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

Abstract

To further improve the charge-discharge characteristics of a porous silicon material.SOLUTION: The porous silicon material according to the present disclosure comprises a Si phase and a conductive phase including a Si-Mo compound in the amount of 20 mass% or less. The mass ratio of SiO2 to the conductive phase, So / Ms, is 2.5 or less, and the porosity determined by mercury intrusion porosimetry is 80 vol% or less.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present specification relates to a porous silicon material, an electricity storage device, and a method for producing the porous silicon material. Disclose. [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 Mo 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-Mo compounds, which makes it possible to further suppress deterioration in charge-discharge characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-096585 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, Mo, 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 Mo 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 20 mass % or less of a Si-Mo compound, The mass ratio So / Ms of SiO2 to the conductive phase is 2.5 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 Mo 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 and 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-Mo 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-Mo 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-Mo phase diagram in the 25at% Si phase. [Figure 2] Cross-sectional schematic diagram of the microstructure formation process during the cooling process of a melt droplet. [Figure 3] Al-Si-Mo 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 4. [Figure 6] Pore ​​size distribution measurement results for Experimental Example 2 measured by mercury intrusion porosimetry. [Figure 7] 1 shows a cross-sectional secondary electron image and element distribution of the porous silicon material of Experimental Example 2. 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 Mo 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-Mo alloys also includes a eutectic composition in which Al and Si phases simultaneously crystallize from the melt. Figure 1 shows an example of this, the Al-Si-Mo phase diagram for 25 at% Al. Figure 2 shows a schematic diagram of the microstructure formation process during the cooling process of a eutectic composition melt droplet. In this system, when a eutectic composition melt (Figures 1(1) and 2(1)) is cooled, the C40 phase is first formed, followed by the precipitation of MoSi2 particles and Si in the liquid phase (Figures 1(2) and 2(2)). When this Al melt, in which Si and MoSi2 particles are dispersed, is cooled, the Al solidifies, forming a skeletal structure in which the Si and MoSi2 particles are connected. This allows the fabrication of a porous material with a characteristic nanostructure in which a conductive phase containing MoSi2 particles segregates along the Si skeletal structure (Figures 1(3) and 2(3)). Since MoSi2 has a strength more than twice that of Si and a higher electrical conductivity than Si, the presence of MoSi2 as both a reinforcing phase and a conductive phase can improve the pressure resistance of the composite and further reduce its resistivity. For these reasons, it is preferable that the raw material composition be one that produces a eutectic structure of Al and Si phases and a Si-Mo compound phase such as MoSi2. The raw material composition may be set within a range that produces the 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 for the Al-Si-Mo system with 20 at% Si (Figure 3A), 25 at% Si (Figure 3B), and 30 at% Si (Figure 3C). As shown in Figure 3, at low Si content compositions, all Si forms MoSi2, preventing stable Si existence. However, above a certain Si content, Al (pore source), Si, and MoSi2 (conductive phase) coexist. To reduce the resistivity of porous Si without sacrificing battery capacity, a structure in which the conductive phase segregates within the Si framework is important. However, achieving this requires careful consideration of the order of phase precipitation. In Al-Mo-Si, Si and MoSi2 precipitate first from the melt. When the Al melt solidifies, the Si and MoSi2 dispersed in the melt connect to form the framework, achieving the ideal structure. The amount of Si phase is important because anode capacity depends on the amount of Si phase contained in the porous structure. The equilibrium diagrams in Figures 1 and 3 were obtained by calculation using the CALPHAD method.

