Method for manufacturing porous silicon material, porous silicon material, and energy storage device

Abstract

The present invention provides a porous silicon material in which the decrease in conductivity and charge / discharge efficiency is suppressed. [Solution] The method for producing a porous silicon material includes a precursor step of melting and rapidly solidifying raw materials containing Al, Si, and Ni to obtain a silicon alloy precursor, and a porosity-forming step of removing the Al component contained in the silicon alloy by alkaline treatment to obtain a porous silicon material.

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing a porous silicon material, a porous silicon material, and an energy storage device. [Background technology]

[0002] Conventionally, porous silicon materials have been proposed as a negative electrode material for energy storage devices, obtained by melting and rapidly solidifying raw materials containing Al, Si, and Cr, and then removing the Al component by acid treatment from a silicon alloy (see, for example, Patent Document 1). In this porous silicon material, the framework is strengthened by Cr silicide distributed along the silicon framework, which is said to suppress the deterioration of charge and discharge characteristics. In addition, porous silicon materials have been proposed as a negative electrode material for energy storage devices, obtained by fabricating an amorphous alloy from raw materials containing Al, Si, and Ni, crystallizing it at a high temperature such as 300°C or higher, and then removing the Al component by acid treatment (see, for example, Non-Patent Document 1). This porous silicon material has a structure in which the surface of the Si ligament is coated with a thin Ni layer, and is said to have excellent electrical conductivity and cycle stability. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-092861 [Non-patent literature]

[0004] [Non-Patent Document 1] J. Hyun et al. J. Power Sources 495 (2021) pp.29802 [Overview of the project] [Problems that the invention aims to solve]

[0005] Incidentally, in Patent Document 1, Cr silicide has the function of acting as a conductive phase and suppressing expansion and contraction during charging and discharging. By suppressing expansion and contraction during charging and discharging, it is possible to suppress the pulverization of porous silicon materials and the resulting decrease in charge-discharge efficiency (Coulomb efficiency). Therefore, when producing porous silicon materials using Al-Si-Cr alloy, it is desirable to increase the amount of Cr silicide from the viewpoint of improving conductivity and suppressing the decrease in charge-discharge efficiency. However, if the amount of Cr in the raw material is increased to increase Cr silicide, conductivity may decrease due to the generation of a large amount of insulating amorphous SiO2 as a by-product during acid treatment, making it difficult to suppress the decrease in conductivity while suppressing the decrease in charge-discharge efficiency. The Al-Si-Ni alloy used in Non-Patent Document 1 has a room-temperature stable phase consisting of a Si phase, an Al phase, and an Al-Ni compound phase (e.g., NiAl3 phase) and does not contain silicide, so it is effective in avoiding the generation of amorphous SiO2 as a by-product that can occur during acid treatment. However, in porous silicon materials obtained by acid treatment of Al-Si-Ni alloy as in Non-Patent Document 1, the amount of Ni-containing phase, which is the conductive phase, was sometimes small.

[0006] This disclosure is made to solve these problems, and its main objective is to provide a porous silicon material in which the decrease in conductivity and charge / discharge efficiency is suppressed. [Means for solving the problem]

[0007] Through diligent research to achieve the above-mentioned objectives, the present inventors discovered that by melting and rapidly solidifying raw materials containing Al, Si, and Ni to produce a silicon alloy, and then removing the Al component from this silicon alloy by alkali treatment, a porous silicon material with suppressed decreases in conductivity and charge / discharge efficiency can be obtained, thus completing this disclosure.

[0008] In other words, the method for manufacturing the porous silicon material of this disclosure is A precursor step involves melting raw materials containing Al, Si, and Ni, rapidly cooling and solidifying them, and obtaining a silicon alloy precursor. A porosity-forming step of removing the Al component contained in the silicon alloy by alkali treatment to obtain a porous silicon material; It includes the following.

[0009] The porous silicon material of the present disclosure may contain a Si phase, a conductive phase containing Ni, and a SiO2 phase. When the total of the Si phase, the conductive phase, and the SiO2 phase is 100% by mass, the conductive phase is contained in the range of 20% to 70% by mass, the content of the SiO2 phase is 10% by mass or less, and the porosity determined by the mercury intrusion method is 45% by volume or more and the average pore diameter is 100 nm or less.

[0010] The energy storage device of the present disclosure a negative electrode containing the above-mentioned porous silicon material as a negative electrode active material; a positive electrode containing a positive electrode active material; an ion conduction medium interposed between the negative electrode and the positive electrode to conduct carrier ions; It is provided with the following.

Advantages of the Invention

[0011] In the present disclosure, a porous silicon material with suppressed reduction in conductivity and charge-discharge efficiency can be provided. The reason for obtaining such an effect is presumed as follows. For example, when an Al-Si-Ni-based alloy is melted and rapidly solidified, it separates into an Al phase, a Si phase, and an Al-Ni compound phase (for example, a NiAl3 phase) due to phase separation. Then, an alkali treatment is performed. Since the Al component is more likely to elute than the Ni component in the alkali treatment, a large amount of the Al-Ni compound phase remains as it is or as a Ni phase even after the removal of the Al phase. The Al-Ni compound phase and the Ni phase function as a conductive phase and also as a swelling relaxation phase. Therefore, in the present disclosure, it is presumed that a porous silicon material with suppressed reduction in conductivity and charge-discharge efficiency can be realized. Further, in the Al-Si-Ni-based alloy, a silicide phase is unlikely to occur during phase separation, so SiO2, which is an insulating phase, is unlikely to occur in the subsequent removal treatment of the Al component, and it is presumed that the reduction in conductivity is further suppressed.

Brief Description of the Drawings

[0012] [Figure 1] Al-Si-Ni phase diagram in 6.2 at% Ni. [Figure 2] Al-Si-Ni phase diagrams in 2, 5, and 10 at% Ni. [Figure 3] An explanatory diagram showing the eutectic composition range of Al-Si-Ni alloys. [Figure 4] An explanatory diagram showing an example of the structure of the energy storage device 10. [Figure 5] XRD measurement results of porous silicon materials and their precursors in Experimental Examples 2 and 6. [Figure 6] Measurement results of pore size distribution in porous silicon materials for experimental examples 2 and 6. [Figure 7] Surface SEM observation results of porous silicon materials and their precursors in Experimental Examples 2 and 6. [Figure 8] Cross-sectional SEM observation results of porous silicon materials in Experimental Examples 2 and 6. [Figure 9] Cross-sectional SEM observation results of porous silicon materials in Experimental Examples 1 and 3. [Figure 10] XRD measurement results of porous silicon material and its precursor in Experimental Example 9. [Figure 11] An explanatory diagram of raw material compositions capable of generating coexisting structures. [Figure 12] XRD measurement results for porous silicon materials in experimental examples 10-25. [Figure 13] SEM / EDX measurement results for the porous silicon material in Experimental Example 21. [Figure 14] SEM / EDX measurement results for the porous silicon material in Experimental Example 15. [Modes for carrying out the invention]

[0013] (Method for manufacturing porous silicon material) The method for producing a porous silicon material according to this disclosure includes a precursor step and a porosity-forming step. In the precursor step, raw materials containing Al, Si, and Ni are melted and rapidly cooled and solidified to obtain a silicon alloy precursor. In the porosity-forming step, the Al component contained in the silicon alloy is removed by alkaline treatment to obtain a porous silicon material. First, the raw material composition will be described.

[0014] In the equilibrium phase diagram (eutectic system) of a binary Al-Si alloy, when the molten state of a eutectic composition with a minimal liquidus line is cooled and solidified, the Al and Si phases crystallize simultaneously, forming a fibrous (lamellar) eutectic structure. At this time, the size of the formed lamellar structure becomes finer as the cooling rate increases, for example, 10 2 At cooling rates above K / s, nanoscale structures are formed. If only the Al element can be selectively removed from this eutectic structure through removal treatments such as acid treatment, a porous material consisting of a Si framework that retains the characteristics of the eutectic structure can be obtained.

[0015] In the case of a ternary Al-Si-Ni system, phase separation occurs, resulting in the formation of an Al phase (pore source), a Si phase, and a NiAl3 phase (conductive phase and expansion relaxation phase). In this system, since silicide phases are generally not formed during phase separation, it has the advantage of being less prone to generating amorphous SiO2, which causes irreversible capacitance and inhibits electrical conductivity, during the removal process, compared to Al-Si-Cr systems where silicide phases are formed during phase separation.

