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

Abstract

To provide a material containing Si in which the lowering of charge / discharge characteristics is more reduced.SOLUTION: A porous silicon material of the present disclosure is in a particulate state. Si and a transition metal silicide containing a transition metal element M coexist in a skeleton part. The Si phase in the whole particle accounts for 55 mass% or more, and the transition metal silicide accounts for 1 mass% or more and 45 mass% or less. The porous silicon material contains 20 vol.% or more of pores of 1 μm or less by a mercury indentation method, and has a two-layer structure in which the transition metal silicide is segregated in either a core part of the center or a shell part of the surface layer.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] This specification discloses porous silicon materials, energy storage devices, and methods for manufacturing porous silicon materials. [Background technology]

[0002] Conventionally, for silicon anode materials, porous silicon has been proposed obtained by mixing 70 parts by mass of bulk Si and 30 parts by mass of Al powder, then atomizing the mixture with a molten alloy under an argon atmosphere using a gas atomization method with helium gas, and finally removing the Al with hydrochloric acid (see, for example, Patent Document 1). This porous silicon is said to completely suppress pulverization due to the expansion and contraction of the active material volume during charging and discharging, as well as peeling of the active material from the current collector and lack of contact with the conductive material. Furthermore, as a method for manufacturing silicon materials, a Si alloy of Si with intermediate alloying elements such as Mg, Co, Cr, Cu, Fe, etc. is separated into a second phase in which the intermediate alloying elements and molten elements are substituted in a molten metal containing predetermined molten metal elements, and Si fine particles, and a porous silicon material is obtained by removing the second phase (see, for example, Patent Document 2). This porous silicon material is said to be able to have high capacity and high cycle characteristics. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2004-214054 [Patent Document 2] Japanese Patent Publication No. 2012-82125 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, the manufacturing method described in Patent Document 1 uses an alloy containing Si and Al to create porosity, but it was not yet sufficient to suppress defects based on the expansion and contraction of Si. The manufacturing method for porous silicon material in Patent Document 2 requires melting a silicon alloy containing intermediate alloying elements and replacing them in molten metal containing predetermined molten elements, i.e., high-temperature processing, and a simpler manufacturing process was desired.

[0005] This disclosure has been made in view of these challenges, and its main objective is to provide a porous silicon material, an energy storage device, and a method for manufacturing a porous silicon material that can further suppress the deterioration of charge and discharge characteristics. [Means for solving the problem]

[0006] Through diligent research to achieve the above-mentioned objectives, the present inventors have discovered that by producing an Al-Si-M ternary alloy (where M is a transition metal element) that generates a SiAl alloy or Al alloy via a peritectic reaction, and then removing the Al-containing compound, it is possible to obtain a porous silicon material with a stronger structure that can further suppress the deterioration of charge-discharge characteristics. This has led to the completion of the porous silicon material, energy storage device, and method for manufacturing the porous silicon material described herein.

[0007] In other words, the porous silicon material of this disclosure is It is particulate, and the transition metal silicide containing the transition metal element M and Si coexist in the framework. The Si phase in the entire particle is 55% by mass or more, and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less. It contains 20% or more by volume of pores smaller than 1 μm, obtained by mercury intrusion method. It has a two-layer structure in which the transition metal silicide is segregated in either the central core portion or the surface shell portion.

[0008] The energy storage device disclosed herein is A positive electrode containing a positive electrode active material, A negative electrode containing the aforementioned porous silicon material as the negative electrode active material, An ion-conducting medium interposed between the positive electrode and the negative electrode, which conducts lithium ions, It is something that is provided.

[0009] The method for manufacturing the porous silicon material disclosed herein is: A precursor step to obtain particulate silicon alloy precursors having a two-layer structure in which Si and transition metal silicides coexist in the pore-forming framework and the transition metal silicides are segregated in either the particle center or the surface framework, by melting and rapidly solidifying the raw materials of an Al-Si-M ternary alloy (where M is a transition metal element) that produces a SiAl alloy or Al alloy via a peritectic reaction, and having particulate silicon alloy precursors having a two-layer structure in which Si and transition metal silicides coexist in the framework that forms the pores, and the transition metal silicides are segregated in either the particle center or the surface framework, A porosizing step to obtain a particulate porous silicon material having a two-layer structure in which the Al component contained in the silicon alloy is removed, the material contains 20% or more by volume of pores of 1 μm or less by mercury intrusion, the transition metal silicide and Si coexist in the skeletal portion, the Si phase in the entire particle is 55% by mass or more and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less, and the transition metal silicide is segregated in either the core portion at the center of the particle or the shell portion on the surface, It includes. [Effects of the Invention]

[0010] This disclosure makes it possible to further suppress the deterioration of charge-discharge characteristics in materials containing Si. The reason for this effect is presumed to be as follows. For example, silicon anodes for lithium-ion secondary batteries have a theoretical capacity of 4199 mAh / g, which is about 10 times higher than 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 absorbed lithium ions is Li 4.4The silicon material expands in volume to approximately four times its original size compared to silicon before lithium storage. High-capacity silicon negative electrodes can experience a loss of current collection ability and reduced capacity due to expansion and contraction during charging and discharging. The porous silicon material of this disclosure has nanopores, which can alleviate the stress associated with the expansion and contraction of silicon during charging and discharging. Simultaneously, by introducing a transition metal silicide phase that is inert to carrier ions into the silicon framework, the framework strength is improved, preventing the silicon pore structure from collapsing due to stress associated with expansion and contraction during charging and discharging. Therefore, further improvement in cycle characteristics can be expected. Furthermore, since it has a two-layer particle structure with a core portion and a shell portion, and transition metal silicide segregated in either portion, the mechanical strength of the particles can be further increased, and the collapse of pores during roll pressing or other processes during electrode fabrication can be suppressed. Therefore, in this disclosure, high-performance energy storage devices can be easily obtained, with reduced volume expansion and contraction and improved cycle characteristics. [Brief explanation of the drawing]

[0011] [Figure 1] Al-Si-Cr phase diagram in 25at%Si. [Figure 2] A schematic diagram illustrating the microstructure formation process of the outermost layer during the cooling process of molten droplets. [Figure 3] Al-Si-Cr phase diagram in Al85-ySi15Cry. [Figure 4] Al-Si-Cr phase diagram in Al80-ySi20Cry. [Figure 5] Al-Si-Cr phase diagram in Al75-ySi25Cry. [Figure 6] Al-Si-Cr phase diagram in Al70-ySi30Cry. [Figure 7] Al-Si-Cr phase diagram in Al60-ySi40Cry. [Figure 8] Diagram illustrating the range of eutectic composition in Al-Si-Cr systems. [Figure 9] An explanatory diagram showing an example of the structure of the energy storage device 10. [Figure 10]XRD measurement results of porous silicon after acid treatment in Experimental Examples 1-6. [Figure 11] STEM image of a cross-section of porous silicon from Experimental Example 2. [Figure 12] STEM images of the core and shell portions of the cross-section of porous silicon in Experimental Example 2. [Figure 13] Pore ​​distribution measurement results for the core and shell portions of Experimental Example 2. [Figure 14] Measurement results of the skeletal size of the core and shell parts in Experimental Example 2. [Figure 15] Measurement results of the change in porosity with respect to pressure in experimental examples 1, 2, 4, 5, and 7. [Figure 16] SEM images of the appearance of experimental examples 2 and 7 under a pressure of 100 MPa. [Modes for carrying out the invention]

[0012] (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 of an Al-Si-M ternary alloy (where M is a transition metal element) that produces a SiAl alloy or Al alloy via a peritectic reaction is melted and rapidly cooled and 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. Here, first, we will describe the raw material composition, with Cr as an example of a transition metal element M that produces a SiAl alloy or Al alloy via a peritectic reaction.

