Abstract
The electrochemical stability of electrolytes is essential to the working potential of supercapacitors. Ionic liquids (ILs) are being considered as safe alternatives to current organic electrolytes and attracting extensive interests owing to their inflammability, widened potential windows, and superior ionic conductivity. Novel supercapacitors with IL electrolytes exhibit attractive energy density and can be utilized in various energy storage systems. Most previous studies focused on electrochemical performances, while rare attentions were devoted to energy storage process details or mechanisms. This review comprehensively summarizes the latest progress on formulated IL electrolytes for different types of supercapacitors, with an emphasis on the intrinsic understanding of the related energy storage mechanisms. Subsequently, comparisons of various IL-based liquid-state electrolytes as well as the state-of-the-art advancements in optimizing ILs electrolytes are introduced. The authors attempt to reveal the inherent correlation between the usage of IL electrolytes and the properties of supercapacitors via referenced works. Some emerging applications of ionogel electrolytes based on conventional polymers and poly(IL)s for flexible supercapacitors are also presented, including the existing problems. In addition, challenges and future perspectives of research in this field are highlighted.
Highlights
Electrochemical energy storage systems have attracted extensive interests in recent years due to their widespread applications in smart electronics, electric vehicles, as well as hybrid load-leveling systems for intermittent green sources
A layered structure is formed at the planar electrodes, while a monolayer structure is formed on porous electrodes with proper pore size and a multilayer structure is formed on porous electrodes with wider pores
The properties and special interfacial nanostructures of self-assembly Surface-active ILs (SAILs) were demonstrated, which would be favorable for EDL structures and charge storage of electric double-layer capacitors (EDLCs)
Summary
Electrochemical energy storage systems have attracted extensive interests in recent years due to their widespread applications in smart electronics, electric vehicles, as well as hybrid load-leveling systems for intermittent green sources. Among various energy storage devices, lithium-ion batteries (LIBs) and supercapacitors (SCs) are the two most extensively used energy storage systems (Ding et al, 2018; Li M. et al, 2018). Commercial LIBs with low power density and potential safety issues cannot meet the growing energy needs. SCs with rapid power output and long cycle life can be used independently or supplement batteries in many fields such as electric buses, light rail, wearable electronics, and energy storage systems for intermittent renewable energy sources, triggering growing tremendous interests (Conway and Pell, 2003; Simon and Gogotsi, 2008; Shao et al, 2018). It is highly desirable to understand the mechanisms of supercapacitors and develop advanced supercapacitors
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