Method for producing nanosilicon, negative electrode active material for lithium-ion battery, negative electrode for lithium-ion battery, and lithium-ion battery

Abstract

A method for producing nanosilicon including (a) a step for subjecting an aluminosilicate having an Al2O3 content of 3-40 mass% to reduction treatment within a range where the ratio of the number of aluminum atoms included in the aluminosilicate and the number of magnesium atoms used as a reducing agent in the reduction treatment step is 1:3.5-1:65, and (b) a step for subjecting the reduced aluminosilicate obtained in step (a) to acid treatment.

Description

[Correction based on Rule 91 07.10.2022] Method for producing nanosilicon, anode active material for lithium ion battery, anode for lithium ion battery, and lithium ion battery

[0001] The present invention relates to a method for producing nanosilicon, specifically, a method for efficiently producing purified nanosilicon by leaving a certain amount of alumina (AlO) in an aluminosilicate mineral, adjusting the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step within an appropriate range, and reducing the alumina (AlO) in an aluminosilicate mineral to a certain amount of aluminum atoms.

[0002] There are various methods for producing nanosilicon, but one known method uses halloysite (alumina mineral), a nanotube mineral, as a raw material: A specific size of halloysite is subjected to a thermal acid treatment to extract alumina, and the resulting nanosilica is mixed with NaCl or other alkali metal chloride and / or alkaline earth metal chloride as a heat absorber and magnesium powder as a reducing agent, and the mixture is reduced by heating to 600-1000°C at a specific heating rate in an argon atmosphere, followed by treatment with dilute acid and then hydrofluoric acid to obtain nanosilicon (Patent Document 1). However, this method leaves unreduced nanosilica behind, and some of the nanosilicon obtained by the reduction reaction is re-oxidized to silica during subsequent water washing or acid treatment, necessitating a final process of hydrofluoric acid treatment to extract the silica.

[0003] Chinese Patent Publication No. 105905908

[0004] However, the inventors' experiments have shown that the alumina content after thermal acid treatment using the above-mentioned existing technology is 0% by mass, and even when the alumina content is 2% by mass, the efficiency of reducing SiO to Si is low and a large amount of amorphous SiO is present. In light of this, it is presumed that a large amount of amorphous SiO is also present in the silicon obtained using the above-mentioned existing technology. Furthermore, the need for hydrofluoric acid treatment has significant disadvantages in terms of safety and cost. Therefore, the problem to be solved by the present invention is to provide a method for producing high-purity nanosilicon that does not require a hydrofluoric acid treatment step by suppressing the generation of amorphous silica and leaving alumina, thereby suppressing the generation of by-products such as spinel, which is a compound of aluminum or magnesium that may be synthesized during the reduction reaction; a negative electrode active material containing nanosilicon produced by the method; a lithium-ion battery negative electrode containing the negative electrode active material; and a lithium-ion battery containing the negative electrode.

[0005] Through repeated experiments under various conditions, the inventors discovered that intentionally leaving some or all of the Al2O3 behind, rather than extracting all of the Al2O3 from the raw aluminosilicate mineral to produce nanosilicon, makes the reduction process more efficient and enables stable purification of nanosilicon. Furthermore, by adjusting the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step within an appropriate range, the generation of impurities such as aluminum and magnesium compounds can be suppressed, leading to the completion of the present invention. Specifically, this application provides the following inventions: 1. A method for producing nanosilicon, comprising: (a) a step of reducing an aluminosilicate having an Al2O3 content of 3 to 40 mass% in such a way that the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step is in the range of 1:3.5 to 1:65; and (b) a step of acid-treating the reduced aluminosilicate obtained in step (a). 2. The production method according to 1 above, wherein the ratio of the numbers of atoms is 1:3.7 to 1:45. 3. The production method according to 1 above, wherein the ratio of the numbers of atoms is 1:3.7 to 1:30. 4. The production method according to any one of 1 to 3 above, wherein the aluminosilicate used in step (a) is halloysite or is obtained by dealumination of halloysite. 5. The production method according to any one of 1 to 4 above, wherein the aluminosilicate used in step (a) has an Al2O3 content adjusted to 3 to 40 mass% by dealumination. 6. The production method according to 5 above, wherein the dealumination comprises an acid treatment selected from the group consisting of hot sulfuric acid treatment, sulfuric acid treatment, hydrochloric acid treatment, hot hydrochloric acid treatment, nitric acid treatment, hot nitric acid treatment, and combinations thereof. 7. The production method according to any one of 1 to 6 above, wherein the reduced aluminosilicate obtained in step (a) contains 6 to 39 mass% Al2O3. 8. 8. The production method according to any one of 1 to 7 above, wherein the reduction treatment in step (a) comprises mixing magnesium powder as a reducing agent, one or more heat-absorbing agents selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, and the aluminosilicate, and heating and reducing the resulting mixture in an argon gas or nitrogen gas atmosphere.9. The production method according to any one of 1 to 8 above, wherein the acid treatment in step (b) uses at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. 10. The production method according to any one of 1 to 9 above, wherein the acid used in step (b) is hydrochloric acid and has a concentration of 0.5 to 2.0 mol / L. 11. A method for producing nanosilicon, comprising: treating an aluminosilicate with hot sulfuric acid to obtain an aluminosilicate having an Al2O3 content of 3 to 40 mass%, and reducing the aluminosilicate; and treating the reduced aluminosilicate obtained in the above step with acid. 12. A negative electrode active material for a lithium ion battery, comprising nanosilicon obtained by the production method according to any one of 1 to 11 above. 13. 13. A negative electrode active material for a lithium ion battery, comprising nanosilicon that is free of an amorphous SiO2 halo and free of a spinel peak as confirmed by X-ray diffraction analysis, and has a primary particle size of 10 to 15 nm as measured by a field emission scanning electron microscope. 14. A negative electrode for a lithium ion battery, comprising the negative electrode active material as described in 12 or 13. 15. A lithium ion battery, comprising the negative electrode for a lithium ion battery as described in 14.

