Chelating Additives Reversing the Lithium Migration Direction in Ionic Liquid Electrolytes

Abstract

Binary electrolytes of Li salt in ionic liquid (IL) are interesting systems for battery application. Typical for these systems is a strong Li-anion coordination causing vehicular Li+ transport in negatively charged Li-anion clusters and negative Li+ transference numbers. We investigate the influence of the additives tetrahydrofuran, monoglyme and triglyme on the Li+ migration behavior in 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­amide (EMImTFSA) based mixtures via electrophoretic NMR. When adding coordinating additives, we partly observe a reversal of the lithium migration direction and an electrophoretic drift of the neutral molecule. Using the same O:Li ratio, this effect is strongly depending on the chelating ability of the respective additive. Strongly coordinating additives form a chelate with the Li ion and render its drift velocity and its transference number positive, while the nonchelating additive tetrahydrofuran has a weaker effect on the Li drift. By choice of a suitable additive, it is therefore possible to decompose negatively charged lithium-anion clusters while maintaining ionic liquid-like properties. These findings provide a mechanistic explanation for the beneficial effect of additives on Li+ transport in IL-based battery cells.

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study

“…The effect may be reduced by the addition of a molecular chelating agent to the IL electrolyte. 52 , 53 …”

Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations

“…Thus, strong anion coordination is a "double-edge sword" and salt-in-IL systems must balance the attributes gained from the anion-derived SEI with the losses incurred by suboptimal lithium transport. Manipulating the ratio of IL salt to lithium salt, as well as incorporating solvent molecules [37], chelating agents [27,28], or co-anions [38,39] could serve as a design pathway for future concentrated electrolyte blends that interpolate in between the solvent-in-salt and IL regimes. The trouble is that as more components comprise the electrolyte formulation, the parameter space undergoes a combinatoric explosion.…”

“…To explain such an observation, the existence of ionic agglomerates containing more IL-anions than alkali cations was postulated [24], and the molecular resolution of atomistic simulations confirmed this hypothesis [26]. While experiments are limited by the high viscosity of highly-concentrated IL electrolytes, MD simulations extended the analysis to high alkali-salt molar fractions, showing the ubiquitous tendency of these systems to percolate into fully-connected networks, and of the alkali-ion transference number to reverse sign and approach +1 [26][27][28]. Both communities took an additional step towards rationalising these puzzling observations by independently measuring the alkalication effective charge, i.e., the time averaged charge carried by alkali-cation-containing clusters [24,26].…”

“…While this experimental and computational understanding is helpful to guide the formulation of electrolyte design rules [27,28], it lacks the support of a thermodynamic theory of ion clustering that can provide a conceptual, mathematical framework for the phenomena observed in experiments and simulations. Some preliminary thermodynamic theories have attempted to capture ionic associations in super-concentrated electrolytes through ion pair formation [29] or by neglecting correlations and only accounting for free ions [30].…”

“…chelating agents [27,28], or co-anions [38,39], theoretical guidance can be invaluable in sweeping through volumes of design space that even high-throughput experimentation and molecular simulation cannot cover.…”