Influence of Inversion on Mg Mobility and Electrochemistry in Spinels

Magnesium oxide and sulfide spinels have recently attracted interest as cathode and electrolyte materials for energy-dense Mg batteries, but their observed electrochemical performance depends strongly on synthesis conditions. In this study, using first-principles calculations and percolation theory, we explore the extent to which spinel inversion influences Mg<sup>2+</sup> ionic mobility in MgMn<sub>2</sub>O<sub>4</sub> as a prototypical cathode, and MgIn<sub>2</sub>S<sub>4</sub> as a potential solid electrolyte. We find that spinel inversion and the resulting changes of the local cation ordering give rise to both increased and decreased Mg<sup>2+</sup> migration barriers, along specific migration pathways, in the oxide as well as the sulfide. To quantify the impact of spinel inversion on macroscopic Mg<sup>2+</sup> transport, we determine the percolation thresholds in both MgMn<sub>2</sub>O<sub>4</sub> and MgIn<sub>2</sub>S<sub>4</sub>. Furthermore, we analyze the impact of inversion on the electrochemical properties of the MgMn<sub>2</sub>O<sub>4</sub> cathode via changes in the phase behavior, average Mg insertion voltages and extractable capacities, at varying degrees of inversion. In conclusion, our results confirm that inversion is a major performance limiting factor of Mg spinels and that synthesis techniques or compositions that stabilize the well-ordered spinel structure are crucial for the success of Mg spinels in multivalent batteries.