Ion vacancies and transport in 1-methylimidazolium triflate organic ionic plastic crystal

Organic ionic plastic crystals (OIPCs) are an important family of materials that have shown exciting possibilities as solid electrolytes for lithium ion batteries and other electrochemical devices. In this study we demonstrate for the first time that, although the X-ray shows sharp diffraction peaks, both cation and anion clearly exhibit significant ion diffusion in solid phase I. Two phases with ion diffusivities differing by 2 orders of magnitude can be identified. The populations of the cation and anion in both phases are found to be unequal, hinting at the existence of (negatively charged) cation vacancies in the plastic crystal phase and a positively charged grain boundary phase. These interesting properties of ion vacancies and unequal populations of cation and anion are likely to be ubiquitous in other OIPCs, and it is of paramount importance to be aware of these features to correctly understand the structure-property relationships of this important material family.