Computational studies of defect mediated ion transport mechanisms in organic ionic plastic crystals

Organic ionic plastic crystals (OIPCs) have drawn attention as one of excellent solid electrolytes due to its high ionic conductivities and plastic-like mechanical properties. Due to its crystalline nature, defects mediated ion transport mechanisms were suggested. Experiments showed the relationship between ion conductivities and the defect volumes. Also, grain boundary diffusion mechanisms were suggested from NMR data. However, the length and lifetime scales of defects were so short that it was challenging to investigate the transport mechanism at a molecular level via experimental techniques. In this study, we conduct atomistic molecular dynamics simulations to understand the effect of defects on the dynamics of ions in OIPCs. Two types of defects are considered: 1) point vacancies and 2) grain boundaries (GBs). With point vacancies, the mobilities of all species are facilitated. In addition, matrix ions show enhanced dynamic heterogeneities in the presence 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. In the future study, correlated motions between Li⁺ and anions will be investigated.