Method for producing porous silicon material

Abstract

To suppress degradation in charge / discharge characteristics furthermore in a Si-containing material.SOLUTION: A method for producing a porous silicon material of the present disclosure includes: a precursor step of obtaining a precursor of a silicon alloy by melting raw materials including Si, Al and Co and quenching and solidifying the raw materials; and a porosification step of removing an Al component included in the silicon alloy to obtain a porous silicon material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] Disclosed herein are methods for producing porous silicon materials, porous silicon materials, and power storage devices. [Background technology]

[0002] Previously, a porous silicon negative electrode material was proposed by mixing 70 parts by mass of block silicon with 30 parts by mass 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 silicon alloy containing intermediate alloying elements, such as magnesium, cobalt, chromium, copper, and iron, into a second phase in which the intermediate alloying elements replace the molten metal elements and silicon fine particles in a molten metal containing selected molten metal elements, and then removing the second phase to obtain a porous silicon material (see, for example, Patent Document 2). This porous silicon material is said to have high capacity and high cycle performance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-214054 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-82125 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the manufacturing method of Patent Document 1, an alloy containing Si and Al is used to make the material porous, but this method 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 predetermined molten metal element, i.e., a high-temperature treatment is required, and a simple manufacturing process has been desired.

[0005] The present disclosure has been made in consideration of these problems, and a main object of the present disclosure 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. [Means for solving the problem]

[0006] As a result of intensive research to achieve the above-mentioned object, the present inventors have found that by preparing a silicon alloy containing Co in addition to Al in a compounding ratio close to the eutectic composition and then removing the Al by an acid and / or alkali treatment, a porous silicon material can be obtained that can further suppress deterioration in charge-discharge characteristics, and have completed the method for producing a porous silicon material, the porous silicon material, and the electricity storage device of the present disclosure.

[0007] 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:

[0008] The porous silicon material of the present disclosure comprises: The pore size distribution range determined by mercury intrusion porosimetry is between 1 nm and 1000 nm, the material contains skeletal silicon with a three-dimensional network structure having voids, contains Co, and has an average porosity between 50 vol% and 90 vol%.

[0009] 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. [Effects of the Invention]

[0010] The present disclosure can further suppress the deterioration of charge-discharge characteristics in materials containing Si. The reason for this effect is presumed to be as follows. For example, 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 ordinary graphite, 372 mAh / g, and further increases in capacity and energy density are expected. On the other hand, silicon that has absorbed lithium ions absorbs Li 4.4 The porous silicon material is Si, and its volume expands to approximately four times that of silicon before lithium absorption. In the present disclosure, the Al components other than silicon contained in the silicon alloy are selectively removed using an acid or alkali that dissolves them, making it possible to easily produce a porous silicon material with small pore size and high porosity. It is also 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 and is therefore believed to further suppress capacity degradation associated with skeleton fluctuations due to repeated charge and discharge. When such porous silicon materials with small pore size and high porosity are used in energy storage devices such as lithium-ion secondary batteries, volume expansion and contraction are significantly reduced, thereby improving charge and discharge characteristics, such as charge and discharge cycle characteristics, and thus making it easy to obtain high-performance energy storage devices. [Brief explanation of the drawings]

[0011] [Figure 1] Al-Si phase diagram. [Figure 2] Al-Si-Co phase diagram with 5 at% Co. [Figure 3] Al-Si-Co phase diagram with 15 at% Co. [Figure 4] FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10. [Figure 5] 1 is a surface SEM image of a silicon particle in Experimental Example 1 before it is made porous. [Figure 6] 1 is a cross-sectional SEM image of porous silicon after porosity formation in Experimental Example 1. [Figure 7] 1 shows XRD measurement results before and after acid treatment in Experimental Example 1. [Figure 8] 1 shows the pore distribution curve of the porous silicon material of Experimental Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Method of manufacturing 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.

