Na-Fe-P Stoichiometry-Driven Heterostructure Design With a P₂O₇-Rich Buffering Phase for Stable NASICON-Type...

Na₄Fe₃(PO₄)₂P₂O₇ (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 NaFePO₄ (m-NFP) during synthesis. Here, we induced the in situ formation of a Na_3.12Fe_2.44(P₂O₇)₂ (N_3.12F_2.44PO) 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_3.12F_2.44PO heterostructure exhibits remarkable structural robustness, with the P₂O₇-rich N_3.12F_2.44PO 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_3.12F_2.44PO phase, offering a viable route to heterogeneous NASICON-type cathodes with exceptional cycling durability for sodium-ion batteries.