Interfacial reactivity and interphase growth of argyrodite solid electrolytes at lithium metal electrodes

Lithium superionic conductors with the argyrodite structure Li6PS5X (X = Cl, Br, I) are considered as suitable candidates for the fabrication of all-solid-state batteries (SSB) facilitating Li metal anodes. The use of metal anodes is required to achieve SSB with high energy densities, however, the thermodynamic stability of the different argyrodites in contact with Li metal has not been systematically investigated yet. The stability against lithium metal is of practical interest for long-term stability of SSB utilizing argyrodites. Here, data on the stability of Li6PS5X (X = Cl, Br, I) in contact with Li metal are reported, obtained from an in situ X-ray photoemission technique in combination with time-resolved impedance spectroscopy. In contact with Li metal, Li6PS5X decomposes into an interphase composed of Li3P, Li2S and LiX, which serves as an SEI and results in an increasing interfacial resistance. The growth of the SEI and the resulting resistance evolution is further analyzed in terms of its kinetics and is compared to other thiophosphate superionic conductors. Li6PS5I is found to show particularly strong SEI formation with severe resistance growth.