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

The evolving oxygen state plays key roles in the performance and stability of high-energy batteries involving oxygen redox reactions. 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– with strong covalency) molecules and compared directly with Li₂O₂ (O^–) and the oxidized oxygen state in representative Na/Li-ion battery electrodes. Our results confirm again that the critical mRIXS feature around the 523.7 eV emission energy is from intrinsically oxidized oxygen, but not from the highly covalent oxygen state (CO₂). The comparison of the mRIXS profile of the four different oxygen states, i.e., O^2–, O^–, O^n– (0 < n < 2), and O⁰, reveals that oxygen redox states in batteries have distinct widths and positions along the excitation energy compared with Li₂O₂ and O₂. The nature of the oxidized oxygen state in oxide electrodes is thus beyond a simple molecular configuration of either peroxide or O₂.