Porous silicon material, power storage device, and method for producing porous silicon material

Abstract

To enhance charge and discharge properties of a porous silicon material.SOLUTION: A porous silicon material disclosed herein comprises Si, Al, and a transition metal element M, has a specific surface area of 30 cm2 / g or less according to nitrogen adsorption, is in particulate form, and has a carbon coating layer on the surface.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Disclosed herein are porous silicon materials, power storage devices, and methods for making the porous silicon materials. [Background technology]

[0002] Conventionally, porous silicon materials used in electricity storage devices have been produced by a manufacturing method including a precursor process in which a raw material containing Cr in the range of 1 at% to 20 at% and Al in the range of 40 at% to 90 at% and the remainder Si, where Si, Al, and Cr together make up 100 at%, is melted and rapidly solidified to obtain a silicon alloy precursor, and a porosity process in which the Al component contained in the silicon alloy is removed to obtain a porous silicon material (see, for example, Patent Document 1). Among these porous silicon materials, those containing Si can further suppress deterioration of charge / discharge characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-92861 Summary of the Invention [Problem to be solved by the invention]

[0004] Silicon anodes have a theoretical capacity 10 times greater than that of graphite anodes currently in practical use, but suffer from poor cycle characteristics due to their large expansion and contraction during charge and discharge. The porous silicon material in Patent Document 1 contains Al, Cr, etc., which increases its strength and further prevents deterioration of charge and discharge characteristics, but this is still insufficient, and further improvement is desired.

[0005] The present disclosure has been made in view of such problems, and a main object thereof is to provide a porous silicon material, a power storage device, and a method for manufacturing a porous silicon material that can further improve the charge-discharge characteristics of the porous silicon material.

Means for Solving the Problems

[0006] As a result of intensive studies to achieve the above object, the present inventors have found that in a porous material containing Al, Si, and transition metal element M, by reducing the specific surface area while providing a carbon coating layer on the outer surface thereof, the charge-discharge characteristics of the particulate porous silicon material can be further improved, and the porous silicon material, the power storage device, and the method for manufacturing the porous silicon material of the present disclosure have been completed.

[0007] That is, the porous silicon material of the present disclosure contains Si, Al, and transition metal element M, has a specific surface area by nitrogen adsorption of 30 cm 2 / g or less, is particulate and has a carbon coating layer on the surface.

[0008] The power storage device of the present disclosure includes a positive electrode containing a positive electrode active material, a negative electrode containing the above-described porous silicon material as a negative electrode active material, an ion conduction medium interposed between the positive electrode and the negative electrode and conducting lithium ions, and is provided with the above.

[0009] The method for manufacturing the porous silicon material of the present disclosure A porous body step of rapidly cooling a master alloy melt having a basic composition formula of Al 100-x-y Si x M y (where 10 ≤ x ≤ 40, 0 < y ≤ 5, 0 < x / y ≤ 0.2 is satisfied) to phase-separate into a conductive phase containing one or more of Si, Al, transition metal silicide, transition metal aluminum, and aluminum transition metal silicide, and producing a porous precursor by selectively removing the Al component. a forming step of forming a carbon coating layer on the outer surface of the porous precursor by performing thermal CVD by heating the porous precursor in the presence of a hydrocarbon gas from room temperature to a temperature range of 650°C to 850°C; It includes: [Effects of the Invention]

[0010] The present disclosure can further improve the charge-discharge characteristics of porous silicon materials. The reason for this effect is believed to be as follows. For example, while electrodes using Si as the active material can achieve a larger capacity than graphite electrodes, the volumetric change of Si during charge and discharge can sometimes result in a decrease in rate characteristics and capacity retention. In response to this, providing a carbon coating layer on the outer surface of porous particles and appropriately reducing the specific surface area can further suppress particle shape change and further reduce the resistance of the Si particles, thereby improving both cycle characteristics and rate characteristics. It is believed that this reduces expansion and contraction during charge and discharge, further improving charge-discharge characteristics such as the capacity retention rate and charge-discharge efficiency during charge-discharge cycles. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10. [Figure 2] FIG. 1 is an illustration of an example of porous silicon having a carbon coating layer. [Figure 3] FIG. 1 is an explanatory diagram of carbon coating treatment conditions 1 and 2. [Figure 4] Raman measurement results of carbon-coated porous silicon treated at 650°C. [Figure 5] Raman measurement results of carbon-coated porous silicon treated at 850°C. [Figure 6] 1 is a diagram showing the relationship between heat treatment temperature and specific surface area. [Figure 7] FIG. 1 is a graph showing the relationship between the amount of carbon coating in porous silicon and the powder resistance value. [Figure 8] FIG. 1 is a diagram showing the relationship between heating temperature and powder resistance value in porous silicon. [Figure 9]TEM images of porous silicon produced under processing conditions 1 and 2. [Figure 10] FIG. 1 is a graph showing the relationship between the amount of carbon coating calculated from the specific surface area and the film thickness. [Figure 11] Measurement results of cycle characteristics with and without carbon coating and treatment conditions. [Figure 12] Evaluation results of rate characteristics with and without carbon coating. [Figure 13] Evaluation results of the presence or absence of carbon coating and its effect on the initial Coulombic efficiency. [Figure 14] Evaluation results of the effect of carbon coating amount on high-rate capacity retention. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present disclosure will be described with reference to the drawings. Fig. 1 is an explanatory diagram showing an example of the structure of an electricity storage device 10. Fig. 2 is an explanatory diagram of an example of porous silicon having a carbon coating layer. Fig. 3 is an explanatory diagram of carbon coating treatment condition 1 (Fig. 3A) and treatment condition 2 (Fig. 3B).

[0013] (Method of manufacturing porous silicon material) The method for producing a porous silicon material according to the present disclosure includes a porosity-inducing step and a formation step. In the porosity-inducing step, a precursor treatment and a porosity-inducing treatment are performed. In the precursor treatment, a raw material of an Al-Si-M ternary alloy (M is a transition metal element) that undergoes a peritectic reaction to produce a SiAl alloy or an Al alloy is melted and rapidly solidified to obtain a silicon alloy precursor. Note that the precursor may be prepared separately and the precursor treatment may be omitted from the porosity-inducing step. In the porosity-inducing treatment, the Al component contained in the silicon alloy is removed to obtain a porous precursor of porous silicon whose main component is Si.

