Mesoscopic Inhomogeneities in Concentrated Electrolytes

A mesoscopic theory for water-in-salt electrolytes combining density functional and field-theoretic methods is developed in order to explain the unexpectedly large period of the oscillatory decay of the disjoining pressure observed in recent experiments for the lithium bis(trifluoromethylsulfonyl)-imide (LiTFSI) salt [T. S. Groves et al., J. Phys. Chem. Lett. 2021, 12, 1702]. We assumed spherical ions with different diameters and implicit solvent, inducing strong, short-range attraction between ions of the same sign. For this highly simplified model, we calculated correlation functions. Our results indicate that mesoscopic inhomogeneities can occur when the sum of the Coulomb and the water-mediated interactions between like ions is attractive at short and repulsive at large distances. We adjusted the attractive part of the potential to the water-in-LiTFSI electrolyte and obtained both the period and the decay rate of the correlations, in semiquantitative agreement with the experiment. In particular, the decay length of the correlations increases nearly linearly with the volume fraction of ions.

Automated summary

In this study, a mesoscopic theory combining density functional and field-theoretic methods was developed to explain the unusually long period of oscillatory decay observed in recent experiments for the lithium bis(trifluoromethylsulfonyl)-imide (LiTFSI) salt. The calculation results of a simplified model with spherical ions and implicit solvent showed that mesoscopic inhomogeneities can occur when the sum of the Coulomb and water-mediated interactions between like ions is attractive at short distances and repulsive at large distances. By adjusting the attractive part of the potential to fit the water-in-LiTFSI electrolyte, the period and decay rate of the correlations were obtained, showing semi-quantitative agreement with the experimental results. The decay length of the correlations was found to increase nearly linearly with the volume fraction of ions.