Method for producing porous silicon material, porous silicon material and power storage device

Abstract

To further suppress deterioration of charge-discharge characteristics of a material including Si.SOLUTION: There is provided a method of producing a porous silicon material according to the present invention, comprising a precursor preparation step of melting and rapidly cooling / solidifying a raw material including Si, Al and Mo to obtain a precursor of a silicon alloy, and a step of making the precursor porous to obtain a porous silicon material by removing an Al component included in the silicon alloy.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 discovered that by preparing a silicon alloy containing Al and Mo and removing Al, 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 disclosed herein.

[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 Mo 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 in the range of 1 nm to 1000 nm, the skeletal silicon has a three-dimensional network structure having voids, the SiMo compound and / or AlSiMo compound is contained, and the porosity is in the range of 30 vol% to 80 vol%. It is something.

[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, elements or compounds other than silicon, silicon molybdenum compounds, and aluminum silicon molybdenum compounds are selectively dissolved and removed from a silicon alloy containing Al and Mo to produce a porous silicon material with small pore size and high porosity. This process requires only the use of a solvent capable of dissolving elements or compounds other than silicon, silicon molybdenum compounds, and aluminum silicon molybdenum compounds, and does not require large-scale equipment, allowing the porous silicon material to be easily synthesized in air. Furthermore, the resulting porous silicon material has a silicon skeleton reinforced with silicon molybdenum compounds or aluminum silicon molybdenum compounds, and has an average pore diameter of, for example, 250 nm or less. When such porous silicon materials with small pore size, high porosity, and reinforced skeletons are used in energy storage devices such as lithium-ion secondary batteries, volume expansion and contraction are greatly reduced, improving charge / discharge cycle characteristics and making it easy to obtain high-performance energy storage devices. [Brief explanation of the drawings]

[0011] [Figure 1] Al-Si-Mo equilibrium phase diagram. [Figure 2]FIG. 1 is an explanatory diagram showing an example of the configuration of an electricity storage device 10. [Figure 3] 1 is a surface SEM image of the porous silicon material of Experimental Example 1. [Figure 4] 1 is a cross-sectional SEM image of the porous silicon material of Experimental Example 1. [Figure 5] 1 shows a cross-sectional TEM image and an EDS mapping image of the porous silicon material of Experimental Example 1. [Figure 6] 1 shows XRD measurement results of the porous silicon material of Experimental Example 1. [Figure 7] 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 Mo 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-Mo equilibrium phase diagram (25 at% Si). Generally, when a melt with a eutectic composition where the liquidus line is at its minimum in the equilibrium phase diagram of a eutectic AB alloy is solidified, a eutectic structure is formed in which the A and B phases are separated into fibrous (lamellar) phases. The size of the resulting lamellar structure 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 a solvent-based removal process (e.g., 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 Mo as the B elements, the equilibrium phase diagram shows that near the Al-Si-Mo ternary eutectic composition, SiMo compounds, AlSiMo compounds, and Si crystallize, while primary Al and AlMo compounds also crystallize or precipitate. The solidification structure forms a lamellar structure consisting of SiMo compounds, AlSiMo compounds, AlMo compounds, eutectic Si, primary Si, and primary Al. Since primary Al and AlMo compounds are thought to be the compounds that dissolve in the acid, it is presumed that a skeletal porous body consisting of the remaining SiMo compounds, AlSiMo compounds, eutectic Si, and primary Si will be obtained after acid treatment.

