Destabilized Passivation Layer on Magnesium-Based Intermetallics as Potential Anode Active Materials for Magnesium Ion Batteries.

Passivation of magnesium metal anode is one of the critical challenges for the development of magnesium batteries. Here we investigated the passivation process of an intermetallic anode: MgBi synthesized by solid-state and thin film process. The MgBi composite electrode shows excellent reversibility in magnesium bis(trifluoromethansulfonylamide) dissolved in acetonitrile, while MgSb, which has same crystal structure and similar chemical properties, is electrochemically inactive. We also fabricated the MgBi thin film electrodes, which show reversibility with low overpotential not only in the acetonitrile solution but also glyme-based solutions. Surface layer corresponding to the decomposed TFSA anion is slightly suppressed in the case of the MgBi thin film electrode, compared with Mg metal. Comparative study of hydrolysis process of the MgBi and the MgSb suggests that the both intermetallic anodes are not completely passivated. The bond valence sum mapping of the MgBi indicates that the fast Mg diffusion pathway between 2 tetrahedral sites is formed. The electrochemical properties of the MgBi anode is mainly due to the less passivation surface with the fast Mg diffusion pathways.