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

Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (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 [P 2 O 7 ] distortion at high voltage. To address this challenge, we introduce a distortion criterion and optimize the local crystal field environment by incorporating Cr 3+ into Fe3 sites adjacent to [P 2 O 7 ]. This substitution elongates Fe1–O bonds, enhances Fe1 activity, and suppresses [P 2 O 7 ] 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 c -axis changes during high-voltage operation, a quantitative method for assessing [P 2 O 7 ] distortion is proposed and confirmed by operando 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.