Production method for porous silicon material, porous silicon material, and electricity storing device

Abstract

To further suppress the deterioration of charging and discharging properties in a material containing Si.SOLUTION: A production method for a porous silicon material according to the present disclosure includes: a precursor step of melting a raw material containing Si, Al, and W and rapid cooling the molten raw material for solidification to obtain a precursor for a silicon alloy; and a porosity forming step of removing an Al component contained 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] Conventionally, in a silicon negative electrode material, a porous silicon has been proposed in which 70 parts by mass of lump Si and 30 parts by mass of Al powder are mixed, and then the mixture is granulated by gas atomization using a molten alloy and helium gas under an argon atmosphere, and then Al is removed with hydrochloric acid (see, for example, Patent Document 1). It is said that this porous silicon can completely suppress pulverization due to expansion and contraction of the active material volume during charging and discharging, peeling of the active material from the current collector, and lack of contact with the conductive material. In addition, as a method for producing a silicon material, a method has been proposed in which an intermediate alloy element containing Mg, Co, Cr, Cu, Fe, etc. and a Si alloy of Si are separated into a second phase in which the intermediate alloy element and the molten metal element are replaced in a molten metal containing a predetermined molten metal element, and Si fine particles, and the second phase is removed to obtain a porous silicon material (see, for example, Patent Document 2). It is said that this porous silicon material can have high capacity and high cycle characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-214054 A [Patent Document 2] JP 2012-82125 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the manufacturing method of the above-mentioned Patent Document 1, an alloy containing Si and Al is used to make the material porous, but the method is still insufficient in suppressing defects due to 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, that is, a high-temperature treatment is required, and a simple manufacturing process is desired.

[0005] The present disclosure has been made in consideration of such problems, and has as its main object to provide a method for producing a porous silicon material, a porous silicon material, and an electricity storage device that can further suppress the deterioration of charge-discharge characteristics. [Means for solving the problem]

[0006] As a result of intensive research to achieve the above-mentioned object, the inventors have discovered that by preparing a silicon alloy containing Al and W and removing the Al component, it is possible to obtain a porous silicon material that has a stronger structure and is able to further suppress deterioration in the charge-discharge characteristics, and have completed the manufacturing method for the porous silicon material, the porous silicon material, and the electricity storage device disclosed herein.

[0007] That is, the method for producing the porous silicon material of the present disclosure includes: a precursor process in which a raw material containing Si, Al, and W is melted and rapidly solidified to obtain a precursor of a silicon alloy; a porosity forming step of removing an Al component contained in the silicon alloy to obtain a porous silicon material; It includes.

[0008] The porous silicon material of the present disclosure comprises: The average pore diameter determined by mercury intrusion porosimetry is in the range of 300 nm or less, it contains skeletal silicon with a three-dimensional mesh structure having voids, it contains one or more of a SiW compound and an AlSiW compound, and the porosity is in the range of 30 volume % or more and 85 volume % or less.

[0009] The electricity storage device of the present disclosure comprises: a positive electrode including a positive electrode active material; A negative electrode containing the above-mentioned porous silicon material as a negative electrode active material; an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions; It is equipped with the following: Effect of the Invention

[0010] This 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 3579 mAh / g, which is about 10 times the theoretical capacity of general graphite, 372 mAh / g, and further increases in capacity and energy density are expected. On the other hand, silicon that has absorbed lithium ions has a high charge / discharge characteristic of Li 15 Si4, and the volume expands to about 300% of silicon before lithium absorption. In the present disclosure, by selectively dissolving elements or compounds other than Si, SiW compounds, and AlSiW compounds, porous silicon with small pore size and large porosity can be produced. In addition, by simply removing elements or compounds other than Si, SiW compounds, and AlSiW compounds, porous silicon can be easily provided in the atmosphere without requiring large-scale equipment. In addition, the obtained porous silicon material has a three-dimensional network structure of skeletal silicon having voids reinforced with at least one of SiW compounds and AlSiW compounds, and has an average pore diameter of 300 nm or less. When such a porous silicon material with small pore size, large porosity, and reinforced skeleton is used in an electricity storage device such as a lithium ion secondary battery, the volume expansion and contraction is alleviated, and the cycle characteristics are improved, making it easy to obtain a high-performance electricity storage device. [Brief description of the drawings]

[0011] [Figure 1] Ternary isothermal equilibrium diagram of the Al-Si-W system at 1500℃ and 1000℃. [Diagram 2]Schematic diagram of the microstructure formation process during cooling of a molten droplet. [Diagram 3] Binary equilibrium phase diagrams for the Al-Si, Al-W, and Si-W systems. [Figure 4] FIG. 2 is an explanatory diagram showing an example of the structure of an electricity storage device 10. [Diagram 5] 1 is an SEM image of the porous silicon surface of Experimental Example 1. [Figure 6] 1 is an SEM image of a cross section of porous silicon in Experimental Example 1. [Figure 7] 1 shows XRD measurement results after acid treatment in Experimental Example 1. [Figure 8] 3 is a pore distribution curve of the porous silicon material of Experimental Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] (Method of manufacturing porous silicon material) The manufacturing method of the porous silicon material of the present disclosure includes a precursor process and a porosity process. In the precursor process, a raw material containing Si, Al, and W is melted and rapidly solidified to obtain a precursor of a silicon alloy. In the porosity process, the Al component contained in the silicon alloy is removed to obtain a porous silicon material. First, the raw material composition will be described.

