## 3 Results and Discussion
### Electrochemical Stability  
#### 3.1.1 Preliminary LSV Tests  
The electrochemical stability window (ESW) of electrolytes is a crucial property to evaluate their applicability in real devices. LSV tests have been carried out on Na/Pt cells for the preliminary study of the anodic stability of each electrolyte formulation and the determination of the anodic potential cut-off in the following analysis. The LSV-specific current curves (normalized with respect to the geometric working electrode area) relative to the cell voltage are shown in Figure 1. All samples exhibit an electrochemical stability above 4.5 V vs. Na\({}^{+}\)/Na\({}^{\circ}\) backed up by a very low current flow (<5 mA cm\({}^{-2}\)) up to the anodic  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Ionic Liquid**} & \multirow{2}{*}{**Thermal Stability/\({}^{\circ}\)C**} & \multicolumn{2}{c}{\(\sigma\)/ **S cm\({}^{-1}\)**} \\ \cline{3-4}  & & \(-\)**20 \({}^{\circ}\)C** & **20 \({}^{\circ}\)C** \\ \hline EMIFSI & 150 & (1.1 \(\pm\) 0.1) \(\times\)\(10^{-3}\) & (8.6 \(\pm\) 0.5) \(\times\)\(10^{-3}\) \\ EMITFSI & 275 & (3.8 \(\pm\) 0.2) \(\times\)\(10^{-4}\) & (3.8 \(\pm\) 0.2) \(\times\)\(10^{-3}\) \\ N\({}_{1114}\)FSI & 150 & (2.8 \(\pm\) 0.2) \(\times\)\(10^{-7}\) & (1.8 \(\pm\) 0.1) \(\times\)\(10^{-3}\) \\ N\({}_{1114}\)TFSI & 225 & (1.6 \(\pm\) 0.1) \(\times\)\(10^{-7}\) & (5.3 \(\pm\) 0.3) \(\times\)\(10^{-4}\) \\ N\({}_{1114}\)M\({}_{14}\) & 225 & (7.0 \(\pm\) 0.4) \(\times\)\(10^{-7}\) & (8.2 \(\pm\) 0.5) \(\times\)\(10^{-5}\) \\ PIP\({}_{13}\)TFSI & 250 & (5.6 \(\pm\) 0.3) \(\times\)\(10^{-7}\) & (5.0 \(\pm\) 0.3) \(\times\)\(10^{-4}\) \\ PIP\({}_{14}\)TFSI & 225 & (9.9 \(\pm\) 0.5) \(\times\)\(10^{-8}\) & (3.1 \(\pm\) 0.2) \(\times\)\(10^{-4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Thermal stability and ionic conductivity (\(\sigma\)) values of 0.1 NaTFSI-0.9 IL electrolytes. The data are taken from ref. [17].  
break down voltage; this is highlighted by a steep current increase due to the massive degradation of the electrolyte at high voltages. A few IL samples show weak features (i.e., not exceeding 10 \(\mu\)A cm\({}^{-2}\)) located from 2.0 to 3.0 V, which is probably ascribable to the irreversible oxidation of minor contaminants at the platinum working electrode. However, apart from this negligible electrochemical signal, no appreciable current increase is observed up to the massive degradation of the electrolyte. The electrolytes containing EMIFSI, N\({}_{1114}\)FSI, PIP\({}_{13}\)TFSI and PIP\({}_{14}\)TFSI as the ionic liquid are very stable up to about 5 V (vs. Na\({}^{+}\)/Na\({}^{\circ}\)). Meanwhile, the LSV curve of the N\({}_{1114}\)IM\({}_{14}\) electrolyte does not exhibit a sudden current increase but rather, a moderate raise around 5.3 V. This behavior, previously observed in other IM\({}_{14}\) ionic liquid electrolytes [22; 23], is likely due to the ability of the IM\({}_{14}\) anion species to form an insulating byproduct upon irreversible oxidation. The accumulation of these insoluble degradation species likely leads to a resistive and less electronically conductive passive layer on the working electrode surface. Based on LSV, a cut-off voltage equal to 4.6 V was adopted for the subsequent CV tests.  
Figure 1: Current-voltage traces obtained through LSV tests run on Na/Pt cells containing EMI (panel (**A**)), N\({}_{1114}\)\({}^{+}\) (panel (**B**)) and PIP\({}_{1A}\)\({}^{+}\) (panel (**C**)) electrolytes. Ionic liquid: LiClO\({}_{4}\) mole ratio = 9:1. Scan rate: 1 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The voltage values are referred to the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple.