Building Less Tortuous Ion-Conduction Pathways Using Block Copolymer Electrolytes with a Well-Defined Cubic Symmetry

Abstract

We investigated the effects of morphology on the ion transport properties of polymer electrolytes. A single sulfonated block copolymer displayed a series of ordered morphologies with three-dimensional symmetry, i.e., cubic, Fddd (O-70), and Fm (3) over barm (fcc) phases, upon the addition of ionic liquids. The unique phase behavior was understood on the basis of the selective swelling of the sulfonated blocks by ionic liquids and the changes in segregation strength with the modulation of the ionic interactions in the ionic phases. The type of three-dimensional lattice was revealed to play an important role in determining the ion transport properties of ionic liquid-containing sulfonated block copolymers. For example, the sample with disordered spherical lattices exhibited the highest tortuosity (similar to 2) for ion conduction, indicative of the considerable diffusion barriers in the conducting phases. On the contrary, the samples with well-defined orthorhombic and face-centered cubic symmetries, i.e., O-70 and fcc phases, revealed the reductions in tortuosity to 1.52 and 1.17, respectively, attributed to the large grain sizes and large cross-sectional areas of the conducting pathways at grain boundaries. The least tortuous ion conduction seen for fcc phases was particularly remarkable. This unprecedented understanding of the advantage of three-dimensional morphologies with cubic symmetry will open a new avenue toward designing future polymer electrolytes with improved ion transport properties for their uses in diverse applications such as high-temperature fuel cells, batteries, and electro-active actuators.