Method for producing porous silicon

Abstract

This method for producing porous silicon comprises a granulation step S2, a carbon coating film formation step S3, a reduction step S4, and a cleaning step S5. The granulation step S2 is a step for obtaining a granule in which primary particles containing a silicon oxide powder are aggregated. The carbon coating film formation step S3 is a step for forming a carbon coating film on the granule surface to obtain a precursor particle in which the surface is coated with a carbon coating film and silicon oxide powder is the primary particle. The reduction step S4 is a step for bringing the precursor particle into contact with Mg vapor to obtain a reduced particle in which the surface is coated with a carbon coating film and an Si- and MgO-containing mixture is the primary particle. The cleaning step S5 is a step for removing MgO from the reduced particle to obtain a porous silicon particle in which the surface is coated with a carbon coating film and pore-bearing porous silicon is the primary particle.

Description

Manufacturing method for porous silicon

[0001] This disclosure relates to a method for producing porous silicon.

[0002] The method for producing porous silicon disclosed in Non-Patent Document 1 comprises a reduction step and a washing step. In the reduction step, Mg vapor generated by heating metallic Mg is used to remove SiO contained in porous diatomaceous earth. ２ The first step involves reducing the substance to produce an intermediate product containing Si and MgO. The second step involves washing the intermediate product with acid to remove the MgO and obtain porous silicon.

[0003] Nature 2007, 446, 172-175

[0004] Porous silicon can be used as an electrode material for energy storage devices such as lithium-ion secondary batteries. For example, porous silicon is suitable as a negative electrode material for lithium-ion secondary batteries because it is advantageous in terms of expansion and contraction during charging and discharging. The object of the present invention is to provide porous silicon suitable for electrode materials.

[0005] The present disclosure is a method for producing porous silicon used as an electrode material, comprising: a granulation step of obtaining a granulated body in which primary particles containing silicon oxide powder are aggregated; a carbon film formation step of obtaining precursor particles in which the powder is used as primary particles and the surface is covered with the carbon film by forming a carbon film on the surface of the granulated body; a reduction step of obtaining reduced particles in which a mixture containing Si and MgO is used as primary particles and the surface is covered with the carbon film by contacting the precursor particles with Mg vapor; and a washing step of obtaining porous silicon particles in which porous silicon having pores is used as primary particles and the surface is covered with the carbon film by removing MgO from the reduced particles.

[0006] In one embodiment of a method for producing porous silicon, the granulation step is a step of obtaining a granulated body in which the powder is made into primary particles and the surface of the primary particles is covered with the carbon source by spray drying a precursor slurry containing silicon oxide powder, a carbon source, and a dispersion medium, and the carbon film formation step is a step of forming a carbon film derived from the carbon source on the surface of the primary particles of the granulated body by firing the granulated body.

[0007] In one embodiment of a method for producing porous silicon, the granulation step is a step of obtaining a granulated body in which the powder is the primary particle by spray-drying a precursor slurry containing silicon oxide powder and a dispersion medium, and includes a slurry preparation step of preparing the precursor slurry to be used in the granulation step, and the slurry preparation step includes a grinding step of obtaining a mixture containing the powder and the dispersion medium by wet grinding with the dispersion medium added to mother particles of silicon oxide.

[0008] The average particle size (D50) of the powder is, for example, 0.3 μm or more and 2.5 μm or less. The average particle size (D50) of the granulated material is, for example, 3 μm or more and 20 μm or less. In one embodiment of the method for producing porous silicon, the aspect ratio of the granulated material is closer to 1 than the aspect ratio of the powder.

[0009] According to the present invention, porous silicon suitable for electrode materials can be provided.

[0010] Figure 1 is a flowchart of the method for manufacturing porous silicon according to the embodiment. Figure 2 is a schematic diagram of each step in the method for manufacturing porous silicon according to the embodiment. Figure 3 is the temperature-Mg vapor pressure curve of reaction equation (1). Figure 4 is an explanatory diagram of the reduction step. Figure 5 is a schematic cross-sectional view of the negative electrode of the energy storage device. Figure 6 is a schematic cross-sectional view of the negative electrode of the energy storage device.

[0011] An embodiment of the present invention will be described below. As shown in Figure 1, the method for producing porous silicon according to this embodiment includes a slurry preparation step (S1), a granulation step (S2), a carbon film formation step (S3), a reduction step (S4), and a washing step (S5). By going through each of the above steps in order, porous silicon particles are obtained. Details of each step will be described below.

[0012] <Slurry Preparation Process (S1)> The slurry preparation process is a process for preparing a precursor slurry containing silicon dioxide powder, a carbon source, and a dispersion medium. The details of each component constituting the precursor slurry are described below.

[0013] [Silicon Oxide Powder] Silicon oxide powder contains silicon oxide as its main component. Examples of silicon oxide include SiO₂, SiO₂ ２ Examples include the following. Silicon oxide is preferably SiO. The powder may also contain components other than silicon oxide. The proportion of silicon oxide in the powder is, for example, 50% by mass or more, preferably 90% by mass or more, and more preferably 99% or more.

[0014] The average particle size (D50) of the above powder is, for example, 0.01 μm or more and 5 μm or less. Preferably, the average particle size (D50) of the above powder is 0.3 μm or more, and more preferably 0.6 μm or more. Also, preferably, the average particle size (D50) of the above powder is 2.5 μm or less, and more preferably 1.3 μm or less. In this specification, "average particle size (D50)" means the median diameter measured by laser diffraction scattering method. By setting the average particle size (D50) of the silicon dioxide powder to 0.3 μm or more, the amount of adsorbed oxygen of the porous silicon particles used as electrode material for batteries can be suppressed to a predetermined value or less. A low oxygen content contributes to improved battery performance. Furthermore, by setting the average particle size (D50) of the silicon dioxide powder to 2.5 μm or less, it becomes possible to suitably produce granules in the granulation process (S2) performed after the slurry preparation process (S1).

