JP7456922B2 - Manufacturing method of negative electrode active material raw material - Google Patents

Description

The present invention relates to a method for producing a negative electrode active material raw material.

A compound called silicon clathrate is known in which other metals are included in a polyhedral space formed by Si. Among silicon clathrates, research has been reported mainly on silicon clathrate I and silicon clathrate II.

Silicon clathrate I is a dodecahedron in which one Na atom is included in 20 Si atoms, and a tetradecahedron in which one Na atom is included in 24 Si atoms, which share a surface. It is represented by the compositional formula of Na ₈ Si ₄₆ . Na is present in all the polyhedral cages constituting the silicon clathrate I.

Silicon clathrate II is composed of a dodecahedron of Si and a hexahedron of Si, and is expressed by the composition formula Na _x Si ₁₃₆ . Here, x satisfies 0≦x≦24. That is, Na may or may not be present in the polyhedral cage constituting silicon clathrate II.

Non-Patent Document 1 describes a method for producing silicon clathrate I and silicon clathrate II from sodium silicide. Specifically, under reduced pressure conditions of less than 10 ^-4 Torr (that is, less than 1.3 × 10 ^-2 Pa), sodium silicide is heated to 400°C or more to remove Na as vapor, resulting in silicon class It is stated that clathrate I and silicon clathrate II were produced. Due to the difference in heating temperature, the production ratio of silicon clathrate I and silicon clathrate II changes, and as the heating temperature increases, Na detaches from silicon clathrate I, and the structure of silicon clathrate I changes. It is also described that Si crystals, which have a general diamond structure, are generated by changing the .
Furthermore, for silicon clathrate II, Na _22.56 Si ₁₃₆ , Na _17.12 Si ₁₃₆ , Na _18.72 Si ₁₃₆ , Na _7.20 Si ₁₃₆ , Na _11.04 Si ₁₃₆ , Na _1.52 Si ₁₃₆ , Na _23.36 Si ₁₃₆ , Na _24.00 Si ₁₃₆ , Na _20.48 Si ₁₃₆ , Na _16.00 Si ₁₃₆ , and Na _14.80 Si ₁₃₆ have been produced.

Patent Document 1 also describes a method for manufacturing silicon clathrate. Specifically, a Na-Si alloy manufactured using a silicon wafer and Na is heated at 400°C for 3 hours under reduced pressure conditions of 10 ^-2 Pa or less to remove Na, thereby converting silicon clathrate I. It is also described that silicon clathrate II was produced.

In addition, silicon clathrate II in which Na included in silicon clathrate II is replaced with Li, K, Rb, Cs, or Ba, and silicon clathrate II in which Si in silicon clathrate II is partially replaced with Ga or Ge. Rate II has also been reported.

H. Horie, T. Kikudome, K. Teramura, and S.Yamanaka, Journal of Solid State Chemistry, 182, 2009, pp.129-135

JP2012-224488A

Silicon clathrate II maintains its structure even if the included Na is removed. The inventor of the present invention paid attention to this point and thought of using silicon clathrate II from which the included Na has been released as a negative electrode active material of a lithium ion secondary battery.

All of the silicon clathrate IIs mentioned above are manufactured using sodium silicide as a raw material. In other words, the sodium silicide can be used as a raw material for the negative electrode active material. A known method for manufacturing sodium silicide is to heat metallic sodium or sodium hydride with silicon and react the sodium with silicon to produce sodium silicide in a molten salt state.

The inventor of the present invention actually reacted sodium and silicon as described above to obtain a reaction product containing sodium silicide.
Since the reaction product thus obtained is in the form of a hard lump, it was necessary to crush the reaction product using a high-power crusher in order to use it for producing a negative electrode active material. In other words, the reaction product cannot be used as it is as a raw material for a negative electrode active material, and large-scale manufacturing equipment including a high-output pulverizer is required to use it as a raw material for a negative electrode active material. Therefore, there is a problem in that the reaction product is not suitable for industrial production of negative electrode active materials.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technology that can suitably industrially produce a negative electrode active material raw material containing sodium silicide.

The inventor of the present invention actually performed X-ray diffraction measurements on the above reaction product using a powder X-ray diffraction device, and found that the reaction product contained a large amount of crystalline silicon in addition to sodium silicide. was. This result did not change even when the reaction was carried out under conditions rich in sodium with respect to silicon, for example, under conditions where the molar ratio of Na:Si was 1.6:1.

