PRODUCTION METHOD OF Na-Si ALLOY AND PRODUCTION METHOD OF NEGATIVE ELECTRODE ACTIVE MATERIAL COMPRISING SILICON CLATHRATE II

Abstract

To provide a production method of Na-Si alloy that can obtain Na-Si alloy having high crystallinity while being excellent in production efficiency.SOLUTION: A production method of Na-Si alloy according to one aspect of the present disclosure comprises: a pulverization mixing step of pulverizing metallic sodium in the presence of powdered Si material to obtain a mixture of pulverized metallic sodium and powdered Si material; and an alloying step of heating the mixture to a first temperature to obtain Na-Si alloy, wherein in the pulverization mixing step, metallic sodium is pulverized until a surface element ratio (at1 / at2) of the number of sodium atoms (at1) to the number of silicon atoms (at2) on the surface of the mixture becomes less than 5 / 95.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a Na—Si alloy and a method for producing a negative electrode active material containing silicon clathrate II. [Background technology]

[0002] A clathrate compound is a compound in which a guest atom is 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) is known, in which a guest atom, sodium, is encapsulated in a cubic crystal structure formed by a dodecahedron of Si atoms and a hexahedron of Si atoms sharing faces.

[0003] Silicon clathrate II can be heat-treated to remove the sodium encapsulated within it while maintaining its cage-like structure, and so its use as an anode active material in secondary batteries is being considered.

[0004] Silicon clathrate II is produced using a Na-Si alloy containing Na and Si as a raw material. Patent Document 1 discloses a method for producing a raw material for a negative electrode active material, which includes a mixing step of mixing a metallic sodium dispersion with powdered silicon in a hydrocarbon-based dispersion medium to obtain a Na-Si mixed raw material, and a reaction step of heating the Na-Si mixed raw material at a temperature below the melting point of sodium silicide to obtain sodium silicide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-083924 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the production method disclosed in Patent Document 1 requires a step of washing the hydrocarbon-based dispersion medium, and there is room for improvement in terms of production efficiency.

[0007] The Na-Si alloy, which is the raw material for silicon clathrate II, is required to have high crystallinity. By using a highly crystalline Na-Si alloy as the raw material, the generation of amorphous silicon during the clathration process is suppressed, and silicon clathrate II can be obtained in high yield.

[0008] The present disclosure provides a method for producing a Na-Si alloy that is highly efficient in production and that provides a highly crystalline Na-Si alloy. The present disclosure also provides a method for producing a negative electrode active material containing silicon clathrate II, the method comprising the step of obtaining a Na-Si alloy by the Na-Si alloy production method. [Means for solving the problem]

[0009] One aspect of the present disclosure provides the following method for producing a Na—Si alloy and method for producing a negative electrode active material containing silicon clathrate II. [1] a grinding and mixing step of grinding metallic sodium in the presence of a powdered Si material to obtain a mixture of ground metallic sodium and powdered Si material; an alloying step of heating the mixture to a first temperature to obtain a Na-Si alloy; Equipped with A method for producing a Na-Si alloy, in which metallic sodium is pulverized in a pulverizing and mixing step until the surface element ratio (at1 / at2) of the number of sodium atoms (at1) to the number of silicon atoms (at2) on the surface of the mixture is less than 5 / 95. [2] The method for producing a Na-Si alloy according to [1], wherein the first temperature is 280°C or higher and 400°C or lower. [3] The method for producing a negative electrode active material containing silicon clathrate II according to [1], wherein the first temperature is 280°C or higher and 360°C or lower. [4] A method for producing a negative electrode active material containing silicon clathrate II, comprising a step of obtaining a Na—Si alloy by the method for producing a Na—Si alloy according to any one of [1] to [3], a containing step of containing the mixture and the Na getter agent in the same container after the pulverizing and mixing step, The method for producing a negative electrode active material containing silicon clathrate II includes a silicon clathrate generation step of reducing the pressure inside the container while keeping the Na-Si alloy contained in the container after the alloying step and heating the container to a second temperature, thereby causing Na vaporized from the Na-Si alloy to react with a Na getter agent to generate silicon clathrate II. [5] In the containing step, the mixture is arranged in a layer in a first containing section that holds the mixture in the container; The method for producing a negative electrode active material containing silicon clathrate II according to [4], wherein the layer thickness of the mixture arranged in layers is 10 mm or less. [6] The method for producing a negative electrode active material containing silicon clathrate II according to [4] or [5], wherein in the accommodation step, a plurality of first accommodation sections for holding the mixture and a plurality of second accommodation sections for holding the Na getter agent are alternately arranged in a first direction in a container. [Effects of the Invention]

