Understanding transport mechanisms in ionic liquid/carbonate solvent electrolyte blendsElectronic supplementary information (ESI) available. See DOI: 10.1039/c8cp01485j

• Main Authors: Oldiges, K, Diddens, D, Ebrahiminia, M, Hooper, J. B, Cekic-Laskovic, I, Heuer, A, Bedrov, D, Winter, M, Brunklaus, G
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c8cp01485j

To unravel mechanistic details of the ion transport in liquid electrolytes, blends of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr 14 TFSI), ethylene carbonate (EC) and dimethyl carbonate (DMC) with the conducting salts lithium hexafluorophosphate (LiPF 6 ) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) were investigated as a function of the IL concentration. Electrochemical impedance, Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) and Raman spectroscopy supported by Molecular Dynamics (MD) simulations allowed the structural and dynamic correlations of the ion motions to be probed. Remarkably, we identified that though the individual correlations among different ion types exhibit a clear concentration dependence, their net effect is nearly constant throughout the entire concentration range, resulting in approximately equal transport and transference numbers, despite a monitored cross-over from carbonate-based lithium coordination to a TFSI-based ion coordination. In addition, though dynamical ion correlation could be found, the absolute values of the ionic conductivity are essentially determined by the overall viscosity of the electrolyte. The IL/carbonate blends with a Pyr 14 TFSI fraction of ∼10 wt% are found to be promising electrolyte solvents, with ionic conductivities and lithium ion transference numbers comparable to those of standard carbonate-based electrolytes while the thermal and electrochemical stabilities are considerably improved. In contrast, the choice of the conducting salt only marginally affects the transport properties. Organic carbonate-based electrolytes blended with ionic liquids exhibit improved electrochemical properties, as demonstrated by experiments and MD simulations.