Defect-Assisted High Anion Conductivity in Diethyldimethylammonium d-Camphorsulfonate Plastic Crystal: A Size Effect of Target Ions

We demonstrate anion (chloride and fluoride ions) transfer in an organic ionic plastic crystal (OIPC), diethyldimethylammonium d-camphorsulfonate, and the role of defects in ion conduction. The phase transitions, crystalline structures, dynamics, and ion transfer mechanisms of the pure material and the doped mixtures were investigated using a combination of differential scanning calorimetry, X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy. The doped mixtures show minor modifications in thermal behaviors and solid phase structures to the host material. The ion mobility of the pure material in the plastic phase was assigned mainly to cations. The fluoride salt-doped mixture has drastically enhanced conductivity at all tested temperatures, and the chloride salt-doped mixture displays a strong temperature-dependent behavior. Ionic conductivity measurements suggest transfer mechanisms through crystalline phases. The pure material and doped mixtures exhibit similar defect volumes and concentrations, and both factors are phase-dependent, as determined using positron annihilation lifetime spectroscopy. The conductivity displays dependence on not only the defect volume but also the target ion volume. The relationships between the defect volume and the conductivity qualitatively follow the Cohen–Turnbull free volume model, while critical volumes were very large. For the first time in OIPCs, the size of the target ions was found to significantly influence ionic conductivity.