Clarifying effects of in-plane cationic-ordering degree on anionic redox chemistry in Na-ion battery layered oxide cathodes

Triggering anionic redox reaction (ARR) has been an effective approach to increasing the energy density of sodium-ion battery layered oxide cathodes. However, there are still challenges in achieving highly active and stable ARR. Here, using layered oxide Nax[LiyMn1-y]O2 with ribbon-like superstructure as model compounds, we show that there are effects of in-plane cationic-ordering degree on the ARR activity and stability. A designed Nax[LiyMn1-y]O2 cathode with high cationic-ordering degree exhibits higher ARR capacity and better cycling stability than the one with low cationic-ordering degree. Combined with in-situ X-ray diffraction and other ex-situ characterizations, we reveal that the stable ARR enabled by high cationic-ordering degree can favor reversible structure evolutions and suppress surface/interface side reactions. Furthermore, the positive role of high-degree ordering distribution of in-plane cations in stabilizing the ARR structure is also confirmed by theoretical calculations. This work not only further clarifies the relation between ARR and superstructure but also provides an accessible way to optimize layered oxide cathodes with ARR activity.