Porous silicon material, energy storage device, and method for manufacturing porous silicon material

Abstract

To provide a novel porous silicon material of which resistance is reduced, an electricity storage device and a method for producing a porous silicon material.SOLUTION: A porous silicon material of the present disclosure has a skeleton in which Si and Cr silicide including at least an AlSiCr compound coexist, and when the sum of the amounts of Si, Al, and Cr is taken as 100 at%, the amount of oxygen is 25 at% or less, the O / Cr ratio, which is the ratio of the amount of oxygen to the amount of Cr, is 10 or less, and the porosity, as determined by mercury intrusion porosimetry, is 55 vol% or more.SELECTED DRAWING: None

Description

Technical Field

[0001] This specification discloses a porous silicon material, a power storage device, and a method for manufacturing a porous silicon material.

Background Art

[0002] Conventionally, as a negative electrode material for silicon, a porous silicon material obtained by removing Al by treating an AlSiCu alloy powder with hydrochloric acid or the like has been proposed (for example, Non-Patent Document 1). In this porous silicon material, by incorporating copper into the porous silicon material, it is said that good electronic conductivity of silicon can be ensured and volume expansion during charge and discharge can be suppressed.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the porous silicon material of Non-Patent Document 1 described above, although electronic conductivity is ensured by copper, there is a problem that a relatively large amount of copper is required to further reduce the resistivity. For this reason, a new porous silicon material with further reduced resistance has been demanded.

[0005] This disclosure has been made in view of such problems, and the main object is to provide a new porous silicon material with further reduced resistance, a power storage device, and a method for manufacturing a porous silicon material.

Means for Solving the Problems

[0006] As a result of intensive research to achieve the above object, the inventors of the present invention melted a raw material of an Al-Si-Cr ternary alloy having a composition in which Al, Si, and Cr silicide containing at least an AlSiCr compound appear at room temperature in an equilibrium phase diagram, rapidly solidified it to produce a precursor of a silicon alloy, and removed the Al component. It has been found that a porous silicon material having a skeleton in which Si and Cr silicide coexist and having a low resistivity can be obtained. At that time, oxygen may be incorporated into the porous silicon material as the Al component is removed, but it has been found that the resistivity can be further reduced by suppressing the amount of oxygen, and the porous silicon material, the power storage device, and the manufacturing method of the porous silicon material of the present disclosure have been completed.

[0007] That is, the porous silicon material of the present disclosure has a skeleton in which Si and Cr silicide containing at least an AlSiCr compound coexist. When the total amount of Si, Al, and Cr is 100 at%, the amount of oxygen is 25 at% or less, and the O / Cr ratio, which is the ratio of the amount of oxygen to the amount of Cr, is 10 or less. The porosity determined by the mercury intrusion method is 55% by volume or more.

[0008] The power storage device of the present disclosure a positive electrode containing a positive electrode active material, a negative electrode containing the above-mentioned 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, and is provided with.

[0009] The manufacturing method of the porous silicon material of the present disclosure is the manufacturing method of the above-mentioned porous silicon material, a precursor step of melting a raw material of an Al-Si-Cr ternary alloy having a composition in which Al, Si, and Cr silicide containing at least an AlSiCr compound appear at room temperature in an equilibrium phase diagram and rapidly solidifying it to obtain a precursor of a silicon alloy, a porosification step of removing the Al component contained in the silicon alloy to obtain the porous silicon material, and includes.

Advantages of the Invention

[0010] The present disclosure can provide a novel porous silicon material with a lower resistivity, a power storage device, and a method for manufacturing the porous silicon material. The reason for obtaining such an effect is presumed as follows. For example, in an equilibrium phase diagram, a raw material of an Al-Si-Cr ternary alloy having a composition in which Al, Si, and a Cr silicide containing at least an AlSiCr compound appear at room temperature is melted and rapidly solidified to produce a precursor of a silicon alloy. When the Al component is removed, a porous silicon material having a skeleton in which Si and a Cr silicide coexist is obtained. In such a porous silicon material, since a Cr silicide, which is a conductive phase, exists along the skeleton serving as a conduction path, it is presumed that efficient electrical conduction is performed and the resistivity can be reduced. At that time, when components other than the Al phase are partially removed during the Al removal treatment, oxygen is incorporated into the porous silicon material, and an insulating phase such as SiO2 (for example, amorphous silica) is formed, which may inhibit electrical conduction. However, by removing Al under milder conditions or the like, the amount of oxygen incorporated is reduced, so that the inhibition of electrical conduction is suppressed and the resistivity can be further reduced.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0012] (Method for manufacturing a porous silicon material) The manufacturing method of the porous silicon material of the present disclosure includes a precursor process and a porosification process. In the precursor process, a treatment is performed in which a raw material of an Al-Si-Cr ternary alloy having a composition in which an Al phase, a Si phase, and a Cr silicide phase including at least an AlSiCr compound appear at room temperature in an equilibrium phase diagram is melted and rapidly solidified to obtain a precursor of a silicon alloy. The Cr silicide phase includes at least an AlSiCr compound containing Al, Si, and Cr. The Cr silicide may include a SiCr compound containing Si and Cr. In the porosification process, a treatment is performed to remove a part of the Al component contained in the silicon alloy to obtain a porous silicon material. The porous silicon material obtained by this manufacturing method of the porous silicon material has, for example, a structure having a skeleton in which Si and a Cr silicide containing at least an AlSiCr compound coexist. Such a structure is also referred to as a coexisting type structure. Cr silicide phases such as CrSi2, Cr(Al,Si)2, and Al 13 Si4Cr4 have conductivity. Therefore, by coexisting Cr silicide as a conductive phase in the Si skeleton, which is a conduction path, the conductivity can be improved. In addition, since the strength of Cr silicide is about twice that of Si, an effect of improving the skeleton strength is also expected by coexisting Cr silicide as a strengthening phase in the Si skeleton.

[0013] Figure 1 is the Al-Si-Cr phase diagram at 25 at% Si. Figure 2 is a schematic diagram of the microstructure formation process of the outermost surface layer during the cooling process of the molten droplets. The formation of the coexisting structure due to the addition of Cr affects the precipitation sequence of each phase in the Al-Si-Cr system. Here, the case of cooling the alloy melt by the gas atomization method will be described as an example. In the gas atomization method, the high-temperature melt is injected into the high-pressure gas from the hole at the tip of the nozzle for cooling. At this time, when the melt (Figure 1(1)) is cooled to the temperature of Figure 1(2), CrSi2 first crystallizes from the surface of the droplet with a lower temperature (Figure 2(2)). However, CrSi2 gradually coarsens in the coexistence temperature region with the melt (Figure 1(3)) (Figure 2(3)). At this time, if CrSi2 coarsens too much, the inert CrSi2 will cover the periphery of the generated silicon powder in a shell shape (Figure 2(3-2)), which is not preferable because the conduction path of carrier ions (Li) on the active material surface decreases. However, in the AlSiCr phase diagram, when cooled to the temperature of Figure 1(4), CrSi2 is decomposed (peritectic reaction), and at the same time, Si and Al 13 Si4Cr4 precipitates (eutectic reaction), so it is more preferable because an active carrier ion conduction path (i.e., Si phase) is formed on the active material surface. Furthermore, this peritectic reaction generally refers to a reaction in which the melt and the solid phase react to form another solid phase, such as L + CrSi2 ⇒ Al 13 Si4Cr4. Since the reaction proceeds from the surface of the solid phase particles, as shown in Figure 2(4), a structure is formed in which the periphery of the fine CrSi2 particles is covered with a shell of Al 13 Si4Cr4 and Si. Thus, in the temperature region of Figure 1(5), a reaction occurs in which CrSi2 (which may actually be Cr(Al,Si)2 in some cases) decomposes into Al 13 Cr4Si4 and Si. Therefore, a state occurs in which cluster particles in which Si and Al 13 Cr4Si4 nanoparticles coexist at extremely close distances are dispersed in the melt (Figure 2(5)). Then, when the Al melt finally solidifies, the Si and Al of the cluster 13The Cr4Si4 nanoparticles are linked together to form a structure in which Si and Cr silicide coexist in the framework, and by removing Al from this by acid treatment, a composite porous material with a coexisting structure is obtained. If the cooling rate is sufficiently high, it is thought that the material will cool before the peritectic reaction is completed, and some of the CrSi2 near the particle surface will remain as a non-equilibrium phase. During the cooling process described above, the peritectic reaction causes Al to surround the fine CrSi2 particles, 13 The formation of a structure in which the Si4Cr4 and Si cores are covered results in a finer microstructure. 13 In addition to the fineness of Si4Cr4, the Al alloy that is the source of the pores is also refined. 13 Since the difference in precipitation temperatures between Si4Cr4 and Al is relatively small (up to 350°C), the crystal growth of these particles is small, and a nano-sized microstructure can be achieved. The solidification structure forms a lamellar structure consisting of SiCr compounds, AlSiCr compounds, AlCr compounds, eutectic Si, primary Si, and primary Al. Compounds that dissolve in acid include primary Al, AlSiCr compounds, and AlCr compounds, and it is presumed that the structure obtained after acid treatment will be a skeletal porous body consisting of the remaining SiCr compounds, eutectic Si, primary Si, as well as undissolved primary Al, AlSiCr compounds, and AlCr compounds.

