Sodium-ion batteries (SIBs) are widely pursued as pragmatic alternative to the Li-ion battery technology having close operational similarity along with natural resource abundance. In this sector, apart from classical cationic redox-based cathodes, various new cathodes have been unveiled based on anionic (oxygen) redox activity. In this work, we will showcase two such case studies of novel cathode composition for SIBs exploiting anionic redox activity.

(1) P2-type manganese-rich layered oxides offers high capacity along with fast rate kinetics. To improve their electrochemical properties, reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations can be explored. We have tested a unique Mn-rich layer-spinel composite, Na_0.7(Li_1/18Mn_11/18Ni_3/18Fe_2/18_1/18)O₂–xNa₂MoO₄, leading to a synergistic effect of layered P2 and spinel phases. This layer/spinel biphasic composite was stabilised through Mo doping and its electrochemical activity was studied at different voltage windows. Upon cycling between 1.5–4.5 V, this Mo-based composite exhibited a high specific capacity of 183 mAh.g^-1 involving both cationic and anionic (O^2-/O₂^n-) redox activity. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2´´ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis. The structural and electrochemical properties of this new composite cathode will be described (Fig. a) (manuscript submitted).

(2) The second part will dwell on anionic redox activity of Na-based mixed-metal oxide cathode [Na_0.7Mn_0.6Ni_0.3Co_0.1O₂]. Combining cationic and anionic redox activity, it exhibited a reversible capacity of 120 mAh/g with anionic redox activity centered around 4.3 V (Fig. b). The underlying redox mechanism has been investigated using in-situ XRD and Raman spectroscopy showing peroxo bond formation (Fig. c). The structural and electrochemical properties of this new cathode composition will be explained (manuscript submitted).