[0013] Figure 1 shows the Al-Si phase diagram. Figure 2 shows the Al-Si-Co phase diagram with 5 at% Co. Figure 3 shows the Al-Si-Co phase diagram with 15 at% Co. Generally, in the equilibrium phase diagram of a eutectic AB 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 and characterized by a eutectic structure can be obtained. The microstructure can be changed 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 phase 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.

[0014] (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 refinement 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 content is preferably, for example, 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. A lower Co content is preferable from the viewpoint of element abundance. On the other hand, a higher Co content is preferable from the viewpoint of additive effects. The Al compounding ratio is preferably 55 at% or more, more preferably 60 at% or more. The Al compounding ratio is preferably 82.5 at% or less, and may be 80 at% or less. A silicon alloy containing Al or Co in such a range is preferable because it can further increase the porosity and obtain voids of a more suitable shape and size. If the Al content is high, when the alloy is melted to form it and then rapidly cooled, a single phase of Al precipitates in large amounts, allowing many voids to be formed. The cooling rate is preferably more rapid, for example, 10 2 ℃ / s or more 10 8 The temperature may be in the range of °C / s or less. 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 may have a predetermined range, such as a hypoeutectic composition or a hypereutectic composition. For example, it may be in the range of ±5 mass% of the eutectic composition.

[0015] In this process, any melting method may 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 may be granulated. In this granulation process, a molten silicon alloy raw material may be cast into a mold, and the resulting ingot may be crushed and granulated. The silicon alloy may be granulated by one or more of gas atomization, water atomization, and roll quenching. Gas atomization and water atomization produce alloy powder. On the other hand, roll quenching produces a thin strip alloy, which may then be crushed and granulated. 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 as a method 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.

[0016] 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. 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 appropriately selected depending on the properties required for 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 an average value. The particles obtained by this granulation process will have the average particle size of the aggregate of porous particles to be ultimately obtained.

[0017] 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, and is, for example, preferably in the range of 10% by mass or less, more preferably 5% by mass or less, of the entire silicon alloy.

[0018] (Porous process) The porous forming step 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 step, 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, and examples of such acids include hydrochloric acid, sulfuric acid, and sodium hydroxide. This acid or alkali is preferably in the form of an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it is capable of removing Al and its compounds, and Co and its compounds, and can be, for example, in the range of 1 mol / L to 5 mol / L. This removal treatment may be performed by heating at, for example, 30°C to 60°C. Furthermore, the removal treatment preferably 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.

[0019] In the porous forming step, 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 are present in a smaller amount 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. In this step, a porous silicon material containing a SiCo compound, a CoAl compound, and an AlSiCo compound may be obtained. Examples of SiCo compounds include Si x Co y(x and y are arbitrary numbers), and examples thereof include SiCo2. Examples of CoAl compounds include Co2Al9 and CoAl3. Examples of AlSiCo compounds include Al a Si b Co c It may be a compound (a, b, c are any numbers), Co5Al 14 Si2, Co6Al 11 Examples include Si6 and Co3Al3Si4. The SiCo compound and AlSiCo compound may be one that is poorly soluble in acid or alkali.

[0020] 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, the porous silicon material preferably contains 2.5 mol% or more of SiCo compounds, and may contain 5 mol% or more. Furthermore, the porous silicon material preferably contains 20 mol% or less of SiCo compounds, and may contain 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, the porous silicon material may contain 5 mol% or more, or may contain 10 mol% or more of AlSiCo compounds. Furthermore, the porous silicon material may contain 20 mol% or less, or may contain 10 mol% or less of AlSiCo compounds. In this step, Co5Al 14 Si2, Co6Al 11 It is preferable to obtain a porous silicon material in which one or more of Si6 and Co3Al3Si4 remain.

[0021] The porosity forming step may produce a porous silicon material having an average porosity in the range of 50% by volume or more and 95% by volume or less. This average porosity is a value measured with 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.

[0022] (porous silicon material) The porous silicon material of the present disclosure may be 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 to 1000 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 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 as determined by mercury intrusion porosimetry may be in the range of 10 nm to 1000 nm or in the range of 50 nm to 250 nm.

