Multiple Ether‐Functionalized Phosphonium Ionic Liquids as Highly Fluid Electrolytes

Abstract

Dedicated to the memory of Reiner Wintringer, our lab technician.Ionic liquids (ILs) are promising electrolytes, although their often high viscosity remains a serious drawback. The latter can be addressed by the introduction of multiple ether functionalization. Based on the highly atom efficient synthesis of tris(2ethoxyethyl) phosphine, several new phosphonium ionic liquids were prepared, which allows studying the influence of the ether side chains. Their most important physicochemical properties have been determined and will be interpreted using established approaches like ionicity, hole theory, and the Walden plot. There is striking evidence that the properties of phosphonium ionic liquids with the methanesulfonate anion are dominated by aggregation, whereas the two triple ether functionalized ILs with the highest fluidity show almost ideal behavior with other factors being dominant. It is furthermore found that the deviation from ideality is not significantly changed upon introduction of the ether side chains, although a very beneficial impact on the fluidity of ILs is observed. Multiple ether functionalization therefore proves as a powerful tool to overcome the disadvantages of phosphonium ionic liquids with large cations.

Ether functionalisation, ion conformation and the optimisation of macroscopic properties in ionic liquids

“…Tris(2ethoxyethyl)phosphine and tris(2-ethoxyethyl)methylphosphonium iodide were prepared as described previously. 24 Water in direct contact with the samples was purified using an Arium s advance EDI Type 2 water purification system (Sartorius, Göttingen, Germany). Thermal transitions, densities, viscosities, ionic conductivities and self-diffusion coefficients were measured as described previously.…”

“…Thermal transitions, densities, viscosities, ionic conductivities and self-diffusion coefficients were measured as described previously. 24 Self-diffusion coefficients were calibrated against water. To improve the fitting procedure, viscosities and ionic conductivities were measured in steps of 5 1C.…”

“…The formatted Gaussian checkpoint file containing the wavefunction was analysed using the quantitative molecular surface analysis tool in the Multiwfn software package as described in our preceding publication. 24,30,31 For the ion volume calculations, convergence criteria were tightened to 10 À11 Hartree for the RMS change in the density matrix.…”

“…Of these, [P5551][NTf 2 ] and [P(2O2) 3 1][NTf 2 ] are already known and our results are in good agreement with previous data. 24 This particular set of ionic liquids was chosen to specifically highlight how changes in molecular structure (i.e., design elements) lead to changes in IL bulk properties, Fig. 1b.…”

“…when replacing a methyl with an ethyl group. 24 However, dispersion interactions between the side chains increases with alkyl chain length and these can eventually lead to aggregation and a decrease of ion mobility. 19 The competing and cooperative effects from a single functional group are difficult to disentangle; hence investigated model systems must be carefully chosen so as to change as few parameters at a time as possible.…”