JP2023092861A - Method of producing porous silicon material, porous silicon material, and storage device - Google Patents

Abstract

To provide a method for producing a porous silicon material that allows the decrease in charge-discharge characteristics to be more suppressed in materials containing Si.SOLUTION: A method for producing a porous silicon material of the present disclosure includes a precursor step of obtaining a precursor of a silicon alloy by melting and rapidly solidifying a raw material containing 1 at% to 20 at% Cr, 40 at% to 90 at% Al, and the remainder Si, when the total content of Si, Al, and Cr is 100 at%, and a porosity-increasing step of obtaining a porous silicon material by removing the Al component contained in the silicon alloy.SELECTED DRAWING: None

Description

Disclosed herein are methods of making porous silicon materials, porous silicon materials, and electrical storage devices.

Conventionally, in a silicon negative electrode material, after mixing 70 parts by mass of bulk Si and 30 parts by mass of Al powder, the alloy was melted in an argon atmosphere and granulated by a gas atomization method using helium gas, and then Al was removed with hydrochloric acid. Porous silicon obtained by the method has been proposed (see, for example, Patent Document 1). It is said that this porous silicon can completely suppress pulverization due to expansion and contraction of the volume of the active material during charging and discharging, separation of the active material from the current collector, and lack of contact with the conductive material. In addition, as a method for producing a silicon material, an intermediate alloy element containing Mg, Co, Cr, Cu, Fe, etc., and a Si alloy with Si are mixed in a molten metal containing a predetermined molten metal element as an intermediate alloy element and a molten metal element. It is proposed to obtain a porous silicon material by separating a second phase substituted with Si particles and removing the second phase (see, for example, Patent Document 2). It is said that this porous silicon material can have high capacity and high cycle characteristics.

JP 2004-214054 A JP 2012-82125 A

However, in the manufacturing method of Patent Document 1 described above, although an alloy containing Si and Al is used to form a porous structure, it is still not sufficient to suppress problems due to expansion and contraction of Si. In the method for producing a porous silicon material of Patent Document 2, a silicon alloy containing an intermediate alloy element is melted and replaced in a molten metal containing a predetermined molten metal element, that is, a treatment at a high temperature is required, and a simple manufacturing process is required. was sought.

The present disclosure has been made in view of such problems, and the main purpose of the present disclosure is to provide a method for producing a porous silicon material, a porous silicon material, and an electricity storage device that can further suppress deterioration in charge and discharge characteristics. and

As a result of intensive research to achieve the above-described object, the present inventors have produced a silicon alloy containing Al and Cr, and have found that when the compound containing Al is removed, it has a stronger structure. The present inventors have found that it is possible to obtain a porous silicon material capable of further suppressing deterioration in properties, and have completed the method for producing a porous silicon material, the porous silicon material, and the electric storage device of the present disclosure.

That is, the method for producing a porous silicon material of the present disclosure includes:
When the total of Si, Al, and Cr is 100 at%, a raw material containing Cr in a range of 1 at% or more and 20 at% or less, Al in a range of 40 at% or more and 90 at% or less, and the balance being Si is melted. a precursor step of obtaining a silicon alloy precursor by rapid cooling and solidification;
a porosification step of removing the Al component contained in the silicon alloy to obtain a porous silicon material;
includes.

The porous silicon material of the present disclosure is
An average pore diameter determined by a mercury intrusion method is in the range of 60 nm or less, contains skeleton-like silicon with a three-dimensional network structure having voids, contains a SiCr compound and/or an AlSiCr compound, and has a porosity of 30% by volume or more85 It is in the range of vol% or less.

The power storage device of the present disclosure is
a positive electrode comprising a positive electrode active material;
a negative electrode containing the porous silicon material described above as a negative electrode active material;
an ion conducting medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
is provided.

INDUSTRIAL APPLICABILITY The present disclosure can further suppress deterioration in charge-discharge characteristics in materials containing Si. The reason why such an effect is obtained is presumed as follows. For example, a silicon negative electrode for a lithium ion secondary battery has a theoretical capacity of 4199 mAh/g, which is about 10 times higher than the theoretical capacity of general graphite, 372 mAh/g. is expected. On the other hand, silicon that occludes lithium ions is Li _4.4 Si, and its volume expands to about four times that of silicon before lithium ions are occluded. In the present disclosure, by selectively dissolving elements or compounds other than Si and Si—Cr compounds, porous silicon with small pore size and high porosity can be produced. In addition, only by removing elements or compounds other than the SiCr compound, porous silicon can be easily provided in the air without the need for large-scale equipment. The obtained porous silicon material has a three-dimensional network structure of skeletal silicon with voids reinforced with a SiCr compound, and an average pore size of 60 nm or less. Such a porous silicon material with a small pore size, a large porosity, and a reinforced skeleton, when used in an electricity storage device such as a lithium-ion secondary battery, can reduce volumetric expansion and contraction, and can be cycled. An electricity storage device with high performance such as improved characteristics can be easily obtained.

Al-Si-Cr phase diagram at 25 at% Si. FIG. 4 is a schematic diagram of a microstructure forming process of the outermost surface layer in the cooling process of the melt droplet. Al-Si-Cr phase diagram in Al _85-y Si ₁₅ Cr _y . Al-Si-Cr phase diagram in Al _80-y Si ₂₀ Cr _y . Al-Si-Cr phase diagram in Al _75-y Si ₂₅ Cr _y . Al-Si-Cr phase diagram in Al _70-y Si ₃₀ Cr _y . Al-Si-Cr phase diagram in Al _60-y Si ₄₀ Cr _y . Explanatory drawing of the range of the eutectic composition in the Al-Si-Cr system composition. FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10; 5 is a SEM image of a cross section of porous silicon of Experimental Example 1. FIG. 4 is a SEM image of a cross section of porous silicon of Experimental Example 2. FIG. XRD measurement results before and after acid treatment in Experimental Examples 1 and 2. Pore distribution curves of the porous silicon materials of Experimental Examples 1, 2 and 16. FIG. 3 is a relationship diagram of Si phase content with respect to x/y in Experimental Examples 3 to 11; FIG. ₂ is a relational diagram between the ratio of CrSi 2 phase and the amount of change in porosity after pressing.

(Method for producing porous silicon material)
A method for producing a porous silicon material of the present disclosure includes a precursor step and a porosification step. In the precursor step, a raw material containing Si, Al, and Cr is melted and rapidly solidified to obtain a silicon alloy precursor. In the porosification step, the Al component contained in the silicon alloy is removed to obtain a porous silicon material. First, the raw material composition will be described.

