JP7601745B2 - Method for producing porous silicon material, porous silicon material and power storage device - Google Patents

Description

This specification discloses a method for producing a porous silicon material, a porous silicon material, and an electricity storage device.

Conventionally, a porous silicon has been proposed as a silicon anode material, in which 70 parts by mass of lump Si and 30 parts by mass of Al powder are mixed, and then granulated by gas atomization using alloy melt and helium gas under an argon atmosphere, and then Al is removed with hydrochloric acid (see, for example, Patent Document 1). It is said that this porous silicon can completely suppress pulverization due to expansion and contraction of the active material volume during charging and discharging, peeling 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, a method has been proposed in which an intermediate alloy element containing Mg, Co, Cr, Cu, Fe, etc. and a Si alloy of Si are separated into a second phase in which the intermediate alloy element and the molten metal element are replaced in a molten metal containing a predetermined molten metal element, and Si fine particles, and the second phase is removed to obtain a porous silicon material (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, an alloy containing Si and Al is used to make the material porous, but this is still insufficient in suppressing defects caused by the expansion and contraction of Si. In the manufacturing method of the porous silicon material of Patent Document 2, a silicon alloy containing an intermediate alloy element is melted and replaced with a molten metal containing a specified molten metal element, i.e., high-temperature processing is required, and a simple manufacturing process is required.

The present disclosure has been made in consideration of these problems, and has as its main objective the provision of a method for producing a porous silicon material, a porous silicon material, and an electricity storage device that can further suppress the deterioration of charge/discharge characteristics.

As a result of intensive research to achieve the above-mentioned object, the inventors discovered that by producing a silicon alloy containing Al and Ti and removing the compound containing Al, a porous silicon material can be obtained that can further suppress the deterioration of the charge-discharge characteristics, and thus completed the manufacturing method for the porous silicon material, the porous silicon material, and the electricity storage device disclosed herein.

That is, the method for producing the porous silicon material of the present disclosure includes:
a precursor process for obtaining a precursor of a silicon alloy by melting and rapidly solidifying a raw material containing Ti in a range of 5 at % to 15 at % and Al in a range of 10 at % to 50 at % and the remainder being Si, where the total of Si, Ti and Al is 100 at %;
a porosity forming step of removing an Al component contained in the silicon alloy to obtain a porous silicon material;
It includes.

The porous silicon material of the present disclosure comprises:
The pore size distribution range determined by mercury intrusion porosimetry is in the range of 1 nm to 250 nm, the skeletal silicon has a three-dimensional mesh structure having voids, the SiTi compound and/or the AlSiTi compound, and the average porosity is in the range of 10 vol. % to 50 vol. %.

The electricity storage device of the present disclosure comprises:
a positive electrode including a positive electrode active material;
A negative electrode containing the above-mentioned porous silicon material as a negative electrode active material;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
It is equipped with the following:

The present disclosure can further suppress the deterioration of charge/discharge characteristics in materials containing Si. The reason for obtaining such an effect is presumed to be as follows. For example, the theoretical capacity of a silicon negative electrode for a lithium ion secondary battery is 4199 mAh/g, which is about 10 times the theoretical capacity of 372 mAh/g of general graphite, and further high capacity and high energy density are expected. On the other hand, silicon that has absorbed lithium ions is Li _4.4 Si, and its volume expands to about four times that of silicon before lithium absorption. In the present disclosure, elements or compounds other than silicon, silicon titanium, and aluminum silicon titanium compounds are selectively dissolved to produce porous silicon with small pore size and large porosity. By simply removing elements or compounds other than silicon, silicon titanium, and aluminum silicon titanium compounds, porous silicon can be easily provided in the atmosphere without the need for large-scale equipment. In addition, the obtained porous silicon material has a cage-shaped silicon skeleton reinforced with silicon titanium compounds and aluminum silicon titanium compounds, and has an average pore diameter of 250 nm or less. Such porous silicon materials with small pore size, high porosity, and reinforced skeletons make it easy to obtain high-performance electricity storage devices such as lithium-ion batteries by mitigating volume expansion and contraction and improving cycle characteristics.

Al-Si-Ti phase diagram at 10 at % Ti. Al-Si-Ti phase diagram at 60 at% Si. FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10. 1 is a SEM image of the surface of the porous silicon of Experimental Example 3. 1 is a cross-sectional SEM image of porous silicon of Experimental Example 3. 1 shows XRD measurement results after acid treatment in Experimental Example 1. 13 shows XRD measurement results after acid treatment in Experimental Example 2. 13 shows XRD measurement results after acid treatment in Experimental Example 3. 3 is a pore distribution curve of the porous silicon material of Experimental Example 3.