[0023] 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. It is presumed that the SiCo compounds and AlSiCo compounds reinforce the silicon skeleton. Co may also be in solid solution with Si. Examples of CoAl compounds include Co2Al9 and CoAl3. Examples of AlSiCo compounds include Co5Al. 14 Si2, Co6Al 11Examples of suitable porous silicon materials include Si6 and Co3Al3Si4. The porous silicon material preferably contains 1 mol% to 15 mol% of a SiCo compound. The porous silicon material preferably contains 1 mol% to 20 mol% of an AlSiCo compound.

[0024] The porous silicon material has an average porosity in the range of 50% by volume or more and 95% by volume or less. 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 with a mercury porosimeter.

[0025] 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, when the total of Si, Al, and Co, excluding oxygen and unavoidable impurities, is taken as 100 at%. The 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 taken as 100 at%, with the remainder being Si. The remainder may include unavoidable impurities. In this porous silicon material, the Co content is more preferably 2 at% or more, and may be 3 at% or more. The Co content is more preferably 12.5 at% or less, and may be 10 at% or less. The Al content in the porous silicon material is more preferably 2 at% or more, and may be 3 at% or more. The Al content is more preferably 17.5 at% or less, and may be 15 at% or less. When the raw material composition of this porous silicon material is Al-20 at% Si-5 at% Co, the porous silicon material after removing Al becomes 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 is as small as possible.

[0026] (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 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% by volume to 50% by volume. 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% by volume to 50% by volume, an electrode fabricated with a porosity of 50% by volume to 95% by volume and then compressed to this range exhibits better charge / discharge characteristics due to the shape of the pores. 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.

[0027] The electrode for a power storage device may be formed by forming the porous silicon material on a current collector and then adhering the material to the current collector. This electrode can be produced 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 content of the porous silicon material is preferably as high as possible, 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 electron-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, and metals (e.g., copper, nickel, aluminum, silver, and gold), 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), either alone or in combination. 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, and the like can be added to water to form a slurry of the active material with a latex such as SBR. Examples of application methods include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any of these can be used to obtain a desired thickness and shape.The current collector may be appropriately selected according to the potential of the active material, etc. For example, in addition to aluminum, titanium, stainless steel, nickel, iron, copper, fired carbon, conductive polymers, conductive glass, etc., for the purpose of improving adhesion, conductivity and oxidation resistance, those obtained by treating the surface of aluminum or copper with carbon, nickel, titanium, silver, etc. can be used. It is also possible to oxidize-treat these surfaces. Examples of the shape of the current collector include foil shape, film shape, sheet shape, net shape, punched or expanded shape, lath body, porous body, foam body, and formed body of fiber group. The thickness of the current collector is, for example, 1 to 500 μm. The formation amount of the active material composite may be appropriately set according to the desired performance required for the power storage device.

[0028] In this electrode, the electrode active material may contain an active material other than the porous silicon material in addition to the porous silicon material within the range where both low restraint pressure and capacity retention rate can be achieved. For example, as the electrode active material, a carbonaceous material, Li4Co5O 12 etc. may be included. However, from the viewpoint of further increasing the battery capacity, it is preferable that the porous silicon material occupies 50% by mass or more, preferably 90% by mass or more, with the entire electrode active material being 100% by mass.

[0029] (Power storage device) The power storage device of the present disclosure includes an electrode having the above-described porous silicon material. This power storage device may include a positive electrode, a negative electrode, and an ion conduction medium interposed between the positive electrode and the negative electrode for conducting carrier ions. The porous silicon material can be used as a negative electrode active material. This power storage device may be any of a lithium ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, etc. can be used. Specifically, transition metal sulfides such as CoS2, CoS3, MoS3, FeS2, basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same below) and Li (1-x)Lithium manganese composite oxides such as Mn2O4, with the basic composition formula Li (1-x) Lithium cobalt composite oxides such as CoO2, with the basic composition formula Li (1-x) Lithium nickel composite oxide such as NiO2, the basic composition formula is Li (1-x) Ni a Co b Mn c Lithium nickel cobalt manganese composite oxides with a basic composition formula of LiV2O3, etc., and transition metal oxides with a basic composition formula of V2O5, etc., can be used. Among these, lithium transition metal composite oxides, such as LiCoO2, LiNiO2, LiMnO2, and Li (1-x) Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc. are preferred. The term "basic composition formula" means that other elements such as Al and Mg may also be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors, lithium ion capacitors, etc. Examples of carbonaceous materials include activated 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. Activated carbons as carbonaceous materials have a specific surface area of ​​1000 m 2 / g or more, and 1500m 2 / g or more is more preferable. 2 / g or more, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m 2 / g or less, and 2 The conductive material, binder, solvent, current collector, and the like used in the positive electrode can be appropriately selected from those exemplified for the electrode described above.

