Realizing high-performance Na3V2(PO4)2O2F cathode for sodium-ion batteries via Nb-doping

Na₃V₂(PO₄)₂O₂F (NVPOF) has received considerable interest as a promising cathode material for sodium-ion batteries because of its high working voltage and good structural/thermal stability. However, the sluggish electrode reaction resulting from its low intrinsic electronic conductivity significantly restricts its electrochemical performance and thus its practical application. Herein, Nb-doped Na₃V_2−xNb_x(PO₄)₂O₂F/graphene (rGO) composites (x = 0, 0.05, 0.1) were prepared using a solvothermal method followed by calcination. Compared to the un-doped NVPOF/rGO, doping V-site with high-valence Nb element (Nb⁵⁺) (Na₃V_1.95Nb_0.05(PO₄)₂O₂F/rGO (NVN05POF/rGO)) can result in the generated V⁴⁺/V³⁺ mixed-valence, ensuring the lower bandgap and thus the increased intrinsic electronic conductivity. Besides, the expanded lattice space favors the Na⁺ migration. With the structure feature where NVN05POF particles are attached to the rGO sheets, the electrode reaction kinetics is further accelerated owing to the well-constructed electron conductive network. As a consequence, the as-prepared NVN05POF/rGO sample exhibits a high specific capacity of ∼72 mAh·g⁻¹ at 10C (capacity retention of 65.2% (vs. 0.5C)) and excellent long-term cycling stability with the capacity fading rate of ∼0.099% per cycle in 500 cycles at 5C.