First-principles assessment of chemical lithiation of sulfide solid electrolytes and its impact on their transport, electronic and mechanical properties

Abstract

Sulfide-based solid electrolytes (SEs) have received considerable attention, but their practical application is hindered by the lack of chemical stability. When in contact with Li metal, sulfide SEs may undergo decomposition induced by lithiation, which could be largely responsible for the high interfacial impedance observed between them. While the underlying mechanism is still uncertain, this first-principles study examines the structural and compositional evolution of sulfide materials, Li3PS4 and Li10GeP2S12, associated with chemical lithiation and its impact on their transport, electronic and mechanical properties. Our calculations demonstrate that the lithiation reaction is thermodynamically favorable until reaching full lithiation in which PS43− and GeS44− tetrahedral anions are completely decomposed to S2−, P3- and Ge4− atomic ions. This process is also found to involve the transition formation of small P(Ge) clusters and more importantly an increase in the relative density which may cause void formation and thus structural failure near the Li metal/SE interface. Moreover, our analyses highlight that, even when fully lithiated, the sulfide materials could retain the high Li-ion conductivity as well as the ability to block electron transport. This suggests that the chemical decomposition of sulfide SEs would not be the only factor that is responsible for the increase of interfacial impedance during electrochemical cycling.