Harmonized sodium coordination engineering for high-energy phosphate cathodes

Abstract

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 Na4Fe3(PO4)2P2O7-type cathodes. Combined experimental and theoretical analyses reveal that precise V3+ 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 Na3.4Fe2.4V0.6(PO4)2P2O7 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. Fe-based polyanionic cathodes are promising for Na-ion batteries but are limited by inactive Na sites and irreversible Na loss. Here the authors employ targeted V3+ substitution to tune the Na+ coordination environment, activate inert sites and stabilize high-voltage redox for high-performance Na-ion batteries.