(Method of Producing Porous Silicon Material)
The manufacturing method of the porous silicon material of the present disclosure includes a precursor process and a porosity process. In the precursor process, a raw material containing Si, Al, and Ti is melted and rapidly solidified to obtain a precursor of a silicon alloy. In the porosity process, the Al component contained in the silicon alloy is removed to obtain a porous silicon material. First, the raw material composition will be described.

Figure 1 is an Al-Si-Ti phase diagram at 10 at% Ti. Figure 2 is an Al-Si-Ti phase diagram at 60 at% Si. In general, when a melt of a eutectic composition with a minimum liquidus line in the equilibrium diagram of a eutectic A-B alloy is solidified, a eutectic structure is formed in which the A phase and the B phase are separated into fibers (lamellae). At this time, the size of the lamellar structure formed becomes finer as the cooling rate increases, and a nano-sized structure is formed at a cooling rate of 1000°C/s or more. If only the A element can be selectively removed from this eutectic structure by acid treatment, a material consisting of the B element characterized by a eutectic structure can be obtained. The microstructure changes by adjusting the composition of the A element and the B element near the eutectic composition, resulting in a porous body having a structure in which the crystallized phase consisting of the B element is connected. Considering Al as the A element in the alloy and Si and Ti as the B element, the equilibrium diagram shows that near the Al-Si-Ti ternary eutectic composition, SiTi compounds, AlSiTi compounds, and Si crystallize, while primary crystal Al and AlTi compounds crystallize or precipitate. The solidification structure forms a lamellar structure consisting of SiTi compounds, AlSiTi compounds, AlTi compounds, eutectic Si, primary crystal Si, and primary crystal Al. Compounds that dissolve in acid include primary crystal Al and AlTi compounds, so it is presumed that the structure obtained after acid treatment will be a skeletal porous body consisting of the remaining SiTi compounds, AlSiTi compounds, eutectic Si, and primary crystal Si.

(Precursor Process)
In the precursor process, it is preferable to use a raw material containing Ti in the range of 5 at% to 15 at% and Al in the range of 10 at% to 50 at% and the remainder being Si, when the total of Si, Ti and Al is 100 at%. The raw material may contain unavoidable impurities. The unavoidable impurities are components that inevitably remain during the purification of any of Si, Ti and Al, and examples thereof include Fe, C, Cu, Ni and P. It is preferable that the amount of unavoidable impurities is as small as possible, and for example, when the total of Si, Ti and Al is 100 at%, it is preferably 5 at% or less, and more preferably 2 at% or less. The compounding ratio of Ti is, for example, preferably 6 at% or more, and may be 7.5 at% or more. The compounding ratio of Ti is preferably 14 at% or less, and may be 12.5 at% or less. The compounding ratio of Al is preferably 50 at% or less, more preferably less than 50 at%, further preferably 40 at% or less, and may be 30 at% or less. The compounding ratio of Al is preferably 10 at% or more, more preferably 15 at% or more, and may be 20 at% or more. In a silicon alloy containing Al or Ti in such a range, the porosity can be further increased and voids of a more suitable shape and size can be obtained, which is preferable. If the content of Al is high, when the alloy is melted to form an alloy and then rapidly cooled, a single phase of Al is largely precipitated, so that many voids can be formed. This cooling rate is preferably more rapid, and may be, for example, in the range of 10 ² ° C./s to 10 ⁸ ° C./s from the molten state.

In this process, 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 process, the alloy obtained from the raw material may be granulated. In this granulation process, the molten metal of the silicon alloy raw material may be cast into a mold, and the obtained ingot may be crushed and granulated. In addition, the method of granulating the silicon alloy may be one or more of the gas atomization method, water atomization method, and roll quenching method. In the gas atomization method and the water atomization method, an alloy powder is obtained. On the other hand, in the roll quenching method, a thin strip alloy is obtained, so that it may be crushed and powdered after that. 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 process. Of these, the gas atomization method is more preferable as the method of granulating the silicon alloy. In the gas atomization, it is preferable to perform the molten metal in an Ar atmosphere, and it is preferable to perform the granulation in an Ar or He atmosphere.

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

In this precursor process, a raw material containing Al, Ti, and Si as well as a second element containing one or more of Ca, Cu, Mg, Na, Sr, and P may be used. Of these, the second element is preferably one or more of Ca, Na, and Sr. The content of the second element is preferably less than the content of Al and Ti, and is preferably in the range of 10 mass% or less, more preferably 5 mass% or less, relative to the entire silicon alloy.

