Fluorine substitution enabling pseudocapacitive intercalation of sodium ions in niobium oxyfluoride

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-Nb2O5-xFy subset of 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-Nb2O5-xFy subset of 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-Nb(2)O(5-x)Fy(3)subset of C-NBs exhibit a hierarchical nanoarchitecture with T-Nb2O5- xFy 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-Nb2O5-xFy subset of C-NBs composite, a sodium-ion capacitor fabricated with T-Nb2O5-xFy subset of 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 similar to 250 and 18 000 W kg(-1), respectively.