Synthesis and characterisation of fluorine-free ionic liquids for electrochemical applications

In this work, two fluorine-free ionic liquids (ILs) were developed, subsequently doped with 20mol% of the lithium salt of the corresponding anion, thus obtaining the respective electrolytes. The synthetized ILs included an imidazolium-based and a pyrrolidinium-based formulations, both functionalised with an ether side chain, combined with a fluorine-free anion. The anion, whose determination and selection represented the most complex part of this work, was based on a triazine derivative, functionalised with two ethoxy side chains and a hydroxyl group, which was then deprotonated to generate the desired anionic precursor. Having the same anion, the imidazolium-based exhibited greater thermal and electrochemical stability, while the pyrrolidinium-based demonstrated better transport properties. When comparing the results with literature data on fluorine-free and fluorinated ILs, the most relevant findings obtained from the various tests were: a slight reduction in ionic conductivity between ILs and their corresponding electrolytes, equal to -8% for the pyrrolidinium and -17% for the imidazolium one, at 20°C, and a wide electrochemical stability window, exceeding 6 V for all the systems. Nevertheless, this latter property must be validated employing different scan rates and electrode configurations. All other measured properties fall within the range of fluorine-free ILs, while they remain lower than those of fluorinated ones. Although no single property clearly outperforms state-of-the-art systems, the synthesis of novel ILs can serve as building blocks for the ongoing research for new fluorine-free anions derived from cheaper raw materials, with promising applications in next-generation energy storage systems.