Characterization of The Solid Electrolyte Interphase with Cryogenic Ion and Electron Microscopy

Characterizing the nanoscale structure and chemistry of energy storage materials is critical due to their significant impact on battery performance. However, conventional sample preparation methods required for high-resolution imaging often fundamentally alter reactive battery components. In this work, the authors employ cryogenic focused ion beam milling (FIB) in a plasma-focused ion beam/scanning electron microscope system to prepare sensitive lithium metal specimens, allowing them to assess potential ion beam damage. Cryogenic transmission electron microscopy reveals that while the lithium itself is not damaged during cryo-FIB milling, lithium oxide shells form around the sample in the instrument chamber, evidenced by diffraction data from thin lamellae prepared at two different thicknesses. Cryogenic electron energy loss spectroscopy further confirms the oxidation of lithium during sample preparation. Consulting the Ellingham diagram indicates that lithium can react with trace oxygen gas in the FIB/SEM chamber at cryogenic temperatures; notably, liquid oxygen exposure does not contribute to lithium oxidation here. This approach allowed the examination of Li metal batteries with vitrified liquid electrolytes and facilitated the discovery of an elusive solid-electrolyte interphase (SEI) component, lithium fluoride. This has not been observed when using conventional sample preparation techniques involving rinsing. Furthermore, diffraction data reveals the presence of short-range order at different regions in the SEI structures, presumably influencing and controlling lithium metal growth. Overall, these results highlight the valuable role cryogenic lift-out and cryogenic scanning transmission electron microscopy can play in enabling nano- to atomic-scale characterization of energy storage devices containing reactive materials or solid-liquid interfaces.