Collective Ion Dynamics in Ionic Plastic Crystals: The Origin of Conductivity Suppression

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 [P_1,2,2,4][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.