Charging and diffusional aspects of Li+ insertion in electrochromic a-WO3

Electrochemical analysis of a-LixWO3 films of 400 nm thick prepared by electron beam evaporation technique over ITO substrates are reported. The chemical potential variation with the fraction of inserted lithium, μ(x), was obtained by chronopotentiometry measurements in quasiequilibrium conditions and the chemical capacitance, Cch∝∂x/∂μ, was determined. A power-law dependence Cch∝xa (a≈0.65) was observed for x>10−3, indicating a wide distribution of site energies. Additionally, impedance measurements at different stages of insertion gave information on the variation of the chemical diffusion coefficient Dch with potential (composition). Impedance spectra were analyzed by means of a spatially restricted diffusion model. Dch was found to vary in the range 10−10–10−12 cm2 s−1 nearly proportional to the inverse of composition, Dch∝1/x. This suggests that ion transport process is considerably influenced by the extent of intercalation.