Nonaqeous Electrolytes for Magnesium-Oxygen Batteries

Because of their high theoretical energy densities, metal-O₂ batteries have long been considered as an improvement upon current battery technologies. Mg-O₂ batteries have one of the highest predicted volumetric energy densities [1]. However, rechargeable Mg-O₂ batteries are not possible to realize with aqueous electrolytes due to non-conducting reaction products that form on the Mg anode. Non-aqueous Mg-conducting electrolytes have been studied in the context of Mg-ion and Mg-sulfur batteries [2]. Our goal is to develop a non-aqeuous electrolyte that is compatible with an O₂ cathode and stable for the MgO (2.95V vs. Mg/Mg⁺⁺) and MgO₂ (2.91V vs. Mg/Mg⁺⁺) formation reactions.

In this presentation, we will discuss our latest progress in electrolytes for rechargeable Mg-O₂ batteries. Figure 1 shows a cyclic voltammogram measured in magnesium borohydride (Mg(BH4)₂) in dimethoxyethane (DME). The oxidative stability of Mg(BH4)₂ in DME was found to be below 2V on a Pt working electrode. Interestingly, ionic liquids are known to have wide electrochemical windows. The oxidative stability of 1-Butyl-3-methylimidazolium bistrifluoromethylsulfonylimide (BmimTFSI) in DME was measured to be over 3V vs. Mg/Mg⁺⁺ (Figure 2). However, addition of BmimTFSI to the Mg(BH₄)/DME solution reduced the reductive stability window considerably and Mg deposition/dissolution was not observed. Mg deposition and dissolution experiments are underway for non-aqueous systems with ionic liquids as the supporting electrolyte.

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

[2] J. Muldoon et al., "Electrolyte roadblocks to a magnesium rechargeable battery," Energy & Environmental Science, 5, 5941 (2012).