[0013] Figure 1 is the Al-Si-Cr phase diagram for 25at%Si. Figure 2 is a schematic diagram of the microstructure formation process of the outermost layer during the cooling process of a molten droplet. Since the strength of CrSi2, a transition metal silicide as a SiCr compound, is about twice that of Si, the structural strength can be improved by coexisting CrSi2 as a reinforcing phase in the Si framework. The refinement of pores by Cr addition is influenced by the characteristics of the Al-Si-Cr phase diagram. Here, we will explain the case of cooling a composite liquid by gas atomization as an example. In gas atomization, a high-temperature molten liquid is injected into a high-pressure gas from a hole at the tip of a nozzle to cool it. At this time, when the molten liquid (Figure 1(1)) is cooled to the temperature shown in Figure 1(2), CrSi2 crystallizes first from the surface of the droplet where the temperature is lower (Figure 2(2)), but CrSi2 gradually coarses in the temperature range where it coexists with the molten liquid (Figure 1(3)) (Figure 2(3)). In this process, if the CrSi2 becomes too coarse, the resulting silicon powder will be surrounded by a shell of inert CrSi2 (Figure 2(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 AlSiCr phase diagram, when cooled to the temperature shown in Figure 1(4), the CrSi2 decomposes (peritectic reaction) and simultaneously Si and Al decompose. 13 Since Si4Cr4 precipitates (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 This refers to a reaction in which a molten liquid and a solid phase react to form another solid phase, as in Si4Cr4. However, since the reaction proceeds from the surface of the solid phase particles, as shown in Figure 2(4), Al forms around the fine CrSi2 particles. 13A structure in which the shell of Si4Cr4 and Si is easily formed, and a finer microstructure is realized. That is, in addition to the fineness of Si particles and the strengthening phase CrSi2, 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 these particles is small, and a nanostructured microstructure can be realized. According to such a mechanism, a particulate Al-Si-M ternary alloy having a two-layer structure in which a transition metal silicide is segregated either in the core part or the shell part of the particle center is obtained. The solidification structure forms a lamellar structure composed of SiCr compounds, AlSiCr compounds, AlCr compounds, eutectic Si, primary Si, and primary Al. Compounds eluted in acid include primary Al, AlSiCr compounds, and AlCr compounds, and it is speculated that the structure after acid treatment is a skeletal porous body composed of the remaining SiCr compounds, eutectic Si, and primary Si.

[0014] Figures 3 to 7 are phase diagrams of the Al-Si-Cr alloy when the contents of Si and Cr are changed. Figure 3 is the Al-Si-Cr phase diagram in 85-y Si 15 Cr y terms of Al. Figure 4 is the Al-Si-Cr phase diagram in 80-y Si 20 Cr y terms of Al. Figure 5 is the Al-Si-Cr phase diagram in 75-y Si 25 Cr y terms of Al. Figure 6 is the Al-Si-Cr phase diagram in 70-y Si 30 Cr y terms of Al. Figure 7 is the Al-Si-Cr phase diagram in 60-y Si 40 Cr yThis is the Al-Si-Cr phase diagram. Figure 8 is an explanatory diagram of the range of eutectic composition in the Al-Si-Cr system. Generally, in order to achieve strength improvement through nanocomposite formation, it is necessary for the reinforcing phase, such as the CrSi2 phase, to be as fine as possible. To achieve this, it is preferable that the crystallization temperatures of Si and the reinforcing phase, silicide, are close in the phase diagram of the Al-Si-Cr system alloy. That is, the ideal composition range is one in which Si and the reinforcing phase, silicide, precipitate simultaneously in a eutectic reaction. As shown in Figures 3 to 7, Al 100-x-y Si x Cr y In the basic composition, when the Si content x is less than 18 at%, the composition does not show a eutectic reaction in which Si and silicide crystallize. However, in compositions 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 There are compositions that produce the eutectic reaction Si4Cr4 (eutectic reaction 2). Figure 8 shows the relationship between the amount of Si x and the amount of Cr y that produce these eutectic reactions 1 and 2. In the compositional region shown by the shaded area, in addition to the fact that Si and CrSi2 crystallize at relatively close temperatures, CrSi2 + L ⇒ Al 13 Because a peritectic reaction of Si4Cr4 occurs, a finer pore structure can be realized, and it is presumed that this is a more preferable composition range, especially for realizing porous silicon materials with a core-shell structure. Such a composition range is based on the basic composition formula Al 100-x-y Si x M y In this case, the range is 18≦x≦40 and 0.1≦y≦15. Also, according to Figure 8, the amount of this transition metal element M is preferably 0.1at% or more, more preferably 0.5at% or more, and more preferably 1at% or more, relative to the total amount of Al and Si.

[0015] (Precursor process) In the precursor process, raw materials for an Al-Si-M ternary alloy (where M is a transition metal element) that produces a SiAl alloy or Al alloy via a peritectic reaction are melted and rapidly cooled and solidified, and Si and transition metal silicide (Si) are used to form the framework that creates the pores. a M bA particulate precursor of a silicon alloy is obtained, having a two-layer structure in which (a and b are arbitrary numbers) coexist and transition metal silicide is segregated in either the particle center or the surface skeletal portion. In this step, when the total amount of Si, Al, and M is set to 100 at%, a raw material may be used that contains M in the range of 0.1 at% to 20 at%, Al in the range of 40 at% to 90 at%, and the remainder being Si. In the raw material composition, the transition metal element M is preferably 0.1 at% or more, more preferably 0.5 at% or more, and more preferably 1 at% or more. Furthermore, as described above, the raw material composition is based on the basic composition formula Al 100-x-y Si x M y In this case, it is more preferable from the viewpoint of forming a core-shell structure that the ranges are 18≦x≦40 and 0.1≦y≦15. The raw materials may contain unavoidable impurities. Unavoidable impurities are components that inevitably remain during the purification of Si, M, or Al, and examples include Fe, C, Cu, Ni, and P. It is preferable that the amount of unavoidable impurities be as small as possible; for example, when the total amount of Si, Cr, and Al is 100 at%, it is preferable that it be 5 at% or less, and more preferably 2 at% or less. The blending ratio of M is, for example, preferably 2 at% or more, and may be 3 at% or more. Also, the blending ratio of M is preferably 15 at% or less, and may be 12.5 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 Si content is preferably 15 at% or more, more preferably 18 at% or more, and may be 20 at% or more or 25 at% or more. Alternatively, the Si content is preferably 59 at% or less, more preferably 50 at% or less, and may be 40 at% or less or 30 at% or less. Silicon alloys containing Al and M within these ranges are preferable because they allow for a higher porosity and the acquisition of voids of more suitable shapes and sizes. When the Al content is high, rapid cooling after melting to form an alloy causes a large amount of single-phase Al to precipitate, thus forming many voids. This cooling rate is preferably rapid, for example, 102 ℃ / s or more 10 8 The range may be less than or equal to °C / s.