Based on the analysis results, the inventor of the present invention found that according to the above production method, the reaction to produce sodium silicide from sodium and silicon did not proceed sufficiently, and as a result, the reaction product obtained by the production method was It was assumed that a large amount of unreacted crystalline silicon remained. If a large amount of unreacted crystalline silicon remains in the reaction product, it is assumed that a large amount of crystalline silicon will be carried into the negative electrode active material obtained using the reaction product as a raw material. Although crystalline silicon functions as a negative electrode active material in power storage devices such as lithium ion secondary batteries, it cannot be said to have excellent durability as a negative electrode active material. Therefore, a negative electrode active material containing a large amount of crystalline silicon may impede improvement in the durability of a negative electrode or a power storage device that includes the negative electrode active material. In other words, a negative electrode active material with a high content of crystalline silicon cannot be said to be a suitable negative electrode active material, and the above-mentioned reaction product with a high content of crystalline silicon cannot be said to be a suitable negative electrode active material raw material either.

The inventors of the present invention attempted to crush the reaction product obtained by the above method and heat it again. When they then performed X-ray diffraction measurement on the reaction product obtained after crushing and reheating, no peaks derived from crystalline silicon were observed in the reaction product, and it was inferred that the reaction of sodium silicide formed from the crystalline silicon and sodium remaining in the reaction product by heating after crushing had progressed sufficiently.

As described above, the reaction product obtained through pulverization and reheating after initial heating does not contain or hardly contains crystalline silicon, and therefore can be said to be suitable as a raw material for the negative electrode active material.
In addition, since no peak derived from crystalline silicon was observed in the reaction product obtained after pulverization and reheating, it was found that by pulverizing the reaction product after the initial heating, silicon and sodium were sufficiently removed. It is presumed that the mixture was mixed, and as a result, the reaction that produced sodium silicide from silicon and sodium proceeded suitably. In other words, among the three steps of heating, pulverization, and reheating, the pulverization step can be said to be effective for advancing the reaction in which sodium silicide is produced from silicon and sodium.

However, as mentioned above, the reaction product containing sodium silicide and crystalline silicon is a solid mass. Therefore, the process of pulverizing the reaction product requires a lot of cost. In other words, the pulverization process is not suitable for industrial production of negative electrode active material raw materials. The inventor of the present invention conducted further studies based on the above knowledge and completed the present invention.

That is, the method for producing a negative electrode active material raw material of the present invention that solves the above problems is as follows:
A metal sodium dispersion in which fine particulate metal sodium is dispersed in a hydrocarbon stabilizer that is non-reactive with the metal sodium, and powdered silicon are mixed in a hydrocarbon dispersion medium. A mixing step to obtain a Na-Si mixed raw material;
This is a method for producing a negative electrode active material raw material, comprising a reaction step of heating the Na--Si mixed raw material at a temperature below the melting point of sodium silicide to obtain the sodium silicide.

According to the method for producing a negative electrode active material raw material of the present invention, it is possible to suitably industrially produce a negative electrode active material raw material containing sodium silicide.

2 is an X-ray diffraction chart of the negative electrode active material raw material of Example 1. 2 is an X-ray diffraction chart of a negative electrode active material raw material of Comparative Example 1 and a negative electrode active material raw material of Comparative Example 2. 3 is an X-ray diffraction chart of a negative electrode active material raw material of Comparative Example 3.

The best mode for carrying out the present invention will be described below. Note that, unless otherwise specified, the numerical range "x to y" described herein includes the lower limit x and the upper limit y. A numerical range can be constructed by arbitrarily combining these upper and lower limit values, as well as the numerical values listed in the examples. Further, numerical values arbitrarily selected from within the numerical range can be used as the upper and lower limits.

Hereinafter, the method for producing the negative electrode active material raw material of the present invention may be referred to as the production method of the present invention, as necessary, and the negative electrode active material raw material produced by the manufacturing method of the present invention is referred to as the negative electrode active material raw material of the present invention. It is sometimes called. According to the production method of the present invention, sodium silicide, which is a raw material for silicon clathrate II, can be produced. Note that silicon clathrate II from which Na has been removed can be used as a negative electrode active material for power storage devices such as lithium ion secondary batteries.

