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

Abstract

To provide a Mg-based negative electrode material that has excellent electrochemical characteristic.SOLUTION: Provided is a Mg-based negative electrode material excellent in electrochemical characteristic, which is a Mg-based composite material containing one or more of carbon, carbide, oxide and nitride, and in which the average crystal grain size of the Mg base material is 1000 μm or less and the particles are dispersed in the Mg mother phase or the crystal grain boundary, or in both, the cycle measurement test result is 50 times or more, the over-voltage in the electrochemical deposition dissolution test is 30 mV or less, and a current value of ±10 mAcm-2 or more is exhibited at an electrode potential of ±0.5 V in the deposition dissolution test.SELECTED DRAWING: Figure 4A

Description

[Technical Field]

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

[0002] Magnesium (Mg) metal has attracted attention as a potential anode material for high-energy-density batteries because it exhibits one of the highest theoretical capacity densities among practically used metals and does not form dendritic deposits during electrodeposition. However, unlike lithium (Li) and sodium (Na) metal anodes, magnesium metal anodes are less likely to form an ion-conductive film at the interface with the electrolyte and are prone to passivation, making them less susceptible to reversible dissolution-precipitation behavior.

[0003] Typically, storage batteries, including secondary batteries, include a cathode active material and an electrolyte in addition to an anode material. Therefore, as a means to solve the above-mentioned problems, improvements have been made to components other than the anode material. For example, as disclosed in Patent Documents 1 and 2 and Non-Patent Document 1, cathode active materials and electrolytes effective for Mg secondary batteries have been developed. Turning to the anode material itself, its constituent materials can be broadly divided into two types: Mg metal anode materials (and Mg-based alloy anode materials) and intermetallic compounds containing Mg. Patent Documents 3 and 4 attempt to solve the above-mentioned problems by utilizing an Mg-Bi intermetallic compound (Mg3Bi2) or an Mg-Sn intermetallic compound (Mg2Sn) as the negative electrode material. However, intermetallic compound negative electrodes have the problem of a higher potential and lower capacity density compared to Mg metal negative electrodes (and Mg-based alloy negative electrodes). Furthermore, in order to form these intermetallic compounds, it is necessary to add high concentrations of Bi (bismuth) or Sn (tin), which have higher densities than Mg, which is undesirable from the perspective of reducing the weight of the battery.

[0004] On the other hand, Patent Document 5 discloses an Mg metal negative electrode and an Mg-based alloy negative electrode in which Mg metal is used as the negative electrode without utilizing an intermetallic compound. Patent Document 5 lists Mg-Al alloys, Mg-Zn alloys, and Mg-Mn alloys as Mg-based alloys that constitute the negative electrode, but only an Mg-6 mass% Al alloy is shown in the examples. In this alloy, from the viewpoint of thermal equilibrium and metal structure, the high concentration of Al added results in the formation of Mg. 17 Al 12 Many intermetallic compounds, such as Al, are densely dispersed within the Mg matrix. Of course, since Al is consumed in the formation of intermetallic compounds, it is self-evident that solute elements do not segregate at the grain boundaries. Patent Document 6 discloses Mg metal anodes and Mg-based alloy anodes suitable for Mg secondary batteries containing 90% or more Mg by mass. Examples of possible minor components include aluminum (Al), zinc (Zn), manganese (Mn), silicon (Si), calcium (Ca), iron (Fe), copper (Cu), and nickel (Ni). However, the internal microstructure of Mg alloys varies significantly depending on the manufacturing method. For example, solute elements cannot be segregated to grain boundaries without the use of wrought processing techniques such as extrusion and rolling. Furthermore, the maximum solid solubility of Si, Fe, Cu, and Ni in Mg is 0.01 mol% or less, which is not enough to affect mechanical or functional properties. Therefore, they are generally treated as unavoidable impurities. [Prior art documents] [Patent documents]

