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

Polyanionic transition metal polyphosphate (TMPO)-type Na₃V₂(PO₄)₂O₂F (NVPO₂F) 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 NVPO₂F (NVPO_2−xCl_xF) cathode material for SIBs. Density functional theory investigation reveals that Cl doping tunes the localized electronic density and structure in NVPO₂F lattice, causing the electron redistribution on vanadium center and dangling anions. Hence, the NVPO_2−xCl_xF 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 V⁴⁺/V⁵⁺ redox couple for NVPO_2−xCl_xF cathode is verified. The NVPO_2−xCl_xF 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−xCl_xF//3DC@Se full cell. This study demonstrates a brand-new strategy to prepare advanced cathode materials for superior SIBs.