Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 23, Issue 4, Pages 2622-2629Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/d0cp06381a
Keywords
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Funding
- Japan Society for the Promotion of Science (JSPS) KAKENHI [20H02837, 20H02823, 18H03926]
- JST ALCA-SPRING, Japan [JPMJAL1301]
- Grants-in-Aid for Scientific Research [20H02823, 20H02837] Funding Source: KAKEN
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By investigating the effects of Li+-solvent and Li+-anion interactions on Li+ transference numbers in liquid electrolytes, insight into the design principles of single-ion conducting liquid electrolytes has been gained, which contributed to improving Li+ ion transport efficiency.
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 (tPPLi = 0.90). Dynamic ion correlation studies suggested that the high tPPLi 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 tPPLi 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.
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