(Invited) Intrinsic and Interfacial Ion Transport Properties of Nanostructured Polymer Electrolytes for Lithium Metal Batteries

Abstract

Polymer electrolytes have demonstrated promise as solid electrolyte materials to enable lithium-metal batteries. However, majority of studies have focused only on thick samples (100’s of microns). It is important to note interfaces play a critical role on the performance of batteries, thus investigating polymer electrolytes in the context of nanothin films (5-100 nm) will lead to better understanding on how surfaces and interfaces influences charge transport properties. Here, we report on ion transport characteristics of nanothin films of PEO and LiTFSI blends as a function of salt concentration, temperature and film thickness. Ion transport measurements were successfully performed using impedance spectroscopy on films fabricated on custom-designed nanofabricated interdigitated electrode (IDE) devices. Importantly, thickness dependence study of ion transport shows a monotonic decrease in ionic conductivity 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. In addition, these measurements reveal previously inaccessible diffusional processes that are critical to fully characterizing transport properties. We also extend our work towards nanostructured block copolymer nanothin film electrolytes (polystyrene-b-polyethylene oxide with LiTFSI). By carefully tuning the substrate surface energy, we can judiciously control block copolymer self-assembly and orientation to reveal previously inaccessible interfacial contributions that dictate the limits of ion-transport. Overall, our work shows that nanothin films in concert with IDEs is a powerful and versatile platform for studying a wide variety of confinement and surface functionalization effects on ion transport in a controlled and precise manner.