Occupied Electronic States of Li in Li, Li₂O₂, and Li₂O Analyzed by Soft X-ray Emission Spectroscopy

Lithium metal and lithium oxides are components of lithium-oxygen (Li-O₂) batteries. To accurately identify Li compounds and understand the degradation mechanism, fundamental knowledge of the electron structures of constituent elements is vital. However, experimentally derived occupied states of Li have been missing due to the intrinsic difficulties in their detection. Herein, using soft X-ray emission spectroscopy, ultrahigh-energy-resolution spectra of Li-K were collected for three critical Li compounds: Li, Li₂O₂, and Li₂O. Large chemical shifts to lower energies and peak broadening were observed in compound-specific Li-K and O-K spectra. Theoretical calculations confirm that these changes derive from the characteristic electronic configurations of 1s and 2p states with core-level shifts in Li⁺. The large chemical shift (∼4.6 eV) between the Li and Li₂O peaks was utilized to visualize the chemical state mapping of the Li metal/oxide phase, facilitating the identification of chemical phases in Li compounds.