[0013] In general, when a melt with a eutectic composition where the liquidus line is at a minimum in the equilibrium diagram of a eutectic AB alloy is solidified, a eutectic structure is formed in which the A and B phases are separated into fibers (lamellae). At this time, the size of the lamellar structure formed becomes finer as the cooling rate increases, and a nano-sized structure is formed at a cooling rate of 1000°C / s or more. If only the A element can be selectively removed from this eutectic structure by acid treatment or the like, a material consisting of the B element with a eutectic structure as a characteristic can be obtained. The microstructure changes by adjusting the composition of the A and B elements near the eutectic composition, and a porous body with a structure in which the crystallized phase consisting of the B element is connected is obtained. Considering Al as the A element in the alloy and Si and W as the B elements, it is thought that in the vicinity of the ternary eutectic composition of Al-Si-W, SiW compounds, AlSiW compounds, and Si crystallize, and primary Al and AlW compounds crystallize or precipitate. The solidification structure forms a lamellar structure consisting of SiW compounds, AlSiW compounds, AlW compounds, eutectic Si, primary Si, and primary Al. Because primary Al and AlW 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 SiW compounds, AlSiW compounds, eutectic Si, and primary Si.

[0014] The feature of the equilibrium diagram of the Al-Si-W system influences the refinement of the pores by adding W. Here, the case where the alloy melt is cooled by the gas atomization method is explained as an example. FIG. 1 shows an example of the ternary isothermal equilibrium diagram of the Al-W-Si system at 1500°C (FIG. 1A) and 1000°C (FIG. 1B). FIG. 2 shows a schematic diagram of the fine structure formation process in the cooling process of the melt droplet. FIG. 3 shows the binary equilibrium diagrams of the Al-Si system (FIG. 3A), the Al-W system (FIG. 3B), and the Si-W system (FIG. 3C) for reference. As shown in FIG. 3, the melting points of Al, Si, and W are 660°C, 1414°C, and 3422°C, respectively. As shown in FIG. 1, if the composition is 0.5 at% to 10 at%, Al is 30 at% to 85 at%, and the balance is Si (the composition in the preferred range surrounded by the dashed line), it is considered that the melt state is reached at 1500°C. In the gas atomization method, high-temperature molten liquid is sprayed from a hole at the tip of a nozzle into a high-pressure gas to cool it. As a result, the atomized powder is rapidly solidified from the AlSiW molten state. This solidification form is different from the solidification reaction in the equilibrium state shown in Figure 1 in the strict sense, but it is considered to be roughly the solidification form shown in Figure 2. For example, in the composition in the preferred range, it is estimated that the molten state is reached at 1500°C from Figures 1A, 3B, and 3C, but it is considered that primary Si and W(Si,Al)2 phases crystallize in the Al solution at 1000°C from Figures 1B, 3B, and 3C. In other words, it is considered that the solidification ends in the reaction form L(molten) (Figure 2A) ⇒ L + primary Si + W(Si,Al)2 (Figure 2B) ⇒ W(Si,Al)2 + primary Al + primary Si + eutectic Al + skeletal eutectic Si (Figure 2C). The crystal size of primary Al and eutectic Al is finer than that of binary AlSi alloys due to the presence of W(Si,Al)2 and primary Si. Primary Al and eutectic Al matrix are dissolved by acid treatment, and the pores of the resulting porous material are finer than those of binary AlSi acid-treated porous Si. In addition, since Si-W-Al porous materials contain the W(Si,Al)2 phase, which has a higher electrical conductivity and Li-ion diffusion coefficient than Si, it is presumed that the electrical conductivity and rate characteristics are improved.

[0015] (Precursor process) In the precursor process, it is preferable to use a raw material containing W in the range of 0.5 at% to 15 at% and Al in the range of 10 at% to 85 at% and the remainder being Si, when the total of Si, W and Al is 100 at%. The raw material may contain unavoidable impurities. The unavoidable impurities are components that inevitably remain during the purification of any of Si, W and Al, and examples thereof include Fe, C, Cu, Ni and P. It is preferable that the amount of unavoidable impurities is as small as possible, and for example, when the total of Si, W and Al is 100 at%, it is preferably 5 at% or less, and more preferably 2 at% or less. For example, the compounding ratio of W is preferably 1 at% or more, and may be 2 at% or more. The compounding ratio of W is preferably 10 at% or less, and may be 5 at% or less. The compounding ratio of Al is preferably 30 at% or more, more preferably 50 at% or more, and may be 60 at% or more. The compounding ratio of Al is preferably 85 at% or less, more preferably 80 at% or less, further preferably 77.5 at% or less, and may be 75 at% or less. The compounding ratio of W and Al may be, for example, 10 at% or more and 80 at% or less in total. The compounding ratio of Si is, for example, preferably 10 at% or more, more preferably 20 at% or more, and may be 25 at% or more or 30 at% or more. The compounding ratio of Si is, for example, preferably 85 at% or less, more preferably 70 at% or less, and may be 50 at% or less or 30 at% or less. In the precursor process, a raw material containing W and Al in a total range of 20 mass% to 85 mass% and preferably 30 mass% to 80 mass% when the total of Si, W, and Al is 100 mass% may be used. A silicon alloy containing Al or W in such a range is preferable because it can increase the porosity and obtain voids of a more suitable shape and size. If the content of Al is high, when the alloy is melted and then rapidly cooled, a single phase of Al is precipitated in a large amount, so that many voids can be formed. The cooling rate is preferably as rapid as possible, for example, within 10 minutes from the molten state. 2 ℃ / s or more 10 8 ° C. / s or less.

