Cathodes of current lithium-ion batteries (LIB) consist of the electrochemically-active material, conductive additives, and binders that are designed to be chemically and mechanically durable during lithium intercalation and deintercalation. By utilizing conjugated polymers as binders, we can add electrical conductivity to this list of properties, allowing for reduced resistive losses and faster charging. We specifically show that dihexyl-substituted poly(3-4-propylenedioxy-thiophene) (PPrODOT-Hx2) can serve as a binder at relevant electrochemical potentials. Combined with conventional cathode materials, the addition of PPrODOT-Hx2 has shown improved rate capability indicative of enhanced electronic and ionic conductivities. Additionally, a family of synthetically tunable random copolymers of PPrODOT-Hx2 with varying extents of oligoether (OE) side-chains are studied to achieve even higher ionic conductivities. To better understand the improved conductivities, it is important to understand where the dopant counterion sits in the polymer film. To examine structural change upon electrochemical doping, we utilize grazing incidence wide-angle x-ray scattering. Here, we observe that the peak corresponding to the lamellar distance shift to smaller q, while the peak corresponding to the pi-pi stacking distance shifts to larger q. These results are consistent with a rearrangement of the unit cell to accommodate the intercalation of the anion between the side-chains of the polymers. With no or low amounts of OE side-chains in the polymer, we see a higher degree of crystallinity upon doping, which leads to a higher electronic conductivity. The higher OE content polymers leads to poor crystallinity in the doped state and lower electronic conductivity. On the other hand, OE side-chains are well-known to be Li<sup>+</sup> ion conductors in a disordered form. Indeed, we find that polymers with higher OE fractions and more disorder show higher ionic conductivity. We also observe that there is almost a linear increase in swelling of polymer by the electrolyte with increasing OE content, up to a 2x mass increase in the polymers with the highest OE content. This suggests that the interplay between intrinsic disorder and electrolyte uptake are important for tuning the balance between electronic and ionic conductivity in conjugated polymer binders.

Investigating the structural changes of semiconducting polymer binders for energy storage to understand electronic and ionic mobilities

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.