JP2023094056A - Electrode for power storage device, power storage device and production method of electrode for power storage device - Google Patents

Abstract

To further increase an energy density as to a material including Si.SOLUTION: An electrode for a power storage device herein disclosed comprises a porous silicon material having a plate-like integral shape with pores and containing at least Si and Al. The production method of an electrode for a power storage device herein disclosed comprises: a precursor step of melting and quench-solidifying a raw material containing at least Si and Al, in which Al is in a range more than 20 at% and less than 70 at% to 100 at% of Si and Al in total, and the rest mainly contains Si, thereby obtaining a plate-like silicon alloy precursor; and a turning-into electrode step of removing Al component included in the silicon alloy to gain a porous silicon material having a plate-like shape as part of an electrode for a power storage device.SELECTED DRAWING: None

Description

Disclosed herein are an electrode for an electricity storage device, an electricity storage device, and a method for manufacturing an electrode for an electricity storage device.

Conventionally, as a silicon negative electrode material, a granulating step of dissolving and granulating a silicon alloy containing 50% by mass or more of Al as the first element and 50% by mass or less of Si, and Porous silicon particles obtained by a production method including a porosification step of removing elements to obtain porous silicon particles have been proposed (see, for example, Patent Document 1). The porous silicon particles have an average particle diameter in the range of 0.1 μm or more and 100 μm or less, contain skeletal silicon having a three-dimensional network structure with voids, and have an average porosity in the range of 50 volume % or more and 95 volume % or less. and contains 85% by mass or more of Si and 15% by mass or less of Al in terms of the ratio of elements other than oxygen.

JP 2021-123517 A

However, the porous silicon particles of Patent Literature 1 described above are relatively fine particles and have a high porosity, which makes it difficult to apply a coating to form an electrode. In addition, the porous silicon particles require a conductive material, a binder, and the like, and there are problems such as a further decrease in energy density and the like.

The present disclosure has been made in view of such problems, and a main object thereof is to provide an electricity storage device electrode that can further increase energy density, an electricity storage device, and a method for manufacturing an electricity storage device electrode.

As a result of intensive research to achieve the above object, the present inventors melted a silicon alloy containing at least Al and Si, quenched it into a plate, and then removed the Al-containing compound to form a binder. The present inventors have found that a plate-shaped porous silicon material can be used as an electrode without using a silicon material, and have completed the electricity storage device electrode, the electricity storage device, and the method for manufacturing the electricity storage device electrode of the present disclosure.

That is, the electricity storage device electrode of the present disclosure is
A porous silicon material containing at least Si and Al and having voids and having a plate-like integral shape is used as an electrode active material.

The power storage device of the present disclosure is
a positive electrode comprising a positive electrode active material;
a negative electrode that is the electrode for the electricity storage device described above;
an ion conducting medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
is provided.

The method for manufacturing an electrode for an electricity storage device of the present disclosure includes:
A raw material containing at least Si and Al and containing Al in a range of more than 20 at% and less than 70 at% when the total of Si and Al is 100 at%, and the balance being Si is melted and rapidly solidified into a plate-like shape. a precursor step of obtaining a silicon alloy precursor;
an electrode forming step of removing the Al component contained in the silicon alloy to obtain a porous silicon material having a plate-like shape as a part of an electrode for an electricity storage device;
includes.

The present disclosure can further increase the energy density in materials containing Si. The reason why such an effect is obtained is presumed as follows. For example, the manufacturing method of the present disclosure can selectively dissolve aluminum from silicon and aluminum alloys to produce porous silicon with small pore sizes and high porosity. Moreover, since the porous silicon material obtained by the present disclosure is a self-supporting film, it can be used as an electrode as it is without adding a conductive material or a binder. Therefore, it becomes possible to provide an electrode with a high energy density. In addition, by optimizing the size and mixing ratio of the constituent silicon, when used in a lithium-ion battery, volumetric expansion and contraction are mitigated, and cycle characteristics are improved, making it possible to easily obtain a high-performance electricity storage device. .

FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10; Al-Si binary phase diagram. Explanatory drawing which shows an example of a single roll quenching apparatus. Evaluation results of the porous silicon material of Experimental Example 3. Evaluation results of the porous silicon material of Experimental Example 7. Evaluation results of the electrode of Experimental Example 3. Evaluation results of the electrode of Experimental Example 4. Appearance photograph of the electrode which formed the current collector.

(Electrodes for power storage devices)
The electricity storage device electrode of the present disclosure has, as an electrode active material, a porous silicon material that contains at least Si and Al, has voids, and has a plate-like integral shape. Furthermore, the electricity storage device electrode may be an integral body and may not contain a binder. This electrode becomes either a positive electrode or a negative electrode based on the potential of the counter electrode with respect to the potential of the electrode active material. When lithium is used as a carrier, it is preferably a negative electrode. This electrode can be used, for example, in lithium ion secondary batteries, hybrid capacitors, air batteries, and the like. The porous silicon material may include skeleton-like silicon having a three-dimensional network structure with voids, and Al may be present inside or outside the skeleton-like silicon.