[0016] In this step, when the raw material is melted, high-frequency crucible melting in an inert gas atmosphere such as Ar is preferable, but any melting method may be used. In the precursor step, the alloy obtained from the raw material may be granulated. In this granulation process, the molten metal of the silicon alloy raw material may be cast into a mold, and the obtained ingot may be crushed and granulated. In addition, the method of granulating the silicon alloy may be one or more of the gas atomization method, water atomization method, and roll quenching method. In the gas atomization method and the water atomization method, an alloy powder is obtained. On the other hand, in the roll quenching method, a thin strip alloy is obtained, so that it may be crushed and powdered after that. Since the powder obtained by the roll quenching method has a fine alloy structure, porous silicon having fine pores can be obtained after the elution process. Among these, the gas atomization method is more preferable as the method of granulating the silicon alloy. In the gas atomization, it is preferable to perform the molten metal in an Ar atmosphere, and it is preferable to perform the granulation in an Ar or He atmosphere.

[0017] In the precursor process, the silicon alloy is preferably granulated to have an average particle size in the range of 0.1 μm to 100 μm. For example, the average particle size of the particles is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 5 μm, and even more preferably 0.5 μm to 3 μm. The silicon alloy particles may be appropriately selected according to the characteristics required for the power storage device. Here, the average particle size of the particles is determined by observing the particles with a scanning electron microscope (SEM), counting the major axis of each particle as the diameter of the particle, and dividing the average value by the number of particles. The particles obtained by this granulation process will be the average particle size of the aggregate of porous particles to be finally obtained.

[0018] In this precursor process, a raw material containing Al, W, and Si as well as a second element containing one or more of Ca, Cu, Mg, Na, Sr, and P may be used. Among 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 W, and is preferably, for example, in the range of 10 mass% or less, and more preferably, in the range of 5 mass% or less, based on the entire silicon alloy.

[0019] (Porous process) In the porosity forming step, a treatment is performed to remove substances other than Si from the silicon alloy particles produced above. Examples of substances other than Si include Al and its compounds, and W and its compounds. In this step, it is preferable to selectively remove the Al component, that is, the Al phase and its compounds, using an acid or alkali. The acid or alkali used is preferably one that dissolves elements and / or compounds other than silicon in the silicon alloy and does not dissolve silicon, and examples of such acids include hydrochloric acid, sulfuric acid, and sodium hydroxide. This acid or alkali is preferably an aqueous solution. The concentration of the acid or alkali is not particularly limited as long as it is within a range that can remove Al and its compounds, and W 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. In addition, the removal treatment is preferably performed by immersing the silicon alloy particles in an acid or alkali solution and stirring for about 1 to 5 hours. The obtained porous silicon material is then washed and dried.

[0020] In the porosity forming step, substances other than Si may be removed in the range of 85% by mass to 100% by mass. For example, Al, W, other oxygen, etc. may remain, but from the viewpoint of charge / discharge capacity, it is preferable that they are less when used as an electrode active material. Furthermore, components such as Al and W 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 one or more of a SiW compound and an AlSiW compound may be obtained. Examples of SiW compounds include Si x W y(x and y are arbitrary numbers), and examples of the AlSiW compound include Al a S b W c The compound (a, b, and c are any numbers) may be used, such as W(Si,Al)2 in which a part of Si in WSi2 is replaced by Al. The SiW compound and the AlSiW compound may be poorly soluble in acid or alkali.

[0021] In the porosity forming step, it is preferable to obtain a porous silicon material containing at least one of SiW compounds and AlSiW compounds, the total of which is in the range of 5% by mass to 40% by mass, relative to the entire Si phase. In addition, in the porous silicon material, the SiW compounds and AlSiW compounds are preferably contained in a total of 10% by mass or more, and may be contained in a total of 20% by mass or more. In addition, in the porous silicon material, the SiW compounds and AlSiW compounds are preferably contained in a total of 37.5% by mass or less, and may be contained in a total of 35% by mass or less. In this step, it is preferable to obtain a porous silicon material in which SiW compounds such as WSi2 and AlSiW compounds such as W(Al,Si)2 remain, and it is preferable to obtain a porous silicon material in which AlSiW compounds such as W(Al,Si)2 remain.

[0022] In the porosification step, a porous silicon material having a porosity in the range of 30 volume % or more and 85 volume % or less may be obtained. This porosity is a value measured by a mercury porosimeter. This porosity is preferably 35 volume % or more, and may be 40 volume % or more, for example. The porosity is preferably 80 volume % or less, more preferably 75 volume % or less, and may be 50 volume % or less, for example. A larger porosity is more responsive to volume changes when carrier ions are absorbed, and a smaller porosity is preferable because the amount of Si present per unit volume increases.

[0023] (Porous silicon material) The porous silicon material of the present disclosure may be one produced by the above-mentioned manufacturing method. Here, the physical properties of the porous silicon material are the same as those of the above-mentioned manufacturing method, and detailed description thereof will be omitted. The porous silicon material has an average pore diameter in the range of 300 nm or less, as determined by mercury intrusion porosimetry. The average pore diameter is preferably in the range of 5 nm to 300 nm. The average pore diameter may be 10 nm or more, or may be 20 nm or more. The average pore diameter is preferably 250 nm or less, and may be 200 nm or less. A small pore diameter is preferable because the pores are less likely to be crushed. Also, a large pore is preferable because the volume change when carrier ions are occluded can be more suppressed. The distribution range of the pore diameter may be in the range of 1 nm to 1 μm, or in the range of 1 nm to 500 nm. When the porous silicon material is used in a lithium ion secondary battery, the pores can be impregnated with a nonaqueous electrolyte, but the smaller the pores, the more uniform the lithium ions will be when alloyed, reducing stress concentration and preventing deterioration of the electrode itself, so it is desirable for the pores to be small. From this perspective, the above-mentioned average pore size is preferably 50 nm or less, more preferably 30 nm or less.

