Overcoming Sluggish Kinetics in Na₄Fe₃(PO₄)₂P₂O₇ via Synergistic Zr Doping and Iron Defect Engineering for High-Performance Sodium-Ion Batteries

The Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode material shows promising potential for sodium-ion batteries (SIBs) due to its cost-effectiveness and high theoretical capacity. However, the unavoidable formation of NaFePO₄ impurities during synthesis and its poor intrinsic electrical conductivity have restricted its large-scale practical application. Herein, we propose a comprehensive strategy integrating Zr doping, Fe-defect engineering, and carbon coating to synergistically optimize the electrochemical performance of NFPP. Zr⁴⁺ doping induces lattice distortion in the NFPP framework, creating additional interstitial sites for Na⁺ migration and accelerating ion transport kinetics. Meanwhile, the introduction of Fe-defects modifies the local electronic structure by generating defect states near the Fermi level, which lowers the energy barrier for Fe²⁺/Fe³⁺ redox reactions. A homogeneous carbon layer deposited on the particle surface enhances electrical conductivity and mitigates mechanical degradation. The Na₄Fe_2.92Zr_0.02(PO₄)₂(P₂O₇) achieves a high capacity of 91 mAh g^-1 at 50 C and 95.24% capacity retention after 4500 cycles at 10 C. This work provides a paradigm for rational design of polyanionic cathode materials, demonstrating that atomic-level compositional tuning and structural engineering can overcome the intrinsic limitations of NFPP for practical SIB applications.