Direct Observation of the Interfacial Instability of the Fast Ionic Conductor Li10GeP2S12 at the Lithium Metal Anode

The very high ionic conductivity of Li10GeP2S12 (LGPS) makes it a potential solid electrolyte for lithium all-solid-state batteries. Besides the high ionic conductivity, another key requirement is the stability of the solid electrolyte against degradation reactions with the electrodes; here, we analyze the reaction of LGPS with lithium metal. In situ X-ray photoelectron spectroscopy (XPS), in combination with time-resolved electrochemical measurements offers detailed information on the chemical reactions at the Li/LGPS interface. The decomposition of Li10GeP2S12 leads to the formation of an interphase composed of Li3P, Li2S, and Li-Ge alloy, which is in perfect agreement with theoretical predictions, and an increase of the interfacial resistance. These results highlight the necessity to perform long-term, time-resolved electrochemical measurements when evaluating potential new solid electrolytes for solid-state batteries. The kinetics of this interphase growth-comparable to SEI formation on lithium anodes in liquid electrolytes seems to be governed by diffusion across the interphase, as a square root time dependence is observed.