Most importantly, adding the molecular sieve (Figure ), leads to the appearance of two small peaks in the negative-going scan (-1.4 V, -1.5 V) and one in the positive-going scan (-1.3 V) marked by stars in Figure . The position of these new peaks is very similar to those observed in measurements where Na was deposited from the same IL (type I) on a Au(111) surface, as described in more detail in the SI. Therefore, we suggest that these peaks could be related to Na, which is an impurity resulting from an ion exchange of Na from the molecular sieve with the cation of the IL. A similar effect has been reported previously. On the other hand, in an experiment where Na[TFSI] was added with a concentration of 5 mM, not all of the here-reported peaks (especially those at potentials larger than 0.5 V) were observed and it was concluded that Na is not responsible for the newly observed features after the treatment of the IL with a molecular sieve. An aspect which has been neglected is that in the experiment where Na[TFSI] had been added to the system, water was also present, which could have reacted with the Na on the surface and thus the desorption peaks are not visible in the CV. However, more detailed studies are required to elucidate the impact of trace amounts of water on the interaction possible Na impurities with the electrode. The addition of a molecular sieve to batches showing type II also has a significant impact on the electrochemical properties, as illustrated in the SI (Figure ). In that case, the peak pair C2/A2 disappears almost completely while the other features apparent in the CV recorded in the IL without any pre-treatment do not change significantly. Possible reasons for the change in C2/A2 is discussed below. Next, we explored the effect of adding water to the as-received IL on the electrochemical properties. Here, approximately 150-200 ppm water was added to batches 3 and 4, resulting in total water contents of 270 ppm and 170 ppm, respectively. The 1 st (black solid line) and 5 th CV (red solid line) are shown in Figure . The CVs for the as-received batches (without additional water) are shown for comparison (dotted lines).