The method for producing the negative electrode active material raw material of the present invention includes:
A metal sodium dispersion in which fine particulate metal sodium is dispersed in a hydrocarbon stabilizer that is non-reactive with the metal sodium, and powdered silicon are mixed in a hydrocarbon dispersion medium. A mixing step 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 the sodium silicide.

One of the technical significances of the manufacturing method of the present invention is that the reaction for producing sodium silicide proceeds in a state where sodium and silicon are sufficiently mixed. In the production method of the present invention, by mixing a metallic sodium dispersion and powdered silicon in a hydrocarbon dispersion medium, it is possible to obtain a Na-Si mixed raw material in which sodium and silicon are sufficiently mixed. It is possible. Note that the metal sodium dispersion is a dispersion in which fine particulate metal sodium is dispersed in a hydrocarbon stabilizer that is non-reactive with respect to the metal sodium, and is in the form of a paste as a whole.

According to the manufacturing method of the present invention, by heating such a Na-Si mixed raw material, the reaction that generates sodium silicide from sodium and silicon progresses suitably, resulting in a negative electrode in which no or almost no crystalline silicon remains. It is possible to obtain active material raw materials.

Another technical significance of the manufacturing method of the present invention is that by performing the reaction between sodium and silicon at a temperature lower than the melting point of sodium silicide, generation of molten salt-like sodium silicide is suppressed. It is in. As a result, it is possible to obtain powdered sodium silicide that can be easily crushed, instead of a hard block of sodium silicide that requires a large amount of energy to crush. Therefore, according to the manufacturing method of the present invention, a high-output crusher or the like is not required, and the cost required for manufacturing equipment is reduced, as well as the number of manufacturing steps.

Due to their cooperation, the production method of the present invention is advantageous for industrial production of negative electrode active material raw materials.
Hereinafter, the manufacturing method of the present invention will be explained for each element.

As described above, the production method of the present invention includes a mixing step and a reaction step.
The significance of the mixing step is to obtain a Na-Si mixed raw material in which sodium and silicon are sufficiently mixed so that the reaction to generate sodium silicide proceeds favorably. In the manufacturing method of the present invention, the above-mentioned metallic sodium dispersion and powdered silicon are used, and these are mixed in a hydrocarbon-based dispersion medium. Note that the Na-Si mixed raw material essentially requires fine particle metallic sodium and powdered silicon, and does not need to contain a hydrocarbon-based dispersion medium or a hydrocarbon-based stabilizer that constitutes the metallic sodium dispersion.

Metallic sodium dispersion and powdered silicon are commercially available, so a stable supply is expected. Furthermore, since the hydrocarbon dispersion medium has no reactivity with metallic sodium or silicon, by using the hydrocarbon dispersion medium as a dispersion medium, a metallic sodium dispersion containing metallic sodium and powdered silicon can be mixed. It is possible to carry out the process safely.

As the metal sodium dispersion, for example, those described in JP-A-2020-50922 can be used. The metal sodium dispersion is also called Sodium Dispersion (SD), and generally refers to a dispersion of fine particulate metal sodium with an average particle size of about 10 μm in a hydrocarbon stabilizer such as mineral oil. . In addition, in the manufacturing method of the present invention, the average particle diameter of metallic sodium and the type of hydrocarbon stabilizer are not particularly limited.

Preferred ranges for the average particle diameter of the metallic sodium contained in the metallic sodium dispersion include a range of 100 μm or less, 50 μm or less, or 10 μm or less. The definition of the average particle diameter of metallic sodium will be described later.

As the hydrocarbon stabilizer contained in the metallic sodium dispersion, it is preferable to use mineral oil which is a hydrocarbon and is liquid at room temperature so as to function as a dispersion medium for metallic sodium. The number of carbon atoms and structure (chain type, cyclic type, etc.) of the hydrocarbon stabilizer are not particularly limited, but as disclosed in JP-A-2020-50922, it may be composed of an alkane having 20 or more carbon atoms. An example is normal paraffin oil.

