Porous silicon material, electricity storage device and production method of porous silicon material

Abstract

To further improve the charge and discharge characteristics of a porous silicon material.SOLUTION: The porous silicon material of the present disclosure contains a Si phase and a conductive phase containing 15 mass% or less of a Si-V compound, in which SiO2 is 20 mass% or less, a mass ratio So / Vs of SiO2 to the conductive phase is 1.5 or less, and a porosity determined by a mercury injection method is 80 vol.% or less.SELECTED DRAWING: Figure 4

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, a method for producing porous silicon material has been proposed that includes a precursor process in which a raw material containing Al, Si, and V 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). In this production method, the silicon skeleton is strengthened by fine Si-V compounds, which makes it possible to further suppress deterioration in charge-discharge characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-154884 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the manufacturing method of Patent Document 1 mentioned above, although the inclusion of Al, V, etc. increases the strength and can further suppress the deterioration of charge-discharge characteristics, it is still not sufficient, and further improvement is desired.

[0005] The present disclosure has been made in consideration of the above-mentioned problems, and has as its main object to provide a porous silicon material, an electricity 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 problem]

[0006] As a result of intensive research to achieve the above-mentioned object, the present inventors have discovered that the charge-discharge characteristics can be further improved by preparing a silicon alloy containing Al, Si, and V and optimizing the conditions for removing Al, and have completed the porous silicon material, electricity storage device, and method for producing the porous silicon material of the present disclosure.

[0007] That is, the porous silicon material of the present disclosure has: A conductive phase containing a Si phase and 15 mass % or less of Si-V compounds, SiO2 is 20 mass% or less, The mass ratio So / Vs of SiO2 to the conductive phase is 1.5 or less, The porosity determined by mercury intrusion porosimetry is 80% by volume or less.

[0008] The electricity storage device of the present disclosure includes: a positive electrode including a positive electrode active material; a negative electrode containing the porous silicon material as a negative electrode active material; an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions; It is equipped with the following.

[0009] The method for producing a porous silicon material of the present disclosure includes: a precursor process in which a raw material containing Al, Si, and V is melted and rapidly solidified to obtain a precursor of a silicon alloy; a porosity forming step of removing the Al component contained in the silicon alloy with an acid having a concentration of less than 0.5 mol / L to obtain a porous silicon material; 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, silicon negative electrodes for lithium-ion secondary batteries exhibit a higher theoretical capacity than graphite and the like, but volume changes during charge / discharge can make it difficult to maintain their structure. This porous silicon material has fine pores, such as nano-sized ones, and the silicon skeleton is strengthened by fine Si-V compounds, thereby strengthening the structure and further suppressing deterioration of charge / discharge characteristics. On the other hand, this porous silicon material undergoes an Al removal process, but this process can sometimes oxidize Si, resulting in increased resistivity. In the present disclosure, by adjusting the Al removal process conditions, a conductive phase containing Si-V compounds is sufficiently present in the structure and Si oxidation is further suppressed, thereby reducing resistivity and further improving charge / discharge characteristics. [Brief explanation of the drawings]

[0011] [Figure 1] Al-Si-V phase diagram at 2.5at%V. [Figure 2] Cross-sectional schematic diagram of the microstructure formation process during the cooling process of a melt droplet. [Figure 3] Al-Si-V phase diagram. [Figure 4] FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10. [Figure 5] XRD patterns of porous silicon materials of Experimental Examples 1 to 3. [Figure 6] Photographs of the supernatant liquid of the acid treatment in the porosity-forming step of Experimental Examples 1 to 3. [Figure 7] Pore ​​size distribution measurement results for Experimental Examples 1 to 3 measured by mercury intrusion porosimetry. [Figure 8] 1 shows a cross-sectional secondary electron image and element distribution of the porous silicon material of Experimental Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Method of manufacturing porous silicon material) The method for producing a porous silicon material according to the present disclosure includes a precursor process and a porosity process. In the precursor process, a raw material containing Al, Si, and V is melted and rapidly solidified to obtain a silicon alloy precursor. In the porosity process, the Al component contained in the silicon alloy is removed with an acid having a concentration of less than 0.5 mol / L to obtain a porous silicon material. First, the raw material composition will be described.

[0013] In the equilibrium phase diagram (eutectic system) of a binary Al-Si alloy, when a melt of a eutectic composition where the liquidus line is at its minimum is cooled and solidified, the Al phase and the Si phase simultaneously crystallize and form a fibrous (lamellar) phase-separated eutectic structure. At this time, the size of the formed lamellar structure becomes finer as the cooling rate increases, and for example, 10 2 At cooling rates of 1000 K / s or faster, a nano-sized structure is formed. If it is possible to selectively remove only the Al element from this eutectic structure by acid treatment or other methods, it is possible to obtain a porous material consisting of a Si skeleton that retains the characteristics of the eutectic structure.

