JP6958823B2 - Electrodes and magnesium secondary batteries with an alloy layer of magnesium and bismuth - Google Patents

Description

The present invention relates to an electrode provided with an alloy layer of magnesium and bismuth, and a magnesium secondary battery provided with the electrode.

In recent years, lithium ion secondary batteries have been put into practical use as secondary batteries and are used in various applications such as electronic devices. However, it is difficult for lithium-ion secondary batteries to cope with future in-vehicle applications and large-scale applications, and other secondary batteries are being developed. Therefore, a magnesium secondary battery having a characteristic that surpasses that of a lithium ion secondary battery in terms of electric capacity per volume has been actively developed. Magnesium not only has about twice the capacitance per volume of lithium, but also has a melting point of 650 ° C, which is higher than that of lithium at 186 ° C. It has been pointed out that lithium-ion secondary batteries are heated and ignited due to a short circuit inside the battery, and one of the causes is the low melting point of lithium. In this respect, magnesium has a higher melting point than lithium, so it is highly safe. Magnesium is more abundant on the earth than lithium, which is a rare metal, and is also abundant in resources. However, the conventional magnesium negative electrode has a problem that since an oxide film which is an insulating layer is formed on the surface thereof, it is difficult for electricity to flow and the overvoltage becomes large, so that the original battery capacity characteristics cannot be exhibited.

Therefore, some proposals have been made to solve the above problem. For example, as a method of suppressing the formation of an oxide film by improving the electrolyte, a method of removing the oxide film on the surface of the magnesium negative electrode by using EtMgBr / THF as an electrolytic solution has been proposed (Non-Patent Document 1). However, this method has a problem that it cannot be charged and discharged for a long period of time because it is not resistant to the potential on the oxidation side and the electrolytic solution is decomposed. Further, the positive electrode materials that can be used are limited, and only low-potential batteries having a voltage of about 1 V have been reported. As a method for improving the negative electrode, a method of using an intermetallic compound of magnesium for the negative electrode has been proposed, and it has been proposed to use an intermetallic compound prepared with a molar ratio of magnesium to bismuth of 3: 2 (Patented). Document 1, Non-Patent Document 2). However, in the case of this method, the magnesium-bismuth intermetallic compound has a magnesium content of about 16% by mass, so that the amount of magnesium that can be used is small. Moreover, since the intermetallic compound is brittle, there is a problem in durability when it is used as it is as an electrode. When a binder is used, it is necessary to add a conductive auxiliary agent in order to increase the durability, and when the weight of the binder and the conductive auxiliary agent is taken into consideration, the magnesium content is further reduced to about 13%. Therefore, there is a problem that the contribution of magnesium to the electrochemical energy per battery is small.

Japanese Patent Application Laid-Open No. 2014-512637

D.Aurbach, Z.Lu, A.Schechter, Y.Gofer, H.Gizbar, R.Turgeman, Y.Cohen, M.Moshkovich & E.Levi “Prototype systems for rechargeable magnesium batteries”, Nature, 407, 724-727 ( 2000). Timothy S. Arthur, Nikhilendra Singh, Masaki Matsui, “Electrodeposited Bi, Sb and Bi1-xSbxalloys as anodes for Mg-ion batteries”, Electrochemistry Communications, 16,103-106 (2012).

The subject of the present invention is to solve the above-mentioned problems, an electrode capable of producing a high-potential and high-capacity magnesium secondary battery in which the formation of an oxide film is suppressed, and a high-potential and high-capacity magnesium in which the formation of an oxide film is suppressed. The purpose is to provide secondary batteries.