[0010] The present disclosure provides a method for producing a Na-Si alloy that is highly efficient in production and that can yield a highly crystalline Na-Si alloy. The present disclosure also provides a method for producing a negative electrode active material containing silicon clathrate II, the method comprising the step of obtaining a Na-Si alloy by the method for producing the Na-Si alloy. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of an example of a grinding device used in the grinding and mixing step. [Figure 2] Figures 2(a) to 2(c) are schematic diagrams showing the case where metallic sodium is crushed alone, and Figures 2(d) to 2(f) are schematic diagrams showing the case where metallic sodium is crushed in the presence of powdered Si material. [Figure 3] FIG. 3 is a schematic cross-sectional view showing a method for producing a negative electrode active material containing silicon clathrate II according to one embodiment. [Figure 4] FIG. 4 is a schematic cross-sectional view showing a state after a silicon clathrate generating step in a method for producing a negative electrode active material containing silicon clathrate II according to another embodiment. [Figure 5] 5(a) is a mapping of the sodium element on the surface of the mixture of Example 1. FIG. 5(b) is a mapping of the sodium element on the surface of the mixture of Comparative Example 2. [Figure 6] 6(a) to 6(c) are X-ray diffraction charts of Example 1, Example 2 and Comparative Example 1, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described in detail, with reference to the drawings as needed. However, the present disclosure is not limited to the following embodiments.

[0013] The numerical range "x to y" described in this disclosure includes the lower limit x and the upper limit y. These upper and lower limit values, as well as the numerical values ​​listed in the examples, can be arbitrarily combined to form a numerical range. Furthermore, the upper and lower limit values ​​can be arbitrarily selected from within the numerical range.

[0014] [Method of manufacturing a negative electrode active material containing silicon clathrate II] The method for producing a negative electrode active material containing silicon clathrate II according to this embodiment (hereinafter simply referred to as "the production method according to this embodiment") includes a step of obtaining a Na-Si alloy by a method for producing a Na-Si alloy, which includes the following pulverizing and mixing steps, a containing step, and an alloying step. The production method according to this embodiment also includes the following silicon clathrate production step after the following alloying step. The produced silicon clathrate II can be suitably used as a negative electrode active material. In the pulverizing and mixing step, metallic sodium is pulverized and the pulverized metallic sodium is mixed with powdered Si material until the surface element ratio (at1 / at2) of the number of sodium atoms (at1) to the number of silicon atoms (at2) on the surface of the mixture obtained after the step is completed becomes less than 5 / 95.

[0015] Grinding and mixing step: A step of grinding metallic sodium in the presence of powdered Si material to obtain a mixture of ground metallic sodium and powdered Si material. Storage step: a step of storing the mixture obtained in the pulverizing and mixing step and the Na getter agent in the same container. Alloying step: A step of obtaining a Na-Si alloy by heating the mixture contained in the container in the containing step to a first temperature. Silicon clathrate generation step: After the alloying step, the pressure in the container is reduced while the Na-Si alloy is still contained in the container, and the container is heated to a second temperature, whereby Na vaporized from the Na-Si alloy reacts with the Na getter agent to generate silicon clathrate II.