[0030] 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, the resulting battery's electrical capacity, and battery output. Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. Among these, a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO) is preferred from the standpoint 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 supporting salt is dissolved at a concentration of 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be more stable. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to this non-aqueous electrolyte.

[0031] Instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of the ion-conducting polymer 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, a combination of an ion-conducting polymer and a nonaqueous electrolyte can also be used. In addition to ion-conducting polymers, other ion-conducting media include inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, and inorganic solid powders bound by an organic binder.

[0032] The power storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery, and examples thereof 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.

[0033] The shape of this power storage device is not particularly limited, and examples include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, etc. It may also be applied to large-sized ones used in electric vehicles or the like. Fig. 4 is an explanatory diagram showing an example of the structure of the power storage device 10. This power 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.

[0034] (All-solid-state lithium-ion secondary battery) This power storage device is preferably an all-solid-state lithium-ion secondary battery. In an all-solid-state battery, changes in performance due to the electrolyte can be more effectively suppressed, and furthermore, safety can be enhanced, which is preferable. This all-solid-state lithium-ion secondary battery may include a positive electrode containing a positive electrode active material, a negative electrode which is the electrode for the power storage device described above, and a solid electrolyte interposed between the positive electrode and the negative electrode and conducting lithium ions. The positive electrode can use any of those shown in the power storage device described above. Also, the negative electrode can use the electrode for the power storage device described above.

[0035] The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La, and Zr. This solid electrolyte has a basic composition of Li 7.0+x-y (La 3-x , A x )(Zr 2-y , T y )O 12 and may be of this kind. However, A is one or more of Sr and Ca, T is one or more of Nb and Ta, and it satisfies 0 < x ≦ 1.0 and 0 < y < 0.75. Alternatively, the solid electrolyte has a basic composition (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 or (Li 7-3z+x-y M z )(La 3-x A x )(Zr 2-y T y )O 12 14 3.25 0.25 0.75 0.5 0.5 2 / 3 3x 1 / 3-2x 12 1.3 0.3 1.7 11 10 4.4 6 10 10

[0001] く、(Li

[0002] M )(La A )(Zr T )O

[0003] x )(Y 2-y T y )O 12 The garnet-type oxide may be represented by the formula (1). 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 be satisfied. In this basic composition formula, it is more preferable that 0.05≦x≦0.1 be satisfied. Furthermore, in this basic composition formula, it is more preferable that 0.1≦y≦0.8 be satisfied. In these ranges, the ionic conductivity can be more favorably achieved.

[0036] Alternatively, the solid electrolyte may be, for example, a common LiN or LISICON. 14 Zn(GeO4)4, Li sulfide 3.25 Ge 0.25 P 0.75 S4, perovskite-type La 0.5 Li 0.5 CoO3, (La 2 / 3 Li 3x □ 1 / 3-2x ) CoO3 (□: atomic vacancy), garnet-type Li7La3Zr2O 12 , LiCo2(PO4)3, also known as NASICON type, Li 1.3 M 0.3 Co 1.7 (PO3)4 (M = Sc, Al), etc. Also, Li7P3S obtained from glass ceramics with a composition of 80Li2S 20P2S5 (mol%) 11 Furthermore, Li, a sulfide-based material with high conductivity, 10Examples 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 that use SiO2, GeO2, B2O3, or P2O5 as the glass-based substance and Li2O as the network modifier. Examples of thiolithium solid electrolytes include Li2S-GeS2, Li2S-GeS2-ZnS, Li2S-Ga2S2, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S-GeS2-SbS5, Li2S-GeS2-Al2S3, Li2S-SiS2, Li2S-P2S5, Li2S-Al2S3, LiS-SiS2-Al2S3, Li2S-SiS2-P2S5, etc. These solid electrolytes may be formed into a plate shape and placed between the positive electrode and the negative electrode.

