Doping‐Induced Ultrafast Kinetics of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathode Enabling Wide Temperature Sodium‐Ion Battery

Abstract Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is a promising cathode material for sodium‐ion batteries (SIBs) because of its low cost and relatively stable cycling performance. However, NFPP behaves neither satisfied rate capability at low temperature nor long‐term stable cycling performance at high temperature due to its inherent low conductivity and structural degradations upon cycling, which hinders its broad applications under extreme conditions. Herein, a wide‐temperature Na 3.96 K 0.04 Fe 3 (PO 4 ) 2 P 2 O 7 (NKFPP) is developed via a small amount of K doping. The doping not only reduces the migration energy barrier of Na + , but also inhibits structural rearrangement and anion cluster deformation during the desodiation from the Na1 and Na4 sites, which are verified by comprehensive characterizations. As a result, the obtained NKFPP has a discharge capacity of 87.1 mAh g −1 at 40C (1C = 129 mA g −1 ) and 30 °C, and maintains 75.41% capacity retention for 10 000 cycles. Even at 10C and 60 °C, the NKFPP delivers a high discharge capacity of 94.3 mAh g −1 with capacity retention of 90.12% for 1000 cycles. Moreover, the K‐doped NFPP exhibits obviously better rate capability than the undoped sample at −20 °C. Therefore, this work provides guidance for further research and development of high‐performance, wide‐temperature‐range polyanion phosphate cathode materials.