Role of Point Defects in Spinel Mg Chalcogenide Conductors

Close-packed chalcogenide spinels, such as MgSc$_2$Se$_4$, MgIn$_2$S$_4$ and\nMgSc$_2$S$_4$, show potential as solid electrolytes in Mg batteries, but are\naffected by non-negligible electronic conductivity, which contributes to\nself-discharge when used in an electrochemical storage device. Using\nfirst-principles calculations, we evaluate the energy of point defects as\nfunction of synthesis conditions and Fermi level to identify the origins of the\nundesired electronic conductivity. Our results suggest that Mg-vacancies and\nMg-metal anti-sites (where Mg is exchanged with Sc or In) are the dominant\npoint defects that can occur in the systems under consideration. While we find\nanion-excess conditions and slow cooling to likely create conditions for low\nelectronic conductivity, the spinels are likely to exhibit significant $n$-type\nconductivity under anion-poor environments, which are often present during high\ntemperature synthesis. Finally, we explore extrinsic aliovalent doping to\npotentially mitigate the electronic conductivity in these chalcogenide spinels.\nThe computational strategy is general and can be easily extended to other solid\nelectrolytes (and electrodes) to aid in the optimization of the electronic\nproperties of the corresponding frameworks.\n