Tailoring Ionic Conductivity of Polymeric Ionic Liquid Block Copolymers through Morphology Control

Block copolymers (BCPs) composed of polymeric ionic liquids (PIL) and neutral blocks are promising materials for electrochemical energy storage and conversion. They offer the potential to combine high ionic conductivity with mechanical robustness. However, recent studies have shown that the ionic conductivity of lamellar phases is significantly lower than expected based on the conductivity of the PIL homopolymer. To investigate the factors contributing to this reduced ionic conductivity, researchers varied both the morphology and segregation strength in a model system. The primary reason for the decreased ionic conductivity in lamellar phases, particularly at intermediate to strong segregation, is the presence of transport-blocking defects at grain boundaries. Additionally, the elevated glass transition temperature (Tg) of the PIL domains, in comparison to the PIL homopolymer, is influenced by the high Tg of the nonionic domains. Diffuse interfaces between the PIL and nonionic blocks do not have a measurable impact on ion transport. Ionic conductivity can be improved by adjusting the molecular weight to create a structure with only short-range order. This can be achieved either by decreasing the molecular weight to promote weak segregation or by increasing it to eliminate ordering into well-defined grains. The latter morphology results in ionic conductivity that approaches that of a PIL homopolymer.