Overcoming Sluggish Kinetics in Na<sub>4</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>P<sub>2</sub>O<sub>7</sub> via Synergistic Zr Doping and Iron Defect Engineering for High-Performance Sodium-Ion Batteries
Yue Wang, Xue Zhang, Xuejie Wang, Jianhui Zhong, Jiaguo Yu, Tao Liu
Abstract
The Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (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 NaFePO 4 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. Zr 4+ 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 Fe 2+ /Fe 3+ redox reactions. A homogeneous carbon layer deposited on the particle surface enhances electrical conductivity and mitigates mechanical degradation. The Na 4 Fe 2.92 Zr 0.02 (PO 4 ) 2 (P 2 O 7 ) 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.