[0016] (Precursor process) In the precursor process, it is preferable to use a raw material containing Al and Mo in a total range of 30 at% to 85 at% when the total of Si, Al, and Mo is taken as 100 at%, with the remainder being Si. Alternatively, in the precursor process, it is preferable to use a raw material containing Al in a range of 30 at% to 85 at% when the total of Si, Al, and Mo is taken as 100 at%, and Mo in a range of 0.5 at% to 10 at% when the total of Si, Al, and Mo is taken as 100 at%, with the remainder being Si. As the raw material, pure metals or alloys may be used. Note that the raw material may contain unavoidable impurities. Examples of unavoidable impurities include components that inevitably remain during the refining of Si, Al, or Mo, such as Fe, C, Cu, Ni, and P. It is preferable to have fewer unavoidable impurities, for example, 5 at% or less, and more preferably 2 at% or less, when the total of Si, Al, and Mo is taken as 100 at%. The total compounding ratio of Al and Mo is, for example, preferably 50 at% or more and may be 60 at% or more. The total compounding ratio of Al and Mo is, for example, preferably 80 at% or less, more preferably 78 at% or less, and may be 76 at% or less. The compounding ratio of Mo is, for example, preferably 1 at% or more and may be 2.5 at% or more. The compounding ratio of Mo is, for example, preferably 7.5 at% or less and may be 5 at% or less. The compounding ratio of Al is preferably 50 at% or more and may be 65 at% or more. The compounding ratio of Al is preferably 80 at% or less, more preferably 77 at% or less, and may be 75 at% or less. In the present disclosure, unless otherwise specified, the compounding ratio of each component refers to the compounding ratio of Si, Al, and Mo relative to the total. Silicon alloys containing Al and Mo in these ranges are preferred because they can increase the porosity and obtain voids with a more suitable shape and size. If the Al content is high, when the alloy is melted and then rapidly cooled, a large amount of Al single phase precipitates, resulting in the formation of many voids. In this process, it is preferable to use a raw material with a composition that allows for the formation of a eutectic structure of Si and one or more of Al, Mo, AlMo compound, SiMo compound, and AlSiMo compound.For example, according to the equilibrium diagram in Fig. 1, when Si is 25 at%, it is believed that a eutectic structure of Si and Al can be obtained when Mo is in the range of 0 to 12 at%. In this step, when the total of Si, Al, and Mo is 100 mass%, a raw material may be used that contains Al and Mo in a total range of 20 mass% to 85 mass%, preferably 30 mass% to 80 mass%, with the remainder being Si.

[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 Mo 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 Mo, 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. Mo 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 include inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid, and organic acids such as formic acid, acetic acid, and citric acid, with hydrochloric acid being preferred. The acid is preferably an aqueous solution. The concentration of the acid is not particularly limited as long as it can remove the Al components, and can be, for example, from 0.01 mol / L to 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 20 mass% or less of a Si-Mo compound. 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 17.5 mass% or less, or 17 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 35 mass% or less of SiO2. The SiO2 content is preferably lower, more preferably 30 mass% or less, and may be 25 mass% or less, 20 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 / Ms of SiO2 (So) to the conductive phase (Ms) of 2.5 or less. A smaller mass ratio is preferable because the resistivity of the material can be further reduced. This mass ratio is preferably 2.0 or less, more preferably 1.75 or less, and may be 1.5 or less. This mass ratio may be 0.1 or more, or 0.5 or more. This porosity step produces 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, 70 volume % or less, or 65 volume % or less. This porosity may be 30 volume % or more, 35 volume % or more, or 40 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 30 nm to 70 nm. This porosity step preferably produces a porous silicon material having a Mo content in the range of 1 at % to 10 at %, where the total of Al, Si, and Mo is 100 at %.

[0022] The porous silicon material may be obtained in the porosity forming step, which contains a Si phase and a Si-Mo 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-Mo compound phase is presumed to reduce the resistivity and to be responsible for reinforcing the silicon bone. Examples of the Si-Mo compound include Si xMo y (x and y are arbitrary numbers), and examples thereof include Si2Mo and Si3Mo5. The Si-Mo compound phase is a (Si,Al) phase in which part of Si is replaced by Al. x Mo y The Si—Mo compound phase may be one that is poorly soluble in acid. The Si phase or the Si—Mo 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 production method. Here, the physical properties of the porous silicon material are the same as those of the above-described production method, and detailed description thereof will be omitted.

[0025] This porous silicon material includes a conductive phase containing a Si phase and a Si-Mo compound. This porous silicon material may have a Si bone structure formed of the Si phase and a reinforcing phase that is a Si-Mo 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-Mo compound phase may be in the form of nanoparticles, arranged between the Si bone structures and acting as pillars supporting the Si bone structures. 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-Mo compound phase is preferably MoSi2. Since MoSi2 has a higher strength than Si, it functions well as a reinforcing phase.

