Atomistic Mechanisms of Mg Insertion Reactions in Group XIV Anodes for Mg-Ion Batteries.

Magnesium (Mg) metal has been widely explored as an anode material for Mg-ion batteries (MIBs) owing to its large specific capacity and dendrite-free operation. However critical challenges, such as the formation of passivation layers during battery operation and anode-electrolyte-cathode incompatibilities, limit the practical application of Mg-metal anodes for MIBs. Motivated by the promise of group XIV elements (namely Si, Ge and Sn) as anodes for lithium- and sodium-ion batteries, here we conduct systematic first principles calculations to explore the thermodynamics and kinetics of group XIV anodes for Mg batteries, and to identify the atomistic mechanisms of the electrochemical insertion reactions of Mg ions. We confirm the formation of amorphous Mgx X phases (where X = Si, Ge, Sn) in anodes via the breaking of the stronger X-X bonding network replaced by weaker Mg-X bonding. Mg ions have higher diffusivities in Ge and Sn anodes than in Si, resulting from weaker Ge-Ge and Sn-Sn bonding networks. In addition, we identify thermodynamic instabilities of Mgx X that require a small overpotential to avoid aggregation (plating) of Mg at anode/electrolyte interfaces. Such comprehensive first principles calculations demonstrate that amorphous Ge and crystalline Sn can be potentially effective anodes for practical applications in Mg-ion batteries.