[0014] (precursor treatment) In the precursor treatment, the basic composition formula is Al 100-x-y Si x M yBy rapidly cooling a master alloy melt (where 10 ≤ x ≤ 40, 0 < y ≤ 5, 0 < x / y ≤ 0.2), precursor particles are produced that are phase-separated into Si, Al, and a conductive phase containing one or more of transition metal silicide, transition metal aluminum, and aluminum transition metal silicide. In this process, when the total of Si, Al, and M is 100 at%, it may be possible to use a raw material that contains M in the range of 0.1 at% or more and 20 at% or less, Al in the range of 40 at% or more and 90 at% or less, and the balance being Si. For the x in the above basic composition formula, a range of 10 or more and 40 or less is preferable, 35 or less is preferable, and it may be 30 or less. Also, x is more preferably 20 or more, and it may be 30 or more. For the y in the above basic composition formula, 0.1 or more is more preferable, 0.5 or more is even more preferable, and it may be 1 or more. This y is more preferably 4 or less, even more preferably 3 or less, and it may be 2 or less. Note that the raw material may contain unavoidable impurities. Unavoidable impurities are components that inevitably remain during the purification of any of Si, M, or Al, and examples include Fe, C, Cu, Ni, P, etc. It is preferable that the unavoidable impurities be less, and for example, when the total of Si, Cr, and Al is 100 at%, 5 at% or less is preferable, and 2 at% or less is more preferable. The blending ratio of M is preferably, for example, 2 at% or more, and it may be 3 at% or more. Also, the blending ratio of M is preferably 15 at% or less, and it may be 12.5 at% or less. The blending ratio of Al is preferably 50 at% or more, and it may be 55 at% or more, 60 at% or more, etc. Also, the blending ratio of Al is preferably 85 at% or less, more preferably 80 at% or less, and it may be 77.5 at% or less. The blending ratio of Si is preferably, for example, 15 at% or more, more preferably 18 at% or more, and it may be 20 at% or more, 25 at% or more, etc. Also, the blending ratio of Si is preferably, for example, 59 at% or less, more preferably 50 at% or less, and it may be 40 at% or less, 30 at% or less, etc. In a silicon alloy containing Al and M within such ranges, it is preferable because the porosity can be increased and voids of a more suitable shape and size can be obtained. When the content of Al is high, after melting into an alloy and rapidly cooling, a large single phase of Al precipitates, so many voids can be formed.This cooling rate is preferably more rapid, for example, 10 min from the molten state. 2 ℃ / s or more 10 8 °C / s or less.

[0015] In this process, when melting the raw materials, high-frequency crucible melting in an inert gas atmosphere such as Ar is preferred, but any melting method can be used. To produce a master alloy, the raw material powder must be melted. High-frequency melting is preferable for producing a uniform sample, but simple heating and melting using an electric furnace or electron beam are also acceptable. In the precursor process, the alloy obtained from the raw materials may be granulated. This granulation process may involve casting a molten silicon alloy raw material into a mold and crushing the resulting ingot into particles. The silicon alloy may also be granulated by one or more of the following methods: gas atomization, water atomization, single-roll milling, etc., using the molten silicon alloy. Gas atomization and water atomization methods produce alloy powder. The single-roll milling method produces a foil-like sample, which may then be crushed and granulated. Of these, gas atomization is more preferred for granulating the silicon alloy. In gas atomization, the molten metal is preferably produced in an Ar atmosphere, and the granulation is preferably carried out in an Ar or He atmosphere.

[0016] In the precursor treatment, the silicon alloy is preferably granulated to a particle size in the range of 0.5 μm to 3 μm. The silicon alloy particles preferably have an average particle size in the range of 0.5 μm to 3 μm, and may have an average particle size in the range of 1 μm to 2.5 μm, or 1.5 μm to 2 μm. The silicon alloy particles may be appropriately selected depending on the characteristics required for the energy storage device. The average particle size of the particles is determined by observing the particles with a scanning electron microscope (SEM), tallying the major axis of each particle as the diameter, and dividing this by the number of particles to obtain an average value. The particles obtained by this granulation treatment will have the average particle size of the aggregate of porous particles to be obtained.

[0017] In this precursor treatment, a raw material containing Al, Cr, and Si as well as a second element including one or more of Ca, Cu, Mg, Na, Sr, and P may be used. Of these, the second element is preferably one or more of Ca, Na, and Sr. The content of the second element is preferably less than the content of Al and Cr, for example, preferably in the range of 10% by mass or less, more preferably 5% by mass or less, of the entire silicon alloy.

[0018] (porosity treatment) In the porosity forming step, the Al component contained in the silicon alloy is removed to produce porous precursor particles. This process may produce a particulate porous silicon material containing 20% ​​by volume or more of pores of 1 μm or less measured by mercury porosimetry, in which transition metal silicide and Si coexist in the framework, with the Si phase accounting for 55% by mass or more and the transition metal silicide accounting for 1% by mass to 45% by mass of the entire particle. Examples of the Al component removed by this process include Al and its compounds. This process preferably uses an acid or alkali to selectively remove the Al component, i.e., the Al phase and its compounds. The acid or alkali used is preferably one that elutes elements and / or compounds other than silicon in the silicon alloy but does not elute silicon, such as hydrochloric acid, sulfuric acid, or sodium hydroxide. This acid or alkali is preferably an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it is within a range that can remove Al and its compounds, and M and its compounds, and can be, for example, in the range of 1 mol / L to 5 mol / L. This removal treatment may be performed by heating at, for example, 30°C to 60°C. Preferably, the removal treatment involves immersing the silicon alloy particles in an acid or alkaline solution and stirring for about 1 to 5 hours. The resulting porous silicon material is then washed and dried.

[0019] In the porosity treatment, substances other than Si may be removed in a range of 85% by mass to 100% by mass. For example, Al, M, and other oxygen may remain, but when used as an electrode active material, it is preferable that they are present in a smaller amount from the viewpoint of charge / discharge capacity. Furthermore, components such as Al and M are preferably contained in a predetermined amount or more from the viewpoint of reinforcing the silicon skeleton and improving durability. This process may be used to obtain a porous silicon material containing a SiM compound. Examples of SiM compounds include transition metal silicides such as Si a M b (a and b are arbitrary numbers), and examples thereof include MSi2. MSi2 as the SiM compound may include M(Si,Al)2, in which a part of Si is substituted with Al. Also, examples of AlSiM compounds include Al a Si b M c It may be a compound (a, b, c are any numbers), Al 13 Examples include Si4Cr4, etc. The SiM compound may be one that is poorly soluble in acid or alkali.

[0020] The porosity treatment preferably produces a porous silicon material containing 55% by mass or more of Si phase and 1% to 45% by mass or less of transition metal silicide. The porous silicon material preferably contains 2% by mass or more of transition metal silicide, and may contain 3% by mass or more of transition metal silicide. The porous silicon material preferably contains 40% by mass or less of transition metal silicide, and may contain 30% by mass or less of transition metal silicide. This step preferably produces a porous silicon material in which the transition metal silicide remains.

