The effects of polarization on the rotational diffusion of ions in Organic Ionic Plastic Crystals

The organic ionic plastic crystals (OIPCs) have been regarded as a great solid-state electrolyte due to their high stability and conductivity. Conventional solid electrolytes have high viscosity such that it is hard to achieve high electrical conductivity. However, ions in OIPCs maintain a long-range ordered crystalline lattice structure but may rotate simultaneously. The rotational motions facilitate the diffusion of dopant ions, which also enhances the conductivity. Therefore, it is important to investigate the rotational diffusion in OIPCs at a molecular level, where molecular simulations can be a great tool. But polarization effects have not been taken into account because of high computational costs and lacks of proper models. In this work, we investigate how the polarization affects the rotational diffusion of OIPCs. We perform atomistic molecular dynamics simulations of 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF₆]) at temperatures from 75 to 500K under NPT conditions with and without electronic polarization. Polarization increases the density of [MMIM][PF₆] slightly. However, we find polarization induces a faster rotational relaxation, which will certainly enhance the diffusion and the conductivity of dopants in OIPCs.