FIG. 1 is an Al--Si--Cr phase diagram at 25 at % Si. FIG. 2 is a schematic diagram of the microstructure formation process of the outermost surface layer in the cooling process of the melt droplet. Since the strength of CrSi ₂ , which is a SiCr compound, is about twice that of Si, the skeleton strength can be improved by coexisting CrSi ₂ as a reinforcing phase in the Si skeleton. The refinement of pores by adding Cr is influenced by the characteristics of the phase diagram of the Al--Si--Cr system. Here, a case of cooling the alloying liquid by the gas atomization method will be described as an example. In the gas atomization method, a high-temperature melt is injected into high-pressure gas through a hole at the tip of a nozzle to cool it. At this time, when the melt (Fig. 1 (1)) is cooled to the temperature shown in Fig. 1 (2), CrSi ₂ crystallizes first from the surface of the droplet with the lower temperature (Fig. 2 (2)), but CrSi ₂ gradually coarsens in the coexistence temperature region with the melt (Fig. 1(3)) (Fig. 2(3)). At that time, if CrSi ₂ becomes too coarse, the generated silicon powder is covered with shell-like inert CrSi ₂ (FIG. 2 (3-2)), and carrier ions (Li) are formed on the surface of the active material. This is not preferable because it reduces the number of conduction paths. However, in the _phase diagram of the AlSiCr system, _CrSi2 is decomposed ( _peritectic reaction) when cooled to the temperature shown in FIG _. is more preferable because active carrier ion conduction paths (that is, Si phase) are formed on the surface of the active material. Furthermore, this peritectic reaction generally refers to a reaction in which the melt reacts with the solid phase to generate another solid phase, such as L + CrSi ₂ ⇒ _{Al 13} Si ₄ Cr ₄ , but from the surface of the solid phase particle As the reaction progresses, as shown in FIG. 2(4), a structure in which the fine CrSi ₂ particles are covered with Al ₁₃ Si ₄ Cr ₄ and Si shells tends to form, and a finer microstructure is formed. Realized. That is, in addition to the fineness of Si particles and the strengthening phase CrSi ₂ , the fineness of the Al alloy, which is the source of pores, is also made finer. Furthermore, since the precipitation temperature difference between the strengthening phase, Si, and the specific oxidation removal phase (Al) is relatively small (up to 350° C.), crystal growth of these particles is small, and a nano-sized microstructure can be realized. The solidification structure forms a lamellar structure composed of SiCr compound, AlSiCr compound, AlCr compound, eutectic Si, primary crystal Si, and primary crystal Al. Compounds that dissolve in acid include primary crystal Al, AlSiCr compounds, and AlCr compounds, and the structure after acid treatment is a framework-like porous body composed of the remaining SiCr compound, eutectic Si, and primary crystal Si. It is assumed that

3 to 7 are phase diagrams of Al--Si--Cr alloys when the contents of Si and Cr are varied. FIG. 3 is an Al--Si--Cr phase diagram in Al _85-y Si ₁₅ Cr _y . FIG. 4 is an Al--Si--Cr phase diagram in Al _80-y Si ₂₀ Cr _y . FIG. 5 is an Al--Si--Cr phase diagram in Al _75-y Si ₂₅ Cr _y . FIG. 6 is an Al--Si--Cr phase diagram in Al _70-y Si ₃₀ Cr _y . FIG. 7 is an Al--Si--Cr phase diagram in Al _60-y Si ₄₀ Cr _y . FIG. 8 is an explanatory diagram of the eutectic composition range in the Al—Si—Cr system composition. In general, in order to improve the strength by nanocompositing, it is necessary that the strengthening phase such as the _CrSi phase is as fine as possible. In the figure, it is preferable that the crystallization temperatures of Si and silicide, which is a strengthening phase, are close to each other. In other words, the ideal composition range is such that Si and silicide, which is a strengthening phase, precipitate simultaneously through a eutectic reaction. As shown in FIGS. 3 to 7, in the basic composition of Al _100-xy Si _x Cr _y , when the Si content x is less than 20 at %, Si and silicide do not crystallize by eutectic reaction, In a composition where the Si content x is 20 at % or more, particularly 25 at % or more, depending on the Cr content y, L⇒Si+CrSi ₂ (eutectic reaction 1) and L⇒Si+Al ₁₃ Si ₄ Cr ₄ (eutectic reaction 2). There are compositions that produce eutectic reactions. FIG. 8 is a diagram showing the relationship between the amount of Si x and the amount of Cr y that cause these eutectic reactions 1 and 2. In the shaded composition region, Si and _CrSi2 crystallize at relatively close temperatures. In addition, since the peritectic reaction of CrSi ₂ +L ⇒ Al ₁₃ Si ₄ Cr ₄ occurs, a finer reconsidered structure can be realized, and in order to realize a porous silicon material with particularly good cycle characteristics, a more preferable composition presumed to be within the range.

(Precursor step)
In the precursor step, when the total of Si, Cr, and Al is 100 at%, Cr is contained in a range of 1 at% or more and 20 at% or less, Al is contained in a range of 40 at% or more and 90 at% or less, and the balance is Si. It is preferable to use a raw material with The raw material may contain unavoidable impurities. The unavoidable impurities are components that inevitably remain during the purification of any one of Si, Cr, and Al, and examples thereof include Fe, C, Cu, Ni, and P. The amount of unavoidable impurities is preferably as small as possible. For example, when the total of Si, Cr, and Al is 100 at %, the content is preferably 5 at % or less, more preferably 2 at % or less. The compounding ratio of Cr is, for example, preferably 2 at % or more, and may be 3 at % or more. Moreover, the compounding ratio of Cr is preferably 15 at % or less, and may be 12.5 at % or less. The compounding ratio of Al is preferably 50 at % or more, and may be 55 at % or more or 60 at % or more. Moreover, the compounding ratio of Al is preferably 85 at % or less, more preferably 80 at % or less, and may be 77.5 at % or less. The compounding ratio of Si is, for example, preferably 9 at % or more, more preferably 12 at % or more, and may be 20 at % or more or 25 at % or more. The compounding ratio of Si is, for example, preferably 59 at % or less, more preferably 50 at % or less, and may be 40 at % or less or 30 at % or less. A silicon alloy containing Al or Cr within such a range is preferable because the porosity can be further increased and pores having a more suitable shape and size can be obtained. When the Al content is high, a single phase of Al precipitates largely when the alloy is rapidly cooled after being melted to form an alloy, so that many voids can be formed. The cooling rate is preferably more rapid, and may be, for example, in the range of 10 ² ° C./s or more and 10 ⁸ ° C./s or less from the molten state.

