Active Sodium Occupancy Ratio‐A Guiding Descriptor for Predicting Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Sodium Storage Performance

ABSTRACT Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (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.