Mesoscopic inhomogeneities in concentrated electrolytes

TL;DR

This paper develops a mesoscopic theory combining density functional and field-theoretic methods to explain long-range oscillatory decay in water-in-salt electrolytes, matching experimental observations.

Contribution

It introduces a simplified model that captures mesoscopic inhomogeneities in concentrated electrolytes, explaining experimental decay lengths.

Findings

Correlation functions show mesoscopic inhomogeneities occur under specific interaction conditions.

Decay length of correlations increases nearly linearly with ion volume fraction.

Model reproduces experimental period and decay rate of oscillatory decay.

Abstract

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 LiTFSI (lithium bis(trifluoromethylsulfonyl)-imide) salt [T. S. Groves et. al., J. Phys. Chem. Lett. {\bf 12},1702 (2021)]. 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 a semiquantitative agreement with the experiment. In particular, the decay length of the correlations increases nearly linearly with the volume fraction of ions.