[0015] [Carbon Source] The carbon source is, for example, an organic compound. Examples of organic compounds include sugars such as glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, and ascorbic acid, organic acids such as citric acid, polyethers, polyhydric alcohols, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl chloride, cellulose, starch, gelatin, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, and polyvinyl acetate. Examples of polyhydric alcohols include polyethylene glycol, polypropylene glycol, polyglycerin, and glycerin. The carbon source may be used alone or in combination of two or more.

[0016] The carbon source is preferably a sugar such as fructose. By using a sugar as the carbon source, it functions as a binder for forming granules in the granulation process described later. Therefore, granules can be formed in the granulation process even without adding a separate binder to the precursor slurry, or even if only a small amount of a separate binder is added.

[0017] The carbon source content in the precursor slurry is adjusted so that the mass of carbon in the carbon source is a specific amount. For example, when the silicon content in the powder in the precursor slurry is 100 parts by mass, the carbon content in the carbon source in the precursor slurry is 3.7 parts by mass or more and 11.3 parts by mass or less. The carbon content is preferably 5.5 parts by mass or more, and more preferably 6.7 parts by mass or more. Also, the carbon content is preferably 9.3 parts by mass or less, and more preferably 8.2 parts by mass or less. The silicon content refers to the mass ratio of silicon atoms contained in the precursor slurry. The carbon content refers to the mass ratio of carbon atoms constituting the carbon source contained in the precursor slurry.

[0018] Increasing the carbon content improves the conductivity of the porous silicon particles produced. Conversely, decreasing the carbon content increases the proportion of silicon in the porous silicon particles produced, resulting in a larger volume per unit area of ​​the porous silicon particles.

[0019] Furthermore, when a carbon source is used as a binder, it is preferable to determine the carbon source content based on the average particle size (D50) of the powder. In other words, it is preferable to determine the carbon source content based on the specific surface area of ​​the powder. For example, suppose the average particle size (D50) of the powder is 0.01 μm or more and 5 μm or less, and the carbon source is a sugar such as fructose. In this case, the carbon source content in the precursor slurry is, for example, 3.7 parts by mass or more, preferably 5.5 parts by mass or more, and more preferably 6.7 parts by mass or more, when the powder content in the precursor slurry is 100 parts by mass. The carbon source content is, for example, 11.3 parts by mass or less. By setting the carbon source content within the above range, granules can be formed in the granulation process even without separately adding a binder to the precursor slurry, or even if only a small amount of binder is separately added.

[0020] Furthermore, when a carbon source is used as a binder, the average particle size of the granules formed in the granulation process can be controlled by adjusting the carbon source content in the precursor slurry. For example, it is preferable to adjust the carbon source content in the precursor slurry so that the amount of carbon coating covering the surface of the primary particles on the precursor particles formed in the carbon coating formation process (S3) described later, i.e., the particles obtained by calcining the granules formed in the granulation process, is at least a predetermined volume percent. In this case, by increasing the amount of carbon source functioning as a binder, it is possible to reduce the proportion of fine powder particles with a particle size of 1 μm or less in the entire granule formed in the granulation process to zero or to a very small amount. A low proportion of fine powder makes the granule easier to handle. Also, the particle size distribution of the various particles formed in each process after the granulation process depends on the particle size distribution of the granules. Therefore, if the proportion of fine powder in the entire granule is small, the proportion of fine powder in the various particles formed in each process after the granulation process will also be small.

[0021] [Dispersion medium] The dispersion medium is water, or a mixed solvent of water and a non-aqueous solvent. The water is not particularly limited, but preferred examples include ion-exchanged water, which is water treated with an ion-exchange resin, and ultrapure water, which is water treated by a reverse osmosis membrane water purification system. Examples of non-aqueous solvents that make up the mixed solvent include solvents that are miscible with water, such as lower alcohols, acetone, tetrahydrofuran, ethylene glycol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, acetonitrile, and dimethyl sulfoxide. The non-aqueous solvent may be used alone or in combination of two or more types.

[0022] The volume percentage of water in the mixed solvent is preferably, for example, 50% by volume or more and 99.9% by volume or less, and more preferably 60% by volume or more and 99% by volume or less. The content of the dispersion medium in the precursor slurry is not particularly limited, but for example, it is an amount such that the solid content ratio is 10% by mass or more and 65% by mass or less.

[0023] [Other Components] The precursor slurry may contain other components besides those mentioned above, as needed. Examples of other components include dispersants and binders. It is also possible to add sintering aids to the precursor slurry. For example, by adding silica nanoparticles such as colloidal silica to the precursor slurry, it is possible to promote the fusion of primary particles when the granulated material is fired in the subsequent carbon film formation process, thereby increasing the particle bonding strength of the granulated material.

[0024] [Specific Example of Slurry Preparation Process] As shown in Figures 1 and 2, an example of the slurry preparation process (S1) includes a grinding process (S1A) and a mixing process (S1B).

[0025] The grinding process involves adding a dispersion medium to silicon dioxide mother particles and performing wet grinding to obtain a mixture containing silicon dioxide powder and a dispersion medium. Wet grinding can be carried out using a grinding device such as a bead mill. It is preferable to perform the grinding process under conditions where the lump-like mother particles are volume-ground. For example, in the first half of the grinding process, the ground particles produced by surface grinding using frictional or shearing forces are further ground under conditions where they are volume-ground by impact or compressive forces in the second half of the grinding process. Performing the grinding process under volume-ground conditions reduces the aspect ratio of the resulting powder (primary particles). Using powder with a small aspect ratio reduces the aspect ratio of the resulting porous silicon particles. The specific method of wet grinding in the grinding process is not particularly limited, and known methods applicable to grinding silicon dioxide can be used.