[0016] The method for producing a Na—Si alloy according to this embodiment provides a highly crystalline Na—Si alloy while maintaining excellent production efficiency. The reasons for this effect are as follows. FIG. 1 is a schematic diagram of an example of a milling device used in the milling and mixing step. The milling device 7 includes a rotating blade 3, a container 4, and a drive unit 5 for driving the rotating blade. Metallic sodium 1 and powdered Si material 2 are fed into the milling device 7. FIGS. 2(a) to 2(c) are schematic diagrams illustrating the case where metallic sodium 1 is milled alone. FIGS. 2(d) to 2(f) are schematic diagrams illustrating the case where metallic sodium 1 is milled in the presence of powdered Si material. When attempting to mill metallic sodium 1 alone using the milling device 7, metallic sodium 1, being a soft and highly ductile metal, adheres to the walls of the rotating blade 3 and the container 4, preventing milling. Furthermore, the milled metallic sodium 1 re-agglomerates, preventing milling. On the other hand, when metallic sodium 1 is milled in the presence of powdered Si material 2, the powdered Si material 2 adheres to the surface of metallic sodium 1. The powdered Si material 2 adhering to the surface of the metallic sodium 1 prevents the metallic sodium 1 from adhering to the rotating blade 3 or the wall surface of the container 4, thereby efficiently pulverizing the metallic sodium. Furthermore, the Si material 2 adhering to the surface of the metallic sodium 1 suppresses re-agglomeration of the pulverized metallic sodium 1, thereby efficiently pulverizing the metallic sodium. In the pulverizing and mixing step, the pulverizing and mixing is continued until the surface element ratio of the mixture consisting of the pulverized metallic sodium and the powdered Si material, i.e., the surface element ratio (at1 / at2) of the number of sodium atoms (at1) to the number of silicon atoms (at2), becomes less than 5 / 95. By continuing the pulverizing and mixing step in this way so that the mixture obtained in the pulverized and mixing step is a mixture in which the pulverized metallic sodium and the powdered Si material are sufficiently dispersed, the remaining unpulverized metallic sodium in the mixture is suppressed, and the uneven distribution of metallic sodium in the mixture is also suppressed. As a result, the Na-Si alloy obtained in the alloying step of heating the mixture of the pulverized metallic sodium and the powdered Si material to the first temperature has high crystallinity.

[0017] Furthermore, the mixture obtained in the milling and mixing process has fine particles of Si material uniformly distributed on the surface of the small-sized crushed metallic sodium. This allows the reaction between the metallic sodium and the Si material to proceed smoothly. As a result, a highly crystalline Na-Si alloy can be obtained without using a high heating temperature.

[0018] In the manufacturing method according to the present embodiment, the silicon clathrate generation step is carried out immediately after the alloying step without removing the Na-Si alloy obtained in the alloying step and the Na getter agent accommodated in the accommodation step from the container after the alloying step is completed, which results in excellent manufacturing efficiency.

[0019] Furthermore, in the manufacturing method according to this embodiment, since the silicon clathrate generation step is performed consecutively to the alloying step, the Na getter agent is accommodated in a container in the accommodation step, and the Na getter agent remains accommodated in the container during the alloying step. The inter-sample distance between the mixture obtained in the pulverizing and mixing step and the Na getter agent in the container is adjusted to suppress reaction between the Na getter agent and Na during the alloying step. The inter-sample distance is, for example, 0.14 μm or more. Alternatively, the inter-sample distance may be, for example, 20 mm or less. This inter-sample distance is set based on the diffusion distance of Na atoms calculated from the mean free path relationship shown in Equation (1) below. Specifically, the diffusion distance of Na atoms during the alloying step is calculated based on the heating temperature of the raw materials required for the alloying reaction to obtain a Na-Si alloy and the pressure inside the container, and the inter-sample distance during the accommodation step is set so that the inter-sample distance between the raw materials and the Na getter agent is greater than the calculated diffusion distance of Na atoms. Furthermore, since the heating temperature required for the silicon clathrate production process is fixed at a predetermined temperature, the diffusion distance of Na atoms in the silicon clathrate production process is adjusted by controlling the pressure inside the container. Specifically, the pressure inside the container in the silicon clathrate production process is set lower than the pressure inside the container in the alloying process so that the diffusion distance of Na atoms in the silicon clathrate production process is equal to or greater than the sample-to-sample distance set in the accommodation process. This promotes the adsorption of Na atoms to the Na getter agent placed at a predetermined sample-to-sample distance from the Na-Si alloy in the silicon clathrate production process. In this embodiment, the mixture of pulverized metallic sodium and powdered Si material heated to the first temperature in the alloying step is sufficiently pulverized and mixed in the preceding pulverizing and mixing step. Therefore, in the alloying step, it is not necessary to heat the mixture to a temperature higher than the eutectic melting temperature of Si. The heating temperature can be, for example, a low temperature of 400°C or lower, which is lower than the eutectic melting temperature of Si. The production method according to this embodiment can obtain a highly crystalline Na-Si alloy even when heated at a low temperature of 400°C or lower, thereby producing silicon clathrate II with a high yield.