[0016] The raw material composition is preferably one that yields a porous silicon material with a coexistence structure in which the NiAl3 phase and the conductive phase based on it are distributed along the Si framework. To realize a porous Si material with a coexistence structure from a phase-separated alloy, the crystallization order of each phase is important, and appropriate composition selection based on the phase diagram is necessary. When the melt of an Al-Si-Ni alloy is cooled, the Al phase, Si phase, and NiAl3 phase crystallize, but the order changes depending on the Ni content, for example. For example, when the Ni content is 1.2 at%, Al and Si crystallize first, and when the temperature is further reduced, the remaining melt solidifies into NiAl3, so a coexistence structure is not formed. On the other hand, when the Ni content is increased to 3.6 at% or more, Si and NiAl3 particles crystallize first, and these particles are dispersed in the melt (also called the coexistence structure precursor state). When the melt solidifies into the Al phase in that state, the Si and NiAl3 particles link together, forming a coexistence structure. As an example, Figure 1 shows the Al-Si-Ni phase diagram for 6.2 at% Ni. Figure 1 shows the Al-Si-Ni phase diagram for 6.2 at% Ni (Figure 1A), as well as a schematic diagram of particulate porous silicon material 20 as an example of a porous silicon material obtained by rapidly solidifying the molten material along the arrows in Figure 1A and then removing the Al component by alkali treatment (Figure 1B). As will be described in detail later, the porous silicon material 20 has a framework 25 composed of a Si framework formed of Si phase 22 and a conductive phase 24 distributed along the Si framework, and voids 26. The conductive phase 24 is, for example, based on NiAl3, and is either the NiAl3 phase or the Ni phase.

[0017] To investigate the compositions that result in a coexisting structure, the Al-Si-Ni phase diagram was calculated using the CALPHAD method, and the results are shown in Figure 2. Figure 2A shows the Al-Si-Ni phase diagram for 2at%Ni, Figure 2B for 5at%Ni, and Figure 2C for 10at%Ni. As shown in Figure 2, it was found that when Ni is 2at% or more, the crystallization order becomes suitable for a coexisting structure. In the shaded composition range in Figure 3, the desired crystallization order occurs (liquid phase L, Si, and NiAl3 coexist), so a coexisting structure can be expected. In particular, in eutectic compositions or compositions near them (for example, compositions with a Si content within ±5at% of the eutectic composition), Si and NiAl3 crystallize simultaneously or almost simultaneously, so grain growth can be suppressed and a fine structure can be realized. Therefore, the composition range enclosed by the dashed line in Figure 3 is more preferable. The raw material composition may be set within the range that results in the desired phase configuration, based on the equilibrium phase diagram and taking into account the effect of rapid cooling as needed. A raw material composition is more preferable if the liquidus temperature is within ±100°C of the eutectic temperature, as this makes it less likely for extremely coarse particles to grow.

[0018] (Precursor process) In the precursor step, it is preferable to use a raw material containing Si in the range of 1 at% to 88 at% and Ni in the range of 2 at% to 25 at% when the total amount of Al, Si, and Ni is set to 100 at%. It is also preferable to use a raw material that satisfies 12.5 - 0.5y ≤ x ≤ 100 - 4y when the Si content ratio is x at% and the Ni content ratio is ya at% (see Figure 3). In addition to Al, Si, and Ni, the raw material may also contain unavoidable impurities. Unavoidable impurities are components that inevitably remain during the purification of any of Al, Si, or Ni, and examples include Fe, C, Cu, and P. It is preferable that the amount of unavoidable impurities be as low as possible; for example, when the total amount of Al, Si, and Ni is set to 100 at%, it is preferable that it be 5 at% or less, and more preferably 2 at% or less. The Al content ratio may be 30 at% or more, 40 at% or more, or 50 at% or more. Furthermore, the Al content ratio may be 90 at% or less, 89 at% or less, or 85 at% or less. The Si content ratio may be 5 at% or more, or 10 at% or more. Furthermore, the Si content ratio may be 50 at% or less, 40 at% or less, or 20 at% or less. The Ni content ratio may be 2.5 at% or more, or 3 at% or more. Furthermore, the Ni content ratio may be 20 at% or less, or 10 at% or less. Silicon alloys containing Al, Si, and Ni within these ranges are preferable because they allow for a higher porosity and the acquisition of a more suitable skeleton and pores of a suitable shape and size. In this process, the general formula Al 100-x-y Si x Ni y(However, a silicon alloy represented by (where 0 < x < 100, 0 < y < 100) may be used as the master alloy. In this step, it is preferable to use a silicon alloy containing Al with a predetermined composition at which a eutectic structure can be obtained. The predetermined composition may be a eutectic composition, may be near the eutectic composition, or may have a predetermined width such as a part of a hypoeutectic composition or a hypereutectic composition. The silicon alloy may, for example, contain Al, Si, and Ni within the range of ±5 at% each with respect to the eutectic composition. It is preferable that no peak of the Si-Ni compound (also referred to as Ni silicide) is confirmed by XRD measurement for this silicon alloy.)

[0019] In this step, when melting the raw material, any melting method may be used, but high-frequency crucible melting in an inert gas atmosphere such as Ar is preferable. Also, when rapidly solidifying the melted raw material, any rapid cooling method may be used, but the cooling rate is preferably more rapid quenching. For example, from the molten state, it may be in the range of 10 2 °C / s or more and 10 8 °C / s or less. As a method of rapid solidification, for example, the molten silicon alloy (melted raw material) may be cast into a mold and rapidly cooled, but it is preferable to rapidly cool the molten silicon alloy (melt) by one or more of gas atomization method, water atomization method, and roll quenching method. In the precursor step, the silicon alloy obtained from the raw material may be granulated. In this granulation process, the ingot obtained by mold casting may be crushed and granulated. Also, in the above-mentioned gas atomization method and water atomization method, alloy powders are obtained, so they may be used for granulation. Also, in the above-mentioned roll quenching method, a strip alloy is obtained, so it may be crushed into powder (granulated) thereafter. The powder obtained by the roll quenching method has a fine alloy structure, so porous silicon having fine pores can be obtained after the elution treatment. Among these, the gas atomization method is more preferable as the method for granulating the silicon alloy. In gas atomization, it is preferable to perform it in an Ar atmosphere when making it into a melt, and it is preferable to perform it under an Ar or He atmosphere during granulation.)

[0020] In the precursor step, it is preferable to atomize the silicon alloy into particles with an average particle size in the range of 0.1 μm to 100 μm. These particles are preferably, for example, 0.5 μm to 10 μm, more preferably 1 μm to 5 μm, and even more preferably 1 μm to 3 μm. The silicon alloy particles can be appropriately selected according to the characteristics required for the energy storage device. Here, the average particle size is determined by observing the particles with a scanning electron microscope (SEM), summing the major axis of each particle as its diameter, dividing by the number of particles, and averaging the result. The particles obtained in this atomization process will be the average particle size of the final aggregate of porous particles to be obtained.

[0021] In this precursor step, in addition to Al, Si, and Ni, a raw material containing one or more secondary elements from Ca, Cu, Mg, Na, Sr, and P may be used. Of these, one or more of Ca, Na, and Sr are preferred as the secondary elements. The amount of the secondary element is preferably less than the amount of Al, Si, and Ni, for example, it is preferably in the range of 10% by mass or less, and more preferably in the range of 5% by mass or less, relative to the total silicon alloy.

[0022] After the precursor process, a heat treatment process may be performed to heat-treat the precursor. In this heat treatment process, a room-temperature stable phase (e.g., Al3Ni or Si) may be generated and stabilized from a high-temperature stable phase (e.g., Al-Si-Ni ternary silicide) that remains metastable at room temperature. In Al-Si-Ni alloys, alloying Al with Ni is more stable than alloying Ni with Si, so Si-Ni binary silicide is not generated. However, in composition ranges with a high Ni content (e.g., more than 10 at% Ni), a metastable high-temperature stable phase, Al-Si-Ni ternary silicide, may remain at room temperature due to non-equilibrium rapid solidification. By removing such silicide, the generation of SiO2 can be suppressed, and the decrease in conductivity can be suppressed. The heat treatment temperature is preferably 200°C or higher, more preferably 250°C or higher. The heat treatment temperature is preferably 400°C or lower, more preferably 350°C or lower. The heat treatment atmosphere is preferably an inert atmosphere such as nitrogen or argon, or a reducing atmosphere such as argon / hydrogen. The heat treatment time is preferably 10 minutes or more, and more preferably 30 minutes or more. The heat treatment time is preferably 120 minutes or less, and more preferably 60 minutes or less.

