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

Abstract

It suppresses the decrease in conductivity and charge / discharge efficiency. [Solution] The porous silicon material has a Si phase and a conductive phase which is a silicide phase containing Cr and V and may also contain Al, and may also have an SiO2 phase. When the total of the Si phase, conductive phase and SiO2 phase is considered to be 100% by mass, the conductive phase is contained in an amount of 12% by mass or more and 60% by mass or less, the SiO2 phase content is 12% by mass or less, the porosity determined by the mercury intrusion method is 45% by volume or more and the average pore diameter is 100 nm or less.

Description

Technical Field

[0001] The present invention relates to a porous silicon material, an electrode, a power storage device, and a method for producing a porous silicon material.

Background Art

[0002] Conventionally, as a negative electrode material for a power storage device, for example, a porous silicon material obtained by removing an Al component from a silicon alloy obtained by melting a raw material containing Al, Si, and Cr in a predetermined ratio and rapidly solidifying it (see, for example, Patent Document 1), or a porous silicon material obtained by removing an Al component from a silicon alloy obtained by melting a raw material containing Al, Si, and V and rapidly solidifying it (see, for example, Patent Document 2) has been proposed. In these porous silicon materials, the silicon skeleton is strengthened by Cr silicide or V silicide that also functions as a conductive phase, and it is said that the deterioration of charge-discharge characteristics can be more suppressed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, in Patent Documents 1 and 2, silicide has functions as a conductive phase and as a function of suppressing expansion and contraction during charge and discharge. By suppressing the expansion and contraction during charge and discharge, it is possible to suppress the pulverization of the porous silicon material and the accompanying decrease in charge and discharge efficiency (Coulomb efficiency). Therefore, when producing a porous silicon material, it is desirable to increase the amount of silicide from the viewpoints of improving conductivity and suppressing a decrease in charge and discharge efficiency. However, if the amount of Cr or V in the raw material is increased to increase the amount of silicide in Patent Document 1 or Patent Document 2, part of the silicide may dissolve when removing the Al component from the silicon alloy, and a large amount of insulating amorphous SiO2 may be by-produced, resulting in a decrease in conductivity. It has been difficult to suppress a decrease in charge and discharge efficiency while suppressing a decrease in conductivity.

[0005] The present disclosure has been made to solve such problems, and the main object is to suppress a decrease in conductivity and charge and discharge efficiency.

Means for Solving the Problems

[0006] As a result of intensive studies to achieve the above-described object, the present inventors have conceived of producing a porous silicon material by removing the Al component from a silicon alloy obtained by melting and rapidly solidifying a raw material containing Al, Si, Cr, and V in a predetermined ratio. The porous silicon material thus obtained has a relatively large amount of a conductive phase of silicide containing Cr and V, and has a relatively small content of the SiO2 phase, and it has been found that it is possible to suppress a decrease in conductivity and charge and discharge efficiency, leading to the completion of the present disclosure.

[0007] That is, the porous silicon material of the present disclosure has a Si phase and a conductive phase which is a silicide phase containing Cr and V and optionally containing Al, may have a SiO2 phase, and when the total of the Si phase, the conductive phase, and the SiO2 phase is 100% by mass, the conductive phase is contained in the range of 12% by mass or more and 60% by mass or less, the content of the SiO2 phase is 12% by mass or less, and the porosity determined by mercury intrusion porosimetry is 45% by volume or more and the average pore diameter is 100 nm or less.

[0008] The electrodes disclosed herein are The material includes the porous silicon material described above and satisfies at least one of the following conditions: the expansion rate during the initial charge is 7% or less, or the thickness change rate during each charge / discharge cycle other than the initial charge is 4% or less.

[0009] The energy storage device disclosed herein is The negative electrode is an electrode containing the porous silicon material described above, A positive electrode containing a positive electrode active material, An ion-conducting medium interposed between the negative electrode and the positive electrode, which conducts carrier ions, It is something that is provided.

[0010] The method for manufacturing the porous silicon material disclosed herein is: A precursor step to obtain a silicon alloy precursor is obtained by melting and rapidly solidifying a raw material containing 10 at% to 50 at% Si, 0.5 at% to 5 at% Cr, and 1 at% to 10 at% V, when the total amount of Al, Si, Cr, and V is 100 at%. A porosizing step to obtain a porous silicon material by removing the Al component contained in the aforementioned silicon alloy, It includes. [Effects of the Invention]

[0011] This disclosure makes it possible to suppress the decrease in conductivity and charge / discharge efficiency. The reason for achieving such effects is presumed to be as follows. For example, when Cr and V are used in combination rather than individually, the by-product formation of SiO2 when the Al component is removed from the silicon alloy is suppressed. Therefore, it is possible to realize a porous silicon material that contains a relatively large amount of conductive phase and a relatively small amount of SiO2, and as a result, it is presumed that the decrease in conductivity and charge / discharge efficiency can be suppressed. [Brief explanation of the drawing]

[0012] [Figure 1] Schematic cross-sectional view of porous silicon material. [Figure 2] Al-Si-V phase diagram at 2.5 at%V. [Figure 3] Schematic diagram of the microstructure formation process during the cooling of molten droplets of Al-Si-V alloy. [Figure 4] Al-Si-Cr phase diagram in 25at%Si. [Figure 5] A schematic diagram of the microstructure formation process during the cooling of molten droplets of Al-Si-Cr alloy. [Figure 6] An explanatory diagram showing an example of the structure of the energy storage device 10. [Figure 7] Cross-sectional SEM image of the porous silicon material from Experimental Example 1. [Figure 8] A graph showing the relationship between conductive phase quantity and resistivity. [Figure 9] Graphs showing the change in electrode thickness using porous silicon material in experimental examples 2, 4, and 5. [Figure 10] A graph showing the relationship between the amount of conductive phase and the initial expansion rate. [Figure 11] XRD measurement results of porous silicon materials and their precursors in Experimental Examples 13 and 14. [Figure 12] Cross-sectional SEM image of the porous silicon material from Experimental Example 14. [Modes for carrying out the invention]

[0013] (Method for manufacturing porous silicon material) The method for producing a porous silicon material according to this disclosure includes a precursor step and a porosity-forming step. In the precursor step, a raw material containing Al, Si, Cr, and V is melted and rapidly solidified to obtain a silicon alloy precursor. In the porosity-forming step, the Al component contained in the silicon alloy is removed to obtain a porous silicon material. The porous silicon material obtained by this method for producing a porous silicon material preferably has both structures (also called a coexistence-type structure) and structures (also called a dispersion-type structure) in which Cr silicides are distributed along the silicon skeleton and V silicides are distributed between the silicon skeletons. Figure 1 shows a schematic cross-sectional view of the porous silicon material. In addition to the porous silicon material with a coexistence-type structure (Figure 1A), Figure 1 also shows a porous silicon material without silicides (Figure 1B), a porous silicon material with a coexistence-type structure (Figure 1C), and a porous silicon material with a dispersion-type structure (Figure 1D). Porous silicon materials with a coexistence + dispersion structure may have V silicides distributed not only between the silicide frameworks but also on the surface of the porous silicon particles. Furthermore, Cr silicide may contain V or Al. Similarly, V silicide may contain Cr or Al. First, the raw material composition will be explained.

[0014] In the equilibrium phase diagram (eutectic system) of a binary Al-Si alloy, when the molten state of a eutectic composition with a minimal liquidus line is cooled and solidified, the Al and Si phases crystallize simultaneously, forming a fibrous (lamellar) eutectic structure. At this time, the size of the formed lamellar structure becomes finer as the cooling rate increases, for example, 10 2 At cooling rates above K / s, nanoscale structures are formed. If only the Al element can be selectively removed from this eutectic structure by acid treatment or other means, a porous material consisting of a Si framework that retains the characteristics of the eutectic structure can be obtained.