In this step, it is preferable to use raw materials in which x/y is in the range of 3 or more and 10 or less in the basic composition formula Al _100-xy Si _x Cr _y . The Si powder is black to dark brown. On the other hand, CrSi ₂ is a silvery intermetallic compound and its powder is black. When x/y is 3 or more, the Si material has the same dark brown color as silicon, which is preferable because it is not oxidized. If an oxide is mixed in, the actual battery capacity of Si per unit mass is lowered, which is not preferable. That is, x/y is more preferably 3 or more. On the other hand, if the amount of Cr is small, the reinforcement of the skeleton by the SiCr compound becomes weak, so x/y is preferably 10 or less, more preferably 8.5 or less, and even more preferably 6 or less.

In this step, when the raw material is melted, high-frequency crucible melting in an inert gas atmosphere such as Ar is preferable, but any melting method may be used. In the precursor step, the alloy obtained from the raw material may be granulated. In this granulation treatment, the molten metal of the raw material of the silicon alloy may be cast in a mold, and the obtained ingot may be crushed and granulated. In addition, as a method for granulating the silicon alloy, one or more of the gas atomization method, water atomization method, roll quenching method, and the like may be used for the molten silicon alloy. An alloy powder is obtained by the gas atomization method and the water atomization method. On the other hand, since the ribbon alloy can be obtained by the roll quenching method, it may be pulverized to powder afterward. Since the powder obtained by the roll quenching method has a fine alloy structure, porous silicon having fine pores can be obtained after the elution treatment. Among these methods, the gas atomization method is more preferable as the method for granulating the silicon alloy. Gas atomization is preferably carried out in an Ar atmosphere when forming a molten metal, and is preferably carried out in an Ar or He atmosphere when forming particles.

In the precursor step, it is preferable to granulate the silicon alloy with an average particle size in the range of 0.1 μm or more and 100 μm or less. For example, the particles preferably have an average particle diameter of 0.5 μm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less, and even more preferably 1 μm or more and 3 μm or less. The silicon alloy particles may be appropriately selected according to the properties required for the electricity storage device. Here, the average particle diameter of the particles is obtained by observing the particles with a scanning electron microscope (SEM), totaling the major diameter of each particle as the diameter of the particle, and dividing by the number of particles to obtain an average value. . The particles obtained by this granulation treatment have the average particle size of the aggregate of porous particles to be finally obtained.

In this precursor step, a raw material containing Al, Cr, and Si and a second element containing one or more of Ca, Cu, Mg, Na, Sr, and P may be used. Among these, one or more of Ca, Na and Sr is preferable as the second element. The content of the second element is preferably less than the content of Al and Cr, for example, relative to the entire silicon alloy. A range of 10% by mass or less is preferred, and a range of 5% by mass or less is more preferred.

(Porosification step)
In the porosification step, a process for removing substances other than Si from the silicon alloy particles produced above is performed. Substances other than Si include, for example, Al and its compounds, and Cr and its compounds. In this step, it is preferable to selectively remove the Al component, that is, the Al phase and its compounds with acid or alkali. The acid or alkali to be used is preferably one that dissolves elements and/or compounds other than silicon in the silicon alloy and does not dissolve silicon, and examples thereof include hydrochloric acid, sulfuric acid, and sodium hydroxide. This acid or alkali is preferably an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it can remove Al and its compounds, and Cr and its compounds. This removal treatment may be performed by heating at, for example, 30.degree. C. to 60.degree. The removal treatment is preferably carried out by immersing the silicon alloy particles in an acid or alkaline solution and stirring for about 1 to 5 hours. The porous silicon material obtained is then washed and dried.

In the porosification step, substances other than Si may be removed in a range of 85% by mass or more and 100% by mass or less. For example, Al, Cr, and other oxygen may remain, but when used as an electrode active material, the smaller the amount, the better, from the viewpoint of the charge/discharge capacity. In addition, it is preferable that components such as Al and Cr are contained in a predetermined amount or more from the viewpoint of reinforcing the silicon skeleton and improving durability. This step may result in a porous silicon material comprising a SiCr compound. Examples of the SiCr compound include Si _x Cr _y (where x and y are arbitrary numbers) such as CrSi ₂ . CrSi ₂ as the SiCr compound may include Cr(Si, Al) ₂ in which part of Si is substituted with Al. The AlSiCr compound may be, for example, an Al _a Si _b Cr _c compound (where a, b, and c are arbitrary numbers), such as Al ₁₃ Si ₄ Cr ₄ . The SiCr compound may be sparingly soluble in acid or alkali.

In the porosification step, it is preferable to obtain a porous silicon material containing a SiCr compound in the range of 5 mol % or more and 20 mol % or less with respect to the Si phase. The SiCr compound content in the porous silicon material is preferably 7.5 mol % or more, and may be 10 mol % or more. The SiCr compound content in the porous silicon material is preferably 15 mol % or less, and may be 12.5 mol % or less. This step preferably results in a porous silicon material with residual CrSi ₂ . Moreover, in this step, it is preferable to obtain a porous silicon material containing an AlSiCr compound in an amount of 10 mol % or less, more preferably 5 mol % or less, and still more preferably 3 mol % or less.

In the porosification step, a porous silicon material having a porosity in the range of 30% by volume or more and 85% by volume or less may be obtained. This porosity is a value measured with a mercury porosimeter. The porosity is, for example, preferably 35% by volume or more, and may be 40% by volume or more. Also, the porosity is, for example, preferably 80% by volume or less, more preferably 77.5% by volume or less, and may be 75% by volume or less. If the porosity is larger, it is easier to respond to the volume change when carrier ions are occluded, and if it is smaller, the amount of Si present per unit volume increases, which is preferable.

(porous silicon material)
The porous silicon material of the present disclosure may be made by the manufacturing method described above. Here, the physical properties of the porous silicon material are the same as those of the above-described manufacturing method, and detailed description thereof will be omitted. This porous silicon material has an average pore size of 60 nm or less as determined by mercury porosimetry. The 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 may be 20 nm or more. Moreover, the pore diameter is preferably 50 nm or less, and may be 45 nm or less. When the pore size is small, the pores are less likely to collapse, which is preferable. Moreover, when the pores are large, it is preferable because the volume change can be further suppressed when the carrier ions are occluded.

The porous silicon material includes skeleton-like silicon having a three-dimensional network structure with voids, and includes a SiCr compound and/or an AlSiCr compound. This SiCr compound and AlSiCr compound are presumed to reinforce the silicon skeleton. This porous silicon material preferably contains a SiCr compound in the range of 5 mol % or more and 20 mol % or less with respect to the Si phase. Moreover, this porous silicon material preferably contains a SiCr compound in the range of 1% by mass or more and 15% by mass or less. Also, the porous silicon material preferably contains an AlSiCr compound in the range of 0.1% by mass or more and 10% by mass or less. The amount of the SiCr compound and the AlSiCr compound is preferably larger from the viewpoint of reinforcing the skeleton, and preferably smaller from the viewpoint of the charge/discharge capacity of the electricity storage device.