[0021] The porosity treatment may be performed to obtain a porous silicon material having a porosity in the range of 30% by volume or more and 85% by volume or less. This porosity is a value measured using a mercury porosimeter. The porosity is preferably, for example, 35% by volume or more, and may be 40% by volume or more. The porosity is preferably, for example, 80% by volume or less, more preferably 77.5% by volume or less, and may be 75% by volume or less. A larger porosity is more responsive to volume changes when carrier ions are absorbed, while a smaller porosity is preferable because it increases the amount of Si present per unit volume.

[0022] (Formation process) In the formation process, a carbon coating layer is formed on the outer surface of the porous precursor by thermal CVD. In this process, the porous precursor and hydrocarbon gas are coexisted and heated from room temperature to a temperature range of 650°C to 850°C, resulting in thermal CVD to form a carbon coating layer on the outer surface of the porous precursor. As shown in Figure 3, the process conditions include process condition 1 (Figure 3A), in which heating is performed in the presence of hydrocarbon gas (the carbon source) and the amount of carbon coating is adjusted by controlling the treatment time, and process condition 2 (Figure 3B), in which the porous precursor is moved through the heated hydrocarbon gas (the carbon source) and the amount of carbon coating is adjusted by controlling the movement speed. Of these, process condition 1 is preferred. As shown in Figure 3, process condition 2 results in the formation of a carbon coating inside the pores, while process condition 1 results in the formation of a carbon coating on the outer surface of the porous particles, which can further suppress the collapse of the Si structure over long periods of cycles, as shown in Figure 2. Under process condition 1, the porous precursor may be placed on a carbon boat and placed in a circular furnace. The furnace atmosphere is then purged with an inert gas (e.g., Ar gas) and heated to 600–1000°C. In this state, it is preferable to flow an Ar / hydrocarbon gas at a rate of 10 mL / min to 500 mL / min. Under process condition 2, for example, the furnace atmosphere is previously purged with Ar / hydrocarbon gas, and a mobile heating unit heated to 600–1000°C is moved to the sample location while the Ar / hydrocarbon gas is flowing. This heats the porous precursor and forms a carbon coating layer on the particle surface. Examples of hydrocarbon gases that can be used as carbon sources include methane, ethylene, and acetylene. Methane has a high decomposition temperature, requiring temperatures of around 1000°C for graphite formation, but can produce highly crystalline graphite films. Ethylene, on the other hand, can be used to form films at low temperatures of around 650°C, but the resulting films are not highly crystalline. By changing the reaction time within the range of 5 to 120 minutes, the carbon coating amount can be controlled within the range of 1 to 30 mass %. The carbon coating amount can be determined from the increase or decrease in weight due to the thermal CVD treatment.

[0023] In the forming step, it is preferable to form a carbon coating layer containing graphite, and it is more preferable that the entire carbon coating layer is graphite. Furthermore, in the forming step, it is preferable to form a carbon coating layer having a thickness in the range of 1 nm to 500 nm. A thicker thickness is preferable because it produces a carbon coating effect, while a thinner thickness is preferable from the viewpoint of capacity per unit mass. Furthermore, in the forming step, it is preferable to form a carbon coating layer having a carbon content in the range of 0.1 mass % to 20 mass % of the entire porous silicon material. A higher carbon content is preferable because it produces a coating effect, while a lower carbon content is preferable from the viewpoint of capacity.

[0024] (porous silicon material) The porous silicon material of the present disclosure may be one produced by the above-mentioned production method. Here, the physical properties of the porous silicon material are the same as those of the above-mentioned production method, and detailed description thereof will be omitted. The porous silicon material contains Si, Al, and a transition metal element M, and has a specific surface area of ​​30 cm2 as determined by nitrogen adsorption. 2 / g or less, and is in the form of particles with a carbon coating layer on the surface.

[0025] The porous silicon material may be in the form of particles, and may have a structure in which a transition metal silicide containing a transition metal element M and Si coexist in the skeleton. Alternatively, the skeleton of the porous particles may contain a main phase of Si and a conductive phase of an AlSiCr alloy. This conductive phase may be MSi2, M(Si,Al)2 or Al 13 The porous silicon material may be M4Si4, and its content may be 6 mol % or less of the total, more preferably 2 mol % or less. In this porous silicon material, the transition metal element M is Cr, and the transition metal elements M may be CrSi2, Cr(Si,Al)2, and Al 13 It may contain one or more of Cr4Si4.

[0026] The Si phase may be 55% by mass or more and the transition metal silicide may be 1% by mass or more and 45% by mass or less in the entire particle. The porosity of the porous silicon material is preferably 45% by volume or more, and may be 50% by volume or more, or may be 55% by volume or more. Alternatively, the porosity is preferably 75% by volume or less, and may be 70% by volume or less, or may be 65% by volume or less. The pore diameter of the porous silicon material is preferably 1 nm or more, more preferably 5 nm or more, and may be 10 nm or more. The pore diameter is preferably 300 nm or less, more preferably 200 nm or less, and may be 150 nm or less. The average pore diameter of the porous silicon material is preferably 5 nm or more, and may be 10 nm or more, or may be 25 nm or more. The average pore diameter is preferably 150 nm or less, and may be 100 nm or less, or may be 75 nm or less.

[0027] In the porous silicon material, the carbon coating layer preferably has a thickness in the range of 1 nm to 500 nm. This carbon coating layer may cover the entire outer surface or may cover only a portion of the outer surface. This thickness can be obtained, for example, by observing the cross section of the particle with an electron microscope. The thickness of the carbon coating layer may be 2 nm or more, 5 nm or more, or 10 nm or more. Also, this thickness may be 250 nm or less, 100 nm or less, or 50 nm or less. Furthermore, the amount of the carbon coating layer is preferably in the range of 0.1 mass % to 20 mass %. The carbon coating amount may be 1 mass % or more, 5 mass % or more, or 10 mass % or more. Also, the carbon coating amount may be 17.5 mass % or less, 15 mass % or less, or 10 mass % or less.

[0028] The porous silicon material may have an Al content in the range of 4 at% to 15 at% and an M content in the range of 1 at% to 7 at% when the total of Si, Al, and M is taken as 100 at%. In the porous silicon material, the Al content may be 5 at% or more, or 8 at% or more. The Al content may also be 14 at% or less, or 12.5 at% or less. This elemental composition is measured using energy dispersive X-ray spectroscopy (EDX).