[0015] Even in the equilibrium phase diagram of a ternary Al-Si-V alloy, there exists a eutectic composition in which the Al and Si phases crystallize simultaneously from the melt. Figure 2 shows an example of this, the Al-Si-V phase diagram at 2.5 at% V. Figure 3 shows a schematic diagram of the microstructure formation process during the cooling of a eutectic melt droplet. In this system, when the eutectic melt (Figure 2(1), Figure 3(1)) is cooled, VSi2 crystallizes as the primary crystal, and VSi2 particles are dispersed in the liquid phase (Figure 2(2), Figure 3(2)). When this is cooled, the Al and Si phases crystallize simultaneously from the melt between the VSi2 particles, forming a structure in which the VSi2-reinforced phase is dispersed and composited within the Si framework (Figure 2(3), Figure 3(3)). By removing the Al component from this structure, a porous silicon material with a dispersed structure can be obtained. Since VSi2 has about twice the strength of Si, the pressure resistance of the composite material can be improved by coexisting with VSi2 as a reinforcing phase. For this reason, the raw material composition is preferably such that a eutectic structure of Al phase and Si phase is obtained, and a Si-V compound phase such as the VSi2 phase is obtained. The raw material composition may be set within the range that results in the desired phase configuration, taking into account the effect of rapid cooling as necessary, based on the equilibrium phase diagram. As described in Patent Document 2, regarding the phase diagrams of the Al-Si-V system at 2.5at%V, 10at%V, and 20at%V, in compositions with a small amount of Si, all Si forms the VSi2 phase, so the Si phase cannot exist stably. However, in composition ranges where the amount of Si is above a certain value, a eutectic composition of Al phase + Si phase exists, and it is presumed that the Al phase + Si phase + VSi2 phase becomes stable at low temperatures. It was also found that the amount of Si required for Si stabilization increases as the amount of V increases. The amount of Si phase is important because the negative electrode capacity depends on the amount of Si phase contained in the porous material. The equilibrium phase diagram in Figure 2 was obtained by calculation using the CALPHAD method.

[0016] In the equilibrium phase diagram of a ternary Al-Si-Cr alloy, there is a eutectic composition 1 in which the Si phase and CrSi2 phase crystallize simultaneously from the melt, and a eutectic composition 1 in which the Si phase and Al phase crystallize simultaneously from the melt. 13There exists a eutectic composition 2 in which the Si4Cr4 phase crystallizes simultaneously. Figure 4 shows an example of this, an Al-Si-Cr phase diagram for 25at%Si. Figure 5 shows a schematic diagram of the microstructure formation process of the outermost surface layer during the cooling process of a molten droplet in a composition between eutectic composition 1 and eutectic composition 2. In this system, when the molten material (Figure 4(1), Figure 5(1)) is cooled to the temperature shown in Figure 4(2), CrSi2 crystallizes first from the surface of the droplet where the temperature is lower (Figure 5(2)). However, CrSi2 gradually coarses in the temperature range where it coexists with the molten material (Figure 4(3)) (Figure 5(3)). At that time, if the CrSi2 coarses too much, an inert CrSi2 shell surrounds the generated silicon powder (Figure 5(3-2)), which is undesirable because it reduces the number of conduction paths for carrier ions (Li) on the active material surface. However, in the phase diagram of the AlSiCr system, when cooled to the temperature shown in Figure 4(4), CrSi2 decomposes (peritectic reaction), and at the same time, Si and Al are released from the melt. 13 Since Si4Cr4 crystallizes (eutectic reaction), a conduction path for active carrier ions (i.e., Si phase) is formed on the surface of the active material, which is more preferable. Furthermore, this peritectic reaction generally proceeds as L + CrSi2 ⇒ Al 13 Like Si4Cr4, it exhibits a reaction in which a molten and solid phase react to form another solid phase, but since the reaction proceeds from the surface of the solid phase particles, as shown in Figure 5(4), Al forms around the fine CrSi2 particles. 13 A structure is formed in which Si4Cr4 and Si shells cover each other. Thus, in the temperature range shown in Figure 4(5), CrSi2 (which may actually be Cr(Al,Si)2) becomes Al 13 A reaction occurs in which Cr4Si4 and Si decompose, so Si and Al are formed around the position of the initially generated CrSi2 crystal nucleus. 13 A state is created in which cluster particles of Cr4Si4 nanoparticles coexist in extremely close proximity and are dispersed in the molten liquid (Figure 5(5)). Then, as the Al molten liquid solidifies at the end, the Si and Al in the clusters separate. 13Cr4Si4 nanoparticles are connected, and a structure in which Si and Cr silicide coexist is formed in the skeletal part. By removing Al from this, a composite porous material with a coexisting structure can be obtained. Since the strength of CrSi2, which is a SiCr compound, is about twice that of Si, the skeletal strength can be improved by coexisting CrSi2 as a strengthening phase in the Si skeleton. When the cooling rate is sufficiently high, since it is cooled before the above peritectic reaction is completed, it is considered that a part of CrSi2 near the particle surface remains as a non-equilibrium phase. In the above cooling process, in the peritectic reaction, Al around the fine CrSi2 particles 13 A structure in which nuclei of Si4Cr4 and Si are covered is formed, so a finer microstructure is realized. That is, Si particles, CrSi2, Al 13 In addition to the fineness of Si4Cr4, the Al alloy serving as the pore source is also refined. Furthermore, since the precipitation temperature differences of the strengthening phase, Si, and the specific oxidation removal phase (Al) are relatively small (~350 °C), the crystal growth of those particles is small, and a nanostructured microstructure can be realized. As described in Patent Document 1, regarding the Al-Si-Cr phase diagram at 15% Si, 20% Si, 25% Si, 30% Si, and 40% Si, generally, in order to achieve strength improvement by nanocomposite formation, it is necessary that the strengthening phase such as the CrSi2 phase is as fine as possible. For its realization, in the phase diagram of the Al-Si-Cr alloy system, it is preferable that the crystallization temperatures of Si and the silicide as the strengthening phase are close. That is, the composition range in which Si and the silicide as the strengthening phase are simultaneously precipitated by the eutectic reaction is ideal. Al 100-x-y Si x Cr y In the basic composition of, when the Si content x is 20 at% or more, particularly 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 addition to Si and CrSi2 crystallizing at relatively close temperatures, in the composition region where the peritectic reaction of CrSi2 + L ⇒ Al 13 Si4Cr4 occurs, it is speculated that a finer pore structure can be realized.

[0017] The equilibrium phase diagram for a quaternary Al-Si-Cr-V alloy may be inferred from the equilibrium phase diagrams for ternary Al-Si-V alloys and ternary Al-Si-Cr alloys. For example, when producing a porous silicon material with a coexistence + dispersion structure, the raw material composition may be set within the range that yields the desired phase configuration, based on the equilibrium phase diagrams for Al-Si-Cr alloys that yield a coexistence structure and Al-Si-V alloys that yield a dispersion structure, and taking into account the effects of rapid cooling as needed.

