Na⁺-conducting, binary electrolytic mixtures, based on 1-ethyl-3-methyl-imidazolium, trimethyl-butyl-ammonium, and N-alkyl-N-methyl-piperidinium ionic liquid (IL) families, were designed and investigated. The anions were selected among the per(fluoroalkylsulfonyl)imide families. Sodium bis(trifluoromethylsulfonyl)imide, NaTFSI, was selected as the salt. The NaTFSI-IL electrolytes, addressed to safer sodium battery systems, were studied and compared in terms of ionic conductivity and thermal stability as a function of the temperature, the nature of the anion and the cation aliphatic side chain length. Room temperature conductivities of interest for sodium batteries, i.e., largely overcoming 10⁻⁴ or 10⁻³ S cm⁻¹, are displayed. Similar conduction values are exhibited by the EMI-based samples even below −10 °C, making these electrolyte mixtures potentially appealing also for low temperature applications. The NaTFSI-IL electrolytes, with the exception of the FSI-ones, are found to be thermally stable up to 275 °C, depending on the nature of the cation and/or anion, thus extending their applicability above 100 °C and remarkably increasing the reliability and safety of the final device, especially in the case of prolonged overheating.

In this work a purification route of precursors for ionic liquids tailored to electrochemical energy storage systems is reported and described. The study was carried out on the N-butyl-N-methylpyrrolidinium bromide (PYR₁₄Br) precursor, which represents the intermediate product of the synthesis process of the N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) hydrophobic ionic liquid. The target is to develop an easy and cost-effective approach for efficiently purifying several kinds of ionic liquid precursors and determining their purity content. The PYR₁₄Br precursor was synthesized through an eco-friendly preparation procedure, which requires water as the only processing solvent, and purified through sorbent materials, such as activated charcoal and alumina. The effect of the treatment/nature/content of sorbents and processing temperature/time was investigated. The impurity content was detected by UV-VIS spectrophotometry measurements. Additionally, a correlation between the measured absorbance and the content of impurities within the precursor was obtained. The purity level of the precursor was seen to play a key role in the electrochemical performance of the ionic liquids.

Sodium-conducting electrolytes, based on the EMIFSI, EMITFSI, N1114FSI, N1114TFSI, N1114IM14, PIP13TFSI and PIP14TFSI ionic liquids, were investigated in terms of electrochemical stability through voltammetry techniques with the aim of evaluating their feasibility in Na-ion devices. Both the anodic and cathodic sides were studied. The effect of contaminants, such as water and/or molecular oxygen, on the electrochemical robustness of the electrolytes was also investigated. Preliminary cyclic voltammetry and charge-discharge tests were carried out in Na/hard carbon and Na/α-NaMnO2 half cells using selected ionic liquid electrolytes. The results are presented and discussed in the present paper.