Sulfur doping induced anionic oxidation of niobium-pentoxide-based anode for ultralong-life and high energy-density Na-ion capacitors

Sodium-ion supercapacitors (SICs) have attracted increasing scientific attention for mid-to-large-scale energy storage applications due to their high energy and power densities. Herein, an ultra-flexible and free-standing hybrid anode material consisting of sulfur-doped Nb2O5 quantum dots (similar to 3 nm) uniformly embedded within nitrogen and sulfur co-doped microporous carbon nanofiber (S-Nb2O5@NS-PCNF) is successfully fabricated by electrospinning followed by a sulfidation treatment. The designed 3D microporous network not only offers a continuous conducting framework for electron-transport, but also provides more accessible channels for rapid Na-ions migration. Furthermore, the S-doping induced anionic oxidation of Nb2O5 (O2-2--> O-) and S-doping in microporous carbon nanofibers result in the formation of numerous oxygen vacancies and defects for enhanced electrical conductivity and surface pseudocapacitance. In particular, the oxygen vacancies induced by the S-doping on Nb2O5 have been firstly demonstrated. This S-Nb2O5@NS-PCNF film electrode exhibits superior rate capability (124 mAh g(-1) at 4 A g(-1)) and ultralong cycling life (173 mAh g(-1) after 10000 cycles at 2 A g(-1)). The SIC full-cell comprising a S-Nb2O5@NS-PCNF anode and an activated carbon cathode delivers a maximum energy density of 112 Wh kg(-1) at 80 W kg(-1) and a ultralong-term cycling stability. This strategy provides a promising application for highly efficient energy storage systems.