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Organic ionic plastic crystals (OIPCs), which consist of organic molecular ions, are considered excellent candidates for solid electrolytes due to their high ionic conductivity in solid phases. Molecular ions undergo either rotational or conformational relaxation at certain temperatures in OIPCs. There have been molecular simulations to understand the rotational motion. The polarizability of ions was, however, often ignored in simulations due to the high computational cost. Since the polarizability may affect the translational diffusion, the ionic conductivity, and the phase transition of ionic liquids, it should be of interest to investigate how the polarizability would affect the rotational diffusion of ions in solid phases. In this work, we perform extensive atomistic molecular dynamics simulations for two different kinds of OIPCs, 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF6]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). We employ various simulation models for ions by turning on and off the polarization in their interaction potentials. We find that the polarizability hardly affects the density, the crystalline structure, and the phase transition of both OIPCs. However, a certain rotational motion, especially the rotational diffusion of PF 6 − in [MMIM][PF6] OIPCs, is enhanced by a factor of up to four when the polarizability is turned on. The PF 6 − in [MMIM][PF6] OIPCs undergoes rotational hopping motions more significantly due to polarizability. We find that the rotational diffusion of a certain ion can be enhanced only when the polarization results in a significant change in the dipole moment of the neighboring ions around the ion. [ABSTRACT FROM AUTHOR]

Organic ionic plastic crystals (OIPCs) are one of promising solid‐state electrolytes (SSEs) for all‐solid‐state batteries. OIPCs consist of organic ions that form crystalline solid phases. It is a challenging but critical task in the development of SSEs to improve the ion conductivity of lithium ions in such crystalline solid phases. We discuss our recent simulation studies on the transport mechanism of ions in OIPCs and show that the dynamic heterogeneity should be a key to the transport mechanism. We aim to show that both translational and rotational dynamic heterogeneities should play an important role in understanding the ionic conductivity in SSEs. We found from our simulations that the introduction of dopants and defects into OIPCs facilitated both the dynamic heterogeneity and the ion conductivity significantly. We also found that the polarizability of matrix organic ions could contribute to the rotational and conformational relaxations of organic ions in OIPCs. [ABSTRACT FROM AUTHOR]