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

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 MgO_2, 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.