Mg-based alloy negative electrode material, Mg negative electrode material, and Mg secondary battery using the same

Abstract

To provide a Mg-based alloy negative electrode material having excellent electrochemical properties.SOLUTION: In a Mg-based alloy negative electrode material with excellent electrochemical properties, the thickness of a Mg-based alloy with addition of one of the elements Al, Ag, Bi, Ca, Sn, Mn, Li, RE (rare earth), and Zn that form a solid solution in Mg, or a Mg-based alloy in which impurity elements are unavoidably present is 7 μm or more and 0.5 mm or less, there is no residual stress in the Mg matrix, the average crystal grain size of the Mg matrix is 5 μm or more, and the Mg-based alloy negative electrode material shows a current value of ±10 mAcm-2 or more when the overvoltage in the electrochemical deposition-dissolution test is 30 mV or less and the electrode potential in the deposition-dissolution test is ±0.5 V in a cycle measurement test for 50 times or more.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an Mg-based alloy negative electrode material having excellent electrochemical properties and an Mg secondary battery using this Mg-based alloy negative electrode material. The present invention also relates to an Mg negative electrode material having excellent electrochemical properties and an Mg secondary battery using this Mg negative electrode material. [Background technology]

[0002] Mg metal anodes have attracted attention as a potential anode material for high-energy-density batteries because they exhibit high theoretical capacity densities among practical metals and do not form dendritic deposits during electrodeposition. However, unlike Li (lithium) and Na (sodium) metal anodes, Mg (magnesium) metal anodes are less likely to form ion-conductive coatings at the anode electrolyte interface and are prone to passivation, making them less susceptible to reversible dissolution-precipitation behavior.

[0003] Typically, storage batteries, including secondary batteries, are composed of a negative electrode material, a positive electrode active material, and an electrolyte. To address the aforementioned issues, there are positive electrode active materials and electrolytes that are effective for Mg secondary batteries, as disclosed in, for example, Patent Documents 1 and 2 and Non-Patent Document 1. Conversely, negative electrode materials can be broadly divided into two types: Mg metal negative electrode materials (and Mg-based alloy negative electrode materials) and intermetallic compounds containing Mg. Patent Documents 3 and 4 attempt to address these issues by utilizing Mg-Bi intermetallic compounds (Mg3Bi2) and Mg-Sn intermetallic compounds (Mg2Sn). However, intermetallic compound negative electrodes have problems such as a higher potential and lower capacity density compared to Mg metal negative electrodes (and Mg-based alloy negative electrode materials). Furthermore, forming these intermetallic compounds requires the addition of high concentrations of Bi (bismuth) and Sn (tin), which have higher densities than Mg, which is undesirable from the perspective of weight reduction.

[0004] On the other hand, Patent Document 5 discloses an Mg metal negative electrode and an Mg-based alloy negative electrode material in which Mg metal is used as the negative electrode without utilizing an intermetallic compound. This is characterized by using an Mg-Al alloy, an Mg-Zn alloy, or an Mg-Mn alloy as a secondary battery negative electrode material, but only an Mg-6 mass% Al alloy is given as an example. From the viewpoint of thermal equilibrium and metal structure, the high concentration of Al added results in the deterioration of Mg 17 Al 12 Numerous intermetallic compounds, such as , are densely dispersed within the Mg matrix. However, the hardness and deformability of the intermetallic compounds themselves differ significantly from those of the Mg matrix. Therefore, during wrought processing, the interface between the intermetallic compounds and the Mg matrix becomes the origin of microvoids that cause fracture, which is undesirable from the perspective of foil formation. Furthermore, in terms of electrochemical properties, the interface between the intermetallic compounds and the Mg matrix is ​​incompatible at the atomic level, which can easily create energy differences and act as active sites for dissolution and precipitation behavior.

[0005] Patent Document 6 also discloses Mg metal anodes and Mg-based alloy anode materials suitable for Mg secondary batteries containing 90% or more Mg by mass. The auxiliary components are characterized by being one of Al (aluminum), Zn (zinc), Mn (manganese), Si (silicon), Ca (calcium), Fe (iron), Cu (copper), and Ni (nickel). However, because elements are added in excess of the solid solution amount for Mg, intermetallic compounds disperse within the Mg matrix, as in Patent Document 5. This prevents the desired electrochemical properties from being achieved. Of course, without the use of wrought processing techniques such as extrusion and rolling, the thickness of the bulk material cannot be reduced, which is undesirable from the perspective of reducing the weight of secondary batteries. Furthermore, the maximum solid solution amounts of Si, Fe, Cu, and Ni in Mg are 0.01 mol% or less, which is not enough to affect mechanical or functional properties, and therefore they are generally treated as unavoidable impurities.