[0006] Without being bound by any theory, the present invention utilizes two routes: a conventional reducing agent such as Mg is brought into contact with not only the SiO present in the aluminosilicate but also the intentionally left-in alumina, reducing the alumina to Al, and then using the resulting aluminum to reduce silica. This route (i.e., a route using two reducing agents: an externally added reducing agent such as Mg, and a new reducing agent, Al, produced by reduction with the reducing agent) improves the efficiency of the reduction process and enables stable purification of silicon without the Si produced by reduction being re-oxidized to SiO. Furthermore, if there is an excess of alumina relative to the magnesium reducing agent, impurities such as spinel, a compound containing aluminum and magnesium that cannot be removed with acid, are generated. The presence of impurities such as spinel cannot be removed by subsequent acid treatment and remain as impurities, reducing the purity of the nanosilicon. By appropriately adjusting the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment process, efficient reduction can be achieved, and the formation of impurities such as spinel, which are formed by the reaction of aluminum and magnesium and cannot be removed by acid treatment, can be suppressed. According to the present invention, by reducing aluminosilicate with aluminum and magnesium at an appropriate ratio, the efficiency of reducing SiO2 to Si is increased, the formation of aluminum-magnesium compounds such as amorphous silica and spinel is suppressed, and the nanosilicon obtained by reduction can be prevented from being re-oxidized, which is thought to increase the yield of nanosilicon. According to the manufacturing method of the present invention, nanosilicon can be obtained without a hydrofluoric acid treatment process. The obtained nanosilicon is highly pure and has a small particle size, so it can be used as a negative electrode material for lithium-ion batteries with excellent cycle characteristics, or as a lithium-ion battery containing the same.

[0007] 1 shows X-ray diffraction patterns of the solids obtained in Examples 1 to 3. 2 shows X-ray diffraction patterns of the solids obtained in Comparative Examples 1 to 3. 3 shows a photograph of the solid obtained in Example 1 observed with a field emission scanning electron microscope.

[0008] <Step (a): Reduction Treatment of Aluminosilicate Containing 3 to 40 Mass % of Al2O3> Aluminosilicate is a salt formed by substituting a portion of Si in silicate with Al, and xM I 2O·yAl2O3·zSiO2·nH2O (where the metal is M II (Some examples include n = 0, where n is 0). Aluminosilicates exist in large quantities in nature as minerals such as micas, feldspars, zeolites, and clays. Among these, halloysite, a clay mineral, is characterized by its non-toxicity and high safety. The aluminosilicate raw material used in the present invention is not particularly limited, but halloysite, which has a small primary particle size and a high specific surface area in its natural state, is preferred. The high specific surface area allows for effective reduction treatment. Aluminosilicate structures vary, including chain, layer, network, and cylindrical. In the present invention, however, it is preferable to use aluminosilicates powdered using a jet mill or roller mill, etc., from the viewpoint of ease of reduction reaction. The average particle size of the aluminosilicate powder is not particularly limited, but from the viewpoints of ease of reduction treatment and cost, a particle size of 1 to 20 μm is preferred, and a particle size of 2 to 5 μm is more preferred. The average particle size of the aluminosilicate is the particle size of the aggregated particles, and can be measured under dry powder conditions using a laser diffraction particle size distribution measuring device (e.g., Mastersizer 3000, manufactured by Malvern Instruments) using a dry disperser (e.g., Aero S, manufactured by Malvern Instruments).

