Method for producing negative electrode active material containing silicon clathrate ii

Abstract

To provide a method for producing a negative electrode active material having a high content ratio of silicon clathrate II.SOLUTION: The method for producing a negative electrode active material containing silicon clathrate II according to one aspect of the present disclosure includes: a getter-agent classification step for obtaining an AlF3 classified material by removing fine particles and coarse particles from AlF3 raw material powder; a mixing step for obtaining a mixture by mixing the AlF3 classified material and a Na-Si alloy; a silicon-clathrate generation step for generating silicon clathrate II by heating the mixture to cause Na vaporized from the Na-Si alloy to react with remaining AlF3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to 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 negative electrode active material, which includes an alloying step of reacting a Na source and a Si source to produce a Na-Si alloy containing Na and Si, and a silicon clathrate production step of heating the Na-Si alloy to reduce the amount of Na in the Na-Si alloy and produce silicon clathrate II, in which porous Si having a BET specific surface area within a specific range is used as the Si source. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2023-043948 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, in a negative electrode active material containing silicon clathrate II, it is required that the ratio of amorphous silicon and I-type silicon clathrate (so-called silicon clathrate I) is small, and the ratio of silicon clathrate II is large.

[0007] However, in the method disclosed in Patent Document 1, the proportion of silicon clathrate II in the obtained negative electrode active material is small, and there is room for improvement.

[0008] The present disclosure provides a method for producing a negative electrode active material with a high content of silicon clathrate II. [Means for solving the problem]

[0009] One aspect of the present disclosure provides the following method for producing a negative electrode active material containing silicon clathrate II. [1] a getter agent classification step of removing fine particles and coarse particles from the AlF3 raw material powder to obtain an AlF3 classified material to be used as a getter agent; a mixing step of mixing the AlF fraction and the Na-Si alloy to obtain a mixture; a silicon clathrate generation step of heating the mixture to react Na vaporized from the Na-Si alloy with the AlF fraction to generate silicon clathrate II; A method for producing a negative electrode active material containing silicon clathrate II, comprising: [2] The method for producing a negative electrode active material containing silicon clathrate II according to [1], wherein the average particle diameter D50 of the AlF3 fraction is 30 to 70 μm. [3] The method for producing a negative electrode active material containing silicon clathrate II according to [1] or [2], wherein the average particle diameter D10 of the AlF3 fraction is 20 to 40 μm. [4] The method for producing a negative electrode active material containing silicon clathrate II according to any one of [1] to [3], wherein the average particle diameter D90 of the AlF3 fraction is 60 to 90 μm. [5] A method for producing a negative electrode active material containing silicon clathrate II according to any one of [1] to [4], further comprising an alloying step of heating raw materials containing a Na material and a Si material to obtain a Na-Si alloy. [Effects of the Invention]

[0010] According to the present disclosure, a method for producing a negative electrode active material with a high content of silicon clathrate II is provided. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a flowchart showing a method for producing a negative electrode active material containing silicon clathrate II according to this embodiment. [Figure 2] FIG. 2 is an X-ray diffraction chart of Example 1, Comparative Example 1, and Comparative Example 2. 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 also simply referred to as the "production method according to this embodiment") includes the following alloying step, getter agent classification step, mixing step, and silicon clathrate production step.

[0015] Alloying process: A process for obtaining a Na-Si alloy by heating raw materials containing Na and Si (step S1 in the flowchart in Figure 1). Getter agent classification step: A step of removing fine particles and coarse particles from the AlF raw material powder to obtain an AlF classified material to be used as a getter agent (step S2 in the flowchart in Figure 1). Mixing step: A step of mixing the AlF fraction and the Na-Si alloy to obtain a mixture (step S3 in the flowchart of FIG. 1). Silicon clathrate generation step: A step in which the mixture is heated to react Na vaporized from the Na-Si alloy with the AlF fraction to generate silicon clathrate II (step S4 in the flowchart in Figure 1).

