Topological suppression of octahedral LiF networks enables fast Li⁺ transport in LiF-rich interphases

LiF-rich solid electrolyte interphases (SEIs) are widely associated with stable battery cycling, despite the intrinsically poor Li⁺ conductivity of crystalline LiF. Here, we investigate how crystallization and the temporal dynamics of local Li-F coordination states regulate ion transport in LiF-Li₂CO₃ inorganic interphase models. A fine-tuned neuroevolution potential enables 10 ns machine-learning molecular dynamics (MLMD) simulations of systems containing approximately 13,000 atoms. Cooling pure LiF from 1000 to 800 K induces rock-salt-like ordering, a pronounced increase in octahedral coordination, and a three-order-of-magnitude decrease in the long-time Li⁺ diffusivity. Motif-resolved survival and interconversion analyses reveal that this transport suppression is associated not only with the instantaneous octahedral population, but also with the emergence of long-lived octahedral states and an approximately 200-fold reduction in tetrahedral-octahedral exchange. Representative climbing-image nudged elastic band pathways yield migration barriers of 0.75 and 1.65 eV for tetrahedral-like and octahedral coordination environments, respectively, supporting the greater kinetic constraint imposed by octahedral coordination. Incorporation of Li₂CO₃ suppresses LiF crystallization, disrupts persistent octahedral networks, and restores dynamically exchangeable non-octahedral environments. The resulting diffusivity varies non-monotonically with carbonate content, while the highest-diffusivity composition shifts toward a higher carbonate content upon cooling. These results extend static coordination descriptions by identifying coordination-state persistence and exchange kinetics as dynamic atomistic descriptors of ion transport in heterogeneous LiF-rich inorganic interphases.