Cycling Durability of Electrochromic W-Ti Oxide Thin Films : Optical Transmittance Data Signal Dual Degradation Modes

Electrochemical degradation of electrochromic W oxide and W–Ti oxide thin films, prepared by reactive DC magnetron sputtering, was studied by voltammetric cycling in potential intervals from 1.5–4.0 to 2.0–4.0 V vs Li/Li+ in an electrolyte of lithium perchlorate in propylene carbonate. Cycling-dependent evolution of the upper and lower limits for the optical modulation range was critically dependent on potential interval. This phenomenon was analyzed through an extension of a previously formulated model for power-law dynamics; it was discovered that the upper and lower limits for the optical modulation range varied in distinctly different ways, and that only data acquired in the interval 2.0–4.0 V vs Li/Li+ could be reconciled with dispersive chemical kinetics. We thus find that our data on optical transmittance degradation lends strong support to the existence of dual degradation modes for electrochromism upon extended electrochemical cycling. This result is of great importance for the development of highly durable electrochromic devices such as smart windows for energy-efficient buildings. The present article is a sequel to a recent publication of ours [J. Electrochem. Soc., 166, H795 (2019)]