The porous silicon material may have a pore diameter distribution range of 1 nm or more and 500 nm or less as determined by a mercury intrusion method. The pore diameter may be 10 nm or more, 50 nm or more, or 100 nm or more. The pore diameter is preferably 250 nm or less, may be 200 nm or less, or may be 150 nm or less. In this porous silicon material, the average pore diameter determined by mercury porosimetry may be in the range of 5 nm or more and 500 nm or less, or may be in the range of 10 nm or more and 250 nm or less.

The porous silicon material may have a porosity ranging from 10% by volume to 50% by volume. The porosity is preferably 40% by volume or less, more preferably 35% by volume or less, and may be 30% by volume or less. The porosity is preferably 15% by volume or more, more preferably 20% by volume or more, and may be 25% by volume or more. This porosity is preferably larger from the viewpoint of volume change of silicon, and preferably smaller from the viewpoint of charge/discharge capacity of the electricity storage device. This porosity is a value measured with a mercury porosimeter.

The porous silicon material may include two types of structures: coarse grains (microscale) containing primary crystal silicon that crystallizes when cooled, and fine grains (nanoscale) containing eutectic silicon with Al. . With this structure, it is speculated that stress due to volume expansion during charging and discharging can be relaxed. Coarse grains can be, for example, granular parts in the range of 1 μm or more and 5 μm or less. Further, the fine particles can be, for example, granular parts in the range of 10 nm or more and 100 nm or less.

The porous silicon material preferably contains 75 at % or more of Si, excluding oxygen and unavoidable impurities, when the total of Si and Al is 100 at %. The Si content is more preferably 80 at % or higher, more preferably 85 at % or higher, and may be 90 at % or higher or 98 at % or higher. The content of Si is preferably higher from the viewpoint of charge/discharge capacity, and may be lower from the viewpoint of relative skeleton reinforcement. The Al content is preferably in the range of 0.5 at % or more and 15 at % or less, preferably 12.5 at % or less, more preferably 10 at % or less, and may be 7.5 at % or less. Also, 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 Al is preferably higher from the viewpoint of reinforcing the skeleton, and is preferably lower from the viewpoint of the charge/discharge capacity of the electricity storage device because it is a component that does not charge and discharge. Furthermore, the porous silicon material may contain one or more of Ti, Sn, Ca, Cu, Mg, Na, Sr and P as the second element in a range of 15% by mass or less. Also, the porous silicon material may contain unavoidable impurities in addition to Si and Al. In addition, it is preferable that the amount of the second element and unavoidable impurities be as small as possible.

In the electric storage device electrode, the thickness of the porous silicon material plate is preferably in the range of 10 μm to 50 μm. A thickness of 10 μm or more can exhibit the capacity of a high-power battery. In addition, when the thickness is 50 μm or less, a sufficient capacity can be obtained. This thickness is more preferably in the range of 20 μm or more and 40 μm or less. The thickness of the porous silicon material may be appropriately selected according to the required battery characteristics.

This electricity storage device electrode may be one in which a current collector is crimped or vapor-deposited on one surface of a porous silicon material plate. The current collector may be formed, for example, on the uneven free surface side of the surface of the porous silicon material. Further, the electricity storage device electrode may be formed by vapor-depositing a current collector component on one surface of a porous silicon material and then press-bonding a current collector foil. Current collectors include, for example, aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymers, conductive glass, and the like. or copper whose surface is treated with carbon, nickel, titanium, silver, or the like can be used. For these, it is also possible to oxidize the surface. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and fiber group formed bodies. The current collector used has a thickness of, for example, 1 to 500 μm.

(storage device)
The power 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. A porous silicon material can be used as a negative electrode active material. This power storage device may be any one of a lithium ion secondary battery, a hybrid capacitor, an air battery, and the like. In the positive electrode, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used as a positive electrode active material. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ and FeS ₂ , and basic compositional formulas of Li _(1-x) MnO ₂ (0<x<1, etc., the same applies hereinafter) and Li _{(1 -x)} Lithium-manganese composite oxides such as Mn ₂ O ₄ , Lithium-cobalt composite oxides such as Li _(1-x) CoO ₂ with a basic composition formula, Li _(1-x) NiO ₂ with a basic composition formula, etc. a lithium-nickel composite oxide having a basic composition formula of Li _(1-x) Ni _a Co _b Mn _c O ₂ (a+b+c=1) or the like, a basic composition formula of LiV ₂ O ₃ Lithium vanadium composite oxides such as V ₂ O 5 , transition metal oxides having a basic composition formula of V 2 O ₅ , and the like can be used. Among these, transition metal composite oxides of lithium such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ and Li _(1-x) Ni _1/3 Co _1/3 Mn _1/3 O ₂ are preferred. The term "basic composition formula" means that other elements such as Al and Mg may be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors, lithium ion capacitors, and the like. Carbonaceous materials include, for example, activated carbons, cokes, vitreous carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m ² /g or more, more preferably 1500 m ² /g or more. When the specific surface area is 1000 m ² /g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m ² /g or less, more preferably 2000 m ² /g or less, for ease of production.