[0005] [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

[0006] [Non-Patent Document 1] Bandai et al., Phys. Chem. Chem. Phys., 2019, 21, 12100 doi: 10.1039 / c9cp01400d [Non-patent document 2] Bandai, ACS Appl. Mater. Interfaces, 2020, 12, 39135 doi: 10.1021 / acsami.0c09948 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide an Mg-based composite material in which Mg and an Mg-based alloy contain particles of one or more types of oxide, nitride, carbide, and carbon, the thickness of which is 1 mm or less, and in which the particles are dispersed in either or both of the grain boundaries and the Mg parent phase, and which has excellent electrochemical properties, and an Mg secondary battery using the Mg-based negative electrode material. [Means for solving the problem]

[0008] The first aspect of the present invention provides an Mg-based composite negative electrode material, which contains particles of one or more types of material selected from the group consisting of carbide, nitride, oxide, and carbon, and which are dispersed in either or both of the Mg matrix and grain boundaries.

[0009] A second aspect of the present invention is the Mg-based composite material according to the first aspect, wherein the oxide is Al2O3 (alumina), Y2O3 (yttria), ZrO2 (zirconia), TiO2 (titania), SiO2 (silica), or CuO (copper oxide); the nitride is GaN (gallium nitride) or LiN (lithium nitride), The carbide is TiC (titanium carbide) or SiC (silicon carbide), The carbon may be C (graphite), C 60 We provide Mg-based negative electrode materials that are either fullerenes or carbon nanotubes (CNTs). A third aspect of the present invention provides an Mg-based negative electrode material according to the first or second aspect of the present invention, wherein the Mg-based composite material comprises an Mg-based alloy selected from the group consisting of an Mg-Al-based alloy, an Mg-Zn-based alloy, an Mg-Ca-based alloy, an Mg-Bi-based alloy, and an Mg-Mn-based alloy. A fourth aspect of the present invention provides an Mg-based negative electrode material, which is the Mg-based composite material according to any one of the first to third aspects, and has a thickness of 1 mm or less. A fifth aspect of the present invention provides an Mg-based negative electrode material, which is an Mg-based composite material according to any one of the first to fourth aspects, characterized in that the average size of the Mg matrix measured by the intercept method is 10 μm or more, and the dispersed volume fraction of particles of one or more types of carbide, nitride, oxide, and carbon is 25% or less of the total Mg-based composite material.

[0010] A sixth aspect of the present invention provides an Mg-based negative electrode material, which is the Mg-based composite material according to any one of the first to fifth aspects, and which exhibits cycle characteristics of 50 or more cycles in cycle measurement using a three-electrode cell. A seventh aspect of the present invention provides an Mg-based negative electrode material according to any one of the first to sixth aspects, characterized in that the material exhibits an overvoltage of 30 mV or less in an electrochemical deposition-dissolution test of Mg metal. The eighth aspect of the present invention is the Mg-based negative electrode material according to any one of the first to seventh aspects, wherein in an electrochemical deposition and dissolution test of Mg metal, the negative electrode material has a current density of ±10 mAcm when the electrode potential is ±0.5 V. -2 The present invention provides an Mg-based negative electrode material that exhibits the above current density.

[0011] A ninth aspect of the present invention provides an Mg secondary battery comprising the Mg-based negative electrode material according to any one of the first to eighth aspects, an electrolyte, and a positive electrode. A tenth aspect of the present invention provides the Mg secondary battery according to the ninth aspect, characterized in that the battery exhibits cycle characteristics of 10 or more cycles in cycle measurement. [Brief explanation of the drawings]