[0014] (Precursor process) In the precursor process, it is preferable to use a raw material containing Al and Mo in a total range of 30 at% to 85 at% when the total of Si, Al, and Mo is taken as 100 at%, with the remainder being Si. Alternatively, in the precursor process, it is preferable to use a raw material containing Al in a range of 30 at% to 85 at% when the total of Si, Al, and Mo is taken as 100 at%, and Mo in a range of 0.5 at% to 10 at% when the total of Si, Al, and Mo is taken as 100 at%, with the remainder being Si. As the raw material, pure metals or alloys may be used. Note that the raw material may contain unavoidable impurities. Examples of unavoidable impurities include components that inevitably remain during the refining of Si, Al, or Mo, such as Fe, C, Cu, Ni, and P. It is preferable to have fewer unavoidable impurities, for example, 5 at% or less, and more preferably 2 at% or less, when the total of Si, Al, and Mo is taken as 100 at%. The total compounding ratio of Al and Mo is, for example, preferably 50 at% or more and may be 60 at% or more. The total compounding ratio of Al and Mo is, for example, preferably 80 at% or less, more preferably 78 at% or less, and may be 76 at% or less. The compounding ratio of Mo is, for example, preferably 1 at% or more and may be 2.5 at% or more. The compounding ratio of Mo is, for example, preferably 7.5 at% or less and may be 5 at% or less. The compounding ratio of Al is preferably 50 at% or more and may be 65 at% or more. The compounding ratio of Al is preferably 80 at% or less, more preferably 77 at% or less, and may be 75 at% or less. In the present disclosure, unless otherwise specified, the compounding ratio of each component refers to the compounding ratio of Si, Al, and Mo relative to the total. Silicon alloys containing Al and Mo in these ranges are preferred because they can increase the porosity and obtain voids with a more suitable shape and size. If the Al content is high, when the alloy is melted and then rapidly cooled, a large amount of Al single phase precipitates, resulting in the formation of many voids. In this process, it is preferable to use a raw material with a composition that allows for the formation of a eutectic structure of Si and one or more of Al, Mo, AlMo compound, SiMo compound, and AlSiMo compound.For example, according to the equilibrium diagram in Fig. 1, when Si is 25 at%, it is believed that a eutectic structure of Si and Al can be obtained when Mo is in the range of 0 to 12 at%. In this step, when the total of Si, Al, and Mo is 100 mass%, a raw material may be used that contains Al and Mo in a total range of 20 mass% to 85 mass%, preferably 30 mass% to 80 mass%, with the remainder being Si.

[0015] In this step, when melting the raw materials, any melting method may be used, but high-frequency crucible melting in an inert gas atmosphere such as Ar is preferred. When rapidly cooling and solidifying the molten raw materials, any rapid cooling method may be used, but the cooling rate is preferably more rapid, for example, 10 2 ℃ / s or more 10 8 The solidification rate may be in the range of °C / s or less. Rapid solidification may be achieved, for example, by casting a molten silicon alloy (a molten raw material) into a mold and quenching it. However, it is preferable to quench the molten silicon alloy (melt) by one or more of gas atomization, water atomization, and roll quenching. In the precursor process, the silicon alloy obtained from the raw material may be granulated. In this granulation process, an ingot obtained by mold casting may be crushed and granulated. Furthermore, since the gas atomization and water atomization methods described above produce alloy powder, this may be used for granulation. Furthermore, since the roll quenching method described above produces a thin strip alloy, this may be subsequently crushed and granulated (granulated). Since the powder obtained by the roll quenching method has a fine alloy structure, porous silicon with fine pores can be obtained after the leaching process. Among these, gas atomization is more preferable as a method for granulating the silicon alloy. In gas atomization, the molten silicon alloy is preferably produced in an Ar atmosphere, and the granulation process 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 0.1 μm to 10 μm, more preferably 0.5 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 Si, Al, and Mo as well as a second element including one or more of Ca, Cu, Mg, Na, Sr, and P may be used. Of these, the second element is preferably one or more of Ca, Na, and Sr. The content of the second element is preferably less than the content of Al and Mo, 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) In the porosity forming step, substances other than Si are removed from the silicon alloy precursor prepared above. Examples of substances other than Si include Al and its compounds, Mo and its compounds, etc. 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 thereof 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 can remove Al and its compounds, and Mo 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 is preferably performed by immersing the silicon alloy precursor 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 85% by mass to 100% by mass. For example, Al, Mo, other oxygen, etc. 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 Mo 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 SiMo compound and / or an AlSiMo compound may be obtained. There is no need to distinguish between SiMo compounds and AlSiMo compounds, but examples of SiMo compounds include, for example, Si x Mo y (x and y are arbitrary numbers), and examples thereof include MoSi2. Also, Mo(Si,Al)2, in which a part of Si in MoSi2 is substituted with Al, may be used. MoSi2 and Mo(Si,Al)2 may have a C11B structure. Also, examples of AlSiMo compounds include, for example, Al a Si b Mo c Compounds (a, b, and c are any numbers) may be used, and Mo 1 / 3(Al,Si) 2 / 3 Examples include: Mo 1 / 3 (Al,Si) 2 / 3 The SiMo compound and the AlSiMo compound may have a C40 structure. The SiMo compound and the AlSiMo compound may be poorly soluble in acid or alkali. The porous silicon material obtained in this step may contain a MoAl compound. Examples of the MoAl compound include Al4Mo, Al5Mo, and Al 12 Examples include Mo.