[0006] On the other hand, the use of thinner anode materials is essential for reducing the total weight and improving the efficiency of secondary batteries. Patent Document 7 proposes improving charge / discharge characteristics by applying a polymer layer of 1 μm or more to a 400 μm-thick anode. Patent Document 8 proposes solving the problem by applying a metal with a high ionization tendency to bulk magnesium using a chemical plating method. Patent Document 9 also proposes improving impedance characteristics by applying a surface treatment method to a 100 μm-thick anode to create a new surface. Both of these proposals require additional processes, such as coating, plating, and surface treatment, on the bulk material.

[0007] Another improvement is to reduce the thickness of the negative electrode material. Patent Document 10 discloses an electrochemically excellent negative electrode with a thickness of 10 to 100 μm, in which the crystal structure is changed from a hexagonal close-packed structure to a body-centered cubic lattice structure by adding lithium. However, adding high concentrations of lithium to magnesium is extremely dangerous from the viewpoint of the casting process.

[0008] Based on these prior examples, the inventors have made the following proposal. Patent Document 11 discloses an anode material characterized by the segregation of added elements at grain boundaries using plastic processing methods such as rolling. The amount of added elements is within the solid solubility range of each element, and the added element is one of the following elements that are solid soluble in Mg: Al, Ag, Bi, Ca, Sn, Mn, Li, RE (rare earth), and Zn. Meanwhile, the internal microstructure of metallic materials varies significantly depending on the plastic processing conditions, including temperature and deformation amount. In particular, differences in the crystal grain size constituting the matrix of metallic materials are known to significantly affect the mechanical properties of the entire bulk material, such as hardness, strength, and toughness. Non-Patent Document 1 proposes that reducing the crystal grain size of the Mg matrix may improve electrochemical properties. However, to the inventors' knowledge, there are no published or disclosed examples relating the internal microstructure and electrochemical properties other than those related to crystal grain size. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-150924 [Patent Document 2] JP 2016-96024 A [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-512637 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-515728 [Patent Document 5] Japanese Patent Application Laid-Open No. 2012-221670 [Patent Document 6] JP 2014-143170 A [Patent Document 7] Japanese Patent Application Laid-Open No. 2015-115233 [Patent Document 8] WO2017-187700 publication [Patent Document 9] JP 2014-143170 A [Patent Document 10] Japanese Patent Application Laid-Open No. 2014-164901 [Patent Document 11] PCT / JP2021 / 4058 publication

[0010] [Non-Patent Document 1] Bandai, Somekawa, ChemComm 56 (2020) 12122 Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to provide an Mg-based alloy negative electrode material and an Mg negative electrode material that have excellent electrochemical properties and can be cycled 50 times or more in cycle measurements using a bipolar cell, and an Mg secondary battery using the same. [Means for solving the problem]

[0012] [1] The Mg-based alloy negative electrode material of the present invention is an Mg-based alloy containing at least one element selected from the group consisting of Al, Ag, Bi, Ca, Sn, Mn, Li, RE (rare earth), and Zn, in an amount of 0.05 mol% or more and 1 mol% or less, among elements that dissolve in Mg, with the remainder being Mg and unavoidable components, and has a thickness of 7 μm or more and 0.5 mm or less. The lower limit of the thickness of the Mg-based alloy negative electrode material is more preferably 10 μm or more, and even more preferably 15 μm or more.

[0013] [2] In the Mg-based alloy negative electrode material [1] of the present invention, it is preferable that no residual stress exists in the Mg matrix. [3] In the Mg-based alloy negative electrode material [1] or [2] of the present invention, the average crystal grain size of the Mg matrix measured by the intercept method is preferably 5 μm or more. [4] In the Mg-based alloy negative electrode materials [1] to [3] of the present invention, it is preferable that the standard deviation of the Mg matrix crystal grain size measured by the intercept method is within the average crystal grain size. [5] In the Mg-based alloy negative electrode materials [1] to [4] of the present invention, preferably, deformation twins are not present in the Mg matrix. [6] In the Mg-based alloy negative electrode materials [1] to [5] of the present invention, it is preferable that the negative electrode materials exhibit cycle characteristics of 50 or more cycles in cycle measurements using a bipolar cell. [7] In the Mg-based alloy negative electrode materials [1] to [6] of the present invention, it is preferable that they exhibit an overvoltage of 30 mV or less in an electrochemical deposition-dissolution test of Mg metal. [8] In the Mg-based alloy negative electrode materials [1] to [7] of the present invention, preferably, in an electrochemical deposition and dissolution test of Mg metal, the negative electrode current is within ±10 mAcm when the electrode potential is ±0.5 V. -2 It is advisable to show the above current density.