[0012] [Figure 1] This is an example of microstructure observation of Mg-Al2O3 (alumina) according to an embodiment of the present invention, and shows the case where it was performed using the electron backscatter diffraction technique. [Figure 2] This is an example of microstructure observation of Mg-Y2O3 (yttria) showing one embodiment of the present invention, and shows the case where it was performed by electron backscatter diffraction technique. [Figure 3] Example of the appearance of Mg-based negative electrode material after electrochemical testing. [Figure 4A] 1 shows the results of the coulombic efficiency of an Mg-based negative electrode material according to an embodiment of the present invention, in the case of pure Mg. [Figure 4B] 1 shows the results of the Coulomb efficiency of an Mg-based negative electrode material according to one embodiment of the present invention, in the case of Mg-CNT (carbon nanotube). [Figure 5A] 1 shows the cycle voltage time results of an Mg-based negative electrode material according to an embodiment of the present invention, in the case of an oxide composite material. [Figure 5B] 1 shows the cycle voltage time results of an Mg-based negative electrode material according to an embodiment of the present invention, in the case of Mg-CNT (carbon nanotube). [Figure 6] 1 is a schematic diagram showing a schematic configuration of an Mg secondary battery in which the Mg-based negative electrode material of the present invention is used. DETAILED DESCRIPTION OF THE INVENTION

[0013] The Mg-based material for achieving the effects of the present invention has at least one of oxide, nitride, carbide, and carbon dispersed in either or both of the Mg matrix and grain boundaries of Mg and Mg-based alloys. Examples of oxides include Al2O3 (alumina), Y2O3 (yttria), ZrO2 (zirconia), TiO2 (titania), SiO2 (silica), and CuO (copper oxide). Examples of nitrides include GaN (gallium nitride) and Li3N (lithium nitride). Examples of carbides include TiC (titanium carbide) and SiC (silicon carbide). Examples of carbon include C (graphite) and C 60(fullerene), CNT (carbon nanotube), etc. The content of these compounds is 25% or less, preferably 20% or less, and more preferably 15% or less, of the entire Mg-based composite. If the dispersion amount exceeds 25%, the wettability and adhesion with Mg decrease, making peeling at the Mg interface more likely to occur during stretching, making it difficult to create a bulk body of 1 mm or less. Furthermore, if the dispersion amount is too high, the above-mentioned compounds will inhibit the unique electrochemical properties of Mg, making it difficult to obtain excellent electrochemical properties.

[0014] The thickness of the Mg-based bulk body after wrought processing is preferably 1 mm or less, more preferably 0.5 mm or less, and even more preferably 0.3 mm or less. Wrought processing refers to processes that impart shear strain to a bulk body, such as extrusion, rolling, and forging. 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 1 mm results in a decrease in efficiency. Furthermore, the weight of the secondary battery increases, reducing the benefit of weight reduction. Furthermore, the crystal grain size of the Mg matrix after wrought processing is preferably 1000 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less. Grain boundaries are likely to serve as sites for Mg dissolution. Therefore, when an Mg-based composite with a thickness of 1 mm (= 1000 μm) and a grain size exceeding 1000 μm is used as a negative electrode, the volume fraction of the grain boundaries present within the negative electrode is extremely small. This makes it difficult for uniform dissolution behavior to occur, making it difficult to obtain the desired electrochemical characteristics. The grain size can be measured using the JIS-based sectioning method (also known as the intercepting method; see JIS H0501 and G0551) or by automatic measurement using the accompanying analysis software (e.g., EDAX-TSL ver. 7) from images acquired by electron backscatter diffraction. When the grain size is small or the grain boundaries are unclear, it is difficult to use the sectioning method, so measurements can also be made using bright-field images obtained with a transmission electron microscope.

[0015] The dispersion state of compound particles such as oxides, nitrides, and carbides, as well as carbon particles, dispersed in the Mg-based composite after wrought processing can be confirmed using an optical microscope or electron backscatter diffraction. The number density of dispersed particles, i.e., the dispersion ratio, is preferably measured using the point counting method using images acquired by an optical microscope. Alternatively, the dispersion ratio can be obtained from images acquired by electron backscatter diffraction using the accompanying analysis software.

