Mesoscopic Inhomogeneities in Concentrated Electrolytes

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 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.

A screening of results on the decay length in concentrated electrolytes

“…In contrast to previous work that reported scaling exponents n < 3, Ciach and Patsahan 42,43 arrived at an exponent n = 3 using a self-consistent Gaussian approximation based on the Brazovskii approach. 51,52 Unlike previous theoretical approaches, these authors took into account the local variance of the charge density.…”

“…41 The cubic scaling in ref. [41][42][43] emerged in the dampened oscillatory decay of the charge-charge correlation function. In experiments, however, the force exhibiting large screening lengths with cubic scaling decayed monotonically with distance between the surfaces.…”

“…The underscreening experiments (Section 2.1) rekindled interest in ionic screening, and several approaches have been developed in an attempt to explain the unusually large screening lengths. 34,35,[40][41][42][43] Rotenberg et al 40 considered primitive (RPM) and 'semi-primitive' (charged hard spheres and hard-sphere solvent) models. They applied MSA to compute the chargecharge correlation functions and found that, in line with previous work, 19,20 the screening length increased with concentration for concentrations above the Kirkwood cross-over.…”

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

“…salt-in-ILs (SiILs) [7][8][9][10][11][12][13], which increases the energy that can be stored in the device. The promise of IL-based technologies has caused significant interest in understanding ILs, and other concentrated electrolytes, such as water-in-salt electrolytes (WiSEs) [14][15][16][17][18][19][20][21][22][23][24]. Indeed, owing to the high concentration and lack of high dielectric solvent, the energy of electrostatic interactions between nearest neighbors is much larger than thermal energy [3].…”