Crystal-Field Manipulated [PO] Distortion for Fast Kinetics of NaFe(PO)(PO) Cathode for Sodium-Ion Batteries.

NaFe(PO)(PO) (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 [PO] distortion at high voltage. To address this challenge, we introduce a distortion criterion and optimize the local crystal field environment by incorporating Cr into Fe3 sites adjacent to [PO]. This substitution elongates Fe1-O bonds, enhances Fe1 activity, and suppresses [PO] 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 -axis changes during high-voltage operation, a quantitative method for assessing [PO] distortion is proposed and confirmed by X-ray diffraction (XRD). The optimized NFPP-0.15Cr exhibits exceptional rate performance (91.74 mAh g 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.