[0024] The porous silicon material may include skeletal silicon having a three-dimensional network structure with voids, and may include at least one of a SiW compound and an AlSiW compound. It is presumed that the SiW compound and the AlSiW compound are responsible for reinforcing the silicon skeleton. The porous silicon material preferably includes one or more of a SiW compound and an AlSiW compound with respect to the entire Si phase, and the total amount is preferably in the range of 5% by mass to 40% by mass. The porous silicon material preferably contains a total of 10% by mass or more of a SiW compound and an AlSiW compound, and may contain 20% by mass or more. The porous silicon material preferably contains a total of 37.5% by mass or less of a SiW compound and an AlSiW compound, and may contain 35% by mass or less. The skeletal silicon may have a diameter in the range of 10 nm to 1 μm, for example. The skeletal silicon may have a structure in which silicon fine particles are bonded, for example.

[0025] The porous silicon material has a porosity of 30 volume % or more and 85 volume % or less, as determined by mercury intrusion porosimetry. The porosity is preferably, for example, 35 volume % or more, and may be 40 volume % or more. The porosity is preferably, for example, 80 volume % or less, more preferably 75 volume % or less, and may be 50 volume % or less. A larger porosity is more responsive to volume changes when carrier ions are absorbed, and a smaller porosity is preferable, since the amount of Si present per unit volume increases.

[0026] The porous silicon material preferably has an average particle size of 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and may be 5 μm or more. The average particle size of the particles is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. The porous silicon material preferably contains 70 at% or more of Si when the total of Si, Al, and W is 100 at%, excluding oxygen and unavoidable impurities. The Si content is more preferably 75 at% or more, more preferably 80 at% or more, and may be 85 at% or more. The Si content is preferably higher from the viewpoint of charge / discharge capacity, and may be lower from the viewpoint of relative skeletal reinforcement. The Al content is preferably in the range of 0 at% to 15 at%, preferably 12.5 at% or less, more preferably 10 at% or less, and may be 7.5 at% or less. The Al content is more preferably 1 at% or more, more preferably 2 at% or more, and may be 2.5 at% or more. The content of W is preferably in the range of 0.5 at% or more and 15 at% or less. The content of W is more preferably 12.5 at% or less, more preferably 10 at% or less, and may be 7.5 at% or less. The content of W is more preferably 1 at% or more, more preferably 2.5 at% or more, and even more preferably 5 at% or more. The content of Al and W is preferably higher from the viewpoint of supplementing the strength of the skeleton, and is preferably lower from the viewpoint of the charge / discharge capacity of the power storage device, since they are components that do not charge and discharge. Furthermore, the porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr, and P as a second element in the range of 15 mass% or less. Furthermore, the porous silicon material may contain inevitable impurities in addition to Si, Al, and W. It is preferable that the second element and inevitable impurities are less. This porous silicon material may contain Si at a ratio of 50 mass% or more of elements excluding oxygen.

[0027] (Electrodes for electricity storage devices) The electrode for a power storage device includes the porous silicon material described above as an electrode active material. This electrode is either a positive electrode or a negative electrode based on the potential of the counter electrode relative to the potential of the electrode active material, but when lithium is used as a carrier, it is preferable to use the electrode as a negative electrode. This electrode can be used, for example, in a lithium ion secondary battery, a hybrid capacitor, an air battery, etc. The electrode for a power storage device may be one in which the porosity of the porous silicon material is compressed to a range of 5 volume % or more and 50 volume % or less. In this electrode, the porosity of the porous silicon material may be reduced by compressing it during production. For example, when porous silicon particles are used as a negative electrode active material for a lithium ion secondary battery, the smaller the pores are, the more uniform the alloying of lithium ions is, so that stress concentration is reduced and deterioration of the electrode itself can be prevented. The porosity of the porous silicon material after compression may be appropriately adjusted according to the characteristics required for the electrode for a power storage device, and may be, for example, 5 volume % or more or 10 volume % or more. The porosity of the porous silicon material after compression may be, for example, 30 volume % or less or 20 volume % or less.

[0028] The electrode for the power storage device may be one in which the porous silicon material described above is formed on a current collector and fixed on the current collector. This electrode can be produced by a process in which the porous silicon material is mixed with a conductive material or a binder in a solvent as necessary to form a paste and then coated on the current collector, or a process in which the porous silicon material is mixed with a conductive material or a binder as necessary and then pressed onto the current collector. In this electrode, the content of the porous silicon material is preferably higher, and is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance, and for example, a mixture of one or more of graphite such as natural graphite (scale graphite, scale graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon nanotubes, carbon whiskers, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used. The binder plays a role of binding the active material particles and the conductive material particles, and may be, for example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine rubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR), or the like, either alone or as a mixture of two or more kinds. In addition, a cellulose-based aqueous binder or an aqueous dispersion of styrene butadiene rubber (SBR) may also be used. As the solvent, for example, an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran may be used. In addition, a dispersant, a thickener, or the like may be added to water, and the active material may be slurried with latex such as SBR. Examples of the application method include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain any thickness and shape.The current collector may be appropriately selected depending on the potential of the active material, and may be, for example, aluminum, titanium, stainless steel, nickel, iron, copper, baked carbon, conductive polymer, conductive glass, or aluminum or copper whose surface is treated with carbon, nickel, titanium, silver, or the like for the purpose of improving adhesion, conductivity, and oxidation resistance. The surfaces of these may also be subjected to oxidation treatment. The shape of the current collector may be a foil, film, sheet, net, punched or expanded, lath, porous, foamed, or fiber group. The thickness of the current collector may be, for example, 1 to 500 μm. The amount of the active material complex formed may be appropriately set depending on the desired performance required for the power storage device.

