Rechargeable magnesium battery can be a candidate of the next-generation batteries because magnesium has a higher volumetric capacity, is more abundant in terms of the resources, and is less reactive in air than lithium. We previously found that a redox-active organic compound, 2,5-di­methoxy-1,4-benzo­quinone (DMBQ), has the potential to serve as the positive electrode material for the Mg system¹. Recently, we have investigated two kinds of electrolytes, sulfone-based and glyme-based electrolytes^2-3, for the rechargeable magnesium-organic batteries. In the case of monoglyme-based electrolyte, the discharge voltage of the Mg-DMBQ cell significantly increases, not decreases, after a few cycles, for which, we found, a decrease in the overpotential for the Mg dissolution upon cycling is responsible³. However, what caused the decrease in the overpotential on the Mg negative electrode is not clear. In the present study, we report the impact of the water content in the monoglyme-based electrolyte to the rechargeable Mg-organic battery.

     Magnesium bis(trifluoromethane sulfonyl)amide (Mg(TFSA)₂) and monoglyme were purchased from Kishida Chemical and used without further purification. The electrolyte was prepared by dissolving 0.5 mol L^-1 of Mg(TFSA)₂ in monoglyme. The water content of the as-prepared electrolyte was 40 ppm. Two other electrolyte solutions were prepared in which pure water was deliberately added to 100 and 1000 ppm. The electrochemical cells were set up and tested based on our previous study³.

     The specific conductivities of water-added 0.5 mol L^-1 Mg(TFSA)₂/monoglyme did not shift from that of the as-prepared. The discharge and charge curves of the cells in the electrolytes at various water contents showed that in all three cases the first discharge curve has two potential plateaus below 0.7 V. In the case of the water-added electrolytes, the discharge voltage during the second cycle significantly increased, while in the as-prepared electrolyte the discharge voltage remained below 0.7 V even after several cycles. The drastic dissolution of Mg from the surface of the negative electrode was observed by SEM only in the water-added electrolytes. A certain amount of water plays an important role in preparing the surface of the electrode to let Mg dissolve in the electrolyte.

1. H. Sano et al., Chem. Lett., 41, 1594 (2012).

2. H. Senoh et al., J. Electrochem. Soc., 161, A1 (2014).

3. H. Senoh et al., ECS Transactions, in press.