[0014] The equilibrium phase diagram for ternary Al-Si-V alloys also includes a eutectic composition in which both Al and Si phases simultaneously crystallize from the melt. Figure 1 shows an example of this, the Al-Si-V phase diagram for 2.5 at% V. Figure 2 shows a schematic diagram of the microstructure formation process during the cooling process of a eutectic melt droplet. In this system, when a eutectic melt (Figures 1(1) and 2(1)) is cooled, VSi2 crystallizes as the primary crystal, resulting in VSi2 particles dispersed in the liquid phase (Figures 1(2) and 2(2)). Upon cooling, Al and Si phases simultaneously crystallize from the melt between the VSi2 particles, resulting in a porous material with a VSi2 reinforcing phase dispersed within the Si framework (Figures 1(3) and 2(3)). Because the strength of VSi2 is approximately twice that of Si, the presence of VSi2 as a reinforcing phase can improve the pressure resistance of composites. For these reasons, it is preferable that the raw material composition be such that a eutectic structure of the Al phase and the Si phase is obtained, and that a Si-V compound phase such as the VSi2 phase is obtained. The raw material composition may be set within a range that results in a desired phase structure based on the equilibrium diagram, taking into account the effect of rapid cooling as necessary.

[0015] Figure 3 shows the phase diagrams of the Al-Si-V system at 2.5 at% V, 10 at% V, and 20 at% V. Figure 3A shows the phase diagram for the Al-Si-V system at 2.5 at% V, Figure 3B shows the phase diagram for the Al-Si-V system at 10 at% V, and Figure 3C shows the phase diagram for the Al-Si-V system at 20 at% V. At low Si content, all of the Si forms the VSi2 phase, preventing the stable existence of the Si phase. However, above a certain Si content, a eutectic composition of Al + Si phase exists, and it is presumed that the Al + Si + VSi2 phase becomes stable at low temperatures. From the phase diagrams in Figures 3A-C, it can be seen that the amount of Si required for Si stabilization increases with increasing V content. The amount of Si phase is important because the negative electrode capacity depends on the amount of Si phase contained in the porous structure. The equilibrium phase diagrams in Figures 1 and 3 were obtained by calculation using the CALPHAD method.

[0016] (Precursor process) In the precursor process, when the total of Al, Si, and V is 100 at%, it is preferable to use a raw material containing Si in the range of 10 at% or more and 60 at% or less, and V in the range of 1 at% or more and 10 at% or less. The raw material may contain inevitable impurities in addition to Al, Si, and V. Inevitable impurities are components that inevitably remain during the purification of any of Al, Si, and V, and examples include Fe, C, Cu, Ni, P, and the like. It is preferable that the inevitable impurities are less, for example, when the total of Al, Si, and V is 100 at%, 5 at% or less is preferable, and 2 at% or less is more preferable. The blending ratio of Al may be 30 at% or more, 40 at% or more, or 50 at% or more. Also, the blending ratio of Al may be 90 at% or less, 89 at% or less, or 85 at% or less. The blending ratio of Si may be 12.5 at% or more or 15 at% or more. Also, the blending ratio of Si may be 50 at% or less or 40 at% or less. The blending ratio of V may be 1.5 at% or more or 2 at% or more. Also, the blending ratio of V may be 7.5 at% or less or 5 at% or less. In a silicon alloy containing Al, Si, and V within such ranges, it is preferable because the porosity can be increased and pores with a more suitable shape and size can be obtained. In this process, a silicon alloy represented by the general formula Al 100-x-y Si x V y (where 0 < x < 100, 0 < y < 100) may be used as the master alloy. In this process, it is preferable to use a silicon alloy containing Al with a predetermined composition that can obtain a eutectic structure. The predetermined composition may be the eutectic composition, near the eutectic composition, or may have a predetermined width such as a part of the hypoeutectic composition or the hypereutectic composition. For example, it may include a range of ±5 mass% with respect to the eutectic composition.

[0017] In this step, when melting the raw materials, any melting method may be used, but high-frequency crucible melting in an inert gas atmosphere such as Ar is preferred. When rapidly cooling and solidifying the molten raw materials, any rapid cooling method may be used, but the cooling rate is preferably more rapid, for example, 10 2 ℃ / s or more 10 8 The solidification rate may be in the range of °C / s or less. Rapid solidification may be achieved, for example, by casting a molten silicon alloy (a molten raw material) into a mold and quenching it. However, it is preferable to quench the molten silicon alloy (melt) by one or more of gas atomization, water atomization, and roll quenching. In the precursor process, the silicon alloy obtained from the raw material may be granulated. In this granulation process, an ingot obtained by mold casting may be crushed and granulated. Furthermore, since the gas atomization and water atomization methods described above produce alloy powder, this may be used for granulation. Furthermore, since the roll quenching method described above produces a thin strip alloy, this may be subsequently crushed and granulated (granulated). Since the powder obtained by the roll quenching method has a fine alloy structure, porous silicon with fine pores can be obtained after the leaching process. Among these, gas atomization is more preferable as a method for granulating the silicon alloy. In gas atomization, the molten silicon alloy is preferably produced in an Ar atmosphere, and the granulation process is preferably carried out in an Ar or He atmosphere.

[0018] In the precursor process, the silicon alloy is preferably granulated to an average particle size in the range of 0.1 μm to 100 μm. The average particle size of these particles is preferably 0.5 μm to 10 μm, more preferably 1 μm to 5 μm, and even more preferably 1 μm to 3 μm. The silicon alloy particles may be appropriately selected depending on the properties required for the energy storage device. Here, 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 of that particle, and dividing this by the number of particles to obtain an average value. The particles obtained by this granulation process will have the average particle size of the aggregate of porous particles to be ultimately obtained.