(Porosification process)
In the porosity forming step, a process is performed to remove substances other than Si from the silicon alloy particles produced above. Examples of substances other than Si include Al and its compounds, Ti and its compounds, etc. In this process, it is preferable to selectively remove the Al component, i.e., the Al phase and its compounds, using an acid or alkali. The acid or alkali used is preferably one that dissolves elements and/or compounds other than silicon in the silicon alloy and does not dissolve silicon, and examples of such acids 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 is within a range that can remove Al and its compounds, Ti and its compounds, and can be, for example, in the range of 1 mol/L to 5 mol/L. This removal process may be performed by heating at, for example, 30°C to 60°C. In addition, the removal process is preferably performed by immersing the silicon alloy particles in an acid or alkali solution and stirring for about 1 to 5 hours. The obtained porous silicon material is then washed and dried.

In the porous forming process, substances other than Si may be removed in the range of 85 mass% to 100 mass%. For example, Al, Ti, other oxygen, etc. may remain, but when used as an electrode active material, it is preferable that they are less in terms of charge/discharge capacity. In addition, components such as Al and Ti are preferably contained in a predetermined amount or more in terms of reinforcing the silicon skeleton and improving durability. In this process, a porous silicon material containing a SiTi compound and/or an AlSiTi compound may be obtained. Examples of SiTi compounds include Si _x Ti _y (x and y are arbitrary numbers), such as Si ₃ Ti ₅ and Si ₄ Ti _5. Examples of AlSiTi compounds include Al _a Si _b Ti _c compounds (a, b, and c are arbitrary numbers), such as Ti ₃₃ Al ₁₅ Si ₅₂ (τ ₂ ). The SiTi compound or AlSiTi compound may be one that is poorly soluble in acid or alkali.

In the porosity forming step, it is preferable to obtain a porous silicon material containing SiTi compounds in a range of 10 mass% or less when the total of Si, SiTi compounds and AlSiTi compounds is 100 mass%. In addition, in the porous silicon material, it is preferable to contain SiTi compounds in an amount of 0.5 mass% or more, and it may be contained in an amount of 1 mass% or more. In addition, in the porous silicon material, it is preferable to contain SiTi compounds in an amount of 7.5 mass% or less, and it may be contained in an amount of 5 mass% or less. In addition, in this step, it is preferable to obtain a porous silicon material containing AlSiTi compounds in a range of 50 mass% or less when the total of Si, SiTi compounds and AlSiTi compounds is 100 mass%. In addition, in the porous silicon material, it is preferable to contain AlSiTi compounds in an amount of 1 mass% or more, and it may be contained in an amount of 5 mass% or more. In addition, in the porous silicon material, it is preferable to contain AlSiTi compounds in an amount of 40 mass% or less, and it may be contained in an amount of 30 mass% or less. This step preferably results in a porous silicon material in which Si ₃ Ti ₅ and Ti ₃₃ Al ₁₅ Si ₅₂ remain.

In the porosification process, a porous silicon material having an average porosity in the range of 10 volume % to 50 volume % may be obtained. This average porosity is a value measured with a mercury porosimeter. This average porosity is preferably 15 volume % or more, and may be 20 volume % or more, for example. The average porosity is preferably less than 50 volume %, more preferably 49 volume % or less, and may be 40 volume % or less or 30 volume % or less, for example. A larger average porosity is more responsive to volume changes when carrier ions are absorbed, and a smaller average porosity is preferable because it increases the amount of Si present per unit volume.

(Porous silicon material)
The porous silicon material of the present disclosure may be one produced by the above-mentioned manufacturing method. Here, the detailed description of each physical property of the porous silicon material is omitted, assuming that it is the same as the above-mentioned manufacturing method. The porous silicon material has a pore size distribution range of 1 nm to 500 nm as determined by mercury intrusion porosimetry. The pore size may be 10 nm or more, 50 nm or more, or 100 nm or more. The pore size is preferably 250 nm or less, may be 200 nm or less, or may be 150 nm or less. In the porous silicon material, the average pore size as determined by mercury intrusion porosimetry may be in the range of 5 nm to 500 nm, or may be in the range of 10 nm to 250 nm.

The porous silicon material contains skeletal silicon with a three-dimensional network structure having voids, and contains SiTi compounds and/or AlSiTi compounds. It is presumed that the SiTi compounds and AlSiTi compounds are responsible for reinforcing the silicon skeleton. The porous silicon material preferably contains SiTi compounds in the range of 10 mass% or less. The porous silicon material preferably contains AlSiTi compounds in the range of 50 mass% or less. It is preferable that the amount of the SiTi compounds and AlSiTi compounds is higher from the viewpoint of supplementing the reinforcement of the skeleton, and it is preferable that the amount is lower from the viewpoint of the charge/discharge capacity of the power storage device.

