Heterogeneous Dynamics of Molecular Ions in Grain Boundaries of 1,3-Dimethylimidazolium Hexafluorophosphate Plastic Crystals

Organic ionic plastic crystals (OIPCs) are soft solid electrolytes consisting of charged molecular ions. Although they form crystalline structures, they show high ionic conductivities and plastic-like mechanical properties. To understand their high ionic conductivity even in the crystalline phase, the translational and rotational behaviors of ions have been widely studied. Among them, defects such as point vacancies or grain boundaries are considered to enhance the transport of ions in OIPCs. For example, experimental studies revealed that conductivities and defect concentrations increased together with increasing temperatures. In addition, various experimental and simulation works found that fast and slow diffusing ions coexisted in OIPCs. However, the ion transport mechanism remains elusive at a molecular level partly because it is hard in experiments to systematically control the types of defects. Therefore, we perform molecular dynamics simulations to understand the effect of defects on the dynamics of ions in lithium-doped OIPCs. Two types of defects are considered: 1) point vacancies and 2) grain boundaries (GBs). With point vacancies, the mobilities of all species of ions are facilitated in a heterogeneous fashion. However, the transference number of Li⁺ decreases with increasing concentrations of point vacancies. In the case of GBs, disordered structures at boundaries are observed. We find that Li⁺ diffuses along the boundaries and coordinates with more anions than Li⁺ in bulk crystals without any vacancies. We also find that the time scales of the dynamic heterogeneity of the ions are sensitive to the types of defects.