A Mechanistic Understanding of Lithium Ion Transport in Ternary Ionic Liquid Electrolytes

Abstract

With regard to methodologically enhancing the electrolyte functionality, and hence the ionic conductivity as a crucial indicator, a profound understanding of the correlated ion motion is essential. For binary ionic liquid systems it has been shown that the motional coupling between ions is determined to a large extent by the conservation of momentum, concurring with simple theoretical relations for transference numbers that serve as a guideline for optimizing the electrolyte [1,2]. Inspired by this work, we perform Molecular Dynamics (MD) simulations to unravel the complex, and rather counterintuitive, interplay of dynamical electrostatic correlations in a ternary ionic liquid - lithium salt mixture, which contribute to the overall conductivity. Considering the environmental sustainability of the electrolyte mixture in the battery context as main scope of application, we choose three anions with reduced toxic fluorine content for this study (bis(trifluormethanesulfonyl)imide (TFSI), trifluormethanesulfonyl-N-cyanoamide (TFSAM) and dicyanamide (DCA)) and study the influence of anion structure on transport behavior. From systematic analysis of the distance- and time-dependent correlational quantities we try to identify key parameters that quantify the microscopic transport properties and hence might serve as a constructive optimization concept. [1] H. Kashyap et al., J. Phys. Chem. B, 2011, 115 (45), pp 13212–13221 [2] D. Diddens, V. Lesch and A. Heuer, Correlated Motion in Ionic Liquids. To be submitted., 2019