High-contrast electron microscopy imaging of polymer materials for Li-batteries

Abstract text (incl. figure legends and references)
Introduction

Ion-conducting polymers have attracted significant interest as electrolyte membrane materials for solid-state lithium batteries because of their excellent safety, mechanical stability and flexibility. The most commonly used ion-conducting polymer is polyethylene oxide (PEO) because the excellent solubility of lithium salts leads to high ionic conductivity. Block copolymers containing PEO covalently linked to hydrophobic polymer blocks such as polystyrene, polyisoprene or polybutadiene allow designing ordered nanoscale membranes with high ionic conductivity and mechanical stability via self-assembly. Self-assembly can be controlled via block lengths to tailor nanodomain phase morphology and macroscopic orientation. Electron microscopy allows visualizing the order and orientation of the conductive block copolymer domains to optimize the conductivity of the block copolymer membranes.

Objectives

To correlate measured ionic conductivities of block copolymer membranes to block copolymer morphologies requires an in-depth structural characterization over length scales from nanometers to hundreds of micrometers, which can be provided by electron microscopy combining SEM and TEM images. Using cryo-ultramicrotomy we aim to show that LiTFSi-filled PEO-block copolymer membranes can be dry sectioned under humidity exclusion.

Materials & methods

As block copolymers we investigated poly(butadiene-b-ethylene oxide) (PB-PEO) with/without added LiTFSi salt. Samples were solvent-cast into films. These were subsequently dry sectioned by cryo-ultramicrotomy and investigated by a JEOL F200 cryo TEM and Thermo Fisher Apreo Volumescope.

Results

We obtained thin sections of LiTFSi-loaded PB-PEO using cryo-ultramicrotomy with thickness of about 80 nm. The thin sections can be imaged at high voltages of 200 kV with STEM without staining and at 30kV with STEM in SEM. Thin sections where prepared under cryo conditions and transferred to the TEM using a cryo holder. After investigation of the sections in TEM, they were transferred to the SEM without humidity control. We find that there is no structural change during air contact.

Images of loaded and unloaded samples can be compared to identify salt induced changes of the morphology.

The block copolymer domains in the ribbons can be imaged by TEM by high- and by SEM by low-voltage STEM without staining. Electron microscopy provides crucial information about the domain morphology, ordering and orientation within the membrane.

Conclusion

We demonstrate that ultramicrotomed thin sections of block copolymers can be imaged with high contrast by STEM at voltages of 200 kV in TEM and 30 kV in SEM without staining. This allows to visualize the LiTFSi-conduction paths across the block copolymer membranes, which is of high importance to improve the conductivity of solid polymer electrolytes in Li-battery applications.

Bright field (figure 1) and dark field (figure 2) image of LiTFSi loaded PB-PEO block-co-polymer