Mechanistic Insights into Solid Electrolyte Interphase Evolution in Polymer-Based Lithium Metal Batteries

Abstract Understanding the evolution of the solid electrolyte interphase (SEI) at a lithium (Li)-solid polymer electrolyte (SPE) interface is pivotal for the development of safer Li metal batteries. Most of the existing studies rely on ex situ tracking of the SEI, which limits the information on its sequential growth. Here, we investigate the dynamic evolution of SEI at the Li-polyethylene oxide (PEO)-based SPE interface using in situ virtual electrode plating X-ray photoelectron spectroscopy (VEP-XPS) combined with coulometric titration time analysis (CTTA) and kinetic modeling. VEP-XPS enables controlled Li deposition through field-induced Li+ migration and reduction, allowing the direct observation of interphase chemistry under conditions that mimics electrochemical plating. The SEI formation initiated by the rapid decomposition of TFSI– of LiTFSI salt in SPE, producing LiF-rich species, followed by the formation of inorganic and polymer-derived products and Li plating. This has been verified by CTTA measurements by quantifying the Li consumption during the early stages of the interface growth, and hence the transition from electrolyte reduction to metallic Li nucleation. Scharifker–Hills framework-based kinetic modeling establishes this sequential SEI formation, leading toward Li nucleation and plating. Hence, this study proposes a systematic pathway toward dissecting SEI formation—crucial information for efficient solid-state battery development.