[0019] In this precursor process, a raw material containing Al, Si, and V as well as one or more second elements selected from 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, Si, and V, and is, for example, preferably in the range of 10% by mass or less, more preferably 5% by mass or less, based on the total silicon alloy.

[0020] (Porous process) The porous forming step involves removing Al components, i.e., Al phases and their compounds, from the silicon alloy. Examples of Al components include Al and its compounds. V and its compounds may also be removed in this step. In this step, the Al components contained in the silicon alloy are preferably selectively removed using an acid with a concentration of less than 0.5 mol / L. The acid used is preferably one that dissolves the Al components in the silicon alloy but does not dissolve the Si components, i.e., the Si phase and its compounds. Examples of the acid used include inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid, and organic acids such as formic acid, acetic acid, and citric acid. Among these, hydrochloric acid is preferred. The acid is preferably an aqueous solution. The acid concentration is not particularly limited as long as it can remove the Al components. For example, it can be in the range of 0.01 mol / L or more but less than 0.5 mol / L. The concentration is more preferably 0.05 mol / L or more, and may be 0.1 mol / L or more. The concentration is more preferably 0.4 mol / L or less, and may be 0.2 mol / L or less. This removal treatment is preferably carried out under milder conditions, for example, at a temperature in the range of room temperature (25°C) to 80°C, or at 30°C or below, or 25°C or below. Furthermore, the removal treatment is preferably carried out by immersing the silicon alloy particles in acid for 0.1 to 48 hours, and stirring may be performed. The immersion time may be adjusted depending on the acid concentration, and may be 24 hours or less, 12 hours or less, or 5 hours or less. The obtained porous silicon material is then washed and dried.

[0021] The porosity-imparting step is intended to obtain a porous silicon material containing a Si phase and a conductive phase containing 15 mass% or less of Si-V compounds. It is preferable that the conductive phase be present in an appropriate amount within the structure from the viewpoint of resistivity. The conductive phase may be 14 mass% or less, or 10 mass% or less. The conductive phase may be 5 mass% or more, or 7.5 mass% or more. The porosity-imparting step is intended to obtain a porous silicon material containing 20 mass% or less of SiO2. The SiO2 content is preferably lower, more preferably 16 mass% or less, and may be 15 mass% or less, 12 mass% or less, or 10 mass% or less. The SiO2 content may be 0.1 mass% or more, 1 mass% or more, or 2 mass% or more. The porosity-imparting step is intended to obtain a porous silicon material having a mass ratio So / Vs of SiO2 (So) to the conductive phase (Vs) of 1.5 or less. A smaller mass ratio is preferable because the resistivity of the material can be further reduced. This mass ratio is preferably 1.4 or less, more preferably 1.2 or less, and may be 1.1 or less. This mass ratio may be 0.1 or more, or 0.5 or more. This porosity step is intended to produce a porous silicon material having a porosity of 80 volume % or less as determined by mercury intrusion porosimetry. This porosity may be, for example, 75 volume % or less, 50 volume % or more, or 60 volume % or more. This porosity step preferably produces a porous silicon material having an average pore diameter as determined by mercury intrusion porosimetry in the range of 50 nm to 70 nm. This porosity step preferably produces a porous silicon material having a V content in the range of 1 at % to 10 at %, where the total of Al, Si, and V is 100 at %.

[0022] The porous silicon material may be obtained in the porosity forming step, which contains a Si phase and a Si-V compound phase. The Si phase is presumed to form the silicon bone and to be responsible for the charging and discharging of the power storage device. The Si-V compound phase is presumed to reduce the resistivity and to be responsible for reinforcing the silicon bone. Examples of Si-V compounds include Si x V y(x and y are arbitrary numbers), and examples include Si2V and Si3V5. The Si-V compound phase is a (Si,Al) phase in which part of Si is replaced by Al. x V y The Si-V compound phase may be poorly soluble in acid. The Si phase or the Si-V compound phase may contain Al as a solid solution.

[0023] The porosity-imparting step may be intended to obtain a porous silicon material having a porosity of 80% by volume or less as determined by mercury intrusion porosimetry. A larger porosity is preferable because it is more responsive to volume changes during occlusion of carrier ions. This porosity may be the porosity of pores of 1 μm or less as determined by mercury intrusion porosimetry. The porous silicon material obtained by the porosity-imparting step may appropriately have the characteristics described below for porous silicon materials.

[0024] (porous silicon material) The porous silicon material of the present disclosure may be one produced by the above-described manufacturing method. Here, the physical properties of the porous silicon material are the same as those of the above-described manufacturing method, and detailed description thereof will be omitted.

[0025] This porous silicon material includes a conductive phase containing a Si phase and a Si-V compound. This porous silicon material may have a Si bone structure formed of the Si phase and a reinforcing phase that is a Si-V compound phase. The Si bone structure may retain the characteristics of the eutectic structure of the precursor silicon alloy, or may form a three-dimensional network structure with voids. The Si-V compound phase may be in the form of nanoparticles, arranged between the Si bones and serving as pillars supporting the Si bones. It is believed that by incorporating a conductive phase as a reinforcing phase in addition to the Si bone structure, this porous silicon material can reduce the resistivity and improve its pressure resistance. In this porous silicon material, the Si-V compound phase is preferably VSi2. VSi2 has a strength approximately twice that of Si, making it highly functional as a reinforcing phase.

