Potentiostatically pretreated electrochromic tungsten oxide films with enhanced durability: Electrochemical processes at interfaces of indium–tin oxide

Highlights: • The electrochemistry of indium-tin-oxide (ITO) films was studied. • High applied potentials induce a dissolution reaction at an ITO electrode. • In and Sn migrate into an overlying tungsten oxide electrochromic film. • The resulting film combination shows enhanced electrochemical stability. -- Abstract: Recent work has shown that electrochromic WO₃ films, backed by In₂O₃:Sn (ITO) and immersed in a lithium-ion-conducting electrolyte, can attain unprecedented electrochemical cycling durability after potentiostatic pretreatment at high voltage. Here we demonstrate that this intriguing feature is associated with changes in the properties of the ITO film. Specifically, we studied thin films of ITO and WO₃/ITO immersed in an electrolyte of LiClO₄ in propylene carbonate at potentials up to 6.0 V vs. Li/Li⁺ by cyclic voltammetry and impedance spectroscopy and present evidence that electrochemical reactions occur under these conditions. X-ray photoemission spectroscopy indicated that the ITO film was partly dissolved at high voltages and that the dissolution reaction promoted diffusion of In and Sn into the WO₃ film.