Nanothin film conductivity measurements reveal interfacial influence on ion transport in polymer electrolytes

The interfacial region where ion-transporting polymer chains are anchored to a hard, insulating phase is a major factor dictating the limits of ion-conduction in nanostructure-forming electrolytes. In this work, we investigate the effect of an end-grafted poly(ethylene oxide) (20 kg mol^-1) surface on the ionic conductivity σ of PEO and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt mixtures. Specifically, we characterize nanothin films in the range of ca. 10 to 250 nm, which amplify the contributions from the polymer/substrate interface that dictate any deviations from expected bulk conductivity σ_bulk values. Conductivity measurements reveal a monotonic decrease in σ upon decreasing film thickness at all values of r (r = molar ratio of Li⁺ to EO units). The reduction from bulk-like σ occurs for film thicknesses approximately 100 nm and below for all values of r. This trend in conductivity arises from the presence of the underlying grafted-PEO layer. Through a thickness dependence normalized conductivity study, we observe nanoscale constraints leading to deviation from intrinsic conductivity of bulk PEO-LiTFSI electrolytes. These nanoscale constraints correspond to an immobile interfacial zone whose thickness h_int ranges from 9.5 ± 1.4 nm at r = 0.01 to 2.9 ± 1.5 nm at r = 0.15 in our nanothin films that impedes ion transport. Overall, we have presented a robust platform that facilitates probing the role of polymer-grafted surfaces on the σ of polymer electrolytes.