Temperature-dependent conductivity, viscosity, and density of four ionic liquids (ILs), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][NTf2]), and 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]), were measured with high precision from +80 °C down to −35 °C, if possible. Fitting parameters for the Vogel−Fulcher−Tammann (VFT) equation were obtained for conductivity and viscosity data, and obtained data were analyzed with the help of the fractional Walden rule and the Walden plot. Excellent linear behavior is observed for all ILs; however, the average slope is not unity as expected for the ideal Walden rule, but 0.92 ± 0.02. The so-called ideal KCl line that is used to compare ILs within the Walden plot is discussed, as literature data for aqueous KCl solutions show that its assumed ideality has to be modified.

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So-called “corrosion” of the aluminum current collector in the electrolyte 1 M LiTFSI in ethylene carbonate : diethyl carbonate, EC:DEC (3:7, by wt) has been investigated by electrochemical and analytical methods. In fact, Al “corrosion” in this electrolyte is actually an anodic Al dissolution reaction. In addition to Al dissolution various electrolyte degradation processes were identified. A combination of a specially developed on-line ICP-OES method and in situ EQCM measurements revealed that before the dissolution of aluminum starts, an “activation” process takes place for ca. 6 hours, which is accompanied by strong electrolyte oxidation. The electrolyte decomposition reactions were investigated by ex situ IC measurements which showed that the LiTFSI decomposed and that F− is the main decomposition product. ex situ GC-MS measurements revealed that also the solvent decomposes and CO2 as well as ethoxyethanol are formed as degradation products.

Temperature-dependent conductivity and viscosity data of over ten new fluoroborate-based ionic liquids (ILs) were measured in a temperature range spanning about 100 K. Data are presented and evaluated according to the fractional Walden rule and Angell’s fragility concept. All ILs show excellent linear relationships for their Walden plots with similar slopes in the range from about 0.90 to about 0.94. It was found empirically that the slopes of the Walden plots reflect the ratio of the corresponding Arrhenius activation energies for the ILs’ temperature-dependent viscosities and molar conductivities. Further analysis of viscosity data of ILs leads to the conclusion that all investigated ILs, including some more common ones, can be classified as highly fragile, very weak liquids, reaching even the limiting value estimated by Vilgis.

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