[0016] The manufacturing method according to this embodiment can obtain a higher proportion of silicon clathrate II in the silicon material obtained in the silicon clathrate generation process compared to conventional methods. The reason for this effect is as follows: In the silicon clathrate generation process, AlF powder acts as a Na getter and reacts with Na vaporized from the Na-Si alloy to form NaAlF. This promotes clathration of the Na-Si alloy, resulting in the generation of silicon clathrate II. Here, if the AlF3 powder used as a getter agent in the silicon clathrate production process contains many coarse AlF3 particles, Na vaporized from the Na-Si alloy will preferentially react near the surfaces of the coarse AlF3 particles to form Na3AlF6, and the AlF3 inside the coarse particles will no longer contribute to the reaction with Na. Therefore, if the getter agent contains many coarse AlF3 particles, the partial pressure of Na in the reaction system will gradually increase as the reaction in the silicon clathrate production process progresses, resulting in an increase in the amount of silicon clathrate I produced. On the other hand, if the AlF powder used as a getter agent in the silicon clathrate production process contains many fine AlF particles, the surface area of ​​the getter agent increases, increasing the reactivity between the Na vaporized from the Na-Si alloy and AlF. Therefore, if the getter agent contains many fine particles, the partial pressure of Na in the reaction system becomes too low as the reaction in the silicon clathrate production process progresses, causing the crystal structure of the Na-Si alloy to collapse. As a result, the amount of amorphous silicon produced increases. In the manufacturing method according to this embodiment, fine particles and coarse particles are removed from the AlF3 particles in the getter agent classification step, thereby obtaining a negative electrode material with a high ratio of silicon clathrate II.

[0017] Each step of the manufacturing method according to this embodiment will be described in detail below.

[0018] (Alloying process) The alloying step will be described in detail below.

[0019] In the alloying step, raw materials containing a Na material and a Si material are heated to cause an alloying reaction between the Na material and the Si material, thereby obtaining a Na-Si alloy.

[0020] Examples of Si materials include silicon, amorphous silicon, polysilicon, ferrosilicon, and porous silicon.

[0021] Examples of Na materials include metallic sodium and sodium hydride.

[0022] The amount of the Si material to be mixed may be 80 to 120 parts by mass with respect to 100 parts by mass of the Na material.

[0023] The content of the Na material and the Si material may be 90 mass % or more, 95 mass % or more, 99 mass % or more, or 100 mass % based on the total amount of the raw materials.

[0024] The heating temperature in the alloying process is preferably 800 °C or lower, 600 °C or lower, 450 °C or lower, less than 400 °C, 380 °C or lower, or 360 °C or lower. There is no particular lower limit for the heating temperature in the alloying process, but in order to efficiently carry out the reaction in the silicon class rate generation process, the heating temperature in the alloying process is preferably 300 °C or higher, 310 °C or higher, 320 °C or higher, or 340 °C or higher.

[0025] The time for heating the raw materials may be, for example, 1 to 40 hours. More preferably, the heating time may be 2 to 11 hours.

[0026] The alloying process is preferably carried out under an inert atmosphere such as an Ar atmosphere.

[0027] The Na-Si alloy obtained in the alloying process may have an excess of Na over Si, that is, the composition of Na and Si may be represented by Na , ,

[0031] ,

[0028] , , , , , , , , , , z ,

[0030] , ,

[0032] ,

[0029] Si(1 < z).

[0028] In the Na-Si alloy, other elements other than Na and Si may be present within the scope not 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.

[0029] (Getter agent classification step) Hereinafter, the getter agent classification step will be described in detail.

[0030] In the getter agent classification step, an AlF3 classified product (hereinafter, also simply referred to as "AlF3 classified product") from which fine particles and coarse particles are removed from the AlF3 raw material powder is obtained.

[0031] Examples of the method for removing fine particles from the AlF3 raw material powder include using a sieve. The mesh size of such a sieve may be, for example, 40 μm or less, 35 μm or less, or 30 μm or less.

[0032] The content of fine particles contained in the AlF3 fraction may be 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0% by mass, based on the total amount of the AlF3 fraction.

[0033] One method for removing coarse particles from the AlF3 raw material powder is to use a sieve, the mesh size of which may be, for example, 53 μm or more, 75 μm or more, or 100 μm or more.

[0034] The content of coarse particles contained in the AlF3 fraction may be 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0% by mass, based on the total amount of the AlF3 fraction.

[0035] The order in which the fine particles and the coarse particles are removed from the AlF3 raw material powder is not particularly limited, and the fine particles and the coarse particles may be removed simultaneously from the AlF3 raw material powder.

[0036] The average particle size D50 of the AlF3 fraction is preferably 30 μm or more, more preferably 35 μm or more, and even more preferably 40 μm or more in order to suppress the generation of amorphous silicon. The average particle size D50 of the AlF3 fraction is preferably 70 μm or less, more preferably 65 μm or less, and even more preferably 60 μm or less in order to suppress the generation of silicon clathrate I.

