Interfacial engineering of NaFePO₄/Na₂FePO₄F heterostructure cathode with enhanced kinetics for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are promising for large-scale energy storage due to the abundance of sodium. Among potential cathodes, maricite NaFePO₄ (m-NFP) offers high theoretical capacity and low cost but is electrochemically inactive, while Na₂FePO₄F (NFPF) possesses favorable Na⁺ diffusion channels yet suffers from limited structural stability. To address these challenges, we develop a heterostructured NFP/NFPF composite coated with N, B-codoped carbon (NFP/F@C) via a facile solid-state method. The composite synergistically integrates the robust framework of m-NFP with the high ionic conductivity of NFPF through a well-defined heterointerface. Consequently, even at a high rate of 5C, the NFP/F@C cathode delivers a high discharge capacity of 63.9 mAh g^-1 and outstanding cycling stability (94.2% capacity retention after 1000 cycles). Enhanced charge-transfer kinetics and Na⁺ diffusion are confirmed by electrochemical analyses. Ex situ XRD reveals highly reversible structural evolution, contrasting with the irreversible amorphization of pristine m-NFP. DFT calculations further demonstrate that the heterointerface narrows the band gap to 0.22 eV and lowers the Na⁺ migration barrier to 0.32 eV. A full cell paired with commercial hard carbon achieves a stable energy density of 215.3 Wh kg^-1, highlighting its practical viability. This work offers an effective interfacial-engineering strategy for designing high-performance, low-cost polyanionic cathodes for advanced SIBs.