Full Energy Range Resonant Inelastic X-Ray Scattering of O2 and CO2: Direct Comparison with Oxygen Redox State in Batteries

The varying oxygen state plays key roles in the performance and stability of various electrochemical systems. However, the nature of the non-divalent state remains elusive with speculations under active debates. A direct comparison between these unconventional states on their full oxygen spectroscopic profile is critical but remains missing. Here, high-efficiency full energy range O-K mapping of resonant inelastic x-ray scattering (mRIXS) was collected from O₂ (O⁰) and CO₂ (O^2-) gas molecules. The results are compared directly with Li₂O₂ (O^1-) and more importantly, the oxidized oxygen (O^n-, 0-n-2) state in representative Na-ion and Li-ion battery electrodes. All the mRIXS features of O₂ and CO₂ are interpreted, and we focus on the contrasts of two characteristic features among all oxidized oxygen species, especially on the striking 523.7 eV emission feature. The full mRIXS profile reveals that oxygen redox states in batteries have distinct distributions along the excitation energy compared with Li₂O₂ and O₂. This work provides not only the first full range mRIXS results of O₂ and CO₂, but also the direct comparison of four different oxygen states, i.e., O^2-, O^1-, O^n-(0-n-2), and O⁰. Our results indicate that the nature of the oxidized oxygen state in oxide electrodes is beyond a simple molecular configuration of either peroxide or O₂.