[0009] The raw aluminosilicate of the present invention has an Al2O3 content of 3 to 40 mass%. Using an aluminosilicate containing Al2O3 in this range increases the efficiency of the subsequent reduction and reduces residual SiO2. This also prevents oxidation of Si in the subsequent acid treatment step (b). The alumina content is preferably 5 to 40 mass%, more preferably 6 to 39 mass%, and even more preferably 6 to 20 mass%. It is said that the Al2O3 content originally contained in halloysite is approximately 35 to 40 mass%, regardless of the source of the halloysite. Therefore, when using halloysite as the raw aluminosilicate, untreated halloysite can be used as is, or the Al2O3 content can be adjusted by appropriate means to fall within the preferred range.

[0010] The Al2O3 content in aluminosilicate can be adjusted by dealumination, specifically hot sulfuric acid treatment. Specifically, the amount of Al2O3 can be controlled by adjusting the sulfuric acid concentration, the mass ratio of sulfuric acid to aluminosilicate, the reaction temperature, the reaction time, and other factors. For example, treating 100 g of halloysite with 1,200 g of 25% sulfuric acid at 90°C for 4 hours yields halloysite with an Al2O3 content of 18%. Under the same conditions, the Al2O3 content can be reduced to 15% by treating for 5 hours, 6% by treating for 7 hours, and 2% by treating for 14 hours. After the hot sulfuric acid treatment, it is typically preferable to wash the material with water according to a conventional method. Following the disclosure of Example 1 in Chinese Patent Publication No. 105905908, when 5 g of halloysite was treated with 500 g of 2 mol / L (17.2% by mass) sulfuric acid at 100°C for 10 hours, the Al2O3 content was reduced to 0%. The dealumination treatment can be carried out not only by hot sulfuric acid treatment but also by sulfuric acid treatment, hydrochloric acid treatment, hot hydrochloric acid treatment, nitric acid treatment, and hot nitric acid treatment.

[0011] In the reduction treatment, magnesium can be used as a reducing agent. It is generally assumed that all of the aluminosilicate raw material except for Al2O3 is SiO2, and it is preferable to use magnesium powder in a molar ratio of 1 to 3 relative to that amount. Furthermore, the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step is preferably 1:3.5 to 1:65, more preferably 1:3.7 to 1:45, and even more preferably 1:3.7 to 1:30. If the number of magnesium atoms used as a reducing agent in the reduction treatment step is too low relative to the number of aluminum atoms contained in the aluminosilicate, compounds of aluminum and magnesium, such as spinel, that cannot be removed with acid will be formed, while if the ratio is too high, amorphous silica will be formed.

[0012] Alkali metal chlorides and alkaline earth metal chlorides act as heat absorbers. That is, the reaction between SiO2 in the aluminosilicate and a reducing agent, such as Mg, is an exothermic reaction, and the Si obtained after reduction melts and agglomerates due to the heat of reaction. In the present invention, by performing the reduction treatment in the presence of an alkali metal chloride or an alkaline earth metal chloride, the increase in reaction temperature can be suppressed, preventing the reaction temperature from exceeding the melting point of silicon, and as a result, agglomeration of the produced silicon can be suppressed. Examples of alkali metal chlorides include NaCl and KCl. From the viewpoints of availability and cost, NaCl is preferred. Examples of alkaline earth metal chlorides include CaCl2 and MgCl2. From the viewpoints of availability and cost, CaCl2 is preferred.

[0013] As the endothermic agent, alkali metal chlorides are preferred from the viewpoints of availability and cost, with NaCl being particularly preferred. To effectively suppress aggregation, it is advisable to use an amount of endothermic agent sufficient to prevent the endothermic agent from reaching the melting point of silicon due to the heat of reaction. However, if the amount is too large, the reduction reaction will be difficult to proceed, so it is preferable to use an appropriate amount. For example, it is desirable to use an endothermic agent with a mass ratio of 1 or more, preferably 9 or more, relative to the raw aluminosilicate. Since increasing the amount makes the reduction reaction difficult to proceed, the upper limit is preferably a mass ratio of 12 or less. Using magnesium powder or aluminum powder in a molar ratio of 1 to 3 relative to the SiO2 present in the raw aluminosilicate and NaCl in a mass ratio of 8 to 12 relative to the aluminosilicate is preferred in terms of the effectiveness and cost of the reduction treatment.

[0014] The reduction treatment can be carried out by heating the aluminosilicate, with a specific amount of residual alumina, in the presence of the reducing agent and the endothermic agent under an argon or nitrogen gas atmosphere. Those skilled in the art can appropriately determine the reduction conditions. The heating is carried out, for example, at a temperature ranging from 500 to 1000°C, preferably from 500 to 800°C. The heating time is, for example, from 1 to 24 hours, preferably from 2 to 6 hours. From the perspective of the effectiveness and cost of the reduction treatment, heating at 500 to 800°C under an argon gas atmosphere for a heating time of 6 hours or less, for example, approximately 3 hours, is preferred. As described above, the present invention can be considered a method for extracting silicon by reducing an aluminosilicate mineral, and can also be referred to as a method for producing purified or smelted nanosilicon. After reduction, it is typically preferable to wash the aluminosilicate with water by a conventional method to remove the endothermic agent. After washing with water, the washing water can be replaced with ethanol, and then heated to remove the ethanol. This allows for more thorough water removal.

