Magnesium-bismuth alloy electrodes

Abstract

To provide a magnesium alloy electrode that can suppress the formation of an oxide film and reduce overvoltage.SOLUTION: Provided is a magnesium-bismuth alloy electrode whose surface is coated with a crown ether polymer.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a magnesium-bismuth alloy electrode, a magnesium secondary battery including the magnesium-bismuth alloy electrode as a negative electrode, and a method for inhibiting the formation of an oxide film on a magnesium-bismuth alloy electrode. [Background technology]

[0002] In recent years, lithium-ion secondary batteries have been put into practical use as secondary batteries and are used in a variety of applications, including electronic devices. However, lithium-ion secondary batteries are unlikely to meet the demands of future automotive and large-scale applications, and other secondary battery technologies are being developed. Therefore, magnesium secondary batteries, which have characteristics that surpass lithium-ion secondary batteries in terms of electrical capacity per volume, are being actively developed. Magnesium not only has approximately twice the electrical capacity per volume of lithium, but also has a higher melting point of 650°C compared to lithium's 186°C. Lithium-ion secondary batteries have been known to overheat and catch fire due to internal short circuits, and one of the causes of this is believed to be the low melting point of lithium. In this regard, magnesium, due to its higher melting point, is safer. Furthermore, magnesium is more abundant on Earth than lithium, a rare metal, making it a more abundant resource. However, conventional magnesium anodes suffer from the problem of forming an insulating oxide film on their surfaces, which reduces electrical flow and increases overvoltage, thereby hindering the battery's inherent capacity.

[0003] Several proposals have been made to solve the above problems. For example, one method for suppressing the formation of an oxide film by improving the electrolyte is to remove the oxide film on the surface of a magnesium negative electrode using EtMgBr / THF as an electrolyte (Non-Patent Document 1). However, this method has the problem that it is not resistant to the oxidizing potential and decomposes the electrolyte, making long-term charge / discharge impossible. Furthermore, the usable positive electrode materials are limited, and only batteries with a low potential of approximately 1 V have been reported. To improve the negative electrode, a method has been proposed in which a magnesium intermetallic compound is used for the negative electrode, and an intermetallic compound prepared with a magnesium to bismuth molar ratio of 3:2 has been proposed (Patent Document 1, Non-Patent Document 2). However, with this method, the magnesium-bismuth intermetallic compound contains approximately 16% by mass of magnesium, so the amount of magnesium that can be used is limited. Furthermore, the intermetallic compound is brittle, so its use as an electrode without modification poses durability problems.

[0004] Therefore, the present inventors have focused on the problems of the magnesium-bismuth intermetallic compound (Mg3Bi2) and reported that the formation of an oxide film is suppressed in alloys with a lower bismuth content, making them usable as electrodes for high-potential, high-capacity magnesium secondary batteries (Patent Document 2).The present inventors have also reported that the formation of an oxide film can be suppressed by using a nitrile-based organic solvent in the electrolyte and adding a crown ether (Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2014-512637 [Patent Document 2] Patent No. 6958812 [Patent Document 3] Patent Publication No. 2021-77459 [Non-patent literature]

[0006] [Non-Patent Document 1] 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). [Non-patent document 2] 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). Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a magnesium alloy electrode that can suppress the formation of an oxide film and reduce overvoltage. [Means for solving the problem]

[0008] The inventors previously proposed a magnesium-bismuth alloy electrode as an electrode capable of suppressing oxide film formation and reducing overvoltage, even in an electrode containing magnesium. However, upon further investigation into a method for further increasing the magnesium content while suppressing oxide film formation, they discovered that coating the surface of a magnesium-bismuth alloy with a crown ether polymer suppresses oxide film formation and reduces overvoltage, even when the magnesium content is increased. While the conventional magnesium-bismuth alloy electrode developed by the inventors also exhibits the improved effects of suppressing oxide film formation and reducing overvoltage, a significant improvement in overvoltage requires a bismuth content of approximately 50% by mass in the magnesium alloy electrode. In this case, the theoretical energy density is approximately half that of magnesium, making it difficult to utilize the inherent energy density of magnesium. Furthermore, the use of a large amount of bismuth increases electrode costs. According to the present invention, even when the bismuth content is reduced to approximately 30% by mass and the magnesium content is reduced to approximately 70% by mass, oxide film formation can be suppressed and overvoltage can be reduced, resulting in a magnesium alloy electrode with high energy density. Furthermore, the amount of bismuth used can be reduced, thereby keeping electrode costs low. This is thought to be because the metal coordination properties of crown ethers make it possible to promote the dissolution-precipitation reaction of magnesium with a smaller amount of bismuth. Furthermore, in addition to the effect of suppressing the formation of an oxide film, the amount of electrolyte decomposition products adhering to the electrode surface can also be reduced, making it possible to maintain a reduced overvoltage even with repeated charge-discharge cycles and enabling charge-discharge over a long period of time. This is how the present invention was completed.