[0026] The mixing step is a step of preparing a precursor slurry by adding a carbon source to the mixture after the pulverization step. In the mixing step, the solid content ratio of the precursor slurry may be adjusted by further adding a dispersion medium. By adjusting the solid content ratio of the precursor slurry, the particle size of the granulated body can be controlled. For example, by increasing the solid content ratio of the precursor slurry, the average particle diameter of the granulated body can be increased. Also, for example, by increasing the solid content ratio of the precursor slurry, the width of the particle size distribution of the granulated body can be narrowed. Furthermore, by setting the solid content ratio of the precursor slurry to a predetermined value or more, the proportion of fine powder having a particle size of 1 μm or less in the entire granulated body formed in the granulation step can be made zero or suppressed to a very small amount (for example, 5% by mass or less).

[0027] Also, when preparing a precursor slurry containing other components, the other components may be mixed in the mixing step. The specific mixing method in the mixing step is not particularly limited, and a known method applicable when preparing a slurry can be used.

[0028] Note that the slurry preparation step is not limited to the above method. For example, in the pulverization step, wet pulverization may be performed with a carbon source added in advance. In this case, the mixing step can be omitted. Also, a precursor slurry may be prepared by separately preparing powders of silicon oxide, a carbon source, and a dispersion medium and mixing them. In this case, for example, a commercially available powder adjusted to a predetermined particle size in advance is used as the powder of silicon oxide.

[0029] <Granulation Step (S2)> As shown in FIG. 2, the granulation step is a step of obtaining a granulated body by spray-drying a precursor slurry. The granulated body is a secondary particle in which the powder of silicon oxide contained in the precursor slurry is used as a primary particle and the surface of the primary particle is covered with a carbon source. In other words, the granulated body is a secondary particle in which primary particles composed of powders of silicon oxide are aggregated.

[0030] As a spraying method in spray drying, for example, there are spraying methods using a disk type, a pressure nozzle, a pressure two-fluid nozzle, a pressure four-fluid nozzle, etc. The spraying temperature in spray drying is, for example, 150°C or higher and 250°C or lower.

[0031] The average particle diameter (D50) of the granulated product obtained by the granulation step is, for example, 3 μm or more and 20 μm or less. The average particle diameter (D50) of the granulated product is preferably 5 μm or more, more preferably 8 μm or more, and still more preferably 10 μm or more. Also, the average particle diameter (D50) of the granulated product is preferably 15 μm or less, more preferably 13 μm or less.

[0032] The aspect ratio of the granulated product obtained by the granulation step is, for example, 1.0 or more and 1.5 or less. The aspect ratio of the granulated product is preferably 1.0 or more and 1.2 or less, and more preferably 1.0 or more and 1.1 or less. The above aspect ratio is, for example, an average value. In one example, the aspect ratio of the granulated product is closer to 1 than the aspect ratio of the silicon oxide powder constituting the granulated product.

[0033] The average particle diameter and the width of the particle diameter distribution of the granulated product can be controlled by adjusting the solid content ratio of the precursor slurry, the liquid feeding amount or the nozzle pressure when spraying the precursor slurry. For example, increasing the liquid feeding amount when spraying the precursor slurry increases the particle diameter of the granulated product, and increasing the nozzle pressure decreases the particle diameter of the granulated product.

[0034] <Carbon Coating Formation Step (S3)> As shown in FIG. 2, the carbon coating formation step is a step of obtaining precursor particles in which a carbon coating derived from a carbon source is formed on the surface of the primary particles of the granulated product by firing the granulated product. The carbon coating formation step of the present embodiment is a step of changing the carbon source adhering to the surface of the primary particles of the granulated product into a carbon coating. The precursor particles obtained by the carbon coating formation step of the present embodiment include primary particles made of silicon oxide and a carbon coating covering the surface of the primary particles.

[0035] The firing temperature in the carbon film formation process is, for example, 500°C to 1100°C. When silicon dioxide is SiO, the disproportionation reaction of SiO at high temperatures (SiO → Si + SiO) ２ To suppress this, the firing temperature is preferably between 500°C and 1000°C.

[0036] The firing time in the carbon film formation process is, for example, 0.5 hours or more and 12 hours or less. The atmosphere in the carbon film formation process is, for example, a non-oxidizing atmosphere. A non-oxidizing atmosphere is, for example, nitrogen (N ２ ), an inert atmosphere such as argon (Ar), and hydrogen (H ２ Examples include reducing atmospheres containing reducing gases such as ).

[0037] The firing temperature should be the temperature necessary to carbonize the carbon source and form a carbon film. Furthermore, if the firing temperature is set below 1000°C, it is possible to suppress the excessive hydrophobicity of the formed carbon film. As a result, the dispersibility of porous silicon particles in aqueous solvents is improved in the electrode manufacturing process using porous silicon particles produced by this manufacturing method.

[0038] In the carbon film formation process, the particle size of the precursor particles and the particle size of the primary particles of the precursor particles do not change significantly from those of the granulated material. Therefore, the average particle size (D50) of the precursor particles is approximately equal to the average particle size (D50) of the granulated material described above. Furthermore, the average particle size (D50) of the primary particles of the precursor particles is approximately equal to the average particle size (D50) of the primary particles of the granulated material.

