Magnesium battery negative electrode material

Abstract

The objective is to provide a negative electrode material for magnesium batteries that can also utilize recycled materials as raw materials. [Solution] The material is characterized by the following mass percentages: Al: 8.0-10.0%, Si: 0.1-1.0%, Fe: 0.2% or less, Cu: 0.4% or less, Zn: 0.5% or less, Mn: 0.4% or less, with the remainder being Mg and unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a negative electrode material for magnesium batteries, and particularly relates to a negative electrode material that can use recycled materials as part of its raw materials.

Background Art

[0002] Batteries using magnesium have been widely studied as primary batteries and secondary batteries. Magnesium batteries use a magnesium alloy for the negative electrode, and an oxide, a carbon material or the like for the positive electrode. Compared with lithium, magnesium has a large electric capacity per unit volume and a high melting point, and thus is expected to have high safety. In addition, magnesium is more abundant as a resource than lithium, and its widespread use in the future is anticipated. However, the lower electrochemical activity of magnesium alloys compared to lithium metals is a technical challenge.

[0003] As magnesium alloys used as negative electrode materials, alloys of magnesium and aluminum have been studied so far. For example, Patent Document 1 describes that although it is a primary battery, an AZ31 (Mg-3%Al-1%Zn) alloy is used as the negative electrode material. Non-Patent Document 1 reports the effect of Cu addition on electrochemical activity using a Mg-Al-Ca based alloy. This Non-Patent Document 1 is a report by some of the inventors of the present application, and 99.9% high-purity aluminum and 99.9% high-purity calcium are used in the production of the magnesium alloy.

[0004] In recent years, the utilization of recycled materials has been studied from the perspectives of reducing environmental impact and effective use of resources. For example, for aluminum used in the production of magnesium alloys, it is said that recycled materials can be produced with about 3% of the electric power compared to virgin materials newly refined from bauxite. Aluminum is highly recyclable, but during collection from the market, it inevitably becomes contaminated with other metals and alloys of different compositions. Therefore, it is desirable to be able to separate these contaminating components. However, in manufacturing processes such as remelting aluminum, the inclusion of Si and Fe components is unavoidable, and there is a growing expectation for the utilization of recycled materials containing these components. Furthermore, if the acceptable range of impurities in magnesium, calcium, etc., used in the manufacture of negative electrode materials is broadened, costs can be reduced accordingly. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-167365 [Non-patent literature]

[0006] [Non-Patent Document 1] Shunsaku Fukui, et al., "Effect of Cu Addition on the Electrochemical Activity of Mg-Al-Ca Negative Electrode Strips for Magnesium Batteries by Rapid Cooling and Solidification Method," Light Metals, Vol. 74, No. 1 (2024), pp. 14-17. [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a negative electrode material for magnesium batteries that can also utilize recycled materials as raw materials. [Means for solving the problem]

[0008] The negative electrode material for magnesium batteries according to the present invention is characterized by having the following mass percentages: Al: 8.0-10.0%, Si: 0.1-1.0%, Fe: 0.2% or less, Cu: 0.4% or less, Zn: 0.5% or less, Mn: 0.4% or less, with the remainder being Mg and unavoidable impurities.

[0009] The reason for expressing Si: 0.1~1.0%, Fe: 0.2% or less, Cu: 0.4% or less, Zn: 0.5% or less, and Mn: 0.4% or less here is that these components are acceptable to be included in Mg-Al-Ca-based anode materials as long as they fall within these ranges. In particular, the electrochemical activity is superior when the silicon component is actively added.

[0010] For example, aluminum matrix alloys referred to as 90% Al material are supplied as aluminum materials obtained from recycled materials containing 8-9% Si. Our research has revealed that materials containing a certain amount of Si exhibit superior electrochemical activity. Therefore, the Si component is in the range of 0.1 to 1.0%, preferably in the range of 0.2 to 1.0%. More preferably, the range is 0.3 to 1.0%. Furthermore, investigations into the effects of components other than Si revealed that the composition may be as follows (by mass%): Al: 8.0-10.0%, Si: 1.0% or less, and containing one or more of Mn: 0.1-0.4%, Cu: 0.1-0.4%, and Zn: 0.1-0.5%, with the remainder consisting of Mg and unavoidable impurities. Expanding the acceptable range for these components will lower the manufacturing cost of the negative electrode material. [Effects of the Invention]