[0029] In this electrode, the electrode active material may contain an active material other than the porous silicon material in addition to the porous silicon material, within a range in which both a low confining pressure and a capacity retention rate can be achieved. For example, the electrode active material may contain a carbonaceous material. However, from the viewpoint of further increasing the battery capacity, it is preferable that the porous silicon material accounts for, for example, 50 mass% or more, preferably 90 mass% or more, of the entire electrode active material as 100 mass%.

[0030] (Electricity storage device) The electricity storage device of the present disclosure includes an electrode having the porous silicon material described above. The electricity storage device may include a positive electrode, a negative electrode, and an ion-conducting medium that is interposed between the positive electrode and the negative electrode and conducts carrier ions. The porous silicon material can be used as the negative electrode active material. The electricity storage device may be any of a lithium ion secondary battery, a hybrid capacitor, an air battery, and the like. Of these, the electricity storage device is preferably a lithium secondary battery, particularly a lithium ion secondary battery. Here, the case where the electricity storage device is a lithium ion secondary battery and the negative electrode is the above-mentioned electrode for the electricity storage device will be mainly described.

[0031] The positive electrode may be one in which a positive electrode active material layer containing a positive electrode active material is formed on a current collector. For example, the positive electrode may be one in which a positive electrode active material is formed on a current collector and fixed on the current collector. This positive electrode may be produced by a process of mixing a positive electrode active material with a conductive material and a binder as needed and a solvent to form a paste and applying it on a current collector, or by a process of mixing a positive electrode active material with a conductive material and a binder as needed and pressing it onto a current collector. In the positive electrode, as the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, phosphate compounds containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as NbS2, NbS3, MoS3, FeS2, basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same below), and lithium manganese composite oxides such as Li (1-x) Mn2O4, basic composition formula Li (1-x) CoO2, etc., lithium cobalt composite oxides, basic composition formula Li (1-x) NiO2, etc., lithium nickel composite oxides, basic composition formula Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), etc., lithium nickel cobalt manganese composite oxides, basic composition formula LiV2O3, etc., lithium vanadium composite oxides, basic composition formula V2O5, etc., transition metal oxides, basic composition formula Li (1-x) FePO4, etc., lithium transition metal phosphate compounds, etc. can be used. Among these, lithium transition metal composite oxides, for example, LiCoO2, LiNiO2, LiMnO2, Li (1-x) Ni 1 / 3 Co 1 / 3 Mn 1 / 3O2 and lithium transition metal phosphate compounds, such as LiFePO4, are preferred. The lithium transition metal composite oxide may have a layered rock salt type crystal phase, and the lithium transition metal phosphate compound may have an olivine type crystal phase. The "basic composition formula" means that other elements such as Al and Mg may be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors and lithium ion capacitors. Examples of carbonaceous materials include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. Activated carbons as carbonaceous materials have a specific surface area of ​​1000 m 2 / g or more, and 1500m 2 / g or more. 2 The specific surface area of ​​this activated carbon is 3000 m2 for ease of preparation. 2 / g or less, and 2 / g or less is more preferable. The conductive material, binder, solvent, current collector, etc. used in the positive electrode can be appropriately used from those exemplified in the above-mentioned electrode. As will be described in detail later, the positive electrode active material layer may contain a solid electrolyte.

[0032] The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charging and discharging, but also allows the viscosity of the electrolyte, the electric capacity of the resulting battery, and the battery output to be well balanced. Examples of the supporting salt include LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiSbF6, LiSiF6, LiAlF4, LiSCN, LiClO4, LiCl, LiF, LiBr, LiI, and LiAlCl4. Among these, it is preferred to use one or more salts selected from the group consisting of inorganic salts such as LiPF6, LiBF4, LiAsF6, and LiClO4, and organic salts such as LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3 in combination in terms of electrical properties.The concentration of the supporting salt in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 5 mol / L or less, more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and when the concentration is 5 mol / L or less, the electrolyte can be more stabilized. In addition, a phosphorus-based or halogen-based flame retardant may be added to the non-aqueous electrolyte.

[0033] Also, instead of the liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. As the ion-conducting polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, vinylidene fluoride, and a supporting salt can be used. Furthermore, an ion-conducting polymer can be used in combination with a non-aqueous electrolyte. As the ion-conducting medium, in addition to the ion-conducting polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder can be used.

[0034] The power storage device may have a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of 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.

