Ionic Conductivities of Block Copolymer Electrolytes with Various Conducting Pathways: Sample Preparation and Processing Considerations

Abstract

We explored the relationship between ionic conductivities and morphology in a lithium perchlorate-doped poly(styrene-b-ethylene oxide) (PS–PEO) system using ac impedance spectroscopy, in situ small-angle X-ray scattering, and transmission electron microscopy. To aid in morphological analysis, a flow alignment technique was used to orient nanoscale domains in order to facilitate characterization of nanostructures such as hexagonally perforated lamellae (HPL), hexagonally packed cylinders (HEX), and lamellae (LAM). Over the PEO volume fraction (fPEO) from 0.70 to 0.75, the neat PS–PEO exhibits a morphological transition from HPL to HEX, while the salt-doped PS–PEO shows morphological transitions from LAM to HPL to HEX. Additionally, experiments on hot-pressed specimens show that 3-D conducting pathways (HPL and HEX) exhibit much higher normalized conductivities than the corresponding 2-D conducting pathway (LAM) even after accounting for nonrandom domain orientations and slight PEO molecular weight changes across samples. Our results further suggest that using block copolymer electrolytes with 3-D conducting pathways can prevent a decrease in through-plane conductivities caused by the partial nanostructure alignment during sample preparation.