[0015] <Step (b): Acid Treatment of the Reduction-Treated Solid Material> This step removes by-products of the reduction reaction, such as alumina, magnesia, and their reaction products. The acid used in this step is not particularly limited as long as it can achieve the above-mentioned objective. For example, at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid can be used. Among these, hydrochloric acid is preferred because it is relatively safe and the neutralized salt can be easily removed by washing with water. From the standpoints of effectiveness and safety, the acid concentration is preferably 0.3 to 8 mol / L, more preferably 0.5 to 2.0 mol / L. In particular, from the standpoints of sufficient reaction progress and higher safety, it is preferable to use hydrochloric acid at a concentration of 0.5 to 2.0 mol / L. The amount of acid is not particularly limited, and it is best to use an amount that can sufficiently dissolve by-products such as alumina and magnesia from the reduction step. After the acid treatment, it is typically preferable to wash with water using a conventional method. After washing with water, the washing water may be replaced with ethanol, or the ethanol may be removed by heating. This allows for more thorough water removal.

[0016] [Anode Material for Lithium-Ion Batteries and Lithium-Ion Batteries] The nanosilicon of the present invention can be suitably used as an anode active material for lithium-ion batteries, and can be suitably used in lithium-ion batteries containing the anode material. A lithium-ion battery is configured with a positive electrode in which a cathode active material is laminated on a cathode current collector, and a negative electrode in which a cathode active material layer is laminated on a negative current collector, with a secondary battery separator and an electrolyte interposed between them. The anode for a lithium-ion battery of the present invention preferably comprises an anode active material layer containing an anode active material, an electrolyte, and a solvent, and an anode current collector. The anode active material layer can consist solely of the anode active material of the present invention, or can consist of a combination of the anode active material of the present invention and a known anode active material. In some cases, it may further contain known materials such as a binder, a conductive material, and an electrolyte. Known anode active materials that can be used in combination with the anode active material of the present invention include carbon-based materials such as graphite, hard carbon, and soft carbon. When a known negative electrode active material is used in combination, for example, at a mass ratio of the negative electrode active material of the present invention to the known negative electrode active material of 1:2 to 1:30, if the ratio of the negative electrode active material of the present invention is too low, the increase in negative electrode capacity will be small, while if the ratio is too high, repeated charge / discharge cycles will result in a decrease in the negative electrode capacity (deterioration in cycle characteristics). Furthermore, if the negative electrode active material of the present invention is carbon-coated using sucrose or other raw materials, the capacity of the negative electrode active material of the present invention can be efficiently utilized. In this case, the mass ratio represents the ratio of the total mass of the net mass of the negative electrode active material of the present invention and the carbon coating to the mass of the known negative electrode active material. Examples of binders include known solvent-dried binders for lithium-ion batteries, such as carboxymethyl cellulose, styrene butadiene rubber latex, starch, polyvinylidene fluoride, polyvinyl alcohol, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, and polypropylene. When a binder is used, it is preferably used in an amount of 1 / 100 to 1 / 5 of the total amount of the negative electrode active material. Examples of conductive materials include acetylene black, graphite, ketjen black, and carbon black. When a conductive material is used, it is preferably used in an amount of 1 / 100 to 1 / 3 of the total amount of the negative electrode active material.Examples of electrolytes include lithium salts of inorganic anions such as LiPF, LiBF, LiSbF, LiAsF, LiClO, and LiN(FSO), and lithium salts of organic anions such as LiN(CFSO), LiN(CFSO), and LiC(CFSO). Examples of solvents include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, acetonitrile, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and dipropyl carbonate. The solvents may be used alone or in combination of two or more. Examples of additives to the solvent include vinylene carbonate, fluorovinylene carbonate, methylvinylene carbonate, fluoromethylvinylene carbonate, ethylvinylene carbonate, propylvinylene carbonate, butylvinylene carbonate, dimethylvinylene carbonate, diethylvinylene carbonate, dipropylvinylene carbonate, vinylene acetate, vinylene butyrate, vinylene hexanate, vinylene crotonate, catechol carbonate, propane sultone, butane sultone, etc. The additives may be used alone or in combination of two or more.