[0035] The shape of the electricity storage device is not particularly limited, but examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type. The electricity storage device may also be applied to large devices used in electric vehicles and the like. FIG. 4 is an explanatory diagram showing an example of the structure of an electricity storage device 10. The electricity storage device 10 has a positive electrode 12, a negative electrode 15, and an electrolyte-containing separator 18 containing an electrolyte as an ion-conducting medium. The positive electrode 12 has a positive electrode active material layer 13 containing a positive electrode active material, and a current collector 14. The negative electrode 15 has a negative electrode active material layer 16 containing a negative electrode active material, and a current collector 17. The negative electrode active material contained in the negative electrode active material layer 16 is the above-mentioned porous silicon material 21, and has voids 23.

[0036] This electricity storage device preferably has a higher capacity retention rate when subjected to charge / discharge cycles, for example, a capacity retention rate at 50 cycles of preferably 80% or more, more preferably 85% or more, and even more preferably 87.5% or more. This electricity storage device also preferably has a higher discharge capacity when subjected to 2C discharge, for example, preferably 700mAh / g or more, more preferably 800mAh / g or more, and even more preferably 1000mAh / g or more.

[0037] This power storage device may be an all-solid-state battery. For example, in the above-mentioned power storage device 10, a solid electrolyte may be used as the ion-conducting medium instead of the electrolyte-containing separator 18. In the all-solid-state battery, the positive electrode active material is preferably a lithium metal composite oxide having a layered rock salt type crystal phase or a lithium metal phosphate compound having an olivine type crystal phase, among the above-mentioned materials. These positive electrode active materials have a small expansion and contraction rate of the active material due to charging and discharging, and are particularly advantageous in all-solid-state batteries in which interfacial contact between particles is important. In the all-solid-state battery, it is preferable that the positive electrode active material layer containing the positive electrode active material contains a solid electrolyte. As the solid electrolyte contained in the positive electrode active material layer, inorganic solid electrolytes such as oxide solid electrolytes and sulfide solid electrolytes are preferable, and sulfide solid electrolytes are more preferable. As the sulfide solid electrolyte, for example, a solid electrolyte containing Li, P, and S as constituent elements can be used. Specifically, Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, LiI-LiBr-Li2S-P2S5, LiI-Li2S-P2S5, LiI-Li2O-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5-GeS2, and the like can be mentioned. Among these, in particular, sulfide solid electrolytes containing Li2S-P2S5 are more preferable. Only one type of solid electrolyte may be used alone, or two or more types may be mixed and used. When the sulfide solid electrolyte is contained in the positive electrode active material layer, a coating layer such as a lithium niobate layer may be provided on the surface of the positive electrode active material from the viewpoint of suppressing the formation of a high resistance layer at the interface between the positive electrode active material and the sulfide solid electrolyte. In this case, the conductive material, binder, solvent, current collector, etc. used in the positive electrode can be appropriately used as exemplified in the above-mentioned electrode. In the all-solid-state battery, the ion conductive medium may be a solid electrolyte, and for example, the solid electrolyte exemplified as the solid electrolyte contained in the positive electrode active material layer can be suitably used. The solid electrolyte of the ion conductive medium may be the same as or different from the solid electrolyte contained in the positive electrode active material layer.

[0038] As described above in detail, the present disclosure can further suppress the deterioration of charge / discharge characteristics in materials containing Si. The reason why such an effect is obtained is presumed to be as follows. For example, by removing a compound containing Al from an Al-Si-W ternary alloy, a high-strength porous silicon material having nano-sized pores and a skeleton reinforced with a nano-sized SiW compound or AlSiW compound can be easily mass-produced. The nanopores relieve the stress associated with the expansion and contraction of silicon during charging and discharging. At the same time, by introducing a SiW compound or an AlSiW compound that is inactive against carrier ions into the silicon skeleton, the skeleton strength can be improved, and the silicon pore structure can be prevented from being crushed by the stress associated with the expansion and contraction during charging and discharging, and the resulting pulverization, peeling from the current collector, and lack of contact with the conductive material can be prevented, thereby improving the charge / discharge cycle characteristics. In addition, by adding W to the AlSi alloy, the pore size can be made finer than that of porous silicon produced by rapidly solidifying and acid-treating a similar AlSi binary alloy, and the effect of suppressing the collapse of the pore structure due to residual stress can be improved. In addition, since SiW compounds and AlSiW compounds have higher electrical conductivity than Si, the inclusion of these compounds is expected to improve rate characteristics.

[0039] It goes without saying that the present disclosure is in no way limited to the above-described embodiment, and can be embodied in various forms as long as it falls within the technical scope of the present disclosure.

[0040] The present disclosure may be any one of the following [1] to

[10] . [1] A method for producing a porous silicon material, comprising: a precursor process in which a raw material containing Si, Al, and W is melted and rapidly solidified to obtain a silicon alloy precursor; and a porosity process in which the Al component contained in the silicon alloy is removed to obtain a porous silicon material. [2] The method for producing a porous silicon material according to [1], wherein in the precursor step, a raw material is used which contains W in the range of 0.5 at % or more and 10 at % or less, Al in the range of 30 at % or more and 85 at % or less, and the remainder is Si, where Si, Al, and W are taken as 100 at % in total. [3] The method for producing a porous silicon material according to [1] or [2], wherein in the porosity-providing step, the Al component is selectively removed with an acid or an alkali. [4] The method for producing a porous silicon material according to any one of [1] to [3], wherein in the porosity-providing step, the porous silicon material containing one or more of a SiW compound and an AlSiW compound is obtained. [5] The method for producing a porous silicon material according to any one of [1] to [4], wherein the porous silicon material has a porosity in the range of 30 volume % or more and 85 volume % or less, in the porosity-making step. [6] The method for producing a porous silicon material according to any one of [1] to [5], 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. [7] The average pore size determined by mercury intrusion porosimetry is in the range of 300 nm or less, the material contains skeletal silicon with a three-dimensional network structure having voids, and contains one or more of a SiW compound and an AlSiW compound, and the porosity is in the range of 30 volume % to 85 volume %. Porous silicon material. [8] The porous silicon material according to [7], wherein the total amount of the SiW compound and the AlSiW compound is in the range of 5 mass % to 40 mass % based on the total amount of the Si phase. [9] The porous silicon material according to [7] or [8], wherein, when the total of Si, Al, and W is taken as 100 at%, W is contained in a range of 0.5 at% or more and 15 at% or less, Al is contained in a range of 0 at% or more and 15 at% or less, and the remainder is Si.