[0020] The porosity forming step may obtain a porous silicon material having a porosity in the range of 30% by volume or more and 85% by volume or less. This porosity is a value measured with a mercury porosimeter. The porosity is, for example, preferably 35% by volume or more, and may be 40% by volume or more. The porosity is, for example, preferably 80% by volume or less, more preferably 70% by volume or less, and may be 60% by volume or less. A larger porosity is more responsive to volume changes during occlusion of carrier ions, while a smaller porosity is preferable because it increases the amount of Si present per unit volume. The porosity forming step may obtain a porous silicon material having pores with an average pore diameter of 1 μm or less as determined by mercury intrusion porosimetry.

[0021] (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. This pore size distribution range may be 2 nm or more, 3 nm or more, or 5 nm or more. Furthermore, this pore size distribution range may be 500 nm or less, 300 nm or less, or 150 nm or less. In this porous silicon material, the average pore size as determined by mercury intrusion porosimetry is preferably 250 nm or less, more preferably 100 nm or less, even more preferably 50 nm or less, and may be 30 nm or less. This average pore size may be 2 nm or more, 5 nm or more, or 10 nm or more.

[0022] The porous silicon material contains skeletal silicon with a three-dimensional network structure having voids, and contains SiMo compounds and / or AlSiMo compounds. The SiMo compounds and AlSiMo compounds are presumed to reinforce the silicon skeleton. Mo and Al may be solid-solubilized in Si. Examples of SiMo compounds include MoSi2. Examples of AlSiMo compounds include Mo. 1 / 3 (Al,Si) 2 / 3 The skeletal silicon may be an aggregate of silicon particles or may be formed in a cage shape.

[0023] The porous silicon material has a porosity in the range of 30% to 80% by volume. This porosity is preferably 35% by volume or more, and may be 40% by volume or more. The porosity is preferably 70% by volume or less, and may be 60% by volume or less. The porosity increases or decreases depending on, for example, the Al content in the raw material state. This porosity is a value measured with a mercury porosimeter. A larger porosity is preferable from the viewpoint of suppressing volumetric changes in the material, and a smaller porosity is preferable from the viewpoint of the charge / discharge capacity of the electricity storage device.

[0024] 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 100 μm or less, more preferably 10 μm or less, even more preferably 5 μm or less, and may be 3 μm or less. The porous silicon material preferably contains 50 at% or more of Si, when the total of Si, Al, and Mo, excluding oxygen and unavoidable impurities, is 100 at%. The Si content may be 70 at% or more, or 80 at% or more, with Al and Mo being the remainder. A higher Si content is preferable from the viewpoint of charge / discharge capacity, but a lower Si content may be preferable from the viewpoint of relative skeletal reinforcement. The porous silicon material preferably contains Mo 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 Mo is 100 at%. The Al and Mo contents are preferably higher from the viewpoint of reinforcing the skeleton, and lower from the viewpoint of the charge / discharge capacity of the electricity storage device, since these are components that do not charge or discharge. The remainder may contain unavoidable impurities. In this porous silicon material, the Mo content is more preferably 2 at% or more, and may be 3 at% or more. The Mo content is more preferably 12.5 at% or less, and may be 10 at% or less. The Al content 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. The porous silicon material may contain 50 mass% or more of Si, when the total of Si, Al, and Mo is 100 mass%. 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. The porous silicon material may also contain unavoidable impurities in addition to Si, Al, and Mo. It is preferable that the amount of the second element and the unavoidable impurities is as small as possible. In this disclosure, the content of each component refers to the total content of Si, Al, and Mo, unless otherwise specified.

