Relation of Ionic Conductivity to Solvent Rotation Times in Dinitrile Plastic Crystal Solvents

In a much-cited paper, Armand and coworkers showed that lithium bistrifluoromethyl sulfonylimide (LiTFSI) and other salts dissolve in the well-known plastic crystal, succinonitrile, (CN-CH₂-CH₂-CN) to give a highly conducting solid solution which they proposed as a novel approach to lithium electrolyte solid state battery technology. Although succinonitrile has been much studied as a molecular rotator phase, there was no direct reference made to its reorientational motion in their paper. In a previous paper we have shown that the time scale for magnetic fluctuations and reorientational relaxation times in dinitrile solvent mixtures (which can be studied over wide temperature and relaxation time ranges) are in close correspondence. Here we use this finding to study the relation between conductivity relaxation times of the LiTFSI solutions and the reorientation times of the solvent molecules. We find that, while the solvent molecule reorientation times accord well with the conductivity relaxation time, the lithium ion is in an environment that fluctuates more slowly. Unfortunately, this will lead to concentration polarization problems in any electrochemical device application. Ways to avoid this problem while maintaining the plastic crystal advantage are suggested.