The porous silicon material has an average porosity in the range of 10 volume % or more and 50 volume % or less. This average porosity is preferably 40 volume % or less, more preferably 35 volume % or less, and may be 30 volume % or less. The average porosity is preferably 15 volume % or more, more preferably 20 volume % or more, and may be 25 volume % or more. From the viewpoint of the volume change of the material, it is preferable that this average porosity is larger, and from the viewpoint of the charge/discharge capacity of the electricity storage device, it is preferable that this average porosity is smaller. This average porosity is a value measured with a mercury porosimeter.

The porous silicon material preferably has an average particle size of 0.1 μm or more, more preferably 1 μm or more, and may be 5 μm or more. 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. The porous silicon material preferably contains 70 at% or more of Si when the total of Si, Al, and Ti is 100 at%, excluding oxygen and unavoidable impurities. The Si content is more preferably 75 at% or more, even more preferably 80 at% or more, and may be 85 at% or more. The Si content is preferably higher from the viewpoint of charge/discharge capacity, and may be lower from the viewpoint of relative skeletal reinforcement. The Al content is preferably in the range of 0.5 at% to 15 at%, more preferably 12.5 at% or less, more preferably 10 at% or less, and may be 7.5 at% or less. 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 Ti content is preferably in the range of 0.5 at% or more and 25 at% or less. The Ti content is more preferably 20 at% or less, even more preferably 17.5 at% or less, and may be 17 at% or less. The Ti content is more preferably 1 at% or more, more preferably 5 at% or more, even more preferably 10 at% or more, and may be 12% or more. The Al and Ti contents are preferably higher from the viewpoint of supplementing the reinforcement of the skeleton, and are preferably lower from the viewpoint of the charge/discharge capacity of the power storage device, since they 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 the second element in the range of 15 mass% or less. The porous silicon material may also contain unavoidable impurities in addition to Si, Al, and Ti. It is preferable that the second element and the unavoidable impurities are less.

(Electrodes for Electricity Storage Devices)
The electrode for the power storage device is provided with the porous silicon material described above as an electrode active material. This electrode is either a positive electrode or a negative electrode based on the potential of the counter electrode relative to the potential of the electrode active material, but when lithium is used as a carrier, it is preferable to use the electrode as a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, air batteries, etc. The electrode for the power storage device may be one in which the average porosity of the porous silicon material is 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 it during production. For example, when porous silicon particles are used as the negative electrode active material of a lithium ion secondary battery, the smaller the pores are, the more uniform the alloying of lithium ions is, so that stress concentration is reduced and deterioration of the electrode itself can be prevented. The average porosity of the porous silicon material after compression may be appropriately adjusted according to the characteristics required for the electrode for the power storage device, and may be, for example, 5 volume % or more or 10 volume % or more. The average porosity of the compressed porous silicon material may be, for example, 30 vol. % or less, or 20 vol. % or less.

The electrode for the power storage device may be formed by forming the porous silicon material on a current collector and fixing it to the current collector. This electrode can be produced by mixing the porous silicon material with a conductive material or a binder in a solvent as necessary to form a paste and applying it to the current collector, or by mixing the porous silicon material with a conductive material or a binder as necessary 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 electronic conductive material that does not adversely affect the battery performance, and for example, a mixture of one or more of graphite such as natural graphite (scale graphite, scale graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metal (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 can be, for example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine rubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR), etc., used alone or as a mixture of two or more kinds. In addition, a cellulose-based aqueous binder or an aqueous dispersion of styrene butadiene rubber (SBR) can also be used. As the solvent, for example, an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran can be used. In addition, a dispersant, a thickener, etc. can be added to water, and the active material can be slurried with latex such as SBR. Examples of application methods include roller coating such as applicator rolls, screen coating, doctor blade method, spin coating, bar coater, etc., and any of these can be used to form any thickness and shape. The current collector may be appropriately selected depending on the potential of the active material, and may be, for example, aluminum, titanium, stainless steel, nickel, iron, copper, baked carbon, conductive polymer, conductive glass, or aluminum or copper whose surface is treated with carbon, nickel, titanium, silver, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. The surfaces of these materials may also be subjected to oxidation treatment. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fiber group formed bodies. The thickness of the current collector is, for example, 1 to 500 μm. The amount of the active material complex formed may be appropriately set depending on the desired performance required for the power storage device.

In this electrode, the electrode active material may contain an active material other than the porous silicon material in addition to the porous silicon material, within a range in which _{both a low confining pressure and a capacity retention rate can be achieved. For example, the electrode active material may contain a carbonaceous material or Li4Ti5O12 .} However, from the viewpoint of further increasing the battery capacity, it is preferable that the porous silicon material accounts for, for example, 50 mass% or _more , preferably 90 mass% or more, of the entire electrode active material as 100 mass%.

