Salt Electrolytes for Vanadium Pentaoxide-Based Electrochromic Devices

Electrochromic (EC) materials are those that change their optical absorption under an external voltage related to changes in their oxidation state. They are used in displays for electronic paper, smart windows, and switchable mirrors. Among the different EC materials, vanadium oxide (V2O5) is unique as it can act as both a cathodic and an anodic EC layer. Typically, lithium-based electrolytes are used with V2O5, but they pose environmental and processing challenges. This study evaluates earth-abundant alkali and alkaline-earth metal salts as potential electrolytes for a V2O5-based EC device. The V2O5 films are grown by DC reactive magnetron sputtering followed by vacuum annealing, which produces a polycrystalline nanoneedle morphology with high surface area. The EC properties of these films are studied using electrolytes based on sodium, potassium, and calcium chlorides, which are safer and more eco-friendly compared to lithium-based electrolytes. The study demonstrates stable and efficient EC performance with high coloration efficiency and a yellowish-green (bleached state)-to-gray (colored state) transition. The films present fast colored/bleached times, with calcium ion (Ca2+) being the most efficient. This calcium chloride electrolyte is further used to fabricate a solid-state EC device with fluorine-doped tin oxide as the counter electrode. The results from this study help in understanding the kinetics of the EC behavior of V2O5 and demonstrate the feasibility of developing Li-free electrolytes for EC devices.