Anion effects on Li ion transference number and dynamic ion correlations in glyme–Li salt equimolar mixtures

Abstract

To achieve single-ion conducting liquid electrolytes for the rapid charge and discharge of Li secondary batteries, improvement in the Li⁺ transference number of the electrolytes is integral. Few studies have established a feasible design for achieving Li⁺ transference numbers approaching unity in liquid electrolytes consisting of low-molecular-weight salts and solvents. Previously, we studied the effects of Li⁺–solvent interactions on the Li⁺ transference number in glyme- and sulfolane-based molten Li salt solvates and clarified the relationship between this transference number and correlated ion motions. In this study, to deepen our insight into the design principles of single-ion conducting liquid electrolytes, we focused on the effects of Li⁺–anion interactions on Li ion transport in glyme–Li salt equimolar mixtures with different counter anions. Interestingly, the equimolar triglyme (G3)–lithium trifluoroacetate (Li[TFA]) mixture ([Li(G3)][TFA]) demonstrated a high Li⁺ transference number, estimated via the potentiostatic polarization method (t^PP_Li = 0.90). Dynamic ion correlation studies suggested that the high t^PP_Li could be mainly ascribed to the strongly coupled Li⁺–anion motions in the electrolytes. Furthermore, high-energy X-ray total scattering measurements combined with all-atom molecular dynamics simulations showed that Li⁺ ions and [TFA] anions aggregated into ionic clusters with a relatively long-range ion-ordered structure. Therefore, the collective motions of the Li ions and anions in the form of highly aggregated ion clusters, which likely diminish rather than enhance ionic conductivity, play a significant role in achieving high t^PP_Li in liquid electrolytes. Based on the dynamic ion correlations, a potential design approach is discussed to accomplish single-ion conducting liquid electrolytes with high ionic conductivity.