Discharge and Recharge Characteristics in an Mg/O2 Battery with a Mixed-Phase Discharge Product

By combining a multivalent cation with the high capacity of a gas-breathing positive electrode, the energy density of a non-aqueous Mg/O₂ battery is projected to surpass that of other ‘beyond-Li-ion’ chemistries. Nevertheless, research into Mg/O₂ systems has been limited. A rechargeable, non-aqueous Mg/O₂ cell that operates at room temperature will be discussed and used to clarify fundamental electrochemical characteristics, including the nature of the product phase(s) and the reaction mechanism for discharge. The discharge product differs from alkali-metal-based chemistries in that it is a mixed phase, primarily comprising crystalline MgO, with a substantial minority of MgO₂. The open-circuit cell voltage is 2.0 V, lower than the theoretically expected ~2.9 V. Both the low voltage and the two-phase discharge product are consistent with a multi-step discharge reaction in which a superoxide (O₂^–) intermediate forms at ~2 V vs. Mg/Mg²⁺. Chemical disproportionation reactions subsequently yield MgO and MgO₂, but do not contribute to the cell’s electrical energy output. During the charging step, MgO₂ is preferentially decomposed. Bypassing this multi-step mechanism in favor of direct electrochemical MgO_x formation would allow for higher discharge potential and consequently higher energy density.