Electrolyte Solutions for Magnesium Rechageable Batteries

Innovative rechargeable batteries beyond the present lithium-ion batteries (LIB) have been investigated for the environmental and energy problem. Among the batteries, rechargeable magnesium metal batteries have attracted much attention, since magnesium metal as negative electrode can bring high volumetric energy density owing to the two-electron reaction, low electrode potential, and lightness. Until now, the electrolyte solutions used for rechargeable magnesium metal batteries are tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME) based electrolyte solutions. The boiling points of THF and DME are 66°C and 85°C, respectively. In order to eliminate the water in the rechargeable magnesium metal battery system, at least 100°C is required for the working temperature. However, THF or DME based electrolyte solutions do not meet the criteria. Therefore, we focused on the triethylene glycol dimethyl ether (triglyme) as a solvent since the boiling point of triglyme is 216°C. As a magnesium salt, magnesium(II) bis(trifluoromethane sulfonyl) amide (Mg(TFSA)2) was selected since TFSA- is a large anion and Mg(TFSA)2 is expected to be highly dissociative in triglyme. By using this system, we can increase the working temperature more than 100°C for rechargeable magnesium metal battery system[1]. As a proof-of-concept, Orikasa et al. reported that a rechargeable magnesium metal battery composed of insertion-type MgFeSiO4 positive electrode can work at 100°C in triglyme-based electrolyte solution [2]. This result indicated that we can fabricate high safety rechargeable magnesium metal batteries at ambient temperature by selecting appropriate positive electrodes. Then, the electrochemical behavior of electrolyte solution at ambient temperature is also important. In this conference, we reports the reversible magnesium metal deposition and dissolution in the Mg(TFSA)2/glymes solution at ambient temperature. In addition, we report new magnesium salt in triglyme. [1] T. Fukutsuka, K. Asaka, A. Inoo, R. Yasui, K. Miyazaki, T. Abe, K. Nishio, and Y. Uchimoto, Chemistry Letters, 11, 1788-1790 (2014). [2] Y. Orikasa, T. Masese, Y. Koyama, T. Mori, M. Hattori, K. Yamamoto, T. Okado, Z. D. Huang, T. Minato, C. Tassel, J. Kim, Y. Kobayashi, T. Abe, H. Kageyama, and Y. Uchimoto, Sci. Rep. 2014, doi: 10.1038/srep05622.