Previously, we investigated the purity of the as-received ILs with NMR and infrared (IR) spectroscopy,, which did not hint at impurities that could be resolved with these techniques. To elucidate other possible contaminants, we performed XPS measurements under UHV conditions, which are given and discussed in detail in the SI. The spectra for both batches are similar, and possible impurities could not be identified. Their concentration could, however, still be below the detection limit of the instrument. The relevance of using XPS to study the quality of the electrolyte will, nevertheless, be illustrated further below.