Molecular Insights into the Interfacial Phenomena at the Li Metal | Polymer Solid‐State Electrolyte in Anode‐Free Configuration During Li Plating‐Stripping via Advanced Operando ATR‐FTIR Spectroscopy

Solid-state batteries are regarded as safe and high-energy-density candidates for next-generation energy storage. However, gaining a mechanistic understanding of the interfacial phenomena under real electrochemically working conditions remains a major challenge for cells containing solid-state electrolytes. This work presents an in-house built attenuated total reflection fourier-transform infrared (ATR-FTIR) spectroscopy cell equipped with an internal temperature-control unit. This cell is used for operando characterization of interfacial processes between plated Li and polymer during Li plating/stripping. As a proof of concept, a polymer electrolyte (cr-PEO10LiTFSI) containing poly(ethylene oxide), Li bis-(trifluoromethanesulfonyl)imide and crosslink-initiator benzophenone (BP) is introduced on a copper mesh as current collector at 60 °C. The developed ATR-FTIR spectroscopy setup provides detailed insights into the electrolyte degradation and reveals the crystallinity transformation of PEO at the interface during plating. Moreover, for the first time, the degradation of BP is observed. This compound, often overlooked in electrolyte systems due to its low concentration, is found to play a significant role in the interfacial electrochemistry process. Overall, this study provides a comprehensive overview of the characterization on the PEO electrolyte-lithium metal interface and introduces a novel perspective on the reaction of BP as a crosslinking initiator in the solid-state batteries.