SEI Formation Mechanisms at Lithium Metal Interfaces in Perfluoroether-Extended LiTFSI-Grafted Polymer Electrolytes

Abstract

Interfacial electron and ion transport play a critical role in electrochemical energy storage systems. In lithium metal batteries, electrolyte reduction at the anode forms the solid electrolyte interphase (SEI), which governs cycling stability and lithium-ion transport. Single-ion conducting polymer electrolytes (SICPEs) grafted with CF3(CF2)2O(CF2)2SO2NLiSO2CF3 (LiPBTFSI) show high ionic conductivity and lithium-ion transference, but their SEI formation mechanisms remain poorly understood. In this study, classical molecular dynamics simulations conducted with the ReaxFF potential (ReaxFF-MD) and ab initio molecular dynamics (AIMD) simulations were combined to investigate SEI formation in LiPBTFSI-grafted aromatic SICPEs. ReaxFF-MD results show that stronger electric fields promote SEI growth and drive the reduction of CwOxFy intermediates to CwFy, a precursor to LizFy. AIMD simulations reveal that the SEI consists of both inorganic fragments (lithium oxides and fluorides) and organic residues derived from the polymer backbone, with the polymer moieties preferentially aligning parallel to the lithium surface due to π–metal interactions. Furthermore, the ReaxFF-MD results suggest that the inorganic species preferentially accumulate at the interface between lithium metal and mPET-based polymer electrolytes. Different reduction pathways ultimately yield C–C or C–C–O species. These findings provide molecular-level understanding of the SEI formation in LiPBTFSI-grafted aromatic SICPEs and offer guidance for designing stable lithium metal batteries with high performance.