[0014] [9] The Mg secondary battery of the present invention is an Mg secondary battery that is composed of the above Mg-based alloy negative electrode materials [1] to [8], an electrolyte, and a positive electrode.

[0015]

[10] The Mg negative electrode material of the present invention is composed of Mg and unavoidable components, and has a thickness of 7 μm or more and 0.5 mm or less. The lower limit of the thickness of the Mg negative electrode material is more preferably 10 μm or more, and even more preferably 15 μm or more.

[0016]

[11] In the Mg negative electrode material

[10] of the present invention, it is preferable that there is no residual stress in the Mg matrix.

[12] In the Mg negative electrode material

[10] or

[11] of the present invention, the average crystal grain size of the Mg matrix measured by the intercept method is preferably 5 μm or more.

[13] In the Mg negative electrode materials

[10] to

[12] of the present invention, it is preferable that the standard deviation of the Mg matrix crystal grain size measured by the intercept method is within the average crystal grain size.

[14] In the Mg negative electrode materials

[10] to

[13] of the present invention, preferably, deformation twins are not present in the Mg matrix.

[15] The Mg negative electrode material of the present invention

[10] to

[14] preferably exhibits cycle characteristics of 50 or more cycles in cycle measurements using a bipolar cell.

[16] The Mg negative electrode materials of the present invention

[10] to

[15] preferably exhibit an overvoltage of 30 mV or less in an electrochemical deposition-dissolution test of Mg metal.

[17] In the Mg negative electrode materials

[10] to

[16] of the present invention, preferably, in an electrochemical deposition / dissolution test of Mg metal, the current density is within ±10 mA cm when the electrode potential is ±0.5 V. -2 It is advisable to show the above current density.

[0017]

[18] The Mg secondary battery of the present invention is a Mg secondary battery composed of the above Mg negative electrode materials

[10] to

[17] , an electrolyte, and a positive electrode. [Brief explanation of the drawings]

[0018] [Figure 1] The photograph shows the appearance after rolling. [Figure 2]This is an example of microstructure observation of an Mg-based alloy negative electrode material according to one embodiment of the present invention, and an image obtained by electron backscatter diffraction is shown as a KAM image. [Figure 3] This is an example of microstructure observation of an Mg alloy negative electrode material, which is a comparative example of the present invention, and an image obtained by electron backscatter diffraction is shown as a KAM image. [Figure 4] 1 shows an example of microstructure observation of an Mg alloy negative electrode material according to an embodiment of the present invention, and shows an image obtained by an optical microscope. [Figure 5] 1 shows cycle voltage-current results in an example of electrochemical deposition-dissolution test of an Mg-based alloy negative electrode material, which shows an example of the present invention and a comparative example. [Figure 6] 1 shows the results of coulombic efficiency of an Mg-based alloy negative electrode material showing an example of the present invention and a comparative example. [Figure 7] 1 is a schematic diagram showing the general configuration of an Mg secondary battery in which the Mg-based alloy negative electrode material of the present invention is used. DETAILED DESCRIPTION OF THE INVENTION

[0019] The Mg-based alloy material for achieving the effects of the present invention is composed of Mg-Amol%X, where X is an element that dissolves in Mg at 0.05 mol% or more, and X is at least one element selected from the group consisting of Al, Ag, Bi, Ca, Sn, Mn, Li, RE (rare earth), and Zn. Rare earth refers to homologous elements such as Y (yttrium), Gd (gadolinium), Ce (cerium), and Sc (scandium). The value of A is equal to or less than the maximum solubility value in Mg, preferably 0.02 mol% to 1 mol%, more preferably 0.02 mol% to 0.5 mol%, and even more preferably 0.02 mol% to 0.3 mol%. When A is less than 0.02 mol%, the added solute element does not affect the mechanical or electrochemical properties and exists as an impurity element. Furthermore, when pure Mg is used as a Mg-based alloy material, the remainder consists of Mg, with unavoidable components such as iron, nickel, silicon, and copper, and it is generally desirable that the purity of the Mg be 99% or higher.