[0037] The all-solid-state lithium-ion secondary battery may also include a restraining member that restrains a stack of a positive electrode, a solid electrolyte, and a negative electrode in the stacking direction. This restraining member may include, for example, a pair of plate-like members that sandwich the stack from both ends of the stack in the stacking direction, a rod-like member that connects the pair of plate-like members, and an adjustment member that is connected to the rod-like members and adjusts the gap between the pair of plate-like members by a screw structure or the like.

[0038] As detailed above, the present disclosure can further suppress the deterioration of charge-discharge characteristics in materials containing Si. The reason for this effect is presumed to be as follows. For example, a silicon negative electrode for a lithium-ion secondary battery has a theoretical capacity of 4199 mAh / g, which is about 10 times the theoretical capacity of ordinary graphite, 372 mAh / g, and further increases in capacity and energy density are expected. On the other hand, silicon that has absorbed lithium ions absorbs Li 4.4The material is Si, and its volume expands to approximately four times that of silicon before lithium absorption. According to the present disclosure, by using an acid or alkali that dissolves Al components other than silicon contained in a silicon alloy, these components can be selectively removed, easily producing a porous silicon material with small pore size and high porosity. It is also 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 and thus is believed to further suppress capacity degradation associated with skeleton fluctuations due to repeated charge and discharge. When used in energy storage devices such as lithium-ion secondary batteries, porous silicon materials with small pore size and high porosity significantly reduce volume expansion and contraction, thereby improving charge and discharge characteristics, such as charge and discharge cycle characteristics, and thus facilitating the production of high-performance energy storage devices. It is also believed that the inclusion of Co when rapidly cooling after melting Si-Al-based raw materials can suppress coarsening of Si, thereby enabling the creation of a finer Si structure. Furthermore, for example, in the case of a eutectic composition, it is presumed that the phases that are formed when Co is added are CoAl alloys, but no silicides. Silicides have properties such as being difficult to dissolve in acid, and remain in the porous body, but it is presumed that the fact that they are not formed is also an advantage.

[0039] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure. [Example]

[0040] Specific examples of fabricating the porous silicon and electricity storage device according to the present disclosure will be described below as experimental examples. Experimental Example 1 is an example of the present disclosure, and Experimental Example 2 is a comparative example.

[0041] [Preparation of porous silicon materials] (Experimental Example 1) 70% by mass of 10 mm square block Al, 19.6% by mass of block Si, and 10.3% by mass of block Co were weighed and mixed, and melted by high-frequency heating in an Ar inert atmosphere to obtain a molten alloy. This molten alloy was then gas atomized using Ar inert gas to obtain AlSiCo alloy powder with an average particle size of 3 μm (precursor process). The quenching rate was 10 6 The temperature was set at °C / s. X-ray diffraction was performed on the obtained powder, and the presence of Al and Si phases as crystalline phases was confirmed. The atomic composition was Al-20 at% Si-5 at% Co. Next, the obtained alloy powder was placed in 3 mol / L hydrochloric acid diluted with pure water, stirred at room temperature of 25 °C for 1 hour, and then filtered while thoroughly washing, 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.

[0042] (Experimental Example 2) The silicon material in Experimental Example 4 was Si powder with an average particle size of 5 μm.

