Boosting sodium-ion battery performance with vanadium substituted Fe, Ni dual doped fluorophosphate cathode over a wide temperature range

Recently, polyanionic material has been identified as the most attractive choice for the cathode in sodium-ion batteries. However, the poor electronic conductivity of Na₃V₂(PO₄)₂F₃ (NVPF) limits its electrochemical performance, while the presence of vanadium increases material costs. To address these challenges, we have synthesized carbon-coated Fe, Ni dual-doped NVPF (Na₃V_1.9Fe_0.01Ni_0.09(PO₄)₂F₃) via a rota tumbler assisted sol-gel process and reported for the first time. Comprehensive studies reveal that dual doping significantly affects the structural, morphological, and electrochemical properties of the material. Rietveld refinement shows that doping adjusts the crystal structure, enlarging Na⁺ diffusion pathways and enhancing diffusion kinetics. Na₃V_1.9Fe_0.01Ni_0.09(PO₄)₂F₃ exhibits a superior capacity of 115.58 mAhg⁻¹ at 0.1C, 92.86 mAhg⁻¹ at 2C, 87.79 % cyclic retention at 2C after 500 cycles. The optimized material demonstrates robust performance across a wide temperature range (55 °C to −21.1 °C). Furthermore, full-cell constructed using Na₃V_1.9Fe_0.01Ni_0.09(PO₄)₂F₃ as cathode and hard carbon as anode delivers an impressive capacity of 87.01 mAhg⁻¹ at 1C and a retention of 94.50 % for 100 cycles at 0.5C. Moreover, reducing the vanadium content in the cathode helps lower the overall manufacturing costs. Our research demonstrates that Na₃V_1.9Fe_0.01Ni_0.09(PO₄)₂F₃ is a promising option for a high-performance cathode in sodium-ion batteries.