Influence of Water on the Properties of Protic Ionic Liquids

Abstract

Protic ionic liquids (PILs) represent an interesting subclass of Ionic Liquids (ILs), with properties suitable for the realization of different kinds of electrochemical energy storage devices. [1-2] PIL have been proposed as electrolytes for electrochemical double layer capacitors (EDLCs) and lithium-ion batteries (LIBs) [ref]. In the case of EDLCs the use of these ILs appears not to be particularly advantageous due to the presence of a proton on their structure, limiting the operative voltage, and their relative high viscosity, limiting the power performance of these devices. In the case of LIBs, to the contrary, the presence of the proton has a positive influence on the lithium coordination and on the performance at high current densities of IL-based LIBs. [3]. The influence of the water content on the chemical-physical properties of PILs have been investigated in the past, and it has been found that even a few hundred ppm of residual water are strongly changing the properties of these ILs, e.g. they might significantly lower the melting point of PILs. Nevertheless, the impact of the water content on the electrochemical performance of electrochemical energy storage devices containing PILs as electrolytes has not been investigated in detail. These important aspects should be investigated because it could supply valuable information about the development of PIL-based devices. In this paper we report about the influence of water on the properties of the PIL butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PyrH4TFSI) containing different amounts of water (0, 1, 2, 3.8 wt%). The viscosity, ionic conductivity, density and electrochemical stability of these PIL have been investigated in detail. Furthermore, the mixtures were utilized as electrolytes for EDLCs containing activated carbon-based electrodes, as well as for LIBs containing graphite anodes and lithium iron phosphate (LFP) cathodes. The results of this study indicate that small amounts of water have a significant impact on the overall transport properties and stability of PyrH4TFSI. Moreover, they also show that the performance of EDLCs and LIBs containing PyrH4TFSI as electrolyte are considerably changing depending on the water content of the PIL. Literature: [1] Menne, Electrochem. Commun. 2013, 31, 39-41.[2] Vogl, Journal of Materials Chemistry A 2016, 4, 10472-10478.[3] Brandt, Electrochim. Acta 2013, 108, 226-231. Figure 1