Active Sodium Occupancy Ratio-A Guiding Descriptor for Predicting Na₄Fe₃(PO₄)₂P₂O₇ Sodium Storage Performance

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) has recently attracted extensive attention as a highly promising cathode material for sodium-ion batteries. However, the intrinsic relationship between its local structure and electrochemical properties remains poorly understood, which is crucial for developing high-performance NFPP. Herein, we propose and validate the Na1/Na4 occupancy ratio as a key descriptor for directly predicting electrochemical performance of NFPP. By precisely controlling the Na/Fe stoichiometric, a low Na1/Na4 occupancy ratio can be obtained, which activates dormant Na1 sites and reduces the Na⁺ migration barrier, thereby optimizing ion-diffusion pathways. NFPP prepared following this principle exhibits outstanding performance at Na1/Na4 ratio of 0.90, achieving a high specific capacity of 80.1 mAh g^-1 at an ultra-high rate of 50 C and retaining 85.7% of its initial capacity after 20 000 cycles. This work reveals a close correlation between the Na1/Na4 occupancy ratio and the electrochemical performance of NFPP, providing clear design guidelines for developing advanced NFPP cathodes.