[0039] <Reduction Process (S4)> As shown in Figure 2, the reduction process is a process in which reduced particles are obtained by contacting precursor particles with Mg vapor to obtain a mixture containing Si and MgO as primary particles. In the reduction process, the primary particles made of silicon oxide that constitute the precursor particles are reduced to SiO or SiO by contacting Mg vapor. ２A reduction reaction occurs to reduce it, changing to primary particles composed of a mixture containing Si and MgO. Therefore, the reduction step can also be said to be a step of changing each of the primary particles constituting the precursor particles from primary particles composed of silicon oxide to primary particles composed of a mixture containing Si and MgO. The reduced particles include primary particles composed of a mixture containing Si and MgO and a carbon coating covering the surface of the primary particles.

[0040] In the reduction step, the particle diameter of the reduced particles and the particle diameter of the primary particles of the reduced particles hardly change from the precursor particles. Therefore, the average particle diameter (D50) of the reduced particles becomes substantially equal to the average particle diameter (D50) of the granulated particles and the precursor particles described above. Also, the average particle diameter (D50) of the primary particles of the reduced particles becomes substantially equal to the average particle diameter (D50) of the primary particles of the granulated particles and the precursor particles.

[0041] Next, the details of the reduction reaction for reducing SiO or SiO ２ by Mg vapor will be described. [Pressure conditions] The pressure condition for the above reduction reaction is the Mg vapor pressure below the equilibrium pressure of the following reaction formula (1).

[0042]

[0043] Reaction formula (1) shows a reversible decomposition reaction in which Mg ２ Si is decomposed into Mg vapor and Si. The temperature-Mg vapor pressure curve of reaction formula (1) is shown in FIG. 3. The Mg vapor pressure below the equilibrium pressure of reaction formula (1) is a Mg vapor pressure lower than the curve shown in FIG. 3. At the Mg vapor pressure below the equilibrium pressure of reaction formula (1), the forward reaction in which Mg ２ Si is decomposed into Mg vapor and Si proceeds, and the reverse reaction in which Mg ２ Si is generated from Mg vapor and Si does not proceed or hardly proceeds. Therefore, it is possible to suppress the reaction between Si generated from SiO or SiO ２ by the reduction reaction and Mg vapor to generate Mg ２ Si.

[0044] [Method for generating Mg vapor] The method for generating Mg vapor is not particularly limited, as long as it is possible to bring Mg vapor into contact with the precursor particles while satisfying the above pressure conditions. One example of a method for generating Mg vapor is to heat a reducing agent that serves as an Mg source, such as an Mg alloy or metallic Mg.

[0045] Among these methods, the method using a Mg alloy in which the equilibrium pressure of the reaction that generates Mg vapor is less than or equal to the equilibrium pressure of reaction equation (1) (hereinafter referred to as low equilibrium pressure Mg alloy) is particularly preferred. In the reaction system in which Mg vapor is generated from the low equilibrium pressure Mg alloy, the Mg vapor pressure will not exceed the equilibrium pressure of reaction equation (1). Therefore, the reaction that generates Mg vapor from the low equilibrium pressure Mg alloy and the reaction in which Mg vapor is brought into contact with precursor particles can be carried out in the same reaction system. In this case, the equipment used in the reduction process can be simplified.

[0046] Examples of low equilibrium pressure Mg alloys include Mg ２ Si, MgCa alloy, MgCu ２ MgNi ２ Examples include MgSn alloys and MgZn alloys. Table 1 below shows the equilibrium pressures at 600°C, 700°C, and 800°C for the reaction that generates Mg vapor from low equilibrium pressure Mg alloys. Note that the Mg vapor pressure values ​​in Table 1 are expressed on a common logarithmic scale. Mg in Table 1 ２ The numerical value related to Si represents the equilibrium pressure in reaction equation (1).

[0047] Among low equilibrium pressure Mg alloys, those in which the equilibrium pressure of the reaction that generates Mg vapor is close to the equilibrium pressure of reaction equation (1) are preferred.

[0048]

[0049] Table 1 also shows, for reference, the equilibrium pressures at each of the above temperatures for the reaction that generates Mg vapor from metallic Mg. As shown in Table 1, the equilibrium pressure of the reaction that generates Mg vapor from metallic Mg is higher than the equilibrium pressure of reaction equation (1). Therefore, the Mg vapor pressure in the reaction system that generates Mg vapor from metallic Mg will be higher than the equilibrium pressure of reaction equation (1). When adopting a method of generating Mg vapor from metallic Mg, a treatment is performed to lower the Mg vapor pressure in the reaction system. As an example of such a treatment, one method is to separately place a primary metal such as Ca, Cu, Sn, Ni, or Zn, which bonds more readily with Mg than Si, into the reaction system. The same applies when adopting a method of generating Mg vapor from an Mg alloy in which the equilibrium pressure of the reaction that generates Mg vapor exceeds the equilibrium pressure of reaction equation (1).

[0050] The amount of reducing agent that serves as a source of Mg, such as Mg alloy and metallic Mg, is charged, and the amount of SiO or SiO contained in the precursor particles ２ This is the amount that generates more than 1 molar equivalent of Mg vapor relative to the number of moles of SiO. In other words, the amount of reducing agent charged is the amount of all SiO or SiO contained in the precursor particles. ２ It is an amount greater than or equal to the amount that can reduce it. For example, SiO or SiO contained in the precursor particles ２ The amount of reducing agent required to generate 1 molar equivalent of Mg vapor relative to the number of moles of Mg is defined as the standard amount. In this case, the amount of reducing agent charged is at least 1 times the standard amount, preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.3 times or more. Alternatively, the amount of reducing agent charged may be, for example, 2 times or less the standard amount.