[0025] The porous silicon material preferably has a Si phase and a MoSi2 phase. The porous silicon material may contain 50% by mass or more of the Si phase when the total of the Si phase and the MoSi2 phase is 100% by mass. The ratio of the Si phase is, for example, preferably 70% by mass or more and may be 80% by mass or more. The ratio of the Si phase is, for example, preferably 99% by mass or less and may be 95% by mass or less. The ratio of the MoSi2 phase is, for example, preferably 1% by mass or more and may be 5% by mass or more. The ratio of the MoSi2 phase is, for example, preferably 30% by mass or less and may be 20% by mass or less. These ratios can be determined by XRD measurement. The Si phase may contain Al and / or Mo as a solid solution. The Si phase preferably has a diamond structure. The MoSi2 phase may contain Al as a solid solution. The MoSi2 phase preferably has a C11B structure.

[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, 5% by volume or more, 10% by volume or more, or 20% by volume or more. The average porosity of the compressed porous silicon material may be, for example, 40% by volume or less, 30% by volume or less, or 20% 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 their surfaces. Regarding the shape of the current collector, examples include foil-like, film-like, sheet-like, net-like, punched or expanded ones, lath bodies, porous bodies, foams, formed bodies of fiber groups, etc. 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 a range where both a low confinement pressure and a capacity retention rate can be achieved. For example, as the electrode active material, a carbonaceous material, SiO, Li4Ti5O 12 etc. may be included. However, from the viewpoint of further increasing the battery capacity, assuming the total electrode active material is 100% by mass, for example, the porous silicon material preferably occupies 50% by mass or more, preferably 90% by mass or more.

[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 one of a lithium-ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, sulfides containing a transition metal element, oxides containing lithium and a transition metal element, etc. can be used. Specifically, transition metal sulfides such as CoS2, CoS3, MoS3, FeS2, the 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, square type, etc. It may also be applied to large-sized ones used in electric vehicles and the like. Fig. 2 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] It is preferable that this power storage device has a higher capacity retention rate when performing charge and discharge cycles. For example, the capacity retention rate at 10 cycles is preferably 90% or more, more preferably 92.5% or more, and even more preferably 95% or more.

[0035] (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 suppressed, and furthermore, the 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 above-mentioned power storage device. Also, the negative electrode can use the electrode for the power storage device described above.

[0036] 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 such. However, A is one or more of Sr and Ca, T is one or more of Nb and Ta, and it satisfies 0 < x ≦ 1.0 and 0 < y < 0.75. Alternatively, the solid electrolyte has a basic composition (Li 7-3z+x-yM z )(La 3-x A x )(Zr 2-y T y )O 12 Ya, (Li 7-3z+x-y M z )(La 3-x A 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.

[0037] 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.

[0038] 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.

[0039] 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 porous silicon material is Si, and its volume expands to approximately four times that of silicon before lithium absorption. According to the present disclosure, elements and / or compounds other than silicon (e.g., Al component substances) contained in the silicon alloy can be selectively removed using an acid or alkali that dissolves these elements, making it easy to produce a porous silicon material with small pore size and high porosity. Furthermore, the microstructure contains a compound containing Mo, which strengthens the silicon skeleton, thereby further suppressing capacity degradation associated with skeleton fluctuations due to repeated charge and discharge. When such porous silicon material with small pore size and high porosity is used in an energy storage device such as a lithium-ion secondary battery, volume expansion and contraction are greatly reduced, thereby improving charge and discharge characteristics such as charge and discharge cycle characteristics, and thus making it easy to obtain a high-performance energy storage device.

[0040] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure. [Example]

[0041] 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.

[0042] [Preparation of porous silicon materials] (Experimental Example 1) 68.2 wt% of 10 mm square Al, 23.7 wt% of lump Si, and 8.1 wt% of lump Mo were weighed and mixed, and melted by high-frequency heating in an Ar inert atmosphere to form a molten alloy. This molten alloy was used to obtain AlSiMo alloy powder with an average particle size of 3 μm by gas atomization using Ar inert gas (precursor process). The quenching rate was 10 6The quenching temperature was set at °C / s. The AlSiMo alloy powder contained Si, MoSi2, and Al. The atomic composition of the AlSiMo alloy powder was Al-24.4 at% Si-2.4 at% Mo. Next, the obtained quenched 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 (powder) of Experimental Example 1 was produced.

