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

Organic ionic plastic crystals (OIPCs) appear as promisingmaterialsto replace traditional liquid electrolytes, especially for use insolid-state batteries. However, OIPCs show low conductive propertiesrelative to liquid electrolytes, which presents an obstacle for theirwidespread applications. Recent studies revealed very high ion mobilityin the solid phases of OIPCs; yet, the ionic conductivity is significantly(& SIM;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, andNMR diffusion measurements in the liquid and solid phases of diethyl(methyl)(isobutyl)phosphonium-hexafluorophosphate[P-1,P-2,P-2,P-4][PF6]. The results confirmed significantdecrease in conductivity of the solid phases of this OIPC throughion-ion correlations. Surprisingly, these ionic correlationssuppress charge displacement on rather long time scales comparableto the time of ion diffusion on the & SIM;1.5 nm length scale. Weascribe the observed phenomena to momentum conservation in the motionof mobile anions and emphasize that a microscopic understanding ofthese correlations might enable design of OIPCs with strongly enhancedionic conductivity.