Miscibility of Li₄GeO₄ into Li₃PS₄ Solid Electrolytes from First-Principles Methods

Lithium-based oxysulfide solid electrolytes are currently being researched for all-solid-state batteries, as they offer a combination of high electrochemical stabilities and elevated Li-ion conductivities, thus combining the best characteristics of sulfide and oxide solid electrolytes. In this work, we investigated the miscibility of Li3PS4 and Li4GeO4 polyanions. Building upon a robust multiscale first-principles methodology, we explore the configurational disorder arising from the mixing of GeO44– and PS43– in the (1 – x)Li3PS4–xLi4GeO4 tie line. The computed phase diagram reveals a stable ordered phase with the composition Li7PO4GeS4. In Li7PO4GeS4, the sulfur atoms initially bound to pentavalent phosphorus as PS43– are fully exchanged with oxygen atoms (from GeO44–), giving rise to GeS44– moieties. Ab initio molecular dynamics simulations predicted a high Li-ion conductivity of ∼13.31 mS cm–1 for Li7PO4GeS4 at 573 K. These results reveal the structural complexity and flexibility of these polyanion systems.