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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 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 liquid and solid phases of Hexafluorophosphate - Diethyl(methyl)(isobutyl)phosphonium [PF6][P1,2,2,4]. The results confirmed significant decrease in conductivity of 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 motion of mobile anions and emphasize that microscopic understanding of these correlations might enable design of OIPCs with strongly enhanced ionic conductivity. 

Hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) has recently been shown to be a promising solid state organic ionic plastic crystal with potential application in advanced alkali metal batteries. This study provides a detailed exploration of the structural and dynamic behavior of [HMG][FSI] mixtures with the sodium salt NaFSI across the whole composition range from 0 to 100 mol%. All mixtures are solids at room temperature. A combination of differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD) and multinuclear solid state NMR spectroscopy is employed to identify a partial phase diagram. The 25 mol% NaFSI/75 mol% [HMG][FSI] composition presents as the eutectic composition with the eutectic transition temperature at 44 °C. Both DSC and SXRD strongly support the formation of a new compound near 50 mol% NaFSI. Interestingly, the 53 mol% NaFSI [HMG][FSI] composition was consistently found to display features of a pure compound whereas the 50 mol% materials always showed a second phase. Many of the compositions examined showed unusual metastable behaviour. Moreover, the ion dynamics as determined by NMR, indicate that the Na + and FSI − anions are signifcantly more mobile than the HMG cation in the liquid state (including the metastable state) for these materials.