[0016] The particles added to Mg-based composites do not dissolve or react with molten Mg, making them unsuitable for casting methods such as gravity casting, sand casting, and die casting. Therefore, Mg powder must be mixed and stirred with the compound powder or carbon powder, followed by bonding and sintering at warm or hot temperatures. All powders must be sized between 0.5 and 1000 microns, preferably between 0.5 and 500 microns, and more preferably between 0.5 and 100 microns. Starting powders exceeding 1000 microns reduce the surface area between the powders, hindering bonding and sintering and preventing the production of a sound bulk body. On the other hand, powder sizes less than 0.5 microns offer the advantage of increased surface area, but are undesirable from a work safety perspective, as the Mg powder is more susceptible to oxygen and may ignite during mixing and stirring. However, the bonding and sintering of powders may be achieved not only by the powder sintering method described above, but also by utilizing the wrought processing method to bond the powders and obtain an Mg-based composite material.Also, a billet may be created using a semi-melted solidification process, and then the Mg-based composite material may be obtained by wrought processing.

[0017] The electrolyte used in this embodiment will be described. The electrolyte contains a magnesium salt and an organic solvent. The magnesium salt dissociates when dissolved in the organic solvent, forming a magnesium complex cation coordinated with the organic solvent. This complex cation is responsible for the electrochemical magnesium deposition and dissolution activity, so good electrochemical properties tend to be obtained when a magnesium salt with higher dissociation properties is used. Electrolytes that exhibit good electrochemical properties include those containing a fluoroalkoxy borate magnesium salt or a fluoroalkoxy aluminate magnesium salt blended with organic ethers.

[0018] The Mg salt used in this embodiment has a fluoroalkoxyborate anion or a fluoroalkoxyaluminate anion as a counter anion. The fluoroalkoxy group of the Mg salt used in this embodiment is not particularly limited. The Mg salt used in this embodiment is tetrakis(hexafluoroisopropoxy)borate Mg salt or tetrakis(hexafluoroisopropoxy)aluminate Mg salt.

[0019] The electrochemical properties of Mg alloys are described below. In cycle measurements using a bipolar cell, the alloys preferably exhibit cycle performance of 5 or more, more preferably 25 or more, and even more preferably 50 or more. A cycle performance of less than 5 cycles is problematic due to side reactions with the electrolyte, making them unsuitable for use as a negative electrode. Furthermore, the overvoltage that can be achieved in chemical deposition / dissolution tests is preferably 100 mV or less, more preferably 80 mV or less, and even more preferably 50 mV or less. Overvoltages of 80 mV or more result in significant loss of battery voltage, making them unsuitable for use as a negative electrode. The electrolyte used in cycle measurement tests and electrochemical deposition / dissolution tests is preferably an electrolyte containing 0.1 mol / L to 1.5 mol / L of fluoroalkoxyborate magnesium salt or fluoroalkoxyaluminate magnesium salt as a solute, blended with an organic ether. However, alkyl magnesium chloride, magnesium chloride, magnesium bistrifluoromethanesulfonylimide salt, magnesium hexamethyldisilazide salt, etc. may also be used as the solute.

[0020] 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]

[0021] Commercially available pure Mg powder (powder diameter: 180 μm) and commercially available Al2O3 powder (powder diameter: 2–3 μm) were used. The Al2O3 powder was weighed to correspond to an area ratio of 10% of Al2O3 particles dispersed in the Mg matrix of the Mg-based composite, and dry-mixed with the Mg powder in a mortar. To fill the Mg and Al2O3 mixed powder, a commercially available Mg alloy (Mg-3Al-1Zn; AZ31) material with an outer diameter of 40 mm and a length of 70 mm was machined to create a cup-shaped extrusion billet. After the mixed powder was filled into the extrusion billet, it was sealed with an Mg alloy (AZ31) material with a diameter of 20 mm and a thickness of 5 mm. After that, the material was kept in a container set at 250°C for more than 30 minutes, and then hot-extended by extrusion at an extrusion ratio of 16:1 to produce an extruded material with a diameter of 10 mm and a length of 500 mm or more.