[0014] 3 to 7 are phase diagrams of Al-Si-Cr alloys when the contents of Si and Cr are changed. 85-y S 15 Cr y FIG. 4 shows the Al-Si-Cr phase diagram. 80-y S 20 Cr y FIG. 5 shows the Al-Si-Cr phase diagram. 75-y S 25 Cr y FIG. 6 shows the Al-Si-Cr phase diagram. 70-y S 30 Cr y FIG. 7 shows the Al-Si-Cr phase diagram. 60-y S 40 Cr yis the Al-Si-Cr phase diagram. Fig. 8 is an explanatory diagram of the eutectic composition range in the Al-Si-Cr system composition. To realize a coexisting structure, for example, as described above, it is necessary to go through a state where cluster particles of Si and Cr silicide are dispersed in the Al melt or a state close to it, and the composition range in which Si and Cr silicide precipitate simultaneously by eutectic reaction is ideal. As shown in Figs. 3 to 7, in the basic composition of Al 100-x-y Si x Cr y , when the Si content x is less than 18 at%, the composition in which Si and silicide crystallize by eutectic reaction is not shown. On the other hand, in the composition where the Si content x is 18 at% or more, especially 25 at% or more, depending on the Cr content y, L⇒Si + CrSi2 (eutectic reaction 1) and L⇒Si + Al 13 Si4Cr4 (eutectic reaction 2) occur in the composition where the eutectic reaction occurs. Fig. 8 is a relationship diagram of the Si content x and the Cr content y in which these eutectic reactions 1 and 2 occur. In the composition region indicated by the slanted lines, in addition to Si and CrSi2 crystallizing at relatively close temperatures, since the peritectic reaction of CrSi2 + L⇒Al 13 Si4Cr4 occurs, a finer pore structure can be realized, and it is presumed to be a more preferable composition range. Such a composition range is in the basic composition formula Al 100-x-y Si x Cr y , in the range of 18 ≦ x ≦ 40 and 0.1 ≦ y ≦ 15. Also, based on Fig. 8, Cr is preferably 0.1 at% or more, more preferably 0.5 at% or more, and even more preferably 1 at% or more with respect to the whole of Al and Si. Further, in Fig. 8, among the composition regions indicated by the slanted lines, in the composition region where Cr is 10 at% or less, that is, the composition region surrounded by the thick line and colored gray, since there are not too many silicides that do not contribute to the charge-discharge reaction, it is more preferable from the viewpoint of increasing the capacity of the energy storage device.

[0015] (Precursor process) In the precursor process, a raw material of an Al-Si-Cr ternary alloy having a composition in which an Al phase, a Si phase, and a Cr silicide phase appear at room temperature is melted and rapidly solidified to obtain a precursor of a silicon alloy. The precursor of the silicon alloy may be, for example, one in which Si and Cr silicide coexist in a skeleton portion forming pores. In this process, when the total of Si, Al, and Cr is 100 at%, a raw material containing Cr in the range of 0.1 at% or more and 20 at% or less, Al in the range of 40 at% or more and 90 at% or less, and the balance being Si may be used. In the raw material composition, Cr is preferably 0.1 at% or more, more preferably 0.5 at% or more, and even more preferably 1 at% or more. Further, as described above, the raw material composition has a basic composition formula Al 100-x-y Si x Cr yAmong them, it is more preferable that 18 ≦ x ≦ 40 and 0.1 ≦ y ≦ 15. Among these, it is more preferable to satisfy x / y ≧ 8. The value of x / y may be 10 or more, or may be 20 or more. Also, the value of x / y may be 50 or less, or may be 40 or less. Note that the raw material may include inevitable impurities. Inevitable impurities are components that inevitably remain during the purification of any of Si, Cr, and Al, 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 Si, Cr, and Al is 100 at%, 5 at% or less is preferable, and 2 at% or less is more preferable. The blending ratio of Cr is preferably 0.1 at% or more, for example, and may be 0.5 at% or more. Also, the blending ratio of Cr is preferably 10 at% or less, and may be 5 at% or less, or may be 3 at% or less. The blending ratio of Al is preferably 50 at% or more, and may be 55 at% or more, or 60 at% or more. Also, the blending ratio of Al is preferably 85 at% or less, more preferably 80 at% or less, and may be 77.5 at% or less. The blending ratio of Si is preferably 15 at% or more, for example, more preferably 18 at% or more, and may be 20 at% or more, or 25 at% or more. Also, the blending ratio of Si is preferably 59 at% or less, more preferably 50 at% or less, and may be 40 at% or less, or 30 at% or less. In a silicon alloy containing Al and Cr within such ranges, the porosity can be increased more, and voids having a more suitable shape and size can be obtained, which is preferable. When the content of Al is large, after melting into an alloy and rapidly cooling, a large amount of single-phase Al precipitates, so that many voids can be formed. This cooling rate is preferably more rapid quenching. For example, from the molten state, it may be in the range of 10 2 °C / s or more and 10 8 °C / s or less.

[0016] In this process, when melting the raw materials, high-frequency crucible melting in an inert gas atmosphere such as Ar is preferred, but any melting method may be used. To produce the master alloy, it is necessary to dissolve the raw material powder. High-frequency melting is more preferred to produce a uniform sample, but simple heating melting in an electric furnace or the like, or melting using an electron beam or the like may also be used. In the precursor process, the alloy obtained from the raw materials may be granulated. In this granulation process, the molten metal of the silicon alloy raw material may be cast into a mold, and the obtained ingot may be crushed and granulated. Further, as a method for granulating the silicon alloy, one or more of a gas atomization method, a water atomization method, a roll quenching method, etc. may be used. Alloy powders are obtained by the gas atomization method and the water atomization method. On the other hand, since a strip alloy is obtained by the roll quenching method, it may be pulverized into powder thereafter. The powder obtained by the roll quenching method has a fine alloy structure, so porous silicon having fine pores can be obtained after the elution treatment. Among these, the gas atomization method is more preferred as a method for granulating the silicon alloy. In gas atomization, it is preferably carried out in an Ar atmosphere when making the molten metal, and preferably carried out under an Ar or He atmosphere during granulation.

[0017] In the precursor process, it is preferable to granulate the silicon alloy within a particle size range of 0.1 μm or more and 100 μm or less. These silicon alloy particles preferably have an average particle size in the range of 0.5 μm or more and 10 μm or less, and may be in the range of 1 μm or more and 5 μm or less, or 2 μm or more and 5 μm or less. In this process, the particle size of the silicon alloy particles may be set to 100 μm or less, 10 μm or less, or 5 μm or less by classification. Also, in this process, the particle size of the silicon alloy particles may be set to 0.1 μm or more, 0.5 μm or more, 1 μm or more, or 2 μm or more by classification. The silicon alloy particles may be appropriately selected according to the characteristics required for the power storage device. Here, the average particle size of the particles is determined as a value obtained by observing the particles with a scanning electron microscope (SEM), aggregating the major axis of each particle as the diameter of the particle, and averaging by dividing by the number of particles. The particles obtained by this granulation treatment will ultimately be the average particle size of the aggregate of porous particles to be obtained.

[0018] In this precursor process, a raw material containing a second element including at least one of Ca, Mg, Na, Sr, and P in addition to Al, Cr, and Si may be used. Among these, as the second element, at least one of Ca, Na, and Sr is preferable. The second element is preferably less than the content of Al or Cr. For example, a range of 10 mass% or less with respect to the entire silicon alloy is preferable, and a range of 5 mass% or less is more preferable. In this precursor process, a raw material having a composition in which a part of Cr is replaced with another transition metal element M may be used. In that case, the transition metal element M may be less than half of Cr in molar ratio, may be 20% or less, or may be 10% or less. Examples of the transition metal element M include Ti, V, Nb, Mo, Ni, Mn, Fe, Co, Zn, and the like.

[0019] (Porous process) In the porous process, a removal treatment for removing the Al component contained in the silicon alloy prepared above is performed. Examples of the Al component removed in this process include Al and its compounds.

[0020] This porosity-forming process is preferably carried out under the condition that the increased amount of oxygen is 7 at% or less. The increased amount of oxygen may be 5 at% or less, 4 at% or less, or 2 at% or less. The increased amount of oxygen may be even smaller, for example, 1.8 at% or less, 1.5 at% or less, or 1.3 at% or less. The increased amount of oxygen may be 0.1 at% or more, 0.5 at% or more, or 0.9 at% or more. This increased amount of oxygen can be calculated from the ratio of the amount of oxygen (element ratio) and the amount of Si (element ratio) after the removal treatment, based on the amount of Si (element ratio) before the removal treatment, assuming that the amount of Si remains unchanged before and after the removal treatment. Specifically, the increased amount of oxygen GO [at%] is the value obtained from the formula GO = O2 × S1 / S2 - O1. However, let the element ratios of Al, Si, Cr, and O determined by EDX for the silicon alloy before the removal treatment be A1 [at%], S1 [at%], C1 [at%], and O1 [at%] (where A1 + S1 + C1 = 100), respectively, and the element ratios of Al, Si, and Cr determined by EDX for the porous silicon material after the removal treatment be A2 [at%], S2 [at%], and C2 [at%], and O2 [at%] (where A2 + S2 + C2 = 100), respectively. For example, since Si does not dissolve in hydrochloric acid, when the removal treatment is carried out by acid treatment with hydrochloric acid, it can be assumed that the amount of Si remains unchanged before and after the removal treatment.

