Na–Fe–P Stoichiometry‐Driven Heterostructure Design With a P ₂ O ₇ ‐Rich Buffering Phase for Stable NASICON‐Type Sodium‐Ion Cathodes

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is a NASICON‐type iron‐based polyanionic cathode that is attracting increasing interest for sodium‐ion batteries because of its low cost, robust framework, three‐dimensional Na + transport channels, and high operating voltage. However, its electrochemical performance is severely compromised by the formation of electrochemically inactive maricite NaFePO 4 (m‐NFP) during synthesis. Here, we induced the in situ formation of a Na 3.12 Fe 2.44 (P 2 O 7 ) 2 (N 3.12 F 2.44 PO) phase within NFPP through stoichiometry engineering to construct a heterogeneous composite. This strategy effectively suppresses m‐NFP formation and improves reversible capacity, while retaining the intrinsic cost advantage of NFPP without incorporating additional elements or extra synthetic steps. The optimized NFPP/N 3.12 F 2.44 PO heterostructure exhibits remarkable structural robustness, with the P 2 O 7 ‐rich N 3.12 F 2.44 PO phase acting as a structural buffer to mitigate lattice deformation, enabling an ultralong cycling life of 10,000 cycles at 40C (1C = 129 mA g −1 ) with 83.6% capacity retention. By mapping phase evolution across a controlled Na‐Fe‐P compositional window, we further reveal that Na and Fe contents govern the formation propensity of the N 3.12 F 2.44 PO phase, offering a viable route to heterogeneous NASICON‐type cathodes with exceptional cycling durability for sodium‐ion batteries.