[0020] The internal microstructure and thickness of the above-mentioned Mg-based alloy material are controlled by using rolling and other wrought processes. While rolling is used as an example here, any method capable of imparting strain to the material, such as extrusion or pressing, can be used. Of course, wrought processes are not limited to these, and any method capable of controlling the internal structure and simultaneously adjusting the thickness during solidification, such as rapid solidification and twin-roll casting, can be used. The thickness of Mg-based alloys (pure Mg and binary Mg alloys) created by these methods is preferably 0.5 mm or less, more preferably 0.3 mm or less, and even more preferably 0.15 mm or less. When used as a negative electrode material, the greater the surface area in contact with the electrolyte, the better the electrochemical efficiency; therefore, a thickness exceeding 0.5 mm results in a decrease in efficiency. Furthermore, the weight of the secondary battery increases, reducing the benefit of weight reduction.

[0021] Next, we describe the characteristics of the internal microstructure of Mg-based alloy foils with thicknesses of 0.5 mm or less. The average grain size of the Mg matrix is ​​preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. In metallurgy and electrochemistry, grain refinement by introducing a large number of grain boundaries is desirable to obtain excellent electrochemical properties, and the finer the grain size, the better. However, in the case of thin foil samples, the number of grains present in the cross-sectional (i.e., thickness) direction is limited to a few to several tens. To ensure cycle performance of 50 or more cycles, local dissolution of the grain boundaries must be considered. Furthermore, the standard deviation of the grain size is preferably the same size or less (100% or less) as the average grain size, more preferably 75% or less, and even more preferably 50% or less. When coarse and fine crystal grains are mixed, charge-discharge cycling is difficult to achieve uniformly across the entire surface of the negative electrode. The heterogeneous microstructure regions consisting of coarse and fine crystal grains are prone to becoming short-circuit sites for precipitation and dissolution, making it difficult to achieve the desired cycle and electrochemical characteristics. It is recommended to measure crystal grain size using the sectional method (also known as the cut method; see JIS H0501 and G0551) based on JIS standards. However, when the crystal grain size is small or the grain boundaries are unclear, it is difficult to use the sectional method. Therefore, bright-field images or electron backscatter diffraction images obtained by a transmission electron microscope can also be used for measurement.

[0022] Refining the size of the Mg matrix is ​​an effective way to improve electrochemical properties (Non-Patent Document 1). Reducing the processing temperature and increasing the amount of processing strain applied per cycle are extremely effective methods for refining the Mg matrix using plastic processing, and these methods are widely used. However, in bulk materials created using low-temperature processing or large strain processing, dislocations introduced by the plastic processing do not recover and remain in the matrix, resulting in residual stress. In the creation of metallic materials, not limited to Mg-based alloys, there is a close relationship between grain size refinement and the presence of residual stress. However, residual stress is an aggregate of lattice defects and is therefore energetically unstable. Therefore, it easily becomes an activation site during charge / discharge and acts as a negative structural factor, which is undesirable from the perspective of improving electrochemical properties. Therefore, it is necessary to control the microstructure other than grain size.

[0023] To reduce residual stress, it is effective to increase the processing temperature while simultaneously reducing the amount of strain applied per processing. Alternatively, it is effective to reheat samples with inherent residual stress created under low-temperature or large-strain conditions at temperatures above the recrystallization temperature. The temperature is preferably 200°C or higher, more preferably 300°C or higher, and even more preferably 400°C or higher. The holding time is preferably 0.25 hours to 24 hours. If the holding time is less than 0.25 hours, the residual strain is not sufficiently diffused, making it difficult to obtain the desired microstructure. Furthermore, holding time exceeding 24 hours is undesirable from the viewpoint of work efficiency. Residual stress is measured by acquiring an electron backscatter diffraction image and then using the accompanying analysis software to obtain a kernel average misorientation (KAM) image or dislocation density image. A smaller KAM value indicates a smaller residual stress, but an average KAM value of 1° or higher can be considered to indicate the presence of residual stress. As another method, if one focuses on a single Mg crystal grain and there is a difference of 0.5 degrees or more in the KAM value, it can be assumed that residual stress exists. When observation by electron backscatter diffraction is difficult, evaluation can also be performed using the Wilamson-Hall method, which is based on X-ray diffraction.