[0051] [Temperature Conditions] The temperature of the reduction reaction described above is the temperature at which Mg vapor is present at or below the equilibrium pressure of reaction equation (1). For example, the temperature of the reduction reaction described above is 900°C or lower, 800°C or lower, or 700°C or lower. Alternatively, the temperature of the reduction reaction described above is 500°C or higher, 600°C or higher, or 650°C or higher. By adjusting the temperature of the reduction reaction described above, the pore size distribution of the porous silicon particles finally obtained can be controlled.

[0052] When the temperature of the reduction reaction described above is lowered, the pore size of the pores in the resulting porous silicon particles becomes smaller. This increases the specific surface area of ​​the resulting porous silicon particles. Conversely, when the temperature of the reduction reaction described above is higher, the pore size of the pores in the resulting porous silicon particles becomes larger. This decreases the specific surface area of ​​the resulting porous silicon particles.

[0053] Therefore, the temperature of the reduction reaction should be set based on the physical properties required for the porous silicon particles obtained in the end. For example, when producing porous silicon particles with small pore sizes, it is preferable to lower the temperature of the reduction reaction. Also, when producing porous silicon particles with reduced oxygen content due to surface oxidation, it is preferable to increase the temperature of the reduction reaction to reduce the specific surface area of ​​the porous silicon particles.

[0054] [Reaction Time] The reaction time for the reduction reaction described above can be appropriately selected according to the pressure and temperature conditions described above. For example, the reaction time for the reduction reaction described above is between 3 hours and 24 hours.

[0055] [Specific Examples of the Reduction Process] The reduction process can be carried out under reduced pressure. Reduced pressure means a pressure of less than 1 atmosphere (101325 Pa). For example, the reduced pressure is preferably 100 Pa or less, and more preferably 20 Pa or less.

[0056] Referring to Figure 4, a specific example of the reduction process carried out under reduced pressure will be described. The stainless steel reaction vessel 10 is a vessel configured to allow the flow of gas between the inside and outside of the vessel. A raw material dish 11 and a reduction dish 12 are housed inside the reaction vessel 10. The raw material dish 11 is placed on legs 11a that rise upright from the bottom of the reaction vessel 10. The raw material dish 11 is a mesh-like dish that allows for airflow. Precursor particles A are placed in the raw material dish 11.

[0057] The reducing dish 12 is located below the raw material dish 11 at the bottom of the reaction vessel 10. Powdered low equilibrium pressure Mg alloy B is placed in the reducing dish 12. Therefore, within the reaction vessel 10, the precursor particles A and the low equilibrium pressure Mg alloy B are arranged in a non-contact state with each other. In addition, a mixture of metallic Mg and a first metal such as Ca, Cu, Sn, Ni, or Zn may be used instead of the low equilibrium pressure Mg alloy B.

[0058] The reaction vessel 10 containing precursor particles A and low equilibrium pressure Mg alloy B is placed in the vacuum furnace 13. The vacuum furnace 13 is then depressurized and heated to a temperature at which Mg vapor is generated from the low equilibrium pressure Mg alloy B. The Mg vapor generated from the low equilibrium pressure Mg alloy B diffuses into the reaction vessel 10. The Mg vapor then comes into contact with the precursor particles A in the raw material dish 11, and the SiO or SiO contained in the precursor particles A is released. ２ The material is reduced to Si and MgO. After a predetermined reaction time has elapsed, the reaction vessel 10 is removed from the vacuum furnace 13, and the reduced particles containing Si and MgO that have been generated in the raw material dish 11 of the reaction vessel 10 are recovered.

[0059] Lowering the pressure in the reduction process makes it easier for Mg vapor to diffuse into the reaction system. This increases the opportunities for contact between precursor particles A and Mg vapor, and as a result, the reduction reaction in the reduction process proceeds more easily. Furthermore, because the reduction reaction proceeds more easily, the temperature required to carry out the reduction process can be lowered.

[0060] Furthermore, the reduction process can also be carried out under atmospheric pressure. By increasing the temperature of the reduction reaction, Mg vapor can be present at a pressure below the equilibrium pressure of reaction equation (1), even under atmospheric pressure. The temperature when the reduction process is carried out under atmospheric pressure is, for example, between 850°C and 900°C.

[0061] When the reduction process is carried out under atmospheric pressure, it is processed at a high temperature, resulting in a smaller specific surface area of ​​the resulting porous silicon particles, as described above. Therefore, when producing porous silicon particles with suppressed oxygen content increases due to surface oxidation, carrying out the reduction process under atmospheric pressure is suitable. Furthermore, when the reduction process is carried out under atmospheric pressure, vacuum equipment for reducing the pressure within the reaction system is unnecessary. This allows for a reduction in the size of the manufacturing equipment. In addition, because vacuum equipment is unnecessary, productivity can be improved by continuously carrying out the carbon film formation process and the reduction process using a continuous firing furnace such as a roller hearth kiln.

[0062] <Cleaning Process (S5)> As shown in Figure 2, the cleaning process is a process to obtain porous silicon particles, in which porous silicon is the primary particle, by removing MgO from the reduced particles. In the cleaning process, the primary particles, which consist of a mixture containing Si and MgO that constitute the reduced particles, are changed into primary particles made of porous silicon having pores by the removal of MgO. Therefore, the cleaning process can also be described as a process to change each of the primary particles constituting the reduced particles from primary particles made of a mixture containing Si and MgO to primary particles made of porous silicon. The porous silicon particles consist of primary particles made of porous silicon and a carbon film covering the surface of the primary particles.

[0063] An example of a washing process includes a first washing process and a second washing process. By going through the first and second washing processes, porous silicon particles are obtained. The first washing process is a process of treating the reduced particles obtained in the reduction process with an acid. In the first washing process, magnesium salts are generated from the MgO contained in the reduced particles, and the generated magnesium salts are washed away. The acid used in the first washing process is not particularly limited and can be any acid capable of generating magnesium salts from MgO. Examples of acids that can be used in the first washing process include hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the acid used in the first washing process can be appropriately selected depending on the type of acid. As an example, the reaction equation when hydrochloric acid is used is shown below.

[0064] MgO+2HCl→MgCl２ +H ２ The second washing step is a step in which any magnesium salts, etc., remaining from the first washing step are washed away using a rinsing solution. An example of a rinsing solution is water or an acidic aqueous solution with a pH of 6 or less. The acid contained in the acidic aqueous solution is preferably an acid that can be removed with an alcohol such as ethanol, for example, an organic acid such as acetic acid. An example of an acidic aqueous solution is a 1% acetic acid aqueous solution. If necessary, a further washing treatment using an alcohol such as ethanol may be performed after the second washing step.

[0065] Another example of a rinsing solution is a polar organic solvent such as ethanol, isopropyl alcohol, or acetone. In this case, the second washing step is performed using only the polar organic solvent, without washing with water or an acidic aqueous solution with a pH of 6 or lower.

[0066] The oxygen content of the resulting porous silicon particles can be controlled by adjusting the processing conditions of the first and second washing steps. For example, if the processing temperature is set to 0°C in either or both of the first and second washing steps, the oxygen content of the resulting porous silicon particles will be lower compared to processing at room temperature. Also, if an acidic aqueous solution is used as the rinsing solution in the second washing step, the oxygen content of the resulting porous silicon particles will be lower compared to using water.

[0067] <Porous Silicon Particles> The porous silicon particles produced by the manufacturing method of this embodiment include primary particles made of porous silicon and a carbon coating covering the surface of the primary particles. In other words, porous silicon particles are secondary particles in which primary particles of porous silicon coated with a carbon coating are aggregated and integrated.

[0068] The silicon content of the porous silicon particles is, for example, 95% by mass or more and 99% by mass or less. Preferably, the silicon content is 96% by mass or more, and more preferably 97% by mass or more. Also, preferably, the silicon content is 98.5% by mass or less, and more preferably 98% by mass or less. The silicon content refers to the mass ratio of silicon atoms contained in the porous silicon particles.

[0069] The carbon content of the porous silicon particles is, for example, 1% by mass or more and 3% by mass or less. Preferably, the carbon content is 1.5% by mass or more, and more preferably 1.8% by mass or more. Also, preferably, the carbon content is 2.5% by mass or less, and more preferably 2.2% by mass or less. The carbon content refers to the mass ratio of carbon atoms contained in the porous silicon particles.

[0070] The average particle size (D50) of the porous silicon particles is, for example, 3 μm or more and 20 μm or less. Preferably, the average particle size (D50) of the porous silicon particles is 5 μm or more, more preferably 8 μm or more, and more preferably 10 μm or more. Also, preferably the average particle size (D50) of the porous silicon particles is 15 μm or less, and more preferably 13 μm or less.

[0071] The average particle diameter (D50) of the primary particles of porous silicon particles is, for example, 0.01 μm or more and 5 μm or less. The average particle diameter (D50) of the primary particles of porous silicon particles is preferably 0.3 μm or more, and more preferably 0.6 μm or more. Furthermore, the average particle diameter (D50) of the primary particles of porous silicon particles is preferably 2.5 μm or less, and more preferably 1.3 μm or less.

[0072] The aspect ratio of the porous silicon particles is, for example, 1.0 to 1.5. Preferably, the aspect ratio of the porous silicon particles is 1.0 to 1.2, and more preferably 1.0 to 1.1. The above aspect ratio is, for example, an average value.

[0073] The primary particles that make up porous silicon particles have pores that are connected in three dimensions. The total pore size of porous silicon particles is, for example, 0.2 cm. ３ / g or more 1.0cm ３ The amount is less than or equal to / g. The total pore size of the porous silicon particles is preferably 0.3 cm². ３ It is 0.4 cm or more per g, and more preferably 0.4 cm ３ The amount is 1 / g or more. Furthermore, the total pore size of the porous silicon particles is preferably 0.8 cm². ３ It is less than or equal to / g, and more preferably 0.6 cm ３It is less than or equal to / g. In this specification, pore size means the number of pores per unit mass calculated based on the BJH (Barret-Joyner-Halenda) method.

[0074] The specific surface area of ​​porous silicon particles is, for example, 100 m². ２ / g or more 250m ２ It is less than / g. The specific surface area of ​​the porous silicon particles is preferably 120 m². ２ It is 150 m or more per gram, and more preferably 150 m ２ It is 1 / g or more. Furthermore, the specific surface area of ​​the porous silicon particles is preferably 220 m². ２ It is less than or equal to / g, and more preferably 200m ２ It is less than or equal to / g. In this specification, the specific surface area of ​​porous silicon particles is N by the BET (Brunauer-Emmett-Teller) method. ２ This refers to the BET surface area per unit mass measured by adsorption.

[0075] Porous silicon particles may contain other components besides silicon and carbon. Examples of other components include oxygen and magnesium compounds. The proportion of other components is, for example, 0% by mass or more and 10% by mass or less, preferably 0% by mass or more and 5% by mass or less.

[0076] The porous silicon particles preferably have an oxygen content of 0% by mass or 8.0% by mass or less. The oxygen content refers to the mass percentage of oxygen atoms contained in the porous silicon particles. In porous silicon particles, oxygen is contained, for example, as a surface oxide formed on the surface of the primary particles of the porous silicon particles.