[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. Melting of the raw material powder is necessary for the production of the master alloy. High-frequency melting is more preferable for producing a uniform sample, but simple heating and melting in an electric furnace or melting using an electron beam may also be used. In the precursor process, the alloy obtained from the raw materials may be atomized. In this atomization process, the molten silicon alloy raw material may be cast into a mold, and the resulting ingot may be crushed to form particles. Furthermore, the method for atomizing the silicon alloy may be one or more of the following: gas atomization, water atomization, etc. Both gas atomization and water atomization yield alloy powder. Of these, gas atomization is more preferable for atomizing the silicon alloy. In gas atomization, it is preferable to use an Ar atmosphere when creating the molten metal, and to use an Ar or He atmosphere when atomizing.

[0017] In the precursor step, it is preferable to atomize the silicon alloy particles to a size of 0.5 μm to 3 μm. These silicon alloy particles preferably have an average particle size of 0.5 μm to 3 μm, but may also be in the range of 1 μm to 2.5 μm or 1.5 μm to 2 μm. The silicon alloy particles should 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), accumulating 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 represent the average particle size of the final aggregate of porous particles to be obtained.

[0018] In this precursor step, a raw material containing a second element, in addition to Al, Cr, and Si, may be used, which includes one or more of Ca, Cu, Mg, Na, Sr, and P. Of these, one or more of Ca, Na, and Sr are preferred as the second element. The amount of the second element is preferably less than the amount of Al and Cr, for example, 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.

[0019] (Porous process) In the porosity creation process, the Al component contained in the silicon alloy prepared above is removed to obtain a particulate porous silicon material having a two-layer structure in which pores of 1 μm or less are present in 20 volume percent or more by mercury intrusion, transition metal silicide and Si coexist in the skeletal portion, the Si phase in the entire particle is in the range of 55 mass percent or more and transition metal silicide is in the range of 1 mass percent or more and 45 mass percent or less, and the transition metal silicide is segregated in either the core portion at the center of the particle or the shell portion on the surface. Examples of the Al component to be removed in this process include Al and its compounds. In this process, it is preferable to selectively remove the Al component, i.e., the Al phase and its compounds, with an acid or alkali. The acid or alkali used is preferably one that dissolves elements and / or compounds other than silicon in the silicon alloy without dissolving silicon, 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 Al and its compounds, M and its compounds, etc., and can be, for example, in the range of 1 mol / L to 5 mol / L. This removal process may involve heating, for example, at 30°C to 60°C. Alternatively, the removal process may involve immersing the silicon alloy particles in an acid or alkaline solution and stirring for 1 to 5 hours. The resulting porous silicon material is then washed and dried.

[0020] In the porosity creation process, substances other than Si may be removed in a range of 85% to 100% by mass. For example, Al, M, 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 and M are preferable to be included in amounts greater than a predetermined amount from the viewpoint of reinforcing the silicon skeleton and improving durability. In this process, a porous silicon material containing a SiM compound may be obtained. As the SiM compound, for example, a transition metal silicide such as Si a M b (a and b are arbitrary numbers), and MSi2 is one example. MSi2 as a SiM compound may include M(Si,Al)2, in which some of the Si is substituted with Al. Also, as an AlSiM compound, for example, Al a Si b M c It can also be a compound (where a, b, and c are any number), Al 13 Examples include Si4Cr4. The SiM compound may be sparingly soluble in acids or alkalis.

[0021] In the porosity creation process, it is preferable to obtain a porous silicon material in which the Si phase is 55% by mass or more and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less. Furthermore, in the porous silicon material, it is preferable to contain 2% by mass or more of transition metal silicide, and it may also contain 3% by mass or more. Furthermore, in the porous silicon material, it is preferable to contain 40% by mass or less of transition metal silicide, and it may also contain 30% by mass or less. In this process, it is preferable to obtain a porous silicon material in which transition metal silicide remains.

[0022] In the porosity creation process, a porous silicon material with a porosity in the range of 30% to 85% by volume may be obtained. This porosity is measured using a mercury porosimeter. This porosity is preferably 35% or more by volume, and may be 40% or more by volume. Furthermore, the porosity is preferably 80% or less by volume, more preferably 77.5% or less by volume, and may be 75% or less by volume. A higher porosity is preferable because it responds more easily to volume changes during carrier ion absorption, while a lower porosity is preferable because it increases the amount of Si present per unit volume.

[0023] (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 will be omitted, assuming they are the same as those of the manufacturing method described above. This porous silicon material is particulate, in which transition metal silicide containing a transition metal element M and Si coexist in the framework, with the Si phase being 55% by mass or more and the transition metal silicide being in the range of 1% by mass or more and 45% by mass or less in the entire particle. Furthermore, this porous silicon material contains 20% by volume or more of pores of 1 μm or less by the mercury intrusion method, and has a two-layer structure in which transition metal silicide is segregated in either the central core portion or the surface shell portion. It is preferable that the porous silicon material has a structure in which transition metal silicide is segregated in the shell portion. When transition metal silicide is segregated in the outer shell portion, it is easier to increase the mechanical strength of the particles, which is preferable. In addition, this porous silicon material may have a two-layer structure in which Al is segregated in either the central core portion or the surface shell portion. This porous silicon material preferably has a structure in which Al is segregated in the shell portion.

[0024] In this porous silicon material, the core portion preferably has a skeleton width in the range of 5 nm to 30 nm, more preferably 10 nm to 20 nm, and even more preferably 12.5 nm to 17.5 nm, as obtained from electron microscope images. Here, the skeleton width refers to the length between voids in the Si skeleton present in the core portion. This skeleton width is obtained by observing the cross-section of the particles of the porous silicon material with an electron microscope, obtaining an image, and then processing this image. Examples of electron microscopes include SEM, STEM, and TEM. Furthermore, the core portion preferably contains pores in the range of 4 nm to 85 nm in an area of ​​50% to 80% by volume, as obtained from electron microscope images. Regarding pore size, each pore present in the electron microscope image is approximated as an ellipse, its major axis and minor axis are measured, and the average value of these is referred to. In this core portion, the pore size is more preferably in the range of 10 nm to 50 nm, and may be in the range of 20 nm to 40 nm. Furthermore, the porosity may be in the range of 60% by volume or more and 70% by volume or less.

[0025] Furthermore, in this porous silicon material, the shell portion preferably has a larger skeleton width than the core portion, as obtained from electron microscope images, preferably in the range of 10 nm to 40 nm, and may be in the range of 15 nm to 25 nm. Here, the skeleton width of the shell portion refers to the length between voids in the Si skeleton present in the shell portion. In addition, the shell portion preferably contains pores in the range of 4 nm to 120 nm in the range of 50 volume% to 80 volume%, as obtained from electron microscope images. In this shell portion, the pore size is more preferably in the range of 10 nm to 100 nm, and may be in the range of 30 nm to 80 nm. In addition, the porosity may be in the range of 60 volume% to 70 volume%. Furthermore, in this shell portion, the transition metal silicide content relative to the total amount of transition metal silicide contained in the porous silicon material is preferably 55 mol% or more, more preferably 60 mol% or more, and even more preferably 70 mol% or more. Furthermore, this content may be 99 mol% or less, 95 mol% or less, or 90 mol% or less. Transition metal silicide (Si) contained in the shell portion a M b (where a and b are arbitrary numbers) A larger number of these factors is preferable, as it can increase the mechanical strength of the particles.

