Cycling mechanism of Li₂MnO₃: Li–CO₂ batteries and commonality on oxygen redox in cathode materials

Li₂MnO₃ has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li₂MnO₃, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li₂MnO₃, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li₂MnO₃ surface, which inspires Li-CO₂/air batteries with Li₂MnO₃ acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li₂MnO₃ questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.