[0023] (Porous process) In the porosity creation process, a removal treatment is performed to remove the Al component, i.e., the Al phase and its compounds, from the silicon alloy prepared above. Examples of the Al component include Al and its compounds. In this process, Ni and its compounds may also be removed. In this process, the Al component contained in the silicon alloy can be removed by alkaline treatment, but it is preferable to selectively remove the Al component contained in the silicon alloy. A strong alkali is preferred to be used, which dissolves the Al component in the silicon alloy but does not dissolve the Si component, i.e., the Si phase and its compounds. Examples of alkalis that can be used include sodium hydroxide, potassium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide. The alkaline treatment is preferably carried out using an aqueous solution of alkali. The alkali concentration of the aqueous solution is not particularly limited as long as it can remove the Al component, and may be, for example, 3 mol / L or less, 2 mol / L or less, or 1 mol / L or less. The alkali concentration may be, for example, 0.01 mol / L or higher, 0.02 mol / L or higher, or 0.03 mol / L or higher. The alkali treatment is preferably carried out under milder conditions, for example, in the range of room temperature (25°C) to 80°C, but may also be 30°C or lower, or 25°C or lower. The alkali treatment may also be carried out by immersing the silicon alloy in an alkali solution, and stirring may be performed as needed. The alkali treatment time may be set appropriately depending on the type and concentration of the alkali, but for example, it is preferably within 10 hours, more preferably within 5 hours, and even more preferably within 2 hours. The alkali treatment time may be, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more. The obtained porous silicon material is then washed and dried. In the porosization step, a porous silicon material in which no Al peak is detected by X-ray diffraction (XRD) measurement may be obtained. That is, in the porosization step, the alkali treatment may be carried out until the Al phase is gone. Furthermore, this porosity-forming process may also yield a porous silicon material in which Ni peaks (peak tops appearing at 2θ = 44-45° and 2θ = 50-51°) can be confirmed by XRD measurement.The Ni peak may be a broad peak. In this porosity-forming process, a porous silicon material may be obtained in which no Si-Ni compound peak is detected by XRD measurement. In this porosity-forming process, a porous silicon material may be obtained in which the following method is described later.

[0024] (Porous silicone material) The porous silicon material of this disclosure may be manufactured by the manufacturing method described above. Here, the detailed explanation of the physical properties of the porous silicon material is omitted, assuming that they are the same as those of the manufacturing method described above.

[0025] This porous silicon material comprises a Si phase and a conductive phase containing Ni. The conductive phase may contain Ni, and may be one or more of the following: Ni phase, Al-Ni compound phase, Ni silicide phase, and Ni phase, with Ni phase and one or more of the Al-Ni compound phase being preferred. The conductive phase may be a single Ni phase. The Ni phase may be an amorphous phase. The Al-Ni compound phase may be, for example, Ni x Al y (x, y are arbitrary numbers) The material may also be a phase, such as a NiAl3 phase. This porous silicon material may have a coexisting structure, specifically, a Si skeleton formed of a Si phase and a conductive phase distributed along the Si skeleton. The Si skeleton may retain the characteristics of the eutectic structure of the precursor silicon alloy, or it may form a three-dimensional network structure with voids. The conductive phase may be nanoparticles and may be distributed along the Si skeleton. This porous silicon material is thought to reduce the resistivity of the porous silicon material and mitigate expansion by introducing a conductive phase containing Ni in addition to the Si skeleton of the Si phase. The porous silicon material may have a skeleton 25 composed of a Si skeleton formed of a Si phase 22 and a conductive phase 24 distributed along the Si skeleton, and voids 26, as shown in the porous silicon material 20 in Figure 1B. In this porous silicon material 20, since the conductive phase 24 is present along the skeleton 25, it is thought that the resistivity can be reduced with a smaller amount of conductive phase. The porous silicon material may also contain an SiO2 phase.

[0026] This porous silicon material contains 20% to 70% by mass of the conductive phase and SiO2 phase, with the total composition of the Si phase, conductive phase, and SiO2 phase being 100% by mass. The conductive phase containing Ni is preferably abundant from the viewpoint of suppressing a decrease in conductivity and charge / discharge efficiency, for example, 25% or more by mass is preferred, and it may also be 30% or more by mass or 50% or more by mass. From the viewpoint of ensuring the amount of Si phase and increasing charge / discharge capacity, the conductive phase should not be too abundant, for example, 65% or less by mass is preferred, and it may also be 50% or less by mass or 30% or less by mass. The SiO2 phase content is preferably low from the viewpoint of suppressing a decrease in conductivity, preferably 7% or less by mass, and more preferably 6% or less by mass. The SiO2 phase content may be 0.1% or more by mass, 1% or more by mass, or 2% or more by mass. This porous silicon material may also have a ratio Ms / Mc of the mass of the SiO2 phase to the mass Mc[g] of the conductive phase containing Ni, which is 1 or less. From the viewpoint of suppressing a decrease in conductivity, a lower Ms / Mc value is preferable, for example, 0.5 or less is preferable, and 0.3 or less is more preferable. The Ms / Mc value may also be, for example, 0.01 or more, 0.05 or more, or 0.1 or more. The proportions of the Si phase, conductive phase, and SiO2 phase in a porous silicon material can be determined from X-ray diffraction (XRD) measurement and compositional analysis by SEM / EDX. Specifically, first, the contained phases (Al phase, Si phase, NiAl3 phase, etc.) are confirmed by XRD. Although the SiO2 peak does not appear in XRD, if O is detected by compositional analysis, it is determined that the SiO2 phase is present. If O is detected by compositional analysis, it is assumed that all O is bonded to Si, and the remaining elements (Si, Al, Ni) excluding SiO2 are allocated to each contained phase to determine the proportion of each contained phase. After alkali treatment, basically only the Si phase and Ni phase remain as contained phases other than oxides (there is no NiAl3 phase), so the proportion of each contained phase is determined by assuming that Al is solid-dissolved in Si. This porous silicon material may contain Ni in an amount of 20% to 70% by mass. Furthermore, the porous silicon material may contain one or more of Ca, Cu, Mg, Na, Sr, and P as a secondary element in an amount of 15% by mass or less. The amount of the secondary element is preferably less.

[0027] This porous silicon material may contain Si in a range of 30 at% to 90 at% and Ni in a range of 5 at% to 70 at% when the total amount of Al, Si, and Ni is considered to be 100 at%. From the viewpoint of increasing charge and discharge capacity, a higher Si content is preferable, for example, 40 at% or more is preferable, 50 at% or more is more preferable, and 60 at% or more is even preferable. From the viewpoint of increasing conductivity, a lower Si content is preferable, for example, 85 at% or less is preferable, and 82 at% or less is more preferable. From the viewpoint of increasing conductivity, a higher Ni content is preferable, for example, 10 at% or more is preferable, and 15 at% or more is more preferable. From the viewpoint of increasing charge and discharge capacity, a lower Ni content is preferable, for example, 50 at% or less is preferable, it may be 40 at% or less, or 30 at% or less. This porous silicon material is preferably low in oxygen (O). When the total amount of Al, Si, and Ni is considered to be 100 at%, the oxygen content is preferably 12 at% or less, more preferably 10 at% or less, and even more preferably 7 at% or less. This oxygen content may be, for example, 1 at% or more, 2 at% or more, or 3 at% or more. The content of each element can be determined by compositional analysis using SEM / EDX.

[0028] This porous silicon material has a porosity (preferably the porosity of pores with a diameter of 1 μm or less) of 45 volume% or more, as determined by the mercury intrusion method. In porous silicon materials, it is preferable to have a larger number of fine pores, such as 1 μm or less, from the viewpoint of distributing the stress on individual pores. This porosity may be 50 volume% or more, or 60 volume% or more. Furthermore, this porosity may be 90 volume% or less, 80 volume% or less, or 75 volume% or less. A higher porosity is preferable because it can better suppress the volume change during carrier ion storage. Also, a lower porosity is preferable because it allows for a larger amount of Si phase per unit volume.