The positive electrode of the electric storage device is formed by mixing the above-described positive electrode active material with a conductive material, a binder, and a solvent as necessary, forming a paste, and applying the positive electrode active material on the current collector. It can be produced by a process of mixing with a material or a binder and crimping it to a current collector. In this electrode, the content of the positive electrode active material is preferably more, preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. One or a mixture of two or more of black, carbon whiskers, needle coke, carbon fibers, 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 is, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Cellulose-based or styrene-butadiene rubber (SBR) aqueous dispersions, which are water-based binders, can also be used. Examples of solvents that can be used include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. . Alternatively, a dispersant, a thickener, or the like may be added to water, and the active material may be slurried with a latex such as SBR. Application methods include, for example, roller coating such as applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any thickness and shape can be obtained using any of these methods. As the current collector and the like used for the positive electrode, those exemplified in the electrode for the electricity storage device described above can be appropriately used.

A non-aqueous electrolytic solution containing a supporting salt, a non-aqueous gel electrolytic solution, or the like can be used as the ion-conducting medium. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, the carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -chain carbonates such as butyl carbonate, di-i-propyl carbonate and t-butyl-i-propyl carbonate, cyclic esters such as γ-butyl lactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; Dioxolanes such as sulfolanes, 1,3-dioxolane, and methyldioxolane are included. Among these, a combination of cyclic carbonates and chain carbonates is preferred. According to this combination, not only is the cycle characteristic, which represents the battery characteristics in repeated charging and discharging, excellent, but also the viscosity of the electrolytic solution, the resulting electric capacity of the battery, the battery output, etc. can be well-balanced. can. _The supporting salt is, for example, _LiPF6 , _LiBF4 , _LiAsF6 , LiCF3SO3, LiN( _{CF3SO2 ) 2} , LiC( _{CF3SO2 ) 3 ,} LiSbF6, _LiSiF6 , _LiAlF4 , _LiSCN , LiClO ₄ , LiCl, LiF, LiBr, LiI, _LiAlCl4 and the like. Among them, from the group consisting of inorganic salts such as _LiPF6 , _LiBF4 , _{LiAsF6 and LiClO4} , and _organic salts such as _LiCF3SO3 , LiN( _{CF3SO2 ) 2} and LiC( _CF3SO2 ) ₃ From the viewpoint of electrical properties, it is preferable to use one selected salt or a combination of two or more selected salts. 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 supporting electrolyte is dissolved in a concentration of 0.1 mol/L or more, a sufficient current density can be obtained, and when it is 5 mol/L or less, the electrolytic solution can be made more stable. In addition, a phosphorus-based or halogen-based flame retardant may be added to the non-aqueous electrolyte.

A solid ion-conducting polymer can also be used as the ion-conducting medium instead of the liquid ion-conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, and vinylidene fluoride and a supporting salt can be used. Furthermore, an ion conductive polymer and a non-aqueous electrolytic solution can be used in combination. 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.

The electricity storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. Porous membranes are mentioned. These may be used alone, or may be used in combination.

The shape of the electricity storage device is not particularly limited, and examples thereof include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular shapes. Also, it may be applied to a large-sized one used for an electric vehicle or the like. FIG. 1 is an explanatory diagram showing an example of the structure of the electricity storage device 10. As shown in FIG. This electricity storage device 10 has a positive electrode 12 , a negative electrode 15 and an ion conducting medium 18 . The positive electrode 12 has a positive electrode active material 13 and a current collector 14 . The negative electrode 15 has a negative electrode active material 16 and a current collector 17 . The negative electrode active material 16 is the above-described porous silicon material 21 which is an integrated plate-like body, and has voids 23 .

(all solid state lithium ion secondary battery)
This electric storage device is preferably an all-solid lithium ion secondary battery. The all-solid-state battery is preferable because it is possible to further suppress changes in performance due to the electrolytic solution and to further improve safety. This all-solid-state lithium-ion secondary battery includes a positive electrode containing a positive electrode active material, a negative electrode that is the above-described electricity storage device electrode, and a solid electrolyte that is interposed between the positive electrode and the negative electrode and conducts lithium ions. It can be a thing. For the positive electrode, any one of the power storage devices described above can be used. Moreover, the electrode for electrical storage devices mentioned above can be used for a negative electrode.

The solid electrolyte may be, for example, a garnet-type oxide containing at least Li, La and Zr. This solid electrolyte may have a basic composition of Li7.0 _+xy (La3 _-x , _Ax )(Zr2 _-y , _Ty ) _O12 . However, A is at least one of Sr and Ca, and T is at least one of Nb and Ta, satisfying 0<x≦1.0 and 0<y<0.75. Alternatively, the solid electrolyte has a basic composition of ( _{Li7-3z+xyMz )} ( _{La3-xAx )} ( _{Zr2-yTy ) O12} or ( _{Li7-3z+xyMz )} (La _3- xA _x )(Y _2-y T _y )O ₁₂ may be a garnet-type oxide. However, in the formula, element M is one or more of Al and Ga, element A is one or more of Ca and Sr, T is one or more of Nb and Ta, and 0 ≤ z ≤ 0.2, 0 ≤ x ≤0.2, 0≤y≤2. In this basic composition formula, it is more preferable to satisfy 0.05≦z≦0.1. In this basic composition formula, it is more preferable to satisfy 0.05≦x≦0.1. Moreover, in this basic composition formula, it is more preferable to satisfy 0.1≦y≦0.8. Within such a range, the ionic conductivity can be made more suitable.