[0020]

number

[0021] Each step of the manufacturing method according to this embodiment will be described in detail below.

[0022] (Crushing and mixing process) The grinding and mixing step will be described in detail below.

[0023] Examples of Si materials include silicon, amorphous silicon, polysilicon, ferrosilicon, and porous silicon.

[0024] The particle size of the Si material may be, for example, 0.1 μm to 5 mm. The particle size can be expressed as D50 when measured using a general laser diffraction particle size distribution measuring device.

[0025] The metallic sodium before pulverization may have a shape of, for example, an approximate cube or an approximate rectangular parallelepiped, with the length of one side being, for example, 1 mm to 30 mm.

[0026] The particle size of the metallic sodium after pulverization may be, for example, 0.01 to 500 μm.

[0027] The amount of Si material mixed with 100 parts by mass of metallic sodium may be, for example, 80 to 120 parts by mass.

[0028] The content of the powdered Si material and metallic sodium contained in the composition subjected to the pulverizing and mixing step may be 90 mass % or more, 95 mass % or more, 99 mass % or more, or 100 mass % based on the total amount of the composition.

[0029] The grinder used in the grinding and mixing step is not particularly limited, but an example thereof is Wonder Crusher WC-3 manufactured by Osaka Chemical Co., Ltd.

[0030] The rotation speed of the pulverizer may be, for example, 2800 to 28000 rpm.

[0031] The time for pulverizing metallic sodium may be, for example, 1 to 10 minutes. The higher the rotation speed of the pulverizer, the shorter the pulverization time can be. On the other hand, the lower the rotation speed of the pulverizer, the longer the pulverization time must be.

[0032] The temperature at which metallic sodium is crushed may be room temperature, for example, 5 to 35°C.

[0033] The surface element ratio (at1 / at2) of the mixture is preferably 4 / 96 or less, more preferably 3 / 97 or less, even more preferably 2 / 98 or less, and particularly preferably 1 / 99 or less. By pulverizing and mixing metallic sodium and powdered Si material until such a surface element ratio (at1 / at2) is achieved, the crystallinity of the Na-Si alloy obtained in the alloying step can be further increased.

[0034] The surface element ratio (at1 / at2) is measured by the following method. For example, a scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM-EDX) is used to perform element mapping of the mixture of the pulverized metallic sodium and the powdered Si material to obtain the element ratio of sodium and silicon. The surface element ratio (at1 / at2) is then calculated from the obtained element ratio.

[0035] The grinding and mixing step may be carried out under an inert gas atmosphere.

[0036] (Storage process) The accommodation step will be described in detail below. FIG. 3 is a schematic cross-sectional view showing the accommodation step and the alloying step according to this embodiment. In the manufacturing method according to this 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 the function of discharging gas from the container to the outside and the function of introducing gas into the container. A raw material basket 12 (first accommodation section) is arranged in the container 10. In the accommodation step, as shown in FIG. 3(a), the raw material basket 12 accommodates the mixture 14 obtained in the pulverizing and mixing step as a raw material for producing a Na-Si alloy in the alloying step, in a layered state (with a constant thickness). In addition, a Na getter agent 18 is accommodated at the bottom of the container 10 in a layered state (with a constant thickness). In other words, the mixture 14 and the Na getter agent 18 are accommodated in the same container 10. The mixture 14 and the Na getter agent 18 are arranged in the same container 10, separated by a predetermined sample distance L. The inter-sample distance L is, for example, the shortest distance between the mixture 14 and the Na getter agent.

[0037] The inter-sample distance L between the mixture 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 even more preferably 0.5 mm or more. By setting the inter-sample distance L to such a value, the yield of silicon clathrate II can be further improved. The inter-sample distance L between the mixture 14 and the Na getter agent 18 inside the container 10 is preferably 17 mm or less, more preferably 7 mm or less, and even more preferably 3 mm or less. By setting the inter-sample distance L to such a value, the amount of mixture 14 that can be accommodated in the container 10 can be increased. As a result, the yield of silicon clathrate II can be improved.

