Imaging Crystal Domains and Orientation in Block Copolymer Electrolytes with 4D-STEM

Nanostructured block copolymer electrolytes are capable of simultaneously transporting ions and suppressing lithium dendrites in lithium metal batteries [1]. Poly (styrene-block-ethylene oxide) (PS-b-PEO) is one of most promising candidates for next generation polymer solid electrolytes, due to its two-phase microstructure and good performance [2]. The electrolyte self-assembles into a lamellar morphology, with alternating layers of ion-conducting PEO-rich domains and mechanically reinforcing PS-rich domains [2]. In this block copolymer morphology, PEO-rich domains are semi-crystalline and PS-rich domains are amorphous. The salt is preferentially located in the PEO lamellae, and consequently, understanding the nature of the crystallinity inside the PEO domains is important. Traditionally, electron microscopy studies of PS-b-PEO used heavy element staining (like OsO4 and RuO4) to increase z-contrast in these beam-sensitive materials and protect the morphology from the electron beam damage [3]. However, the information from stained samples is then limited to only the distribution of PS and PEO. Information about local crystallinity and crystal information, especially the spatial distributions of PEO crystal orientations inside the lamellar microphase remains unresolved. To better understand how lithium-ions transport inside the PEO phase, it is important to improve the existing structural model with real experimental data about the crystalline morphology. 4D-STEM (four-dimensional scanning transmission electron microscopy) is a technique by which a nearly parallel electron nanoprobe is rastered across a sample (two dimensions in real space), recording the diffraction pattern (two dimensions in reciprocal space) at each scan point. [4] The diffraction patterns acquired form a 4D dataset that can reconstruct real space information about the phase distribution and orientation of the polymer materials. In this work, we applied 4D-STEM to PS-b-PEO while the sample was maintained near 100K. By carefully selecting the acquisition parameters to maximize the diffracted signal while controlling beam damage, we successfully achieved both resolution in real space and signal-to-noise ratio (SNR) in reciprocal space to enable us to visualize the semi-crystalline regions using the py4DSTEM software package [5]. The quantification of lattice vectors provides information-rich maps that show the orientations, spatial distribution, and crystallinity in PS-b-PEO simultaneously. That structural information is not resolvable by using conventional TEM or STEM imaging of chemical-stained block copolymers. To our knowledge, this is the first time that grain size and crystalline orientation within a semi-crystalline block co-polymer has ever been imaged. The establishment of such analysis method shades the light on the discovery of buried structure-property corrections in semi-crystalline block copolymers [6]. (a) RuO4 stained PS-b-PEO HAADF image at 300kV; the Ru referentially segregate to the PEO layer to make it bright; (b) Virtual dark field image (virtual detector selected using the amorphous ring) from the 4D-STEM acquisition showing the amorphous PS as the bright phase as compared with the PEO (300kV, TEAM I); (c) Flow line map where color represents the different orientations revealing orientation domains inside the PEO-rich region (d) A schematic model for the semi-crystalline structure. (e) Mean diffraction pattern from 4D-STEM dataset (f) Bragg Vector Map (containing all the diffraction spots detected from the 4D-STEM dataset).