Boosting sodium-ion mobility in Na3V2(PO4)2F3 through anion engineering with Br− substitution

Abstract

Enhancing Na-ion intercalation in Na₃V₂(PO₄)₂F₃ (NVPF) without compromising cycling stability is crucial for the development of high-performance sodium-ion batteries. This study demonstrates that partial substitution of F⁻ with less electronegative Br⁻ in NVPF facilitates electrochemical activation of the third Na ion while preserving structural integrity. A series of Br-substituted NVPF hollow microspheres, Na₃V₂(PO₄)₂F_3(1−x)Br_3x (x = 0.1, 0.2, 0.3, 0.4), were synthesized and evaluated. Among them, NVPF-Br (x = 0.2) exhibited the highest electrochemical performance, delivering a high discharge capacity of 169 mA h g⁻¹ at 64 mA g⁻¹ within a 1.2–4.3 V window and retaining 134 mA h g⁻¹ with 100% coulombic efficiency after 150 cycles. Structural analysis revealed that optimised Br substitution causes the structural transformation from orthorhombic to tetragonal structure. This structural rearrangement expands the 2D tunnel framework, enabling faster Na-ion diffusion and reducing migration activation energy, as confirmed by density functional theory (DFT) and nudged elastic band (NEB) calculation. Moreover, a stable cathode electrolyte interphase (CEI) layer, minimal irreversible capacity loss, and suppressed carbonate formation contributed to improved long-term cycling stability. These findings demonstrate the role of Br substitution in optimizing NVPF cathodes for next-generation sodium-ion batteries.