Transport and electrochemical properties of the potential ternary electrolytes MPPyrr-TFSI + (DMSO or γ-butyrolactone, or acetronitile) + Li-TFSI

One of the most popular applications of ionic liquids (ILs) is as electrolyte component for electrochemical devices. In this context, and with the aim to complement previous studies, mixtures of 1-methyl-1-propylpyrrolidinium bis(trifluoromethyl sulfonyl)imide (MPPyrr-TFSI) with one of these three different solvents, acetonitrile (AN), γ-butyrolactone (GBL) or dimethyl sulfoxide (DMSO), in a proportion around 50 wt%_IL-50 wt%, were prepared. Then, different concentrations of the lithium salt, Li-TFSI, was added to the binary mixtures, obtaining three potential electrolytes based in that IL. Thermal properties of those mixtures and diffusion coefficient of the [Li]⁺, [MPPyrr]⁺ and [TFSI]⁻ ions, for selected ternary mixtures, were experimentally measured using the NMR-DOSY technique. To elucidate the [Li]⁺ cation contribution to charge transport, we have determined its diffusion constant for many different Li salt concentrations in the ternary mixture with DMSO as solvent. By using density, viscosity and electrical conductivity of those same mixtures presented in a previous paper, theoretical values for the molar conductivities, calculated through Nerst-Einstein equation taking into account the obtained data of diffusion coefficients, were calculated. The resulting values are in strong agreement with those measured experimentally. In addition, the electrochemical window of some selected mixtures was measured, and its electrochemical performance in coin cell configuration with LiFePO₄ as cathode and Li metal as anode were studied. Results were compared with those obtained with a common carbonate based Li-TFSI electrolyte. As a result, a very promising ternary mixture to be part of future IL based electrolytes for the next battery generation is proposed.