[0018] (Precursor process) In the precursor step, it is preferable to use a raw material containing, when the total amount of Al, Si, Cr, and V is set to 100 at%, Si in the range of 10 at% to 50 at%, Cr in the range of 0.5 at% to 5 at%, and V in the range of 1 at% to 10 at%. In addition to Al, Si, Cr, and V, the raw material may also contain unavoidable impurities. Unavoidable impurities are components that inevitably remain during the purification of any of Al, Si, Cr, or V, and examples include Fe, C, Cu, Ni, and P. It is preferable that the amount of unavoidable impurities be as low as possible; for example, when the total amount of Al, Si, Cr, and V is set to 100 at%, it is preferable that it be 5 at% or less, and more preferably 2 at% or less. The Al content may be 30 at% or more, 40 at% or more, or 50 at% or more. Furthermore, the Al content may be 90 at% or less, 80 at% or less, or 70 at% or less. The Si content ratio may be 15 at% or more, or 20 at% or more. Alternatively, the Si content ratio may be 45 at% or less, or 40 at% or less. The Cr content ratio may be 1 at% or more, or 1.5 at% or more. Alternatively, the Cr content ratio may be 3 at% or less, 2.5 at% or less, or 2 at% or less. The V content ratio may be 1.5 at% or more, or 2 at% or more. Alternatively, the V content ratio may be 7.5 at% or less, or 5 at% or less. The ratio of the sum of the V content ratio Av[at%] and the Cr content ratio Ac[at%] to the Si content ratio As[at%], (Av+Ac) / As, may be 0.05 or more, 0.1 or more, or 0.15 or more. This ratio (Av+Ac) / As may be 0.5 or less, 0.3 or less, or 0.25 or less. The ratio of Cr (Ac[at%]) to V (Av[at%]) (Ac / Av) may be less than 1, 0.5 or less, or 0.4 or less. This ratio Ac / Av may be 0.1 or more, 0.2 or more, or 0.3 or more. Silicon alloys containing Al, Si, Cr, and V within these ranges are preferable because they allow for a higher porosity and the acquisition of pores of a more suitable shape and size. In this process, the general formula Al100-x-y-z Si x Cr y V z (However, a silicon alloy represented by 0 < x < 100, 0 < y < 100, 0 < z < 100, 0 < x + y + z < 100) may be used as the master alloy. In this step, it is preferable to use a silicon alloy containing Al with a predetermined composition where a eutectic structure can be obtained. The predetermined composition may be a eutectic composition, may be near the eutectic composition, or may have a predetermined width such as a part of a hypoeutectic composition or a hypereutectic composition. For example, it may contain Al, Si, Cr, and V within the range of ±5% by mass with respect to the eutectic composition.

[0019] In this step, when melting the raw materials, any melting method may be used, but high-frequency crucible melting in an inert gas atmosphere such as Ar is preferable. Also, when rapidly solidifying the melted raw materials, any rapid cooling method may be used, but the cooling rate is preferably more rapid cooling. For example, from the molten state, it may be in the range of 10 2 ℃ / s or more and 10 8 ℃ / s or less. As a method of rapid solidification, for example, the molten silicon alloy (molten raw materials) may be cast into a mold and rapidly cooled, but it is preferable to rapidly cool the molten silicon alloy (melt) by one or more of the gas atomization method, water atomization method, and roll quenching method. In the precursor step, the silicon alloy obtained from the raw materials may be granulated. In this granulation process, the ingot obtained by mold casting may be crushed and granulated. Also, in the above-mentioned gas atomization method and water atomization method, alloy powders are obtained, so they may be used for granulation. Also, in the above-mentioned roll quenching method, a strip alloy is obtained, so it may be pulverized into powder (granulated) 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 preferable as a method of granulating the silicon alloy. In gas atomization, it is preferable to perform it in an Ar atmosphere when making it into a melt, and it is preferable to perform it under an Ar or He atmosphere during granulation.

[0020] In the precursor step, it is preferable to atomize the silicon alloy into particles with an average particle size in the range of 0.1 μm to 100 μm. These particles are preferably, for example, 0.5 μm to 10 μm, more preferably 1 μm to 5 μm, and even more preferably 1 μm to 3 μm. The silicon alloy particles can be appropriately selected according to the characteristics required for the energy storage device. Here, the average particle size is determined by observing the particles with a scanning electron microscope (SEM), summing the major axis of each particle as its diameter, dividing by the number of particles, and averaging the result. The particles obtained in this atomization process will be the average particle size of the final aggregate of porous particles to be obtained.

[0021] In this precursor step, in addition to Al, Si, Cr, and V, a raw material containing one or more secondary elements from Ca, Cu, Mg, Na, Sr, and P may be used. Of these, one or more of Ca, Na, and Sr are preferred as the secondary elements. The amount of the secondary element is preferably less than the amount of Al, Si, Cr, and V, for example, it is preferably in the range of 10% by mass or less, and more preferably in the range of 5% by mass or less, relative to the total silicon alloy.

[0022] In this precursor step, a silicon alloy having a Si phase, a conductive phase which is a silicide phase containing Cr and V, and an Al phase may be obtained. The conductive phase may, for example, contain Cr silicide and V silicide, or contain one or more of Cr silicide and V silicide and Cr,V silicide, or contain only Cr,V silicide. The Cr silicide may be a Si-Cr compound containing Si and Cr, or an Al-Si-Cr compound containing Al, Si and Cr. The Cr silicide may be, for example, Cr x Si y (x and y are arbitrary numbers), and examples include CrSi2. Cr silicide is a Cr compound in which some of the Si is replaced by Al. x (Al,Si) yIt may also be Cr(Al,Si)4, for example. V silicide may be a Si-V compound containing Si and V, or an Al-Si-V compound containing Al, Si and V. V silicide may be, for example, V x Si y (x and y are arbitrary numbers) can also be used, and examples include VSi2 and V5Si3. V silicides are V silicides in which part of the Si is replaced with Al. x (Al,Si) y This may also be the case. The Cr,V silicide may be a Si-Cr-V compound containing Si, Cr, and V, or an Al-Si-Cr-V compound containing Al, Si, Cr, and V. The Cr silicide, V silicide, and Cr,V silicide may be sparingly soluble in acids or alkalis. Al may be solid-dissolved in the Si phase, Cr silicide phase, V silicide phase, and Cr,V silicide phase. The Al phase is partially or completely removed in the porous formation process described later.

[0023] (Porous process) In the porosity creation process, the Al component is removed from the silicon alloy prepared above. Examples of the Al component include Al and its compounds. In this process, Cr and its compounds, V and its compounds, etc., may also be removed. In this process, it is preferable to selectively remove the Al component, i.e., the Al phase and its compounds, using an acid or alkali. The acid or alkali used is preferably one that dissolves the Al component in the silicon alloy without dissolving the Si component, i.e., the Si phase and its compounds, and examples include hydrochloric acid, sulfuric acid, and sodium hydroxide. It is preferable that the acid or alkali be an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it can remove the Al component, and can be, for example, in the range of 0.01 mol / L to 5 mol / L. The concentration of the acid or alkali may be, for example, 0.05 mol / L or more, or 0.1 mol / L or more. Also, the concentration of the acid or alkali may be, for example, 1 mol / L or less, or 0.5 mol / L or less. The temperature of this removal process may be, for example, in the range of room temperature (25°C) to 80°C, but may also be 30°C or lower, or 25°C or lower. The removal process may also be carried out by immersing the silicon alloy particles in an acid or alkali solution, and stirring may be performed as necessary. The duration of the removal process may be set appropriately depending on the type and concentration of the acid or alkali, but for example, it is preferably within 48 hours, but may also be within 24 hours or within 10 hours. The duration of the removal process may be, for example, 1 hour or more, or 2 hours or more. The obtained porous silicon material is then washed and dried. In the porosization process, a porous silicon material in which no Al peak is detected by X-ray diffraction (XRD) measurement may be obtained. In other words, in the porosization process, alkali treatment may be carried out until the Al phase is eliminated.

[0024] In the porosity process, substances other than Si may be removed in a range of 85% to 100% by mass. For example, Al, Cr, V, and other oxygen may remain, but from the viewpoint of charge / discharge capacity, it is preferable to have less of them when used as an electrode active material. Furthermore, components such as Al, C, and V are preferable to be included in amounts greater than a predetermined amount from the viewpoint of reinforcing the silicon skeleton and improving durability. In the porosity process, the following porous silicon material may be obtained.

[0025] (Porous silicone material) The porous silicon material of this disclosure may be manufactured by the manufacturing method described above. Here, the detailed explanation of the physical properties of the porous silicon material is omitted, assuming that they are the same as those of the manufacturing method described above.