While investigating a method for suppressing the formation of an oxide film on an electrode in a magnesium secondary battery, the present inventors focused on the magnesium electrode itself from among the elements constituting the magnesium secondary battery and started development. .. When magnesium was alloyed with bismuth at an arbitrary ratio and its electrochemical characteristics were evaluated as the negative electrode of a magnesium secondary battery, _{unlike the conventional intermetallic compound (Mg 3} Bi ₂ ), an alloy with a smaller amount of bismuth was obtained. It was found that the formation of an oxide film is suppressed and that it can be used as an electrode for a magnesium secondary battery with high potential and high capacity. Further, the magnesium-bismuth alloy electrodes reported so far are brittle intermetallic compounds (Mg ₃ Bi ₂ ) and have poor electrode molding processing characteristics, and conductive carbon and a binder (binder) can be used to make the electrodes. ) Etc., and the content of magnesium, which has a high capacity, is significantly reduced in order to function as an electrode (Mg content of 13% by mass or less). The present inventors have found that the composition in the alloy in high state of Mg-Bi solid solution or Mg-Mg ₃ Bi ₂ eutectic found that an excellent electrode material molding characteristics.

That is, the present invention is specified by the following matters.
(1) An electrode including an alloy layer of magnesium (Mg) and bismuth (Bi), wherein the alloy layer is a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; x contains 0.001 to 0.0112) or a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; where x is 0.001 to 0.0112) and magnesium (Mg). ) And bismuth (Bi) containing an intermetallic compound (Mg ₃ Bi ₂ ).
(2) The electrode according to (1) above, wherein the content of bismuth in the alloy of magnesium (Mg) and bismuth (Bi) is 1 to 58.9% by mass with respect to the entire alloy. ..
(3) A magnesium secondary battery comprising the electrode according to (1) or (2) above as a negative electrode, and further including an electrolyte layer and a positive electrode.

The electrode of the present invention can increase the magnesium content while suppressing the formation of an oxide film. In addition, it can be processed into a form that can be used as an electrode without the need for a binder or a conductive auxiliary agent. Since the magnesium secondary battery of the present invention can suppress the formation of an oxide film, it is possible to suppress an overvoltage and obtain a secondary battery having a high potential and a high capacity.

It is a figure which shows the cyclic voltammetry of the magnesium-bismuth alloy which has the bismuth content of 2 mass% in Example 1. FIG. It is a figure which shows the cyclic voltammetry of the magnesium-bismuth alloy which has the bismuth content of 30 mass% in Example 1. FIG. It is a figure which shows the cyclic voltammetry of the magnesium-bismuth alloy which has the bismuth content of 50% by mass in Example 1. FIG. It is a figure which shows the cyclic voltammetry of the magnesium which has the bismuth content of 0 mass% in the comparative example 1. FIG. It is a figure which shows the structure of the magnesium secondary battery produced in Example 2. It is a figure which shows the result of charge / discharge measurement of the magnesium secondary battery produced in Example 2. It is a figure which shows the result of charge / discharge measurement of the magnesium secondary battery produced in the comparative example 2.

