Harmonized sodium coordination engineering for high-energy phosphate cathodes

Fe-based polyanionic cathodes are promising for large-scale Na-ion batteries owing to their stability, safety and elemental abundance, however their capacity remains limited by electrochemically inactive Na sites and irreversible Na loss. Here we identify that the Na + coordination environment critically influences the Na-site accessibility and redox activity in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 -type cathodes. Combined experimental and theoretical analyses reveal that precise V 3+ substitution at the Fe2 site harmonizes Na + coordination geometry and softens the polyanionic framework, thereby activating previously inert Na sites and stabilizing high-voltage redox reactions above 4 V. The optimized Na 3.4 Fe 2.4 V 0.6 (PO 4 ) 2 P 2 O 7 achieves full Na + utilization (3.4 Na + , 150.7 mAh g −1 ) and a 52% increase in energy density (487 Wh kg −1 ), approaching the practical limit of Li-ion phosphate cathodes. It also demonstrates exceptional durability over 10,000 cycles in the 2.1-4.5 V range and stable pouch-cell performance. These findings provide a coordination-based strategy to overcome intrinsic capacity limitations in phosphate cathodes, enabling high-energy, durable Na-ion batteries.