Lithium‐Metal Anode Instability of the Superionic Halide Solid Electrolytes and the Implications for Solid‐State Batteries

Owing to high ionic conductivity and good oxidation stability, halide-based solid electrolytes regain interest for application in solid-state batteries. While stability at the cathode interface seems to be given, the stability against the lithium metal anode has not been explored yet. Herein, the formation of a reaction layer between Li₃ InCl₆ (Li₃ YCl₆ ) and lithium is studied by sputter deposition of lithium metal and subsequent in situ X-ray photoelectron spectroscopy as well as by impedance spectroscopy. The interface is thermodynamically unstable and results in a continuously growing interphase resistance. Additionally, the interface between Li₃ InCl₆ and Li₆ PS₅ Cl is characterized by impedance spectroscopy to discern whether a combined use as cathode electrolyte and separator electrolyte, respectively, might enable long-term stable and low impedance operation. In fact, oxidation stable halide-based lithium superionic conductors cannot be used against Li, but may be promising candidates as cathode electrolytes.