Vanadium oxide (V2O5) thin films due to layered VO5 pyramids in orthorhombic crystal structure, visible transparency (Eg =2.5 eV) and varied oxidation states have multifunctional applications. Most emergent is the V2O5 based pseudo-capacitive energy storage with high pulsed power capability. The V2O5 exhibits energy storage functionality in high pH aqueous medium. This is detrimental to stability under charge-discharge due to electrode loss via mass transport. We demonstrate energy storage function in V2O5 supercapacitors using the ionic liquid gel electrolyte, address the electrode stability issue and enable flat solid-state cell assembly. With visible V2O5 and gel electrolyte transparency, supercapcitor device over conducting glass has potential for integration with transparent electronics. We used liquid synthesis to realize V2O5 films in layered structure. By inducing Li+ ion intercalation in V2O5 via LiClO4 dopant in ionic liquid gel electrolyte, we investigated its effect on the active V2O5 layer thickness and over energy storage, power density performance of supercapacitors.
The V2O5 film electrodes were deposited by spin coating of HVO4+ sol and air annealing at 300°C for 3h. Using Raman spectra peaks at 144, 283, 403, 526, 700 and 993 cm-1 layered crystalline V2O5 structural phase was established. Optical spectra show 75% transmittance in 700-480 nm range. The ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) with 0.2M LiClO4 dopant formed as gel in PVdF/acetone solution was applied between two V2O5 electrodes enabling large interfacial contact by infiltration to fabricate supercapacitor device. The energy storage mechanism is derived from oxidation-reduction peaks observed in cyclic voltammetry (CV) plots. Basically, V2O5 surface reduction from V(5+) to V(4+) state at cathode upon charging is shown to involve intercalation of Li+ ions accompanied by oxidation at anode. Studies on V2O5 electrode thickness show additional energy storage from electrical double layer charging. The supercapcitor cells in solid-state platform showed energy density of ~ 4.8-7.3 Wh/kg scaling with cell voltage. Specific capacitance values ~ 35-40 F/g show voltage scan rate dependence implying diffusive ion transport lags continuous electron exchange. This reflects on the performance during charging and discharge phase. The discharge rates in V2O5 cells are higher in aqueous medium compared to other oxides, and with ionic liquid, we obtained high power density in the range 20-70 KW/kg. These energy-power values with added transparency factor are attractive for powering compatible electronics. The energy-power parameters of single V2O5 cell can be further enhanced by multiple stacking enabled by the solid-state design. This paper will report on detailed V2O5 electrode, synthesis, structural and electrochemical properties and the energy storage performance of supercapacitor with emphasis on storage mechanism with ionic liquid electrolyte.