Monika SCHÖNHOFF1, Pinchas NÜRNBERG1, Jaschar ATIK2, Martin LORENZ1, Florian ACKERMANN1, Martin WINTER1,2, Elie PAILLARD1,3
1University of Münster, Münster, Germany
2Helmholtz Institute Münster, IEK-12, Forschungszentrum Ju?lich GmbH,, Mu?nster,, Germany
3Politecnico di Milano, Department of Energy, Milan, Italy
In ionic liquid (IL) based electrolytes, relevant for current energy storage applications, ion transport is limited by strong ion-ion correlations, generally yielding inverse Haven ratios (ionicities) below 1. In particular, Li+ is subject to vehicular transport in anionic clusters, inducing migration into the wrong direction of the electric field, as these clusters bear a net negative charge. This yields negative Li transference numbers and requires compensation of the migrational Li flux in the electric field by a diffusive Li ion flux in a strong concentration gradient.
Here, we evaluate several strategies to overcome the detrimental Li-anion correlations. The results are mainly based on species-selective NMR transport experiments, namely PFG-NMR, providing diffusion coefficients, and electrophoretic NMR, providing drift velocities in an electric field.
In Li salt in IL systems a high Li salt concentration in combination with asymmetric anions can improve the transport mechanisms, as it influences the Li-anion coordination numbers and thus reduces the Li-anion correlation. However, in most cases the Li transference number remains negative, though it may almost approach the zero line with increasing salt concentration.
In solvate ionic liquids on the other hand, the formation of a solvate cation, i.e. a Li+-glyme complex, reduces the Li-anion correlation. Here, anticorrelations between the solvate cation and the anion dominate the transport behavior and act to enhance ionicity. These anticorrelations are a consequence of local volume conservation, which is a consequence of the low compressibility of the system.
Further exploiting the beneficial Li-oligoether interaction, organic cations with a Li-coordinating chain were designed. 1H NMR and Raman spectra show that IL cations with seven or more ether-oxygens in the side chain induce Li coordination to organic cations. Here, an unusual behavior of an inverse Haven ratio > 1 is found, suggesting an ionicity larger than that of an ideal electrolyte with uncorrelated ion motion. This superionic behavior is consistently found in NMR transport experiments as well as in molecular dynamics (MD) simulations. Key for this achievement is the formation of long-lived Li-IL cation complexes, which invert the Li drift direction, yielding positive Li+ ion mobilities for the first time in a single solvent IL-based electrolyte. Onsager correlation coefficients derived from MD simulations demonstrate that the main contributions to the inverse Haven ratio, which induce superionicity, arise from an enhanced Li-IL cation correlation and a sign inversion of the Li-anion correlation coefficient. Thus, the concept of coordinating cations not only corrects the unfortunate anionic drift direction of Li in ILs, but even exploits strong ion correlations towards superionic transport.
The effectiveness of this concept is finally compared to the effect of coordinating solvents in salt-in-IL systems, which are not covalently bound to the cation, but reduce the Li-anion correlation by solvate formation.