[0009] That is, the present invention is specified by the following items. (1) A magnesium-bismuth alloy electrode in which the surface of the magnesium-bismuth alloy is coated with a crown ether polymer. (2) The magnesium-bismuth alloy electrode according to (1) above, wherein the magnesium content in the magnesium-bismuth alloy is 50 mass % or more. (3) The magnesium-bismuth alloy electrode according to (1) or (2) above, characterized in that the crown ring forming the skeleton of the crown ether polymer is at least one selected from a 15-membered ring, an 18-membered ring, a 21-membered ring, and a 24-membered ring. (4) A magnesium secondary battery comprising the electrode according to any one of (1) to (3) above as a negative electrode, an electrolyte layer, and a positive electrode. (5) A method for inhibiting the formation of an oxide film on a magnesium-bismuth alloy electrode, comprising coating the surface of the magnesium-bismuth alloy with a crown ether polymer. [Effects of the Invention]

[0010] The present invention can provide a magnesium alloy electrode that can suppress the formation of an oxide film and reduce overvoltage, and by using this magnesium alloy electrode, it is possible to provide a magnesium secondary battery that suppresses overvoltage and has a high potential and a high capacity. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the synthesis process of the crown ether polymer in Example 1. [Figure 2] FIG. 2(a) is a schematic diagram showing the coating process in Example 1, and FIG. 2(b) is a photograph of the surface of the Mg—Bi alloy electrode after coating. [Figure 3] FIG. 3 is a diagram showing an L-shaped three-electrode glass cell used in the CV measurement in Example 1. [Figure 4] FIG. 4 shows the cyclic voltammetry of the Poly24C8-coated Mg—Bi alloy electrode of Example 1. [Figure 5] FIG. 5 is a photograph of the surface of the Poly24C8-coated Mg—Bi alloy electrode of Example 1 after the test. [Figure 6] FIG. 6 is a diagram showing the cyclic voltammetry of the Mg—Bi alloy electrode of Comparative Example 1. [Figure 7]FIG. 7 is a photograph of the surface of the Mg—Bi alloy electrode of Comparative Example 1 after the test. [Figure 8] FIG. 8 shows the cyclic voltammetry of the Poly24C8-coated Mg—Bi alloy electrode of Example 2. [Figure 9] FIG. 9 is a photograph of the surface of the Poly24C8-coated Mg—Bi alloy electrode of Example 2 after the test. [Figure 10] FIG. 10 is a diagram showing the cyclic voltammetry of the Mg—Bi alloy electrode of Comparative Example 2. [Figure 11] FIG. 11 is a photograph of the surface of the Mg—Bi alloy electrode of Comparative Example 2 after the test. [Figure 12] FIG. 12 is a graph showing the results of a charge-discharge test using a coin battery using the Poly24C8-coated Mg—Bi alloy electrode of Example 1. [Figure 13] FIG. 13 is a diagram showing the results of a charge-discharge test of a coin battery using the Mg—Bi alloy electrode of Comparative Example 1. [Figure 14] FIG. 14 is a graph showing the change in discharge capacity per cycle for a coin battery using the Poly24C8-coated Mg—Bi alloy electrode of Example 1 and a coin battery using the Mg—Bi alloy electrode of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] The magnesium-bismuth alloy electrode of the present invention is characterized in that the surface of the magnesium-bismuth alloy is coated with