Sulfone-Based Electrolyte Solutions for Rechargeable Magnesium Batteries Using 2,5-Dimethoxy-1,4-Benzoquinone Positive Electrode

Rechargeable magnesium batteries may be an option for replacing the rechargeable lithium batteries because magnesium is much more abundant in the earth’s crust and is more widely distributed than lithium. To achieve rechargeable magnesium batteries, it is important to consider the combination of the electrolyte and the positive electrode with the Mg negative electrode. We previously reported that an organic molecule, 2,5-di­methoxy-1,4-benzoquinone (DMBQ), acts as a positive electrode material in a Mg electrolyte solution, Mg(ClO₄)₂/GBL, in a half-cell setup¹, in which reversible Mg deposition/dissolution does not occur on the negative electrode.

In the present study, we investigated three sulfones (RR´SO₂) for use as a solvent to dissolve the Mg solute in rechargeable magnesium batteries; viz., sulfolane (SL), ethyl-i-propyl sulfone (EiPS) and di-n-propyl sulfone (DnPS). As the Mg electrolyte, we used magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)₂) because it can tolerate a high potential and the large TFSA anion, a soft Lewis base, should facilitate its dissociation from the Mg²⁺cation.

The thermal stability of the electrolyte solutions examined by DSC indicated that these solutions remain stable as electrolytes over a wide temperature range < 250˚C. The specific conductivity of these solutions was 1~2 mS cm^‑1 at 30˚C, which is about an order of magnitude lower than that of the electrolyte solutions used in conventional rechargeable lithium batteries, ~10 mS cm^‑1. The potential window of the electrolyte solutions examined by LSV indicated that all three solutions remained stable between 0 and 2.0 V vs. Mg reference electrode (Mg_quasi). From the viewpoint of the width of the potential window as well as both the thermal stability and specific conductivity, Mg(TFSA)₂/SL was the best electrolyte solution among the three tested in this study.

The charge-discharge properties of the DMBQ electrode were investigated in the three-electrode cell between 0 and 2.0 V vs. Mg_quasi. The first discharge curves showed two potential plateaus that were often observed for quinone-based electrodes in Li, Na and Mg systems. The DMBQ electrode was successfully oxidized in the subsequent charge process. From EDX measurement and XRD analysis, it was found that DMBQ crystal reversibly accommodates and releases Mg²⁺in the sulfone electrolyte solutions during cycles.

The electrochemical properties of the metallic Mg electrode in the sulfone electrolyte solutions were investigated by cyclic voltammetry at room temperature in the three-electrode cell. The reduction-oxidation current of the metallic Mg electrode was observed. SEM observations and XRD analysis suggest that Mg²⁺in the electrolyte solution is indeed reduced to Mg(0) and deposited on the electrode during a cathodic sweep.

The properties of a two-electrode cell consisting of Mg|Mg(TFSA)₂/SL|DMBQ were examined as a rechargeable Mg battery at 30˚C. The cycle trend of the cell is shown in Figure 1. The discharge capacity increases in the initial seven cycles to reach ca. 100 mAh g(DMBQ)^‑1. Although the capacity substantially decreases upon cycling, it retains 20% of the maximum capacity after 50 cycles.

1.      H. Sano, H. Senoh, M. Yao, H. Sakaebe, and T. Kiyobayashi, Chem. Lett., 41, 1594-1596 (2012)