Atomic-scale roughness of Li metal surface evident in soft X-ray absorption spectroscopy*

Realizing Li metal electrodes depends on fundamental understanding and efficient control of surface properties, which requires reliable characterization of the Li metal surface. Controlled experiments of Li $K$-edge soft X-ray absorption spectroscopy (XAS) reveal evidence of steady oxidation of the Li metal surface even under ultrahigh vacuum (UHV) conditions. The XAS of the short-lived Li metal surface, prepared by \textit{in-situ} scratching, exhibits a prominent peak at 55.6~eV, more intense and at a slightly higher energy than the first peak expected for bulk Li metal at 55~eV. First-principles XAS calculations explain the origin of both the increased intensity and energy shift. This required the use of surface structural models with under-coordinated Li atoms and an estimated 4~\AA~inelastic mean-free-path for Auger electrons, implying extreme surface sensitivity of the measurements to the first 2-3 atomic layers. This work provides a benchmark on both experiment and theory for further studies of Li and other reactive metal surfaces, which are currently under scrutiny for next-generation energy storage devices.