Pure ionic liquids (ILs) and IL mixtures in organic solvents have been investigated for higher operating voltages around 3.0–4.0 V. ILs have design flexibility due to the numerous possible combinations of anions and cations. Current research on ILs as electrolytes has focused on several ILs, including imidazolium and pyrrolidinium. At early stages, various ILs have been studied as salts of electrolyte with organic solvents like acetonitrile and propylene carbonate. Neat ILs have been applied for high-performance electrolyte, and some of them have been used as electrolyte (1-ethyl 3-methylimidazolium tetrafluoroborate). These liquid electrolytes need additional encapsulation; therefore, SCs applied ILs face difficulty in integration and manufacturing flexible devices. These drawbacks can be solved by adopting a polymer electrolyte because the ILs maintain the conductivity even when solidified, unlike a typical organic electrolyte. Common polymer matrixes such as PVdF, PMMA, and PVA have been suggested to embed ILs. Poly(ionic liquid) (PIL) is also studied. PIL is a polymer electrolyte containing a polymer backbone and an IL species in the monomer repeat unit. PIL-based polymer electrolytes have high ionic conductivity, wide electrochemical windows, and high thermal stability.
Part of the book: Supercapacitors