First Principles Insights into Amorphous Mg2Sn Alloy Anode for Mg-ion Batteries

Rechargeable Mg-ion batteries (MIBs) are an advantageous alternative solution to Li-ion batteries in many ways. Mg is safer and abundant in the Earth, and has a high electrochemical capacity owing to its divalent nature. It is yet relatively less studied largely due to primal success of Li-base batteries and challenges associated with the design of MIBs including high performance electrode materials. Herein, using first principles calculation, we study the electrochemical and mechanical properties of the most viable alloy anode Mg₂Sn with special attention to its amorphous phase—unavoidable phase forming during cyclic Sn magnesiation in MIBs due to volume changes. We create amorphous Mg₂Sn via simulated annealing technique using ab initio molecular dynamics. We find while Mg₂Sn undergoes a substantial atomic-level structural changes during the crystal-to-amorphous transformation, its polycrystalline properties degrade slightly and become softer by only 20 % compared to the crystal phase. Moreover, we predict competitive electrochemical properties for the amorphous phase assuming it goes under similar reaction path as the average electronic charge on Mg ions almost remain unaffected. This work thus not only demonstrate that a-Mg₂Sn phase could be a bypass to combat the challenges associated with the crystal cracking during volume change, but also serves as first step to better understand the widely used Mg₂Sn alloy anode in MIBs.