Thermodynamic investigation of phase transformation in Sn anode for magnesium batteries

Metallic Mg anodes are incompatible with conventional electrolytes, such as Mg(BF4)(2) or Mg(ClO4)(2), due to the formation of a passivation layer that blocks the transport of Mg ions, thus limiting the selection of electrolytes and cathodes. Alloying anode materials for Mg batteries, such as Sn and its intermetallics, have recently been proposed as a new class of anode materials for Mg-ion batteries to address the issues of incompatibility with the conventional electrolytes. However, the large changes in the volume of the Mg-Sn alloy during cycling lead to poor Coulombic efficiency and rapid capacity degradation. The underlying reasons for how the structural changes hamper electrochemical performance remain unclear. In this work, we perform a theoretical study of the Mg-Sn alloys to have a deeper insight into the alloying process and the phase transformation in the Sn anode. This work is the first in-depth computational study that combines density functional theory and cluster expansion to investigate the phase transition process in the Mg-Sn system that includes Mg2Sn, alpha-Sn, and beta-Sn structures. We considered three possible routes for the transformation pathway from Mg2Sn to beta-Sn: Mg2Sn ->alpha-Sn -> beta-Sn, Mg2Sn ->beta-Sn, and Mg2Sn -> amorphous phase ->beta-Sn. Our study shows that the transformation of Sn between its alpha- and beta-phases hinders the alloying process. This hindrance, together with the amorphization of the alloy, is revealed to be the key factor to understand the poor electrochemical performance of the Mg-Sn alloy.