw materials were prepared in the same manner as in Example 1, and the raw materials were placed in a first stainless steel container. After evacuating the inside of the first container, Ar was flowed to bake the raw materials. The baking conditions were a heating temperature of 330 °C and a heating time of 3 hours under atmospheric pressure. Thereby, a Na-Si alloy was obtained. After baking, the first container was cooled, and the Na-Si alloy was taken out from the cooled first container. Subsequently, a second stainless steel container was prepared, and Fe3O4 was placed in layers at the bottom of the second container as a Na getter agent. The amount of the Na getter agent was arranged to be 284 parts by mass with respect to 100 parts by mass of the Na raw material. A stainless steel mesh basket was placed on the Na getter agent. The Na-Si alloy taken out from the first container was placed in the basket. The sample distance between the Na getter agent and the Na-Si alloy was set to 1 mm. Then, the second container was covered and placed in a heating furnace. The Na-Si alloy was baked at a pressure of 20 Pa, a heating temperature of 350 °C, and a heating time of 10 hours. Thereby, a silicon material containing silicon clathrate II was obtained. The container was cooled to room temperature, the pressure was increased to atmospheric pressure, and the silicon material was taken out from the container.

[0073] Regarding the series of operations of charging the raw materials in this comparative example, synthesizing and taking out the Na-Si alloy, charging the Na getter agent and the Na-Si alloy, clathrating, and taking out the silicon material, a graph with the elapsed time on the horizontal axis and the temperature inside the container on the vertical axis is shown in Fig. 4(a). Also, regarding the series of operations, a graph with the elapsed time on the horizontal axis and the pressure inside the container on the vertical axis is shown in Fig. 4(b).

[0074] [Yield of silicon clathrate] (Examples 1 to 10, Comparative Example 1) For the silicon materials obtained in each example and comparative example, X-ray diffraction measurement was performed using a powder X-ray diffractometer. From the results of the X-ray diffraction measurement, the yield of silicon clathrate II and the yield of type I silicon clathrate in the silicon material were determined. The results are shown in Table 1. The diffraction charts of the silicon materials of Example 1 and Comparative Example 1 are shown in Fig. 5.

[0075]

Table 1

Explanation of symbols

[0076] 10... Container, 14... Raw material, 15... Na-Si alloy, 18... Na getter agent, L... Sample interval distance.

Claims

1. A housing step of housing a raw material containing a Na material and a Si material and a Na getter agent in the same container while separating them by a predetermined sample interval distance; An alloying step of obtaining a Na—Si alloy by heating the raw material housed in the container in the housing step to a first temperature; A silicon class rate generation step of reacting Na vaporized from the Na—Si alloy with the Na getter agent to generate silicon class rate II while reducing the pressure in the container and heating the container to a second temperature while keeping the Na—Si alloy obtained in the alloying step housed in the container; A method for producing a negative electrode active material containing silicon class rate II, comprising:

2. In the alloying step, the pressure in the container is controlled to a first pressure such that the diffusion distance of Na is less than the sample interval distance; The method for producing a negative electrode active material containing silicon class rate II according to claim 1, wherein in the silicon class rate generation step, the pressure in the container is controlled to a second pressure lower than the first pressure such that the diffusion distance of Na is equal to or greater than the sample interval distance.

3. The method for producing a negative electrode active material containing silicon class rate II according to claim 1 or 2, wherein the first temperature is 280° C. or higher and 400° C. or lower.

4. The sample interval distance is 7 mm or less, and The method for producing a negative electrode active material containing silicon class rate II according to claim 1 or 2, wherein the first temperature is 280° C. or higher and 360° C. or lower.

5. In the housing step, in a first housing portion that holds the raw material in the container, the raw material is arranged in layers, and The method for producing a negative electrode active material containing silicon class rate II according to claim 1 or 2, wherein the layer thickness of the raw material arranged in layers is 10 mm or less.

6. The method for producing a negative electrode active material containing silicon class rate II according to claim 1 or 2, wherein in the housing step, in the container, a plurality of first housing portions that hold the raw material and a plurality of second housing portions that hold the Na getter agent are alternately arranged in a first direction.

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