[0026] This porous silicon material has a Si phase and a conductive phase which is a silicide phase containing Cr and V. The Si phase is presumed to form the silicon skeleton and to be responsible for charging and discharging the energy storage device. The conductive phase is presumed to improve conductivity and to reinforce the silicon skeleton. The conductive phase may, for example, contain Cr silicide and V silicide, or contain one or more of Cr silicide and V silicide and Cr,V silicide, or contain only Cr,V silicide. The Cr silicide may be a Si-Cr compound containing Si and Cr, or an Al-Si-Cr compound containing Al, Si and Cr. The Cr silicide is, for example, Cr x Si y (x and y are arbitrary numbers), and examples include CrSi2. Cr silicide is a Cr compound in which some of the Si is replaced by Al. x (Al,Si) y It may also be, for example, Cr(Al,Si)4. The V silicide may be a Si-V compound containing Si and V, or an Al-Si-V compound containing Al, Si and V. As for the V silicide, for example, V x Si y(x and y are arbitrary numbers) can also be used, and examples include VSi2 and V5Si3. V silicides are V silicides in which part of the Si is replaced with Al. x (Al,Si) y This may also be the case. The Cr,V silicide may be a Si-Cr-V compound containing Si, Cr, and V, or an Al-Si-Cr-V compound containing Al, Si, Cr, and V. Al may be solid-dissolved in the Si phase, Cr silicide phase, V silicide phase, and CrV silicide phase. The Cr silicide, V silicide, and Cr,V silicide may be amorphous, and may not show clear peaks in XRD diffraction. The porous silicon material may have an SiO2 phase in addition to the Si phase and the conductive phase.

[0027] In this porous silicon material, when the total composition of the Si phase, conductive phase, and SiO2 phase is considered as 100% by mass, the conductive phase is contained in an amount of 12% to 60% by mass, and the SiO2 phase content is 12% by mass or less. In this porous silicon material, a higher content of the conductive phase is preferable from the viewpoint of suppressing a decrease in conductivity and charge / discharge efficiency. For example, 15% by mass or more is preferable, 20% by mass or more is more preferable, 25% by mass or more is even more preferable, and it may be 30% by mass or more. In this porous silicon material, a moderate content of the conductive phase is preferable from the viewpoint of suppressing a decrease in initial discharge capacity. For example, 60% by mass or less is preferable, it may be 50% by mass or less, and it may be 40% by mass or less. In this porous silicon material, the conductive phase is Cr(Al,Si)4 and VSi2, and the total content of these may be 20% by mass or more (preferably 30% by mass or more) and 60% by mass or less. In this porous silicon material, the SiO2 phase content is preferably low from the viewpoint of suppressing a decrease in conductivity and charge / discharge efficiency, and may be 11% by mass or less, or 10% by mass or less. The SiO2 phase content may be, for example, 5% by mass or more, or 7% by mass or more. In this porous silicon material, the ratio Ms / Mc, which is the ratio of the mass of the SiO2 phase to the mass Mc[g] of the conductive phase, may be 1 or less. From the viewpoint of suppressing a decrease in conductivity, a low Ms / Mc value is preferable, for example, 0.7 or less is preferable, 0.5 or less is more preferable, and 0.4 or less is even more preferable. The Ms / Mc value may be, for example, 0.1 or more, or 0.15 or more. The proportions of the Si phase, conductive phase, and SiO2 phase in the porous silicon material can be determined from X-ray diffraction (XRD) measurement and compositional analysis by SEM / EDX. Specifically, first, the contained phases are confirmed by XRD. For example, if Si and (Cr,V)(Al,Si)2 are detected, it is determined that these phases are present. If the SiO2 peak does not appear in XRD, but O is detected by compositional analysis, it is determined that an amorphous SiO2 phase is present. When O is detected by compositional analysis, it is assumed that all the O has bonded with Si to form SiO2, and the amount of SiO2 is determined.Next, the amount of (Cr,V)(Si,Al)2 is determined from the amounts of V and Cr. In the acid treatment method, it is empirically known that even when Al is completely removed by phase separation of an Al-Si binary alloy, about 5% of Al always remains and is incorporated into the Si phase (95%Si-5%Al). Therefore, assuming that the remaining Al after removing the 5% of Al dissolved in Si is contained in (Cr,V)(Si,Al)2, the amount of (Cr,V)(Si,Al)2 is determined. Finally, the remaining Si and 5% of Al are taken as the amount of Si phase. In this way, the proportion of each contained phase is determined. This porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr, and P as a secondary element in a range of 15 mass% or less. It is preferable that the amount of the secondary element be as small as possible.

[0028] As shown in Figure 1A, this porous silicon material may have a Si skeleton formed of a Si phase, a Cr silicide phase, and a V silicide phase. The Si skeleton may retain the characteristics of the eutectic structure of the precursor silicon alloy, or it may form a three-dimensional network structure with voids. The Cr silicide phase may be distributed along the silicon skeleton and form a skeleton together with the silicon skeleton. The V silicide phase may be distributed between the silicon skeletons. The V silicide phase may be in the form of nanoparticles, or it may be placed between the Si skeletons and act as pillars supporting the Si skeleton. Al may be solid-dissolved in the Cr silicide phase and the V silicide phase.

[0029] This porous silicon material has a porosity (preferably the porosity of pores with a diameter of 1 μm or less) of 45 volume% or more, as determined by the mercury intrusion method. In porous silicon materials, it is preferable to have a larger number of fine pores, such as those with a diameter of 1 μm or less, from the viewpoint of distributing the stress on individual pores. This porosity may be 50 volume% or more, or 55 volume% or more. Furthermore, this porosity may be 75 volume% or less, or 70 volume% or less, or 60 volume% or less. A higher porosity is preferable because it can better suppress the volume change during carrier ion adsorption. Also, a lower porosity is preferable because it allows for a larger amount of Si phase per unit volume.

[0030] This porous silicon material has an average pore diameter of 100 nm or less, determined by the mercury intrusion method. In this porous silicon material, the average pore diameter determined by the mercury intrusion method may be 30 nm or more, 50 nm or more, or 70 nm or more. This average pore diameter may be 95 nm or less, or 90 nm or less. Smaller pore diameters are preferable because the pores are less likely to collapse. Larger pores are preferable because they can further suppress volume changes when carrier ions are absorbed. This porous silicon material may have a pore diameter of 1 μm or less, determined by the mercury intrusion method, that is, the pore diameter distribution range may be 1 μm or less. This pore diameter distribution range may be 1 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more. Furthermore, the distribution range of this pore size may be 1000 nm or less, 500 nm or less, 300 nm or less, or 250 nm or less.

[0031] This porous silicon material may contain 60 at% or more of Si, 1 at% to 12 at% of Cr, and 1 at% to 12 at% of V, with an oxygen content of 12 at% or less, when the total amount of Al, Si, Cr, and V is set to 100 at%. From the viewpoint of charge / discharge capacity, a higher Si content is preferable, for example, 65 at% or more is preferable, and it may be 70 at% or more. From the viewpoint of conductivity and charge / discharge efficiency, a lower Si content is preferable, for example, 98 at% or less is preferable, it may be 95 at% or less, and it may be 90 at% or less. From the viewpoint of conductivity and charge / discharge efficiency, a higher Cr content is preferable, for example, 1.2 at% or more is preferable, it may be 1.5 at% or more. From the viewpoint of charge / discharge capacity, a lower Cr content is preferable, for example, 10 at% or less is preferable, it may be 5 at% or less, and it may be 4 at% or less. From the viewpoint of conductivity and charge / discharge efficiency, a higher V content is preferable, for example, 2 at% or more is preferable, and 3 at% or more is more preferable. From the viewpoint of charge / discharge capacity, a lower V content is preferable, for example, 10 at% or less is preferable, and it may be 9 at% or less. In porous silicon materials, the ratio of the Cr content Bc[at%] to the V content Bv[at%] Bc / Bv may be 1 or less, 0.7 or less, or 0.5 or less. This ratio Bc / Bv may be 0.1 or more, or 0.2 or more. Furthermore, porous silicon materials may contain one or more of Ca, Cu, Mg, Na, Sr, and P as secondary elements in a range of 15 mass% or less. It is preferable that elements other than Si be present in smaller amounts.

[0032] The porous silicon material preferably has an average particle size of 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and may be 5 μm or more. Furthermore, the particles preferably have an average particle size of 100 μm or less, more preferably 10 μm or less, may be 5 μm or less, and may be 3 μm or less.

