Electrochemical and energy storage properties of layer-by-layer assembled vanadium oxide electrode-based solid-state supercapacitor in n+-SnO2:F/n-V2O5 heterostructure device form using ionic liquid gel electrolyte

Abstract

Vanadium oxide film electrodes synthesized by layer-by-layer assembly using sol–gel spin casting and variable 3 & 1 h annealing process over SnO2:F film coated glass substrates are investigated for supercapacitor energy storage using ionic liquid gel-electrolyte. The X-ray photoelectron spectroscopy analysis of V2p3/2 core-level and O1s peak show short-term (1 h) annealing forms vanadium as V2O5 alongside multivalent and oxygen deficient phases, whereas synthesis by 3-h annealing forms stochiometric V2O5 film. In stochiometric V2O5, capacitive contributions are dominantly from redox processes with peaks identified from V5+/V4+, and V4+/V3+ valance states change and partially from electrical double layer (EDL) yielding high specific capacitance of 346.9 Fg−1. Supercapacitor with V2O5 in mixed valence and oxygen defects phases shows enhanced EDL contribution alongside Faradaic yielding 316.2 Fg−1 specific capacitance. These mechanistic differences are analyzed for ionic diffusion limitations. The linear charge/discharge curves at 0.04–0.15 A/g current density show 85–90% Coulomb efficiency and steady energy density 6.8–5.5 Whkg−1 as specific power increases from 1.9 to 5.7 kWkg−1. Oxygen-deficient V2O5-based supercapacitor shows higher 19.2 Whkg−1 energy density declining to 8.9 Whkg−1 with specific power change from 1.1 to 4.6 kWkg−1. Raman spectra show during charge/discharge, the V5+- V3+ redox is mediated by V4.67+, V4.57+, and V3.33+ intermediate valence states. The pseudocapacitive energy storage depends on charge transfer across n+-SnO2:F/n-V2O5 heterostructure with different band alignments for stochiometric and oxygen deficient V2O5. The conduction band shift to the flat band position determines the potential range and extent of V5+ -V3+ redox reaction.