a crown ether polymer. In the present invention, "magnesium-bismuth alloy" means "an alloy of magnesium and bismuth." A crown ether polymer is a polymer composed of a crown ether skeleton. A crown ether is a macrocyclic compound having heteroatoms such as oxygen, nitrogen, and sulfur as electron-donating atoms. The crown ether that constitutes the skeleton is not particularly limited, but examples include crown ethers such as crown ethers, benzocrown ethers, and dibenzocrown ethers; azacrown ethers such as diazacrown ethers in which some of the oxygen atoms of a crown ether are replaced with nitrogen atoms; and thiacrown ethers in which some of the oxygen atoms of a crown ether are replaced with sulfur atoms.Specific examples thereof include 12-crown-4-ether, 14-crown-4-ether, 15-crown-5-ether, 18-crown-6-ether, 21-crown-7-ether, 24-crown-8-ether, 27-crown-9-ether, naphthyl-12-crown-4-ether, dibenzo-14-crown-4-ether, benzo-12-crown-4-ether, benzo-15-crown-5-ether, benzo-18-crown-6-ether, dibenzo-12-crown-4-ether, dibenzo-15-crown-5-ether, dibenzo-18-crown-6-ether, dibenzo-21-crown-7-ether, dibenzo-24-crown-8-ether, dibenzo-27-crown-9-ether, dicyclohexyl Examples of the crown ether include benzo-18-crown-6-ether, azacrown ethers such as 1-aza-15-crown-5-ether, 1-aza-18-crown-6-ether, benzo-1-aza-18-crown-6-ether, 4,10-diaza-12-crown-4-ether, 4,10-diaza-15-crown-5-ether, 4,13-diaza-18-crown-6-ether, and 5,6,14,15-dibenzo-1,4-dioxa-8,12-diazacyclopentadeca-5,14-diene, and thiacrown ethers such as 1-thia-15-crown-5-ether, 1-thia-18-crown-6-ether, and 1,4,8,11-tetrathiacyclotetradecane. The crown ether of the present invention may have a substituent. The crown ether constituting the skeleton may be one type or two or more types. The crown ether constituting the skeleton is preferably a crown ether in which the crown ring is at least one selected from a 15-membered ring, an 18-membered ring, a 21-membered ring, and a 24-membered ring. The crown ether polymer in the present invention is not particularly limited as long as it is a polymer having a crown ether skeleton, but from the viewpoint of coordination with magnesium ions, the crown ring constituting the skeleton of the crown ether polymer is preferably at least one selected from a 15-membered ring, an 18-membered ring, a 21-membered ring, and a 24-membered ring.Furthermore, from the viewpoint of the balance between coordination with magnesium ions and release, at least one ring selected from 18-membered rings, 21-membered rings, and 24-membered rings is preferred. Crown ethers in which the electron-donating atom is oxygen are also preferred. The method for synthesizing the crown ether polymer is not particularly limited, and for example, the polymer can be synthesized by using a crown ether monomer, introducing a polymerizable group, and carrying out a polymerization reaction. Other methods for synthesis include metathesis reactions, click reactions, polycondensation, thiol-ene reactions, coupling reactions, and the like. The crown ether polymer in the present invention is not particularly limited as long as it contains a crown ether in the main chain, and examples thereof include structures in which at least two crown ethers are linked together via alkyl, alkene (internal olefin), aromatic, oxazoline, sulfide (thioether), disulfide, sulfonyl, ether, ester, amide, imide, urethane, or carbonate.

