Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode

Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na(2)MO(3) (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na(2)MO(3) are poorly established. Here using two polymorphs of Na(2)RuO(3), we demonstrate the critical role of honeycomb-type cation ordering in Na(2)MO(3). Ordered Na(2)RuO(3) with honeycomb-ordered [Na(1/3)Ru(2/3)]O(2) slabs delivers a capacity of 180 mAh g(−1) (1.3-electron reaction), whereas disordered Na(2)RuO(3) only delivers 135 mAh g(−1) (1.0-electron reaction). We clarify that the large extra capacity of ordered Na(2)RuO(3) is enabled by a spontaneously ordered intermediate Na(1)RuO(3) phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na(2)MO(3) cathodes.