Alternatively, as the solid electrolyte, for example, general Li ₃ N, Li ₁₄ Zn(GeO ₄ ) ₄ called LISICON, sulfide Li _3.25 Ge _0.25 P _0.75 S ₄ , perovskite La _0.5 Li _0.5 TiO ₃ , (La _2/3 Li _3x □ _1/3-2x )TiO ₃ (□: atomic vacancies), garnet-type Li ₇ La ₃ Zr ₂ O ₁₂ , NASICON-type LiTi ₂ (PO ₄ ) ₃ , Li _1.3M0.3Ti1.7 ( _PO3 ) _{4 (} M=Sc, Al) _and the like. In addition, Li ₇ P ₃ S ₁₁ obtained from glass having a composition of 80Li ₂ S·20P ₂ S ₅ (mol %), which is a glass-ceramic, and Li ₁₀ Ge ₂ PS, which is a sulfide-based substance with high electrical conductivity. ₂ are also mentioned. Li ₂ S--SiS ₂ , Li ₂ S--SiS ₂ --LiI, Li ₂ S--SiS ₂ --Li ₃ PO ₄ , Li ₂ S--SiS ₂ --Li ₄ SiO ₄ , and Li ₂ S-- as glass-based inorganic solid electrolytes. P ₂ S ₅ , Li ₃ PO ₄ —Li ₄ SiO ₄ , Li ₃ BO ₄ —Li ₄ SiO ₄ , and SiO ₂ , GeO ₂ , B ₂ O ₃ , and P ₂ O ₅ as glass-based materials, and Li ₂ O Examples include those used as network modifiers. Li ₂ S--GeS ₂ system, Li ₂ S--GeS ₂ --ZnS system, Li ₂ S--Ga ₂ S ₂ system, Li ₂ S--GeS ₂ --Ga ₂ S ₃ system, and Li ₂ S as thiolysicone solid electrolytes. _{-GeS2- P2S5 system ,} Li2S _{- GeS2} - _SbS5 system _{, Li2S - GeS2} - _Al2S3 system, _Li2S - _SiS2 system, _Li2SP2S5 system, Li ₂ S--Al ₂ S ₃ system, LiS--SiS ₂ --Al ₂ S ₃ system, and Li ₂ S--SiS ₂ --P ₂ S ₅ system. These solid electrolytes may be formed in a plate shape and arranged between the positive electrode and the negative electrode.

Further, the all-solid-state lithium ion secondary battery may be provided with a restraining member that restrains the laminate in which the positive electrode, the solid electrolyte, and the negative electrode are laminated in the stacking direction. This restraining member includes, for example, a pair of plate-like portions that sandwich the laminate from both ends in the stacking direction of the laminate, a rod-like portion that connects the pair of plate-like portions, and a screw structure that is connected to the rod-like portions. An adjusting portion for adjusting the interval between the pair of plate-like portions may also be provided.

(Manufacturing method of electrode for power storage device)
A method for manufacturing an electricity storage device electrode according to the present disclosure may be used to manufacture the electricity storage device electrode described above. Here, the physical properties of the porous silicon material are the same as those of the electricity storage device electrode described above, and detailed description thereof will be omitted. The manufacturing method of the porous silicon material of the present disclosure includes a precursor step and an electrode forming step. In the precursor step, a raw material containing at least Si and Al, containing Al in a range of more than 20 at% and less than 70 at% when the total of Si and Al is 100 at%, and the balance being Si, is melted and rapidly cooled. A process for solidifying to obtain a plate-shaped silicon alloy precursor is performed. Further, in the electrode formation step, processing is performed to obtain a porous silicon material having a plate-like shape as a part of the electricity storage device electrode by removing the Al component contained in the silicon alloy. First, the raw material composition will be described.

FIG. 2 is a phase diagram of the Al—Si binary system. In general, in the equilibrium phase diagram of the eutectic Si—Al alloy, when a melt with a eutectic composition in which the liquidus line becomes the minimum is solidified, the Si phase and the Al phase are separated into fibrous (lamellar) states. A eutectic structure is formed. 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 K/s or higher. If it is possible to selectively remove only the Al element from this eutectic structure, it is possible to obtain a material composed of the Si element characterized by the eutectic structure. In the case of Si-Al alloy, 13Si-87Al is near the eutectic composition. In this case, it is difficult to obtain a self-supporting film by dissolving Al, but when an alloy is produced around 50Si-50Al, eutectic Sj and primary crystal Si that does not alloy with Al grow. This primary crystal Si becomes a skeleton that keeps the electrode in a sheet form. During the electrochemical reaction, the pores formed by the eutectic Si act to relax the expansion and contraction of the electrode.