[0013] The magnesium-bismuth alloy of the present invention includes a solid solution of magnesium and bismuth, or includes a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth. Here, including a solid solution of magnesium and bismuth and an intermetallic compound of magnesium and bismuth also includes a case where the solid solution of magnesium and bismuth and the intermetallic compound are present in the alloy in the form of a eutectic structure. It is known from the phase diagram that binary alloys of magnesium (Mg) and bismuth (Bi) form metallic Mg and Mg-Bi solid solutions up to the solubility limit of 8.87 mass% (1.12 at%), form Mg-Bi solid solutions and Mg-Mg3Bi2 eutectic crystals from 8.87 mass% (1.12 at%) to 58.9 mass% (14.3 at%), which is the eutectic point, form Mg-Bi solid solutions and Mg-Mg3Bi2 eutectic crystals, form Mg-Mg3Bi2 eutectic crystals and Mg3Bi2 intermetallic compounds from 58.9 mass% (14.3 at%) to 82.2 mass% (35 at%), and form Mg3Bi2 intermetallic compounds, Bi-Mg solid solutions, and metallic Bi at 82.2 mass% (35 at%) or higher. The morphology of the structure varies depending on the manufacturing conditions. The magnesium-bismuth alloy includes a magnesium-bismuth solid solution, a magnesium-bismuth solid solution, and a magnesium-bismuth intermetallic compound. That is, the composition of the magnesium-bismuth alloy includes a form in which metallic Mg and an Mg-Bi solid solution coexist, a form in which an Mg-Bi solid solution and an Mg-Mg3Bi2 eutectic coexist, and a form in which an Mg-Mg3Bi2 eutectic and an Mg3Bi2 intermetallic compound coexist. Because the battery reaction involves dissolution and precipitation of metallic Mg and an Mg-Bi solid solution, the bismuth content of the magnesium-bismuth alloy is preferably 58.9% by mass or less, more preferably 50% by mass or less, based on the total magnesium alloy, so that no solid solution is formed in the alloy. From the viewpoint of the workability of the magnesium alloy, the bismuth content of the magnesium-bismuth alloy is preferably 1 to 50% by mass, based on the total magnesium alloy. Furthermore, from the viewpoint of the effect of suppressing oxide film formation, the bismuth content of the magnesium-bismuth alloy is preferably 1 to 58.9% by mass, more preferably 1 to 50% by mass, based on the total magnesium alloy.Furthermore, the bismuth content of the magnesium-bismuth alloy is preferably 1 to 50 mass%, more preferably 10 to 50 mass%, and even more preferably 10 to 40 mass%, from the viewpoint of increasing the energy density. The magnesium content of the magnesium-bismuth alloy of the present invention is preferably 50 mass% or more, and more preferably 60 mass% or more. The magnesium content is also preferably 50 to 99 mass%, and more preferably 60 to 90 mass%. The solid solution of magnesium and bismuth (Mg. 1-x Bi x In the formula (2), x is preferably in the range of 0.001 or more, which is the lower limit for the alloy to exist substantially as a solid solution, to 0.0112 or less, which is the solid solubility limit. Furthermore, the present invention does not exclude cases in which the magnesium-bismuth alloy contains metal elements other than magnesium and bismuth. The magnesium-bismuth alloy of the present invention, including unavoidable impurities, may contain, for example, 1% by mass or less of any one element selected from the group consisting of Al, Zn, Mn, Ca, Ce, La, Si, C, Be, Zr, Sn, Sb, Li, Na, Ag, Cu, Ni, Fe, Pb, Y, Nd, Gd, Sr, and Dy, or 2% by mass or less in total of any two or more elements selected from these metal elements. The magnesium-bismuth alloy of the present invention can be produced, for example, by a casting method or the like.

[0014] The magnesium-bismuth alloy of the present invention has sufficient strength and can be easily processed into shapes such as plates and columns. Therefore, the processed magnesium-bismuth alloy itself can be used as an electrode, or it can be attached to a current collector and used as an electrode. Therefore, unlike when using granular electrode materials, electrodes can be produced without the need for binders or conductive additives, and the magnesium content in the electrode can be reduced. Alternatively, an electrode can be produced by processing the magnesium-bismuth alloy into granular shapes such as flakes, flattened, spindle-shaped, or spherical shapes, mixing them with a binder, and fixing them on a current collector. The magnesium-bismuth alloy electrode of the present invention includes a magnesium-bismuth alloy layer. The term "magnesium-bismuth alloy layer" refers to a layer containing the magnesium-bismuth alloy of the present invention. This term includes cases where the magnesium-bismuth alloy of the present invention is processed into shapes such as plates and columns, and the magnesium-bismuth alloy itself forms the magnesium-bismuth alloy layer, as well as cases where magnesium-bismuth alloy particles are fixed with a binder or the like to form the magnesium-bismuth alloy layer. The magnesium-bismuth alloy electrode of the present invention is sufficient as long as it has the above-mentioned magnesium alloy layer, and includes an electrode constituted only by the magnesium alloy layer, and an electrode constituted by the magnesium alloy layer together with other components such as a current collector.