[0029] Furthermore, this porous silicon material has an average pore diameter of 100 nm or less, determined by the mercury intrusion method. In this porous silicon material, the average pore diameter determined by the mercury intrusion method may be 30 nm or more, 50 nm or more, or 70 nm or more. This average pore diameter may be 90 nm or less, or 80 nm or less. Smaller pore diameters are preferable because the pores are less likely to collapse. Also, larger pores are preferable because they can further suppress volume changes when carrier ions are absorbed. This porous silicon material may have a pore diameter of 1 μm or less, determined by the mercury intrusion method, that is, the pore diameter distribution range may be in the range of 1 μm or less. This pore diameter distribution range may be 1 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more. Furthermore, the distribution range of this pore size may be 1000 nm or less, 500 nm or less, 300 nm or less, or 250 nm or less.

[0030] 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. Furthermore, the particles preferably have an average particle size of 100 μm or less, more preferably 10 μm or less, may be 5 μm or less, and may be 3 μm or less.

[0031] The porous silicon material preferably has a resistivity of 10 Ωcm or less, more preferably 5 Ωcm or less, and even more preferably 3 Ωcm or less. This resistivity may be, for example, 0.1 Ωcm or more, or 0.5 Ωcm or more. The resistivity can be evaluated by a four-terminal method using copper electrodes attached to a porous silicon material that has been pressure-molded at 200 MPa.

[0032] Preferably, the porous silicon material has an initial discharge capacity of 1000 mAh / g or more per unit mass of porous silicon material when charged and discharged at a current density of 0.05 C in an evaluation cell comprising an electrode containing the porous silicon material as an electrode active material and a lithium metal counter electrode. More preferably, this initial discharge capacity is 1500 mAh / g or more, and even more preferably 2000 mAh / g or more. This initial discharge capacity may be, for example, 2500 mAh / g or less. Furthermore, preferably, the porous silicon material has an initial charge-discharge efficiency of 80% or more when charged and discharged at a current density of 0.5 C in the above evaluation cell. More preferably, this initial charge-discharge efficiency is 82% or more. This initial charge-discharge efficiency may be, for example, 95% or less. Furthermore, preferably, the porous silicon material has a capacity retention rate of more than 45% after 100 cycles at a current density of 0.5 C in the above evaluation cell. This capacity retention rate is more preferably 50% or more, and even more preferably 70% or more.

[0033] (Electrodes for energy storage devices) The electrode for the energy storage device is equipped with the porous silicon material described above as the electrode active material. This electrode becomes either a positive or negative electrode based on the potential of the counter electrode relative to the potential of the electrode active material, but it is preferable to make it a negative electrode when lithium is used as the carrier. This electrode can be used, for example, in lithium-ion secondary batteries, hybrid capacitors, air batteries, etc. The electrode for the energy storage device may be made by compressing the porous silicon material to a porosity of 5 to 50 volume%. In this electrode, the porosity of the porous silicon material may be reduced by compression during manufacturing. Compared to a porous silicon material made with a porosity of 50 to 95 volume% and then compressed to this range, it exhibits better charge and discharge characteristics depending on the shape of the voids, etc. The porosity of the compressed porous silicon material can be adjusted as appropriate according to the characteristics required for electrodes in energy storage devices, for example, it may be 5% by volume or more, 10% by volume or more, or 20% by volume or more. Alternatively, the porosity of the compressed porous silicon material may be 40% by volume or less, 30% by volume or less, or 20% by volume or less.

[0034] The electrode for the energy storage device may be formed by creating the porous silicon material described above on a current collector and fixing it to the current collector. This electrode can be manufactured by either mixing the porous silicon material with a conductive material and a binder in a solvent as needed to form a paste and applying it to the current collector, or by mixing the porous silicon material with a conductive material and a binder as needed and pressing it onto the current collector. In this electrode, the content of the porous silicon material is preferably higher, preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more. The content of the porous silicon material may be, for example, 80% by mass or less, or 70% by mass or less. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. For example, one or more of the following can be used: graphite such as natural graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjen black, carbon whiskers, needle coke, carbon fiber, or metals (copper, nickel, aluminum, silver, gold, etc.). The binder plays the role of binding the active material particles and conductive material particles together. For example, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber, or thermoplastic resins such as polypropylene and polyethylene, polyimide (PI), ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, and natural butyl rubber (NBR) can be used alone or as a mixture of two or more. Water-based binders such as cellulose-based or aqueous dispersions of styrene-butadiene rubber (SBR) can also be used. As solvents, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. Alternatively, a dispersant, thickener, etc., may be added to water, and the active material may be slurryed with a latex such as SBR.As coating methods, for example, roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. can be mentioned, and any thickness and shape can be achieved using any of these. 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 polymer, conductive glass, etc., for the purpose of improving adhesion, conductivity and oxidation resistance, those obtained by treating the surface of aluminum or copper with carbon, nickel, titanium, silver, etc. can be used. It is also possible to oxidize-treat these surfaces. Regarding the shape of the current collector, examples include foil shape, film shape, sheet shape, net shape, punched or expanded shape, lath body, porous body, foam body, formed body of fiber group, 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.

[0035] In this electrode, the electrode active material may contain, in addition to the porous silicon material, an active material other than the porous silicon material. For example, as the electrode active material, a carbonaceous material, Li4Ti5O 12 etc. may be included. However, from the viewpoint of further increasing the battery capacity, taking the total electrode active material as 100% by mass, for example, the porous silicon material preferably occupies 50% by mass or more, preferably 90% by mass or more.

[0036] (Power storage device) The energy storage device of the present disclosure includes an electrode having the porous silicon material described above. This energy storage device may include a negative electrode, a positive electrode, and an ion conductive medium interposed between the negative electrode and the positive electrode and conducting carrier ions. The porous silicon material can be used as a negative electrode active material. This energy storage device may be any of a lithium ion secondary battery, a hybrid capacitor, an air battery, etc. In the positive electrode, as the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, lithium manganese composite oxides with a basic composition formula of Li (1-x) MnO2 (0 < x < 1, etc., the same hereinafter), lithium manganese composite oxides such as Li (1-x) Mn2O4, lithium cobalt composite oxides with a basic composition formula of Li (1-x) CoO2, lithium nickel composite oxides with a basic composition formula of Li (1-x) NiO2, lithium nickel cobalt manganese composite oxides with a basic composition formula of Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), lithium vanadium composite oxides with a basic composition formula of LiV2O3, transition metal oxides with a basic composition formula of V2O5, 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 / 3 O2, etc. are preferable. Note that 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 a capacitor or a lithium ion capacitor. Examples of the carbonaceous material include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, polyacenes, etc. Among these, activated carbons showing a high specific surface area are preferable. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m 2 / g or more, and 1500 m2 It is more preferable that the amount is greater than or equal to / g. The specific surface area is 1000m². 2 At concentrations of 1 / g or higher, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m² due to its ease of manufacture. 2 It is preferable that it be less than / g, and 2000m 2 It is more preferable that the value be less than or equal to / g. The conductive material, binder, solvent, and current collector used in the positive electrode can be any of the examples given above for the electrode.

[0037] As the ion-conducting medium, non-aqueous electrolytes containing supporting salts or non-aqueous gel electrolytes can be used. Examples of solvents for non-aqueous electrolytes include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can be used individually or in combination. Specifically, examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; linear 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; linear 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; sulforanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Of these, a combination of cyclic carbonates and linear carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge cycles, but also allows for a balanced relationship between the viscosity of the electrolyte, the electrical capacity of the resulting battery, and the battery output. Examples of supporting salts include LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiSbF6, LiSiF6, LiAlF4, LiSCN, LiClO4, LiCl, LiF, LiBr, LiI, and LiAlCl4. From the viewpoint of electrical characteristics, it is preferable to use a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF6, LiBF4, LiAsF6, and LiClO4, and organic salts such as LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3.The supporting salt is preferably at a concentration of 0.1 mol / L or more and 5 mol / L or less in the non-aqueous electrolyte, and more preferably at a concentration of 0.5 mol / L or more and 2 mol / L or less. A concentration of 0.1 mol / L or higher allows for sufficient current density, while a concentration of 5 mol / L or lower allows for greater stability of the electrolyte. Furthermore, flame retardants such as phosphorus-based or halogen-based agents may be added to this non-aqueous electrolyte.