[0022] Except for the added powder, the Mg powder was filled into a billet and extruded in exactly the same manner as in the above example. The Mg powder had an average powder diameter of 180 μm, and the compound powders including yttria and carbon powders had average powder diameters of 1 to 5 μm.

[0023] Figures 1 and 2 show examples of microstructure observations of Mg-Al2O3 composite and Mg-Y2O3 obtained by electron backscatter diffraction. Figure 1 shows an example of the microstructure observation of Mg-Al2O3 (alumina) according to one embodiment of the present invention. (A) is a microstructure photograph obtained by electron backscatter diffraction, and (B) is a diagram showing the relationship between crystal orientation and shading, showing the Miller indices (0001), (1-21-0), and (011-0) for the hexagonal crystal. Here, the "1-" in the Miller indices represents an overline of 1. (C) is also a diagram showing the relationship between crystal orientation and shading, showing the Miller indices (0001), (1-21-0), and (101-0) for the hexagonal crystal. Figure 2 shows an example of the microstructure of Mg-Y2O3 (yttria) according to one embodiment of the present invention. (A) is a microstructure photograph taken by electron backscatter diffraction, (B) is a diagram showing the relationship between crystal orientation and shading, showing the Miller indices (0001), (1-21-0), and (011-0) for a hexagonal crystal. (C) is also a diagram showing the relationship between crystal orientation and shading, showing the Miller indices (001), (010), and (111) for a cubic crystal. A lump of the same contrast is a single crystal grain made of the Mg phase, and its size is 20 μm. Regardless of the type of compound or carbon added, the average crystal grain size is 100 μm or less in all cases.

[0024] The Mg powder was filled into a billet and extruded in exactly the same manner as in the above example, except that only Mg powder was used without adding powders of compounds or carbon. The average powder diameter of the Mg powder was 180 μm. The average size of the crystal grains consisting of the Mg phase after extrusion was measured by electron backscatter diffraction and was found to be 20 μm. Hereinafter, this will be referred to as pure Mg. (Electrochemical evaluation)

[0025] 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 carried out at a current density of 1000 kJ / s.

[0026] The constant current voltage measurement profile of the composite is shown in Figures 4A and 4B, and the Coulombic efficiency of the magnesium precipitation dissolution reaction is shown in Figures 5A and 5B. In the constant-current voltage measurement profile shown in Figure 4A, the magnesium precipitation 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, with 0 V vs. Mg as the reference. Furthermore, for both the precipitation and dissolution reactions, the deviation from 0 V vs. Mg corresponds to the overpotential. The Coulomb efficiency can be calculated by taking the ratio of the current required for magnesium precipitation to the current required for magnesium dissolution in one precipitation-dissolution cycle. As can be seen from Figure 4B, the composite exhibited magnesium precipitation-dissolution characteristics similar to those of pure magnesium metal, indicating that the composite components did not inhibit the magnesium precipitation-dissolution reaction. In particular, when CNTs (carbon nanotubes) were added to the composite, the precipitation-dissolution overpotential was lower than that of pure Mg, promoting the precipitation-dissolution reaction.

[0027] Figures 5A and 5B show the Coulombic efficiency calculated from constant current voltage measurements of various composite materials. Coulombic efficiency differs depending on the type of composite material, and composites of oxides such as Al2O3 and SiO2 had a lower Coulombic efficiency than pure Mg material. On the other hand, when CNT was added to the composite, a higher Coulombic efficiency was shown than pure Mg material. While a decrease in Coulombic efficiency was confirmed after 30 cycles for pure Mg material, it was found that the magnesium precipitation and dissolution reaction occurred extremely stably for over 100 cycles for the CNT composite.

[0028] 6 is a schematic diagram showing the general configuration of an Mg secondary battery using the Mg-based negative electrode material of the present invention. As shown in the figure, 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 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.

[0029] The negative electrode 12 is made of the Mg-based 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.

[0030] [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)

[0031] 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.

[0032] 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.

