Boosting ionic conductivity in antiperovskite Li3OCl via defect engineering: Interstitial versus vacancy

Ion transport in solid materials is frequently facilitated by defects. It is thus attractive to increase ionic conductivity by introducing proper types of defects. In antiperovskite ${\mathrm{Li}}_{3}\mathrm{OCl}$, a promising solid-state electrolyte material, a Li interstitial is predicted to be superior to Li vacancy for Li ion transport. However, the Li interstitial is difficult to dope into ${\mathrm{Li}}_{3}\mathrm{OCl}$ and it typically comes with a charge-compensating ${\mathrm{O}}_{\mathrm{Cl}}^{\ensuremath{'}}$ substitutional defect, which is a Li trap and thus hampers Li ion transport. In this study, a protocol to enhance Li interstitial formation via S doping is proposed. With a ${\mathrm{S}}_{\mathrm{Cl}}^{\ensuremath{'}}$ charge-compensating defect, the Li interstitial becomes more stable than Li vacancy. At the same time, thanks to a cancellation of two different contributions, ${\mathrm{S}}_{\mathrm{Cl}}^{\ensuremath{'}}$ has little effect on the Li local electrostatic environment and thus ion transport. As a result, Li interstitial conductivity can be boosted from $3.822\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ to 1.286 mS/cm upon S doping, which leads to a 1-order-of-magnitude increase of the overall ionic conductivity. Such a significant performance enhancement via defect engineering opens an alternative avenue for the design of battery materials.