Evidence for a Mixed-Phase Discharge Product and Superoxide-Mediated Discharge Pathway in a Mg/O2 Battery
We will discuss our progress in exploring discharge and recharge characteristics of non-aqueous Mg/O2 cells. Although Mg/O2 batteries have been demostrated using non-aqueous electrolytes in the past, the nature of the discharge product is not well understood. Figure 1 shows the discharge product on a porous-carbon positive electrode after first discharge. We have elucidated the Mg/O2 discharge-product composition using several characterization techniques, including scanning electron microscopy, energy dispersive spectroscopy, Auger electron spectroscopy, X-ray diffraction and Raman spectroscopy. The discharge product predominantly comprises MgO. In addition to MgO, there is evidence of a substantial minority of MgO2, as well as trace Cl. The energy efficiency for the first cycle is 42%, lower than other metal/O2 chemistries,5,6 but comparable to prior elevated-temperature Mg/O2 cells.2
The open-circuit potential (OCP) prior to first discharge of the non-aqueous Mg/O2 cell is 2.0V, which is lower than would be expected for the direct electrochemical formation of MgO or MgO2 (2.95V and 2.91V vs. Mg/Mg2+, respectively). This observation is consistent with discharge-product formation via a superoxide-ion intermediate, which forms from molecular O2 at 2.03 V vs. Mg/Mg2+. Superoxide then drives an O2-releasing precipitation of MgO2, which subsequently disproportionates to O2 and MgO – chemical reactions that do not affect the cell voltage, and rationalize the observed discharge-product composition.
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