[0026] The porous silicon material includes a Si phase and a conductive phase containing 15% by mass or less of an Si-V compound. From the viewpoint of resistivity, it is preferable for the conductive phase to be present in an appropriate amount within the structure. The conductive phase may be 14% by mass or less, or 10% by mass or less. The conductive phase may be 5% by mass or more, or 7.5% by mass or more. The porous silicon material has an SiO2 content of 20% by mass or less. A lower SiO2 content is preferable, more preferably 16% by mass or less, and may be 15% by mass or less, 12% by mass or less, or 10% by mass or less. The SiO2 content may be 0.1% by mass or more, 1% by mass or more, or 2% by mass or more. The porous silicon material has a mass ratio So / Vs of SiO2 (So) to the conductive phase (Vs) of 1.5 or less. A smaller mass ratio is preferable because it further reduces the resistivity of the material. This mass ratio is preferably 1.4 or less, more preferably 1.2 or less, and may be 1.1 or less. This mass ratio may also be 0.1 or more, or 0.5 or more. In this porous silicon material, the V content is preferably in the range of 1 at% or more and 10 at% or less, where the total of Al, Si, and V is taken as 100 at%. This V content may be 8 at% or less, or may be 5 at% or less. This V content may also be 2 at% or more, or may be 2.5 at% or more.

[0027] From the viewpoint of the charge / discharge capacity of the power storage device, it is preferable that the porous silicon material contains a large amount of Si phase, and from the viewpoint of bone reinforcement, it is preferable that the porous silicon material contains a large amount of Si-V compound phase. The porous silicon material may have a Si phase ratio of 30 mass% or more, 35 mass% or more, or 50 mass% or more. The Si phase ratio may also be 85 mass% or less, or 80 mass% or less. The porous silicon material may have a Si-V compound phase (preferably VSi2 phase) ratio of 2 mass% or more, 4 mass% or more, or 5 mass% or more. The Si-V compound phase ratio may also be 15 mass% or less, 12 mass% or less, or 10 mass% or less. The Si phase ratio and the Si-V compound phase ratio can be determined by X-ray diffraction (XRD) measurement. The porous silicon material may be composed of the Si phase and the Si-V compound phase, with no other phases detected by XRD.

[0028] This porous silicon material has a porosity (preferably the porosity of pores with a diameter of 1 μm or less) of 80% by volume or less as determined by mercury intrusion porosimetry. From the viewpoint of dispersing the stress acting on each pore, it is preferable for the porous silicon material to have a greater number of fine pores, such as pores of 1 μm or less. This porosity may be 50% by volume or more, or 60% by volume or more. Also, this porosity may be 75% by volume or less, or 70% by volume or less. A larger porosity is preferable because it can better suppress volume changes during absorption of carrier ions. A smaller porosity is preferable because it increases the amount of Si phase present per unit volume.

[0029] Furthermore, this porous silicon material preferably has an average pore diameter, as determined by mercury intrusion porosimetry, in the range of 50 nm to 70 nm. This porous silicon material may have a pore diameter, as determined by mercury intrusion porosimetry, of 1 μm or less, i.e., a pore diameter distribution range of 1 μm or less. This pore diameter may be 1 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more. This pore diameter may be 1000 nm or less, 500 nm or less, 300 nm or less, or 250 nm or less. In this porous silicon material, the average pore diameter, as determined by mercury intrusion porosimetry, may be 10 nm or more, or 20 nm or more. This average pore diameter may be 60 nm or less, or 55 nm or less. Smaller pore diameters are preferred because the pores are less likely to collapse. Furthermore, large pores are preferable because they can further suppress volume changes when carrier ions are occluded.

[0030] This porous silicon material may contain Al in a range of more than 0 at% to 10 at% or less, where the total of Si, V, and Al, excluding oxygen and unavoidable impurities, is 100 at%. The Al content may be 1 at% or more, 3 at% or more, or 4 at% or more. The Al content may be 9 at% or less, or 8 at% or less. This porous silicon material may have a Si content of 50 at% or more, 60 at% or more, or 70 at% or more. The Si content may be 95 at% or less, 90 at% or less, or 87 at% or less. This porous silicon material may have a V content of 1 at% or more, 3 at% or more, or 5 at% or more. The V content may be 25 at% or less, 20 at% or less, or 19 at% or less. The porous silicon material may also contain, as a second element, one or more of Ca, Cu, Mg, Na, Sr, and P in an amount of 15 mass % or less. It is preferable that the amount of elements other than Si is as small as possible.

[0031] When this porous silicon material is used as an electrode and subjected to a confining pressure of 1 GPa, the change in porosity is preferably less than 20 vol%, more preferably less than 15 vol%, and even more preferably less than 10 vol%. From the viewpoint of bone strength, it is preferable that the decrease in porosity of the porous silicon material when subjected to a confining pressure is as small as possible.

[0032] The porous silicon material preferably has an average particle size of 0.1 μm or more, more preferably 1 μm or more, and may be 5 μm or more, and the particles preferably have an average particle size of 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less.