[0024] Furthermore, when Mg-based alloys are plastically processed at low temperatures, deformation twins form within the Mg matrix due to the lack of slip systems resulting from the crystal structure. However, compared to typical grain boundaries, the interface energy of deformation twins is stable, so from the perspective of improving electrochemical properties, the introduction of a high density of twin interfaces within the Mg matrix is ​​undesirable. Therefore, the interface length is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less of the total interface length obtained by electron backscatter diffraction.

[0025] The electrochemical properties of Mg alloys are described below. In cycle measurements using a bipolar cell, the alloys preferably exhibit cycle characteristics of 5 or more, more preferably 25 or more, and even more preferably 50 or more. If the cycle characteristics are less than 5, there is a problem of side reactions with the electrolyte, and the alloys cannot be used as negative electrodes. Furthermore, the overvoltage that can be obtained in chemical deposition and dissolution tests is preferably 50 mV or less, more preferably 30 mV or less, and even more preferably 20 mV or less. If the overvoltage is 50 mV or more, the battery voltage loss is large, and the alloys cannot be used as negative electrodes. In electrochemical deposition and dissolution tests, the current value is preferably ±10 mAcm when the electrode potential is ±0.5 V. -2 More preferably, ±20 mAcm -2 More preferably, ±25 mAcm -2 The above values ​​are shown. Current value is ±10mAcm -2 If the concentration is less than this, the rate of the electrical reaction is determined by the negative electrode, and the material cannot be used as a negative electrode. The electrolyte used in cycle measurement tests and electrochemical deposition / dissolution tests should preferably be a solution containing 0.1 mol / L to 1.5 mol / L of fluoroalkoxy magnesium borate or fluoroalkoxy magnesium aluminate as the solute, blended with an organic ether, but alkyl magnesium chloride, magnesium chloride, magnesium bistrifluoromethanesulfonylimide salt, magnesium hexamethyldisilazide salt, etc. may also be used as the solute.

[0026] The magnesium tetrakis(hexafluoroisopropoxy)borate used in this embodiment can also be synthesized by reacting magnesium borohydride with hexafluoroisopropanol. However, electrolytes containing magnesium tetrakis(hexafluoroisopropoxy)borate, synthesized from magnesium borohydride as a raw material, and organic ethers as a solute do not exhibit sufficient performance in cycle measurements of electrochemical deposition-dissolution tests using a bipolar cell unless approximately 5 to 10 preliminary cycles are performed. In contrast, the electrolytes synthesized in this embodiment containing magnesium tetrakis(hexafluoroisopropoxy)borate as a solute and organic ethers as a solute and blended with organic ethers exhibit an overvoltage of approximately 80 mV from the first cycle in cycle measurements of electrochemical deposition-dissolution tests using a bipolar cell. (Materials Creation) [Example]

[0027] Commercially available pure Mg (99.96 mass%) casting material was extruded to control casting defects and plate shape. The extrusion process was performed at 300°C with an extrusion ratio of 30:1 to produce plate-shaped extrusions measuring 2 mm thick and 500 mm or longer (hereafter referred to as "extruded material"). The extruded material was cut into 100 mm lengths and rolled. Prior to rolling, the extruded material was held in an electric furnace set at 400°C for 15 minutes or longer, and then rolled to a thickness of 0.05 mm using a rolling mill with a roll temperature set at 300°C or higher. A photograph of the appearance of the resulting rolled material and a photograph of measurements taken with a micrometer are shown in Figure 1.

[0028] Figure 2 shows an example of the microstructure of a rolled pure Mg material obtained using electron backscatter diffraction (EBSD). This image was converted into a kernel average misorientation (KAM) image using EDAX analysis software. Grain boundaries, where the misorientation between adjacent grains is 15° or greater, are indicated by black lines. Each cluster represents a single grain, denoted as G in the image, and its average size (=average grain size) is 22.2 μm. The image also shows that there is little variation in grain size, with a standard deviation of 13.2 μm. Furthermore, the contrast within the Mg matrix qualitatively indicates residual stress; the darker the contrast within a grain, the greater the residual stress (see, for example, Figure 3 for Comparative Example 1). However, in the rolled material shown in the example, only a small proportion of grains exhibiting these characteristics (the average KAM value is 0.94°), demonstrating low residual stress. Furthermore, as illustrated by the arrows, focusing on a single grain reveals contrast within the matrix. Although the presence of residual stress is suggested, the KAM value is 0.2 degrees, which indicates that the residual stress is extremely small. Furthermore, an interface orientation analysis using EDAX analysis software showed that no deformation twins were present in the figure.