[0026] The porous silicon material includes a Si phase and a conductive phase containing 20% ​​by mass or less of a Si-Mo compound. From the viewpoint of resistivity, it is preferable that the conductive phase be present in an appropriate amount within the structure. The conductive phase may be 17.5% by mass or less, or 17% 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 preferably has an SiO2 content of 35% by mass or less. A lower SiO2 content is preferable, more preferably 30% by mass or less, and may be 25% by mass or less, 20% 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 / Ms of SiO2 (So) to the conductive phase (Ms) of 2.5 or less. A smaller mass ratio is preferable because the resistivity of the material can be further reduced. This mass ratio is preferably 2.0 or less, more preferably 1.75 or less, and may be 1.5 or less. This mass ratio may also be 0.1 or more, or 0.5 or more. This porous silicon material preferably has a Mo content in the range of 1 at% or more and 10 at% or less, where the total of Al, Si, and Mo is taken as 100 at%. This Mo content may be 8 at% or less, or may be 5 at% or less. This Mo 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-Mo 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-Mo compound phase (preferably MoSi2 phase) ratio of 2 mass% or more, 4 mass% or more, or 5 mass% or more. The Si-Mo 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-Mo compound phase ratio can be determined by X-ray diffraction (XRD) measurement. The porous silicon material may be composed of a Si phase and a Si-Mo 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 30 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 15 at% or less, where the total of Si, Mo, 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 12.5 at% or less, or 10 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 88 at% or less. This porous silicon material may have a Mo content of 1 at% or more, 1.5 at% or more, or 4 at% or more. The Mo content may be 10 at% or less, 7.5 at% or less, or 5 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 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 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% 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 the conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PModF), and fluororubber; 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. It is also possible to oxidize-treat these 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, and formed body of fiber group. 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 constraint 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 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 TiS2, TiS3, MoS3, FeS2, basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same hereinafter) 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 LiMoO (a+b+c=1), lithium vanadium composite oxides with a basic composition formula of LiMoO, and transition metal oxides with a basic composition formula of MoO can be used. Among these, lithium transition metal composite oxides, such as LiCoO, LiNiO, LiMnO, 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 electricity storage device is not particularly limited, and examples thereof include coin, button, sheet, laminate, cylindrical, flat, and rectangular shapes. The device may also be applied to large devices used in electric vehicles and the like. FIG. 4 is an explanatory diagram showing an example of the structure of an electricity storage device 10. The electricity storage device 10 has a positive electrode 12, a negative electrode 15, and an ion-conducting 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 above-described porous silicon material 21 and voids 23, and the porous silicon material 21 includes a Si phase and a conductive phase containing a Si-Mo compound.

[0041] It is preferable that this electricity storage device has a higher capacity retention rate when subjected to charge / discharge cycles. For example, the capacity retention rate after 50 cycles at a current density of 0.05 C is preferably 90% or more, more preferably 92% or more, and even more preferably 95% or more.

[0042] (All-solid-state lithium-ion secondary battery) This electricity storage device is preferably an all-solid-state lithium ion secondary battery. All-solid-state batteries are preferable because they can further suppress changes in performance due to the electrolyte solution and further enhance safety. This all-solid-state lithium ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode that is the above-mentioned electrode for the electricity storage device, and a solid electrolyte that conducts lithium ions and is interposed between the positive electrode and the negative electrode. The positive electrode can be any of those shown in the above-mentioned electricity storage device. Furthermore, the negative electrode can be the above-mentioned electrode for the electricity storage device.

[0043] The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La, and Zr. 7.0+x-y (La 3-x ,A x )(Zr 2-y ,T y )O 12It 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 (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 or (Li 7-3z+x-y M z )(La 3-x A x )(Y 2-y T y )O 12 and may be a garnet-type oxide represented by. However, in the formula, the element M is one or more of Al and Ga, the element A is one or more of Ca and Sr, and T is one or more of Nb and Ta, and it may be such that 0 ≤ z ≤ 0.2, 0 ≤ x ≤ 0.2, and 0 ≤ y ≤ 2. In this basic composition formula, it is more preferable to satisfy 0.05 ≤ z ≤ 0.1. In this basic composition formula, it is more preferable to satisfy 0.05 ≤ x ≤ 0.1. Also, in this basic composition formula, it is more preferable to satisfy 0.1 ≤ y ≤ 0.8. In such a range, the ionic conductivity can be made more suitable.