(Electricity storage device)
The electricity 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. The porous silicon material can be used as the negative electrode active material. The electricity storage device may be any of a lithium ion secondary battery, a hybrid capacitor, and an air battery. 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 the positive electrode active material. Specifically, transition metal sulfides such as _TiS2 , _TiS3 , _MoS3 , and _FeS2 , lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0<x<1, etc., the same applies below) and Li ₍₁ -x _{) Mn2O4} , lithium cobalt composite oxides having a basic composition formula such as Li( _1-x ) _CoO2 , lithium nickel composite oxides having a basic composition formula such as Li(1- _{x ) NiO2} , _lithium nickel cobalt _manganese composite oxides having a basic composition formula such as Li(1-x _{) NiaCobMncO2} (a+b+c=1) _, lithium vanadium composite oxides having _a basic composition formula such as _LiV2O3 , and transition metal oxides having a basic composition formula such as _V2O5 can be used. Among these, lithium transition metal composite oxides, 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 and lithium ion capacitors. Examples of carbonaceous materials include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m ² /g or more, and more preferably 1500 m ² /g or more. When the specific surface area is 1000 ^m2 /g or more, the discharge capacity can be further increased. From the viewpoint of ease of preparation, the specific surface area of the activated carbon is preferably 3000 ^m2 /g or less, and more preferably 2000 ^m2 /g or less. The conductive material, binder, solvent, current collector, etc. used in the positive electrode can be appropriately selected from those exemplified in the above-mentioned electrode.

The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charging and discharging, but also provides a well-balanced electrolyte viscosity, battery capacity, battery output, etc. 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 . Among these, it is preferable to use one or more salts selected from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiClO ₄ and organic salts such as LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , and LiC(CF ₃ SO ₂ ) ₃ in combination from the viewpoint of electrical properties. The concentration of this supporting salt in the non-aqueous electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, and more preferably 0.5 mol/L or more and 2 mol/L or less. When the concentration at which the supporting salt is dissolved is 0.1 mol/L or more, a sufficient current density can be obtained, and when it is 5 mol/L or less, the electrolyte can be more stabilized. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to this non-aqueous electrolyte.

In addition, instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. As the ion-conducting polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, vinylidene fluoride, and a supporting salt can be used. Furthermore, an ion-conducting polymer can be used in combination with a non-aqueous electrolyte. As the ion-conducting medium, in addition to an 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 power storage device may have 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 lithium secondary batteries, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

The shape of this electricity storage device is not particularly limited, but examples include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type. It may also be applied to large devices used in electric vehicles and the like. FIG. 3 is an explanatory diagram showing an example of the structure of an electricity storage device 10. This electricity storage device 10 has a positive electrode 12, a negative electrode 15, and an ion conductive medium 18. The positive electrode 12 has a positive electrode active material 13 and a current collector 14. The negative electrode 15 has a negative electrode active material 16 and a current collector 17. The negative electrode active material 16 is the porous silicon material 21 described above, and has voids 23.

(All-solid-state lithium-ion secondary battery)
The power storage device is preferably an all-solid-state lithium ion secondary battery. In the case of an all-solid-state battery, the change in performance due to the electrolyte can be further suppressed, and safety can be further improved, which is preferable. The all-solid-state lithium ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode which is the above-mentioned electrode for the power storage device, and a solid electrolyte which is interposed between the positive electrode and the negative electrode and conducts lithium ions. The positive electrode may be any of those shown in the above-mentioned power storage device. The negative electrode may be the above-mentioned electrode for the power storage device.

The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La, and Zr. The solid electrolyte may have a basic composition of _Li7.0+xy (La3 _-x , _Ax )(Zr2 _-y , _Ty ) _O12 . However, A is one or more of Sr and Ca, and T is one or more of Nb and Ta, and satisfies 0<x≦1.0 and 0<y<0.75. Alternatively, the solid electrolyte may be a garnet-type _oxide represented by a basic composition of ( _{Li7-3z+ xyMz} )( _{La3-xAx ) (} Zr2 _{- yTy} ) _O12 or ( _Li7-3z +xyMz)( _{La3 -} xAx)(Y2- _yTy ) _O12 . However, in the formula, element M may be one or more of Al and Ga, element A may be one or more of Ca and Sr, and T may be one or more of Nb and Ta, and 0≦z≦0.2, 0≦x≦0.2, and 0≦y≦2 may be satisfied. In this basic composition formula, it is more preferable that 0.05≦z≦0.1 is satisfied. In this basic composition formula, it is more preferable that 0.05≦x≦0.1 is satisfied. In addition, it is more preferable that 0.1≦y≦0.8 is satisfied. In such a range, the ion conductivity can be more suitable.

