# Ionic Liquid Electrolytes for Safer and More Reliable Sodium Battery Systems
## 3 Results and Discussion
hindering the formation of the crystalline lattice and, therefore, producing a decrease in the melting temperature of the ionic liquid, i.e., the PIP14 TFSI electrolyte exhibits a melting point of approximately 20 \({}^{\circ}\)C lower than that of the PIP13 TFSI one, once more in agreement with the results obtained for the pure ILs [13]. The ionic conductivity data, determined at selected temperatures, of the 0.1 NaTFSI-0.9 IL sodium electrolytes are summarized in Table 1. At \(-20\)\({}^{\circ}\)C only the EMI-based samples exhibit conduction values (i.e., from \(10^{-4}\) to \(10^{-3}\) S cm\({}^{-1}\)) of interest for practical devices, making these electrolytes appealing for low temperature applications. At 20 \({}^{\circ}\)C all investigated NaTFSI-IL electrolytes, with the exception of the NaTFSI-0N1114 IM14 sample, largely overcome \(10^{-4}\) or \(10^{-3}\) S cm\({}^{-1}\). The EMI-based samples display higher conductivity values than those containing (N1114)\({}^{+}\) and piperidinium cations, due to the lower viscous drag ascribable to the smaller size and planar configuration of the imidazolium cation [4]. At 80 \({}^{\circ}\)C, the conductivity value of all sodium IL electrolytes largely exceeds \(10^{-3}\) S cm\({}^{-1}\); in particular, that of EMI- and FSI-based electrolytes widely overcomes \(10^{-2}\) S cm\({}^{-1}\). The results, obtained from isothermal TGA measurements, are illustrated in Figure 2a (samples containing the FSI anion) and Figure 2b (TFSI and IM14 anions), respectively. The time evolution of the sample temperature is highlighted by the stepped profiles (grey traces). All investigated samples show an initial loss in weight ranging from 0.4% to 0.6% due to the removal of absorbed H\({}_{2}\)O (accentuated by the presence of the NaTFSI salt) during handling (sample loading into the pans). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ionic** **liquid**} & \multirow{2}{*}{**Thermal**} & \multicolumn{3}{c}{\(\sigma\)/S cm\({}^{-1}\)} \\ \cline{3-6}  & & \(-20\)\({}^{\circ}\)C & \(20\)\({}^{\circ}\)C & \(50\)\({}^{\circ}\)C & \(80\)\({}^{\circ}\)C \\ \hline EMIFSI & 150 & \((1.1\pm 0.1)\times 10^{-3}\) & \((8.6\pm 0.5)\times 10^{-3}\) & \((1.8\pm 0.1)\times 10^{-2}\) & \((3.2\pm 0.2)\times 10^{-2}\) \\ EMITFSI & 275 & \((3.8\pm 0.2)\times 10^{-4}\) & \((3.8\pm 0.2)\times 10^{-3}\) & \((8.9\pm 0.5)\times 10^{-3}\) & \((1.8\pm 0.2)\times 10^{-2}\) \\ N\({}_{1114}\)FSI & 150 & \((2.8\pm 0.2)\times 10^{-7}\) & \((1.8\pm 0.1)\times 10^{-3}\) & \((5.4\pm 0.3)\times 10^{-3}\) & \((1.2\pm 0.1)\times 10^{-2}\) \\ N\({}_{1114}\)TFSI & 225 & \((1.6\pm 0.1)\times 10^{-7}\) & \((5.3\pm 0.3)\times 10^{-4}\) & \((2.5\pm 0.2)\times 10^{-3}\) & \((6.5\pm 0.4)\times 10^{-3}\) \\ N\({}_{1114}\)IM\({}_{14}\) & 225 & \((7.0\pm 0.4)\times 10^{-7}\) & \((8.2\pm 0.5)\times 10^{-5}\) & \((5.5\pm 0.3)\times 10^{-4}\) & \((2.1\pm 0.1)\times 10^{-3}\) \\ PIP13 TFSI & 250 & \((5.6\pm 0.3)\times 10^{-7}\) & \((5.0\pm 0.3)\times 10^{-4}\) & \((2.0\pm 0.1)\times 10^{-3}\) & \((4.9\pm 0.3)\times 10^{-3}\) \\ PIP14 TFSI & 225 & \((9.9\pm 0.5)\times 10^{-8}\) & \((3.1\pm 0.2)\times 10^{-4}\) & \((1.6\pm 0.1)\times 10^{-3}\) & \((3.9\pm 0.2)\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermal stability and ionic conductivity (\(\sigma\)) values of 0.1 NaTFSI-0.9 IL sodium electrolytes. Figure 2: Isothermal TGA trace of 0.1 NaTFSI-0.9 IL sodium electrolytes: (**a**) FSI-based electrolytes; (**b**) TFSI- and IM14-based electrolytes. The sample temperature vs. time profile is represented by the stepwise grey trace. The binary mixtures containing the FSI anion exhibit thermal stability up to 150 \({}^{\circ}\)C and reveal a progressive weight loss above this temperature (more accentuated in the passage from 175 to 200 \({}^{\circ}\)C). The EMIPSI electrolyte shows a lower degradation than that containing N114 FSI, due to the higher stability of the (EMI)\({}^{+}\) cation compared to (N1114)\({}^{+}\)[4].