Thermophysical and Electrochemical Properties of Ethereal Functionalised Cyclic Alkylammonium‐based Ionic Liquids as Potential Electrolytes for Electrochemical Applications

Abstract

A series of hydrophobic room temperature ionic liquids (ILs) based on ethereal functionalised pyrrolidinium, piperidinium and azepanium cations bearing the bis[(trifluoromethyl)sulfonyl]imide, [TFSI]−, anion were synthesized and characterized. Their physicochemical properties such as density, viscosity and electrolytic conductivity, and thermal properties including phase transition behaviour and decomposition temperature have been measured. All of the ILs showed low melting point, low viscosity and good conductivity and the latter properties have been discussed in terms of the IL fragility, an important electrolyte feature of the transport properties of glass‐forming ILs. Furthermore, the studied [TFSI]−‐based ILs generally exhibit good electrochemical stabilities and, by coupling electrochemical experiments and DFT calculations, the effect of ether functionalisation at the IL cation on the electrochemical stability of the IL is discussed. Preliminary investigations into the Li‐redox chemistry at a Cu working electrode are also reported as a function of ether‐functionality within the pyrrolidinium‐based IL family. Overall, the results show that these ionic liquids are suitable for electrochemical devices such as battery systems, fuel cells or supercapacitors.

Acyclic and Cyclic Alkyl and Ether‐Functionalised Sulfonium Ionic Liquids Based on the [TFSI]− and [FSI]− Anions as Potential Electrolytes for Electrochemical Applications

“…[35] This reduction can be attributed to a reduction in molecular symmetry, increased rotational freedom due to the electron donating effect of the ether group and subsequent reduction in lattice energy. [14] Therein, this small pre-peak transitions are attributed to crystal-crystal transitions between metastable solid phases. Figure 3 shows the DSC thermogram for the [S 11G1 ][TFSI] IL which exhibits three distinct types of phase transition behaviour.…”

“…The calculated a p for the nine ILs lies within the range of (5.98 10 À4 -6.83 10 À4 ) K À1 between 293.15 K-363.15 K. This calculated range is commensurate with values previously 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 reported a p values for ILs displaying comparable physical properties: a series of alkyl/ether-functionalised cyclic ammonium-[TFSI] based ILs, a p = (5.7 10 À4 -7.5 10 À4 ) K À1 ; [14] S,S-diethyl-S-methylsulfonium-[TFSI], a p = 6.49 10 À4 K À1 ; [22] Nbutyl-N,N,N-trimethylammonium-[TFSI], a p = 6.32 10 À4 K À1 ; [22] 1-butyl-1-methylpyrrolidinium-[FSI], a p = 6.01 10 À4 K À1 . [44] …”

“…[46] Increasing the length of the alkyl chain (i. there is a balance on the resulting IL transport properties between the relative increase in cation size (molecular weight) and the promotional effects of introducing higher degrees of ether functionality. [14] The identity of the anion also has a profound effect on the IL viscosity. The delocalised charge of the [TFSI] À anion often yields ILs with only moderate viscosities and is one of the main reasons why it is used so often.…”

“…[24] As reported in Table 1, good agreements are generally observed between our data with literature, [13,23,24,26,28] except in the case of the data reported by Orita et al [26] IL shows the largest ionic conductivity of 8.53 mS · cm À1 at 298.15 K, congruent with the low viscosity of this IL. [13,14] [14] is related to the consequent reduction in ionic mobility of the bulkier [THT G2 ] + cation, impeding the conduction of charge relative to the smaller [THT 4 ] + cation. Furthermore, the ILs with etheral functionality exhibit a higher conductivity than their alkyl-functionalised counterparts, also relating to the lower viscosities of these ILs Table 3.…”

“…[11] The substitution of alkyl groups with a similarly sized, flexible ether group onto the cation structure has been shown previously to reduce the viscosity of the IL. [14] Due to high charge delocalization and weak coordinating nature of fluorinated sulfonyl anions, namely, bis{(trifluoromethyl)sulfonyl}imide [TFSI] À , and bis(fluorosulfonyl)imide [FSI] À , are often used to obtain ILs with even lower viscosity, moderate transport properties and generally excellent hydrophobicity. [14] Due to high charge delocalization and weak coordinating nature of fluorinated sulfonyl anions, namely, bis{(trifluoromethyl)sulfonyl}imide [TFSI] À , and bis(fluorosulfonyl)imide [FSI] À , are often used to obtain ILs with even lower viscosity, moderate transport properties and generally excellent hydrophobicity.…”