[0021] This porosity formation step may be performed under conditions such that the reduction amount of Cr is 60% or less of the reduction amount of Cr when Cr is completely removed. The reduction amount of Cr may be 55% or less, or 50% or less of the reduction amount of Cr when Cr is completely removed. Although the smaller the reduction amount of Cr is, the more preferable it is, the reduction amount of Cr may be 5% or more, 20% or more, or 30% or more of the reduction amount of Cr when Cr is completely removed. Further, in the porosity formation step, it may be performed under conditions such that the reduction amount of Cr is 0.5 at% or more less than the reduction amount of Cr when Cr is completely removed. The reduction amount of Cr may be made 0.6 at% or more less, or 0.7 at% or more less than the reduction amount of Cr when Cr is completely removed. Further, the reduction amount of Cr may be made 1.0 at% or less, 0.9 at% or less, or 0.8 at% or less less than the reduction amount of Cr when Cr is completely removed. The reduction amount of Cr may be, for example, -0.6 at% or less. Although the smaller the reduction amount of Cr is, the more preferable it is, the reduction amount of Cr may be, for example, -0.1 at% or more, -0.3 at% or more, or -0.4 at% or more. This porosity formation step may be performed under conditions such that the reduction amount of Al exceeds 90% of the reduction amount of Al when Al is completely removed, or may be performed under conditions such that it is 95 at% or more. The reduction amount of Al may be, for example, -40 at% or more and -75 at% or less, or -50 at% or more and -70 at% or less.

[0022] Here, when the amount of Si can be assumed to be unchanged before and after the removal treatment, for example, the decrease amounts of Cr and Al may be calculated from the ratios of the amounts (element ratios) of Cr and Al and the amount (element ratio) of Si after the removal treatment to the amount of Si (element ratio) before the removal treatment, based on the amount of Si before the removal treatment (element ratio). Specifically, the decrease amount LC [at%] of Cr is taken as the value obtained by the formula LC = C2 × S1 / S2 - C1, and the decrease amount LA [at%] of Al is taken as the value obtained from the formula LA = A2 × S1 / S2 - A1. However, let the element ratios of Al, Si, and Cr determined by EDX for the silicon alloy before the removal treatment be A1 [at%], S1 [at%], and C1 [at%] (where A1 + S1 + C1 = 100), respectively, and let the element ratios of Al, Si, and Cr determined by EDX for the porous silicon material after the removal treatment be A2 [at%], S2 [at%], and C2 [at%] (where A2 + S2 + C2 = 100), respectively. Note that, for example, since Si does not dissolve in hydrochloric acid, when the removal treatment is performed by acid treatment with hydrochloric acid, the amount of Si can be assumed to be unchanged before and after the removal treatment.

[0023] In the porosity process, it is preferable to selectively remove the Al component, i.e., the Al phase and its compounds, with an acid or an alkali. The removal treatment using an acid is also referred to as an acid treatment, and the removal treatment using an alkali is also referred to as an alkali treatment. The acid or alkali to be used preferably elutes elements and / or compounds other than silicon in the silicon alloy and does not elute silicon, and examples thereof include hydrochloric acid, sulfuric acid, sodium hydroxide, etc. This acid or alkali is preferably made into an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it can remove the Al component. For example, it may be 0.01 mol / L or more, or 0.02 mol / L or more, or 0.05 mol / L or more. The concentration of the acid or alkali may be, for example, 5 mol / L or less, or 3 mol / L or less, or 1.5 mol / L or less. From the viewpoint of suppressing the incorporation of oxygen into the porous silicon material, the concentration of the acid or alkali is preferably low. For example, less than 1 mol / L is preferable, less than 0.5 mol / L is more preferable, less than 0.2 mol / L is even more preferable, and it may be 0.1 mol / L or less. The temperature during the removal treatment may be, for example, less than 40°C, or 35°C or lower, or 30°C or lower. Also, the temperature during the removal treatment may be, for example, 0°C or higher, or 5°C or higher, or 10°C or higher. The time for performing the removal treatment may be, for example, 1 hour or more, or 2 hours or more. Also, the time for performing the removal treatment may be, for example, 24 hours or less, or 12 hours or less, or 10 hours or less. This removal treatment is preferably performed by immersing the silicon alloy in an acid or alkali solution and stirring as necessary. The obtained porous silicon material is then washed and dried.

[0024] (porous silicon material) The porous silicon material of the present disclosure may be produced by the manufacturing method described above. This porous silicon material has a skeleton in which Si coexists with Cr silicide containing at least an AlSiCr compound. Since Cr silicide is a semiconductor with a relatively high conductivity, an effect of reducing resistance can be expected. In particular, since such Cr silicide coexists with Si and Cr silicide exists along the skeleton serving as a conduction path, it is presumed that electric conduction is efficiently performed and the resistivity can be further reduced. The Cr silicide phase may be a CrSi compound or an AlCrSi compound, but an AlCrSi compound is preferable. The Cr silicide phase may be one or more of CrSi2, Cr(Si,Al)2, Al 13 Cr4Si4, and may be one or more of Cr(Si,Al)2, Al 13 Cr4Si4. The Cr silicide may exist in an amorphous form. This porous silicon material may contain an Al phase. Since Al has a high conductivity as a metal phase, an effect of reducing resistance can be expected. Further, since Al has a charge-discharge capacity as an active material of an energy storage device, an effect of alleviating a decrease in capacity due to the coexistence of a conductive phase can be expected.

[0025] When the total of the amounts of Si, Al, and Cr is 100 at%, this porous silicon material has an oxygen content of 25 at% or less, and an O / Cr ratio, which is the ratio of the oxygen content to the Cr content, of 10 or less. The lower the oxygen content and the lower the ratio of the oxygen content to the Cr content, the lower the resistivity tends to be, which is presumably because there is less insulating phase such as SiO2. The above-mentioned oxygen content is preferably 20 at% or less, may be 18 at% or less, may be 15 at% or less, or may be 10 at% or less. Also, this oxygen content may be 3 at% or more, may be 5 at% or more, or may be 10 at% or more. Also, the above-mentioned O / Cr ratio may be 9 or less, may be 8 or less, or may be 5 or less. This O / Cr ratio may be 2 or more or may be 3 or more. It is preferable that this porous silicon material satisfies at least one of (1) the above-mentioned oxygen content being 10 at% or less and (2) the O / Cr ratio being 5 or less. In those that satisfy any of these, the resistivity can be further reduced.

[0026] When the total of the amounts of Si, Al, and Cr is 100 at%, this porous silicon material may have a Cr content of more than 0 at% and 5 at% or less, and an Al / Cr ratio, which is the ratio of the Al content to the Cr content, of 2 or more and 7 or less. This Al / Cr ratio may be 6 or less, may be 5 or less, or may be 4 or less. This Al / Cr ratio may also be 3 or more. Incidentally, the Al / Cr ratio may be 30 or less, may be 20 or less, or may be 10 or less.

[0027] In this porous silicon material, from the perspective of reducing resistivity, it is preferable that the Al content is high. For example, when the total amount of Si, Al, and Cr is 100 at%, the Al amount may be 6 at% or more, 7 at% or more, or 10 at% or more. This Al amount may be, for example, 50 at% or less, 30 at% or less, 25 at% or less, 20 at% or less, or 15 at% or less. Also, in the porous silicon material, from the perspective of enhancing strength, it is preferable that the Cr content is high. For example, when the total amount of Si, Al, and Cr is 100 at%, the Cr amount may be 0.5 at% or more, 0.7 at% or more, or 1 at% or more. This Cr amount may be, for example, 10 at% or less, 7 at% or less, or 5 at% or less. When the total amount of Si, Al, and Cr in this porous silicon material is 100 at%, the Si amount may be 93 at% or less, 92 at% or less, or 90 at% or less. This Si amount may be 50 at% or more, 70 at% or more, or 75 at% or more.

