What is the Role of Relative Humidity on Conductivity in Polymer Electrolytes?

Ion transport properties are widely studied in polymer electrolytes under rigorously dry and highly swollen conditions. 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 ion conduction in LiTFSI-doped polyethers under 0-80% relative humidity conditions (0-15% water by volume). Our results suggest that Li⁺ conduction within this limit can be divided into slow and fast regimes. Under dry conditions, Li⁺ conduction is dictated by the relative motions of polymer ether oxygen solvation cages. Under humid conditions, Li⁺ ions are preferentially solvated by water molecules. At low water contents, the average Li⁺ is weakly hydrated (two or less water molecules), maintaining strong interactions with its solvation cage and therefore exhibiting a negligible increase in conduction relative to dry conditions. At higher water contents, Li⁺ ions form complete hydration shells, residing within water-rich domains that promote fast conduction, significantly increasing the overall conductivity. By contrast, TFSI^- has comparatively weak interactions with its environment, exhibiting only a marginal increase in conduction at all hydrations probed due to the increase in local dynamics.