The hydrocarbon dispersion medium may be a hydrocarbon and a liquid at least in the mixing step so as to function as a dispersion medium for mixing the metal sodium dispersion and powdered silicon. The number of carbon atoms and structure (chain type, cyclic type, etc.) of the hydrocarbon dispersion medium are also not particularly limited, and are determined as appropriate depending on the temperature of the mixing step, the scale of the reaction system in which the mixing step and reaction step are performed, etc. Just do it.
As mentioned above, a hydrocarbon with a relatively large number of carbon atoms is generally used as a hydrocarbon stabilizer contained in a metal sodium dispersion, but in some cases, a hydrocarbon dispersion medium and the hydrocarbon stabilizer are used. The hydrogen stabilizer may be the same.

Considering the cost required for the mixing step, the mixing step is preferably performed at room temperature, and the hydrocarbon dispersion medium used in the mixing step is preferably liquid at room temperature.

Furthermore, taking into consideration the significance of the mixing process described above, a hydrocarbon-based dispersion medium is not particularly necessary for the reaction in which sodium and silicon produce sodium silicide, and it can even be said that it is not necessary.

That is, in the production method of the present invention, the hydrocarbon dispersion medium used in the mixing step is preferably removed before the reaction step. Similarly, since the hydrocarbon stabilizer is unnecessary in the reaction process, it is preferable to remove it before the reaction process.
In other words, the production method of the present invention preferably includes a step of removing the hydrocarbon dispersion medium and/or the hydrocarbon stabilizer after the mixing step and before the reaction step.
Considering the cost required for the production method of the present invention, it is preferable to remove the hydrocarbon dispersion medium by volatilization. Therefore, it is preferable to use a hydrocarbon dispersion medium that can volatilize at a temperature lower than the heating temperature in the reaction process, and it is particularly preferable to use a hydrocarbon dispersion medium that can volatilize at a relatively low temperature, for example, around room temperature. I can get it.
Note that the hydrocarbon stabilizer is preferably removed by volatilization if it can be volatilized at a relatively low temperature like the hydrocarbon dispersion medium. Hydrocarbon stabilizers that are difficult to volatilize at relatively low temperatures can be removed by washing as described below.

Taking these into consideration, preferred melting points of the hydrocarbon solvent are, for example, in the range of 20°C or less, 15°C or less, or 10°C or less. Preferred boiling points of the hydrocarbon solvent are, for example, in the range of 30°C to 400°C, 50°C to 300°C, or 55°C to 200°C. The preferred ranges of the melting point and boiling point of the hydrocarbon stabilizer are not limited to these, but the melting point and boiling point of the hydrocarbon stabilizer do not have to be within the above ranges.

In the mixing step, in order to more uniformly disperse and mix the metal sodium dispersion and powdered silicon, it is preferable to use a hydrocarbon dispersion medium with a low viscosity.
Preferred ranges for the viscosity of the hydrocarbon dispersion medium include 50 mPa·s or less, 10 mPa·s or less, or 1 mPa·s or less. The lower limit of the viscosity of the hydrocarbon dispersion medium is not particularly limited, and it is sufficient that it exceeds 0 mPa·s.

Considering the various conditions mentioned above, preferable hydrocarbon dispersion media include alkanes having 5 to 17 carbon atoms, cycloalkanes having 5 to 10 carbon atoms, alkenes having 5 to 10 carbon atoms, cycloalkenes having 5 to 10 carbon atoms, Examples include aromatic hydrocarbons having 6 to 9 carbon atoms. Considering safety and ease of handling, it is particularly preferable to use alkanes having 5 to 11 carbon atoms or aromatic hydrocarbons having 6 to 9 carbon atoms.

The mixer used in the mixing step, its output, mixing time, etc. may be appropriately set depending on the composition and scale of the reaction system. For example, a general stirrer can be used as the mixer.

In order to mix the metallic sodium dispersion and the powdered silicon more uniformly, it is preferable to control the particle diameter of the metallic sodium dispersion and the particle diameter of the silicon within appropriate ranges. Specifically, as the metal sodium dispersion and the powdered silicon, it is preferable to use those with a small average particle size.
Among these, the average particle diameter of silicon can be expressed as D50 when measured with a general laser diffraction particle size distribution analyzer.
Moreover, the average particle diameter of the metallic sodium contained in the metallic sodium dispersion can be measured by image analysis. Specifically, based on the projected area of metallic sodium obtained by image analysis of a microscopic photograph, the diameter of a sphere having the same projected area as the projected area is calculated, and the diameter is calculated as the particle diameter of metallic sodium. It's fine if you consider it. Then, a plurality of particle diameters of the metal sodium may be obtained and the average value thereof may be calculated.
In addition, when using a commercially available product as the metal sodium dispersion, the test result value can be regarded as the average particle diameter of the metal sodium dispersion.