[0029] The porous silicon material has a Si phase content of 55% or more by mass and a transition metal silicide phase content of 1% to 45% by mass in the entire particle. The values ​​of each phase are determined from the peak intensity ratio of X-ray diffraction. From the viewpoint of replenishment and discharge capacity, the Si phase content is preferably higher, more preferably 60% by mass or more, 70% by mass or more, or 80% by mass or more. From the viewpoint of improving strength, the transition metal silicide phase content is preferably higher, more preferably 2% by mass or more, 10% by mass or more, or 20% by mass or more. Furthermore, from the viewpoint of charge and discharge capacity, the transition metal silicide phase content is preferably lower, more preferably 40% by mass or less, 30% by mass or less, or 25% by mass or less.

[0030] The porous silicon material may have a diameter in the range of 0.5 μm to 3 μm, and the length of the core portion may be in the range of 40% to 60% of the particle diameter. The diameter of this porous silicon material may be 1 μm or more, or may be in the range of 2 μm or less. The particle diameter is a value obtained by SEM observation. The particle diameter may be set appropriately depending on the performance required for the electricity storage device.

[0031] In the porous silicon material, the transition metal silicide is preferably one or more of MSi2 and M(Si,Al)2 in which Al is solid-solved, and the content of the transition metal silicide is preferably 2% by mass or more and 30% by mass or less. A transition metal silicide content of 2% by mass or more can sufficiently strengthen the skeleton, while a transition metal silicide content of 30% by mass or less is preferable because it relatively reduces the reinforcing phase not involved in charge and discharge, thereby further increasing the charge and discharge capacity. In this porous silicon material, the average pore diameter measured by mercury intrusion porosimetry is preferably 60 nm or less. This pore diameter is preferably 5 nm or more and 300 nm or less. This pore diameter may be 10 nm or more or may be 20 nm or more. Furthermore, this pore diameter is preferably 50 nm or less, or may be 45 nm or less. A small pore diameter is preferable because the pores are less likely to collapse. Furthermore, a large pore is preferable because it can further suppress volume change when carrier ions are absorbed. The transition metal element M is not particularly limited as long as it can produce a SiAl alloy or an Al alloy through a peritectic reaction, and examples thereof include Cr.

[0032] The porous silicon material includes skeletal silicon with a three-dimensional network structure having voids, and may include an SiM compound and / or an AlSiM compound. The SiM compound includes a transition metal silicide. The SiM compound and the AlSiM compound are presumed to reinforce the silicon skeleton. The porous silicon material preferably contains the SiM compound in a range of 5 mol % to 20 mol % relative to the Si phase. The porous silicon material preferably contains the SiM compound in a range of 1 mass % to 15 mass %. The porous silicon material preferably contains the AlSiM compound in a range of 0.1 mass % to 10 mass %. A larger amount of the SiM compound and the AlSiM compound is preferable from the viewpoint of supplementing the reinforcement of the skeleton, and a smaller amount is preferable from the viewpoint of the charge / discharge capacity of the power storage device.

[0033] The porous silicon material may have a porosity in the range of 30% by volume or more and 85% by volume or less, as determined by mercury intrusion porosimetry. This porosity is, for example, preferably 35% by volume or more, and may be 40% by volume or more. The porosity is, for example, preferably 80% by volume or less, more preferably 77.5% by volume or less, and may be 75% by volume or less. A larger porosity is more responsive to volume changes during occlusion of carrier ions, while a smaller porosity is preferable, since it increases the amount of Si present per unit volume.

[0034] The porous silicon material preferably has 20% or more by volume of pores of 1 μm or less measured by mercury porosimetry. It is preferable that the porous silicon material has a greater number of such fine pores. It is more preferable that the porous silicon material has 30% or more by volume of pores of 1 μm or less measured by mercury porosimetry, and even more preferable that the porous silicon material has 40% or more by volume of pores of 1 μm or less measured by mercury porosimetry. Furthermore, the porous silicon material may have 90% or less by volume of pores of 1 μm or less measured by mercury porosimetry.

[0035] Porous silicon material has a specific surface area of ​​30 cm 2 / g or less. A lower specific surface area is preferable from the viewpoint of improving the coulomb efficiency. This specific surface area is 27.5 cm 2 / g or less, or 25cm 2 / g or less. The specific surface area is 1 cm 2 / g or more, or 10cm 2 / g or more. This specific surface area is determined by nitrogen adsorption.

[0036] This porous silicon material preferably contains 70 at% or more of Si when the total of Si, Al, and the transition metal element M, excluding oxygen and unavoidable impurities, is taken as 100 at%. The Si content is more preferably 75 at% or more, even more preferably 80 at% or more, and may be 85 at% or more. A higher Si content is preferable from the viewpoint of charge / discharge capacity, but a lower Si content may be used from the viewpoint of relative skeletal reinforcement. The Al content is preferably in the range of 4 at% to 15 at% or less, preferably 12.5 at% or less, more preferably 10 at% or less, and may be 7.5 at% or less. The Al content may be 5 at% or more, or may be 7.5 at% or more. The M content is preferably in the range of 1 at% to 7 at% or less. The M content is more preferably 6 at% or less, and may be 5 at% or less. The M content is more preferably 2 at% or more, more preferably 3 at% or more, and may be 4 at% or more. The content of Al and M is preferably higher from the viewpoint of reinforcing the skeleton, and is preferably lower from the viewpoint of the charge / discharge capacity of the electricity storage device, since these are components that do not charge or discharge. Furthermore, the porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr, and P as a second element in a range of 15 mass % or less. Furthermore, the porous silicon material may contain inevitable impurities in addition to Si, Al, and M. It is preferable that the amount of the second element and inevitable impurities is as small as possible.

[0037] (Electrodes for electricity storage devices) An electrode for a power storage device includes the porous silicon material described above as an electrode active material. This electrode functions as either a positive electrode or a negative electrode depending on the potential of the electrode active material relative to the potential of the counter electrode. When lithium is used as a carrier, the electrode is preferably a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, and air batteries. The electrode for a power storage device may be one in which the porosity of the porous silicon material is compressed to a range of 5% to 50% by volume. In this electrode, the porosity of the porous silicon material may be reduced by compression during fabrication. For example, when porous silicon particles are used as a negative electrode active material in a lithium ion secondary battery, smaller pores allow lithium ions to alloy more uniformly, thereby reducing stress concentration and preventing deterioration of the electrode itself. The porosity of the compressed porous silicon material can be appropriately adjusted depending on the properties required of the power storage device electrode, and may be, for example, 5% by volume or more or 10% by volume or more. The porosity of the compressed porous silicon material may also be, for example, 30% by volume or less or 20% by volume or less.