[0015] In the present invention, the method for coating the surface of a magnesium-bismuth alloy with a crown ether polymer is not particularly limited. For example, a coating can be formed on the surface of the magnesium-bismuth alloy by applying the crown ether polymer to the surface of the magnesium-bismuth alloy. When coating, if necessary, a coating solution may be prepared by dissolving the crown ether polymer in a solvent, and then the coating solution may be applied and dried. Alternatively, the magnesium-bismuth alloy may be immersed in the coating solution, removed, and then dried. In the magnesium-bismuth alloy electrode and method for inhibiting the formation of an oxide film on the magnesium-bismuth alloy electrode of the present invention, coating the surface of the magnesium-bismuth alloy with a crown ether polymer can inhibit the formation of an oxide film on the surface when the magnesium-bismuth alloy is used as an electrode, thereby reducing overvoltage. In the present invention, "coating" does not necessarily mean that the entire surface of the magnesium-bismuth alloy is coated with a crown ether polymer, but rather, it is sufficient that the crown ether polymer coating is formed on the surface of the magnesium-bismuth alloy to the extent that the effects of the present invention, namely, the inhibition of oxide film formation on the electrode and the resulting reduction in overvoltage, are achieved. A preferred coating amount of the crown ether polymer is, for example, 40 to 100 μg / cm. 2 Examples include:

[0016] The battery of the present invention is characterized by comprising the electrode of the present invention as a negative electrode, an electrolyte layer, and a positive electrode. The positive electrode, electrolyte, and other components, as needed, of the battery of the present invention can be those used in conventional magnesium secondary batteries. For example, the positive electrode may be one in which a positive electrode active material is fixed on a current collector with a binder, and may contain a conductive additive as needed. Examples of positive electrode active materials include oxide-based positive electrodes such as sulfur or sulfur compounds, V2O5, sulfur-doped V2O5, MnO2, MnO3, MnO3, and other MnO3-based positive electrodes, sulfide-based positive electrodes such as Mo6S8, and Mg 1.03 Mn 0.97 SiO4, MgCoSiO4, MgFeSiO4, MgMnSiO4, Mo9Se 11, FePO4, etc. Examples of binders include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, and resin materials such as styrene butadiene rubber and carboxymethyl cellulose. Examples of conductive additives include graphite such as amorphous carbon, natural graphite, and artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and carbonaceous materials such as carbon nanotubes.

[0017] Examples of electrolytes include magnesium perchlorate (Mg(ClO4)2), Grignard reagents (RMgX (R is an organic group, and X is a halogen)), magnesium halides such as magnesium bromide (MgBr2), magnesium nitrate (Mg(NO3)2), magnesium bistrifluoromethanesulfonimide (Mg(TFSI)2), Mg(SO2CF3)2, magnesium borofluoride (Mg(BF4)2), magnesium trifluoromethylsulfonate (Mg(CF3SO3)2), and magnesium hexafluorophosphate (Mg(PF6)2). As the electrolyte solvent, known nonaqueous electrolyte solvents can be used, such as acetonitrile (AN), diethoxyethane, diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraglyme dimethyl ether (tetraglyme), tetrahydrofuran (THF), propylene carbonate (PC), ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, etc. As the separator, any material that is used as a separator for a nonaqueous electrolyte secondary battery can be used, such as a porous membrane or nonwoven fabric having high-rate discharge performance, which can be used alone or in combination.Examples of materials that can be used to form the separator include polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, cellulose, and cellulose derivatives (such as carboxymethyl cellulose). [Example]

