Mg-doped single-crystalline Na2+2xFe2-x(SO4)3 cathode materials with greatly improved rate performance and stable cyclability for low-cost Na-ion batteries

Alluaudite-type Na 2+2x Fe 2-2x (SO 4 ) 3 (NFS) compounds, as a promising polyanionic cathode material for sodium-ion batteries (SIBs), have attracted considerable attention owing to their cost-effectiveness and elevated operating voltage. Nevertheless, practical implementation of NFS has been hindered by inherent limitations including insufficient crystallinity and compromised electronic/ionic conductivity. Herein, we propose a pioneering dual-strategy integrating cation substitution with crystal engineering to synthesize Mg 2+ -doped single-crystalline NFS enveloped by flash graphene (denoted as [email protected] ). Through synchrotron radiation characterization complemented by density functional theory (DFT) simulations, we verified the preferential occupation of magnesium at the target iron site. Notably, while preserving the fundamental alluaudite framework, this strategic doping induced a band structure modulation with reduced bandgap and diminished Na⁺ diffusion barriers, synergistically enhancing charge transfer kinetics. The optimized [email protected] cathode delivers remarkable rate performance (63 mAh g -1 at 100C vs 21 mAh g -1 for the undoped counterpart) and unprecedented cyclability, retaining 92.8 % capacity after 7,000 cycles at 50C. In addition, the pillar effect of Mg 2+ in the crystal, as evidenced by in-situ XRD results, greatly inhibited the migration of Fe 3+ during cycling, maintaining the stable structure and resulting in outstanding cycling stability under drastic conditions. These findings establish a universal crystal engineering paradigm for developing high-performance polyanionic cathodes in energy storage systems.