[0026] The porous silicon material may have an Al content of 4 at% to 15 at% and an M content of 1 at% to 7 at% when the total amount of Si, Al, and M is set to 100 at%. In the porous silicon material, the Al content may be 5 at% or more, or 8 at% or more. Alternatively, the Al content may be 14 at% or less, or 12.5 at% or less. This elemental composition is determined by measurement results obtained by energy-dispersive X-ray spectroscopy (EDX).

[0027] The porous silicon material has a Si phase of 55% by mass or more and a transition metal silicide phase of 1% by mass or more and 45% by mass or less in the total particle composition. The values ​​for each phase are determined from the peak intensity ratio of X-ray diffraction. From the viewpoint of charge-discharge capacity, a higher Si phase content is preferable, with 60% by mass or more, 70% by mass or more, or 80% by mass or more being more preferable. From the viewpoint of strength improvement, a higher transition metal silicide phase content is preferable, with 2% by mass or more, 10% by mass or more, or 20% by mass or more being more preferable. Furthermore, from the viewpoint of charge-discharge capacity, a lower transition metal silicide phase content is preferable, with 40% by mass or less, 30% by mass or less, or 25% by mass or less being more preferable.

[0028] The porous silicon material may have a diameter in the range of 0.5 μm to 3 μm, and the length of the core portion may occupy a range of 40% to 60% of the particle diameter. The diameter of this porous silicon material may be 1 μm or more, or 2 μm or less. The particle diameter shall be the value obtained by SEM observation. The particle diameter may be set appropriately according to the performance required for the energy storage device.

[0029] In porous silicon materials, it is preferable that the transition metal silicide is one or more of M(Si,Al)2, which is a solid solution of MSi2 and Al, and that the content of the transition metal silicide is 2% by mass or more and 30% by mass or less. When the content of the transition metal silicide is 2% by mass or more, sufficient strengthening of the framework can be achieved, and when it is 30% by mass or less, the strengthening phase that does not participate in charging and discharging can be relatively reduced, thereby further increasing the charge and discharge capacity, which is preferable. In this porous silicon material, it is preferable that the average pore diameter determined by the mercury intrusion method is in the range of 60 nm or less. This pore diameter is preferably in the range of 5 nm or more and 300 nm or less. This pore diameter may be 10 nm or more, or 20 nm or more. Furthermore, this pore diameter is preferably 50 nm or less, and may be 45 nm or less. Smaller pore diameters are preferable because the pores are less likely to collapse. Also, larger pores are preferable because volume changes can be further suppressed when carrier ions are intercalated. The transition metal M is not particularly limited as long as it undergoes a peritectic reaction to produce a SiAl alloy or Al alloy, but examples include Cr.

[0030] The porous silicon material contains a three-dimensional network structure of silicon skeleton having voids, and may also contain a SiM compound and / or an AlSiM compound. The SiM compound contains a transition metal silicide. It is presumed that the SiM compound and AlSiM compound are responsible for reinforcing the silicon skeleton. The porous silicon material preferably contains the SiM compound in an amount of 5 mol% to 20 mol% relative to the Si phase. Furthermore, the porous silicon material preferably contains the SiM compound in an amount of 1% to 15% by mass. Furthermore, the porous silicon material preferably contains the AlSiM compound in an amount of 0.1% to 10% by mass. From the viewpoint of supplementing the reinforcement of the skeleton, a higher amount of the SiM compound and AlSiM compound is preferable, and from the viewpoint of the charge / discharge capacity of the energy storage device, a lower amount is preferable.

[0031] The porous silicon material may have a porosity in the range of 30% to 85% by volume, as determined by the mercury intrusion method. This porosity is preferably, for example, 35% or more by volume, and may be 40% or more by volume. Furthermore, the porosity is preferably, for example, 80% or less by volume, more preferably 77.5% or less by volume, and may be 75% or less by volume. A higher porosity is preferable because it responds more readily to volume changes during carrier ion absorption, while a lower porosity is preferable because it increases the amount of Si present per unit volume.

[0032] The porous silicon material preferably has 20% or more by volume of pores smaller than 1 μm, obtained by the mercury intrusion method. It is preferable that the porous silicon material has a greater number of such fine pores. More preferably, the porous silicon material has 30% or more by volume of pores smaller than 1 μm, obtained by the mercury intrusion method, and even more preferably, 40% or more by volume. Furthermore, the porous silicon material may have 90% or less by volume of pores smaller than 1 μm, obtained by the mercury intrusion method.

[0033] When a porous silicon material is subjected to a confinement pressure of 1 GPa as an electrode, it is preferable that the change in pore size is less than 25 volume%, more preferably less than 20 volume%, and even more preferably less than 10 volume%. From the viewpoint of skeletal strength, it is preferable that the decrease in pore size is smaller when subjected to a confinement pressure.

[0034] This porous silicon material preferably contains 70 at% or more Si when the total amount of Si, Al, and transition metal M, excluding oxygen and unavoidable impurities, is set to 100 at%. The Si content is more preferably 75 at% or more, even more preferably 80 at% or more, and may be 85 at% or more. A higher Si content is preferable from the viewpoint of charge / discharge capacity, and a lower Si content is preferable from the viewpoint of relative skeletal reinforcement. The Al content is preferably in the range of 4 at% to 15 at%, preferably 12.5 at% or less, more preferably 10 at% or less, and may be 7.5 at% or less. The Al content may also be 5 at% or more, or 7.5 at% or more. The M content is preferably in the range of 1 at% to 7 at%. The M content is more preferably 6 at% or less, and may be 5 at% or less. The M content is also more preferably 2 at% or more, even more preferably 3 at% or more, and may be 4 at% or more. The content of Al and M is preferably higher from the viewpoint of reinforcing the framework, and preferably lower from the viewpoint of the charge / discharge capacity of the energy storage device, since these are components that do not charge or discharge. Furthermore, the porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr, and P as secondary elements in a range of 15% by mass or less. In addition, the porous silicon material may contain unavoidable impurities in addition to Si, Al, and M. It is preferable that the amount of secondary elements and unavoidable impurities be as low as possible.

[0035] (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 of porous silicon material in which the porosity is compressed to a range of 5 volume% to 50 volume%. In this electrode, the porosity of the porous silicon material may be reduced by compression during manufacturing. For example, when porous silicon particles are used as the negative electrode active material in a lithium-ion secondary battery, the smaller the pores, the more uniformly the lithium ions are alloyed, thus reducing stress concentration and preventing deterioration of the electrode itself. The porosity of the porous silicon material after compression can be appropriately adjusted according to the characteristics required for the electrode for the energy storage device, for example, it may be 5 volume% or more, or 10 volume% or more. Alternatively, the porosity of the porous silicon material after compression may be, for example, 30 volume% or less, or 20 volume% or less.

