Determination of the Structure of the Solid Electrolyte Interphase Using Cryogenic Scanning Electron Nanobeam Diffraction

Cryogenic electron microscopy is seeing increasing use in describing solid-liquid interfaces, specifically the solid-electrolyte interphase in battery systems. We have employed cryogenic focused ion beam milling (FIB) in a plasma-focused ion beam/scanning electron microscope system to prepare sensitive lithium metal specimens, allowing us to assess potential ion beam damage, understand the process of lithium oxide formation of the sample surfaces and suppress the formation of ice contamination. As a result, we reproducibly create thin sections of battery materials for subsequent cryogenic electron microscopy observation. This approach has 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, scanning electron nanobeam diffraction data reveals the presence of short-range order at different regions in the SEI structures, influencing and controlling lithium metal growth and ion and electron transport. We can correlate specific SEI structural motifs and layers with enhanced battery performance, providing important insights into the design of batteries with high cycling stability. Furthermore, we show that prior studies that utilize phase-contrast high-resolution electron microscopy likely report incorrect information regarding SEI structure due to the effects of electron beam irradiation. 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.