Crystal-Field Manipulated [P<sub>2</sub>O<sub>7</sub>] Distortion for Fast Kinetics of Na<sub>4</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>(P<sub>2</sub>O<sub>7</sub>) Cathode for Sodium-Ion Batteries
Weishun Jian, Xinyu Hu, Jinqiang Gao, Jingyao Zeng, Yu Mei, Haoji Wang, Ningyun Hong, Jiangnan Huang, Kai Wang, Wentao Deng, Guoqiang Zou, Hongshuai Hou, Hongyi Chen, Xiaobo Ji
Abstract
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.