Influence of a Temperature Driven Structural Phase Transition on the Ionic Conductivity of Na6Mn(SO4)4 Vanthoffite System

Sodium Ion Batteries have emerged as practicable successors of the Li-ion battery technology with respect to performance, availability and safety. However, to address the ever-growing demand of improving energy density and capacity, it is important to invest into the research of materials that can serve as efficient battery components and ensure high operational voltages along with long term stability. Polyanionic sulfate materials are known for their high cathodic efficiency owing to their energy densities that arise from electronegativity of sulfate anions. The Vanthoffite class of compounds, with their open framework skeletons, have been explored experimentally as good ionic conductors, which show a temperature dependent ionic conductivity. A superionic phase transition was found in the Mn based system at 445 °C. This structural transition is reversible and is a cumulative effect of the changes in the local topology. Our theoretical calculations provide a mechanistic insight into this phase transition and also establishes Na6Mn(SO4)4 to be an efficient cathode material for sodium ion batteries.