Organic ionic plastic crystal (OIPC) is an emerging solid electrolyte with high ionic conductivity and plastic-like mechanical properties. They are composed of organic ions, which can show additional rotational motions even in crystal phases. Many studies suggested that these local orientational motions may enhance the dynamics of ions. On the other hands, defects mediated ion transport mechanisms were also suggested due to its crystalline nature. Experimental studies revealed the relationship between ion conductivity and the defect volumes. Thus, we conduct atomistic molecular dynamics simulations to understand the effect of defects on the dynamics of ions in 1,3-dimethylimidazolium hexafluorophosphate ([MMIM][PF<sub>6</sub>]). In this work, we employ two types of defects: 1) point vacancies and 2) grain boundaries (GBs). We generate point vacancies by removing pairs of ions from supercells. To construct GBs, one layer of ions normal to the x, y, and z axis at supercells are removed and equilibrated. We calculate the mean squared displacement of ions in the presence of various types of defects. We also estimate the non-Gaussian parameters to quantify the heterogenous dynamics of ions. With point vacancies, the mobilities of all species of ions are facilitated. In addition, matrix ions show enhanced dynamic heterogeneity in the presence of point vacancies. In the case of GBs, disordered structures at boundaries are observed. Due to this, matrix ions at GBs show heterogenous dynamics. However, only Li<sup>&lt;font size="1"&gt;+&lt;/font&gt;</sup> ions at [100] GBs show faster dynamics compared to systems without any point vacancies.

Computational studies of defect mediated ion transport mechanisms in 1,3-dimethylimidazolium hexafluorophosphate

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