Network Structure and Strong Microphase Separation for High Ion Conductivity in Polymerized Ionic Liquid Block Copolymers

A series of strongly microphase-separated polymerized ionic liquid (PIL) diblock copolymers, poly(styrene-b-1-((2-acryloyloxy)ethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (poly(S-b-AEBIm-TFSI)), were synthesized to explore relationships between morphology and ionic conductivity. Using small-angle X-ray scattering and transmission electron microscopy, a variety of self-assembled nanostructures including hexagonally packed cylinders, lamellae, and coexisting lamellae and network morphologies were observed by varying PIL composition (6.6–23.6 PIL mol %). At comparable PIL composition, this acrylate-based PIL block copolymer with strong microphase separation exhibited ∼1.5–2 orders of magnitude higher ionic conductivity than a methacrylate-based PIL block copolymer with weak microphase separation. Remarkably, we achieved high ionic conductivity (0.88 mS cm–1 at 150 °C) and a morphology factor (normalized ionic conductivity, f) of ∼1 through the morphological transition from lamellar to a coexistence of lamellar and three-dimensional network morphologies with increasing PIL composition in anhydrous single-ion conducting PIL block copolymers, which highlights a good agreement with the model predictions. In addition to strong microphase separation and the connectivity of conducting microdomains, the orientation of conducting microdomains and the compatibility between polymer backbone and IL moiety of PIL also significantly affect the ionic conductivity. This study provides avenues to controlling the extent of microphase separation, morphology, and ion transport properties in PIL block copolymers for energy conversion and storage applications.