[0033] The porous silicon material preferably has a resistivity of 500 Ωcm or less, more preferably 400 Ωcm or less, and even more preferably 300 Ωcm or less. This resistivity may be, for example, 10 Ωcm or more, 50 Ωcm or more, or 100 Ωcm or more. The resistivity can be evaluated by a four-terminal method using copper electrodes attached to a porous silicon material that has been pressure-molded at 200 MPa.

[0034] In electrodes containing the porous silicon material as an electrode active material, it is preferable that the expansion rate during the initial charge is 8% or less. This expansion rate is more preferably 7% or less, and even more preferably 6% or less. This expansion rate may also be, for example, 3% or more, or 5% or more. In electrodes containing the porous silicon material as an electrode active material, it is preferable that the thickness change rate in each charge-discharge cycle other than the initial one is 4% or less. This thickness change rate may also be, for example, 1% or more, or 3% or more.

[0035] Preferably, the porous silicon material has an initial discharge capacity of 1000 mAh / g or more per unit mass of porous silicon material when charged and discharged at a current density of 0.05 C in an evaluation cell comprising an electrode containing the porous silicon material as an electrode active material and a lithium metal counter electrode. More preferably, this initial discharge capacity is 1500 mAh / g or more, and even more preferably 1700 mAh / g or more. This initial discharge capacity may be, for example, 2500 mAh / g or less. Furthermore, preferably, the porous silicon material has an initial charge-discharge efficiency of 80% or more when charged and discharged at a current density of 0.5 C in the above evaluation cell. More preferably, this initial charge-discharge efficiency is 85% or more. This initial charge-discharge efficiency may be, for example, 95% or less. Furthermore, preferably, the porous silicon material has a capacity retention rate of 50% or more after 30 cycles at a current density of 0.5 C in the above evaluation cell. More preferably, this capacity retention rate is 60% or more, and even more preferably 70% or more.

[0036] (Electrodes for energy storage devices) The electrode for the energy storage device is equipped with the porous silicon material described above as the electrode active material. This electrode becomes either a positive or negative electrode based on the potential of the counter electrode relative to the potential of the electrode active material, but it is preferable to make it a negative electrode when lithium is used as the carrier. This electrode can be used, for example, in lithium-ion secondary batteries, hybrid capacitors, air batteries, etc. The electrode for the energy storage device may be made by compressing the porous silicon material to a porosity of 5 to 50 volume%. In this electrode, the porosity of the porous silicon material may be reduced by compression during manufacturing. Compared to a porous silicon material made with a porosity of 50 to 95 volume% and then compressed to this range, it exhibits better charge and discharge characteristics depending on the shape of the voids, etc. The porosity of the compressed porous silicon material can be adjusted as appropriate according to the characteristics required for electrodes in energy storage devices, for example, it may be 5% by volume or more, 10% by volume or more, or 20% by volume or more. Alternatively, the porosity of the compressed porous silicon material may be 40% by volume or less, 30% by volume or less, or 20% by volume or less.

[0037] The electrode for the energy storage device may be formed by creating the porous silicon material described above on a current collector and fixing it to the current collector. This electrode can be manufactured by either mixing the porous silicon material with a conductive material and a binder in a solvent as needed to form a paste and applying it to the current collector, or by 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 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more. The content of the porous silicon material may be, for example, 80% by mass or less, or 70% by mass or less. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. For example, one or more types of graphite such as natural graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjen black, carbon whiskers, needle coke, carbon fiber, or metals (copper, nickel, aluminum, silver, gold, etc.) can be used. The binder plays the role of binding the active material particles and conductive material particles together. For example, fluororesins 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, and natural butyl rubber (NBR) can be used alone or as a mixture of two or more. In addition, aqueous binders such as cellulose-based or aqueous dispersions of styrene-butadiene rubber (SBR) can also be used. As solvents, 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 and a thickener may be added to water, and the active material may be slurryed with a latex such as SBR.Coating methods include roller coating using applicator rolls, screen coating, doctor blade method, spin coating, and bar coating, and any thickness and shape can be achieved using any of these methods. The current collector can be appropriately selected according to the potential of the active material, but examples include aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymers, conductive glass, etc., as well as aluminum and copper whose surfaces have been treated with carbon, nickel, titanium, or silver for the purpose of improving adhesion, conductivity, and oxidation resistance. It is also possible to oxidize the surface of these materials. The shape of the current collector can be foil, film, sheet, net, punched or expanded, lath, porous, foam, or fiber cluster. The thickness of the current collector can be, for example, 1 to 500 μm. The amount of active material composite formed can be appropriately set according to the desired performance required for the energy storage device.

[0038] In this electrode, the electrode active material may include a porous silicon material as well as an active material other than a porous silicon material. For example, carbonaceous materials or Li4Ti5O 12 These may also be included. However, from the viewpoint of further increasing the battery capacity, it is preferable that, with the total electrode active material being 100% by mass, porous silicon material accounts for 50% or more by mass, preferably 90% or more by mass.

[0039] The electrode is preferably one in which the expansion rate during the initial charge is 8% or less. This expansion rate is more preferably 7% or less, and even more preferably 6% or less. This expansion rate may be, for example, 3% or more, or 5% or more. The electrode is preferably one in which the thickness change rate during each charge-discharge cycle other than the initial charge is 4% or less. This thickness change rate may be, for example, 1% or more, or 3% or more.

[0040] (Energy storage device) The energy storage device of the present disclosure includes an electrode having the porous silicon material described above. This energy storage device may include a negative electrode, a positive electrode, and an ion conduction medium interposed between the negative electrode and the positive electrode to conduct carrier ions. The porous silicon material can be used as a negative electrode active material. This energy storage device may be any of a lithium-ion secondary battery, a hybrid capacitor, an air battery, and the like. In the positive electrode, as the positive electrode active material, sulfides containing a transition metal element, oxides containing lithium and a transition metal element, and the like can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, lithium manganese composite oxides having a basic composition formula of Li (1-x) MnO2 (0 < x < 1, etc., the same hereinafter), lithium manganese composite oxides such as Li (1-x) Mn2O4, lithium cobalt composite oxides having a basic composition formula of Li (1-x) CoO2, lithium nickel composite oxides having a basic composition formula of Li (1-x) NiO2, lithium nickel cobalt manganese composite oxides having a basic composition formula of Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), lithium vanadium composite oxides having a basic composition formula of LiV2O3, transition metal oxides having a basic composition formula of V2O5, and the like 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 and the like are preferable. Note that the "basic composition formula" means that other elements such as Al and Mg may be included. Alternatively, the positive electrode active material may be a carbonaceous material used in a capacitor or a lithium-ion capacitor. Examples of the carbonaceous material include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, polyacenes, and the like. Among these, activated carbons having a high specific surface area are preferable. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m 2 / g or more, and 1500 m2 It is more preferable that the amount is greater than or equal to / g. The specific surface area is 1000m². 2 At concentrations of 1 / g or higher, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m² due to its ease of manufacture. 2 It is preferable that it be less than or equal to / g, and 2000m 2 It is more preferable that the value be less than or equal to / g. The conductive material, binder, solvent, and current collector used in the positive electrode can be any of the examples given above for the electrode.

[0041] As the ion-conducting medium, non-aqueous electrolytes containing supporting salts or non-aqueous gel electrolytes can be used. Examples of solvents for non-aqueous electrolytes include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can be used individually or in combination. Specifically, examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; linear carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; linear 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; sulforanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Of these, a combination of cyclic carbonates and linear carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge cycles, but also allows for a balanced relationship between the viscosity of the electrolyte, the electrical capacity of the resulting battery, and the battery output. Examples of supporting salts include LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiSbF6, LiSiF6, LiAlF4, LiSCN, LiClO4, LiCl, LiF, LiBr, LiI, and LiAlCl4. From the viewpoint of electrical characteristics, it is preferable 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.The supporting salt is preferably at a concentration of 0.1 mol / L or more and 5 mol / L or less in the non-aqueous electrolyte, and more preferably at a concentration of 0.5 mol / L or more and 2 mol / L or less. A concentration of 0.1 mol / L or higher allows for sufficient current density, while a concentration of 5 mol / L or lower allows for greater stability of the electrolyte. Furthermore, flame retardants such as phosphorus-based or halogen-based agents may be added to this non-aqueous electrolyte.

