Solid electrolytes for solid-state-supercapacitors and their interface chemistry: gel polymers, ionic liquids, ceramics, and beyond

Abstract

Conventional supercapacitors are limited by liquid electrolytes, which pose safety risks, have narrow electrochemical stability windows, are prone to leakage, and offer limited mechanical flexibility. Solid-state and quasi-solid-state electrolytes have appeared as promising alternatives, particularly for flexible and wearable energy storage devices. This review comprehensively summarizes recent developments in polymer electrolytes, inorganic ceramics, ionic gels, and organic–inorganic composites, highlighting their roles in ionic conductivity, electrochemical stability, operating voltage, and device performance. Polymer electrolytes, which combine polymer flexibility with thermal stability, mechanical strength, and enhanced ion transport facilitated by the incorporation of inorganic fillers, receive special attention due to their interface compatibility with solid electrodes. Key strategies such as filler incorporation, polymer crystallinity suppression, in situ polymerization and interfacial engineering to improve electrode–electrolyte contact are critically discussed. Furthermore, advanced in situ and operando characterization techniques are highlighted for their ability to probe buried solid–solid interfaces, track interfacial degradation, and monitor ion transport dynamics in real time. Finally, the review outlines the current challenges and future directions for designing high-performance, safe, and flexible solid-state supercapacitors, providing valuable insights for the development of next-generation energy storage technologies.