[0028] When the total of the Si phase, Cr silicide phase, SiO2 phase, and Al phase is 100 mol%, the content of the Cr silicide phase may be 10 mol% or less, 7 mol% or less, 6 mol% or less, or 2 mol% or less. The content of this Cr silicide phase may be 0.1 mol% or more, 0.5 mol% or more, or 1 mol% or more. This porous silicon material may have a Si phase content of 96 mol% or less, 90 mol% or less, or 85 mol% or less. The content of this Si phase may be 65 mol% or more, 70 mol% or more, or 75 mol% or more. This porous silicon material may have a SiO2 phase content of 15 mol% or less, 12 mol% or less, or 10 mol% or less. The content of this SiO2 phase may be 0.5 mol% or more, 1 mol% or more, or 3 mol% or more. This porous silicon material preferably does not contain an Al phase. Not containing an Al phase means that no peak of the Al phase is confirmed in X-ray diffraction measurement (XRD). Even if this porous silicon material contains an Al phase, 30 mol% or less is preferable, 10 mol% or less is more preferable, 5 mol% or less is still more preferable, and 3 mol% or less is even more preferable. The contents of the Si phase, Cr silicide phase, SiO2 phase, and Al phase may be determined, for example, as follows. The porous silicon material after heat treatment at 600 °C is subjected to XRD measurement to obtain the ratios of the Si phase, Cr(Al,Si)2 phase, and Al phase, and further, the ratio of the SiO2 phase to the Cr(Al,Si)2 phase is obtained from the elemental ratio of Cr and O, and the contents of each phase may be calculated with the total of the Si phase, Cr(Al,Si)2 phase, Al phase, and SiO2 phase being 100 mol%. When it is confirmed in XRD that no Al phase is contained, it may be calculated from the elemental ratios of Al, Si, Cr, and O on the assumption that the phases present in the sample are Si, Cr(Al,Si)2, and SiO2. At that time, it may be calculated assuming that 5 at% of Al is contained in (substituted in) the Si phase and all the remaining Al is contained in the Cr(Al,Si)2 phase. In addition, the inventors of the present invention, Al 80 Si 20When an AlSi alloy without Cr such as [description] is acid-treated, only the Si phase is confirmed in the XRD results and the Al phase is not confirmed, but the composition analysis results confirm that about 5 at% of Al is present. The same results are obtained when using AlSi alloys with different compositions, and it is considered that Al is dissolved in the Si phase. It is speculated that in the AlSiCr system as well, it can be assumed that the Si phase contains 5 at% of Al.

[0029] In this porous silicon material, the porosity is 55% by volume or more. This porosity is the value measured by a mercury porosimeter. This porosity may be, for example, 60% by volume or more, 65% by volume or more, or 70% by volume or more. Also, the porosity may be, for example, 85% by volume or less, 80% by volume or less, or 75% by volume or less. A larger porosity is more likely to respond to the volume change during the occlusion of carrier ions, and a smaller porosity means a larger amount of Si present per unit volume, which is preferable.

[0030] In this porous silicon material, the average pore diameter may be 100 nm or less. Also, in this porous silicon material, the pore diameter range may be 1 nm or more and 300 nm or less. The average pore diameter and the pore diameter are the values measured by a mercury porosimeter. The average pore diameter may be 65 nm or less, or 60 nm or less. Also, the average pore diameter may be 10 nm or more, 20 nm or more, or 30 nm or more. The pore diameter range may be, for example, 5 nm or more, 10 nm or more, or 20 nm or more. Also, this pore diameter range may be, for example, 200 nm or less, or 100 nm or less. A smaller pore diameter is preferable because the pores are less likely to collapse. Also, a larger pore diameter is preferable because it can better suppress the volume change when carrier ions are occluded.

[0031] This porous silicon material may have a resistivity of 10,000 Ωcm or less. The resistivity of the porous silicon material shall be the resistivity obtained by the four-terminal method. When the porous silicon material is in powder form, the porous silicon powder is pressed at 200 MPa to produce a compacted powder, and the resistivity obtained by the four-terminal method for this compacted powder shall be taken as the resistivity of the porous silicon material. This resistivity may be 2,000 Ωcm or less, 1,000 Ωcm or less, 500 Ωcm or less, or 200 Ωcm or less.

[0032] The porous silicon material may include skeletal silicon having a three-dimensional network structure with voids and contain SiCr compounds and / or AlSiCr compounds as Cr silicides. The Cr silicides are presumed to be responsible for reinforcing the silicon skeleton. From the perspective of reinforcing the skeleton, it is preferable to have more Cr silicides, and from the perspective of the charge-discharge capacity of the energy storage device, it is preferable to have fewer Cr silicides.

[0033] The porous silicon material preferably has pores of 1 μm or less by mercury intrusion porosimetry of 20% by volume or more. In the porous silicon material, it is preferable to have more such fine pores. It is more preferable that the porous silicon material has pores of 1 μm or less by mercury intrusion porosimetry of 30% by volume or more, still more preferable to have 40% by volume or more, and even more preferable to have 45% by volume or more. Also, the porous silicon material may have pores of 1 μm or less by mercury intrusion porosimetry of 90% by volume or less.

[0034] When the porous silicon material is subjected to a confinement pressure of 1 GPa as an electrode, it is preferable that the amount of pore change is less than 25% by volume, more preferably less than 20% by volume, and still more preferably less than 10% by volume. From the perspective of the skeleton strength, it is preferable that the amount of pore reduction is smaller when the porous silicon material is subjected to the confinement pressure.

[0035] This porous silicon material may contain one or more of Ca, Mg, Na, Sr, and P as the second element in the range of 15% by mass or less. Further, the porous silicon material may contain inevitable impurities in addition to Si, Al, and Cr. Note that it is preferable that the amount of the second element and inevitable impurities is smaller. Further, in this porous silicon material, a part of Cr may be substituted with another transition metal element M. In that case, the transition metal element M may be less than or equal to half of Cr in molar ratio, may be 20% or less, or may be 10% or less. Examples of the transition metal element M include Ti, V, Nb, Mo, Ni, Mn, Fe, Co, and Zn.

[0036] (Electrode for energy storage device) The electrode for an energy storage device includes the above-described porous silicon material as an electrode active material. This electrode becomes either a positive electrode or a negative electrode based on the potential of the counter electrode with respect to the potential of the electrode active material. However, when lithium is used as a carrier, it is preferably used as a negative electrode. This electrode can be used, for example, in a lithium-ion secondary battery, a hybrid capacitor, an air battery, or the like. The electrode for an energy storage device may have a porosity of the porous silicon material compressed in the range of 5% by volume or more and 50% by volume or less. In this electrode, the porosity of the porous silicon material may be decreased by compression during production. For example, when particles of porous silicon are used as the negative electrode active material of a lithium-ion secondary battery, the smaller the pores, the more uniformly lithium ions alloy when alloying, so stress concentration is reduced and deterioration of the electrode itself can be prevented. The porosity of the porous silicon material after compression may be appropriately adjusted according to the characteristics required for the electrode for an energy storage device. For example, it may be 5% by volume or more or 10% by volume or more. Further, the porosity of the porous silicon material after compression may be, for example, 30% by volume or less or 20% by volume or less.

[0037] The electrode for the energy storage device may be formed by forming the above-described porous silicon material on the current collector and fixing it on the current collector. This electrode can be produced by a process of mixing the porous silicon material with a conductive material and a binder and a solvent as needed to form a paste and applying it onto the current collector, or by a process of mixing the porous silicon material with a conductive material and a binder as needed and pressing it onto the current collector. In this electrode, the content of the porous silicon material is preferably higher, preferably 70% by mass or more, more preferably 80% by mass or more, and still 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, one or a mixture of two or more of graphite such as natural graphite (scaly graphite, flaky graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, metals (copper, nickel, aluminum, silver, gold, etc.) can be used. The binder serves to connect and hold the active material particles and the conductive material particles. For example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR), etc. can be used alone or as a mixture of two or more. Also, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber (SBR) aqueous binder can be used. As the solvent, for example, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. Alternatively, a dispersant, a thickener, etc. can be added to water, and the active material can be slurried with a latex such as SBR. Examples of the coating method include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc., and any thickness and shape can be obtained using any of these methods.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.

[0038] 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, assuming the total electrode active material is 100% by mass, for example, the porous silicon material preferably occupies 50% by mass or more, preferably 90% by mass or more.

[0039] (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 for conducting carrier ions. The porous silicon material can be used as a negative electrode active material. This power storage device may be any of a lithium-ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as CrS2, CrS3, MoS3, FeS2, 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 oxides such as NiO2, with the basic composition formula Li (1-x) Ni a Co b Mn c Lithium nickel cobalt manganese composite oxides such as LiNiCoMnO2 (a + b + c = 1), lithium vanadium composite oxides such as LiV2O3, transition metal oxides such as V2O5, etc. can be used. Among these, lithium transition metal composite oxides, for example, LiCoO2, LiNiO2, LiMnO2, Li (1-x) Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc. are preferred. Note that the "basic composition formula" means that it may contain other elements such as Al and Mg. 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, polyacenes, etc. Among these, activated carbons showing a high specific surface area are preferred. The activated carbon as a carbonaceous material preferably has a specific surface area of 1000 m 2 / g or more, more preferably 1500 m 2 / g or more. When the specific surface area is 1000 m 2 / g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m 2 / g or less, more preferably 2000 m 2 / g or less, from the viewpoint of ease of production. Conductive materials, binders, solvents, current collectors, etc. used for the positive electrode can be appropriately used those exemplified in the above-described electrodes.