[0017] Nanosilicon obtained by the manufacturing method of the present invention can be used for a variety of purposes, but is preferably used as an anode material for lithium-ion batteries. The nanosilicon obtained by the manufacturing method of the present invention typically has a small primary particle diameter of approximately 10-15 nm and uniform particle size, as shown in Figure 3. The nanosilicon obtained by the manufacturing method of the present invention also does not exhibit an amorphous SiO halo or spinel peaks as confirmed by X-ray diffraction analysis. In other words, the present invention produces high-purity nanosilicon. When using silicon as an anode material for lithium-ion batteries, a problem arises: pulverization due to volume changes caused by the inflow and outflow of lithium ions, resulting in a decrease in capacity. However, small silicon particles prevent pulverization due to volume changes, allowing for the production of a lithium-ion battery anode with excellent cycle characteristics. Uniform particle size increases the voids between particles, resulting in superior cycle characteristics. In this specification, "%" represents "mass %" unless otherwise specified.

[0018] <Pretreatment 1 - Hot Sulfuric Acid Treatment> 1200 g of a 25% aqueous sulfuric acid solution was heated to 90°C, and 100 g of dry-milled halloysite powder (trade name DRAGONITE-HP, manufactured by Applied Minerals, average particle size 12 μm) was added while stirring. The mixture was covered with a lid to prevent the water from evaporating and the sulfuric acid concentration from changing significantly, and the temperature was maintained at 90°C while stirring, allowing the reaction to proceed for 5 hours. The remaining solid matter was then collected on filter paper by suction filtration. Further distilled water was added to the collected solid matter, and suction filtration was continued. The solid matter was washed with water until the ionic conductivity of the filtrate reached 30 μS / cm or less. This removed sulfuric acid and water-soluble reaction products from the solid matter. The solid matter was dried at 120°C for 24 hours and then pulverized using an airflow pulverizer (SJ-100CB, manufactured by Nisshin Engineering Inc.) to obtain a white aluminosilicate powder with an average particle size of approximately 3 μm. This aluminosilicate was measured using a fluorescent X-ray analyzer (ZSX Primus II manufactured by Rigaku Corporation) by a glass bead calibration curve method in accordance with JIS R 2216: Method for fluorescent X-ray analysis of refractory products, and it was confirmed that it contained 15% Al2O3.

[0019] <Pretreatment 2 - Hot sulfuric acid treatment> A white powder of aluminosilicate was obtained by the same treatment as pretreatment 1, except that the reaction time was 14 hours. This aluminosilicate was measured in the same manner as pretreatment 1, and it was confirmed that it contained 2% Al2O3.

[0020] <Pretreatment 3 - Hot sulfuric acid treatment> A white powder of aluminosilicate was obtained by the same treatment as pretreatment 1, except that the reaction time was 7 hours. This aluminosilicate was measured in the same manner as pretreatment 1, and it was confirmed that it contained 6% Al2O3.

[0021] <Pretreatment 4 - No hot sulfuric acid treatment> Halloysite powder was dry-milled without reacting it with hot sulfuric acid (product name DRAGONITE-HP, manufactured by Applied Minerals, average particle size 12 μm) and then pulverized in an airflow pulverizer (SJ-100CB manufactured by Nisshin Engineering Inc.) to obtain a white powder of aluminosilicate with an average particle size of approximately 3 μm. This aluminosilicate was measured in the same manner as in pretreatment 1 and was confirmed to contain 39% AlO.

[0022] Example 1 Mixing with NaCl and Drying 4.88 g of the aluminosilicate prepared in Pretreatment 1 was mixed with 41.48 g of NaCl and 137 mL of distilled water, and the mixture was stirred at room temperature for 1 hour. Thereafter, the solvent was evaporated under reduced pressure using a rotary evaporator, and the mixture was further dried at 120°C for 12 hours.

[0023] (Mixing with Mg) The obtained dried material was pulverized in an agate mortar to form a powder. 8.94 g of the obtained powder was taken out, and 0.72 g of Mg powder (SiO:Mg ≈ 1:2.2 (molar ratio)) was added as a reducing agent (calculated as SiO except for AlO) to the powder and mixed under an Ar gas atmosphere. At this time, the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:10.7.

[0024] (Reduction Reaction) The entire mixture obtained was placed in an alumina crucible and placed in an electric furnace. After that, the pressure inside the electric furnace was reduced, and Ar gas was injected three times ((reduced pressure → Ar gas injection) × 3) to replace the atmosphere inside the electric furnace with Ar gas. With Ar gas flowing continuously inside the electric furnace, the electric furnace was heated to 580°C at a heating rate of 10°C / min, and then heated to 600°C at a heating rate of 1°C / min. After maintaining the Ar gas flow at 600°C for 3 hours, the temperature was lowered to 40°C. Then, the Ar gas inside the electric furnace was gradually replaced with air, and the alumina crucible was removed.

[0025] (Water washing) A large amount of water was added to the solid material removed from the alumina crucible, and the resulting solution was subjected to ultrasonic waves for 10 minutes, followed by centrifugation and removal of the supernatant. This procedure (water addition → ultrasonic waves → centrifugation) was repeated three times to remove NaCl. The solid material was then collected on filter paper by suction filtration, and ethanol was added over the solid material. The water present in the solid material was then replaced with ethanol by suction filtration. The solid material on the filter paper was then heated under reduced pressure at 60°C for 3 hours to remove the ethanol, yielding a brown solid material.