[0038] The electrode for a power storage device may be formed by forming the porous silicon material on a current collector and then adhering the material to the current collector. This electrode can be produced by either mixing the porous silicon material with a solvent and, if necessary, a conductive material and a binder to form a paste, which is then applied to the current collector, or by mixing the porous silicon material with a conductive material and a binder if necessary and then pressing the mixture onto the current collector. In this electrode, the content of the porous silicon material is preferably as high as possible, preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. The conductive material is not particularly limited as long as it is an electron-conductive material that does not adversely affect battery performance. For example, a mixture of one or more of graphite, such as natural graphite (e.g., scaly graphite or flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, and metals (e.g., copper, nickel, aluminum, silver, and gold), can be used. The binder serves to bind the active material particles and conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber; thermoplastic resins such as polypropylene and polyethylene; ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, and natural butyl rubber (NBR), either alone or in combination. Water-based binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR) can also be used. Examples of solvents that can be used include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, dispersants, thickeners, and the like can be added to water to form a slurry of the active material with a latex such as SBR. Examples of application methods include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any of these can be used to obtain a desired thickness and shape.The current collector may be appropriately selected according to the potential of the active material or the like. For example, in addition to aluminum, titanium, stainless steel, nickel, iron, copper, fired carbon, conductive polymers, conductive glass, etc., for the purpose of improving adhesion, conductivity and oxidation resistance, those obtained by treating the surface of aluminum or copper with carbon, nickel, titanium, silver, etc. can be used. It is also possible to oxidize-treat their surfaces. Regarding the shape of the current collector, examples include foil shape, film shape, sheet shape, net shape, punched or expanded shape, lath body, porous body, foam body, formed body of fiber group, etc. The thickness of the current collector is, for example, 1 to 500 μm. The formation amount of the active material composite may be appropriately set according to the desired performance required for the power storage device.

[0039] (Power storage device) The power storage device of the present disclosure includes an electrode having the porous silicon material described above. This power storage device may include a positive electrode, a negative electrode, and an ion conduction medium interposed between the positive electrode and the negative electrode for conducting carrier ions. The porous silicon material can be used as a negative electrode active material. This power storage device may be any one of a lithium ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as CrS2, CrS3, MoS3, FeS2, lithium manganese composite oxides with the basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same below), Li (1-x) Mn2O4, etc., lithium cobalt composite oxides with the basic composition formula Li (1-x) CoO2, etc., lithium nickel composite oxides with the basic composition formula Li (1-x) NiO2, etc., lithium nickel cobalt manganese composite oxides with the basic composition formula Li (1-x) Ni a Co b Mn cLithium nickel cobalt manganese composite oxides with a basic composition formula of LiV2O3, etc., and transition metal oxides with a basic composition formula of V2O5, etc., can be used. Among these, lithium transition metal composite oxides, such as LiCoO2, LiNiO2, LiMnO2, and Li (1-x) Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc. are preferred. The term "basic composition formula" means that other elements such as Al and Mg may also be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors, lithium ion capacitors, etc. Examples of carbonaceous materials include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. Activated carbons as carbonaceous materials have a specific surface area of ​​1000 m 2 / g or more, and 1500m 2 / g or more is more preferable. 2 / g or more, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m 2 / g or less, and 2 The conductive material, binder, solvent, current collector, and the like used in the positive electrode can be appropriately selected from those exemplified for the electrode described above.

[0040] The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent battery characteristics during repeated charge and discharge, but also allows for a well-balanced electrolyte viscosity, the resulting battery's electrical capacity, and battery output. Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. Among these, a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO) is preferred from the standpoint of electrical properties.The concentration of this supporting salt in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the supporting salt is dissolved at a concentration of 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be more stable. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to this non-aqueous electrolyte.

[0041] Instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of the ion-conducting polymer include polymer gels composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride and a supporting salt. Furthermore, a combination of an ion-conducting polymer and a nonaqueous electrolyte can also be used. In addition to ion-conducting polymers, other ion-conducting media include inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, and inorganic solid powders bound by an organic binder.

[0042] The power storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery, and examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0043] The shape of this power storage device is not particularly limited, and examples include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, etc. It may also be applied to large-sized ones used in electric vehicles, etc. The power storage device 10 shown in FIG. 1 has a positive electrode 12, a negative electrode 15, and an ion conductive medium 18. The positive electrode 12 has a positive electrode active material layer 13 and a current collector 14. The negative electrode 15 has a negative electrode active material layer 16 and a current collector 17. The negative electrode active material layer 16 includes the porous silicon material 21 described above and voids 23, and the porous silicon material 21 has a Si skeleton, a conductive layer containing a transition metal element M, and a carbon coating layer covering the outer surface.

[0044] (All-solid-state lithium-ion secondary battery) This power storage device is preferably an all-solid-state lithium-ion secondary battery. In an all-solid-state battery, changes in performance due to the electrolyte can be more effectively suppressed, and furthermore, safety can be enhanced, which is preferable. This all-solid-state lithium-ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode using the above-described porous silicon material as a negative electrode active material, and a solid electrolyte interposed between the positive electrode and the negative electrode for conducting lithium ions. The positive electrode can use any of those shown in the above-described power storage device. Also, the negative electrode can use the above-described porous silicon material as the negative electrode active material.

[0045] The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La, and Zr. This solid electrolyte has a basic composition of Li 7.0+x-y (La 3-x , A x )(Zr 2-y , T y )O 12 and may be of this form. However, A is one or more of Sr and Ca, T is one or more of Nb and Ta, and it satisfies 0 < x ≦ 1.0 and 0 < y < 0.75. Alternatively, the solid electrolyte has a basic composition (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 or, (Li7-3z+x-y M z )(La 3-x A x )(Y 2-y T y )O 12 The garnet-type oxide may be represented by the formula (1). In the formula, the transition metal element M may be one or more of Al and Ga, the element A may be one or more of Ca and Sr, and T may be one or more of Nb and Ta, and 0≦z≦0.2, 0≦x≦0.2, and 0≦y≦2 may be satisfied. In this basic composition formula, it is more preferable that 0.05≦z≦0.1 is satisfied. In this basic composition formula, it is more preferable that 0.05≦x≦0.1 is satisfied. Furthermore, in this basic composition formula, it is more preferable that 0.1≦y≦0.8 is satisfied. In these ranges, the ionic conductivity can be more favorably achieved.

