Interface Stability of Argyrodite Li6PS5Cl toward LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4 in Bulk All-Solid-State Batteries

Argyrodite Li6PS5Cl is a good candidate for being a solid electrolyte for bulk all-solid-state Li-ion batteries because of its high ionic conductivity and its good processability. However, the interface stability of sulfide-based electrolytes toward active materials (negative or positive electrodes) is known to be lower than that of oxide-based electrolytes. In this work, we investigate the interface stability of argyrodite toward several positive electrode materials: LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4. All-solid-state half-cells were cycled, and the interface mechanisms were characterized by Auger electron spectroscopy and X-ray photoelectron spectroscopy. We show that Li6PS5Cl is oxidized into elemental sulfur, lithium polysulfides, P2Sx (x ≥ 5), phosphates, and LiCl at the interface with the positive electrode active materials. In spite of this interface reactivity, good capacity retention was observed over 300 cycles. Li6PS5Cl shows some reversible electrochemical activity (redox processes) that might contribute to the reversible capacity of the battery.