[0026] (HCl Treatment) The obtained brown solid was gradually added to 100 mL of 1 M hydrochloric acid solution. After the entire amount was added, the mixture was allowed to stand for 4 hours.

[0027] (Water washing) A large amount of water was then added, and after centrifugation, the supernatant was removed. This operation (water addition → ultrasonic → centrifugation) was repeated three times, and the solid matter was collected on filter paper by suction filtration. Ethanol was then added to the collected solid matter, and suction filtration was performed to replace the water present in the solid matter with ethanol. The solid matter on the filter paper was then heated at 60°C under reduced pressure for 3 hours to remove the ethanol, and a brown solid matter was obtained.

[0028] X-ray diffraction analysis of the resulting brown solid revealed a clear silicon peak (Figure 1). The halo at 2θ = 20-25°, which typically appears when amorphous SiO2 is present, was not observed. A slight quartz peak, likely derived from natural aluminosilicate, was also observed. A slight peak from the aluminum sample holder was also observed. Observation with a field-emission scanning electron microscope (JEOL JSM-6700F) revealed that the primary silicon particles had a diameter of approximately 10-15 nm (Figure 3). SEM-EDX analysis of Si, Al, and O (SEM: Hitachi High-Technologies SU3500 scanning electron microscope, EDX: Horiba, Ltd. EMAXEvolution EX-370 X-MAX20 energy dispersive X-ray analyzer) determined the atomic concentration (%) of residual oxygen, which was 21.49%. The results are shown in Table 1.

[0029] Example 2 A brown solid was obtained in the same manner as in Example 1, except that the starting material was the aluminosilicate obtained in Pretreatment 3, the amount of NaCl was 46.07 g, the amount of distilled water was 148 mL, and the amount of powder mixed with NaCl, dried, and then pulverized in the Mg mixing step was 8.85 g (the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:29.7). X-ray diffraction analysis of the obtained brown solid revealed a clear silicon peak (Figure 1). The halo at 2θ = 20-25° that typically appears when amorphous SiO2 is present was not observed. A slight quartz peak, likely derived from natural aluminosilicate, was also observed. A slight peak from the aluminum sample holder was also confirmed.

[0030] Example 3: A brown solid was obtained in the same manner as in Example 1, except that the starting material was the aluminosilicate obtained in Pretreatment 4, 29.96 g of NaCl, 103 mL of distilled water, 9.30 g of powder mixed with NaCl, dried, and then pulverized in the Mg mixing step, and 0.94 g of Mg powder [SiO:Mg ≈ 1:2.90 (molar ratio)] (all calculations were based on SiO except for AlO) (the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:3.9), and the amount of 1 M hydrochloric acid solution used in the HCl treatment step was 160 mL. X-ray diffraction analysis of the obtained brown solid revealed a clear silicon peak (Figure 1). The halo at 2θ = 20-25°, which typically appears when amorphous SiO is present, was not observed. A slight quartz peak, likely derived from natural aluminosilicate, is observed, as well as a slight peak from the aluminum sample holder.

[0031] Example 4: 10.4 g of the brown solid (nano silicon) obtained in Example 1, 5.2 g of sucrose, 59.1 g of methanol, and 25.3 g of distilled water were crushed in a mortar to prepare a suspension. The suspension was dried at an inlet temperature of 100°C using a micromist spray dryer (manufactured by GF Corporation) to obtain a powder. The powder was dried under reduced pressure at 60°C for 2 hours and then heated at 800°C in an Ar atmosphere for 2 hours to carbon-coat the brown solid (nano silicon). 0.85 g of the carbon-coated brown solid (nano silicon) was mixed with 7.65 g of graphite, resulting in a mass ratio of silicon:graphite of 1:9 for the active material of the negative electrode material. The resulting active material mixture was mixed with 1.00 g of acetylene black as a conductive material, 20 g of an aqueous solution of carboxymethyl cellulose (1% mass percent concentration) as a binder, 0.74 g of styrene-butadiene rubber latex (40.4 mass percent concentration), and 1.20 g of distilled water while stirring to obtain a suspension with a mass percent concentration of 31.8%. The suspension was applied to Cu foil, dried at 120 ° C for 10 hours using a vacuum dryer, and punched out to a diameter of 17 mm. A basic evaluation cell (half cell) was created using this as the working electrode (negative electrode) under the conditions shown in Table 1, and the single-electrode characteristics were measured under the charge / discharge conditions shown in Table 1. The single-electrode characteristics are shown in Tables 3 and 4.