Preferred average particle diameters of sodium metal include the following ranges: 0.01 to 100 μm, 0.05 to 50 μm, and 0.1 to 20 μm. Preferred average particle diameters of silicon are exemplified in the range of 0.1 μm to 5 mm, 1 μm to 1 mm, or 3 μm to 100 μm.

Furthermore, there is a suitable range for the amount of the hydrocarbon-based dispersion medium relative to the metallic sodium dispersion and the powdered silicon. If the amount of the hydrocarbon-based dispersion medium is too small, it may take a long time or require a large amount of energy to uniformly mix the metallic sodium dispersion and the powdered silicon.
As a preferred range of the amount of the hydrocarbon-based dispersion medium relative to the metallic sodium dispersion and the powdered silicon, when the total mass of the metallic sodium dispersion and the powdered silicon is 100 parts by mass, each range of 20 parts by mass or more, 50 parts by mass or more, 75 parts by mass or more, or 100 parts by mass or more can be exemplified. Note that, although there is no particular upper limit to the preferred range of the amount of the hydrocarbon-based dispersion medium relative to the metallic sodium dispersion and the powdered silicon, in consideration of the cost, it is preferably 1000 parts by mass or less.

In the production method of the present invention, the amounts of the metallic sodium dispersion and silicon used in the mixing step are not particularly limited, but since the sodium silicide that is the target of the reaction step is ideally NaSi, the Na:Si molar ratio is It is preferable that the ratio is around 1:1. Furthermore, it is known that the reaction of producing silicon clathrate II as a negative electrode active material from sodium silicide contained in the negative electrode active material raw material of the present invention proceeds suitably in the presence of metallic sodium. It is also preferred that the molar ratio of sodium be slightly excessive.
Furthermore, as described later, the negative electrode active material raw material of the present invention obtained by the production method of the present invention does not contain or hardly contains crystalline Si. Therefore, in the production method of the present invention, unlike the conventional production method described above, that is, a method in which sodium silicide in the form of a molten salt is produced by reacting sodium and silicon, it is necessary to make the reaction system excessively rich in sodium. There is no.

Considering these, in the mixing step in the production method of the present invention, the preferred range of Na:Si contained in the Na-Si mixed raw material is within the range of 1:1 to 3:1 in terms of molar ratio, 1:1 to 2 Examples of the range include: 1:1, 1:1 to 1.5:1, and 1:1 to 1.2:1.

By the way, as mentioned above, commercially available metal sodium dispersions use hydrocarbon stabilizers that are difficult to volatilize at low temperatures, such as room temperature, such as normal paraffin oil composed of alkanes with 20 or more carbon atoms. may be used. Since this type of hydrocarbon stabilizer is difficult to remove by volatilization, it is preferable to remove it by washing. Specifically, the production method of the present invention includes a cleaning step of washing and filtering the Na-Si mixed raw material, which is a mixture of a metallic sodium dispersion and powdered silicon, after the mixing step and before the reaction step. It is preferable to do so. After filtration, it is also preferable to evaporate the solvent used for washing, if necessary.

As the solvent used in the cleaning step, it is preferable to use a solvent that has no reactivity with metallic sodium or silicon, and specifically, it is preferable to use the hydrocarbon dispersion medium described above. The hydrocarbon dispersion medium used in the mixing step and the hydrocarbon dispersion medium used in the cleaning step may be the same or different, but it is suitable for industrial production of negative electrode active material raw materials. From this point of view, it is more preferable to use the same one.
Note that filtration in this specification means separating metallic sodium and silicon from a cleaning solvent such as a hydrocarbon dispersion medium and a hydrocarbon stabilizer to be removed. These may be separated using a filter medium, or may be separated using known solid-liquid separation or liquid separation techniques.

In the reaction step in the production method of the present invention, the Na--Si mixed raw material that has undergone the above mixing step and optionally a washing step is heated at a temperature below the melting point of sodium silicide. The heating temperature in the reaction step is, of course, a temperature sufficient to cause sodium and silicon to react to generate sodium silicide.