[0038] The main surfaces of the layered mixture 14 and the layered Na getter agent 18 face each other. In a plan view, the area of ​​the main surface of the mixture 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%.

[0039] The layer thickness t of the mixture 14 contained in the raw material basket 12 in layers is preferably 10 mm or less, more preferably 5 mm or less, and even more preferably 4 mm or less, which promotes the reaction between the Na vapor and the Na getter agent 18 in the silicon clathrate production step, and further improves the yield of silicon clathrate II.

[0040] The Na getter agent refers to a material capable of reacting with zero-valent Na. The Na getter agent refers to a material capable of reacting with zero-valent Na and having a vapor pressure lower than the vapor pressure of metallic sodium.

[0041] Examples of the Na getter agent include oxides of metals other than alkali metals and alkaline earth metals, sulfides of metals other than alkali metals and alkaline earth metals, and halides of metals other than alkali metals and alkaline earth metals.

[0042] Examples of Na getter agents include WO2, WO3, MoO3, ZnO, FeO, VO, V2O3, TiO2, SiO, SiO2, Al2O3, WS2, MoS2, ZnS, FeS, TiS2, SiS2, Al2S3, and Fe3O4, and among these, 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.

[0043] As the Na getter agent, one of the above materials may be used alone, or two or more of the above materials may be used in combination.

[0044] The amount of the Na getter agent contained in the container may be 235 to 620 parts by mass relative to 100 parts by mass of the Na material. The Na getter agent may be contained in a layer in a mesh basket placed in the container 10.

[0045] The container 10 may be made of, for example, stainless steel.

[0046] The raw material basket 12 may be a mesh basket. In the manufacturing method according to this embodiment, the raw materials are contained in a basket, but the material containing the mixture 14 is not limited to a basket. For example, the mixture 14 may be placed on a mesh plate. The material of the raw material basket 12 may be, for example, stainless steel.

[0047] (Alloying process) The alloying step will be described in detail below.

[0048] In the alloying process, as shown in FIG. 3(b), the mixture 14 in the container 10 is heated to a first temperature, causing an alloying reaction between the metallic sodium in the mixture 14 and the Si material, thereby obtaining a Na-Si alloy 15.

[0049] 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 metallic sodium and the Si material in mixture 14 can be promoted. The first temperature is 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.

[0050] In the alloying step, the pressure in container 10 (hereinafter also referred to as "the first pressure") may be, for example, 10 5 Pa or higher.

[0051] The heating time of mixture 14 may be, for example, 1 to 40 hours. More preferably, the heating time may be 2 to 11 hours.

[0052] The alloying step may be carried out in an inert gas atmosphere by introducing an inert gas into container 10.

[0053] The Na-Si alloy obtained in the alloying step may be one in which Na is present in excess of Si, that is, the composition of Na and Si is represented by Na z Si (1 < z).

[0054] In the Na-Si alloy, other elements other 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.

[0055] (Silicon class rate generation step) Hereinafter, the silicon class rate generation step will be described in detail.

[0056] The silicon clathrate generation process is a silicon clathrate generation process that is performed immediately after the alloying process, without removing the Na-Si alloy obtained in the alloying process and the Na getter agent contained in the containing process from the container after the alloying process is completed. In the silicon clathrate generation process, the Na-Si alloy in the container is heated at a second temperature. When the Na-Si alloy in the container is heated 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. This allows the reaction formula to proceed favorably to the right. Furthermore, in the manufacturing method according to this embodiment, the reaction proceeds under relatively low Na partial pressure conditions. Therefore, there is an advantage in that the generation of silicon clathrate I is suppressed and silicon clathrate II is preferentially produced. In the silicon clathrate generation process, a silicon material 16 containing silicon clathrate II is obtained, as shown in FIG. 3(c). Na-Si alloy ←→ silicon clathrate II + Na(g)

[0057] At the start of the silicon clathrate production process, the Na—Si alloy obtained in the alloying process and the Na getter agent are contained in the container 10. The inter-sample distance between the Na—Si alloy and the Na getter agent in the container 10 is the same as the inter-sample distance L between the raw material and the Na getter agent in the containing process.

