Polymer Electrolytes for Energy Storage and Electrochromic Energy Devices: From Design Strategy to Multifunctional Applications

Polymer electrolytes are key enabling materials for advanced energy‐storage and electro‐optical devices because they couple ion transport with interfacial stabilization, mechanical compliance, and operational safety within a single platform. While their roles in batteries and supercapacitors are well established, recent advances have extended their importance to multifunctional electrochromic systems, including electrochromic supercapacitors, electrochromic batteries, and smart windows, where electrochemical operation is directly coupled with reversible optical modulation. These systems impose tighter design constraints, requiring simultaneous control of ionic conductivity, electrochemical stability, optical transparency, switching uniformity, processability, and environmental durability. This review establishes the general design logic of polymer electrolytes in conventional energy‐storage devices by defining their core functions, major classes, and governing design variables. Representative systems are organized into hydrogels, organogels/ionogels, and free‐standing solid or semi‐solid electrolytes, highlighting how polymer chemistry, salt–solvent interactions, network architecture, and processing strategy govern device‐level behavior. This review discusses how these variables are reweighted in multifunctional electrochromic systems under coupled electrochemical and optical demands. By linking electrolyte classification with device‐dependent design criteria, this review provides a transferable framework for polymer‐electrolyte design across both conventional energy‐storage technologies and next‐generation electro‐optical systems.