Collective ion dynamics in Organic Ionic Plastic Crystals

Organic ionic plastic crystals (OIPCs) appear to be promising materials to replace conventional liquid electrolytes in solid state batteries. OIPCs, in contrast to liquid electrolytes, exhibit low conductivity, preventing them to be widely used. Recent investigations have shown that despite of high ion mobility even in their solid phases, the DC-conductivity is significantly suppressed by strong ion-ion interactions [1, 2]. In this study we conducted broadband dielectric spectroscopy, light scattering, and NMR diffusion investigations in the sample Hexafluorophosphate - Diethyl(methyl)(isobutyl)phosphonium [PF₆][P_1,2,2,4] in order to better understand the causes of the ion-ion correlations in OIPCs. We discovered two types of ion dynamic in OIPCs: i) at short times and length scale the ion dynamic in crystalline phase is similar to regular ionic liquids, ii) while at longer scale 1.5-2 nm, the ion-ion correlations suppress charge displacement and DC conductivity. DC conductivity regime is achieved only when ions are diffusing more than 1.5-2 nm.