Various mixed Fe / V-oxides can be used as anodes in Li rocking chair batteries, however, their 1 small optical modulation during the insertion / extraction of Li ions makes them candidates for the counter electrodes in electrochromic (EC) devices. The sol-gel route in combination with dip-coating deposition was used for the preparation of Fe / V-oxide films with molar ratios Fe:V5 0.1:1, 1:2, 1:1 and 2:1. X-ray diffraction combined with Fourier transform infrared (FT-IR) spectroscopy studies of films and powders reveal that heating of xerogel films at 4008C produces films with nanosized FeVO (Fe:V5 1:1) and Fe V O 4 2 4 1 3 (Fe:V5 1:2) grains, while the corresponding crystalline powders were obtained at 5008C (8 h). Charge capacities (Q) of Fe / V-oxide films (300 and 4008C) were determined using cyclic voltammetry (CV) from 1.5 to 2 1.5 V vs. Ag /AgCl (4.8 to 1.8 V vs. Li) in 1 M LiClO / propylene carbonate (PC) electrolyte. Our results 4 22
revealed that Q values of Fe / V-oxide films are up to 20 mC cm depending on the thickness (40-100 nm), temperature of heating and the Fe:V molar ratio (1:2, 1:1). During the first 300 cycles the cycling stability of the Fe-containing films is better than that of V O crystalline films. UV-visible spectra of charged / discharged 2 5 films revealed that these films, similar to V O films, exhibit a mixed anodic / cathodic electrochromism. It was 2 5
established that with regard to the colouring / bleaching changes of V O crystalline films, the Fe / V-oxide films 2 5
exhibit smaller cathodic colouring at wavelengths l . 600 nm and higher visible transmittance. IR 31 spectroscopy of charged / discharged Fe / V-oxide films confirmed that the reduction of Fe prevents the 51 31 overreduction of V to V , which takes place in V O films cycled in the same potential range.