  Recent debates on the oxygen redox behaviors in battery electrodes have
triggered a pressing demand for the reliable detection and understanding of
non-divalent oxygen states beyond conventional absorption spectroscopy. Here,
enabled by high-efficiency mapping of resonant inelastic X-ray scattering
(mRIXS) coupled with first-principles calculations, we report distinct mRIXS
features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are
successfully reproduced and interpreted theoretically. mRIXS signals are
dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized
oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific
intra-band excitonic feature in mRIXS. Such a feature displays a specific
emission energy in mRIXS, which disentangles the oxidized oxygen states from
the dominating transition-metal/oxygen hybridization features in absorption
spectroscopy, thus providing critical hints for both detecting and
understanding the oxygen redox reactions in transition-metal oxide based
battery materials.
