Ionic Liquids and Electrochemistry
Ionic liquids are liquids composed solely of ions, containing no neutral molecules such as water or other solvents. For example, sodium chloride, which consists of sodium ions and chloride ions, is a solid (ionic crystal) at room temperature. However, above its melting point of 801°C, it melts to become an ionic liquid (or molten salt). Depending on the combination of cations and anions, some salts can become ionic liquids even at room temperature due to a decrease in their melting point. Salts with melting points below room temperature were traditionally called room-temperature molten salts, but in recent years, they are more commonly referred to simply as ionic liquids.
Generally, when the charge density of cations and anions is low, the electrostatic interactions between them weaken, leading to a lower melting point. For this reason, many ionic liquids are obtained by combining bulky ions. For example, the melting point of 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide, shown in Figure 1, is reported to be –18°C. The first ionic liquid reported in the literature is considered to be ethylammonium nitrate, discovered by Walden in 1914, with a recorded melting point of 13–14°C. Since then, countless ionic liquids have been discovered through various combinations of cations and anions. Because the properties of an ionic liquid depend on the combination of its cation and anion, efforts have been made to design ionic liquids for specific purposes and applications. As such, ionic liquids have garnered attention as "designable liquids."
Electrochemistry is the academic field that forms the basis of technologies for interconverting electrical and chemical energy through electrode reactions. Lithium-ion batteries, widely used in ICT devices such as smartphones and laptops, as well as in electric vehicles, are one type of electrochemical device. Many lithium-ion batteries use electrolytes containing flammable organic solvents, which poses a risk of the battery catching fire or exploding in an accident. Since ionic liquids have low volatility and low flammability, using them as electrolytes is expected to improve the safety of lithium-ion batteries.
On the other hand, many aspects of electrochemistry in ionic liquids remain unresolved. For example, even considering ion transport alone, the influence of electrostatic interactions between ions is particularly pronounced, unlike in conventional solution systems. Researching electrochemistry in ionic liquids has the potential to uncover unprecedented new scientific principles and technologies.