Insights into the Electrochemical Stability and Lithium Conductivity of Li4MS4 (M = Si, Ge, and Sn).

The design of solid electrolytes with a wide electrochemical stability window and high Li-ion conductivity is a prerequisite for the realization of all solid-state Li batteries, which promises to enable extraordinary levels of safety for the battery system and may potentially revolutionize the energy storage field. Among all the promising inorganic solid electrolytes, Li4MS4 (M = Si, Ge, and Sn) with a crystal structure of Pnma symmetry have recently been recommended due to their good air stability and ionic conductivity. Here, we employ ab initio simulations to conduct a systematic investigation of the electrochemical stability and Li conductivity of Li4MS4 compounds. Our computation results reveal that the edge-sharing and face-sharing characteristics of LiS4 and LiS6 polyhedra not only facilitate the formation of a percolating Li diffusion network but would severely destabilize the crystal structure as well, thus resulting in a rather narrow electrochemical window. Although the stronger M-S bonds manifested in Li4SiS4 can benefit the overall stability, unfortunately, it also contributes to a more rugged energy landscape that inhibits Li diffusion. Li4SnS4 with a less densely packed lattice exhibits a substantially lower energy for Li ions to be accommodated at interstitial sites, which is a trigger for high Li conductivity in the bulk material, reaching 11 mS/cm at room temperature. The fast Li diffusion occurs through the concerted migration of multiple Li ions at lattice and interstitial sites. These findings open up new possibilities for the rational design of Li4MS4 (M = Si, Ge, and Sn) solid electrolytes for next-generation Li batteries.