JP7807776B2 - Mg-based alloy negative electrode material, Mg negative electrode material, and Mg secondary battery using the same - Google Patents

Description

The present invention relates to an Mg-based alloy negative electrode material with 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 with excellent electrochemical properties and an Mg secondary battery using this Mg negative electrode material.

Mg metal anodes exhibit a high theoretical capacity density among practical metals and do not form dendritic deposits during electrodeposition, making them attractive as a potential anode material for high-energy density batteries. However, unlike Li (lithium) and Na (sodium) metal anodes, Mg (magnesium) metal anodes are less likely to form an ionically conductive film at the anode electrolyte interface and are more likely to become passivated, making them less susceptible to reversible dissolution-precipitation behavior.

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 Mg-containing intermetallic compounds. Patent Documents 3 and 4 attempt to address these issues by utilizing Mg-Bi intermetallic compounds (Mg ₃ Bi ₂ ) and Mg-Sn intermetallic compounds (Mg ₂ Sn). However, intermetallic compound negative electrodes have the problems of 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) or Sn (tin), which have higher densities than Mg, which is undesirable from the perspective of weight reduction.

On the other hand, Patent Document 5 discloses Mg metal negative electrodes and Mg-based alloy negative electrode materials in which Mg metal is used as the negative electrode without utilizing intermetallic compounds. While the secondary battery negative electrode materials are characterized by the use of Mg-Al alloys, Mg-Zn alloys, and Mg-Mn alloys, only an Mg-6 mass% Al alloy is cited as an example. From the viewpoint of thermal equilibrium and metal structure, due to the high concentration of Al added, numerous intermetallic compounds, typified by Mg ₁₇ Al _12, are densely dispersed within the Mg matrix. However, since the hardness and ease of deformation of the intermetallic compounds themselves differ significantly from those of the Mg matrix, 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 inconsistent at the atomic level, which may easily generate energy differences and become active sites for dissolution and precipitation behavior.

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 amounts greater than the solid solution amount for Mg, intermetallic compounds disperse within the Mg matrix, as in Patent Document 5. As a result, the desired electrochemical properties cannot be achieved. Of course, without using 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 impurity elements.

On the other hand, the use of thinner anode materials is essential to reducing the total weight of secondary batteries and improving their efficiency. Patent Document 7 aims to improve charge/discharge characteristics by applying a polymer layer of 1 μm or more thick to a 400 μm thick anode. Patent Document 8 attempts to solve the problem by applying a metal with a high ionization tendency to bulk magnesium using a chemical plating method. Furthermore, Patent Document 9 aims to improve impedance characteristics by performing a surface treatment method on a 100 μm thick anode to create a new surface. Both methods require additional processes, such as coating, plating, and surface treatment, on the bulk material.

Another improvement is to reduce the thickness of the negative electrode material. Patent Document 10 discloses an electrochemically superior 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 perspective of casting operations.

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 that makes up the parent phase 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 parent phase may improve electrochemical properties. However, to the inventors' knowledge, there are no published or disclosed examples relating internal microstructures and electrochemical properties other than those related to crystal grain size.

JP 2012-150924 A JP 2016-96024 A JP 2014-512637 A JP 2015-515728 A JP 2012-221670 A JP 2014-143170 A JP 2015-115233 A WO2017-187700 publication JP 2014-143170 A JP 2014-164901 A PCT/JP2021/4058 publication

Bandai, Somekawa, ChemComm 56 (2020) 12122

The objective of the present invention is to provide an Mg-based alloy negative electrode material, an Mg negative electrode material, and an Mg secondary battery using these materials, which have excellent electrochemical properties and can be cycled 50 times or more in cycle measurements using a bipolar cell.

[1] The Mg-based alloy negative electrode material of the present invention is an Mg-based alloy containing at least one element selected from 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 form a solid solution 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.

[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 parent phase 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, it is preferable that they show a current density of ±10 mA cm ⁻² or more when the electrode potential is ±0.5 V in an electrochemical deposition-dissolution test of Mg metal.

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

[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.

[11] In the Mg negative electrode material [10] of the present invention, preferably, 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 parent phase 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] In the Mg negative electrode materials [10] to [14] 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.
[16] In the Mg negative electrode materials [10] to [15] 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.
[17] In the Mg negative electrode materials [10] to [16] of the present invention, it is preferable that they show a current density of ±10 mA cm ⁻² or more when the electrode potential is ±0.5 V in an electrochemical deposition-dissolution test of Mg metal.

[18] The Mg secondary battery of the present invention is an Mg secondary battery composed of any of the above Mg negative electrode materials [10] to [17], an electrolyte, and a positive electrode.