[0058] The pressure inside the container 10 in the silicon clathrate production step (hereinafter also referred to as "second pressure") is adjusted to a value lower than the first pressure so that the diffusion distance of Na is equal to or greater than the inter-sample distance between the raw material and the Na getter agent set in the accommodation step. This can improve the yield of silicon clathrate II. The second pressure is, for example, 10 5 Less than 10 Pa 4 Pa or less, 10 3 Pa or less, 10 2 The second pressure may be, for example, 10 Pa or less, or 10 Pa or less. -2 Pa or more, or 10 -1 It may be Pa or more.

[0059] 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, it is possible to suppress the formation of Si crystals with a diamond structure. As a result, the resulting silicon clathrate II can be suitably used as a negative electrode active material for secondary batteries.

[0060] The silicon clathrate production step may be carried out in an inert gas atmosphere by introducing an inert gas into the vessel 10 .

[0061] In the manufacturing method according to this embodiment, a silicon material 16 containing silicon clathrate II obtained in the silicon clathrate generation step and a new Na getter agent may be allowed to coexist in the container 10, and the silicon material 16 may be heated to further reduce the amount of Na remaining in the silicon clathrate II.

[0062] Silicon clathrate 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. A lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

[0063] When silicon clathrate II is used as the negative electrode active material, the composition formula of silicon clathrate II is Na x Si 136 The value of x may be 0 or more. It is preferable that x is 10 or less, more preferably 7 or less, even more preferably 5 or less, even more preferably 3 or less, particularly preferably 2 or less, and most preferably 1 or less. This allows charge carriers such as lithium to move into the polyhedral cages of silicon clathrate II from which Na has been released. As a result, the degree of expansion of the negative electrode active material is suppressed.

[0064] Silicon clathrate II can be used, for example, in thermoelectric, light emitting and light absorbing device applications.

[0065] (Cleaning process) The silicon material 16 containing silicon clathrate II obtained in the silicon clathrate production step may have by-products other than silicon clathrate II attached thereto, such as Na or NaOH. The manufacturing method according to this embodiment may further include a cleaning step of cleaning the silicon material 16 in order to remove by-products attached to the silicon material 16 obtained in the silicon clathrate production step.

[0066] In the washing step, the silicon material 16 is washed using a solvent capable of dissolving the by-products. For example, an acidic aqueous solution is used as the solvent. The acid concentration in the acidic aqueous solution is preferably 0.1 to 10 mass %, more preferably 0.5 to 5 mass %, and even more preferably 1 to 4 mass %.

[0067] After the washing step, it is preferable to remove water from the silicon material 16 by filtering and drying.

[0068] The silicon material 16 is preferably pulverized and classified to form a powder with a certain particle size distribution. The 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 refers to D50 when a sample is measured using a general laser diffraction particle size distribution analyzer.

[0069] Although the method for producing silicon clathrate II according to one embodiment has been described above, the method for producing silicon clathrate II according to the present disclosure is not limited to the above embodiment. For example, in the above embodiment, the layered mixture and the layered Na getter agent are each placed horizontally in the container with their main surfaces aligned horizontally, but the layered mixture and the layered Na getter agent may also be placed vertically in the container with their main surfaces aligned vertically.

[0070] In the above embodiment, the accommodation step involves placing one layered mixture and one layered Na getter agent in the reactor vessel. However, this is not limiting. Alternatively, the accommodation step may involve preparing multiple first accommodation sections, each holding a layered mixture, and multiple second accommodation sections, each holding a layered Na getter agent, and arranging these first accommodation sections and second accommodation sections alternately in the first direction within the same vessel. Figure 4 is a schematic cross-sectional view showing the state after the silicon clathrate production step when two layered mixtures and three layered Na getter agents are accommodated in the accommodation step. Silicon material 16 containing silicon clathrate II can be obtained in two mixture baskets 12. To efficiently utilize the volume within the vessel 10, multiple first accommodation sections for holding the mixture are accommodated within the vessel, thereby increasing the amount of silicon clathrate II that can be obtained in one production run. As a result, production efficiency can be further improved.

