| Ionic liquids (ILs) are salts with a melting temperature (T m) at or below room temperature. Depending on their structure, they can be nonvolatile, nonflammable, and highly stable, both thermally and electrochemically. These properties make ILs attractive alternatives to the organic solvent-based electrolytes currently in use. However, ILs have a number of drawbacks, including a relatively low ionic conductivity and a Tm greater than that of many solvents which often precludes their use at low (lower than -20 °C) temperature. This work investigates two potential means of addressing these problems: the addition of aprotic solvents to ILs and mixtures containing two ILs. The use of these mixtures in electrochemical double-layer capacitors (EDLCs) is examined and links between properties and capacitor performance quantified.;The physicochemical properties (density, viscosity, ionic conductivity, and thermal phase behavior) of the pure ILs, IL-aprotic solvent mixtures, and IL-IL mixtures containing varying ions were initially investigated. While pure ILs are often highly viscous, with a correspondingly low ionic conductivity, the IL-solvent mixtures containing small amounts (20 mol%) of aprotic solvents are much more conductive and less viscous than the pure ILs. In addition, the presence of solvent molecules inhibits the crystallization of the ILs, thus retaining the liquid phase for these electrolytes to a much lower temperature. The interactions between the ILs and solvent appear to be quite weak, however, allowing the solvent to readily volatilize at higher temperatures. Thus, IL-IL mixtures were also studied as a route to modify the properties of IL-based electrolytes without reintroducing volatility concerns. Mixing two ILs can result in significant variations in the viscosity, density, and ionic conductivity as compared to that of the pure ILs. Furthermore, the compositions with mixed anions often remain amorphous mixtures, rather than crystallizing, thereby potentially allowing IL-IL mixed electrolytes to be used at low temperatures.;In addition to modifying the properties and phase behavior of electrolytes, the presence of mixed cations and/or anions in an EDLC electrolyte may influence both the energy and power density of a device. The addition of smaller ions to IL-based electrolytes was expected to result in increased specific energy due to the utilization of smaller pores and/or increased specific power due to the higher mobility of smaller ions. However, these effects are not observed for many of the IL-IL electrolytes, suggesting that factors other than ion size influence the performance of IL-based EDLCs. In addition, the noted differences in performance did not correlate with the performance that would be predicted based upon the current understanding of electrolyte optimization for capacitors. Specifically, increases in specific energy may be accompanied by increases in viscosity and decreases in conductivity (counter to what one would expect), while increases in specific power may be observed for mixtures that have no changes in physicochemical properties. In toto, these results suggest that the relationships between electrolyte properties and EDLC performance are complex and require further attention from the research community. |
