# PyT\({}_{\mathbf{1,x}}\)TFSI Ionic Liquids (x = 1-8): A Computational Chemistry Study
## 3 Results
### Electrochemical Stability  
The computed reduction and oxidation potentials of the ionic pairs are shown in Figure 3.  
The electrochemical stability, both in reduction and in oxidation, of the Pyr\({}_{1,x}\)TFSI ionic liquids are apparently barely affected by the elongation of the later alkyl chain on the pyrrolidinium cation. Remarkably the electrochemical stability of the isolated ions is much smaller compared to ionic couples. This discrepancy likely suggests a remarkable impact on the effective oxidation and reduction onset potential of the ionic association in liquid electrolytes.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Dissociation Reaction** & **Gibbs Energy of Dissociation at 298 K** \\  & \(\Delta_{diss}\sigma_{298\text{/}k}^{0}\)**/**kJ mol\({}^{-1}\)** \\ \hline Pyr\({}_{1,1}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 2.6 \\ Pyr\({}_{1,2}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 4.7 \\ Pyr\({}_{1,3}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 5.9 \\ Pyr\({}_{1,4}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 8.7 \\ Pyr\({}_{1,5}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 3.0 \\ Pyr\({}_{1,6}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 1.8 \\ Pyr\({}_{1,7}\)TFSI \(\rightarrow\) Pyr\({}_{1,1}\)\({}^{+}\) + TFSI\({}^{-}\) & 0.3 \\ Pyr\({}_{1,8}\)TFSI \(\rightarrow\) Li\({}^{+}\) + TFSI\({}^{-}\) & 0.4 \\ LiTFSI \(\rightarrow\) Li\({}^{+}\) + TFSI\({}^{-}\) & 540 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the Gibbs energy of dissociation of the various ionic liquids.  
Figure 3: Predicted electrochemical stability windows for the Pyr\({}_{1,x}\)TFSI (x = 1–8) ionic liquids.  
These computational estimates can be compared with the few available experimental electrochemical stability values reported in the literature by means of voltameric runs [9; 10; 29; 30; 31; 32; 33; 34; 35]. The cathodic stability of the Pyr1,NTFSI solvents (N = 3-7) ranges between 0 and \(-\)0.2 V vs. Li\({}^{+}\)/Li: our predictions are in excellent agreement with these experimental determinations, thus confirming the reliability of the methodology. Turning to the anodic stability, for Pyr1,4 TFSI, many reports observed values ranging between 4.2 to 5.5 V vs. Li\({}^{+}\)/Li, whereas less is known for the other molecules in the series. In this view, our predictions are the first systematic report. It should be noted that the electrochemical oxidation and reduction potentials are strongly dependent on the nature of the working electrode surface/composition (e.g., Pt, Pd, stainless steel, carbon coated copper, copper, aluminum) and well as on the voltameric experimental conditions (scan rate, potential holds, temperature, nature of the counter electrode, nature of the reference electrode). In the case of reduction potentials, a further critical point is the overlap of the degradation chemistry of the ionic liquid molecules with the plating reaction of alkaline metals on the working electrode when lithium/sodium counter electrodes are used.  
Overall, our analysis confirms that all Pyr1,xTFSI have an electrochemical stability that is suitable for application as electrolyte solvents in aprotic batteries (both Na- or Li-based). In fact, the typical positive electrodes working potentials ranges between 3.5 and 4.5 V vs. Li [36] whereas negative electrodes operate at potentials between 1.5 and few mV vs. Li (e.g., Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\), graphite) [37; 38].