(Precursor step)
In the precursor step, it is preferable to use a raw material containing more than 20 at % and less than 70 at % of Al, with the balance being Si, when the total of Si and Al is 100 at %. The raw material may contain unavoidable impurities. The unavoidable impurities are components that inevitably remain during the purification of either Si or Al, and examples thereof include Fe, C, Cu, Ni, and P. The amount of unavoidable impurities is preferably as small as possible. For example, when the total of Si, Ti, and Al is 100 at %, the content is preferably 5 at % or less, more preferably 2 at % or less. The compounding ratio of Al is preferably 60 at % or less, more preferably 50 at % or less, and may be 40 at % or less. Moreover, the compounding ratio of Al is preferably 25 at % or more, more preferably 30 at % or more, and may be 40 at % or more. A silicon alloy containing Al in such a range is preferable because the porosity can be further increased and pores with a more suitable shape and size can be obtained. When the Al content is high, a single phase of Al precipitates largely when the alloy is rapidly cooled after being melted to form an alloy, so that many voids can be formed. This cooling rate is preferably more rapid, and may be, for example, in the range of 10 ² ° C./s or more and 10 ⁸ ° C./s or less from the molten state.

In this step, when the raw material is melted, high-frequency melting in an inert gas atmosphere such as Ar is preferable, but any melting method may be used. In the precursor step, the molten silicon alloy may be cooled by a roll quenching method to obtain a plate-like (including thin film-like) precursor. Since the precursor obtained by the roll quenching method has a fine alloy structure, porous silicon having fine pores can be obtained after the elution treatment.

In this precursor step, a single roll quenching apparatus shown in FIG. 3 may be used. In this step, a plate-like precursor may be obtained by injecting a molten liquid obtained by melting the raw material onto a cooling roll in a decompressed chamber containing an oxygen removing material and cooling the same. When the liquid is quenched with a cooling roll, the parameters that affect the cooling rate include the temperature and thermal conductivity of the melt, the hole diameter d at the tip of the nozzle, the gap size g between the roll and the nozzle, the roll rotation speed r, and the like. . The hole diameter d of the tip of the nozzle can be, for example, in the range of 0.1 mm or more and 5 mm or less, and may be in the range of 0.2 mm or more and 2 mm or less. The gap size g can range from 0.1 mm to 5 mm, and may range from 0.2 mm to 2 mm. The roll rotation speed r can be in the range of 1000 rpm or more and 5000 rpm or less, and may be in the range of 2000 rpm or more and 4000 rpm or less. The chamber is preferably evacuated to 10 ^-3 Pa or less. Also, it is preferable to introduce an inert gas into the chamber. The inert gas is preferably Ar, for example. The internal pressure may be, for example, 30 cmHg or higher, or 40 cmHg or higher. Moreover, the internal pressure is preferably 100 cmHg or less. It is possible to obtain a rapidly solidified precursor by heating the raw material until it melts, injecting Ar gas, and spraying the melt from the tip of the nozzle toward the cooling roll.

In the precursor step, a plate-shaped precursor having a thickness in the range of 10 μm to 50 μm may be obtained. This thickness may be 20 μm or more and 40 μm or less. The thickness of the precursor may be appropriately selected according to the performance required for the electricity storage device. In the precursor step, a plate-like precursor containing no binder may be obtained.

In this precursor step, in addition to Al and Si, a raw material containing a second element containing one or more of Ti, Sn, Ca, Cu, Mg, Na, Sr and P may be used. Among these, one or more of Ca, Na and Sr is preferable as the second element. The content of the second element is preferably less than the content of Al and Ti, for example, relative to the entire silicon alloy. A range of 10% by mass or less is preferred, and a range of 5% by mass or less is more preferred.

(Electrode formation process)
In the electrode forming step, a process for removing substances other than Si from the plate-shaped precursor prepared above is performed. Substances other than Si include, for example, Al and compounds thereof. In this step, it is preferable to selectively remove the Al component, that is, the Al phase and its compounds with acid or alkali. The acid or alkali to be used is preferably one that dissolves elements and/or compounds other than silicon in the silicon alloy and does not dissolve silicon, and examples thereof 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 can remove Al and its compounds, and can be, for example, in the range of 1 mol/L or more and 5 mol/L or less. This removal treatment may be performed by heating at, for example, 30.degree. C. to 60.degree. The porous silicon material obtained is then washed and dried.

In the electrode forming step, substances other than Si may be removed in a range of 85% by mass or more and 100% by mass or less. For example, Al and other oxygen may remain, but when used as an electrode active material, the smaller the amount, the better, from the viewpoint of the charge/discharge capacity. In addition, it is preferable that a component such as Al is contained in a predetermined amount or more from the viewpoint of reinforcing the silicon skeleton and improving durability. In this electrode forming step, it is preferable to obtain a porous silicon material containing Al in the range of 0.5 at % or more and 15 at % or less when the total of Si and Al is 100 at %. In this porous silicon material, Al may be contained in an amount of 1.0 at % or more, or may be contained in an amount of 2.0 at % or more. Also, Al may be contained in a range of 10 atomic % or less, or 7.5 atomic % or less.

