Interfacial Degradation Mechanisms between Li Metal and a Li ₃ InCl ₆ Hybrid Electrolyte in the Presence of a Poly(ethylene oxide) Electrolyte Interlayer

To develop safe, room-temperature operating, and high-energy-density lithium (Li) metal based batteries, Li3InCl6 halide solid electrolytes have shown promise when formulated in situ within an inert polymer matrix, creating flexible self-standing thin films. However, their practical application with Li metal remains limited because of chemical instability at the Li | electrolyte interface, leading to resistive interphase formation and active material consumption. This study presents a comprehensive analysis of the degradation processes occurring during prolonged Li contact (up to 1000 h) with a hybrid Li3InCl6 film, employing multiple characterization techniques, including electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and cryo-focused ion beam scanning electron microscopy. Long-term Li | electrolyte contact induces conductive pathway formation at the Li | electrolyte interface, significantly reducing the overall interfacial resistance and resulting in a stabilized interface. However, this stabilization comes at the cost of poor cycling performance. To address this limitation, poly(ethylene oxide) electrolyte was investigated as a potential interlayer between Li and the hybrid electrolyte. Both short- and long-term studies demonstrated that while the polymer layer initially passivates the Li metal electrode, it ultimately fails to prevent Li3InCl6 degradation. The degradation process involves diffusion of chlorine and indium based species throughout the entire polymer thickness, indicating that this approach does not provide a complete solution to the interfacial stability challenge. Therefore, this study reveals significant insights into Li metal stability and halide degradation mechanisms as well as their dynamics, which permits in turn to envision mitigation strategies to ultimately develop a reliable Li | electrolyte interface to be used in battery applications.