[0071] Furthermore, in the above embodiment, the silicon clathrate generation step is performed consecutively to the alloying step, but the silicon clathrate generation step does not have to be performed consecutively. In that case, the Na getter agent does not have to be stored in the container in the storing step. Furthermore, the Na getter agent does not have to be stored in the container in the alloying step. If the Na getter agent is not present in the container in the alloying step, the Na-Si alloy may be removed from the container in an inert atmosphere after the alloying step, and the removed Na-Si alloy and the Na getter agent may be stored in a separate container and heat-treated to synthesize silicon clathrate II.

[0072] [Method of manufacturing 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 containing silicon clathrate II according to the above embodiment, and a step of manufacturing a negative electrode for a battery using the manufactured negative electrode active material.

[0073] [Secondary battery manufacturing method] 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 the battery. [Example]

[0074] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0075] [Example 1] (Production of Na-Si alloy) As metallic sodium, 6.60 g of Na ingot was chopped into approximately 10 mm square pieces. As the Si material, 6.72 g of silicon powder (particle size: 5 μm) was prepared. The particle size of the silicon powder was measured using a laser diffraction particle size analyzer. The silicon powder and the Na ingot chopped in the preparation process were placed in a grinding device (Osaka Chemical Co., Ltd., Wonder Crusher WC-3), and the metallic sodium was ground and mixed under the silicon powder (grinding and mixing process). Grinding and mixing were performed in an argon-substituted glove box (dew point: -80°C) at room temperature (25°C), at a rotation speed of 28,000 rpm, for 5 minutes, and at atmospheric pressure. This resulted in a mixture of crushed Na ingot and silicon powder. The state of the Na ingot after grinding and mixing was visually observed to confirm the presence or absence of agglomerates larger than 1 mm square. The results are shown in Table 1.

[0076] As shown in Figure 3(a), Fe3O4 was placed in a layer on the bottom of a stainless steel container as a Na getter (containing step). The amount of Na getter was 284 parts by mass per 100 parts by mass of Na ingot. A stainless steel mesh basket was placed on top of the Na getter. A mixture of crushed Na ingot and silicon powder was placed in the basket in a layer. The distance between the Na getter and the mixture was set to 1 mm. The container was then covered and placed in a furnace. After evacuating the container, argon was flowed in to sinter the mixture. The sintering conditions were atmospheric pressure (first pressure), a heating temperature of 330°C (first temperature), and a heating time of 3 hours. This resulted in a Na-Si alloy (alloying step).

[0077] (Silicon material manufacturing) A Na-Si alloy was obtained in the same manner as in the production of the Na-Si alloy described above. Then, without removing the Na-Si alloy or Na getter agent from the container, the pressure inside the container was reduced, and the Na-Si alloy was sintered at a pressure of 20 Pa (second pressure), a heating temperature of 350°C (second temperature), and a heating time of 10 hours. This resulted in a silicon material containing silicon clathrate II (silicon clathrate generation step). The container was cooled to room temperature, the pressure was increased to atmospheric pressure, and the silicon material was removed from the container.

[0078] [Example 2] A Na—Si alloy and a silicon material were produced in the same manner as in Example 1, except that the rotation speed in the grinding and mixing step was 14,000 rpm and the grinding time was 10 minutes.

[0079] [Comparative Example 1] A Na—Si alloy and a silicon material were produced in the same manner as in Example 1, except that the rotation speed in the grinding and mixing step was 14,000 rpm and the grinding time was 1.5 minutes.

[0080] Comparative Example 2 As in Example 1, 6.60 g of Na ingot and 6.72 g of silicon powder (particle size: 5 μm) were prepared. With the Na ingot and silicon powder mixed, the container was heated to 130°C to melt the Na ingot, and the molten Na ingot and silicon powder were cooled to 25°C over 10 minutes while being stirred with a three-one motor. Stirring was performed in an argon-substituted glove box (dew point: -80°C) at a rotation speed of 250 rpm. This yielded a mixture of Na ingot and silicon powder. The state of the Na ingot after stirring was visually observed. The results are shown in Table 1.

[0081] Using the obtained mixture, a Na—Si alloy and a silicon material were produced in the same manner as in Example 1.