[0042] Furthermore, instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of ion-conducting polymers include polymer gels composed of polymers such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, and vinylidene fluoride, along with supporting salts. In addition, a combination of an ion-conducting polymer and a non-aqueous electrolyte can also be used. Moreover, in addition to ion-conducting polymers, inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, or inorganic solid powders bound together by an organic binder can be used as the ion-conducting medium.

[0043] The energy storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as its composition can withstand the operating range of lithium secondary batteries, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric or polyphenylene sulfide nonwoven fabric, and thin microporous membranes of olefin resins such as polyethylene or polypropylene. These may be used individually or in combination.

[0044] The shape of this energy storage device is not particularly limited, but examples include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular shapes. It may also be applied to larger devices used in electric vehicles, etc. Figure 6 is an explanatory diagram showing an example of the structure of the energy storage device 10. This energy storage device 10 has a positive electrode 12, a negative electrode 15, and an ion conducting medium 18. The positive electrode 12 has a positive electrode active material 13 and a current collector 14. The negative electrode 15 has a negative electrode active material 16 and a current collector 17. The negative electrode active material 16 is the porous silicon material 21 described above and has voids 23.

[0045] The energy storage device preferably has a higher discharge capacity. For example, the initial discharge capacity when charging and discharging at a current density of 0.05C is preferably 1000mAh / g or more, more preferably 1500mAh / g or more, and even more preferably 1700mAh / g or more per unit mass of porous silicon material. This initial discharge capacity may be, for example, 2500mAh / g or less. The energy storage device preferably has a higher charge and discharge efficiency. For example, the initial charge and discharge efficiency when charging and discharging at a current density of 0.5C is preferably 80% or more, and more preferably 85% or more. The initial charge and discharge efficiency may be, for example, 95% or less. The energy storage device preferably has a higher capacity retention rate when performing charge and discharge cycles. For example, the capacity retention rate after 30 cycles at a current density of 0.5C is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0046] The energy storage device preferably has a smaller negative electrode expansion rate during the initial charge, for example, 8% or less, more preferably 7% or less, and even more preferably 6% or less. This expansion rate may be, for example, 3% or more, or 5% or more. The energy storage device also preferably has a smaller negative electrode thickness change rate in each charge-discharge cycle other than the initial charge, for example, 4% or less. This thickness change rate may be, for example, 1% or more, or 3% or more.

[0047] As detailed above, this disclosure can suppress the decrease in conductivity and charge / discharge efficiency. The reason for obtaining such effects is presumed to be as follows. For example, when Cr and V are used in combination rather than individually, the by-product formation of SiO2 when the Al component is removed from the silicon alloy is suppressed. Therefore, it is possible to realize a porous silicon material that contains a relatively large amount of conductive phase and a relatively small amount of SiO2, and as a result, it is presumed that the decrease in conductivity and charge / discharge efficiency can be suppressed. Furthermore, when the electrode area is increased in order to increase the capacity of the battery, the pressure pressing down on the electrodes tends to decrease, so suppressing electrode expansion is particularly important. In this disclosure, the conductive phase is thought to function as an expansion-relaxing phase, and an effect of suppressing electrode expansion can also be expected. Furthermore, since chromium silicide has a smaller volume change during charging and discharging than silicon, an expansion-suppressing effect can also be expected with its introduction. However, chromium silicide is easily dissolved by acid treatment, and as a result of dissolution, amorphous SiO2, which is an insulator, is produced, so if too much is introduced, the conductivity and Coulomb efficiency may decrease. In this disclosure, it is believed that by adding V, which is more acid-resistant than Cr, to a material containing a small amount of Cr, high-strength and highly conductive chromium silicide is distributed along the framework, reducing resistivity, and vanadium silicide distributed between the framework layers mitigates expansion without impairing conductivity or Coulomb efficiency, thereby achieving small expansion and high conductivity.

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

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

[11] . [1] A porous silicon material having a Si phase and a conductive phase which is a silicide phase containing Cr and V and may also contain Al, and which may also have an SiO2 phase, wherein when the total of the Si phase, the conductive phase and the SiO2 phase is considered to be 100% by mass, the conductive phase is contained in an amount of 12% by mass or more and 60% by mass or less, the content of the SiO2 phase is 12% by mass or less, and the porosity determined by the mercury intrusion method is 45% by volume or more and the average pore diameter is 100 nm or less. [2] The porous silicon material according to [1], comprising the conductive phase in an amount of 30% by mass or more and 60% by mass or less, wherein the conductive phase comprises a Cr(Al,Si)4 phase and a VSi2 phase. [3] The porous silicon material according to [1] or [2], wherein the ratio of the SiO2 phase to the conductive phase is 0.4 or less. [4] A porous silicon material according to any one of [1] to [3], wherein when the total amount of Al, Si, Cr, and V is 100 at%, Si is in the range of 60 at% or more, Cr is in the range of 1 at% to 12 at% and V is in the range of 1 at% to 12 at% and the oxygen content is 12 at% or less. [5] A porous silicon material according to any one of [1] to [4], wherein the resistivity is 500 Ωcm or less. [6] An electrode comprising a porous silicon material described in any one of [1] to [5], wherein the expansion rate during the first charge is 7% or less, or the thickness change rate in each charge / discharge cycle other than the first is 4% or less. [7] A power storage device comprising a negative electrode which is an electrode containing a porous silicon material as described in any one of [1] to [5], a positive electrode containing a positive electrode active material, and an ion conducting medium interposed between the negative electrode and the positive electrode for conducting carrier ions. [8] A storage device as described in [7] that satisfies one or more of the following conditions (1) to (5). (1) The initial discharge capacity is 1000 mAh / g or more per porous silicon material. (2) The initial charge / discharge efficiency is 80% or higher. (3) The capacity retention rate after 30 cycles is 70% or more. (4) The expansion rate of the negative electrode during the initial charge is 7% or less. (5) The rate of change in the thickness of the negative electrode in each charge-discharge cycle other than the first one is 4% or less. [9] A method for producing a porous silicon material, comprising: a precursor step of melting and rapidly solidifying a raw material containing 10 at% to 50 at% of Si, 0.5 at% to 5 at% of Cr, and 1 at% to 10 at% of V, when the total amount of Al, Si, Cr, and V is 100 at%; and a porosity step of removing the Al component contained in the silicon alloy to obtain a porous silicon material.

[10] The method for producing a porous silicon material according to [9], wherein the Al component is removed by acid treatment or alkali treatment in the porosity-forming step.

[11] A porous silicon material in which, when the total amount of Al, Si, Cr, and V is set to 100 at%, Si is in the range of 60 at% or more, Cr is in the range of 1 at% to 12 at% and V is in the range of 1 at% to 12 at%; the oxygen content is 12 at% or less; the porosity determined by the mercury intrusion method is 45 volume% or more and the average pore diameter is 100 nm or less. [Examples]

[0050] The following describes specific examples of how the porous silicon material of this disclosure was fabricated. Experimental Examples 1-4 and 13-14 correspond to the examples, and Experimental Examples 5-12 correspond to the comparative examples.