[0029] [Comparative Example 1] Using the extruded material from Example 1, a rolled material was created under all the same conditions, except that the electric furnace temperature was set to 200°C and the roll temperature was below 200°C. Figure 3 shows an example of the microstructure of a pure Mg rolled material obtained by electron backscatter diffraction. As with Figure 2, the image was converted into a KAM image using EDAX analysis software, with grain boundaries indicated by black lines. The average grain size was 13.1 μm, with a standard deviation of 13.8 μm. Compared to Figure 2, the appearance of the Mg matrix is ​​significantly different, with numerous grains exhibiting black contrast. The average KAM value was 1.21°, confirming a residual stress that is more than 30% higher than that of the rolled material from Example 1. Furthermore, a clear contrast (indicated by an arrow) was formed within each grain, and the KAM value was 0.8°, again confirming the presence of residual stress. This is due to the lower rolling temperature compared to the example, which resulted in insufficient lattice and grain boundary diffusion and failure to recover dislocations. Furthermore, the formation of deformation twins, as indicated by the white lines in the figure, was confirmed by interface orientation analysis using EDAX analysis software. The proportion of deformation twin interfaces to the total interface length was 10%. [Example]

[0030] The rolled material of Example 1 was cut into 10 × 10 mm 2 After cutting into strips, the strips were placed in an electric furnace set to 300°C for one hour (hereafter referred to as the heat-treated material). Figure 4 shows an example of the microstructure of the heat-treated pure Mg material, observed using an optical microscope. The appearance of the Mg matrix has changed significantly from Figure 3, with coarsening of the average size and homogenization (a decrease in standard deviation) evident. Reheating at 300°C or higher is effective in reducing residual stress and homogenizing the structure. (Electrochemical property evaluation)

[0031] Next, we will describe the characteristics obtained through electrochemical evaluation. All operations, including preparation of the electrolyte and assembly of the two-electrode cell, were carried out in a glove box under an argon atmosphere. Tetrakis(hexafluoroisopropoxy)borate magnesium salt was mixed with diethylene glycol dimethyl ether to prepare the electrolyte. A two-electrode cell was assembled using an Mg alloy as the working electrode, a glass fiber filter as the separator, 0.2 mL of the above electrolyte solution was dropped onto the filter, and a porous carbon material as the counter electrode. Current was 0.5 mAcm. -2 A constant-current voltage test was conducted at a current density of 0 V vs. Mg. Figure 5 shows the constant-current voltage measurement profiles for the materials of Example 1 and Comparative Example 1. In the constant-current voltage measurement profiles, with 0 V vs. Mg as the reference, the magnesium deposition reaction occurs in the region where a negative voltage is applied, and the magnesium dissolution reaction occurs in the region where a positive voltage is applied. Furthermore, for both the deposition and dissolution reactions, the deviation from 0 V vs. Mg corresponds to the overpotential. In one cycle of deposition-dissolution, the coulombic efficiency can be calculated by taking the ratio of the amount of current required for magnesium deposition to the amount of current required for magnesium dissolution.

[0032] Figure 6 shows the Coulombic efficiency calculated from constant-current voltage measurements of various composite materials. Although a decrease in Coulombic efficiency was observed in the initial cycles for the material in Example 1, it was found that the magnesium precipitation and dissolution reaction occurred extremely stably over 100 cycles. On the other hand, for the material in Comparative Example 1, the sample short-circuited after about 40 cycles, making it impossible to evaluate the electrochemical characteristics thereafter.

[0033] 7 is a schematic diagram showing the general configuration of an Mg secondary battery using the Mg-based alloy negative electrode material of the present invention. As shown in Fig. 7, the Mg secondary battery 1 includes a positive electrode 11, a negative electrode 12, an electrolyte 13, and a container 14.

