Dynamic chloride frameworks enable dualistic Li-ion transport in Li2ZrCl6: Single-ion hopping and cooperative diffusion

Li 2 ZrCl 6 (LZC) is a promising chloride fast-ion conductor, yet the microscopic origin of its high Li + conductivity remains unclear. Here we combine density functional theory, machine-learning-potential molecular dynamics (MLP-MD) and CI-NEB calculations to probe Li-ion transport in crystalline LZC. We find a critical coupling between Li migration and low-frequency anharmonic Zr–Cl framework vibrations: dynamic chloride fluctuations intermittently open Cl-defined migration bottlenecks. Li transport shows a scale-dependent cooperative–single-ion duality. At short times, motion proceeds mainly through intermittent single-ion hopping rather than concerted multi-ion jumps; over the nanosecond statistical window of our simulations, discrete hops become correlated through Li–Li interactions and framework fluctuations, yielding statistically correlated diffusion. Static CI-NEB barriers (0.15–0.40 eV) show that low-barrier single-ion paths are accessible, whereas representative two-ion concerted paths are less favourable. Modest sensitivity to non-stoichiometry indicates that fast diffusion does not require one predefined Li–Li configuration. These results establish a dualistic transport paradigm in LZC: chloride-framework gating turns local single-ion hops into long-time correlated diffusion, highlighting soft anion-framework dynamics as a useful handle for designing halide solid electrolytes.