[0033] (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, however, it is preferably a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, air batteries, and the like. 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. Compared to an electrode fabricated with a porosity of 5% to 50% by volume, an electrode fabricated with a porosity of 50% to 95% by volume and then compressed to this range exhibits better charge / discharge characteristics due to the shape of the pores, etc. For example, when porous silicon particles are used as a negative electrode active material for a lithium ion secondary battery, the smaller the pores, the more uniform the lithium ions will be when alloyed, reducing stress concentration and preventing deterioration of the electrode itself. The porosity of the compressed porous silicon material may be adjusted as appropriate depending on the properties required of the electrode for an electrical storage device, and may be, for example, 5% by volume or more, 10% by volume or more, or 20% by volume or more. The porosity of the compressed porous silicon material may be, for example, 40% by volume or less, 30% by volume or less, or 20% by volume or less.

[0034] 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 fluorine-containing rubber; thermoplastic resins such as polypropylene and polyethylene; polyimide (PI), 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 polymer, 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. For these, it is also possible to perform surface oxidation treatment. 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.

[0035] In this electrode, the electrode active material may contain an active material other than the porous silicon material in addition to the porous silicon material within a range where both low restraint pressure and capacity retention rate can be achieved. For example, as the electrode active material, a carbonaceous material, Li4Ti5O 12 etc. may be included. However, from the viewpoint of further increasing the battery capacity, taking the total electrode active material as 100% by mass, for example, the porous silicon material preferably occupies 50% by mass or more, preferably 90% by mass or more.

[0036] (Power storage device) The power storage device of the present disclosure includes an electrode having the above-described porous silicon material. 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 and 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, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, the basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same below) and Li (1-x)Lithium manganese composite oxides such as Mn2O4, with the basic composition formula Li (1-x) Lithium cobalt composite oxides such as CoO2, with the basic composition formula Li (1-x) Lithium nickel composite oxide such as NiO2, the basic composition formula is Li (1-x) Ni a Co b Mn c Lithium 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The shape of this power storage device is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, etc. Further, it may be applied to large-sized ones used in electric vehicles and the like. FIG. 4 is an explanatory diagram showing an example of the structure of the power storage device 10. This power storage device 10 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 13 and a current collector 14. The negative electrode 15 has a negative electrode active material 16 and a current collector 17. The negative electrode active material 16 is the porous silicon material 21 described above and has voids 23.

[0041] It is preferable that this power storage device has a higher capacity retention rate when performing charge-discharge cycles. For example, the capacity retention rate at 50 cycles with a current density of 0.05C is preferably 80% or more, more preferably 82% or more, and even more preferably 85% or more.

[0042] (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 an electrolytic solution can be more suppressed, and further, the 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 which is the electrode for the power storage device described above, and a solid electrolyte interposed between the positive electrode and the negative electrode and conducting lithium ions as well. The positive electrode can use any of those shown in the above-described power storage device. Further, the negative electrode can use the electrode for the power storage device described above.

[0043] 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 such. 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 (Li7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 Ya, (Li 7-3z+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, element M may be one or more of Al and Ga, 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 be satisfied. In this basic composition formula, it is more preferable that 0.05≦x≦0.1 be satisfied. Furthermore, in this basic composition formula, it is more preferable that 0.1≦y≦0.8 be satisfied. In these ranges, the ionic conductivity can be more favorably achieved.

[0044] 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 TiO3, (La 2 / 3 Li 3x □ 1 / 3-2x )TiO3 (□: atomic vacancy), garnet-type Li7La3Zr2O 12 , LiTi2(PO4)3, called NASICON type, Li 1.3 M 0.3 Ti 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.

[0045] The all-solid-state lithium-ion secondary battery may also include a restraining member that restrains a stack of a positive electrode, a solid electrolyte, and a negative electrode in the stacking direction. This restraining member may include, for example, a pair of plate-like members that sandwich the stack from both ends of the stack in the stacking direction, a rod-like member that connects the pair of plate-like members, and an adjustment member that is connected to the rod-like members and adjusts the gap between the pair of plate-like members by a screw structure or the like.

[0046] 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, silicon negative electrodes for lithium-ion secondary batteries exhibit a higher theoretical capacity than graphite and the like, but volume changes during charge / discharge can make it difficult to maintain their structure. This porous silicon material has fine pores, such as nano-sized ones, and the silicon skeleton is strengthened by fine Si-V compounds, thereby strengthening the structure and further suppressing deterioration of charge / discharge characteristics. On the other hand, this porous silicon material is subjected to an Al removal process, but during this process, Si may be oxidized, resulting in an increase in resistivity. In the present disclosure, by adjusting the Al removal process conditions, a conductive phase containing Si-V compounds is sufficiently present in the structure and Si oxidation is further suppressed, thereby reducing resistivity and further improving charge / discharge characteristics.

[0047] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[0048] For example, the present disclosure may be any of the following [1] to

