Solid-liquid electrolyte interphases in quasi-solid-state lithium-sulfur batteries with argyrodite-type solid...

Quasi-solid-state lithium-sulfur batteries offer a promising route to combine the high energy density of lithium-sulfur chemistry with improved interfacial stability. However, their performance is limited by the formation of resistive solid-liquid electrolyte interphases at the interface between the solid electrolyte separator and the liquid catholyte. In this work, we investigate solid-liquid electrolyte interphase formation at the interface between an argyrodite-type sulfide solid electrolyte Li_5.5PS_4.5Cl_1.5 and two representative liquid electrolytes: a conventional ether-based electrolyte (LiTFSI in DOL:DME) and an ionic liquid (LiTFSI in EMIMTFSI). Using a combination of time-resolved electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and focussed ion beam scanning electron microscopy, we reveal substantial differences in interphase chemistry, morphology, and transport properties. The ether-based electrolyte undergoes continuous chemical reaction with the sulfide solid electrolyte, forming a thick, inhomogeneous, and highly resistive interphase (∼2000 Ω cm² after 250 h), driven in part by dissolution of polysulfide species. In contrast, the ionic liquid electrolyte forms a significantly thinner, layered interphase with a comparatively low area-specific resistance (∼150-220 Ω cm²), which remains constant over extended time and during electrochemical cycling. These findings demonstrate that choice of catholyte plays an important role in governing solid-liquid electrolyte interphase formation, and highlights ionic liquids as viable catholytes for stable, low-resistance interfaces in quasi-solid-state lithium-sulfur batteries with argyrodite-type solid electrolyte separators.