Distinct Oxygen Redox Activities in Li2MO3 (M = Mn, Ru, Ir)

Li₂MO₃ (M = transition metal) systems are parent compounds of Li-rich materials and widely considered to offer oxygen redox for high-energy batteries. However, recent clarifications have revealed that, among the three representative Li₂MO₃ (M = Mn, Ru, Ir) compounds, no reversible oxygen redox takes place in the Mn and Ir systems. Here, we reevaluate the redox reactions in Li₂RuO₃ through advanced spectroscopy, which shows both Ru redox and highly reversible O redox (96% initial-cycle reversibility, 80% retained after 10 cycles, and 77% after 50 cycles). This is in sharp contrast with the Li₂MnO₃ and Li₂IrO₃ systems and concludes the three distinct oxygen behaviors in the Li₂MO₃ systems during charging: (i) only irreversible oxygen oxidation in Li₂MnO₃; (ii) reversible Ru and O redox in Li₂RuO₃; (iii) only cationic redox in Li₂IrO₃. This work suggests the critical role of transition metals and their coupling to oxygen for maintaining reversible oxygen redox activities for high-energy batteries.