JP7745368B2 - Method for producing porous silicon material - Google Patents

Description

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

Previously, a proposed silicon anode material was a porous silicon obtained by mixing 70 parts by weight of block silicon with 30 parts by weight of aluminum powder, then granulating the mixture with a molten alloy in an argon atmosphere using a gas atomization method using helium gas, and then removing the aluminum with hydrochloric acid (see, for example, Patent Document 1). This porous silicon is said to completely prevent pulverization due to the expansion and contraction of the active material volume during charging and discharging, as well as peeling of the active material from the current collector and loss of contact with the conductive material. Another proposed method for producing a silicon material involves separating a Si alloy of an intermediate alloying element, such as Mg, Co, Cr, Cu, or Fe, with Si in a molten alloy containing a specified molten alloy element into a second phase in which the intermediate alloying element replaces the molten alloy element and Si fine particles, and then removing the second phase (see, for example, Patent Document 2). This porous silicon material is said to have high capacity and high cycle characteristics.

Japanese Patent Application Laid-Open No. 2004-214054 JP 2012-82125 A

However, while the manufacturing method described in Patent Document 1 uses an alloy containing Si and Al to create porosity, it is still insufficient in preventing defects caused by the expansion and contraction of Si. The manufacturing method for porous silicon material described in Patent Document 2 requires melting a silicon alloy containing intermediate alloying elements and replacing it with a molten metal containing a specified molten metal element, i.e., high-temperature processing, and a simpler manufacturing process was desired.

The present disclosure has been made in consideration of these issues, and its primary objective is to provide a method for manufacturing a porous silicon material, a porous silicon material, and an electricity storage device that can further suppress deterioration in charge-discharge characteristics.

After extensive research to achieve the above-mentioned objectives, the inventors discovered that by preparing a silicon alloy containing Co in addition to Al in a composition ratio close to the eutectic composition and then removing the Al by acid and/or alkali treatment, a porous silicon material can be obtained that can further suppress deterioration in charge-discharge characteristics. This led to the completion of the disclosed method for producing a porous silicon material, the porous silicon material, and the electricity storage device.

That is, the method for producing a porous silicon material according to the present disclosure includes the steps of:
a precursor process in which a raw material containing Si, Al, and Co is melted and rapidly solidified to obtain a precursor of a silicon alloy;
a porosity forming step of removing Al components 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 1000 nm, the skeletal silicon has a three-dimensional network structure with voids, contains Co, and the average porosity is in the range of 50 vol% to 90 vol%.

The electricity storage device of the present disclosure includes:
a positive electrode including a positive electrode active material;
a negative electrode containing the 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 this effect is believed to be as follows. For example, silicon anodes for lithium-ion secondary batteries have a theoretical capacity of 4199 mAh/g, approximately 10 times the theoretical capacity of 372 mAh/g of conventional graphite, and further increases in capacity and energy density are expected. Meanwhile, silicon that absorbs lithium ions is Li _4.4 Si, and its volume expands to approximately four times that of the silicon before lithium absorption. The present disclosure selectively removes Al components other than silicon contained in silicon alloys using an acid or alkali that dissolves them, thereby easily producing porous silicon materials with small pore sizes and high porosity. Furthermore, it is believed that the solid solution of Co in the Si skeleton strengthens the porous body. Furthermore, the microstructure contains a Co-containing compound, which strengthens the silicon skeleton, thereby further suppressing capacity degradation due to skeleton changes caused by repeated charge-discharge cycles. When a porous silicon material with such small pore size and large porosity is used in an electricity storage device such as a lithium ion secondary battery, volume expansion and contraction are significantly alleviated, thereby improving charge/discharge characteristics such as charge/discharge cycle characteristics, and making it easy to obtain a high-performance electricity storage device.

Al-Si phase diagram. Al—Si—Co phase diagram with 5 at % Co. Al—Si—Co phase diagram with 15 at % Co. 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 a silicon particle before being made porous in Experimental Example 1. 1 is a cross-sectional SEM image of porous silicon after porosity formation in Experimental Example 1. 1 shows XRD measurement results before and after acid treatment in Experimental Example 1. 1 shows the pore distribution curve of the porous silicon material of Experimental Example 1.

