The sol‐gel route was used for the preparation of Fe/V oxide films with a molar ratio Fe:V = 1:2. Transmission electron microscopy (TEM) measurements of films prepared at 300°C showed that the films consist of a FeVO 4 ‐ II phase with the grains (1 nm) embedded in an amorphous phase. TEM measurements of films heated at 400°C agreed with the X‐ray diffraction results, confirming Fe 2 V 4 O 13 grains with dimensions up to 50 nm. Charge capacities per unit thickness ( Q ins d − 1 / Q ext d − 1 ) were determined in a 1 M LiClO 4 / propylene carbonate (PC) electrolyte using cyclic voltammetry (1.5 to −1.5 V vs. Ag/AgCl, 4.8 to 1.8 V vs. Li). The IR spectroscopic result is the assignment of the bands between 1000 and 600 cm − 1 to V‐O (terminal) and V‐O‐V/V‐O‐Fe (bridging) stretching modes of VO 4 tetrahedra. Ex situ IR spectra of Fe 2 V 4 O 13 films (400°C) charged/discharged to various extents showed that at low charging ( Q ins d − 1 ( − 0.13 mC cm − 2 nm − 1 ) terminal and bridging modes are affected by lithium insertion. Higher chargings ( Q ins d − 1 ( − 0.33 mC cm − 2 nm − 1 ) result in the amorphization of the films and in the appearance of a broad absorption below 600 nm ascribed to the Li + ‐ O modes. The optical response of films is evaluated in terms of photopic transmittance and chromaticity coordinates. Results confirmed that films are suitable for use in electrochromic transmissive (smart) windows.
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Šurca et al. (2000) studied this question.