We investigated the penetration depth profiles of lithium ions in titanium oxide thin film during electrochromic reactions. The penetration depth profiles were obtained using time-of-flight secondary ion mass spectrometry (TOF-SIMS) for various states associated with coloring and bleaching reactions.
It was found that the amount of penetrated lithium ions in the coloring process decreased sharply with increasing depth for samples in which lithium ions were inserted by applying a voltage, Vi, for 50 s in electrolyte. Over 70 % of the total lithium ions were detected within 10 nm from the surface of the film. The amount of ions in each depth was represented as a function of 1/d 2 (d, depth), and the penetration depth of the lithium ions increased by increasing Vi. Furthermore, we found that the de-intercalation of the lithium ions by applying a reverse voltage arise preferentially near the surface of the film. The de-intercalated (released) lithium ion ratio seems to decrease with increasing the penetration depth.
Hence, we expected that an electrochromic device using nanorod-structured titanium oxide film would improve reproducibility owing to a small number of unreleased ions. A film composed of nanorods was made by oblique evaporation. The electrochromic device using the nanorod-structured film showed excellent reproducibility for small value of driving voltage. The number of possible switching increased by a factor of 30 according to the driving voltage.
In this paper, we discuss the details of the relationship between the lithium ion penetration depth profile and the electrochromic reactions.