JP7384346B2 - Insulation suppression electrolyte and insulation suppression method for magnesium secondary batteries - Google Patents

Description

The present invention provides an electrolytic solution for a magnesium secondary battery that can improve the cycle characteristics and increase the current value of a magnesium secondary battery, a magnesium secondary battery using the electrolytic solution, and a magnesium secondary battery using the electrolytic solution. The present invention relates to a magnesium alloy electrode for use in a magnesium secondary battery, and a method for suppressing oxide film formation on a magnesium alloy 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 will be difficult for lithium ion secondary batteries to meet future automotive and large-scale applications, and other secondary batteries are being developed. Therefore, development of magnesium secondary batteries, which have characteristics superior to lithium ion secondary batteries in electric capacity per volume, is being actively conducted. Magnesium not only has about twice the electric capacity per volume as lithium, but also has a higher melting point of 650°C compared to 186°C for lithium. It has been pointed out that lithium ion secondary batteries have the problem of overheating and ignition due to short circuits inside the battery, and one of the causes of this is the low melting point of lithium. In this respect, magnesium has a higher melting point than lithium, so it is safer. In addition, magnesium is more abundant on earth than lithium, which is a rare metal, and is an abundant resource. However, conventional magnesium negative electrodes have a problem in that because an oxide film, which is an insulating layer, is formed on the surface of the negative electrode, it is difficult for electricity to flow, resulting in a large overvoltage, and the battery cannot exhibit its original capacity characteristics.

Therefore, several proposals have been made to solve the above problems. For example, as a method of suppressing the formation of an oxide film by improving the electrolyte, a method has been proposed in which the oxide film on the surface of a magnesium negative electrode is removed using EtMgBr/THF as an electrolyte (Non-Patent Document 1). However, this method has a problem in that it is not resistant to the potential on the oxidation side and decomposition of the electrolyte occurs, making it impossible to charge and discharge over a long period of time. In addition, a method has been proposed in which the formation of an oxide film on the negative electrode surface is suppressed by using an intermetallic compound (Mg ₃ Bi ₂ ) prepared with a molar ratio of magnesium and bismuth of 3:2 in the negative electrode ( Patent Document 1, Non-Patent Document 2). However, this method has the problem that the magnesium content of the negative electrode cannot be increased and sufficient battery capacity cannot be obtained.

The present inventors focused on the problems of the above-mentioned magnesium-bismuth intermetallic compound (Mg ₃ Bi ₂ ), and found that an alloy with a smaller amount of bismuth suppresses the formation of an oxide film, and produces a high-potential and high-capacity magnesium alloy. It has been reported that it can be used as an electrode for secondary batteries (Patent Document 2). Such a magnesium-bismuth alloy electrode is an electrode material that has a sufficiently high magnesium content and has excellent molding properties.

Special table 2014-512637 publication International Publication No. 2018/168995

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, “ElectrodepositedBi, Sb and Bi1-xSbx alloys as anodes for Mg-ion batteries”, Electrochemistry Communications, 16, 103-106 (2012).

The present inventors have found that by increasing the magnesium content to 50% by mass compared to a magnesium-bismuth intermetallic compound (Mg ₃ Bi ₂ ) with a magnesium content of 16% by mass, a higher energy density and a lower overvoltage can be achieved. It has succeeded in providing a reduced magnesium negative electrode (Patent Document 2). However, if the proportion of magnesium is increased in order to further increase the energy density, the effect of suppressing the oxide film by bismuth decreases, leading to an increase in overvoltage in the battery, so there is a need to improve this trade-off. Therefore, an object of the present invention is to suppress the formation of an oxide film even when a magnesium alloy electrode with a high magnesium content is used as the negative electrode of a magnesium secondary battery, thereby reducing the gap between oxidation and reduction of the negative electrode. The purpose is to reduce overvoltage.