(Method for producing porous silicon material)
The method for producing a porous silicon material according to the present disclosure includes a precursor process and a porosity process. In the precursor process, a raw material containing Si, Al, and Co is melted and rapidly solidified to obtain a silicon alloy precursor. 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 phase diagram. Figure 2 is an Al-Si-Co phase diagram with 5 at% Co. Figure 3 is an Al-Si-Co phase diagram with 15 at% Co. Generally, in the equilibrium phase diagram of a eutectic A-B alloy, when a melt with a eutectic composition where the liquidus line is at its minimum is solidified, a eutectic structure is formed in which the A and B phases are separated into fibrous (lamellar) phases. The size of the lamellar structure formed becomes finer as the cooling rate increases, and at cooling rates of 1000°C/s or higher, a nano-sized structure is formed. 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 and B elements near the eutectic composition, resulting in a porous body with 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 Co as the B elements, the equilibrium diagram shows that near the Al-Si-Co ternary eutectic composition, SiCo compounds, AlSiCo compounds, and Si crystallize, while primary Al and AlCo compounds also crystallize or precipitate. The solidification structure forms a lamellar structure consisting of SiCo compounds, AlSiCo compounds, AlCo compounds, eutectic Si, primary Si, and primary Al. Since primary Al and AlCo compounds are thought to be the compounds that dissolve in the acid, it is presumed that the structure obtained after acid treatment will be a skeletal porous body consisting of the remaining SiCo compounds, AlSiCo compounds, eutectic Si, and primary Si.

(Precursor process)
In the precursor process, it is preferable to use a raw material containing Co in a range of 1 at% to 15 at% and Al in a range of 50 at% to 85 at% with the remainder being Si, when the total of Si, Al, and Co is taken as 100 at%. The raw material may contain unavoidable impurities. Examples of unavoidable impurities include components that inevitably remain during the refining of Si, Al, or Co, such as Fe, C, Cu, Ni, and P. The amount of unavoidable impurities is preferably as small as possible, and for example, when the total of Si, Al, and Co is taken as 100 at%, it is preferably 5 at% or less, and more preferably 2 at% or less. The Co compounding ratio is preferably, for example, 2 at% or more, and may be 3 at% or more. The Co compounding ratio is preferably 12.5 at% or less, and may be 10 at% or less. A lower Co compounding ratio is preferable from the viewpoint of element abundance. On the other hand, a higher Co compounding ratio is preferable from the viewpoint of additive effect. The Al content is preferably 55 at% or more, more preferably 60 at% or more. The Al content is preferably 82.5 at% or less, and may be 80 at% or less. Silicon alloys containing Al and Co within these ranges are preferred because they can increase the porosity and obtain voids of a more suitable shape and size. A high Al content leads to the precipitation of a large single phase of Al when the alloy is melted and rapidly cooled, thereby forming many voids. The cooling rate is preferably more rapid, and may be, for example, from 10 ² °C/s to 10 ⁸ °C/s from the molten state. In this process, it is preferable to use a silicon alloy containing Al in a range that results in a eutectic composition. The eutectic composition is 87.6 mass% Al and 12.6 mass% Si, but it may be close to the eutectic composition or have a certain range, such as a hypoeutectic composition or a hypereutectic composition. For example, it may include a range of ±5 mass% of the eutectic composition.

In this process, any melting method can be used to melt the raw materials, but high-frequency crucible melting in an inert gas atmosphere such as Ar is preferred. In the precursor process, the alloy obtained from the raw materials can be granulated. This granulation process can involve casting a molten silicon alloy raw material into a mold and crushing the resulting ingot into particles. The silicon alloy can also be granulated by one or more of the following methods: gas atomization, water atomization, and roll quenching. Gas atomization and water atomization produce alloy powder. On the other hand, roll quenching produces a thin ribbon alloy, which can then be crushed and powdered. The powder obtained by roll quenching has a fine alloy structure, so porous silicon with fine pores can be obtained after the leaching process. Of these, gas atomization is more preferred for granulating the silicon alloy. In gas atomization, the molten alloy is preferably produced in an Ar atmosphere, and the granulation is preferably carried out in an Ar or He atmosphere.