[0036] 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, which is then applied 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 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. For example, one or more of the following can be used: graphite such as natural graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjenblack, carbon whiskers, needle coke, carbon fiber, or a mixture of two or more metals (copper, nickel, aluminum, silver, gold, etc.). The binder serves to bind 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 individually or as a mixture of two or more. Water-based binders such as cellulose-based or aqueous dispersions of styrene-butadiene rubber (SBR) can also be used. As a solvent, 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, for example, roller coating using an applicator roll, 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 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. Examples of the shape of the current collector 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.

[0037] 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 the 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, it is preferable that the porous silicon material occupies 50% by mass or more, preferably 90% by mass or more, with the entire electrode active material being 100% by mass.

[0038] (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 one of a lithium ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, etc. can be used. Specifically, transition metal sulfides such as CrS2, CrS3, MoS3, FeS2, 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 being Li (1-x) Lithium cobalt composite oxides such as CoO2, with the basic composition formula being Li (1-x) Lithium nickel composite oxides such as NiO2, with the basic composition formula being Li (1-x) Ni a Co b Mn c Lithium nickel cobalt manganese composite oxides such as O2(a+b+c=1), lithium vanadium composite oxides with a basic composition formula such as LiV2O3, and transition metal oxides with a basic composition formula such as V2O5 can be used. Among these, lithium transition metal composite oxides such as LiCoO2, LiNiO2, LiMnO2, Li (1-x) Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2 is preferred. Note that the "basic composition formula" means that other elements such as Al and Mg may also be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors, lithium-ion capacitors, etc. Examples of carbonaceous materials include activated carbon, coke, glassy carbon, graphite, non-graphitizable carbon, pyrolytic carbon, carbon fibers, carbon nanotubes, and polyacenes. Of these, activated carbon, which exhibits a high specific surface area, is preferred. Activated carbon as a carbonaceous material has a specific surface area of ​​1000 m². 2 Preferably, it is 1500m or more per gram. 2 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] The shape of this energy storage device is not particularly limited, but examples include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flattened, and rectangular shapes. It may also be applied to larger devices used in electric vehicles, etc. Figure 9 is an explanatory diagram showing an example of the structure of an 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 a central core portion 22, an outer shell portion 23, and voids 24.

[0043] This energy storage device preferably has a higher capacity retention rate when undergoing charge-discharge cycles. For example, a capacity retention rate of 95% or higher after 10 cycles is preferred, 97.5% or higher is more preferred, and 98% or higher is even more preferred.

[0044] (All-solid-state lithium-ion rechargeable battery) This energy storage device is preferably an all-solid-state lithium-ion secondary battery. All-solid-state batteries are preferable because they can further suppress changes in performance due to the electrolyte and further enhance safety. This all-solid-state lithium-ion secondary battery may comprise a positive electrode containing a positive electrode active material, a negative electrode using the porous silicon material described above as the negative electrode active material, and a solid electrolyte interposed between the positive and negative electrodes to conduct lithium ions. The positive electrode can be any of those shown in the energy storage device described above. The negative electrode can also be the porous silicon material described above as the negative electrode active material.

[0045] The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La, and Zr. This solid electrolyte has a basic composition of Li 7.0+x-y (La 3-x ,A x )(Zr 2-y ,T y )O 12It may be such. However, A is one or more of Sr and Ca, T is one or more of Nb and Ta, and it satisfies 0 < x ≤ 1.0 and 0 < y < 0.75. Alternatively, the solid electrolyte has a basic composition (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 or (Li 7-3z+x-y M z )(La 3-x A x )(Y 2-y T y )O 12 and may be a garnet-type oxide represented by 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, 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. Within such a range, the ionic conductivity can be made more suitable.

[0046] Alternatively, as the solid electrolyte, for example, common ones such as Li3N, Li called LISICON 14 Zn(GeO4)4, sulfide Li 3.25 Ge 0.25 P 0.75 S^4, 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-ceramics11 Furthermore, Li, a sulfide-based substance with high conductivity. 10 Ge2PS2 is another example. Examples of glass-based inorganic solid electrolytes include Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li4SiO4, Li2S-P2S5, Li3PO4-Li4SiO4, Li3BO4-Li4SiO4, and those using SiO2, GeO2, B2O3, and P2O5 as glass-based materials with Li2O as a network modifier. Other examples of thiolysicone solid electrolytes include Li2S-GeS2 systems, Li2S-GeS2-ZnS systems, Li2S-Ga2S2 systems, Li2S-GeS2-Ga2S3 systems, Li2S-GeS2-P2S5 systems, Li2S-GeS2-SbS5 systems, Li2S-GeS2-Al2S3 systems, Li2S-SiS2 systems, Li2S-P2S5 systems, Li2S-Al2S3 systems, LiS-SiS2-Al2S3 systems, and Li2S-SiS2-P2S5 systems. These solid electrolytes may be formed in a plate shape and placed between the positive and negative electrodes.

[0047] Furthermore, the all-solid-state lithium-ion secondary battery may be equipped with a restraining member that restrains the laminate, which is made up of a positive electrode, a solid electrolyte, and a negative electrode, in the stacking direction. This restraining member may, for example, include a pair of plate-like parts that sandwich the laminate from both ends in the stacking direction, a rod-like part that connects the pair of plate-like parts, and an adjustment part connected to the rod-like part that adjusts the distance between the pair of plate-like parts by a screw structure or the like.

[0048] As detailed above, this disclosure can further suppress the decrease in charge-discharge characteristics in Si-containing materials. The reason for this effect is presumed to be as follows. For example, silicon anodes for lithium-ion secondary batteries have a theoretical capacity of 4199 mAh / g, which is about 10 times higher than 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 absorbed lithium ions is Li 4.4The silicon material expands in volume to approximately four times its original size compared to silicon before lithium storage. High-capacity silicon negative electrodes can experience a loss of current collection ability and reduced capacity due to expansion and contraction during charging and discharging. The porous silicon material of this disclosure has nanopores, which can alleviate the stress associated with the expansion and contraction of silicon during charging and discharging. At the same time, by introducing a transition metal silicide phase that is inert to carrier ions into the silicon framework, the framework strength can be improved, preventing the silicon pore structure from collapsing due to stress associated with expansion and contraction during charging and discharging. Therefore, further improvement in cycle characteristics can be expected. Furthermore, since it has a two-layer particle structure with a core portion and a shell portion, and transition metal silicide segregated in either portion, the mechanical strength of the particles can be further increased, and the collapse of pores during roll pressing or other processes during electrode fabrication can be suppressed.

[0049] Furthermore, during charging and discharging of the Si negative electrode, an SEI layer is formed on the negative electrode surface due to reactions between the electrolyte and the negative electrode. However, repeated expansion and contraction of Si leads to repeated destruction and regeneration of the SEI layer, and the resulting fragments of the peeled-off SEI layer degrade the cycle characteristics. To improve this, it is necessary to reduce the contact area between the electrolyte and Si. The transition metal silicide phase coated in a core-shell type also acts as a protective layer that prevents direct contact between the electrolyte and Si. Since the outer shell is coated with a high-strength silicide, it prevents the particles from expanding outward and prevents peeling of the SEI layer. Thus, in this disclosure, cycle characteristics can be improved by coexisting an expansion relaxation space and a protective layer for the SEI. Moreover, although the transition metal silicide is a p-type semiconductor, the introduction of M(Si,Al)2, in which some of the Si is replaced with Al, as a strengthening phase improves conductivity, and thus an improvement in rate characteristics can be expected. Furthermore, the increased strength of the particles can suppress the collapse of pores during roll pressing and other processes when manufacturing composite electrodes. Therefore, this disclosure makes it possible to easily obtain high-performance energy storage devices that mitigate volume expansion and contraction and improve cycle characteristics.

