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.