Ionic liquid (IL) based electrolytes are promising for a use in Li ion batteries, due to their low vapor pressure and inflammability, accompanied with a moderate conductivity. However, strong ion interactions, in particular Li-anion coordination, influence Li+ dynamic behavior. In typical salt-in-IL electrolytes, this leads to a correlated Li-anion migration, which promotes Li migration in the undesired direction. This behavior is directly observable via electrophoretic NMR (eNMR). Various efforts were made recently to decouple the Li and anion dynamics, for example the introduction of asymmetric anions and the increase of the Li salt concentration, or the addition of suitable additives. In this work, Li coordinating oligoether cations with one, seven or twelve ether-oxygens in the side chain are employed. Salt-in-IL electrolytes based on these coordinating cations exhibit the desired positive Li migration direction for n ≥ 7 ether-oxygens, independently of the Li salt concentration. Employing Pulsed Field Gradient (PFG)-NMR diffusion experiments and eNMR measurements, a detailed picture of the Li-cation interactions in these electrolytes is given. Furthermore, the calculation of the inverse Haven ratio shows a superionic behavior of the electrolytes at high Li salt concentration, which has not been observed before in salt-in-IL electrolytes. This superionicity is based on the presence of Li-Li, Li-cation and cation-cation correlations and strong Li-anion anticorrelations.

Solvating cation induced reversing of Li+ migration direction in ionic liquid-in-salt electrolytes observed via electrophoretic NMR

ACS Spring Meeting 2021

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