The porous silicon material has a porosity of 30% by volume or more and 85% by volume or less as determined by mercury porosimetry. The porosity is, for example, preferably 35% by volume or more, and may be 40% by volume or more. Also, the porosity is, for example, preferably 80% by volume or less, more preferably 77.5% by volume or less, and may be 75% by volume or less. If the porosity is larger, it is easier to respond to the volume change when carrier ions are occluded, and if it is smaller, the amount of Si present per unit volume increases, which is preferable.

The porous silicon material preferably has pores of 1 μm or less measured by mercury porosimetry in an amount of 20% by volume or more. Porous silicon materials preferably have more such fine pores. The porous silicon material preferably contains 30% by volume or more, more preferably 40% by volume or more, of pores of 1 μm or less measured by mercury porosimetry. In addition, the porous silicon material may contain 90% by volume or less of pores having a diameter of 1 μm or less as determined by mercury porosimetry.

The porous silicon material preferably has a pore size change of less than 25% by volume, more preferably less than 20% by volume, and less than 10% by volume when subjected to a confining pressure of 1 GPa as an electrode. is more preferred. From the viewpoint of skeleton strength, it is preferable that the porous silicon material has a smaller reduction in pores when subjected to a confining pressure.

The average particle diameter of the porous silicon material is preferably 0.1 μm or more, more preferably 1 μm or more, and may be 5 μm or more. In addition, the particles preferably have an average particle size of 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. This porous silicon material preferably contains 70 at % or more of Si, excluding oxygen and unavoidable impurities, when the total of Si, Al and Cr is 100 at %. The Si content is more preferably 75 at % or higher, still more preferably 80 at % or higher, and may be 85 at % or higher. The content of Si is preferably higher from the viewpoint of charge/discharge capacity, and may be lower from the viewpoint of relative skeleton reinforcement. The Al content is preferably in the range of 0.5 at % or more and 15 at % or less, preferably 12.5 at % or less, more preferably 10 at % or less, and may be 7.5 at % or less. Also, the Al content is more preferably 1 at % or more, more preferably 2 at % or more, and may be 2.5 at % or more. The Cr content is preferably in the range of 0.5 atomic % or more and 20 atomic % or less. The Cr content is more preferably 17.5 at % or less, still more preferably 15 at % or less, and may be 12 at % or less. Also, the Cr content is more preferably 1 at % or more, more preferably 4.5 at % or more, and even more preferably 8 at % or more. The content of Al and Cr is preferably higher from the viewpoint of reinforcing the skeleton, and is preferably lower from the viewpoint of the charge/discharge capacity of the electricity storage device because they are components that do not charge and discharge. Furthermore, the porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr and P as the second element in a range of 15% by mass or less. Also, the porous silicon material may contain unavoidable impurities in addition to Si, Al, and Cr. In addition, it is preferable that the amount of the second element and unavoidable impurities be as small as possible.

(Electrodes for power storage devices)
An electric storage device electrode includes the porous silicon material described above as an electrode active material. This electrode becomes either a positive electrode or a negative electrode based on the potential of the counter electrode with respect to the potential of the electrode active material. When lithium is used as a carrier, it is preferably a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, air batteries, and the like. In the electric storage device electrode, the porosity of the porous silicon material may be compressed to a range of 5 volume % or more and 50 volume % or less. In this electrode, the porosity of the porous silicon material may be reduced by compressing during fabrication. For example, when porous silicon particles are used as the negative electrode active material of a lithium ion secondary battery, the smaller the pores, the more uniformly the lithium ions are alloyed, so the stress concentration is reduced, and the electrode itself deterioration can be prevented. The porosity of the porous silicon material after compression may be appropriately adjusted according to the characteristics required for the electrode for an electricity storage device, and may be, for example, 5% by volume or more, or 10% by volume or more. Moreover, the porosity of the porous silicon material after compression may be, for example, 30% by volume or less, or 20% by volume or less.

The electricity storage device electrode may be formed by forming the above-described porous silicon material on a current collector and adhering it to the current collector. This electrode is produced by mixing a porous silicon material with a conductive material, a binder, and a solvent as necessary, making a paste, and applying the porous silicon material on a current collector. It can be produced by a process of mixing with a binder and crimping onto a current collector. In this electrode, the content of the porous silicon material is preferably larger, 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. One or a mixture of two or more of black, carbon whiskers, needle coke, carbon fibers, metals (copper, nickel, aluminum, silver, gold, etc.) can be used. The binder plays a role of binding the active material particles and the conductive material particles, and is, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Cellulose-based or styrene-butadiene rubber (SBR) aqueous dispersions, which are water-based binders, can also be used. Examples of solvents that can be used include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. . Alternatively, a dispersant, a thickener, or the like may be added to water, and the active material may be slurried with a latex such as SBR. Application methods include, for example, roller coating such as applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any thickness and shape can be obtained using any of these methods. The current collector may be appropriately selected depending on the potential of the active material, and examples thereof include aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymer, conductive glass, etc. For the purpose of improving adhesion, electrical conductivity and oxidation resistance, it is possible to use aluminum, copper or the like whose surface has been treated with carbon, nickel, titanium, silver or the like. For these, it is also possible to oxidize the surface. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and fiber group formed bodies. The current collector used has a thickness of, for example, 1 to 500 μm. The amount of the active material complex formed may be appropriately set according to the desired performance required for the electricity storage device.

In this electrode, the electrode active material may contain, in addition to the porous silicon material, an active material other than the porous silicon material within a range in which both the low confining pressure and the capacity retention rate can be achieved. For example, the electrode active material may contain a carbonaceous material, Li ₄ Cr ₅ O ₁₂ , or the like. However, from the viewpoint of further increasing the battery capacity, the porous silicon material preferably accounts for 50% by mass or more, preferably 90% by mass or more, for example, when the entire electrode active material is 100% by mass.