[0018] The present invention will be specifically described below with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0019] [Synthesis of crown ether polymer (Poly24C8)] (1) A mixed solution of dibenzo-24-crown-8-ether (3.00 g: 6.69 mmol), hexamethylenetetramine (7.50 g: 53.5 mmol), and trifluoroacetic acid (30 mL: 0.39 mol) was refluxed at 60°C for 16 hours. Pure water (20 mL) was added to the reaction mixture, and the mixture was extracted with dichloromethane. The organic layer was washed with pure water. The organic layer was dried over magnesium sulfate, and the filtered solution was concentrated and vacuum-dried. The mixture was then suction-filtered and air-dried to obtain BFB24C8 (yield 2.83 g: 94.2%). (2) BFB24C8 (2.83 g, 5.60 mmol) was dissolved in a mixed solvent of tetrahydrofuran (THF) (150 mL) and methanol (MeOH) (50 mL), and sodium tetraborate (0.84672 g, 22.4 mmol) was slowly added. The mixture was refluxed at 60 °C for 18 hours. Purified water (50 mL) was added, and the mixture was extracted with dichloromethane. The organic layer was washed with purified water. The organic layer was dried over magnesium sulfate, and the filtered solution was concentrated and dried under vacuum to obtain BHMB24C8 (yield 2.96 g, 100%). (3) To a solution of BHMB24C8 (2.96 g, 5.81 mmol) in THF (50 mL) was slowly added NaH (2.34 g, 58.1 mmol) under ice-bath stirring. Once the mixture was stable, a solution of 11-bromo-1-undecene (5.42 g, 23.2 mmol) in THF (20 mL) was added, and the mixture was stirred at 60 °C for 18 hours. The reaction was quenched by slowly adding purified water. The mixture was extracted with dichloromethane, the organic layer was dried over magnesium sulfate, and the filtered solution was concentrated. The concentrate was crudely purified by column chromatography (silica gel, dichloromethane / ethyl acetate = 1 / 50; v / v) and reprecipitated using dichloromethane and ethyl acetate to obtain BHMB24C8 (yield 1.50 g, 100%). (4) Grubbs' catalyst (13.9 mg) was added to a mixture of monomer BUOMB24C8 (0.2762 g, 3.39 mmol) and dichloromethane (20 mL), and the mixture was refluxed at 40°C for 16 hours using an oil bath. The reaction mixture was concentrated, redissolved in dichloromethane (3 mL), and reprecipitated with methanol (30 mL) to obtain Poly24C8 (0.221 g). Figure 1 shows the synthesis process of steps (1) to (4) above.

[0020] [Example 1] [Preparation of Poly24C8-coated magnesium-bismuth alloy (Mg-Bi alloy) electrodes] A coating solution was prepared by mixing Poly24C8 (0.0735 g) and chloroform (0.7400 g). The coating solution was applied to the top surface of a cylindrical Mg-Bi alloy (70% by mass magnesium, 30% by mass bismuth) using a Pasteur pipette and allowed to dry naturally. The cross-sectional diameter of the cylindrical Mg-Bi alloy was 7 mm and the height was 3 mm. The polymer coating amount was 88.7 μg / cm. 2 Figure 2 shows a schematic diagram of the coating process and a photograph of the Mg-Bi alloy electrode surface after coating.

[0021] [Cyclic voltammetry (CV)] The electrochemical behavior of the Poly24C8-coated Mg-Bi alloy prepared above was measured in the negative electrode electrolyte. The test electrode was a Poly24C8-coated Mg-Bi alloy, the reference electrode was a silver wire (Ag), the counter electrode was a magnesium ribbon, and the electrolyte was a 0.5 M Mg(TFSA)2 / G3 solution in triethylene glycol dimethyl ether (G3) solvent. CV was performed using a three-electrode glass cell. TFSA stands for bistrifluoromethanesulfonylamide. The test electrode and counter electrode were polished with emery paper #600-2000 before use (except for the test electrode, which was polished before polymer coating). The reference electrode was polished with emery paper #2000 before use. The scan range was -3.5 to 0.0 V. The measurement temperature was room temperature, the scan rate was 10 mV / s, the sampling interval was 0.1 s, and the potential sweep direction was 0 V → oxidation → reduction. The L-shaped three-electrode glass cell used is shown in Figure 3. Figure 4 shows the cyclic voltammetry of the Poly24C8-coated Mg-Bi alloy electrode of Example 1. Figure 5 shows a photograph of the electrode surface after the test of Example 1.

[0022] [Comparative Example 1] CV was performed in the same manner as in Example 1, except that a cylindrical Mg-Bi alloy (70% by mass of magnesium, 30% by mass of bismuth) similar to that in Example 1 was used as the test electrode without being coated with Poly24C8. The results are shown in Figure 6. Figure 7 shows a photograph of the electrode surface after testing in Comparative Example 1. Although the overvoltage in Comparative Example 1 was also reduced compared to conventional magnesium electrodes and magnesium-bismuth intermetallic compound electrodes, the cycle characteristics showed a maximum current value at 20 cycles, and the current value decreased thereafter. On the other hand, in Example 1, the maximum current value was maintained even at 100 cycles, with the overvoltage reduced compared to Comparative Example 1. Furthermore, in Example 1, the electrode surface after testing was less darkened than in Comparative Example 1, indicating less decomposition of the electrolyte.

