They have been measured for 1M LiPF 6 in EC:EMC 3:7 wt% (LP57) and 2M LiPF 6 in EC:EMC 3:7 wt% electrolytes at room temperature. The results are presented in Fig. . Unprocessed and full wavenumber interval spectra can be found in the supplementary information. In the FTIR spectra characteristic C=O bond stretching vibrations are found in the region 1700 to 1800 cm -1 and correspond to the C=O stretching bands free ethylene carbonate at 1806 cm -1 and ethylene carbonate coordinated to lithium ion at 1772 cm -1 , as well as C=O stretching of free ethyl methyl carbonate at 1744 cm -1 and coordinated ethyl methyl carbonate at 1716 cm -1 ,. In the spectral region of 800 to 900 cm -1 , infrared spectra have a characteristic P-F bond stretching mode at 838 cm -1 and the EMC band at around 873 cm -1 . The increase of LiPF6 concentration results in an increase of the two shoulders centered around 820 and 860 cm -1 (both blue-and red-shifted as compared to free PF6peak), that have been attributed to the presence of Li + -P F - 6 ion pairs/aggregates. The peaks of C=O and P-F stretching vibrations were fitted using the Voigt function (50% Lorentzian-50% Gaussian), and fitted peak positions, heights and areas are presented in a table S1 in the supplementary information. For the analysis of the FTIR spectra we start by analysing the ratio of free EC (denoted A F ) and EC, coordinated by lithium (denoted A C ). The results are A F A C = 0.9 and 0.5 for 1 M (MD: 0.95 M) and 2 M (MD: 1.93 M). For EMC the experimentally determined ratios are A F A C = 1.4 and 0.8, respectively. Finally, for PF 6 one observes ratios of 3.5 and 2.3. On this basis, for all three cases the CN can be estimated as