[0043] (Experimental Example 2) The silicon material in Experimental Example 2 was Si powder with an average particle size of 5 μm.

[0044] (Physical property measurement of porous silicon materials) The obtained porous silicon material was dissolved in HF and HNO3 and subjected to elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II). It was also observed using a scanning electron microscope (SEM, Hitachi S-4300). It was also observed and analyzed using a transmission electron microscope (TEM, JEOL JEM-2100F) and an energy dispersive X-ray analyzer (EDS, JEOL JED-2300). XRD measurements were performed using an X-ray diffractometer (Rigaku RINT-TTR) with a Cu tube in the 2θ range of 20° to 100° at a rate of 5° / min. The ratio of the Si phase and MoSi2 phase was calculated as a mass percentage from the XRD peak intensity ratio of each phase using an alumina standard. The pore size distribution was measured using a mercury porosimeter (Quantachrome POWERMASTER 60GT). Since the mercury porosimeter also detects voids between particles, the voids between particles were determined from the atomized powder (precursor of the silicon alloy) before acid treatment, and the pore diameter and pore volume of the pore portion were determined.

[0045] (Results and Discussion) Figure 6 shows the XRD measurement results for the porous silicon material of Experimental Example 1 (XRD results for Al-24.4 at% Si-2.4 at% Mo atomized powder after acid treatment). As shown in Figure 6, after the acid treatment, sharp peaks indicating the Si phase and the MoSi2 phase were detected. This indicates that the powder obtained after the acid treatment contains the Si phase and the MoSi2 phase as crystalline phases.

[0046] Figure 3 is a surface SEM image of the porous silicon material of Experimental Example 1, with an enlarged view shown on the right. Figure 4 is a cross-sectional SEM image of the porous silicon material of Experimental Example 1, with an enlarged view shown on the right. Figure 5 is a cross-sectional TEM image (Figures 5A and 5B) and EDS mapping images (Figures 5C, 5D, and 5E) of the porous silicon material of Experimental Example 1. It was found that voids were formed by eluting and removing the Al component from the precursor AlSiMo alloy powder, and a porous silicon material was obtained. From the SEM and TEM observations shown in Figures 3 to 5, pores ranging in size from several nm to approximately 200 nm were observed in the porous material of Experimental Example 1.

[0047] Figure 7 shows the pore distribution curve of the porous silicon of Experimental Example 1. As shown in Figure 7, the pore distribution of the porous silicon material obtained using a mercury porosimeter was 5 to 150 nm, which was similar to the pore distribution observed using SEM and TEM. The pore diameter obtained using the mercury porosimeter was most common in the vicinity of 25 nm, the average pore diameter was 25 nm, and the porosity was 46%.

[0048] Table 1 summarizes the elemental composition of the melted raw material, the elemental composition of the porous silicon material, the compound composition of the porous silicon material, the porosity, and the average pore size for each experimental example. The compound composition of the porous silicon material was measured by XRD, and Si and MoSi2 phases were identified. Meanwhile, the elemental composition of the porous silicon material was measured by ICP-OES, and not only Si and Mo but also Al was detected. The cross-sectional TEM and EDS results shown in Figure 5 also detected Si, Mo, and Al in the skeleton. From these results, it was inferred that the silicon skeleton contained a SiAlMo compound in which Al and Mo were solid-solved in Si. Furthermore, MoSi2 was clearly detected in the XRD shown in Figure 6, indicating that the skeleton contained not only the Si phase in which Al and Mo were solid-solved in Si but also the MoSi2 phase, a SiMo intermetallic compound. Furthermore, the mass ratio of the Si phase to the MoSi2 phase determined by XRD was found to be 90 / 10.