[0011] The anode material for magnesium batteries according to the present invention is an Mg-Al-Ca alloy with excellent flame retardancy and improved electrochemical activity through grain refinement. By adding Si, an anode material with even better electrochemical activity and improved mechanical properties can be obtained. In particular, the ability to use recycled magnesium and aluminum in the production of the negative electrode material makes it promising for reducing environmental impact and as an energy-saving material. [Brief explanation of the drawing]

[0012] [Figure 1]The table of the alloy components used for evaluation is shown. [Figure 2] The charge-discharge cycle test results are shown. [Figure 3] The appearance of the negative electrode surface after the test is shown. [Figure 4] The X-ray diffraction results are shown. [Figure 5] The basal plane orientation degree is shown. [Figure 6] The Knoop hardness test results are shown. [Figure 7] The alloy compositions used for comparative evaluation are shown. [Figure 8] The cycle characteristic evaluation results are shown. [Figure 9] The cycle characteristic evaluation results are shown. [Figure 10] The evaluation results of the rolled material are shown. [Figure 11] The cycle test conditions are shown. [Figure 12] The rolling conditions are shown. [Mode for Carrying Out the Invention]

[0013] Hereinafter, test specimens were prepared and evaluated, and the description is provided below. The raw materials used were various high-purity metals (99.9%) of magnesium, aluminum, and calcium, as well as a 90%Al master alloy which is a recycled material. These were weighed to achieve the target composition, melted in a crucible made of SUS430 stainless steel alloy, and cast in a cylindrical mold. The component composition of the test specimens used for evaluation is shown in the table of Figure 1. The content of each component is in mass% and is a value obtained by ICP emission spectroscopic analysis. 99.9% grade materials were used for Mg and Ca, only Alloy 1 used the 90%Al master alloy, and Comparative Alloy 1 used 99.9% pure aluminum. The ribbon was produced using a single-roll rapid solidification apparatus. The billet for melting was cut to about 20 g, inserted into a nozzle spaced 1.4 mm away from the cooling roll, and heated using a high-frequency induction coil. Subsequently, argon gas at a pressure of 0.08 MPa was injected onto a φ200 mm × 55 mm oxygen-free copper cooling roll rotating at a peripheral speed of 25 m / s.

[0014] As shown in the table in Figure 1, Alloy 1 of Example 1 uses a recycled 90% Al matrix alloy, and therefore contains higher amounts of Si, Fe, Cu, Zn, and Mn compared to Comparative Alloy 1.

[0015] Using alloy 1 and comparative alloy 1, thin strip-shaped test specimens were prepared as described above, and their electrochemical properties were evaluated as follows. Electrochemical properties were evaluated by constant current charge-discharge testing. The negative electrode consisted of a sample cut into strips measuring 7 mm x 25 mm, the positive electrode was an activated carbon electrode with aluminum foil as the current collector, the reference electrode was high-purity magnesium foil, and the electrolyte was a solution of succinic anhydride (SA) and magnesium-bis-trifluoromethylsulfonylamide (MgTFSA2), both cyclic acid anhydrides, dissolved in NN dimethylacetamide (DMAc). The sample was activated by applying an oxidation current of 18 μA × 4h and a reduction current of 6 μA × 12h, and then subjected to 20 charge-discharge cycles by applying a discharge current of 48 μA × 1.5h and a charge current of 16 μA × 4.5h. The results of the constant current charge-discharge test are shown in Figure 2, and the appearance of the negative electrode surface after the cell was disassembled following the test is shown in Figure 3. It was revealed that alloy 1 from Example 1 exhibited more stable charge-discharge cycles and superior charge-discharge characteristics compared to comparative alloy 1, which was made using high-purity raw materials. Comparing the appearance of the negative electrode surface after testing shown in Figure 3, alloy 1 according to the present invention [Figure 3(a)] had fewer defects than alloy 3(b) which uses high-purity material.

[0016] Figure 4 shows the X-ray diffraction results, and Figure 5 shows the degree of basal orientation. In magnesium anode materials, the (0002) plane, which is the basal plane having a hexagonal close-packed crystal structure, has been reported to be electrochemically inert. Furthermore, it is presumed that preferential orientation to the (0002) plane results in low electrochemical activity. Based on the X-ray diffraction results, the ratio of the (0002) plane to the pyramidal crystal plane of magnesium shown in Figure 4 was defined as the degree of basal orientation, and is shown in Figure 5. TIFF2026165190000002.tif9166 This indicates that alloy 1 had a lower degree of basal orientation and higher electrochemical activity than comparative alloy 1. Figure 6 shows the results of the Knoop hardness test. Alloy 1 was found to have higher hardness than comparative alloy 1. Microstructural observation using FE-SEM revealed that Si-derived precipitates were found at the grain boundaries, suggesting that the hardness improved due to precipitation strengthening.