[0037] The average particle size D10 of the AlF3 fraction is preferably 10 μm or more, more preferably 20 μm or more, in order to suppress the generation of amorphous silicon. The average particle size D10 of the AlF3 fraction may be, for example, 40 μm or less.

[0038] The average particle size D90 of the AlF3 fraction may be, for example, 60 μm or more. In order to suppress the generation of silicon clathrate I, the average particle size D90 of the AlF3 fraction is preferably 100 μm or less, and more preferably 90 μm or less.

[0039] The average particle size D100 of the AlF3 fraction may be, for example, 140 μm or more. The average particle size D100 of the AlF3 fraction may be, for example, 170 μm or more.

[0040] The average particle sizes D10, D50, D90 and D100 of the AlF3 fraction are measured, for example, by a laser diffraction particle size distribution analyzer.

[0041] (Mixing process) The mixing step will be described in detail below.

[0042] The mixing step is a step of mixing the AlF3 fraction obtained in the getter agent classification step with the Na-Si alloy obtained in the alloying step to obtain a mixture.

[0043] The AlF3 fraction and the Na-Si alloy may be mixed by stirring, for example.

[0044] The amount of the AlF3 fraction may be 50 parts by mass or more, or 60 parts by mass or more, and may be 70 parts by mass or less, or 100 parts by mass or less, relative to 100 parts by mass of the Na-Si alloy.

[0045] (Silicon clathrate production process) The silicon clathrate production process will be described in detail below.

[0046] In the silicon clathrate production process, the mixture obtained in the mixing process is heated. This generates gaseous Na(g), as shown in the following reaction formula, and the AlF3 particles capture the gaseous Na(g). This allows the reaction formula below to proceed favorably to the right. In the silicon clathrate production process, a silicon material containing silicon clathrate II is obtained. Na-Si alloy ←→ silicon clathrate II + Na(g)

[0047] The temperature to which the mixture is heated may be 100°C to 500°C, 200°C to 400°C, 270°C to 360°C, 270°C to 310°C, or 270°C to 300°C. If the temperature to which the mixture is heated is low, the content of silicon clathrate I in the negative electrode active material can be reduced and the content of silicon clathrate II can be increased. It is presumed that if the temperature to which the mixture is heated is high, stable silicon clathrate I is more likely to be produced. On the other hand, if the temperature to which the mixture is heated is high, there is an advantage in that the reaction time can be shortened.

[0048] The temperature at which the mixture is heated is preferably 400° C. or less. If the temperature at which the mixture is heated is 400° C. or less, the formation of Si crystals with a diamond structure can be suppressed, and the resulting silicon material can be endowed with properties suitable for use as a negative electrode active material in non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries.

[0049] The mixture may be heated for, for example, 20 to 60 hours.

[0050] The silicon clathrate production step is preferably carried out in an inert atmosphere such as an Ar atmosphere.

[0051] 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.

[0052] When silicon clathrate II is used as the negative electrode active material, the composition formula of silicon clathrate II is Na x Si 136The 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.

[0053] Silicon clathrate II can be used, for example, in thermoelectric, light emitting and light absorbing device applications.

[0054] (Cleaning process) The silicon material 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, NaOH, etc. The production method according to this embodiment may further include a cleaning step of cleaning the silicon material obtained in the silicon clathrate production step to remove by-products attached to the silicon material.

[0055] In the washing step, the silicon material 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% by mass, more preferably 0.5 to 5% by mass, and even more preferably 1 to 4% by mass.

[0056] After the washing step, it is preferable to remove water from the silicon material by filtering and drying.

[0057] The silicon material is preferably pulverized and classified to obtain a powder with a certain particle size distribution. The average particle size of the silicon material 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 the D50 measured when a sample is measured using a laser diffraction particle size distribution analyzer.

[0058] 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. The order in which the getter agent classification step and the alloying step are performed is not particularly limited.

[0059] [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.

[0060] [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]

[0061] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0062] Example 1 A Na ingot chopped into approximately 5 mm squares was prepared as the Na material. Silicon powder was prepared as the Si material. The Na ingot and silicon powder were mixed in a Wonder Crusher. The silicon powder was mixed in an amount of 102 parts by mass per 100 parts by mass of Na ingot. One-third of the total amount of Na ingot was mixed, and the mixture was mixed three times for one minute at a Wonder Crusher intensity of 5. This yielded a raw material containing Na and Si materials.