The electrode of the present invention is an electrode provided with an alloy layer of magnesium and bismuth, wherein the alloy contains a solid solution of magnesium and bismuth, or a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth. And. Further, in the present invention, the inclusion of the solid solution of magnesium and bismuth and the intermetallic compound of magnesium and bismuth also includes the case where the solid solution of magnesium and bismuth and the intermetallic compound are present in the alloy in a symcyclic structure. From the state diagram of the binary alloy of magnesium (Mg) and bismuth (Bi), the Bi content in the alloy is up to 8.87 mass% (1.12 at%), which is the solid solution limit, as metal Mg and Mg-Bi. _{A solid solution is formed, and an Mg-Bi solid solution and an Mg-Mg 3} Bi ₂ eutectic are formed from 8.87% by mass (1.12 at%) to 58.9% by mass (14.3 at%), which is the eutectic point. Then, from 58.9 mass% (14.3 at%) to 82.2 mass% (35 at%), Mg-Mg ₃ Bi ₂ eutectic and Mg ₃ Bi ₂ intermetallic compound are formed, and 82.2 mass%. _{It is known that an Mg 3} Bi ₂ metal-to-metal compound forms a Bi-Mg solid solution and a metal Bi at (35 at%) or more, and the structure morphology differs depending on the production conditions. The magnesium-bismuth alloy in the present invention (hereinafter, also referred to as “magnesium alloy in the present invention”) includes a solid solution of magnesium and bismuth, or a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth. That is, the composition of the magnesium alloy in the present invention includes a form in which the metal Mg and the Mg-Bi solid solution coexist, a form in which the Mg-Bi solid solution and the Mg-Mg ₃ Bi ₂ eutectic coexist, and an Mg-Mg ₃ Bi ₂ eutectic. Includes a form in which and Mg ₃ Bi _{2 intermetallic compound coexist.} Regarding the bismuth content of the magnesium alloy in the present invention, since the battery reaction involves dissolution-precipitation of the metal Mg and Mg-Bi solid solution, the solid solution is not formed in the alloy from the viewpoint of increasing the electric capacity per volume. It is preferably 58.9% by mass or less, more preferably 50% by mass or less, based on the entire magnesium alloy. The content of bismuth in the magnesium alloy in the present invention is preferably 1 to 50% by mass with respect to the entire magnesium alloy from the viewpoint of processability of the magnesium alloy. The bismuth content of the magnesium alloy in the present invention is preferably 1 to 58.9% by mass, preferably 1 to 50% by mass, based on the entire magnesium alloy, from the viewpoint of the effect of suppressing the formation of an oxide film. More preferably, it is more preferably 30 to 50% by mass. In the solid solution of magnesium and bismuth (Mg _1-x Bi _x ) in the present invention, x is preferably in the range of 0.001 or more, which is the lower limit that can substantially exist as a solid solution, and 0.0112 or less, which is the solid solution limit. The alloy layer in the electrode of the present invention contains a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; where x is 0.001 to 0.0112), or with magnesium (Mg). Contains a solid solution with bismuth (Bi) (Mg _1-x Bi _x ; where x is 0.001 to 0.0112) and an intermetallic compound of magnesium (Mg) and bismuth (Bi) (Mg ₃ Bi ₂ ). ..

Since the magnesium alloy in the present invention has sufficient strength and can be easily processed into a plate-like or columnar shape, the processed magnesium alloy itself can be used as an electrode or attached to a current collector as an electrode. Can be used. Therefore, unlike the case of using a granular electrode material, the electrode can be manufactured without the need for a binder or a conductive auxiliary agent, so that the electrode can be manufactured without reducing the magnesium content in the electrode. Further, the electrode may be produced by processing a magnesium alloy into particles having a scale shape, a flat shape, a spindle shape, a spherical shape, or the like, mixing the magnesium alloy with a binder, and fixing the electrode on the current collector. The "magnesium alloy layer" in the present invention means a layer containing a magnesium alloy in the present invention, and the magnesium alloy in the present invention is processed into a plate shape, a columnar shape, or the like, and the magnesium alloy itself forms a magnesium alloy layer. This includes the case where the magnesium alloy particles are fixed with a binder or the like to form a magnesium alloy layer. The "electrode having a magnesium alloy layer" in the present invention means that the electrode may have the magnesium alloy layer, and when the electrode is composed of only the magnesium alloy layer, the magnesium alloy layer and the current collector or the like may be used. This includes the case where the electrode is configured together with other components.

The battery of the present invention is characterized by including the electrode of the present invention as a negative electrode, and further including an electrolyte layer and a positive electrode. As the positive electrode, the electrolyte, and optionally other components in the battery of the present invention, those used in conventional magnesium secondary batteries can be used. For example, as the positive electrode, a positive electrode active material may be fixed on the current collector by a binder, and a conductive auxiliary agent may be contained if necessary. As the positive electrode active material, for example, sulfur or sulfur _compounds, V 2 _{O 5,} sulfur-doped _V 2 _{O 5} or the like _V 2 _{O 5} system, MnO _{2 system,} MnO 3, MgMnO oxides such as MnO ₃ system, such as ₃ may be mentioned a system of positive, sulfide-based positive electrode of _Mo 6 _{S 8} type and the _{like, Mg 1.03 Mn 0.97 SiO 4,} MgCoSiO 4, MgFeSiO 4, MgMnSiO 4, Mo 9 Se 11, FePO 4 and .. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, and resin materials such as styrene-butadiene rubber and carboxymethyl cellulose. Examples of the conductive auxiliary agent include graphite such as atypical carbon, natural graphite and artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black, and carbon nanotubes. Carbonaceous substances and the like can be mentioned.

