Unveiling the Interfacial Dynamics of Crosslinked Biodegradable Polycaprolactone Solid Polymer Electrolytes and Li Metal During Lithium Plating and Stripping via Operando ATR-FTIR Spectroscopy.

Polycaprolactones (PCL) have gained attention as a biodegradable and environmentally sustainable solid polymer electrolyte (SPE) for solid-state battery applications. While poly(ethylene oxide) (PEO) systems are widely studied, the interfacial mechanisms of PCL-based SPEs in contact with Li metal remain insufficiently explored. In this work, operando attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was employed to monitor the chemical environment at the interface of Li metal and crosslinked PCL-based SPEs (cl-PCL5LiTFSI) during Li plating/stripping, enabling both spectral assignment and electrochemical comparison between the crosslinked and non-crosslinked systems. During galvanostatic cycling experiments, cells with cl-PCL5LiTFSI SPEs exhibited increased cycling stability compared to cells containing PCL5LiTFSI SPEs, which can be attributed to the enhanced mechanical strength of the polymer network. Overall, the results show the development of localized ordered PCL domains and the redistribution of TFSI- anion during Li plating/stripping, revealing the possible degradation and dynamic changes occurring at the SPE|Li metal interface. The phenomenon of interfacial evolution of the SPEs was visualized through converting specific characteristic bands of PCL and LiTFSI from operando spectra into penetration depth-dependent band ratio maps.