# Ionic Liquid Electrolytes for Safer and More Reliable Sodium Battery Systems
## 3 Results and Discussion  
The investigated ionic liquid electrolytes have exhibited a purity level above 99.5 wt.%; in particular, the content of lithium, halide and moisture is below 2 ppm. The ion transport properties have been studied as a function of the temperature and the obtained results are reported in Figure 1a (EMI-based electrolytes), 1b (N\({}_{11114}\)-based) and 1c (PIP\({}_{1A}\)-based), respectively. The conductivity-temperature diagram of the 0.1 NaTFSI-0.9 IL electrolytes containing TFSI-based ionic liquids (EMITFSI, N\({}_{1114}\)TFSI, PIP\({}_{14}\)TFSI) exhibits ion conduction values around \(10^{-7}\) S cm\({}^{-1}\) (i.e., typical of a crystalline solid state) [4; 5; 9] from \(-40\)\({}^{\circ}\)C up to the melting point, evidenced by a step increase (from two to three orders of magnitude) of the ionic conductivity. Conversely, the mixtures containing FSI-based ILs (EMIFSI, N\({}_{1114}\)FSI) show, in the low-temperature range, an almost linear increase in conduction, likely attributable to an ion mobility gain due to solid-solid phase transitions occurring prior the melting [12]. A similar trend is also observed for the 0.1 NaTFSI-0.9 PIP\({}_{13}\)TFSI sample, in agreement with the conductivity results reported elsewhere for pure PIP\({}_{13}\)TFSI [13]. A two-orders-of-magnitude jump (not registered for the FSI-based electrolytes) is, however, detected around the melting temperature. A comparison among the conductivity-temperature plots evidences that solid-liquid transitions, recorded in the FSI-based electrolytes, occur at lower temperatures with respect to those observed in the TFSI ones. The melting point of an IL material is determined by the strength of its crystal lattice which, in turn, is controlled by competitive factors, such as surface charge density, intermolecular forces (Van der Waals) and molecular symmetry [14], which have opposite effect. Therefore, the lower Van der Waals forces, due to the smaller steric hindrance of the FSI anion, might counterbalance the higher surface charge density (which, conversely, tends to increase the melting point), thus decreasing the melting temperature with respect to the TFSI electrolytes [4]. No sudden conductivity raise is seen for the 0.1 NaTFSI-0.9N\({}_{1114}\)M\({}_{14}\) electrolyte, indicating the absence of any solid-liquid phase transition in the investigated temperature range. This is likely attributable to the high asymmetry of the IM\({}_{14}\) anion with respect to FSI and TFSI (symmetrical), which strongly hinders the ion packing and, therefore, the IL crystallization. In the molten state, the 0.1 NaTFSI-0.9 IL samples display a VTF behavior, typical of the ionic liquid electrolytes [4]. Further increases in temperature beyond that of the melting lead to a progressive raise in conductivity, due to the convective thermal motions within the ionic liquid media. The ionic conduction values, in the liquid state, progressively decrease with increasing the steric footprint of the anion in the following order: FSI > TFSI > IM\({}_{14}\). This behavior, reported in the literature for other IL electrolytes [4], is due to the progressive increase in viscosity (linked to the size of the ions), which slows down the mobility of the ions within the IL material [4]. In the molten state, a moderate decrease in conduction is observed for the 0.1 NaTFSI-0.9 PIP\({}_{14}\)TFSI electrolyte with respect to 0.1 NaTFSI-0.9 PIP\({}_{13}\)TFSI, attributable to the longer cation alkyl chain (n-butyl compared to n-propyl), which increases the steric bulk and, therefore, the viscosity of the piperidinium IL [13]. Conversely, the increase in alkyl chain length leads to a greater asymmetry of the cation,  
Figure 1: Ionic conductivity vs. temperature dependence of 0.1 NaTFSI-0.9 IL sodium electrolytes: (a) EMI-based electrolytes; (b) N\({}_{1114}\)-based electrolytes; (c) PIP\({}_{1A}\)-based electrolytes.