[0050] 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.

[0051] For example, this disclosure may be any of the following [1] to

[10] . [1] Particulate, in which transition metal silicides containing transition metal elements M and Si coexist in the framework, The Si phase in the entire particle is 55% by mass or more, and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less. It contains 20% or more by volume of pores smaller than 1 μm, obtained by mercury intrusion method. It has a two-layer structure in which the transition metal silicide is segregated in either the central core portion or the surface shell portion. Porous silicon material. [2] The core portion, as obtained from electron microscope images, has a skeleton width in the range of 5 nm to 30 nm and contains pores in the range of 4 nm to 85 nm in the range of 50 volume% to 80 volume%, The aforementioned shell portion is a porous silicon material according to [1], wherein the width of the skeleton obtained from an electron microscope image is in the range of 10 nm to 40 nm, and pores in the range of 4 nm to 120 nm are included in a range of 50 volume% to 80 volume%. [3] The porous silicon material according to [1] or [2], having a structure in which the transition metal silicide is segregated in the shell portion. [4] The porous silicon material according to any one of [1] to [3], wherein the Al content is in the range of 4 at% to 15 at% and the M content is in the range of 1 at% to 7 at%. [5] The porous silicon material according to any one of [1] to [4], wherein the porous silicon material has a diameter in the range of 0.5 μm to 3 μm, and the length of the core portion is in the range of 40% to 60% of the particle diameter. [6] The porous silicon material is one or more of M(Si, Al)2 in which the transition metal silicide is MSi2 and Al is solid-dissolved, and the content of the transition metal silicide is 2% by mass or more and 30% by mass or less. The porous silicon material according to any one of [1] to [5]. [7] A positive electrode including a positive electrode active material, [1]~[6] The porous silicon material according to any one of the above 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 provided with. [8] Melting a raw material of an Al-Si-M ternary alloy (M is a transition metal element) that forms a SiAl alloy or an Al alloy through a peritectic reaction and rapidly solidifying it, so that Si and a transition metal silicide coexist in the skeletal part forming pores, and a precursor step of obtaining a particulate precursor of a silicon alloy having a two-layer structure in which the transition metal silicide is segregated in either the core part of the particle center or the skeletal part of the surface layer, Removing the Al component contained in the silicon alloy, containing 20% by volume or more of pores of 1 μm or less by mercury intrusion porosimetry, the transition metal silicide and Si coexisting in the skeletal part, the Si phase in the whole particle being 55% by mass or more and the transition metal silicide being in the range of 1% by mass or more and 45% by mass or less, and a transition metal silicide being segregated in either the core part of the particle center or the shell part of the surface layer. A method for producing a particulate porous silicon material having a two-layer structure. And a method for producing a porous silicon material including. [9] In the precursor step, in the basic composition formula Al 100-x-y Si x M y The raw materials in the range of 18 ≦ x ≦ 40 and 0.1 ≦ y ≦ 15 are used. The method for producing a porous silicon material according to [8].

[10] In the porousization step, the Al component is selectively removed by an acid or an alkali. The method for producing a porous silicon material according to [8] or [9].

Example

[0052] The following describes specific examples of the porous silicon and energy storage devices fabricated according to this disclosure as experimental examples. Experimental Examples 1 to 6 correspond to embodiments of this disclosure, and Experimental Examples 7 to 8 correspond to comparative examples.

[0053] [Fabrication of porous silicon materials] The raw materials are Al, Si, and Cr, and the basic composition formula is Al 100-x-y Si x Cr y The material was weighed to achieve a composition of (x=10~30, y=2~15) 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 system was purged with Ar gas. Melting of the raw material powder was necessary for the preparation of the master alloy, and high-frequency induction melting was performed to produce a uniform sample. The obtained master alloy was heated to 1000-1300°C in an Ar atmosphere to melt, and then rapidly solidified at a rate of 102 K / sec or higher using the gas atomization method to obtain AlSiCr alloy powder (precursor step). The diameter of this alloy powder, as measured by SEM observation, was in the range of 0.5 μm to 3 mm. The obtained alloy was immersed in a 3N hydrochloric acid aqueous solution and treated at 80°C for 5-10 hours to separate Al and Al 13 Si4Cr4 and other elements were selectively removed. The residue was transferred to a filtration filter, and after removing the acid from the treatment by pressure filtration, it was washed four or more times with distilled water, and the washing water was removed by the same pressure filtration method to obtain core-shell type porous silicon (porosity formation process).

[0054] (Experimental Examples 1-6) The base alloy composition Al is set to x=25 and y=1.5 in the basic composition formula above. 73.5 Si 25 Cr 1.5 The porous silicon material obtained from this was designated as Experimental Example 1. Furthermore, the base alloy composition Al was set to x=25, y=3. 72 Si 25 A porous silicon material obtained from Cr3 was used as Experimental Example 2. Furthermore, the base alloy composition Al was set to x=25 and y=5. 70 Si 25 A porous silicon material obtained from Cr5 was designated as Experimental Example 3. Furthermore, the base alloy composition Al was set to x=30 and y=1. 69 Si 30The porous silicon material obtained from Cr1 was designated as Experimental Example 4. Furthermore, the base alloy composition Al was set to x=30 and y=3. 67 Si 25 A porous silicon material obtained from Cr3 was designated as Experimental Example 5. Furthermore, the base alloy composition Al was set to x=30 and y=7.5. 62.5 Si 30 Cr 7.5 The porous silicon material obtained from this was designated as Experimental Example 6.

[0055] (Experimental Examples 7 and 8) The basic chemical formula for a binary system is Al 100-x Si x Let x = 25 for the base alloy composition Al 75 Si 25 Experimental Example 7 involved a porous silicon material obtained through the same process as in Experimental Example 1, which included rapid cooling and solidification by gas atomization followed by acid treatment for porosity. Experimental Example 8 involved Si powder with an average particle size of 5 μm.

[0056] (Measurement of physical properties of porous silicon materials) Porous silicon powder after acid treatment was observed and elementally analyzed using a scanning electron microscope (SEM, HITACHI S-4300) and energy-dispersive X-ray spectroscopy (EDAX, HITACHI S-4300). Similarly, elemental mapping and microstructure observation were performed using a scanning transmission electron microscope (STEM, JEOL JEM2100F). In addition, X-ray diffraction measurements were performed using an X-ray diffractometer (Rigaku RINT-TTR) with a Cu tube at a rate of 5° / min in the range of 2θ = 10° to 80°. Si phase, Al phase, CrSi2 phase, Al 13 The proportion of each phase in the Cr4Si4 phase was calculated by mass percentage from the XRD peak intensity ratio of each phase using alumina standards. Pore distribution was also measured using a mercury porosimeter (Powermaster 60GT, Quantachrome). The porous silicon powder after acid treatment was dissolved in HF and HNO3, and elemental analysis was performed using ICP emission spectroscopy (ICP-OES, PS3520UVDDII II, Hitachi High-Tech Science).