[0017] Comparing the composition of alloy 1 with comparative alloy 1, it is clear that in addition to Si, Fe, Cu, Zn, and Mn are all present in large quantities. Therefore, in order to investigate the effect of each component on the electrochemical properties, alloys with the compositions shown in the table in Figure 7 were prepared using various master alloys, and test specimens were prepared and evaluated in the same manner as described above (for ease of comparison, alloy 1 and comparative alloy 1 are also shown in Figure 7). The evaluation criteria are shown in Figure 11. In this study, a charge-discharge cycle test was conducted using a two-electrode cell with a separator interposed between the counter electrode and the working electrode, without using a reference electrode. Charging was performed slowly at 20 μA for 4.5 hours, followed by discharging at 60 μA for 1.5 hours, and this cycle was repeated. Under these conditions, repeated charging and discharging degrades the negative electrode material (Mg alloy), increasing its resistance, which in turn increases the amplitude of the voltage during charging and discharging. In this embodiment, the upper limit was set to 2.8V and the lower limit to 1.0V, and the test was configured to terminate when either the upper or lower limit was exceeded.

[0018] The test results are shown in Figures 8 and 9. Figure 8 shows the test results for alloy 2 (Example 2) and comparative alloy 1 (Comparative Example 1). While comparative alloy 1 broke down after 15 cycles, alloy 2 remained intact for approximately 38 cycles, although the amplitude of the breakdown gradually increased. Alloy 2 has a higher Si content (0.51%) compared to comparative alloy 1, and also has higher amounts of Fe, Cu, Zn, and Mn than comparative alloy 1.

[0019] As shown in Figure 7, alloy 3 (Example 3) had a Mn content of 0.110%, and the effect of Mn was investigated, but there was no significant difference compared to comparative alloy 1. As shown in Figure 7, alloy 4 (Example 4) had a Cu content of 0.28%, and the effect of Cu was investigated, yielding better results than comparative alloy 1. Alloy 5 (Example 5) contains 0.29% Si, and Alloy 6 (Example 6) contains 0.53% Si. Both performed well for 50 cycles or more, and Alloy 6 performed particularly well up to 83 cycles. This indicates that electrochemical properties improve when Si content is in the range of 0.1 to 1.0%. Alloy 7 (Example 7) contained 0.41% Zn, which was also better than comparative alloy 1.

[0020] The thickness of the negative electrode material may be adjusted depending on the application. Therefore, a thin strip with a thickness of approximately 100 μm was roll-rolled under the conditions shown in Figure 12 to reduce its thickness to approximately 60 μm. The results of evaluating this as a negative electrode material are shown in Figure 10. In comparative alloy 1, the cycle characteristics were improved by the rolling process. In alloys 2, 5, and 6, it can be seen that rolling processes result in better cycle characteristics than those before rolling. In particular, when comparing the cycle characteristics of alloy 6 (Si: 0.53%) shown in Figure 9 with those of alloy 6 (rolled) with the same Si content but rolled, as shown in Figure 10, the amplitude of alloy 6 (rolled) is more stable, indicating that the electrochemical properties are further improved by the rolling process.

Claims

1. A negative electrode material for a magnesium battery, characterized by the following mass percentages: Al: 8.0-10.0%, Si: 0.1-1.0%, Fe: 0.2% or less, Cu: 0.4% or less, Zn: 0.5% or less, Mn: 0.4% or less, with the remainder being Mg and unavoidable impurities.

2. The negative electrode material for magnesium batteries according to claim 1, characterized in that the Si content is in the range of 0.2 to 1.0%.

3. A negative electrode material for a magnesium battery, characterized in that, in mass percent, it contains Al: 8.0 to 10.0%, Si: 1.0% or less, and one or more of Mn: 0.1 to 0.4%, Cu: 0.1 to 0.4%, and Zn: 0.1 to 0.5%, with the remainder being Mg and unavoidable impurities.

Images