Coupling of bowl-like VS2 nanosheet arrays and carbon nanofiber enables ultrafast Na+-Storage and robust flexibility for sodium-ion hybrid capacitors

Abstract

Sodium-ion hybrid capacitors (SIHCs) show great promise due to their combination of high energy-storage properties and low cost of Na resources. Yet most anodes in SIHCs suffer from sluggish Na+-diffusion kinetics and relatively low specific capacity, which restrict their widespread development. Herein, bowl-like VS2 nanosheet arrays are uniformly and robustly anchored on a carbon nanofiber (CNF) substrate through a one-step solvothermal method. The bowl-like VS2 is featured with an ultrathin thickness (several atomic layers), intrinsically metallic conductivity and an interconnected array architecture. This unique structural design can not only facilitate ultrafast Na+ ion-diffusion, but also suppress capacity decay of VS2 that is caused by the conversion process below 0.3 ​V. DFT calculations demonstrate a faster Na+ transport rate and a lower diffusion energy barrier for the VS2/C interface than for the VS2/VS2 bilayer. The quasi-solid-state SIHC full cell that integrates the CNF@VS2 anode and the graphene sheets-wrapped carbon nanofiber (CNF@GS) cathode, displays prominent energy//power densities in the voltage range of 0.0–4.0 ​V (116 ​Wh kg−1 ​at 0.4 ​kW ​kg−1; 66 ​Wh kg−1 ​at 40 ​kW ​kg−1 based on the total mass of both electrodes), outperforming recent SIHCs.