Evidence for a Mixed-Phase Discharge Product and Superoxide-Mediated Discharge Pathway in a Mg/O2 Battery

Abstract

Metal/O2 batteries have the potential to achieve extremely high energy densities, and metal/O2 batteries based on magnesium are predicted to have one of the highest.1 Shiga et al. have demonstrated rechargeable, non-aqueous Mg/O2 cells that operate above room temperature.2 , 3 The need for elevated temperatures can be negated by using a Mg2+ electrolyte that can deposit and dissolve Mg at room temperature. We have produced Mg/O2 test cells using a modified-Grignard-reagent electrolyte, which has oxidative stability higher than 4V vs. Mg/Mg2+ and is viable at room temperature.4 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. (1) Zu, C.-X.; Li, H. Energy Environ. Sci. 2011, 4, 2614. (2) Shiga, T.; Hase, Y.; Kato, Y.; Inoue, M.; Takechi, K. Chem. Commun. (Camb) 2013, 49, 9152. (3) Shiga, T.; Hase, Y.; Yagi, Y.; Takahashi, N.; Takechi, K. J. Phys. Chem. Lett. 2014, 5, 1648. (4) Nelson, E. G.; Brody, S. I.; Kampf, J. F.; Bartlett, B. M. J. Mater. Chem. A 2014. (5) Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. Nat. Mater. 2013, 12, 228. (6) Ren, X.; Wu, Y. J. Am. Chem. Soc. 2013, 135, 2923. Figure 1