In the precursor process, the silicon alloy is preferably granulated to an average particle size in the range of 0.1 μm to 100 μm. For example, the average particle size of these 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 selected appropriately depending on the characteristics required of the energy storage device. Here, the average particle size of the particles is determined by observing the particles with a scanning electron microscope (SEM), tallying the major axis of each particle as the diameter of that particle, and dividing this by the number of particles to obtain the average value. The particles obtained by this granulation process will have the average particle size of the porous particle aggregate that is ultimately desired.

In this precursor process, a raw material containing a second element including, in addition to the first element and Si, 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 or Co; for example, it is preferably in the range of 10% by mass or less, more preferably 5% by mass or less, of the total silicon alloy.

(Porous process)
The porous formation process involves removing substances other than Si from the silicon alloy particles prepared above. Examples of substances other than Si include Al and its compounds, Co and its compounds, and the like. 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 preferably elutes elements and/or compounds other than silicon in the silicon alloy but does not elute silicon, such as hydrochloric acid, sulfuric acid, or 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 Co and its compounds, and can be, for example, between 1 mol/L and 5 mol/L. This removal process may be performed by heating the solution at, for example, 30°C to 60°C. Preferably, the removal process involves immersing the silicon alloy particles in an acid or alkali solution and stirring for approximately 1 to 5 hours. The resulting porous silicon material is then washed and dried.

In the porous formation process, substances other than Si may be removed in a range of 70% by mass to 100% by mass. For example, Al, Co, and other oxygen may remain, but when used as an electrode active material, it is preferable that they remain as little as possible from the viewpoint of charge/discharge capacity. Furthermore, components such as Al and Co are preferably contained in a predetermined amount or more from the viewpoint of reinforcing the silicon skeleton and improving durability. This process may also produce a porous silicon material containing a SiCo compound, a CoAl compound, and an AlSiCo compound. Examples of SiCo compounds include Si _x Co _y (x and y are arbitrary numbers), such as SiCo ₂ . Examples of CoAl compounds include Co ₂ Al ₉ and CoAl ₃ . The AlSiCo compound may be, for example, an Al _a Si _b Co _c compound (a, b, and c are any numbers), such as Co ₅ Al ₁₄ Si ₂ , Co ₆ Al ₁₁ Si ₆ , and Co ₃ Al ₃ Si _4. The SiCo compound or AlSiCo 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 SiCo compounds in a range of 1 mol% to 15 mol% when the total of Si, SiCo compounds, and AlSiCo compounds is taken as 100 mol%. Furthermore, in the porous silicon material, the SiCo compounds are preferably contained in an amount of 2.5 mol% or more, and may be contained in an amount of 5 mol% or more. Furthermore, in the porous silicon material, the SiCo compounds are preferably contained in an amount of 20 mol% or less, and may be contained in an amount of 15 mol% or less. Furthermore, in this step, it is preferable to obtain a porous silicon material containing AlSiCo compounds in a range of 1 mol% to 20 mol% when the total of Si, SiCo compounds, and AlSiCo compounds is taken as 100 mol%. Furthermore, in the porous silicon material, the AlSiCo compounds may be contained in an amount of 5 mol% or more, or may be contained in an amount of 10 mol% or more. The porous silicon material may contain 20 _mol % or less of the AlSiCo compound, or 10 mol % or less of _the AlSiCo compound. In this step, it is preferable to obtain a porous silicon material in which _one or more _{of Co5Al14Si2} , _{Co6Al11Si6 ,} and _Co3Al3Si4 remain _.

The porosity forming process may produce a porous silicon material with an average porosity in the range of 50% to 95% by volume. This average porosity is a value measured using a mercury porosimeter. The average porosity is preferably, for example, 54% by volume or more, and may be 60% by volume or more. The average porosity is also preferably, for example, 90% by volume or less, and may be 85% by volume or less.