[0038] Furthermore, instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of ion-conducting polymers include polymer gels composed of polymers such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, and vinylidene fluoride, along with supporting salts. In addition, a combination of an ion-conducting polymer and a non-aqueous electrolyte can also be used. Moreover, in addition to ion-conducting polymers, inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, or inorganic solid powders bound together by an organic binder can be used as the ion-conducting medium.

[0039] The energy storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as its composition can withstand the operating range of lithium secondary batteries, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric or polyphenylene sulfide nonwoven fabric, and thin microporous membranes of olefin resins such as polyethylene or polypropylene. These may be used individually or in combination.

[0040] The shape of this energy storage device is not particularly limited, but examples include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flattened, and rectangular shapes. It may also be applied to larger devices used in electric vehicles, etc. Figure 4 is an explanatory diagram showing an example of the structure of the energy storage device 10. This energy storage device 10 has a positive electrode 12, a negative electrode 15, and an ion conducting medium 18. The positive electrode 12 has a positive electrode active material 13 and a current collector 14. The negative electrode 15 has a negative electrode active material 16 and a current collector 17. The negative electrode active material 16 is the porous silicon material 20 described above and has voids 26.

[0041] This energy storage device preferably has a higher discharge capacity. For example, when charging and discharging is performed at a current density of 0.05C, the initial discharge capacity is preferably 1000mAh / g or more, more preferably 1500mAh / g or more, and even more preferably 2000mAh / g or more per unit mass of porous silicon material.

[0042] This energy storage device preferably has a higher charge and discharge efficiency. For example, when charging and discharging is performed at a current density of 0.5C, the initial charge and discharge efficiency is preferably 80% or higher, and more preferably 82% or higher. The initial charge and discharge efficiency may be, for example, 95% or lower.

[0043] This energy storage device preferably has a higher capacity retention rate when performing charge-discharge cycles. For example, it is preferable that the capacity retention rate after 100 cycles at a current density of 0.5C exceeds 45%, more preferably 50% or more, and even more preferably 52% or more.

[0044] As detailed above, this disclosure provides a porous silicon material in which the decrease in conductivity and charge / discharge efficiency is suppressed. The reason for obtaining such effects is presumed to be as follows. For example, when an Al-Si-Ni alloy is melted and rapidly solidified, it separates into an Al phase, a Si phase, and an Al-Ni compound phase during phase separation. Subsequently, an alkaline treatment is performed, but since the Al component dissolves more easily than the Ni component during the alkaline treatment, the Al-Ni compound phase remains as is or in large quantities as the Ni phase even after the Al phase is removed. The Al-Ni compound phase and the Ni phase function as both a conductive phase and an expansion relaxation phase. Therefore, it is presumed that this disclosure can realize a porous silicon material in which the decrease in conductivity and charge / discharge efficiency is suppressed. Furthermore, in Al-Si-Ni alloys, the silicide phase is less likely to be generated during phase separation, so it is presumed that SiO2, which is an insulating phase, is less likely to be generated in the subsequent Al component removal treatment, and the decrease in conductivity is further suppressed. Furthermore, when increasing the electrode area to increase the battery capacity, the pressure holding the electrodes tends to decrease, making it particularly important to suppress electrode expansion. In this disclosure, the conductive phase containing Ni is thought to function as an expansion-relaxing phase, and is therefore expected to have the effect of suppressing electrode expansion.

[0045] It goes without saying that this disclosure is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure.

[0046] For example, in the embodiments described above, regarding the raw material composition, the case in which a ternary Al-Si-Ni system undergoes phase separation into an Al phase (pore source), a Si phase, and a NiAl3 phase, and silicide is not generated during phase separation, was mainly explained, but the invention is not limited to this. For example, from the viewpoint of improving the conductivity and expansion relaxation properties of porous silicon materials, it is conceivable to increase the proportion of Ni in the raw material composition. However, in composition ranges with a high Ni content, various Ni silicides, such as the Al-Si-Ni ternary silicide, which is a metastable high-temperature stable phase, may remain at room temperature due to non-equilibrium rapid solidification. Such Ni silicides may be used as a conductive phase or an expansion relaxation phase. However, since the Ni silicide phase can cause a decrease in capacity and Coulomb efficiency, it is preferable to have a low content. The raw material composition of the porous silicon material may be such that a porous silicon material with a coexistence structure in which at least one of the Ni phase and the Ni silicide phase is distributed along the Si skeleton can be obtained. Examples of Ni silicides include NiSi2, Ni2(Al,Si)3, and Ni3Al 5.5 Si 1.5 These are some examples.

[0047] Figure 11 shows an explanatory diagram of raw material compositions capable of generating coexisting structures. Figure 11A identifies raw material compositions capable of generating coexisting structures (raw material compositions in which a state exists where the liquid phase (L), Si, and NiAl3 coexist in the equilibrium phase diagram) using the Al-Si-Ni phase diagram and summarizes them in a triangular composition diagram. Figure 11B is the Al-Si-Ni phase diagram at Ni 15 at%, Figure 11C is the Al-Si-Ni phase diagram at Ni 10 at%, and Figure 11D is the Al-Si-Ni phase diagram at Ni 6 at%. As shown in Figure 11A, in the Ni-excessive composition region, Al and Si react with Ni to form the silicide phase, so the Al phase (pore source) and Si phase (active material source) cannot exist individually, and it is considered that porous Si material cannot be formed in the first place (Si phase non-formation region). In the region where the Ni content is 40 at% or less, Al and Si phases are formed, but since there is no precursor state for the coexisting structure until the Ni content reaches about 20 at%, it is considered that the desired coexisting structure cannot be realized (porous Si formation range). When the Ni content decreases to about 20 at%, it is possible to take on a precursor state for the coexisting structure, and it is considered that the coexisting structure can be realized (coexisting structure formation range). The raw material composition is preferably one that can form the coexisting structure. The raw material composition that can form the coexisting structure can be determined from the Al-Si-Ni equilibrium phase diagram. The Al-Si-Ni phase diagram may be calculated using the CALPHAD method. In this coexisting structure formation range, as the Ni content increases, the phase diagram changes to a multi-stage phase diagram that goes through the decomposition of the high-temperature stable phase to the precursor state for the coexisting structure, and the silicide phase formed at high temperature may remain. From the viewpoint of suppressing the retention of such silicide phase, the Ni content in the raw material composition may be 20 at% or less, 12 at% or less, 8 at% or less, etc. In the raw material composition, the Ni content ratio may be set to 3 at% or more, or 4 at% or more, from the viewpoint of increasing the proportion of Ni phases that function as expansion relaxation phases, etc. In the raw material composition, the Ni / Si ratio, which is the molar ratio of Ni to Si, may be set to 1.5 or less, 1 or less, or 0.7 or less, from the viewpoint of suppressing the silicide phase. In the raw material composition, the Ni / Si ratio may also be set to 0.05 or more, 0.1 or more, or 0.15 or more, from the viewpoint of increasing the proportion of Ni phases that function as expansion relaxation phases, etc.

[0048] For example, although the above-described embodiment mainly described the case where the porous silicon material does not contain silicide, it may also contain silicide. The porous silicon material may be, for example, a coexistence structure in which at least one of the Ni phase and the Ni silicide phase is distributed along the Si skeleton. The Ni phase and the Ni silicide phase are also considered to function as a conductive phase or an expansion relaxation layer. In the porous silicon material, the Ni phase content may be 10% by mass or more, 15% by mass or more, 20% by mass or more, or 60% by mass or less, 50% by mass or less, or 40% by mass or less. In the porous silicon material, the silicide phase content may be 70% by mass or less, 50% by mass or less, 20% by mass or less, or 10% by mass or less. In the porous silicon material, the silicon phase content may be 30% by mass or more, 50% by mass or more, or 60% by mass or more.