Examples of the electrolyte include magnesium halide such as magnesium perchlorate (Mg (ClO ₄ ) ₂ ), Grinard reagent (RMgX (R is an organic group, X is a halogen)), magnesium bromide (MgBr ₂ ), and the like. , Magnesium Nitrate (Mg (NO ₃ ) ₂ ), Magnesium Bistrifluoromethanesulfonimide (Mg (TFSI) ₂ ), Mg (SO ₂ CF ₃ ) ₂ , Magnesium Booxide (Mg (BF ₄ ) ₂ ), Trifluoromethyl Examples thereof include magnesium sulfonate (Mg (CF ₃ SO ₃ ) ₂ ) and magnesium hexafluorophosphate (Mg (PF ₆ ) _2). As the electrolyte solvent, a known non-aqueous electrolyte solvent can be used, and for example, acetonitrile (AN), dietkietan, diethylene glycol dimethyl ether (diglime), triethylene glycol dimethyl ether (triglime), tetraglyme dimethyl ether (tetraglime), tetrahydrofuran ( THF), propylene carbonate (PC), ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyllactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1 , 3-Dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and the like.

[Example 1]
(Pretreatment of electrodes)
The magnesium-bismuth alloy (Mg-Bi alloy) in the present invention having a bismuth (Bi) content of 2% by mass, 30% by mass, and 50% by mass was cut into a cylinder having a diameter of 7 mm and a thickness of 2 mm, and each of them was cut out. Both sides of the metal piece were polished in the order of emery paper # 220, 600, 2000 to obtain a test piece.

(Cyclic voltammetry)
A tripolar beaker cell with each test piece as a test electrode, a reference electrode as an Ag wire (manufactured by Nikola Co., Ltd., purity 99.99%), and a counter electrode as a magnesium ribbon (manufactured by Nikola Co., Ltd., purity 99.99%). Cyclic voltammetry was performed using this. As the electrolytic solution, 0.5 M magnesium bistrifluoromethanesulfonimide [Mg (TFSA) ₂ ] / triglime was used. The scanning range was −3.5 to 1.0 V, the scanning speed was unified to 10 mV / s, and 5 cycles were performed. The results of the Mg-Bi alloy in which the bismuth contents are 2% by mass, 30% by mass, and 50% by mass, respectively, are shown in FIGS. 1 to 3. In the figure, the dotted line 1 is the result of the first cycle, and the broken line 5 is the result of the fifth cycle.

[Comparative Example 1]
A magnesium metal containing no bismuth was cut into a cylinder having a diameter of 7 mm and a thickness of 2 mm, and a metal piece was prepared and processed in the same manner as in the examples to prepare a test piece. Cyclic voltammetry was performed using the prepared test pieces in the same manner as in Examples. The results are shown in FIG.

As shown in FIG. 4, when the magnesium test piece of the comparative example is used, the potential difference at the rise of the oxidation-reduction response due to the dissolution-precipitation of magnesium is 2 V or more. This is due to the oxide film formed on the surface of the magnesium electrode. On the other hand, when the Mg-Bi alloy of the example was used for the test piece, as shown in FIGS. 1 to 3, the potential difference at the rising edge of the oxidation-reduction response was small, and the potential difference was particularly high when the Bi content was 30% by mass or more. No oxidation-reduction response has been observed. As described above, it can be seen that the Mg-Bi alloy of the example suppressed the formation of the oxide film. As a result, charging / discharging from a higher potential becomes possible, and a magnesium secondary battery having a high capacity can be provided. The magnesium content of the Mg-Bi alloy of the examples is 98 to 50% by mass, which is much higher than, for example, 16% by mass of the magnesium content in _{the Mg 3} Bi _{2 intermetallic compound.}

