Enhancing Kinetic Performance and Structural Stability of Na₄Fe₃(PO₄)₂(P₂O₇) Cathode via La Doping Defect Engineering

Due to the strong electronegativity of P in the anion group and the strong P─O covalent bond, NFPP exhibits low electronic and ionic conductivity, hindering its rate capability. A doping modification strategy of selecting La³⁺ with a large ion radius at Na site has been designed, and the nano-micro architectural Na_4-3xLa_x□_2xFe₃(PO₄)₂(P₂O₇)/C (0≤x≤0.04) cathode material with Na vacancies is successfully synthesized via a scalable preparation route. Introducing positively charged substitutional point defects and charged vacancies through doping La³⁺ not only broadens the Na⁺ transport channels but also reduces lattice stress and stabilizes the crystal bulk structure during long-term cycling for La³⁺ as pillars. Additionally, high valence La³⁺ doping enhances the effective charge carrier concentration and improves material conductivity. Consequently, the kinetic performance of Na⁺ migration is significantly enhanced. The optimal Na_3.91La_0.03□_0.06Fe₃(PO₄)₂(P₂O₇)/C (NFPP/C-La3) exhibits the best electrochemical performance. The synthesized NFPP/C-La3 exhibits excellent rate performance (99.45 mAh g^-1 at 20 C) and long-term cycle stability (92.36% of capacity retention over 1000 cycles at 10 C). These results provide the importance and prospect of the high valence ion doping for NFPP/C with high rate stability.