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High‐performance electrochemical applications have expedited the research in high‐power devices. As such, supercapacitors, including electrical double‐layer capacitors (EDLCs) and pseudocapacitors, have gained significant attention due to their high power density, long cycle life, and fast charging capabilities. Yet, no device lasts forever. It is essential to understand the mechanisms behind performance degradation and aging so that these bottlenecks can be addressed and tailored solutions can be developed. Herein, the factors contributing to the aging and degradation of supercapacitors, including electrode materials, electrolytes, and other aspects of the system, such as pore blocking, electrode compositions, functional groups, and corrosion of current collectors are examined. The monitoring and characterizing of the performance degradation of supercapacitors, including electrochemical methods, in situ, and ex situ techniques are explored. In addition, the degradation mechanisms of different types of electrolytes and electrode materials and the effects of aging from an industrial application standpoint are analyzed. Next, how electrode degradations and electrolyte decompositions can lead to failure, and pore blocking, electrode composition, and other factors that affect the device's lifespan are examined. Finally, the future directions and challenges for reducing supercapacitors' performance degradation, including developing new materials and methods for characterizing and monitoring the devices are summarized.

promising technologies. [1] EDLCs display high power densities and extremely high lifetime which makes them the technology of choice for a variety of applications requiring a fast and continuous delivery of energy. However, as the energy density of these devices is rather limited (5-8 Wh kg −1 ), their use is currently limited to applications that require a relatively low amount of energy. [2] As indicated by several studies, an increase in energy density would lead to a drastic increase in the number of possible applications for EDLCs. [3] Furthermore, it would enable the use of these devices as replacement for established battery systems in fields where high power is required. [4] This latter application is particularly interesting because nowadays the use of batteries in high-power applications is typically leading to increased cell degradation and, consequently, to increased cost due to early system failure. For this reason, in the available electric vehicles, EDLCs are often applied as a support to reduce the load on the main battery system during rapid acceleration and power uptake by regenerative braking. It has been shown that this hybrid energy storage solution results in improved performance while extending the lifetime of the battery system. [5] Nonetheless, further improvements are urgently needed to extend the battery life even more.For these reasons, current research focuses on the development of EDLCs with improved energy density. To reach this goal, the electrode capacitance needs to be maximized and, at the same time, the operating voltage of these systems needs to be significantly increased. While commercially available devices are limited to operating voltages of up to 3.0 V, an increase to 3.2 V or more would increase the energy density significantly. The operating voltage of EDLCs is mainly determined by the onset of electrolyte decomposition at the electrode surface. [6] The state-of-the-art electrolytes consist of solutions containing ammonium-based conducting salt, e.g. tetraethylammonium tetrafluoroborate (TEABF 4 ), dissolved in acetonitrile (ACN). [7] These electrolytes display high conductivity but guarantee high stability only if used in devices with a maximum voltage of 3 V. [8] With the aim to overcome this limitation and realize high-voltage EDLCs, a variety of novel solvents, salts, and ionic liquids have been investigated in the last years. [9] Although Two of the main weaknesses of modern electric double-layer capacitors are the rather limited ranges of operating voltage and temperature in which these devices do not suffer from the occurrence of irreversible decomposition processes. These parameters are strongly interconnected and lowering the operating voltage when increasing the temperature is unavoidable, so as to protect the electric double-layer capacitor from damage. With the aim to maintain the operating voltage as high as possible at elevated temperatures, in this study, the application of ethyl isopropyl sulfone as an electrolyte solvent for electric double-layer ...

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