Fluorine substitution enabling pseudocapacitive intercalation of sodium ions in niobium oxyfluoride

Abstract

Low electrical conductivity and sluggish charge storage kinetics are the key issues of orthorhombic niobium pentoxide (T-Nb₂O₅) for sodium-ion batteries. Here, we report on an approach for improving the electrochemical properties of T-Nb₂O₅ using fluorine substitution and carbon modification strategies. The obtained orthorhombic niobium oxyfluoride/carbon nanobelt composite (T-Nb₂O_5−xF_y⊂C-NBs) displayed significantly improved electrochemical properties with sodium ion storage capacity as high as 292 mA h g⁻¹ at 0.05 A g⁻¹, along with an excellent cycling stability over 10 000 cycles at 1 A g⁻¹ (0.002% capacity decay per cycle) as measured using a half cell. An intercalation-pseudocapacitance mechanism (1.0–3.0 V vs. Na/Na⁺) for storing sodium ions was observed in T-Nb₂O_5−xF_y⊂C-NBs, along with a conversion reaction mechanism (<0.2 V vs. Na/Na⁺), leading to an improved energy storage performance and faster kinetics. Density functional theory calculations revealed that the fluorine-substituted niobium oxyfluoride possesses energetically more favourable sodiation sites and lower diffusion barriers compared to the pristine T-Nb₂O₅. Characterisation results confirmed that the self-assembled T-Nb₂O_5−xF_y⊂C-NBs exhibit a hierarchical nanoarchitecture with T-Nb₂O_5−xF_y nanoslabs uniformly embedded in a carbon nanobelt matrix to form arrays, enabling excellent electron conductivity and electron/ion transport, as well as structural stability against cycling. Benefitting from both the compositional and structural advantages of the T-Nb₂O_5−xF_y⊂C-NBs composite, a sodium-ion capacitor fabricated with T-Nb₂O_5−xF_y⊂C-NBs as the anode and a commercial activated carbon as the cathode delivered energy densities of 86.8 and 32.1 W h kg⁻¹ at power densities of ∼250 and 18 000 W kg⁻¹, respectively.