Large-Area, Uniform, Aligned Arrays of Na3 (VO)2 (PO4 )2 F on Carbon Nanofiber for Quasi-Solid-State Sodium-Ion Hybrid Capacitors.

Sodium-vanadium fluorophosphate (Na3 V2 O2 x (PO4 )2 F3-2 x , NVPF, 0 ≤ x ≤ 1) is considered to be a promising Na-storage cathode material due to its high operation potentials (3.6-4 V) and minor volume variation (1.8%) during Na+ -intercalation. Research about NVPF is mainly focused on powder-type samples, while its ordered array architecture is rarely reported. In this work, large-area and uniform Na3 (VO)2 (PO4 )2 F cuboid arrays are vertically grown on carbon nanofiber (CNF) substrates for the first time. Owing to faster electron/ion transport and larger electrolyte-electrode contact area, the as-prepared NVPF array electrode exhibits much improved Na-storage properties compared to its powder counterpart. Importantly, a quasi-solid-state sodium-ion hybrid capacitor (SIHC) is constructed based on the NVPF array as an intercalative battery cathode and porous CNF as a capacitive supercapacitor anode together with the P(VDF-HFP)-based polymer electrolyte. This novel hybrid system delivers an attractive energy density of ≈227 W h kg-1 (based on total mass of two electrodes), and still remains as high as 107 Wh kg-1 at a high specific power of 4936 W kg-1 , which pushes the energy output of sodium hybrid capacitors toward a new limit. In addition, the growth mechanism of NVPF arrays is investigated in detail.