In the electrode forming step, a porous silicon material having a porosity in the range of 10% by volume or more and 50% by volume or less may be obtained. This porosity is a value measured with a mercury porosimeter. This porosity is, for example, preferably 15% by volume or more, and may be 20% by volume or more. Moreover, the porosity is preferably 50% by volume or less, and may be 40% by volume or less or 30% by volume or less. If the porosity is larger, it is more likely to respond to volume changes when carrier ions are occluded.

In the electrode formation step, a current collector may be crimped or vapor-deposited onto at least one surface of the plate-like porous silicon material. In this treatment, the components of the current collector may be vapor-deposited on one surface of the plate-like porous silicon material, and then the current collector may be crimped. Any of the above-described current collectors may be appropriately used as the current collector. The thickness of the current collector may be, for example, in the range of 1 μm or more and 50 μm or less.

As detailed above, the present disclosure can further increase the energy density in materials containing Si. The reason why such an effect is obtained is presumed as follows. For example, the manufacturing method of the present disclosure can selectively dissolve aluminum from silicon and aluminum alloys to produce porous silicon with small pore sizes and high porosity. Moreover, since this porous silicon material is a self-supporting film, it can be used as an electrode as it is without adding a conductive material or a binder. Therefore, it becomes possible to provide an electrode with a high energy density. In addition, by optimizing the size and mixing ratio of the constituent silicon, when used in a lithium-ion battery, volumetric expansion and contraction are mitigated, and cycle characteristics are improved, making it possible to easily obtain a high-performance electricity storage device. .

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

Hereinafter, specific examples of fabricating the porous silicon and the electricity storage device of the present disclosure will be described as experimental examples. Experimental Examples 2-5 and 8-10 are examples of the present disclosure, and Experimental Examples 1 and 6-7 are comparative examples.

(Production of ribbon-shaped Al-Si ingot)
First, an Al--Si ingot was produced. Table 1 summarizes the composition of the ingots. Raw materials Al (Kojundo Chemical, 99.5% granular) and Si (Kojundo Chemical, 99.999% granular) were weighed to obtain the desired composition (Table 1) (Experimental Examples 1 to 7). A weighed raw material was set in a copper hearth in a chamber of an arc melting furnace (Nisshin Giken NEV-ACD1). Furthermore, a Ti lump was also set as an oxygen getter. After the inside of the chamber was evacuated to 10 ⁻³ Pa or less, Ar was introduced to 40 cmHg, and after melting the Ti ingot by arc discharge to remove residual oxygen, the raw material was melted to obtain an Al—Si alloy ingot. . At this time, in order to improve the uniformity of the sample, the process of turning over the ingot and remelting was repeated three times or more.

Next, the synthesized ingot was subjected to a rapid solidification treatment using a single-roll liquid rapid cooling apparatus shown in FIG. When quenching a liquid with a single roll, the temperature and thermal conductivity of the melt, the hole diameter d at the tip of the nozzle, the gap size g between the roll and the nozzle, and the roll rotation speed r can be considered as parameters that affect the cooling rate. . The temperature and thermal conductivity of the melt are parameters dependent on the sample, but the rest can be adjusted by setting the instrument. This time, (1) the hole diameter d of the nozzle tip was adjusted to 0.5 mm, (2) the gap size g was 0.5 mm, and (3) the roll rotation speed was adjusted to 3000 rpm. The produced ingot was placed in a quartz nozzle and connected to a high-frequency melting device. After the inside of the chamber was evacuated to 10 ⁻³ Pa or less, Ar gas was introduced to 50 cmHg. Gradually increase the current value, heat the sample while checking the temperature with a radiation thermometer until the sample melts, spray Ar gas and spray the melt from the tip of the nozzle toward the copper roll, and quickly solidify the ribbon sample. Obtained.

The bulk cycle characteristics of silicon cannot be said to be good due to the expansion of the electrode when Experimental Example 7 is used as similar data. On the other hand, it is known that if a thin film (for example, 5 μm or less) is formed by a sputtering method or the like, cycle characteristics are relatively improved. However, it is unsuitable for practical use due to its small capacitance per unit area and low conductivity (cannot be added with a conductive material). In other words, the high output type requires high conductivity instead of a thin film. General electrodes are required to have cycle characteristics even if the conductivity is low. At present, the thickness of graphite negative electrodes in high-power batteries is about 50 μm. For silicon with ten times the capacity, the thickness would be 5 μm. When a silicon electrode of this thickness is deposited by sputtering, it has low conductivity and cannot be used as a substitute for graphite. On the other hand, the porous silicon electrode of the present disclosure can be graphitized by impregnating higher alcohol (such as furfuryl alcohol) into the gaps and thermally decomposing and polymerizing it. As a result, an electrode with high conductivity can be obtained. A thickness of 40 μm is sufficient for the thickness of the porous silicon electrode, considering the capacity ratio between the positive electrode and the negative electrode and the handling during the electrode manufacturing process. Furthermore, considering cycle characteristics, the film thickness is more preferably 30 μm or less. In this example, all the electrodes were produced with a film thickness of 30 μm.