The present inventors have conducted intensive studies to perform reversible charging and discharging while suppressing the formation of an oxide film on the magnesium surface even when the magnesium content is increased. It was discovered that by using an organic solvent and adding a crown ether that has magnesium coordinating properties, the surface of magnesium can be modified and the oxide film can be removed. As a result, in addition to the oxide film suppression effect of bismuth, the oxide film removal effect of the coordination compound in the nitrile electrolyte acts synergistically, and even in alloy materials with bismuth that has a high magnesium content. It has become possible to achieve both energy density and overvoltage reduction effect (high potential of the battery). The main points of the present invention are as follows.
Point 1. By using a nitrile-based organic solvent in the electrolyte and adding crown ether that coordinates to magnesium, it is possible to remove the oxide film on the surface of the magnesium-bismuth alloy negative electrode.
Point 2. Even if the oxide film suppression effect of intermetallic compounds decreases due to increased magnesium content, overvoltage can be reduced due to the film removal effect of the electrolytic solution.

That is, the present invention relates to the following.
(1) An electrolytic solution for a magnesium secondary battery, characterized by containing a magnesium salt, a nitrile solvent, and a crown ether.
(2) The electrolytic solution for a magnesium secondary battery as described in (1) above, wherein the crown ether is a crown ether selected from 18C6, 21C7, 24C8 and derivatives thereof.
(3) The electrolytic solution for a magnesium secondary battery as described in (1) or (2) above, wherein the nitrile solvent is selected from butyronitrile, acetonitrile, propionitrile, and isobutyronitrile.
(4) A magnesium secondary battery, characterized in that the electrolyte layer contains the electrolytic solution according to any one of (1) to (3) above, and further includes a positive electrode and a negative electrode.
(5) The magnesium secondary battery according to (4) above, wherein the negative electrode is a magnesium-bismuth alloy.
(6) The magnesium according to (4) or (5) above, wherein the magnesium content of the magnesium-bismuth alloy is 50% by mass or more and/or the bismuth content is 10 to 50% by mass. Secondary battery.
(7) A magnesium alloy electrode containing 50% by mass or more of magnesium, characterized in that it is used in a magnesium secondary battery equipped with an electrolyte layer containing the electrolyte solution according to any one of (1) to (3) above. The magnesium alloy electrode.
(8) A method for suppressing oxide film formation on a magnesium alloy electrode, which comprises adding a crown ether to an electrolytic solution containing a magnesium salt and a nitrile solvent.

By using the magnesium-bismuth alloy material reported by the present inventors as a negative electrode and the electrolyte of the present invention, it is possible to suppress the formation of insulation, and a high potential and high capacity magnesium secondary Batteries can be developed at low cost.

FIG. 2 is a diagram showing an outline of a test cell used in Examples. It is a figure which shows the result of cyclic voltammetry when A31 was used as a test electrode. FIG. 3 is a diagram showing the results of cyclic voltammetry when Mg-Bi Alloy 1 (Mg 50% by mass, Bi 50% by mass) was used as a test electrode. FIG. 3 is a diagram showing the results of cyclic voltammetry when Mg-Bi Alloy 2 (Mg 70% by mass, Bi 30% by mass) was used as a test electrode. A shows the results of cyclic voltammetry in an electrolytic solution to which 18C6 is not added, and B shows the results of cyclic voltammetry in an electrolytic solution to which 18C6 is added. It is a figure which shows the result of cyclic voltammetry when replacing with the electrolyte solution of this invention and using triglyme of a comparative example.

