Boron-Doping Induced Electron Delocalization in Fluorophosphate Cathode: Enhanced Na-Ion Diffusivity and Sodium-Ion Full Cell Performance

Na₃V₂(PO₄)₂O₂F (NVPOF) is widely accepted as advanced cathode material for sodium-ion batteries with high application prospects ascribing to its considerable specific capacity and high working voltage. However, challenges in the full realization of its theoretical potential lie in the novel structural design to accelerate its Na⁺ diffusivity. Herein, considering the important role of polyanion groups in constituting Na⁺ diffusion tunnels, boron (B) is doped at the P-site to obtain Na₃V₂(P_2−xB_xO₈)O₂F (NVP_2−xB_xOF). As evidenced by density functional theory modeling, B-doping induces a dramatic decrease in the bandgap. Delocalization of electrons on the O anions in BO₄ tetrahedra is observed in NVP_2−xB_xOF, which dramatically lowers the electrostatic resistance experienced by Na⁺. As a result, the Na⁺ diffusivity in the NVP_2−xB_xOF cathode has accelerated up to 11 times higher, which secures a high rate property (67.2 mAh g⁻¹ at 60 C) and long cycle stability (95.9% capacity retention at 108.6 mAh g⁻¹ at 10 C after 1000 cycles). The assembled NVP_1.90B_0.10OF//Se-C full cell demonstrates exceptional power/energy density (213.3 W kg⁻¹ @ 426.4 Wh kg⁻¹ and 17970 W kg⁻¹ @ 119.8 Wh kg⁻¹) and outstanding capability to withstand long cycles (90.1% capacity retention after 1000 cycles at 105.3 mAh g⁻¹ at 10 C).