(Porous silicon material)
The porous silicon material of the present disclosure may be one produced by the above-described manufacturing method. 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. The porous silicon material has a pore size distribution range of 1 nm or more and 1000 nm or less, 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 may also be 500 nm or less, 300 nm or less, or 250 nm or less. In this porous silicon material, the average pore size determined by mercury intrusion porosimetry may be in the range of 10 nm or more and 1000 nm or less, or in the range of 50 nm or more and 250 nm or less.

The porous silicon material contains skeletal silicon with a three-dimensional network structure having voids and contains Co. When containing Co, this porous silicon material may contain SiCo compounds, CoAl compounds, and AlSiCo compounds. The SiCo compounds and AlSiCo compounds are presumed to reinforce the silicon skeleton. Co may also be solid-dissolved in Si. Examples of _CoAl compounds include _Co2Al9 and _CoAl3 . Examples of _AlSiCo compounds include _{Co5Al14Si2 , Co6Al11Si6} , and _{Co3Al3Si4 .} This porous silicon material preferably contains the SiCo compound in a range of 1 mol% _to 15 _mol %. Furthermore, the porous silicon material preferably contains the AlSiCo compound in a range of 1 mol% to ₂₀ mol%.

The porous silicon material has an average porosity in the range of 50% to 95% by volume. This average porosity is preferably 54% by volume or more, and more preferably 60% by volume or more. The average porosity varies depending on, for example, the Al content in the raw material. This average porosity is a value measured using 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. This porous silicon material preferably contains 45 at% or more of Si, excluding oxygen and unavoidable impurities, when the total of Si, Al, and Co is 100 at%. This Si content is preferably higher, and may be 50 at% or more, or the remainder may be Al and Co. The porous silicon material preferably contains Co in the range of 1 at% to 15 at% and Al in the range of 1 at% to 20 at% when the total of Si, Al, and Co is 100 at%, with the remainder being Si. The remainder may include unavoidable impurities. In this porous silicon material, the Co content is preferably 2 at% or more, and may be 3 at% or more. The Co content is preferably 12.5 at% or less, and may be 10 at% or less. The Al content in the porous silicon material is preferably 2 at% or more, and may be 3 at% or more. The Al content is preferably 17.5 at% or less, and may be 15 at% or less. If the raw material composition of this porous silicon material is Al-20 at% Si-5 at% Co, the porous silicon material after removing the Al will be Si-10.1 at%-5.7 at% Co. The porous silicon material may also contain one or more of Ca, Cu, Mg, Na, Sr, and P as a second element in a range of 15 mass% or less. It is preferable that the amount of elements other than Si be as small as possible.

(Electrodes for electricity storage devices)
An electrode for a power storage device includes the porous silicon material described above as an electrode active material. This electrode functions as either a positive electrode or a negative electrode depending on the potential of the electrode active material relative to the potential of the counter electrode. When lithium is used as a carrier, however, it is preferably used as a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, air batteries, and the like. The electrode for a power storage device may be one in which the average porosity of the porous silicon material is compressed to a range of 0.1 vol% to 50 vol%. In this electrode, the porosity of the porous silicon material may be reduced by compression during fabrication. Compared to an electrode fabricated with a porosity of 0.1 vol% to 50 vol%, an electrode fabricated with a porosity of 50 vol% to 95 vol% and then compressed to this range exhibits better charge/discharge characteristics due to the shape of the pores, etc. For example, when porous silicon particles are used as a negative electrode active material for a lithium ion secondary battery, the smaller the pores, the more uniform the lithium ions will be when alloyed, reducing stress concentration and preventing deterioration of the electrode itself. The average porosity of the compressed porous silicon material may be adjusted as appropriate depending on the properties required of the electrode for an electrical storage device, and may be, for example, 10% by volume or more, 20% by volume or more, or may be, for example, 40% by volume or less, or 30% by volume or less.

