Effects of alkali ion dopants on the diffusion mechanisms and thermal stabilities of imidazolium-based organic ionic plastic crystal*

Organic ionic plastic crystals (OIPCs), in which ions show rotational disorder with its crystalline structure maintained, are one of excellent candidates for the solid-state electrolytes of rechargeable batteries. Doping alkali ions in the OIPCs matters because one may affect both the thermal stability and the ionic conductivity, both of which are desired properties for the solid-state electrolytes. In this work, we perform molecular dynamics simulations to investigate the effects of alkali ion-doping on the melting temperatures and the translational diffusion of ions in OIPCs. In our simulations, we consider lithium ion (Li⁺), sodium ion (Na⁺), and potassium ion (K⁺) as doping agents on 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF₆]) OIPCs. We find from our results that as alkali ions of smaller size are doped, the larger domain of the crystal becomes disordered. Because larger distortion disables the crystal to maintain its lattice structure, Li⁺ doped crystal shows the lowest melting temperature. Also, the translational diffusion mechanism of alkali ions depends on the kind of alkali ion dopants. Li⁺ undergoes continuous diffusion through amorphous regime, whereas K⁺ undergoes hopping diffusion. Na⁺ shows an intermediate diffusion feature between Li⁺ and K⁺.