The photograph shows the appearance after rolling. 1 is an example of microstructure observation of an Mg-based alloy negative electrode material according to an embodiment of the present invention, and shows an image obtained by electron backscatter diffraction as a KAM image. This is an example of microstructure observation of an Mg alloy negative electrode material, which shows a comparative example of the present invention, and an image obtained by electron backscatter diffraction is shown as a KAM image. 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. 1 shows cycle voltage-current results in an example of electrochemical deposition-dissolution test of an Mg-based alloy negative electrode material showing an example of the present invention and a comparative example. 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. 1 is a schematic diagram showing a general configuration of an Mg secondary battery in which the Mg-based alloy negative electrode material of the present invention is used.

The Mg-based alloy material for achieving the effects of the present invention consists of Mg-A mol% X, where X is an element that dissolves in Mg at 0.05 mol% or more, and X is at least one 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 an Mg-based alloy material, it consists of inevitable components such as iron, nickel, silicon, and copper, with the remainder being Mg, and it is generally desirable for the purity of the Mg to be 99% or higher.

The internal microstructure and thickness of the above Mg-based alloy material are controlled 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, is acceptable. Of course, wrought processes are not the only option; any method capable of controlling the internal structure and simultaneously adjusting thickness during solidification, such as rapid solidification and twin-roll casting, is also acceptable. The thickness of Mg-based alloys (pure Mg and binary Mg alloys) created using 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 greater the electrochemical efficiency; therefore, a thickness exceeding 0.5 mm will result in a decrease in efficiency. Furthermore, the weight of the secondary battery increases, reducing the benefits of weight reduction.

Next, we describe the characteristics of the internal microstructure of Mg-based alloys foiled to a thickness 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 occur uniformly across the entire negative electrode surface, and heterogeneous microstructure regions consisting of coarse or fine crystal grain sizes are likely to become short-circuit sites for precipitation and dissolution, making it impossible to obtain the desired cycle characteristics and electrochemical characteristics. It is desirable to measure the crystal grain size using the section method (also known as the intercept method; see JIS H0501 and G0551) based on JIS standards. However, when the crystal grain size is small or the crystal grain boundaries are unclear, it is difficult to use the section method. Therefore, measurement can be performed using bright-field images or electron backscatter diffraction images obtained by a transmission electron microscope.

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 and widely used methods for refining the Mg matrix using plastic processing. 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, existing as residual stress. In the creation of metallic materials, not just 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 is prone to become an activation site during charge and discharge, acting 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.

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. This 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 will not diffuse sufficiently, making it difficult to obtain the desired microstructure. Furthermore, if the holding time exceeds 24 hours, the holding time is too long, which is undesirable from the standpoint of work efficiency. Residual stress is measured by acquiring an electron backscatter diffraction image and then using the accompanying analysis software to generate a kernel average misorientation (KAM) image or dislocation density image. The smaller the KAM value, the smaller the residual stress, but if the average KAM value is 1 degree or more, it can be assumed that residual stress is present. Another method is to focus on a single Mg crystal grain, and if there is a difference in KAM value of 0.5 degrees or more, it can also be assumed that residual stress is present. When observation using electron backscatter diffraction is difficult, evaluation can also be performed using the Williamson-Hall method, which is based on X-ray diffraction.

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 interfacial energy of deformation twins is stable, so from the perspective of improving electrochemical properties, it is undesirable to introduce a high density of twin interfaces into the Mg matrix. Therefore, the total interface length obtained by electron backscatter diffraction is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less.

The electrochemical properties of Mg alloys are described below. In cycle measurements using a bipolar cell, the alloy preferably exhibits 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, the alloy cannot be used as a negative electrode due to side reactions with the electrolyte. Furthermore, the overvoltage that can be obtained in a chemical deposition/dissolution test 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 significant, making the alloy unsuitable for use as a negative electrode. In an electrochemical deposition/dissolution test, the current value is preferably ±10 mAcm ^{−2 or more, more preferably ±20 mAcm −2 or} more, and even more preferably ±25 mAcm ⁻² or more when the electrode potential is ±0.5 V. If the current value is less than ±10 mAcm ⁻² , the electric reaction is rate-limited by the negative electrode, making the alloy unsuitable for use as a negative electrode. The electrolyte used in the cycle measurement test and the electrochemical deposition/dissolution test is preferably an electrolytic solution in which a fluoroalkoxy magnesium borate salt or a fluoroalkoxy magnesium aluminate salt is used as a solute at a concentration of 0.1 mol/L to 1.5 mol/L and blended with an organic ether, but alkyl magnesium chloride, magnesium chloride, magnesium bistrifluoromethanesulfonylimide salt, magnesium hexamethyldisilazide salt, or the like may be used as a substitute for the solute.

The magnesium tetrakis(hexafluoroisopropoxy)borate salt used in this embodiment can also be synthesized by reacting magnesium borohydride salt with hexafluoroisopropanol. However, electrolytes containing magnesium tetrakis(hexafluoroisopropoxy)borate salt synthesized from magnesium borohydride salt as a raw material and blended with organic ethers as a solute do not exhibit sufficient characteristics 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 salt 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)

Commercially available pure Mg (99.96 mass%) casting material was extruded to eliminate casting defects and achieve plate-shaped control. Extrusion 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 to 400°C for at least 15 minutes, and then rolled to a thickness of 0.05 mm using a rolling mill with a roll temperature set to at least 300°C. A photograph of the appearance of the resulting rolled material and a photograph of measurements taken with a micrometer are shown in Figure 1.

