Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts

Abstract: Investigations of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li K-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li-K sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of in-situ prepared samples, we report the intrinsic Li-K sXAS of Li-metal that displays a prominent leading peak, which has never been revealed before. Furthermore, theoretical simulations show that the Li-K sXAS is strongly affected by the response of the valence electrons to the core-hole due to the low number of valence electrons in Li. We successfully reproduce the Li-K sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, resolution, and especially contributions from transitions which are forbidden in the so-called single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li-K sXAS of Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li surface chemistry in both metallic and ionic states.