CaF₂: A novel electrolyte for all solid-state electrochromic devices

The energy consumption in building ventilation, air, and heating conditioning systems, accounts for about 25% of the overall energy consumption in modern society. Therefore, cutting carbon emissions and reducing energy consumption is a growing priority in building construction. Electrochromic devices (ECDs) are considered to be a highly promising energy-saving technology, due to their simple structure, active control, and low energy input characteristics. At present, H⁺, OH^- and Li⁺ are the main electrolyte ions used for ECDs. However, H⁺ and OH^- based electrolytes have a high erosive effect on the material surface and have a relatively short lifetime. Li⁺-based electrolytes are limited due to their high cost and safety concerns. In this study, inspired by prior research on Ca²⁺ batteries and supercapacitors, CaF₂ films were prepared by electron beam evaporation as a Ca²⁺-based electrolyte layer to construct ECDs. The structure, morphology, and optical properties of CaF₂ films were characterized. ECDs with the structure of ITO (indium tin oxide) glass/WO₃/CaF₂/NiO/ITO show short switching times (22.8 s for the coloring process, 2.8 s for the bleaching process). Additionally, optical modulation of the ECDs is about 38.8% at 750 nm. These findings indicate that Ca²⁺ based ECDs have the potential to become a competitive and attractive choice for large-scale commercial smart windows.