[0043] (Physical property measurement of porous silicon materials) The acid-treated porous silicon powder was dissolved in HF and HNO3 and subjected to elemental analysis using inductively coupled plasma (ICP) optical emission spectroscopy (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II). Observation and elemental analysis were also performed using a scanning electron microscope (SEM, Hitachi S-4300) and energy dispersive X-ray analyzer (EDAX, Hitachi S-4300). The pore distribution was measured using a mercury porosimeter (Quantachrome POWERMASTER 60GT).

[0044] (Fabrication of Lithium Secondary Batteries Using Non-Aqueous Electrolyte) Each of the silicon materials used in 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. This slurry was then 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 approximately 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 placed on top of it as a counter electrode to form a laminate. Next, an electrolyte solution of 1 mol / L 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 Tom cell 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.

[0045] (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.

[0046] As shown in Figure 6, it was confirmed that the acid treatment eluted Al components, such as primary Al, from the Si alloy, resulting in 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 obtained 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%.

[0047] As shown in Figure 7, X-ray diffraction detected peaks for Al and Si in the atomized powder, but because the Si peak was broad, it was inferred that Co was dissolved in the Si, resulting in an amorphous structure. After acid treatment, the Al peak was no longer detected in the porous silicon particles, and instead, a Si peak was confirmed, representing the Co solid solution with an amorphous structure. It was also inferred that the Al phase and AlCo compounds were eluted by the acid treatment, and these areas became pores.

[0048] The capacity retention rate is the ratio of the discharge capacity Q1 at the first cycle to the discharge capacity Q at the tenth cycle. 10 Using and, Q 10The capacity retention rate was calculated using the formula / Q1 × 100. Experimental Example 1 exhibited a smaller initial discharge capacity than Experimental Example 2 due to the presence of Al and Co and the relatively small amount of Si phase. Experimental Example 2 exhibited a reduced capacity retention rate of 28% after 10 cycles. This is presumably due to the lack of porosity in silicon particles, which was unable to absorb volume changes, resulting in electrode failure. On the other hand, the lithium secondary battery of Experimental Example 1 exhibited a favorable capacity retention rate of 92%, demonstrating the stability of the electrode. It was found that when Co is further added to a porous silicon material exhibiting a porosity of 50% by volume or more, as in Experimental Example 1, the solid solution of Co in Si and the inclusion of a Co compound can reinforce the Si skeleton, further suppressing the deterioration of cycle characteristics and increasing the capacity retention rate. It is presumed that the inclusion of Co narrows the phase width of the Si liquid, thereby suppressing coarsening of the Si. Thus, it is presumed that a finer Si structure can be created by preparing a porous silicon material based on an Al-Si-Co ternary alloy. Furthermore, for example, a CoAl alloy can be produced by adding Co to a eutectic composition, but silicide is not produced. Silicides have properties such as being difficult to dissolve in acid and remain in the porous body, but it was inferred that the absence of this formation is also an advantage.

[0049] [Table 1]

[0050] [Table 2]

[0051] It goes without saying that the present invention is not limited to the above-described experimental examples, and can be embodied in various forms as long as they fall within the technical scope of the present invention. [Industrial Applicability]

[0052] The present disclosure is applicable to the technical field of secondary batteries. [Explanation of symbols]

[0053] 10 Energy storage device, 12 Positive electrode, 13 Positive electrode active material, 14 Current collector, 15 Negative electrode, 16 Negative electrode active material, 17 Current collector, 18 Ion conducting medium, 21 Porous silicon material, 23 Void.

Claims

1. 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%.

2. 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.

3. 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. 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. A method for producing a porous silicon material described in any one of claims 1 to 4, wherein the porous silicon material has a pore size distribution range determined by mercury intrusion porosimetry in the range of 1 nm to 1000 nm, contains skeletal silicon with a three-dimensional mesh structure having voids, contains Co, and has an average porosity in the range of 50 vol% to 95 vol%.

6. A method for producing a porous silicon material described in any one of claims 1 to 5, wherein the porous silicon material contains one or more of a SiCo compound and an AlSiCo compound.

7. A method for producing a porous silicon material described in any one of claims 1 to 6, wherein the porous silicon material has an average porosity of 60 volume % or more.

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