[0046] Alternatively, the solid electrolyte may be, for example, a common LiN or LISICON. 14 Zn(GeO4)4, Li sulfide 3.25 Ge 0.25 P 0.75 S4, perovskite-type La 0.5 Li 0.5 CrO3, (La 2 / 3 Li 3x □ 1 / 3-2x )CrO3 (□: atomic vacancy), garnet-type Li7La3Zr2O 12 , LiCr2(PO4)3, called NASICON type, Li 1.3 M 0.3 Cr 1.7 (PO3)4 (M = Sc, Al), etc. Also, Li7P3S obtained from glass ceramics with a composition of 80Li2S 20P2S5 (mol%) 11 Furthermore, Li, a sulfide-based material with high conductivity, 10Examples of glass-based inorganic solid electrolytes include Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li4SiO4, Li2S-P2S5, Li3PO4-Li4SiO4, Li3BO4-Li4SiO4, and those that use SiO2, GeO2, B2O3, or P2O5 as the glass-based substance and Li2O as the network modifier. Examples of thiolithium solid electrolytes include Li2S-GeS2, Li2S-GeS2-ZnS, Li2S-Ga2S2, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S-GeS2-SbS5, Li2S-GeS2-Al2S3, Li2S-SiS2, Li2S-P2S5, Li2S-Al2S3, LiS-SiS2-Al2S3, Li2S-SiS2-P2S5, etc. These solid electrolytes may be formed into a plate shape and placed between the positive electrode and the negative electrode.

[0047] As described above in detail, the present disclosure can further improve the charge-discharge characteristics of porous silicon materials. The reason for this effect is believed to be as follows. For example, while electrodes using Si as the active material can achieve a larger capacity than graphite electrodes, the volume change of Si during charge and discharge can sometimes result in a decrease in rate characteristics and capacity retention. In response to this, providing a carbon coating layer on the outer surface of porous particles to more appropriately reduce the specific surface area can further suppress particle shape change and further reduce the resistance of the Si particles, thereby improving both cycle characteristics and rate characteristics. It is believed that this reduces expansion and contraction during charge and discharge, further improving charge-discharge characteristics such as the capacity retention rate and charge-discharge efficiency during charge-discharge cycles.

[0048] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

[0049] For example, the present disclosure may be any of the following [1] to

[10] . [1] It contains Si, Al, and transition metal element M, and has a specific surface area of 30 cm 2 / g or less due to nitrogen adsorption, is particulate and has a carbon coating layer on its surface, a porous silicon material. [2] The porous silicon material according to [1], wherein the carbon coating layer has a thickness in the range of 1 nm or more and 500 nm or less. [3] The porous silicon material according to [1] or [2], wherein the carbon coating layer is formed on the outer peripheral surface of the silicon particles. [4] The transition metal element M is Cr, and the porous silicon material according to any one or more of [1] to [3] contains one or more of CrSi2, Cr(Si,Al)2, and Al 13 Cr4Si4. [5] The porous silicon material according to any one or more of [1] to [4], wherein the carbon coating layer contains graphite. [6] A positive electrode containing a positive electrode active material, a negative electrode containing the porous silicon material according to any one or more of [1] to [5] as a negative electrode active material, an ion conduction medium interposed between the positive electrode and the negative electrode and conducting lithium ions, [[ID=​​​​​​​​​​​​​​[8] The method for producing a porous silicon material according to [7], wherein the forming step forms the carbon coating layer containing graphite. [9] The method for producing a porous silicon material according to [7] or [8], wherein the forming step forms the carbon coating layer to a thickness in the range of 1 nm to 500 nm.

[10] The method for producing a porous silicon material according to any one or more of [7] to [9], wherein in the forming step, the carbon coating layer is formed so that the carbon content relative to the entire porous silicon material is in the range of 0.1 mass % to 20 mass %. [Example]

[0050] Specific examples of fabricating porous silicon and electricity storage devices according to the present disclosure are described below as experimental examples. Experimental Examples 1 to 4 correspond to working examples of the present disclosure, Experimental Examples 5 to 10 correspond to comparative examples, and Experimental Examples 11 to 13 correspond to reference examples.

[0051] [Preparation of porous silicon materials] The raw materials of Al, Si and Cr are treated as the basic composition formula Al 100-x-y Si x Cr y The alloys were weighed to have a composition of (x = 10 to 40, y = 0 to 10) and melted in an arc melting furnace. Before melting, the temperature in the arc melting furnace was 8 × 10 -3 The pressure was reduced to below 100 Pa, and then the mixture was purged with Ar gas. To produce the master alloy, the raw material powders needed to be melted, and high-frequency melting was performed to produce a uniform sample. The resulting master alloy was heated to 1000–1300°C in an Ar atmosphere and melted. It was then rapidly solidified using a gas atomization method at a rate of 102 K / sec or more to obtain AlSiCr alloy powder (precursor processing). The diameter of this alloy powder, as measured by SEM observation, was in the range of 0.5 μm to 3 mm. The resulting alloy was immersed in a 0.1–3 N hydrochloric acid solution and treated at a temperature between room temperature and 80°C for 1–48 hours to selectively remove Al and other elements. The residue was transferred to a filter, and after removing the acid by pressure filtration, it was washed with distilled water at least four times. The washing water was removed by the same pressure filtration method, and the mixture was dried in a vacuum dryer at 100°C to obtain porous silicon (porosity processing).

[0052] (Graphite film formation) To coat the prepared porous silicon with a graphite film, thermal CVD was performed under the following two conditions. The obtained porous silicon powder was placed on a carbon boat and placed in a tube furnace. The furnace atmosphere was replaced with inert Ar gas and heated to 600–1000°C. Under this condition, Ar / hydrocarbon gas was flowed at a rate of 300 mL / min to induce a reaction, forming a graphite protective layer on the porous particle surface (Processing Condition 1). Furthermore, while the furnace atmosphere had previously been replaced with Ar / hydrocarbon gas, a mobile heating unit heated to 600–1000°C was moved to the sample location while the Ar / hydrocarbon gas was flowing. This heated the porous silicon powder and formed a graphite coating on the particle surface (Processing Condition 2). Ethylene was used as the hydrocarbon gas serving as the graphite source, and the film was formed at a low temperature of approximately 650°C. The graphite coating amount was controlled within the range of 1–30 mass% by varying the reaction time within the range of 5–120 minutes. Assuming that the weight of the sample increased due to the generation of graphite during the thermal CVD treatment, the amount of graphite coating was evaluated from the change in weight before and after the treatment.