[0040] As the ionic conductive medium, a non-aqueous electrolyte containing a supporting salt, a non-aqueous gel electrolyte, or the like can be used. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, and these can be used alone or in combination. Specifically, 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-butyrolactone 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; dioxolanes such as 1,3-dioxolane and methyldioxolane, etc. Among these, a combination of cyclic carbonates and chain carbonates is preferred. According to this combination, not only are the cycle characteristics representing the battery characteristics during repeated charge and discharge excellent, but also the viscosity of the electrolyte, the electric capacity of the resulting battery, the battery output, etc. can be made well-balanced. Examples of the supporting salt include LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiSbF6, LiSiF6, LiAlF4, LiSCN, LiClO4, LiCl, LiF, LiBr, LiI, LiAlCl4, etc. Among these, it is preferable from the viewpoint of electrical properties to use a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF6, LiBF4, LiAsF6, and LiClO4, and organic salts such as LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3.This supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration for dissolving the supporting salt is 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 made more stable. Further, a flame retardant such as a phosphorus-based or halogen-based one may be added to this non-aqueous electrolyte.

[0041] Also, instead of the liquid ion conduction medium, a solid ion conductive polymer can be used as the ion conduction medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, vinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. Also, as the ion conduction medium, in addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder can be utilized.

[0042] The power storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the usage range of lithium secondary batteries. Examples include polymer non-woven fabrics such as polypropylene non-woven fabrics and polyphenylene sulfide non-woven fabrics, and thin microporous membranes of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination of multiple ones.

[0043] The shape of this power storage device is not particularly limited, and examples include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, etc. Further, it may be applied to large-sized ones used in electric vehicles and the like. FIG. 9 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 a porous silicon material 20 which is the above-described porous silicon material, and has a skeleton 25 in which Si22 and Cr silicide 24 coexist, and voids 26. In this porous silicon material 20, as shown in FIG. 10A, since the Cr silicide 24 which is a conductive phase exists along the skeleton 25 which is a conduction path, the resistivity can be lowered with a smaller amount of the conductive phase, and it is considered that a reduction in battery capacity can also be suppressed. The Al-Si-Cr-based master alloy composition is suitable for producing the porous silicon material 20 having such a coexisting structure. For example, a part of Cr in the Al-Si-Cr-based master alloy composition may be replaced with another transition metal element M (such as Ti, V, Nb, Mo, Ni, Mn, Fe, Co, Zn). In that case, in addition to having the above-described coexisting structure, the porous silicon material 20 may have a dispersed structure in which the silicide of the transition metal element M is dispersed between the skeletons 25. By the way, in the porous silicon material, when the oxygen amount exceeds 25 at% or the O / Cr ratio exceeds 10, etc., as in the porous silicon material 20F of FIG. 10B, the Cr silicide 24 which is the conductive phase of the skeleton 25 is segmented by an insulating phase such as SiO2 28, and the resistance may increase. However, in the porous silicon material 20, when the total of the amounts of Si, Al, and Cr is 100 at%, since the oxygen amount is 25 at% or less and the O / Cr ratio which is the ratio of the oxygen amount to the Cr amount is 10 or less, such an increase in resistance is also suppressed.

[0044] This energy storage device preferably has a higher initial charge-discharge efficiency. For example, 1000 mAh / g or more is preferable, 1200 mAh / g or more is more preferable, and 1500 mAh / g or more is even more preferable. This energy storage device preferably has a high initial charge-discharge efficiency. For example, 60% or more is preferable, 65% or more is more preferable, and 70% or more is even more preferable. This energy storage device preferably has a high capacity retention rate after cycling. For example, the capacity retention rate after 50 cycles is preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more. Also, for example, the capacity retention rate after 100 cycles is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0045] (All-solid-state lithium-ion secondary battery) This energy storage device is preferably an all-solid-state lithium-ion secondary battery. In an all-solid-state battery, changes in performance due to the electrolyte can be more effectively suppressed, and furthermore, safety can be enhanced, which is preferable. This all-solid-state lithium-ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode using the above-described porous silicon material as the negative electrode active material, and a solid electrolyte interposed between the positive electrode and the negative electrode for conducting lithium ions. The positive electrode can use any of those shown in the above-described energy storage device. Also, the negative electrode can use the above-described porous silicon material as the negative electrode active material.

[0046] 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 (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y Ty )O 12 or (Li 7-3z+x-y M z )(La 3-x A x )(Y 2-y T y )O 12 It may be a garnet-type oxide represented by the formula. However, in the formula, element M is one or more of Al and Ga, 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.

[0047] Alternatively, as the solid electrolyte, for example, general ones such as Li3N, Li called LISICON 14 Zn(GeO4)4, Li of sulfide 3.25 Ge 0.25 P 0.75 S4, perovskite-type La 0.5 Li 0.5 CrO3, (La 2 / 3 Li 3x □ 1 / 3-2x )CrO3 (□: atomic vacancy), garnet-type Li7La3Zr2O 12 , LiCr2(PO4)3 called NASICON type, Li 1.3 M 0.3 Cr 1.7 (PO3)4 (M = Sc, Al), etc. can be mentioned. Also, Li7P3S obtained from a glass of 80Li2S·20P2S5 (mol%) composition which is a glass-ceramics 11 , and furthermore, a substance having a high conductivity in the sulfide system, Li 10Examples also include Ge2PS2. Glassy inorganic solid electrolytes include Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li4SiO4, Li2S-P2S5, Li3PO4-Li4SiO4, Li3BO4-Li4SiO4, and those using SiO2, GeO2, B2O3, P2O5 as glassy substances and Li2O as a network modifier. Also, thio-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 disposed between the positive electrode and the negative electrode.

[0048] In addition, the all-solid-state lithium-ion secondary battery may include a restraining member that restrains the laminate in which the positive electrode, the solid electrolyte, and the negative electrode are laminated in the lamination direction. This restraining member may include, for example, a pair of plate-like portions that sandwich the laminate from both ends in the lamination direction of the laminate, a rod-like portion that connects the pair of plate-like portions, and an adjustment portion that is connected to the rod-like portion and adjusts the distance between the pair of plate-like portions by a screw structure or the like.

[0049] As described in detail above, the present disclosure can provide a novel porous silicon material with a lower resistivity, a power storage device, and a method for manufacturing the porous silicon material. The reason for obtaining such an effect is speculated as follows. For example, in an equilibrium phase diagram, a raw material of an Al-Si-Cr ternary alloy having a composition in which Al, Si, and a Cr silicide containing at least an AlSiCr compound appear at room temperature is melted and rapidly solidified to produce a precursor of a silicon alloy. When the Al component is removed, a porous silicon material having a skeleton in which Si and a Cr silicide coexist is obtained. In such a porous silicon material, since a Cr silicide, which is a conductive phase, exists along the skeleton serving as a conduction path, it is speculated that electric conduction is efficiently performed and the resistivity can be reduced. At that time, as the Al component is removed, oxygen may be taken into the porous silicon material to form an insulating phase such as SiO2, which may inhibit electric conduction. However, it is speculated that by reducing the amount of oxygen taken in, the inhibition of electric conduction is suppressed and the resistivity can be further reduced. In addition, since the Cr silicide contains an AlSiCr alloy, it is speculated that the resistivity can be effectively reduced with a smaller amount of conductive phase. The AlSiCr alloy is easily dissolved by acid treatment. As a result, amorphous silica may be generated during the removal treatment, which may reduce the conductivity. However, in the present disclosure, since the amount of amorphous silica generated can be reduced, the resistivity can be reduced even with a smaller amount of conductive phase, and it is speculated that the battery capacity of the negative electrode is not impaired. Further, as a result of reducing the resistivity, an improvement in rate characteristics can be expected, and it is speculated that a higher-performance battery can be manufactured.

[0050] In addition, the method for manufacturing the porous silicon material of the present disclosure uses an Al-Si-Cr ternary alloy as the master alloy. The Al-Si-Cr ternary alloy phase-separates into an Al phase, a Si phase, and a Cr silicide phase at room temperature. Since the crystallization sequence is such that the Si phase and the Cr silicide phase crystallize first, and then the Al phase crystallizes, during the process of cooling the alloy melt, a state occurs in which cluster particles of Si and Cr silicide are dispersed in the Al melt. When the Al melt solidifies, a structure in which the cluster particles are connected to each other is formed. From this, it is considered that by selectively removing the Al component, a composite porous body in which Cr silicide is segregated in the Si skeleton portion is formed. Since Cr silicide is a semiconductor with a relatively high conductivity, a reduction in resistivity can be expected due to the coexistence of Cr silicide. Conventionally, from the viewpoint of alleviating the expansion and contraction of silicon, the porosity of silicon particles has been studied. Generally, however, porous silicon tends to have a higher resistance than non-porous silicon. In order to solve this problem, reducing the resistance by compounding with a conductive phase has also been studied. However, in general methods, while the resistance decreases as the amount of the conductive phase increases, the battery capacity is often sacrificed. On the other hand, in the present disclosure using an Al-Si-Cr ternary alloy, since Cr silicide, which is a conductive phase, exists along the silicon skeleton, which is a conduction path, it is speculated that the resistivity can be reduced with a smaller amount of the conductive phase, and moreover, the battery capacity is not impaired.

