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

Abstract

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 demonstrated 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.