(103c) 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).