[0032]

[0033] Comparative Example 1 A brown solid was obtained in the same manner as in Example 1, except that the starting materials were the aluminosilicate obtained in Pretreatment 2, 48.06 g of NaCl, 154 mL of distilled water, and the amount of powder mixed with NaCl, dried, and then pulverized in the Mg mixing step was 8.81 g (in this case, the ratio of the number of aluminum atoms contained in the aluminosilicate after mixing with Mg to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:93.0). X-ray diffraction analysis of the obtained brown solid revealed a clear silicon peak ( FIG. 2 ). A halo at 2θ = 20 to 25°, which appears when amorphous SiO is present, was also observed. SEM-EDX analysis of Si, Al, and O was performed (SEM: Hitachi High-Technologies SU3500 scanning electron microscope, EDX: Horiba, Ltd. EMAXEvolution EX-370 X-MAX20 energy dispersive X-ray analyzer) to measure the atomic concentration (%), and the residual oxygen atomic concentration was found to be 51.71%. The results are shown in Table 1.

[0034] Comparative Example 2: A brown powder was obtained using the same procedure as in Comparative Example 1, except that the starting material was the aluminosilicate obtained in Pretreatment 2, and the reduction treatment time was 24 hours at 600°C in an Ar gas atmosphere. After mixing with Mg, the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:93.0. X-ray diffraction analysis of the obtained brown powder revealed a clear silicon peak (Figure 2). A halo at 2θ = 20-25°, which appears when amorphous SiO2 is present, was observed. Observation with a field-emission scanning electron microscope (JEOL JSM-6700F) revealed primary silicon particles ranging from 20 to 60 nm. SEM-EDX analysis of Si, Al, and O (SEM: Hitachi High-Technologies Corporation Scanning Electron Microscope SU3500, EDX: Horiba, Ltd. Energy Dispersive X-ray Spectrometer EMAXEvolution EX-370 X-MAX20) was performed to measure the atomic concentration (%) of residual oxygen, which was found to be 56.65%. The results are shown in Table 1.

[0035] Comparative Example 3: A brown powder was obtained in the same manner as in Example 3, except that the amount of Mg powder added as a reducing agent in the mixing step with Mg was 0.72 g [SiO:Mg ≈ 1:2.2 (molar ratio)] (all calculations were based on SiO except for AlO). (In this case, the ratio of the number of aluminum atoms contained in the aluminosilicate after mixing with Mg to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:3.0.) X-ray diffraction analysis of the obtained brown solid showed a clear silicon peak (Figure 1). The halo at 2θ = 20-25° that appears when amorphous SiO is present was not observed. Quartz peaks, likely derived from natural aluminosilicate, were observed. Spinel (MgAlO) peaks were also observed.

[0036] Comparative Example 4 A basic evaluation cell (half cell) was prepared under the same conditions as in Example 4, except that the silicon used in the working electrode was commercially available carbon-coated 100-nanometer silicon, and the single-electrode characteristics were measured under the charge-discharge conditions shown in Table 1. The single-electrode characteristics are shown in Tables 3 and 4.

[0037]

[0038]

[0039]

