Unveiling SEI Formation Dynamics of PEO: LiTFSI with Lithium Metal: An In Situ Approach Combining SIMS, XPS, and CTTA

Abstract Electrolyte interfaces with freshly plated lithium metal are crucial for the development of reservoir‐free all‐solid‐state batteries (ASSBs). This study provides an in‐depth characterization of the polyethylene oxide (PEO) lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) interphase with lithium metal resembling reservoir‐free cell conditions. Using an in situ setup in both secondary ion mass spectrometry (SIMS) and X‐ray photoelectron spectroscopy (XPS), the dynamic evolution of the solid electrolyte interphase (SEI) layer is investigated at 60 °C. Lithium metal plating is conducted with an electron flood gun, ensuring controlled lithium deposition and enabling direct identification of intermediate reaction products. Time‐of‐flight‐ (ToF‐) and Orbitrap‐SIMS are employed to provide high‐resolution insights into the formation of inorganic and organic SEI components coming from PEO and LiTFSI degradation after plating. XPS further reveals chemical transformations occurring at the interface. Additionally, coulometric titration time analysis (CTTA) captures the kinetic aspect of PEO: LiTFSI reaction with lithium metal, highlighting the influence of molecular weight and salt concentration on initial reaction speed and long‐term stability. By integrating high‐resolution SIMS analysis with surface‐sensitive XPS and electrochemical methods, a comprehensive approach is offered to better understand the dynamic behavior of the polymer electrolyte | lithium metal interphase under conditions relevant to reservoir‐free battery concepts.