Lithium ions protected orbital symmetry enables reversible oxygen redox in layered manganese-based cathodes

With a high capacity at low cost, manganese oxides utilizing oxygen redox chemistry are desirable candidates for replacing nickel- and cobalt-based cathodes. However, they suffer from rapid oxygen activity fading, commonly related to bonding interactions between the transition metals and oxygen, while receiving limited attention on the role of Li ions. Here, we demonstrated the protective role of Li ions in maintaining oxygen orbital symmetry to improve oxygen redox reversibility. The orbital symmetry breaking induced by Li ion migration leads to a disproportionation of oxygen during charging. This results in some oxygen becoming overoxidized and labile, compromising the structure and cycling stability. This mechanism is validated through experiments on P3-Na0.6Li0.2Mn0.8O2 and other layered manganese-based cathodes, as well via dynamic simulations. Finally, employing P2-Na0.75Li0.25Mn0.75O2, we propose a design strategy for stabilizing Li ions via electrostatic field disordering, enhancing the capacity retention over 150 cycles from 46% to 72%. The insights open an avenue towards the rational design of high-performance manganese-based cathodes.