[0040] The X-ray diffraction analysis results shown in Figure 2 confirmed that when the amount of Al2O3 was adjusted to 2% by pretreatment 2 and the ratio of aluminum atoms in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:93.0, a large amount of amorphous SiO2 was present, enough to confirm the presence of an amorphous SiO2 halo (Comparative Examples 1 and 2). The reduction time in Example 1 was 3 hours, while the reduction time in Comparative Example 2 was 24 hours. Even when the reduction time was extended to 24 hours, the amorphous SiO2 halo was clearly observed (Comparative Example 2, Figure 2). The X-ray diffraction analysis results shown in Figure 2 also confirmed that when the amount of Al2O3 was adjusted to 39% by pretreatment 4 and the ratio of aluminum atoms in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was 1:3.0, a spinel (MgAl2O4) peak was observed (Comparative Example 3). In contrast, when the Al2O3 content was adjusted to 6%, 15%, and 39% by pretreatments 1, 3, and 4, and the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step was adjusted to 1:29.7, 1:10.7, and 1:3.9, respectively, and then reduction treatment was performed (Examples 1 to 3), no amorphous SiO2 halo or spinel (MgAl2O4) peak was observed after a reduction time of 3 hours (Figure 1). Furthermore, the primary particle size of the silicon in Example 1 was 10 to 15 nm, smaller than that of Comparative Example 2. When attempting to obtain high-purity silicon, in Comparative Examples 1 and 2, the amorphous SiO2 halo was present to a degree that could be confirmed by X-ray diffraction analysis, and therefore a process of removing the amorphous SiO2 using hydrofluoric acid was required to obtain high-purity silicon. Furthermore, when the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step is 1:3.0, as in Comparative Example 3, and the amount of aluminum is excessive relative to the amount of magnesium, spinel (MgAlO) is produced that cannot be removed even by acid treatment.However, by carrying out the reduction treatment in Examples 1 to 3, while leaving an appropriate amount of alumina and maintaining an appropriate ratio between the number of aluminum atoms contained in the aluminosilicate and the number of magnesium atoms used as a reducing agent in the reduction treatment step, high-purity silicon was obtained, with no amorphous SiO halo or spinel peaks as determined by X-ray diffraction analysis. The mass percent of oxygen calculated from the atomic concentrations of O, Al, and Si in Example 1 is 13.49 mass%. Assuming a silicon density of 2.33, a SiO density of 2.3, and a primary particle diameter of nanosilicon of 10 nm, the thickness of the SiO coating present on the primary particles of nanosilicon in Example 1 is approximately 0.5 nm. For example, even when the nanosilicon of Example 1 is used as an anode material for a lithium-ion battery, this SiO thickness does not hinder the movement of lithium ions or electrons into and out of the nanosilicon. Therefore, according to the present invention, high-purity nanosilicon suitable for use as anode material in lithium-ion batteries can be obtained without the need for the highly dangerous and costly process of removing amorphous SiO2 using hydrofluoric acid. Actual basic evaluation cells (half cells) were fabricated and subjected to charge-discharge cycle tests. A lithium-ion battery (Example 4) in which silicon obtained according to the present invention was carbon-coated and incorporated into the working electrode (negative electrode) was compared with a lithium-ion battery (Comparative Example 4) in which commercially available 100-nanometer silicon was carbon-coated and incorporated into the working electrode. Perhaps due to the higher amount of carbon coating on the silicon in Example 4, Example 4 exhibited a lower initial capacity than Comparative Example 4, but demonstrated superior capacity retention during cycle testing. Battery capacity also increased in the Example from the fifth cycle onward. When silicon is used as anode material in lithium-ion batteries, a problem arises: pulverization due to volume changes caused by the inflow and outflow of lithium ions, resulting in a decrease in capacity. However, the size of the silicon particles obtained in the present invention is very small, at 10 to 15 nanometers, and is less likely to be pulverized due to volume change, which is thought to be why the capacity retention rate was excellent in cycle tests.

Claims

1. (a) A step of subjecting an aluminosilicate having an Al content of 3 to 40% by mass to a reduction treatment in a range where the ratio of the number of aluminum atoms contained in the aluminosilicate to the number of magnesium atoms used as a reducing agent in the reduction treatment step is from 1:3.5 to 1:65; and 2 O 3 (b) A step of subjecting the reduced aluminosilicate obtained in step (a) to an acid treatment; A method for producing nanosilicon, comprising:

2. The production method according to claim 1, wherein the ratio of the number of atoms is from 1:3.7 to 1:

45.

3. The production method according to claim 1, wherein the ratio of the number of atoms is from 1:3.7 to 1:

30.

4. The production method according to any one of claims 1 to 3, wherein the aluminosilicate used in step (a) is halloysite or is obtained by dealumination of halloysite.

5. The production method according to claim 1, wherein the aluminosilicate used in step (a) is adjusted to have an Al 2 O 3 content of 3 to 40% by mass by dealumination treatment.

6. The production method according to claim 5, wherein the dealumination treatment includes an acid treatment selected from the group consisting of hot sulfuric acid treatment, sulfuric acid treatment, hydrochloric acid treatment, hot hydrochloric acid treatment, nitric acid treatment, hot nitric acid treatment, and combinations thereof.

7. The production method according to claim 1, wherein the reduced aluminosilicate obtained in step (a) contains 6 to 39% by mass of Al 2 O 3

8. The reduction treatment in step (a) includes mixing magnesium powder as a reducing agent, at least one selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides as an endothermic agent, and the aluminosilicate, and heating and reducing the obtained mixture in an argon gas or nitrogen gas atmosphere. The manufacturing method according to claim 1.

9. The acid treatment in step (b) uses at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. The manufacturing method according to claim 1.

10. The acid used in step (b) is hydrochloric acid, and its concentration is 0.5 to 2.0 mol / liter. The manufacturing method according to claim 1.

11. By subjecting the aluminosilicate to hot sulfuric acid treatment, an aluminosilicate with an Al 2 O 3 content of 3 to 40% by mass is obtained, and the step of reducing the aluminosilicate; and The step of acid-treating the reduced aluminosilicate obtained in the above step; A method for manufacturing nanosilicon, comprising:

12. A negative electrode active material for a lithium ion battery containing nanosilicon obtained by the manufacturing method according to claim 1.

13. There is no amorphous SiO 2 halo confirmed by X-ray diffraction analysis, and there is no peak of spinel confirmed by X-ray diffraction analysis, and the primary particle diameter measured by a field emission scanning electron microscope is 10 to 15 nm. A negative electrode active material for a lithium ion battery containing nanosilicon.

14. A negative electrode for a lithium ion battery containing the negative electrode active material according to claim 12 or 13.

15. A lithium ion battery containing the negative electrode for a lithium ion battery according to claim 14.