An electrode for an electricity storage device may be formed by forming the porous silicon material on a current collector and adhering it to the current collector. This electrode can be fabricated by either mixing the porous silicon material with a solvent and, if necessary, a conductive material and a binder to form a paste, which is then applied to the current collector, or by mixing the porous silicon material with a conductive material and a binder if necessary and then pressing the mixture onto the current collector. In this electrode, the porous silicon material content is preferably higher, preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. For example, a mixture of one or more of graphite, such as natural graphite (e.g., scaly graphite or flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, or metals (e.g., copper, nickel, aluminum, silver, gold, etc.), can be used. The binder serves to bind the active material particles and conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber; thermoplastic resins such as polypropylene and polyethylene; ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, and natural butyl rubber (NBR), which can be used alone or in a mixture of two or more. Water-based binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR) can also be used. Examples of solvents that can be used include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, dispersants, thickeners, etc. can be added to water to form a slurry of the active material with a latex such as SBR. Application methods include, for example, roller coating (e.g., applicator roll), screen coating, doctor blade coating, spin coating, and bar coating. Any of these can be used to obtain the desired thickness and shape. The current collector can be selected appropriately depending on the potential of the active material. Materials that can be used include aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymers, and conductive glass. Materials such as aluminum and copper whose surfaces are treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance can also be used. These surfaces can also be subjected to oxidation treatment. Current collector shapes include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fiber-grouped. The thickness of the current collector is, for example, 1 to 500 μm. The amount of the active material composite formed can be appropriately determined depending on the desired performance of 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, as long as 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 _Li4Co5O12 . However, from the viewpoint of further increasing the battery capacity, it is preferable that the porous silicon material accounts for, for example, 50% by mass or more, preferably 90% by mass or more, of the entire electrode active material taken as 100% by mass.

(Electricity storage device)
The electricity storage device of the present disclosure includes an electrode having the porous silicon material described above. This electricity storage device may include a positive electrode, a negative electrode, and an ion-conducting medium interposed between the positive electrode and the negative electrode and conducting carrier ions. The porous silicon material can be used as the negative electrode active material. This electricity storage device may be any of a lithium-ion secondary battery, a hybrid capacitor, an air battery, and the like. In the positive electrode, the positive electrode active material may be a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like. Specifically, transition metal sulfides such as _CoS2 , _CoS3 , _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) or 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, lithium ion capacitors, and the like. 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 a carbonaceous material preferably has a specific surface area of 1000 m ² /g or more, and more preferably 1500 ^{m 2} /g or more. A specific surface area of 1000 m ² /g or more can further increase the discharge capacity. From the viewpoint of ease of production, the specific surface area of this activated carbon is preferably 3000 ^{m 2} /g or less, and more preferably 2000 m ² /g or less. The conductive material, binder, solvent, current collector, etc. used in the positive electrode can be appropriately selected from those exemplified for the electrode described above.

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 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 battery characteristics during repeated charge and discharge, but also allows for 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 a combination of one or more salts selected from _the group consisting of inorganic salts such as _LiPF6 , _LiBF4 , _LiAsF6 , and _LiClO4 , and organic salts such as _LiCF3SO3 , LiN( _{CF3SO2 ) 2} , and LiC( _CF3SO2 ) ₃ , 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, more preferably 0.5 mol/L or more and 2 mol/L or less. When the concentration of the dissolving supporting salt is 0.1 mol/L or more, a sufficient current density can be obtained, and when it is 5 mol/L or less, the electrolyte can be more stable. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to the non-aqueous electrolyte.

In place of a liquid ion-conducting medium, a solid ion-conducting polymer can also be used as the ion-conducting medium. Examples of ion-conducting polymers that can be used include polymer gels composed of a polymer, such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride, and a supporting salt. Furthermore, ion-conducting polymers can also be used in combination with nonaqueous electrolyte solutions. In addition to ion-conducting polymers, other ion-conducting media that can be used include inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, and inorganic solid powders bound with an organic binder.

The power storage device may include a separator between the negative electrode and positive electrode. There are no particular limitations on the separator, as long as its composition can withstand the range of use of lithium secondary batteries. Examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric and polyphenylene sulfide nonwoven fabric, and thin microporous membranes made 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, button, sheet, laminate, cylindrical, flat, and rectangular shapes. It may also be applied to large devices used in electric vehicles and the like. Figure 4 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-conducting medium 18. The positive electrode 12 has a positive electrode active material 13 and a current collector 14. The negative electrode 15 has a negative electrode active material 16 and a current collector 17. The negative electrode active material 16 is the porous silicon material 21 described above, and has voids 23.