(storage device)
The power storage device of the present disclosure includes an electrode having the porous silicon material described above. The electricity storage device may include a positive electrode, a negative electrode, and an ion-conducting medium that is interposed between the positive electrode and the negative electrode and conducts carrier ions. A 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, and the like. In the positive electrode, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used as a positive electrode active material. Specifically, transition metal sulfides such as CrS ₂ , CrS ₃ , MoS ₃ and FeS ₂ , and basic compositional formulas such as Li _(1-x) MnO ₂ (0<x<1, etc., hereinafter the same) and Li _{(1 -x)} Lithium-manganese composite oxides such as Mn ₂ O ₄ , Lithium-cobalt composite oxides such as Li _(1-x) CoO ₂ with a basic composition formula, Li _(1-x) NiO ₂ with a basic composition formula, etc. a lithium-nickel composite oxide having a basic composition formula of Li _(1-x) Ni _a Co _b Mn _c O ₂ (a+b+c=1) or the like, a basic composition formula of LiV ₂ O ₃ Lithium vanadium composite oxides such as V ₂ O 5 , transition metal oxides having a basic composition formula of V 2 O ₅ , and the like can be used. Among these, transition metal composite oxides of lithium such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ and Li _(1-x) Ni _1/3 Co _1/3 Mn _1/3 O ₂ are preferred. The term "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 capacitors, lithium ion capacitors, and the like. Carbonaceous materials include, for example, activated carbons, cokes, vitreous carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m ² /g or more, more preferably 1500 m ² /g or more. When the specific surface area is 1000 m ² /g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m ² /g or less, more preferably 2000 m ² /g or less, for ease of production. As the conductive material, binder, solvent, current collector, and the like used for the positive electrode, those exemplified for the electrode described above can be appropriately used.

A non-aqueous electrolytic solution containing a supporting salt, a non-aqueous gel electrolytic solution, or the like can be used as the ion-conducting medium. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, the carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -chain carbonates such as butyl carbonate, di-i-propyl carbonate and t-butyl-i-propyl carbonate, cyclic esters such as γ-butyl lactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; Dioxolanes such as sulfolanes, 1,3-dioxolane, and methyldioxolane are included. Among these, a combination of cyclic carbonates and chain carbonates is preferred. According to this combination, not only is the cycle characteristic, which represents the battery characteristics in repeated charging and discharging, excellent, but also the viscosity of the electrolytic solution, the resulting electric capacity of the battery, the battery output, etc. can be well-balanced. can. _The supporting salt is, for example, _LiPF6 , _LiBF4 , _LiAsF6 , LiCF3SO3, LiN( _{CF3SO2 ) 2} , LiC( _{CF3SO2 ) 3 ,} LiSbF6, _LiSiF6 , _LiAlF4 , _LiSCN , LiClO ₄ , LiCl, LiF, LiBr, LiI, _LiAlCl4 and the like. Among them, 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 From the viewpoint of electrical properties, it is preferable to use one selected salt or a combination of two or more selected salts. The concentration of the supporting salt in the non-aqueous electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, more preferably 0.5 mol/L or more and 2 mol/L or less. When the supporting electrolyte is dissolved in a concentration of 0.1 mol/L or more, a sufficient current density can be obtained, and when it is 5 mol/L or less, the electrolytic solution can be made more stable. In addition, a phosphorus-based or halogen-based flame retardant may be added to the non-aqueous electrolyte.

A solid ion-conducting polymer can also be used as the ion-conducting medium instead of the liquid ion-conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, and vinylidene fluoride and a supporting salt can be used. Furthermore, an ion conductive polymer and a non-aqueous electrolytic solution can be used in combination. As the ion conducting medium, in addition to the ion conducting polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder can be used.

The electricity storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. Porous membranes are mentioned. These may be used alone, or may be used in combination.

The shape of the electricity storage device is not particularly limited, and examples thereof include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular shapes. Also, it may be applied to a large-sized one used for an electric vehicle or the like. FIG. 9 is an explanatory diagram showing an example of the structure of the electricity storage device 10. As shown in FIG. This electricity storage device 10 has a positive electrode 12 , a negative electrode 15 and an ion conducting medium 18 . The positive electrode 12 has a positive electrode active material 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 above-described porous silicon material 21 and has voids 23 .

This electricity storage device preferably has a higher capacity retention rate when subjected to charge-discharge cycles. For example, the capacity retention rate in 10 cycles is preferably 95% or more, more preferably 97.5% or more, and 98%. It is more preferable that it is above.

(all solid state lithium ion secondary battery)
This electric storage device is preferably an all-solid lithium ion secondary battery. The all-solid-state battery is preferable because it is possible to further suppress changes in performance due to the electrolytic solution and to further improve safety. This all-solid-state lithium-ion secondary battery includes a positive electrode containing a positive electrode active material, a negative electrode that is the above-described electricity storage device electrode, and a solid electrolyte that is interposed between the positive electrode and the negative electrode and conducts lithium ions. It can be a thing. For the positive electrode, any one of the power storage devices described above can be used. Moreover, the electrode for electrical storage devices mentioned above can be used for a negative electrode.

The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La and Zr. This solid electrolyte may have a basic composition of Li7.0 _+xy (La3 _-x , _Ax )(Zr2 _-y , _Ty ) _O12 . However, A is at least one of Sr and Ca, and T is at least one of Nb and Ta, satisfying 0<x≦1.0 and 0<y<0.75. Alternatively, the solid electrolyte has a basic composition of ( _{Li7-3z+xyMz )} ( _{La3-xAx )} ( _{Zr2-yTy ) O12} or ( _{Li7-3z+xyMz )} (La _3- xA _x )(Y _2-y T _y )O ₁₂ may be a garnet-type oxide. 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 0 ≤ z ≤ 0.2, 0 ≤ x ≤0.2, 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. Moreover, 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.

Alternatively, as the solid electrolyte, for example, general Li ₃ N, Li ₁₄ Zn(GeO ₄ ) ₄ called LISICON, sulfide Li _3.25 Ge _0.25 P _0.75 S ₄ , perovskite La _0.5 Li _0.5 CrO ₃ , (La _2/3 Li _3x □ _1/3-2x )CrO ₃ (□: atomic vacancies), garnet-type Li ₇ La ₃ Zr ₂ O ₁₂ , NASICON-type LiCr ₂ (PO ₄ ) ₃ , Li _1.3M0.3Cr1.7 ( _PO3 ) ₄ (M=Sc, Al) _and the _like . In addition, Li ₇ P ₃ S ₁₁ obtained from glass having a composition of 80Li ₂ S·20P ₂ S ₅ (mol %), which is a glass-ceramic, and Li ₁₀ Ge ₂ PS, which is a sulfide-based substance with high electrical conductivity. ₂ are also mentioned. Li ₂ S--SiS ₂ , Li ₂ S--SiS ₂ --LiI, Li ₂ S--SiS ₂ --Li ₃ PO ₄ , Li ₂ S--SiS ₂ --Li ₄ SiO ₄ , and Li ₂ S-- as glass-based inorganic solid electrolytes. P ₂ S ₅ , Li ₃ PO ₄ —Li ₄ SiO ₄ , Li ₃ BO ₄ —Li ₄ SiO ₄ , and SiO ₂ , GeO ₂ , B ₂ O ₃ , and P ₂ O ₅ as glass-based materials, and Li ₂ O Examples include those used as network modifiers. Li ₂ S--GeS ₂ system, Li ₂ S--GeS ₂ --ZnS system, Li ₂ S--Ga ₂ S ₂ system, Li ₂ S--GeS ₂ --Ga ₂ S ₃ system, and Li ₂ S as thiolysicone solid electrolytes. _{-GeS2- P2S5 system ,} Li2S _{- GeS2} - _SbS5 system _{, Li2S - GeS2} - _Al2S3 system, _Li2S - _SiS2 system, _Li2SP2S5 system, Li ₂ S--Al ₂ S ₃ system, LiS--SiS ₂ --Al ₂ S ₃ system, and Li ₂ S--SiS ₂ --P ₂ S ₅ system. These solid electrolytes may be formed in a plate shape and arranged between the positive electrode and the negative electrode.

