Hopping-Dominated Li+ Transport in Deeply Supercooled Polymer-in-Salt Electrolytes

Deeply supercooled solvent-free lithium electrolytes can exhibit Li-ion transference numbers approaching unity under anion-blocking conditions, yet the microscopic origin of this behavior remains unclear. Here, we use long, statistically replicated molecular dynamics simulations to investigate Li-ion transport in deeply supercooled lithium (fluorosulfonyl)(trifluoromethanesulfonyl)amide, Li[FTA], and its poly(methyl methacrylate)-based polymer-in-salt electrolytes. Li+ ions exhibit translational mobility nearly an order of magnitude higher than that of FTA- anions and display pronounced non-Gaussian displacement statistics indicative of intermittent hopping. Spatially resolved analyses reveal heterogeneous dynamics: Li+ motion is suppressed near the polymer interface but enhanced in polymer-dilute regions, where Li-anion coordination is weakened and hopping events become more frequent. These results suggest that hopping-mediated, spatially heterogeneous Li+ motion is a key microscopic origin of near-unity transference numbers in deeply supercooled polymer-in-salt electrolytes and provide design principles for high-transference-number solvent-free systems.