Oxygen reaction activities in battery materials Li₂MO₃ studied through resonant inelastic soft x-ray spectroscopy (RIXS)

Recent clarification through high efficiency mapping of resonant inelastic X-ray scattering (mRIXS) have revealed that, oxygen reaction activities are distinctive among the three representative Li-rich parent compounds, Li₂MO₃ (M=Mn, Ru, Ir), i.e., only irreversible oxygen oxidation reaction for Mn (3d) system, separate reversible Ru and O redox reactions for Ru (4d) system, and only divalent cationic redox reaction for Ir (5d) systems. Here, through quantification of transition metal redox and characterizations of the lattice oxygen state, we finally provide a direct comparison among the three representative Li2MO3 compounds. Firstly, Li-rich configuration does not naturally trigger reversible oxygen redox, as there is no oxygen redox observed in Mn and Ir systems. It indicates that extra critical TM factors are required to maintain the reversible oxygen redox. Secondly, compared with Ru and Ir systems with both Ru/O redox involved and only Ir redox involved, separately, it indicates that oxygen redox is not necessarily enabled until the Li extraction amount which cationic redox could compensate is exhausted. Thirdly, comparison between irreversible oxygen oxidation process in Li2MnO3 and the high reversible oxygen redox activities in Ru systems, suggests the critical role for stabilization effect of TMs-O hybridization in oxygen activities, which has been emphasized in the ground-breaking works of the oxygen redox but got largely overlooked later. This work suggests the important role of transition metals and their coupling and hybridization effect to oxygen for maintaining reversible oxygen redox activities.