[Example 2]
(Synthesis of positive electrode active material)
The molar ratio of the crown ether compound represented by the sulfur _{(S 8)} and formula (I) is 1: to 0.5, sulfur _{(S 8) (0.5g, 1.95mmol} ) in a reaction vessel And stirred at 155 ° C. for 10 minutes. After the sulfur was dissolved, the crown ether compound (0.820 g, 0.98 mmol) was added, and the mixture was heated to 160 to 165 ° C. and stirred. Initially, the liquid reaction mixture solidifies in about 20 minutes, but the reaction is continued as it is, and a heating reaction is carried out at about 160 ° C. for 60 minutes. Further, by heating to 170 ° C. and stirring, a yellowish brown rubber-like sulfur-containing polymer was quantitatively obtained.

(Preparation of positive electrode)
The synthesized sulfur-containing polymer, Ketjen black, and polytetrafluoroethylene were mixed by a dry method at a weight ratio of 3: 6: 1, and the mixture was pressure-bonded to a current collector (SUS) to prepare a positive electrode.

(Creation and evaluation of magnesium secondary battery)
The positive electrode prepared above, _{a separator impregnated with Mg (TFSA) 2} / triethylene glycol dimethyl ether (Triglyme) as an electrolyte, and a magnesium-bismuth alloy (containing 50 wt% bismuth) plate of the present invention as a negative electrode are used in the figure. A magnesium secondary battery was prepared by arranging it as shown in No. 5, and charge / discharge measurement was performed at room temperature. The result of charge / discharge measurement is shown in FIG.

[Comparative Example 2]
Using the positive electrode prepared above, _{a separator impregnated with Mg (TFSA) 2} / triethylene glycol dimethyl ether (Triglyme) as an electrolyte, and a magnesium plate for the negative electrode, the magnesium secondary is arranged as shown in FIG. A battery was prepared and charge / discharge measurement was performed at room temperature. The result of charge / discharge measurement is shown in FIG.

As a result of charge / discharge measurement of the magnesium-sulfur battery using the Mg-Bi alloy of the present invention as the negative electrode, a high potential discharge behavior was obtained as compared with the normal negative electrode of pure magnesium metal. The effect of suppressing the oxide film of magnesium on the negative electrode was confirmed. Along with this, the discharge capacity has almost reached the theoretical capacity of sulfur.

Since the electrode of the present invention suppresses the formation of an oxide film on the electrode surface and has a high magnesium content, it can be suitably used for a magnesium secondary battery, suppresses overvoltage, and has a high potential and a high capacity. The next battery can be obtained. Further, since the electrode of the present invention uses a magnesium alloy as an electrode material, it has sufficient strength and durability as an electrode. Since the magnesium secondary battery of the present invention suppresses the formation of an oxide film on the electrodes, it is possible to sufficiently exhibit the electrical characteristics of magnesium and obtain a secondary battery having a high potential and a high capacity. can. Therefore, the magnesium secondary battery of the present invention is suitable for in-vehicle use and large-scale use.

Claims (3)

An electrode including an alloy layer of magnesium (Mg) and bismuth (Bi), wherein the alloy layer is a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; where x is 0). .001 to 0.0112) or a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; where x is 0.001 to 0.0112), magnesium (Mg) and bismuth An electrode characterized by containing an intermetallic compound (Mg ₃ Bi _{2) with (Bi).} The electrode according to claim 1, wherein the content of bismuth in the alloy of magnesium (Mg) and bismuth (Bi) is 1 to 58.9% by mass with respect to the entire alloy. A magnesium secondary battery comprising the electrode according to claim 1 or 2 as a negative electrode, and further comprising an electrolyte layer and a positive electrode.

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