(373b) Probing the Dynamic Structure of Block Copolymer Electrolytes Under Dc Polarization with in-Situ Small Angle X-Ray Scattering

Microphase separated block copolymer electrolytes have been widely studied to enable rechargeable lithium metal batteries. Operation of a battery requires flowing ionic current through the electrolyte. At current densities relevant for high power applications (e.g. electric vehicles), significant salt concentration gradients develop between the anode and cathode of the cell. In liquid electrolytes, solvent rearrangement happens easily to accommodate the redistribution of the salt. For rigid block copolymer electrolytes, the process by which the polymer and salt rearrange themselves is not straightforward. In this work, we employ in-situ small angle X-ray scattering (SAXS) to examine an electrochemical cell consisting of a polystyrene-block-poly(ethylene oxide) (SEO) electrolyte between two lithium electrodes to study block copolymer rearrangement as salt concentration gradients develop and relax. The X-ray beam is scanned between the electrodes to obtain spatially resolved scattering data, which has not been previously achieved. We have applied this technique to several systems, including a lamellar SEO with total molecular weight of 40 kg mol^-1 and a weakly segregated SEO with molecular weight of 3 kg mol^-1. We observe appreciable gradients in the periodicity of the polymer (domain spacing increases near the salt rich electrode and decreases at the opposite electrode) in both systems, as well as morphological changes in the weakly segregated block copolymer electrolyte. Interestingly, we observe that the extent of swelling/contraction of the PEO domains depends strongly on grain orientation. We will discuss how these data improve, and complicate, our understanding of how block copolymer electrolytes facilitate the flow of ionic current.