The present disclosure provides an electrolytic solution for a magnesium secondary battery (hereinafter referred to as "the present electrolytic solution") characterized by containing a magnesium salt, a nitrile solvent, and a crown ether, and an electrolytic solution containing the present electrolytic solution in an electrolyte layer, and further comprising: A magnesium secondary battery characterized by having a positive electrode and a negative electrode (hereinafter referred to as "this magnesium secondary battery"), a magnesium alloy electrode containing 50% by mass or more of magnesium, and an electrolyte layer containing the present electrolyte solution. A crown ether is added to the magnesium alloy electrode (hereinafter referred to as "the present magnesium alloy electrode") and an electrolyte containing a magnesium salt and a nitrile solvent, which is characterized in that it is used in a magnesium secondary battery equipped with The present invention relates to a method for suppressing oxide film formation on a magnesium alloy electrode (hereinafter referred to as "this method for suppressing oxide film formation"). In this specification, the term "magnesium secondary battery" refers to a secondary battery comprising magnesium metal or a magnesium alloy as a negative electrode. The present invention is based on the finding that the formation of an oxide film on the surface of a magnesium alloy electrode can be suppressed by adding crown ether to an electrolytic solution using a nitrile solvent.

In this electrolytic solution, the magnesium salt is not particularly limited as long as it dissociates in a nitrile solvent to release magnesium ions (does not form an organomagnesium complex). For example, magnesium perchlorate (Mg ClO ₄ ) ₂ ), magnesium halides such as magnesium bromide (MgBr ₂ ), magnesium nitrate (Mg(NO ₃ ) ₂ ), magnesium bistrifluoromethanesulfonimide (Mg(TFSI) ₂ ), Mg(SO ₂ CF ₃ ) ₂ , magnesium borofluoride (Mg(BF ₄ ) ₂ ), magnesium trifluoromethylsulfonate (Mg(CF ₃ SO ₃ ) ₂ ), magnesium hexafluorophosphate (Mg(PF ₆ ) ₂ ), magnesium bis(fluorosulfonyl ) imide (Mg(FSI) ₂ ) and the like. The concentration of the magnesium salt may be such that the magnesium ion concentration in the electrolyte is 0.01 to 2M, preferably 0.1 to 1M, more preferably 0.3 to 0.7M.

As the nitrile solvent that can be used in this electrolytic solution, nitriles having an alkyl group having 1 to 6 carbon atoms are preferred, and among them, butyronitrile, acetonitrile, propionitrile, and isobutyronitrile are preferred, and butyronitrile, isobutyronitrile, and isobutyronitrile are preferred. Nitriles are more preferred, and these nitriles may be used alone or in combination. Examples of crown ethers include 15-crown-5, 18-crown-6, 21-crown-7, and 24-crown-8, and these crown ethers may be used alone or in combination. It may also be used. In this electrolytic solution, the content of the nitrile solvent is 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or substantially 100% by mass, and the content of the crown ether is The content is preferably 0.1 to 2 equivalents, more preferably 0.5 to 1 equivalent relative to the magnesium ion concentration in the electrolytic solution. Preferably, the electrolyte is a non-aqueous electrolyte. The present electrolyte may or may not contain diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and/or tetraglyme dimethyl ether (tetraglyme). In producing this electrolytic solution, the magnesium salt, nitrile solvent, and crown ether may be mixed in any order.

In the present secondary battery, the present electrolyte can be used as an electrolyte layer as it is or by impregnating a separator. Here, as the separator, any material can be used as long as it is used as a separator for nonaqueous electrolyte secondary batteries. For example, a porous membrane or nonwoven fabric having high rate discharge performance may be used alone or Can be used together. Materials constituting the separator include, for example, polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Copolymer, vinylidene fluoride-perfluorovinylether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexane Fluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, Examples include vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, cellulose and cellulose derivatives (carboxymethylcellulose, etc.).

