Two-State Bursty Dynamics of Li+-Anion Solvation Shells in Ionic Liquid Electrolytes

Understanding lithium-ion transport in ionic liquids (ILs) requires connecting local coordination dynamics to ion mobility. Using polarizable-force-field molecular dynamics simulations of Li+/Pyr14+ systems with three anions (FSI-, TFSI-, BETI-) across 298-423 K, we show that the structural motion of Li+-anion solvation shells is non-Poissonian and bursty rather than simply random. We extend a two-state bursty dynamics framework to this anion series, partitioning the trajectory into a transient, exchange-rich soft state and a more persistent hard state. Likelihood-based model comparisons, supported by a burstiness analysis, show that the state-duration distributions are comparatively short-tailed and near-exponential, whereas the within-state pair-survival functions remain nonsingle-exponential and heavy-tailed over finite windows. The temperature-dependent state-transition rates indicate larger kinetic barriers for hard-state transitions and for bulkier anions, and the soft state─characterized by lower coordination numbers and more frequent anion exchange─shows a larger effective single-ion mobility. Threshold-sensitivity and sampling-uncertainty analyses support the qualitative two-state picture while clarifying its limits. These findings identify the soft-state fraction as a candidate local kinetic descriptor that links solvation-shell dynamics to single-ion Li+ mobility─a connection that future conductivity, transference, and viscosity measurements can test directly.