Alternatively, examples of solid electrolytes include common _Li3N , _Li14Zn ( _GeO4 ) ₄ known _as LISICON, the sulfide _{Li3.25Ge0.25P0.75S4} , perovskite-type _{La0.5Li0.5TiO3 ,} ( _La2 / _3Li3x □ _1/3-2x ) _TiO3 ( _□ : atomic _vacancy ), garnet-type _Li7La3Zr2O12 , NASICON-type _LiTi2 ( _PO4 ) _{3 , Li1.3M0.3Ti1.7} ( _PO3 ) ₄ ( _{M = Sc} , Al _{) ,} etc. Other examples include 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 ₂ , which is a sulfide-based substance having high electrical conductivity. 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 in which _SiO2 , _{GeO2 , B2O3} , _{or P2O5} is _the glass _- based substance _{and Li2O} is the network modifier _. In addition, examples of thio-liquid oxide solid electrolytes include _Li2S -GeS2 _- based, _Li2S -GeS2 _- ZnS-based, Li2S- _{Ga2S2 -} based, _Li2S -GeS2 _{-Ga2S3-based, Li2S-GeS2 - P2S5 - based , Li2S - GeS2} - _{SbS5 -} based, _Li2S -GeS2- _Al2S3 -based, _Li2S - _{SiS2 -} based, _Li2S - _{P2S5 -} based, _Li2S - _{Al2S3 -} based, LiS- _SiS2 - _{Al2S3 - based} , _Li2S - _SiS2 - _P2S5 -based, and the _like . These solid electrolytes may be formed into a plate shape and disposed between the positive electrode and the negative electrode.

The all-solid-state lithium-ion secondary battery may also include a restraining member that restrains the stack of the positive electrode, solid electrolyte, and negative electrode in the stacking direction. This restraining member may include, for example, a pair of plate-shaped parts that sandwich the stack from both ends of the stack in the stacking direction, a rod-shaped part that connects the pair of plate-shaped parts, and an adjustment part that is connected to the rod-shaped part and adjusts the distance between the pair of plate-shaped parts by a screw structure or the like.

As described above in detail, the present disclosure can further suppress the deterioration of charge/discharge characteristics in materials containing Si. The reason for obtaining such an effect is presumed to be as follows. For example, the theoretical capacity of a silicon negative electrode for a lithium ion secondary battery is 4199 mAh/g, which is about 10 times the theoretical capacity of general graphite, 372 mAh/g, and further high capacity and high energy density are expected. On the other hand, silicon that has absorbed lithium ions is Li _4.4 Si, and its volume expands to about four times that of silicon before lithium absorption. According to the present disclosure, the Al component material other than silicon contained in the silicon alloy can be selectively removed by using an acid or alkali that dissolves it, and a porous silicon material with a small pore size and a large porosity can be easily produced. In addition, a compound containing Ti is included as a microstructure, and this compound strengthens the silicon skeleton, so that the capacity deterioration caused by the skeleton fluctuation due to repeated charging and discharging can be further suppressed. When a porous silicon material having such a small pore size and large porosity is used in an electricity storage device such as a lithium ion secondary battery, volume expansion and contraction is greatly alleviated, and therefore, for example, charge/discharge characteristics such as charge/discharge cycle characteristics are improved, making it easy to obtain a high-performance electricity storage device.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in various forms as long as they fall within the technical scope of the present disclosure.

Below, specific examples of the fabrication of the porous silicon and electricity storage device of the present disclosure are described as experimental examples. Experimental Examples 1 to 3 are working examples of the present disclosure, and Experimental Example 4 is a comparative example.

[Preparation of porous silicon material]
(Experimental Example 1)
45.5% by mass of 10 mm square lump Al, 37.7% by mass of lump Si and 16.8% by mass of sponge-like Ti were weighed and mixed, and melted in an Ar inert atmosphere by high-frequency heating to obtain a molten alloy. This molten alloy was gas atomized using Ar inert gas to obtain an AlSiTi alloy powder with an average particle size of 3 μm (precursor process). The quenching rate was 10 ⁶ ° C./s. The AlSiTi alloy powder contained Si, τ ₂ (Ti ₃₃ Al ₁₅ Si ₅₂ ), (Al, Si) ₃ Ti, Si ₃ Ti ₅ , and Al. The atomic composition of the AlSiTi alloy powder was Al-40 at % Si-10 at % Ti. When the powder obtained after the acid treatment was subjected to X-ray diffraction, the presence of Si, (Al, Si) ₃ Ti, and Al phase and Si phase as crystalline phases was confirmed. Next, the obtained alloy powder was placed in 3 mol/L hydrochloric acid diluted in pure water, stirred at room temperature of 25° C. for 1 hour, filtered while being thoroughly washed, and dried in a vacuum drying oven at 30° C. for 2 hours (porosity forming step). In this manner, the porous silicon material of Experimental Example 1 was produced.

