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 also analyze 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 of these features can be rationalized by assuming that conduction is due to the self-diffusion of a minority of ionized dinitrile molecules. Doping with lithium salts slows down the collective molecular dynamics while leaving the intramolecular relaxation motion unaffected. All samples exhibit a very broad melting transition and exist in a mixed liquid plus plastic state near room temperature. Some mixtures undergo phase segregation below 233 K, the transition temperature between the plastic and the fully ordered solid phase in pure succinonitrile, resulting in the appearance of a space-charge relaxation loss. Phase separation therefore plays an important role in pristine and lithium-doped succinonitrile mixtures.