(All-solid-state lithium-ion secondary battery)
This electricity storage device is preferably an all-solid-state lithium ion secondary battery. All-solid-state batteries are preferable because they can further suppress changes in performance due to the electrolyte solution and further enhance safety. This all-solid-state lithium ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode that is the above-mentioned electrode for the electricity storage device, and a solid electrolyte that conducts lithium ions and is interposed between the positive electrode and the negative electrode. The positive electrode can be any of those shown in the above-mentioned electricity storage device. Furthermore, the negative electrode can be the above-mentioned electrode for the electricity storage device.

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 , where A is one or more of Sr and Ca, T is one or more of Nb and Ta, and 0<x≦1.0 and 0<y<0.75 are satisfied. Alternatively, the solid electrolyte may be a garnet-type oxide having 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, the element M may be one or more of Al and Ga, the 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. Furthermore, it is more preferable that 0.1≦y≦0.8 is satisfied. In such ranges, the ionic conductivity can be made more suitable.

Alternatively, examples of solid electrolytes include common _Li3N , _Li14Zn ( _GeO4 ) ₄ called LISICON, sulfide _{Li3.25Ge0.25P0.75S4} , perovskite-type _{La0.5Li0.5CoO3} , (La2 _{/ 3Li3x □ 1/3-2x} )CoO3 (□: atomic vacancy) _, garnet- _{type Li7La3Zr2O12} , NASICON-type _{LiCo2 ( PO4 ) 3 , Li1.3M0.3Co1.7(PO3)4 ( M = Sc , Al} ), and _{the like} . Other examples include Li ₇ P ₃ S ₁₁ obtained from glass with a composition of 80Li ₂ S·20P ₂ S ₅ (mol %), which is a glass ceramic, and Li ₁₀ Ge ₂ PS ₂ , a sulfide-based substance with high 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 thiolicon solid electrolytes include _Li2S -GeS2 _systems , _Li2S - _GeS2 -ZnS systems, Li2S- _Ga2S2 systems, _Li2S - _GeS2 - _{Ga2S3 systems} , _Li2S - _GeS2 - _{P2S5 systems} , _Li2S - _GeS2 - _{SbS5 systems} , _Li2S - _GeS2 - _{Al2S3 systems} , _Li2S -SiS2 _systems , Li2S-P2S5 _systems , _Li2S - _{Al2S3 systems} , LiS- _{SiS2 -Al2S3 systems , and Li2S - SiS2} - _{P2S5 systems} . These solid electrolytes may be formed into a plate shape and placed 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 members that sandwich the stack from both ends of the stack in the stacking direction, a rod-shaped member that connects the pair of plate-shaped members, and an adjustment member that is connected to the rod-shaped members and adjusts the spacing between the pair of plate-shaped members using 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 this effect is believed to be as follows. For example, silicon anodes for lithium-ion secondary batteries have a theoretical capacity of 4199 mAh/g, approximately 10 times the theoretical capacity of 372 mAh/g of conventional graphite, and further increases in capacity and energy density are expected. Meanwhile, silicon that has absorbed lithium ions is Li _4.4 Si, and its volume expands to approximately four times that of the silicon before lithium absorption. According to the present disclosure, Al components other than silicon contained in silicon alloys can be selectively removed using an acid or alkali that dissolves them, making it easy to produce porous silicon materials with small pore sizes and high porosity. Furthermore, it is believed that the solid solution of Co in the Si skeleton strengthens the porous body. Furthermore, the microstructure contains a Co-containing compound, which strengthens the silicon skeleton, thereby further suppressing capacity degradation due to skeleton changes caused by repeated charge-discharge cycles. When porous silicon materials with such small pore sizes and large porosity are used in energy storage devices such as lithium-ion secondary batteries, volume expansion and contraction are greatly alleviated, thereby improving charge/discharge characteristics, such as charge/discharge cycle characteristics, and thus making it easy to obtain high-performance energy storage devices. Furthermore, it is believed that the inclusion of Co when rapidly cooling a Si-Al-based raw material after melting it can suppress coarsening of the Si, allowing for the creation of a finer Si structure. Furthermore, it is believed that, for example, in a eutectic composition, the addition of Co produces a CoAl alloy, but no silicide. Silicides have properties such as being difficult to dissolve in acid, and remain in the porous body, but the absence of such formation is also believed to be an advantage.