Further, the all-solid-state lithium ion secondary battery may be provided with a restraining member that restrains the laminate in which the positive electrode, the solid electrolyte, and the negative electrode are laminated in the stacking direction. This restraining member includes, for example, a pair of plate-like portions that sandwich the laminate from both ends in the stacking direction of the laminate, a rod-like portion that connects the pair of plate-like portions, and a screw structure that is connected to the rod-like portions. An adjusting portion for adjusting the interval between the pair of plate-like portions may also be provided.

As described in detail above, the present disclosure can further suppress deterioration in charge-discharge characteristics in materials containing Si. The reason why such an effect is obtained is presumed as follows. For example, a high-strength porous silicon material having nano-sized pores and having a skeleton reinforced with a nano-sized SiCr compound by removing Al-containing compounds from an Al-Si-Cr ternary alloy. can be easily mass-produced. The nanopores relax the stress associated with the expansion and contraction of silicon during charging and discharging. At the same time, by introducing an SiCr compound that is inert to carrier ions into the silicon skeleton, the strength of the skeleton can be improved, and the pore structure of silicon can be prevented from being crushed by the stress associated with expansion and contraction during charging and discharging. Therefore, charge-discharge cycle characteristics can be improved. In addition, by adding Cr to the AlSi alloy, the pore size can be made finer than the porous silicon produced by rapidly solidifying and acid-treating a similar AlSi binary alloy, so residual stress It is possible to improve the effect of suppressing collapse of the pore structure by.

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

Hereinafter, specific examples of fabricating the porous silicon and the electricity storage device of the present disclosure will be described as experimental examples. Experimental Examples 1-13 are examples of the present disclosure, and Experimental Examples 14-17 are comparative examples.

[Preparation of porous silicon material]
(Experimental Examples 1 and 2)
Raw materials of Al, Si and Cr were weighed so as to have a composition of the basic composition formula Al _100-xy Si _x Cr _y (x=10-40, y=2-15) and melted in an arc melting furnace. Before melting, the inside of the arc melting furnace was evacuated to 8×10 ^-3 Pa or less and then replaced with Ar gas. The production of the mother alloy requires melting of the raw material powder, and high-frequency melting was performed to produce a uniform sample. The obtained mother alloy was heated to 1000 to 1300° C. in an Ar atmosphere to melt, and was subjected to rapid solidification at a rate of 102 K/sec or higher using gas atomization to obtain AlSiCr alloy powder (precursor step). The obtained alloy was immersed in a 3N hydrochloric acid aqueous solution and treated at 50° C. for 5 hours to selectively remove Al. The residue is transferred to a filtration filter, and after removing the acid after treatment by pressure filtration, it is washed with distilled water four times or more, and the washing water is removed by the same pressure filtration method to obtain porous silicon. (Porosification step). A porous silicon material obtained from the master alloy composition Al ₇₂ Si ₂₅ Cr ₃ where x=25 and y=3 in the above basic composition formula was used as Experimental Example 1. Experimental Example 2 is a porous silicon material obtained from a master alloy composition of Al _62.5 Si ₃₀ Cr _7.5 with x=30 and y=7.5.

(Experimental Examples 3 to 13)
Porous silicon obtained through the same steps as in Experimental Example 1 except that the mother alloy composition of x = 15 and y = 5 in the above basic composition formula was rapidly solidified by a single roll method and then made porous by acid treatment. Experimental Example 3 was used as the material. Experimental Example 4 is a porous silicon material obtained through the same steps as in Experimental Example 3 from the master alloy composition where x=15 and y=7.5 in the above basic composition formula. Experimental Example 5 is a porous silicon material obtained from a master alloy composition of x=15 and y=10 in the above basic composition formula through the same steps as in Experimental Example 3. Experimental Example 6 is a porous silicon material obtained by performing the same steps as in Experimental Example 3 from the master alloy composition where x=20 and y=5 in the above basic composition formula. Experimental Example 7 is a porous silicon material obtained by performing the same steps as in Experimental Example 3 from the master alloy composition where x=20 and y=10 in the above basic composition formula. Experimental Example 8 is a porous silicon material obtained through the same process as in Experimental Example 3 from the master alloy composition where x=20 and y=12.5 in the above basic composition formula. Experimental Example 9 is a porous silicon material obtained through the same process as in Experimental Example 3 from the master alloy composition of x=25 and y=5 in the above basic composition formula. Experimental Example 10 is a porous silicon material obtained through the same process as in Experimental Example 3 from the master alloy composition of x=25 and y=10 in the above basic composition formula. Experimental Example 11 is a porous silicon material obtained from a master alloy composition of x=25 and y=12.5 in the above basic composition formula through the same steps as in Experimental Example 3. Experimental Example 12 is a porous silicon material obtained from a master alloy composition of x=12 and y=3 in the above basic composition formula through the same steps as in Experimental Example 3. Experimental Example 13 is a porous silicon material obtained through the same process as in Experimental Example 3 from the master alloy composition of x=18 and y=3 in the above basic composition formula.

(Experimental Examples 14-17)
The same process as in Experimental Example 1 was carried out, in which the master alloy composition Al ₈₈ Si ₁₂ with x=12 in the binary basic composition formula Al _100-x Si _x was rapidly solidified by gas atomization and then acid-treated to make it porous. The porous silicon material thus obtained was designated as Experimental Example 14. Experimental example 15 is a porous silicon material obtained through the same process as in experimental example 14 from a master alloy composition of x=20 in the binary basic composition formula. Experimental example 16 is a porous silicon material obtained through the same process as in experimental example 14 from a master alloy composition of x=26 in the binary basic composition formula. Experimental Example 17 is Si powder having an average particle size of 5 μm.