(Electrochemical evaluation)
1 mg of a sample was weighed and placed on a copper current collector foil to obtain a working electrode. 1M-LiPF ₆ dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) = 3/7 (volume ratio) was used as the electrolyte, and lithium metal was applied to the counter electrode via a polyethylene microporous membrane separator. Using. Using the obtained test cell, charging and discharging were performed at a current density of 1/20C, a lower limit potential of 50 mV, and an upper limit potential of 2V.

(Electrode observation)
After inserting 3000 mAh/g of Li, the battery was disassembled in an argon atmosphere glove box, and the silicon negative electrode was washed with dimethyl carbonate (DMC) three times. The sample was prepared by drying it under reduced pressure at room temperature. Using a holder not exposed to air, this was introduced into an SEM to observe the cross section of the electrode. In Experimental Example 1, the battery resistance was large, and SEM observation was performed after Li was inserted up to 2800 mAh/g.

(Preparation of current collector integrated electrode)
A current collector-integrated electrode was prepared by forming a copper foil as a current collector on one surface of the porous silicon ribbon after the acid treatment of Experimental Examples 2-4. A copper film having a thickness of 20 to 100 nm was formed by sputtering on the free surface of the Al—Si alloy ribbon cooled by a single roll (the surface with large irregularities, the lower portion of the initial electrode in FIG. 6 and the upper portion of the initial electrode in FIG. 7). , and this surface was uniaxially pressurized and crimped to a copper foil as a current collector. In this state, the electrode was immersed in a 0.5N-HCl aqueous solution for 5 hours to prepare an electrode in which the porous silicon electrode and the copper current collector were integrated (Experimental Examples 8 to 10).

(Results and discussion)
4 is the evaluation result of the porous silicon material of Experimental Example 3, FIG. 4A is the XRD chart of the raw material alloy, FIG. 4B is the XRD chart of the porous silicon ribbon after acid treatment, and FIG. 4C is the raw material. 4D is an SEM photograph of the porous silicon ribbon after acid treatment, and FIG. 4E is a SEM photograph of the surface of the porous silicon ribbon after acid treatment. FIG. 5 shows evaluation results of the porous silicon material of Experimental Example 7, and FIGS. 5A to 5E are the same as FIGS. 4A to 4E. FIG. 6 shows evaluation results of the electrode of Experimental Example 3. FIG. 6A is a charge/discharge curve, FIG. 6B is an initial cross-sectional SEM image, and FIG. 6C is a cross-sectional SEM image when 3000 mAh/g of Li is inserted. 7 shows the evaluation results of the electrode of Experimental Example 4. FIG. 7A is a charge/discharge curve, FIG. 7B is an initial cross-sectional SEM image, and FIG. 7C is a cross-sectional SEM image when 3000 mAh/g of Li is inserted. 8A and 8B are photographs of the external appearance of an electrode on which a current collector is formed. FIG. 8A is for Experimental Example 9, and FIG. Table 1 summarizes the charged composition, weighed value, alloy shape after single roll cooling, crystal phase and crystallinity after acid treatment, shape after acid treatment, and the like of each sample. In addition, Table 1 shows the initial Li insertion capacity A (mAh / g), the initial Li removal capacity B (mAh / g), the initial charging efficiency (B / A × 100; %), 3000 mAh / g as the charge / discharge evaluation results. The electrode expansion rate (volume %) at the time of Li insertion is summarized. Table 2 shows the charging composition of the porous silicon materials of Experimental Examples 8 to 10, the initial Li insertion capacity A (mAh / g), the initial Li desorption capacity B (mAh / g), the initial charging efficiency (B / A × 100 ;%) were summarized.

(Consideration on shape)
As shown in Table 1, Experimental Examples 1 to 5 with the charged compositions of Al70Si30, Al60Si40, Al50Si50, Al40Si60 and Al30Si70 had long ribbon-like or ribbon-like shapes after cooling and acid treatment. Also, in Experimental Examples 1 to 5, highly crystalline silicon was detected, indicating high crystallinity. Furthermore, in Experimental Examples 1 to 5, when observing the cross section, there are two coarse grains (microscale) containing primary crystal silicon that crystallizes during cooling and fine grains (nanoscale) containing eutectic silicon with Al. Different types of tissue were observed. On the other hand, in Experimental Examples 6 and 7, in which the Al content was low, the particles were scaly or powdery, had poor shape retention, and had low crystallinity. Further, in Experimental Examples 6 and 7, when the cross section was observed, fine grains were not observed, and a shape in which coarse grains (microscale) were connected was recognized.

(Experimental example 3: charged element ratio Al50Si50)
Experimental Example 3 was examined more specifically regarding the shape and the like. The Al—Si alloy after single roll cooling had a mixed phase of Al and Si, and after acid treatment, had a Si single phase. The lattice constant was 5.428 Å, equal to the literature value (5.43 Å). Also, the sample after the acid treatment changed from a ribbon shape to a strip shape. It is speculated that the reason for this is that since the silicon content in the alloy is small, the connection of the silicon skeleton is weak, making it difficult to maintain the ribbon shape after aluminum removal. Two types of structures were observed in the electrode after acid treatment. The coarse grains (microscale) were primary crystal silicon crystallized during cooling, and the fine grains (nanoscale) were eutectic silicon with aluminum.

