Honeycomb superstructure enabled by Mg doping for enhanced anionic redox reversibility in layered oxide cathodes

Layered oxides show great promise as cathode materials for sodium-ion batteries, attributed to their high theoretical specific capacity. Activating anionic redox reactions (ARR) is considered an effective strategy for further increasing the energy density. However, ARR is often accompanied by lattice oxygen loss and irreversible structural phase transformation. Herein, a honeycomb superstructure is constructed by introducing an appropriate amount of Mg at Mn sites within the transition metal (TM) layer of P2-Na0.72Li0.23Mn0.77O2. Thus, the collaborative regulation of lattice oxygen stability and phase structure stability is realized. First, upon Mg incorporation into the TM layer, a well-defined MgMn6 honeycomb superstructure is constructed. The honeycomb superstructure effectively suppresses phase transformation arising from Mn migration within the TM plane and simultaneously mitigates oxygen release associated with vacancy clustering. Second, the incorporation of Mg2+ with a larger ionic radius than Mn3+ and Mn4+ expands the interlayer spacing, thereby facilitating Na+ diffusion kinetics. Consequently, the as-optimized cathode achieves 86.5% capacity retention over 100 cycles at 1 C and 72.1 mAh g−1 at 5 C, demonstrating outstanding cyclability and rate capability. These findings provide a feasible modification strategy for improving anionic redox reversibility and energy density in layered oxide cathodes.