Oxysulfide solid electrolytes: The impact of oxygen in sulfides

Sulfide solid electrolytes have gained significant attention because they are crucial to the development of high-performance all-solid-state batteries (ASSBs). However, their practical application is hindered by chemical instability and incompatible interfaces with electrodes, highlighting the need for advanced technological solutions. Oxygen doping into sulfide solid electrolytes has emerged as a promising approach for balancing high ionic conductivity and robust stability. However, the underlying chemistry of oxysulfide solid electrolytes remains insufficiently understood. This review discusses the challenges faced by sulfide solid electrolytes and highlights the advantages of oxysulfide counterparts in addressing these limitations. It covers synthesis and characterization methods for oxysulfide solid electrolytes, categorized by their crystallinity and parent sulfide structures. This review also examines the fundamental mechanisms by which oxygen doping enhances ionic conductivities, lowers activation energies for lithium migration, and improves chemical and electrochemical stabilities. In addition, this review summarizes the battery performance of state-of-the-art oxysulfide solid electrolytes. Finally, future research directions are proposed to advance the viability and safety of next-generation ASSBs that incorporate oxysulfide solid electrolytes.