[10] An electricity storage device comprising: a positive electrode containing a positive electrode active material; a negative electrode containing the porous silicon material according to any one of [7] to [9] as a negative electrode active material; and an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions. EXAMPLES

[0041] Specific examples of the porous silicon and electricity storage device according to the present disclosure that have been fabricated will be described below as experimental examples. Experimental example 1 is an embodiment of the present disclosure, and Experimental examples 2 and 3 are comparative examples.

[0042] [Preparation of porous silicon materials] (Experimental Example 1) 62.5% by mass of Al in a lump of about 10 mm square, 24.3% by mass of Si, and 8.0% by mass of W were prepared, mixed, and melted in an Ar inert atmosphere by high-frequency heating to obtain a molten alloy. This molten alloy was gas atomized using Ar inert gas to obtain an AlSiW alloy powder consisting of Si, W(Al,Si)2, and Al with an average particle size of 3 μm. The atomic composition is Al-25 at% Si-2.5 at% W. Next, the obtained quenched alloy powder was placed in 3 mol / L hydrochloric acid diluted in pure water, stirred at 25 ° C for 1 hour, filtered while being thoroughly washed, and dried in a vacuum drying furnace at 30 ° C for 2 hours. In this way, the porous silicon material of Experimental Example 1 was produced.

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

[0044] (Experimental Example 3) 74.5% by mass of Al and 25.5% by mass of Si were prepared in a block shape of about 10 mm square, mixed, melted in an N2 inert atmosphere by high-frequency heating to obtain a molten alloy, and gas atomized using N2 inert gas to obtain an AlSi alloy powder consisting of Si and Al with an average particle size of 3 μm. The atomic composition is Al-25 at% Si. Next, the obtained quenched alloy powder was placed in 3 mol / L hydrochloric acid diluted in pure water, stirred at 25 ° C for 1 hour, filtered while being thoroughly washed, and dried in a vacuum drying furnace at 30 ° C for 2 hours. In this way, the porous silicon material of Experimental Example 3 was produced.

[0045] (Physical property measurement of porous silicon materials) The porous silicon powder after the acid treatment was observed with a scanning electron microscope (SEM, Hitachi S-4300). In addition, an X-ray diffraction measurement was performed with a Cu tube in the range of 2θ = 20 ° to 100 ° at a speed of 5 ° / min using an X-ray diffractometer (Rigaku RINT-TTR). The ratio of each phase, Si phase, Al phase, and W (Al, Si) 2 phase, was calculated by mass ratio from the XRD peak intensity ratio of each phase. In addition, the pore distribution was measured with a mercury porosimeter (Quantachrome POREMASTER 60GT). Since the mercury porosimeter detects the voids between particles, the pore diameter and pore volume of the pore part were obtained by determining the voids between particles from the atomized powder before the acid treatment and subtracting the voids between particles from the voids after the acid treatment. The porous silicon powder after the acid treatment was dissolved in HF and HNO3, and elemental analysis was performed by inductively coupled plasma optical emission spectrometry (ICP-OES, Hitachi High-Tech Science PS3520UVDDII II).

[0046] FIG. 5 is an SEM image of the porous silicon surface of Experimental Example 1, and FIG. 6 is an SEM image of the porous silicon cross section of Experimental Example 1. FIG. 7 is an XRD measurement result after acid treatment of Experimental Example 1. FIG. 8 is a pore distribution curve of the porous silicon material of Experimental Example 1. As shown in FIGS. 5 and 6, it was found that vacancies were formed by completely dissolving primary crystal Al in the Si compound with acid, and porous Si particles were obtained. In addition, from the SEM photographs shown in FIGS. 5 and 6, pores with a pore size of several nm to about 200 nm were observed in Experimental Example 1. As shown in FIG. 7, it was found that the Al-25at%Si-2.5at%W atomized powder had a Si phase and a W(Al,Si)2 phase detected after acid treatment. In the powder obtained after acid treatment, the presence of the Si phase and W(Al,Si)2 was confirmed as a crystalline phase. The pore distribution of the porous Si powder obtained by mercury porosimetry in FIG. 8 was 5 to 150 nm, with the most numerous pore size being around 15 nm, the average pore size being 25 nm, and the porosity being 46%.

[0047] Table 1 summarizes the composition ratio (at%) of the dissolved raw material, the composition ratio (at%) after acid treatment, and the compound composition ratio (mass%), porosity (volume%), and average pore size (nm) obtained from XRD in Experimental Examples 1 to 3. After acid treatment, Al was dissolved, and Si, W(Al,Si)2 phases were identified from the XRD results. The mass ratio of Si:W(Al,Si)2 obtained from XRD was 73:27.