[0051] [Fabrication of porous silicon materials] The raw materials for Al, Si, Cr, and V are based on the basic composition formula Al 100-x-y-z Si x Cr y V z The material was weighed to achieve a composition of (x=10~40, y=0~2, z=0~6) and melted in an arc melting furnace. Before melting, the inside of the arc melting furnace was 8 × 10 -3 After reducing the pressure to below Pa, the mixture was purged with Ar gas. The preparation of the master alloy required the melting of the raw material powder, and high-frequency induction melting was performed to produce a uniform sample. The resulting master alloy was heated to 1000-1300°C in an argon atmosphere to melt, and then atomized using the gas atomization method. 2AlSi alloy powder was obtained by rapid solidification at a rate of °C / sec or higher (precursor step). The obtained AlSi alloy powder (also called atomized powder) was immersed in 0.1 mol / L hydrochloric acid and treated at room temperature for 1 to 48 hours to selectively remove Al until the Al phase could no longer be detected by XRD measurement. The residue was transferred to a filtration filter, and the supernatant liquid after treatment was removed by vacuum filtration. The residue was then washed four or more times with distilled water, and the washing water was removed by the same vacuum filtration method to obtain composite porous silicon (porosization step).

[0052] (Experimental Examples 1-4) Experimental Example 1 involved a porous silicon material obtained using the above basic composition formula with x=30, y=1, and z=2. Experimental Example 2 involved a porous silicon material obtained using the above basic composition formula with x=30, y=1, and z=4. Experimental Example 3 involved a porous silicon material obtained using the above basic composition formula with x=30, y=2. Experimental Example 4 involved a porous silicon material obtained using the above basic composition formula with x=30, y=2, and z=6.

[0053] (Experimental Examples 5-7) Experimental Example 5 was obtained using the above basic composition formula with x=30, y=1, and z=0 to obtain a porous silicon material. Experimental Example 6 was obtained using the above basic composition formula with x=30, y=3, and z=0 to obtain a porous silicon material. Experimental Example 7 was obtained using the above basic composition formula with x=30, y=7.5, and z=0 to obtain a porous silicon material.

[0054] (Experimental Examples 8-10) Experimental Example 8 was obtained using the above basic composition formula with x=30, y=0, and z=3 to obtain a porous silicon material. Experimental Example 9 was obtained using the above basic composition formula with x=30, y=0, and z=5 to obtain a porous silicon material. Experimental Example 10 was obtained using the above basic composition formula with x=25, y=0, and z=5 to obtain a porous silicon material.

[0055] (Experimental Example 11) Without fabricating a porous silicon material, we used Si powder with an average particle size of 5 μm (SIE23PB from Koshu Chemical) as is for Experimental Example 11.

[0056] (Experimental Example 12) The porous silicon material obtained by setting x=20, y=0, and z=0 in the above basic composition formula was designated as Experimental Example 12.

[0057] (Experimental Examples 13, 14) Experimental Example 13 was a porous silicon material obtained by setting x=30, y=3.5, and z=4 in the above basic composition formula. Experimental Example 14 was a porous silicon material prepared in the same manner as Experimental Example 13, except that the atomized powder was immersed in a 0.05 mol / L sodium hydroxide aqueous solution instead of immersing it in 0.1 mol / L hydrochloric acid.

[0058] [X-ray diffraction measurement] X-ray diffraction (XRD) measurements were performed on porous silicon materials and their precursors in Experimental Examples 1-10. An X-ray diffractometer (Rigaku RINT-TTR) was used for the measurements, with a Cu tube, in the range of 2θ = 10° to 80°, at a rate of 5° / min.

[0059] [SEM observation, compositional analysis] For the cross-sections of the porous silicon materials in Experimental Examples 1-10, the backscattered electron composition images were observed using a scanning electron microscope (SEM, HITACHI S-4300). Furthermore, the porous silicon materials in Experimental Examples 1-10 were subjected to compositional analysis using an energy-dispersive X-ray spectrometer (EDX) attached to the SEM. Specifically, after press-molding the porous silicon material into a disc shape at 1 GPa, five random locations within the pressurized surface were observed using SEM (Hitachi High-Tech S-3600N, 1000x magnification). Compositional analysis was performed across the entire observation surface, and the average value was determined as the elemental ratio. Furthermore, using the results of the compositional analysis and XRD measurements, the proportions of the Si phase, conductive phase (Cr,V silicide), and SiO2 phase in the porous silicon material were investigated.

[0060] [Resistivity measurement] The resistivity of porous silicon materials in Experimental Examples 1-10 and 12 was measured. Sample powders of the porous silicon material were pressure-molded at 200 MPa, and the resistivity was evaluated using a four-terminal method with copper electrodes attached.

[0061] [Pore distribution measurement] For the porous silicon materials in Experimental Examples 1-10, the pore distribution was measured using a mercury porosimeter (POWERMASTER60GT manufactured by Cantachrome).

[0062] [Evaluation of battery characteristics] Evaluation cells were fabricated using silicon materials from Experimental Examples 1-2, 4-7, and 11-12 as negative electrode active materials, and the initial discharge capacity, charge / discharge efficiency, and cycle capacity retention rate were evaluated. A negative electrode slurry was prepared by mixing 60% by mass of silicon material as the negative electrode active material, 20% by mass of carbon black as the conductive material, and 20% by mass of polyimide binder as the binder, and adjusting the viscosity with N-methylpyrrolidone. Next, this negative electrode slurry was applied to copper current collector foil and dried to prepare composite electrodes. The electrodes were punched out in a 16 mm diameter circle, a porous polyethylene separator was placed between the negative electrode and metallic lithium as the counter electrode, and an electrolyte was injected to manufacture a lithium secondary battery using a Tom cell type small battery cell. As the electrolyte, a mixed solvent was used, consisting of 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 ethylmethyl carbonate (EMC), to which LiPF6 was added at a concentration of 1 mol / L. For the obtained lithium secondary battery, the initial discharge capacity at 0.05C was determined by charging and discharging at a current density of 0.05C in the battery voltage range of 0.005V to 1.5V. Then, the initial charge-discharge efficiency and the capacity retention rate after 30 cycles at 0.5C were determined by repeating the charge-discharge at a current density of 0.5C in the same battery voltage range for 30 cycles. In the evaluation of battery characteristics, discharge is considered to represent oxidation of the porous silicon anode (desorption of lithium).

[0063] [Evaluation of electrode thickness changes] By setting a displacement system on the top cover of the battery cell and reading the change in the cover's position while repeatedly charging and discharging, the amount of electrode displacement was measured, and the change in electrode thickness was evaluated. Then, the initial expansion rate (also simply called the expansion rate), which is the rate of change during the first charge, and the cycle-dependent thickness change rate, which is the average value of the displacement range during 20 charge-discharge cycles, were determined.

[0064] [Results and Discussion] Figure 7 is a cross-sectional SEM image of the porous silicon material from Experimental Example 1. Figure 8 is a graph showing the relationship between the conductive phase content and resistivity. Figure 9 is a graph showing the thickness change of electrodes using the porous silicon material from Experimental Examples 2, 4, and 5. Figure 10 is a graph showing the relationship between the conductive phase content and the initial expansion rate. Table 1 summarizes the base alloy composition and the properties of the porous silicon material (elemental ratio, oxygen content, SiO2 phase concentration, conductive phase concentration, SiO2 phase / conductive phase (ratio of SiO2 phase concentration to conductive phase concentration)), porosity, average pore diameter, and resistivity) for Experimental Examples 1 to 10. Table 2 summarizes the base alloy composition, silicon material properties (conductive phase concentration, resistivity), battery characteristics (initial discharge capacity per active material, initial charge / discharge efficiency, and capacity retention rate after 30 cycles), and electrode thickness change (initial expansion rate, thickness change rate during cycles) for Experimental Examples 1 to 2, 4 to 7, and 11 to 12.

[0065] XRD measurements of the phase composition of the precursor and porous silicon materials revealed diffraction peaks of Al, Si, and VSi2 in the atomized powders of Experimental Examples 1-4. However, the diffraction peak of Al decreased with increasing acid treatment time. The porosity treatment was considered complete when the Al peak was no longer visible. However, compositional analysis revealed that even after treatment was complete, the Al content was not zero, and Al existed as a solid solution. Al, Si, Cr, V, and O were confirmed in all samples. However, no diffraction peaks of the oxide phase were observed by XRD. This was presumed to be because the amorphous SiO2 phase was formed as a result of partial dissolution of CrSi2 and VSi2. SiO2 is a highly resistive insulator, and an increase in SiO2 is undesirable because it increases the resistivity of the negative electrode. In addition, Cr was detected in the compositional analysis, but no diffraction peak corresponding to CrSi2 was observed. Since CrSi2 existed in an amorphous state in experimental examples 5-7 of the Al-Si-Cr system, it was inferred that CrSi2 also existed in an amorphous state in experimental examples 1-4 of the Al-Si-Cr-V system.