The present secondary battery is characterized in that it includes the electrolyte layer described above, and further includes a negative electrode and a positive electrode. The negative electrode used in the present secondary battery is preferably a magnesium alloy electrode containing 50% by mass or more of magnesium (the present magnesium alloy electrode), and more preferably the present magnesium alloy electrode is a magnesium-bismuth alloy. The bismuth content of the magnesium-bismuth alloy is preferably 10 to 50% by mass, more preferably 20 to 50% by mass. In a preferred embodiment, the magnesium-bismuth alloy is characterized in that it contains a solid solution of magnesium and bismuth, or a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth. Here, the expression "containing a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth" also includes a case where a solid solution of magnesium and bismuth and an intermetallic compound exist in the alloy in a eutectic structure. According to the phase diagram, a binary alloy of magnesium (Mg) and bismuth (Bi) contains metallic Mg and Mg-Bi until the Bi content in the alloy reaches the solid solubility limit of 8.87 mass% (1.12 at%). Forms a solid solution, and forms Mg-Bi solid solution and Mg-Mg ₃ Bi ₂ eutectic from 8.87 mass % (1.12 at %) to 58.9 mass % (14.3 at %) which is the eutectic point However, from 58.9 mass % (14.3 at %) to 82.2 mass % (35 at %), an Mg-Mg ₃ Bi ₂ eutectic and Mg ₃ Bi ₂ intermetallic compound are formed, and 82.2 mass % (35 at%) or more, it is known that a Mg _{3 Bi} intermetallic compound, a Bi--Mg solid solution, and metal Bi are formed, and the morphology of the structure differs depending on the manufacturing conditions. The above magnesium-bismuth 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. That is, the composition of the magnesium-bismuth alloy is such that metal Mg and Mg-Bi solid solution coexist, Mg-Bi solid solution and Mg-Mg ₃ Bi ₂ eutectic coexist, and Mg-Mg ₃ Bi ₂ eutectic. This includes forms in which Mg ₃ Bi and Mg ₃ Bi intermetallic compounds coexist. The content of bismuth in the above magnesium-bismuth alloy is such that a solid solution is not formed in the alloy in order to increase the electric capacity per volume, since the battery reaction involves dissolution and precipitation of metal Mg and Mg-Bi solid solution. 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-bismuth alloy is preferably 1 to 50% by mass based on the entire magnesium alloy from the viewpoint of workability of the magnesium alloy. Further, the content of bismuth in the magnesium-bismuth alloy is preferably 1 to 58.9% by mass, and 1 to 50% by mass based on the entire magnesium alloy, from the viewpoint of suppressing the formation of an oxide film. The content is more preferably 30 to 50% by mass. In the solid solution of magnesium and bismuth (Mg _1-x Bi _x ) in the above magnesium-bismuth alloy, x has a range of 0.001 or more, which is the lower limit that can substantially exist as a solid solution, and 0.0112, which is the solid solution limit. preferable. The alloy layer in the negative electrode of this secondary battery contains a solid solution of magnesium (Mg) and bismuth (Bi) (Mg _1-x Bi _x ; where x is 0.001 to 0.0112), or contains magnesium (Mg ) and 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 ) May include. The present magnesium alloy electrode may also be manufactured using a magnesium alloy that exhibits a magnesium dissolution-precipitation reaction and preferably an alloying-dealloying reaction when used with the present electrolyte. In addition to the above-mentioned magnesium-bismuth alloy, the magnesium alloy here includes magnesium and one or more metals selected from Al, Zn, Zr, Mn, Fe, Si, Cu, Ni, and Ca. Examples of alloys include AZ31, AZ61, AZ80, ZK60, AZ91D, AM60B, AM50A, AM20, AS41B, AS21, and AE42. The present magnesium alloy electrode can be selected by performing cyclic voltammetry in the present electrolyte using a magnesium alloy plate manufactured from these alloys as a test electrode to confirm the presence or absence of a magnesium dissolution-precipitation reaction.

