Solvent effects on Li ion transference number and dynamic ion correlations in glyme- and sulfolane-based molten Li salt solvates

The Li + transference number of electrolytes is one of the key factors contributing to the enhancement in the charge-discharge performance of Li secondary batteries. However, a design principle to achieve a high Li + transference number has not been established for liquid electrolytes. To understand the factors governing the Li + transference number t Li , we investigated the influence of the ion-solvent interactions, Li ion coordination, and correlations of ion motions on the Li + transference number in glyme (G n , n = 1-4)- and sulfolane (SL)-based molten Li salt solvate electrolytes with lithium bis(trifluoromethansulfonyl)amide (LiTFSA). For the 1 : 1 tetraglyme-LiTFSA molten complex, [Li(G4)][TFSA], the Li + transference number estimated using the potentiostatic polarisation method ( t PP Li = 0.028) was considerably lower than that estimated using the self-diffusion coefficient data with pulsed filed gradient (PFG)-NMR ( t NMR Li = 0.52). The dynamic ion correlations ( i.e. , cation-cation, anion-anion, and cation-anion cross-correlations) were determined from the experimental data on the basis of Roling and Bedrov's concentrated solution theory, and the results suggest that the strongly negative cross-correlations of the ion motions (especially for cation-cation motions) are responsible for the extremely low t PP Li of [Li(G4)][TFSA]. In contrast, t PP Li is larger than t NMR Li in the SL-based electrolytes. The high t PP Li of the SL-based electrolytes was ascribed to the substantially weaker anti-correlations of cation-cation and cation-anion motions. Whereas the translational motions of the long-lived [Li(glyme)] + and [TFSA] − dominate the ionic conduction for [Li(G4)][TFSA], Li ion hopping/exchange conduction was reported to be prevalent in the SL-based electrolytes. The unique Li ion conduction mechanism is considered to contribute to the less correlated cation-cation and cation-anion motions in SL-based electrolytes. Ion-solvent interactions and Li ion coordination structure have a significant impact on dynamic ion correlations and Li ion transference number of molten Li salt solvate electrolytes.