[0053] (Experimental Examples 1 to 4) The mother alloy composition in the above basic composition formula where x=30 and y=1 is Al 69 Si 30 Experimental Example 1 was a porous silicon material in which the graphite coating amount was 6 mass % under treatment condition 1, and Experimental Example 2 was a porous silicon material that had undergone the same process as Experimental Example 1 except that the graphite coating amount was 17 mass %. 74.5 Si 25 Cr 1.5 The porous silicon material of Experimental Example 3 was obtained by the same process as Experimental Example 1, except that the amount of graphite coating was 13 mass %. 67 Si 30 A porous silicon material of Experimental Example 4 was obtained by the same process as Experimental Example 1, except that Cr3 was used and the graphite coating amount was 13 mass %.

[0054] (Experimental Examples 5-10) The mother alloy composition in the above basic composition formula where x=30 and y=3 is Al67 Si 30 Experimental Example 5 was a porous silicon material obtained by the same process as Experimental Example 1 except that the carbon coating was omitted and the graphite coating amount was 13 mass % under Processing Condition 2. Experimental Example 6 was a porous silicon material obtained by the same process as Experimental Example 1 except that the graphite coating amount was 13 mass % under Processing Condition 2. 74.5 Si 25 Cr 1.5 The porous silicon material of Experimental Example 8 was obtained by the same process as Experimental Example 1 except that the carbon coating was omitted. 80 Si 20 The porous silicon material of Experimental Example 9 was obtained by the same process as Experimental Example 1 except that the carbon coating was omitted. 80 Si 20 A porous silicon material of Experimental Example 10 was obtained through the same steps as in Experimental Example 1, except that the amount of graphite coating was 13 mass %.

[0055] (Physical property measurement of porous silicon materials) The acid-treated porous silicon powder was observed and analyzed using a scanning electron microscope (SEM, Hitachi S-4300) and an energy dispersive X-ray analyzer (EDAX, Hitachi S-4300). Similarly, a scanning transmission electron microscope (STEM, JEOL JEM2100F) was used to perform elemental image mapping and structural observation. Furthermore, an X-ray diffraction measurement was performed using an X-ray diffractometer (Rigaku RINT-TTR) with a Cu tube in the range of 2θ = 10° to 80° at a rate of 5° / min. Furthermore, a Raman spectrum measurement was performed using a JASCO NRS-3300 laser Raman spectrometer with an excitation wavelength of 532 nm and a laser power of 0.01 mW or 0.001 mW. The pore size distribution and specific surface area were also evaluated. The pore size distribution was evaluated using mercury intrusion porosimetry and nitrogen adsorption. Mercury intrusion porosimetry was performed using a mercury porosimeter (Quantachrome Powermaster 60GT). Specific surface area measurements were performed using a gas adsorption / desorption analyzer (Quantachrome Autosorb-1) to measure nitrogen adsorption / desorption. Nitrogen adsorption / desorption measurements were performed at liquid nitrogen temperature (77 K) with a P / P0 ratio in the range of 0 to 1. The desorption side of this adsorption / desorption isotherm was used to calculate the pore size distribution curve using the BJH method. The acid-treated porous silicon powder was dissolved in HF and HNO3, and elemental analysis was performed using ICP optical emission spectroscopy (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II).

[0056] Table 1 summarizes the master alloy composition of the porous precursor before the carbon coating layer was formed, the Si phase and Cr(Al,Si)2 contents (mass%) evaluated from the XRD pattern, the average pore diameter (nm) evaluated by mercury porosimetry, the porosity (volume%) evaluated by mercury porosimetry, and the compositional analysis results (at%) evaluated by EDX. Although an SiO2 XRD peak was not observed, the contents of each phase were calculated from the compositional analysis assuming that the layer consisted of Si phase, Cr(Al,Si)2 phase, and SiO2 phase. A certain amount of Si content is necessary to ensure the capacity of the negative electrode. The compositions of Experimental Examples 1, 3, 4, and 11-13 had Si contents of 58-97 mass%, indicating that the main phase was Si. Furthermore, while a small pore size is advantageous for crush resistance, these compositions were found to achieve an average pore diameter of 25-60 nm. Furthermore, compositional analysis of the prepared samples indicated that they contained 4-15 at% Al.

[0057] [Table 1]

[0058] (Results and Discussion) Master alloy composition Al 69 Si 30 We measured the Raman spectrum of a sample in which atomized Cr1 powder was made porous by acid treatment and then coated with a 13 mass% carbon coating layer. Figure 4 shows the results of Raman measurement of carbon-coated porous silicon treated at 650°C. Figure 5 shows the results of Raman measurement of carbon-coated porous silicon treated at 850°C. In the sample before coating, only amorphous silicon peaks were observed. In the sample treated by CVD at 650°C or higher, the silicon peak intensity changed, indicating increased silicon crystallinity. Graphite peaks also appeared, confirming the formation of a graphite layer.

[0059] One possible way to improve Coulomb efficiency is to reduce the specific surface area while maintaining high conductivity. Figure 6 shows the relationship between the heat treatment temperature and the specific surface area of ​​porous silicon. When the fabricated porous silicon was heated and its specific surface area was measured, we confirmed that the specific surface area tended to decrease with increasing heat treatment temperature. However, increasing the heat treatment temperature tends to decrease the powder's resistance, so simple heat treatment alone does not improve Coulomb efficiency. Here, the formation of a graphite coating can compensate for the decreased conductivity. To confirm the effectiveness of the carbon coating, a graphite-coated sample powder was placed on a piece of paper and inserted into two probes connected to a tester at a fixed distance (1 cm) to measure the resistance. Figure 7 shows the relationship between the amount of carbon coating and the powder resistance of porous silicon. Figure 8 shows the relationship between the heating temperature and the powder resistance of porous silicon. Samples without graphite coating showed high resistance values, exceeding the measurement range of 300 MΩ. It was found that the powder resistance was affected by the thermal CVD processing temperature and the amount of coating. In the sample processed at 650°C, the resistance did not decrease even with a large coating amount. This is thought to be due to the graphite film quality as well as the increased crystallinity of the Cr silicide. It was found that the resistance significantly decreased when the processing temperature was increased to 850°C or higher. Comparing the samples processed at 850°C, it was confirmed that the resistance generally decreased with a larger coating amount. Furthermore, it was revealed that the resistivity of the sample deposited under CVD processing condition 1 decreased with a smaller coating amount compared to the sample deposited under processing condition 2. At the same time, the specific surface area of ​​the sample prepared under processing condition 2 was higher than that of a sample simply heated, which is disadvantageous for improving Coulomb efficiency, but it was confirmed that the specific surface area of ​​the sample prepared under processing condition 1 decreased.

