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

Abstract 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.