(Experimental example 7: charge element ratio Al10Si90)
The Al—Si alloy after single-roll cooling becomes a mixed phase of Al and Si, and becomes a single Si phase after acid treatment, but it was assumed that the crystallinity was low because the XRD peak intensity was low. The lattice constant of Si is 5.421 Å, which is smaller than the literature value (5.43 Å), and it was presumed that holes were formed by substitution of aluminum at silicon sites. Moreover, it was inferred that the crystallinity was low for the same reason. After single-roll cooling, it became flake-like rather than ribbon-like. It was speculated that the reason for this is that silicon is an element with low rolling properties. The electrode after the acid treatment was found to have a shape in which coarse particles (microscale) were connected, unlike Experimental Examples 1 to 5 and the like.

(Discussion on electrochemical properties)
As shown in Table 1, in Experimental Examples 2 to 7, the initial Li insertion capacity was 3000 mAh/g or more, the initial Li desorption capacity was 2500 mAh/g or more, and the initial charge/discharge efficiency was 70% or more. Both were expensive. On the other hand, in Experimental Example 1, the Li insertion limit was 2800 mAh/g, and the insertion capacity, desorption capacity, and initial charge/discharge efficiency were all relatively low. In Experimental Example 1, it was observed that this was caused by the high amount of Al and the high resistance. Further, the expansion rate of the electrode was good in Experimental Examples 1 to 5, with the expansion only up to 20% by volume. On the other hand, in Experimental Examples 6 and 7, the volume expansion was 50% by volume or more, and the volume change was large. Experimental Examples 1 to 5 had a structure in which microscale particles and nanoscale particles were mixed, and this structure was presumed to suppress the volume change during Li absorption/desorption. On the other hand, in Experimental Examples 6 and 7, different from Experimental Examples 1 to 5, nanoscale particles derived from primary crystal silicon were not mixed, so it was presumed that the volume expansion was large.

(Study on current collector integrated electrode)
As shown in Table 2, it was found that bonding a copper current collector to one surface of the porous silicon ribbon did not adversely affect the electrochemical properties. In addition, as shown in FIG. 8, in Experimental Examples 2 to 4, it was found that a flexible electrode body could be produced without requiring a binder. Therefore, it was found that by using this porous silicon ribbon as it is as an electrode active material, it is possible to provide an electrode with a high energy density that does not contain additives such as binders.

It goes without saying that the present disclosure is not limited to the experimental examples described above, and can be implemented in various modes as long as they fall within the technical scope of the present disclosure.

INDUSTRIAL APPLICABILITY The present disclosure can be used in the technical field of secondary batteries.

10 electricity storage device, 12 positive electrode, 13 positive electrode active material, 14 current collector, 15 negative electrode, 16 negative electrode active material, 17 current collector, 18 ion conducting medium, 21 porous silicon material, 23 voids.

Claims (13)

An electric storage device electrode comprising, as an electrode active material, a porous silicon material containing at least Si and Al, having voids, and having a plate-like integral shape. 2. The electricity storage device electrode according to claim 1, wherein said porous silicon material contains Si in a range of 98 at % or more when the total of Si and Al is 100 at %. 3. The electricity storage device electrode according to claim 1, wherein the porosity is in the range of 10% by volume or more and 50% by volume or less. The electricity storage device electrode according to any one of claims 1 to 3, which does not contain a binder. The electricity storage device electrode according to any one of claims 1 to 4, wherein a current collector is crimped or vapor-deposited on one surface of the porous silicon material plate. The electricity storage device electrode according to any one of claims 1 to 5, wherein said porous silicon material plate-like body has a thickness ranging from 10 µm to 50 µm. a positive electrode comprising a positive electrode active material;
a negative electrode that is an electrode for an electricity storage device according to any one of claims 1 to 6;
an ion conducting medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
A storage device with A raw material containing at least Si and Al and containing Al in a range of more than 20 at% and less than 70 at% when the total of Si and Al is 100 at%, and the balance being Si is melted and rapidly solidified into a plate-like shape. a precursor step of obtaining a silicon alloy precursor;
an electrode forming step of removing the Al component contained in the silicon alloy to obtain a porous silicon material having a plate-like shape as a part of an electrode for an electricity storage device;
A method for manufacturing an electrode for an electricity storage device, comprising: 9. The electricity storage device according to claim 8, wherein in the precursor step, the melt obtained by melting the raw material is injected onto a cooling roll in a decompressed chamber containing an oxygen removing material and cooled to obtain the plate-like precursor. method of manufacturing an electrode for 10. The method for producing an electric storage device electrode according to claim 8, wherein in said precursor step, a plate-shaped precursor having a thickness in the range of 10 [mu]m to 50 [mu]m is obtained. 11. The method for producing an electricity storage device electrode according to claim 8, wherein in said precursor step, said plate-like precursor containing no binder is obtained. 12. The method for producing a porous silicon material according to claim 8, wherein said electrode forming step selectively removes an Al component with an acid or an alkali. 13. The method for producing an electric storage device electrode according to any one of claims 8 to 12, wherein in the electrode forming step, a current collector is crimped or vapor-deposited onto at least one surface of the plate-shaped porous silicon material. .

Images