  Realizing reversible reduction-oxidation (Redox) reactions of lattice oxygen
in batteries is a promising way to improve the energy and power density.
However, conventional oxygen absorption spectroscopy fails to distinguish the
critical oxygen chemistry in oxide-based battery electrodes. Therefore,
high-efficiency full-range mapping of resonant inelastic X-ray scattering
(mRIXS) has been developed as a reliable probe of oxygen redox reactions. Here,
based on mRIXS results collected from a series of Li1.17Ni0.21Co0.08Mn0.54O2
electrodes at different electrochemical states and its comparison with
peroxides, we provide a comprehensive analysis of five components observed in
the mRIXS results. While almost all the components change upon electrochemical
cycling, comparison with peroxide species show that only two evolving features
correspond to the critical oxidized oxygen states. One is a specific feature at
531.0eV excitation and 523.7eV emission energy, the other is a low-energy loss
feature. We show that both features evolve with electrochemical cycling of
Li1.17Ni0.21Co0.08Mn0.54O2 electrodes, and could be used for characterizing
oxygen redox states in battery electrodes. This work is the first benchmark for
a complete assignment of all the important mRIXS features collected from
battery materials, which provides guidelines for future studies in
characterization, analysis and theoretical calculation for probing and
understanding oxygen redox reactions.
