Anomalous lithium ion diffusion into CeO2·TiO2 thin film by film thickness variations

s CeO 2 ·TiO 2 thin film is considered as an excellent candidate for a passive ion storage layer due to its good electrochemical stability and comparatively great charge capacitance. When cerium-titanium oxide thin film is adopted as an ion storage layer against cathodic tungsten oxide layer, the electrochromic device shows long term durability and cyclability. Therefore, many researchers investigated the composition and crystallinity effects to the charge density. In our study, we prepared CeO 2 ·TiO 2 thin by sol–gel dip-coating method, varying thickness by controlling withdrawal speeds. As investigating results of cyclic voltammetry and chronocoulometry, we found that there are three regions in the film thicknesses: (1) fast lithium ion diffusion region under 100 nm, (2) slow diffusion region in the range of 100 to 150 nm, and (3) fast and great charge capacitance region over 150 nm. In region 1, lithium ions diffuse very fast and reach into indium-tin oxide (ITO) layers, and slow diffusion region follows in region 2, probably due to the remains or impurities within the film, and in region 3, lithium ion diffusion gets fast again, accompanied with charge capacitance increase with thickness.