It goes without saying that the present disclosure is in no way 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.

Below, specific examples of fabricating the porous silicon and electricity storage device of the present disclosure are described as experimental examples. Experimental Example 1 is an example of the present disclosure, and Experimental Example 2 is a comparative example.

[Preparation of porous silicon material]
(Experimental Example 1)
70% by mass of 10 mm square chunks of Al, 19.6% by mass of chunks of Si, and 10.3% by mass of chunks of Co were weighed and mixed, and then melted by high-frequency heating in an Ar inert atmosphere to form a molten alloy. This molten alloy was then gas atomized using Ar inert gas to obtain an AlSiCo alloy powder with an average particle size of 3 μm (precursor process). The quenching rate was 10 ^° C/s. X-ray diffraction analysis of the resulting powder confirmed the presence of Al and Si phases as crystalline phases. The atomic composition was Al-20 at% Si-5 at% Co. Next, the resulting alloy powder was placed in 3 mol/L hydrochloric acid diluted with pure water, stirred at room temperature (25°C) for 1 hour, then thoroughly washed and filtered, and dried in a vacuum drying oven at 30°C for 2 hours (porosity formation process). In this way, the porous silicon material of Experimental Example 1 was produced.

(Experimental Example 2)
The silicon material in Experimental Example 4 was Si powder with an average particle size of 5 μm.

(Measurement of the physical properties 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 PS3520UVDDIII II). Observation and elemental analysis were also performed using a scanning electron microscope (SEM, Hitachi S-4300) and energy dispersive X-ray analysis (EDAX, Hitachi S-4300). The pore distribution was also measured using a mercury porosimeter (Quantachrome POWERMASTER 60GT).

(Fabrication of a lithium secondary battery using a non-aqueous electrolyte)
Each of the silicon materials of Experimental Examples 1 and 2 was used as the negative electrode active material. 60% by mass of this material, 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 20 μm thick copper foil, dried, and rolled to prepare a 50 μm thick negative electrode. It was estimated that this rolling process reduced the porosity of the porous silicon material to about 20% by volume while maintaining its three-dimensional network structure. The prepared negative electrode was punched 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 on top of it as a counter electrode to form a laminate. Next, an electrolyte solution containing ₁ mol/L of LiPF6 added to a mixed solvent of fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1.5:3:4:3 was poured into the laminate to produce a lithium secondary battery, a small Tomcell-type battery cell. The resulting lithium secondary battery was repeatedly charged and discharged for 10 cycles at a current density of 0.2 C over a battery voltage range of 0.005 V to 1.5 V.

(Results and Discussion)
FIG. 5 is a surface SEM image of a silicon particle in Experimental Example 1 before being made porous, with an enlarged view shown on the right. FIG. 6 is a cross-sectional SEM image of the porous silicon in Experimental Example 1 after being made porous, with an enlarged view shown on the right. FIG. 7 shows the results of XRD measurements before and after the acid treatment in Experimental Example 1. FIG. 8 is a pore distribution curve of the porous silicon material in Experimental Example 1. Table 1 summarizes the elemental compositions (mass %) and atomic compositions (at %) of the raw materials and porous bodies in Experimental Examples 1 and 2. Table 2 also summarizes the initial discharge capacities (mAh/g), discharge capacities (mAh/g) after 10 cycles, and capacity retention rates (%) for Experimental Examples 1 and 2. Table 2 also shows the porosity (volume %) and average pore diameter (nm) measured by a mercury porosimeter.