[0049] [Table 1]

[0050] (Fabrication of Lithium-ion 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 wt. % of this material, 20 wt. % of acetylene black with an average particle size of 2 μm as a conductive material, and 20 wt. % 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 an 18 μm-thick copper foil, dried, and rolled to prepare a 25 μm-thick negative electrode. It was estimated that this rolling process reduced the porosity of the porous silicon material to approximately 40% by volume while maintaining its three-dimensional network structure. The prepared negative electrode was punched into a 16 mm-diameter circle, 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 10 times at a current density of 0.2 C over a battery voltage range of 0.005 V to 1.5 V.

[0051] (Results and Discussion) The initial discharge capacity (mAh / g), discharge capacity after 10 cycles (mAh / g) and capacity retention rate (%) of Experimental Examples 1 and 2 are shown in Table 2. The capacity retention rate is calculated by dividing the initial discharge capacity Q1 by the discharge capacity Q2 at the 10th cycle. 10 Using and, (Q 10The capacity retention rate was calculated using the formula: (Q1 / Q1) × 100. Experimental Example 1 exhibited a smaller initial discharge capacity than Experimental Example 2 because a silicide phase (MoSi2 phase) was present and the Si phase was relatively small. Experimental Example 2 exhibited a reduced capacity retention rate of 28% after 10 cycles. This is presumably because silicon particles without voids were unable to absorb volume changes, resulting in electrode defects. On the other hand, the lithium secondary battery of Experimental Example 1 exhibited a favorable capacity retention rate of 97%, demonstrating the stability of the electrode. It was found that in porous silicon materials containing Mo, as in Experimental Example 1, the solid solution of Al or Mo in Si and the inclusion of SiMo compounds and / or AlSiMo can reinforce the Si skeleton, further suppressing the deterioration of cycle characteristics and increasing the capacity retention rate. It was also presumed that the inclusion of Mo causes fine precipitation of primary crystal Al, resulting in finer pores. Thus, it was presumed that a finer Si structure can be created by preparing a porous silicon material based on an Al-Si-Mo ternary alloy.

[0052] [Table 2]

[0053] It goes without saying that the present disclosure is not 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 disclosure. [Industrial Applicability]

[0054] The present disclosure is applicable to the technical field of secondary batteries. [Explanation of symbols]

[0055] 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 Mo 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; Including, In the precursor step, the raw material contains Al in a range of 50 at% or more and 80 at% or less, and Mo in a range of 1 at% or more and 5 at% or less, when the total of Si, Al, and Mo is taken as 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 a SiMo compound and / or an AlSiMo compound.

4. The method for producing a porous silicon material according to any one of claims 1 to 3, wherein the porous silicon material obtained in the porosity forming step has a porosity in the range of 30 vol% to 85 vol%.

5. 5. The method for producing a porous silicon material according to claim 1, wherein in the precursor step, the molten silicon alloy is rapidly solidified by any one of a gas atomization method, a water atomization method, and a roll quenching method, to obtain the precursor granulated to have an average particle size in the range of 0.1 μm to 100 μm.

6. A method for producing a porous silicon material described in any one of claims 1 to 5, wherein the porous silicon material obtained in the porosity-making process has an average pore diameter of 100 nm or less as determined by mercury porosimetry.

7. A method for producing a porous silicon material described in any one of claims 1 to 6, wherein in the precursor process, a raw material containing Al in the range of 65 at% or more and 75 at% or less when the total of Si, Al and Mo is 100 at% is used.

8. 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 having voids, the SiMo compound and / or the AlSiMo compound is contained, and the porosity is in the range of 30 vol% to 80 vol%, When the total of Si, Al, and Mo is taken as 100 at%, Al is contained in a range of 3 at% to 15 at% and Mo is contained in a range of 3 at% to 10 at%. Porous silicon material.

9. 9. The porous silicon material of claim 8, wherein the average pore size is 250 nm or less.

10. Si phase and MoSi 2 10. The porous silicon material according to claim 8 or 9, comprising a phase.

11. The porous silicon material according to claim 8, wherein the average pore diameter determined by mercury intrusion porosimetry is 100 nm or less.

12. a positive electrode including a positive electrode active material; A negative electrode containing the porous silicon material according to any one of claims 8 to 11 as a negative electrode active material; an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions; An electricity storage device comprising:

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