Probing Coexisting Ordered Phases with X-ray Scattering Techniques in Block Copolymer Electrolytes*

Block copolymers have been studied for applications in lithium metal solid-state battery electrolytes due to their ability to microphase separate and decouple their ion conducting and mechanical properties. The addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt to poly(styrene)-block-poly(ethylene oxide) (SEO) has been shown to induce microphase separation and lead to the formation of ordered nanostructures. By tuning the composition, molecular weight, and salt concentration of the block copolymer electrolyte, complex morphologies such as coexisting ordered phases can be formed. Of particular interest is understanding the driving forces behind the formation of these phases as well as the salt distribution within the coexisting microphases. X-ray scattering techniques, such as small angle X-ray scattering (SAXS) and resonant soft X-ray scattering (RSoXS), were used to elucidate the nanostructures and salt distribution in these systems. This information can be used to predict block copolymer morphology in dynamic systems, such as those in an operating battery, where the introduction of salt concentration gradients can affect the polymer nanostructure across the cell.