Motif-Based Design of an Oxysulfide Class of Lithium Superionic Conductors: Toward Improved Stability and Record-High Li-Ion Conductivity

A sulfur-based solid-state electrolyte with the highest Li-ion conductivity shows promise toward next-generation all-solid-state lithium batteries. However, the moisture-sensitivity and (electro)chemical instability restrict them from a real battery setup. In contrast, moisture-resistant oxygen analogues exhibit poor Li conductivity. To overcome these well-known problems, we theoretically develop the chemistry of local structural motifs to build an unprecedented oxygen–sulfur mixed framework Li10(MS4)(PO4)2 (M = Ge and Sn), which combines the moisture stability with high Li conductivity. Especially, the Li10(MS4)(PO4)2 oxysulfide framework exhibits isotropic three-dimensional Li diffusion associated with a lesser Li-migration barrier (0.10 ± 0.02 eV) compared to its sulfur analogue Li10GeP2S12 (0.18 eV). Furthermore, oxysulfides exhibit a wider electrochemical stability window compared to Li10GeP2S12. As a reference for experimentalists, we also tabulate the expected decomposition products at the interface while considering a number of high-performing cathodes in combination with the Li10(MS4)(PO4)2 electrolyte to discuss their compatibility. We emphasize that identification of specific moieties for oxygenation and sulfuration leads to a design principle for a unique oxysulfide class of Li superionic conductors (Li-SICs). The novel concept of higher O content without limiting the Li-ion conductivity could open up a new avenue of broad compositional spaces for stable Li-SICs.