Preferred temperatures for the reaction step are specifically within the range of 100°C to 750°C, within the range of 200°C to 750°C, within the range of 300°C to 700°C, and within the range of 450°C to 650°C. I can give an example.

The atmosphere of the reaction system in the reaction step is not particularly limited, but it is preferable that the atmosphere contains little water, carbon, oxygen, etc., which are highly reactive with metallic sodium and silicon, and it is preferable that the atmosphere is an inert gas atmosphere such as nitrogen gas or argon gas, or a reduced pressure atmosphere. Considering the cost required for manufacturing equipment, etc., an inert gas atmosphere is desirable.

The manufacturing method of the present invention may include a step of crushing and/or classifying the negative electrode active material raw material after the reaction step, if necessary. A mortar, a cutter mill, or the like may be used to crush the negative electrode active material raw material. Further, a sieve may be used to classify the negative electrode active material raw material.

The negative electrode active material raw material of the present invention obtained by the production method of the present invention has sodium silicide produced in the reaction step as a main component. The term "main component" as used herein means that its content is 90% or more by mass. The negative electrode active material raw material can be used as a raw material for synthesizing silicon clathrate II by containing sodium silicide. Silicon clathrate II may be synthesized by a conventional method, and it is preferable to remove Na during or after the synthesis.

The negative electrode active material synthesized using the negative electrode active material raw material of the present invention can be 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. I can do it. Note that 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.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be embodied in various forms with modifications and improvements that can be made by those skilled in the art without departing from the spirit of the present invention.

The present invention will be explained in more detail below with reference to examples and comparative examples. Note that the present invention is not limited to these examples.

(Example 1)
- Mixing process As the metallic sodium dispersion, one in which powdered metallic sodium was dispersed in mineral oil as a hydrocarbon stabilizer was used. The metallic sodium dispersion contains 25.0% by weight of metallic sodium and has a mass of 83.1 g. Note that the average particle diameter of the metallic sodium contained in the metallic sodium dispersion is 10 μm.
As the powdered silicon, 24 g of silicon powder with an average particle size of 10 μm was used.
Using 44 ml of hexane as a hydrocarbon dispersion medium, the above-described metal sodium dispersion and powdered silicone hydrocarbon dispersion medium were charged, and the mixture was stirred for 10 minutes at 25° C. under a nitrogen atmosphere using a mechanical stirrer. As a result, a Na--Si mixed raw material in which the metallic sodium dispersion and powdered silicon were uniformly or substantially uniformly mixed was obtained. In the Na--Si mixed raw material, it can be said that the powdered silicon and the particulate metallic sodium contained in the metallic sodium dispersion are also uniformly or substantially uniformly mixed.

・Cleaning process The Na-Si mixed raw material obtained in the mixing process contains a metal sodium dispersion, powdered silicon, and a hydrocarbon dispersion medium, and the metal sodium dispersion also contains a mineral as a hydrocarbon stabilizer. It is thought to contain oil. In the cleaning step in the production method of Example 1, a cleaning step was performed to remove the hydrocarbon dispersion medium and the hydrocarbon stabilizer from the Na--Si mixed raw material. Note that it is assumed that the hydrocarbon stabilizer is still attached to the fine particulate metallic sodium in the Na-Si mixed raw material.

First, a metal sodium dispersion containing fine particulate metal sodium and a hydrocarbon stabilizer and powdered silicon are separated from the hydrocarbon dispersion medium by filtration to remove the hydrocarbons contained in the Na-Si mixed raw material. The system dispersion medium was removed. At this time, it is thought that a part of the hydrocarbon stabilizer is also removed together with the hydrocarbon dispersion medium, but it is thought that a part of the hydrocarbon stabilizer still remains attached to the metal sodium.
After removing the hydrocarbon dispersion medium described above, 44 ml of a cleaning solvent was added to the filtration residue containing fine particulate metallic sodium and powdered silicon, and the mixture was stirred with a mechanical stirrer at 25°C for 10 minutes under a nitrogen atmosphere. Thereafter, washing to remove fine particulate metal sodium and powdered silicon by filtration was repeated twice. Note that hexane, which is the same as the hydrocarbon dispersion medium, was used as the cleaning solvent.

