First-Principles Study of Magnesium Peroxide Nucleation for Mg-Air Battery

Recently, rechargeable non-aqueous Mg-air batteries have gained a lot of interest as the next-generation energy storage device due to the high theoretical volumetric density (3832 Ah L-1 for Mg anode vs. 2062 Ah L-1 for Li), low cost and safety. The field of Mg-air batteries is in the initial stage of development having a limited number of experimental and theoretical reports, in which mainly a carbon cathode is used; however, the information regarding the structural form of carbon is still missing. In this work, using first-principles density functional theory (DFT) calculations, we demonstrate the possibility of graphene and graphite as a cathode material towards Mg-air batteries by studying the initial MgO and MgO2 nucleation processes on the surfaces of graphene and graphite. The calculated free energy diagrams for the redox reactions of oxygen are used to identify the rate-determining step controlling the overpotentials for initial nucleation of MgO and MgO2. We observe that graphene and graphite surfaces show similar reactivity towards the nucleation of MgO or MgO2, and the overpotential of the controlling steps for MgO2 nucleation is comparatively less than that of MgO nucleation, which is supported by a recent experimental study, where a higher discharge voltage was observed in a cell having a mixed MgO/MgO2 discharge product than MgO-based cells. Furthermore, the preferable formation of MgO2 cluster compared to MgO on the graphene surface during the ab initio molecular dynamic (AIMD) simulations confirms the selectivity of MgO2 formation over MgO as the final discharge product. We believe that our study will be helpful in understanding the initial nucleation processes during the oxygen reduction reaction (ORR) mechanism and development of suitable cathodes for the future Mg-air batteries.