Beyond the Paddle-Wheel Mechanism: Hop Function Analysis of Ion Transport in Organic Ionic Plastic Crystals

The paddle-wheel mechanism has long been invoked to explain ion transport in organic ionic plastic crystals (OIPCs), wherein rotational motion of matrix ions is assumed to facilitate ion hopping. Here, we critically examine the paddle-wheel mechanism using molecular dynamics (MD) simulations combined with hop function analysis for a representative OIPC, Li-doped 1,3-dimethylimidazolium hexafluorophosphate ([MMIM][PF₆]). While matrix ions ([MMIM]⁺ and PF₆^-) exhibit translation-rotation coupling consistent with the paddle-wheel mechanism, Li⁺ ion transport, central to the ion conductivity of solid-state electrolytes (SSEs), is decoupled from the rotational dynamics of neighbor ions. Instead, the hop function analysis reveals that the collective rearrangement of the third, fourth, and fifth nearest PF₆^- anions around Li⁺ forms the transition-state configuration governing Li⁺ hopping. This process constitutes the primary Li⁺ ion transport mechanism, replacing the conventional paddle-wheel mechanism. Our results establish the hop function analysis as a robust framework for disentangling ion transport mechanisms in complex solid-state electrolytes and call for a reassessment of long-standing mechanistic assumptions in OIPCs.