Structural and electrochromic properties of nanosized Fe/V-oxide films with FeVo(4) and Fe2V4O13 grains: Comparative studies with crystalline V2O5

Various mixed Fe/V-oxides can be used as anodes in Li+ rocking chair batter ies, however, their small optical modulation during the insertion/extractio n of Li+ ions makes them candidates for the counter electrodes in electroch romic (EC) devices. The sol-gel route in combination with dip-coating depos ition was used for the preparation of Fe/V-oxide films with molar ratios Fe :V= 0.1:1, 1:2. 1:1 and 2:1. X-ray diffraction combined with Fourier transf orm infrared (FT-IR) spectroscopy studies of films and powders reveal that heating of xerogel films at 400 degrees C produces films with nanosized FeV O4 (Fe:V=I:I) and Fe2V4O13 (Fe:V=I:2) grains, while the corresponding cryst alline powders were obtained at 500 degrees C (8 h). Charge capacities (Q) of Fe/V-oxide films (300 and 400 degrees C) were determined using cyclic vo ltammetry (CV) from 1.5 to - 1.5 V vs. Ag/AgCl (4.8 to 1.8 V vs. Li) in 1 M LiClO4/propylene carbonate (PC) electrolyte. Our results revealed that Q v alues of Fe/V-oxide films are up to 20 mC cm(-2) depending on the thickness (40-100 nm), temperature of heating and the Fe:V molar ratio (1:2, 1:1). D uring the first 300 cycles the cycling stability of the Fe-containing films is better than that of V2O5 crystalline films. UV-visible spectra of charg ed/discharged films revealed that these films, similar to V2O5 films, exhib it a mixed anodic/cathodic electrochromism. It was established that with re gard to the colouring/bleaching changes of V2O5 crystalline tilms, the FelV -oxide films exhibit smaller cathodic colouring at wavelengths lambda>600 n m and higher visible transmittance. 1R spectroscopy of charged/discharged F e/V-oxide films confirmed that the reduction of Fe3+ prevents the overreduc tion of V5+ to V3+, which takes place in V2O5 films cycled in the same pote ntial range. (C) 2000 Elsevier Science Ltd. All rights reserved.