After washing three times, the filtration residue was dried at 25°C for 3 hours under a reduced pressure of -758 mmHg to evaporate hexane, the washing solvent, and form a powdered Na-Si containing metallic sodium and silicon as main components. A mixed raw material was obtained.

・Reaction process The entire amount of the Na-Si mixed raw material after the cleaning process is placed in a reaction vessel, and heated at 600°C for 10 hours in a muffle furnace while flowing argon gas at a rate of 100 ml/min into the reaction vessel. , sodium and silicon were reacted. The reaction product obtained after heating and cooling was visually recognized to be in the form of mutually aggregated particles. Since the melting point of sodium silicide is around 800° C., it is presumed that the sodium silicide produced by the reaction between sodium and silicon did not become a molten salt in the reaction step in the production method of Example 1.
In a glove box under an argon atmosphere, the reaction product was scraped off from the reaction container using a spatula and lightly crushed in a mortar. The reaction product after crushing was used as the negative electrode active material raw material of Example 1.
In addition, since the mass of the negative electrode active material raw material of Example 1 was 44.2 g, the yield in the manufacturing method of Example 1 was about 99% on a mass basis.

(Comparative example 1)
140 g of powdered silicon and 125 ± 1 g of block-shaped sodium were placed in a reaction vessel, heated in a muffle furnace at 880°C for 1 hour while flowing argon gas through the reaction vessel at a rate of 100 ml/min, and then cooled. A reaction product in the form of a hard lump was obtained. In a glove box under an argon atmosphere, the reaction product was taken out by hitting the reaction container with a hammer, and then the reaction product was crushed with a hammer and further crushed with a crusher. As the crusher, Wonder Crusher WC-3 manufactured by Osaka Chemical Co., Ltd. was used, and the crushing time was 20 seconds with memory 5.

The reaction product after heating and pulverization was placed in a reaction vessel, and reheated at 600° C. for 10 hours in a muffle furnace while flowing argon gas into the reaction vessel at a rate of 100 ml/min. The reaction product obtained after reheating and cooling was visually recognized to be in the form of mutually aggregated particles. In a glove box under an argon atmosphere, the reaction product was scraped off from the reaction container using a spatula and lightly crushed in a mortar. The reaction product after crushing was used as the negative electrode active material raw material of Comparative Example 1.
Note that since the mass of the negative electrode active material raw material in Comparative Example 1 was 250 g, the yield in the manufacturing method of Comparative Example 1 was about 94% on a mass basis.

(Comparative example 2)
The manufacturing method of Comparative Example 2 is the same as that of Comparative Example 1 except that reheating was not performed. A negative electrode active material raw material of Comparative Example 2 was obtained by the manufacturing method of Comparative Example 2.

(Comparative example 3)
- Mixing Step The mixing step of Comparative Example 3 was carried out in the same manner as the mixing step of Example 1 to obtain a Na--Si mixed raw material in which the metallic sodium dispersion and powdered silicon were mixed uniformly or substantially uniformly.

-Volatilization step The Na-Si mixed raw material obtained in the mixing step contains a metallic sodium dispersion and powdered silicon, and the metallic sodium dispersion contains a hydrocarbon stabilizer. In the volatilization step, the hydrocarbon stabilizer contained in the metal sodium dispersion was removed by volatilization from the Na--Si mixed raw material.
Specifically, the Na-Si mixed raw material is placed in a reaction vessel and held at 450°C for 5 hours under a reduced pressure of -758 mmHg to volatilize the mineral oil, which is a hydrocarbon stabilizer, and convert it into metallic sodium. A powdered Na--Si mixed raw material containing silicon as a main component was obtained.

・Reaction process After the volatilization process, the entire amount of the Na-Si mixed raw material is heated in a muffle furnace at 600°C for 10 hours while flowing argon gas at a rate of 100 ml/min into the reaction vessel, thereby converting it into sodium and silicon. reacted.
In a glove box under an argon atmosphere, the reaction product was scraped off from the reaction container using a spatula and lightly crushed in a mortar. The reaction product after crushing was used as the negative electrode active material raw material of Comparative Example 3.
In addition, since the mass of the negative electrode active material raw material of Comparative Example 3 was 52.3 g, the yield in the manufacturing method of Comparative Example 3 was about 117% on a mass basis.

