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

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Abstract

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.

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

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Supplementary files

Article information


Submitted
23 Apr 2020
Accepted
01 Jun 2020
First published
29 Jun 2020

This article is Open Access

Phys. Chem. Chem. Phys., 2020, Advance Article
Article type
Paper

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

K. Shigenobu, K. Dokko, M. Watanabe and K. Ueno, Phys. Chem. Chem. Phys., 2020, Advance Article , DOI: 10.1039/D0CP02181D

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