Solid State Supercapacitors for Energy Storage: Materials, Device Engineering, Multifunctionality, and Emerging...

Solid-state supercapacitors (SSS) are emerging electrochemical energy-storage devices that combine high power capability, long cycle life, improved safety, and compatibility with compact and flexible formats. This review examines SSS from an electrical-engineering-oriented perspective, linking materials development with device design and emerging system-level applications. First, the roles of electrode materials, solid and quasi-solid electrolytes, current collectors, substrates, and electrode/electrolyte interfaces are discussed in relation to ion transport, charge transfer, voltage window, equivalent series resistance, and mechanical reliability. Second, recent progress in hierarchical electrodes, interface engineering, flexible architectures, self-charging systems, electrochromic devices, sensing-integrated platforms, and wearable configurations is critically assessed. Third, the review introduces an application-translation framework that connects material and device advances with engineering metrics such as power density, response time, cycling durability, leakage behavior, converter compatibility, module scalability, hybrid energy-storage operation, renewable-energy buffering, and grid-edge support. Persistent challenges, including limited ionic conductivity, interfacial resistance, constrained voltage windows, low practical mass loading, packaging instability, scale-up inconsistency, and nonstandardized testing, are also highlighted. Finally, future directions are proposed toward multifunctional hybrid materials, manufacturable device architectures, digital diagnostics, and intelligent integration of SSS within resilient and sustainable electrical energy systems.