[0034] In the positive electrode 11, a positive electrode active material (not shown) is held by a positive electrode current collector (not shown). The positive electrode current collector has the function of donating electrons to the positive electrode active material during discharge. Materials used as the positive electrode current collector include nickel, iron, stainless steel, titanium, and aluminum, which are relatively corrosion-resistant and inexpensive. The material used as the positive electrode active material is not particularly limited as long as it can insert and extract Mg ions, but MgFeSiO4, MgMn2O4, or V2O5 are preferably used. A specific example of the positive electrode 11 configuration is a configuration in which V2O5 is coated on stainless steel.

[0035] The negative electrode 12 is made of the Mg-based alloy negative electrode material of the present invention. The electrolyte 13 is held by a separator (not shown) and generates ionic conductivity between the positive electrode 11 and the negative electrode 12. The electrolyte 13 contains Mg ions. During discharge, the Mg ions undergo a reduction reaction (for example, the reaction of formula (1) described below) at the positive electrode 11 and an oxidation reaction (for example, the reaction of formula (2) described below) at the negative electrode 12. During charge, the Mg ions undergo an oxidation reaction (for example, the reaction of formula (3) described below) at the positive electrode 11 and a reduction reaction (for example, the reaction of formula (4) described below) at the negative electrode 12. These oxidation-reduction reactions enable the Mg secondary battery to be charged and discharged.

[0036] [C1] V2O5+Mg 2+ +2e- → MgV2O5… Formula (1) Mg → Mg 2+ +2e- … Formula (2) MgV2O5 → V2O5+Mg 2+ +2e- … Formula (3) Mg 2+ +2e- → Mg … Equation (4)

[0037] The positive electrode 11, the negative electrode 12, and the electrolyte 13 are sealed in a container 14. The material of the container 14 is not particularly limited as long as it does not leak the electrolyte and is corrosion-resistant, but a container formed by pressing a metal plate such as iron and having a plating layer of nickel or the like formed on the entire inner and outer surfaces for corrosion resistance is preferably used.

[0038] The electrolytic solution 13 according to this embodiment may contain an organic solvent as a main solvent and a magnesium salt, or may be an inorganic solvent having magnesium ion conductivity.

[0039] The above-described embodiment of the present invention merely describes one example of an Mg secondary battery, and should not be construed as limiting, and also includes technical matters that are obvious in the technical field of Mg secondary batteries. [Industrial Applicability]

[0040] The Mg-based alloy of the present invention exhibits excellent electrochemical properties and can therefore be used as an Mg-based alloy negative electrode material for Mg secondary batteries as well as for Mg primary batteries. The Mg-based alloy negative electrode material of the present invention can be used in Mg secondary batteries. Furthermore, since Mg has a low density and is a thin material, an Mg-based alloy foil having the composition of the Mg-based alloy negative electrode material of the present invention can be used as a lightweight foil material that can replace aluminum foil. [Explanation of symbols]

[0041] 1...Mg secondary battery 11...Positive electrode 12...Negative electrode 13...Electrolyte 14...Container G...Crystal grain

Claims

1. The Mg negative electrode material is composed of Mg and inevitable components and has a thickness of 7 μm or more and 0.5 mm or less, The residual stress in the Mg matrix of the Mg negative electrode material is An electron backscatter diffraction image is obtained, and the average KAM (kernel average misorientation) value of the Mg crystal grains is less than 1 degree, or when focusing on one Mg crystal grain, there is a difference in the KAM value of less than 0.5 degrees. A magnesium negative electrode material characterized by:

2. 2. The Mg negative electrode material according to claim 1, wherein the average crystal grain size of the Mg matrix measured by the intercept method is 5 μm or more.

3. 3. The Mg negative electrode material according to claim 1, wherein the standard deviation of the Mg parent phase crystal grain size measured by the intercept method is within the average crystal grain size.

4. 4. The Mg negative electrode material according to claim 1, wherein no deformation twin crystals are present in the Mg matrix.

5. 5. The Mg negative electrode material according to claim 1, which exhibits cycle characteristics of 50 or more cycles in cycle measurements using a bipolar cell.

6. 6. The Mg negative electrode material according to claim 1, which exhibits an overvoltage of 30 mV or less in an electrochemical deposition-dissolution test of Mg metal.

7. In the electrochemical deposition and dissolution test of Mg metal, when the electrode potential is ±0.5 V, ±10 mA cm －２ 7. The Mg negative electrode material according to claim 1, which exhibits a current density of at least 1000 kJ / cm2.

8. 8. A Mg secondary battery comprising the Mg negative electrode material according to claim 1, an electrolyte, and a positive electrode.

Images