Influence of\nWater Sorption on Ionic Conductivity\nin Polyether Electrolytes at Low Hydration

Ion-containing polymers are subject to a wide range of\nhydration\nconditions across electrochemical and water treatment applications.\nSignificant work on dry polymer electrolytes for batteries and highly\nswollen membranes for water purification has informed our understanding\nof ion transport under extreme conditions. However, knowledge of intermediate\nconditions (i.e., low hydration) is essential to emerging applications\n(e.g., electrolyzers, fuel cells, and lithium extraction). Ion transport\nunder low levels of hydration is distinct from the extreme conditions\ntypically investigated, and the relevant physics cannot be extrapolated\nfrom existing knowledge, stifling materials design. In this study,\nwe conducted ion transport measurements in LiTFSI-doped polyethers\nthat were systematically hydrated from dry conditions. A semiautomated\napparatus that performs parallel measurements of water uptake and\nionic conductivity in thin-film polymers under controlled humidity\nwas developed. For the materials and swelling range considered in\nthis study (i.e., <0.07 g water/g dry polymer electrolyte), ionic\nconductivity depends nonlinearly on water uptake, with the initial\nsorbed water weakly affecting conductivity. With additional increases\nin swelling, more significant increases in conductivity were observed.\nRemarkably, changes in conductivity induced by water sorption were\ncorrelated with the number of water molecules per lithium ion, with\nthe normalized molar conductivity of different samples effectively\ncollapsing onto one another until this unit of hydration exceeded\nthe solvation number of lithium ions under aqueous conditions. These\nresults provide important knowledge regarding the effects of trace\nwater contamination on conductivity measurements in polymer electrolytes\nand demonstrate that the lithium-ion solvation number marks a key\ntransition point regarding the influence of water on ion transport\nin ion-containing polymers.