[10] . [1] A conductive phase containing a Si phase and 15 mass% or less of Si-V compounds, SiO2 is 20 mass% or less, The mass ratio So / Vs of SiO2 to the conductive phase is 1.5 or less, The porosity determined by mercury intrusion porosimetry is 80% by volume or less. Porous silicon material. [2] The conductive phase is 10% by mass or less, The porous silicon material according to [1], wherein the SiO2 content is 10 mass % or less. [3] The porous silicon material according to [1] or [2], wherein the V content is in the range of 1 at % to 10 at % when the total of Al, Si, and V is taken as 100 at %. [4] The porous silicon material according to any one of [1] to [3], wherein the porosity is 50% by volume or more. [5] The porous silicon material according to any one of [1] to [4], wherein the average pore diameter determined by mercury intrusion porosimetry is in the range of 50 nm to 70 nm. [6] a positive electrode including a positive electrode active material; a negative electrode containing the porous silicon material according to any one of [1] to [5] 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] a precursor process in which a raw material containing Al, Si, and V is melted and rapidly solidified to obtain a precursor of a silicon alloy; a porosity forming step of removing the Al component contained in the silicon alloy with an acid having a concentration of less than 0.5 mol / L to obtain a porous silicon material; A method for producing a porous silicon material, comprising: [8] The method for producing a porous silicon material according to [7], wherein the porous silicon material obtained in the porosity-forming step includes a conductive phase containing a Si phase and 15 mass % or less of Si-V compounds, SiO2 is 20 mass % or less, the mass ratio So / Vs of SiO2 to the conductive phase is 1.5 or less, and the porosity determined by mercury intrusion porosimetry is 80 volume % or less. [9] The method for producing a porous silicon material according to [7] or [8], wherein the Al component is removed with hydrochloric acid having a concentration of 0.2 mol / L or less in the porosity-forming step.

[10] The method for producing a porous silicon material according to any one of [7] to [9], wherein the porous silicon material obtained in the porosity-forming step has the conductive phase at 10 mass % or less and the SiO2 at 10 mass % or less. [Example]

[0049] 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 3 are working examples of the present disclosure, Experimental Examples 4 to 6 are reference examples, and Experimental Examples 7 and 8 are comparative examples.

[0050] [Preparation of porous silicon materials] The raw materials of Al, Si and V are treated as the basic composition formula Al 100-x-y Si x V y The alloys were weighed to have a composition of (x = 10 to 40, y = 0 to 5) and melted in an arc melting furnace. -3 After reducing the pressure to below 1 Pa, the pressure was replaced with Ar gas. To prepare the master alloy, it was necessary to melt the raw material powder, and high-frequency melting was performed to prepare a uniform sample. The obtained master alloy was heated to 1000-1300°C in an Ar atmosphere to melt it, and then gas atomized to 1000°C. 2 The resulting alloy was rapidly solidified at a rate of 1000 K / sec or more to obtain an AlSiV alloy powder (precursor process). The resulting alloy was immersed in a 0.1-3 mol / L aqueous hydrochloric acid solution and treated at a temperature between room temperature and 80°C for 1-48 hours to selectively remove Al. The residue was transferred to a filter, and the acid was removed by pressure filtration. The residue was then washed with distilled water at least four times, and the washing water was removed by the same pressure filtration method to obtain porous silicon (porosity process).

[0051] (Experimental Examples 1 to 3) Experimental Example 1 was a porous silicon material obtained by setting x = 30 and y = 2.5 in the above basic composition formula and removing Al with 0.1 mol / L hydrochloric acid at room temperature (25°C). Experimental Example 2 was a porous silicon material obtained by setting x = 25 and y = 2.5 in the above basic composition formula and removing Al with 0.1 mol / L hydrochloric acid at room temperature. Experimental Example 3 was a porous silicon material obtained by setting x = 20 and y = 2.5 in the above basic composition formula and removing Al with 0.1 mol / L hydrochloric acid at room temperature.

[0052] (Experimental Examples 4 to 6) Experimental Example 4 was a porous silicon material obtained by setting x = 30 and y = 1 in the above basic composition formula and removing Al with 3 mol / L hydrochloric acid at 50°C. Experimental Example 5 was a porous silicon material obtained by setting x = 25 and y = 2.5 in the above basic composition formula and removing Al with 3 mol / L hydrochloric acid at 50°C. Experimental Example 6 was a porous silicon material obtained by setting x = 20 and y = 2.5 in the above basic composition formula and removing Al with 3 mol / L hydrochloric acid at 50°C.

[0053] (Experimental Examples 7-8) No porous silicon material was prepared, and Si powder (SIE23PB manufactured by Kojundo Chemical Co., Ltd.) with an average particle size of 5 μm was used as is as Experimental Example 7. A porous silicon material was obtained from an Al-Si binary master alloy composition in which x = 20 and y = 0 in the above basic composition formula through the same process as Experimental Example 1 as Experimental Example 8.

[0054] [X-ray diffraction measurement] X-ray diffraction (XRD) measurements were performed on the porous silicon materials of Experimental Examples 1 to 6 and 8. For the X-ray diffraction measurements, an X-ray diffractometer (Rigaku Corporation, RINT-TTR) was used, and X-ray diffraction measurements were performed using a Cu tube in the range of 2θ = 10° to 80° at a rate of 5° / min. X-ray diffraction measurements were also performed on samples in which the acid treatment conditions for the precursor were varied. Furthermore, the abundance ratios of the Si phase and the VSi2 phase (conductive phase) were evaluated using the reference intensity ratio method, using the intensities of the strongest peak of the Si phase (~28.3°) and the strongest peak of the VSi2 phase (~41.8°).

[0055] [SEM observation, composition analysis] Secondary electron images were observed for the cross sections of the porous silicon materials of Experimental Examples 1 to 6 using a scanning electron microscope (SEM, Hitachi S-4300). Furthermore, composition analysis was performed for the porous silicon materials of Experimental Examples 1 to 6 using an energy dispersive X-ray analyzer (EDX) attached to the SEM. Specifically, the powder sample was press-molded into a disk shape at 500 MPa using a carbide mold, and then five random locations on the pressed surface were observed using an SEM (Hitachi High-Tech S-3600N, 1000x magnification). Composition analysis was performed across the entire observation surface, and the average value was calculated as the composition ratio.

