## 3 Results and Discussion
### Vibrational Properties  
The three possible structures of conformers identified by [PERSON] [28] were re-optimized by means of DFT calculations at the B3 LYP/6-31G** level. The relative energies of the three conformers, the vibrational frequencies and infrared intensities are reported in Tables S1 and S2 of the Supplementary Materials. The difference between the conformers comes from the different FSSC dihedral angle, which is equal to 153.2 (anti configuration), \(-\)40.6 (gauche) and 6.1 (syn)\({}^{\circ}\) for Conf_A, Conf_B and Conf_C, which have relative energies of 0.0, +1.1 and +2.9 kJ/mol (see Table S1 of the Supplementary Materials). These values are in line with those reported in the only study about conformers of FTFSI, which is available in the literature [28]. The temperature dependence of the mid-infrared spectra of the two ILs are reported in Figure 6, together with the calculated absorbance of the three conformers of FTFSI. The far-infrared spectrum could be measured only for N1114-FTFSI and is reported in Figure 7, in comparison with the computational results. Figures 6 and 7 display a good correspondence between the expected vibrational lines of the FTFSI conformers and the measured absorbance lines in the whole energy range, provided that the frequencies of the calculated absorptions are scaled by a factor of 1.04 for the mid-infrared or 1.065 for the far-infrared. This procedure is usually performed for comparing calculated and experimentally measured spectra. Figure 5: Anodic cyclic voltammetries of the 0.2 LiTFSI-0.8N1114-FTFSI (panel **(A)**) and 0.2 LiTFSI-0.8N122(2O1)-FTFSI (panel **(B)**) ionic liquid electrolytes. Carbon and lithium metal are used as the working electrode and the counter electrode, respectively. Scan rate: 1 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The current density value was obtained by normalizing the current flow with respect to the geometrical surface area of the working electrode. Analogously to the case of many ionic liquids containing other fluorinated anions, like TFSI or FSI, the infrared spectrum is dominated by the absorption lines of the anion, except above 1400 cm\({}^{-1}\), where one can find NH, CH or OH bending and stretching bands, usually coming from the cations. Moreover, a limited spectral region between 800 and 1050 cm\({}^{-1}\) is usually free from the vibrations of the anions, thus containing vibrations only belonging to the cations. Indeed, these spectral regions are those presenting different absorption bands, due to the occurrence of different cations in the two ILs. Figures 6 and 7 show that despite there is a clear narrowing of all absorption lines at low temperatures, there are no disappearing bands or abrupt changes inthe spectra. This fact further suggests that no crystallization of the ILs occurs even at the lowest temperatures investigated here, which is in agreement with the previously reported DSC results and further extending the investigated temperature range. Moreover, the vibrational bands of the three conformers of FTSI are very close to each other, and it is not possible to identify clear spectral marks of any of the conformers of FTSI. This is different from the TFSI anion, for which clear marks of the two cis and trans conformers are visible in the infrared spectra [41]. Instead, for FSI, the infrared spectra of its two conformers are rather superimposed, and the lines found in the experimental spectra come from the superposition  
Figure 6: Temperature dependence of the mid-infrared spectrum of N122(2 OI)-FTSI (panel (a)) and N1114-FTSI (panel (b)) and comparison with the calculated absorbance of the three conformers of the FTSI anion (panel (c)); a scaling factor of 1.04 was used for the calculated frequencies. Figure 7: Temperature dependence of the far-infrared spectrum of N1114-FTSI (upper part) and comparison with the calculated absorbance of the three conformers of the FTSI anion (lower part); a scaling factor of 1.065 was used for the calculated frequencies. of the contribution from the two rotamers [42].