[0057] (Results of composition analysis) First, we will explain the composition range suitable for core-shell particle formation using the phase diagrams of Al-Si-Cr alloys calculated by the CALPHAD method shown in Figures 3-7. In the core-shell structure, CrSi2 is initially nucleated and grows in the melt, then decomposes, directly forming Al 13 The reaction must proceed in the following order: Si4Cr4 and Si are formed, and finally, the melt solidifies and Al crystallizes. When Si is 15 at% or more, sufficient CrSi2 is formed, which is preferable. When Si is 20 at% or less, the formation of CrSi2 can be further suppressed. Furthermore, in the range where Si is 20 at% or more and Cr is 3.5 at% or less (see Figure 4), the formation of Al4Cr as an intermediate reaction in the above decomposition reaction can be further suppressed, which is preferable. Thus, by narrowing down the appropriate composition range, it was found that the region enclosed by the dashed lines in Figures 3-7, i.e., the composition range shown by the shaded area in Figure 8, is appropriate.

[0058] (XRD evaluation results) Next, the generated phases contained in the prepared samples were investigated. Figure 10 shows the XRD measurement results of porous silicon after acid treatment in Experimental Examples 1-6. The master alloy composition is Al. 73.5 Si 25 Cr 1.5 (Experimental Example 1), Al 72 Si 25 Cr3 (Experimental Example 2), Al 70 Si 25 Cr5 (Experimental Example 3), Al 69 Si 30 Cr1 (Experimental Example 4), Al 67 Si 30 Cr3 (Experimental Example 5) and Al 62.5 Si 30 Cr 7.5 When the sample from (Experimental Example 6) was rapidly cooled by gas atomization and then treated with acid, only the crystalline phases of Si and CrSi2 were observed in all compositions, and the stable phase in the equilibrium phase diagram, Al, was not observed. 13 Si4Cr4 was not found. That is, Al 13It was found that Si4Cr4 dissolves in acid treatment. The CrSi2 peak is shifted, and structural analysis revealed that some of the Si sites are replaced with Al, resulting in Cr(Al,Si)2. Al and Al are stable phases in the equilibrium phase diagram. 13 Si4Cr4 was removed by acid treatment, and its diffraction peaks were not observed. CrSi2 is introduced to strengthen the framework, but if its proportion is too high, the proportion of Si, the negative electrode active material, decreases, leading to a reduction in capacity. For these powder samples, α-alumina was added as a standard sample to quantitatively evaluate the proportion of the contained phases.

[0059] Table 1 summarizes the master alloy composition, Si phase and Cr(Al,Si)2 content (mass%) evaluated from XRD patterns, average pore size (nm) and porosity (volume%) evaluated by mercury porosimeter measurement, and compositional analysis results (at%) evaluated by EDX. A certain amount of Si content is necessary to ensure the capacity of the negative electrode. In the compositions of Experimental Examples 1-6, Si content of 58-97 mass% was obtained, indicating that Si was the main phase. Furthermore, a smaller pore size is advantageous for resistance to crushing, and it was found that average pore sizes of 25-60 nm were obtained with these compositions. In addition, compositional analysis of the prepared samples showed that they also contained 4-15 at% Al.

[0060] [Table 1]

[0061] (STEM evaluation results) Figure 11 shows a scanning transmission electron microscope (STEM) image of a cross-section of porous silicon from Experimental Example 2, with Figure 11(a) being a magnified portion of Figure 11(b) and Figure 11(c) showing the elemental distribution of Al, Si, and Cr. It was found that porous silicon particles could be obtained by forming voids by dissolving Al and Al alloy phases in the silicon compound with acid. Furthermore, elemental mapping results showed that the locations of Si and Cr overlapped, indicating that the framework (core) portion has a coexisting structure of Si and Cr silicides. In Experimental Examples 1-6, Al, Cr, and Si were detected in the particle shell portion, indicating that transition metal silicides were segregated in the shell portion, and that a two-layer core-shell structure with different compositions was formed. The content of transition metal silicides in the shell portion derived from this elemental distribution was estimated to be 94.6 mol% of the total porous silicon material. The content of Al in the shell portion derived from this elemental distribution was estimated to be 81 at% of the total porous silicon material. Similarly, in the sample of Experimental Example 4, the content of transition metal silicides in the shell portion was estimated to be 70.8 mol% of the total porous silicon material. Furthermore, the Al content of the shell portion of Experimental Example 4, derived from this elemental distribution, was estimated to be 61.3 at% of the total porous silicon material.

[0062] (Results of core shell size evaluation using image processing) To further analyze the fine structure of the obtained particles, the contrast of the STEM images was adjusted and the images were binarized. Figure 12 is an explanatory diagram of the core and shell portions in the STEM image of the cross-section of porous silicon from Experimental Example 2. Based on this image, the size of the skeleton, pore size, and porosity of the central and surface portions were evaluated using the image processing software ImageJ [Rasband, WS, ImageJ, US National Institutes of Health, Bethesda, Maryland, USA, http: / / rsb.info.nih.gov / ij / , 1997-2007. and Abramoff, MD, Magelhaes, PJ, Ram, SJ "Image Processing with ImageJ". Biophotonics International, vol.11, p.36-42 (2004).].

[0063] Regarding pore size, each pore was approximated as an ellipse, its major and minor axes were measured, and the average value was taken as the pore size. The pore size distribution and average pore size for the core and shell portions were then determined. Figure 13 shows the pore distribution measurement results for the core portion (a) and shell portion (b) of Experimental Example 2. As shown in Figure 13, the core portion had an average pore size distribution of 4 to 85 nm, and the average pore size, averaged by the area ratio distribution of each pore, was 35.8 nm. Similarly, for the shell portion, a distribution range of 4 to 120 nm and an average size of 63.6 nm were confirmed.

[0064] Regarding the size of the skeleton (septum), for example, the thickness was randomly measured at 500 points each in the skeleton of the core and shell parts shown in Figure 12, and the distribution range and average value were determined. Figure 14 shows the results of the skeleton size measurements of the core (a) and shell (b) parts in Experimental Example 2. The core's skeletal structure was distributed in the range of 8 to 29 nm, with an average size of 14.8 nm. In contrast, the shell's structure was distributed in the range of 11 to 39 nm, with an average size of 19.7 nm.

[0065] Table 2 summarizes the width, skeleton size, pore size, porosity, silicide content, and Al content of the core and shell portions of Experimental Example 2, as determined from image analysis. In Table 2, the width of the core portion refers to the diameter of the particle's center. On the other hand, the width of the shell portion refers to the width from the interface between the core and shell portions to the particle surface. The analyzed porosity was slightly smaller than that shown in Table 1. In image analysis, the porosity is evaluated from the area ratio of the cross-sectional portion, assuming that the pore distribution is uniform in the depth direction of the TEM image. However, since TEM also includes information in the depth direction, the skeleton portion located at the back of the image may be counted at the front of the image, potentially leading to an underestimation of the porosity. The width of the core portion was 40-60% of the particle diameter. Furthermore, the shell portion of Experimental Example 2 contained 94.6 mol% silicide of the entire porous silicon material, which was higher than the 5.4 mol% in the core portion. A similar result was observed in Experimental Example 4. Therefore, it was found that the porous silicon material has a two-layer structure in which transition metal silicide is segregated in the shell portion. Similarly, in Experimental Example 2, the shell portion contained 81 at% Al, which was significantly higher than the 19 at% in the core portion. The same was true in Experimental Example 4. Therefore, it was found that the porous silicon material has a two-layer structure in which Al is segregated in the shell portion.

