Honeycomb Ordering to Trigger Stable Oxygen Redox Chemistry in Layered Cathode

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. Among the various positive electrode materials that have been investigated, sodium-excess metal oxides Na 2 M O 3 ( M : transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. Therefore, to establish the general strategies for enhancing the capacity of Na 2 M O 3 are primarily important. In this work, using two polymorphs of Na 2 RuO 3 , we demonstrate the decisive role of honeycomb-type cation ordering in Na 2 M O 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) [1]. 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 [2]. The highly stabilized intermediate phase with honeycomb-type cation ordering in the [Na 1/3 M 2/3 ]O 2 slab 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 M O 3 cathodes. [1] M. Tamaru, et al. , M. Okubo & A. Yamada, Electrochem. Commun. (2013) 33, 23-26. [2] B. Mortemard de Boisse, et al. , M. Okubo & A. Yamada, Nat. Commun. (2016) 7 , 11397. Figure 1