[0060] The thickness of the graphite film on the surface of the prepared sample was measured using a TEM. Figure 9 shows TEM images of porous silicon prepared under processing conditions 1 and 2, with Figures 9A and 9B being for processing condition 1 and Figure 9C being for processing condition 2. Figure 10 shows the relationship between the amount of carbon film calculated from the specific surface area and the film thickness. The TEM measurement was carried out on porous silicon prepared under processing conditions 1 and 2, with Figures 9A and 9B being for processing condition 1 and Figure 9C being for processing condition 2. 69 Si 30The porous silicon used was Cr1, treated at 650°C, and coated with 13% carbon by mass. If the graphite film completely covers the inside of the pores, the thickness of the graphite film can be evaluated from the specific surface area of ​​the sample after carbon coating. The density of graphite is 2.0 g / cm 3 If this is the case, the film thickness evaluated from the specific surface area and the measured film thickness for the sample prepared under processing condition 2 matched, indicating that the sample was thinly coated all the way to the inside. In contrast, for the sample prepared under processing condition 1, the film thickness on the surface of the sample was thicker than the film thickness evaluated from the specific surface area, indicating that the graphite film was segregated to the specific surface area of ​​the sample. It is presumed that under processing condition 2, the graphite film formed simultaneously with heating prevents the inner wall of the sample from clogging, resulting in a larger specific surface area.

[0061] To evaluate battery performance, composite electrodes were fabricated. A slurry was prepared by mixing 5% by mass of polyimide binder with the active material in N-methylpyrrolidone (NMP) as the solvent. No conductive material was added to increase the electrode capacity. The electrode composite was applied to a copper current collector foil using the doctor blade method, dried at 150°C, and then densified using a roll press. The composite electrodes were then heat-treated at 400°C in Ar gas to obtain composite electrodes. The electrodes were punched out with a 16 mm diameter punch and used in a Li half-battery cell containing a 1 mol% LiPF6 electrolyte to evaluate charge-discharge characteristics. The graphite coating sometimes peeled off when touched after deposition, and the adhesion strength between the current collector foil and the active material tended to be lower than when the coating was not applied. This tendency was more pronounced in samples fabricated under treatment condition 2 than under treatment condition 1.

[0062] The resulting evaluation cells were subjected to 100 charge-discharge cycles at a current density of 0.2 C over a battery voltage range of 0.005 V to 1.5 V. Figure 11 shows the results of measurements of cycle performance with and without a graphite coating and treatment conditions. Without carbon coating, the Cr-free composition lost nearly zero capacity after approximately 50 cycles, whereas the Cr-containing composition maintained a capacity of over 500 mAh / g even after 100 cycles. Carbon coating under condition 2 did not improve cycle performance due to a decrease in adhesion strength. In contrast, the carbon-coated sample under treatment condition 1 maintained a high capacity of nearly 1000 mAh / g even after 100 cycles. However, even for samples prepared under treatment condition 1, the Cr-free active material was found to be unable to maintain sufficient capacity. Because carbon coating reduces adhesion, further battery performance was evaluated using samples with an increased polyimide binder content of 15 mass%.

[0063] Figure 12 shows the results of evaluating the rate characteristics with and without carbon coating. Figure 13 shows the results of evaluating the effect of the presence or absence of carbon coating on the initial coulombic efficiency. Figure 14 shows the results of evaluating the effect of the amount of carbon coating on the high-rate capacity retention rate. Without carbon coating, the capacity at low rates was high, but above 0.5C, the decrease in capacity became significant. In contrast, in the case of carbon-coated samples with small capacities, the theoretical capacity decreased, so the capacity at low rates was smaller than that without carbon coating, but when charged and discharged at high rates, the capacity increased compared to the case without coating. Furthermore, as shown in Figure 13, samples with various master alloy compositions (Al 100-x-y Si x Cr y ), it was confirmed that the initial coulombic efficiency tended to increase when carbon was coated. Furthermore, as shown in Figure 14, when the capacity retention rate at 0.5C was plotted against the amount of carbon coating, using the capacity when charged and discharged at a low rate of 0.025C as the reference, it was found that when the amount of carbon coating was 20 mass% or less, the capacity retention rate was improved compared to when no carbon coating was used.

[0064] [Table 2]

[0065] It goes without saying that the present disclosure is not limited to the above-described experimental examples, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure. [Industrial Applicability]

[0066] The present disclosure is applicable to the technical field of secondary batteries. [Explanation of symbols]

[0067] 10 Energy storage device, 12 Positive electrode, 13 Positive electrode active material layer, 14 Current collector, 15 Negative electrode, 16 Negative electrode active material layer, 17 Current collector, 18 Ion conducting medium, 21 Porous silicon material, 23 Void.

Claims

1. containing Si, Al, and a transition metal element M; The specific surface area by nitrogen adsorption is 30 cm 2 / g or less, It is particulate and has a carbon coating layer on its surface. Porous silicon material.

2. The porous silicon material according to claim 1 , wherein the carbon coating layer has a thickness in the range of 1 nm to 500 nm.

3. The porous silicon material according to claim 1 , wherein the carbon coating layer is formed on the outer peripheral surfaces of silicon particles.

4. the transition metal element M is Cr, CrSi 2 , Cr(Si,Al) 2 and Al 13 Cr 4 Si 4 3. The porous silicon material according to claim 1, comprising one or more of:

5. The porous silicon material according to claim 1 or 2, wherein the carbon coating layer comprises graphite.

6. a positive electrode including a positive electrode active material; a negative electrode containing the porous silicon material according to claim 1 or 2 as a negative electrode active material; an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions; An electricity storage device comprising:

7. The basic composition formula is Al 100-x-y Si x M y a porous body process in which a molten master alloy (where 10≦x≦40, 0<y≦5, 0<x / y≦0.2) is rapidly cooled to separate it into a conductive phase containing Si, Al, and one or more of a transition metal silicide, a transition metal aluminum, and an aluminum transition metal silicide, and the Al component is selectively removed to produce a porous precursor; a forming step of forming a carbon coating layer on the outer surface of the porous precursor by performing thermal CVD by heating the porous precursor in the presence of a hydrocarbon gas from room temperature to a temperature range of 650°C or higher and 850°C or lower; A method for producing a porous silicon material, comprising:

8. The method for producing a porous silicon material according to claim 7 , wherein the forming step forms the carbon coating layer containing graphite.

9. The method for producing a porous silicon material according to claim 7 or 8, wherein the carbon coating layer is formed to a thickness in the range of 1 nm to 500 nm in the forming step.

10. 9. The method for producing a porous silicon material according to claim 7, wherein in the forming step, the carbon coating layer is formed so that the carbon content relative to the entire porous silicon material is in the range of 0.1 mass % to 20 mass %.

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