Figure 2 shows an example of the microstructure of a rolled pure Mg material obtained using electron backscatter diffraction. 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 degrees or greater, are indicated by black lines. Each cluster represents a single grain, denoted by G in the image, with an average grain size of 22.2 μm. The image also shows 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, the rolled material shown in the example only exhibits a small proportion of grains exhibiting these characteristics (average KAM value of 0.94 degrees), demonstrating low residual stress. Furthermore, as shown by the arrow, focusing on a single crystal grain reveals a contrast within the matrix. This suggests the presence of residual stress, but the KAM value is 0.2 degrees, indicating that the residual stress is extremely small. Furthermore, interface orientation analysis using EDAX analysis software revealed no deformation twins in the image.

[Comparative Example 1]
Using the extruded material of Example 1, rolled materials were 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 (EBSD). As with Figure 2, the image was converted into a KAM image using EDAX analysis software, with grain boundaries clearly 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 degrees, confirming a residual stress more than 30% higher than that of the rolled material of Example 1. Furthermore, a clear contrast (indicated by an arrow) was formed within each grain, and the KAM value was 0.8 degrees, again confirming the presence of residual stress. This is due to the lower rolling temperature compared to the example, resulting 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%.

The rolled material of Example 1 was cut into 10 x 10 mm2 ^pieces and then held in an electric furnace set to 300°C for 1 hour (hereinafter referred to as the heat-treated material). Figure 4 shows an example of the microstructure of the heat-treated pure Mg material, obtained by optical microscopy. The appearance of the Mg matrix phase has changed significantly from Figure 3, and coarsening of the average size and homogenization (a decrease in standard deviation) can be confirmed. Reheating at 300°C or higher is effective in reducing residual stress and homogenizing the structure.
(Electrochemical property evaluation)

Next, the characteristics obtained through electrochemical evaluation are described. All operations, including preparation of the electrolyte and assembly of the two-electrode cell, were performed in a glove box under an argon atmosphere. Tetrakis(hexafluoroisopropoxy)borate magnesium salt was blended 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, and a porous carbon material as the counter electrode. A constant-current voltage test was performed at a current density of 0.5 mA ^{cm −2} . Figure 5 shows the constant-current voltage measurement profiles of the materials in Example 1 and Comparative Example 1. In the constant-current voltage measurement profiles, with 0 V vs. Mg as the reference, a magnesium deposition reaction occurs in the region where a negative voltage is applied, and a magnesium dissolution reaction occurs in the region where a positive voltage is applied. Furthermore, for both the deposition and dissolution reactions, deviations from 0 V vs. Mg correspond to overvoltages. 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.

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

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

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 MgFeSiO ₄ , MgMn ₂ O ₄ , or V ₂ O ₅ is preferably used. A specific configuration of the positive electrode 11 may be, for example, a configuration in which V ₂ O ₅ is coated on stainless steel.

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 (e.g., the reaction of formula (1) described below) at the positive electrode 11 and an oxidation reaction (e.g., the reaction of formula (2) described below) at the negative electrode 12. During charge, the Mg ions undergo an oxidation reaction (e.g., the reaction of formula (3) described below) at the positive electrode 11 and a reduction reaction (e.g., 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.

[Chemical formula 1]
V ₂ O ₅ +Mg ²⁺ +2e- → MgV ₂ O ₅ ... Formula (1)
Mg → Mg ²⁺ +2e- … Formula (2)
MgV ₂ O ₅ → V ₂ O ₅ +Mg ²⁺ +2e-... Formula (3)
Mg ²⁺ +2e- → Mg… Formula (4)

The positive electrode 11, negative electrode 12, and electrolyte 13 are sealed in a container 14. There are no particular restrictions on the material of the container 14, as long as it is corrosion-resistant and does not leak the electrolyte. However, a container formed by pressing a metal plate such as iron, with a plating layer of nickel or the like formed on the entire inner and outer surfaces for corrosion resistance, is preferably used.

The electrolyte solution 13 according to this embodiment may contain an organic solvent as the main solvent and a magnesium salt, or may be an inorganic solvent with magnesium ion conductivity.

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

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.

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

Claims (8)

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. 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. 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. The Mg negative electrode material according to claim 1 , wherein no deformation twin crystals are present in the Mg matrix. 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. 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. The Mg negative electrode material according to any one of claims 1 to 6 , characterized in that it shows a current density of ±10 mA cm ^-2 or more when the electrode potential is ±0.5 V in an electrochemical deposition-dissolution test of Mg metal. 8. A Mg secondary battery comprising the Mg negative electrode material according to claim 1 , an electrolyte, and a positive electrode.