[0048] [Table 1]

[0049] (Preparation of lithium secondary batteries using non-aqueous electrolyte) Lithium ion secondary batteries were fabricated using the silicon materials of Experimental Examples 1 to 3 as the negative electrode active material, and battery characteristics such as discharge capacity were evaluated. 60% by mass of the negative electrode active 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 were mixed, and N-methylpyrrolidone was added and stirred to prepare a slurry. Next, this slurry was applied to a copper foil with a thickness of 18 μm, dried, and rolled to prepare a negative electrode with a thickness of 25 μm. The fabricated negative electrode was punched out into a circle with a diameter of 16 mm, and metallic lithium was layered on this negative electrode as a counter electrode with a porous polyethylene separator sandwiched therebetween, and an electrolyte was poured into the negative electrode to produce a lithium secondary battery, which is a Tom cell type small battery cell. The electrolyte was prepared by adding LiPF6 at a concentration of 1 mol / L to a mixed solvent of fluoroethylene carbonate / ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (FEC / EC / DMC / EMC) with a volume ratio of 1.5:1.5:4:3. The obtained lithium secondary battery was repeatedly charged and discharged at a current density of 0.1C in the battery voltage range of 0.005 V to 1.5 V for 50 cycles, and the rate characteristics were evaluated at 0.1C, 1C, and 2C.

[0050] (Characteristics of lithium secondary batteries using non-aqueous electrolytes) Table 2 shows the composition of the mother alloy of the negative electrode active material, the initial discharge capacity, the discharge capacity after 50 cycles, and the capacity retention rate after 50 cycles. In Experimental Example 1, the capacity retention rate was good at 88%, whereas in Experimental Example 2, the capacity retention rate was low at 5.9%. On the other hand, the initial discharge capacity of Experimental Example 3 was 2570 mAh / g, and the capacity retention rate after 50 cycles was 81%. Since the capacity associated with the insertion of Li in the SiW compound and the AlSiW compound is smaller than that of Si, the discharge capacity of Experimental Example 1 was reduced by the addition of W compared to Experimental Example 3, but it was presumed that the capacity retention rate after 50 cycles of Experimental Example 1 was improved compared to Experimental Example 3 due to the reduction in pore size and the strengthening of the skeleton by the SiW compound and the AlSiW compound.

[0051] [Table 2]

[0052] Table 3 shows the mother alloy composition of the negative electrode active material and the discharge capacity at 0.1C, 1C, and 2C as the rate characteristics. The rate characteristics were evaluated by 10 charge / discharge cycles at 0.1C, followed by 5 charge / discharge cycles at 1C and 2C, and the average value of each discharge capacity is shown in Table 3. In Experimental Example 3, the discharge capacity was 2440mAh / g at 0.1C, but decreased as the rate increased, to 1012mAh / g at 1C rate and 526mAh / g at 2C rate. On the other hand, in Experimental Example 1, the discharge capacity was 1500mAh / g at 0.1C, but was maintained at 1134mAh / g at 1C rate and 1043mAh / g at 2C rate, so that the rate characteristics were greatly improved compared to Experimental Example 3. It was presumed that this was because the pores became finer by the addition of W, and the SiW compound and AlSiW compound had a higher conductivity and a higher diffusion coefficient of Li ions than Si.

[0053] [Table 3]

[0054] It goes without saying that the present disclosure is in no way 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. [Industrial Applicability]

[0055] The present disclosure is applicable to the technical field of secondary batteries. [Explanation of symbols]

[0056] 10 electricity storage device, 12 positive electrode, 13 positive electrode active material layer, 14 current collector, 15 negative electrode, 16 negative electrode active material layer, 17 current collector, 18 electrolyte-containing separator, 21 porous silicon material, 23 void.

Claims

1. a precursor process in which a raw material containing Si, Al, and W is melted and rapidly solidified to obtain a precursor of a silicon alloy; a porosity forming step of removing an Al component contained in the silicon alloy to obtain a porous silicon material; A method for producing a porous silicon material comprising:

2. 2. The method for producing a porous silicon material according to claim 1, wherein in the precursor step, a raw material is used which contains W in the range of 0.5 at % to 10 at %, Al in the range of 30 at % to 85 at %, and the remainder is Si, when the total of Si, Al, and W is 100 at %.

3. The method for producing a porous silicon material according to claim 1 or 2, wherein in the porosity forming step, an Al component is selectively removed with an acid or an alkali.

4. 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 one or more of a SiW compound and an AlSiW compound.

5. The method for producing a porous silicon material according to claim 1 or 2, wherein in the porosity forming step, the porous silicon material obtained has a porosity in the range of 30 volume % or more and 85 volume % or less.

6. 3. 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.

7. The average pore diameter determined by mercury intrusion porosimetry is in the range of 300 nm or less, the skeletal silicon has a three-dimensional network structure having voids, and the porosity is in the range of 30 volume % or more and 85 volume % or less. Porous silicon material.

8. 8. The porous silicon material according to claim 7, wherein the total content of the SiW compound and the AlSiW compound is 5 mass % or more and 40 mass % or less with respect to the entire Si phase.

9. 9. The porous silicon material according to claim 7 or 8, wherein, when the total of Si, Al and W is 100 at%, W is contained in the range of 0.5 at% or more and 15 at% or less, Al is contained in the range of 0 at% or more and 15 at% or less, and the remainder is Si.

10. a positive electrode including a positive electrode active material; A negative electrode comprising the porous silicon material according to claim 7 or 8 as a negative electrode active material; an ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions; A power storage device comprising:

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