Crystal-Field Manipulated [P₂O₇] Distortion for Fast Kinetics of Na₄Fe₃(PO₄)₂(P₂O₇) Cathode for Sodium-Ion Batteries

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material for sodium-ion batteries with cost-effectiveness and structural stability. However, its electrochemical behaviors are seriously hindered by its [P₂O₇] distortion at high voltage. To address this challenge, we introduce a distortion criterion and optimize the local crystal field environment by incorporating Cr³⁺ into Fe3 sites adjacent to [P₂O₇]. This substitution elongates Fe1-O bonds, enhances Fe1 activity, and suppresses [P₂O₇] distortion, facilitating fast Na⁺ diffusion and structural reversibility, as validated by X-ray absorption fine structure (XAFS) and density functional theory (DFT) calculations. Based on <i>c</i>-axis changes during high-voltage operation, a quantitative method for assessing [P₂O₇] distortion is proposed and confirmed by <i>operando</i> X-ray diffraction (XRD). The optimized NFPP-0.15Cr exhibits exceptional rate performance (91.74 mAh g^-1 at 50C), long-term cycling stability (88.81% capacity retention after 10,000 cycles at 50C), and wide temperature tolerance (-40 to 60 °C). This study provides a strategic approach for designing high-performance iron-based mixed phosphate cathodes, advancing their practical application in sodium-ion batteries.