Development of a Mg/O ReaxFF Potential to describe the Passivation Processes in Magnesium-Ion Batteries

One of the key challenges preventing the breakthrough of magnesium-ion batteries (MIB) is the formation of a passivating boundary layer at the Mg anode. To describe the initial steps of Mg anode degradation by O2 impurities, we have developed a Mg/O ReaxFF reactive force field description capable of accurately modeling the bulk, surface, adsorption, and diffusion properties of both metallic Mg and the salt MgO. We show that O2 immediately dissociates upon first contact with the Mg anode (modeled as Mg(0001), Mg(10m10)A, and Mg(10m11)), heating the surface to several 1000 K. The high temperature assists the further oxidation and forms a rocksalt interphase intersected by several grain boundaries. Among the Mg surface terminations, Mg(10m10)A is the most reactive, forming an MgO layer with a thickness of up to 25 Å. We also demonstrate the recrystallization of an amorphous MgO particle, which is obtained performing grand-canonical Monte Carlo simulations, into the rocksalt structure by thermal annealing at elevated temperatures. Our force field can be used to model the ongoing reactions in Mg-air batteries and constitutes an important step towards modeling the solid--electrolyte interface formation at the Mg anode.