[0051] Furthermore, the present disclosure is also expected to further suppress the deterioration of charge and discharge characteristics in materials containing Si. The reason for obtaining such an effect is speculated as follows. For example, the silicon negative electrode for a lithium-ion secondary battery has a theoretical capacity of 4199 mAh / g, which is about 10 times that of the theoretical capacity of general graphite, 372 mAh / g, and further increases in capacity and energy density are expected. On the other hand, silicon that has occluded lithium is Li 4.4It is Si, and its volume expands up to about 4 times that of silicon before lithium intercalation. A high-capacity Si negative electrode may have its current collection property damaged due to expansion and contraction during charge and discharge, resulting in a decrease in capacity. In the porous silicon material of the present disclosure, it has nanopores, which can relieve the stress associated with the expansion and contraction of silicon during charge and discharge. At the same time, by introducing a Cr silicide phase that is inert to carrier ions into the silicon skeleton, the skeleton strength can be improved, and it is possible to prevent the pore structure of silicon from being crushed by the stress associated with expansion and contraction during charge and discharge. Therefore, further improvement in cycle characteristics can be expected. Furthermore, by improving the strength of the particles, it is possible to suppress the crushing of pores by roll pressing or the like during the production of the composite electrode. Therefore, in the present disclosure, it is possible to easily obtain a high-performance power storage device such as by relaxing volume expansion and contraction and improving cycle characteristics.

[0052] Note that the present disclosure is not limited to the above-described embodiments at all, and it goes without saying that various embodiments can be implemented as long as they belong to the technical scope of the present disclosure.

[0053] For example, the present disclosure may be as shown in any one of the following [1] to

[10] . [1] A porous silicon material having a skeleton in which Si and Cr silicide containing at least an AlSiCr compound coexist, when the total of the Si amount, the Al amount, and the Cr amount is 100 at%, the oxygen amount is 25 at% or less, the O / Cr ratio which is the ratio of the oxygen amount to the Cr amount is 10 or less, and the porosity determined by mercury intrusion porosimetry is 55% by volume or more. [2] The porous silicon material according to [1], satisfying one or more of the following (1) to (2). (1) The oxygen amount is 10 at% or less. (2) The O / Cr ratio is 5 or less. [3] The porous silicon material according to [1] or [2], wherein the Cr amount is more than 0 at% and 5 at% or less, and the Al / Cr ratio which is the ratio of the Al amount to the Cr amount is 2 or more and 7 or less. [4] The porous silicon material according to any one of [1] to [3], wherein the average pore diameter determined by mercury intrusion porosimetry is 60 nm or less. [5] The Cr silicide is one or more of CrSi2, Cr(Si,Al)2, and Al 13 Cr4Si4, and the content of the Cr silicide is 6 mol% or less. The porous silicon material according to any one of [1] to [4]. [6] The porous silicon material according to any one of [1] to [5], having a resistivity of 1000 Ωcm or less. [7] A positive electrode containing a positive electrode active material, [1] to [6] A negative electrode containing the porous silicon material according to any one of [1] to [6] as a negative electrode active material, An ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions, And a power storage device comprising the same. [8] A method for producing the porous silicon material according to any one of [1] to [6], In a precursor step of melting a raw material of an Al-Si-Cr ternary alloy having a composition in which Al, Si, and at least a Cr silicide containing an AlSiCr compound appear at room temperature in an equilibrium phase diagram and rapidly solidifying to obtain a precursor of a silicon alloy, A pore formation step of removing the Al component contained in the silicon alloy to obtain the porous silicon material, And a method for producing a porous silicon material. [9] In the precursor step, using the raw material satisfying 18 ≦ x ≦ 40, y ≧ 0.1, and x / y ≧ 8 in the basic composition formula Al 100-x-y Si x Cr y The method for producing a porous silicon material according to [8].

[10] In the pore formation step, removing the Al component with hydrochloric acid having a concentration of less than 0.5 mol / L. The method for producing a porous silicon material according to [8] or [9].

Examples

[0054] Hereinafter, examples of specifically producing the porous silicon and the power storage device of the present disclosure will be described as experimental examples. Experimental examples 1 to 8, 11 to 12, 14 to 27, and 29 correspond to the examples of the present disclosure, and experimental examples 9 to 10, 13, and 28 correspond to comparative examples.

[0055] [Fabrication of Porous Silicon Material] Raw materials of Al, Si, and Cr were weighed to have a composition of the basic composition formula Al 100-x-y Si x Cr y (x = 15 - 40, y = 0 - 10), and melted in an arc melting furnace. Before melting, the inside of the arc melting furnace was evacuated to 8×10 -3 Pa or less and then replaced with Ar gas. For the production of the master alloy, melting of the raw material powder was necessary, and high-frequency melting was performed to produce a uniform sample. The obtained master alloy was heated and melted at 1000 - 1300 °C in an Ar atmosphere, and subjected to rapid solidification treatment at a rate of 10 2 K / sec or more using the gas atomization method to obtain AlSiCr alloy powder (precursor process). The obtained alloy was immersed in a hydrochloric acid aqueous solution of 0.1 - 3 mol / L, and acid treatment (removal treatment) was performed at an arbitrary temperature from room temperature (25 °C) to 80 °C for 1 - 48 hours to selectively remove the Al component. The residue was transferred to a filtration filter, and after removing the acid after treatment by the pressure filtration method, it was washed with distilled water four or more times, and the washing water was removed by the same pressure filtration method to obtain porous silicon (porous process).

[0056] (Experimental Examples 1 - 4) The porous silicon material prepared under the conditions of x = 30 and y = 1 in the above basic composition formula for the master alloy composition, hydrochloric acid concentration of 0.1 mol / L, acid treatment temperature of 25 °C, and acid treatment time of 22 hours was used as Experimental Example 1. The porous silicon material prepared in the same manner as Experimental Example 1 except that the master alloy composition was x = 25 and y = 1.5 in the above basic composition formula and the acid treatment time was 20 hours was used as Experimental Example 2. The porous silicon material prepared in the same manner as Experimental Example 1 except that the master alloy composition was x = 30 and y = 3 in the above basic composition formula and the acid treatment time was 20 hours was used as Experimental Example 3. The porous silicon material prepared in the same manner as Experimental Example 1 except that the master alloy composition was x = 25 and y = 3 in the above basic composition formula and the acid treatment time was 20 hours was used as Experimental Example 4.

[0057] (Experimental Examples 5 - 9) The master alloy composition was set to the composition of x = 30 and y = 1 in the above basic composition formula, and the porous silicon material prepared under the conditions of a hydrochloric acid concentration of 3 mol / L, an acid treatment temperature of 80 °C, and an acid treatment time of 5 hours was taken as Experimental Example 5. An experimental porous silicon material prepared in the same manner as Experimental Example 5 was taken as Experimental Example 6, except that the master alloy composition was set to the composition of x = 25 and y = 1.5 in the above basic composition formula. An experimental porous silicon material prepared in the same manner as Experimental Example 5 was taken as Experimental Example 7, except that the master alloy composition was set to the composition of x = 30 and y = 3 in the above basic composition formula. An experimental porous silicon material prepared in the same manner as Experimental Example 5 was taken as Experimental Example 8, except that the master alloy composition was set to the composition of x = 25 and y = 3 in the above basic composition formula. An experimental porous silicon material prepared in the same manner as Experimental Example 5 was taken as Experimental Example 9, except that the master alloy composition was set to the composition of x = 20 and y = 0 in the above basic composition formula.

[0058] (Experimental Example 10) Si powder with an average particle size of 5 μm (SIE23PB manufactured by high-purity chemicals) was directly taken as Experimental Example 10.

[0059] (Experimental Examples 11 - 29) The master alloy composition was set to x = 30 and y = 1 in the above basic composition formula, and the porous silicon material prepared under the conditions of hydrochloric acid concentration of 0.1 mol / L, acid treatment temperature of 25 °C, and acid treatment time of 18 hours was taken as Experimental Example 11. The porous silicon material prepared in the same manner as Experimental Example 11 except that the acid treatment time was 24 hours was taken as Experimental Example 12. The porous silicon material prepared in the same manner as Experimental Example 11 except that the hydrochloric acid concentration was 1 mol / L and the acid treatment time was 2 hours was taken as Experimental Example 13. The porous silicon materials prepared in the same manner as Experimental Example 13 except that the acid treatment times were 5 hours, 8 hours, 10 hours, 12 hours, 18 hours, and 24 hours were taken as Experimental Example 14, Experimental Example 15, Experimental Example 16, Experimental Example 17, Experimental Example 18, and Experimental Example 19, respectively. The porous silicon material prepared in the same manner as Experimental Example 11 except that the hydrochloric acid concentration was 3 mol / L and the acid treatment time was 2 hours was taken as Experimental Example 20. The porous silicon materials prepared in the same manner as Experimental Example 20 except that the acid treatment times were 5 hours, 12 hours, and 24 hours were taken as Experimental Example 21, Experimental Example 22, and Experimental Example 23, respectively. The porous silicon material prepared in the same manner as Experimental Example 11 except that the hydrochloric acid concentration was 3 mol / L, the acid treatment temperature was 80 °C, and the acid treatment time was 1 hour was taken as Experimental Example 24. The porous silicon materials prepared in the same manner as Experimental Example 24 except that the acid treatment times were 2.5 hours, 4 hours, and 7.3 hours were taken as Experimental Example 25, Experimental Example 26, and Experimental Example 27, respectively. The porous silicon materials prepared in the same manner as Experimental Example 11 except that the acid treatment times were 5 hours and 20 hours were taken as Experimental Example 28 and Experimental Example 29, respectively.