As shown in Figure 6, it was confirmed that the acid treatment dissolved primary Al and other Al components from the Si alloy, forming porous silicon particles. Furthermore, when the cross-section of the particles was observed, a skeletal silicon structure with a three-dimensional network structure of Si was confirmed. Furthermore, it was found that this silicon skeleton had relatively thick partition walls and a solid skeleton. From the cross-sectional SEM image shown in Figure 6, the pore size of the porous silicon particles was estimated to be less than 200 nm. Furthermore, the pore distribution of the porous silicon particles determined using a mercury porosimeter was 5 to 200 nm, as shown in Figure 8, with peak pore sizes at 5 to 10 nm and 40 to 70 nm, and the porosity was 54%.

As shown in Figure 7, X-ray diffraction detected peaks for Al and Si in the atomized powder, but the Si peak was broad, suggesting that Co was dissolved in the Si, creating an amorphous structure. After acid treatment, the Al peak was no longer detected in the porous silicon particles, and instead, a Si peak representing Co dissolved in the amorphous structure was confirmed. It was also suggested that the acid treatment eluted the Al phase and AlCo compounds, creating pores in these areas.

The capacity retention rate was calculated using the discharge capacity _Q1 at the first cycle and the discharge capacity _Q10 at the tenth cycle according to the formula _Q10 / _Q1 × 100. In Experimental Example 1, Al and Co were present and the Si phase was relatively small, so the initial discharge capacity was smaller than that of Experimental Example 2. In Experimental Example 2, the capacity retention rate decreased to 28% after 10 cycles. This is presumably because silicon particles without voids cannot absorb volume changes, causing electrode defects. On the other hand, the lithium secondary battery of Experimental Example 1 exhibited a good capacity retention rate of 92%, demonstrating the electrode's stability. It was found that when Co is further contained in a porous silicon material exhibiting a porosity of 50% by volume or more, the Co solid-solubilization in Si and the inclusion of a Co compound can reinforce the Si skeleton, further suppressing deterioration in cycle characteristics and increasing the capacity retention rate. It was speculated that the inclusion of Co would likely narrow the phase width of the Si liquid, thereby suppressing coarsening of the Si. Thus, it was speculated that creating a porous silicon material based on an Al-Si-Co ternary alloy would enable the creation of a finer Si structure. Furthermore, for example, a CoAl alloy is one example of a phase that is produced by adding Co to a eutectic composition, but silicide is not produced. Silicide has properties such as being difficult to dissolve in acid, and remains in the porous body, but it was speculated that not producing silicide is also an advantage.

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

This disclosure is applicable to 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 conductive medium, 21: Porous silicon material, 23: Voids.

Claims (7)

a precursor process in which a raw material containing Si, Al, and Co is melted and rapidly solidified to obtain a precursor of a silicon alloy;
a porosity forming step of removing Al components contained in the silicon alloy to obtain a porous silicon material ,
In the precursor step, when the total of Si, Al, and Co is 100 at%, a raw material containing Co in a range of 1 at% to 15 at% and Al in a range of 50 at% to 85 at% and the remainder being Si is used,
In the porosity forming step, a process is performed to obtain the porous silicon material having an average porosity in the range of 50% by volume or more and 95% by volume or less,
A method for producing a porous silicon material, wherein the porous silicon material contains Si in the range of 45 at% or more, Co in the range of 1 at% to 15 at% and Al in the range of 1 at% to 20 at% when the total of Si, Al and Co is 100 at%. The method for producing a porous silicon material according to claim 1 , wherein the porosity-forming step selectively removes the Al component with an acid or an alkali. The method for producing a porous silicon material according to claim 1 or 2 , wherein the porous silicon material obtained in the porosity-forming step contains Co. 4. The method for producing a porous silicon material according to 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 a gas atomization method, a water atomization method, and a roll quenching method. 5. The method for producing a porous silicon material according to claim 1, wherein the porous silicon material has a pore size distribution range of 1 nm to 1000 nm as determined by mercury intrusion porosimetry, contains skeletal silicon with a three-dimensional network structure having voids, contains Co, and has an average porosity of 50 vol% to 95 vol%. The method for producing a porous silicon material according to any one of claims 1 to 5 , wherein the porous silicon material contains one or more of a SiCo compound and an AlSiCo compound. The method for producing a porous silicon material according to any one of claims 1 to 6 , wherein the porous silicon material has an average porosity of 60% by volume or more.