(Physical property measurement of porous silicon material)
The acid-treated porous silicon powder was observed by a scanning electron microscope (SEM, S-4300 manufactured by HITACHI) and energy dispersive X-ray analysis (EDAX, S-4300 manufactured by HITACHI), and subjected to elemental analysis. Also, using an X-ray diffraction apparatus (RINT-TTR manufactured by Rigaku Co., Ltd.), X-ray diffraction measurement was performed with a Cu tube at a rate of 5°/min in the range of 2θ=10° to 80°. The ratio of each phase of Si phase, Al phase, CrSi ₂ phase, and Al ₁₃ Cr ₄ Si ₄ phase was calculated as a mass ratio from the XRD peak intensity ratio of each phase using an alumina standard material. Also, the pore size distribution was measured with a mercury porosimeter (POWERMASTER 60GT manufactured by Quantachrome). The acid-treated porous silicon powder was dissolved in HF and HNO ₃ and subjected to elemental analysis by ICP emission spectrometry (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II).

10 is a secondary electron image (SEM image) of the cross section of the porous silicon of Experimental Example 1. FIG. 11 is a secondary electron image (SEM image) of the cross section of the porous silicon of Experimental Example 2. FIG. It has been found that by eluting Al and Al alloy phases in the silicon compound with acid, voids are formed and porous silicon particles are obtained.

12 shows XRD measurement results before and after acid treatment in Experimental Examples 1 and 2. FIG. Experimental Examples 1 and 2 before acid treatment show the measurement results of samples heat-treated in air at 400°C, and Experimental Examples 1 and 2 after acid treatment show the results of samples heat-treated in air at 200°C. Measurement results are shown. In the powder obtained by rapidly solidifying the mother alloys of Experimental Examples 1 and 2 by gas atomization, only diffraction peaks of Al and Si were confirmed in the XRD pattern, indicating that the strengthening phase was present in an amorphous state. When this atomized powder was heat-treated at 400° C. or higher, Al ₁₃ Si ₄ Cr ₄ was crystallized as shown in FIG. 8, matching the phase expected from the equilibrium diagram. Also, in the sample after the acid treatment, only the diffraction peak of Si was confirmed, but it was confirmed that the CrSi ₂ phase crystallized as a strengthening phase by heat treatment at 200° C. or higher. CrSi ₂ is introduced to strengthen the skeleton, but if the ratio is too high, the ratio of Si, which is the negative electrode active material, will decrease, resulting in a decrease in capacity. When α-alumina was added as a standard sample to these powder samples and the ratio of the contained phase was quantitatively evaluated, the Si contents of Experimental Examples 1 and 2, which were made porous by acid treatment, were 76% by mass and 58% by mass, respectively. %Met. Also, the proportions of the strengthening phase CrSi ₂ were 24% and 42% by weight, respectively. Al ₁₃ Si ₄ Cr ₄ was not confirmed in the sample after the acid treatment, and it was presumed that it was dissolved by the acid treatment. Thus, in some cases, the strengthening phase was amorphous after quenching, but in other cases, the strengthening phase was crystallized even without heat treatment. In this example, heat treatment is performed for phase identification, but since the skeleton reinforcing effect can be confirmed even when the strengthening phase is in an amorphous state, heat treatment is not particularly necessary for the negative electrode material. However, it was presumed that the heat treatment may improve the crystallinity of Si, thereby improving the battery characteristics, so that the heat treatment may be used.

13 shows pore distribution curves of the porous silicon materials of Experimental Examples 1, 2 and 16. FIG. After rapid solidification of the master alloy, the pore size distribution was evaluated with a mercury porosimeter for samples made porous by acid treatment. Also, the average pore diameter was determined from the volume fraction of pores of each size in the pore diameter distribution data. In general, powders of porous materials contain pores within particles and pores between particles, but the data on the pore size distribution cannot distinguish between the two. In this example, intra-particle and inter-particle pores were distinguished by comparing pore size distribution data for samples before and after acid treatment. In the binary alloy of Experimental Example 16, the intraparticle pore distribution peak was present near 100 nm, and the average pore diameter was 121 nm. On the other hand, in Experimental Examples 1 and 2 in which Cr was added, it was found that the center of the pore size distribution peak shifted to 30 nm or less, and the average pore size also decreased to 35 nm or less. If the expansion/contraction change during charge/discharge of Si is estimated to be four times, pores of 75% by volume or more are required to completely mitigate the volume expansion during full charge. When the porosity is smaller than this value, the Si capacity per unit mass is improved, which is preferable. When the porosity is less than 75% by volume, the Si content increases, resulting in a high capacity, while at the same time reducing volume expansion to some extent.

Furthermore, the results of studies on the effect of master alloy composition on the pore structure and phases produced will be described. The amount of Si and CrSi ₂ produced varies depending on the ratio x/y of the amount x of Si and the amount y of Cr in the master alloy. In order to grasp the Si content, which is important for the negative electrode capacity, master alloys with various x/y values were prepared. Table 1 summarizes the relationship between x/y and the Si content. FIG. 14 is a relationship diagram of the Si phase content with respect to x/y in Experimental Examples 3-11. As shown in FIG. 14, it was found that the Si content tends to decrease as the value of x/y decreases, although the Si content also depends on the Si content. In order to make the most of the large battery capacity of Si, the Si content is preferably 50% by mass or more, and was presumed to be more preferably 70% by mass or more.

Table 1 also shows the oxidizability determined from the color of the sample after acid treatment. Only Si and CrSi ₂ could be identified by XRD in the porous bodies of Experimental Examples 3-13. Generally, Si powder is black to dark brown. On the other hand, CrSi ₂ is a silvery intermetallic compound and its powder is black. When x/y was 3 or more, the color of the prepared sample was dark brown like silicon. On the other hand, when x/y was less than 3, the color of the sample changed from white to light gray. This suggests that the sample contains an amorphous oxide phase that cannot be detected by XRD. As a result of composition analysis by ICP, it was found that these samples contained 25 to 50 at % of oxygen. If an oxide is mixed in, the actual battery capacity of Si per unit mass is lowered, which is not preferable. That is, x/y is more preferably 3 or more. On the other hand, even when the value of x/y was changed, the average pore size did not change much, and was found to be about 40 to 60 nm.

Furthermore, composition analysis was performed on the porous silicon materials of Experimental Examples 1 to 13 by EDX. As described above, a sample with a small x/y ratio contains a large amount of oxygen. In all experimental examples, the amount of Si was about 75 to 96 at % and the amount of Cr was about 3 to 15 at. By XRD of the porous body, only Si phase and CrSi ₂ phase were confirmed, and no phase containing Al was confirmed. The measured diffraction peak position of CrSi ₂ is slightly different from that of pure CrSi ₂ , indicating that Al exists as a Cr(Si, Sl) ₂ phase in solid solution in the generated phase. It was inferred that

Next, the necessary porosity will be explained. As described above, when Si completely reacts with Li, the volume expands four times. In that case, for example, if the volume of Si is assumed to be 25 cm ³ , the volume will be 100 cm ³ , so a porosity of 75 cm ³ or 75% by volume is required. This is the case where the amount of Si in the active material is 100% by mass, but when the amount of Si changes, the necessary porosity also changes. Table 2 summarizes the required porosity for each Si content. In order to use a high Si content, a Si content of 50% by weight or more is preferred, in which case the required porosity is 60% by volume. That is, the preferred porosity range is 60% to 75% by volume. However, since the Si skeleton has a certain degree of strength, even if the porosity is less than 60% by volume, the pore structure will not immediately break.