[0060] (XRD measurement) For the atomized powder before acid treatment and the porous silicon powder after acid treatment, X-ray diffraction measurement was performed using an X-ray diffractometer (Ultima IV manufactured by Rigaku Corporation), with a Cu tube target, in the range of 2θ = 20° to 60° at a rate of 20° / min. The ratios of the Si phase, Al phase, Al 13 Cr4Si4 phase, Cr(Al,Si)2 phase were calculated as mass ratios from the XRD peak intensity ratios of each phase using an alumina standard substance. The ratios of each phase were the values measured for the sample heat-treated at 600 °C to crystallize the amorphous phase.

[0061] (SEM / EDX measurement) Regarding the atomized powder before acid treatment and the porous silicon powder after acid treatment, they were pressure-molded into a disc shape using a cemented carbide die, and the pressure surface was observed with a scanning electron microscope (SEM, S-4300 manufactured by HITACHI) and energy-dispersive X-ray analysis (EDX, S-4300 manufactured by HITACHI), and a composition analysis was performed.

[0062] (SEM Observation) Regarding the porous silicon powder after acid treatment, it was embedded in resin, and the cross-section was exposed by polishing, and SEM observation was performed using the above scanning electron microscope.

[0063] (Pore Size Distribution Measurement) Regarding the porous silicon powder after acid treatment, the pore size distribution was measured with a mercury porosimeter (POREMASTER60GT manufactured by Quantachrome). Also, the average pore size was determined from the values of the volume fraction of pores of each size in the pore size distribution data. Generally, the powder of porous materials contains pores inside the particles and pores between the particles, but in the pore size distribution data, the two cannot be distinguished. Here, by comparing with the pore size distribution measurement results of the atomized powder before acid treatment-induced porosification, the pore fraction below 300 nm was judged as the pores inside the particles to distinguish the two, and the pore fraction and average pore size inside the particles were determined. Although the illustration is omitted, in the pore size distribution, there were peaks at 700 - 800 nm and around 100 nm. Based on the pore size distribution data of the atomized powder before acid treatment and the comparison with SEM, the peak around 100 nm was judged as the peak of the pores inside the particles, and using 300 nm, which is the boundary of the two peaks, as a reference, the pores below 300 nm were judged as the pores inside the particles as described above. In addition, by comparing the pore size distribution data of the sample before acid treatment and the sample after acid treatment, the pores inside the particles and the pores between the particles can be distinguished, and the pore fraction and average pore size inside the particles can be determined.

[0064] (STEM / EDX Measurement) Regarding the porous silicon powder after acid treatment, elemental mapping and microstructure observation were performed using a scanning transmission electron microscope (STEM, JEM2100F manufactured by JEOL).

[0065] (Resistivity Measurement) For the porous silicon powder after acid treatment, it was pressed at 200 MPa to produce a pressed powder body of 3×1×1 mm, and a copper wire was baked with silver paste, and the resistivity was evaluated by the four-terminal method. Incidentally, when the porosity before and after pressing was evaluated by a mercury porosimeter, it was inferred that there was almost no change in the porosity under a pressure of 200 MPa and the pore structure of the porous silicon powder was maintained. The resistivity measurement was carried out at 25°C.

[0066] (Battery characteristic evaluation) The battery characteristics were evaluated using the porous silicon powder after acid treatment. 60 parts by mass of the porous silicon powder, 20 parts by mass of the carbon black conductive assistant, and 20 parts by mass of the polyimide binder were mixed, and the slurry whose viscosity was adjusted with N-methylpyrrolidone was applied onto a copper current collector foil and dried to fabricate a composite electrode. A metal lithium was stacked as a counter electrode with a porous polyethylene separator interposed therebetween on the negative electrode fabricated by punching out the composite electrode with a punch having a diameter of 16 mm, and an electrolyte was injected to manufacture a lithium secondary battery using a Tom cell type small battery cell. The electrolyte used was a mixture solvent containing 1.5 parts by volume of fluoroethylene carbonate (FEC), 3 parts by volume of ethylene carbonate (EC), 4 parts by volume of dimethyl carbonate (DMC), and 3 parts by volume of ethyl methyl carbonate (EMC), to which LiPF6 was added at a concentration of 1 mol / L. With respect to the obtained lithium secondary battery, charge and discharge were repeatedly performed 100 cycles at a current density of 0.2C in the range of a battery voltage of 0.005V to 1.5V. Here, the reaction in which the porous silicon powder occludes lithium is referred to as charging, and the reaction in which the porous silicon powder releases lithium is referred to as discharging. Then, the initial discharge capacity, the initial charge-discharge efficiency, the initial areal capacity, the capacity retention rate after 50 cycles, and the capacity retention rate after 100 cycles were determined. Here, the capacity referred to herein is the discharge capacity (specific capacity) per mass of the active material contained in the negative electrode (the mass of the substance excluding the binder and the conductive assistant on the copper current collector foil). The initial discharge capacity was taken as the capacity per mass of the porous silicon powder. The initial charge-discharge efficiency was taken as the ratio of the discharge capacity of the discharge following the initial charge to the initial charge capacity. The initial areal capacity was taken as the discharge capacity of the total mass per area of the negative electrode. The capacity retention rate after 50 cycles and the capacity retention rate after 100 cycles were taken as the ratios of the discharge capacities at the 50th cycle and the 100th cycle, respectively, to the initial discharge capacity. Further, with respect to the lithium secondary battery fabricated in the same manner as above, in the range of a battery voltage of 0.01V to 1.5V, the current per mass of the porous silicon powder was changed to 0.1A / g, 0.2A / g, 0.5A / g, 1A / g, 2A / g, 0.1A / g every 10 cycles to perform charge and discharge, and the rate characteristics were evaluated.

[0067] (Results and Discussion) The experimental results of Experimental Examples 1 to 29 were summarized in Tables 1 to 3. Hereinafter, the experimental results were discussed. First, regarding the crystal phase of the atomized powder, it was examined using the XRD measurement results. As an example, in Fig. 11, Al which is the precursor of the porous silicon material of Experimental Example 2 73.5 Si 25 Cr 1.5 The XRD pattern of the atomized powder is shown (Fig. 11(a)). Fig. 11 also shows the XRD pattern of the sample obtained by heat-treating the atomized powder at 200 °C (Fig. 11(b)). In the atomized powder, only diffraction peaks of the Si phase and the Al phase were observed. However, when heat-treated at 200 °C or higher, diffraction peaks of the Al 13 Cr4Si4 phase appeared in addition to the Al and Si phases, indicating that the room-temperature equilibrium phase in the phase diagram exists. From this, it was inferred that due to rapid cooling, the silicide phase in the atomized powder exists in an amorphous state. Also, as a result of elemental analysis, it was found that almost no oxygen is contained at the stage of the atomized powder.

[0068] Regarding the influence of the acid treatment conditions (hydrochloric acid concentration, treatment time, temperature) on the composition of the product, the EDX measurement results of the porous silicon materials of Experimental Examples 5, 13 to 27 were used for examination. Fig. 12 shows the changes in the amounts of oxygen and chromium in the Al 69 Si 30 Cr1 atomized powder with the acid treatment time. In the process of increasing the treatment time and becoming porous, the amount of oxygen increased and the chromium content tended to decrease. Regarding the dissolution of Cr, it was more remarkable when treated with a high-concentration acid. In 3 mol / L high-concentration hydrochloric acid, about half of the Cr in the master alloy composition dissolved. On the other hand, when treated with 0.1 mol / L low-concentration hydrochloric acid, it was found that the dissolution of Cr could be suppressed to about half. Along with the acid treatment, the concentration of the acid contained after the acid treatment also increased, and the amount was linked to the change in the Cr elution amount, indicating that oxidation occurred with the dissolution of the chromium silicide phase. However, by XRD, no oxide phase could be confirmed even after heating, suggesting that amorphous SiO2 was formed. Regarding oxygen as well, the content decreased the more it was treated with low-concentration hydrochloric acid, and in particular, it was found that 0.1 mol / L low-concentration hydrochloric acid was effective.

[0069] Regarding the dissolution of Al due to acid treatment, it was examined using the XRD measurement results of the porous silicon materials in Experimental Examples 11, 28, and 29. In FIG. 13, Al 69 Si 30 As an example showing the change in the contained phases with the treatment time when the Cr1 atomized powder was treated with a 0.1 mol / L hydrochloric acid aqueous solution, the XRD patterns of the porous silicon materials in Experimental Examples 11, 28, and 29 were shown. A strong diffraction peak of Al was confirmed after 5 hours of treatment. However, as the treatment time increased, the peak intensity of Al decreased, indicating that the dissolution of Al proceeded. After 20 hours of treatment, the diffraction peak of Al completely disappeared, indicating that the porous treatment was completed. However, when the sample at this stage was subjected to compositional analysis, it was found that more than 5 at% of Al remained, and the amount hardly changed even when the acid treatment time was increased. Therefore, it was found that a certain amount of Al remained in the form of a solid solution or the like even after the porous treatment.