[0056] [Pore distribution measurement] The pore distribution of the porous silicon materials of Experimental Examples 1 to 6 was measured using a mercury porosimeter (Quantachrome POWERMASTER60GT).

[0057] [Resistivity measurement of porous silicon materials] The resistivity was measured for the porous silicon of Experimental Examples 1 to 6. A sample powder of the porous silicon material was pressed at 200 MPa, and copper electrodes were attached to it to measure the resistivity by the four-terminal method.

[0058] [Evaluation of battery performance using evaluation cells with non-aqueous electrolyte] Lithium-ion secondary batteries were fabricated using the silicon materials of Experimental Examples 1-4 and 7-8 as negative electrode active materials, and the discharge capacity and capacity retention were evaluated. 60% by mass of the negative electrode active material was mixed with 20% by mass of acetylene black (average particle size 2 μm) as a conductive material and 20% by mass of polyimide as a binder, and N-methylpyrrolidone was added and stirred to prepare a negative electrode slurry. Next, this negative electrode slurry was applied to a 20 μm-thick copper foil, dried, and rolled to prepare a 50 μm-thick negative electrode. The fabricated negative electrode was punched into a 16 mm diameter circle, and a porous polyethylene separator was sandwiched between the negative electrode and metallic lithium was stacked on top of it as a counter electrode. An electrolyte consisting of 1 mol / L LiPF6 in a 1.5 / 3 / 4 / 3 volumetric ratio mixed solvent of fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) was added. A compact Tom cell-type battery cell was then used to fabricate an evaluation cell. The resulting evaluation cell was repeatedly charged and discharged 50 times at a current density of 0.05 C over a battery voltage range of 0.005 V to 1.5 V, and also at a current density of 0.5 C.

[0059] (Results and Discussion) FIG. 5 shows XRD patterns of the porous silicon materials of Experimental Examples 1 to 3. FIG. 6 is a photograph of the supernatant liquid of the acid treatment in the porosity forming step of Experimental Examples 1 to 3. FIG. 7 shows the pore size distribution results measured by mercury intrusion porosimetry for Experimental Examples 1 to 3. FIG. 8 shows a cross-sectional secondary electron image and element distribution of the porous silicon material of Experimental Example 1. Table 1 also summarizes the master alloy composition, which is the starting composition for Experimental Examples 1 to 6, the acid concentration (mol / L) for the porosity forming treatment, the element ratio (at%) after the acid treatment, the oxygen content (at%), the concentrations (mass%) of the SiO2 phase and the conductive phase, the mass ratio So / Vs of the SiO2 phase to the conductive phase, and the resistivity (Ωcm) of the compact of the porous silicon material. Table 2 also summarizes the master alloy composition, porosity (volume %) measured by mercury intrusion porosimetry, and average pore size (nm) for Experimental Examples 1 to 3. Table 3 also summarizes the master alloy composition, conductive phase concentration (mass%), resistivity (Ωcm), initial discharge capacity per unit active material at 0.05C (mAh / g), initial charge / discharge efficiency (%), capacity retention rate after 50 cycles (%), and discharge capacity at 0.5C (mAh / g) for Experimental Examples 1 to 4 and 7 to 8.

[0060] First, we consider the crystalline phase of the precursor atomized powder. 67.5 Si 30 V 2.5The master alloy was atomized and the resulting samples were analyzed by X-ray diffraction. Diffraction peaks for Si, Al, and VSi2 were observed. This was consistent with the crystalline phase predicted from the equilibrium phase diagram. The Al diffraction peak became smaller with increasing acid treatment time and eventually disappeared. When the hydrochloric acid concentration was 3 mol / L, the Al diffraction peak completely disappeared within 5 hours. On the other hand, when treated with a dilute solution of 0.1 mol / L, it was confirmed that Al could be completely removed after treatment for 30 hours or more. As shown in Figure 5, the porous silicon materials of Experiments 1 to 3 contained not only the main Si phase but also the conductive VSi2 phase. Furthermore, as shown in Figure 6, when treated with a high-concentration hydrochloric acid solution of 3 mol / L, the supernatant after acid treatment turned light blue, indicating that V partially dissolved. On the other hand, when treated with a dilute solution of 0.1 mol / L, the supernatant was not colored, suggesting that V dissolution was suppressed. In this way, it was suggested that by setting the acid concentration and treatment time under mild conditions, it is possible to leave the conductive phase, VSi2, in the porous structure, and further reduce the resistance of the particles.

[0061] Next, we investigated the composition of the porous silicon materials. As shown in Table 1, 4 to 12 at% Al was detected in all compositions. However, XRD did not reveal any Al-derived phases, but EDX detected Al, suggesting that Al was dissolved in the Si phase or VSi2 conductive phase. Furthermore, for the same V content (y), the proportion of Si phase in the porous silicon tended to increase with increasing Si content (x). As shown in Table 3, a decrease in the Si phase reduces the Li charge / discharge capacity, so from a capacity perspective, a higher Si phase is preferable. However, an increase in the Si phase reduces the VSi2 conductive phase, so from the perspectives of resistance and strength, some conductive phase is preferable. As shown in Table 1, the V content of the same master alloy tended to increase when treated with 0.1 mol / L hydrochloric acid compared to when treated with 3 mol / L hydrochloric acid. Furthermore, the O content also tended to decrease in the sample treated with 0.1 mol / L hydrochloric acid. However, XRD did not reveal any diffraction peaks of oxide phases. This is thought to be due to the partial dissolution of VSi2, resulting in the formation of an amorphous SiO2 phase. SiO2 is an insulator with high resistivity, and its coexistence is undesirable as it increases the resistivity of the anode.