[0077] <Applications of Porous Silicon Particles> Porous silicon particles can be used as electrode materials for energy storage devices. Examples of electrode materials for energy storage devices include negative electrode materials for energy storage devices. Examples of energy storage devices include secondary batteries such as lithium-ion secondary batteries. Furthermore, energy storage devices may be all-solid-state batteries or semi-solid-state batteries, or electric double-layer capacitors.

[0078] (Function) Next, the function of this embodiment will be described. Firstly, in the manufacturing method of this embodiment, firstly, in the granulation step, silicon oxide powder is used as primary particles and a granulated body is formed in which the surface of the primary particles is covered with a carbon source. Subsequently, in the carbon film formation step, the carbon source of the granulated body is changed into a carbon film, and in the reduction step and the washing step, the primary particles of the granulated body are changed into primary particles of porous silicon, thereby producing porous silicon particles.

[0079] According to the above manufacturing method, porous silicon particles are obtained, which include primary particles made of porous silicon and a carbon film covering the surface of the primary particles. In the porous silicon particles, the carbon film is located between the primary particles. As a result, conductive paths are formed within the porous silicon particles via the carbon film. Furthermore, in the porous silicon particles, the carbon film is also located on the surface of the particles, i.e., on the surface of the primary particles located on the surface of the particles. As a result, conductive paths can also be formed between the porous silicon particles via the carbon film. Therefore, in electrodes using porous silicon particles, conductive paths via the carbon film are densely formed both within and between the porous silicon particles. Consequently, by using porous silicon particles as an electrode material, the conductivity of the electrode can be improved.

[0080] Secondly, in the manufacturing method of this embodiment, as described above, after forming a carbon film in the carbon film formation step, the primary particles are made porous in the reduction step and the washing step. In this case, no carbon film is formed in the pores of the porous primary particles. Therefore, it is possible to suppress the reduction in pore size or collapse of pores caused by the formation of a carbon film in the pores. Consequently, porous silicon particles can more reliably obtain the effect of suppressing expansion and contraction during charging and discharging based on the pores of the primary particles made of porous silicon. Therefore, by using porous silicon particles as an electrode material, the volume change of the electrode during charging and discharging can be reduced.

[0081] In addition, making the primary particles porous after forming the carbon film also contributes to reducing the oxygen content of the porous silicon particles. In other words, since the primary particles are not porous during firing to form the carbon film, the heat applied during firing does not cause surface oxidation inside the pores of the primary particles, as would occur when the carbon film is formed on the surface of the primary particles after they have been porous. This suppresses the increase in oxygen content caused by surface oxidation inside the pores.

[0082] Thirdly, in the granulation process, a precursor slurry is spray-dried to form a granule containing primary particles as secondary particles. Then, while maintaining the shape of the secondary particles formed in the granulation process, porous silicon particles are produced by performing a carbon film formation process, a reduction process, and a washing process. Since the granule obtained by spray drying is close to a perfect sphere, the porous silicon particles produced are also close to a perfect sphere, that is, have a small aspect ratio. By using porous silicon particles with a small aspect ratio as an electrode material, the ionic conductivity of the electrode can be improved.

[0083] Figures 5 and 6 are schematic diagrams of the negative electrode 20 of the energy storage device. The negative electrode 20 comprises a negative electrode current collector 21 and a negative electrode active material layer 22 formed on one side surface of the negative electrode current collector 21. The negative electrode active material layer 22 is formed by mixing porous silicon particles P as the negative electrode active material, other components constituting the negative electrode active material layer 22, and a dispersion medium to create a slurry-like negative electrode mixture, which is then applied to the surface of the negative electrode current collector 21 and dried.

[0084] As shown in the enlarged view of Figure 5, when the porous silicon particles P have a large aspect ratio, the porous silicon particles P are stacked on the negative electrode current collector 21 with their major axis oriented in line with the plane direction of the negative electrode current collector 21. In this case, the distance required for charge carriers L, such as lithium ions, to travel around the porous silicon particles P when they move in the thickness direction of the negative electrode active material layer 22 becomes large. In contrast, as shown in the enlarged view of Figure 6, when the porous silicon particles P have an aspect ratio close to 1, the distance required for charge carriers L to travel around the porous silicon particles P can be shortened. By shortening the conduction path of the charge carrier in the thickness direction of the negative electrode active material layer 22, the ionic conductivity of the negative electrode active material layer 22 is improved.

[0085] (Effects) Next, the effects of this embodiment will be described. (1) The method for producing porous silicon includes a granulation step, a carbon film formation step, a reduction step, and a washing step. The granulation step is a step to obtain a granulated body in which primary particles containing silicon oxide powder are aggregated. The carbon film formation step is a step to obtain precursor particles in which silicon oxide powder is used as the primary particle and the surface is covered with a carbon film by forming a carbon film on the surface of the granulated body. The reduction step is a step to obtain reduced particles in which a mixture containing Si and MgO is used as the primary particle and the surface is covered with a carbon film by contacting the precursor particles with Mg vapor. The washing step is a step to obtain porous silicon particles in which porous silicon having pores and having a carbon film on the surface is used as the primary particle by removing MgO from the reduced particles.

[0086] By using porous silicon particles obtained with the above configuration, the conductivity of the electrode can be improved, and the volume change during charging and discharging of the electrode can be reduced. Therefore, with the above configuration, porous silicon suitable for electrode materials can be manufactured.

[0087] (2) The granulation process is a process in which a precursor slurry containing silicon dioxide powder, a carbon source, and a dispersion medium is spray-dried to obtain a granulated body in which the silicon dioxide powder is used as primary particles and the surface of the primary particles is covered with the carbon source. The carbon film formation process is a process in which a carbon film derived from the carbon source is formed on the surface of the primary particles of the granulated body by firing the granulated body.

