## 3 Results and Discussion  
### Differential Scanning Calorimetry (DSC)  
DSC curves of pure IL and binary mixtures of [C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI] + EC are shown in Figure 1a. While the pure IL exhibits single peaks upon heating (at 9 \({}^{\circ}\)C) and cooling (at \(-11\)\({}^{\circ}\)C), corresponding to melting and crystallisation processes, respectively, the DSC heating profiles of the mixtures are characterised by two endothermic peaks for all EC concentrations, except for the x = 0.5 mixture, which shows a single peak at \(-19\)\({}^{\circ}\)C. This melting point, decreasing upon the addition of EC, has been observed for various IL-carbonate mixtures [25]. The remaining mixtures, which exhibit two endothermic peaks upon heating, interestingly present the first endothermic peak also at \(-19\)\({}^{\circ}\)C. This behaviour suggests the possible formation of different polymorphs depending on the EC concentration. The onset temperatures and the enthalpies of every transition obtained from DSC curves are listed in Table 2. A typical supercooling effect, characteristic of ILs, is also observed in all these samples, with crystallisation temperatures below 0 \({}^{\circ}\)C in all the cases and with T\({}_{\rm m}\)-T\({}_{\rm c}\)\(>\) 20 \({}^{\circ}\)C.  
Figure 1b shows the phase diagram, where the 0.5 molar ratio mixture can be considered the eutectic composition. This behaviour is in good agreement with the findings of [PERSON] et al. [26] and [PERSON] et al. [25] who studied mixtures of pyrrolidinium-based ionic liquids and 1-Ethyl-3-methylimidazolium bis(trifluoromenthanesulfonyl)azanide, respectively, with EC as a solvent, and reported similar eutectic composition. Furthermore, it can be observed that the melting enthalpy increases with the addition of EC, obtaining lower values than the corresponding pure EC [27]. From the eutectic IL + EC mixture, ternary mixtures (IL + EC + LiTFSI) with three different salt concentrations, 0.2, 0.5, and 1 m (mol (kg IL + EC mixture)\({}^{-1}\)) were prepared. Figure 2 shows the DSC curves of ternary mixtures of 0.5 [C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI] + 0.5 EC and LiTFSI. The addition of lithium salt practically vanishes phase transitions over the entire studied temperature range, except for the mixture with the lowest lithium salt concentration, for which smooth cold crystallisation and melting peaks are observed. This vanishing is the typical behaviour after the salt addition, as has been widely reported previously [19; 20; 21]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \(\mathbf{T_{m}}\) & \(\mathbf{T_{c}}\) & \(\mathbf{T_{cc}}\) & \(\mathbf{\Delta H_{m}/\mathrm{J}~{}\mathrm{g}^{-1}}\) & \(\mathbf{\Delta H_{cc}}\); \(\mathbf{\Delta H_{c}/\mathrm{J}~{}\mathrm{g}^{-1}}\) \\ \hline Mixtures (1 \(-\) x) EC + x [C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI] & & & \\
1.0 & 9 & \(-11\) & \(-\) & 36 & 33 \\
0.7 & \(-18\) \(|\)\(-\)8 & \(-37\) \(|\)\(-\)49 & \(-\) & 42 & 35 \\
0.5 & \(-19\) & & 52 & 51 & 33 \\
0.4 & \(-19\) \(|\)\(-\)10 & \(-\) & \(-61\) & 63 & 53 \\
0.3 & \(-19\) \(|\)\(-\)3 & \(-\) & \(-59\) & 73 & 60 \\
0.25 & \(-19\) \(|\)0 & \(-\) & \(-\)58 & 77 & 60 \\
0.2 & \(-19\) \(|\)5 & \(-\)59 & \(-44\) & 87 & 13/50 \\
0.15 & \(-19\) \(|\)8 & \(-51\) & \(-60\) & 92 & 7/63 \\
0.1 & \(-19\) \(|\)15 & \(-42\) & & 100 & 85 \\ Ternary mixtures ([C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI]/EC/LiTFSI) & & & & \\
0.2 m & \(-25\) & \(-\) & \(-39\) & 2.6 & 2.6 \\
0.5 m & \(-\) & \(-\) & \(-\) & \(-\) & \(-\) \\
1 m & \(-\) & \(-\) & \(-\) & \(-\) & \(-\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Temperature (in \({}^{\circ}\)C) of melting \(T_{m}\), freezing \(T_{c}\), and cold crystallisation \(T_{cc}\) enthalpy of melting (\(\Delta H_{\mathrm{m}}\)) and freezing (\(\Delta H_{c}\)) obtained from DSC curves. Figure 1: (a) DSC curves of binary mixtures [C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI] with ethylene carbonate for different concentrations (expressed in molar fraction of IL). (b) Phase diagram of [C\({}_{3}\)C\({}_{1}\)Pyrr][TFSI] and EC.