Recent advances in solid-state supercapacitors: From emerging materials to advanced applications

Solid-state supercapacitors (SSCs) hold great promise for next-generation energy storage applications, particularly portable and wearable electronics, implementable medical devices, the Internet of Things (IoT), and smart textiles. This review is intended to present the broad picture of SSC technology by covering various kinds of all-solid-state and flexible solid-state supercapacitors. The review begins with introducing a brief history of the development of supercapacitors and then discusses the fundamentals, charge storage mechanisms, and the performance evaluation methods of SSCs. The next chapter provides an overview of the solid-state electrolytes, notably solid polymer electrolytes, inorganic electrolytes, and redox-active solid electrolytes. In this study, a particular focus is given to the electrode fabrication methods and some emerging electrode materials, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), metal nitrides (MNs), MXenes, polyoxometalates (POMs), and black phosphorus (BP). Subsequently, the performance of SSCs with different configurations based on the cell design (ie, symmetric, asymmetric) and the electrode type (ie, freestanding, fiber, interdigitated, flexible) is discussed. This review also presents a comprehensive summary of the latest innovations and state-of-the-art applications of SSCs, including electrochromic, self-healing, shape memory, thermally chargeable, piezoelectric-, photo-SSCs. The final section highlights the future directions and critical technological challenges in the field of SSCs.