Localized Electron Density Redistribution in Fluorophosphate Cathode: Dangling Anion Regulation and Enhanced Na‐Ion Diffusivity for Sodium‐Ion Batteries

Abstract Polyanionic transition metal polyphosphate (TMPO)‐type Na 3 V 2 (PO 4 ) 2 O 2 F (NVPO 2 F) is promising as cathode for large‐scale sodium‐ion batteries (SIBs) on account of its considerable capacity and highly stable structure. However, the redox of transition metal and phase transitions along with the (de)intercalation of Na + lead to its slow kinetics and inferior rate performance. Herein, chlorine (Cl) is applied as a heteropical dopant to obtain Cl‐doped NVPO 2 F (NVPO 2−x Cl x F) cathode material for SIBs. Density functional theory investigation reveals that Cl doping tunes the localized electronic density and structure in NVPO 2 F lattice, causing the electron redistribution on vanadium center and dangling anions. Hence, the NVPO 2−x Cl x F cathode exhibits a revised redox behavior of vanadium for Na + extraction/insertion, increases Na + diffusion rate, as well as lowers charge transfer resistance. A Na + storage mechanism of reversible transformations between three phases and V 4+ /V 5+ redox couple for NVPO 2−x Cl x F cathode is verified. The NVPO 2−x Cl x F cathode reveals a high rate capacity of ≈63 mAh g −1 at 30C and great cycle stability over 1000 cycles at 10C. More importantly, outstanding rate property (314 Wh kg −1 at 5850 W kg −1 ) and cycling capability are obtained for the NVPO 2−x Cl x F//3DC@Se full cell. This study demonstrates a brand‐new strategy to prepare advanced cathode materials for superior SIBs.