As the positive electrode and other components as necessary in this secondary battery, those used in conventional magnesium secondary batteries can be used. For example, the positive electrode may include one in which a positive electrode active material is fixed on a current collector with a binder, and may contain a conductive additive as necessary. Examples of positive electrode active materials include sulfur or sulfur compounds, V ₂ O ₅ , V ₂ O ₅ series such as sulfur-doped V ₂ O ₅ , MnO ₂ series, MnO ₃ series such as MnO ₃ , MgMnO ₃ and other oxides. Sulfide-based positive electrodes such as _{Mo6S8 - based} positive electrodes, _{Mg1.03Mn0.97SiO4 , MgCoSiO4} , _MgFeSiO4 , _{MgMnSiO4 , Mo9Se11} , FePO4 _, etc. . Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, resin materials such as styrene-butadiene rubber, and carboxymethyl cellulose. Examples of conductive aids include amorphous carbon, graphite such as 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. Examples include carbonaceous substances.

The magnesium salt and nitrile solvent in the method for suppressing oxide film formation may be any magnesium salt or nitrile solvent that can be used in the electrolyte, and the crown ether may be, for example, 15-crown-5, 18- Crown-6, 21-crown-7, 24-crown-8, etc. can be mentioned. Examples of the concentration of the above-mentioned magnesium salt include a concentration such that the magnesium ion concentration in the electrolytic solution is 0.01 to 2M, preferably 0.1 to 1M, more preferably 0.3 to 0.7M, and the above-mentioned The amount of crown ether added is preferably 0.1 to 2 equivalents, more preferably 0.5 to 1 equivalent relative to the magnesium ion concentration in the nitrile solvent.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the technical scope of the present invention is not limited to these examples.

1. Materials and Methods 1-1 Pretreatment of Test Electrode Both sides of a cylindrical metal piece with a diameter of 7 mm and a thickness of 2 mm made of the following alloy were polished with emery paper #220 to #600 to #2000, and then polished with pure acetone. It was washed and used as a working electrode in cyclic voltammetry. Mg-Bi Alloy 1 and Mg-Bi Alloy 2 were manufactured by the method described in Patent Document 2.
・AZ31 (Mg 96% by mass, Al 3% by mass, Zn 1% by mass)
・Mg-Bi alloy 1 (Mg 50% by mass, Bi 50% by mass)
・Mg-Bi alloy 2 (Mg 70% by mass, Bi 30% by mass)

1-2 Test Cell A three-electrode beaker cell was used as a test cell for cyclic voltammetry. An overview is shown in Figure 1. The test electrode, reference electrode, counter electrode, and electrolyte were as follows.
Test electrode; Mg alloy plate reference electrode pretreated in 1-1 above; Ag line counter electrode; Mg ribbon electrolyte; 0.5M Mg(TFSA) ₂ /butyronitrile (BN)
0.5M Mg(TFSA) ₂ /butyronitrile (BN)
+0.25M 18-crown 6-ether (18C6)
*Mg(TFSA) ₂ ; Magnesium bistrifluoromethanesulfonimide

1-3 Electrochemical measurement (cyclic voltammetry)
The above sample was verified by cyclic voltammetry. The scanning range was -3.0 to 0.0V. The scanning rate was 10 mV/s, and the number of reduction-oxidation cycles was 100.

2. Result 2-1 AZ31
FIG. 2 shows the results of the 50th cycle of cyclic voltammetry using AZ31 as a test electrode. When a nitrile electrolyte without crown ether added was used, the dissolution-precipitation reaction of magnesium in the test electrode disappeared, but by adding Mg-coordinating crown ether (18C6), it was completely suppressed. Although the coating was not removed and overvoltage was generated, we were able to confirm the dissolution-precipitation reaction of magnesium. The results here show that even in Mg electrodes that do not contain Bi, the oxide film can be removed by adding crown ether to the nitrile electrolyte.

2-2 Mg-Bi alloy 1 (Mg 50% by mass, Bi 50% by mass)
FIG. 3 shows the results of the 50th cycle of cyclic voltammetry using Mg-Bi Alloy 1 (Mg 50% by mass, Bi 50% by mass) as a test electrode. When a magnesium-bismuth alloy electrode was used as the test electrode, no major change in overvoltage reduction was observed, but the addition of crown ether increased the current value by about twice, and the dissolution-precipitation reaction of magnesium was observed. It was suggested that this was accelerated.

