Ionic liquids (ILs) are liquids consisting entirely of ions and can be further defined as molten salts having melting points lower than 100 °C. One of the most important research areas for IL utilization is undoubtedly their energy application, especially for energy storage and conversion materials and devices, because there is a continuously increasing demand for clean and sustainable energy. In this article, various application of ILs are reviewed by focusing on their use as electrolyte materials for Li/Na ion batteries, Li-sulfur batteries, Li-oxygen batteries, and nonhumidified fuel cells and as carbon precursors for electrode catalysts of fuel cells and electrode materials for batteries and supercapacitors. Due to their characteristic properties such as nonvolatility, high thermal stability, and high ionic conductivity, ILs appear to meet the rigorous demands/criteria of these various applications. However, for further development, specific applications for which these characteristic properties become unique (i.e., not easily achieved by other materials) must be explored. Thus, through strong demands for research and consideration of ILs unique properties, we will be able to identify indispensable applications for ILs.

Ionic liquids (ILs) are ambient temperature molten salts that have attracted considerable attention because of their negligible volatility, thermal stability, nonflammability, and high ionic conductivity. These remarkable properties result essentially from their ionic nature. Thus, the concept of ionicity of ILs (i.e. how ionic they are) is of great significance for characterising their properties. Here, we show the methodologies used to assess the ionic nature in ILs. On the basis of quantitative estimation of the ionicity, their dependence on ionic structure, polarity scales, and physicochemical properties is reviewed. The ionicity of certain ILs is also predicted from their physicochemical properties. The effects of different classes of ILs (e.g., protic ILs and lithium ILs) and binary systems consisting of ILs and other components on the ionicity are also discussed.

To demonstrate a new family of ionic liquids (ILs), i.e., “solvate” ionic liquids, the properties (thermal, transport, and electrochemical properties, Lewis basicity, and ionicity) of equimolar molten mixtures of glymes (triglyme (G3) and tetraglyme (G4)) and nine different lithium salts (LiX) were investigated. By exploring the anion-dependent properties and comparing them with the reported data on common aprotic ILs, two different classes of liquid regimes, i.e., ordinary concentrated solutions and “solvate” ILs, were found in the glyme–Li salt equimolar mixtures ([Li(glyme)]X) depending on the anionic structures. The class a given [Li(glyme)]X belonged to was governed by competitive interactions between the glymes and Li cations and between the counteranions (X) and Li cations. [Li(glyme)]X with weakly Lewis basic anions can form long-lived [Li(glyme)]⁺ complex cations. Thus, they behaved as typical ionic liquids. The lithium “solvate” ILs based on [Li(glyme)]X have many desirable properties for lithium-conducting electrolytes, including high ionicity, a high lithium transference number, high Li cation concentration, and high oxidative stability, in addition to the common properties of ionic liquids. The concept of “solvate” ionic liquids can be utilized in an unlimited number of combinations of other metal salts and ligands, and will thus open a new field of research on ionic liquids.

Novel Brønsted acid–base ionic liquids, derived from a simple combination of a wide variety of organic amines with bis(trifluoromethanesulfonyl) amide are electroactive for H₂ oxidation and O₂ reduction at a Pt electrode under non-humidifying conditions, which shows the prospect of the use of protic ionic liquids as new materials for anhydrous proton conductors at elevated temperatures.