[0049] For example, this disclosure may be any of the following [1] to

[10] . [1] A method for producing a porous silicon material, comprising: a precursor step of melting and rapidly solidifying raw materials containing Al, Si, and Ni to obtain a silicon alloy precursor; and a porosity step of removing the Al component contained in the silicon alloy by alkaline treatment to obtain a porous silicon material. [2] The method for producing a porous silicon material according to [1], wherein the precursor step uses a raw material containing Si in an amount of 1 at% to 88 at% and Ni in an amount of 2 at% to 25 at% when the total amount of Al, Si and Ni is 100 at%. [3] The method for producing a porous silicon material according to [1] or [2], wherein the alkali treatment is performed using NaOH as the alkali in the porosity step. [4] The method for producing a porous silicon material according to any one of [1] to [3], wherein the alkali treatment is performed using an aqueous solution with an alkali concentration of 3 mol / L or less in the porosity step. [5] A method for producing a porous silicon material according to any one of [1] to [4], wherein the alkali treatment is performed within a range of 10 hours or less in the porosification step, and a porous silicon material is obtained in which no Al peak is observed by XRD measurement. [6] A porous silicon material comprising a Si phase and a conductive phase containing Ni, which may also contain an SiO2 phase, wherein when the total amount of the Si phase, conductive phase and SiO2 phase is 100% by mass, the conductive phase is contained in an amount of 20% by mass or more and 70% by mass or less, the SiO2 phase content is 10% by mass or less, and the porosity determined by the mercury intrusion method is 45% by volume or more and the average pore diameter is 100 nm or less. [7] The porous silicon material according to [6], wherein when the total amount of Al, Si, and Ni is 100 at%, Si is contained in an amount of 30 at% to 90 at% and Ni is contained in an amount of 5 at% to 70 at%. [8] A porous silicon material according to [6] or [7], wherein the resistivity is 10 Ωcm or less. A power storage device comprising: a negative electrode containing a porous silicon material described in any one of [9] [6] to [8] as a negative electrode active material; a positive electrode containing a positive electrode active material; and an ion conducting medium interposed between the negative electrode and the positive electrode for conducting carrier ions.

[10] The energy storage device described in [9], wherein the initial discharge capacity per unit mass of porous silicon material is 1000 mAh / g or more and the initial charge-discharge efficiency is 80% or more. [Examples]

[0050] The following describes specific examples of the porous silicon material and energy storage devices fabricated according to this disclosure. Experimental Examples 1-3 and 9 are embodiments of this disclosure, and Experimental Examples 4-8 are comparative examples. Experimental Examples 10, 13-25 are embodiments of this disclosure, and Experimental Examples 11-12 and 26 are comparative examples.

[0051] [Fabrication of porous silicon materials] Al, Si, Ni raw materials 100-x-y Si x Ni yThe material was weighed to achieve a composition of (x=10~40, y=3~20) and melted in an arc melting furnace. Before melting, the inside of the arc melting furnace was 8 × 10 -3 After reducing the pressure to below Pa, the mixture was purged with Ar gas. The preparation of the master alloy required dissolving the raw material powder, and high-frequency induction melting was performed to produce a uniform sample. The resulting master alloy was heated to 1000-1300°C in an argon atmosphere to melt, and then melted by gas atomization. 2 The alloy powder was rapidly solidified at a rate of K / sec or higher to obtain a precursor, which is an Al-Si-Ni alloy powder (precursor step). The obtained alloy powder was immersed in 0.1 mol / L hydrochloric acid or 0.05 mol / L sodium hydroxide aqueous solution and treated at room temperature for 1 to 48 hours to selectively remove Al until the Al phase could no longer be detected by XRD measurement. Regarding the treatment time, first, the state in which no hydrogen gas was generated was determined, and the XRD of the sample treated at that time was measured. If Al was not detected, the treatment was stopped there. If Al was detected, the treatment time was increased by 1 hour at a time until the total time until Al could no longer be detected was defined as the treatment time. The residue was transferred to a filtration filter, and the supernatant after treatment was removed by vacuum filtration. After that, it was washed four or more times with distilled water, and the washing water was removed by the same vacuum filtration method to obtain a composite porous silicon material (porosization step).

[0052] (Experimental Examples 1-3) Experimental Example 1 involved setting the above basic composition formula to x=14.5 and y=3.6, and removing Al with a 0.05 mol / L sodium hydroxide aqueous solution to obtain a porous silicon material. Experimental Example 2 involved setting the above basic composition formula to x=30 and y=6, and removing Al with a 0.05 mol / L sodium hydroxide aqueous solution to obtain a porous silicon material. Experimental Example 3 involved setting the above basic composition formula to x=18 and y=15, and removing Al with a 0.05 mol / L sodium hydroxide aqueous solution to obtain a porous silicon material.

[0053] (Experimental Examples 4-5) The raw materials are Al, Si, and Cr. 67 Si 30Experimental Example 4 was a porous silicon material obtained in the same manner as in Experimental Example 1, except that the composition was weighed to be Cr3 and Al was removed with a 0.1 mol / L hydrochloric acid aqueous solution. The raw materials for Al, Si, and Cr were Al 62.5 Si 30 Cr 7.5 A porous silicon material was obtained in the same manner as in Experimental Example 1, except that the composition was measured and Al was removed using a 0.1 mol / L hydrochloric acid aqueous solution. This porous silicon material was designated as Experimental Example 5.

[0054] (Experimental Example 6) The porous silicon material obtained by setting x=30 and y=6 in the above basic composition formula, and removing Al with a 0.1 mol / L hydrochloric acid aqueous solution, was designated as Experimental Example 6.

[0055] (Experimental Example 7) Without fabricating a porous silicon material, we used Si powder with an average particle size of 5 μm (SIE23PB from Koshu Chemical) as is for Experimental Example 7.

[0056] (Experimental Example 8) The porous silicon material obtained by setting x=20 and y=0 in the above basic composition formula, and removing Al with a 0.1 mol / L hydrochloric acid aqueous solution, was designated as Experimental Example 8.

[0057] (Experimental Example 9) A porous silicon material obtained in the same manner as in Experimental Example 3 was designated as Experimental Example 9, except that a heat treatment was performed in which the material was heated at 350°C for 1 hour under an argon atmosphere before the Al removal treatment.

[0058] [Pore distribution measurement] The pore distribution of the fabricated porous silicon material was measured using a mercury porosimeter (POWERMASTER60GT manufactured by Cantachrome).

[0059] [SEM observation, compositional analysis] The surface and cross-section of the fabricated porous silicon materials were observed using a scanning electron microscope (SEM, HITACHI S-4300) to obtain secondary electron images and backscattered electron composition images. Furthermore, the porous silicon materials of Experimental Examples 1-6 were subjected to compositional analysis using an energy-dispersive X-ray spectrometer (EDX) attached to the SEM. Specifically, powder samples were pressure-molded into disc shapes using a cemented carbide mold at 500 MPa. Five random locations within the pressure surface were then observed using SEM (Hitachi High-Tech S-3600N, 1000x magnification), and the compositional analysis of the entire observation surface was performed. The average value was then determined as the compositional ratio.

[0060] [X-ray diffraction measurement] X-ray diffraction (XRD) measurements were performed on the fabricated porous silicon materials and their precursors. An X-ray diffractometer (Rigaku RINT-TTR) was used for the measurements, with a Cu tube, in the range of 2θ = 10° to 80°, at a rate of 5° / min. Furthermore, using the XRD measurement results and compositional analysis results, the concentrations of the SiO2 phase and the conductive phase (Ni phase and NiAl3 phase in Experimental Examples 1-3 and 6, and Cr(Al,Si)2 phase in Experimental Examples 4-5) were determined for the porous silicon materials.

[0061] [Resistivity measurement] The resistivity of the fabricated porous silicon was measured. The porous silicon powder sample was pressure-molded at 200 MPa, and the resistivity was evaluated using a four-terminal method with copper electrodes attached.

[0062] [Evaluation of battery characteristics] An evaluation cell was fabricated using the prepared porous silicon material as the negative electrode active material, and the discharge capacity, charge / discharge efficiency, and cycle capacity retention rate were evaluated. A negative electrode slurry was prepared by mixing 60% by mass of silicon material as the negative electrode active material, 20% by mass of carbon black as the conductive material, and 20% by mass of polyimide binder as the binder, and then adding N-methylpyrrolidone and stirring. Next, this negative electrode slurry was applied to a copper current collector foil and dried to prepare an composite electrode. The electrode was punched out in a 16 mm diameter circle, a porous polyethylene separator was placed between the negative electrode and metallic lithium as the counter electrode, and an electrolyte was poured in to manufacture a lithium secondary battery using a Tom cell type small battery cell. As the electrolyte, a mixed solvent was used, consisting of 1.5 parts by volume of fluoroethylene carbonate (FEC), 3 parts by volume of ethylene carbonate (EC), 4 parts by volume of dimethyl carbonate (DMC), and 3 parts by volume of ethylmethyl carbonate (EMC), to which LiPF6 was added at a concentration of 1 mol / L. For the obtained lithium secondary battery, the initial discharge capacity at 0.05C was determined by charging and discharging at a current density of 0.05C in the battery voltage range of 0.005V to 1.5V. Then, the initial charge-discharge efficiency and the capacity retention rate after 100 cycles at 0.5C were determined by repeating the charge-discharge process at a current density of 0.5C in the same battery voltage range for 100 cycles. In the evaluation of battery characteristics, discharge is considered to represent oxidation of the porous silicon anode (desorption of lithium).