[0082] [Measurement of surface element ratio (at1 / at2)] For the mixtures obtained in the milling and mixing steps of each Example and Comparative Example, the surface element ratio (at1 / at2), which is the ratio of the number of sodium atoms (at1) to the number of silicon atoms (at2) on the surface, was measured. Specifically, element mapping was performed using a scanning electron microscope (SEM: Carl Zeiss Gemini500, EDX detector: Oxford Instruments Ultim max170) equipped with energy dispersive X-ray spectroscopy to obtain the element ratio of sodium to silicon, and the surface element ratio (at1 / at2) was calculated. The acceleration voltage was 5 kV, the working distance was 8.6 mm, and the magnification was 5000x. The results are shown in Table 1. Figure 5(a) shows a mapping of sodium on the surface of the mixture of Example 1. Figure 5(b) shows a mapping of sodium on the surface of the mixture of Comparative Example 2. It can be confirmed that the mixture of Example 1 does not contain large clumps of sodium, and that sodium and silicon are evenly distributed throughout the mixture.

[0083] [X-ray diffraction measurement] The Na—Si alloys obtained in each example and comparative example were analyzed using a powder X-ray diffractometer using Cu-Kα. The half-widths of the peaks observed at 2θ=33.7° from the X-ray diffraction measurement results are shown in Table 1. Figures 6(a) to 6(c) show X-ray diffraction charts for Example 1, Example 2, and Comparative Example 1, respectively.

[0084] [Yield of silicon clathrate] The silicon materials obtained in each of the examples and comparative examples were subjected to X-ray diffraction measurement using a powder X-ray diffractometer. The yields of amorphous silicon, silicon clathrate type I, and silicon clathrate type II in the silicon material were determined from the results of the X-ray diffraction measurement. The results are shown in Table 1. The yield of silicon clathrate type I in each of the examples and comparative examples was 0% by mass.

[0085] [Table 1]

[0086] It can be seen that the Na-Si alloys of each Example have a narrower half-width of the peak (2θ=33.7°) than the Na-Si alloys of each Comparative Example. This indicates that in each Example, the Na ingot and silicon powder were mixed more thoroughly in the mixing and grinding process than in each Comparative Example, and a highly crystalline Na-Si alloy was obtained. In each Example, the use of a highly crystalline Na-Si alloy suppressed the generation of amorphous silicon, and silicon clathrate type II was obtained in a high yield. [Explanation of symbols]

[0087] 1...metallic sodium, 2...Si material, 12...raw material basket (first storage section), 14...mixture, 15...Na-Si alloy, 18...Na getter agent, I, II...silicon clathrate.

Claims

1. a grinding and mixing step of grinding metallic sodium in the presence of a powdered Si material to obtain a mixture of the ground metallic sodium and the powdered Si material; an alloying step of heating the mixture to a first temperature to obtain a Na—Si alloy; Equipped with In the pulverizing and mixing step, the metallic sodium is pulverized until a surface element ratio (at1 / at2) of the number of sodium atoms (at1) to the number of silicon atoms (at2) on the surface of the mixture is less than 5 / 95.

2. 2. The method for producing a Na-Si alloy according to claim 1, wherein the first temperature is 280°C or higher and 400°C or lower.

3. 2. The method for producing a Na-Si alloy according to claim 1, wherein the first temperature is 280°C or higher and 360°C or lower.

4. A method for producing a negative electrode active material containing silicon clathrate II, comprising a step of obtaining a Na—Si alloy by the method for producing a Na—Si alloy according to any one of claims 1 to 3, a containing step of containing the mixture and a Na getter agent in the same container after the pulverizing and mixing step, the method for producing a negative electrode active material containing silicon clathrate II includes a silicon clathrate production step of reducing the pressure inside the container and heating the container to a second temperature while keeping the Na—Si alloy contained in the container after the alloying step, thereby causing Na vaporized from the Na—Si alloy to react with the Na getter agent to produce silicon clathrate II.

5. In the containing step, the mixture is arranged in a layered manner in a first containing section that holds the mixture in the container, 5. The method for producing a negative electrode active material containing silicon clathrate II according to claim 4, wherein the layer thickness of the mixture arranged in layers is 10 mm or less.

6. 5. The method for producing a negative electrode active material containing silicon clathrate II according to claim 4, wherein in the containing step, a plurality of first storage sections for holding the mixture and a plurality of second storage sections for holding the Na getter agent are alternately arranged in a first direction within the container.

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