Thin Film Ion Transport and Morphology of Poly(ethylene oxide) and Lithium Salt Mixtures
Authors: Dong Ban , Kambe Yu , Dolejsi Moshe , Nealey Paul F. , Patel Shrayesh N. 
Published: February 2020

With the ability to tune ion transport and mechanical properties, polymer electrolytes have demonstrated promise as solid electrolyte materials for lithium-metal anode batteries. In particular, polyethylene oxide (PEO) and lithium salt mixtures have been the most widely studied polymer electrolyte system.1 Moreover, block copolymer electrolytes (e.g. block polystyrene-block-PEO) have yielded more advanced nanostructured architectures with increased mechanical robustness and efficient ion transport.2 Driven by their potential applications, enormous efforts have been devoted to elucidating mechanisms for ion transport and the formation of lithium dendrites. However, majority of reported studies have focused only on thick bulk samples (100’s of microns thick films). This is significant to note as interfaces3 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 (ultra)-thin film regime (5-500 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 thin films.4 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.

Here, we report on our initial study on charge transport and morphology of a model polymer electrolyte system (mixtures of PEO and LiTFSI, lithium bis(trifluoromethanesulfonyl)imide)) in the thin film regime (<200 nm). We specifically probe the structure-property relationships as function of salt concentration, temperature, and film thickness. We utilized synchrotron-based grazing-incidence wide angle X-ray scattering (GIWAXS) to probe the effects of salt concentration and film thickness on the semicrystalline morphology. At room temperature, the PEO crystallites become more ordered, but the relative degree of crystallinity (rDoC) of PEO gradually decreases with increasing salt concentration. The film becomes completely amorphous at the highest salt concentration r = 0.15, where r is the molar ratio of lithium ions to ethylene oxide repeat units. This trend is consistent with the established theories that adding Li salt disrupts the crystallite structure of PEO and reduces the overall degree of crystallinity. Ionic transport measurements were performed using a.c. impedance spectroscopy on PEO-LiTFSI thin films on custom-designed interdigitated electrode devices (IDE). Above the melting point of PEO (≈ º60 C), the temperature dependence of ionic conductivity of all samples can be well-described using Vogel-Tammann-Fulcher (VTF) model. This suggests that segmental motion of PEO chains facilitate ionic transport in PEO-LiTFSI thin films. Ionic conductivity is found to increase with increasing salt concentration at first but decreases at high salt concentration. This is likely due to the reduced dissociation rate and increased glass transition temperature at high concentration, similar to the behavior of bulk samples.1,2 Our results have demonstrated the first successful fabrication and characterization of ionic transport in PEO-Li salt thin films. This provides an important step toward exploiting thin film configuration to reveal many morphological and interfacial effects on ion conduction mechanism of polymer electrolytes and will be subject of investigations in our future studies.

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