Unlocking Ultra-Long Cycling Stability in Fluorophosphate Cathodes via Electrostatic Interaction Regulation and Enhanced V─O Covalency.

Abstract

Polyanionic Na₃(VO)₂(PO₄)₂F is a promising cathode for sodium-ion batteries (SIBs) due to its stable structural framework and high operating voltage. However, its practical application is hindered by low electronic conductivity and sluggish Na⁺ diffusion kinetics, which originate from the strong Coulombic attraction between Na⁺ and the framework anions, and the Na⁺-Na⁺ repulsion. In this study, we propose a novel anion engineering strategy involving simultaneous Br doping and Na vacancy. Theoretical and experimental analyses reveal that the partial substitution of O^2- with less electronegative Br^- induces local charge redistribution, which enhances V 3d─O 2p orbital hybridization and strengthens V─O covalent bonds, improving structural stability and narrowing bandgap. The resulting charge compensation creates sodium vacancies that alleviate electrostatic repulsion among Na⁺ ions, facilitating Na⁺ diffusion. Moreover, Br doping expands interlayer spacing and mitigates charge transfer resistance. Consequently, the electrode exhibits exceptional long-term cyclability (62.07 mAh g^-1 after 90,000 cycles at 20 C) and superior rate capability (85.93 mAh g^-1 at 100 C). The full cell paired with a hard carbon achieves high energy density and excellent cycling stability. This work provides a feasible and effective anionic doping approach for designing long-life SIBs.