Encapsulation of Na3(VO)2(PO4)2F into carbon nanofiber as an superior cathode material for flexible sodium-ion capacitors with high-energy-density and low-self-discharge

As a member of Na-super-ionic conductor (NASICON) family, Na-3(VO)(2)(PO4)(2)F (NVPF) is reported to possess higher redox potential and larger specific capacity than its analogue Na3V2(PO4)(3) (NVP). However, the rate and cycling performance of NVPF is significantly lower than that of NVP. Moreover, almost reported NVPF materials are bulk-type powders, which can not satisfy the requirement of flexible energy-storage devices. Herein, NVPF nanoparticles are uniformly encapsulated into the carbon nanofiber (CNF) by a simple electrospinning & annealing strategy. In the CNF@NVPF architecture, the CNF framework can provide structurally stable hosts for fast Na+ extraction/insertion and effective electron-transport channels. As a result, mechanical flexibility (multiple bending, rolling and twisting) and high Na-storage properties such as superior rate capability (20C) and high cycle stability (96% retention after 2000 cycles), are achieved. By employing CNF@NVPF as the battery-type cathode and ZnO-activated porous carbon nanofiber (pCNF) as the capacitor-type anode, a novel sodium-ion capacitor (SIC) is constructed with both high energy and powder densities (182.8 Wh kg(-1) at 340 W kg(-1) and 85.3 Wh kg(-1) at 5898 W kg(-1) based on the total mass of both electrodes), apparently superior to recently SIC counterparts in literature. Furthermore, the satisfactory mechanical flexibility and the low self-discharge rate are demonstrated in the pouch cell.