Effects of alkali ion dopants on the transport mechanisms and the thermal stabilities of imidazolium-based organic ionic plastic crystals

Abstract

Various dopant alkali ions have been introduced into organic ionic plastic crystals (OIPCs) in order to design solid electrolytes with the desired thermal stability and ionic conductivity. We performed extensive molecular dynamics simulations to investigate at the molecular level how dopant alkali ions affect the rotational and the translational diffusion of ions and the thermal stability of OIPCs. We introduced lithium (Li⁺), sodium (Na⁺), and potassium (K⁺) ions as dopants into 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF₆]) OIPCs at the molecular level. We found that as smaller alkali ions are doped, larger domains of the crystals are disrupted. This makes it harder for OIPCs doped with smaller alkali ions to maintain their crystal structure such that the melting temperature of OIPCs decreases and phase transitions between rotator phases change. The size of dopant alkali ions also affects the rotational diffusion of matrix ions of [MMIM]⁺ and PF₆⁻: the rotational diffusion of matrix ions near Li⁺ ions becomes more heterogeneous and facilitated than those near other kinds of alkali ions. We also find that alkali ions of different kinds diffuse translationally in OIPCs via different transport mechanisms: while the Li⁺ ion undergoes continuous (anion-associated) diffusion through an amorphous region, the K⁺ ion hops between neighbor lattice sites. To investigate the effects of the relative size between matrix cations and dopant ions on translational diffusions, we also simulate OIPCs with longer alkyl chains such as 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF₆]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) crystals. We find that as the size of imidazolium cations increases, the hopping diffusion of the K⁺ ion becomes suppressed and the K⁺ ion is more likely to diffuse through amorphous domains.