Characterizing Discharge in Non-Aqueous Magnesium-Oxygen Batteries

Metal-O₂ batteries are considered as an improvement upon current battery technologies due to their high theoretical energy densities. In particular, Mg-O₂ batteries have one of the highest predicted theoretical specific energy densities [1]. Mg-O₂ batteries have been previously demonstrated using aqueous electrolytes.  However, aqueous chemistries are generally non-rechargeable. and more recent studies have targeted secondary systems based on non-aqueous Mg-conducting electrolytes in the context of Mg-ion and Mg-sulfur batteries [2]. Although Shiga et. al. reported the first rechargeable Mg-O₂ battery using a non-aqueous electrolyte [3], the nature of the discharge product remains poorly understood. Our goal is to understand the discharge characteristics of Mg-O₂ batteries using non-aqueous electrolytes.

Here we discuss our progress in characterizing the discharge products in Mg-O₂ batteries. Figure 1 shows a discharge curve for such a battery using 0.05M Mg(BH₄)₂ in dimethoxyethane as the electrolyte. This electrolyte has been shown to reversibly deposit and dissolve Mg [4]. A voltage plateau of 1.1 V is observed, similar to previously reported non-aqueous Mg-O₂ battery [3]. The measured cell voltage is low compared to the theoretical voltage expected from formation of MgO and MgO₂ (2.95V and 2.91V vs. Mg/Mg²⁺, respectively) [5]. The carbon cathodes were characterized by x-ray diffraction and Raman spectroscopy. It was observed that MgCO₃ was the dominant Mg compound. MgCO₃ formation is likely due to reactions with the carbon cathode or with the electrolyte.

[1] Zu, C.-X., & Li, H. "Thermodynamic analysis on energy densities of batteries". Energy & Environmental Science, 4 (2011) 2614.

[2] Muldoon, J., Bucur, C. B., Oliver, A. G., Sugimoto, T., Matsui, M., Kim, H. S., et al. "Electrolyte roadblocks to a magnesium rechargeable battery". Energy & Environmental Science, 5 (2012) 5941.

[3] T. Shiga, Y. Hase, Y. Kato, M. Inoue, K. Takechi, " A rechargeable non-aqueous Mg-O2 battery". Chemistry Communications, 49 (2013) 9152.

[4] R. Mohtadi, M. Matsui, T.S. Arthur, S.-J. Hwang, “Magnesium borohydride: from hydrogen storage to magnesium battery.” Angewandte Chemie International Edition in English 51 (2012) 9780. 

[5] Vannerberg, N. “Peroxides, Superoxides, and Ozonides of the Metals of Group Ia, IIa, and IIb”, Progress in Inorganic Chemistry, 4^th ed.