Different ion-based electrolytes for electrochromic devices: A review

Initially, electrochromic (EC) technology was an attractive research topic aiming at the development of green buildings and smart displays. Nowadays, EC technology has been extended in various fields of energy storage systems, wearable devices, military camouflage, intelligent thermal control, etc. Electrochromic devices (ECDs) have been considered promising candidates for next-generation multi-functional devices due to their prominent advantages in integrated intelligence, low energy consumption, flexibility, displayable, and adjustable features. Electrochromism refers to the reversible change in color, transmittance, or reflectance of materials via a low voltage. Generally, the EC process lies in the dual adjustment of electrons and ions, so the transmission kinetics of electrolyte ions plays an extremely key role in EC performance. Although Li-ion-based ECDs are still mainstream, the emerging ion-based ECDs (such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, etc) with improved performance have been reported one after another in recent years. This review summarized existing studies about advanced EC design by driving different electrolyte ions and analyzed their positive influence and side effect on EC performance. Meanwhile, the corresponding preparation method, structure design, and ions transport mechanism were discussed in detail. Finally, the facing challenges and development direction of related filed in the future were proposed. Hopefully, this review can provide useful help for the suitable selection of electrolyte cations for different EC systems and the promotion of diverse ion-based ECDs in practical application.