[0044] Alternatively, as the solid electrolyte, for example, general ones such as Li3N, Li called LISICON 14 Zn(GeO4)4, sulfide Li 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 , NASICON-type LiTi2(PO4)3, Li 1.3 M 0.3 Ti 1.7 (PO3)4 (M = Sc, Al), etc. can be mentioned. Also, Li7P3S obtained from a glass having a composition of 80Li2S·20P2S5 (mol%) which is a glass-ceramic11 Furthermore, Li, a sulfide-based material with high conductivity, 10 Examples 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-Mo 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-Mo 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 20 mass% or less of a Si-Mo compound, The mass ratio So / Ms of SiO2 to the conductive phase is 2.5 or less, The porosity determined by mercury intrusion porosimetry is 80% by volume or less. Porous silicon material. [2] The conductive phase is 5% by mass or more, The porous silicon material according to [1], wherein the SiO2 content is 35 mass % or less. [3] The porous silicon material according to [1] or [2], wherein the Mo content is in the range of 1 at % to 10 at % when the total of Al, Si, and Mo 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 30 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 Mo 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-providing step includes a conductive phase containing a Si phase and 20 mass % or less of a Si-Mo compound, wherein the mass ratio So / Ms of SiO2 to the conductive phase is 2.5 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 is one in which the conductive phase is 5 mass % or more and the SiO2 is 35 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 4 are working examples of the present disclosure, Experimental Examples 6 and 7 are reference examples, and Experimental Examples 5 and 9 are comparative examples.

[0050] [Preparation of porous silicon materials] The raw materials of Al, Si and Mo are treated as follows: 100-x-y Six Mo 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 AlSiMo 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 4) Experimental Example 1 was a porous silicon material obtained by setting x = 30 and y = 1 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 = 30 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. Experimental Example 4 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 5 to 8) Experimental Example 5 was a porous silicon material 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. Experimental Example 6 was a porous silicon material obtained by changing x = 30 and y = 2.5 in the above basic composition formula and removing Al with 3 mol / L hydrochloric acid at 50°C. Experimental Example 7 was a porous silicon material obtained by changing 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 8 was a porous silicon material obtained by changing 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 Example 9) As Experimental Example 9, no porous silicon material was prepared, and Si powder (SIE23PB manufactured by Kojundo Chemical Co., Ltd.) having an average particle size of 5 μm was used as it was.

[0054] [X-ray diffraction measurement] X-ray diffraction (XRD) measurements were performed on the porous silicon materials of Experimental Examples 1 to 8. X-ray diffraction measurements were performed using an X-ray diffractometer (Rigaku Corporation, RINT-TTR) with a Cu tube in the 2θ range of 10° to 80° at a rate of 5° / min. X-ray diffraction measurements were also performed on samples obtained by varying the acid treatment conditions for the precursor. Furthermore, the abundance ratios of the Si phase and the MoSi2 phase (conductive phase) were evaluated using the reference intensity ratio method, using the intensities of the strongest peak of the Si phase (at about 28.3°) and the strongest peak of the MoSi2 phase (at about 41.8°).

[0055] [SEM observation, composition analysis] Secondary electron images were observed using a scanning electron microscope (SEM, Hitachi S-4300) for the cross sections of the porous silicon materials of Experimental Examples 1 to 8. Furthermore, composition analysis was performed on 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 on the entire observation surface, and the average value was calculated as the composition ratio.

[0056] [Pore distribution measurement] The pore distribution of the silicon materials of Experimental Examples 1 to 9 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 8. 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 to 9 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 placed on top of it as a counter electrode. An electrolyte consisting of 1 mol / L LiPF6 in a 1.5 / 3 / 4 / 3 volumetric mixture of fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) was then poured into a Tom cell-type small battery cell 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 4. Fig. 6 shows the pore size distribution measurement results for Experimental Example 2 measured by mercury intrusion porosimetry. Fig. 7 shows a cross-sectional secondary electron image and element distribution of the porous silicon material of Experimental Example 2. Table 1 also shows the master alloy composition, which is the starting composition for Experimental Examples 1 to 8, the acid concentration (mol / L) for the porosity 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 / Ms of the SiO2 phase to the conductive phase, the porosity (volume%), the average pore size (nm), and the resistivity (Ωcm) of the molded body of the porous silicon material. Table 2 also summarizes the master alloy composition, acid treatment concentration, 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 6 and 9.

[0060] First, we consider the crystalline phase of the precursor atomized powder. 67.5 Si 30 Mo 2.5 The master alloy was atomized and the resulting sample was analyzed by X-ray diffraction. Diffraction peaks for Si, Al, and MoSi2 were observed. These peaks were consistent with the crystalline phase predicted from the equilibrium phase diagram. The Al diffraction peaks became smaller with increasing acid treatment time and eventually disappeared. When the hydrochloric acid concentration was 3 mol / L, the Al diffraction peaks completely disappeared within 5 hours. However, 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. When treated with a high-concentration hydrochloric acid solution of 3 mol / L, the supernatant after acid treatment was lightly colored, and Mo partially dissolved. On the other hand, when treated with a dilute solution of 0.1 mol / L, the supernatant was almost colorless, suggesting that Mo dissolution was suppressed.

