Research progress in low-temperature lithium-ion capacitors

Recently, lithium-ion capacitors (LICs) have developed rapidly and have been applied in many fields, such as power storage and new energy transportation. LICs utilize the cathode materials of electrical double-layer capacitors (EDLCs) and the anode materials of lithium-ion batteries (LIBs). This produces a unique energy storage mechanism different from those of LIBs and EDLCs, i.e., charge transfer and Li⁺ insertion and desorption. Consequently, LICs combine the advantages of LIBs and EDLCs with high energy density, high power density, and long cycle life. However, because of this unique energy storage protocol, LICs inherit the poor low-temperature performance of LIBs, severely limiting their widespread application. Sometimes, the electrolyte becomes more viscous or even solidifies, affecting normal ion transportation and charge transfer. An increase in impedance prevents the normal operation of LICs, severely limiting their all-weather applications. Improving the low-temperature performance of LICs has become an urgent issue and has received widespread attention from researchers. Electrodes and electrolytes are the main components of LICs, and numerous studies have shown that their relationship directly determines the energy storage process of LICs at low temperatures. Therefore, this article reviews the recent research progress on the design and fabrication of low-temperature LICs in terms of electrodes and electrolytes. First, the research on key electrode materials for high-performance low-temperature LICs is discussed, including chemical modification, surface modification, ion insertion, and the development of new electrode materials for rapid intercalation of traditional carbon-based materials. Second, the electrolyte system that matches the electrode material is critically reviewed. The fundamental reasons for the poor performance of LICs in low-temperature environments are comprehensively explained from the chemical properties, physical states, and reaction mechanisms, providing a sufficient theoretical basis for searching electrolyte systems with better low-temperature performance. Third, starting from the main components of the electrolyte–lithium salts, solvents, and additives, this article summarizes the past year’s progress on low-temperature electrolytes in LICs. Emphasis is placed on additives for LIC electrolytes, which, as the essence of the entire electrolyte system, are the most controllable factor throughout the electrolyte system and currently the most available factor for selection. They can reduce the viscosity of the electrolyte with minimal content and improve the low-temperature charging and discharging ability of LICs. Commonly used low-temperature additives such as fluoroethylene carbonate (FEC), vinylene carbonate (VC), ropylene sulfite (PS) and lithium difluorooxalate borate (LiODFB) demonstrate excellent low-temperature performance. Finally, the article summarizes the research progress of the new generation of low-temperature electrolytes and provides a tentative outlook toward next-generation LICs with a wide temperature range.