Dual surface/bulk engineering of Nb2O5 for high-rate sodium storage

Orthorhombic Nb₂O₅ is a highly promising fast-charging anode material for sodium-ion capacitors. However, its poor intrinsic electronic/ionic conductivity limits its performance. Here, we developed a one-step heat treatment method to create an N-doped carbon coating on the outside and S-doped Nb₂O₅ on the inside (CN-SCN). Ionic liquids are used as the source of C/N/S, which synergistically enhance the surface and bulk electronic/ionic conductivity. The N-doped carbon coating on the surface exhibits excellent electronic conductivity and a low ion-diffusion barrier, thanks to the high nitrogen ratio and extremely low content (<2 wt%). Auger electron spectroscopy analysis confirms that S atoms detach from the carbon chain of the ionic liquids and enter the bulk Nb₂O₅, resulting in S-doped Nb₂O₅, significantly facilitating reaction kinetics. The CN-SCN electrodes exhibit outstanding rate capability, achieving a capacity of up to 94 mAh g⁻¹ even at a high current rate of 50 C. When paired with activated carbon as the positive electrode, the sodium-ion capacitor with the CN-SCN anode exhibits a high-energy density of up to 59 Wh kg⁻¹ and a long cycle life with 73% capacity retention after 10,000 cycles. This work opens up possibilities for low-cost and large-scale production of high-rate Nb₂O₅ for sodium-storage applications.