(680e) Ion Transport Properties of Ultra-Thin Film Polymer Electrolytes

With the ability to tune charge transport and mechanical properties, polymer electrolytes have demonstrated promise as solid electrolyte materials for lithium-metal anode batteries. However, majority of reported studies have focused only on bulk thick samples (100’s of microns films). This is significant to note as interfaces play a critical role in the performance of batteries, but researchers have mostly inferred interfacial effects using bulk properties. As a consequence, we focus on investigating polymer electrolytes in the ultrathin film regime (5-300 nm), which can be used as a platform to directly study fundamental interfacial effects. In addition, structural characteristics limiting charge transport in polymer electrolytes such as domain orientation and interconnectivity can be more easily probed using ultrathin films. Ultimately, investigating polymer electrolyte in the context of thin films will lead to better fundamental understanding of charge transport in polymer electrolytes and the charge transfer limitations at the electrode/electrolyte interfaces.

In this work, we report the fabrication, structure and ion transport characteristics of ultra-thin films through a model system of poly(ethylene oxide) PEO and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) blends as a function of salt concentration, temperature and film thickness. PEO-LiTFSI ultrathin films are fabricated on top of a grafted PEO brush layer to circumvent thin film dewetting effect on substrates. Ion transport measurements were successfully performed using electrochemical impedance spectroscopy on PEO-LiTFSI ultra-thin films fabricated on top of custom-designed nanofabricated interdigitated electrode (IDE) devices. The thin film conductivity is found to increase from r = 0.01 to 0.05 but reduces for r equal or larger than 0.05, consistent with the well-established Vogel-Tammann-Fulcher (VTF) model for ion transport. Importantly, thickness dependence study of ion transport shows a monotonic decrease in PEO-LiTFSI thin film conductivity fabricated on the grafted PEO brush upon decreasing film thickness from 250 nm to ca. 10 nm, and the effect is stronger at low salt concentrations. The decrease of ionic conductivity at thinner films originate from the increasing fraction of the immobilized layer near the polymer/substrate interface. Our results suggest that using thin film configuration is a promising strategy to probe morphological and interfacial effects on ion conducting mechanism of polymer electrolytes.