Regioregularity and Molecular Weight Effects in Redox-Active Poly(3-hexylthiophene)-block-poly(ethylene oxide) Electrode Binders
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Copyright 2018 American Chemical Society. Simultaneous electron- and ion-conducting polymeric binders for battery electrodes offer a multifunctional alternative to commonly used poly(vinylidene fluoride). One example is poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO), which conducts electrons and ions in the P3HT and PEO domains, respectively. Notably, P3HT stores charge by doping and dedoping, which further adds to the overall capacity of the battery electrode. Conjugated P3HT has been extensively studied for various solid state applications (e.g., photovoltaic cells, field-effect transistors, and light-emitting diodes), where the performance is strongly affected by regioregularity and molecular weight. However, in electrochemical systems such as in batteries, the effects of regioregularity and molecular weight on the charge storage performance are not understood for P3HT-b-PEO. Here, by comparing different P3HT-b-PEO block copolymers of varying P3HT regioregularity (86-97%) and molecular weight (8-19 kg mol -1 ), we demonstrate a strong correlation between regioregularity and molecular weight with electrochemical properties (i.e., capacity and redox potential). We show that the charge storage capacity of P3HT-b-PEO significantly increases with increasing regioregularity. The changes in capacity and redox potential are attributed to P3HT's backbone conformation, planarity, and chain packing. This points to the importance of these two parameters in the design of simultaneous electron- and ion-conducting polymer binders for battery electrodes.