Organic ionic plastic crystals (OIPCs) appear as promising
materials
to replace traditional liquid electrolytes, especially for use in
solid-state batteries. However, OIPCs show low conductive properties
relative to liquid electrolytes, which presents an obstacle for their
widespread applications. Recent studies revealed very high ion mobility
in the solid phases of OIPCs; yet, the ionic conductivity is significantly
(∼100 times) suppressed because of strong ion–ion correlations.
To understand the origin of the ion–ion correlations in OIPCs,
we employed broadband dielectric spectroscopy, light scattering, and
NMR diffusion measurements in the liquid and solid phases of diethyl(methyl)(isobutyl)phosphonium-hexafluorophosphate
[P1,2,2,4][PF6]. The results confirmed significant
decrease in conductivity of the solid phases of this OIPC through
ion–ion correlations. Surprisingly, these ionic correlations
suppress charge displacement on rather long time scales comparable
to the time of ion diffusion on the ∼1.5 nm length scale. We
ascribe the observed phenomena to momentum conservation in the motion
of mobile anions and emphasize that a microscopic understanding of
these correlations might enable design of OIPCs with strongly enhanced
ionic conductivity.