Understanding transport mechanisms in ionic liquid/carbonate solvent electrolyte blends

Abstract

To unravel mechanistic details of the ion transport in liquid electrolytes, blends of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), ethylene carbonate (EC) and dimethyl carbonate (DMC) with the conducting salts lithium hexafluorophosphate (LiPF6) 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 Pyr14TFSI 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.

Enhancement of Functional Properties of Liquid Electrolytes for Lithium‐Ion Batteries by Addition of Pyrrolidinium‐Based Ionic Liquids with Long Alkyl‐Chains

“…[29] In all cases the addition of an ILs increase the viscosity of the solution. Moreover, in line with the conductivities also viscosities of IL-added electrolytes show a trend with the size of the alkyl chain in the pyrrolidinium cation being TC > TB > TA > T and PC > PB > PA > P. These data are partially in line with the observation of Oldiges et al: [42] the ionic conductivity in the mixture of ionic liquids/organic solvent is affected mostly by the increasing of viscosity. This description nicely matches with the here reported reversed trend of various electrolytes viscosities and conductivities.…”

“…These bands are shifted respectively at 904 and 933 cm À 1 when solvent molecules coordinate lithium ions. [42,58] In the case of electrolytes without ionic liquids, the three bands at 893, 904 and 916 cm À 1 are sharp. On the contrary in all IL-added electrolytes the intensity of the bands in the 850-950 cm À 1 region is reduced and all peaks are broader, especially in the band at 893 cm À 1 .…”

“…Specifically, the range 850-950 cm À 1 was deconvoluted in 4 bands, accounting the free and coordinated EC and DMC. [42,58] Considering that in this region few vibrational features due to pyrrolidinium cation are also present and according to the previous works, these contributions have been subtracted. [42,58] In the range 705-765 cm À 1 five bands have been included in the fitting, considering the free and coordinated EC and TFSI and the two conformers (cisoid and transoid) of TFSI ( Figure S4).…”

Structural, Mechanical, and Dynamical Properties of Amorphous Li2CO3 from Molecular Dynamics Simulations

“…Simulations have been conducted using a revised version of the many-body polarizable force field (APPLE&P) force field [21]. The APPLE&P force field has been extensively used to predict ion dynamics in various electrolytes and model SEI systems [22]. Ewald summation was employed for the long-range electrostatic interactions (charge−charge and charge−induced dipoles), with a cutoff radius of 12 Å for non-bonded van der Waals interactions and the real part of Ewald summation.…”

Improved lithium ion dynamics in crosslinked PMMA gel polymer electrolyte

“…22 In addition to experimental measurements, molecular modelling techniques are extremely helpful to gain insights at the microscopic level. Molecular dynamics (MD) simulations are frequently employed to study ion coordination [23][24][25][26] or ion transport mechanism [27][28][29][30][31][32][33] in electrolyte materials, however, these calculations depend on reliable force elds [34][35][36] to obtain accurate results. On the other hand, ab initio quantum chemistry (QC) calculations require no further input than the molecular structure, and offer access to binding energies between different molecular species.…”