2-3 Mg-Bi alloy 2 (Mg 70% by mass, Bi 30% by mass)
FIG. 4 shows the results of the 1st, 10th, 50th, and 100th cycles of cyclic voltammetry using Mg-Bi Alloy 2 (Mg 70% by mass, Bi 30% by mass) as a test electrode. As a result of comparing Figure 4A (without 18C6) and Figure 4B (with 18C6), a significant difference in current value was observed after 50 cycles, and even in electrodes with increased magnesium content, the addition of crown ether Acceleration of the dissolution-precipitation reaction of magnesium was observed, suggesting that it was effective in removing the oxide film from the magnesium negative electrode.

[Comparative example]
In the test cell of 1-2 above, the electrolyte was changed to 0.5M Mg(TFSA) ₂ / triglyme (G3), and Mg-Bi alloy 1 (Mg 50% by mass, Bi 50% by mass) was used as the test electrode. 10 cycles of cyclic voltammetry were performed.

FIG. 5 shows the results of the 1st, 2nd, 5th, and 10th cycles. The results shown in Figure 3 using this electrolyte showed an excellent current value even at the 50th cycle, whereas when triglyme was used as the electrolyte, the current value decreased with each cycle. It was shown that the durability of the oxide film removal effect (overvoltage suppression effect) of triglyme was lower than that of this electrolyte.

According to the present invention, it is possible to suppress the formation of an insulating oxide film on the negative electrode (magnesium alloy electrode) of a magnesium secondary battery. Therefore, it is possible to improve the cycle characteristics and increase the current value, which have been problems with magnesium secondary batteries, and it is possible to develop high-potential and high-capacity magnesium secondary batteries at low cost. The secondary battery manufactured using the electrolyte according to the present invention can be used for large-capacity power storage devices such as those for vehicles and residential areas, where it was difficult to apply conventional lithium secondary batteries, so it can be used for industrial purposes. The possibility is extremely high.

Claims (7)

It contains a magnesium salt, a nitrile solvent, and a crown ether, and the crown ether is a crown ether selected from 18C6, 21C7, 24C8 and derivatives thereof, and the content of the crown ether is 0 with respect to the magnesium ion concentration. .1 to 1 equivalent, and the nitrile solvent is a nitrile having an alkyl group having 1 to 6 carbon atoms. The electrolytic solution for a magnesium secondary battery according to claim 1, wherein the nitrile solvent is selected from butyronitrile, acetonitrile, propionitrile, and isobutyronitrile. An electrolytic solution for use in a magnesium secondary battery in which the negative electrode is a magnesium-bismuth alloy, wherein the magnesium content of the magnesium-bismuth alloy is 50% by mass or more, and/or the bismuth content is 10 to 50% by mass. The electrolytic solution for a magnesium secondary battery according to claim 1 or 2, characterized in that the electrolytic solution is % by mass. A magnesium secondary battery comprising an electrolytic solution according to claim 1 or 2 in an electrolyte layer, and further comprising a positive electrode and a negative electrode. 5. The magnesium secondary battery according to claim 4, wherein the negative electrode is a magnesium-bismuth alloy. The electrolyte layer contains the electrolytic solution according to claim 3, further comprising a positive electrode and a negative electrode, and the negative electrode has a magnesium content of 50% by mass or more and /or a bismuth content of 10 to 50% by mass. A magnesium secondary battery characterized by being made of a magnesium-bismuth alloy . An electrolytic solution containing a magnesium salt and a nitrile solvent, the nitrile solvent being a nitrile having an alkyl group having 1 to 6 carbon atoms, selected from 18C6, 21C7, 24C8 and derivatives thereof. Magnesium having a magnesium content of 50% by mass or more and/or a bismuth content of 10 to 50% by mass, characterized in that 0.1 to 1 equivalent of crown ether is added to the magnesium ion concentration. - Method for suppressing oxide film formation on bismuth alloy electrodes.