[0063] [Results and Discussion] Figure 5 shows the XRD measurement results for Experimental Example 2 (Figure 5(c)), Experimental Example 6 (Figure 5(b)), and their precursor (Figure 5(a)). Figure 5B is an enlarged view of Figure 5A. Figure 6 shows the pore size distribution measurement results for the porous silicon material of Experimental Example 2 (Figure 6B) and Experimental Example 6 (Figure 6A). Figure 7 shows the surface SEM observation results for Experimental Example 2 (Figure 7B), Experimental Example 6 (Figure 7C), and their precursor (Figure 7A). Figure 8 shows the cross-sectional SEM observation results for Experimental Example 2 (Figure 8B) and Experimental Example 6 (Figure 8A). Table 1 summarizes the base alloy composition, porosity treatment details, and properties of the porous silicon material (elemental ratio, oxygen content, SiO2 phase ratio, conductive phase ratio, porosity, average pore diameter, and resistivity) for experimental examples 1-6. Table 2 summarizes the base alloy composition, porosity treatment details, properties of the porous silicon material (conductive phase ratio, resistivity), and battery characteristics (initial discharge capacity per active material, initial charge / discharge efficiency, and capacity retention rate after 100 cycles) for experimental examples 1-5 and 7-8.

[0064] XRD measurements of the phase composition of the precursor and porous silicon material revealed that the Al-Si-Ni alloy (precursor) after rapid quenching contained Al, Si, and NiAl3 phases, as predicted from the equilibrium phase diagram (Figure 5A(a)). No peak of the Ni silicide phase was observed for this precursor. When the Al-Si-Ni alloy powder was treated with an aqueous hydrochloric acid solution, the diffraction peak of Al decreased over time, and after more than 12 hours of treatment, the Al diffraction peak completely disappeared, indicating that Al removal was complete. However, at the same time, the NiAl3 peak also disappeared, and the conductive phase and expansion relaxation phase were also removed (Figure 5B(b)). On the other hand, when treated with an aqueous sodium hydroxide solution, the diffraction peak of the Al phase disappeared within 1 hour, indicating that porosity was completed. In this case as well, the diffraction peak of the NiAl3 phase disappeared, but instead, the diffraction peak of Ni appeared (Figure 5B(c)), indicating that Ni remained dissolved as a conductive phase. The Ni phase peaks were observed around 2θ = 44.2° (for example, within the range of 44.2 ± 1°) and around 2θ = 51.5°. No Ni silicide phase peaks were observed for this porous silicon material. In the Al-Si-Cr system, the amount of oxygen increased with the partial dissolution of the conductive phase, Cr silicide, and amorphous SiO2 was formed. However, in the Al-Si-Ni system, the amount of oxygen was found to be low (Table 1). SiO2 is a highly resistive insulator, and its presence is undesirable because it increases the resistivity of the negative electrode. Compositional analysis revealed that when the Al-Si-Ni alloy was treated with hydrochloric acid, most of the Ni dissolved. When treated with an alkaline solution, the amount of Ni was nearly 10 times greater than when treated with hydrochloric acid, indicating that almost all of the Ni remained undissolved.

[0065] Regarding the pore size distribution of porous powder, when the pore size distribution was measured using a mercury porosimeter, both intragranular and intergranular pores were included, but it was difficult to distinguish the boundary between the two in the sample treated with hydrochloric acid (Figure 6A). Observation of the powder's shape revealed that in the hydrochloric acid-treated particles, spherical particles had disintegrated and aggregated (Figures 7B, 8B). This was presumed to be because the NiAl3 segregated in the skeletal portion dissolved during hydrochloric acid treatment, making it impossible to maintain the skeletal structure. On the other hand, in the alkali-treated sample, intragranular pores and intergranular pores were clearly separated (Figure 6B). Particle shape disintegration was also suppressed (Figures 7C, 8A). In all samples, the porosity was in the range of 60-90 volume%, indicating that the pore size was fine, less than 100 nm. Furthermore, cross-sectional SEM observation of the alkali-treated sample revealed that a porous powder with pores of several tens of nanometers was produced (Figure 8A). Furthermore, compositional analysis revealed a structure in which Ni was segregated in the skeletal portion.

[0066] Regarding the resistivity of porous powders, as shown in Table 1, the Al-Si-Cr system achieved the lowest resistivity when the conductive phase was small, but the resistivity increased sharply as the conductive phase increased. In samples with a high conductive phase, the amount of SiO2 was also high, suggesting that the by-formed insulating phase (SiO2 phase) was inhibiting electrical conduction. On the other hand, in the Al-Si-Ni system, the resistivity value itself was an order of magnitude lower, and it was found that even lower resistivity was achieved in the region with a high conductive phase.

[0067] Regarding battery characteristics, as shown in Table 2, porous samples containing a conductive phase showed a higher capacity retention rate compared to Si powder without pores or porous Si without a conductive phase. In composite porous silicon materials, the Al-Si-Cr system showed decreased Coulomb efficiency and high resistivity at compositions with a conductive phase content of 20% by mass or more, where a swelling suppression effect can be expected. On the other hand, the Al-Si-Ni system achieved high Coulomb efficiency and low resistivity even at compositions with a conductive phase content of 20% by mass or more, where an effect on suppressing the swelling rate can be expected, and furthermore, it was found to maintain a high capacity of 1000 mAh / g or more. As described above, in Experimental Examples 1 to 3, it was confirmed that the amount of expansion can be suppressed without impairing resistivity while maintaining a high capacity of 1000 mAh / g or more. In particular, in Experimental Examples 1 and 3, which used alloys with compositions near the eutectic composition among the Al-Si-Ni systems, the capacity retention rate after cycling was higher, which was found to be particularly preferable. In Experimental Example 1, as shown in Figure 9A, for example, there were no coarse particles and the skeletal structure was uniformly formed, with no areas where the composition was clearly segregated, and a structure in which Ni and Si coexisted in the skeleton was realized. In Experimental Example 3, as shown in Figure 9B, for example, there were no coarse particles and the skeletal structure was uniformly formed. However, in Experimental Example 3, the raw material composition had a high Ni content, and in the atomized powder before alkali treatment, a phase that appeared to be a metastable Al-Si-Ni ternary silicide was observed during rapid cooling and solidification (Figure 10(a)). In Experimental Example 9, this atomized powder was heat-treated at over 350°C before alkali treatment. The metastable phase changed to Si and Al3Ni due to the heat treatment (Figure 10(b)), and by alkali treatment, a porous silicon material composed of Si phase and Ni phase was obtained (Figure 10(c)).

[0068] [Table 1]

[0069] [Table 2]

[0070] (Experimental Examples 10-26) Porous silicon materials obtained in the same manner as in Experimental Example 1, except for the master alloy composition shown in Table 3, were designated as Experimental Examples 10 to 26.

