Self-Diffusion, Phase Behavior, and Li-Ion Conduction in Succinonitrile-Based Plastic Cocrystals

We study the temperature-dependent molecular dynamics, ion conduction and phase behavior of plastic-crystal electrolytes based on the succinonitrile molecule. We employ calorimetry and dielectric spectroscopy to probe binary mixtures of succinonitrile with glutaronitrile or acetonitrile, and analyze also the effect of dissolving lithium salts in these systems. The glutaronitrile-succinonitrile mixture has the highest conductivity and it is the only plastic-crystal system that displays a perfect correlation between the ion drift and the on-site reorientational dynamics. Doping with lithium ions boosts the conductivity but breaks such perfect correlation. All these features can be rationalized assuming that conduction is due to self-diffusion of a minority of ionized dinitrile molecules. Doping with lithium salts slows down the collective