[0033] 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]

[0034] The Mg-based alloy of the present invention exhibits excellent electrochemical properties and can therefore be used as an Mg-based negative electrode material for Mg secondary batteries as well as Mg primary batteries. The Mg-based 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 negative electrode material of the present invention can be used as a lightweight foil material that can replace aluminum foil. [Explanation of symbols]

[0035] 1...Mg secondary battery 11...Positive electrode 12...Negative electrode 13...Electrolyte 14...Container

Claims

1. An Mg-based composite material, comprising particles of one or more types of carbide, nitride, oxide, and carbon, the particles being dispersed in either or both of an Mg matrix phase and grain boundaries, The oxide is Al ２ O ３ (Alumina), Y ２ O ３ (yttria), ZrO ２ (zirconia), TiO ２ (Titania), SiO ２ (silica), or CuO (copper oxide), The nitride is GaN (gallium nitride) or Li ３ N (lithium nitride), the carbide is TiC (titanium carbide) or SiC (silicon carbide), The carbon may be C (graphite), C ６０ (fullerene) or CNT (carbon nanotube), The Mg-based composite material contains an Mg-based alloy in the Mg matrix, the Mg-based alloy being selected from the group consisting of an Mg—Al-based alloy, an Mg—Zn-based alloy, an Mg—Ca-based alloy, an Mg—Bi-based alloy, and an Mg—Mn-based alloy, the thickness of the Mg-based composite material is 1 mm or less, and the average size of the crystal grains in the Mg matrix measured by a section method is 10 μm or more and 100 μm or less, 1. A Mg-based negative electrode material, characterized in that the volume fraction of dispersed particles of one or more types of carbide, nitride, oxide and carbon is 25% or less with respect to the entire Mg-based composite material.

2. The Mg-based negative electrode material according to claim 1, which exhibits cycle characteristics of 50 or more in cycle measurements using a bipolar cell, The cycle measurement using the bipolar cell is The electrolyte is prepared by blending magnesium tetrakis(hexafluoroisopropoxy)borate with diethylene glycol dimethyl ether. A two-electrode cell was assembled using the Mg-based composite as a working electrode, a glass fiber filter as a separator, 0.2 mL of the electrolyte solution was dropped onto the filter, and a porous carbon material as a counter electrode. A constant current voltage test is carried out at a current density of 0.5 mA cm −2 . A magnesium-based negative electrode material characterized by:

3. 2. The Mg-based 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, and The electrochemical deposition and dissolution test of the Mg metal was carried out using an electrolyte prepared by mixing 0.1 mol / L or more and 1.5 mol / L or less of tetrakis(hexafluoroisopropoxy) magnesium borate with diethylene glycol dimethyl ether, and using a two-electrode cell in which an Mg alloy was used as the working electrode, a glass fiber filter was used as the separator, 0.2 mL of the electrolyte was dropped onto the separator, and a porous carbon material was used as the counter electrode. －２ A Mg-based negative electrode material characterized in that it is subjected to a constant current voltage test at a current density of 1000 kJ / cm2.

4. 2. The Mg-based negative electrode material according to claim 1, wherein in an electrochemical deposition and dissolution test of Mg metal, the current density is within ±10 mA cm when the electrode potential is ±0.5 V. －２ The above current density is shown, The electrochemical deposition and dissolution test of the Mg metal was carried out using an electrolyte prepared by mixing 0.1 mol / L or more and 1.5 mol / L or less of tetrakis(hexafluoroisopropoxy) magnesium borate with diethylene glycol dimethyl ether, and using a two-electrode cell in which an Mg alloy was used as the working electrode, a glass fiber filter was used as the separator, 0.2 mL of the electrolyte was dropped onto the separator, and a porous carbon material was used as the counter electrode. －２ A Mg-based negative electrode material characterized in that it is subjected to a constant current voltage test at a current density of 1000 kJ / cm2.

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

6. 6. The Mg secondary battery according to claim 5, characterized in that it exhibits cycle characteristics of 10 or more cycles in cycle measurement.

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