[0061] Next, we investigated the composition of the porous silicon material. To confirm the dissolution state of Mo during acid treatment, the acid-treated samples were pressed into disks using a carbide mold and analyzed for composition by EDX. As shown in Table 1, the Mo content tended to increase when treated with 0.1 mol / L hydrochloric acid compared to when treated with 3 mol / L hydrochloric acid, even when the same master alloy was treated. 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 likely due to the partial dissolution of MoSi2, resulting in the formation of an amorphous SiO2 phase. SiO2 is a highly resistive insulator, and its coexistence is undesirable because it increases the resistivity of the anode.

[0062] To investigate the effect of the acid treatment conditions on the resistivity of the composite porous silicon anode for Experimental Examples 1 to 8, sample powders treated with 0.1 mol / L or 3 mol / L hydrochloric acid aqueous solution were pressed at 200 MPa, and copper electrodes were attached to evaluate the resistivity using the four-terminal method. As shown in Table 1, the resistivity of the samples tended to increase as the Mo content in the master alloy composition increased. Increasing the Mo content in the master alloy increases the amount of MoSi2, which is a conductive phase, and is therefore thought to be advantageous for reducing resistivity. However, in this case, it is presumed that the amount of SiO2 phase, which is a by-product when part of MoSi2 dissolves, increases, resulting in an increase in resistivity. In the sample treated with 3 mol / L hydrochloric acid aqueous solution, the amount of insulating phase was high (>2.5) relative to the conductive phase, and the Al phase, which does not contain a conductive phase, was low. 80 Si 20 In contrast, the resistivity of the sample treated with 0.1 mol / L hydrochloric acid was found to be approximately three orders of magnitude lower than that of the sample without a conductive phase, because the amount of insulating phase relative to the conductive phase was small (≦1.7).

[0063] The pore distribution of the acid-treated powder was evaluated using a mercury porosimeter, and the results are summarized in Table 1. 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 diameter tended to decrease with increasing Mo 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 shown in Figure 7, Mo and Si overlap, confirming the formation of a structure in which MoSi2 coexists with the Si framework, as predicted from the phase diagram.

[0064] As shown in Table 2, the charge-discharge characteristics of the porous samples were evaluated. Compared with the non-porous Si powder, the porous samples exhibited higher capacity retention. Furthermore, while the capacity improved with a lower molybdenum 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 molybdenum silicide phase, which maintains the porous structure. Furthermore, the conductive phase content is preferably 20% by mass or less, more preferably 17.5% by mass or less, the SiO2 phase content is 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less, the mass ratio of the SiO2 phase to the conductive phase (So / Ms) is 2.5 or less, more preferably 2.0 or less, the porosity measured by mercury intrusion porosimetry is in the range of 60-80% by volume, and the average pore diameter measured by mercury intrusion porosimetry is in the range of 30 nm to 70 nm. It was also found that the Mo content is preferably in the range of 1 at % to 10 at % when the total of Al, Si, and Mo is taken as 100 at %.

[0065] [Table 1]

[0066] [Table 2]

[0067] 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]

[0068] The present disclosure is applicable to the technical field of electricity storage devices. [Explanation of symbols]

[0069] 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. A conductive phase containing a Si phase and 20 mass % or less of a Si—Mo compound, SiO for the conductive phase 2 The mass ratio So / Ms is 2.5 or less, The porosity determined by mercury intrusion porosimetry is 80% by volume or less. Porous silicon material.

2. The conductive phase is 5% by mass or more, The SiO 2 2. The porous silicon material according to claim 1, wherein the content of the silicon dioxide is 35% by mass or less.

3. 3. The porous silicon material according to claim 1, wherein the content of Mo is in the range of 1 at % to 10 at % when the total of Al, Si, and Mo 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 30 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 Mo 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 20 mass % or less of a Si—Mo compound is formed, and SiO 2 8. The method for producing a porous silicon material according to claim 7, wherein the porous silicon material has a mass ratio So / Ms of 2.5 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 5 mass % or more, 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 SiO 2 content of 35 mass % or less.

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