<p>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 Na_3.4Fe_2.4V_0.6(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.</p>