Role of point defects in spinel Mg chalcogenide conductors

TL;DR

This study uses first-principles calculations to analyze point defects in Mg chalcogenide spinels, identifying defect types influencing electronic conductivity and suggesting doping strategies to improve their performance as solid electrolytes.

Contribution

It provides a detailed defect analysis in Mg spinel chalcogenides and proposes doping methods to reduce undesired electronic conductivity, aiding solid electrolyte optimization.

Findings

Mg-vacancies and Mg-metal anti-sites are dominant defects.

Anion-excess conditions reduce electronic conductivity.

High-temperature synthesis may induce n-type conductivity.

Abstract

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