Dependence of the estimated diffusion coefficient of Li xWO 3 films on the scan rate of cyclic voltammetry experiments

Abstract

In order to assess the effect of the scan rate on the Li + ion diffusion coefficient as calculated from cyclic voltammetry (CV), a series of CV experiments at various rates (namely 2, 5, 10, 20 and 50 mV/s) has been carried out for the same WO 3 film prepared by electron gun deposition at room temperature. The diffusion coefficient ( D) for lithium ion intercalation into the WO 3 film, calculated from the Randles–Sevcik equation has been compared to that calculated by the Galvanostatic Intermittent Titration Technique (GITT), carried out on the same WO 3 film. It has been found that at high scan rates (above 2 mV/s) the Randles–Sevcik method tends to grossly overestimate diffusion coefficient, possibly due to the occurrence of large potential gradients within the WO 3 film. Compared to the results deduced from GITT, the voltammetric diffusion coefficient was found to be up to 67% higher. It can therefore be concluded that diffusion coefficient values deduced by CV peaks with use of the Randles–Sevcik method must be treated with caution and that low scan rates (2 mV/s or below) are required for credible results.