(Experimental Example 2)
A porous silicon material of Experimental Example 2 was produced in the same manner as in Experimental Example 1, except that 47.2 mass% Al, 19 mass% Si, and 16.6 mass% sponge Ti were used. The atomic composition was Al-50 at% Si-10 at% Ti. An acid treatment was carried out under the same conditions as in Experimental Example 1 to produce the porous silicon material of Experimental Example 2.

(Experimental Example 3)
A porous silicon material of Experimental Example 3 was produced in the same manner as in Experimental Example 1, except that 56 mass% of Al, 56.2 mass% of Si, and 16.7 mass% of sponge Ti were used. The atomic composition was Al-60 at% Si-10 at% Ti. An acid treatment was carried out under the same conditions as in Experimental Example 1 to produce the porous silicon material of Experimental Example 3.

(Experimental Example 4)
The silicon material in Experimental Example 4 was a Si powder having an average particle size of 5 μm.

(Physical property measurement of porous silicon materials)
The acid-treated porous silicon powder was dissolved in HF and _HNO3 , and elemental analysis was performed using ICP optical emission spectroscopy (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II). In addition, observation and elemental analysis were performed using a scanning electron microscope (SEM, Hitachi S-4300) and energy dispersive X-ray analysis (EDAX, Hitachi S-4300). In addition, the pore distribution was measured using a mercury porosimeter (Quantachrome POWERMASTER 60GT).

(Results and Discussion)
Fig. 4 is a surface SEM image of the porous silicon of Experimental Example 3, and Fig. 5 is a cross-sectional SEM image of the porous silicon of Experimental Example 3. As shown in Fig. 4, it was confirmed that Al components such as primary crystal Al were dissolved from the Si alloy by the acid treatment to form porous silicon particles. Furthermore, as shown in Fig. 5, when the cross section of the particle was observed, a structure in which SiTi compounds and AlSiTi compounds were precipitated on skeletal silicon of a three-dimensional network structure of Si was confirmed. It was found that voids were formed by dissolving primary crystal Al and Al compounds in the silicon compound with acid, and porous silicon particles were obtained.

Fig. 6 shows the XRD measurement results after the acid treatment in Experimental Example 1. Fig. 7 shows the XRD measurement results after the acid treatment in Experimental Example 2. Fig. 8 shows the XRD measurement results after the acid treatment in Experimental Example 3. After the acid treatment, Si, _τ2 ( _Ti33Al15Si52 ) was detected in Experimental Examples 1 and ₂ , and Si, _τ2 ( _{Ti33Al15Si52 )} and _Si3Ti5 phases were detected in Experimental Example _3. It was presumed _that the Al phase and (Al,Si) _3Ti were dissolved by the acid treatment, and _these portions became pores.

Figure 9 shows the pore distribution curve of the porous silicon material of Experimental Example 3 measured by a mercury porosimeter. Since the mercury porosimeter also detects voids between particles, the voids between particles were measured from the atomized powder before the acid treatment, and the pore diameter and pore volume of the pores were obtained. The pore size measured from the SEM images shown in Figures 4 and 5 was calculated to be several nm to about 200 nm. The pore distribution of the porous silicon material measured by the mercury porosimeter was in the range of 5 to 200 nm, which was equivalent to the range measured by the SEM. The pore size measured by the mercury porosimeter in Experimental Example 3 was most common in the vicinity of 80 nm, the average pore size was 61 nm, and the porosity was 29 vol.%. In Experimental Examples 1 and 2, the average pore size was 85 nm and 76 nm, and the porosity was 47 vol.% and 37 vol.%, respectively. As the Al blend composition decreased, the porosity and average pore size tended to decrease.

Table 1 shows the elemental composition (at%) of the raw material in Experimental Examples 1 to 4, the elemental composition (at%) after the acid treatment, the compound composition ratio (Si, τ ₂ (Ti ₃₃ Al ₁₅ Si ₅₂ ), Si ₃ Ti ₅ phase) (mol%), the porosity (volume%) by the mercury intrusion method, and the average pore size (nm). After the acid treatment, Al and AlTi compounds were dissolved, and Si was the main component, resulting in a structure of τ ₂ and/or Si ₃ Ti ₅ phase. In Experimental Examples 1 to 3, the compound composition was in the range of 60 to 87 mol of Si, 0 to 4 mol of Si ₃ Ti ₅ , and 9 to 40 mol of τ ₂ (Ti ₃₃ Al ₁₅ Si ₅₂ ). In addition, the porosity of Experimental Examples 1 to 3 was 29 to 47 vol% and the average pore size was 61 nm to 85 nm, indicating high porosity.