[0088] According to the above configuration, in the granulation process, a granulated material is obtained in which the carbon source is located between the primary particles. This allows precursor particles to be obtained in the subsequent carbon coating formation process in which the carbon coating is located between the primary particles. As a result, porous silicon particles with the carbon coating located between the primary particles are obtained.

[0089] (3) A method for producing porous silicon includes a slurry preparation step for preparing a precursor slurry to be used in the granulation step. The slurry preparation step includes a grinding step to obtain a mixture containing powder and a dispersion medium by adding a dispersion medium to mother particles of silicon dioxide and wet grinding.

[0090] By using the porous silicon particles obtained by the above configuration, the ionic conductivity of the electrode can be improved. Therefore, according to the above configuration, porous silicon suitable for electrode materials can be manufactured. Furthermore, in the above configuration, the pulverization process is carried out using the same dispersion medium as that contained in the precursor slurry. In this case, when preparing the precursor slurry using the powder obtained by the pulverization process, there is no need to replace the dispersion medium. Therefore, the process of preparing the precursor slurry can be simplified.

[0091] (Example of modification) This embodiment can be implemented with the following modifications. This embodiment and the following examples of modifications can be combined with each other to the extent that they do not contradict each other technically.

[0092] ○In the grinding process, the dispersion medium used for wet grinding may be different from the dispersion medium used for the precursor slurry. ○The method for grinding the silicon dioxide mother particles in the grinding process is not limited to wet grinding, but may also be dry grinding.

[0093] Regarding the carbon coating process, the method for obtaining precursor particles whose surface is covered with a carbon coating is not limited to the method of forming a granule having a carbon source coating on the surface of primary particles using the precursor slurry containing the carbon source described above, followed by calcination. For example, a method may be used in which a granule without a carbon source coating is formed using a precursor slurry that does not contain a carbon source, and then a carbon coating is formed on the surface of the granule by CVD using hydrocarbon gas. Furthermore, two or more different coating formation methods may be used in appropriate combination.

[0094] ○In the carbon film formation process, the carbon film only needs to be formed on the surface of the granules. When forming granules with a carbon source film using a precursor slurry containing a carbon source, the carbon source film is formed on the entire surface of the primary particles constituting the granules. Therefore, the carbon source film is formed not only on the surface of the granules but also between the primary particles inside the granules. As a result, the carbon film formed from the carbon source film by firing is also formed not only on the surface of the granules but also between the primary particles inside the granules. On the other hand, when forming a carbon film on the surface of a granule by CVD, the carbon film is formed on the surface of the granules among the primary particles constituting the granules, that is, on the parts that form the surface of the secondary particles. And, the carbon film is hardly formed between the primary particles inside the granules. In this case, porous silicon particles with almost no carbon film between the primary particles inside the granules are produced.

[0095] ○After the granulation process, a fine powder removal process may be added in which the granulated material is classified and fine powder particles with a particle size of less than or equal to a predetermined size are removed. Fine powder particles of less than or equal to a predetermined size are, for example, fine powder particles of 1 μm or less.

[0096] A...Precursor particles B...Low equilibrium pressure Mg alloy 10...Reaction vessel 11...Raw material dish 11a...Legs 12...Reduction dish 13...Vacuum furnace

Claims

1. A method for producing porous silicon used as an electrode material, comprising: a granulation step of obtaining a granulated body in which primary particles containing silicon oxide powder are aggregated; a carbon film formation step of forming a carbon film on the surface of the granulated body to obtain precursor particles in which the powder is used as primary particles and the surface is covered with the carbon film; a reduction step of contacting the precursor particles with Mg vapor to obtain reduced particles in which a mixture containing Si and MgO is used as primary particles and the surface is covered with the carbon film; and a washing step of removing MgO from the reduced particles to obtain porous silicon particles in which porous silicon having pores is used as primary particles and the surface is covered with the carbon film.

2. The method for producing porous silicon according to claim 1, wherein the granulation step is a step of obtaining a granulated body in which the powder is made into primary particles and the surface of the primary particles is covered with the carbon source by spray drying a precursor slurry containing silicon oxide powder, a carbon source, and a dispersion medium, and the carbon film formation step is a step of forming a carbon film derived from the carbon source on the surface of the primary particles of the granulated body by firing the granulated body.

3. The granulation step is a step of obtaining a granulated body in which the powder is a primary particle by spray-drying a precursor slurry containing silicon oxide powder and a dispersion medium, and includes a slurry preparation step of preparing the precursor slurry to be used in the granulation step, and the slurry preparation step includes a grinding step of obtaining a mixture containing the powder and the dispersion medium by wet grinding silicon oxide mother particles.

4. The method for producing porous silicon according to claim 3, wherein the slurry preparation step further includes a mixing step of adding the carbon source to the mixture to obtain the precursor slurry.

5. The method for producing porous silicon according to any one of claims 1 to 4, wherein the average particle size (D50) of the powder is 0.3 μm or more and 2.5 μm or less.

6. The method for producing porous silicon according to any one of claims 1 to 4, wherein the average particle size (D50) of the granulated material is 3 μm or more and 20 μm or less.

7. The method for producing porous silicon according to any one of claims 1 to 5, wherein the aspect ratio of the granulated material is closer to 1 than the aspect ratio of the powder.

8. The method for producing porous silicon according to any one of claims 1 to 7, wherein the aspect ratio of the granulated material is 1.0 or more and 1.5 or less.

9. The method for producing porous silicon according to any one of claims 1 to 8, wherein in the reduction step, Mg vapor is brought into contact with the precursor particles under atmospheric pressure.

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