[0023] [Example 2] The Mg-Bi alloy used was a Mg-Bi alloy containing 50 mass% magnesium and 50 mass% bismuth, and the coating amount as a polymer was 44.4 μg / cm 2 Except for this, the electrode was coated with Poly24C8 and subjected to CV in the same manner as in Example 1. The results are shown in Figure 8. Figure 9 shows a photograph of the electrode surface after the test in Example 2.

[0024] Comparative Example 2 CV was performed in the same manner as in Example 2, except that a cylindrical Mg-Bi alloy (50% by mass of magnesium, 50% by mass of bismuth) similar to that in Example 2 was used for the test electrode without being coated with Poly24C8. The results are shown in Figure 10. Figure 11 shows a photograph of the electrode surface after the test in Comparative Example 2. In Comparative Example 2, the cycle characteristics showed that the current value reached a maximum at 40 cycles, and the current value decreased thereafter. On the other hand, in Example 2, the maximum current value was almost maintained even at 100 cycles, with the overvoltage reduced compared to Comparative Example 2. Furthermore, in Example 2, the darkening of the electrode surface after the test was less than in Comparative Example 2, indicating less decomposition of the electrolyte.

[0025] [Evaluation using coin cells (bipolar cells)] A charge-discharge test was performed using a coin cell (CR2032) with a negative electrode made of the Poly24C8-coated Mg-Bi alloy prepared in Example 1 and a coin cell (CR2032) with a negative electrode made of the Mg-Bi alloy not coated with Poly24C8 prepared in Comparative Example 1. The negative electrode consisted of a vanadium pentoxide (VO), ketjen black (KB), and polytetrafluoroethylene (PTFE) mixture in a 6:3:1 ratio. A gel electrolyte containing 0.5M Mg(TFSA) / G3 with a rotaxane network polymer was used as the electrolyte. The network polymer with a rotaxane structure was prepared according to the method disclosed in JP 2016-162543 A. The measurement conditions were C = 0.05 C, 0 to 4 V, and room temperature. The results for the Poly24C8-coated Mg-Bi alloy negative electrode prepared in Example 1 are shown in Figure 12, and the results for the Mg-Bi alloy negative electrode prepared in Comparative Example 1 are shown in Figure 13. In the case of the Mg-Bi alloy negative electrode of Comparative Example 1, the performance rapidly decreased after 5 cycles, but in the case of the Poly24C8-coated Mg-Bi alloy negative electrode of Example 1, the performance did not decrease even after 7 cycles. Figure 14 also shows the change in discharge capacity over each cycle. [Industrial Applicability]

[0026] The electrode of the present invention suppresses the formation of an oxide film on the electrode surface even when the magnesium content is high, improving cycle characteristics, and therefore can be suitably used in magnesium secondary batteries, allowing for the production of secondary batteries that suppress overvoltage and have high potential, high capacity, and excellent cycle characteristics. Furthermore, because the electrode of the present invention uses a magnesium alloy as the 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 electrode, it is possible to fully utilize the electrical properties of magnesium, resulting in a secondary battery that has high potential, high capacity, and excellent cycle characteristics. Therefore, the magnesium secondary battery of the present invention is suitable for in-vehicle and large-scale applications.

Claims

1. A magnesium-bismuth alloy electrode, in which the surface of the magnesium-bismuth alloy is coated with a crown ether polymer, and the crown ring forming the skeleton of the crown ether polymer is at least one selected from a 15-membered ring, an 18-membered ring, a 21-membered ring and a 24-membered ring.

2. 2. The magnesium-bismuth alloy electrode according to claim 1, wherein the magnesium content in the magnesium-bismuth alloy is 50 mass % or more.

3. A magnesium secondary battery comprising the magnesium-bismuth alloy electrode according to claim 1 or 2 as a negative electrode, an electrolyte layer, and a positive electrode.

4. A method for inhibiting the formation of an oxide film on a magnesium-bismuth alloy electrode, comprising coating the surface of the magnesium-bismuth alloy with a crown ether polymer whose skeleton crown ring is at least one selected from 15-membered rings, 18-membered rings, 21-membered rings and 24-membered rings.

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