(Preparation of lithium secondary battery using non-aqueous electrolyte)
Each of the silicon materials of Experimental Examples 1 to 4 was used as the negative electrode active material, and 60% by mass of this, 20% by mass of acetylene black with an average particle size of 2 μm as a conductive material, and 20% by mass of polyimide as a binder were weighed and mixed, and N-methylpyrrolidone was added and stirred to prepare a negative electrode composite slurry. Next, this slurry was applied to a copper foil with a thickness of 20 μm, dried, and rolled to prepare a negative electrode with a thickness of 50 μm. The prepared negative electrode was punched out into a circle with a diameter of 16 mm, and a porous polyethylene separator was sandwiched between this negative electrode and metallic lithium was layered as a counter electrode to form a laminate. Next, an electrolyte solution in which LiPF6 was added at a concentration of ₁ mol/L to a mixed solvent of fluoroethylene carbonate (FEC)/ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) in a volume ratio of 1.5:3:4:3 was injected into the laminate to produce a lithium secondary battery, which is a Tom cell type small battery cell. The resulting lithium secondary battery was repeatedly charged and discharged for 10 cycles at a current density of 0.2 C in a battery voltage range of 0.005 V to 1.5 V.

(Characteristics of lithium secondary batteries using non-aqueous electrolyte)
Table 2 shows the initial discharge capacity (mAh/g), the discharge capacity after 10 cycles (mAh/g), and the capacity retention rate (%) of Experimental Examples 1 to 4. The capacity retention rate was calculated from the formula Q ₁₀ /Q ₁ ×100 using the discharge capacity Q ₁ at the first cycle and the discharge capacity Q ₁₀ at the tenth cycle. In Experimental Examples 1 to 3, the silicide phase and τ ₂ phase were present, and the Si phase was relatively small, so the initial discharge capacity tended to be smaller than that of Experimental Example 4. In addition, in Experimental Examples 1 to 3, the discharge capacity tended to increase with the decrease in the Al amount. On the other hand, in Experimental Example 4, the capacity retention rate decreased to 28% after 10 cycles. This was presumed to be because the silicon particles without voids could not absorb the volume change, and a defect occurred in the electrode. On the other hand, in the lithium secondary batteries of Experimental Examples 1 to 3, the capacity retention rate was extremely good at 98% or more, and it was found that the electrodes were stable. As in Experimental Examples 1 to 3, it was found that when a porous silicon material having a porosity in the range of 10 to 50 volume % further contains a compound phase containing Ti, the deterioration of cycle characteristics can be further suppressed and the capacity retention rate can be increased by, for example, reinforcing the Si skeleton.

It goes without saying that the present invention is in no way limited to the above-mentioned experimental examples, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

This disclosure can be used in the field of secondary battery technology.

10: power 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 conductive medium, 21: porous silicon material, 23: void.

Claims (9)

a precursor process for obtaining a precursor of a silicon alloy by melting and rapidly solidifying a raw material containing Ti in a range of 5 at % to 15 at % and Al in a range of 10 at % to 50 at % and the remainder being Si, where the total of Si, Ti and Al is 100 at %;
a porosity forming step of removing an Al component contained in the silicon alloy to obtain a porous silicon material;
A method for producing a porous silicon material comprising: The method for producing a porous silicon material according to claim 1, wherein the porosity-making step selectively removes the Al component using an acid or alkali. The method for producing a porous silicon material according to claim 1 or 2, wherein the porous silicon material contains a SiTi compound and/or an AlSiTi compound in the porosity forming step. The method for producing a porous silicon material according to any one of claims 1 to 3, wherein the porous silicon material has an average porosity in the range of 10% by volume or more and 50% by volume or less in the porosity forming step. The method for producing a porous silicon material according to any one of claims 1 to 4, wherein in the precursor process, 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. the pore size distribution range determined by mercury intrusion porosimetry is in the range of 1 nm to 500 nm, the skeletal silicon has a three-dimensional network structure having voids, the SiTi compound and/or the AlSiTi compound is contained, and the average porosity is in the range of 10 vol. % to 50 vol. %;
Porous silicon material. 7. The porous silicon material according to claim 6, wherein the _SiTi compound _Si3Ti5 is contained in an amount of 10 mass% or less, and the _{AlSiTi compound Ti33Al15Si52 is contained} in an amount of 50 mass% or less. The porous silicon material according to claim 6 or 7, having an average porosity of 40 volume % or less. a positive electrode including a positive electrode active material;
A negative electrode comprising the porous silicon material according to any one of claims 6 to 8 as a negative electrode active material;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
A power storage device comprising: