Functional gel materials for next-generation electrochromic devices and applications

Flexible, wearable, bistable displays, visualized energy storage devices and large-area smart windows based on electrochromic (EC) technology are regarded as promising next-generation sustainable display technologies, with the potential to improve people's lives by enabling low-energy consumption, vision-friendly, smart display, and energy-efficient building solutions. Recently, gel-based EC devices have gained considerable research interest and have emerged as an effective platform for EC applications due to their unique and enhanced properties. Compared to solid-state and liquid-state EC devices, gel-based EC systems offer superior processability and scalability, improved mechanical properties such as flexibility and stretchability, and high ionic conductivity without leakage or volatility issues. This review summarizes and analyzes the gelation chemistry in EC systems, focusing on their relationship with key EC properties of the device. Ionic conductivity, temperature adaptability, and mechanical characteristics of the gels such as stretchability, self-healing ability, flexibility, and viscosity are foundational for enabling diverse functional EC applications. We introduce the preparation methods of related gels for EC devices and then discuss the factors influencing the properties and the strategies for tuning them, including the control of morphology, network architecture, polymer skeletons, functional groups, and additives within ion gels. Representative and latest applications of gel-based electrolytes in EC devices for various promising displays were then presented. Finally, we critically analyze the remaining challenges that need to be addressed to enable the practical deployment of gel-based EC devices and offer more insights into future directions for advancing EC technologies.