[0070] As described above, since chromium silicide exists in an amorphous state, no peak of chromium silicide was observed in the sample after acid treatment (FIG. 13). Using the samples of the porous silicon materials in Experimental Examples 1 to 4 heat-treated at 600 °C, the phases contained in the samples after acid treatment were examined. In FIG. 14, Al 69 Si 30 Cr1, Al 73.5 Si 25 Cr 1.5 、Al 67 Si 30 Cr3、Al 72 Si 25 The XRD patterns of the samples of the porous silicon materials in Experimental Examples 1 to 4, which are samples obtained by treating each of the Cr1, Al 13 Cr4Si4 phase (see FIG. 11), was crystallized.

[0071] Regarding the pore distribution, it was examined using the measurement results of the mercury porosimeter of the porous silicon materials in Experimental Examples 5 to 8 and the SEM observation results of Experimental Example 5. In Table 2, Al 69Si 30 Cr1, Al 73.5 Si 25 Cr 1.5 , Al 67 Si 30 Cr3, Al 72 Si 25 The porosity and average pore diameter of the porous silicon materials of Experimental Examples 5 to 8, which are samples obtained by subjecting each atomized powder of Cr3 to acid treatment and porosity, were summarized. In Experimental Examples 5 to 8, the porosity was in the range of 65 to 75 vol%, and a tendency was observed that the average pore diameter decreased with an increase in the Cr content. It was found that all samples had fine pore diameters of 100 nm or less. Fig. 15 shows a cross-sectional secondary electron image of the porous silicon material of Experimental Example 8 by SEM. In Fig. 15, a spherical structure in which pores of 100 nm or less were uniformly distributed was observed.

[0072] Next, for the porous silicon material of Experimental Example 8, the elemental distribution of the pore part was measured by STEM / EDX. In Fig. 16, Al 72 Si 25Shown is the STEM / EDX elemental mapping of the porous silicon material of Experimental Example 8, which is a sample obtained by acid-treating and atomizing Cr3 powder to make it porous. Fig. 16A is a STEM image, and Figs. 16B to D are mapping images of Si, Cr, and Al, respectively. Also, Fig. 17 summarizes the electron beam diffraction results of the porous silicon material of Experimental Example 8. As shown in Fig. 16, it was found that each element of Al, Si, and Cr was segregated in the skeletal part. Furthermore, in the electron beam diffraction of the skeletal part, a pattern as shown in Fig. 17 was obtained, and from the values of the interplanar spacing, it was shown that crystalline Si and Cr(Al,Si)2 were present in the skeletal part. From these results, it became clear that, as expected, a coexisting type of composite nanoporous Si had been formed. Also, for the porous silicon materials of Experimental Examples 1 and 3, the elemental distribution in the pore part was measured by STEM / EDX. Fig. 18 shows the STEM / EDX elemental mapping of the porous silicon material of Experimental Example 4. Fig. 19 shows the STEM / EDX elemental mapping of the porous silicon material of Experimental Example 1. Figs. 18A and 19A are cross-sectional secondary electron images by SEM, Figs. 18B and 19B are STEM photographs, Figs. 18C and 19C are superposed images of elemental mapping, and Figs. 18D to F and Figs. 19D to F are mapping images of Si, Cr, and Al, respectively. From Figs. 18 and 19, it also became clear that a coexisting type of composite nanoporous Si had been formed in Experimental Examples 1 and 4.

[0073] Next, using the porous silicon materials of Experimental Examples 1 to 9, the resistivity was examined. Figure 20 shows the influence of the chromium silicide concentration on the resistivity of the porous body. The concentration of chromium silicide was calculated from the elemental ratios of Al, Si, Cr, and O assuming that the phases present in the sample were Si, Cr(Al,Si)2, and SiO2. It was found that the resistivity decreased in the composition containing the conductive phase compared to the case without the conductive phase. Also, the resistivity decreased when treated with 0.1 mol / L hydrochloric acid compared to when treated with 3 mol / L hydrochloric acid. Furthermore, the resistivity was minimized when the chromium silicide concentration was 1 to 2 mol%, and tended to increase as the chromium silicide concentration increased. These behaviors are considered to be related to the amount of SiO2 generated by dissolving chromium silicide during acid treatment. Figure 21 shows the change in the amount of oxygen with respect to the chromium silicide concentration. As the amount of chromium silicide increases, the oxygen concentration also increases, and the amount of oxygen is large in the sample treated with 3 mol / L hydrochloric acid. Chromium silicide acts as a conductive phase and reduces the resistivity, but it was suggested that increasing the amount of chromium silicide by increasing the amount of Cr during charging increases the formation of the SiO2 phase accompanying the dissolution of chromium silicide, and conversely increases the resistivity. From this, it was considered that the ratio of the oxygen content O (O / Cr ratio) correlated with the amount of SiO2 affects the resistivity with respect to the Cr content correlated with the amount of chromium silicide. Figure 22 shows the change in resistivity with respect to the Cr content of the porous body sample. From Figure 22, when sorted by the O / Cr ratio, it was shown that at the same Cr content, the lower the O / Cr ratio, the lower the resistivity. When the O / Cr ratio is about 3 to 4, a low resistance of 1000 Ωcm or less can be achieved, and it was found that a lower chromium silicide concentration suffices to achieve the same resistivity.

[0074] Finally, batteries were fabricated using the porous silicon materials of Experimental Examples 1 to 5 and 9, and their battery characteristics were examined. Table 2 summarizes the battery characteristics of Experimental Examples 1 to 5 and 9. Specifically, the initial discharge capacity, initial charge-discharge efficiency, initial areal capacity, capacity retention rate after 50 cycles, and capacity retention rate after 100 cycles were summarized. A higher capacity retention rate was shown in the porous body samples compared to the Si powder without pores. Furthermore, it was found that the capacity decreased as the composition with more chromium silicide increased, but when the chromium silicide conductive phase was 2 mol% or less, a capacity comparable to that of pure Si could be obtained. In addition, Fig. 23 shows the evaluation results of the rate characteristics of Experimental Examples 1, 3, and 9. From Fig. 23, it was found that in Experimental Examples 1 and 3 with low resistivity, not only was the capacity higher and the capacity retention rate higher than that of Experimental Example 9 with high resistivity, but the rate characteristics were also good.

[0075]

Table 1

[0076]

Table 2

[0077]

Table 3

[0078] Note that the present disclosure is not limited to the above-described experimental examples, and it goes without saying that it can be implemented in various modes as long as it belongs to the technical scope of the present disclosure.

Industrial Applicability

[0079] The present disclosure is applicable to the technical field of secondary batteries.

Explanation of Signs

[0080] 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 conduction medium, 20, 20F Porous silicon material, 22 Si, 24 Cr silicide, 25 Skeleton, 26 Void, 28 SiO2.

Claims

1. A porous silicon material having a skeleton in which Si coexists with Cr silicide containing at least an AlSiCr compound, wherein when the total amount of Si, Al, and Cr is 100 at%, the oxygen amount is 25 at% or less, the O / Cr ratio, which is the ratio of the oxygen amount to the Cr amount, is 10 or less, and the porosity determined by mercury intrusion porosimetry is 55% by volume or more.

2. The porous silicon material according to claim 1, satisfying one or more of the following (1) to (2). (1) The oxygen amount is 10 at% or less. (2) The O / Cr ratio is 5 or less.

3. The porous silicon material according to claim 1 or 2, wherein the Cr amount is more than 0 at% and 5 at% or less, and the Al / Cr ratio, which is the ratio of the Al amount to the Cr amount, is 2 or more and 7 or less.

4. The porous silicon material according to claim 1 or 2, wherein the average pore diameter determined by mercury intrusion porosimetry is 60 nm or less.

5. The Cr silicide is CrSi 2 , Cr(Si,Al) 2 , Al 13 Cr 4 Si 4 or more of these, and the content of the Cr silicide is 6 mol% or less. The porous silicon material according to claim 1 or 2.

6. The porous silicon material according to claim 1 or 2, having a resistivity of 1000 Ωcm or less.

7. A positive electrode containing 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 conduction medium interposed between the positive electrode and the negative electrode and conducting lithium ions, and a power storage device comprising the same.

8. A method for producing the porous silicon material according to claim 1 or 2, In a precursor step of melting a raw material of an Al—Si—Cr ternary alloy having a composition in which Al, Si, and Cr silicide containing at least an AlSiCr compound appear at room temperature in an equilibrium diagram and rapidly solidifying the melt to obtain a precursor of a silicon alloy; In a porous formation step of removing an Al component contained in the silicon alloy by acid treatment or alkali treatment to obtain the porous silicon material; including; In the precursor step, in a basic composition formula Al100−x−ySixCry, the raw material satisfying 18 ≦ x ≦ 40, y ≧ 0.1, and x / y ≧ 8 is used; A method for producing a porous silicon material.

9. In the porous formation step, the Al component is removed with hydrochloric acid having a concentration of less than 0.5 mol / L. The method for producing a porous silicon material according to claim 8.

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