Recent advances and challenges in the development of advanced positive electrode materials for sustainable Na-ion batteries

As there is growing energy demand, the current focus is on the development of low-cost and sustainable energy storage devices. In this regard, the development of rechargeable non-aqueous Na-ion batteries is essential owing to the high availability and economic merits of sodium as compared to lithium. Conventional sodiated transition metal-based oxides NaxMO2 (M = Mn, Ni, Fe, and their combinations) have been considered attractive positive electrode materials for Na-ion batteries based on redox activity of transition metals and exhibit a limited capacity of around 160 mAh/g. Introducing the anionic redox activity-based charge compensation is an effective way to improve the specific capacity and specific energy of transition metal oxides. In this review paper, we will describe recent research progress and perspective of (i) structural aspects of O3 and P2-type metal oxides, (ii) effect of metal oxide synthesis and morphology on the electrochemical performance, (iii) valorization of energy density by introducing the anionic redox activity, (iv) charge storage mechanism and structural evolution during cycling, (v) electrochemical performance of bi-phasic layered oxides, (vi) development of full Na-ion cells involving various oxide cathodes, and (vii) conclusion.