The strength of the pore structure can be improved by introducing silicides into the framework. This will be explained next. In order to compare the strength of the pore structure, the prepared porous body sample was filled in a cemented carbide mold and uniaxially pressed at 1 GPa. Then, the porosity was evaluated from the pore size distribution data of the sample before and after pressurization. For comparison, changes in porosity under pressure were similarly evaluated for porous body samples prepared by subjecting a Cr-free binary AlSi alloy to rapid solidification acid treatment (Experimental Examples 14 to 16). Table 3 summarizes the measurement results of Experimental Examples 6, 9, 12, 14-16. In any sample, after pressurization at 1 GPa, some of the pores were crushed and the porosity was reduced. However, compared to the binary samples with similar amounts of Si charged x, the ternary samples to which Cr is added show a smaller change in porosity, indicating that the introduction of Cr strengthens the pore structure. I found out. Although there is a difference in porosity change behavior depending on the composition, an extra porosity of about 2 to 15% by volume survives compared to the case where Cr is not added, and the porosity is reduced accordingly. It was suggested that That is, it was suggested that in the sample into which Cr was introduced, even a negative electrode active material with a Si content of 50% by mass could withstand pore collapse due to residual stress if it had a porosity of about 45% by volume. .

A discussion will now be made regarding the proportion of the necessary reinforcing phase CrSi ₂ . FIG. 15 is a diagram showing the relationship between the CrSi ₂ phase ratio (mol %) and porosity change after pressing (volume %). In FIG. 15 the effect of the reinforcing phase CrSi ₂ content on the porosity change after pressing at 1 GPa is shown. The degree of collapse of the porosity is affected by the magnitude of the initial porosity. The smaller the initial porosity, the smaller the porosity change after pressurization. When samples with similar porosity were compared, it was confirmed that the porosity change amount tended to decrease as the CrSi ₂ reinforced phase increased.

(Production of lithium ion secondary battery using non-aqueous electrolyte)
Lithium-ion secondary batteries were produced using the silicon materials of Examples 1, 2 and 17 as negative electrode active materials, and the batteries and peculiarities such as discharge capacity were evaluated. 60% by mass of the negative electrode active material, 20% by mass of acetylene black having an average particle size of 2 μm as a conductive material, and 20% by mass of polyimide were mixed, and N-methylpyrrolidone was added and stirred to prepare a slurry. Next, this slurry was applied onto a copper foil having a thickness of 20 μm, dried, and rolled to prepare a negative electrode having a thickness of 50 μm. The prepared negative electrode was punched out into a circle with a diameter of 16 mm. The following batteries were produced. The electrolytic solution was a mixed solvent of fluoroethylene carbonate/ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (FEC/EC/DMC/EMC) in a volume ratio of 1.5:3:4:3, and _LiPF6 at 1 mol/mol. was added at a concentration of L. The obtained lithium secondary battery was repeatedly charged and discharged at a current density of 0.2 C in a battery voltage range of 0.005 V to 1.5 V for 10 cycles.

(Characteristics of lithium-ion secondary battery using non-aqueous electrolyte)
Table 4 summarizes the composition of the master alloy of the negative electrode active material, the initial discharge capacity, the discharge capacity after 10 cycles, and the capacity retention rate after 10 cycles. In Experimental Example 17, the capacity retention rate was as low as 28%, whereas in the secondary batteries of Experimental Examples 1 and 2, it was found that the capacity retention rate was as good as about 99%. However, in the case of x/y=8.33, the discharge capacity itself is relatively high, but in Experimental Example 2, where x/y=4, which is small, the discharge capacity was about half that of Experimental Example 1.

It goes without saying that the present disclosure is not limited to the experimental examples described above, and can be implemented in various modes as long as they fall within the technical scope of the present disclosure.

INDUSTRIAL APPLICABILITY The present disclosure can be used in the technical field of secondary batteries.

10 electricity 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 voids.

Claims (11)

When the total of Si, Al, and Cr is 100 at%, a raw material containing Cr in a range of 1 at% or more and 20 at% or less, Al in a range of 40 at% or more and 90 at% or less, and the balance being Si is melted. a precursor step of obtaining a silicon alloy precursor by rapid cooling and solidification;
a porosification step of removing the Al component contained in the silicon alloy to obtain a porous silicon material;
A method of making a porous silicon material comprising: 2. The method for producing a porous silicon material according to claim 1, wherein in said porosification step, an Al component is selectively removed with acid or alkali. 3. The method for producing a porous silicon material according to claim 1, wherein said porous silicon material containing a SiCr compound and/or an AlSiCr compound is obtained in said porosification step. 4. The method for producing a porous silicon material according to claim 1, wherein said porous silicon material having a porosity in the range of 30% by volume or more and 85% by volume or less is obtained in said porosity making step. The porous porous material according to any one of claims 1 to 4, wherein in the precursor step, a raw material having a basic composition formula Al _100-xy Si _x Cr _y in which x/y is in the range of 3 or more and 10 or less A method of manufacturing a silicon material. Claim 1 or claim 1, wherein in the precursor step, the molten silicon alloy is granulated to have an average particle size in the range of 0.1 μm to 100 μm by any one of gas atomization, water atomization, and roll quenching. 6. The method for producing a porous silicon material according to any one of 5. An average pore diameter determined by a mercury intrusion method is in the range of 60 nm or less, contains skeleton-like silicon with a three-dimensional network structure having voids, contains a SiCr compound and/or an AlSiCr compound, and has a porosity of 30% by volume or more85 is in the range of volume % or less,
Porous silicon material. 8. The porous silicon material according to claim 7, wherein said SiCr compound, _CrSi2 , is contained in a range of 5 mol % or more and 20 mol % or less with respect to the Si phase. 9. The porous silicon material according to claim 7, wherein pores of 1 μm or less measured by mercury intrusion method are 20% by volume or more. 10. The porous silicon material according to any one of claims 7 to 9, wherein the pore diameter determined by mercury porosimetry is in the range of 5 nm or more and 300 nm or less. a positive electrode comprising a positive electrode active material;
a negative electrode comprising the porous silicon material according to any one of claims 7 to 10 as a negative electrode active material;
an ion conducting medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
A storage device with

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