[0063] The raw materials were placed in a stainless steel container. After evacuating the container, Ar was flowed and the raw materials were heated. The heating temperature was 420°C and the heating time was 40 hours. This resulted in a Na-Si alloy (alloying process).

[0064] AlF particles (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "Aluminum Fluoride") were prepared. Fine particles were removed from the AlF particles using a sieve with a mesh size of 32 μm. Next, coarse particles were removed from the AlF particles using a sieve with a mesh size of 75 μm (getter agent classification process).

[0065] The AlF3 particles remaining after removing the fine and coarse particles were mixed with the Na-Si alloy to obtain a mixture (mixing step). The mixture was placed in a stainless steel container. After evacuating the container, Ar was flowed and the mixture was heated. The heating temperature was 350°C and the heating time was 60 hours. This resulted in a silicon material containing silicon clathrate II (silicon clathrate production step). The container was cooled to room temperature, and the silicon material was removed from the container.

[0066] Example 2 A silicon material was obtained in the same manner as in Example 1, except that a sieve with 25 μm openings was used instead of the 32 μm openings when removing fine particles.

[0067] (Comparative Example 1) A Na-Si alloy was obtained in the same manner as in Example 1. AlF particles (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "Aluminum Fluoride") and the Na-Si alloy were mixed to obtain a mixture (mixing step). This mixture was used to obtain a silicon material in the same manner as in Example 1 (silicon clathrate generation step).

[0068] (Comparative Example 2) A Na-Si alloy was obtained in the same manner as in Example 1. AlF particles (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "Aluminum Fluoride") were pulverized in a pulverizer for 24 hours. The pulverized AlF particles were mixed with the Na-Si alloy to obtain a mixture (mixing step). This mixture was used to obtain a silicon material in the same manner as in Example 1 (silicon clathrate generation step).

[0069] (Comparative Example 3) A silicon material was obtained in the same manner as in Comparative Example 2, except that the grinding time was changed to 96 hours.

[0070] [Ratio of silicon clathrate II] X-ray diffraction measurements were performed on the silicon materials obtained in each example and comparative example using a powder X-ray diffractometer. The ratio (yield ratio) of silicon clathrate II to silicon clathrate I in the silicon material was determined from the results of the X-ray diffraction measurements. The results are shown in Table 1. The proportion of amorphous silicon was also confirmed from the results of the X-ray diffraction measurements. Diffraction charts for the silicon materials of Example 1, Comparative Example 1, and Comparative Example 2 are shown in Figure 2. The diffraction chart in Figure 2 confirms that the silicon material of Example 1 has fewer peaks derived from silicon clathrate I than the silicon material of Comparative Example 1. The diffraction chart in Figure 2 confirms that the silicon material of Example 1 has fewer peaks derived from amorphous silicon than the silicon material of Comparative Example 2. In Figure 2, the area between the diffraction chart and the baseline at the bottom of the diffraction chart indicates the amount of amorphous silicon produced. The diffraction chart also confirmed that the silicon material of Example 2 had fewer peaks derived from amorphous silicon and silicon clathrate I, indicating a high proportion of silicon clathrate II. The silicon material of Comparative Example 3 had higher peaks derived from amorphous silicon and a lower proportion of silicon clathrate II.

[0071] [Table 1]

Claims

1. AlF ３ AlF used as a getter agent by removing fine particles and coarse particles from raw material powder ３ a getter agent classification step for obtaining a classified substance; The AlF ３ a mixing step of mixing the classified material and the Na—Si alloy to obtain a mixture; The mixture is heated to convert Na vaporized from the Na—Si alloy into AlF ３ a silicon clathrate production step in which the silicon clathrate is reacted with the fraction to produce silicon clathrate II; A method for producing a negative electrode active material containing silicon clathrate II, comprising:

2. The AlF ３ 2. The method for producing a negative electrode active material containing silicon clathrate II according to claim 1, wherein the average particle diameter D50 of the classified product is 30 to 70 μm.

3. The AlF ３ 3. The method for producing an anode active material containing silicon clathrate II according to claim 1, wherein the average particle diameter D10 of the classified product is 20 to 40 μm.

4. The AlF ３ 3. The method for producing a negative electrode active material containing silicon clathrate II according to claim 1, wherein the average particle diameter D90 of the classified product is 60 to 90 μm.

5. 3. The method for producing a negative electrode active material containing silicon clathrate II according to claim 1, further comprising an alloying step of heating raw materials containing a Na material and a Si material to obtain the Na-Si alloy.

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