Low Resistance Interphase Formation at the PEO‐LiTFSI|LGPS Interface in Lithium Solid‐State Batteries

ABSTRACT Sulfide‐based solid electrolytes (SEs) are among the most promising candidates for solid‐state batteries due to their high ionic conductivity and low grain boundary resistance. However, their chemical and electrochemical instability when paired with lithium metal renders them susceptible to severe degradation reactions. To prevent contact between lithium metal and SEs, polymer electrolyte (PE) interlayers are proposed as a solution, enabling the potential use of SEs in lithium metal solid‐state batteries. These interlayers, however, lead to the formation of rarely studied PE|SE interfaces, which significantly influence battery performance. This study examines the interphase formed and charge transfer process at the PE|SE interface between polyethylene oxide (PEO)‐based PE and Li ₁₀ GeP ₂ S ₁₂ (LGPS) SE using a four‐electrode cell configuration. X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) reveal that thermal degradation at the PEO‐LiTFSI|LGPS interface leads to the formation of a low resistance interphase containing polysulfides (S _n ²⁻ ), sulfites (SO ₃ ²⁻ ), LiF, and P‐[S] _n ‐P type bridged PS ₄ ³⁻ units. Overall, the interface resistance is only 4.4 Ω cm ² at 80°C. Given the diverse chemical and electrochemical interactions of sulfide‐based SEs with PEO‐based PEs, our findings contribute to a deeper understanding of the low‐resistive interphase, aiding in optimizing PE‐SE combinations for lithium metal solid‐state batteries.