Role of hydration on ion transport in the transition from dry to wet salt-doped PEO*

Ion transport properties are widely studied in polymer electrolytes under rigorously dry conditions and in highly swollen hydrogel networks. While our knowledge of ion transport in the extreme cases of dry and highly swollen polymer membranes is relatively well developed, the transition between these extremes, i.e., the low hydration regime, is poorly understood. In this study, we apply atomistic molecular dynamics simulations to probe the role of hydration on salt transport in the transition from dry to poorly hydrated to water-percolated salt-doped PEO. Our results suggest that even small quantities of water decouple ion diffusion from polymer segmental dynamics compared to rigorously dry systems with increasing temperature. We attribute this to the rapid solvation of ions by water over PEO decreasing the impact of segmental motions on ion diffusion. Despite this solvation of lithium by water, we observe significantly faster speedup of water relative to lithium with increasing hydration under the low-hydration regime. We attribute this to competing solvation dynamics between lithium, water, and PEO. In water-percolated networks, we observe good agreement between simulated ion diffusion coefficients and the Mackie-Meares model.