[0066] Regarding the structure of porous silicon materials, for example, as shown in Figure 7, Al 67 Si 30 As shown in the cross-sectional SEM image of the porous silicon material (Experimental Example 1) obtained using Cr1V2 powder as the master alloy, it was found to be a porous powder with pores of several tens of nanometers distributed. Compositional analysis by SEM / EDX revealed spherical particles with Si and V segregated on the surface and inside the porous silicon particles, suggesting that it is VSi2. Furthermore, analysis of the composition of the framework of the porous silicon material showed that Cr was abundant, suggesting that the expected structure with CrSi2 segregated along the framework was realized. In addition, when the proportion of each phase was determined using the compositional analysis results, it became clear that the amount of conductive phase tended to increase as the proportion of V and Cr relative to Si in the master alloy increased.

[0067] Regarding the resistivity of porous silicon materials, as shown in Figure 8, in experimental examples 5-7 of the Al-Si-Cr system, the lowest resistivity was achieved when the conductive phase was small, but the resistivity increased sharply as the conductive phase increased. Since the amount of SiO2 was also large in the samples with a large amount of conductive phase, it was suggested that the by-formed insulating phase inhibited electrical conduction. On the other hand, in experimental examples 8-10 of the Al-Si-V system, the resistivity was higher than in experimental examples 5-7 of the Al-Si-Cr system when the amount of conductive phase was small, but the resistivity did not increase as much as in the Al-Si-Cr system even when the conductive phase was increased. Furthermore, compared to these, it was found that experimental examples 1-4 of the Al-Si-Cr system achieved even lower resistivity in the region with a large amount of conductive phase.

[0068] Regarding the pore distribution of porous silicon materials, as shown in Table 1, the pore ratios determined using a mercury porosimeter for the porous silicon materials in Experimental Examples 1 to 10 ranged from 50 to 75 volume%, and this value tended to decrease as the amount of conductive phase increased. Furthermore, it was found that all samples had fine pore sizes of 100 nm or less.

[0069] Regarding battery characteristics, porous samples containing a conductive phase showed higher capacity retention compared to Si powder without pores or porous Si without a conductive phase. In composite porous silicon, the Al-Si-Cr system showed decreased Coulomb efficiency and high resistivity in compositions with a conductive phase content of 20% by mass or more, which have a high effect in suppressing swelling. On the other hand, the Al-Si-Cr-V system achieved high Coulomb efficiency and low resistivity even in compositions containing 20% ​​by mass or more of the conductive phase, which has a high effect in suppressing swelling, and it was found that high capacities of 1000 mAh / g or more were maintained in compositions with a conductive phase content of less than 60% by mass.

[0070] Regarding the change in electrode thickness, as shown in Figures 9 and 10, the composition of the electrode is Al 69 Si 30In Experimental Example 5, a porous silicon material prepared from a Cr1 master alloy powder, the expansion during initial charging was 8%. In contrast, when the amount of Cr and V in the master alloy was increased to increase the conductive phase, a tendency for the initial expansion rate to decrease was observed. The effect of reducing the expansion rate was found to be high when the conductive phase was 20% by mass or more, and even higher when the conductive phase was 30% by mass or more. From the above, it was found that the decrease in conductivity and charge / discharge efficiency could be suppressed in Experimental Examples 1 to 4. Furthermore, it was also found that electrode expansion could be suppressed in Experimental Examples 1 to 4.

[0071] Regarding the treatment to remove the Al component, as shown in Figure 11, when acid treatment was performed, the peak intensity of the conductive phase was lower than that of the Si phase (Figure 11(b)), whereas when alkali treatment was performed, the peak intensities reversed, and the peak intensity of the conductive phase was higher than that of the Si phase. This was presumed to be because a large amount of silicide dissolved in the acid treatment, while less silicide dissolved in the alkali treatment. Furthermore, as shown in Figure 12, a porous silicon material with a similar pore structure to that obtained with acid treatment was obtained even when alkali treatment was performed. However, it was presumed that in alkali treatment, the dissolution of Cr was suppressed more than in acid treatment, and the reinforcement of the Si framework by Cr silicide was further strengthened. In addition, as shown in Table 3, an initial discharge capacity of 1000 mAh / g or more was obtained even when alkali treatment was performed, and the Coulomb efficiency and cycle characteristics were improved compared to when acid treatment was performed. From this, it was confirmed that both acid treatment and alkali treatment are acceptable for removing the Al component.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] It goes without saying that this disclosure is not limited in any way to the experimental examples described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure. [Industrial applicability]

[0076] This disclosure is applicable to the field of energy storage devices. [Explanation of Symbols]

[0077] 10 Energy storage device, 12 Positive electrode, 13 Positive electrode active material, 14 Current collector, 15 Negative electrode, 16 Negative electrode active material, 17 Current collector, 18 Ion conducting medium, 21 Porous silicon material, 23 Void.

Claims

1. It has a Si phase and a silicide phase which contains Cr and V and may also contain Al, and SiO 2 It may have a phase, the Si phase, the conductive phase and the SiO 2 When the total amount of the phases is considered to be 100% by mass, the conductive phase is contained in a range of 12% by mass or more and 60% by mass or less, and the SiO 2 The phase content is 12% by mass or less, the porosity determined by the mercury intrusion method is 45% by volume or more, and the average pore diameter is 100 nm or less. Porous silicon material.

2. The conductive phase is contained in an amount of 30% by mass or more and 60% by mass or less, and the conductive phase is Cr(Al,Si) 4 Phase and VSi 2 A porous silicon material according to claim 1, comprising a phase.

3. The SiO relative to the conductive phase 2 A porous silicon material according to claim 1 or 2, wherein the phase ratio is 0.4 or less.

4. The porous silicon material according to claim 1 or 2, wherein when the total amount of Al, Si, Cr, and V is set to 100 at%, Si is contained in an amount of 60 at% or more, Cr is contained in an amount of 1 at% to 12 at% or less, and V is contained in an amount of 1 at% to 12 at% or less, and the oxygen content is 12 at% or less.

5. A porous silicon material according to claim 1 or 2, wherein the resistivity is 500 Ωcm or less.

6. An electrode comprising the porous silicon material described in claim 1 or 2, satisfying at least one of the following: the expansion rate during the initial charge is 7% or less, or the thickness change rate during each charge / discharge cycle other than the initial charge is 4% or less.

7. A negative electrode which is an electrode containing the porous silicon material described in claim 1 or 2, A positive electrode containing a positive electrode active material, An ion-conducting medium interposed between the negative electrode and the positive electrode, which conducts carrier ions, Equipped with, Energy storage device.

8. The energy storage device according to claim 7, which satisfies one or more of the following conditions (1) to (5). (1) The initial discharge capacity is 1000 mAh / g or more per porous silicon material. (2) The initial charge / discharge efficiency is 80% or higher. (3) The capacity retention rate after 30 cycles is 70% or more. (4) The expansion rate of the negative electrode during the initial charge is 7% or less. (5) The rate of change in the thickness of the negative electrode in each charge-discharge cycle other than the first one is 4% or less.

9. A precursor step to obtain a silicon alloy precursor by melting and rapidly solidifying a raw material containing 10 at% to 50 at% Si, 0.5 at% to 5 at% Cr, and 1 at% to 10 at% V, when the total amount of Al, Si, Cr, and V is 100 at%; A porosizing step to obtain a porous silicon material by removing the Al component contained in the silicon alloy, A method for producing porous silicon material containing [the specified material].

10. The method for producing a porous silicon material according to claim 9, wherein the Al component is removed by acid treatment or alkali treatment in the porosification step.

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