[0066] [Table 2]

[0067] (Pressure resistance evaluation results) The acid-treated powders from Experimental Examples 1-7 were filled into molds, pressurized at pressures of 2-1000 MPa, and their pressure resistance was evaluated by measuring the porosity with a mercury porosimeter. Figure 15 shows the measurement results of the change in porosity with respect to pressure for Experimental Examples 1, 2, 4, 5, and 7. Figure 16 shows SEM images of the appearance of Experimental Examples 2 and 7 at a pressurization of 100 MPa. As shown in Figure 15, the raw material composition is Al without the introduction of a reinforcing phase. 80 Si 20 In Experimental Example 7, the initial porosity was high, but it decreased rapidly with increasing pressure, and after increasing pressure to 1000 MPa, the raw material composition with the reinforced phase was Al72 Si 25 Compared to Experimental Example 2, which used Cr3, the porosity was lower. Also, as shown in Figure 16, in Experimental Example 7, which did not have a reinforcing phase, the spherical particles were crushed and pulverized at a pressurization level of 100 MPa. On the other hand, in Experimental Example 2, which introduced a reinforcing phase, the particles were hardly destroyed and maintained their spherical shape. Thus, it was found that in Experimental Examples 1 to 6, which contain CrSi2 as a reinforcing phase and have a core-shell structure, the mechanical strength of the particles was improved compared to Experimental Example 7, which did not contain a reinforcing phase.

[0068] (Manufacturing of lithium-ion secondary batteries using non-aqueous electrolytes) Negative electrodes were fabricated using silicon particles from Experimental Examples 2, 6, and 8 as negative electrode active materials, and lithium-ion secondary batteries were evaluated. 60% by mass of each negative electrode active material was mixed with 20% by mass of acetylene black with an average particle size of 2 μm and 20% by mass of polyimide as conductive materials. N-methylpyrrolidone was added, and the mixture was stirred to prepare a slurry. Next, this slurry was applied to a 20 μm thick copper foil, dried, and then rolled to produce a 50 μm thick negative electrode. The fabricated negative electrode was punched out into a 16 mm diameter circle, a porous polyethylene separator was placed between the negative electrode and metallic lithium as the counter electrode, and a non-aqueous electrolyte was poured in to produce a test cell as a Tom cell-type small battery cell. The non-aqueous electrolyte was prepared by adding LiPF6 at a concentration of 1 mol / L to a mixed solvent of fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC) in a volume ratio of 1.5:3:4:3. The resulting lithium secondary battery was subjected to 10 cycles of charge and discharge at a current density of 0.2C within a battery voltage range of 0.005 V to 1.5 V.

[0069] (Evaluation of the characteristics of lithium-ion secondary batteries) Table 3 summarizes the raw material's master alloy composition, initial discharge capacity (mAh / g), discharge capacity after 10 cycles (mAh / g), and the capacity retention rate (%) after 10 cycles, calculated from the discharge capacity after 10 cycles relative to the initial discharge capacity. As shown in Table 3, in Experimental Example 8, which had no voids, the capacity retention rate was low at 28%, while in Experimental Examples 2 and 6, the test cells showed a good capacity retention rate of over 95%. Thus, it was inferred that porous silicon materials, having a core portion rich in the reinforcing phase and a shell portion with high porosity, have high particle strength and can further improve charge-discharge characteristics by responding to volume changes due to lithium ion insertion and removal.

[0070] [Table 3]

[0071] 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]

[0072] This disclosure is applicable to the field of secondary batteries. [Explanation of Symbols]

[0073] 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, 22 Core portion, 23 Shell portion, 24 Void.

Claims

1. A porous silicon material, It is particulate, and the transition metal silicide containing the transition metal element M and Si coexist in the framework. The Si phase in the entire particle is 55% by mass or more, and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less. It contains 20% or more by volume of pores smaller than 1 μm, obtained by mercury intrusion. It has a two-layer structure in which the transition metal silicide is segregated in either the central core portion or the surface shell portion. The porous silicon material has a diameter in the range of 0.5 μm to 3 μm, and the length of the core portion occupies in the range of 40% to 60% of the particle diameter. The transition metal element M is Cr. Porous silicon material.

2. The aforementioned core portion, as obtained from electron microscope images, has a skeleton width in the range of 5 nm to 30 nm, and contains pores in the range of 4 nm to 85 nm in the range of 50 volume% to 80 volume%. The porous silicon material according to claim 1, wherein the shell portion, obtained from an electron microscope image, has a skeleton width in the range of 10 nm to 40 nm and contains pores in the range of 4 nm to 120 nm in an area of ​​50 volume% to 80 volume%.

3. The porous silicon material according to claim 1 or 2, having a structure in which the transition metal silicide is segregated in the shell portion.

4. The porous silicon material according to claim 1 or 2, wherein the Al content is in the range of 4 at% to 15 at% and the M content is in the range of 1 at% to 7 at%.

5. The porous silicon material is such that the transition metal silicide is MSI 2 and M(Si,Al) in which Al is in solid solution 2 The porous silicon material according to claim 1 or 2, wherein one or more of the above are present, and the content of the transition metal silicide is 2% by mass or more and 30% by mass or less.

6. A positive electrode containing a positive electrode active material, A negative electrode comprising the porous silicon material described in claim 1 or 2 as a negative electrode active material, An ion-conducting medium interposed between the positive electrode and the negative electrode, which conducts lithium ions, A power storage device equipped with [a specific feature / ability].

7. A precursor step to obtain particulate silicon alloy precursors having a two-layer structure in which Si and transition metal silicide coexist in the pore-forming framework and the transition metal silicide is segregated in either the particle center or the surface framework, by melting and rapidly solidifying the raw materials of an Al-Si-M ternary alloy (where M is a transition metal element) that produces a SiAl alloy or Al alloy via a peritectic reaction, and having a two-layer structure in which Si and transition metal silicide coexist in the framework that forms the pores, and the transition metal silicide is segregated in either the particle center or the surface framework, The process includes removing the Al component contained in the silicon alloy to obtain a particulate porous silicon material having a two-layer structure in which the transition metal silicide and Si coexist in the skeletal portion, the Si phase in the entire particle is 55% by mass or more and the transition metal silicide is in the range of 1% by mass or more and 45% by mass or less, and the transition metal silicide is segregated in either the core portion at the center of the particle or the shell portion on the surface, and A method for producing a porous silicon material, wherein in the precursor step and the porosizing step, the porous silicon material is obtained in which the diameter is in the range of 0.5 μm to 3 μm, the length of the core portion is in the range of 40% to 60% of the particle diameter, and the transition metal element M is Cr.

8. In the aforementioned precursor step, the basic composition formula is Al 100-x-y Si x M y A method for producing a porous silicon material according to claim 7, wherein the raw materials are in the range of 18 ≤ x ≤ 40 and 0.1 ≤ y ≤ 15.

9. The method for producing a porous silicon material according to claim 7 or 8, wherein the Al component is selectively removed by an acid or alkali in the porosification step.

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