Evaluating the Impact of Conjugation Break Spacer Incorporation in Poly(3,4-propylenedioxythiophene)-Based Cathode Binders for Lithium-Ion Batteries

The driving force behind the significant advancement of conductive polymer binders for lithium-ion batteries (LIBs) stems from the poor binding strength, limited mechanical properties, and absence of electronic conductivity of the commonly used nonconjugated polymer binder, poly(vinylidene fluoride) (PVDF). With a goal to induce stretchability and deformability to the otherwise brittle conjugated backbone, we report here dihexyl-substituted poly(3,4-propylenedioxythiophene)-based (PProDOT-Hx2-based) conjugated polymers wherein conjugation break spacers (CBS, T-X-T) of varying alkyl spacer lengths (X = 6, 8, and 10) and varying contents (5, 10, and 20%) have been randomly incorporated into the PProDOT backbone, generating a family of nine random PProDOT-CBS copolymers. Electrochemical characterization revealed that three out of the nine PProDOT-CBS polymers (5% T-6-T, 5% T-8-T, and 10% T-6-T) are electrochemically stable over long-term cycling of 100 cycles. The electronic conductivity of the PProDOT-CBS polymers is consistent with previous literature reports on CBS polymers, where a decline in charge carrier mobility is observed with an increase in CBS content and spacer length, although no significant difference in ionic conductivity in these polymers was observed. This is supported by GIWAXS studies indicating a decrease in lamellar peak intensity with increasing CBS content and spacer length. Mechanical properties of the three selected PProDOT-CBS polymers were investigated by using the established “film-on-water” technique and a novel “film-on-solvent” technique that we report here for the first time, where the solvent used is the same as employed in the battery electrolyte. Both techniques showcase a generally lower tensile modulus (E) and higher crack onset strain (COS) of the PProDOT-CBS polymers relative to fully conjugated PProDOT-Hx2. Furthermore, significant enhancement in mechanical properties is observed with the “film-on-solvent” method, suggesting that electrolyte-induced swelling has a plasticizing effect on the polymers, accounting for their increased stretchability and deformability. Finally, cell testing of the PProDOT-CBS polymers with NCA cathodes aligned well with the electrochemical and mechanical studies, where a higher crack onset strain was crucial for higher capacity retention during long-term cycling. On the contrary, rate capability measurements proved that higher electronic conductivity is favored over mechanical properties during high rates of discharge. This work illustrates that the strategic introduction of CBS units into conjugated polymer binders is a viable method for the generation of stretchable conductive polymer binders for emerging high-capacity electrodes in LIBs.