[0071] Figure 12 shows the XRD measurement results (XRD patterns) of porous silicon materials for experimental examples 10 to 25. Figure 12A is a magnified view of part A of Figure 11, Figure 12B shows the XRD patterns of experimental examples 10 to 12, Figure 12C shows the XRD patterns of experimental examples 13 to 16, Figure 12D shows the XRD patterns of experimental examples 17 to 20, and Figure 12E shows the XRD patterns of experimental examples 21 to 25. Figure 12A plots the compositions of experimental examples 10 to 25. The compositions of experimental examples 10, 11, and 12 correspond to the region where coexisting structures can be formed, the porous Si formation range, and the Si phase non-formation region, respectively (Figure 12A). In the composition of experimental example 12, a single silicide phase with the same composition as the master alloy was formed. Since this silicide phase does not dissolve in alkali, porosity could not be created, and it was found that porous Si could not be fabricated as expected (Figure 12B). Even with the composition of Experimental Example 11, various silicide phases (Ni3Al 5.5 Si 1.5Ni2(Si,Al)3 and other silicide phases of unknown structure were formed (although the structure was unknown, EDX analysis revealed regions where Al, Si, and Ni were segregated, so it was judged to be a peak of the silicide phase). However, since Al and Si phases were formed, porous Si could be fabricated. However, since the Ni phase was not included, it became clear that the target structure (a structure in which the Ni phase coexists with the Si framework) could not be formed as expected (Figure 12B). On the other hand, the composition of experimental example 10 contained both Si and Ni phases, indicating the possibility of realizing a coexisting structure (Figure 12B). However, since the proportion of Si and Ni phases was small and many silicide phases coexisted, further optimization of the composition was considered. The compositions of experimental examples 13 to 25, which fall within the region in which coexisting structures can be formed, all contained Si and Ni phases as the main phases, indicating the possibility of realizing a coexisting structure. In experimental examples 21 to 25, where Ni was 6%, it was found that they consisted almost entirely of Si and Ni phases (Figure 12E). In experimental examples 17-20, where the Ni content was increased to 10%, a silicide phase was observed, but in small quantities (Figure 12D). In contrast, in experimental examples 13-16, where the Ni content was increased to 15%, the peak of the silicide phase became more pronounced (Figure 12C).

[0072] Figures 13 and 14 show the SEM / EDX measurement results for porous silicon materials in Experimental Examples 21 and 25. Figures 13A and 14A are SEM images, Figures 13B and 14B are Al mapping images, Figures 13C and 14C are Ni mapping images, and Figures 13D and 14D are Si mapping images. In the sample with low Ni content and almost no silicide (Experimental Example 21), Al, Ni, and Si were uniformly distributed in the skeletal structure, indicating that a sample with the desired structure was formed (Figure 13). On the other hand, in the sample with high Ni content and a large amount of silicide phase (Experimental Example 15), Al, Si, and Ni were segregated, and it was found that the coexistence structure was not as clean as in Experimental Example 21 (Figure 14).

[0073] The pore distribution of the porous silicon materials in Experimental Examples 10-25 was evaluated using a mercury porosimeter and summarized in Table 3. In Experimental Examples 10 and 13-25, the porosity ranged from approximately 45-75 volume%, and all samples were found to have fine pore sizes of 100 nm or less. This porosity tended to be smaller than the theoretical porosity estimated assuming complete removal of the Al component from AlSiNi. This was presumed to be because all samples contained a considerable amount of the Al-containing silicide phase.

[0074] The relative proportions of the Si phase, Ni phase, and various silicide phases were evaluated using the reference intensity ratio method from the peak intensity ratios of the XRD in experimental examples 10-25 and summarized in Table 3. In this table, the silicide is Ni3Al 5.5 Si 1.5 This composition includes Ni2(Si,Al)3 and other silicide phases of unknown structure. The reference intensity ratios for each phase were based on values ​​published in the database. However, since the reference intensity ratio for the silicide phase of unknown structure was unknown, the value for the AlSiNi alloy phase (2.0) was used as a substitute. In experimental examples 21-25 using a master alloy with 6 at% Ni, the silicide phase content was less than 5 mass%, while a tendency was observed for the amount of silicide phase to increase as the Ni content in the master alloy increased. In experimental examples 17-20 using a master alloy with 10 at% Ni, the proportion of silicide phase remained below 20 mass%. In experimental examples 13-16 using a master alloy with 15 at% Ni, the amount of silicide phase increased to over 50 mass%. Furthermore, it was observed that, when the Ni content in the master alloy was the same, the proportion of silicide phase tended to increase with higher Si content compositions. If silicide formation is not observed, a higher Ni / Si ratio in the base alloy results in a higher Ni phase content after porosity treatment. However, for example, when using an AlSiNi alloy with 15 at% Ni as the raw material, even with a high Ni / Si ratio, a large amount of Ni was converted into the silicide phase, preventing an increase in the Ni phase content. In contrast, when using a base alloy with 10 at% Ni or less, a higher Ni phase content was achieved even with a low Ni / Si ratio.

[0075] Using the silicon materials from Experimental Examples 10-26 as the negative electrode active material, evaluation cells were fabricated in the same manner as described above, and 30 cycles of charging and discharging were performed with a current density of 0.2C in the battery voltage range of 0.005V to 1.5V. Table 3 summarizes the evaluation results of the battery characteristics. Table 3 also shows the theoretical capacity calculated assuming that the AlSiNi alloy becomes Si phase and Ni phase through porosity treatment. In Experimental Examples 21-25, which used a master alloy with 6at% Ni and had a small amount of silicide phase in the porous silicon material, a capacity close to the theoretical capacity was obtained, and the Coulomb efficiency and capacity retention rate were also relatively high. On the other hand, in Experimental Examples 13-16, which used a master alloy with a large amount of Ni and had a large amount of silicide phase in the porous silicon material, a tendency for the capacity to decrease compared to the theoretical capacity was observed. In the Al-Si-Ni system, Al 80 Si 20 Compared to porous Si with a simple structure fabricated from the master alloy, the capacity retention rate was improved. However, in the Al-Si-Ni alloy, the samples in Experimental Example 12 (in the Si-unformed region) and Experimental Example 11 (in the porous Si formation range) had an extremely high amount of silicide phase, resulting in a capacity lower than that of graphite. In contrast, compositions in the region where coexisting structures can be formed showed a higher capacity compared to graphite, with a capacity retention rate of over 60% after 100 cycles. In particular, it was found that setting the Ni content in the master alloy to 10 at% or less was preferable, as it allowed for both a high capacity of over 1000 mAh / g and a high charge-discharge efficiency of over 75%.

[0076] [Table 3]

[0077] It goes without saying that this disclosure is not limited in any way to the experimental examples described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure. [Industrial applicability]

[0078] This disclosure is applicable to the field of energy storage devices. [Explanation of Symbols]

[0079] 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, 20 Porous silicon material, 22 Si phase, 24 Conductive phase, 25 Skeleton, 26 Void.

Claims

1. A precursor step involves melting raw materials containing Al, Si, and Ni, rapidly cooling and solidifying them to obtain a silicon alloy precursor, A porosizing step to obtain a porous silicon material by removing the Al component contained in the silicon alloy by alkaline treatment, A method for producing porous silicon material containing [the specified material].

2. The method for producing a porous silicon material according to claim 1, wherein the precursor step uses a raw material containing Si in an amount of 1 at% to 88 at% and Ni in an amount of 2 at% to 25 at% when the total amount of Al, Si, and Ni is 100 at%.

3. A method for producing a porous silicon material according to claim 1 or 2, wherein the alkali treatment is performed using NaOH as the alkali in the porosification step.

4. A method for producing a porous silicon material according to claim 1 or 2, wherein the alkali treatment is performed using an aqueous solution with an alkali concentration of 3 mol / L or less in the porosification step.

5. The method for producing a porous silicon material according to claim 1 or 2, wherein the alkali treatment is performed within a range of 10 hours or less in the porosification step to obtain a porous silicon material in which no Al peak is detected by XRD measurement.

6. Si phase, conductive phase containing Ni, and containing SiO 2 It may contain phases such as Si phase, conductive phase and SiO 2 When the total mass of the phases is considered to be 100 mass%, the conductive phase is contained in a range of 20 mass% to 70 mass%, and SiO 2 A porous silicon material having a phase content of 10% by mass or less, a porosity of 45% by volume or more as determined by the mercury intrusion method, and an average pore diameter of 100 nm or less.

7. The porous silicon material according to claim 6, wherein, when the total amount of Al, Si, and Ni is 100 at%, Si is contained in an amount of 30 at% to 90 at% and Ni is contained in an amount of 5 at% to 70 at%.

8. A porous silicon material according to claim 6 or 7, wherein the resistivity is 10 Ωcm or less.

9. A negative electrode containing the porous silicon material described in claim 6 or 7 as an electrode active material, A positive electrode containing a positive electrode active material, An ion-conducting medium interposed between the negative electrode and the positive electrode, which conducts carrier ions, A power storage device equipped with [a specific feature / ability].

10. The energy storage device according to claim 9, wherein the initial discharge capacity per unit mass of porous silicon material is 1000 mAh / g or more, and the initial charge-discharge efficiency is 80% or more.

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