(Evaluation test)
For each of the negative electrode active material raw materials of Example 1 and Comparative Examples 1 to 3, X-ray diffraction measurements were performed using a powder X-ray diffractometer. The X-ray diffraction chart of the negative electrode active material raw material of Example 1 is shown in FIG. 1, the X-ray diffraction chart of the negative electrode active material raw material of Comparative Example 1 and the negative electrode active material raw material of Comparative Example 2 is shown in FIG. An X-ray diffraction chart of the negative electrode active material raw material is shown in FIG. In addition, in FIG. 1, the position of the peak derived from sodium silicide (NaSi in the figure) is also shown. Further, in FIG. 2, the position of a peak derived from sodium silicide (NaSi in the figure) and the position of a peak derived from silicon (Si in the figure) are also shown. Figure 3 shows the position of the peak derived from sodium silicide (NaSi in the figure), the position of the peak derived from SiC (SiC in the figure), and the position of the peak derived from SiC ₂ (SiC ₂ in the figure). Also listed.

As shown in FIG. 2, in the XRD chart of the negative electrode active material raw material of Comparative Example 2, a peak derived from crystalline silicon was observed. This peak was not observed in the negative electrode active material raw material of Comparative Example 1 obtained by reheating.
This proves that in the method for producing the negative electrode active material raw material of Comparative Example 2 in which silicon and bulk sodium are simply heated, the reaction in which sodium silicide is produced from sodium and silicon does not proceed sufficiently. In addition, since no peak derived from crystalline silicon was observed in the negative electrode active material raw material of Comparative Example 1, by crushing and reheating after the first heating, the reaction in which sodium silicide is generated from sodium and silicon can be suppressed. This confirms that the disease is progressing sufficiently.

As shown in FIG. 1, in the XRD chart of the negative electrode active material raw material of Example 1, similarly to the XRD chart of the negative electrode active material raw material of Comparative Example 1, no peaks derived from crystalline silicon were observed. This confirms that the method for producing the negative electrode active material of Example 1 also allows the reaction in which sodium silicide is produced from sodium and silicon to proceed sufficiently. In other words, by subjecting the Na-Si mixed raw material in which a metal sodium dispersion containing fine particulate metal sodium and powdered silicon are mixed to a reaction step and heating it at a temperature below the melting point of sodium silicide, the comparative example The reaction of producing sodium silicide from sodium and silicon proceeds suitably without the need for twice-heating or high-output pulverization as in the production method 1.
This result confirms that the production method of the present invention is suitable for industrial production of negative electrode active material raw materials.

Furthermore, as shown in FIG. 3, a peak derived from SiC and a peak derived from _SiC2 were observed in the XRD chart of the raw material for the negative electrode active material of Comparative Example 3, but these peaks were not observed in the XRD chart of the raw material for the negative electrode active material of Example 1, as shown in FIG.
From this, it can be seen that when the hydrocarbon stabilizer is removed by volatilization rather than by washing, carbon derived from the hydrocarbon stabilizer remains in the Na-Si raw material mixture, and the carbon reacts with silicon. Since SiC and _SiC2 do not function as negative electrode active materials in lithium ion secondary batteries, it is preferable that the negative electrode active material does not contain a large amount of these, and it is also preferable that the negative electrode active material raw material does not contain these.
This result supports the view that the production method of the present invention, which includes a washing step, is particularly suitable for industrial production of raw materials for negative electrode active materials.

Claims (3)

A metal sodium dispersion in which fine particulate metal sodium is dispersed in a hydrocarbon stabilizer that is non-reactive with the metal sodium, and powdered silicon are mixed in a hydrocarbon dispersion medium. A mixing step to obtain a Na-Si mixed raw material;
A method for producing a negative electrode active material raw material, comprising a reaction step of heating the Na-Si mixed raw material at a temperature below the melting point of sodium silicide to obtain the sodium silicide. 2. The method for producing a negative electrode active material raw material according to claim 1, further comprising a cleaning step of cleaning the Na-Si mixed raw material and filtering out the metallic sodium and the silicon after the mixing step and before the reaction step. The method for producing a negative electrode active material raw material according to claim 1 or 2, wherein the hydrocarbon stabilizer is mineral oil.