In the quest for solid-state batteries, salt-containing block copolymers (BCPs), such as polystyrene-b-poly(ethylene oxide) (PS-PEO) and lithium salt, have been widely studied. Despite efforts, there remains no model that can quantitatively predict properties like degree of crystallinity or ionic conductivity in BCP electrolytes. In this work, we revisit effective medium theory, which has been proposed to capture the complex relationship between volume fraction and ion transport properties. This work studies 3 unique morphologies: spheres, cylinders, and lamella, with either PS or PEO majority, for a total of 6 different cases. Although effective medium theory reproduces qualitative tends of ionic conductivity, quantitative prediction is quite poor. Therefore, we posed the hypothesis that geometric features alone are insufficient, but that PS proximity modifies the dynamics in the ionically conductive PEO microdomains. In order to evaluate this hypothesis, a confinement length was introduced and defined as the shortest distance between two non-conductive PS microdomains. Larger confinement lengths thus correspond to less confined conductive domains at the bottleneck in transport/minimum spacing for crystallization. It was found that increasing confinement length allows the PEO phase to act more similarly to neat PEO. This is most clearly evidenced by the exponential dependence of degree of crystallinity of the neat BCPs on confinement length, with conductivity exhibiting a similar relationship. As confinement length is a function of domain size, molecular weight and processing methods that impact domain size are just as important to consider as the volume fraction of each block.
