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. Among the various positive electrode materials that have been investigated, sodium-excess metal oxides Na2 MO3 (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 Na2 MO3 are primarily important. In this work, using two polymorphs of Na2RuO3, we demonstrate the decisive role of honeycomb-type cation ordering in Na2 MO3. Ordered Na2RuO3 with honeycomb-ordered [Na1/3Ru2/3]O2 slabs delivers a capacity of 180 mAh g-1 (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g-1 (1.0-electron reaction) [1]. We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure [2]. The highly stabilized intermediate phase with honeycomb-type cation ordering in the [Na1/3M2/3]O2 slab induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2 MO3 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
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