[0062] As shown in Table 1, the resistivity of the samples in Experimental Examples 1 to 6 tended to increase as the V content in the master alloy composition increased. Increasing the V content in the master alloy increases the amount of VSi2, a conductive phase, which is thought to be beneficial for reducing resistivity. However, in this case, it is presumed that the amount of SiO2 phase, a by-product of VSi2 dissolving, increases, resulting in increased resistivity. The samples treated with a 3 mol / L hydrochloric acid solution had a high amount of insulating phase relative to the conductive phase (>1.5), and all exhibited resistivities greater than 500 Ωcm. However, the samples treated with a 0.1 mol / L hydrochloric acid solution had a low amount of insulating phase relative to the conductive phase (≦1.5), resulting in low resistivities of less than 500 Ωcm.

[0063] The pore size distribution of the acid-treated powder was evaluated using a mercury porosimeter, and the results are summarized in Table 2. The porosity ranged from 65 to 75% by volume and tended to increase with increasing Al content in the master alloy. Furthermore, the average pore size tended to decrease with increasing V content in the master alloy. All samples were found to have fine pores with diameters of 100 nm or less. Furthermore, the pore structure of the acid-treated samples was observed using an electron microscope. A spherical structure with uniformly distributed pores of 100 nm or less was observed. Next, the elemental distribution in the pores was measured using STEM / EDX. As predicted from the phase diagram, as shown in Figure 7, the VSi2 phase segregated as primary crystals on the surface of the porous particles and was dispersed throughout the porous material.

[0064] As shown in Table 3, the charge-discharge characteristics were evaluated, and the porous samples showed higher capacity retention than the non-porous Si powder. Furthermore, while the capacity improved with a lower vanadium silicide content, the presence of the vanadium silicide conductive phase also contributed to the improved capacity retention. This is presumably due to the presence of the vanadium silicide phase maintaining the porous structure. Furthermore, it was found that the conductive phase content was 15% by mass or less, more preferably 14% by mass or less, the SiO2 phase content was 20% by mass or less, more preferably 16% by mass or less, and even more preferably 10% by mass or less, the mass ratio So / Vs of the SiO2 phase to the conductive phase was 1.5 or less, more preferably 1.4 or less, the porosity measured by mercury porosimetry was preferably in the range of 50 to 80% by volume, and the average pore diameter measured by mercury porosimetry was preferably in the range of 50 nm to 70 nm. Furthermore, it was found that the V content was preferably in the range of 1 at% to 10 at% when the total of Al, Si, and V was 100 at%.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] 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]

[0069] The present disclosure is applicable to the technical field of electricity storage devices. [Explanation of symbols]

[0070] 10 Energy storage device, 12 Positive electrode, 13 Positive electrode active material, 14 Current collector, 15 Negative electrode, 16 Negative electrode active material, 17 Current collector, 18 Ion conducting medium, 21 Porous silicon material, 23 Void.

Claims

1. a conductive phase containing a Si phase and 15 mass % or less of an Si-V compound; SiO 2 is 20% by mass or less, SiO for the conductive phase 2 The mass ratio So / Vs is 1.5 or less, The porosity determined by mercury intrusion porosimetry is 80% by volume or less. Porous silicon material.

2. The conductive phase is 10% by mass or less, The SiO 2 The porous silicon material according to claim 1 , wherein the amount of SiO 2 is 10 mass % or less.

3. 3. The porous silicon material according to claim 1, wherein the content of V is in the range of 1 at % to 10 at % when the total of Al, Si, and V is taken as 100 at %.

4. 3. The porous silicon material according to claim 1, wherein the porosity is 50% by volume or more.

5. 3. The porous silicon material according to claim 1, wherein the average pore diameter determined by mercury intrusion porosimetry is in the range of 50 nm to 70 nm.

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. a precursor process in which a raw material containing Al, Si, and V is melted and rapidly solidified to obtain a precursor of a silicon alloy; a porosity forming step of removing the Al component contained in the silicon alloy with an acid having a concentration of less than 0.5 mol / L to obtain a porous silicon material; A method for producing a porous silicon material, comprising:

8. In the porosity forming step, a conductive phase containing a Si phase and 15 mass % or less of an Si-V compound is formed, and SiO 2 is 20 mass % or less relative to the conductive phase, 2 8. The method for producing a porous silicon material according to claim 7, wherein the porous silicon material has a mass ratio So / Vs of 1.5 or less and a porosity determined by mercury intrusion porosimetry of 80% by volume or less.

9. 9. The method for producing a porous silicon material according to claim 7, wherein the Al component is removed with hydrochloric acid having a concentration of 0.2 mol / L or less in the porosity-forming step.

10. In the porosity forming step, the conductive phase is 10 mass % or less, and the SiO 2 The method for producing a porous silicon material according to claim 7 or 8, wherein the porous silicon material has a content of 10 mass % or less.

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