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 tLi, we investigated the influence of the ion–solvent interactions, Li ion coordination, and correlations of ion motions on the Li+ transference number in glyme (Gn, 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 (tPPLi = 0.028) was considerably lower than that estimated using the self-diffusion coefficient data with pulsed filed gradient (PFG)-NMR (tNMRLi = 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 tPPLi of [Li(G4)][TFSA]. In contrast, tPPLi is larger than tNMRLi in the SL-based electrolytes. The high tPPLi 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.